Occupant Safety in Freefall Lifeboats: Full Scale Drop Testing with

Occupant Safety in Freefall Lifeboats:
Full Scale Drop Testing with Instrumented Dummy
P A Forbes*, L van Rooij*, M Philippens**, R Corbeij*, R Skjæveland***
*TNO Science and Industry, Automotive Integrated Safety, Steenovenweg 1, Helmond, The Netherlands
**TNO Defence, Security and Safety, Lange Kleiweg 137, Rijswijk, The Netherlands
***StatoilHydro, Forusbeen 50, Stavanger, Norway
Abstract – Full scale drop tests of freefall lifeboats were conducted with an anthropometric test device (RID3D) installed in the boats
to replicate occupant seating conditions. A total of twelve tests were performed, nine of which used a drop boat while the remaining
three used a skid boat. Each boat provided different seating configurations, where tests were performed to investigate variations in
boat load, drop height, seating location and restraint design. Boat and dummy responses were measured throughout the impact,
including accelerations, forces and moments. Assessment and occupant response criteria such as CDRR, CAR, HIC, Nkm, etc. were
calculated from the recorded responses. The magnitude of these values indicated the sensitivity to varying impact conditions.
Although the dummy showed the greatest sensitivity to drop and loading conditions (i.e. height and boat load respectively),
preliminary restraint design improvements showed a positive reduction in dummy (i.e. occupant) load severity. Optimization of the
restraints for both boat types is required to reduce the occupant loads further, however the work performed here suggests that a 5 or
6-point restraint would provide the best protection. Existing assessment criteria (CAR and CDRR) by definition are unable to account
for such implementations and it is recommended that regionally specific occupant injury assessment criteria, such as head or neck
criteria obtained through the use of an ATD during testing, be adapted in the future.
Keywords: lifeboat, occupant, safety, drop, impact, injury, ATD, dummy, test
NOTATION
AIS
CAR
IARV
PMHS
Abbreviated Injury Scale
Combined Acceleration Response
Injury Assessment Response Value
Post Mortem Human Subjects
ATD
CDRR
IMO
RID3D
Anthropometric Test Device
Combined Dynamic Response Ratio
International Maritime Organization
Rear Impact Dummy (3-dimensional)
INTRODUCTION
Historically, occupant injury and safety research has focussed on the protection of automotive occupants in
high energy vehicular collisions. The field has incorporated techniques such as injury biomechanics, crash
testing, numerical simulation and safety system design. As the transportation occupant safety field continues
to improve, new areas of application for this knowledge emerge to achieve similar goals. Occupant safety in
freefall drop lifeboats is one such area, where boats installed on oil platforms and tankers are dropped into
the water during emergency evacuation scenarios. Boats are dropped in the water in an effort to create
forward momentum, propelling the craft away from the evacuated platform or tanker. The resultant impact
however may have severe consequences on the sustained injury risk of the lifeboat occupants.
Although free-fall lifeboats are currently installed on platforms for emergency evacuations, only a limited
number of studies have been published investigating their free-fall impact. The majority of these studies have
focused on the hydrodynamic impact performance of the boat in an effort to understand variations such as
center of gravity (CG), mass distribution, environmental effects (waves) and hull design [1-5]. All studies
used boat acceleration as the primary response characteristic as measured at various locations throughout the
boat. A handful of studies have considered the impact response of the occupants, but were often limited to
global acceleration based injury criteria using signals obtained from the boat. Only one study could be found
investigating the impact response of a Hybrid III dummy during free-fall impacts, which was focused on its
ability to predict occupant injury potential [6].
The overall purpose of this research was to provide a comprehensive assessment of occupant injury risk
during drop conditions of currently installed free-fall lifeboats, in an effort to improve occupant safety. The
entire research scope has included injury biomechanics, occupant assessment criteria, boat hydrodynamics,
full-scale testing, numerical modelling, sensitivity analysis and optimization, and sled testing. This paper
focuses on the methodologies used for studying occupant responses during full-scale drop testing, where the
RID3D anthropometric test device (ATD) was used to represent the average occupant. A variety of impact
conditions was considered including boat type, drop height, restraint design and seating location. Principal
dummy response parameters, as identified by Skjæveland et al. [7] for lifeboat occupant impact conditions
from available biomechanics literature, were used to evaluate the occupant response sensitivity as a function
of experimental variation. Documented insight into the impact kinematics and load responses for lifeboat
occupants has been obtained, allowing for the validation of simulated drop conditions as described in the
accompanying paper by Forbes et al. [8].
BACKGROUND
Free-fall lifeboat drops
Free-fall lifeboats are classified according to one of two launch conditions: (1) drop boats, which fall
vertically following release and (2) skid boats, which fall following a slide phase within a launch skid (see
Figure 1 (a) and (b)). Both systems are intended to provide forward momentum on impact, moving the boat
away from the platform; hence, the deliberately angled impact. Drop boats possess only two stages of
launch: (1) free fall and (2) impact with the water, while skid boats possess four stages of launch: (1) slide
along skid, (2) rotation at the end of the skid, (3) free-fall and (4) impact with the water.
Z
X
(a) Drop boat
Z
X
(b) Skid boat
(c) Impact phase (both boats)
Figure 1 Boat impact characteristics during free-fall (a) & (b), and water impact phases, (c).
Studies by Boer and Nelson have developed the equations of motion describing the launch phases of both
types of boats. The sliding stage of the skid boat induces initial linear velocities at the start of free-fall that
depend on the geometry, friction and mass of the boat-skid combination. The rotation stage of the skid boat
induces rotational velocities at the start of freefall that depend on the center of gravity, runner length and
friction of the boat-skid combination [3]. Both boats move under the influence of gravity during the free-fall
phase; however the linear and rotational velocities at the beginning of this phase influence the final location
and orientation of the boat at the time of impact.
Impact of the boat with the water can be mathematically described using theories of hydrodynamics [1-5].
The total impact is effected by moment transfer (along and normal to the boats x-axis), water buoyancy,
wave inertia and drag forces. Mechanically, impact occurs first at the front, generating accelerations along its
length (x-axis) and in the direction normal to the length (z-axis). The impact of the boat’s front induces
rotation about its lateral axis, slamming the rear part of the boat on the water. For the purposes of this study
these two distinct impact phases have been defined as the “frontal impact” and “rear slamming” phases.
Nelson et al. [3] showed that the severity of the impact depends heavily on the CG for several reasons. First,
the rotation induced during the free-fall phase (skid boat only) causes the boat to enter the water at a steeper
angle. If the CG is shifted forward the rotation during the free-fall phase increases and the entry angle is
increased. Subsequently, the steeper entry angle reduces the “righting moment arm” created between the boat
CG and the point of impact, allowing the boat to dive deeper, decreasing the peak of acceleration and
increasing the duration. Subsequently the impact rotation of the boat and the severity of the slamming of the
rear of the boat are reduced. Second, if the CG is shifted forward (for example a forward loaded drop or skid
boat) the righting moment arm is also reduced, thereby decreasing the severity of the slamming phase.
However, if the CG is shifted backwards, the opposite occurs and the severity of the slamming phase
increases. Additionally, the magnitude of the boat’s mass has an effect on the impact, where boats with a
larger mass can dive deeper into the water, decelerating the boat in a longer period of time and reducing peak
accelerations. Such factors have been considered within this study, where the loading conditions full load
and 50% aft loading were tested.
Occupant safety
The current regulatory process to certify free-fall lifeboats for the safety of occupants imposed by the
International Maritime Organization (IMO) [9] includes two impact response criteria: (1) the combined
acceleration response (CAR) and (2) the combined dynamic response ratio (CDRR). Both rely on
acceleration signals obtained from the boat hull and represent global approximations of the occupant impact
severity and should not exceed unity. The current IMO standard requires that these two criteria be satisfied in
four different boat occupancy conditions: (1) occupancy of full boat, (2) occupancy forward of boat CG, (3)
occupancy aft of boat CG and (4) occupancy of launching crew. An occupied condition is represented by
securely fastening a 75kg mass to any seat. Research by Nelson et al. [3] showed the importance of
considering all four cases for certification, where a 13.5 percent difference in acceleration was observed
between the forward and aft occupancy loading conditions. The results also suggested that the aft seating
location induced more severe accelerations than the forward seating for all loading conditions.
In another study by Nelson et al. [6] an evaluation of the IMO criteria was made, comparing their predicted
potential for injury against the impact response of a Hybrid III dummy. A series of tests were conducted,
placing the dummy in three differently sized boats, which were launched from three different heights and
loaded in four different occupancy conditions. In all tests, the dummy was placed in the front seat. It was
concluded that while both the CAR and CDRR responses provided an indication of occupant safety, they
could not predict whether or not an injury would occur. Furthermore, it was noted that good coupling of the
occupant to the seat was required for accurate representation of these values, and neither response could
evaluate the effectiveness of this coupling.
While the Nelson et al. [3] study represented the first and only attempt at evaluating specific occupant
loading responses using ATDs, it possessed several limitations. First, all tests were conducted with the
dummy seated in the front seat, while previous studies by Nelson et al. [3], along with the results from this
study, indicate the most severe loading to occur in the rear of the boat. Second, the largest boat considered
was only 12,800 kg fully loaded with a drop height of 25.5 m, while boats considered in this study had a
maximum load of approximately 15,500 kg and a maximum drop height of 30 m. It is noted however, that
these two factors alone do not result in an increase in impact severity and further considerations regarding
hull shape are often required. Third, although considerations for restraint design were discussed, the
effectiveness of various restraint types was not evaluated. Fourth, the effect of seat design and occupant
orientation were also not considered. And finally, the applicability of regionally specific dummy responses
appropriate for occupants in free-fall lifeboat impacts was not considered.
METHODOLOGY
Overview
In an effort to evaluate a variety of lifeboats currently installed on oil platforms, two different types were
used for the full scale testing in this study. One of each boat launch type was chosen – drop and skid – where
the drop boat was noted to generate high accelerations during previous drop tests. The design features of
each boat are presented in Table 1; both possessing similar length, occupancy and mass. Although not shown
here these boats were noted to have significantly different hull shapes.
Table 1 Boat design parameters
Parameter
Drop Boat
Skid Boat
Length (m)
Approx. capacity
12.5
60
12.5
60
Approx. loaded mass, full (kg)
Approx. freefall height (m)
15,500
30
13,600
30
Launch type
Drop
Skid
Seat and restraint systems
The seats installed in the drop boat were oriented along the longitudinal axis of the boat, where the feet point
towards the front, when the boat is level on the water. When seated an occupant is required to take on a
crouched position, generating rotation in the hips. Two different restraints were used during testing with this
seat (see Figure 2):
1. The conventional torso restraint system, consisting of two 2-point belts across the thorax and the
abdomen. This system also included a head strap.
2. An adapted restraint system, consisting of a trial 4-point belt system, using one pelvic belt and 2 shoulder
belts. This restraint system was tested with and without the head strap.
Boat front
(a)
(b)
(c)
Figure 2 Drop boat seat (a); conventional restraint system (b) and adapted restraint system (c).
The seats installed in the skid boat were oriented in an upright rearward facing direction when the boat is level on
the water. The seat has a straight back, forcing the occupants to sit in an upright position similar to fighter pilot
ejection seats. Two different restraint systems were used during testing with this seat (see Figure 3):
1. The conventional 3-point belt system consisting of a single pelvis belt and two shoulder belts. The
shoulder belts join at a single point (sewn together) and split again into two anchor points. For this reason
this restraint has been termed here as a 3-point system. It was noted however that these belts remain loose
throughout the impact due to the lack of any restraint tightening features and acts primarily as a 2-point
belt system.
2. The adopted 4-point belt system consisting of a single pelvis belt and two shoulder belts. All belts could
be tightened individually.
Boat front
(a)
(c)
(d)
Figure 3 Skid boat seat (a), 3-point restraint (b), and 4-point restraint (c).
The orientation and arrangement of the seats with respect to the boat axes was expected to influence the
loading response of the occupant throughout impact. The impact was expected to generate loading on the
occupants in forward, rearward and vertical directions. Each phase (frontal impact and rear slamming) were
expected to generate different occupant loading characteristics due to their combination of different linear
and rotational accelerations. Additionally, the rear slamming phase of both boats would more significantly
affect occupants in the rear of the boat, who are seated furthest from the point of rotation.
Occupant surrogate
The choice of the occupant surrogate for testing was made with an understanding of the impact and expected
loading directions, as well as the seat and restraint conditions used throughout. Considering the main loading
directions (frontal, rear and vertical), available surrogates included the following frontal and rear impact
dummies: Hybrid III, BioRID and RID3D. A Hybrid III or any other frontal-only dummy is not suited for the
rearward impacts expected in both boats, as they are not validated for their attenuation characteristics. The
BioRID and RID3D dummies designed for whiplash applications were more appropriate. However, the
BioRID is only validated for low speed rear impacts 16 km/hr, a maximum acceleration of 5-10 g and has no
frontal validation. The RID3D is validated for higher speed rear and frontal impacts, where in frontal impact
the maximum velocity evaluated is 60 km/hr with an acceleration of 15g [10-17].
The Hybrid III and BioRID dummies are not suitable for the vertical loading expected in both boats. The
Hybrid III possesses a curved lumbar region representing an automotive seated occupant, and is unable to
capture the vertical loads experienced in occupants in an erect-spine seated posture [18]. The BioRID was
simply not designed for such loading conditions. The RID3D however, has a straight lumbar region, similar to
Federal Aviation Administration (FAA) Hybrid III dummy, which is used for evaluating vertical lumbar
loads during pilot ejections [18]. It is noted however that the RID3D itself has not been validated for such
loading conditions.
Considering the seating conditions, the RID3D was the only dummy with the flexibility to be placed within
the crouched position required by the drop boat seats. An adjustable pelvis spine angle gives the dummy the
flexibility to fit within the seat. Furthermore, the straight lumbar spine region used within the RID3D provides
a better representation of the expected initial upright seating conditions from occupants in the skid boat over
the Hybrid III. With the consideration discussed above, the RID3D was chosen for all tests, providing the
optimal balance of all constraints.
Test conditions and test matrix
All testing was performed in calm sea conditions, with the boats dropped from a water based crane. Although
dropping the boats in rougher sea conditions would have been more representative of a potential evacuation,
such conditions were deemed unfeasible for this application. An objective of testing for both boats was to
investigate the occupant impact response due to variations in drop height, load conditions (full or aft
occupancy), seating location and restraint type. However for the skid boat, poor weather combined with the
complexity of the setup (i.e. the additional skid mechanism required during testing) limited the number of
tests and only boat loading conditions and restraint type were tested. Table 2 summarizes the full test matrix.
Table 2 Full-scale boat drop test matrix
TNO Test Number
Boat Type
Dummy position
Drop Height (m)
Load condition
Restraint Condition
F064311
Drop
Rear
30
50% Aft
Torso belt
F064312
F064313
Drop
Drop
Rear
Rear
30
30
50% Aft
50% Aft
Torso belt
4-point
F064314
F064315
Drop
Drop
Rear
Rear
30
15
100%
100%
4-point
4-point
F064316
F064317
Drop
Drop
Rear
Rear
15
30
100%
100%
Torso belt
4-point (no head strap)
F064318
F064319
Drop
Drop
Front
Rear
30
20
100%
100%
Torso belt
4-point (no head strap)
F065011
F065012
Skid
Skid
Rear
Rear
30
30
100%
50% Aft
4-point
4-point
F065013
Skid
Rear
30
50% Aft
2-point
Instrumentation
Instrumentation of both boats was completed using accelerometers and gyrometers mounted in both the front
and rear of the boat. The gyrometer was placed in the approximate center for the seat while the
accelerometers were placed fore and aft from the seat center. The two accelerometers were used to define the
final acceleration as measured at the location of the gyrometer. The coordinate systems for measurement in
both boats were aligned with the horizontal (when the boat was floating), where the x-axis pointed towards
the front of the boat, the z-axis pointed downwards and the y-axis pointed laterally (see Figure 1).
The RID3D dummy was instrumented with the sensors as summarized in Table 3. The directions specified for each
sensor were defined with respect to the standing dummy coordinate system and oriented in the following fashion:
− X-direction, positive: forward from the dummy
− Y-direction, positive: left from the dummy
− Z-direction, positive: up from the dummy
All signals were filtered according to the Society for Automotive Engineers (SAE) J211/1 experimental
instrumentation standard [19]. This included filtering the boat structural acceleration responses using a CFC
60 filter. It is noted however that the current evaluation of lifeboat responses requires the use of a low-pass
20 Hz filter for such signals [9] and that the use of these two filter methods would generate different results,
particularly for boat-based responses such as CDRR and CAR. However, since the focus of this study was on
the response of the occupant surrogate, whose design originates from the automotive industry wherein a
significant amount of work has gone into using the appropriate filters, this aspect was not investigated.
However, for the calculation of CDRR and CAR implemented in this study, the signals were filtered
according to the IMO standard using a low-pass 20 Hz filter [9].
Table 3 Dummy sensors used throughout testing
Sensor measuring
Directions
CFC Filter Class
Pelvis acceleration
T12 acceleration
XYZ
XYZ
180
180
T1 acceleration
Head acceleration
XYZ
XYZ
1000
1000
T12 loads
C1 (Upper neck) loads
X Y Z, forces and torques
X Y Z, forces and torques
600
600
A high speed camera running at 125 frames per second (fps) was used to monitor the dummy movement
during the test. It was positioned to give a full side view of the dummy. Due to the large movements of the
structures within the lifeboat, the camera views were not used for motion analysis purposes.
Occupant impact response analysis
Analysis of the occupant impact responses was implemented using both the high-speed videos and the
dummy response signals. The high-speed videos were used to characterize the general occupant motion
during impact and any differences that exist as a result of experimental variation. The dummy response
signals were analyzed using peak values measured throughout the impact. Comparison of these responses
from the various test provided an indication of the sensitivity to the varying impact conditions. Table 4
summarizes the response signals used as suggested by Skjæveland et al. [7] for the evaluation of occupant
responses in lifeboat impacts. Comparison of the responses to noted dummy injury criteria has not been
performed here but will follow in a future study.
Table 4 Dummy response values chosen for evaluation of drop conditions
Body Region
Pelvis
Dummy Response
Pelvis x-acceleration (g)
Pelvis z-acceleration (g)
Abdomen & Thorax
T12 compression (N)
T12 resultant acceleration (g)
Neck
Upper neck Fx shear (N)
Upper neck Fz tension (N)
Upper neck Fz compression (N)
Upper neck My (MOCy), extension (N)
Upper neck My (MOCy), flexion (N)
Nij
Head
Nkm
HIC36
RESULTS
Impact kinematics
The seating orientation, seat type, seat location and restraint conditions all contributed to the kinematic
responses observed in the impacts. For the drop boat seat type and orientation, the dummy was first loaded
into the bottom of the seat during the frontal impact phase (see Figure 4, 88 msec). The acceleration in the zdirection combined with the subsequent boat rotation pushed the dummy out of the seat. In cases where poor
restraint of the shoulders (i.e. torso belts) was combined with a head restraint, significant neck extension was
observed. During the slamming phase of the impact the dummy was pushed into the back of and upwards
along the seat (see Figure 4, 120 and 188 msec). The initial position of the dummy when entering the
slamming phase combined with its upward motion generated further neck extension. High friction between
the dummy head and seat along with the head restraint prevented the head from moving upwards with the
body. An arrow indicating the approximate whole body loading direction is also included in Figure 4. In
cases where no head restraint was used, the dummy’s head flexed forward during the frontal impact phase,
followed by impact into the seat during the slamming phase. The use of the 4-point restraint helped to better
restrain the dummy shoulders during the frontal impact and rotation phases; however, due to the inability to
provide more secured and optimally located belt attachment points significant shoulder motion was still
observed. In the front seating case, the dummy was loaded primarily during the frontal impact phase, being
pushed downwards and rearwards within the seat to generate a flexed upper neck position. No further
significant loading was noted during the slamming phase.
0 msec
0 msec
88 msec
128 msec
120 msec
354 msec
198 msec
376 msec
Figure 4 Occupant impact kinematics for the drop boat (left) and skid boat (right); both 50% aft load, max drop height,
rear seated and conventional restraint. Chosen still shots represent maximum occupant excursion during the frontal
impact phase, initial impact during the slamming phase and maximum observed load (i.e. neck bending) during the
slamming phase.
For the skid boat seat type and orientation, the dummy was first loaded into the seat back during the frontal
impact phase (see Figure 4, 128 msec). The z-acceleration and boat rotation lifted the dummy out of the seat;
restrained from further motion only by the seatbelts. During the slamming phase of the impact, the dummy
was pushed into the seat bottom (see Figure 4, 354 msec). The subsequent seat impact generated slight neck
flexion (see Figure 4, 376 msec). The use of the shoulder straps in the 4-point belt provided increased
restraint, thereby reducing the upward motion during the frontal impact and rotation phases. Differences in
100% and 50% aft boat loadings for both boats were observed during the frontal impact and rotation phases,
where the 50% aft boat load also produced greater dummy displacements.
Occupant impact responses
Drop boat
Dummy signals measured during the first two tests indicated reasonable repeatability and provided
confidence in the test methodology. In all tests the dummy pelvis acceleration signals produced slightly
higher responses as those measured in the boat, suggesting good pelvic coupling with the seat. For tests
where the dummy was in the rear seating position, a first acceleration peak representative of the frontal
impact phase occurred, followed by an acceleration peak of the opposite sign and greater severity (often 2-3
times) representative of the rear slamming phase. In the test were the dummy was placed in the front seat
however, the dummy acceleration showed a double peak without sign change. The first peak was the more
severe and represented the frontal impact phase, when the dummy is forced into the bottom of the seat, while
the second peak represented the rear slamming phase.
Tests F064314 and 18 provide a comparison of dummy responses when varying seating location (see Table
5). The results indicate that a rear seating location generated extended neck loads while the front seating
location generated flexed neck loads. In general, the rear seating location generated a more severe impact
response with most peak loads reaching or surpassing those from the front seating location. This was
similarly seen in the CDRR and CAR responses, where the rear seating location values were higher.
Interestingly however, the front seating location produced a greater pelvic X-acceleration. This occurred
during the frontal impact phase while the rear seating peak occurred in the rear slamming phase. The
corresponding rear seated pelvic X-acceleration during the frontal impact phase produced a peak load of only
9g, although in the opposite direction.
Table 5 Impact responses, drop boat; variation of occupant seating location
(4-point restraint, 30m drop, 50% aft load)
Impact Response
Rear Seat
(Test F064314)
Front Seat
(Test F063018)
CDRR
1.01
0.84
CAR
1.49
1.16
HIC
Neck Fx Post. (N)
78
143
93
20
Neck Fx Ant. (N)
Neck Fz Tension (N)
290
53
154
55
Neck Fz Comp. (N)
Neck My Exten. (Nm)
945
50
1088
3
Neck My Flex. (Nm)
Nij
2
0.43
53
0.35
Nkm
T12 Fz Comp. (N)
1.25
1486
0.78
1365
T12 Result. Acc. (g)
Pelvis X-Acc. (g)
26
20
26
24
Pelvis Z-Acc. (g)
8
6
The effects of boat load condition and drop height (see Table 6) on occupant responses seated in the rear of
the boat were seen in almost all signals. A significant increase in neck load responses was seen in the 50%
aft load case over the 100% load case, where in the former the neck compression and moment were 1.5-2.0
times that of the other. Once again both the CDRR and CAR limits where exceeded, and sensitivity to the aft
loading could be seen in the responses. The effect of boat drop height generated a similarly significant
response, where a drop height of 15 m produced responses less that half those from the 30 m drop. It was
further noted that a drop height of 15 m produced CAR and CDRR responses under the IMO [9] limit, which
was similarly observed for the 20m drop height test (F064319).
The effects of restraint setup required the comparison of four test scenarios (see Table 7). Drop conditions
involving 50% boat load and 30m drop height were used for the comparison of the old torso restraint and the
new ad-hoc 4-point belt. Analysis of the individual values showed the 4-point belt produced a positive effect
reducing all dummy responses. Drop conditions involving 100% boat load and a 30m drop height were used
for the comparison of the 4-point restraint system with and without head strap. Without the head strap, the
anterior shear force (289 N vs. 566 N) and the axial tension force (52 N vs. 1471 N) in the neck increased
considerably during full flexion of the head. The flexion moment is also higher due to that motion (2 Nm vs.
63 Nm), but it is not as violent as the severe extension moment with the head strap (46 Nm vs. 2 Nm).
Compression of the neck without the head strap was less severe (944 N vs. 822 N) but fell close to the value
obtained with the head strap. The HIC measured in the test without the head strap is high due to the hard
contact with the seat during the rear slamming phase of the boat impact. All conditions produced CDRR and
CAR values exceeding the limits and no obvious sensitivity to these variations.
Table 6 Impact responses, drop boat; variation of boat loading conditions (4-point restraint, rear seating)
Impact Response
50% Aft Boat Load
30m Drop
(Test F064313)
100% Boat Load
30m Drop
(Test F064314)
100% Boat Load
15m Drop
(Test F064315)
CDRR
CAR
1.22
1.70
1.01
1.49
0.50
0.81
HIC
Neck Fx Post. (N)
116
181
78
143
11
99
Neck Fx Ant. (N)
Neck Fz Tension (N)
379
124
290
53
127
52
Neck Fz Comp. (N)
Neck My Exten. (Nm)
1262
79
945
50
353
10
Neck My Flex. (Nm)
Nij
1
0.71
2
0.43
9
0.09
Nkm
T12 Fz Comp. (N)
1.90
1901
1.25
1486
0.32
737
T12 Result. Acc. (g)
Pelvis X-Acc. (g)
27
23
26
20
14
11
Pelvis Z-Acc. (g)
13
8
5
Table 7 Impact responses, drop boat; variation of occupant restraint conditions (30m drop, rear seating)
Impact Response
50% Aft Boat Load
Torso Belt
4-point Belt
(Test F064311/12)
(Test F064313)
100% Boat Load
4-point Belt
4-point Belt no
(Test F064314)
Head-strap
(Test F064317)
CDRR
1.20
1.22
1.01
1.00
CAR
HIC
1.69
140
1.70
116
1.49
78
1.50
700
Neck Fx Post. (N)
Neck Fx Ant. (N)
277
426
181
379
143
290
144
568
Neck Fz Tension (N)
Neck Fz Comp. (N)
160
1600
124
1262
53
945
1504
831
Neck My Exten. (Nm)
Neck My Flex. (Nm)
92
1
79
1
50
2
4
55
Nij
Nkm
0.88
2.27
0.71
1.90
0.43
1.25
0.28
1.33
T12 Fz Comp. (N)
T12 Result. Acc. (g)
1562
29
1901
27
1486
26
1236
27
Pelvis X-Acc. (g)
Pelvis Z-Acc. (g)
25
12
23
13
20
8
24
10
Skid boat
In all tests the pelvis acceleration signals matched the boat accelerations shape and timing measured at that
location. As compared to the drop boat, similar patterns in dummy load responses were observed when
varying boat load and restraint type (see Table 8). The effect of a 50% aft loaded boat generated slightly
higher responses for almost all signals. It was thought however that a less effective restraint may have
provided more significant response differences. The effect of the 4-point restraint was more clearly defined,
decreasing almost all loads throughout impact. The inclusion of a shoulder restraint reduced the dummy’s
vertical displacement during the frontal impact phase, which in turn reduced the severity of the impact in the
rear slamming phase when the dummy was loaded into the bottom of the seat.
Table 8 Impact responses, skid boat; variation of boat load and restraint condition (30m drop)
Impact Response
100% Boat Load
4-point Restraint
(Test F065011)
50% Aft Boat Load
4-point Restraint
(Test F065012)
50% Boat Load
2-point Restraint
(Test F065013)
CDRR
0.56
0.60
0.58
CAR
HIC
0.51
2
0.79
2
0.75
11
Neck Fx Post. (N)
Neck Fx Ant. (N)
109
130
123
132
236.6
151.1
Neck Fz Tension (N)
Neck Fz Comp. (N)
45
216
55
245
66
587
Neck My Exten. (Nm)
Neck My Flex. (Nm)
15
6
18
4
21
15
Nij
Nkm
0.15
0.48
0.16
0.53
0.20
0.62
T12 Fz Comp. (N)
T12 Result. Acc. (g)
495
6
484
6
599
16
Pelvis X-Acc. (g)
Pelvis Z-Acc. (g)
5
4
4
5
7
14
DISCUSSION
The study described here was initiated due to the lack of comprehensive response data available in the
literature of occupant loading during free-fall lifeboat drop conditions. The intent was to provide additional
data in an effort to answer questions regarding boat type, loading conditions, restraint design, seat type and
seating location. Additionally, this study was intended to investigate the regionally specific loads occurring
during the various impact conditions. The results of the front and rear seating during the drop boat tests
showed the difference in directional loading occurring at each seating location where the front seat produced
extended neck rotations and the rear seat produced flexed neck rotations. The kinematics confirmed this
response where the front seated dummy was pushed into the back of the seat as opposed to the rear seated
dummy undergoing a bidirectional motion out of and into the seat back. Interestingly, both produced a
similar neck compression and T12 accelerations peaks. The pelvic X-accelerations of the front seating
location was however higher, occurring earlier in the impact (frontal impact phase). These differences in
timing and load direction highlight the challenges faced in designing the boats with effective restraint
systems for all occupants.
The loading conditions (both boat load and drop height) influenced the boat and occupant impact response
for both boats. A significant increase was observed when increasing the release height of the drop boat by
15m. This was expected as the energy at the point of impact was doubled. The additional effect caused by the
50% aft loading condition further exacerbated the responses. The increase in occupant acceleration was
driven by the rearward shift in boat center of gravity, matching the conclusions made by Nelson et al. [3] on
the basis of hydrodynamic theory. Although load increases were also seen in the skid boat during the 50% aft
loading condition, the high effectiveness of the 4-point restraint system used in this test limited the
sensitivity to this loading condition, i.e. a positive outcome.
Restraint conditions for both boats were explored using old and new-ad-hoc restraint systems. For the drop
boat the inclusion of restraint over the shoulders provided by the 4-point belt system reduced relative torsohead motion, thereby reducing the neck related loading. Furthermore, the routing of these belts closer to the
pelvic region reduced the pelvic and T12 loads. Nonetheless, the high severity responses and observed
relative head-torso motion with the 4-point belt indicated the need for further optimization of this restraint
design. The lack of a head strap in the drop boat was the only test to generate significant neck tension
combined with a flexed upper neck moment, caused by the unrestrained forward motion of the head.
Furthermore, during the slamming phase the dummy was flung into the back of the seat generating
significant head impact and a high HIC. Such responses prompt the debate regarding the use of a head strap.
As seen by many tests, improper restraint of the torso in combination with a head strap generates significant
neck bending. On the other hand the high neck tension and HIC responses in the case without a head suggest
its necessity. The optimal solution for such a system would be the implementation of a more effective
restraint (optimized 4-point with head strap) or the complete redesign of the seating orientation within the
boat. Given the proliferation of this seat system in existing boats, the associated cost of a complete redesign
is prohibitive. The more realistic option would be the optimization of a more effective restraint system such
as the 4-point system. An increased effectiveness may be expected if a 5-point belt with crotch strap were
used for this seating configuration.
Although not shown here for confidentiality reasons, the acceleration measured in the drop boat had a highpeak low-duration response, while the acceleration measured in the skid boat had a low-peak high-duration
response. These differences were primarily driven by the hydrodynamics of the two boats upon impact with
the water after the free-fall period. However, if one were to simulate the application of each pulse on the
alternative boat-seating condition, it could be assumed that the seats in the skid boat (rearward facing vertical
seats) would ensure a reduced occupant load in comparison to the drop boat seats. The rear-facing seats
would distribute the loading across the back of the occupant and induce occupant motion without relative
torso and head motion.
The results of these tests further support the need for more detailed lifeboat occupant safety assessment
criteria than those currently implemented in the IMO standards [9]. Both of these criteria lack sensitivity to
variation in restraint condition or seating orientation. Further exploration of this topic is provided by
Skjæveland et al. [7].
A noted limitation of this study is the use of an ATD unvalidated in lifeboat drop conditions. However, no
port mortem human subject (PMHS) or human volunteer freefall boat occupant impact data was available for
validation of the dummy. Another limitation was the exclusive use of a 50th percentile automotive ATD.
Recent findings suggest a significant population of the workers on oil-rigs represent the larger male
population; a size better aligned with the 95th percentile automotive occupant. As such, any further
optimization and certification of boat safety and restraint systems should consider the variability expected
within the oil-worker population. Finally, this study did not consider occupants wearing evacuation suits.
The suits are worn by oil-workers during drop conditions and possess a significant amount of insulation,
generating gaps between the occupant and the restraint. Additionally, items attached to the front of the suit,
such as an emergency GPS beacon and breathing apparatus can interfere with the use of the restraints with
potentially negative consequences.
Finally, these responses provide significant data for the development and validation of simulated occupant
lifeboat drop conditions. This work has been performed in the accompanying paper by Forbes et al. [8].
CONCLUSIONS
This study provides a comprehensive analysis of occupant impact response during freefall lifeboat drop
conditions, through the use of full scale boat drop tests with an instrumented ATD. A rear impact dummy
(RID3D) was chosen as the appropriate occupant surrogate given its accurate back attenuation characteristics,
omni-directional neck biofidelity and applicable acceleration range. The tests were intended to provide
comprehensive occupant impact responses allowing for an assessment of the occupant load sensitivity to
varying impact parameters such as boat load, drop height, seating location and restraint design. Of the two
boats tested, the drop boat was noted to produce a short duration, high peak acceleration response while the
skid boat produced a long duration, low peak acceleration response. Although the primary reason for these
differences dealt with the hydrodynamic differences of the two boats, the drop boat was noted to produce a
greater righting moment arm increasing the subsequent impact accelerations. Overall the drop boat was
found to produce the more severe impact responses.
The findings indicated dummy impact response sensitivity to all parameter variation; the most severe being
boat load and drop height. In particular, the neck responses played an important role in the severity of the
loads sustained. On the other hand, the boat based occupant impact severity estimates (i.e. CAR and CDRR)
lacked discrimination between different restraint conditions. This suggests that even if restraint design and
optimization could significantly reduce the occupant loads, the CAR and CDRR could over or under predict
the severity of the occupant impact response. In other words, both measures lack any sensitivity to restraint
and seating conditions... It is therefore recommended that the regionally specific occupant loading criteria be
adapted within the IMO procedures when assessing freefall lifeboat occupant safety.
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