New Built Versus Converted

The 11th Annual

FPSO CONGRESS

2010

TECHNICAL CHALLENGES AND COST EFFECTIVE WAYS FOR CONVERSION OF DOUBLE HULL

TANKERS: LESSONS LEARNT FROM CONVERSION OF SINGLE HULL TANKERS

IBIBA EMMANUEL DOUGLAS

DEPARTMENT OF MARINE ENGINEERING

RIVERS STATE UNIVERSITY OF SCIENCE AND TECHNOLOGY

PORT HARCOURT, NIGERIA

Technical Challenges and Cost Effective Ways For Conversion of Double Hull Tankers: Lessons Learnt from Conversion of Single Hull Tankers

Marina Bay Sands, Singapore27th - 30th September, 2010

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CONTENTS• Objectives

• The FPSO: New (Purpose-built) versus converted (Fast Track) Vessel

• Conversion Phases/Processes

• Conversion Requirements/Specifications: Single and Double Hull

• Technical Challenges in FPSO conversion projects

• Integrated design Format

• Application of Design Software Systems

• Regulatory Regimes – Safety and Quality

• Design Methodology for Enhanced Life-cycle Management

(A New Paradigm)

• Techno-economic Appraisal/Analysis

OBJECTIVES• Highlight Technical difficulties in Tanker to FPSO conversion and

discuss solutions• Outline basic differences between single hull and double hull

tankers conversion• Establish efficient Quality-Safety Interface in the specific

processes of the conversion• Examine gainful utilization of single hull conversion experience

for the case of double hull• Explore efficient conversion methodologies from Feasibility

Studies through Engineering to Construction• Introduce a new Design paradigm for enhanced Tanker-FPSO

conversion ( a new frontier for FPSOs optimal conversion)• Introduce efficient combined Techno-economic optimization

procedure for Tanker FPSO conversion

The 11th AnnualFPSO CONGRESS

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The FPSO: New Built versus

ConvertedConverted New BuiltAdvantages Advantages• Marginal field development For higher volume development• Better Construction Schedules Used in highly severed

environment• lower Capex(<1/3 cost of NB) Lower Opex & more flexible for • Low severity environment field life extension• Favourable regulatory regime Optimal Design and latest

standards

in construction safety rulesLimitations Limitations• Stringent selection criteria Longer Project Schedules• Service life extension is limited Much higher Capex


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Technical Challenges in FPSO conversion projectsDesign and Engineering

� Insufficient Design Documentation ie drawings of vessel for conversion

� Adaptation of existing hullform and general arrangements to required specifications and safety and quality standards

� Harmonization of detailed Engineering features

� Complications in matching new features with old structures

Construction

� Lack of accurate as-built drawings and some design data

� Non availability of 3-D as-built models which are required by some design application softwares

� Insufficient fatigue analysis in the design of connections for primary and secondary members: especially for older generation vessels


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Possible solutions to the conversion problemsDesign and Engineering Phase

� Lack of accurate as-built drawings and 3-D models can be overcome by the use of laser scanning technology. This is currently being used in various fields.

� Most of the design associated problems can be resolved by the use of modern engineering application software systems that integrate geometrical and numerical modelling for all aspects of structural and hydrodynamic analysis.

Construction Phase

� Carry detailed fatigue analysis for all connections involving primary and secondary members

� Increase of structural stiffness and corrosion margin

� Use of High Tensile Steel (HTS)


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FPSO Conversion - Integrated Design ApproachProject Specifications

� Hull Type

� Storage Capacity

� Cargo and Ballast Tanks Layout

� Permanent or Disconnectable Systems

� Deck /Topsides Equipment/Facilities Layout

� Materials of Construction

� Mooring Arrangements

� Environmental Conditions of Service Location

� Required Service Life


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Tanker Hull Typesa. Single Hull c. Single Side – Double Bottom

b. Single Bottom – Double Side d. Double Hull


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STRUCTURAL DESIGN PROCEDURES

The structural design/analysis is implemented in 6 main steps namely;

� Determination of hull structural arrangement

� Determination and computation of loads

� Analysis/computation of structural response

� Analysis/computation of limit states

� Evaluation /assessment of structural adequacy

� Optimization of design


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Important Considerations in the Selection of Tanker for Conversion

� Structural drawings �Trading history�Previous thickness measurements�Condition and extent of protective coatings�Classification status including any outstanding conditions of class�Previous repairs�Typical load conditions and purpose of tanks (i.e. cargo, clean ballast, dirty ballast, segregated ballast)

Typical defects:CorrosionBucklingCracks


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Parameters for Intact Stability Code


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GZ

GZ max

GM

300

E30

φ57.27o Heeling angle

INTACT STABILITY CRITERIA


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The main design criteria of the code are:

1. E30 ≥0.055m.rad; E30 is the area under the static stability

curve up to 300

E40 ≥0.09m.rad; corresponding to area up to 400

E40 – E30 ≥0.03m.rad; corresponding to area between 300 and

400

INTACT STABILITY CRITERIA CONT’D


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If the angle of flooding φφφφf is less than 400 φφφφf instead of 40

0 is to be

used in the above rules.

2. h30o ≥0.20m; h30

o is the righting lever at 30

o heel.

3. The maximum righting lever must be at an angle φφφφ ≥250

4. The initial metacentric height GM0 = 0.15m or greater.

WEATHER CRITERIA


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Restoring lever

φ1 = φ3 - φ0

Flooding Angle

LW2

D GZ

AR

AH

B

0 φ 0

LW1

φ3 φ0

φ2 φc

PARAMETERS OF WEATHER CRITERIA


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φφφφ0 – initial heel angle under the action of steady wind with lever

LW1 – constant heeling lever which is independent of the heel

angle.

φφφφ1 – maximum angle of roll on the weather side due to side wave

action.

φφφφ2 – limiting roll angle on the lee-side enclosed to the left and right

sides respectively.

PARAMETERS OF WEATHER CRITERIA CONT’D


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AR, AH – areas enclosed to the left and right sides respectively

between the restoring lever curve and a constant heeling lever lw2

due to wind gust.

The value of lw2 is given as 1.5 lw1.

Components/Items requiring detailed analysis in FPSO design( New built and Converted)

� Hull Midship Section

� Bow Structure (against Slamming pressures)

� Cargo Tanks (against Sloshing pressures)

� Turret

� Station Keeping System

� Fluid Transfer System

� Deck and Topsides

� Helideck – Helicopter Landing Platform

� Fatigue ( for Hull, Supports/Foundations, Connections, Fluid Transfer Systems etc)

� Stability: Intact and Damaged


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S/No Design

Element

Primary Ship Secondary Ship

1 Owner’s

Requirements

INPUT

Specifications missions and General

requirements for primary ship

-specifications/ mission and general

requirements for conversion options

(secondary ships)

OUTPUT

Representation set of owner’s

requirements obtained by

harmonization of owner’s

requirements for all specified

candidate ship; the primary ship

is given full weight

INPUT

Representative set of owner’s requirements

generated for the primary ship design

Owner’s requirement specific to the proposed

conversion

OUTPUT

Important differences harmonized requirements

between for the primary ship and those specific

to the proposed conversion option.

Owner’s Requirements: comparison of primary and converted designs


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Wave Loads on Ships and Offshore Structures are Complex and difficult to analyze

Require efficient Fluid-StructureAnalysis/Computational methods

Reliable and cost effective design and operation of a ship or offshore structure requires proper analysis of waves and other environmentalloads as well as internal loads onthe structure.

HULL GIRDER STRENGTH DETERMINATION


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0

0.5

1

1.5

2

2.5

3

0.2

0.4

0.6

0.8 1

1.2

1.4

1.6

1.8 2

2.2

2.4

2.6

2.8 3

Wave spectra: Pierson and Moskowitz (1964)

Derivation of Seaway Wave Spectrum


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-8

-6

-4

-2

0

2

4

6

8

0 100 200 300 400 500 600 700 800 900 1000

Period Freq Amplitude MeanSq

sec rad/sec

31.42 0.2 1.69 1.43

15.71 0.4 2.20 2.43

10.47 0.6 4.29 9.20

7.85 0.8 7.35 27.00

6.28 1 6.79 23.04

5.24 1.2 5.09 12.94

4.49 1.4 3.35 5.61

3.93 1.6 3.81 7.27

3.49 1.8 3.26 5.30

3.14 2 2.14 2.30

2.86 2.2 3.20 5.12

2.62 2.4 2.03 2.06

2.42 2.6 2.90 4.21

2.24 2.8 1.35 0.91

2.09 3 1.05 0.55

( ) ( )∫ ∑∞

=

δωω≈ωω=

0

N

1j

jjSdS area Spectrum

Superimposition of simple wavesSimple waveWave Superposition

Superimpose waves travelling in various directions (χk) to simulate a “short crested sea”


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( )∑=

δ+ω−=ηN

1j

jjjj txkcosA

-8

-6

-4

-2

0

2

4

6

8

0 100 200 300 400 500 600 700 800 900 1000

( )[ ]∑∑= =

δ+ω−χ+χ=ηM

1k

N

1j

jjkkjk,j tsinycosxkcosA

Determination of Modification Options and

Requirements


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�Determine tanker requiring minimal modification from

list of candidate tankers

�Assess stability and strength characteristics of selected

tankers

�Determine modification requirements

�Carryout preliminary design analysis of proposed

modifications;

i. Hydrostatics/Hydrodynamics Analysis

ii. Structural Analysis

Final Design

� Determine deck and internal hull arrangement

� Determine Hydrostatics/Hydrodynamics

characteristics and carry out stability analysis

� Carry out structural analysis

� Iterate to converge at required design


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Structural Modification Requirements


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�Create extra deck space for process facilities, pipe rack, additional accommodation and office space

Strategies and options for increasing deck space are;i. Increase Hull breadth ii. Increase Hull Length

Either option will require re-evaluation of stability and strength characteristics.

Due to the substantial increase in deck load required, modifications of Tankers to FPSOs require extensive structural analysis

Application of Ship Structural Design SoftwareMost of the software systems for ship structural design utilize a first

principle approach to the design and analysis of Hull Structures.

However, since they are also required to satisfy the requirements of

the classification societies, the they can be used in two phases:

The first phase establishes the initial minimum required scantling for a

vessel using the applicable Classification Society’s rules which includes

consideration of ;

� Hull girder longitudinal bending and shear strength

� Local strength of shell and bulkhead plating and associated stiffeners.

� Strength of main supporting members and fatigue assessment.

This is used generally for conceptual design, feasibility studies and

design definition


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The second phase employs the Finite Element Analysis (FEA) method to assess the strength of major portions of the hull and to confirm the minimum scantlings in the phase. The main use of the second phase is for detailed strength analysis and final design validation.


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Elements of the ABS SafeHull Design

and Analysis Processes

Phase A.� generates midship section geometric model

� Calculates stiffener spacing, plating spacing, unsupported span,

corrosion margin

� Calculates Hull girder section modulus

� Calculates required minimum value for longitudinal scantlings

according to the applicable ABS Rules

� Calculates required minimum value of the main supporting

members and the transverse scantlings according to the Rules.

� Calculates the minimum required scantlings for the double bottom

floors and girders.


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Phase A continues

�Calculates required thickness based on shear force and allowable still

water shear force

�Performs simplified fatigue assessment based on the relevant ABS Rules;

the combined load range and corresponding stress range are determined,

permissible stress range is determined based on the ships length,

structure type, and fatigue classification (S-N)

�Performs both a simplified and detailed (if needed,) sloshing check

�Calculates the required minimum value for the longitudinal scantlings

(according to the Rules) based on sloshing pressure requirement

�Calculates required scantlings of side shell plating and side longitudinals

in the fore body region.

�Calculates the required scantlings of side transverse and stringers in the

fore body region


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Phase B

Finite element analysis

�Generates wave loads and/or external Local point Loads for the 3D

model

�Generates sloshing loads for the 3D model

�Generates ballast loads for the 3D model

�Checks the 3D model against yielding criteria for small panels

�Check the 3D model against buckling criteria for small panels

�Generates tabular reports for small panel yielding and buckling

assessments


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Phase B continues

� Check s the 3D model against buckling criteria for large panels

� Check s the 3D model against buckling criteria for local corrugated

panels

� Checks the 3D model against buckling criteria for large

corrugated panels

� Checks local model against yielding criteria

� Check s local model against buckling criteria


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ECONOMIC EVALUATION

Important Cost considerations in a conversion project

include;

� Revenue loss during off-trade period

� Cost for Initial Vessel Selection Feasibility studies

� Design, Engineering and Certification costs

� Actual Conversion Costs (i.e. Materials and Labour)

� Cost of Towing and site installations and

Commissioning


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Expressing Cost of Items as functions of Time gives for the L-th Cost

Component, the Cumulative costs for k-years as;

CC(l,k)=∑ ��1, �� = ��, 1� + ��, 2� + − − +��, �� =1

For item 1

CC(1,k) = ∑ ��1, �� = ��1,1� + ��1,2� + − − +��1, ��=1

For item 2;

CC(2,k) = ∑ ��2, �� = ��2,1� + ��2,2� + − − +��2, ��=1

For item m;

CC(m,k)=∑ ��, �� = ��, 1� + ��, 2� + − − +��, ��=1

Similarly Total variable Costs for the First year can be expressed as;

� ��, 1��

� =1


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For the Nth

year the Total Variable Costs for all the items will be;

� ��, ��

� =1

Cumulative Variable Cost in V years will be;

CC(j,v) = ∑ {�� =1 ∑ ��, ��=1 }

Total Cost inclusive of initial Costs in V years will be;

CCT(j,v) = CCIN + ∑ ��, �� =1

DERIVATION OF REVENUE FUNCTION INVESTMENT EVALUATION EQUATION


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The Cumulative Revenue function for k-years can be expressed as;

RC(k) =∑ �� = ��1� + ��2� + − − +��=1

Putting CCT(j,v) = f1(t) and RC(k)=f2(t)

Gives the following Investment Evaluation Equation;

f2(t) - f1(t) = 0

Solution of this equation gives the Pay Back Period(PBP)

N


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LAMP

SYSTEM

Finite-Element

Interface

Slamming

Calculation

Wave FrequencyLoads

ImpactPressure

Finite-Element

Analysis

Ship Motions

Calculation

ShipMotions

Ship & Wave

Description

Loading for

FE Analysis

Dynamic Beam

Calculation

ImpactForces

Total MainGirder Loads

Detailed

Stresses

Wave FrequencyPressure

Input

Pre-Processing

General

Post-Processing

• Statistics

• RAOs

• Visualization

• Derived quantities

CONCLUSIONSFPSOs offer clear technical and economic advantages over other

competing options in area of exploiting marginal fields and those

located in deep/ultra deep waters.

Building new FPSOs are generally associated with higher costs and

longer construction periods than converting existing trading

tankers to FPSOs.

It is clear that a poor choice of tanker for conversion, and inefficient

project planning and implementation can erode most of the

benefits of the conversion option

• It is possible and will be beneficial if FPSO conversion designsadopt more holistic approach that incorporates other conversion option


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Conclusions continues

Conversion of trading Tankers to FPSOs to high standards will generally require proper evaluation of the stability and strength characteristics of the converted vessel.

There are presently computer software systems that have the capacity to handle detailed hydrodynamics and structural analysis efficiently and cost effectively.


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Thank you


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Documents

New Built Versus Converted