r
Prediction of Injury Risk for Occupants of
High Speed Pläning Craft
Cameron Bass, University of Virginia, crb7q(vfrginia.edu Joseph Ash, Umversity of Virginia, iash()virgima edu
Robert Salzar, University of Virginia, salzarvirginia.edu
Ron Peterson, L3 Communications, ronaldspeterson(),comcast.net Eric Pierce, Naval Surface Warfare Center Panama City, eric.pierce(navv.mil
ABSTRACT
High speed craft are used by civilian agencies and the military for rescue, for interdiction,
and for rapid insertion and extraction of forces. Ensign et al. (2000) found evidence of a
significant mjury problem in a study of self-reported mjunes of high speed boat operators
of high speed craft. Though repeated vertical spinal impacts with greater than 10 g peak accelerations may occur in such craft, there is currently no completely suitable injury criterion to predict the likelihood of spinal injuries from high speed craft operations. To
assess the injury nsk to high speed craft occupants, sea trials with instrumented occupants
were used to provide data on the relative performance of discomfort methods (RMS and
ISO 2631 Part 1 (1997) VDV) and injury assessment methods (ISO 2631 P5, 2003).
This study found that the RMS of the seat pad accelerations does not account for the effects of discrete severe impact events that are common with high speed planing boats.
The repeated impact standard ISO 2631 Part 5 for seated occupants has been found to be as good as or better than any of the other injury cntena for apphcation to the assessment of
injury criteria in high speed planing craft (cf. Bass, 2005); however, the neural net
dynamics model within the standard has severe limitations when applied to vertical
impacts above +4 g and below O g. A new low-order dynamics metaniodel for predicting
vertical impact to the human spme has been developed using a Madymo (mO, hic)
simulation of a seated occupant under predominantly vertical impact. This model has
been validated using experimental high speed craft operations for impacts with vertical
accelerations greater than 10 g.
INTRODUCTION
High speed planing boats are used in both military and civilian settings for at-sea rescue, for craft interdiction, and for rapid insertion and extraction of forces, often in rough water conditions.
Occupants of thêse high speed craft experience repeated impacts with
amplitudes reaching 10 to 15 g in the vertical direction. Goliwitzer and Peterson (1994)described the effects of repeated shock impacts on occupants during high speed operations in Naval Special Warfare planing boats. Ensign et al. (2000) found compelling evidence
of a significant injury problem in a study of self-reported injuries of high speed boat operators. It was found that 65% of operators sustained a boat-related injury, 89% of which occurred within the first tWo years of operation. The self-reported injury rate of NSW crewmen (2,687 injuries per 100,000 person year exposure) is well over 5 times
higher than that of the Navy average (497 injuries per 100,000 person year exposure). In the mission logs of 12 crewmen, 33% of missions resulted in an injury, while 67% of the
reported injuries resulted in chronic pain, Anecdotally, this injury problem is both acute
and chronic, reducing both the short-term and the long-term effectiveness of the personnel
who are exposed to repeated shock impacts. The most serious injuries included lower
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back, fleck, shoulder, ktiee, and arkie injuries. The spectrum of injuries includes a
substantial number of intervertebral disc injuries (cf. Bass, 2003).
Though repeated vertical spinal impacts with greater than 10 g peak accelerations may
occur m such craft, there is currently no completely suitable injury critenon to predict the
likelihood of spinal injuries from high speed craft operations. This paper examines the problem using two different approaches: 1) sea trial data analysis, and 2) numerical modeling. The first is the analysis and discussion of sea trials using instrumented boats
and seated humans in a high speed Mk V Special Operations Craft (SOC), shown m
Figure la. This includes the evaluation of the relative performance of conventional injury
assessment methods used by the naval architecture commumty and alternative methods
that may better assess the probability of injury. The second is the further development of a
numerical model to simulate the respoflse of a human under typical high speed planing
boat impacts in a seated position.
The curtent repeated impact standard for seated occupants, ISO 2631 Part 5, has been asserted to be as good as or better than any of the other injury criteria for spinal injuries
from high speed craft operations (Bass, 2005) However, repeated vertical spinal impacts with greater than 10 g peak accelerations can occur iñ such craft. l'his study fmds that the
neural net dynamics model within the standard has severe limitations when applied to vertical impacts above +4 g and below O g. A new low-order dynamics metaínodel for
predicting vertical impact to the human spine has been developed using a Madymo (TNO, mc) simulation of a seated occupant under predominantly vertical impact.
Figure 1. (a) Mk V Special Operations Craft (on left). (b) The isolated seat inside (on right).
The simulation uses a Madyrno (TNO, mc) lumped mass human model and is validated
using experimental impact pulses taken during sea trials of a Mk V SOC in January 2003. The model has been validated for high speed planing boat impacts on occupants m a Stidd
Systems v4 rigid seat, hereafter called the non-isolated seat (Bass, 2003).
Thisinvestigation includes the further validation of the Madymo model using impacts from the
January 2003 sea triaL The resulting model was used to evaluate shock mitigation
postures and techniques, and is also being considered as a means to improve the lumbar
dynamics model within the dynanucs model within the existing ISO-263 i Part 5 standard
Additionally, this paper describes conventional methods used by the naval architecture
methods, the relative performance of conventional and alternative method, and a process for integrating new methods into the craft design process.
METHODOLOGY
SEA TRIAL DATA: In January 2003, a Mk V SOC was outfitted with four different
isolated seats, termed vSa, v5b, v5c, and v5d, which were located in the front row of the
craft.
An instrumented sea trial was conducted during a transit from the Naval
Amphibious Base in Little Creek. Virginia to the King's Bay Submarine Base in Georgia
while recording acceleration data in the heave, surge, and sway directions. The sea
conditions during the transit included significant wave heights of 5 to 6 feet with 3 to 4 fóot wind waves. A three-axis accelerometer and a three-axis angular rate sensor Were
löcated on the deck along the centerline of the boat. Three-axis accelerometers were
located on the seat frame (above the isolator) and seat pad (between the occupant and the seat pad) on each of the instrumented seats, as well as on the occupants' backs on the L4 lumbar spme using a tightly worn belt and the occupant helmets Of the four seats, the v5c
seát (Figure lb), which is referred to as the isolated seat, was chosen as the best seat by
experienced occupants and its data is analyzedinthis paper (Peterson, 2003)
In October 2003, a second sea trial was performed to compare the non-isolated (rigid) seat
to the isolated seat..
Both seats were placed in the front row of .a Mk V SOC and
mstrumentedinthe same fashion as m the January 2003 trial The October 2003 sea trial is of special interest as injury criteria can be assessed on their ability to determine the known improvement of the isolated seat over the non-isolated seat (Bass, 2005). The acceleration
data was obtained in seas with a significant wave height of approximately 4 feet, for varying craft headings, in the area around the Naval Amphibious Base in Little Creek,
Virginia A further test was performed off the coast of Monterey, California in January
2005. The test of January 2005 involved the use of isolated seats by volunteers and
dunirnies and a rigid seat that was used with dunmiies The combined sea trials account for 22 hours total duration over six days; the summaiy of these tests is shown in Table 1.
Table 1. Sea Trial Data Characteristics
The acceleration data was sampled using a 12-bit AID converter at 750 Hz with hardware
anti-aliasing at 250 Hz. Measurements from the previous sea trials suggested that, in
practice, the power above a frequency of -200 Hz is limited (Bass, 2005). Following the
trials, data was processed using a phaseless 8-pole Butterworth lowpass filter to 80 Hz as specified in the ISO 2631 Part 5 standard.
3 Sea TÑ.ls Total Duration (hours) Number of Impact Events with Peak
Sig.
Wave Height
(fi) Occupant Seat > 0.5 g
> 2 g >4g
January 2003 4 8,211 982 113
2 iO
Human IsolatedOctober 2003 0.8 1,285 159 6 4 Human Rigid afld
Isolated January 2005 17 9,406 462 - 2 - 10 14111 Dummy and Human Isolated (Human) Rigid and Isolated (Duthmy)
iNJURY CRITERIA: It has been argued that there is no fully suitable criterion for spinal injuries from rpeated shocks at the heave acceleration levels expenenced in planing boat
operations (Bass, 2003, Bass, 2004). Traditional evaluation methods include the root
mean square (RMS) vertical acceleration and the frequency weighted RMS from ISO 2631
Part 1 (1985) Alternative evaluation methods mvestigated in this paper are the power
spectral density (PSD), the newer ISO 2631 Part 1 Vibration Dose Value (VD\T), the peak acceleration, the Dynamir Response Index (DM), and ISO 2631 Part 5 (2003).
The basic RMS is a measure of the mean power within the entire bandwidth of the signal,
and although a lower RMS corresponds to less power within the system, it may not
correlate to crew comfort or injury (Payne, 1976)
The frequency weighted RMSaddresses this issue, however, it is not validated for the higher crest factors which are seen
in planing boat operations A crest factor is defmed as the maximum amplitude of the frequency-weigited signal, divided by the RMS of the frequency-weighted signal (ISO
2631 Part 1, 1997) The newer ISO 2631 Part 1 (1997) standard includes the Vibration
Dose Value (V[?V), used for higher crest factors It is computed as the root-mean-quad of
the frequency-eighted acceleration This better accounts for acceleration peaks that are
potentially mor injurious but have a small effect in an RMS calculation. The power
spectral density (PSD) [10 logto(g2fHz)] is computed for the acceleration profiles over the 0.1 to 100 Hz raage using a 4096 point Hamming window with 50% overlap. The 4-8 Hz bandwidth is exniined because it is generally associated with human torso dynamics and
discomfort (IS9 2631 Part 1,
1997).The average PSD value within this range is
computed and presented.
These 'traditional criteria' used to assess injuÑs are often not based on inju criteria.
For example, RMS vibration in high speed boat impact has not been associatéd or
correlated with injuries in the biomechanics literature For example, ISO 2631 Part 1 is
based on discomfort boundaries, not injury boundaries However, these have been used as
injury mdicators in the design of craft There are, however, two existing spinal injury
cntena that may be appropnate for repeated vertically dominated impacts at levels seen m high speed craft, dynamic response index (DM) and ISO 2631 Part 5
The DR! (cf Payne, 1976) was developed by the U.S. Air Force as a spinal injury measure
of the short duration vertical accelerations in aircraft ejection seats The DitE uses a
simple mechanical spring-damper model of the spme to represent the spinal column
stifThess and dampening characteristics. A DR! value is obtained based on the maximum
spinal compression predicte4 with the model. Stech and Payne (1969) correlated spinal injury rates to DRI values using 364 non-fatal seat ejections from six different aircraft.
They found that DRI values for single impact events of 15.2, 18.0, and 22.8 correspond to 0.5% (low), 5% (medium), and 50% (high) risks of spinal injury.
The ISO 2631 Pat 5 (2003) was developed by the U.S. Ariiiy to assess the risk of spinal injury involved with the vibration and shocks expenenced by occupants of Army tactical ground vehicles (Village 1995). The injury model within this standard is based on a
vertebral body failure assessment from laboratory experiments. The spine dynamics
model is represented by a nonlinear neural network for the heave direction (vertical-z) and a linear lumped parameter model for the surge (forward-x) and sway (lateral-y) directions The heave model vas developed based on the shock transmission of acceleration between
the seat and the human volunteers. Ethical constraints prevented the voluteers from
developed for the standard is limited to the ±1- 4 g range. The standard has not been
validated at the higher impact levels that are seen on high speed planing boats. The
parameter of interest within this standard is the daily equivalent static compressive dose,
Sed, normalized to an exposure time of 8 hours, Sedj(8), in units of M.Pa. This will be
referred to as the Spinal Stress Dose. Añother parameter of interest is the R-value, from which the risk of spinal injwy over the course of a career can be quantified. An R-value
below 0.8 corresponds to a low lifetime risk of injury, and an R-value above 1.2
corresponds to a high lifetime risk of injury.
MADYMO Model: A numerical model was developed to simulate the response of the
human under high speed planing boat impacts and to assess the potential for injuries. The
model is based on a Madymo (TNO, mc) lumped mass human model and was validated
using experimental impact pulses taken during sea trials of an MK V SOC in January 2003
(Bass, 2004). The Madymo model has also been validated for high speed planing boat impacts on occupants in a rigid seat and in a shock isolated seat (Bass, 2004). Previous
work showed good agreement between experiments and Madymo model output.
In this study, comparisons were made between experiments and the ISO 2631 Part 5
dynamic model output vertical accelerations. Further, to determine the transmissibility of the neural network throughout a range of peak amplitudes, positive and negative half-sme waves of varying amplitudes, within and outside of the 4 g design limit, were used as the
mput The transmissibility was found from the ratio of the predicted peak output of the neural network to the input magnitude. The transmissibility of a system can vary with frequency (Rao, 2004); thus, half-sine waves of the varying amplitudes from 1 g to 15 g with frequencies ranging from 10 Hz (half-sine period of 100 ms) to 0.5 Hz (half-sine
period of 2 sec) were used as the input into the 150 2631 Part 5 neural network. Finally, a metamodel for high amplitude heave impacts was developed based on the Madymo model for vertical impacts (Bass, 2005). This metamodel is a simple transfer function that relates
the z-axis deck acceleration time history to peak L4 accelerations necessary for the ISO
2631 Part 5 injury criteria calculation.
RESULTS
SEA TRIAL DATA: Table i includes the results of the conventional injury criteria used by the naval architecture community and the proposed alternative injury criteria used in the analysis of the October 2003 dataset. Further, the results of the conventional and
alternative injury criteria applied to the isolated seat in the January 2003 dataset are given
m Table 2 The nde produced while on the isolated seat m this sea trial was said to be at
an acceptable level for the sea conditions (sea state 3) by eight experienced operators. In contrast, the ride in the existing non-isolated seats was deemed not acceptable at the same
sea conditions. Without long term epidemiology to assess the presence of injuries in the
operators from use of either seat, the values of the injury criteria represent threshold values that could represent tolerable levels to human occupants,
The peak acceleration and DRI require single impacts for evaluation. The ISO-2631 Part 5 spinal stress dose is also considered using a single impact analysis. After evaluation of
the single impacts, the injury criterion values obtained are subsequently binned by deck
peak acceleration to produce a histogram that shows the general trend of the injury
criterion with respect to the deck peak acceleration. The peak acceleration and DRI are evaluated from the heave acceleration of the seat pad, and the ISO 2631 Part 5 spinalstress dose is evaluated using all three directional components at the seat pad.
In áll of the thtasets, it is clear that the RMS value does not discriminate between the
isolated and the rigid seat. The RMS is not a good measure of discrete impacts over long duration waveforms The other measures mcluding the ISO 2631 P1 RMS, VDV, 1/10 highest DR!, ISO-2631 Part 5 and bandwidth-limited PSD show good discrimination between the seas Only DR! and ISO-263 I Part 5 are biomechanically based on injury,
and only ISO-263 1 Part 5 has substantial validation for repeated impacts.
Table 1. Comparison of the non-isolated (rigid) and isolated seat m the October 2003 sea trial using conventional and álternative analyses.
The Wstogram for the peak
acceleration for the October
dataset is given m Figure 7. The
advantage of the isolated seat for
the largest number of impacts
above 0.5g deck impact
acceleration is clear within each
bin. The total veighted percent
increase of the isolated seat over
the non4so1ated seat is 18.7%.
Both the iso1atd and the
non-isolated seats show higher acceleration vaiues
during an
impact event leding to higher
peak acceleratioi values compared to the deck. The trends of the DR! (Figure 8) and ISO
2631 Part 5 (Figure 9) histograms are very similar to the peak acceleration histogram,
giving similar wéighted increases of 18.9% and 18.2% respectively.
a 400 300 = 200 loo o
Table 2. Results of conventional and alternative analysis for the isolated seat in the January 2003
sea tfials. 500 10 = Freqiency of hnpaçts Non-lsolated + Isolated 1 -r i
i
i
o C. a a a. 0.5 1.5 2.5 3.5 4.5 5.5 6.5Bins by Deck Peak Acceleratloñ (g)
Figure 7. HiStogras relating the peak acceleration of individual impacts seen at the seat pad of both the non-isolated. and isolated seats to the peak acceleration of th deck
(October 2003 dataset)
Critena
i30Secàndlnterval. .145SàcondInterva[ EjitiréData Set
I Rigid Isolited I % Iiñ Rigid I Iö1ated I % Iñipr Rigid f Isolated I % Impr
Traditional RMS 062 063 1 6% 076 0 75_ 1 3% 0.38 - 035 86% ISO 2631 P1 RMS I 3.01 2.19 27.2% 4.15 2.58 37.8% 1.41 0.94 33.3% ..Alternative. ISO2631P1VDV I 922 624 323% 1180 702 405% 497 291 414% I/IOWDRI 30 23 233% 47 34 277% 47 30 362% ISO2631P5S I 793 658 170% 110 835 241% 622 418 328% BW-Limited PSD I-28.1 db -34.3 db 22.1% -26.0 db -33.5 db 28.8% -36.4 db -45.0 db 23.6%
-Criteria
-January
Results Traditional RMS 0.47 ISO 2631 P1 ('85) RMS092
Alternative ISO 2631 Pl ('97) VDV 3.80 ISO 2631 P5 Sed (8 br) 5.87 BW-Li.mited PSD - -44.4 db500 400 300 n n C. 200 100 o 500 300 200 loo o 2500 2000 8 1500 Frequency of Impacts Non-Iso1a1ed Iso1ated - L 0.5 1.5 2.5 3.5 4.5 5.5 6.5
Bins by Deck Peak Acceleration (g)
Figure 8. Histogram relating the dynamic response index of individual impacts of both the non-isolated and isolated seats to the peak acceleration of the deck (October 2003
dataset) Frequency of Impacts +Non-lsoIzLed 400 4Isolated 3000 1000 500 o
I
I
0.5 1.5 2.5 3.5 4.5 5.5 6.5Bins by Deck Peak Acceleration (g)
Figure 9. Histogram relating the Spinal Stress Dose of individual impacts of both the non-isolated and the non-isolated seats to the peak acceleration of the deck (October 2003 dataset)
In Figure 10, the peak acceleration from the isolated seat of the January dataset yields a similar histogram to that seen with the isolated seat from the Octóber dataset though the
January dataset contains a large number of impacts per bin. So, though the sample sizes are different, the acceleration peak results from the October dataset are quantitatively
similar to those from the January dataset.
0.5 1.5 25 3.5 4.5 5.5 6.5 7.5 8.5
Blas by Deck Peak Acceleration (g)
lo 8 a. g o. rl,
Figure 10. Histogram relating the peak acceleration of individual impacts at the seat pad nf the isniated seat nf the Tarnlarv and Oetnher datasets tn the neak a're1ratinn nf the dei-k
NUMERICAL MODELING: The experimental resUlts
for peak vertical
impact acceleration response for a range of seat pad acceleration inputs is shown in Figure 1. For the shock isolated seat system present m the craft, peak vertical acceleration measured at the back (IA location) was approximately 20% greater than acceleration measured on the seat pad for impact acceleration peaks ranging from j-1 g to -14 gExperimental b'ack accelerations (L4 location) are compared with ISO 2631 Part neural
network predicted spinal accelerations for two representative impact events in Figure 2;
one within the esign range of the neural network and the other outside the design range.
For the impact with a peak seat pad acceleration of 13.9 g, the neural network peak
vertical acceleration response of 8 8 g substantially underpredicts the measured peak spinal acceleraion of 16.4 g. For the impact event with accelerations within the designrange, howevei, there is good correlation between the predicted peak spmal acceleration
and the peak back acceleration at JA For the penod of 'free fall' with accelerations from -0 5g to -1 0 gJ the ISO 2631 Part5neural net model frequently shows behavior m which
the sign and aceleration magnitude are different from the measured back response (LA
location) as shown in both Figure 2a and Figure 2b.
Figure 1. Comparison of the vertical axis peak measured back acceleration (L-4 location) and the peak vertical seat pad acceleration for representative impacts. Light blue line is i
to i ratio, and dark blue line is 4 g valúe.
15
¡
lo -L Seat InpUt 13.6 g Back: 164g Neural: 8.8 g 18 = 16 '4 12 10 R20.98O2 -Measuréd Back-o- Neural Network
0.5 1
ii
2 2.5 .5lime (sec)
Figure 2. Comparison of peak measured back acceleration (L-4 location) response with ISO 2631 Paçt 5 peak acceleration response for a) 13.6 g Peak Input, b) 3.1 g Peak Input. Two representative half sine inputs and ISO 2631 Part 5 neural net model responses are
shown in Figure 3a and in Figure 3b. With input amplitude of 4 g, the top end of the
y= 1.1872x Thne (sec) Measured Back -c-Neural NetWOrk Seat Input: t9 g Back: 3.1 g Neural: 3.4 g 1.5 2 2.5
li
u u 0 5 IO 15 20design range, the neural network response is 17% larger at 4.7 g; this response is similar to
that seen in the experimental data. However, for a 10 g peak input, the neural net peak
response is approximately 7 g, a 30% decrease m peak response that has a different
character than that seen in the experiments. Neural net output peaks for various inputamplitudes and frequencies are shown in Figure 3ç. For inputs with greater than 5 g peaks
and half sine durations greater than 0.2 seconds, the ISO 2631 Part5 response generally
falls below the level of the mput pulse peaks, behavior that is genencally different than
that seen in the measured data.
Io
16
a o
5 10 15
Peak Back Acceleration (g)
toløg.
a) 16 14 12 10 : 0.0 o 20 0.1 ere 0.2 ere -o- 0.2 ere 0.75 ecc -a--20 cre 1.1 LadFigure 3. Half sine input with ISO 2631 Part 5neural net response a) 4 g input, b) 10 g
input, c) neural net response for a range of amplitudes and frequencies.
In Figure 4a, the ISO 2631 Part5model response is shown against the experimental back
response.
Note that in the experimental data, the duration of the impact event is
correlated with the vertical acceleration. For acceleration peaks above
5
g, the neural net model response underpredicts the expenmental data, with increasmg divergence for larger peak accelerations. In contrast, the low order dynamic metamodel developed during this study as shown in Figure 4b compares well with the experimental data over a range from i0 5 10 15 20
Deck Heave AcceleratIon (gj
Figure 4. a) Predicted ISO 2631 Part 5 peak L-4 acceleration, b) Heave (Z) direction transfer function based on Madymo calculations.
DISCUSSION AN]) CONCLUSIONS
Though the vertically neural network impact dynamics model of ISO 2631 Part 5 mostly
matches experimental data within its range of validity (i e range of training) for positive
impacts of up to 4g, it substantially underestimates lumbar spinal accelerations for larger positive impacts. Ftirthermore, there is evidence of qualitatively inappropriate behavior in
the neural net model for accelerations with peaks between - 05 g and -1 g, within the
9
b)
Madyreo Catculation
J0n501y2005 Esee#rrrontat Data 25
£Full Pulse Madyrno Cata-tation
20 o o 15 10 .
;:
5 b) Output Peak = 7.0 g a) Output Peak = 4.7 g 0 0.2 0.4 0.6 OES Tinte (aec) 0.2 0.4 0.6 0.8 Tinte (eec)stated range of the dynamics model. As acceleration time hitories below O g and above 4 g occur regularly during planing boat operations, the iSO 2631 Part 5 neural net dynamic
response may regularly predict both incorrect response amplitude and incorrect sign. These results indicate that the ISO 2631 Part 5 neural net dynamics model for vertical
impacts shOuld nOt be used outside its design range and should be used with caution within.
its design range In contrast, the low order dynamic metamodel developed during this Study compares well with the experimental data over a. range from O to 14 g and is. applicable for use as a replacement dynamics model that is useful for the assessment of
injury from repeted impact ina Mk V high speed plannmg planing craft
Limitations of this study include validation data that is limited to the data obtained in the sea trials. This data is vertical acceleration dorninatéd and has limited data from all
potential headings relative to the dominant wave direction
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