Bow design for operation in brash ice1145353/FULLTEXT01.pdf · Masterofsciencethesis DepartmentofAeronauticalandVehicleEngineering Bow design for operation in brash ice Kjell Teepen

Master of science thesisDepartment of Aeronautical and Vehicle Engineering

Bow design for operation in brash ice

Kjell Teepen

2nd July 2017

Abstract

Using a vessel for public transport can possibly save large amounts of time in a city asStockholm. The transport is easy during the period of the year where there is no icecover on the waters, however during the time when there is ice, the vessels used facemore extreme conditions. Swedish Steel Yachts (SSY) now wants to have a design fortheir “Shuttle Ferry Concept” intended for operation all year round.

SSY has developed a special way of designing a ship’s hull structure, using thisdesign together with the super duplex stainless steel alloy, SAF2507, SSY hopes to re-volutionize the ship building industry. The aim of this thesis is to deliver a bow designthat is able to combine operation in brash ice with good performance in open waterusing the special SSY design together with the super duplex stainless steel.

This thesis presents to you basic knowledge regarding operation in ice, ice theory,the SSY design concept more in detail and finally a design development of a suitablestructure.

The results from the thesis are shown, mainly as preferred outer geometry andexpected load cases from the ice in the operational area.

Sammanfattning

Genom att använda en båt som kommunalt färdmedel kan man troligen spara storamängder tid i en stad som Stockholm. Transporten sker enkelt på tiden av året då detej finns ett istäcke på vattnet, dock, under tiden det finns is, upplever båtar mycket ex-trema förhållanden. Swedish Steel Yachts (SSY) vill nu ha en design för deras "ShuttleFerry Concept" ämnad för bruk året runt.

SSY har utvecklat ett speciellt sätt att designa deras skrovkonstruktion, genomatt använda denna design, tillsammans med ett super duplext rostfritt stål (SAF2507)hoppas SSY revolutionera sjöfartsbranschen. Målet med detta examensarbete är attleverera en design på ett förskepp som kan kombinera drift i isförhållanden med braegenskaper för öppet vatten, detta skall uppnås genom användning av SSY’s specielladesign tillsammans med det super duplexa rostfria stålet.

Detta examensarbete presenterar grundläggande kunskaper om drift i isförhål-landen, isteori, SSY’s designkoncept mer i detalj och till sist ett designförlopp av fören.

Resultaten från examensarbetet är presenterade genom redovisning av föredragenyttergeometri samt väntade lastfall från driftsområdet.

Contents

1 Glossary and abbreviation 1

2 Introduction 22.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1.1 SSY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.3 Design of the vessel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3.1 The SSY design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4 Operating in ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.4.1 Brash ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.4.2 Grease ice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.5 Area of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.5.1 Ice thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.6 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Goals and structure of this thesis 113.1 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1.1 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2 Structure of this thesis - the logical way . . . . . . . . . . . . . . . . . . 12

4 Limitations 13

5 Operation in ice 145.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.1.1 Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.1.2 Hull shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

5.2 Ice class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.2.1 FSICR Notations . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.2.2 DNV GL Notations . . . . . . . . . . . . . . . . . . . . . . . . . . 185.2.3 Sets of ice class rules . . . . . . . . . . . . . . . . . . . . . . . . . 185.2.4 How to classify a SSY design? . . . . . . . . . . . . . . . . . . . . 195.2.5 Strengthening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.2.6 Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5.3 Loads and resistances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

CONTENTS

6 Ice Theory 226.1 Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

6.1.1 Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226.1.2 Loads due to brash ice . . . . . . . . . . . . . . . . . . . . . . . . 226.1.3 Loads due to ice breaking . . . . . . . . . . . . . . . . . . . . . . 256.1.4 Design Loads from DNV-GL . . . . . . . . . . . . . . . . . . . . . 326.1.5 Jordaan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 386.1.6 Masterson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 406.1.7 Suyuthi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416.1.8 Su B., Riska K., Moan T . . . . . . . . . . . . . . . . . . . . . . . 416.1.9 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

6.2 Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 436.2.1 Brash ice resistance from DNV-GL . . . . . . . . . . . . . . . . . 446.2.2 Myland & Ehlers (Lindqvists) formulas . . . . . . . . . . . . . . . 456.2.3 Riska a.o.; Level ice resistance . . . . . . . . . . . . . . . . . . . . 506.2.4 Open water resistance . . . . . . . . . . . . . . . . . . . . . . . . 54

6.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

7 SSY design 587.1 Hull . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

7.1.1 Conventional hull structure . . . . . . . . . . . . . . . . . . . . . 587.1.2 SSY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597.1.3 Plate thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . 597.1.4 Stringer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7.2 Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617.3 Evaluation of current design . . . . . . . . . . . . . . . . . . . . . . . . . 62

8 Design 658.1 Concept design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

8.1.1 Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 658.1.2 Shuttle Ferry concept . . . . . . . . . . . . . . . . . . . . . . . . . 668.1.3 Modern ice breaker . . . . . . . . . . . . . . . . . . . . . . . . . . 678.1.4 Combination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 688.1.5 Combination with bulbous . . . . . . . . . . . . . . . . . . . . . . 698.1.6 Concept design with other angle . . . . . . . . . . . . . . . . . . . 708.1.7 Evaluation matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

8.2 Basic design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 728.2.1 Stringer design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 738.2.2 Web frame design . . . . . . . . . . . . . . . . . . . . . . . . . . . 738.2.3 Shuttle Ferry Concept . . . . . . . . . . . . . . . . . . . . . . . . 74

8.3 Detailed design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758.3.1 Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758.3.2 Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 838.3.3 SSY concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 868.3.4 Thickness analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 1018.3.5 Conventional design . . . . . . . . . . . . . . . . . . . . . . . . . 104

CONTENTS

8.3.6 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1128.4 Final design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

9 Results and discussion 1179.1 Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1179.2 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

9.2.1 Design parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 1189.2.2 Flexibility of the entire structure . . . . . . . . . . . . . . . . . . 1199.2.3 Fairness of comparison . . . . . . . . . . . . . . . . . . . . . . . . 120

9.3 Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1209.3.1 Load Case 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1219.3.2 Load Case 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1219.3.3 Load Case 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

10 Conclusions 12310.1 Ice theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12310.2 Bow geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12310.3 SSY vs. Conventional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

10.3.1 Stress concentrations . . . . . . . . . . . . . . . . . . . . . . . . . 12410.3.2 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

10.4 Further work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

References 126

Chapter 1

Glossary and abbreviation

Abbreviation Long form∆ Displacement (m3)CFD Computational Fluid Dynamics (Flow simulation software)DNV GL De Norske Veritas Germanischer LloydSAF Sandvik Austenitic-Ferritic (Material name)SSY Swedish Steel YachtsStem The stem is the most forward part of a boat or ship’s bow and is an

extension of the keel itself.Flare Flare is the angle at which a ship’s hull plate or planking departs

from the vertical in an outward direction with increasing height.Buttock A buttock line is a curve indicating the shape of an air foil or naut-

ical equivalent in a vertical plane parallel to the longitudinal axis ofthe craft or vessel.

Stern The stern is the back or aft-most part of a ship.Ice horn The ice horn is a triangular ship constructional part at the stern of

a ship. It serves the helm of the ship to protect against unbrokenand broken ice, especially when driving astern.

Ridge A ridge develops in an ice cover as a result of a stress regime estab-lished within the plane of the ice. A ridge is an extra strong part ofthe ice cover, creating higher loads and increasing the possibility aship gets stuck.

FSICR Finnish-Swedish Ice Class Rules

1

Chapter 2

Introduction

This thesis is an investigation regarding the special SSY way of designing a vessel, us-ing the SAF2507 super duplex stainless steel together with a different way of designingthe load transfer, this technique will now be investigated for the usage in an harsherenvironment. In the Stockholm area a shuttle ferry concept is supposed to operateall year round. This challenges the vessel a lot, it will encounter ice loads during thewinter and at the same time it should be able to perform well during the ice-free periodof the year. A few different geometries are examined and listed with pros and cons.From that, the most suitable one is selected and a structural arrangement is given. Itis then investigated for the different load cases a vessel will encounter in this environ-ment. This leads to a final design which is compared to the traditional way of design-ing a hull, with the traditional technique and steel. From this pros and cons for thedifferent designs are discussed.

2.1 BackgroundThis thesis is done as a request from Swedish Steel Yachts, (SSY) in Gävle, Sweden.The author has been working with guidance from KTH, SSY and Cervino Consulting.

2.1.1 SSYSSY has since the start in 2011 developed a different way of designing ships. This newthinking together with a special kind of steel has up to this moment resulted in one fullscale prototype and a number of ship designs (11.5 – 17.1 m, with concepts up to 50m). The existing prototype has been tested successfully for operation in ice (20 knotsin 4-5 cm), although it does not have a design specifically for ice conditions. SSY nowwants to develop a design of a bow that is capable of operating in brash ice, for givenpreconditions.

2.2 ProblemThe assignment is to create a design of a bow for a ship intended for transporting pas-sengers. Namely the SSY concept of a Shuttle Ferry (1601_1-Sea Shuttle 25) a 25 m

2

CHAPTER 2. INTRODUCTION 3

long vessel with the purpose of acting as a shuttle ferry in Stockholm city and the innerarchipelago. The bow should be designed to be able to operate in brash ice. The bowshould therefore be able to withstand the loads exposed to it when operating in thegiven area and thereby fulfil the criteria stated in Table 2.4. Ice classification for thevessel is hard to perform due to the special design and because the DNV ice regulationsare primarily meant for ice breakers which are much bigger vessels, it will however stillbe presented.

The bow is designed to be able to be integrated into the existing method of hulldesigns by SSY. This structure should be built from a special kind of stainless steel,SAF 2507. The steel used for the SSY technique of designing a hull is both in weightand price comparable to a traditional aluminium hull (the steel weighs more and ismore expensive, but a lesser amount of material, and thereby money is needed to achievethe same weight as the aluminium hull). And again compared to an aluminium hull,the SAF2507 one has much better properties in corrosion, fatigue, strength and hard-ness according to Sandvik [16] and SSY.

The study is done by comparing different geometries and picking the most suit-able for the purpose, later this geometry is given a structure and then the entire struc-ture is evaluated against the same geometry using a traditional structure and the sameamount of material.

2.2.1 PurposeThe purpose of this thesis is to examine how the SSY technique performs in brash icein terms of ice loads, from that a suitable design is suggested.

2.3 Design of the vesselThe initial design provided by SSY of the Shuttle Ferry Concept is shown in Figure 2.1.


Figure 2.1: Shuttle Ferry Concept

The ship characteristics are shown in Table 2.1.

Table 2.1: Ship characteristicsParameter Symbol Value UnitLength between perpendiculars Lpp 24.8 mWaterline length LWL 24 mBeam B 6.75 mDraught (high) D 1.23 mLoaded displacement (LWT + DWT) M 64 000 kgBlock coefficient Cb 0.31

The vessel is equipped with 2 x 590 kW machinery.

2.3.1 The SSY designSpecial kind of steel

The SSY boats are built from a special kind of steel, the stainless steel alloy SAF 2507has been developed by metallurgists at Sandvik AB. The alloy is extremely resistant to


corrosion (which can be caused by salt-water, which in turn causes fracture of a mater-ial). All metal under the waterline is mirror polished.

The steel hull retains its mechanical properties despite tough conditions over along period of time. This is a major advantage compared with aluminium boats, whichmay deteriorate over a number of years losing mechanical properties and are therebyprone to fatigue [16], [21]. Plastic and carbon fiber vessels don’t have this problem,however they are non-recyclable and are prone to cracking and delaminating upon im-pact.

Table 2.2: SAF 2507 properties [16]Parameter Symbol Value UnitDensity ρ 7800 kg/m3

Proof strength Rp0.2 ≥550 MPaRp1.0 ≥ 640 MPa

Tensile strength Rm 800 - 1000 MPa

2.4 Operating in iceNot all ships are built to an ice class. Building a ship to an ice class means that thehull must be thicker, and more scantlings must be in place. Most of the higher classesrequire several forms of rudder and propeller protection. Two rudder pintles are usuallyrequired, and strengthened propeller tips are often required in the higher ice classes.More watertight bulkheads, in addition to those required by a ship’s normal class, areusually required. In addition, heating arrangements for fuel tanks, ballast tanks, andother tanks vital to the ship’s operation may also be required depending on the class[13].

2.4.1 Brash iceThe definition of Brash ice is, "Accumulations of floating ice made up of fragments notmore than 2 meters across, the wreckage of other forms of ice." [3]

Waterways may often become covered and congested with relatively small-size icerubble, called brash ice. Of particular concern to operators of ships with limited ice-breaking capability is that, in frequently transited waterways and navigation channels,relatively thick accumulations of brash ice may develop and greatly hinder the transitof ships. It is not uncommon for a layer of brash ice, accumulated in a frequently nav-igated ice-bound channel, to be several times thicker than a level ice sheet borderingthe channel [22]. A cross-section through a brash ice channel is shown schematically inFig. 2.2


Figure 2.2: Brash ice

2.4.2 Grease iceThe definition of Grease ice is, "Ice at that stage of freezing when the crystals have co-agulated to form a soupy layer on the surface. Grease ice is at a later stage of freezingthan frazil ice (a collection of loose, randomly oriented needle-shaped ice crystals in wa-ter) and reflects little light, giving the sea a matte appearance." [3]

Figure 2.3: Grease ice with some pancake ice

Grease ice is assumed to be the closest definition of the ice that will freeze overnight which the vessel has to be able to break in the morning.

2.5 Area of operationThe area of operation is set to the Stockholm city area and the inner archipelago, Fig-ure 2.4 shows a map of these areas.


Figure 2.4: Area of operation

2.5.1 Ice thicknessProperties of ice

In the table below a number of properties for ice is shown, those can vary a lot depend-ing on for example salinity and temperature, the ones from below are taken from anexperiment in the Baltic sea and will be used throughout the report.

Table 2.3: Ice properties from the Baltic sea [19]Parameter Symbol Value UnitDensity ρ 880 kg/m3

Young’s modulus E 5400 MPaPoisson ratio γ 0.33 -Crushing strength σc 2.30 MPaFlexural Strength σf 0.55 MPaFrictional coefficient µi 0.15 -

Brash ice

From the graphs in Figure 2.5 on the ice thickness in the Stockholm area an estima-tion of the ice floe sizes the vessel will interact with can be made. From the followingreport, [11] the largest possible ice floe in the Stockholm area has been modelled as acylinder with the diameter 1.79 m and a height of 0.21 m. This gives a volume of 0.52m3 and a weight of 477 kg.


Figure 2.5: Ice thickness in the Stockholm area [11]

Grease ice

The grease ice is the ice that has frozen over night and thereby the ice the vessel willhave to break in the morning. From Figure 2.6 it can be seen, that ranging from -20 ◦Fto 30 ◦F (-28 ◦C to -1 ◦C) the ice will grow between 0.1 and 2.55 inches or 0.3 cm to6.5 cm over a period of 12 hours. From this the maximum ice thickness in the morningis set to 7 cm. This value is assumed to be a good guess for the maximum encounteredthickness that will grow during the night, using the extreme value of almost -30 ◦C av-erage during one night is assumed to be a good guess.


Figure 2.6: Overnight ice thickness [8]

Time span during the year with ice coverage

Based on available weather and ice data [18] the worst possible winter conditions fora ferry in Stockholm could include 3-4 months of ice with a maximum thickness of 0.4m. An ice thickness of 0.4 is rare and requires a really severe winter. A severe winterrequires the ferries to run in brash ice channels which eventually get clogged with brashice requiring new channels to be broken. This means that the ferries have to be ableto break relatively thick ice occasionally. However, some winters there is hardly anyice [18] and it is questionable if a ferry should be dimensioned to the most extreme con-ditions that rarely occur. In this thesis the Ferry is assumed to only operate in brashice channels and a thin ice layer frozen over night.

2.6 RequirementsA set of requirements are stated in the table below to show what requirements willhave to be met for the final design,


Table 2.4: RequirementsParameter Symbol Value UnitDesign speed v 20 knotsBrash ice speed vbi 10 knotsGrease ice speed vgi 10 knotsMaximum brash ice floe size Vbi 0.33 m3

mbi 300 kgMaximum grease ice thickness hgi 0.05 m

The design speed of 20 knots is taken from the concept description given by SSY.The brash ice and grease ice speed is set somewhat lower then the design speed at 10knots. For higher velocities the resistance increases a lot which makes it more environ-mental friendly to operate at lower velocities. The loads will also increase a lot if thespeed is to high.

The maximum brash ice floe size and the maximum grease ice thickness are takenfrom section 2.5.1, the dimensions given there are in extreme conditions and will prob-ably never be encountered by this vessel, this in combination with the lesser structuralweight which is favourable for non-ice conditions allows for a decrease in size to theones stated in Table 2.4.

Chapter 3

Goals and structure of this thesis

3.1 GoalsThe goal for this thesis is to give SSY a foundation to a design philosophy on what tothink of when designing a vessel with the purpose of operating in the given area of op-eration. At the same time this basic philosophy is used to propose a bow design to SSYsuitable for operation in the given area. Also the pros and cons for using the SSY tech-nique together with the super duplex stainless steel (SAF2507) are delivered compar-ing this with the effects of using a traditional hull structure with a traditional material(structural steel).

To make the results credible the following questions will have to be answered,

• What is a typical bow shape for a vessel operating in ice?

• Which design rules apply when operating in ice and how can they be used on theSSY design concept?

• Which load cases are to be expected in the area of operation, and how does thehull absorb those loads?

• Is it possible to tell how much the resistance is increasing due to the passage inice?

• How does the SSY distinguish itself from a traditional design?

• What are the pros and cons for the SSY design concept?

• What are the pros and cons for a traditional design?

• How is the SSY design concept affected by the use of the special steel the com-pany is using?

• How is a traditional design affected if it would be built using the same kind ofsteel as SSY is using?

• Does the SSY design have any advantages compared to a traditional design whenoperating in ice?

11

CHAPTER 3. GOALS AND STRUCTURE OF THIS THESIS 12

• Does the special steel have any advantages compared to a traditional steel whenoperating in ice?

3.1.1 ResultsThe results of the thesis are presented in form of numbers in showing maximum stressesencountered and maximum deformation of the structure. The final recommended designis also presented, along with FEM pictures on the stresses and deformations.

3.2 Structure of this thesis - the logical wayThe structure of this thesis after this chapter is as follows, the chapters are put in alogical way which makes it easy for the reader to follow, the list of questions from theprevious section will be answered in a chronological order as the chapters go on.

• Chapter 4: Limitations of this thesis, what will be excluded from the investiga-tion

• Chapter 5: Operation in ice, some basic knowledge regarding the special designof a ship intended for operation in ice and classification rules is presented. Thisgives the reader a theoretical background which is later used in the design Chapter.

• Chapter 6: Ice theory, Loads and resistances, what loads and resistances are ex-pected? Theory from few different sources is presented and discussed upon. Thisgives the reader a theoretical background which is later used in the design Chapter.

• Chapter 7: SSY design, what is special with the SSY design? How does the designdiffer from a conventional design? What is the theory behind the design? Anevaluation of the current design is performed and discussed upon.

• Chapter 8: Design, here will the design process be presented, starting with a Con-ceptual design then a Basic design followed by a Detailed design and finally a Fi-nal design. The theory discussed in Chapter 5,6 and 7 is used as foundation.

• Chapter 9: Results and discussion, here the results are presented, including theFinal design of the bow. A discussion on the results will also be performed.

• Chapter 10: Conclusions, the final chapter will show what conclusion the authorcan draw from this thesis.

• Bibliography: A list of sources

Chapter 4

Limitations

In this project the purpose is to develop a bow design and perform calculations on it.It is thereafter highly recommended to perform an evaluation on the entire vessel usingthe bow design as input to gather results on the overall performance of the vessel.

The focus of the project is to deliver a technical report of a design applicable forthe certain case stated above. Due to the limited amount of time of the project theseareas will need further investigation,

• DesignIs it enough to only have the bow modified or does the ice change the load casesfor the rest of the ship as well?

• Fatigue due to iceDue to the fact that the ice will cover all of the operational area fatigue is a bigpart of this. The vessel will constantly encounter different loads from both thegrease ice and the brash ice were the loads can differ quite a lot.Due to the high quality of the steel fatigue is however not considered an issue.Critical for fatigue might be for example the weld joints.

• StabilityThe stability of the vessel should also be taken in to consideration, from the spe-cifications of the vessel it can be seen that the current draught is only 1.23 m. Ifthe vessel is lifted up from the brash ice it might get a even lower draught andthe roll stability might become an issue.

13

Chapter 5

Operation in ice

At first a pre-study is performed to get to know the subject. This chapter gives inform-ation on how a vessel is designed for operation in ice, what classification rules exist andhow the rules suggest vessels should be strengthened for operation in ice.

In this chapter the first two bullets from the list of goals are answered.

The performance of a merchant vessel in ice is determined by its ability to pro-ceed forward in ice, an ability which usually is measured with travel times through ice-covered areas and the energy consumed in making the transits. Good performance inice is characterized by low ice resistance, high propulsion efficiency and power, result-ing in high thrust and also experience of the crew in manoeuvring the ship through ice.Good ice performance means also that the ship should not get stuck in ice.

The requirement of good ice performance leads to hull shapes that are not op-timal in open water. Especially the seakeeping characteristics may suffer. Further theincreased machinery power and thus price and weight of the machinery together withhigher fuel consumption makes the ice-going ship somewhat less economical in openwater. [14]

5.1 DesignFor a ship to be considered an icebreaker, it requires the three attributes most shipslack: a strengthened hull, an ice-clearing shape and the power to push through sea ice.

5.1.1 SpeedThe thicker the ice, the lower the speed of the vessel. Ice breakers are designed for onepurpose, breaking the ice. Speed does not matter that much, when the velocities aregreater the ice sheet is much thinner.

5.1.2 Hull shapeThe bow shape is not as streamlined as a regular vessel, this is because the purposeis to break ice as efficient as possible. The bow shapes for icebreakers may be described

14

CHAPTER 5. OPERATION IN ICE 15

by the stem, flare, buttock, and water-line angles. These angles contribute to the icebreak-ing, submergence, and clearing efficiency. Recent trends in the design of icebreakers areto increase flare angles, to reduce water-line angles, and to reduce stem and buttockangles [14].

The selection of a midbody shape must consider its effect on resistance, manoeuv-rability, construction cost, and the required deadweight. The midbody may be charac-terized by a flare angle (over the full depth or locally), a parallel midbody, and a lon-gitudinal taper.

The stern design on icebreaking ships is controlled mainly by the number of pro-pellers, which is a function of the required power and operational requirements. Thestern must, to the greatest extent possible, provide protection to the rudder(s) and pro-peller(s). To provide this protection, a number of design options can be selected. Theconventional stern, typical of Canadian Coast Guard icebreakers, is rounded to providegood icebreaking astern performance, and is usually fitted with an ice horn to protectthe rudder. A transom, or ramped, stern is installed on several icebreakers. The ob-jective of this stern is to allow the broken ice pieces to move upward to the surface wellahead of the propeller(s).

The hull shape design of ice breaking ships aims at,

• Minimizing the ice resistance by selecting optimal beam and bow shape;

• Ensuring good manoeuvring characteristics;

• Enabling the ship to go astern as much and as well as the operational descriptionrequires

• Ensuring a proper undisturbed operation of the propeller(s) by minimizing theamount of ice impacting on the propeller(s).

The most important parameters for ice resistance are the beam B and the stem angleφ1, which is the angle between the stem and the waterline as displayed in Figure 5.1.A large beam causes more resistance and thus narrower ships with a large L/B ratio isthe result. For an icebreaker a small beam is not, however, good as the escorted shipsshould get as wide of a channel as possible. Typical beams of the largest icebreakerat present are about 26 m. A smaller stem angle induces larger bending forces whilekeeping the horizontal force component smaller, thus ice breaking ships have quitesmall stem angles, 20◦ to 25◦ is common. Nowadays also the stem is rounded as thisdecreases the crushing at the stem.


Figure 5.1: Definition of stem angle

Figure 5.2: Modern icebreaker design

From the hull shape perspective, the stern shoulder area is crucial for good man-oeuvring characteristics. If the stern shoulders break ice in bending, the ship turns bet-ter as the resisting force for turning this way is minimized. The performance astern isimportant if the ship has to navigate independently. When encountering ridges, theships often gets stuck and in order to be able to proceed, the ship must be able to re-verse and ram again. Good reversing performance is reached by avoiding blunt lines atthe stern. Many merchant ships that are only ice strengthened need not to go astern inice but can count on icebreaker escort in heavier ice conditions. In this case the designof the stern shape is less important.

The bow shape should be such that it allows the ice floes to float towards the sur-face before getting under the bottom. One way to do this is to make a bow plough.When the ice floes follow the buttock lines, they hit the bow plough that pushes thefloes aside. There is an ice thickness limit up to which the bow plough is efficient, thickerice will go under the bottom. The disadvantage of the bow plough is that it increasesthe ice resistance somewhat and also the open water resistance, in ships that are re-quired to do heavy ice breaking the advantages outrank the disadvantages of a bow


plough [13].

Bulbous bow

Present experience shows that most merchant ships need not to break ice as they eithersail in broken channels or follow an icebreaker. Thus ice strengthened ships often havebulbous bow which is not a handicap in broken ice. The reason for this is that brokenice is displaced around the hull in a way that resembles the hydrodynamic flow. Onlyin ice going ships and icebreakers which must break the ice themselves the bulbous bowis not appropriate. By shaping the bulbous bow for ice, much of the additional ice res-istance can be avoided [17].

5.2 Ice classThe ice class determines the ice conditions of which the vessel is approved to operate inaccording to rules. A vessel with an ice class has a sufficiently strong hull, dependingon ice class, suitable hull shape, strong enough engine and technical solutions that fitthe purpose of the vessel. When operating in ice the hull experiences higher loads thenusual which are concentrated to a certain area. The hull therefore needs reinforcementswhere the ice hits the vessel.

There are different ice class denotations depending on which classification societiesor maritime authority assigns them.

5.2.1 FSICR NotationsThe following are the ones from Sweden/Finland, FSICR (Finnish-Swedish Ice ClassRules) [2],

Table 5.1: Ice classes in the BalticFSICR Ice thicknessIA Super > 100 cmIA > 50 cmIB 30 - 50 cmIC 15 - 30 cmII (not strengthened) 10 - 15 cm

IA Super

Ships with such structure, engine output and other properties that are normally cap-able of navigating in difficult ice conditions without the assistance of icebreakers.

IA

Ships with such structure, engine output and other properties that are capable of nav-igating in difficult ice conditions, with the assistance of icebreakers when necessary.


IB

The same as above for ice class IA.

IC

The same as above for ice class IA.

II

Ships that have a steel hull and that are structurally fit for navigation in the open sea,and that, despite not being strengthened for navigation in ice, are capable of navigatingin very light ice conditions with their own propulsion machinery.

III

Ships that do not belong to the ice classes referred to in the sections above.

5.2.2 DNV GL NotationsDNV GL has formed number of different notations for vessels operating in ice [1], someof them are listed below and for some of them the equivalent FSICR notation is givenfor comparison. For a full explanation, the DNV GL guidelines regarding ships fornavigation in ice should be read but basically they provide a full handbook on how todesign a structure for a vessel for each notation.

• ICE-1A*F

• ICE-1A* (FSICR, 1A Super)

• ICE-1A (FSICR, 1A)

• ICE-1B (FSICR, 1B)

• ICE-1C (FSICR, 1C)

• ICE-C

• ICE-E (FSICR, Intended for light localised drift ice in mouths of rivers and) coastalareas.

As discussed before the DNV GL guidelines are written for building a conven-tional vessel, see section 7.1.1 this makes it hard to apply those rules to the SSY design,however, for example a design load could probably be obtained.

5.2.3 Sets of ice class rulesAt present there are a few main sets of ice class rules: the Finnish-Swedish Ice ClassRules (FSICR), DNV, the Russian Maritime Register of Shipping (RMRS) ice rulesand the unified Polar Class (PC) rules of the International Association of ClassificationSocieties (IASC).


5.2.4 How to classify a SSY design?Classification of the SSY vessels is quite hard because the structural arrangement isquite different. In this case a classification is not of interest, however some input fromthe current set of ice class rules can be used.

5.2.5 StrengtheningTraditional ice strengthening of ships is done by adding plating and ordinary stiffenersand primary supporting members. Depending on ice class designed for the extension ofthe ice strengthened area is defined as "x m above LWL (Load Water Line)" and "x mbelow BWL (Ballast Water Line)". The fore foot is the area below the ice strengthenedarea extending from the stem to a position five ordinary stiffeners spaces aft of thepoint where the bow profile departs from the keel line. The upper fore is the area ex-tending from the upper limit of the ice strengthened area to 2 m above and from thestem to a position at least 0,2 L aft of the forward perpendicular. [15]

Figure 5.3: Strengthening

The hull is usually divided into three parts,

• The Bow Area: From the stem to a line parallel with and 0.04 ·L and behindthe front border line for the part of the hull where the water line is parallel withthe centre line.

• The Midship Area: From the rear of the bow to a line parallel with and 0.04 ·Lrear of the stern border line for the part of the hull where the water line is paral-lel with the centre line.

• The Stern Area: From the midship rear border to the most aft part of thestern. [2]


The strengthening for this vessel is as follows (Note that the UIWL and LIWL are putin approximately due to the lack of information on this, the designed draught is how-ever 1.23 m which is almost in the middle of the total height of the vessel),

Figure 5.4: Ice strengthening for the concept vessel

The geometries created in the design part are 8 m long which gives an area ofinvestigation as seen in Figure 5.4. The vertical extension of the ice belt for the low-est ice class is shown in Table 5.2. Having a normal draught of 1.23 m, the verticalextension covers a major part of the bow. From a manufacturing point of view it ismost convenient to have the strengthening over the entire bow, which is chosen for thedesign created.

Table 5.2: Vertical extension of ice beltIce class Region Above UIWL (m) Below LIWL (m)

Bow 0.70ICE 1C Midbody 0.40 0.60

Stern

Discussion

Even though the vessel is not supposed to be ice classed it would be recommended touse some kind of strengthening of the bow to start with. If FEM-investigations latershow that this is not necessary the plate thickness can always be reduced. Because theentire bow would need to be strengthened for a vessel of this size it does not matter ifthe FEM-analysis starts with a slightly thicker bow plating than perhaps is necessary.

The framing of the vessel is also supposed to be strengthened according to theDNV-GL guidelines, however, because of the special conditions the SSY-design (seeChapter 8) provides us this is not necessary.

5.2.6 PerformanceThe most common way of showing a vessels performance in ice is to create a so calledh-v-plot. The plot shows the thickness of ice the vessel can break when operating at a


certain speed.

Turning performance

Turning performance in ice is measured by the diameter of the turning circle (dividedby the ship length).

5.3 Loads and resistancesWhen a vessel operates in ice it will encounter loads and resistances from the ice withlargely varying magnitudes, these magnitudes depend on a lot of different parameters.We have many different types of ice, salinity levels, temperatures, angles of impact, andtypes of impact. All this will be discussed in the next chapter.

Chapter 6

Ice Theory

6.1 Loads

6.1.1 StrategyIn this chapter the third and the fourth bullet from the list of goals are answered. Fromthe theory part it is expected to examine theory regarding how the loads for the ShuttleFerry Concept will be calculated, that will be encountered when travelling through abrash ice channel in the area of operation. These loads are then applied to the designcreated in Chapter 8 to see that the requirements are fulfilled.

Three load cases are assumed to be relevant to investigate considering the purposeof the vessels operational criteria,

• Breaking of thin ice: When the ferry starts its working day in the morning it willhave to break some ice that has frozen during the night.

• Brash ice: Due to the fact that the vessel will operate as a shuttle it will travelbetween two locations during the day. The route will be through an broken icechannel with brash ice in it.

• DNV GL ice load: A design load given by DNV GL from the guidelines for iceoperation. [1]

6.1.2 Loads due to brash iceOne definition of brash ice is, "Accumulations of floating ice made up of fragments notmore than 2 meters across, the wreckage of other forms of ice" [3]. This means thatthe vessel will travel through a channel of broken ice where smaller pieces of ice aregathered. The vessel will hit the pieces of ice at a certain speed encountering differentload magnitudes. For a certain area of operation these differences could be representedby a distribution on the different sizes of pieces of ice and thereby the correspondingloads.

22

CHAPTER 6. ICE THEORY 23

Weibull distribution (3P)

When operating in brash ice the loads experienced can vary a lot in size. One way torepresent the different loads that the bow is exposed to is to use a probabilistic dis-tribution of the magnitude of the loads. This can be represented by for example a 3parameter Weibull distribution as shown in Figure 6.1. From the graph the distribu-tion between the different sizes of the ice floes is illustrated. The majority of the floesare really small with some exceptions, the maximum floe size stated in Table 2.4 wouldbe expected to be found far to the right on the x-axis at a very low frequency of occur-rence.

Figure 6.1: Weibull distribution of loads

From a number of test results performed in [12], Equation 6.1 is developed, byusing a best-fit line and fitting it to the top 20 % peak pressures of each distribution.

Fx(x) = 1− exp(−(x− x0)/α) (6.1)

where x0 and α are constants for a given area, and x is a random quantity denot-ing pressure. The parameter α is the inverse slope of the best-fit line, and x0 is theintercept of this line with the abscissa (the number whose absolute value is the per-pendicular distance of a point from the vertical axis), see Figure 6.2, where the x-axisshows the pressure and the y-axis shows the negative log of probability of exceedance[12].


Figure 6.2: How to obtain x0 and α

Another interesting parameter is the pressure per area, the parameter α is a func-tion of area, represented by the curve α = CaD , where a is the local area of interest,and C and D are constants that depend on the physical characteristics of ice.

In [6] the following parameters for the equation are presented,

α = 1.25a−0.7 (6.2)

In Figure 6.3 the equation above is displayed, as can be seen the maximum pres-sure is around 1.75 MPa given by the minimum area tested on (0.6 m2).


Figure 6.3: Pressure over area curve

From the tests performed in the paper [6], the equation seems reasonable for theice properties in that area. Ice is however dependent on a lot of different parameters,so if this would be interesting for the Stockholm area a actual test will have to be per-formed to be able to confirm the equation parameters.

6.1.3 Loads due to ice breakingOne of the scenarios considered is when the vessel operates in ice that has frozen dur-ing the night to a thin layer of solid ice. When the vessel starts its round in the morn-ing with the first passengers it will need to break the thin ice layer.

The ice load is a statistical quantity and thus the design load value must in prin-ciple be determined assuming a probability level or return period of the load. The loadlevel in FSICR is, however, determined based on the experience from the hull damagescaused by ice. A more ambitious approach would be to define a certain risk or safetylevel. The design load value can be given by selecting a return period of occurrence forthe load, like once per lifetime, once per ice season, or once per voyage. The selectedreturn period of the load must be in balance with the consequences of exceeding the al-lowable structural responses. This procedure of defining the design points requires thus,

• Definition of the loads in probabilistic terms

• Definition of the responses in terms of stresses, elastic deflections or permanentdeflections and


• Definition of the limit states in order to ensure a consequent risk implied by thefailure of different structural components (e.g. shell plating and frames).Figure 6.4 shows how a ship usually operates through ice, here can be seen that

smaller contact zones appear due to the breaking pattern of the ice. These contactzones can be defined as so called High Pressure Zones [5].

Figure 6.4: Ship breaking ice

From what can be seen in Figure 6.5 a certain area of the ice (an ice wedge) isbroken given a set of input variables [19]. The ice braking radius can be given by thefollowing formula,

R = Cll(1.0 + Cvvreln ) (6.3)

where vreln is the relative normal velocity between the ice and the hull node, Cland Cv are two empirical parameters obtained from field measurements, and l is thecharacteristic length of the ice:

l =(

Eh3i

12(1− v2)ρwg

)1/4(6.4)

Figure 6.5: Loads from ice on ship


By using Equation 6.3 above and assuming the empirical parameters to be 1, theplot in Figure 6.6 is obtained.

Figure 6.6: Radius of ice wedge for different velocities

The ice is broken by the procedure shown in Figure 6.7 and Figure 6.8 below. Theship forces the ice down at a certain angle defined by the stem of the ship. The shipthereby applies a force on the ice sheet which then gets crushed by the ship (the crush-ing force obtained by the area of contact times the crushing strength of the ice) [19].The force can be divided into a vertical and horizontal component as seen in Figure6.8.




As seen in Figure 6.9, depending on the angle (β) of the ship side the ice willeither fail in shear and bending or only due to shear.

Figure 6.9: Loads from ice on ship [4]

Shear vs. Bending failure

When the angle of the ship side is 0◦ the ice will only fail in shear. The Crushing strength(Compressive strength) of the ice used in this report is around 2.30 MPa.

If the angle of the ship side is larger then 0◦ the ice will fail due to shear and bend-ing. The Flexural strength of the ice used in this report is around 0.55 MPa.

The values are obtained from Table 2.3From this we can see that the Crushing strength is about 4.2 larger then the flex-

ural strength. This means that it is much easier to break the ice when the incline ofthe ship side is greater then 0◦.

Global loads vs. Local loads

Global loads act on the entire "ships girder", the vessel as a whole.


Local loads are experienced by stiffened panels, girders, beams, & stringers.

Loads at the stem The stem is the most forward part of a vessel. The stem willencounter the first hit of the ice and thereby encounter the largest stresses.

Loads at bow The bow is the structural part right behind the stem and will en-counter loads almost in the same way as the stem, from an ice sheet High PressureZones will occur as a strip along the length of the bow from the stem going aft.

Ice bending failure

The following equation is proposed in [19], where Pf is the bending failure force in N.

Pf = Cfθ

π

2σfh

2i (6.5)

where θ is the opening angle (see Figure 6.5) of the idealized ice wedge, σf is the flex-ural strength of the ice, hi is the thickness of the ice, and Cf is an empirical parameterwhich is obtained from measurements. In Figure 6.10 the characteristics of the bend-ing failure of ice is shown for different ice thickness’s and opening angles. It can beseen that when the opening angle and the ice thickness increase the bending failure in-creases even more.

Figure 6.10: Bending failure of ice for different thickness’s

Forces

The equations below show expressions of the forces displayed in Figure 6.8, if the ver-tical component (Equation 6.10) exceeds the bending failure (Equation 6.5) the icesheet will break.


Ice Crushing ForceThe Force seen in Figure 6.7

Fcr = σcAc (6.6)

Frictional forcesThe frictional forces are divided into a vertical and horizontal component.

fH = µiFcrvrelt /

√(vrelt )2 + (vreln,1)2 (6.7)

fV = µiFcrvreln,1/

√(vrelt )2 + (vreln,1)2 (6.8)

Vertical and Horizontal componentsFrom the ice crushing force and the frictional forces a vertical and horizontal compon-ent can be obtained which is visualized in Figure 6.8 and the equations below,

FH = Fcrsin(φ) + fV cos(φ) (6.9)

FV = Fcrcos(φ)− fV sin(φ) (6.10)

Using the reasoning above, comparing the bending failure and the vertical com-ponent of the force and varying the area of contact it can be seen what area of contactis required for a few different parameters (ice thickness and stem angle) tested below.The velocity components are estimated and kept constant (vreln,1 = 3 m/s and vrelt = 5m/s) during the investigation.

In Figure 6.11 the ice thickness and the opening angle are kept constant at 5 cmand 45 ◦ while the plot shows the bending failure for three different stem angles, 15◦,45◦ and 70◦. Because the thickness of the ice is constant Pf only has one line while FVdepends on the varying stem angle and thereby has three lines.


Figure 6.11: Vertical force needed for different stem angles at 5 cm ice thickness

In Figure 6.12 the stem angle and the opening angle are kept constant at 30◦ and45◦ while the plot shows the bending failure for three different ice thickness’s, 3, 5 and10 cm. This implies that FV only is area dependent, while PF has three different linesbecause the thickness of the ice varies.


Figure 6.12: Vertical force needed for different ice thickness’s at 30 ◦ stem angle

Conclusions

From Figure 6.11 it can be seen that when the stem angle increases, the crushing fail-ure takes place at a smaller area of contact, creating a larger load and thereby easierbreaking the ice. In Figure 6.12 the ice thickness is varied showing what area of contactis needed to break the ice.

6.1.4 Design Loads from DNV-GLAs ice loads arise from contact with an ice edge, it is commonly assumed that the loadacts mostly on a load patch (area of non-zero ice pressure) that is narrow in verticaldirection and long in horizontal direction. In case of an impact with multi-year ice ofrounded shape, the load patch can be of more irregular shape. The load patch is ideal-ized as a rectangular patch for structural response calculation of local shell structureslike plating, main frames, stringers and web frames. This idealization is sketched inFigure 6.14.


Figure 6.13: Idealization of loads

Ice pressure is not measured directly, it is always ice force F that is measured on acertain gauge area Ag and then the pressure is deduced as F/Ag.

In the DNV-GL rules given for "Ships for Navigation in ice" [1] a set of designloads is given (Sec. 3, B. Design Loads).

B 100 Height of the ice load area

An ice strengthened ship is assumed to operate in open sea conditions corresponding toa level ice thickness not exceeding h0. The design the ice height (h) of the area actuallyunder ice pressure at any particular point of time is, however, assumed to be only afraction of the ice thickness. The values for h0 and h are given in the following table,

Table 6.1: Values of h and h0

Ice class h (m) h0 (m)ICE-1A* 1.0 0.35ICE-1A 0.8 0.30ICE-1B 0.6 0.25ICE-1C 0.4 0.22

B 200 Ice pressure

In the subsection, B 200 Ice pressure, the design ice pressure can be calculated by apresented formula based on a nominal ice pressure of 5600 kN/m2,

p = 5600cdc1ca(kN/m2) (6.11)

where cd is a factor which takes account of the influence of the size and engineoutput of the ship. This factor is taken as maximum cd = 1. It is calculated by the for-mula,


cd = ak + b

1000 (6.12)

where,

k =

√(∆fPs)1000 (6.13)

Table 6.2: Values of a and bRegion

Bow Midbody and Sternk ≤ 12 k > 12 k ≤ 12 k > 12

a 30 6 8 2b 230 518 214 286

∆f = displacement (t)Ps = machinery output (kW)c1 = a factor which takes account of the probability that the design ice pressure occursin a certain region of the hull for the ice class in question.

Table 6.3: Values of c1

Ice class RegionBow Midbody Stern

ICE-1A* 1.0 1.0 0.75ICE-1A 1.0 0.85 0.65ICE-1B 1.0 0.7 0.45ICE-1C 1.0 0.5 0.25

ca = a factor which takes account of the probability that the full length of the area un-der consideration will be under pressure at the same time. It is calculated by the for-mula,

ca =√l0la, (6.14)

with a maximum of 1.0 and a minimum of 0.35, l0 = 0.6 m.la is given according to the table below,


Table 6.4: Values of laStructure Type of framing la

Shell transverse frame spacinglongitudinal 1.7 x frame spacing

Frames transverse frame spacinglongitudinal span of frame

Ice stringer span of stringerWeb frame 2 x web frame spacing

This will give a few different pressure values for the different ice class requirementsand areas of the ship investigated.

Result

Using the DNV-GL guidelines from above the design ice pressure is calculated as,

pice = 965 kN/m2 (6.15)

The design pressure can now be used to calculate the shell plating thickness ac-cording to Pt.5 Ch.1 Sec.3 C. Shell Plating

Shell plating

C 100 Vertical extension of ice strengthening for plating For the bow in thelowest ice class (1B and 1C) the extension shall be placed 0.40 m above UIWL and0.70 m below LIWL, see Figure 5.2.

C 200 Plate thickness in ice belt For the longitudinal framing the thickness ofthe shell plating shall be determined by the formula,

t = 21.1s√

p

f2 σF+ tc (mm) (6.16)

s = stiffener spacing i m measured along the plating between ordinary and/or interme-diate stiffeners.p as given in equation 6.16f2 = 0.6 + 0.4

(h/s) , when h/s ≤ 1, 0.33 in our caseh = as given in Table 6.1σf = yield stress of the material (N/mm2)tc = increment for abrasion and corrosion (mm); normally 2 mm. If a special surfacecoating, by experience shown capable to withstand the abrasion of ice, is applied andmaintained, lower values may be approved. In this case the super duplex stainless steelis expected to give no abrasion or corrosion so the parameter is set to 0.


Result

The resulting plate thickness in the ice belt is calculated as,

t = 12.94 mm (6.17)

This is a really thick plate which is not desired in this case, in the Figure belowthe stiffener spacing is plotted against the required plate thickness to show the differentthickness’s and the corresponding spacing of the stiffeners.

Figure 6.14: Stiffener spacing vs. Plate thickness

Discussion

The stiffener spacing in the DNV-GL guides refer to an actual stiffeners being placedbetween girders/stringers which can be seen in Figure 6.15. This is however not thecase in the SSY-design which makes it harder to compare these so called spacings againsteach other.


Figure 6.15: Traditional structure

As can be seen in Figure 6.16 the spacing between the load transfers (red lines) isnot constant which will create problems interpreting the DNV-GL rules, it can eitherbe X1 or Y1 as stiffener spacing.

Figure 6.16: SSY structure

In the calculations above, the stiffener spacing is however chosen as the maximumbetween them, Y1 = 0.675 m. This gives a really large plate thickness compared to nor-mal plate thickness for the SSY vessels, to reduce the thickness naturally the stiffenerspacing has to be reduced as seen in Figure 6.14.

H 100 Stem, baltic ice strengthening

In the subsection H 101 [1] two different stem designs are presented which are suitablefor ice strengthening, see Figure 6.17


Figure 6.17: Stem designs

Scantlings

The DNV-GL rules also state scantlings for the structural parts which the hull struc-ture consist of, this is however not relevant because of the special design SSY uses tobuild there vessels.

Loads from other sourcesOther sources than DNV-GL have also been examined, this is to see if the design loadsfrom DNV-GL differ a lot to the other load formulas obtained by other sources. Thesesources might also give a better design load for the case of brash ice.

6.1.5 JordaanIn two different articles regarding High Pressure Zones [5] and [6] show how the con-tact looks like more in detail between the vessel and the ice. At the points of contactso called High Pressure Zones will occur, this can be seen in Figure 6.18


(a) Contact between ice sheet and vessel seen from the side

(b) Contact between ice sheet and vessel seenfrom above

Figure 6.18: High Pressure Zones

The authors have performed some tests on ice breakers to see what forces are en-countered at certain velocities, however the velocities are of course much lower than on


the SSY vessel. The forces however do very rarely reach above 20 MN, this could givesome indication on what forces our vessel will need to be designed for. It will travelat a much higher speed, but the ice conditions wont even be close to the ones the icebreakers in question operate in.

6.1.6 MastersonIn [9] the authors have performed empirical tests on offshore structures and ship hullsexposed to ice loads. This work resulted in a few different equations where the firsttests were performed on multi-year ice in the Beaufort sea, however in the end an-other test was performed which according to the authors was closer to the conditionsone could expect in the Baltic (even though the results of there equation might still behigher than the once encountered in the Baltic). The result from this is as follows,

p = 4.0A−0.5 (6.18)

This then gives a relationship as shown in Figure 6.19 below.

Figure 6.19: Area vs. Pressure

The linear regression is according to the authors only tested for areas above 1 m2

which was the smallest panel tested on. If however this expression also was to be valid


for areas below 1 m2 this is what it looks like. The reason why this might be interest-ing is due to the fact that the vessel investigated is quite small and the load areas areexpected to be quite small as well.

Discussion

The Masterson expression is expected to show a slightly exaggerated result, this is be-cause of the different ice conditions in the Beaufort sea vs. Stockholm city/inner ar-chipelago. It also has been measured for areas that are slightly larger than the designcreated will have which also makes it a bit questionable.

6.1.7 SuyuthiIn [20] the author shows how a a typical time history of ice induced loads on a vesselcan look like. This is shown in Figure 6.20,

Figure 6.20: Typical ice loads over time

When the hull gets in contact with the ice edge, a crushing failure mechanismtakes place first and consequently the spike is getting higher and higher. Looking care-fully at the spike-like load, there is evidence of a typical saw-tooth shape which indic-ates infrequent crushing when the hull is advancing into the ice. When the accumu-lated force is high enough to initiate bending failure of the ice at a certain distance infront of the contact surface, a sudden drop of the load is observed. Afterwards, thereis no event in the time series record until the next contact of the hull with the next iceedge at which the same spike-like load is repeated.

6.1.8 Su B., Riska K., Moan TIn [19] the authors have performed tests on a real size vessel, Figure 6.21 shows a timehistory of simulated ice loads on a frame on the side of the bow. The vessel was operat-ing in a ice thickness of 0.300 m at an average speed of 5.67 m/s.


Figure 6.21: Typical ice loads over time

From the figure a typical load pattern can be seen, when breaking ice, even thoughthe tests are performed in much thicker ice then the vessel in this thesis is intended forit gives an idea of what to expect in terms of load pattern but not load size.

In Figure 6.22 it is shown how the peak values of the loads are distributed. Themeasurements are taken on the same vessel as above at an ice thickness of 0.125 m atthe same frame as before.

Figure 6.22: Peak distribution of loads


6.1.9 DiscussionAll formulas obtained from the sources above are semi-empirical which makes them dif-ficult to use while they sometimes depend on constants obtained from empirical tests.

6.2 ResistanceIce resistance refers to the time average of all longitudinal forces due to ice acting onthe ship. These ice forces are divided into categories of different origin,

• Breaking forces

• Submergence forces

• Sliding forces

In different ice conditions the relative importance of these components varies, inlevel ice the breaking component is usually the largest but in brash ice or when hittingsmaller ice floes the other two components become more important. The breaking forceis related to the breaking of the ice i.e. to crushing, bending and turning the ice. Sub-mergence is related to pushing ice down along the ship hull whereas the sliding forcesinclude frictional forces. Usually the velocity dependency of the ice resistance is attrib-uted to the last component.

The ice resistance is usually expressed as follows,

RiTOT = Ri +Row (6.19)

that is, the total resistance is the ice resistance plus the open water resistance [13].The ice resistance is then divided into different components as mentioned above,

Ri = RB +RS +RF (6.20)

where the first component is from breaking the ice, the second from submerging theice and the last from friction against the hull due to sliding. Most methods used tocalculate the ice resistance are based on regression on full scale and model scale data.The regression assumes the ice resistance to be linear with ship speed and to consist ofthese three components. Thus the calculation methods for ice resistance are at bestsemi-empirical, and these methods should be used cautiously, especially outside therange of validity. The calculation methods to determine the ice resistance should beused only in the conceptual design phase as these methods cannot account for the de-tails of the hull shape. When the design proceeds, ice model tests should be carried outto finalize the hull shape.

The ice resistance in brash ice can be determined similarly as the ice resistance inlevel ice. The only exception is that the breaking component is different, it exists andis attributed to cohesive forces present in broken ice.


6.2.1 Brash ice resistance from DNV-GLFrom the DNV guidelines [1] the following equations are obtained regarding brash iceresistance in a channel with brash ice and a consolidated layer. The formulas havesome limits regarding ship parameters (Length, Breadth, Draught, etc.) to be valid, theship in this assignment falls outside these parameters, however due to lack of more suit-able theory the formulas still might give a hint on how to design a bow for minimumresistance.

Figure 6.23: Definitions

RCH = C1 + C2 + C3Cµ(HF +HM)2(B + CψHF ) + C4LPARH2F + C5(LT

B2 )AwfL

(6.21)

Where,Cµ = 0.15cos(φ2) + sin(ψ)sin(α) ≥ 0.45,Cψ = 0.047ψ − 2.115 and 0 if ψ ≤ 45◦

HF = 0.26 + (HMB)0.5

HM = 0.6 (for ICE-1C, closest to what is desired) C1 and C2 are set to zero be-

cause they take a consolidated upper layer of brash ice into account. ψ = arctan(tan(φ2)sin(α) )


Figure 6.24: Brash ice resistance

From the figure above the minimum resistance is obtained at α = 15◦ and φ2 =90◦. Having a vertical bow at B/4 of the vessel is however not that good from an openwater water resistance perspective. Another promising point according to the figureis at the lower extremes of each angle, at α = 15◦ and φ2 = 10◦, this gives a totalresistance of 7.88 kN.

6.2.2 Myland & Ehlers (Lindqvists) formulasIn the paper, "influence of bow design on ice breaking resistance" [10] the authors presentsome formulas for calculating resistance to ship hulls due to ice developed by Lindqvist(1989). The resistance is divided into two main components, ice breaking and submer-sion of the ice floes. The submersion component includes the resistance due to ice floessliding along the ship hull, whereas the rotation of the ice floes is not taken into ac-count.


Figure 6.25: Hull parameters

Ice breaking resistance The breaking component is further divided into a bendingcomponent,

RBi = (2764)σfB

(h1.5ice)√E

12(1− ν2)ρwg

(tan(ψi) + µcos(φi)

cos(ψi)sin(αi))(1 + 1

cos(ψi)) (6.22)

and a crushing component,

RC = 0.5σfh2ice(tan(φ) + µ

cos(φ)cos(ψ))

/(1− µsin(φ)

cos(ψ))) (6.23)

where RB is the bending resistance, σf the flexural strength, B the ship breadth,hice the ice thickness, ν the Poisson’s ratio, ρw the density of water, g the gravitationalacceleration, ψ the normal angle, µ the friction coefficient between ship hull and ice,φ the stem angle, α the waterline entrance angle and RC the crushing resistance. Thenormal angle is calculated from the waterline entrance angle and the stem angle ac-cording to,

ψ = atan(tan(phi)/sin(α)) (6.24)

Both components are derived from semi-empirical approximation of the physicalprocess of ice breaking. The formulas consider only roughly the mechanical and geo-metrical parameters.

Submerging resistance The submersion component is estimated by application offull scale experiments and experience gained from model testing. The submersion res-istance is calculated as the sum of the loss of potential energy and the frictional forcesthat are acting between the ship hull and the ice floes,


RS = ρgghiceB(T (B + T )B + 2T + µ(0.7L− T

tan(φ) −B

4tan(α)

+Tcos(φ)cos(ψ)√

1sin(φ)2 + 1

tan(α)2 ))(6.25)

where RS is the submersion resistance, ρg the density difference, T the ship draughtand L the ship length between perpendiculars.

The influence of a plough is not considered in the formula, whereas the ships draftand the density difference between ice and water are taken into account. The total es-timated bottom coverage of the vessel is 70%, since the stern of the vessel is in generalnot completely covered by ice.

Total resistance The main resistance components are extended by speed dependentcomponents based on empirical constants. Thus, the resulting ice resistance is reportedby Lindqvist as,

Rice = (RC +RB)(1 + 1.4 ∗ v√ghice

) +RS(1 + 9.4 ∗ v√gL

) (6.26)

where Rice is the ice resistance and v the ship velocity.

The above formulas are the basic formulas by Lindqvist, they are later adjustedby the authors for a better fit. This is done by dividing the hull into smaller sectionsand adding resistances together. The hull division is done according to the Figure be-low,


Figure 6.26: Re-defined geometry for better results

Results

Figure 6.27: Breaking Resistance


Figure 6.28: Crushing resistance

Figure 6.29: Submerging resistance


Figure 6.30: Total resistance

Discussion

In the Figures above it can be seen that the formulas provide some very extreme res-ults for some angles, whereas other angles give really small values. To try to visualizethe plots better the logarithmic values of the results have been plotted instead.

This tells us that either the values are actually very low at those angles wherevery little is shown or that the formulas are somehow limited to a certain range of angles.

6.2.3 Riska a.o.; Level ice resistanceRiska explains that there exist a wide variation in ice resistance predictions obtainedby different formulations. This variation has been the subject of several studies, e.g.Bachér (1983) and Kämäräinen (1993). Instead of adopting any of the former level iceresistance formulations, a simplified version based mainly on three formulations is de-rived here. The three formulations used are those of Ionov (1988), Lindqvist (1989) andKämäräinen (1993).

The parameter ice resistance depends on may be divided into three groups [2].The first group consists of external variables: ice thickness, hi and ship speed, v. Thetwo other groups contain the shape of the ship (φ,B/T, L/B,Lbow/L, Lpar/L) and thesize of the ship (Lpp, B, T ). This way the ice resistance is,

Ri = f(hi, v, φ,B

T,L

B,LbowL

,LparL

,Lpp, B, T ) = C1 + C2v (6.27)

The constants C1 and C2 dependent of ship particulars must now be determined.This is done by modifying the formulas of Ionov (1988) and Lindqvist (1989). Thespeed dependency is assumed to be linear as no justification from full scale tests toother forms exists within the natural scatter in the data. The waterplane entrance


angle at the bow, α0, or along the waterline, α, are not included in the formulation be-cause their influence on resistance is contradictory in the three references mentionedearlier. This angle is also difficult to define for all but wedge-like waterlines. The flareangle, ψ(= tanφ/sinα) has been suggested to influence the resistance significantly (En-qvist & Mustamäki 1986) but as the angle α is neglected then only the influence fromthe stem angle φ remains.

The equations for the functions C1 and C2 are,

C1 = f11

2TB

+ 1BLparhi + (1 + 0.021φ)(f2Bh

2i + f3Lbowh

2i + f4BLbowhi) (6.28)

C2 = (1 + 0.063φ)(g1h1.5i + g2Bhi) + g3hi(1 + 1.2T

B) B

2√L

(6.29)

where the values for the constants are shown in Table 6.5, these values have beendeveloped based on performances measured on large ice classed ships between 96 and193.7 m long.

Table 6.5: Values of fi and gjf1 = 0.23 kN/m3 g1 = 18.9 kN/(m/s x m1.5)f2 = 4.58 kN/m3 g2 = 0.67 kN/(m/s x m2)f3 = 1.47 kN/m3 g3 = 1.55 kN/(m/s x m2.5)f4 = 0.29 kN/m3

The influence of the stem angle φ on the ice resistance is usually proportional tothe tangent of this angle. The dependence of ice resistance on the stem angle is madesomewhat less dominating in the above equation because many merchant vessels havequite vertical bows without the icebreaking performance suffering that much. The stemangle is to be measured without accounting for the bulb.

In the two figures below it can be seen how the level ice resistance changes for dif-ferent speeds and stem angles at a constant ice thickness of 5 cm.


Figure 6.31: Speed vs. Resistance at 5 cm ice sheet and stem angle 30 ◦

Figure 6.32: Stem angle vs. Resistance at 5 cm ice sheet and speed 20 m/s

From this significant changes in resistance can be seen in Figure 6.31 when thespeed is varied (a change of around 25 kN in the speed range chosen), while the changein stem angle effects the resistance much less (a change of around 0.6 kN in the stemangle range chosen) which can be seen in Figure 6.32.


In Figure 6.33 the speed and the stem angle are varied giving a 3D-plot showingthe resistance for different combination of these variables. From this the same conclu-sion can be drawn as before, when increasing the speed the resistance increases a lot,while when the stem angle changes the resistance only changes very little.

Figure 6.33: Stem angle vs. Resistance at 5 cm ice sheet and speed 20 m/s

Discussion

The results seem quite reasonable here, because the thickness of the ice is really smallthe stem angle should not matter that much. As can be seen in Figure 6.9 the ice breakseasier when bending then when buckling, but in this case the buckling force is reallylow which means that the ice breaks quite well regardless of the stem angle. There is ofcourse a small increase in resistance when the stem angle increases which implies thatthe ice breaking force goes from bending to buckling. The formulas of course neglectthe water resistance which would increase a lot when changing the stem angle.

The large increase in resistance when changing the speed might be quite intuitive,whenever the speed of a vehicle is increased also the resistance is increased. This canbe easily explained using the expression for drag force,

FD = 12ρv

2CdA (6.30)

From this it is seen that the drag force increases by a power of two when the speedincreases (the rest of the variables are kept constant).

The fact that the Riska equations are quite old might not be in the favour of say-ing that these equations give a credible value on the actual resistance encountered bya vessel. Firstly, the experimental values are obtained from a much larger vessel thenthe one used in this report, also the only geometry variable changeable for the bow isthe stem angle, which excludes a lot of geometry options. What it does tell us is the ef-fect of changing a vessels velocity and stem angle, and it also give a hint on what mag-nitudes of the resistance can be expected.


6.2.4 Open water resistanceThe open water resistance is of great importance when choosing the best geometry forthe bow that is going to be developed later in this paper. Due to limitations a CFDanalysis wont be performed, to still get some knowledge about the open water resist-ance the Holtrop & Mennen formulas could be of use.

Holtrop & Mennen

The Holtrop & Mennen formulas [7] are probably the most used when this resistancehas to be calculated analytically. The problem however is that they have a hard timeaccounting for smaller changes in the bow geometry. This makes them complicated touse when the geometries are similar in size but have different geometrical properties.

Holtrop & Mennen developed the following formula to estimate the resistance of aship,

RT = RF (1 + k1) +RAPP +RW +RB +RTR +RA (6.31)

Here, RF is the frictional resistance according to the ITTC 1957 friction formula= 0.5ρV 2SCF . ρ is the fluid density, V is the ship speed, S the wetted area of the hulland CF the frictional coefficient. The frictional resistance is the net fore-and-aft forcesupon the ship due to tangential fluid forces. Frictional resistance accounts for nearly 80percent of total resistance in slow-speed ships like oil tankers and as much as 50 per-cent in high-speed ships like container vessels. Frictional resistance is due to the viscos-ity of the fluid.

The frictional coefficient as calculated according to the ITTC (1957), CF = 0.075/(log10Re−2)2, Re is the Reynolds number. The frictional coefficient can be calculated in a lot ofdifferent ways, however the Holtrop & Mennen formulas seem to be based on the ITTCway of calculating it.

The Reynolds number, Re = ρV L/µ, ρ again representing the fluid density, V, theships speed, L the length of the ship, and µ the viscosity of the fluid.

1 + k1, Form factor describing the viscous resistance of the hull form in relation toRF . k1 can be determined using the Watanabe formula, k1 = −0.095 + 25.5 CB

(LB

)2

√B

TThe equation is visualized in Figure 6.34,

the form factor is calculated as CB = ∆BLT


Figure 6.34: Form factor

Appendage resistance, RAPP = 0.5ρV 2SAPP (1 + k2)CF , where SAPP is the wettedarea of the appendages and 1 + k2 the appendage resistance factor. Appendages includefor example rudders, shafts, fins and keels.

Table 6.6: Values of k2

Rudder behind skeg 1.5 - 2.0Rudder behind stern 1.3 - 1.5Twin-screw balance rudders 2.8Shaft brackets 3.0Skeg 1.5 - 2.0Strut bossings 3.0Hull bossings 2.0Shafts 2.0 - 4.0Stabilizer fins 2.8Dome 2.7Bilge keels 1.4

The equivalent 1 + k2 value for a combination of appendages is determined from,(1 + k2)eq =

∑(1 + k2)SAPP∑SAPP

Wave-making and wave-breaking resistance, RW . In the KTH Naval architecturecourse binder this resistance is obtained by looking at a plot created by Guldhammerand Harvald, shown in Figure 6.35.


Figure 6.35: Form factor coefficient, CR

Using a prismatic coefficient, CP = ∆/(AMLPP ) of 0.5 which is the closest to themeasured one of 0.41 and a Froude number of 1.28, this gives a CR of around 5.6∗10−3.The actual value of ∆/L1/3 is 6.2, using interpolation CR is corrected to 5.56 ∗ 10−3.For reasons explained in the literature CR is calculated as the difference between CTMand CfM , which means that the the CR coefficient is scale dependent and is obtainedby first calculating CTM and then deriving our CR from CR = CTM(1 + k)CfM , the newCR is the same for full scale and model scale (see ITTC-78). By assuming that the size


of the model is 4 m, using ITTC-78RB, Additional pressure resistance due to bulbous bow near the water surfaceRTR, Additional pressure resistance of immersed transom sternRA, Model-ship correlation resistance

6.3 DiscussionFor both the loads and resistances the formulas found are most often semi-empirical,basing constants on results from model or full-scale tests, this shows how hard it ac-tually is to deal with ice. The forms and physical properties of an ice sheet constantlychange, there can be brash ice, pancake ice, level ice, grease ice, ice from salt water, icefrom fresh water and so on. These differences make it really hard to formulate exactformulas for all conditions or for that sake a general condition applicable for all typesof ice. The formulas presented can however show how things change when a certainparameter is modified, it can show what kind of magnitudes to expect on resistances orloads for example. A lot can be learnt from studying them.

For more exact results for a certain ship or some earlier unexplored geometry amodel test could be of great value. Unfortunately there are not that many facilitiesaround that can perform these tests in an basin with the possibility of growing an icesheet and it is also rather expensive.

Figure 6.36: Test basin at Aker Arctic in Helsinki

Chapter 7

SSY design

This chapter will present the SSY design theory, showing the hull structural design andother features that differ from a conventional design. This chapter presents the answerto the fifth bullet from the list of goals.

The SSY design technique has been developed from looking at ships used duringthe Viking period, the Vikings’ long and relatively narrow wooden longships were ex-tremely lightweight and fast. One of their secrets that allowed them to sail over vastoceans was that the Viking longship hull was flexible in construction, which allowed itto absorb the force from powerful waves. In modern times, several Viking ships havebeen built, including the Ormen Långe in Stensund near Trosa in the 1950s. More re-cently, a Viking ship was built on the historic island of Björkö in Lake Mälaren. Thereare extensive archeological remains from a large Viking settlement from around theyear 1000 AD. The island is now known as Birka and has a famous museum. The Birkaship has sailed up to 18 knots.

Håkan Rosén, the founder of the SSY technique has now incorporated the Viking’sflexible hull structure into the modern SSY steel boats. This has been done by design-ing flexible longitudinal stringers (which are patented), which together with Sandviksstainless steel makes it possible to build extremely lightweight boats entirely from stain-less steel.

7.1 Hull

7.1.1 Conventional hull structureTraditionally a hull structure is built up as a typical framework. The loads from thesea are absorbed through a number of structural parts, starting with the outer plates,the plates distribute the loads to stiffeners, the stiffeners to girders/stringers and thenthrough the web frames to the ship as a whole.

58

CHAPTER 7. SSY DESIGN 59

Figure 7.1: Traditional hull structure

7.1.2 SSYThe SSY design is a more flexible construction then the traditional construction, thismeans that the loads encountered by the ship will be absorbed more then the tradi-tional design does.

As explained before the SSY technique uses the flexible hull structure to absorbsea loads in global deformations, using the flexible stringers.

The stringers in question act as a spring to absorb local loads, if the spring wasextremely stiff the energy absorbed by it would not have anywhere to go. But whenthe spring is less stiff the energy can be absorbed by the spring through deformation,elastic or plastic. This is called elastic energy or potential energy.

SSY uses this technique along with the traditional structure build up. At firstthere is an outer plate, the loads encountered by the plate are transferred through flex-ible stringers to web frames. The web frames then act as a rib cage to the human body,it keeps everything in place and allows the body to be flexible [21].

7.1.3 Plate thicknessThin plates are desirable to reduce weight, and also to deform elastically under extremeloads. This is made possible by the extremely high fatigue limits of the SAF2507. Sobasically, thin plates are possible due to the structural arrangement with built-in flexib-ility, and the result is a lighter hull.

Thin plates are however much harder to work with as it really complicates thewelding process. The plate thickness of 3 mm for an already proven SSY concept (P16with a length of 17.2 m) is the analysed minimum thickness achievable for the design


criteria of this boat. Smaller/slower boats would have thinner plates and larger boatswould have thicker.

7.1.4 StringerFrom a previous design produced by SSY the desired width of the stringers was set as150-180 mm.

The thickness of the stringers was around 1.5 mm.

Design

The design of the stringer is basically as shown in Figure 7.2a although there are anumber of cut-outs to lower the weight of the stringer and get a more even distributionof the stresses.

(a) Profile

(b) Stringer on plate

Figure 7.2: Stringer design

The flat outer parts of the stringer in Figure 7.2b show where the stresses on theplate are the highest and where the load transfer between the plate and the stringertakes place.

Web frame

The web frame is the rib cage of the vessel, the web frame profile is shown in Figure7.3a and the connection between the stringers and the web frame is shown in Figure7.3b


(a) Profile

(b) Web frame on stringer

Figure 7.3: Web frame design

The connection between the stringers and the web frames generate stresses thatare concentrated in the bottom of the frame, at the top of the stringer. This is wherethe load transfer takes place between the stringers and the web frames.

The flexibility in the stringers can be compared to the one in a leaf spring as shownin Figure 7.4

Figure 7.4: Leaf spring

To save weight and distribute stresses more evenly, the web frames can also havesome cut-outs.

7.2 SteelThe SSY vessels are built from the super duplex stainless steel alloy SAF2507 developedby Sandvik AB with input from SSY on desired properties.


7.3 Evaluation of current designThe current design by SSY for the shuttle ferry is shown in Figures 7.5 and 7.6

Figure 7.5: Current design

(a) Bow (b) Aft

Figure 7.6: Current bow and aft design

Comparing the design to a modern icebreaker as seen in Figure 7.7 some differ-ences can be seen.


Figure 7.7: Modern icebreaker design

It can be seen, by comparing the two designs that there is quite a difference shapewise. The design that is going to be created should be a trade-off between performanceduring the time of the year were there is no ice and ice operating performance duringthe the few months were there is ice.

From earlier experiences when trying to operate in ice conditions with the SSYvessel "Elvira" the company experienced no problem in the strength of the hull, how-ever when operating at higher speeds the vessel did not break the ice more but she slidup on the ice sheet.

Figure 7.8: Elvira

As can be see in the picture above the hull shape is really similar to the shuttleferry concept and will most definitely act in the same way when experiencing these


kind of conditions. It will also be twice as big as Elvira so it will still be able to op-erate in somewhat harsher conditions due to the extra weight compared to Elvira.

Chapter 8

Design

In this chapter the current design is updated according to the following steps. Fromthe evaluation of the current design a conceptual design is created. After this the con-ceptual design evaluates in a basic design, which later becomes a detailed design beforefinally having a final design. In this chapter the last six bullets from the list of goalsare answered.

8.1 Concept designThe conceptual design includes,

• Possible modification of existing geometry, this depends on the results from theliterature study, depending on how we want to use the vessel to break the ice.

• Design delivered as a few design options regarding geometry without any specificstructural parts.

• Pros and cons of different geometries analysed compared to each other.

• Evaluation of existing ice breaking solutions.

8.1.1 GeometryThe geometry of the concept design does not yet have the final geometry, but five dif-ferent options with pros and cons for each option. This provides the reader a motiva-tion for the choice of geometry, why this geometry is the best for this case. The reasonfor comparing geometries is to show what different types of geometry can be considereddesigning the vessel, this is because the operating profile of the vessel is not only openwater but also ice. The different designs compared will be designs that are aimed fordifferent conditions of operation. First the design of the "Shuttle Ferry" concept is eval-uated, then the design of a modern ice breaker, thirdly a combination of these two,then the combination will have a bulbous added and last a design with a larger stemangle.

The main properties of the geometry are given in the table below,

65

CHAPTER 8. DESIGN 66

Table 8.1: Geometry of bowParameter Symbol Value UnitBeam B 6.75 mHeight H 2.5 mLength LB 8 mDraught T 1.23 mFreeboard f 1.27 m

For each concept pros and cons are listed, these originate from the writers exper-ience and an email discussion with Rickard Lindstrand SSY employee, Lic. Eng. M.Scin Mechanics and Fluid Dynamics.

8.1.2 Shuttle Ferry conceptThis concept is based purely on performance in open waters with no ice cover or brashice. The geometry gives the vessel a high performance by lowering the water resistanceand thereby also lowering the fuel consumption and emissions of for example CO2. Ithowever does not have the ability to act as a good icebreaker, this is mainly because ofthe large radius shown in Figure 8.1.

Figure 8.1: Shuttle Ferry concept 3D geometry


Pros

• Good hydrodynamic propertiesas it has a streamlined design.

• Lower fuel consumption due tothe streamlined shape.

• Excellent open water perform-ance.

• The largest part of the year thisdesign is desirable due to the lackof ice.

Cons

• Limited ice breaking capability.The curvature of the bow willcause the vessel to slide up onthe ice. This is because the angleof the bow is quite small, the ves-sel will thereby break the ice dueto shear failure rather then thedesired bending failure.However when the vessel is plan-ing bending failure is the majorpart, planing in ice might how-ever not be that common.

From the evaluation it can be seen that due to the uncertainties in the ice break-ing capability for the design it is still a possible option. It is however not sure howgood it will perform. Thereby further investigation is needed.

8.1.3 Modern ice breakerThe modern icebreaker is a vessel only intended for the purpose of breaking ice andclearing passage for other vessels. A part of the bow will be strengthened to be able towithstand the ice pressure. The design, starting from the bottom, has a vertical partacting as a so called bow plough. The plough directs the ice to the sides of the ship,lowering resistance.

Figure 8.2: Modern ice breaker 3D geometry


Pros

• Outstanding ice breaking capab-ility, most likely way to good forthe purpose of our vessel.

• Directs ice around the ship, redu-cing resistance.

Cons

• Very high open water resistance.

• Built for breaking the ice, brashice however needs to be pushedaround the vessel which makesthe need of a ice breaker ques-tionable.

• The bow plough does not fill anyfunction for this case becauseoperation most of the time isthrough brash ice.

• The larger part of the year thisdesign is not needed and will in-stead increase fuel consumption alot.

From the evaluation it can be seen that using a modern ice breaker design is notnecessary, it is for sure outstanding in breaking ice but because the main purpose ofthis vessel is NOT to break ice but rather have a design capable of operating in ice,this geometry should not be used.

8.1.4 CombinationThe combination of the two previous designs is expected to give the vessel a bit fromboth parts, by having a larger stem angle the vessel is expected to have a better abilityof breaking the ice and will not slide up on the ice sheet. A small bow plough is usedto direct ice around the ship and not under the ship.


Figure 8.3: Combination 3D geometry

Pros

• Adjusted to a combinationbetween the first and the seconddesign. Better hydrodynamicproperties, more streamlinedthen an ice breaker but still notas good as the Shuttle FerryConcept.

• Moderate fuel consumption.

• A combination that is rathergood both in ice conditions andopen water.

• When needed, the smaller bowplough helps the ship directingice along the sides of the ship.

Cons

• The bow plough is probably notreally necessary, due to increasingresistance.

• Not needed during a larger partof the year, but still lower resist-ance then the ice breaker design,making it more suitable.

From the evaluation it can be seen that the geometry title is accurate, it is a com-bination between the two extreme cases, ice breaker and open water vessel. Because itis supposed to be right in-between and the ice breaker geometry is a bit too much, thiscombination geometry is accepted as one of the geometries to be further investigated.

8.1.5 Combination with bulbousThis geometry has been kept the same as the one above but with a bulbous addedto it. The bulbous is a great feature that has been added quite recently to merchantships that not need to break ice, they sail either in broken ice channels or follow an


icebreaker. By adding the bulbous the broken ice is displaced around the hull in a waythat resembles the hydrodynamic flow, thereby much of the additional ice resistancecan be avoided. [13]

Figure 8.4: Combination 3D geometry with bulbous

ProsSame as the previous design, withthese additions,

• According to [13] icestrengthened ships often havebulbous bow which is not a han-dicap in broken ice.

• The bulbous bow moves thebroken ice that is displacedaround the hull in a way thatresembles the hydrodynamic flow.

ConsSame as the previous design, withthese additions,

• If the vessel hits a large piece ofice it is not sure how the bulbousbow will react.

• Higher material and manufactur-ing costs.

• Weight increase.

From the evaluation it can be seen that this geometry is basically the same as theprevious one, however with an addition of a bulbous bow. The bulbous bow is addedto the geometry to find out if it can make any change in resistance. This design shouldalso be investigated.

8.1.6 Concept design with other angleBecause the ice breaker design is too extreme this geometry is introduced. The thoughtwith this geometry is that it will bend the ice more like a ice breaker but still be stream-lined like a regular vessel.


Figure 8.5: Combination 3D geometry with other stem angle

Pros

• More streamlined then a conven-tional ice breaker but breaks theice like one.

• Gets more of its weight on theice, which creates a greater mo-mentum then for example theConcept design. Using this in-creased momentum the ice willbreak easier.

Cons

• The geometry might have agreater resistance then the con-ceptual one.

• The large angle at the bottom ofthe bow might cause turbulence.

From the evaluation it can be seen that this geometry is rather interesting. Thereare quite some unknowns and it should thereby be investigated more thoroughly.

8.1.7 Evaluation matrixTo evaluate the different designs a so called Evaluation Matrix is created, the values inthe matrix are based on the pros and cons discussed above.


Table 8.2: Evaluation matrixFeature Shuttle Ferry Ice Combination Combination Other stem

Concept breaker with bulb angleTotal resistance 7 4 7 8 7Weight 9 3 6 5 8Ice performance 3 10 6 7 5Open water 9 3 6 6 8performanceTotal 28 20 25 29 28Relative total 0.215 0.154 0.192 0.223 0.215Scale, 10 -> best 0 -> worst

From this it is seen that there are four designs that are quite close to each other.Considering however the fact that the waters in Stockholm are ice covered in averagearound 30% of the year [18], this fraction will also have to be considered in the evalu-ation. Now adding this fact to the evaluation matrix gives the results seen in Table 8.3.The number for the Shuttle Ferry concept is given as a 7 because it is optimal for 70% of the year, whereas the ice breaker is given a 3 because it is optimal for 30 % of theyear. The Combination is put somewhere in between at 5, and the same concept witha bulb is ranked with a 4 due to the added bulb, unnecessary in the summer. The sea-sonal factor is a factor trying to enhance the open water abilities of the vessel due tothe fact that the ice cover in the operational area is as low as it is.

Table 8.3: Evaluation matrixFeature Shuttle Ferry Ice Combination Combination Other stem

Concept breaker with bulb angleSeasonal factor 7 3 5 4 7Total 35 23 30 33 35Relative total 0.224 0.147 0.192 0.212 0.224

This changes the outcome to a draw between the already existing concept and the onewith the change in stem angle. This means that the original concept is a good conceptand does not need changing.

8.2 Basic designThe basic design includes,

• Final bow geometry.

• Overview of structural parts, no calculations.

• Check that design proposition is possible to manufacture.


8.2.1 Stringer designIn this section the stringer design is introduced, dimensions and placements are still notexact but a good guess is presented. After some analysis the stringers will be adjustedto meet the design criteria given in Table 2.4.

Figure 8.6: Stringer design & dimensions

Table 8.4: Stringer dimensionsParameter Value DimensionH 30 [mm]B 240 [mm]w 40 [mm]b 5 [mm]t 5 [mm]

8.2.2 Web frame designIn this section the web frame design is introduced, dimensions and placements are stillnot exact but a good guess is presented. After some analysis the web frames are adjus-ted to meet the design criteria given in Table 2.4.


Figure 8.7: Web frame design & dimensions

Table 8.5: Web frame dimensionsParameter Value DimensionH1 2500 [mm]H2 1500 [mm]B1 6750 [mm]t1 180 [mm]thickness* 50 [mm]*third dimension of Figure 8.59

8.2.3 Shuttle Ferry ConceptThe basic design has been created as seen in Figure 8.8 with four stringers along theships length and four web frames across the width of the ship.


Figure 8.8: Shuttle Ferry concept 3D geometry

8.3 Detailed designThe detailed design includes,

• Bow geometry with structural parts developed to fulfil the special requirements.

• Dimensions of all individual parts of the structure.

• Boundary conditions for the design.

• Structural analysis performed on design.

8.3.1 LoadsA few different loads are examined for the selected geometries, the DNV ice pressure,the load from an 330 kg ice floe and the load from an 5 cm ice sheet . These loads areconsidered as the worst case scenario loads for the operational area of the vessel, andcan thereby seem a bit extreme in some cases.

Load case 1 - DNV

The first load case is defined by using the DNV ice pressure calculated in subsection6.1.4. The calculated ice pressure of 965 kN/m2 (or 0.965 MPa) is applied to the hullstructure according to Figure 8.9 as a line pressure just below the waterline. The heightof this pressure is set to 20 cm.


Figure 8.9: DNV ice pressure

Load case 2 - 330 kg ice floe

The second load case is also chosen from the First requirements in Table 2.4. Here thevessel operates in a open water and hits an 330 kg ice floe at an operating speed of 15knots. The pressure encountered should look something like Figure 8.11.

Figure 8.10: 330 kg ice floe

The force encountered from the impact with a 330 kg ice floe can be calculatedusing kinetic energy and work performed by the impact.

Ek = 12mv

2 (8.1)

where, m = mass of the ice floe, v = speed difference between ice floe and vessel(speed of vessel if ice floe speed is assumed to be 0).

W = Fd (8.2)

where, W = work performed, F = impact force and d = distance to slow down theice floe. In the impact between the ice floe and the vessel the kinetic energy from theice floe (vessel) is converted to work in stopping it. Combining Equation 8.1 and 8.2the following is obtained,


F =12mv

2

d(8.3)

there is a constant ice floe mass of 330 kg, the distance the hull can flex to stopthe ice floe is assumed to be 5 mm, this gives a speed vs. force plot as follows,

Figure 8.11: Speed of vessel vs. impact force

as can be seen from the formula in Equation 8.3 the force increases with the squareof the speed, this means that when the speed increases the impact force increases muchfaster. The impact force here is close to reality but it neglects for example the brittle-ness of the ice floe which probably reduces the force a bit from the ice floe breakingduring impact.

The ice floe is modelled as a cylinder as described in Section 2.5.1 with a diameterof 1.79 m and a height of 0.21 m.


Figure 8.12: Model of ice floe

The ice floe model is shown in Figure 8.13 in real proportion to the vessel. Thearea of contact between the ice floe and the vessel is assumed to be quite small in prac-tice, while in reality it is really hard to guess because the ice floe shapes will vary a lot.

Figure 8.13: Model of ice floe

The height of the area is set to be 0.21 m as modelled, the area can then vary de-pending on how much of the "length" of the floe hits the hull. In Figure 8.14 the areaof impact is given depending on the percentage of the length of the ice floe hitting thehull.


Figure 8.14: Area of impact

Assuming that the area of impact is 8 % (for a given, plausible case) of the ice floeand the speed of the impact is 15 knots, this gives a pressure on the hull of,

p = F

A= 1.965MN

0.0301m2 = 65.34MPa (8.4)

Figure 8.15 displays a plot of the Area of impact vs. the Pressure encountered,showing a great decrease in pressure when the contact area increases.


Figure 8.15: Pressure for a certain percentage of Area

The contact length between two cylinders is calculated as,

b =

√√√√√√√√√4F[

1− ν21

E1+ 1− ν2

2E2

]

πL

(1R1

+ 1R2

) (8.5)

The different radius’s are calculated according to Figure 8.17 as, R1 = 1.79 m, R2= 16.97 m, F is obtained from Equation 8.3, E1 = 5400 MPa, ν1 = 0.33 (from Table2.3, ν is the Poisson ratio), E2 = 200 MPa (from the Sandvik data sheet), ν2 = 0.31(assumed from results on the internet, not found in data sheet) and L = 0.21.


Figure 8.16: Radius’s of ice floe and vessel

The resulting contact length from Equation 8.5 is calculated as 0.0206 m, whichis about 1.2 %. This would imply that the impact pressure according to this model is1.965 MN / 0.0043 m2 = 456.98 MPa which is a lot higher then in the assumption be-fore.

This load case can look something like this,

Figure 8.17: Load case 2 visualization

This load case looks to be the largest one, this case is however also really hard toestimate because of the way an ice floe behaves. The ice floe has been idealized as anperfect cylinder, which of course not is the case. The cylinder in fact might be a totallyunknown shape that hits the vessel in a number off different ways and either with asharp edge or a flat surface. The right way in this case is more likely to estimate themaximum pressure encountered, which has been done in the calculations above. Thelikeliness for the vessel to hit exactly this kind of ice floe would however be really low.

Another factor is of course that the ice floe will break into pieces when the colli-sion occurs which means that the ice floe absorbs part of the kinetic energy as well asthe vessel. This will reduce the pressure on the vessel by a lot.


From section 6.1.6 it can be seen in Figure 6.19 how the authors have measuredpressure caused by ice on a steel structure. In this case the pressure increases a lotwith the decreasing area but it would be really doubtful that the pressure can reach the457 MPa calculated above. A much more reasonable value might be around 10 MPajudging from Figure 6.19 and also considering the angle of attack from the ice floe onthe vessel(which in the worst case scenario is 0 deg which is a direct hit on the stemand ). The likelihood for the vessel to encounter higher pressures are most likely verylow.

Load case 3 - 5 cm ice sheet

The third load case is chosen from the First requirements in Table 2.4. Here the ves-sel operates in a 5 cm thick ice sheet at an operating speed of 15 knots. The loads en-countered look something like Figure 8.18.

Figure 8.18: High Pressure Zones

Basically the same as Load case 2 but at an other location on the hull, the heightof the load will not differ much from Load case 2, however in this case the ice will hitdirectly from the front of the ship instead of the side which would be most common inLoad case 2.

In this load case the vessel will hit a sheet of ice that can be thought of as infin-itely big compared to the vessel, however when the vessel hits the ice sheet, the ice willcrack similar to Figure 6.7. This creates a ice floe a bit similar to the one in the previ-ous load case. The thickness of this floe is however only set as 5 cm instead of 21 cm,which means that this load case will be much less then Load case 2. Around 25 % ofthe 10 MPa estimated for load case 2 if the load is decreasing linearly.

In this case the High Pressure Zones discussed in section 6.1.5 can be of relevance,along the side as shown in Figure 8.18

Discussion

The second load case would by far be the largest one, however this one is also reallyhard to estimate because of the way an ice floe behaves. Either way this must be con-sidered as the dimensioning pressure for the structure.


8.3.2 PreparationHere the result from each individual FEM-study on the different designs is presented.

Boundary conditions

The boundary conditions used in the FEM-model are applicable for all designs, al-though only the SSY concept is shown in Figure 8.20.

• Hull - The hull is assumed to be fixed in 6 DOF at all edges.

• Stringer - The stringers are assumed to be fixed in 6 DOF at both ends of thestringer.

• Web frame - The web frames are assumed to be fixed in 6 DOF at the verticalface at the centre of the vessel and at the horizontal face at the side of the vessel(where the deck structure is attached).

Figure 8.19: Boundary Conditions

The boundary conditions are simplified for this evaluation, neglecting global loadson the vessel such as hogging and sagging, neglecting small displacements and rotationsat the boundaries of the structure.

Mesh

The mesh is performed using a triangular mesh with an element size of 60,0 mm. Giv-ing the following result,


Figure 8.20: Mesh

The element size is usually iterated until the results converge towards a specificnumber, however lowering the mesh size for this structure will create a element numberexceeding the allowed one for the student version of Ansys.

Table 8.6: Meshing statisticsGeomtery Nodes ElementsSSY 221 846 109 321Conventional 230 803 115 616

Evaluation

The results from the analysis are evaluated by looking at some different parameters,

• Total deformation - The map on the total deformation shows how much thestructure deforms at a certain point on the structure. This deformation shall becompared against the span of the area of deformation, but for a panel of 1 m itshould not deform much more than 1-2 mm for the traditional design. The SSYdesign is supposed to flex and thereby a higher deformation tolerance should beallowed.

• Equivalent stress (Von Mises) - The map on the von Mises stresses will showin which zones stress concentration occur, the stresses shall also be comparedagainst the maximum stresses this material can take according to the Sandvikdata sheet [16], commonly known as the yield strength.


– Safety factor - a safety factor of 1.5 is added to ensure that the vessel doesnot suffer any crucial damage when hit by the ice. This means that the max-imum stress measured should be 1.5 times less then the maximum stress thematerial can take.

Ansys settings

A static structural analysis is performed on the geometries created using the AnsysWorkbench. The material data used for the analysis is shown in Table 8.7

Table 8.7: Material data used in AnsysParameter Value DimensionDensity 7800 [kg/m3]Tensile yield strength 550 [MPa]Compressive Yield strength 550 [MPa]Tensile ultimate strength 800 [MPa]

Tuning/Iterating

To full fill the evaluation criteria stated above the dimensions of the structures willhave to be changed. Some of the different structure parts are easy to change the di-mension of, while other require a lot of work,

• The plate thickness is easy to change because its on the outside of the struc-ture.

• The stringers are trapped between the plate and the web frames, making it a lotof work changing them.

• The geometry of the web frames are somewhat easier to change then the stringers,however the easiest with this part is to change the thickness of it.

This means that the design requirements will be met by changing the thickness ofthe hull plate and the web frames.

Challenges/limitations

Due to difficulties in designing a curved structure in CAD, the load analysis is per-formed on only a part of the structure. The difficulties occur when the design is aboutto be meshed in Ansys, due to the fact that the CAD-model does not connect well oris overlapping in points on the structure. This can be because the CAD-file is impor-ted from Creo to Ansys, creating some small failures. The student version of Creo ishowever limited in the simulation tool.

The different load cases have been applied as a nodal pressure on a selection ofnodes from the mesh. A challenge with this is to have the same area selected for bothdesigns, this has been done by looking at the number of nodes selected for one designand trying to use the same number of nodes and the same location for both designs.


8.3.3 SSY conceptAfter looking at the results from the assumed dimensions the designs were re-dimensionedto fit the evaluation criteria given in section 8.3.2 under "Evaluation". This gave the di-mensions shown in the sections below.

The hull thickness is set to 10 mm.

Stringer design


Table 8.8: Stringer dimensionsParameter Value DimensionH 30 [mm]B 240 [mm]w 40 [mm]b 10 [mm]t 5 [mm]


Web frame design



Mass

3599.5 kg

FEM

Resulting in the following results from the FEM-analysis,

Model test

To test the CAD models authenticity a basic load is applied on the structure as can beseen in Figure 8.23


Figure 8.23: Test load applied

The boundary conditions applied are as shown in Figure 8.24. the outer edge ofeach plate, stringer and web frame is fixed in 6 DOF.

Figure 8.24: Boundary Conditions

The total deformation of the simulation is shown in Figure 8.25


(a) Without flange on web frame

(b) With flange on web frame

Figure 8.25: Deformation

The equivalent von Mises stress is shown in Figure 8.26


(a) Without flange on web frame

(b) With flange on web frame

Figure 8.26: von Mises stress

First of all it can be seen that adding a flange to the web frames reduces the max-imum deformation by 79 %,from 49.7 mm to 10.4 mm and the equivalent von Misesstress by 34 %, from 2741 MPa to 1798 MPa.

The results above look very reasonable considering the load and the boundaryconditions applied. From the deformation it can be seen that when not applying adeck structure the web frames seem to buckle quite a lot, while the rest of the struc-ture shows much smaller deformations.

Looking at the equivalent von Mises stress it can be seen how the stresses con-centrate at the contact between the stringers and the web frames. This shows the loadtransfer taking place, from the lowest structural element in the hierarchy (the plate) tothe higher one, the stringer and then the web frame, ultimately ending up as a global


load.

Reference

For a known reference given by SSY an idea is be given on how the structure shouldbe built up in terms of frame work dimensions and spacings. The hard thing howeverusing the round outer geometry (compared to the straighter one SSY is using) createdis that the CAD-program is very difficult to use when sweeping the stringers along thecurved outer hull. When doing this the stringers constantly get twisted around and notgoing perfectly along the hull geometry, creating a variety in actual stringer dimensionsand making it impossible to mesh due to that the stringer in some coordinates is creat-ing intersecting/or close to intersecting geometries error.

A work around is to use a straight outer geometry as shown in Figure 8.27. Thisone is created out of three straight rectangles but can of course be created more "round"to create a lower resistance.

Figure 8.27: Straight geometry framework

Compared to the framework shown in for example Figure 8.24 this set up is cre-ated really easy by doing some patterns along the straight surfaces of the squares wherethe structure is built up from.

Load case 1

This is the DNV load case, applying a line load horizontally on the structure by creat-ing a named selection in Ansys and selecting nodes to apply a pressure of 0.965 MPaon which is shown in Figure 8.28


Figure 8.28: Named selection for applying load

The results of this is shown as a deformation in Figure 8.29 and equivalent vonMises stress in Figure 8.30




Stress maxima/concentrationsFor the first load case the stress maximum is found at the connection between the stringerand the web frame at around 419 MPa as seen in Figure 8.39c. This maximum stress isfound at a point looking like a stress concentration, this could possibly be a reason foreasier failure due to for example cracking. The value however is not higher then thetensile strength of the material.


(a) Zoomed out (b) Stress on outer plate

(c) Zoomed in on stress area

Figure 8.31: Stress analysis

DeformationLooking at the deformation shown in Figure 8.29 a maximum deformation of 2.8 mm isshown, which is acceptable for this load case.

Load case 2

In this load case it is assumed that an 330 kg ice floe hits the structure at a pressureof 10 MPa. Here another named selection of nodes is created, see Figure 8.32 and thepressure is applied on them.







Stress maxima/concentrationsFor the second load case the stress maximum is found on the stringer at around 957MPa as seen in Figure 8.35c. The maximum stress is found on the stringer showing nostress concentrations or unreasonable values.





ConclusionsLooking at the deformation shown in Figure 8.33 the maximum value is quite high,11.41 mm. If this is to high is hard to say without looking at the von Mises stress,shown in Figure 8.34. Here is seen that the maximum stress exceeds the yield strengthof the material at 956.6 MPa vs. 550 MPa. This means that according to this analysisthe load of 10 MPa on the structure is just a bit to high to make it acceptable.

When a flange was added to the web frames, the stiffness increased a lot makingit harder for the load transfer to take place. Hereby the deflection and stress increasedwhen adding the flanges.

To lower the stress on the structure the dimensions of the structural parts can bechanged, comparing to other SSY designs however, this structure already is quite overdimensioned. Traditionally in ship building often brackets are inserted to reduce stressconcentrations, this might be an option here. A small bracket could for example be ad-ded between the stringer and the web frame at the ice belt region discussed in subsec-tion 5.2.5


Load case 3

In this load case it is assumed that the structure hits a ice sheet of 5 cm thickness at apressure of 2.5 MPa. Here another named selection of nodes is created, see Figure 8.36and the pressure is applied on them.






Stress maxima/concentrationsFor the third load case the stress maximum is found on the stringer close to the con-nection to the web frame at around 236 MPa as seen in Figure 8.39c. This maximumstress is found at a point looking somewhat like a stress concentration, this could pos-sibly be a reason for easier failure due to for example cracking. The value however isnot higher then the tensile strength of the material.





DeformationLooking at the deformation and von Mises stress shown in Figure 8.37 and 8.38 a de-formation of 1.24 mm.

Discussion

The dimensions of the structure can seem a bit large, to investigate this a few differ-ent thickness’s are tested to ensure that the spring effect of the stringers works as itshould. They could possibly be to stiff and not deform/convert stress as good as athinner one.


8.3.4 Thickness analysisSmall section

To examine the validity of the chosen thickness’s a small test is performed to see whathappens when different thickness’s change.

Figure 8.40: Set up

A pressure is applied to the outer plate of the structure (from the bottom left inFigure 8.40) and the web frame is set as fixed. This will show the stresses and deform-ations of the stringer and plate, to test the "spring effect" of the stringer a few differentplate and web frame thickness’s are used.

(a) Deformation (b) Stress

Figure 8.41: Plate: 10 mm, stringer: 3 mm

In the Table below the outer plate thickness is reduced from 10 mm to 7, 5 and 3mm. The stringer thickness is kept constant at 3 mm.


Table 8.10: Deformation and stress for different plate thickness’s

ResultPlate thickness 10 mm 7 mm 5 mm 3 mm

Deformation (mm) 9.99 16.94 27.31 62.00Stress (MPa) 252.46 313.83 400.85 380.53

The same evaluation is now performed for the stringer thickness, keeping a platethickness of 5 mm.

Table 8.11: Deformation and stress for different stringer thickness’s

ResultStringer thickness 7 mm 5 mm 3 mm 1 mm


At last the stringer thickness and the plate thickness are kept constant, howeverthe contact width (w in Figure 8.21) between the stringer and the web frame is changed.The starting value is 40 mm, the other values examined are 20 and 60 mm.

Table 8.12: Deformation and stress for different contact widths

ResultContact width 60 mm 40 mm 20 mm

Deformation (mm) 12.58 13.97 15.25Stress (MPa) 153.08 133.24 132.43

Alternative geometry

The same can be tested on the straight rectangle structure discussed in Section 8.3.3.As before the total deformation and the equivalent von Mises stress are looked at asseen in Figure 8.42, the load applied is 0.6 MPa at the two bottom rectangles of thestructure.

(a) Deformation (b) Stress

Figure 8.42: Alternative framework


In the Table below the outer plate thickness is reduced from 10 mm to 7, 5 and 3mm. The rest of the dimensions are according to the reference data from Section 8.3.3.

Table 8.13: Deformation and stress for different plate thickness’s

ResultPlate thickness 10 mm 7 mm 5 mm 3 mm


The same evaluation is now performed for the stringer thickness, keeping the restof the dimensions according to the reference data from Section 8.3.3.

Table 8.14: Deformation and stress for different stringer thickness’s

ResultStringer thickness 6 mm 4.5 mm 3 mm 1.5 mm


At last the same evaluation is now performed for the web frame thickness, keepingthe rest of the dimensions according to the reference data from Section 8.3.3.

Table 8.15: Deformation and stress for different contact widths

Result

Web framethickness 6 mm 4 mm 2 mm

Deformation (mm) 8.07 10.87 17.13Stress (MPa) 1616.2 2856.4 4626.7

Discussion

This examination is performed to see how the properties of the structure change whenthickness’s of the different parts are changed. This, together with trying different spa-cings of the stringers and web frames, would be done with iterative steps to find a op-timal weight and load absorption for the structure. The SSY design is based on theflexibility of the structure, and load absorption through the "spring effect" of the stringers.Having a stringer that is to thick/to stiff can take away this flexibility and thereby thewhole SSY thinking.

What can be seen from the examination of the alternative geometry is that chan-ging the hull and the web frame thickness acts as expected, the thicker the plate/ webframe the lower deformation and stress. However by changing the stringer thickness thestress gets reduced for a thicker stringer, the deformation goes up and down without apattern. What can be said is that the thickness’s are kept within just a few mm andthe results are all within about 0.5 mm, which could imply that the different stringerthickness’s are to close to each other. Although the stress is reduced for the increasingthickness which is very reasonable.


8.3.5 Conventional designTo compare the SSY design against a conventional design the same weight of mater-ial and outer geometry is used to create the structural arrangement shown in Figure8.47. The comparison is done as package versus package, that is the SSY package isthe combination SAF2507 plus the SSY design. Whereas the conventional package isthe combination traditional ship building material plus the traditional way of buildinga hull structure. This way SSY as a company can compare their solution against thetraditional solution.

Figure 8.43: Conventional design

Properties

The properties and parameters of the conventional design used are shown in Table 8.16.

Table 8.16: Properties of conventional designParameter Value Dimension FeatureMass 3595.08 [kg] Conventional designtplate 10.0 [mm] Outer platinghweb 174.2 [mm] Longitudinal girdertweb 11.3 [mm] T-profilewflange 156.8 [mm]tflange 11.3 [mm]hweb 134.0 [mm] Transverse framingtweb 8.7 [mm] T-profilewflange 120.6 [mm]tflange 8.7 [mm]tWebframe 9.0 [mm] Web frame

The longitudinal girders and the transverse framing are both built up using T-profile beams, which can be seen in Figure 8.44.


Figure 8.44: T-profile

The material data is used for the analysis is shown in Table 8.17

Table 8.17: Material data used in AnsysParameter Value DimensionDensity 7850 [kg/m3]Tensile yield strength 250 [MPa]Compressive Yield strength 250 [MPa]Tensile ultimate strength 460 [MPa]

FEM

The three load cases below are applied in the same way as for the SSY concept (seeFigures 8.28, 8.32 and 8.36)

Test load

A load of 1 MPa was applied to the entire area of the hull to check if the load transfertook place as it should.




From the Figures above it can be seen that the load transfer looks convincing.


Load case 1


Figure 8.48: Equivalent von Mises stress

Stress maxima/concentrationsFor the first load case the stress maximum is found at the intersection between theweb frame and the longitudinal stringer at about 72 MPa as seen in Figure 8.49c. This


maximum stress is found at a point looking somewhat like a stress concentration, thiscould possibly be a reason for easier failure due to for example cracking. The valuehowever is not higher then the tensile strength of the material.




DeformationLooking at the deformation shown in Figure 8.47 a maximum deformation of 0.74 mmis shown, which is acceptable for this load case.


Load case 2



Stress maxima/concentrationsFor the second load case the stress maximum is found on the outside of the hull atabout 722 MPa as seen in Figure 8.52c. This maximum stress is found at a point inthe middle of the square built up from the girder and stringer closest to the maximum.The value is higher then the tensile strength of the material which will create a failure.


(a) Zoomed out (b) Stress on inner structure



DeformationLooking at the deformation shown in Figure 8.50 a maximum deformation of 9.8 mm isshown, which is not even close to acceptable.


Load case 3



Stress maxima/concentrationsFor the third load case the stress maximum is found on the outside of the hull at about227 MPa as seen in Figure 8.55c. This maximum stress is found at a point in the middleof the square built up from the girder and stringer closest to the maximum. The valuehowever is not higher then the tensile strength of the material.


(a) Zoomed out (b) Stress on inner structure



DeformationLooking at the deformation shown in Figure 8.53 a maximum deformation of 3.1 mm isshown, which is acceptable.

8.3.6 ComparisonWhat both solutions have in common is that they both create a structure using squaresas shown in Figure 8.56. This means that the maximum deformation and highest vul-nerability for both designs is when the load is applied in the middle of this square.

The maximum stress especially in load case 1 and 3 is more evenly distributedthen the maximum deformation. The stress is high both at the middle of the squareand on the edges, where the load transfer takes place.


(a) SSY design (b) Conventional design

Figure 8.56: Structure build-up

The ice loads in general are loads that can have a wide range of magnitudes, muchdepending on ice thickness, type of ice and volume of the ice source. As discussed abovethe most critical loads are when the collision occurs in the middle of the "square" asmuch forward as possible on the vessel and very concentrated on one specific point.

SSY

The SSY design has one property that the conventional design does not have, the flex-ible stringers, these stringers show in the second and third load case that the stressconcentration is at some point on the top of the stringer, the first load case also hasthis behaviour, however in the intersection with a web frame. This would imply thatthe stringers, due to there flexibility, relieve the rest of the structure on stress and "con-sumes" it by the damping effect of the stringer.

To reduce the maximum stress on the SSY design a small bracket can be placedbetween the stringer and the web frame.

Conventional

The traditional design is more stiff then the SSY design, this can be seen examiningthe stress concentrations apprehended in this design. In the first load case the stressmaximum is at a point where the stringer and girder intersect, which most likely is dueto a stress concentration point which can be eliminated by designing the web frame en-tirely as a T-profile and lowering the height of it. What can be seen in the second andthird load case as well as in the first is that if this stress concentration is neglected isthat the stress maximum all occur on the middle of the square plate as seen in Figure8.56 on the outside of the structure. This implies that the squares are not relieved ofits stresses as the SSY design does by using the flexibility in the stringers. The stressmaxima are instead only on the outer plate.

To reduce the maximum stress on the conventional design the design needs to be


built with a thicker outer plating or more stringers and girders to reduce the stress onthe middle of the square.

Conclusion

Both designs seem to be built up resembling each other quite well in terms of struc-tural arrangements. The SSY design however uses flexible stringers which relieve theouter plate of stresses. By reducing these stresses in combination with the special it ispossible to reduce the weight of the structure.

To lower the maximum stresses on the structure the SSY design can have somesmall brackets added or having cut outs performed on the stringer, adding just a frac-tion of weight to it, whereas the conventional design needs more stringers/girders or athicker outer plating, adding a lot of weight to the structure.

The material plays a big part in for the different designs, this is a factor that hasto be considered. What if the conventional design uses the super duplex stainless steel.

8.4 Final designFinal design includes,

• Changes made to the design after feedback from review of the detailed design

For the final design the weight of the structure is lowered by creating an ice beltregion on the structure, reducing the thickness of the stringers not effected by the iceloads (the ones outside of the ice belt region). The stringers in blue represent the icebelt region as seen in Figure 8.57. This will not effect the FEM part in any particularway, the structure outside the ice belt area could get somewhat higher deflections butnothing major.

Figure 8.57: Final design


Hull thickness

The hull thickness is set to 10 mm.

Stringer design


Table 8.18: Stringer dimensionsParameter Value DimensionH 30 [mm]B 240 [mm]w 160 [mm]b 10 [mm]t 1.5 [mm]ticebelt 5 [mm]

Web frame design




Mass

3521 kg

Chapter 9

Results and discussion

In this chapter the results of the investigations are presented. Results regarding geo-metry and loads are presented and discussed upon.

9.1 GeometryThe outer geometry of the vessel is chosen to be kept the same as the concept provided.The basic reason for this is that the pros of having this design overcome the cons, thelargest part of the year there is no ice which favours the stream lined design. Due tothe fact that the vessel won’t break that much ice, the ice breaker design is not reallydesired, the most important thing is to reduce the brash ice resistance. Looking atevaluation performed in section 8.1.7 chancing the stem angle could be an option thatcould be further investigated, also adding a bulbous to the design could help reducingthe brash ice resistance. The bulbous though, will add more weight and work (labour)to the design.

9.2 DesignThe design created is based on a curved outer geometry which makes it harder to designa structure. As can be seen in Figure 9.1 for a design with straight side all the stringerscan be the same, whereas a curved side needs a different angle for each stringer due tothe non-linear curve, angle-wise.

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CHAPTER 9. RESULTS AND DISCUSSION 118

(a) Perpendicular

(b) Angle

Figure 9.1: Difference between designs

When designing the structure it is essential to be able to easily play around withthe model to find some kind of optimum structure that fits the requirements. Addingor removing the number of stringers is extremely difficult and time consuming. To geta hum of the behaviour a structure with straight sides could be created and playedaround with, this structure is easily created by applying patterns (see Figure 8.27)

9.2.1 Design parametersThe thickness’s of the design created are really thick compared to normal SSY designs,this is because of the difficulties in adding stringers to a curved design in the CADmodel. Instead the FEM has been performed using a slightly over dimensioned modelwhich will effect the results a bit. In section 8.3.4 a small analysis has been performedregarding different thickness’s of the structural parts using a simplified outer geometry.The results from this are that the hull and web frame thickness act linear in termsof thickness vs. stress/deformation. The different stringer thickness’s show about thesame deformation but different stresses.

The reason why the normal SSY designs have much thinner structural parts ismainly because the number of stringers is higher then in the design created in thisthesis. The increased number of stringers most likely increases the total flexibility andflexible sensitivity of the structure (can be compared by having 5 springs on a 1 m2

plate with a high spring constant vs. 50 springs on the same surface with a much lowerspring constant).

In this case it is however favourable having one or more thicker stringers at the icebelt region, increasing stiffness at that part of the structure and thereby allowing forhigher loads on that area. This is visualized in Figure 9.2 where the blue stringers canbe thickened and the grey stringers kept the usual thickness.


(a) Curved design

(b) Straight design

Figure 9.2: Strengthening in ice belt region

9.2.2 Flexibility of the entire structureSSY has had tendencies on there structures being to flexible globally, this can be solvedadding a traditional stringer (fixed beam) in the middle of the structure dividing it intotwo sections and thereby lowering the total deflection of the structure.


9.2.3 Fairness of comparisonThe comparison between the different methods is done in so called packages, the reasonfor this is that the whole concept of SSY is to produce vessels using the special designdeveloped by the company. By holding a world patent for the design and having a veryclose co-operation with Sandvik AB this design package is the only one SSY will use.SSY is also in the process of creating a super duplex stainless steel specially designedfor the company and meant to be used as the building material for their vessels.

To alter with this set up is therefore not done. A legitimate question would how-ever be how the combination conventional hull structure design and super duplex steelwould perform. The same goes for the combination SSY design with a traditional shipbuilding material.

• Conventional hull + SAF2507 the combination of using SAF205 on a con-ventional design is preferable for some reasons, the material works really well inharsh conditions and would thereby be well suited. It is however more expensivethen traditional steel and is hard to work with.

• SSY design + traditional material the combination using the SSY designand a traditional material would not work well. SAF2507 is used due to its abil-ity to not lose its properties when it is exposed to a high cycle of bending. No-ticeable for the SSY design is its flexible stringers, using a traditional materialthe structure will most likely fail in fatigue.

9.3 LoadsTo start with, the maximum deformation and stress for each load case and design areput up for comparison in Table 9.1 to get a good overview.

It has to be said that the designs created are not iterated to an optimal design interms of dimensions of the structural parts. The dimensions are chosen based on look-ing at other similar designs and using about the same dimensions.

Also, according to the discussion in section 8.3.6 the stress maxima on the SSYdesign have been reduced to the value obtained from adding a small bracket to thestructure.

Table 9.1: Result comparisonDesign Load Case 1 Load Case 2 Load Case 3

max def. [mm] max stress [MPa] max def. [mm] max stress [MPa] max def. [mm] max stress [MPa]SSY 2.8 140 11.4 600 1.24 80Conventional 0.74 70 9.8 720 3.1 230

The limits for each material are shown in Table 9.2


Table 9.2: MaterialsMaterial Tensile Yield Strength Tensile Ultimate Strength

(MPa) (MPa)SAF2507 550 800Structural Steel 250 460

9.3.1 Load Case 1For the first load case it can be seen that both designs are within the limit of the yieldstrength of the materials. The stress on the material is higher for the SSY design, how-ever due to the special steel this is acceptable.

9.3.2 Load Case 2The second load case is the crucial load case for the design, both the SSY and the con-ventional design show really bad values here.

This is also the load case which is the hardest to estimate the numerical value ofas discussed in Section 8.3.1. If the 10 MPa estimated is a correct value some differentactions can be taken,

• Increase dimensions of the structure to get the values within the limit.

• A probability analysis has to be performed on how often this size ice floe willoccur and what the likelihood is that a collision will occur with one. If it thenshows that the probability is really low a collision might be considered really un-likely and the maximum load case can be lowered.

• Allow some exceedance of the tensile yield strength, but not so high that thetensile ultimate strength is reached. If this load case would occur in an extremelyrare case, the structure could then be repaired if it has deformed. The structurecracking would of course be really bad.

Due to the smaller area of the load it matters where on the structure the load oc-curs. In Figure 9.3 the same load is applied on a different part of the structure, righton the intersection between a stringer and a web frame. This reduces the maximumdeformation to 3.0 mm and the maximum von Mises stress to 888.2 MPa, this is not asolution, but it shows that the second load case shows a worst case scenario. The worstcase scenario might also never occur during the life time of the vessel, it all depends onthe area of collision and where the collision occurs on the structure.


(a) Deformation (b) von Mises stress

Figure 9.3: Alternative second load case

The 10 MPa load estimated should however be further investigated, as seen thestructure has a hard time handling and logically it feels like it’s on the limit being tohigh. If the load is legitimate it is the critical dimensioning load for the structure.

9.3.3 Load Case 3The third load case is a bit similar to both the first and the second load case. The highpressure zones are represented by load case two at three different positions on a linealong the water line. Just as the first load case also the third load case is well withinthe limits of the materials.

Chapter 10

Conclusions

In this chapter the conclusions drawn from the investigations will be presented. Themain subjects discussed are the physical properties of ice, how the hull shape influencesthis kind of vessel and finally how the SSY design performs vs. the conventional design.

10.1 Ice theoryGenerally ice has different physical properties depending on temperature, salinity andtime frozen. This results in a lot of different types of ice, each with another property.This makes it very hard to generate a specific data sheet that applies to any type ofice, instead it is easier to create data for a specific area. With an increasing size of thisarea the certainty of the ice properties however decrease. The best way to get the de-sired data is therefore probably to perform actual tests in the desired area of operation.

The same goes for the different load cases, the ones used are at a best guess level.For a more precise load value an actual test is to be performed, either as a model testin an ice basin or by applying pressure meters to a ship hull and test the pressureswithin the operational area.

There are also the DNV guidelines for operation in ice to consider, these howeverare only applicable for vessel within a certain parameter range, where the vessel in thisreport not is included.

The ice theory was successfully presented, an overview was given on how a vesseloperates in winter conditions, what load patterns to expect, where the loads originatefrom, how a estimation on the resistance can be given and how the ice properties in-fluence the way a vessel should break the ice. It was also made clear on the challengesone faces when dealing with ice, the difference in ice properties depending on location,weather, salinity etc. and thereby the difficulty in constructing tools for giving preciseanswers regarding loads and resistances.

10.2 Bow geometryBecause the Stockholm area only has ice coverage for a smaller amount of the year thebow geometry has to be a trade off between minimizing resistance (open water and

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CHAPTER 10. CONCLUSIONS 124

ice) and ability to make its way through the ice. In the Stockholm area the waters arecovered by ice at maximum around 1/3 of the year, making the open water perform-ance the dominating factor. This implies that the current SSY design which is designedfor a low open water resistance is the best.

The analysis performed in this report is not done by any calculations due to thelack of good formulas, therefore, to get a good value on the open water and ice resist-ance either an CFD analysis (for the open water case) or a model towing tank test isrecommended.

From the bow geometry a more exact ranking of the different geometries was ex-pected to be done, based on the formulas examined in section 6.2. This was howeverreally hard due to the great similarity of the different geometries in terms of input val-ues to the formulas.

10.3 SSY vs. ConventionalThe pros with having the SSY design are found to be that the material properties ofthe SAF2507 allow for larger deformations and stresses on the structure as can be seenfrom the FEM-investigations. This together with the flexibility and thereby load ab-sorption makes it able for the structure to use its unique properties to absorb loads.The smaller load area of the ice floe compared to the a normal sea load does only focusthe load at a certain area on the structure, utilizing the structure properties basicallythe same way as a spread out load.

Here possibly the spring effect is even more of a pro, especially when the collisionis close to a stringer. Then the stringers spring function can absorb a lot of the loadwhile the traditional stringer just passes it on to the web frame.

When operating in ice the fatigue of the material is of a great concern due to theconsistent impact patter visualized in Figure 6.21. Duplex stainless steels which has ahigh tensile strength usually has a high fatigue limit and high resistance to both fatigueand corrosion fatigue. In the data sheet provided by Sandvik no number on fatigue aregiven, just that the material is really good in fatigue.

From the FEM-analysis performed it is hard to draw any decisive conclusions dueto the non-optimized structural arrangements and the struggles of creating a true rep-resentation of the desired SSY structure as discussed in section 9.2. It has howeverbeen possible to examine the locations of the stress concentrations as discussed below.

10.3.1 Stress concentrationsFor the SSY design the maximum stresses occur at the top of the stringer whereas forthe conventional design they are found in between the stringer and the girder in themiddle of the outer plate.

10.3.2 GeneralA comparison between what SSY offers and what the traditional industry offers wasperformed, in both cases a non-optimized structure was used. The optimization would

CHAPTER 10. CONCLUSIONS 125

refine the design, reducing stress concentrations, finding the ideal distances betweenstructural parts and minimizing the total weight. What has been found for a non-optimizedstructure is where the designs vary in vulnerability.

10.4 Further workThe work performed in this thesis is a fine overview of the challenges one copes withdesigning a vessel with the purpose of operating in ice. There are however still areaswhere more focus can be put. The ones found the most crucial are the following,

• Outer geometry - how does the hull shape create a streamlined flow of waterand ice around the vessel when operating in brash ice? This could possibly besolved doing a CFD-analysis of the scenario or a towing tank test.

• Actual load values - Are the values in the load cases legitimate? This can besolved by performing a test with a vessel already operating in the operationalarea. At a harsh winter day pressure sensors can be attached and a load patterncan be obtained. This load pattern will give more exact results on the maximumloads that the vessel will encounter.

Other areas that could be of interest are for example,

• Resistance of ice vs. speed in ice.

• Fatigue due to ice.

• Stability due to buoyancy of ice.

• How are the loads in the back of the vessel? Should there be strengthened stringersthere as well?

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