COMPUTER SIMULATION OF THE PERFORMANCE OF LIFEJACKETS - A Feasibililty Study

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2 OTH COMPUTER SIMULATION OF THE PERFORMANCE OF LIFEJACKETS - A Feasibililty Study Prepared by Frazer-Nash Consultancy Limited Shelsley House, Randalls Way Leatherhead Surrey KT22 7TX London: HMSO Health and Safety Executive - Offshore Technology Report

3 Cron copyright 1993 Applications for reproduction should be made to HMSO: First published 1993 ISBN This report is published by the Health and Safety Executive as part of a series of reports of ork hich has been supported by funds formerly provided by the Department of Energy and lately by the Executive. Neither the Executive, the Deparment nor the contractors concerned assume any liability for the reports nor do they necessarily reflect the vies or policy of the Executive or the Department. Results, including detailed evaluation and, here relevant, recommendations stemming from their reearch 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 Standing order service Placing a standing order ith HMSO BOOKS enables a customer to receive other itels in this series automatically as published. This saves time, trouble and expense of placing individual orders and avoids the problem of knoing hen to do so. For details please rite to HMSO BOOKS (PC 13A/1). Publications Centre, PO Box 276, London SW8 5DT quoting reference The standing order service also enables customers to receive automatically as published all material of their choice hich additionally saves extensive catalogue research. The scope and selectivity of the service has been extended by ne techniques, and there are more than 3,500 classifications to choose from. A special leaflet describing the service in detail may be obtained on request.

4 CONTENTS PAGE SUMMARY INTRODUCTION THE SIMULATION CONCEPT iii The DYNAMAN Technique The Forces on Bodies in Water Buoyancy Drag Sea Conditions Test Case Summary of Simulation Features DYNAMAN DEMONSTRATION SIMULATION Introduction Description Results SELF-RIGHTING TRIALS Test Facility Lifejackets Test Procedure Results INITIAL SIMULATIONS BASED ON TRIALS Lifejacket models Simulation of Trials

5 5.3 Results COMPARISON BETWEEN TRIALS AND INITIAL SIMULATIONS Inflatable Lifejacket Inherently Buoyant Lifejacket DISCUSSION Righting Mode Inflatable Lifejacket Inherently Buoyant Lifejacket Simulation Improvements BUOYANCY AND DENSITY MEASUREMENT REVISED SIMULATIONS Model Changes Results of Revised Simulations

6 SUMMARY This report describes ork carried out by Frazer-Nash Consultancy Limited (FNC) on behalf of the Offshore Safety Division of the Health and Safety Executive (HSE) under agreement number E/5B/CON/8387/2804. The objective of the ork has been to demonstrate the feasibility of using the DYNAMAN computer simulation technique, developed by FNC, to study the performance of lifejackets. The effects of buoyancy and drag have been implemented in DYNAMAN to allo lifejacket behaviour to be modelled. In addition, the ability to model simulated ave conditions has been included. These features have been quantitatively validated ith simple test cases. A series of in-ater trials has been carried out at the Institute of Naval Medicine (INM) ith a marine manikin to obtain data on the self-righting characteristics of the manikin earing to types of lifejacket. Computer simulations of the trials have been generated to validate the modelling technique. These initial simulations highlighted the need for accurate buoyancy and density data. Further, detailed measurements of the buoyancy and density properties for the marine manikin and lifejackets ere therefore made at INM. These data ere used in revised simulations aimed at achieving a closer match beteen the trials and simulations. The simulations have shon that the DYNAMAN technique can be used to model the self-righting behaviour of lifejackets. Very good correlation can be achieved beteen trials and computer simulations provided that the eight and buoyancy distribution in DYNAMAN himself and in his lifejacket are correctly modelled. Applications of the technique have been identified as: gaining an understanding of the principles of buoyancy aid behaviour, in particular in the case of complex systems such as lifejacket/survival suit combinations; use as a design tool for screening designs at an early stage, reducing the need for expensive prototyping; use as a means of assessing the suitability of a particular aid to meet specific performance requirements; use as part of an approval process, either to help define physical tests or to assess designs in conditions here physical testing is inappropriate. 3

7 1. INTRODUCTION This report describes ork carried out by Frazer-Nash Consultancy Ltd (FNC) on behalf of the Offshore Safety Division of the Health and Safety Executive (HSE) under agreement number E/5B/CON/8387/2804. The purpose of the ork has been to demonstrate the feasibility of using the DYNAMAN computer simulation technique, developed by FNC, to study the performance of lifejackets. Computer simulation could have a number attractions: For many applications the cost of computer simulations is loer than that of corresponding physical tests. The timescales for simulations are often shorter hich can be very important in bringing ne products rapidly to the market place or identifying possible problems during the design process. 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. For studying lifejacket performance, computer simulation ould avoid the ethical and safety issues associated ith the use of human subjects in physical testing. The ork has been carried out in a number of stages as follos: The effects of buoyancy and drag forces ere implemented in DYNA3D. The implementation as validated ith a simple test case. A demonstration simulation as carried out in hich DYNAMAN as used to simulate the behaviour of a man earing a typical lifejacket in simulated ave conditions. The purpose of this stage as to demonstrate the feasibility of using the DYNAMAN technique to model behaviour in a moving sea. A series of self-righting trials as carried out at the Institute of Naval Medicine (INM) using a marine manikin and to types of lifejacket; a typical inherently buoyant jacket and a typical inflatable jacket. These trials provided self-righting data against hich the DYNAMAN technique could be validated. DYNAMAN simulations of the self-righting trials ere carried out. One simulation as run for each lifejacket. The results of the simulations ere compared ith the trials. Assessment of the results highlighted the need for more accurate buoyancy data for the manikin and lifejackets. 4

8 As a result of the previous stage, detailed measurements ere made at INM of the density and volume of each part of the marine manikin and the to lifejackets. This alloed the buoyancy of the manikin and lifejackets to be determined. The computer simulations ere re-run ith revised input data based on the measurements made at INM. The results of the revised simulations ere compared ith the self-righting trial data. Section 2 describes the theoretical basis of the simulation technique and the implementation of buoyancy and drag forces to allo lifejacket behaviour to be modelled. A validation test case is also presented in this section. Section 3 describes the demonstration simulation. In Section 4, the self-righting trials carried out at INM are described. In Section 5 the initial computer simulations based on the trials are described and the results presented. Comparisons beteen the trials and simulations are dran in Section 6 and discussed further in Section 7. The measurements made to determine the buoyancy of manikin and lifejackets are described in Section 8. In Section 9 the revised computer simulations are described. The results are compared ith the self-righting trials in Section 10. Conclusions from the ork, recommendations for further ork and applications of the technique are summarised in Section 11. 5

9 2. THE SIMULATION CONCEPT 2.1 THE DYNAMAN TECHNIQUE The DYNAMAN technique as developed by FNC for modelling the dynamic behaviour of human, or surrogate dummies, under a range of different loadings. DYNAMAN is based on the finite element analysis code DYNA3D (Reference 1). DYNA3D has been ritten specifically for modelling transient events here there are large material or geometric non-linearities. FNC has already successfully used DYNAMAN in ork carried out for the HSE Railay Inspectorate (Reference 2). In particular, the model helped to explain injury patterns in the Cannon Street rail crash. Figure 1 shos a typical DYNAMAN model of the type used to assess rail crash injuries. The dimensions of any part of the body can be adjusted to suit the requirements of the particular simulation and mass and inertia properties of each part can be individually assigned. The limbs are joined together so that each joint is free to bend (as ould be the case ith a human) but rotations are limited to a realistic extent. In a typical DYNAMAN analysis, the model is adjusted to the required position and a dynamic loading is then applied. The load may be applied either direct to DYNAMAN (such as in a blast loading scenario) or to the structure modelled around him (such as in a rail crash). DYNAMAN predicts the resulting motion of the person including any impact beteen the person and his surrounding. From the results of a simulation it is possible to make an assessment of likely injuries, examine the effect of adding padding or restraints, etc. In this project, DYNAMAN has been used to model the dynamic behaviour of unconscious people or surrogate dummies earing buoyancy aids in ater. The results that are of particular interest in this application include the general motion of DYNAMAN, the relative position of the airays to the ater surface, and the time taken for the body to right from a face don position. These are important indicators of the relative performance of different buoyancy aid designs. 2.2 THE FORCES ON BODIES IN WATER In order to model the behaviour of bodies in ater, the forces acting on them must be correctly applied. The form include: buoyancy drag gravity surface tension ind loading Figure 2 illustrates these forces. 6

10 The first three of these forces are believed to be the most significant ones for the analysis of lifejacket self-righting performance. It as agreed ith the HSE that only these effects ould be considered in this ork programme. The ability to apply gravitational forces is a standard feature of DYNAMAN. Hoever, it has been necessary to include the effects of buoyancy and drag to allo lifejacket performance to be modelled. The theory used and its implementation in DYNAMAN is described in the next to sections. 7

11 2.3 BUOYANCY When a body is fully or partially immersed in a fluid the fluid exerts a pressure on it as shon in Figure 3. If the pressure ere uniform over the entire surface there ould be no resultant force on the object. Hoever, here pressure varies (for example, ith depth) there ill be a net resultant force on the object. This force is knon as the buoyancy force. On any small area, A, on the surface of an object, the buoyancy force, hich acts normal to the surface of the object, is given by; F buoy = ρ ater g ha Eqn 1 Where ρ ater g h = density of ater = gravitational acceleration = depth of the centre of the area, A, belo the ater surface. In DYNAMAN this buoyancy force is implemented by applying a pressure P buoy to each small segment of the man or lifejacket hich is belo the surface of the ater. P buoy is given by; 2.4 DRAG P buoy = ρ ater g h Eqn 2 The conventional theory of hydrodynamics assumes that the total drag force acting on an object moving relative to surrounding ater is given by; F drag = ½ ρ ater u 2 C D A Where ρ = density of ater u = velocity of the object relative to the ater C D = drag coefficient A = a reference area of the object (often the presented area of the object). Eqn 3 8

12 C D is an empirically determined factor dependent (among other things) on the shape of the object. Typical values of C D for some simple shapes are given belo: Shape Flat plate Sphere Long cylinder C D In DYNAMAN it is assumed that the total drag force is due to high pressure on parts of the object's surface hich are moving into the fluid and lo pressure on parts of the object's surface moving aay from the fluid as shon in Figure 4. Surfaces moving into the flo are given an increased pressure given by P drag = ½ ρ ater U 2 and surfaces moving aay from the fluid are given a reduced pressure given by P drag = ½ ρ ater U 2 (1 - C p ) The total drag force on the hole object is then given by Equation 3. Eqn 4 Eqn 5 For objects partly immersed in the ater, drag forces act only on the etted surface. Although ind loading is not being considered here, the form acting due to the ind could be calculated in a similar ay and applied to those surfaces not in the ater. 2.5 SEA CONDITIONS To investigate the behaviour of buoyancy aids in rough sea conditions an idealised sinusoidal motion of the sea surface has been included in the DYNAMAN model. The height of the ater surface is given by h = h o + h av sin (v x t + x) Eqn 6 Where, h 0 = h av = = v x = x = t = mean depth of sea ave amplitude ave frequency ave velocity x position time 9

13 In addition to the plane ave travelling on the sea surface, currents in the plane of the sea can also be included in the model. Other, possibly more realistic, sea surface shapes could be incorporated into the model at a later date. For visualisation purposes a sheet of shell elements has been used to represent the sea surface. 10

14 2.6 TEST CASE Description A simple test case has been used to demonstrate and validate the features hich have been added to DYNAMAN. The test model consists of a lo density ball (relative density 0.5) hich is released a distance belo the surface of the ater. The ater surface has a sinusoidal shape. The model is shon in Figure Expected Behaviour The expected behaviour of the ball in this model is that it should initially accelerate upards toards the ater surface due to buoyancy. At some stage the drag force plus the gravitational force ill balance the buoyancy force and the ball should reach a terminal velocity. The ball should then rise at the terminal velocity until it reaches the ater surface. When it reaches the surface the ball should oscillate slightly and then take up a periodic motion ith the same frequency as the surface ave. Since its relative density is 0.5, approximately half of the ball should be visible on the surface of the ater as it oscillates. The expected terminal velocity, u, occurs hen: ie Drag force + Gravitational force = Buoyancy force ½ ρ ater u t 2 C D A + ρ body g v = ρ ater g v Eqn 7 here u t = A = V = ρ = r = = g terminal velocity plan area of the ball = Π r 2 volume of the ball = 4/3 Π r 3 density radius of ball gravitational acceleration Equation 7 can be rearranged to give; Eqn u t = ( ater body ) r g 1 2 ater C D 11

15 In the test case Ρ ater ρ body r C D = = = = 1000 kg/m3 500 kg/m 3 5 mm 1.2 In this case, from Equation 8, the predicted terminal velocity is 233 mm/s. The equations of motion of the ball can be solved numerically to give the expected variation of vertical velocity ith time Results Figure 6 compares the velocity history of the ball obtained from the simulation ith the expected behaviour. It can be seen that the correct temporal variation is obtained and the terminal velocity is 233 mm/s as expected. The vertical motion history of the ball is shon in Figure 7. It can be se that, after a small oscillation in the first cycle, the ball settles don to a smooth, roughly sinusoidal motion as expected. The position the ball takes on the surface of the ater is shon in Figure 8. As expected half the volume is out of the ater. This test case validates the implementation of the buoyancy and drag las and the sinusoidal sea surface described in Sections 2.3, 2.4 and SUMMARY OF SIMULATION FEATURES With the features described above successfully implemented in DYNAMAN, the simulation technique no has the ability to model the folloing features: Properties of each segment of the person dimensions inertial properties effective drag coefficient Properties of the lifejacket shape eight method of attachment effective drag coefficient Properties of the ater 12

16 surface motion (Sinusoidal motion. Amplitude, speed and frequency are specified by the user) current (constant in the horizontal plane) Post-processing allos both the person and the ater surface to be displayed pictorially. In addition, the displacement, velocity and acceleration of any point on DYNAMAN or the lifejacket may be plotted as a function of time. 13

17 3. DYNAMAN DEMONSTRATION SIMULATION 3.1 INTRODUCTION As a demonstration simulation, the behaviour of a 'person' earing a typical lifejacket in simulated aves has been modelled using DYNAMAN. For the demonstration DYNAMAN is configured to behave in a similar manner to a marine manikin representing an unconscious person. The lifejacket used in the simulations is not representative of any particular design but has a representative volume. 3.2 DESCRIPTION Figure 9 shos the initial configuration of the demonstration model. Note that to vies of the model are shon, one from above and one from belo the ater surface. The model consists of three main parts: The man, The lifejacket The ater The standard DYNAMAN torso consists of three sections; the thorax, the abdomen and the pelvis hich can all move relative to each other. Hoever, to represent more closely the stiffness a marine manikin these sections of DYNAMAN have been joined together so that they move as one part. The density data used for DYNAMAN as taken from Reference 3 hich gives densities of the parts of an actual human body. Using these figures the overall relative density of the body is very close to 1.0. The relative density of the lifejacket is very much loer (about 0.02). The overall relative density of the body and lifejacket combined is about 0.8. For the purpose of this simulation a simple representation of a lifejacket as generated to demonstrate qualitatively the body behaviour hen supported by a buoyancy aid. Thus the model has a rather square appearance compared ith a real lifejacket. Some lifejackets hold the head quite snugly hen fitted to prevent the head moving relative to the lifejacket. In the simulation, the head and neck are not alloed to move at all relative to the lifejacket. The removed degrees of freedom can easily be reintroduced for other lifejacket designs. The lifejacket is attached to DYNAMAN by springs hich represent the tapes hich ould be used to tie a jacket on. The ave conditions chosen for the test represent a sell of 600 mm peak to trough, and a period of 4 seconds. In the simulation DYNAMAN is initially lying on his back. The initial position chosen for DYNAMAN is such that the mass of ater displaced by DYNAMAN and the jacket is close to the mass of DYNAMAN to minimise the time taken for the model to reach a realistic floating position. 14

18 3.3 RESULTS Figures 9 to 13 sho the motion of DYNAMAN and the ater surface at intervals of 1.0 second for a total of 9 seconds. Figure 14 shos the motion history of DYNAMAN's nose during the analysis. The folloing features of the motion should be noted from Figures 9 to 14: From the initial position DYNAMAN rises out of the ater. This indicates that in the initial position the buoyancy forces exceed the gravitational forces. As the first ave passes over DYNAMAN the legs and pelvis drop relative to the torso. This happens for to reasons: firstly because the large buoyancy force lifts the lifejacket and thorax causing rotation at the hips, and secondly because the legs and pelvis are slightly denser than ater and so tend to drop anyay. The arms drop for similar reasons. From the initial straight position the joints of the model bend and DYNAMAN takes a very realistic relaxed position in the ater. As the aves pass DYNAMAN rises and falls in the ater. The postures assumed by DYNAMAN are the same at each point in successive aves. As the ater surface rises so the sea covers more of the lifejacket. Similarly, as the ater falls so the jacket rides higher in the ater. Figure 14 gives an indication of DYNAMAN's motion during the analysis. The X-displacement history shos that DYNAMAN moves backards and forards as he slides don the ave surfaces. Overall he moves in the direction of travel of the ave. The Y-displacement history shos very little lateral movement. The Z-displacement history shos that, after the first half cycle, DYNAMAN assumes a cyclic vertical motion in phase ith the ave. Discussions ith experts at INM confirmed that the motion of DYNAMAN in this simulation is representative of the types of motion hich ould be expected of unconscious subjects in ave conditions. 15

19 4. SELF-RIGHTING TRIALS A series of in-ater trials ere carried out at INM using a marine manikin to obtain data on the self righting characteristics of a manikin earing to types of lifejacket. The self-righting trials ere conducted by INM staff. The trials ere observed by FNC. The trials and the results of the trials are discussed in this section. Initial DYNAMAN simulations of the trials are presented in Section TEST FACILITY The INM testing facility consists of a large ater tank ith lifting equipment to aid handling of the manikin. Underater video cameras ere positioned in the tank so that video recordings could be made of the trials from both the head of the manikin and side on to the manikin. The manikin as manually positioned in the ater for the tests by an INM member of staff from inside the ater tank. 4.2 LIFEJACKETS To lifejackets ere used for the trials. The first as a typical inflatable jacket and the second a typical inherently buoyant jacket Inflatable Lifejacket This is a single piece jacket hich is put on in the uninflated state and inflated hen needed by means of a gas discharge or the earer bloing it up. The majority of the buoyancy aid is orn at the front although a collar around the back of the neck fits snugly and can support the head hen in the ater. The jacket is secured to the body ith a single aist strap Inherently Buoyant Jacket The inherently buoyant jacket consisted of a series of nylon bags filled ith buoyant material. A large section forms the front, a slightly smaller on the back, and to very small sections act as epaulettes holding the front and back together. The jacket is secured by long tapes hich pass through loops on the front and back sections of the jacket. The tapes are finally tied across the front of the jacket. It as found that the jacket could be fastened in a number of ays so that the positioning on the body is variable. The jacket does not make contact ith the head and therefore offers no support to the head. 4.3 TEST PROCEDURE The manikin as unclothed for the trials and a 4 litre lung as fitted into the chest representing the full lung capacity of a man. 16

20 A series of trials as carried out ith each of the lifejackets. The lifejackets ere fitted to the manikin out of the ater and tied on as tightly as possible at the beginning of each series of trials. The conventional test procedure for determining lifejacket righting times involves holding the jacketed manikin or person face don in the ater ith legs and arms straight and in line ith the torso and then letting go once settled. It as found difficult for only one person holding the manikin to achieve this starting position in the ater so a different initial position as adopted for the purpose of these trials. The manikin as held face donard ith the arms and legs dropped don so that it as lying on top of the lifejacket. It is felt that this may be a more severe test of the jacket's self righting ability. The manikin as held still in the initial position described above. Once the ater had settled, the manikin as let go. Several trials ere carried out ith each of the lifejackets and videoed for later analysis. 4.4 RESULTS The General Behaviour of the Marine Manikin The manikin used in the trials is intended to represent an unconscious person. The neck is very flexible alloing the head to move in all directions but other joints are fairly stiff. In particular, it as found that in certain positions the joints (particularly the shoulder) could lock-up. When earing a lifejacket in the ater the manikin took up a relaxed position on its back. The torso lay at an angle of about 30 to the ater surface ith the arms lying close to the torso in the same attitude. The legs bent at the knees and hips ith the thighs adopting almost a sitting position Righting Modes Throughout the trials to different modes of self righting could be identified. In one, the hips drop, the jacket rises and the man manikin tries to sit up (the sit-up mode). In the other, the shoulders rotate about a head-hip axis as the lifejacket rises out of the ater to one side of the body and the manikin rolls over (the roll-over mode). Figure 15 illustrates the to modes. In reality any particular righting motion includes a combination of both modes. Hoever, one is usually dominant. A typical response of the manikin in the trials as as follos: When released the hips ere seen to drop (sit up mode). One shoulder rose up out of the ater as the body rotated about the head to hip axis (roll over mode). The body turned through 90 about the hip to hip axis and 90 about the head to hip axis. 17

21 Once righted the manikin took up the position described in Section Self Righting Time The determination of a self righting time is somehat subjective since it is difficult to define precisely a starting and finishing point. This as made particularly difficult because each trial produced slightly different body motions. The most orkable definition of the self righting time as found to be from the time hen the hips began to drop until the time hen the lifejacket as furthest out of the ater ith the head fully visible. From this definition, typical righting times from the trials are summarised in Table 1. Note that, there as considerable scatter in the results for each jacket even though the initial conditions ere nominally the same in each case. Table 1 Trial Self Righting Times Jacket Righting Time (sec) Trial 1 Trial 2 Trial 3 Inflatable lifejacket Inherently Buoyant Life Jacket Lifejacket Position During each of the trials the lifejackets ere seen to move around on the manikin. To some extent this movement ould be restricted in a real situation by clothing since friction form ould be generated beteen the fabric straps and clothing. The inflatable lifejacket remained in position somehat better than the inherently buoyant lifejacket and also held the head more firmly Position of Airays With the equipment available it as not possible to measure the relative position of the ater surface and the airays on the manikin accurately enough to determine an airay motion history. 18

22 5. INITIAL SIMULATIONS BASED ON TRIALS 5.1 LIFEJACKET MODELS In order to simulate the self righting trials, DYNAMAN models of the manikin earing the to life jackets ere required. These models ere generated as discussed belo. It should be noted that in this stage of the ork the to simulations ere set up ith prior knoledge of the manikin's initial position and ith access to the jackets themselves. Hoever, the results presented in this section are from the first iteration of each simulation. That is, no tuning or modification of the models as carried out at this stage to make the simulation fit the trials better Inflatable Lifejacket The inflatable lifejacket consists of to layers of material hich are sealed at the edges and blon up ith gas. The inflated shape of the jacket is difficult to measure and is difficult to generate as a finite element mesh. For this reason the jacket geometry for the simulation as generated by mimicking the inflation process. The jacket material as modelled ith shell elements connected around the periphery and a pressure as applied to their inside surfaces. The resultant shape of the jacket is shon on DYNAMAN in Figure 16. It as found that this method produced a realistic jacket shape. The positioning of the jacket on the body as determined by examining its fit on a real person. The jacket fits snugly around the neck such that an attachment here is not necessary. The loer part of the jacket is pulled into the body by to straps fixed to a belt. These have been modelled in DYNAMAN by stiff spring elements beteen the jacket and abdomen. Very little movement is therefore alloed beteen the jacket and DYNAMAN. The density of the elements forming the jacket has been chosen such that the overall mass of the jacket in the model is the same as the real jacket. The real lifejacket holds the head reasonably securely. In the simulation the head is assumed to be held completely by the jacket Inherently Buoyant Lifejacket The inherently buoyant lifejacket consists of a series of nylon bags filled ith buoyant material. It comprises to large rectangular sections, one of hich forms the front of the jacket, the other sits behind the head. These pieces are joined by to buoyant epaulettes. A model of this jacket has been generated as shon in Figure 17. Its positioning on DYNAMAN as determined by examining a person earing the jacket. In practice the jacket could be tied to the body in a number of ays and the relative motion beteen the earer and jacket ill be dependent on the ability of the earer to fasten it tightly. In the simulation, motion has been prevented beteen the jacket and thorax to simulate the case here the jacket is attached very securely. In contrast to 19

23 the inflatable lifejacket, the inherently buoyant jacket does not hold the head at all. In the simulation the head is therefore alloed to move freely. The density of the jacket as chosen so that the model mass is equal to the actual mass of the jacket. 5.2 SIMULATION OF TRIALS A DYNAMAN simulation as generated and run for each of the life jackets representing as closely as possible the trial conditions. From the video of the trials a typical starting position as determined for DYNAMAN, as shon in Figure 18. DYNAMAN is face don in the ater ith arms and legs hanging don. The relative positions of parts of the body and the ater surface ere taken from sketches made from the video. In the simulation, some simplifying assumptions ere made as follos: The ater as assumed to be stationary although in practice some disturbance as unavoidable, The relative motion of the lifejacket and DYNAMAN as restricted as discussed in Section 5.1, A drag coefficient of 1.0 as assumed for all parts of the model. DYNAMAN as alloed to move freely from the initial position from time zero. The total simulation time as 8 seconds in each case. 5.3 RESULTS The results of the simulations hich ere used to make a comparison beteen the trials and the computer simulations are the general motion of the body and the righting time as defined in Section These are presented for each lifejacket in the folloing sections Inflatable Lifejacket The motion of DYNAMAN in this simulation is shon in Figures 19 and 20. To aid ith determining the self righting time, the vertical position of a point on the centre of the life jacket is plotted in Figure 21. To sho airay motion, the vertical position of DYNAMAN's nose is plotted in Figure 22. The folloing points should be noted: Self righting occurs in one smooth movement. Initially the model starts to turn in a roll-over mode (Section 4.4.2). Hoever, the hips quickly drop and righting continues in a sit-up mode. 20

24 From examination of Figures 19-22, it is estimated that righting begins at about 0.5 seconds and is complete by about 3.5 seconds giving a self righting time of about 3 seconds. At the end of the simulation (8 seconds) the model has reached an equilibrium position. The body lies at about 45 to the horizontal ith the arms dropped vertically and the legs in line ith the torso. The body has turned through just over 90 about a vertical axis. The airay ends up about 100 mm above the ater surface Inherently Buoyant Lifejacket The motion of DYNAMAN earing this jacket is shon in Figure 23 and 24. To aid determination of the self righting time the vertical motion of the centre of the lifejacket is shon in Figure 25. The height of the nose above ater is shon in Figure 26. The folloing points should be noted: The model takes some time to begin to right. Righting occurs mainly in the roll-over mode although toards the end the hips do drop into a sit-up mode. The head moves around considerably during the simulation. It is estimated that righting begins at about 2.25 seconds and is completed at about 7.5 seconds giving a righting time of about 5.25 seconds. There is a noticeable pause from about 4.5 seconds to 5.5 seconds here the manikin has rolled to about 90 and remains at that angle. At the end of the simulation (8 seconds) the model has not yet reached equilibrium. It is believed that, given a longer simulation time, the model ould end face up in a similar position to the final state in the inflatable lifejacket simulation but ith the head to one side. The airay reaches a maximum height of 175 mm above the ater surface but is clearly moving don again at the end of the simulation. 21

25 6. COMPARISON BETWEEN TRIALS AND INITIAL SIMULATIONS Comparison beteen the trials results described in Section 4 and the initial simulations described in Section 5 shos that the simulation of the inflatable lifejacket agrees quite ell ith the trial. Hoever, the simulation of the inherently buoyant lifejacket agrees less ell. The folloing sections compare the trials and simulations for each jacket in turn. 6.1 INFLATABLE LIFEJACKET The folloing comparisons can be made: In both the trial and the simulation, the jacket succeeded in self righting. Righting occurred in a single, smooth movement dominated by the sit-up mode in both the trial and the simulation. Hoever, there as also some roll-over mode apparent. This as more noticeable in the simulation than in the trial. In the trial, self righting as estimated to take about 2 seconds. In the simulation, self righting took longer, about 3 seconds. The final positions ere very similar. Hoever, in the simulation the arms hung more vertically and the legs ere spread apart. In the trial, some movement of the jacket relative to the manikin occurred. In particular, the belt holding the jacket slid up the manikin's torso. In the simulation this could not occur. 6.2 INHERENTLY BUOYANT LIFEJACKET The folloing comparisons can be made: In both the trial and the simulation, the jacket succeeded in self righting. In the trial, the manikin righted in a smooth movement dominated by the sit-up mode ith a small roll-over component. Hoever, in the simulation the reverse as true. The greater part of the motion as in the roll-over mode and as by no ay continuous (see Section 5.3.2). Only at the end of the simulation, here righting as almost complete, did the model begin to sit-up. The trial and simulation give significantly different righting times ( seconds in the trial and 5.25 seconds in the simulation). In the simulation the model did not reach an equilibrium position ithin the 8 seconds of the analysis. Hoever, it is believed that the 22

26 final position ould be similar to that seen in the trial ith the head rolled to one side and the torso tilted ith one shoulder loer in the ater than the other. As ith the inflatable lifejacket the inherently buoyant lifejacket simulation ould probably give the arms more vertical than in the trial and the legs spread further apart. In the trial the inherently buoyant jacket moved around a great deal on the manikin. In particular, the jacket had a noticeable tendency to slip sideays on the manikin and also to ride up toards the head. In the simulation the position of the jacket relative to the torso as fixed and no relative movement occurred. 23

27 7. DISCUSSION It ould appear that the inflatable lifejacket simulation agreed much more closely ith the trials than did the inherently buoyant lifejacket simulation in that the overall motion of the model and the righting time ere closer to those observed in the trial. In the inherently buoyant lifejacket simulation righting as much sloer than as observed in the trials. As noted in Section 5.1, the results presented here represent the first iteration of both models. The folloing sections discuss the factors hich influence righting and then consider in detail changes hich could be made to the simulations to improve the results. 7.1 RIGHTING MODE The sit-up and roll-over righting modes can be explained in terms of the principal forces acting on the lifejacket and the body as it rights. The forces acting are the donard force due to the eight of the body acting at the centre of gravity (typically somehere in the abdomen), and the upard buoyancy force acting at the centre of buoyancy (typically near the centre of the lifejacket). These forces are shon pictorially in Figure 27. The side-on vie in Figure 27 shos ho the forces act to cause the sit-up mode. The lines of action of the gravitational and buoyancy forces are separated by a distance x hich gives rise to a turning moment about a lateral axis. The head on vie in Figure 27 shos ho the forces act to cause the roll-over mode. The fines of action of the gravitational and buoyancy forces are separated by a distance y hich gives rise to a turning moment about a head-to-hip axis. Clearly both of those moments ill be dependent on the magnitude of the gravitational and buoyancy forces. An increase in either ill increase the turning moment and speed up self-righting. Which of the to modes is dominant ill depend on the relative magnitude of the distances x and y and the rotational inertia of the body about the to axes. In general, as x increases (ie the centre of buoyancy moves aay from the centre of gravity) the sit up component of the righting mode ill become more significant, and as y increases the more significant ill be the rolling component of the righting mode. In a self-righting trial the righting motion ill be a complex combination of roll-over and sit up modes. Although the size of the gravitational force ill be constant, the C of G ill move as the body bends. In addition, the centre of buoyancy and the size of the buoyancy force ill change constantly as the volume of ater displaced changes as parts of the body and lifejacket move in and out of the ater. Thus x and y and the magnitude of the buoyancy force ill change throughout the righting process. 24

28 7.2 INFLATABLE LIFEJACKET The overall righting motion for the inflatable lifejacket simulation as close to that seen in the trials. Hoever, righting as a bit slo to start. In the trials the first observed movement of the manikin as the hips dropping (see Section 4.4.2). There as a noticeable delay in the hips dropping in the simulation hich suggests that the centre of gravity of the body may have been too close to the head of DYNAMAN or the centre of buoyancy may have been too close to the hips of DYNAMAN. In addition the righting time in the simulation as 1½ times longer than observed in the trials. This implies that the turning moment as too small or the resistance to motion eg inertia, drag too high. The discrepancy in the positions of the centre of gravity or the centre of buoyancy and the size of the forces could be due to some combination of the folloing: The mass of DYNAMAN may have been different to the manikin such that the gravitational force as incorrect. The density distribution of DYNAMAN may have been different to that in the marine manikin so that DYNAMAN's centre of gravity as nearer the head than in the manikin hich ould reduce the turning moment. The model of the lifejacket may have been smaller than in reality. This ould mean that the buoyancy force and turning moment provided by the jacket as too small. The lifejacket may have been incorrectly positioned on DYNAMAN. If it ere too lo don the chest the centre of buoyancy ould be brought closer to the centre of gravity thus reducing the sit-up moment. Although the jacket as free to move aay from DYNAMAN on springs representing tapes holding the jacket, it as not free to slide up the chest. In the trials this as seen to happen on the manikin as the chest strap slid over the smooth surface of the manikin. The drag forces acting on the body could have been larger than in reality. Hoever, results of the simulation shoed that these forces are small in comparison to the buoyancy forces and probably have a less significant effect on righting time and mode. The other difference beteen the inflatable lifejacket simulation and the trials as the final position of the bodies. In the simulation, DYNAMAN's arms dropped vertically in the ater and the legs ere splayed and relaxed. The arms and legs of the manikin hoever, remained in line and close to the body. The difference in the position arises because the joints of DYNAMAN have less resistance to rotation and do not lock up as the manikin's tended to in some positions. This increased freedom allos hips to rotate, legs to spread, shoulders to relax and arms to drop. 25

29 7.3 INHERENTLY BUOYANT LIFEJACKET The righting time of DYNAMAN in the inherently buoyant lifejacket simulation as clearly greater than measured in the trials ith the marine manikin. The possible reasons for the differences beteen the trials and simulation are as for the inflatable jacket, ie different density distribution, undersize jacket (possibly of a slightly different shape) and jacket attachment. Hoever, ith this jacket hen the body has rolled through 90 it appears to pause in the rolling mode (Figures 28 and 29). This pause occurs hen the back of the jacket enters the ater so that buoyancy forces act each side of the centre of gravity giving a stable system hich is reluctant to continue to roll. This feature could have arisen because of an incorrect distribution of buoyancy beteen the front and back of the jacket in the simulation. At the end of the 8 second simulation DYNAMAN had not reached a stable position. Hoever, the final position ould be much the same as that achieved in the inflatable lifejacket simulation ith one exception. Because the head rolls to one side in the inherently buoyant lifejacket simulation the body floats ith one shoulder further in the ater than the other. This occurred in the trials also. A likely difference beteen the final positions in the simulation and trial ould be the position of the arms and legs due to the greater flexibility of DYNAMAN as discussed in SIMULATION IMPROVEMENTS From the preceding discussion it is evident that the prediction of correct righting motion and times ill require some improvement in the initial simulations. Aspects of the simulation hich could be varied include: Provision of more accurate density data for the marine manikin and lifejackets. This ould enable the correct magnitude of the gravitational force to be calculated. Provision of more accurate volume data for the marine manikin and lifejacket. This ould provide the correct buoyancy forces. Revision of lifejacket position on the body. Introduction of some movement in the ater hich may provide additional turning moments. Modification of the drag coefficient used to determine the fluid resistance to body motion. Of these possible modifications, the first three ere considered likely to have the greatest effect on the behaviour of DYNAMAN. As a result, it as agreed ith HSE that further measurements ould be made on the real manikin and lifejackets to allo more accurate simulations to be generated. Section 8 describes the 26

30 measurements hich ere made hile Section 9 describes revised simulations hich incorporate the ne data. 27

31 8. BUOYANCY AND DENSITY MEASUREMENT As discussed in Section 7.4, a series of measurements ere made at INM after the initial simulations had been carried out to determine the volume and density of each major component of the marine manikin and lifejackets used in the self-righting trials reported in Section 4. The volume and density ere determined by eighing each component in air and then in ater. In the case of buoyant components (ie density less than ater) sink eights ere used in the measurements. The volume and density ere calculated as follos: Eqn 9 Weight in air (W A ) - ρ c g V c + W sa Weight in ater (W ) = Eqn 10 ρ c g V c - ρ g V c W +W s Where ρ c = ρ = g = V c = W s = = W s density of component density of ater gravity volume of component eight of sink eight in air eight of sink eight in ater. From (9) and (10) c = W Ws 1 1 W A W sa and V c = W A WsA cg The marine manikin as disassembled into the folloing components for measurement; 4 litre lung thigh shin and foot 28

32 upper arm loer arm and hand head and torso (ithout lung) pelvis and abdomen. The measured values for these component parts and the to lifejackets are compared in Table 2 ith the values used in the initial simulations. Note that the inherently buoyant jacket is composed of three sections, the front, the back and the shoulder epaulettes as shon in Figure 17. It can be seen from Table 2 that the densities of the individual parts and overall density of DYNAMAN used in the initial simulations ere slightly loer than the values measured or the marine manikin. The table also shos that the volume of the body parts used in the initial simulation ere larger than measured. In addition, the lifejacket densities ere larger and volumes significantly smaller in the initial simulations compared ith the measured values. In summary, the table shos that DYNAMAN as more buoyant and the jackets less buoyant in the initial simulation compared ith the values for the marine manikin and lifejackets measured at INM. Table 2 Density and Buoyancy Measurements Component Initial Simulations Measured Values Relative Density Volume (litres) Relative Density Volume (litres) 4 litre lung Thigh Shin and foot Upper arm Loer arm and hard Head and torso Inflatable lifejacket Inherently Buoyant lifejacket Total ith 4 litre lung

33 9. REVISED SIMULATIONS The to simulations presented in Section 5 ere refined using the measured data presented in Section 8. Modifications ere made to the volume and density of DYNAMAN to more closely represent the marine manikin. The volume of the lifejackets as also revised. The specific changes and the results of the revised simulations are presented in this section. 9.1 MODEL CHANGES DYNAMAN The value of density and volume given in Table 2 ere used to adjust the size and density of the individual components of DYNAMAN. The changes resulted in a 7.8% decrease in total mass of DYNAMAN. The centre of gravity of the revised DYNAMAN moved aay from the head toards the feet by 36 mm. The volume changes gave a 12.8% reduction in DYNAMAN volume and hence an equivalent decrease in the buoyancy force. The change in volume is greater than the change in mass since the marine manikin as found to be slightly denser overall than the DYNAMAN used in the initial simulations Lifejackets As shon in Section 8, the volume measurements made of the lifejackets shoed that the models used in the initial simulations ere both about 25% too small. This difference ill have a significant effect on the turning moment generated by the jackets. The revised shape of the inflatable jacket is shon in Figure 28. The shape as generated by inflating the jacket ithin the simulation as described in Section 5, using a higher inflation pressure than in the initial simulation to achieve the required increase in volume. The revised shape and position of the inherently buoyant lifejacket is shon in Figure 34. This jacket is modelled as a solid as described in Section 5. The dimensions of the front, back and epaulettes ere adjusted to achieve the required volume. In the initial simulations the position of the jackets as determined from the observed position taken hen the manikin as out of the ater as described in Section 5. In the revised simulation, the jackets have been repositioned to represent the position taken hen the manikin is in the ater. The revised inflatable lifejacket simulation has the jacket further aay from the chest as can be seen by comparing Figure 28 ith Figure 16. In the revised inherently buoyant jacket simulation the jacket is positioned higher up DYNAMAN's chest as can be seen by comparing Figures 29 and

34 9.2 RESULTS OF REVISED SIMULATIONS The results of the simulations hich ere used to make a comparison beteen the trials and computer simulations are the general motion of the body and the righting time as defined in Section These are presented for each lifejacket in the folloing sections Inflatable Lifejacket The motion of DYNAMAN in this simulation is shon in Figures 30 and 31. To aid ith determining the self-righting time, the vertical position of a point on the centre of the lifejacket is plotted in Figure 32. To sho airay motion, the vertical position of DYNAMAN's nose is plotted in Figure 33. The folloing points should be noted and compared ith the behaviour described in Section for the initial simulation: Self-righting occurs in one smooth movement. The hips drop quickly and DYNAMAN rights in predominantly the 'sit-up' mode. From the figures it is estimated that righting begins as soon as the simulation starts and is completed in about 1.6 seconds. Within 4 seconds DYNAMAN has reached an equilibrium position. The body lies at about 60 to the horizontal ith the arms dropped vertically and legs in line ith the torso. The body has turned through about 60 about a vertical axis. The airay ends up about 180 mm above the ater surface Inherently Buoyant Lifejacket The motion of DYNAMAN in this simulation is shon in Figures 34 and 35. To aid ith determining the self-righting time the vertical position of a point on the centre of the lifejacket is plotted in Figure 36. To sho airay motion, the vertical position of DYNAMAN's nose is plotted in Figure 37. The folloing points should be noted and compared ith the behaviour described in Section for the initial simulation: Self-righting occurs in one smooth movement. Righting occurs mainly in the roll -over mode although the hips drop more quickly than in the initial simulation. The head moves around considerably during the simulation. It is estimated from Figure 41 that righting begins at about 0.5 seconds and is complete by about 3 seconds giving a righting time of about 2.5 seconds. There is a short pause in the righting at about seconds. Within about 6 seconds, DYNAMAN has reached equilibrium. The body lies at about 45 to the horizontal ith the arm and legs dropped vertically. The body has turned through just over 90 about 31

35 32 a vertical axis. The airay ends up about 250 mm above the ater surface.

36 10. COMPARISON OF TRIALS WITH INITIAL AND REVISED SIMULATIONS A comparison beteen the trials and initial simulations is made in Section 6. It as found that the initial simulations ere slo to right (the inherently buoyant simulation being particularly slo). The simulations ere revised to improve buoyancy and density properties as described in Section 8. The folloing sections compare the trials ith the revised simulations and highlight improvements over the initial simulations INFLATABLE LIFEJACKET The folloing comparisons can be made: In both the trials and revised simulation the jacket succeeded in self-righting. In the trial, self-righting as estimated to take 2 seconds. In the initial simulation self-righting took longer, about 3 seconds. Hoever, in the revised simulation the self-righting time as only 1.6 seconds. This significant decrease in righting time as due to the increase in turning moment resulting from the increase in buoyancy force and the increased separation beteen the centres of gravity and buoyancy due to changes in DYNAMAN's mass distribution and repositioning of the larger lifejacket. Righting occurred in a single, smooth movement in both simulations. In the initial simulation this as dominated by the roll-over mode. Hoever, in the revised simulation it as dominated by the sit-up mode. The reason for this is that in the revised simulation the jacket is positioned significantly further aay from DYNAMAN's chest. As explained in Section 6, this increases the turning moment about the shoulder to shoulder axis compared to that about a head to hip axis and hence increases the Sit-up mode in the righting action. In the trials, the equilibrium position of the manikin in the ater as to lie at about 45 to the horizontal ith the arms in line ith and beside the torso. The legs ere bent ith the knees at about 90 to the thighs hich ere slightly raised relative to the fine of the torso. The knees stayed together. In the initial simulation the torso as seen to take up a similar equilibrium position, hoever the limbs ere spread and relaxed due to differing joint constraints beteen DYNAMAN and the manikin. In the revised simulation, DYNAMAN lay at about 60 to the horizontal ith limbs in the same relaxed position. The increase in the angle of the body to the horizontal occurred due to the increased angle beteen the jacket and DYNAMAN in the revised simulation. 33

37 In the trial some movement of the jacket relative to the manikin occurred. In both simulations such movement could not occur INHERENTLY BUOYANT LIFEJACKET The folloing comparisons can be made: In both the trial and the simulations, the jacket succeeded in self-righting. The righting time in the trial as seconds and in the initial simulation as 5.25 seconds. Hoever, the righting time as reduced to 2.5 seconds in the revised simulation. As ith the inflatable lifejacket, this reduction in righting time as due to the increased turning moment brought about by the revisions made to the DYNAMAN In the trial, the manikin righted in an apparently smooth movement. Hoever, in the initial simulation righting as by no means continuous. The body as seen to stay on its side for several seconds before righting as completed. In the revised simulation the right motion as much improved in that it occurred in a smoother movement although a small delay still occurred hen the body as on its side. The equilibrium position of the manikin in the trials earing the inherently buoyant jacket as similar to its position hen earing the inflatable jacket described in Section In the initial simulation, DYNAMAN did not reach equilibrium after 8 seconds. In the revised simulation, equilibrium as reached about 6 seconds into the simulation. As for the inflatable jacket simulation the body lay at about 45 to the horizontal ith relaxed limbs laying almost vertically. The relaxed final position of the limbs indicate the increased joint flexibility in DYNAMAN compared to a manikin. In the trial the jacket moved around a great deal on the manikin. In particular, the jacket had a noticeable tendency to slip sideays and move up toards the head. In both the initial and revised simulation this motion as prevented SUMMARY The revised simulations have given much better righting times then the initial simulations. The results are summarised in Table 3. The inflatable jacket rights a little faster than measured (1.6 seconds as opposed to 2 seconds) and the inherently buoyant jacket rights ithin the range of observed time (2.5 seconds in the revised simulation as opposed to 2.4 to 3.6 seconds measured). 34

38 Table 3 Self-Righting Times Jacket Trial Initial Simulation Revised Simulation Inflatable secs 3.00 secs 1.6 secs Inherently Buoyant secs 5.25 secs 2.5 sees The reduction in righting time is a direct consequence of the increase in buoyancy of the jackets and the adjustment in density and volume of DYNAMAN. The predicted righting modes differ beteen the initial and revised simulations but it as apparent from the trials that a range of modes can occur and the one hich occurs is very much dependent on the initial position of the lifejacket. This point is demonstrated most clearly by the change from roll-over to sit-up mode observed ith the inflatable jacket in the initial and revised simulation. 35

39 11. CONCLUSIONS, RECOMMENDATIONS AND APPLICATIONS OF THE TECHNIQUE 11.1 CONCLUSIONS The ork carried out in this project has demonstrated that the DYNAMAN technique can be used to investigate the behaviour of humans and manikins earing buoyancy aide. The effects of buoyancy and drag have been implemented in DYNAMAN for this particular modelling application. The validation test cases give quantitatively correct behaviour hen compared ith analytical results. Good results have also been obtained in a demonstration simulation of a lifejacket earer in simulated aves. A series of trials ere carried out at INM ith a marine manikin to obtain data on the self righting characteristics of to lifejackets. Initial DYNAMAN simulations ere then carried out based on these trials and the results compared ith the trial data. The simulations ere not specially tuned to obtain better agreement after the first iteration. The initial simulations produced sloer righting times than measured in the trials. Several reasons for this ere identified. The most significant cause of the different as felt to be inaccurate volume and mass distributions in the simulation. Measurements ere made at INM to obtain data for the manikin and lifejacket. The computer simulations ere revised to incorporate this data. Righting times in the revised simulations ere then much closer to those measured. The computer simulations shoed that self righting in still ater can be modelled ith the DYNAMAN technique. The results demonstrated that good correlation can be achieved beteen trials and simulation provided that the distributions of eight and buoyancy are correct. A major contributor to the buoyancy is (of course) the lifejacket RECOMMENDATIONS The current ork programme has contributed, significantly to the understanding of lifejacket behaviour. Further benefit ould be gained from extra ork. In particular, further ork is recommended in the folloing areas: Investigate further the mechanisms of self righting, including: simulate people, not manikins simulate clothed subjects investigate behaviour in real sea conditions simulate survival suits and suit-jacket combinations simulate buoyancy aids hich inflate in the ater. 36

40 Some of this investigation could use DYNAMAN as it stands. Other aspects may require development of the simulation technique. Investigate the sensitivity of the righting behaviour to, lifejacket attachments person size and shape lifejacket size and shape other factors. This investigation should be carried out by generating and analysing further DYNAMAN models APPLICATIONS OF THE TECHNIQUE Ultimately the technique could have four main arm of application: As a research tool to understand more about the principles of lifejacket behaviour and the ay that different types of design ould ork. This could be particularly valuable in the case of lifejackets for very specialist applications or for looking at more complex systems such as lifejacket/survival suit combinations; The technique could help the lifejacket manufacturer to develop ne products quicker and more cheaply. It could, for example, allo him to try out ne designs on the computer ithout having to go to the expense of prototyping and physical testing until he has some confidence in the design; DYNAMAN could help the lifejacket user - especially those users ith very specific performance requirements - to ensure that they chose a product hich ill do exactly hat they ant it to do; The technique could be used as part of an approval process either to help define appropriate physical tests or to allo a ne design to be assessed for conditions hich ould not be practical to test. 37

41 12. REFERENCES 1. DYNA3D User's Manual (Non-Linear Dynamic Analysis of Structures in Three Dimensions) J.O. Hallquist, D J Benson, Larence Livermore National Laboratory. 2. Dynamic Modelling of Occupant Motion - Final Report. FNC 731/2475R Frazer-Nash Consultancy report to HSE Railay Inspectorate. 3. Properties of Body Segments based on Size and Weigh American Journal of Anatomy,

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