Finite Element Techniques in Ship Structural Design

Eng. 6002 Ship Structures 1

L E C T U R E 1 0 : F I N I T E E L E M E N T T E C H N I Q U E S I N S H I P S T R U C T U R A L D E S I G N

Contents

Introduction

Linear Analysis

Static Non-linear Analysis

Transient Dynamic Analysis

Other Applications

Introduction

The finite element technique was introduced in ship design with a significant delay with respect to other fields (automotive, aircraft and aerospace industry)

the high degree of conventionality of vessels allowed a complete and effective structural scantling based on experienced and consolidated rules.

Introduction

Recently, the development of unconventional vessels and the adoption of light and advanced materials showed the limitations of the range of validity of the existing rules

Increased use of F.E. technique

Introduction

In this lecture, some significant examples relevant to the application of F.E. technique to ship structural design are reported.

Examples make reference to:

1. global and local strength evaluation

2. the collapse of structural components in advanced materials

3. impact simulations with fluid-structure interaction.

Introduction

Particular emphasis is devoted to the fluid-structure interaction, which represents one of the most recent numerical application in the field of ship design.

Linear Analysis: Global Strength Evaluation

Global strength evaluation: evaluation of the stress levels related to the hull girder idealisation,

Considers the main global load effects due to both wave and still water conditions acting on the hull as:

longitudinal bending moment, both hogging and sagging;

shear force;

torque moment.

Linear Analysis : Global Strength Evaluation

the approaches:

the adoption of F.E. technique, using a beam idealisation of the ship

Linear Analysis : Global Strength Evaluation

using a 3D - F.E. model of a part of the ship

Linear Analysis : Global Strength Evaluation

using a 3D - F.E. model representing the whole hull structure:

Linear Analysis : Global Strength Evaluation

the third approach allows a more complete overview of the structural behaviour of the hull,

the designer can obtain useful information about:

global and local deflection;

stress concentration and structural behaviour in the main structural discontinuities of the hull and superstructures;

the contribution level of the different decks to longitudinal bending (especially for passengers vessels);

the distribution of shear stress in the main structural components of the hull;

the stress level in pillar structural elements.

Linear Analysis : Local Strength Evaluation

Local strength evaluation can be interpreted as:

1. the structural analysis of a limited part of the structure subjected to the loads directly applied on it;

2. the analysis of a limited part of the structure or what happens in a well defined structural detail when the whole ship structure is subjected to the global load effects (substructuring).

Linear Analysis : Local Strength Evaluation

the second interpretation represents a very ambitious objective. This is particularly used in strength and/or in fatigue life considerations

Linear Analysis : Local Strength Evaluation

In this context, a further methodology that can offer a great help in local strength evaluation is represented by an adaptive-meshing procedure.

This procedure is practically an automatic refinement of the mesh, that can be guided by the user.

Linear Analysis : Local Strength Evaluation

So, the adaptive method may represent a very useful tool for investigating stress levels in structural details

the actual geometry of the examined structure should be completely defined in the initial mesh. In fact, it is not effective to deal with very fine meshes if the geometric shapes and details do not correspond to the actual structure.

Transient Dynamic Analysis: Drop test simulation

Large impulse loads are experienced by a body during impact with water. This is often designated as slamming.

Both fore and bottom parts of a ship are exposed to slamming, as well as the deck between the two hulls of a catamaran or a surface effect ship

Slamming loads can lead to structural damage as well as induce whipping.

Transient Dynamic Analysis: Drop test simulation

Current trends to produce innovative, lighter and faster ships, increase the probability of slamming and, in addition, lighter structures are more prone to slamming damage than conventional structures.

Both aspects demand a better understanding and treatment of slamming loads

In general, conventional analyses are not able to provide good descriptions or exhaustive models for this kind of phenomena.

Transient Dynamic Analysis: Drop test simulation

The study of hydrodynamic impact between ship panels and a free water surface is traditionally dealt with through:

1. Analytical methods and experiments, these latter being preferably carried out under controlled conditions (drop tests).

2. Direct calculations, by means of suitable finite element codes for transient analysis that include structure-fluid interaction algorithms.

Transient Dynamic Analysis: Drop test simulation

Consider the study of flat ship panels impacting a calm water surface at prescribed falling velocities.

Analytical methods for wedge bodies break down for all but the simplest scenarios

The finite element simulation of the above problem needs a multi-fluid (water & air) approach for the fluid domain, the modelling of the structural properties of the plate, equations of state for the fluids, a fluid-structure coupling algorithm.

Transient Dynamic Analysis: Drop test simulation

For an impact configuration with the panel impacting the calm water surface at 0° , and some initial height

Geometric and material properties are specified for the plate

Transient Dynamic Analysis: Drop test simulation

The fluid domain has about 20x the number of cells as the structural grid. A portion of the elements must be dedicated to both the air domain and water domain. A view of half of the fluid domain is shown:

Transient Dynamic Analysis: Drop test simulation

The adopted equations of state for water and air must be specified and a simulation is run

Timestepsat 5,10,15,20 ms

Vibration Analysis

The finite element technique was introduced in ship design with a significant delay with respect to other fields (automotive, aircraft and aerospace industry)

the high degree of conventionality of vessels allowed a complete and effective structural scantling based on experienced and consolidated rules.

Vibration Analysis

A forced vibration analysis was run using the engine propulsor excitation forces.

The analysis confirmed the vessel’s vibration problem was caused by the propulsor.

Changes were made in the design, and vibration was eliminated

Modeling of Loads: Mass Distribution

Modeling of Loads: Hydrostatic

Still water Height of WL above global reference point

Trim & Heel angle of waterplane

Wave pressures Wavelength

Amplitude

Phase angle & yaw angle

R.B.D.-Modeling of Loads: Hydrostatic

Hydrostatic loads are appliedand the model is automaticallybalanced on the chosen waveor stillwater height.

Modeling of Loads: Tanks

Volume (tank) loads are appliedas a percentage filled or a specificmass or a head.

Canadian Patrol Frigate

100m Fast Ferry

Photo and model courtesy of Rodriquez Engineering, Genoa, Italy

Patrol Craft – USCG Island Class

SWATH Vessels: Cracking Investigation

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T-AGOR 26 (Kilo Moana)

The natural frequency analysis accurately predicted the hull mode measured in full

scale trials