Feasibility of computer simulation of the launch of free ...behaviour of free-fall lifeboats. ......

OTH 92 391

FEASIBILITY OF COMPUTERSIMULATION OF THE LAUNCH

OF FREE-FALL LIFEBOATS

Prepared by

Frazer-Nash Consultancy LimitedShelsley House, Randalls Way

LeatherheadSurrey KT22 7TX

London: HMSO

Health and Safety Executive - Offshore Technology Report

© Crown copyright 1993Applications for reproduction should be made to HMSO

First published 1993ISBN 0-11-882138-5

This report is published by the Health and Safety Executive aspart of a series of reports of work which has been supported byfunds formerly provided by the Department of Energy and latelyby the Executive. Neither the Executive, the Department nor thecontractors concerned assume any liability for the report nor dothey necessarily reflect the views or policy of the Executive orthe Department.

Results, including detailed evaluation and, where relevant,recommendations stemming from their research projects arepublished in the OTH series of reports.

Background information and data arising from these researchprojects are published in the OTI series of reports.

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CONTENTS

23232424

5.4 Comparison with Available Data5.4.1 Injury Levels of Properly Restrained Occupants5.4.2 Role of Harnesses in Different Seating Positions5.4.3 Maladjustment of Harness Straps

202121

22

22

5.3 Demonstration Simulations5.3.1 Case 1 - Correctly Restrained Occupant in Aft of Boat5.3.2 Case 2 - Correctly Restrained Occupant in Fore of Boat5.3.3 Case 3 - Occupant in Aft of Boat Restrained by a

Maladjusted Harness system5.3.4 Case 4 - Correctly Restrained Occupant in Aft with

Input Taken from a Simulation of Launch Kinetics

181819191920

5.1 Effects and Mechanisms5.2 Generation of DYNA3D Occupant Motion Model

5.2.1 DYNAMAN5.2.2 Seat Structure5.2.3 The Restraint System5.2.4 Input Hull Kinetics

18OCCUPANT MOTION5.

1616

4.4 Comparison with Available Data4.5 Discussion of Feasibility of Modelling Structural Response

151515

4.3 Demonstration Simulation4.3.1 Motion4.3.2 Stress

13131415

4.1 Effects and Mechanisms4.2 Generation of DYNA3D Structural Response Model

4.2.1 Description of Boat Representation4.2.2 Water Representation

13STRUCTURAL RESPONSE4.

1011

3.4 Comparison with Available Data3.5 Discussion of Feasibility of Modelling Launch Kinetics

8999

10

3.3 Demonstration simulation3.3.1 Sliding Along Ramp3.3.2 Rotation3.3.3 Free-fall3.3.4 Water Entry

778

3.2 Generation of DYNA3D Launch Kinetics Model3.2.1 DYNA3D Lifeboat Model3.2.2 Application of Forces

44556

3.1 Effects and Mechanisms3.1.1 Sliding Along the Ramp3.1.2 The Rotation Stage3.1.3 Free-fall Stage3.1.4 Water Entry Stage

4LAUNCH KINETICS3.

2OVERVIEW2.

1INTRODUCTION1.

PAGE

iiiSUMMARY

FIGURES 1 - 37

30REFERENCES8.

28282829

RECOMMENDATIONS7.1 Launch Kinetics7.2 Structural Response7.3 Occupant Motion

7.

27CONCLUSIONS6.255.5 Discussion of Feasibility of Modelling Occupant Motion

SUMMARY

The feasibility of using the DYNA3D finite element code to model the launch offree-fall lifeboats has been demonstrated.

In particular, three different types of DYNA3D model have been used to simulate:

• Launch kinetics• Structural response• Occupant motion

The launch kinetics simulation predicts the rigid body motion of a lifeboat during thevarious stages of launch. The predicted behaviour is seen to agree well withbehaviour observed in a real launch.

The structural response simulation predicts stresses and strains in the boat structureduring water impact. The predictions appear sensible but no data have beenavailable with which to compare predicted stress levels.

The occupant motion simulation predicts the motion of passengers within the boatincluding the influence of their harnesses. Results of the simulation agree well withbehaviour observed in a real launch.

The next stage must be to validate all three types of simulation. To do this it will benecessary to obtain detailed experimental data. It will also be necessary to developfurther DYNA3D models based upon the experiments.

Once validated, the simulations could be used in at least three different ways:

• To understand more about the mechanisms involved in free-fall launch• To help optimise boat design with regard to aspects such as weight, cost and

manufacturing method• To assist in the generation or assessment of safety cases or in type approval.

These would be of interest to lifeboat manufacturers, operators and regulatorybodies.

1

1. INTRODUCTION

This report describes work carried out by Frazer-Nash Consultancy Limited (FNC)for MaTSU on behalf of Offshore Safety Division of the Health and SafetyExecutive (HSE) under agreement Number MaTSU/8429/2866.

The purpose of the work has been to demonstrate the feasibility of using thecomputer simulation code DYNA3D to simulate the following aspects of the launchbehaviour of free-fall lifeboats.

• Launch kinetics• Structural response• Occupant motion.

The purpose of the launch kinetics simulation is to predict boat motion during launchunder different conditions. This will allow the effect on the trajectory of the boat ofdifferent boat designs, different ramp heights and angles and different sea conditionsto be investigated.

The purpose of the structural response simulation is to predict the stresses and strainsinduced in the boat’s structure during impact with the water. This will allow the boatstructure to be optimised in terms of hull shape, cost, weight, etc as well as allowingassessments to be made of structural integrity in the event of striking debris duringlaunch.

The purpose of the occupant motion simulation is to assess how boat occupantswould move around during launch and how they might be injured. This allows theboat interior including seats and harnesses to be optimised and ways ofaccommodating injured passengers to be investigated.

Computer simulation of these aspects of lifeboat launch has a number of importantadvantages over alternative approaches such as physical testing.

• The cost of a computer simulation is invariably much lower than that of acorresponding physical test.

• Sometimes a simulation can actually give more understanding of underlyingmechanisms than a physical test since a correct representation of thefundamental physics of a problem is an integral part of any simulation.

• The test conditions can be precisely controlled during a computer simulation,with results being fully reproducible.

• Timescales for simulation are often shorter which can be very important inbringing new products rapidly to the market place or identifying possibleproblems during the design process.

• Simulation can be used to make a cost-effective initial assessment ofproposed new designs. It can avoid devoting resources to expensiveprototyping of designs until there is some confidence in their practicality.

Section 2 describes in overview the approaches adopted in simulating each aspect.Sections 3 - 5 describe the work carried out at each stage and present the results ofDYNA3D simulations which demonstrate the feasibility of using the code to modelthe three aspects of lifeboat launch. Section 6 discusses the conclusions from thework while Section 7 makes recommendations for further work.

2

2. OVERVIEW

The finite element code DYNA3D (Reference 1) was written by the LawrenceLivermore National Laboratory in California for modelling dynamic behaviour ofstructures, particularly under transient loading. The code has been continuallydeveloped during its 15 year history and the range of applications for which it hasbeen successfully used is constantly increasing. The purpose of this project has beento demonstrate that DYNA3D could be used to predict the behaviour of free- falllifeboats and their occupants during launch.

In particular, the following aspects of launch behaviour have been considered:

• Launch kinetics - overall boat motion during launch• Structural response - the stresses and strains developed in the boat’s

structure during launch• Occupant motion - the way in which the passengers move around during

launch.

The important mechanisms and effects are different in each of these three aspects.Hence it has been most appropriate to consider each aspect separately using threedifferent types of DYNA3D simulation.

The work has therefore comprised three separate phases, each considering oneaspects of launch behaviour. The approach adopted in each phase has been similarand has consisted of five activities.

• Consideration of the important effects and mechanisms which need to beincluded in a simulation of the aspect of launch.

• Generation of an appropriate DYNA3D model which includes the importantmechanisms.

• Simulation of one or more scenarios using the DYNA3D model todemonstrate the operation of the simulation and the range of output whichcan be obtained.

• Qualitative comparison of the results of the simulations which data from reallifeboat launches. It should be noted that quantitative validation of themodels was not part of the work programme.

• Assessment of the feasibility of using DYNA3D to simulate the aspect oflaunch based on the previous activities.

Sections 3 - 5 of this report consider each of the three aspects of launch behaviour inturn and describe and discuss the work carried out in each of the above activities.

The qualitative comparison of the simulation results with actual data used a pool ofinformation from the following sources:

• Data supplied by Robert Gordon Institute of Technology (RGIT). Thisincluded some photographs of a free fall lifeboat taken during constructionand video tape of a lifeboat launch from inside and outside the boat.

• Engineering details of a free-fall lifeboat supplied by RGIT with thepermission of the lifeboat manufacturer.

• A paper (Reference 2) which included some results of launching a lifeboatcontaining an instrumented car crash dummy.

• Photographs and measurements taken by FNC on a visit to RGIT’s trainingestablishment in Dundee.

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3. LAUNCH KINETICS

This section describes the work carried out to demonstrate the feasibility of usingDYNA3D to simulate the launch kinetics aspect of free-fall lifeboat launch.

The following sections discuss:

• The effects and mechanisms which need to be taken into account in a launchkinetics simulation.

• The generation of a DYNA3D launch kinetics model.• A demonstration simulation using the launch kinetics model.• Comparison between DYNA3D results and actual behaviour in a real

launch.• Feasibility of using DYNA3D to simulate launch kinetics.

3.1 EFFECTS AND MECHANISMS

In launch kinetics it is the overall behaviour of the boat which is of interest ie thetrajectory taken and accelerations experienced by the boat. For this purpose thelifeboat can be treated as a rigid body (with appropriate mass and inertia) with forcesapplied representing interaction with the launch ramp, gravity and interaction withwater. To understand the mechanisms involved it is convenient to consider fourstages of lifeboat launch:

• sliding along ramp• rotation at the end of the ramp• free-fall• water entry

The states are illustrated in Figure 1. The physics of the stages are discussed in thefollowing sections.

3.1.1 Sliding Along the Ramp

The forces acting during this stage of the launch are shown in Figure 2. Theequations of motion are:-

(2)mz - N cos hL + l N sin hL - mg

(1)mx - N sin hL - lN cos hL

boat masshorizontal, vertical accelerationsnormal reaction force (mg cos hL)coefficient of friction between the boat and ramplaunch anglegravity

======

mx, zNlhL

g

Where,

From equations 1 and 2 it can be seen that the boat accelerations during this stage ofthe launch are dependent on the launch angle, , and coefficient of friction betweenhthe launch ramp and boat, , but are independent of mass.l

4

The velocity of the boat at the end of this stage will depend on the acceleration andthe distance travelled along the ramp during the stage.

3.1.2 The Rotation Stage

The rotation stage beings where the centre of gravity of the boat passes beyond theend of the ramp. A turning moment is generated between the gravitational forceacting at the centre of gravity and the reaction between the boat and the end of theramp as shown in Figure 3.

The equations of motion 1 and 2 from Section 3.1.1 still apply during this stage,however the turning moment causes angular acceleration of the boat according to:

(3) - N (cos + - N (sin + ) Ih hL l sin hL ) dx hL l cos hL dz

Where I = moment of inertia of the boat = angular accelerationhand , are shown on Figure 3.dx dz

Since and change as the boat passes at the end of the ramp, the angulardx dzacceleration will change with time. The rotation stage ends when the boat leaves theramp completely.

In this stage of the launch, the launch kinetics are dependent on the mass, massdistribution in the boat ie moment of inertia of the boat, the coefficient of frictionbetween the ramp and the boat and the boat launch angle.

The linear and angular velocities of the boat at the end of this stage will be dependenton the boat length aft of the centre of gravity.

3.1.3 Free-Fall Stage

Once the boat has left the ramp it will be in free-fall. The only force acting on it willbe gravity if the effects of cross winds are ignored. Under these conditions the boatwill continue to rotate at the same angular velocity with which it left the ramp.

During this stage then,

(5)mz - mg

(4)mx - h

(6)h - h initial + hinitial x time of free-fall

Where = angle at end of rotation phaseh initial

h initial = angular velocity at end of rotation phase

The water entry angle at the end of this stage will depend on the exit angular velocityfrom the rotation stage and the free-fall height. The greater the free-fall height themore acute the water entry angle.

5

3.1.4 Water Entry Stage

As the boat enters the water forces are generated as the water is displaced by theboat. Neglecting the effects of surface tension the sources of these forces can beidentified as:

• buoyancy effects• drag effects.

These effects are shown in Figure 4 and discussed in the following sections.

3.1.4.1 Buoyancy

When a body is fully or partially immersed in a fluid the fluid exerts a pressure on it.If the pressure were uniform over the entire surface there would be no resultant forceon the object. However, where pressure varies (for example, with depth) there willbe a net resultant force on the object.

This force is known as the buoyancy force. The buoyancy force is the net effect ofthe different pressures acting on different parts of the boat’s surface due to theirdifferent depths below the water surface.

On any small area of boat’s surface the pressure due to buoyancy effects is given by

(7)Pbuoy - q water g h

where

depth below water surface.=hgravitational acceleration= gwater density=pwater

Pressure due to buoyancy effects=Pbuoy

3.1.4.2 Drag

Drag forces are difficult to determine as they represent energy loss mechanismswhich are dependent on many factors, eg flow regime, shape, surface texture etc.

Typically drag can result from three effects. Firstly ‘form or pressure drag’ arises aswater is forced to change direction by collision with the boat, ie the water flow is notparallel with the surface. Secondly, energy is lost due to friction between the flowingwater and boat surfaces which are parallel with the flow. These effects together areknown as ‘profile’ drag. Thirdly, ‘water inertia’, represents the energy required todisplace water as the boat enters, ie the waves made. This is a very difficult effect toquantify.

Although the individual drag effects are difficult to predict, their net effect is a totaldrag force which can be approximated by:

(8)F drag - p water (V body - V water )2 CD A12

where:

water density=Vbody -Vwater

“drag coefficient” which represents the net effect of all forms ofdrag.

=CD

6

area of body normal to the flow.=Awater density=p water

This total drag force will act against the direction of travel of the boat as shown inFigure 4.

It is usual to determine values for CD experimentally. CD will depend on hull shapeand direction of water flow relative to the boat. For free-fall launch it will also varyas the amount of boat hull which is below the water surface varies.

3.2 GENERATION OF DYNA3D LAUNCH KINETICS MODEL

As discussed in Section 3.1, the simulation of launch kinetics using DYNA3Drequires a rigid body representation of a boat together with mechanisms for applyingthe various forces. This section describes how a DYNA3D model with these featureshas been generated.

3.2.1 DYNA3D Lifeboat Model

A DYNA3D rigid body model based on a typical free-fall lifeboat has beengenerated. The finite element mesh created is shown in Figure 5. It consist entirelyof 4-noded shell elements.

The model was based on the lifeboat data described in Section 2. The lifeboatdimensions and centre of gravity position were taken from a sketch supplied byRGIT. The boat is assumed to be partly loaded and has a mass of 9.76 tonnes.

The moment of inertia of the boat was unknown. It has been assumed for thesimulation that all the mass of the boat is distributed evenly around the surface of theboat, ie the model is hollow, and the mass is controlled by adjusting the shell elementthickness. This is likely to give a moment of inertia somewhat higher than the actualvalue.

The lifeboat launch rails were modelled with further rigid bodies as shown inFigure 6. The water was not modelled explicitly in this simulation. Instead theforces generated by interaction with the water were calculated and applied to themodel as discussed in the next section. The surface is represented by shell elementsin the figure for visualisation purposes only.

The water was not modelled explicitly in this simulation. Instead the forcesgenerated by interaction with the water were calculated and applied to the model asdiscussed in the next section.

7

3.2.2 Application of Forces

Gravity and contact forces between the boat and launch ramp were applied usingstandard DYNA3D “base acceleration” and “sliding interface” features.

Buoyancy and drag forces were included in the model as outlined below.

3.2.2.1 Buoyancy

In DYNA3D the buoyancy force described in Section 3.1.4.1 is implemented byapplying a pressure P buoy given by Equation (7), to each small segment of the modelwhich is below the surface of the water as shown in Figure 7.

In the program the pressure distribution is integrated over the surface to give the totalbuoyancy force.

3.2.2.2 Drag

In DYNA3D it is assumed that the total drag force described in Section 3.1.4.2 isdue to high pressure on parts of the object’s surface which are moving into the fluidand low pressure on parts of the object’s surface moving away from the fluid asshown in Figure 8.

Drag is applied as pressure on all parts of the boat surface which are below the watersurface. To allow for the variation in drag depending on flow direction over the boat(see Section 3.1.4.2) drag pressure on each small segment of boat surface iscalculated by,

(9)P drag - ½ p water (MX + My M + M2 )Ux2

y2 Uz

2

for segments moving into the water, or by,

(10)P drag - ½ p water (MX + My M + M2 ) (1 - C)Ux2

y2 Uz

2

For segments moving out of the water, where;

an overall drag coefficient.=Cvelocity of segment relative to water in x, y and z directions=Ux, Uy, Uz

multipliers for x, y and z directions=Mx, My, Mz

water density=p water

pressure applied to segment of boat surface=P drag

3.3 DEMONSTRATION SIMULATION

To demonstrate the use of DYNA3D launch kinetics model described in Section 3.2 atypical launch scenario was modelled.

8

In addition to the model details described in Section 3.2 the following were assumed:

19.56 m350

Zero1000 kg m-3

2.00.11.00.5

Launch heightLaunch angle (to horizontalRamp friction coefficient

Water densityOverall drag coefficient, C

x direction drag multiplier, Mx

y direction drag multiplier, My

z direction drag multiplier, Mz

ValueParameter

Table 1Demonstration Simulation Parameters

Figure 9 shows the resultant trajectory of the bow and stern of the boat relative to thesurface of the water. This figure shows the maximum depth to which each pointtravels and the maximum distance that the bow and stern rise out of the water afterinitial entry. The position of the boat at each of these maxima and at several otherstages during launch are shown pictorially in Figure 10.

The stages in the launch of the boat can be identified from the acceleration historiesof the boat shown in Figure 11. These show the boat accelerations in boatcoordinates ie axial accelerations are those in line with bow to stern and normalaccelerations are perpendicular to the boat floor.

The main features of the boat’s motion in the separate stages of launch are discussedin the following sections.

3.3.1 Sliding Along Ramp

• The boat slides along the ramp for about 2 seconds. This stage starts at0 seconds and ends at Point ‘A’ shown in Figure 11..

• In this stage, the predicted accelerations are given by Equations 1 and 2 andare plotted in Figure 12 along with the expected value. As can be seen theyagree well.

3.3.2 Rotation

• The angle, angular velocity and angular acceleration history of the boatduring launch are shown in Figure 13. The boat begins this phase at Point‘A; when the angular acceleration starts and finishes at ‘B’ where it leavesthe ramp completely and the angular acceleration is reduced to zero.

3.3.3 Free-Fall

• The free-fall stage begins at Point ‘B’ and ends at Point ‘C’ marked onFigure 11.

• During free-fall the angular acceleration is zero and the angular velocityremains constant and the final water entry angle is about 520 to thehorizontal.

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• During free-fall the global x-acceleration of the boat is zero and the globalz-acceleration is due to gravity only.

3.3.4 Water Entry

The buoyancy and drag forces calculated by DYNA3D are presented in Figure 14.

These can be compared with the accelerations experienced by the boat presented inFigure 11. The important features of the simulation during this stage are as follows:

• When the boat hits the water large drag forces are immediately applied to theboat. These drag forces decrease as the boat slows down. As the boat risesout of the water, drag forces are developed which pull it back down into thewater as shown by the period of negative normal drag force.

• A large buoyancy force is generated which increases as the boat sinks intothe water and peaks after the drag force. The buoyancy force then rises andfalls as the boat oscillates on the water surface and levels off at a value equalto the boat’s weight.

• The boat rotates rapidly after initial contact with the water and a very largeangular velocity and angular acceleration are generated. The boat rotates toa horizontal position in less than one boat length

• As the boat enters the water the bow sinks to about 3 m below the watersurface at 3.2 seconds shown in Figure 10. During the ensuing rotation thestern sinks to the gunnel at 3.7 seconds shown in Figure 10.

• When the boat reaches the lowest point in the water the buoyancy forcespush it back out. The boat falls back into the water again and then osillatesgently on the surface with a small forward velocity.

• The normal acceleration history shows two periods of negative accelerationat the aft of the boat. In the first instance this occurs due to the large angularacceleration experienced at the aft of the boat and in the second due to theboat falling back into the water.

• The boat continues to travel forward at a low velocity experienced at the aftof the boat.

3.4 COMPARISON WITH AVAILABLE DATA

As indicated in Section 2, RGIT provided a video tape of the launch of a free-falllifeboat. From this video the following characteristics of the launch are apparent:

• rotation during the free-fall phase is clearly visible. From a launch height ofabout 20 m and launch angle of 350, the boat rotates through about 200

before water entry;• the boat pushes water away creating a depression in the water surface and

waves around the outside;• as the boat enters the water the boat bow sinks to several metres. During the

ensuing rotation the stern of the boat sinks into the water as far as thegunnel;

• after the bow has penetrated the water the boat rotates rapidly until the sterncomes into contact with the water. The boat achieves a horizontal attitude inless than one boat length;

• after the initial water entry phase the boat rises out of the water and fallsback in. This happens several times with rapidly decreasing amplitude;

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• the occupants at the rear of the boat experience two jolts. The first when theboat contacts the water and the second as the boat drops back into the waterafter the first bounce. These jolts occur approximately 1 second apart;

• the boat continues to travel forward after launch without engine power;• the boat veers to the right when unpowered.

Comparison of the boat trajectory predicted by DYNA3D (Section 3.3.4) with thevideo results from RGIT shows that the overall trajectories of the simulation and reallaunch are similar although the simulated launch would appear to be underdampedparticularly in the vertical (z) direction. This is highlighted by the number andmagnitude of vertical oscillations which occur after the initial water entry. Theoscillations could be reduced by increasing the value of C.

From recording made inside the boat of the motion of occupants during launch, thepredicted negative accelerations correspond to the ‘bumps’ felt by passengers at theaft of the boat. The time elapsed between these ‘bumps’ in the DYNA3D simulationagrees well with the timings seen in the video.

3.5 DISCUSSION OF FEASIBILITY OF MODELLING LAUNCH KINETICS

The favourable comparison between the simulation and the RGIT video shows thatDYNA3D can be used to model launch kinetics and can give realistic predictions ofboat motion. The feasibility of using DYNA3D to simulate this aspect of free falllifeboat launch has therefore been demonstrated.

Experience gained in the development of the demonstration simulation suggests thatthe model could be improved in a number of ways:

• A more accurate representation of the hull shape and the boat’s moments ofinertia would improve the simulation in terms of motion during launch andalso the final, stable position in the water.

• The drag coefficient and the three directional multipliers (seeSection 3.2.2.2) could probably be tailored to give a closer match betweenthe simulation and the actual launch.

• Currently the directional multipliers apply to drag in the global x, y, zdirections. It would be better if they were modified to apply to an axissystem which moved with the boat.

It would be appropriate to incorporate these improvements into future models beforeattempting to validate the technique quantitatively. To allow the technique to bevalidated it would also be necessary to obtain more detailed trajectory andacceleration data from an actual launch.

The current simulation method uses four coefficients (C, Mx, My and Mz, see Section3.2.2.2) to characterise the boat’s drag behaviour. Appropriate values for thesecoefficients would need to be determined using empirical data. One could not predictthe behaviour of a new hull shape which had never been made and tested using thisapproach. However, data on a particular hull’s behaviour could be determined fromsome relatively simple tests and from launches in benign conditions. Once asimulation based on this data had been developed it could then be used to considermore “interesting” launch conditions for that boat.

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The same semi-empirical approach is used in the automotive and aerospaceindustries. However, in those industries a considerable body of data has been builtup over the years which allows good estimates of drag behaviour to be made prior towind tunnel or other testing. Data relating to drag on different shaped boats oncethey are stable in the water is likely to exist already but data for boats impacting thewater during free-fall launch probably does not.

It may be possible, with further research, to develop an approach to drag modellingfor lifeboat launch which does not rely upon empirically derived coefficients. If thiswere possible then the advantages would be significant since new hull shapes couldbe assessed while they were still at the drawing board stage. It is recommended thatconsideration should be given to commissioning research in this area.

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4. STRUCTURAL RESPONSE

This section describes the work carried out to demonstrate the feasibility of usingDYNA3D to simulate the structural response of a free-fall lifeboat during launch.


• The effects which are of concern in a structural response simulation.• The generation of a DYNA3D structural response model.• A demonstration simulation using the structural response model.• Comparison between DYNA3D results and the behaviour which one would

expect to see in a actual launch.• Feasibility of using DYNA3D to model structural response during water

impact.


In the structural response aspect of the work programme, it is the actual stresses andstrains which are generated in the boat structure on impact with the water which areof interest. In order to predict these stresses and strains the model must be able tocorrectly simulate the loads on the boat arising from water inertia and buoyancywhile accounting for the changing amount of hull surface area in contact with thewater.

This can be achieved directly with DYNA3D by creating a finite element modelrepresentative of the major structural features of the free-fall lifeboat and allowing itto interact with a second block of finite elements with the properties of water. Bymeans of sliding interfaces between the boat and the water, the boat’s behaviour canthen be simulated as it enters the water and is acted upon by the forces generated bydisplacing water, gravity and its own inertia.

4.2 GENERATION OF DYNA3D STRUCTURAL RESPONSE MODEL

The boat modelled during this stage of the work was loosely based upon the datasupplied by RGIT and the boat manufacturer (see Section 2). However, a number ofsimplifications were made in generating the DYNA3D model. Thus, the results ofthis simulation may be typical of free-fall lifeboats but are unlikely to give anaccurate prediction of the behaviour of the particular boat for which data weresupplied due to the number of simplifications made.

Figure 15 shows the DYNA3D structural response model. It comprises a simplifiedfinite element representation of the boat and a region of water. The figure shows afull boat. In fact, because of the existence of a vertical plane of symmetry along theboat’s fore-aft axis, only a half model was generated. The graphics software used togenerate this and other pictures was used to add the “missing half” for visualisationpurposes.

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To minimise computer run times the simulation begins just before impact and lastsfor 0.5 s after impact. The initial conditions for the model were obtained from theoutput of a launch kinetics model (Section 3) and were as follows:

9.3 ms-1

16.3 ms-1

520 to horizontal

Horizontal velocityVertical velocity

Impact angle

ValueParameter

Table 2Structural Response Model Initial Condition

Figure 16 illustrates these initial conditions.

The following sections describe the features of the boat and water.

4.2.1 Description of Boat Representation

Figure 17 shows an exploded view of the major components of the boat model.These are:

• Hull plating• Hull ribs and stringers• Passenger compartment• Deck framing• Deck and superstructure

The hull and superstructure shapes were based upon drawings supplied by a lifeboatmanufacturer (see Section 2). From photographs of the boat in construction it wasseen that the real hull plating is reinforced with small “top hat” section stringers.However, for simplicity these were represented by increasing the effective thicknessof the hull plating in the DYNA3D model. Furthermore the real superstructureincludes glass areas and a large rear door. Again, for simplicity, these were notincluded in the model. This is acceptable for a feasibility study. Clearly for a realanalysis it would be necessary to model these structural details.

The positions and shapes of the hull ribs and stringers and the deck framing wereestimated from the available drawings and photographs. Where the available datawere incomplete engineering judgement was used to define a “sensible” structure.Figure 18 shows the arrangement of the internal framework in the boat model. It isbelieved to be representative of typical boat construction although it may differ in anumber of respects from the actual design of the boat for which data were provided.In particular, the design of the framing in the bow area may be more complex than inthe model.

The whole boat was assumed to be constructed of aluminium alloy with typicalmechanical properties.

The actual boat contains a number of massive items, such as the engine, seating, fueltank, etc. These were not explicitly represented in the model. Instead, their masswas included by increasing the density of the aluminium to achieve the correct totalmass for the boat.

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4.2.2 Water Representation

The water was represented by brick elements using the DYNA3D fluid materialmodel. This material model allows the density and compressibility of water to berepresented. However, as with a real fluid, the material model has no shear stiffness.The viscosity of real water was not included in the model, since this was notconsidered to be a significant effect as far as structural response is concerned.

4.3 DEMONSTRATION SIMULATION

The objectives of this work were to demonstrate the ability of DYNA3D to simulatethe interaction of a free-fall lifeboat with water and also to show the range of resultsthat could be extracted from such a simulation. The results can be generalised intotwo areas:

• Motion (movement of the lifeboat, deformation of the structure)• Stresses.

The results of this simulation are therefore presented under these headings in Sections4.3.1 and 4.3.2.

It should be noted that the model was not an exact representation of the lifeboat forwhich data were supplied. Therefore numerical results, such as stresses, cannot beused for a critical assessment of the actual lifeboat, although the range of valuesderived would be typical of a generic free-fall lifeboat. As the available informationwas limited, critical regions such as the bow (which takes the initial impact) weremodelled in a similar manner to the rest of the boat, whereas in reality there wouldprobably be additional strengthening members if this area. Hence the results in theseregions may not be fully representative.

4.3.1 Motion

The positions of the boat the water during the impact are shown in Figure 19 at equaltime intervals over a 0.4 seconds time span. The bow of the boat can be seen topenetrate into the water and quickly decelerate due to buoyancy and water inertiaeffects causing the lifeboat to rotate and the stern to contact the water shortlyafterwards. This sequence of pictures corresponds closely to the motion of a free-falllifeboat in reality as seen on the video supplied by RGIT.

4.3.2 Stress

Figure 20 shows the Von Mises stress distribution on the deck/superstructure of thelifeboat at the end of the analysis. At this stage the boat was moving forwardsthrough the water, and hence the region of highest stress, as expected, was around thebow where the water was being pushed out of the way by the boat’s motion. Theforce of the water on the boat created pressure and compression on the forwardstructure giving rise to the stresses shown.

The effect of the boat’s geometry is seen approximately half way along its lengthwhere a ‘kink’ in the upper deck creates a local stress raiser. The boat was generallyin compression at this stage as its motion was being resisted by the force of the wateron the bow. The compressive stresses in the deck created out of plane bending

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moments where there was an abrupt angle in the otherwise flat structure, giving riseto the effect shown in Figure 20. Further high stress areas can be seen at similargeometrical features around the cabin and also along the gunnel where loads from thehull transmit out-of-plane forces into the deck.

Figure 21 shows the stress distribution in the passenger compartment at the end ofthe analysis. The effect of the internal stiffening members can be seen as areas ofhigh stress where the ribs meet the internal compartment and the stringers meet theforward bulkhead. These areas were locally stiff and therefore attracted load andcreated higher stresses. This was also an indication that loads were being transferredfrom the hull into the rest of the boat’s structure, ie the stiffening members weredistributing the loads from the water throughout the boat, which is the purpose of anysuch stiffening structure.

This behaviour is seen in more detail in Figure 22 which shows the ribs and stringersalone. Higher stress areas can be seen at local stiff points where members meet orwhere there were stress concentrating features such as sharp corners or changes insection. To predict actual stress levels at these details it would be necessary to modelthem with a finer mesh density.


It was not possible to obtain any data on the stresses actually generated in a lifeboatduring launch.

As discussed in Section 4.3 the behaviour predicted agrees qualitatively with theresponse which one would expect. In particular, local high stresses occur at changesof section and where the structure is locally stiff.

4.5 DISCUSSION OF FEASIBILITY OF MODELLING STRUCTURALRESPONSE

The demonstration simulation described in Section 4.3 shows that DYNA3D can beused to model a lifeboat hitting the water and that sensible looking behaviour ispredicted. The feasibility of modelling this aspect of launch behaviour has thereforebeen demonstrated.

The experience gained during this phase of the work suggests that the model could beimproved in a number of ways before it would be appropriate to consider quantitativevalidation:

• A more faithful representation of the actual boat structure would clearly beneeded. In particular, it is felt that the bow area of the model would need tobe refined.

• A finer water mesh (smaller finite elements) would be expected to give abetter representation of the water loading on the structure. It is felt that themesh used here may have applied a load which was too concentrated atdiscrete points and not sufficiently distributed over the parts of the hull incontact with the water.

• The mesh density used in the demonstration simulation for the boat itself isappropriate for an overview of likely high stress areas. However, to obtainan accurate picture of peak stresses, particularly near changes of section or

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welded joints, it would be necessary to generate further models with finermesh at points of interest.

Quantitative validation of the technique should incorporate these improvements. Itwill also require experimental data such as strain gauge output from an actuallaunch.

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5. OCCUPANT MOTION

This section describes the work carried out to demonstrate the feasibility of usingDYNA3D to simulate the motion of lifeboat occupants during lifeboat launch.


• The important features which need to be taken into account in modellingoccupant motion.

• The generation of a DYNA3D occupant motion model.• Four demonstration simulations using the occupant motion model.• Comparison between simulation results and actual data.• Feasibility of using DYNA3D to model lifeboat occupant motion.


The motion of the occupants during launch is of interest in determining likely injurymechanisms and the effectiveness of body restraints used in the boat. Although amodel of the boat could be included in this analysis, it is unnecessary since only theacceleration histories of the boat are required. These can be applied to the occupantvia his seat. In this phase then, a person and the seat only have to be modelled andacceleration histories applied to the seat to determine occupant motion.

5.2 GENERATION OF DYNA3D OCCUPANT MOTION MODEL

FNC has developed the DYNAMAN technique to allow DYNA3D to be used tomodel the motion of people or crash dummies under various types of loading.

DYNAMAN takes into account all of the important effects such as:

• Shape and size of the person• Seat shape and motion• Effect of harnesses or other restraints.

The DYNAMAN technique has been used to simulate occupant motion in free-falllifeboat launch. The model is shown in Figure 23 and comprises four maincomponents:

• DYNAMAN itself• The Seat Structure• The Restraint Systems• The Input Kinematics of the Lifeboat.

The first two components are common to all four of the simulation cases which aredescribed in Section 5.3. The differences between the cases are in the restraintcharacteristics and the different kinematics experienced in different positions in theboat.

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5.2.1 DYNAMAN

DYNAMAN comprises a number of rigid bodies which represent the various bodysections. The body sections are linked together by springs and dampers representinghuman joints. Each of the body sections and joints can be tailored to suit particularapplication.

The requirements of this particular application were assessed from two sources ofinformation:

• The video recording of occupants in a launch of a lifeboat supplied by RGIT(see Section 2).

• Previous experience of human response modelling.

Based on this information DYNAMAN was configured with the following features;

• Rigid arms to represent muscle tension when the arms are in the crossedposition adopted for launch.

• Limited knee and hip joint movement, to represent leg muscle tension.• Dampers in head and thorax joints to model the dynamic response of a tensed

human body.

Thus the configuration of DYNAMAN in these simulations represented a well bracedoccupant adopting the recommended launch posture. The results of these simulationsmay not represent the response of a “typical” human in an emergency situation.

5.2.2 The Seat Structure

The seat structure was modelled in three parts:

(1) The cushion(2) Sea back and base(3) Foot rest.

All three components were modelled with a rigid mesh of brick elements.Dimensions of the seat structure were not available, therefore the shape of thecushion and overall seat structure was estimated from photographs. Estimates werealso made for the material properties of the cushion material. The seat and footrestwere given representative material properties taken from standard data.

5.2.3 The Restraint System

The restraint system used in the free-fall lifeboat comprises a four point body harness(two lap and two shoulder straps) and a two point head restraint. These six strapswere simulated with six sets of springs and dampers representing the elasticity andenergy absorption characteristics of the webbing.

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Spring and damper elements were included in the following positions (also see Figure23):

• Between seat back and DYNAMAN left and right shoulders• Between lower seat back and DYNAMAN pelvis (left and right)• From the two sides of DYNAMAN head to seat head-rest.

Spring stiffness and damping characteristics were estimated from experience ofautomotive seat belt webbing. The six sets of spring and damper elements weregiven non-linear characteristics in that they only applied force when in tension.Harness webbing will not apply any force when in compression. Slack in the harness(where required) was modelled by an alteration to the standard spring characteristicsuch that the spring could extend to take up the slack before any force was generated.

5.2.4 Input Hull Kinematics

The movement of the hull during launch provides the dynamic input to the occupantrestraint system.

The stage of launch which is of most interest when considering occupant motion isthe water impact phase. During free-fall little movement of the occupant relative tothe boat will occur as both objects are subject to the same force ie gravity. However,when the boat first enters the water the forces on the boat and occupant change. Theboat is decelerated by buoyancy and drag forces from the water, whereas theoccupant is still only subject to the force of gravity. Therefore relative motion willoccur between boat and occupants. In this investigation only the water impact phaseof the boat kinematics has been considered.

Different inputs were used for the four demonstration simulations as described inSection 5.3.

5.3 DEMONSTRATION SIMULATIONS

As indicated in Section 5.1, four demonstration simulations, each representing adifferent scenario were produced. The cases were:

Correctly restrained occupant in aft of boat with input kinematicstaken from a FNC launch kinetics simulation.

Case 4 -

Occupant restrained by a maladjusted harness in aft of lifeboat withinput kinematics taken from Reference 2.

Case 3 -

Correctly restrained occupant in fore of lifeboat with inputkinematics taken from Reference 2.

Case 2 -

Correctly restrained occupant in aft of lifeboat with input kinematicstaken from Reference 2 (see Section 2).

Case 1 -

The details and results of the four cases are described in the following sections.

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5.3.1 Case 1 - Correctly Restrained Occupant in Aft of Boat

For this case it was assumed that the occupant had tightened all of his restraintsfully.

The motion input used for this case was obtained from Reference 2 and representedthe vertical and longitudinal (fore and aft) motion measured in a real launch for aseating position in the aft of a lifeboat. Figure 24 shows the vertical and longitudinalvelocity histories used in this case. No rotational motion data was available fromReference 2 and so rotational motion of the seat could not be included even thoughsome rotation would certainly occur in reality.

Figure 25 shows the motion of DYNAMAN relative to his seat during the first 0.75 sof the simulation.

On the whole there is little movement of the occupant. However, there is some liftingof the occupant’s back and legs in the early stages of the simulation. The lifting ofthe occupant’s body is due to the initial downward acceleration in the aft of the boatwhich occurs as the boat rotates to the horizontal. Once the aft of the boat is incontact with the water there is an overall upward acceleration which forces theoccupant back down into the seat.

The occupant accelerations that were calculated during the simulation are shown inFigures 26 and 27. Both the head and thorax accelerations closely follow those ofthe lifeboat. This was as expected as the occupant is well restrained and willtherefore generally move with the boat.

5.3.2 Case 2 - Correctly Restrained Occupant in Fore of Boat

As with Case 1, this case assumed that the occupant had fully tightened hisrestraints.

The motion input was obtained from Reference 2 and represented vertical andlongitudinal seat motion measured at the front of the lifeboat during an actual launch.Figure 28 shows the vertical and longitudinal velocity histories which were applied tothe seat. As with Case 1, no rotational motion history was available.

The results of this simulation were similar to those of the aft simulation (Case 1)excepting that there is no initial upward movement of the occupant relative to theseat. This movement does not occur as there is no initial downward acceleration ofthe fore of the boat since it is close to the centre of rotation as the boat returns to thehorizontal.

When examining the kinematics of the occupant motion (see Figure 29) it can be seenthat there are only very small movements. The major component of the boatacceleration is vertically upwards, therefore the occupant is forced down into the seatand little movement of the occupant will occur. Deceleration of the boat is the xdirection that would “throw” the occupant forwards is only of the order of 1 - 2 g andis easily restrained by the harness.

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Figure 30 shows the head acceleration and Figure 31 the thorax acceleration. Asobserved in the correctly restrained aft case the occupant accelerations closely followthose of the lifeboat.

The differences between these graphs and the hull deceleration are mainly due to thetwo different coordinate systems (the occupant declarations are in a coordinatesystem local to the body region) and to vibration of the body mass relative to the seat.This vibration in the simulation occurs, as in real life, because the body is attached tothe boat via belts (springs) and is not directly “glued” to the hull.

5.3.3 Case 3 - Occupant in Aft of Boat Restrained by a MaladjustedHarness System

For the third simulation, a case of restraint misuse was chosen. Modes of misusethat were considered were:

• Slack in Harness• Non-use of head restraint• Non-use of harness system altogether• Out of position occupants• Loose objects in occupant compartment• Unseated occupants• More severe launch conditions.

Any of these modes could be investigated using the DYNAMAN technique, but theslack harness case was chosen for this project. 100 mm of slack was assumed. Theaft seat motion used in Case 1 was also used for this case. This allows a directcomparison to be made of the effect of loose restraints.

The effect of the slack in the harness system can be seen in both the pictorial images(Figure 32) and the acceleration plots of the head and thorax (Figures 33 and 34).The slack in the belts allows the occupant to move up relative to the seat, when theaft of the boat is in its initial downward acceleration phase. Once the slack is takenup, the occupant is “yanked” back down into the seat. The delay in the restraint ofthe occupant caused by the slack, leads to an increase in relative velocity betweenoccupant and boat. There is therefore an increase in the force required to stop theoccupant resulting in higher head and thorax accelerations in Figures 33 and 34.

5.3.4 Case 4 - Correctly Restrained Occupant in Aft with Input Takenfrom a Simulation of Launch Kinetics

This case repeated Case 1 except that the seat motion was taken from a DYNA3Dlaunch kinetics model (see Section 3) rather than from Reference 2. This allowedrotational motion to be included in the simulation.

The results of this simulation were similar to Case 1. The occupant lifts away fromthe seat in the initial stage of hull acceleration and is then forced back into the seat(see Figure 35). The acceleration traces of DYNAMAN’s head (Figure 36) andthorax (Figure 37) exhibit similar shapes and peak values to those seen in Case 1, butdifferences can be seen in the timing of these peaks. The differences are most likelydue to deviations between the measured hull accelerations (used in Case 1) and thecalculated values used in this Case.

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Data for comparison with the DYNAMAN results was available from the RGITvideo and from Reference 2 (see Section 2).

The video gave a qualitative indication of occupant motion at the aft of a boatalthough the occupants in the video were probably smaller than the DYNAMANmodel used. No numerical data was available to go with the video.

Reference 2 gave quite detailed numerical results in terms of accelerations.However, these were measured on an instrumented crash dummy and not a person.Also, other details of the experimental set up were not available at the time when thework was carried out.

In view of the above comments it would be surprising if DYNAMAN had predictedexactly the same behaviour as that seen in either the video or Reference 2.

Nevertheless, comparison of DYNAMAN overall occupant motion and head andthorax accelerations with the available data shows good agreement. Discussion withRGIT also confirmed that the sensations experienced by occupants in the front andrear of the boat are different and that a significant jolt would be experienced byoccupants with excessively loose restraints, particularly in the aft of the boat.

Although the simulations could not be considered validated, they do give sufficientlyrealistic predictions that it is possible to make a number of general observations:

• The simulations suggest that correctly restrained occupants in “normal”launch conditions as simulated appear unlikely to be at risk of significantinjuries.

• In the launch scenario considered in the simulations it appears that theharnesses worn by the lifeboat occupants play a more important role inrestraining those occupants in the aft of the boat than those occupants in thefore of the boat.

• The simulations suggest that the potential for injury could be increased if therestraint is not used correctly.

These observations are discussed in more detail in the following sections.

5.4.1 Injury Levels of Properly Restrained Occupants

In the simulation Cases 1, 2 and 4 the occupant is restrained by a harness and headstrap which are tight (ie no slack at all). As a result the occupant moves with hisseat. Unless there is some loose equipment in the boat it is more unlikely that hewould impact with any other occupant or structure in the launch scenario modelled.

The lack of impact with other occupants or the structure and tight restraint meansthat acceleration levels experienced by the occupant are low, certainly well withinnormally accepted injury criteria. This conclusion is no great surprise since it isunderstood that RGIT has conducted training launches similar to the launchsimulation for several years without any significant injuries.

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Two important caveats must, however, be applied. Firstly, although the simulationresults are in broad qualitative agreement with real life experience they are not yetquantitatively validated. It would be unwise at this stage to make quantitativepredictions based on the simulation results. Secondly, only one launch scenario hasbeen considered. Without further work it would be wholly inappropriate to assumethat conclusions drawn for this launch scenario would apply to other situations.

5.4.2 Role of Harnesses in Different Seating Positions

In the simulation cases considered in this work occupants in the front of the boatexperience a significantly different acceleration environment than occupants at theback of the boat.

In front of the boat the forces act primarily to push the occupant into his seat. Thusthe harnesses may need to play little part in restraining him.

In the back of the boat there is an initial phase where the forces act to pull theoccupant out of his seat (or, more correctly, to pull the seat downwards, away fromthe occupant). After this initial phase the forces once again act to push the occupantinto his seat. In the initial phase the harness does come into play to prevent theoccupant coming out of his seat.

One might be tempted to conclude that occupants in the front of the boat may not, infact, need their harnesses. For the precise conditions simulated here that might betrue.

However, it would be necessary to consider other launch scenarios where theacceleration histories in different parts of the boat could be different before it wouldbe wise to adopt this conclusion. In any case, the harness may have a beneficial rolein preventing any lateral boat motion (during launch or once in the water) fromthrowing the occupant around. There may also be important psychologicaladvantages in having occupants strapped in where they may feel more secure andmay be less likely to leave their seats after launch and perhaps get in the way of theboat’s crew.

5.4.3 Maladjustment of Harness Straps

The simulation of an occupant restrained with a loosely adjusted harness exhibitedgreater occupant movement than for an equivalent occupant with a tight harness.The increase in movement implies a greater chance of impact with part of the boatstructure. In addition, comparison of the accelerations in simulation Cases 1 and 3show that higher maximum levels are experienced by occupants with loose restraints.The potential for injury would therefore be higher for the occupant who had nottightened his harness. This observation is in agreement with the generally acceptedview in the automotive world that loose seat belts offer less protection than beltswithout slack.

Following from the comments in Section 5.4.2, it may be that correct (ie tight)harness adjustment is particularly important in the aft of the boat. This could pose apossible problem. If it is assumed that the lifeboat will be filled from front to back(which would seem the most logical method) then those occupants for whom correctharness adjustment is most critical (ie those at the back of the boat) will be the ones

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with least time before launch to ensure that correct adjustment is achieved. It wouldbe necessary to examine in more detail what the boarding procedure for the boatsactual is before one could assess whether this presents a problem in reality.

5.5 DISCUSSION OF FEASIBILITY OF MODELLING OCCUPANTMOTION

DYNAMAN is a well proven technique for modelling the motion of people and thefour demonstration simulations described in Section 5.3 confirm that it can be used tomodel lifeboat occupants.

The accuracy of the simulations could be improved given more precise data. Inparticular, the details of the predicted occupant motion could be influenced by:

• Precise size, shape and mass of the occupant.• Force-extension characteristics of the webbing in the restraints.• More accurate seat padding properties.• Initial tightness of the restraints.• Motion input applied to the seat.

Obtaining more accurate data in these areas from a future experiment would not beexpected to present any significant difficulties.

A more problematic aspect, however, is the degree of muscle tension or rigidity of theoccupants in a launch. Discussion with RGIT suggests that different people willbrace themselves to different extents and even the same person will be likely to reactin different ways on different occasions.

If information could be obtained on the range of likely human response thenDYNAMAN could be configured to assess various different cases. In the absence ofsuch data it may be appropriate to consider two extremes of behaviour (completelylimp or completely stiff). The completely limp case will generally indicate the largestkinematic envelope in which the occupant could move while the completely stiff casemight give an indication of maximum forces on the body.

In addition, it will be necessary to consider the types of injury which could besuffered by occupants and the data which would need to be obtained from thesimulations to allow the likelihood of these injuries to be assessed. In thedemonstration simulations, overall occupant motion and head and thorax accelerationwere obtained. However, the technique is also able to give results such as:

• Contact forces between the body and the inside of the boat,• Forces within the body, such as the load on joints or in the neck,• Various conventional injury criteria such as the Head Injury Criterion (HIC).

It will be necessary to determine which injury criteria should be considered and todefine acceptable levels for these criteria. These acceptable levels will clearly bemuch lower than the levels which would be allowable in a car crash where the aim isthat the occupant should survive even if he would not be expected to walk away fromthe accident.

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6. CONCLUSIONS

DYNA3D has been used to simulate three aspects of free-fall launch:

• Launch kinetics• Structural response• Occupant motion

The important effects and mechanisms are different for each aspect and so it has beenappropriate to develop different types of DYNA3D model for each application. Thedemonstration simulations show that it is possible to use DYNA3D to model all threeaspects of launch behaviour.

Quantitative validation of the models was beyond the scope of the project. However,qualitative comparison with available data has shown that all of the simulationsappear to give realistic results.

In each case it has been possible to identify ways in which the simulations could beimproved. In all cases it would be desirable to have more complete information onthe shape, structural and interior arrangement of the boat. In the simulation oflaunch kinetics and structural response it is also believed that the simulation andmodelling technique could be improved to allow more accurate or detailed predictionsto be obtained.

It is considered that the feasibility of using DYNA3D to model these aspects oflifeboat launch has been demonstrated although the validity of these particulardemonstration simulations is yet to be proven.

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7. RECOMMENDATIONS

Now that the feasibility of using DYNA3D to simulate aspects of lifeboat launch hasbeen demonstrated it is strongly recommended that quantitative validation of thesethree types of simulation should be undertaken. This will probably requireinstrumental test launches to be carried out. It is recommended that organisationswhich might be prepared to participate in this activity should be approached. It willalso be necessary to generate further DYNA3D simulations which incorporate theimprovements suggested in Sections 3.5, 4.5 and 5.5.

Once the validity of the techniques has been confirmed it will be possible to applythem to a wide range of problems. It is believed that the simulations could be used inat least three different ways.

• To understand more about the mechanisms involved in free-fall launch. Thisshould be of interest to regulatory bodies, operators and manufacturers sinceit will help all three groups to understand the real merits and drawbacks offree-fall boats compared with other methods.

• To help optimise boat design with regard to weight, cost, manufacturingmethod, etc. This will be of most interest to manufacturers.

• To assist in the generation or assessment of safety cases or in type approval.This will be of interest mainly to regulatory bodies and operators.

Considering the individual simulations in more detail, once they are validated theycould be used to assess a wide range of scenarios. Some obvious applications aresuggested below.

7.1 LAUNCH KINETICS

Models of the type described in Section 3 could be used to assess the following:

• Range of sea or wind conditions under which launch would be consideredsafe

• Merits of different hull shapes• Merits of different launch heights and angles, ramp lengths, etc• Effects of flexible launch ramps (or assessment of the dangers of not

properly locking ramps in position).

7.2 STRUCTURAL RESPONSE

Models of type described in Section 4 could be used to assess the following:

• Effects of hitting debris on launch• Merits of different hull shapes• Merits of different materials of constructions methods• Minimisation of boat weight.

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7.3 OCCUPANT MOTION

Models of the type described in Section 5 could be used to assess the following:

• Different types of harness• Different seat shapes or configurations• Methods of accommodating injured occupants• Optimisation of boat loading while still allowing each occupant an adequate

space envelope.

Undoubtedly there are a great many further applications of the techniques. Onceagain, however, it must be stressed that validation of the techniques must be the nextactivity.

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8. REFERENCES

1. DYNA3D Users’ Manual, Lawrence Livermore National Laboratory.

2. ‘The Use of an Instrumented Hybrid III Dummy to assess the Ride Characteristicsof Free-Fall Lifeboats’. Surg. Cdr. P.J. Waugh, Offshore Safety: Protection ofLife and the Environment. 20th - 21st May 1992.

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