oth391_simulation of lifboat water entry
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OTH 92 391
FEASIBILITY OF COMPUTER
SIMULATION OF THE LAUNCH
OF FREE-FALL LIFEBOATS
Prepared by
Frazer-Nash Consultancy Limited
Shelsley House, Randalls Way
Leatherhead
Surrey KT22 7TX
London: HMSO
Health and Safety Execu t ive - Offshore Techno logy Report
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© Crown copyright 1993
Applications for reproduction should be made to HMSO
First published 1993
ISBN 0-11-882138-5
This report is published by the Health and Safety Executive as
part of a series of reports of work which has been supported by
funds formerly provided by the Department of Energy and lately
by the Executive. Neither the Executive, the Department nor the
contractors concerned assume any liability for the report nor do
they necessarily reflect the views or policy of the Executive or
the Department.
Results, including detailed evaluation and, where relevant,
recommendations stemming from their research projects are published in the OTH series of reports.
Background information and data arising from these research
projects are published in the OTI series of reports.
HMSO
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CONTENTS
23
23
2424
5.4 Comparison with Available Data
5.4.1 Injury Levels of Properly Restrained Occupants
5.4.2 Role of Harnesses in Different Seating Positions5.4.3 Maladjustment of Harness Straps
20
21
21
22
22
5.3 Demonstration Simulations
5.3.1 Case 1 - Correctly Restrained Occupant in Aft of Boat
5.3.2 Case 2 - Correctly Restrained Occupant in Fore of Boat
5.3.3 Case 3 - Occupant in Aft of Boat Restrained by a
Maladjusted Harness system
5.3.4 Case 4 - Correctly Restrained Occupant in Aft with
Input Taken from a Simulation of Launch Kinetics
18
18
19
19
19
20
5.1 Effects and Mechanisms
5.2 Generation of DYNA3D Occupant Motion Model
5.2.1 DYNAMAN
5.2.2 Seat Structure
5.2.3 The Restraint System
5.2.4 Input Hull Kinetics
18OCCUPANT MOTION5.
16
16
4.4 Comparison with Available Data
4.5 Discussion of Feasibility of Modelling Structural Response
15
15
15
4.3 Demonstration Simulation
4.3.1 Motion
4.3.2 Stress
13
13
1415
4.1 Effects and Mechanisms
4.2 Generation of DYNA3D Structural Response Model
4.2.1 Description of Boat Representation4.2.2 Water Representation
13STRUCTURAL RESPONSE4.
10
11
3.4 Comparison with Available Data
3.5 Discussion of Feasibility of Modelling Launch Kinetics
8
9
9
9
10
3.3 Demonstration simulation
3.3.1 Sliding Along Ramp
3.3.2 Rotation
3.3.3 Free-fall
3.3.4 Water Entry
7
7
8
3.2 Generation of DYNA3D Launch Kinetics Model
3.2.1 DYNA3D Lifeboat Model
3.2.2 Application of Forces
4
4
5
5
6
3.1 Effects and Mechanisms
3.1.1 Sliding Along the Ramp
3.1.2 The Rotation Stage
3.1.3 Free-fall Stage
3.1.4 Water Entry Stage
4LAUNCH KINETICS3.
2OVERVIEW2.
1INTRODUCTION1.
PAGE
iiiSUMMARY
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FIGURES 1 - 37
30REFERENCES8.
28
28
28
29
RECOMMENDATIONS
7.1 Launch Kinetics
7.2 Structural Response
7.3 Occupant Motion
7.
27CONCLUSIONS6.
255.5 Discussion of Feasibility of Modelling Occupant Motion
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SUMMARY
The feasibility of using the DYNA3D finite element code to model the launch of
free-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 the
various stages of launch. The predicted behaviour is seen to agree well with
behaviour observed in a real launch.
The structural response simulation predicts stresses and strains in the boat structure
during 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 boat
including the influence of their harnesses. Results of the simulation agree well with
behaviour observed in a real launch.
The next stage must be to validate all three types of simulation. To do this it will be
necessary to obtain detailed experimental data. It will also be necessary to develop
further 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 regulatory
bodies.
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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 Safety
Executive (HSE) under agreement Number MaTSU/8429/2866.
The purpose of the work has been to demonstrate the feasibility of using the
computer simulation code DYNA3D to simulate the following aspects of the launch
behaviour of free-fall lifeboats.
• Launch kinetics
• Structural response
• Occupant motion.
The purpose of the launch kinetics simulation is to predict boat motion during launch
under different conditions. This will allow the effect on the trajectory of the boat of
different boat designs, different ramp heights and angles and different sea conditions
to be investigated.
The purpose of the structural response simulation is to predict the stresses and strains
induced in the boat’s structure during impact with the water. This will allow the boat
structure to be optimised in terms of hull shape, cost, weight, etc as well as allowing
assessments to be made of structural integrity in the event of striking debris during
launch.
The purpose of the occupant motion simulation is to assess how boat occupants
would move around during launch and how they might be injured. This allows the
boat interior including seats and harnesses to be optimised and ways of
accommodating injured passengers to be investigated.
Computer simulation of these aspects of lifeboat launch has a number of important
advantages over alternative approaches such as physical testing.
• The cost of a computer simulation is invariably much lower than that of a
corresponding physical test.
• Sometimes a simulation can actually give more understanding of underlying
mechanisms than a physical test since a correct representation of the
fundamental 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 in
bringing new products rapidly to the market place or identifying possible
problems during the design process.
• Simulation can be used to make a cost-effective initial assessment of
proposed new designs. It can avoid devoting resources to expensive
prototyping 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 of
DYNA3D simulations which demonstrate the feasibility of using the code to model
the three aspects of lifeboat launch. Section 6 discusses the conclusions from the
work while Section 7 makes recommendations for further work.
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2. OVERVIEW
The finite element code DYNA3D (Reference 1) was written by the Lawrence
Livermore National Laboratory in California for modelling dynamic behaviour of
structures, particularly under transient loading. The code has been continually
developed during its 15 year history and the range of applications for which it has
been successfully used is constantly increasing. The purpose of this project has been
to demonstrate that DYNA3D could be used to predict the behaviour of free- fall
lifeboats 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 three
different types of DYNA3D simulation.
The work has therefore comprised three separate phases, each considering one
aspects of launch behaviour. The approach adopted in each phase has been similar
and has consisted of five activities.
• Consideration of the important effects and mechanisms which need to be
included in a simulation of the aspect of launch.
• Generation of an appropriate DYNA3D model which includes the important
mechanisms.• Simulation of one or more scenarios using the DYNA3D model to
demonstrate the operation of the simulation and the range of output which
can be obtained.
• Qualitative comparison of the results of the simulations which data from real
lifeboat launches. It should be noted that quantitative validation of the
models was not part of the work programme.
• Assessment of the feasibility of using DYNA3D to simulate the aspect of
launch based on the previous activities.
Sections 3 - 5 of this report consider each of the three aspects of launch behaviour in
turn 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 of
information from the following sources:
• Data supplied by Robert Gordon Institute of Technology (RGIT). This
included some photographs of a free fall lifeboat taken during construction
and video tape of a lifeboat launch from inside and outside the boat.
• Engineering details of a free-fall lifeboat supplied by RGIT with the
permission of the lifeboat manufacturer.
• A paper (Reference 2) which included some results of launching a lifeboat
containing 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 using
DYNA3D 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 launch
kinetics 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 the
trajectory taken and accelerations experienced by the boat. For this purpose the
lifeboat can be treated as a rigid body (with appropriate mass and inertia) with forces
applied representing interaction with the launch ramp, gravity and interaction with
water. To understand the mechanisms involved it is convenient to consider four
stages 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 the
following sections.
3.1.1 Sliding Along the Ramp
The forces acting during this stage of the launch are shown in Figure 2. The
equations of motion are:-
(2)mz - N cos hL + l N sin hL - mg
(1)mx - N sin hL - l N cos hL
boat mass
horizontal, vertical accelerations
normal reaction force (mg coshL)
coefficient of friction between the boat and ramp
launch angle
gravity
=
=
=
=
=
=
m
x, z
N
l
hL
g
Where,
From equations 1 and 2 it can be seen that the boat accelerations during this stage of
the launch are dependent on the launch angle, , and coefficient of friction betweenh
the launch ramp and boat, , but are independent of mass.
l
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The velocity of the boat at the end of this stage will depend on the acceleration and
the 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 force
acting at the centre of gravity and the reaction between the boat and the end of the
ramp 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 angular dx dz
acceleration will change with time. The rotation stage ends when the boat leaves the
ramp completely.
In this stage of the launch, the launch kinetics are dependent on the mass, mass
distribution in the boat ie moment of inertia of the boat, the coefficient of friction
between 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 will
be gravity if the effects of cross winds are ignored. Under these conditions the boat
will 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 offree-fall
Where = angle at end of rotation phasehinitial
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 velocity
from the rotation stage and the free-fall height. The greater the free-fall height the
more acute the water entry angle.
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3.1.4 Water Entry Stage
As the boat enters the water forces are generated as the water is displaced by the
boat. Neglecting the effects of surface tension the sources of these forces can be
identified as:
• buoyancy effects
• drag effects.
These effects are shown in Figure 4 and discussed in the following sections.
3.1.4.1 Bu oyancy
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 force
on the object. However, where pressure varies (for example, with depth) there will
be a net resultant force on the object.
This force is known as the buoyancy force. The buoyancy force is the net effect of the different pressures acting on different parts of the boat’s surface due to their
different depths below the water surface.
On any small area of boat’s surface the pressure due to buoyancy effects is given by
(7)P buoy - q water g h
where
depth below water surface.=hgravitational acceleration=g
water density= pwater
Pressure due to buoyancy effects=P buoy
3.1.4.2 Drag
Drag forces are difficult to determine as they represent energy loss mechanisms
which 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 as
water is forced to change direction by collision with the boat, ie the water flow is not
parallel with the surface. Secondly, energy is lost due to friction between the flowing
water and boat surfaces which are parallel with the flow. These effects together are
known 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 to
quantify.
Although the individual drag effects are difficult to predict, their net effect is a total
drag force which can be approximated by:
(8)F drag - p water (V body - V water )2 CD A1
2
where:
water density=V body -Vwater
“drag coefficient” which represents the net effect of all forms of
drag.
=CD
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area of body normal to the flow.=A
water density= p water
This total drag force will act against the direction of travel of the boat as shown in
Figure 4.
It is usual to determine values for CD experimentally. CD will depend on hull shape
and direction of water flow relative to the boat. For free-fall launch it will also vary
as 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 DYNA3D
requires a rigid body representation of a boat together with mechanisms for applying
the various forces. This section describes how a DYNA3D model with these features
has been generated.
3.2.1 DYNA3D Lifeboat Model
A DYNA3D rigid body model based on a typical free-fall lifeboat has been
generated. The finite element mesh created is shown in Figure 5. It consist entirely
of 4-noded shell elements.
The model was based on the lifeboat data described in Section 2. The lifeboat
dimensions and centre of gravity position were taken from a sketch supplied by
RGIT. 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 the
simulation that all the mass of the boat is distributed evenly around the surface of the
boat, ie the model is hollow, and the mass is controlled by adjusting the shell element
thickness. This is likely to give a moment of inertia somewhat higher than the actual
value.
The lifeboat launch rails were modelled with further rigid bodies as shown in
Figure 6. The water was not modelled explicitly in this simulation. Instead the
forces generated by interaction with the water were calculated and applied to the
model as discussed in the next section. The surface is represented by shell elements
in 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 as
discussed in the next section.
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3.2.2 Application of Forces
Gravity and contact forces between the boat and launch ramp were applied using
standard DYNA3D “base acceleration” and “sliding interface” features.
Buoyancy and drag forces were included in the model as outlined below.
3.2.2.1 Bu oyancy
In DYNA3D the buoyancy force described in Section 3.1.4.1 is implemented by
applying a pressure P buoy given by Equation (7), to each small segment of the model
which 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 total
buoyancy force.
3.2.2.2 Drag
In DYNA3D it is assumed that the total drag force described in Section 3.1.4.2 is
due to high pressure on parts of the object’s surface which are moving into the fluid
and low pressure on parts of the object’s surface moving away from the fluid as
shown in Figure 8.
Drag is applied as pressure on all parts of the boat surface which are below the water
surface. 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 is
calculated 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 a
typical launch scenario was modelled.
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In addition to the model details described in Section 3.2 the following were assumed:
19.56 m
350
Zero
1000 kg m-3
2.0
0.1
1.0
0.5
Launch height
Launch angle (to horizontal
Ramp friction coefficient
Water density
Overall drag coefficient, C
x direction drag multiplier, Mx
y direction drag multiplier, My
z direction drag multiplier, Mz
ValueParameter
Table 1
Demonstration Simulation Parameters
Figure 9 shows the resultant trajectory of the bow and stern of the boat relative to the
surface 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 after
initial entry. The position of the boat at each of these maxima and at several other
stages during launch are shown pictorially in Figure 10.
The stages in the launch of the boat can be identified from the acceleration histories
of the boat shown in Figure 11. These show the boat accelerations in boat
coordinates ie axial accelerations are those in line with bow to stern and normal
accelerations are perpendicular to the boat floor.
The main features of the boat’s motion in the separate stages of launch are discussed
in the following sections.
3.3.1 Sliding Along Ramp
• The boat slides along the ramp for about 2 seconds. This stage starts at
0 seconds and ends at Point ‘A’ shown in Figure 11..
• In this stage, the predicted accelerations are given by Equations 1 and 2 and
are plotted in Figure 12 along with the expected value. As can be seen they
agree well.
3.3.2 Rotation
• The angle, angular velocity and angular acceleration history of the boat
during 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 leaves
the 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 on
Figure 11.
• During free-fall the angular acceleration is zero and the angular velocity
remains 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 global
z-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 in
Figure 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 the
boat. These drag forces decrease as the boat slows down. As the boat rises
out of the water, drag forces are developed which pull it back down into the
water as shown by the period of negative normal drag force.
• A large buoyancy force is generated which increases as the boat sinks into
the water and peaks after the drag force. The buoyancy force then rises and
falls 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 large
angular velocity and angular acceleration are generated. The boat rotates to
a horizontal position in less than one boat length
• As the boat enters the water the bow sinks to about 3 m below the water
surface at 3.2 seconds shown in Figure 10. During the ensuing rotation the
stern sinks to the gunnel at 3.7 seconds shown in Figure 10.
• When the boat reaches the lowest point in the water the buoyancy forces
push it back out. The boat falls back into the water again and then osillates
gently 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 angular
acceleration experienced at the aft of the boat and in the second due to the
boat falling back into the water.
• The boat continues to travel forward at a low velocity experienced at the aft
of the boat.
3.4 COMPARISON WITH AVAILABLE DATA
As indicated in Section 2, RGIT provided a video tape of the launch of a free-fall
lifeboat. From this video the following characteristics of the launch are apparent:
• rotation during the free-fall phase is clearly visible. From a launch height of
about 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 the
gunnel;
• after the bow has penetrated the water the boat rotates rapidly until the stern
comes into contact with the water. The boat achieves a horizontal attitude in
less than one boat length;
• after the initial water entry phase the boat rises out of the water and falls
back 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 the
boat contacts the water and the second as the boat drops back into the water
after 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 the
video results from RGIT shows that the overall trajectories of the simulation and real
launch are similar although the simulated launch would appear to be underdamped
particularly in the vertical (z) direction. This is highlighted by the number and
magnitude of vertical oscillations which occur after the initial water entry. The
oscillations could be reduced by increasing the value of C.
From recording made inside the boat of the motion of occupants during launch, the
predicted negative accelerations correspond to the ‘bumps’ felt by passengers at the
aft of the boat. The time elapsed between these ‘bumps’ in the DYNA3D simulation
agrees 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 that
DYNA3D can be used to model launch kinetics and can give realistic predictions of
boat motion. The feasibility of using DYNA3D to simulate this aspect of free fall
lifeboat launch has therefore been demonstrated.
Experience gained in the development of the demonstration simulation suggests that
the model could be improved in a number of ways:
• A more accurate representation of the hull shape and the boat’s moments of
inertia would improve the simulation in terms of motion during launch and
also the final, stable position in the water.
• The drag coefficient and the three directional multipliers (see
Section 3.2.2.2) could probably be tailored to give a closer match between
the simulation and the actual launch.
• Currently the directional multipliers apply to drag in the global x, y, z
directions. It would be better if they were modified to apply to an axis
system which moved with the boat.
It would be appropriate to incorporate these improvements into future models before
attempting to validate the technique quantitatively. To allow the technique to be
validated it would also be necessary to obtain more detailed trajectory and
acceleration data from an actual launch.
The current simulation method uses four coefficients (C, Mx, My and Mz, see Section
3.2.2.2) to characterise the boat’s drag behaviour. Appropriate values for these
coefficients would need to be determined using empirical data. One could not predict
the behaviour of a new hull shape which had never been made and tested using this
approach. However, data on a particular hull’s behaviour could be determined from
some relatively simple tests and from launches in benign conditions. Once a
simulation based on this data had been developed it could then be used to consider
more “interesting” launch conditions for that boat.
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The same semi-empirical approach is used in the automotive and aerospace
industries. However, in those industries a considerable body of data has been built
up over the years which allows good estimates of drag behaviour to be made prior to
wind tunnel or other testing. Data relating to drag on different shaped boats once
they are stable in the water is likely to exist already but data for boats impacting the
water during free-fall launch probably does not.
It may be possible, with further research, to develop an approach to drag modelling
for lifeboat launch which does not rely upon empirically derived coefficients. If this
were possible then the advantages would be significant since new hull shapes could
be assessed while they were still at the drawing board stage. It is recommended that
consideration 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 using
DYNA3D to simulate the structural response of a free-fall lifeboat during launch.
The following sections discuss:
• 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.
4.1 EFFECTS AND MECHANISMS
In the structural response aspect of the work programme, it is the actual stresses and
strains which are generated in the boat structure on impact with the water which are
of interest. In order to predict these stresses and strains the model must be able to
correctly simulate the loads on the boat arising from water inertia and buoyancy
while accounting for the changing amount of hull surface area in contact with the
water.
This can be achieved directly with DYNA3D by creating a finite element model
representative of the major structural features of the free-fall lifeboat and allowing it
to interact with a second block of finite elements with the properties of water. By
means 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 by
displacing 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 data
supplied by RGIT and the boat manufacturer (see Section 2). However, a number of
simplifications were made in generating the DYNA3D model. Thus, the results of
this simulation may be typical of free-fall lifeboats but are unlikely to give an
accurate prediction of the behaviour of the particular boat for which data were
supplied due to the number of simplifications made.
Figure 15 shows the DYNA3D structural response model. It comprises a simplified
finite element representation of the boat and a region of water. The figure shows a
full boat. In fact, because of the existence of a vertical plane of symmetry along the
boat’s fore-aft axis, only a half model was generated. The graphics software used to
generate this and other pictures was used to add the “missing half” for visualisation
purposes.
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To minimise computer run times the simulation begins just before impact and lasts
for 0.5 s after impact. The initial conditions for the model were obtained from the
output of a launch kinetics model (Section 3) and were as follows:
9.3 ms-1
16.3 ms-1
520 to horizontal
Horizontal velocity
Vertical velocity
Impact angle
ValueParameter
Table 2
Structural 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 was
seen that the real hull plating is reinforced with small “top hat” section stringers.
However, for simplicity these were represented by increasing the effective thickness
of the hull plating in the DYNA3D model. Furthermore the real superstructure
includes glass areas and a large rear door. Again, for simplicity, these were not
included in the model. This is acceptable for a feasibility study. Clearly for a real
analysis it would be necessary to model these structural details.
The positions and shapes of the hull ribs and stringers and the deck framing were
estimated from the available drawings and photographs. Where the available data
were incomplete engineering judgement was used to define a “sensible” structure.Figure 18 shows the arrangement of the internal framework in the boat model. It is
believed to be representative of typical boat construction although it may differ in a
number 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 in
the model.
The whole boat was assumed to be constructed of aluminium alloy with typical
mechanical properties.
The actual boat contains a number of massive items, such as the engine, seating, fuel
tank, etc. These were not explicitly represented in the model. Instead, their mass
was included by increasing the density of the aluminium to achieve the correct total
mass for the boat.
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4.2.2 Water Representation
The water was represented by brick elements using the DYNA3D fluid material
model. This material model allows the density and compressibility of water to be
represented. 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 simulate
the interaction of a free-fall lifeboat with water and also to show the range of results
that could be extracted from such a simulation. The results can be generalised into
two areas:
• Motion (movement of the lifeboat, deformation of the structure)
•
Stresses.
The results of this simulation are therefore presented under these headings in Sections
4.3.1 and 4.3.2.
It should be noted that the model was not an exact representation of the lifeboat for
which data were supplied. Therefore numerical results, such as stresses, cannot be
used for a critical assessment of the actual lifeboat, although the range of values
derived would be typical of a generic free-fall lifeboat. As the available information
was limited, critical regions such as the bow (which takes the initial impact) were
modelled in a similar manner to the rest of the boat, whereas in reality there would
probably be additional strengthening members if this area. Hence the results in these
regions 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 equal
time intervals over a 0.4 seconds time span. The bow of the boat can be seen to
penetrate into the water and quickly decelerate due to buoyancy and water inertia
effects causing the lifeboat to rotate and the stern to contact the water shortly
afterwards. This sequence of pictures corresponds closely to the motion of a free-fall
lifeboat 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 the
lifeboat at the end of the analysis. At this stage the boat was moving forwards
through the water, and hence the region of highest stress, as expected, was around the
bow where the water was being pushed out of the way by the boat’s motion. The
force of the water on the boat created pressure and compression on the forward
structure giving rise to the stresses shown.
The effect of the boat’s geometry is seen approximately half way along its length
where a ‘kink’ in the upper deck creates a local stress raiser. The boat was generally
in compression at this stage as its motion was being resisted by the force of the water on 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 rise
to the effect shown in Figure 20. Further high stress areas can be seen at similar
geometrical features around the cabin and also along the gunnel where loads from the
hull transmit out-of-plane forces into the deck.
Figure 21 shows the stress distribution in the passenger compartment at the end of the analysis. The effect of the internal stiffening members can be seen as areas of
high stress where the ribs meet the internal compartment and the stringers meet the
forward bulkhead. These areas were locally stiff and therefore attracted load and
created higher stresses. This was also an indication that loads were being transferred
from the hull into the rest of the boat’s structure, ie the stiffening members were
distributing the loads from the water throughout the boat, which is the purpose of any
such stiffening structure.
This behaviour is seen in more detail in Figure 22 which shows the ribs and stringers
alone. Higher stress areas can be seen at local stiff points where members meet or
where there were stress concentrating features such as sharp corners or changes in
section. To predict actual stress levels at these details it would be necessary to model
them with a finer mesh density.
4.4 COMPARISON WITH AVAILABLE DATA
It was not possible to obtain any data on the stresses actually generated in a lifeboat
during launch.
As discussed in Section 4.3 the behaviour predicted agrees qualitatively with the
response which one would expect. In particular, local high stresses occur at changes
of section and where the structure is locally stiff.
4.5 DISCUSSION OF FEASIBILITY OF MODELLING STRUCTURAL
RESPONSE
The demonstration simulation described in Section 4.3 shows that DYNA3D can be
used to model a lifeboat hitting the water and that sensible looking behaviour is
predicted. The feasibility of modelling this aspect of launch behaviour has therefore
been demonstrated.
The experience gained during this phase of the work suggests that the model could be
improved in a number of ways before it would be appropriate to consider quantitative
validation:
• A more faithful representation of the actual boat structure would clearly be
needed. In particular, it is felt that the bow area of the model would need to
be refined.
• A finer water mesh (smaller finite elements) would be expected to give a
better representation of the water loading on the structure. It is felt that the
mesh used here may have applied a load which was too concentrated at
discrete points and not sufficiently distributed over the parts of the hull in
contact with the water.
• The mesh density used in the demonstration simulation for the boat itself is
appropriate 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 finer
mesh at points of interest.
Quantitative validation of the technique should incorporate these improvements. It
will also require experimental data such as strain gauge output from an actual
launch.
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5. OCCUPANT MOTION
This section describes the work carried out to demonstrate the feasibility of using
DYNA3D to simulate the motion of lifeboat occupants during lifeboat launch.
The following sections discuss:
• The important features which need to be taken into account in modelling
occupant 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.
5.1 EFFECTS AND MECHANISMS
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 a
model of the boat could be included in this analysis, it is unnecessary since only the
acceleration histories of the boat are required. These can be applied to the occupant
via his seat. In this phase then, a person and the seat only have to be modelled and
acceleration 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 to
model 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-fall
lifeboat launch. The model is shown in Figure 23 and comprises four main
components:
•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 are
described in Section 5.3. The differences between the cases are in the restraint
characteristics and the different kinematics experienced in different positions in the
boat.
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5.2.1 DYNAMAN
DYNAMAN comprises a number of rigid bodies which represent the various body
sections. The body sections are linked together by springs and dampers representing
human joints. Each of the body sections and joints can be tailored to suit particular
application.
The requirements of this particular application were assessed from two sources of
information:
• 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 crossed position 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 braced
occupant adopting the recommended launch posture. The results of these simulations
may 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 the
cushion and overall seat structure was estimated from photographs. Estimates were
also made for the material properties of the cushion material. The seat and footrest
were 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 straps
were simulated with six sets of springs and dampers representing the elasticity and
energy absorption characteristics of the webbing.
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Spring and damper elements were included in the following positions (also see Figure
23):
• 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 of
automotive seat belt webbing. The six sets of spring and damper elements were
given 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 characteristic
such 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 is
the water impact phase. During free-fall little movement of the occupant relative to
the 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. The
boat is decelerated by buoyancy and drag forces from the water, whereas the
occupant is still only subject to the force of gravity. Therefore relative motion will
occur between boat and occupants. In this investigation only the water impact phase
of the boat kinematics has been considered.
Different inputs were used for the four demonstration simulations as described in
Section 5.3.
5.3 DEMONSTRATION SIMULATIONS
As indicated in Section 5.1, four demonstration simulations, each representing a
different scenario were produced. The cases were:
Correctly restrained occupant in aft of boat with input kinematics
taken from a FNC launch kinetics simulation.
Case 4 -
Occupant restrained by a maladjusted harness in aft of lifeboat with
input kinematics taken from Reference 2.
Case 3 -
Correctly restrained occupant in fore of lifeboat with input
kinematics taken from Reference 2.
Case 2 -
Correctly restrained occupant in aft of lifeboat with input kinematics
taken 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 restraints
fully.
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 a
seating position in the aft of a lifeboat. Figure 24 shows the vertical and longitudinal
velocity histories used in this case. No rotational motion data was available from
Reference 2 and so rotational motion of the seat could not be included even though
some rotation would certainly occur in reality.
Figure 25 shows the motion of DYNAMAN relative to his seat during the first 0.75 s
of the simulation.
On the whole there is little movement of the occupant. However, there is some lifting
of the occupant’s back and legs in the early stages of the simulation. The lifting of
the occupant’s body is due to the initial downward acceleration in the aft of the boat
which occurs as the boat rotates to the horizontal. Once the aft of the boat is in
contact with the water there is an overall upward acceleration which forces the
occupant back down into the seat.
The occupant accelerations that were calculated during the simulation are shown in
Figures 26 and 27. Both the head and thorax accelerations closely follow those of
the lifeboat. This was as expected as the occupant is well restrained and will
therefore 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 his
restraints.
The motion input was obtained from Reference 2 and represented vertical and
longitudinal 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 to
the 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 the
seat. This movement does not occur as there is no initial downward acceleration of the fore of the boat since it is close to the centre of rotation as the boat returns to the
horizontal.
When examining the kinematics of the occupant motion (see Figure 29) it can be seen
that there are only very small movements. The major component of the boat
acceleration is vertically upwards, therefore the occupant is forced down into the seat
and little movement of the occupant will occur. Deceleration of the boat is the x
direction that would “throw” the occupant forwards is only of the order of 1 - 2 g and
is easily restrained by the harness.
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Figure 30 shows the head acceleration and Figure 31 the thorax acceleration. As
observed in the correctly restrained aft case the occupant accelerations closely follow
those of the lifeboat.
The differences between these graphs and the hull deceleration are mainly due to the
two 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 to
the 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 Maladjusted
Harness System
For the third simulation, a case of restraint misuse was chosen. Modes of misuse
that 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 the
slack harness case was chosen for this project. 100 mm of slack was assumed. The
aft seat motion used in Case 1 was also used for this case. This allows a direct
comparison 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 the
aft of the boat is in its initial downward acceleration phase. Once the slack is taken
up, the occupant is “yanked” back down into the seat. The delay in the restraint of
the occupant caused by the slack, leads to an increase in relative velocity between
occupant and boat. There is therefore an increase in the force required to stop the
occupant resulting in higher head and thorax accelerations in Figures 33 and 34.
5.3.4 Case 4 - Correctly Restrained Occupant in Aft with Input Taken
from a Simulation of Launch Kinetics
This case repeated Case 1 except that the seat motion was taken from a DYNA3D
launch kinetics model (see Section 3) rather than from Reference 2. This allowed
rotational motion to be included in the simulation.
The results of this simulation were similar to Case 1. The occupant lifts away from
the 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) and
thorax (Figure 37) exhibit similar shapes and peak values to those seen in Case 1, but
differences can be seen in the timing of these peaks. The differences are most likely
due to deviations between the measured hull accelerations (used in Case 1) and thecalculated values used in this Case.
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5.4 COMPARISON WITH AVAILABLE DATA
Data for comparison with the DYNAMAN results was available from the RGIT
video 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 DYNAMAN
model 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 the
work was carried out.
In view of the above comments it would be surprising if DYNAMAN had predicted
exactly the same behaviour as that seen in either the video or Reference 2.
Nevertheless, comparison of DYNAMAN overall occupant motion and head and
thorax accelerations with the available data shows good agreement. Discussion with
RGIT also confirmed that the sensations experienced by occupants in the front and
rear of the boat are different and that a significant jolt would be experienced by
occupants with excessively loose restraints, particularly in the aft of the boat.
Although the simulations could not be considered validated, they do give sufficiently
realistic 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 significant
injuries.
• In the launch scenario considered in the simulations it appears that the
harnesses worn by the lifeboat occupants play a more important role in
restraining those occupants in the aft of the boat than those occupants in the
fore of the boat.
• The simulations suggest that the potential for injury could be increased if the
restraint 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 head
strap which are tight (ie no slack at all). As a result the occupant moves with his
seat. Unless there is some loose equipment in the boat it is more unlikely that he
would 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 means
that acceleration levels experienced by the occupant are low, certainly well within
normally accepted injury criteria. This conclusion is no great surprise since it is
understood that RGIT has conducted training launches similar to the launch
simulation for several years without any significant injuries.
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Two important caveats must, however, be applied. Firstly, although the simulation
results are in broad qualitative agreement with real life experience they are not yet
quantitatively validated. It would be unwise at this stage to make quantitative
predictions based on the simulation results. Secondly, only one launch scenario has
been considered. Without further work it would be wholly inappropriate to assume
that 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 boat
experience a significantly different acceleration environment than occupants at the
back of the boat.
In front of the boat the forces act primarily to push the occupant into his seat. Thus
the 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 the
occupant out of his seat (or, more correctly, to pull the seat downwards, away from
the occupant). After this initial phase the forces once again act to push the occupant
into his seat. In the initial phase the harness does come into play to prevent the
occupant coming out of his seat.
One might be tempted to conclude that occupants in the front of the boat may not, in
fact, need their harnesses. For the precise conditions simulated here that might be
true.
However, it would be necessary to consider other launch scenarios where the
acceleration histories in different parts of the boat could be different before it would
be 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) from
throwing the occupant around. There may also be important psychological
advantages in having occupants strapped in where they may feel more secure and
may be less likely to leave their seats after launch and perhaps get in the way of the
boat’s crew.
5.4.3 Maladjustment of Harness Straps
The simulation of an occupant restrained with a loosely adjusted harness exhibited
greater 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 3
show that higher maximum levels are experienced by occupants with loose restraints.
The potential for injury would therefore be higher for the occupant who had not
tightened his harness. This observation is in agreement with the generally accepted
view in the automotive world that loose seat belts offer less protection than belts
without 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 a
possible 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 would
be necessary to examine in more detail what the boarding procedure for the boats
actual is before one could assess whether this presents a problem in reality.
5.5 DISCUSSION OF FEASIBILITY OF MODELLING OCCUPANT
MOTION
DYNAMAN is a well proven technique for modelling the motion of people and the
four demonstration simulations described in Section 5.3 confirm that it can be used to
model lifeboat occupants.
The accuracy of the simulations could be improved given more precise data. In
particular, 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 be
expected to present any significant difficulties.
A more problematic aspect, however, is the degree of muscle tension or rigidity of the
occupants in a launch. Discussion with RGIT suggests that different people will
brace themselves to different extents and even the same person will be likely to react
in different ways on different occasions.
If information could be obtained on the range of likely human response then
DYNAMAN could be configured to assess various different cases. In the absence of
such data it may be appropriate to consider two extremes of behaviour (completely
limp or completely stiff). The completely limp case will generally indicate the largest
kinematic envelope in which the occupant could move while the completely stiff case
might give an indication of maximum forces on the body.
In addition, it will be necessary to consider the types of injury which could be
suffered by occupants and the data which would need to be obtained from the
simulations to allow the likelihood of these injuries to be assessed. In the
demonstration simulations, overall occupant motion and head and thorax acceleration
were 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 to
define acceptable levels for these criteria. These acceptable levels will clearly be
much lower than the levels which would be allowable in a car crash where the aim is
that the occupant should survive even if he would not be expected to walk away from
the 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 been
appropriate to develop different types of DYNA3D model for each application. The
demonstration simulations show that it is possible to use DYNA3D to model all three
aspects 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 simulations
appear to give realistic results.
In each case it has been possible to identify ways in which the simulations could be
improved. In all cases it would be desirable to have more complete information on
the shape, structural and interior arrangement of the boat. In the simulation of
launch kinetics and structural response it is also believed that the simulation and
modelling technique could be improved to allow more accurate or detailed predictions
to be obtained.
It is considered that the feasibility of using DYNA3D to model these aspects of
lifeboat launch has been demonstrated although the validity of these particular
demonstration 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 has
been demonstrated it is strongly recommended that quantitative validation of these
three types of simulation should be undertaken. This will probably require
instrumental test launches to be carried out. It is recommended that organisationswhich might be prepared to participate in this activity should be approached. It will
also be necessary to generate further DYNA3D simulations which incorporate the
improvements suggested in Sections 3.5, 4.5 and 5.5.
Once the validity of the techniques has been confirmed it will be possible to apply
them to a wide range of problems. It is believed that the simulations could be used in
at least three different ways.
• To understand more about the mechanisms involved in free-fall launch. This
should be of interest to regulatory bodies, operators and manufacturers since
it will help all three groups to understand the real merits and drawbacks of free-fall boats compared with other methods.
• To help optimise boat design with regard to weight, cost, manufacturing
method, 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 they
could be used to assess a wide range of scenarios. Some obvious applications are
suggested 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 considered
safe
• 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. Once
again, however, it must be stressed that validation of the techniques must be the next
activity.
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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 Characteristics
of Free-Fall Lifeboats’. Surg. Cdr. P.J. Waugh, Offshore Safety: Protection of Life and the Environment. 20th - 21st May 1992.
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