ANALYSES OF SHIP STRUCTURES USING ANSYS AAC/Tech Paper/ANSYS Conference 2008 Paper... · Analysis of ship structures using ANSYS SeaTech Solutions International (S) Pte Ltd 1 ANALYSES

Analysis of ship structures using ANSYS SeaTech Solutions International (S) Pte Ltd

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ANALYSES OF SHIP STRUCTURES USING ANSYS Suman Kar, D.G. Sarangdhar & G.S. Chopra

SeaTech Solutions International (S) Pte Ltd

E-Mail:[email protected]

Abstract

This paper describes the use of ANSYS Structural in the simulation of the complex ship structure and the

various loading conditions that a ship experiences during its operation.

The ship, during operation experiences complex loading conditions, which is generally divided into few

categories: Linear Static and Dynamic Loads, Thermal Loads and Complex Non-Linear Dynamic Loads.

Generally the loading on the ship is a combination of some or all of the load categories mentioned above,

depending on the type of the ship.

The first part of the paper is dedicated to Static and Linear Dynamic Analysis and its combinations. This is

used to analyze merchant and Naval ships. Static Analysis is usually done to find the overall strength of the

structure. For this, in addition to hydrostatic and hydrodynamic loadings from exterior, the local loads due

to ballast tanks; cargo loads etc are also considered. In the domain of Dynamic Analysis, Vibration

Analysis (Free and forced) is done for all ships/components to check if the structure is dynamically stable

or not. The natural frequencies of the structure are compared with the forcing frequency to check for

resonance.

The second part of the paper deals with Complex Non- Linear Static and Dynamic Analysis, which is either

used for research or for analysis of Ship local components / regions. These analysis includes analysis of

Forward part of the ship due to slamming loads, Analysis of Floating structures due to underwater

Explosions, transient dynamic analysis of heli-decks due to crash landing of helicopters etc and Ultimate

strength Analysis of stiffened ship panels and Midship sections of various ships. Both Geometric and

Material Non-Linearity are extensively incorporated to analyze these components/regions, which gives near

accurate results. Ships, which carry LNG, require a Thermal Analysis to be done to check the structural

integrity of these LNG Carriers. Hence for these ships ANSYS is used for Thermal Analysis.

Keywords: Ships, ANSYS Mechanical, Static Analysis, Vibration Analysis, Transient Dynamic Analysis,

Ultimate Strength Analysis, Thermal Analysis

1. Introduction

Provision of ‘adequate’ strength in a ship at a reasonable cost, has always been one of the most challenging

task for the ship designers. Over the years the classification societies have been providing the necessary

standards to ensure the adequacy of strength against all demands that can be envisaged during service life

of the ship. Earlier, these formulations were largely based on experience of good ships and the scantling

requirements were given in simple tabular forms based on few basic parameters like main particulars of the

ship. Subsequently, from around 1970, there was significant change in the class rule formulations, wherein

the requirements were specified by various formulae for environmental loads and allowable stresses based

on simple principles of structural mechanics. Though these formulations were more scientific, the designer

did not explicitly know the safety margins in the formulation and inherent assumptions made, so it was

difficult to use such formulations for a novel design.

In the last two decades there has been a rapid development in the fields of hydrodynamics and structural

analysis. The advanced methods developed now provide a better understanding of environmental loads

(Demand) and strength of ship (Capacity). As these methods became more and more mature, providing

reliable tools for structural optimization, it was only logical that they were incorporated in the classification

rules for ship and components design and analysis.


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2. Advanced methods for assessment of structural strength

Behavior of ship in a seaway has been traditionally assessed by means of sea keeping model tests. The

procedures for model testing and subsequently scaling of the results for actual ship size are now well

established and are quite reliable. However one of the drawbacks with the experimental methods is the

large cost associated with it. With the advancement in the structural and hydrodynamics theories today it is

possible to predict the ship behavior to an acceptable reliability by means of numerical simulation.

Once the sets of various realistic loads applicable to a vessel are determined, the combined effects of these

loads are assessed to determine the overall strength of the ship structure. Advanced methods based on

Finite Element Method are best suited to analyze complex structures in three dimensions. With the

advancement of Numerical analysis methods and availability of computing facility at affordable costs, use

of such methods provides more reliable and direct assessment.

3. Ship and Allied Structures

Ships and its components are generally assessed for Strength and Vibration under normal operation and

Strength assessment under critical loading condition like collision or underwater explosion etc. Assessment

of strength is now a days based on ‘Direct Strength Analysis’ which eliminates any error in judgment,

which may arise out of several assumptions that otherwise have to be made regarding the interaction

between several structural members. The method takes into account the effect of bending, shear, and axial

and torsional deformations all together. Thus it is very useful for larger Ships with complex structural

arrangements and large variety of cargo / ballast loading conditions. There major aspects of structural

analysis are (i) Creation of structural model for FE analysis (ii) Calculation of loads and transfer to the

structural elements (iii) FE analysis (iv) Extraction of results, checks and report.

3.1 Modeling

The complex ship structure, which may vary from 100 to 300 meters in length: 7 to 30 meters in breadth

and 5 to 30 meters in depth, is modeled completely using ANSYS Pre processor. The plates are modeled

using Shell 63/ Shell 43/Shell 181 Elements. The stiffeners are modeled using Beam 4/Beam 44/ Beam 188

Elements. All the pillars are modeled using Pipe elements and all other structural masses are modeled using

Mass elements.

The modeling of the entire ship structure with all details consumes lot of time and is usually cumbersome.

Hence ANSYS macros and APDL’s are extensively used in modeling. This reduces lot of modeling time

and manual errors.

Ones the model is completed, each of the plates, Stiffeners, Pipes and masses are individually selected and

the real constants calculated and inputted in the tabular form to model each component of the structure with

complete accuracy. This process is also automated at times with the use of ANSYS macros and APDL’s

3.2 Meshing

After the complete structure is modeled, the plates, stiffeners, pipes and masses are individually meshed.

The last step to be completed before meshing the model is to set the meshing controls, i.e. the element

shape, size, the number of divisions per line, etc. Selecting the various parts of the model, one by one finite

element mesh is generated. The critical portions are plates with sharp corners, curvature etc. These areas

can be remeshed with advance mesh control options. "Smart element sizing" is a meshing feature that

creates initial element sizes for free meshing operation.

Proper care has to be taken to have the control over the number of elements and hence the number of

degrees of freedom associated with the structure. This is done to have a control over the solution time.

However, no compromise is made on the accuracy of the results.


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3.3 Loading

Loads are generally estimated using the classification rules or by direct hydrodynamic calculations. The

loads that a ship experiences during its voyage can be roughly divided into two parts.

Static Loads – These consists of loads, which do not vary with time, or even if they vary, the effect of

time could be neglected. The hydrostatic pressure, Weights of the ship components, Cargo and Ballast

loads come under this category. In addition to these wave moments and forces coming due to ship

components are also considered as static loads.

Dynamic Load – These are the loads, which vary with time, and the variation is substantially large

because of which a dynamic analysis is generally required. The hydrodynamic Pressure due to waves,

Wind Loads and other operational Loads like loads due to underwater Explosion, Machinery

operational loads etc., are the loads, which are considered as dynamic loads.

Both the above categories of loads, would act on ship and its components from time to time. Hence it

becomes essential to consider the loads correctly and analyze the structure accordingly. Use of ANSYS

makes the process of application of load very simple and manageable., also the chances of errors in

combining the loads is eliminated.

3.4 Analysis

Ones the loads are determined; different structural analyses can be carried out based on needs. The results

from these analyses are extracted and checked for yielding, buckling and ultimate strength automatically at

desired locations.

Different Analyses that are usually performed are 1) Stress Analysis of Ship Structures and components

2) Vibration Analysis of Ship Structures 3) Ultimate Strength Analysis 4) Transient Dynamic Analysis and

Strength Analysis under Impact loading 6) Thermal Analysis

Before these analyses could be actually performed on the full-scale model, they are always backed up with

a thorough in-house research on simpler structures like plate or plate with stiffeners etc. Suitable ANSYS

models are made and appropriately loaded and boundary conditions are applied. Various analyses are then

performed and the process is thoroughly checked and fine-tuned for mesh size, element type, time step (for

transient dynamic and Non- Linear analysis) and other FE properties. The results of these analyses are

checked with either published or experimental results. Ones these methods are established they are

performed on the complex ship structures and components.

This reduces the chances of errors due to wrong or un-established methods and as the FE parameters are

thoroughly checked, the results are close to the actual results. Further the analysis, which are done on

complex ships structures are also verified by measurements on actual ship structures. Once the results are

verified, the method is finalized for subsequent analyses.

Some of the analyses are briefly described below.

3.4.1 Stress Analysis of Ship Structures and components

Stress analysis is usually performed to find the overall strength of the ship and its components. As there are

always two or more loads acting on the structure at a time and as these loads change, based on the loading

condition, hence it becomes necessary to prepare different load cases and then to analyze all or some of the

load cases independently, as required. The number of load cases to be investigated depends on i) the

number of envisaged cargo and ballast loading conditions and ii) number of wave cases (snapshot load sets)

to be considered for each ship loading case. Sometimes it becomes necessary to carry out the structural

analyses for a large number of load cases, making it absolutely essential to develop automatic processes.

Thus several interfacing software programs, which connect with the FE analysis software, have been

specially developed e.g. for transfer of all load data into the FE model and apply automatically. ANSYS,

and the use of its macro and APDL programming helps a lot in making this process simple and error free. A

simple static analysis is usually performed taking Non-Linearity (if required) into consideration.


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The results are extracted and checked for yielding, buckling and ultimate strength automatically at desired

locations.

Few Stress analysis projects are briefly described below

3.4.1.1 Advanced Analyses of Complex Structural Systems (Bulk carrier) Using FE Codes- The

complex modeling of the complete Bulk Carrier, (Fig. 1) detailing each of its components requires

advanced modeling technique and a keen technical mind. We model Bulk Carriers and other large vessels

and perform direct strength Analysis with varied load cases to check the strength of the vessel.

Fig 1. Bulk Carrier Model (Stress Distribution) Fig 2. VLCC Tanker Model

3.4.1.2 Optimization Of Scantlings For VLCC Tanker By Finite Element Method - On a project for

unification of Ship Scantlings, a proposal of IACS working group, we had carried out calculations of

scantlings, based on IRS own rules and direct structural analysis by finite element method for Panamax

bulk carrier (Fig. 2) and a double hull tanker of 300000 DWT. The efficient use of FE (using ANSYS)

and the results were well appreciated at international forums.

3.4.1.3 Buckling Analysis of Mini Bulk Carrier - The Static (structural stress) analysis of Mini Bulk

Carrier (Fig 3) was carried out to check the strength adequacy due to local (Hydro static and hydro

dynamic pressure and cargo load) and global loads (Still water bending moment and wave bending

moment). For this various pressures, Forces and Moments were all applied simultaneously for the

analysis. The hatch coaming area has been specially investigated for buckling for various axial loads.

The complete project was done in a record time, which would have not been possible without the use

of advanced FE software, ANSYS.

Fig 3. Mini Bulk Carrier (Stress Distribution) Fig 4. FE Model of Fast patrol Vessel


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3.4.1.4 Static And Dynamic Analyses Of Fast Patrol Vessel - A static and free vibration analysis of

the vessel was done to simulate the loads resulting from lightweight, high speed and slamming

pressure. The vessel, made up of steel and aluminum (Fig 4), has to be checked against the water jets

and slamming loads. The static analysis was carried out under these loading conditions. The free

vibration analysis of the entire vessel and superstructure were performed to estimate the natural

frequency and the mode shapes for avoiding resonance with other machinery components.

3.4.1.5 Transverse Strength Analysis of LSTL Hull Structure - The transverse strength analysis is

done to simulate the loads due to oblique wave, rolling of the ship and unsymmetrical cargo loading.

The transverse strength computations of the Landing Ship Tank (Large) [LST (L)], (Fig 5) has been

examined for following loading conditions: a) Lightship b) Transport role ‘A’ & ‘B’ vehicles –

Departure c) Transport role ‘A’ & ‘B’ vehicles – Arrival d) Beaching role ‘A’ & ‘B’ vehicles – Arrival

e) Replenishment role – Departure f) Replenishment role – Arrival. The FE results helped, the

shipyard, a lot to modify the design to withstand these loads.

Fig 5. FE Model of LST (L) -Stress Distribution Fig 6. Effect of Torsional Loads on Acid Carrier

3.4.1.6 Effect Of Torsional Loads On An Acid Carrier Of Wide Hatch Opening - Torsional analysis

is important for vessels with wide hatch openings and single bottom structure (Fig 6). The main

objective of this study was to develop a method to improve the structural adequacy with respect to

warping displacement and stresses, by 3-D finite element method. The analysis was carried out using

ANSYS, for cargo loading, dynamic forces and hydrodynamic torque due to oblique wave with

appropriate boundary conditions. The results were verified with the one, available in literature and

were found satisfactorily matching. Hence suitable modifications were suggested to the client.

3.4.1.7 Flexure analysis of Advanced Offshore Patrol Vessel - The objective of this project was to

determine the axial stress on longitudinal member as well as on super structure due to total hull girder

bending moment. Full ship (Fig 7) has been modeled from frame no.32 to frame no.140 so that whole

superstructure is covered. Total bending moment was applied on both ends of model. The results

showed that the axial stress on one of the deck was more than the critical buckling strength. Hence,

modifications were suggested to the client.

Fig 7. Deflected Shape of AOPV Fig 8. Ship at Campus


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3.4.1.8 Ship at Campus - The Ship at Campus, designed to be used to train Seafarer in general and

engineering training in particular. This 23 m long (950 tones DWT), half ship (Fig 8) will be erected

on a concrete foundation. The main engine will be kept at a separate concrete foundation to avoid

transmission of vibrations to the main hull. ANSYS was used to help the client to design the scantlings

of both ship and the foundation. Finite Element analysis, using ANSYS, was carried out to calculate

the loads on the foundation and strengthen the moon-pool area. The scantlings were optimized by

detailed analysis to safely transmit the required dead weight and lightweight into foundations.

3.4.1.9 3-D static analysis of cold water box for OTEC barge - This structure is connected to the deck

of the barge of an Ocean Thermal Energy Conversion plant, (Fig 9) which generates electricity from

the seawater temperature gradient. The cold water pipes are attached with this structure from 2 km

depth of seabed. The self-weight of the box, hydrostatic, hydrodynamic (wave &current), buoyancy

and pipe weight were considered as the applied loads. The static analysis was carried out using a

simplified model to find out the strength adequacy of the structure under the applied loads.

Fig 9. Cold Water Box on Barge Fig 10. Gun Support Structure

3.4.1.10 Gun Support Structures - The natural frequencies and stresses for stated recoil loads and

loads due to wind, ship motions etc were analyzed using ANSYS for the Gun Support Structure (Fig

10) of a vessel for a foreign agency. Different loading cases were analyzed to ensure the adequacy of

the structure in all possible load combinations. Modifications were suggested based on thorough study

of the results.

3.4.1.11 Design Analysis of Forward part of Hull Structure for Air Defense Ship - The shell

structure forward of FR 16 of ADS (Fig 11) has been analyzed to study adequacy of structural strength

against Bottom Slamming pressure and Bow flare impact pressure. The slamming and bow flare

pressure was calculated as per IRS and LRS Naval rules. The detailed finite element analysis has been

done for various models at different locations. Thorough In-house research was done before finalizing

the method for analysis of such a real time condition.

Fig 11. Forward Part of hull Structure (ADS) Fig 12. Stress Distribution on Skeg


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3.4.1.12 Design Analysis of Skeg Structure - A detail 3-dimensional FE analysis of the skeg

structure (Fig 12) of Air Defense Ship has been carried out to check the structural adequacy of the

proposed design for two different loading conditions. Static Analysis (Linear) of skeg structure has

been done to find out the maximum deflections and combined stresses after docked condition. While

non-linear analysis, has been carried out to check for buckling in the process of docking of the ship.

3.4.1.13 Mast Static and Dynamic analysis for various vessels - The strength of the mast structure

(Fig 13) was estimated due to the wind and ship motion loads. This mast accommodates radar antenna

bases and for the proper functioning of the equipment, the deflection of the mast for operating

frequencies of the radar should be limited, hence a transient dynamic analysis was also done to

determine the deflection due to the ship motion. The structure was modified to restrict the deflection

and resulting stresses within allowable limit. The adequacy of the mast structure subjected to wind

load and ship motion load are checked on the modified structure and the stresses are kept within

allowable limit.

Fig 13. Deflection of Mast Fig 14. Deflection Plot of Helideck

3.4.1.14 Structural Analysis Of Heli-Deck Of Advanced Offshore Patrol Vessel - The main objective

of the structural analysis of Heli-Deck for Advanced Offshore Patrol Vessel (Fig 14) was to determine

the strength of heli-deck for normal landing and emergency crash landing of the helicopter. The

analysis was also carried out based on NES154 standards. The elasto-plastic analysis was carried out,

using ANSYS, based on allowable permanent deflection criteria.

3.4.1.15 Ship Lift System - The Ship Lift System,

shown in Fig 15, is used to transport the naval vessel

from the sea to the repair shed. These whole systems

have upward, longitudinal and transverse movement

during the full operation. The transverse movement at

a junction was found to have problems during shifting

from longitudinal movement at the rail and crossing

piece.

Hence, a FE analysis was done to analyze and re-

design the transfer system so that it could effectively

transfer the ship from sea to desired repair location.

The recommendation suggested was rectified in the

system and tested successfully.

Fig 15. Ship Lift System


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3.4.2 Vibration Analysis of Ship Structures

With the progress made in the field of FE analysis and methods developed to cut down time, it is now

possible to use FE analysis to predict vibration characteristics of ship. It is desirable to perform a Free

vibration analysis during design stage of a vessel to estimate the natural frequencies in various mode

shapes, as any changes later are difficult, if not impossible. This analysis is normally carried out for

important and representative ship loading conditions e.g. Light Ship, Fully loaded departure/arrival and

Ballast departure/arrival conditions, which cover the major variation of loading during voyage. Added mass

is suitably incorporated to simulate the vibration of elements in contact with water.

In respect of the global i.e. hull girder vibration modes; it is customary to investigate the 1,2,3 and 4 noded

vertical, horizontal, torsional modes and the respective natural frequencies of vibration. These are

compared with the major excitation frequencies e.g. propeller blade frequency, engine/ shaft frequency,

main generators frequency etc. to avoid resonance. Similarly it is useful to model local structures for e.g.

engine room, funnel and the super structure/deck houses to avoid potential area of resonance. Necessary

corrective action can be taken at the design stage by rearrangement of structural elements and/or by

increasing the scantlings.

The forced vibration analysis wherein the time varying forces from machinery (propeller, engine etc.) are

also taken into account can provide useful information such as the displacement, velocity and acceleration

at critical locations. These responses can be compared to ensure compliance with acceptable standards.

Few important vibration analysis projects are briefly described below

3.4.2.1 Determination of natural frequencies of a cement carrier - During the sea trail of this cement

carrier, shown in Fig 16, excessive vibration was sensed and also measured by our surveyor. In view of

that, finite element analysis of superstructure, including engine room and steering gear compartment

was carried out to find out the natural frequencies and mode shapes. From the analysis result,

excessive vibration was found at the same locations, which the surveyors had detected during the trial.

The remedial measures to reduce excessive vibration were identified and made. After the modification

the vibration and noise level came down to the acceptable limits

Fig 16. Mode Shape of Cement Carrier Fig 17. Mode Shape of 500 Passenger Vessel

3.4.2.2 Vibration Analysis of 500 Passenger Vessel - Vibration analysis for a passenger vessel

designed for well-known Indian Shipyard, Fig 17, was carried out to estimate the natural frequencies

of the first 40 modes of vibration in various loading conditions. A 3D shell-beam model of the

complete ship in all details was created using the pre-processor of ANSYS 8.0. The vertical, horizontal

and torsional modes of hull girder frequencies for various loading conditions were compared with the

frequencies of engine, shaft and propeller. Suitable recommendations were made to avoid resonance.

3.4.2.3 Vibration Analysis of OPV’s, FAC’s and AOPV’s - Vibration Analysis of Naval Vessels is of

prime importance due to the kind of conditions and the type of service they operate on. We model the

entire vessel, shown in Fig 18, carefully and perform Vibration analysis to find the natural frequency


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of the same and also to check if forcing frequency matches with the natural frequency to create

resonance. Suitable modifications, if any, are then recommended to the yard.

Fig 18. Mode Shape of a Naval Vessel Fig 19. Mode Shape of a Mast

3.4.2.4 Vibration Analysis of Masts fitted in Various Vessels- Masts fitted on ships carry important

navigation and communication components, which are very sensitive to vibrations. And, it is

constantly acted upon by wind and acceleration loads. Hence it becomes very important to check the

vibration and strength of masts for the specified operation. The mast is modeled in complete detail and

free vibration analysis is performed, Fig 19, fitted to check if it could operate safely with the level of

vibrations, which arise due to the running of the main engine, Pumps etc. Modifications if any, are

suggested to the client.

3.4.3 Ultimate Strength Analysis

Collapse of hull girder is an important failure mechanism for ships as it can cause huge losses of life &

property and marine pollution, particularly in case of large ships. Therefore it becomes essential to check

the ultimate hull girder strength against the combined vertical wave bending moments and still water

bending moments, including those in the flooded conditions. The ultimate hull girder strength against

bending can be calculated using either the simplified methods such as that based on incremental-iterative

approach or by nonlinear FE analysis.

In the incremental-iterative approach the stress-strain relationship for all stiffened plate elements are

established separately for when under tension and when under compression. Finite Element Method, which

not only takes into consideration complex geometries associated due to initial imperfections but also the

geometrical non-linearity and the material non-linearity. It is possible to include the effects of internal loads

due to ballast water and cargo. The midship section is modeled including all longitudinal members between

web frames and moments are applied incrementally on the model end. The analysis results in the curvature

vs the ultimate moment carrying capacity curve to provide the ultimate hull girder bending moment

capacity

3.4.3.1 Ultimate Strength of Tankers, bulk Carriers and Container Ships - The aim of this project

was to develop a procedure to determine the ultimate strength of the hull girder of ships considering all

possible modes of progressive failure. The work was carried out as a part of combined research

conducted by the Asian Class Societies. All longitudinal strength members between two web frames

were modeled in detail for the full transverse sections, using shell elements and fine mesh. Static non-

linear elasto-plastic analysis was carried out considering the global vertical bending moments (Sagging

and Hogging) and horizontal bending moments applied at the ends of the structure. The Ultimate

collapse moment was determined for different ships and was compared with Rule allowable bending

moments to provide an indication of the reserve bending strength at the very extreme loads.


10

0

5000

10000

15000

20000

25000

30000

35000

-350 -250 -150 -50 50 150 250 350

Compression Axial Stress (Mpa) Tension

Location fro

m B

ase (m

m)

Fig 20. Stress Distribution (Mid-Ship Section) Fig 21. Variation of Axial Stress along Depth of Ship

3.4.3.2 Determination of ultimate collapse pressure of hatch cover - In certain cases of bulk carrier

total losses, failure of the foreside hatch covers due

to abnormal waves has been sited as initial cause

leading to flooding and subsequent loss. In this

project a method was developed, for determining the

ultimate strength of hatch covers by non-linear

FEM. The load is increased in step increments and

both the region of material plasticity and deflection

are monitored. The practical ultimate strength is

determined up to a point when the hatch cover fails

to perform its assigned function of keeping sea

outside of the ship.

Fig 22. Stress Plot of Hatch Cover

3.4.4 Transient Dynamic and Strength Analyses under Impact Loading

The wave loads discussed above are dynamic in the sense that they are time varying, the frequency of the

cyclic variation being similar to the frequency of encountering waves. This being comparatively low, it is

possible to consider them as quasi-static loads and add to the still water loads. However there are other

types of highly dynamic loads, which are characterized by very high amplitude but short durations usually

lasting up to few milli-seconds e.g. slamming loads, under water shocks, contact/ collision etc. To fully

understand the effects of such loads the use of a non-linear transient dynamic analysis is necessary.

Nonlinear transient dynamic analysis is carried out in time domain, wherein the loads are varied at each

time step. Material and geometrical non-linearities are accounted, thus it is possible to investigate post-

elastic stage and estimate any permanent deformation left after the load history.

This kind of analysis usually requires extensive research due to the extreme sensitivity of the results on the

Finite Element Parameters.

3.4.4.1 Analysis of FSP subjected to Underwater Explosion - Now a day, all naval vessels in addition

to analysis to Normal Operational Loads, needs to checked for Explosive loads as well. We have

already started developing methods to study this complex phenomenon of structures subjected to

Explosive Loads. In a preliminary study we have analyzed a Floating Shock Platform (Fig 23) under

shock loads. The results matched closely with a similar study made by a Naval Department.


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Fig 23. Deflected Shape of FSP due to UNDEX Fig 24. Graph for Dynamic Load Factor

3.4.4.2 Development of Method to Analyze structures due to Impact Loading - Systematic use of

Transient dynamic Analysis has been used to convert complex analysis of impact loading into simple

and practical design guidelines or class requirement. A design method was developed, using FE

Software extensively, for finding strength against slamming. A systematic study of over 2000 cases

lead to introduction of Dynamic Load Factor (DLF), shown in Fig 24, which capture the Dynamic

Load Amplification depending on the natural frequency of plate panels. Further the method provides

guidance for plate thickness required considering in-plane stresses and any value of acceptable

permanent deformations.

Fig 25. Graph used to calculate thickness Fig 26. Stress Plot on Barge after Collision

3.4.4.3 Collision Analysis of Barge with supply vessel - Dynamic simulation of collision analysis

between barge and supply vessels has been carried out by implicit method. A 3-D model has been

created for both the vessels. The supply vessel, with a specified velocity collides at the corner of the

offshore barge. The contact regions between two vessels have been established by contact and target

elements. The geometric and material non-linearity has been taken care off. The effects of collision

have been studied up to 1.5 sec. The results, shown in fig 26, which include elastic deflection,

permanent deformation / damage and stress values, have been checked at the critical collision zone.

3.4.5 Thermal Analysis

With the advent of computing facility and the provision in ANSYS to do a coupled field analysis, lot of

complex loading which previously required lot of time and manual calculations could now be done easily

and more accurately.

3.4.5.1 Thermal Analysis of Thermo-syphon - A method to perform a Thermal analysis of thermo-

syphon loop system using finite element analysis was developed to study the maximum deflections,

Variation of DLF

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50

t L D / TLRS PRESENT

Variation of slamming load with aspect ratio based on FEM

(with axial load value 40% of yield stress)

0

2

4

6

8

10

12

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Aspect Ratio

Pslam/(

�� Y*(t/b)2)

wo/t=0.0 wo/t=0.25 wo/t=0.5

wo/t=0.75 wo/t=1.00


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combined stresses, reaction forces and moments at various locations for different temperature

distributions. The 40m high Heat exchanger structure shown here consists of 3 parts i.e. Heat

Exchanger, Vapor Separator with Column and Thermo- syphon loop pipe.

Fig 27. Stress Plot on a Thermo-Syphon Fig 28. Stress Plot of a LNG Carrier

3.4.5.2 Thermal analysis of LNG Carrier - The role of thermal stresses in connection with brittle

fracture is complicated. There are a fairly large number of low-temperature casualties where they

appear to have played a significant part in the initiation and propagation of the fracture. The structural

as well as thermal analysis has been studied for a LNG carrier designed by Daewoo Shipbuilding and

Marine Engineering Company, Korea. The temperature effects play due to varying sea temperature, air

and the inner shell temperature. The inner shell tends to contract being at very low temperature (-18°

C) and the outer shell tends to expand as it in contact with the higher temperature viz. water at 35° C

and air at 45° C. The effect of internal and external loads and temperature are found out in separate

load cases. Finally both the load cases are combined to get temperature distribution, stress distribution,

deflection of the surfaces and maximum equivalent von-mises stress in the LNG carrier.

3.5 Analysis Results and Discussions

All the Analysis results mentioned above are backed up with rigorous in-house research. Thorough

literature survey is done on a particular analysis for establishing method. All relevant data about the model,

loads, boundary conditions and results are collected. These results are checked using ANSYS modeling and

analysis. All published and experimental results are then compared with the results derived from ANSYS.

Ones the results shows resemblance with that of the experimental results, the proposed method is

established.

These methods are then used to analyze complex ship structures and components. Also data from surveys

of the ship structures and components are verified.

4. Conclusion

Advance methods developed in the field of hydrodynamics and structural analyses have become more

reliable and faster. Thus it is now possible to routinely investigate many issues connected with both

demand and capacity directly for each specific ship.

The use of FE Software ANSYS and its components allow faster and efficient model generation. Almost all

structurally important components could be modeled using ANSYS Preprocessor. The application of varied

types of loads and boundary condition is also simple. Automation of process, with the use of ANSYS

macro and APDL has helped in increasing the scope of such investigation to a large number of operating

conditions explicitly at the design stage so that the ships are more ‘fit for purpose’ then ever before.


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5. References

1. “Vibration analysis and Structural system” by Prof. M. Mukhopadhyay Dept. of OENA IIT

Kharagpur.

2. “Hydro elasticity of ships” by R E D Bishop and W G Price

3. “Development unit note no. 30 examples of ship vibration analysis” by Lloyd’s Register of shipping.

4. IRS Rule Book

5. ANSYS 11.0 Manuals

6. Vorus, William S., Principles of Naval Architecture, Chapter 7 “ vibration”, Published by, The

Society of Naval Architects and Marine Engineers, 1988.

7. Vibration Control in Ships, Published by Veritec, Hovik Norway, 1985.

8. IACS - Common Structural rules for Bulk Carriers 2006

9. Ultimate Limit State Design of Ships Hulls, By J. K. Paik, Ge. Wang, B. J. Kim, A. K. Thayamballi.

10. Guidance for Ultimate Hull Girder Strength Assessment, By Korean Register of Shipping Sept. 2003

11. Estimation of Ultimate Longitudinal Bending Moment of Ships and Box Girder, By M. K. Rahman

and M.Chowdhury, Journal of Ship Research vol. 40 No.3 Sept. 1996.

12. Ultimate Strength of Ship Hulls Under Combined Vertical, Horizontal Bending and Shearing Forces,

By J. K. Paik, A. K. Thayamballi, J. S. Che, Presentation at SNAME transactions vol.104 1996

13. “Underwater Explosion”, by Robert H Cole Dover Publication, Inc, New York

14. “Dynamic analysis of beams and plates subjected to air Blast loading”, J E Slater and R Houlston,

3rd International Modal Analysis Conference, Orlando, Florida, Jan 28-321(1985)

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