Charis Ntontoros (Dodoros) presentation at ANSYS Convergence Conference 2016

ANSYS Convergence Regional Conference in Athens

Charis Ntontoros (Dodoros)

30th of June, 2016

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Introductory Information:

Experienced on Strength Calculations of Ship and Offshore

Structures

Independent Structural Engineer based in Greece

Partner of C-Job & Partners BV

Naval Architecture and Engineering Office based in the

Netherlands

Project References: Passenger, Cargo Ships – Heavy Lift

Vessels with Cranes of Operational Capacity up to 150 tons,

Yachts…

Dredgers, Rock Dumping Vessels, Jack-Up Vessels, Tugs..

Able to Perform Projects from Concept to Detail Design


30-6-2016ANSYS Convergence Regional Conference in Athens

30-6-20163

Current Presentation Demonstrates two Analyses

Performed with Ansys:

Part A:

Static Analysis on Rock-Dumping Fallpipe Tower Structure

Part B:

Vibration Analysis on Thruster Foundation Structure



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A Few Words on the Project:

Clients based on the maritime sector always need to increase

their operational abilities by upgrading their offshore structure

capabilities

Rock-dumping vessels of 26,000 tons loading capacity are used

to install rocks in water depths up to 1,500 meters by means of

flexible fallpipe buckets

Part A: Static Analysis on Rock-Dumping Fallpipe Tower

Structure


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Rock - Dumping Vessel in Figures…


Structure


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Rock - Dumping Vessel in Figures…


Structure

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Fall Pipe Tower


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A Few Words on the Project :

The fall pipe tower, located at the deck of the vessel, is designated to carry the resulting loads from installed equipment during operation

The structure has been designed to operate in the most efficient way in respect to the steel strength characteristics, operational effectiveness and weight optimization

34 different equipment items are installed on the fall pipe tower structure, such as Winches and Pulleys

Several Working and Sailing Scenarios can occur during the lifetime of the vessel

Multiple Load Cases (about 40) have been investigated

All Results are assessed in accordance to the rules and regulations provided by the certified classification societies

Using Ansys, a representation of the Stress Occurrences and Deflections is succeeded


Structure


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Pre-Processing: Creating the Structural Model

Model has been created in SpaceClaim

Model consists of Surfaces and Lines meshed with Plates and

Beam Elements

Some parts were already modeled with solids in other designing

software and later imported in .STEP files in SC

Solids have been converted to Midsurfaces through Midsurface

Tool in SC

All parts of the model have been checked for their connectivity

(Shared Topology)

Shared Node mesh has been achieved.

Thickness property has been assigned in SC

Parts which have been converted from solids to midsurfaces have

kept their thickness property in SC


Structure


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Pre-Processing: Preparing the Calculation Model

Importing to Ansys Mechanical

Preparing Name Selections and Meshing

Multiple Element Size has been Assigned so as to Minimize

Calculation Time


Structure


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Element Types Used:

Shell181, Beam188, Mass21, Conta175, Target170

Tower Structure Consists of Beam Elements

Ship Structure (Hull) Consists of Plate Elements


Structure


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Structure


Tower Meshed with

Beam Elements

Hull Meshed with

Plate Elements

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The beam like structure has been modeled with surfaces for 1

meter height above deck so as to smoothly transit the stresses on

the hull structure

Unrealistic Hot Spots were avoided

Tower’s Beam and Ship’s Plate Element Nodes were Connected

with Contact Elements

MPC Formulation with Coupled U to ROT This option is useful

when you wish to fully constrain one contact side completely to

another.


Structure


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Structure


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Multiple equipment was installed such as Winches, Pulleys etc.

Equipment itself was not modeled

Equipment mass was applied at their actual CoG with point

masses

In order to simulate the rigidity of the equipment the dummy

beams connecting the point mass to the structure were assigned

with rigid behavior


Structure


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Structure


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Boundary Conditions:

The model has been fixed constrained on the nodes of the plate

elements at the lower end


Structure


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Post-Processing: Calculation & Validation of Analysis

Extend of the Model - Validation

Stresses were decreased at the lower part of the model

Stresses were increased close to the boundaries

Extension of the model was finally sufficient

Reaction Forces retrieved were equal to the sum of the applied

loads and self weight of the structure


Structure


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Post-Processing: Calculation & Results

Stress Results:

Equivalent and Shear Stresses as required from the classification

societies

Top/Bottom – Including out of plane bending (conservative approach)

Stiffness Results:

Total Deflection


Structure


Post-Processing: Calculation & Validation of Analysis

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Stress Results: Stiffness Results:


Structure


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Post-Processing: Detailed Analysis, SubModeling &

Reporting

Detailed stress analysis:

Focusing on peak stresses

Averaging Peak Stress values on the extend defined by Rules

Requesting Membrane Equivalent Stresses with User Defined Result

Sub-Modeling:

Sub-models with refined mesh have been created from the global

model in areas of interest

Exporting Stress Plots - Reporting:

Automatically export defined stress plots with “Export Figures” Add-In

downloaded from the Ansys Customer Portal


Structure


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Self-elevating, Jack-Up vessels are commonly used for multiple

purposes in offshore projects, from drilling up to 114 meters

depth, to windmill park installations

Part B: Vibration Analysis on Thruster Foundation Structure


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The Jack-Up Vessel in Figures…



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The several structural areas of such vessels require detailed

engineering analysis

One of these areas are the Thruster Foundations on which the

Rudder Propellers are bolted at



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The Thruster Foundation & Rudder Propeller:



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Thruster foundations are subjected to vibrational loads originated

by the rudder propellers

The Rudder Propellers are designed by the manufacturer to

operate in a certain frequency range

In accordance to the rudder propeller suppliers, the propeller

frequencies are to be “located” in a 25% range away from the

thruster foundation eigenfrequencies (natural frequencies), so as

to avoid excitation



The Thruster Foundation

Imported to Ansys

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A Few Words on the Project :

First, a modal analysis is performed

The frequencies under which the Thruster Foundation excites are then known

Some of the propeller frequencies given by the manufacturer were belonging in the range of the foundation’s eigenfrequencies, identified in the modal analysis

An Harmonic (Vibrational) Analysis was required

With the Harmonic Analysis it is possible to apply loads on a given frequency range

During the Harmonic Analysis the Force produced by the propeller was induced on the foundation structure on the given frequency by the manufacturer

The Foundation Structure was excited by the propeller induced force

The expected peak stresses were then noted



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Pre-Processing:

Modeling and Preparation of the Model in Ansys for calculation, is

very similar to the method followed for the Tower structure project

No further explanation will be provided on this part



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Post-Processing: Modal Analysis

It was known by the Rudder Propeller Manufacturer that the

propeller frequency range was between 15Hz and 25Hz

During the Modal Analysis the first 20 Modes of the Thruster

Foundation have been requested

From the 20 Modes, the Natural Frequency (Eigenfrequency) for

which resonance was noted on the Thruster Foundation was

equal to 21.853Hz and 23.267 Hz

2nd and 4th Mode respectively



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Post-Processing: Modal Analysis



2nd Natural Frequency Noted on

the Foundation Structure, equal

to 21.853Hz

4th Natural Frequency Noted on

the Foundation Structure, equal

to 23.267Hz

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Pre-Processing: Harmonic Analysis (Forced Vibration)

2 type of loads have been applied (Moments and Forces)

A point mass was representing the mass of the Rudder Propeller

Forces, Moments and Point Mass have been Applied on the

location where the Rudder Propeller was bolted at, with a Remote

Point

Both moment and force loads are applied in the same phase

angle in a sinusoidal manner

Moments and Forces have been applied under a frequency range

between 15Hz to 25Hz



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Post-Processing: Harmonic Analysis (Forced Vibration)

Peak stresses are noted at the frequency area close to the

eigenfrequency of the foundation structure

Relevant Graphs are exported from Ansys Mechanical

Peak Stress at 21.800Hz Close to 2nd Natural Frequency

Peak Stress at 23.225Hz Close to 4th Natural Frequency

Max. Peak Stress about 80 Mpa

Can reduce the service life of the vessel



@23.2

25H

z

@21.8

00H

z

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Post-Processing: Harmonic Analysis (Forced Vibration)

Relevant Stress Plots are exported from Ansys Mechanical at the

frequencies where peaks are noted. @ 23.225Hz



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Post-Processing: Conclusions

It is concluded that a fatigue analysis is required for the

assessment of the service life of the structure under the cyclic

loads imposed by rudder propeller

Additional reinforcement will further increase the service live of

the vessel



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!THANK YOU FOR YOU ATTENTION!



18-3-2015Froude Lunchlezing35

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