Motion-induced interruptions aboard ship

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ETOCRN2244 73033 Occupational ergonomics. the journal of Title: the International Society for Occupational Ergonomics and Safety. Publisher: lOS PRESS ISSN: 1359-9364 Year: 7 2007 Volume: Month: Issue: 3 Pages: 183-200 Author name(s): Crossland , P.IEvans, M.J.IGrist, D.llowten Motion-induced interruptions aboard ship: Article title Model development and application to words: ship design

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Occupational Ergonomics 7 (2007) 183- 199 lOS Press

183

Motion-induced interruptions aboard ship: Model development and application to ship design Paul Crosslanda,* , Mark J. Evansa, David Gristb, Mark Lowtenb, Helen Jonesb and Robert S. Bridgerb aQinetiQ Ltd, Gosport, Hampshire, UK bJnstitute of Naval Medicine, Crescent Road, Alverstoke, Hants, UK

Abstract. The most severe direct motion induced effect on the ability of an individual to work in a moving environment probably occurs in gross body tasks requiring balance and co-ordination, be it the crew trying to undertake their task effectively or the passenger trying to walk around the vessel. During rough weather working in the ship becomes more difficult and even the most experienced sailor will experience events where they must stop their activity, be it a specific task or merely standing, and hold on to some suitable point to minimise the risk of injury; these events are called Motion-induced interruptions (Mils). Mlls were recorded during the performance of a series of tasks on board a ship at sea in rough weather. The tasks were: standing facing aft, walking athwartships, a simulated weapon loading task, standing facing athwartships and a simulated fire-fighting task. Complex mathematical models of postural stability exist but currently lack the fidelity to accurately predict Mlls. This paper presents data from an empirical study in which Mlls experienced by subjects on a ship at sea were logged by an observer. Measurements of lateral and vertical acceleration of the deck immediately prior to the Mil were made and thresholds of acceleration for undertaking task were determined. These so called tipping coefficients are presented for use with predictive tools in ship design. Keywords: Ship motion, postural stability, trimaran, ship design

1. Introdu ction

The ship design community has long recognised the widespread difficulties in quantifying how a vessel might perform in waves (seakeeping performance assessment studies), because of the difficulties in quantifying the effects of ship motion on human perfonnance. Some of these issues have been addressed in the collaborative effort of the ABCD working group on human performance at sea. The ABCD working group [ 1] has created a forum for information exchange, collaborative planning and joint sponsorship of research and development work to investigate the effects of ship motions on human performance. The goal of the group research is to develop methods and criteria for assessing the effects of ship motions on performing real tasks in a ship environment. • Address for correspondence: P. Crossland, QinetiQ Ltd, Haslar Marine Technology Park, Haslar Road, Gosport, Hampshire, PO 12 2AG, UK. Tel.: +44 23 92 335172; E-mail: [email protected].

1359-9364/07/$17.00 © 2007 - lOS Press and the authors. All rights reserved

P Crossland eta/. I Motion-induced interruptions aboard ship

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Prior to the work of the group, typical personnel criteria were specified in terms ofRMS roll (or pitch) limits or by limitations on vertical and lateral accelerations see NG/6 [12]. Much of these criteria are based upon engineering experience in existing ship types. A typical criterion for personnel operations on deck is a root mean square (RMS) roll angle of four degrees. Clearly, the deck being inclined to four degrees or the occasional excursion to 12 degrees will not prevent ship personnel from doing their task effectively. It is the accelerations associated with those roll angles that limit deck operations. Using angular limits for certain unconventional craft can be especially misleading as the frequencies associated with the angular motions can be quite different. The ship designer and naval architect need to be able to assess ship performance as a whole including the ability of the human to perform a task effectively. The key aspects to this, so called systems approach, are suitable mathematical models that represent the key areas of human performance degradation in a moving environment; a relationship between these mathematical models and task effectiveness; and criteria against which performance can be measured. The most severe direct motion induced effect on the ability of an individual to work in a moving environment probably occurs in gross body tasks requiring balance and co-ordination, be it the crew trying to undertake their task effectively or the passenger trying to walk around the vessel. During rough weather, working in the ship becomes more difficult and even the most experienced sailor will experience events where they must stop their activity, be it a specific task or merely standing, and hold on to some suitable point to minimise the risk of injury. Human postural stability is maintained by a complex musculo-skeletal system integrated with various control systems in the body. The musculo-skeletal system comprises an active system with over 200 degrees of freedom powered by 750 individual muscles. Exact system identification maybe possible but _ is probably unnecessary for use in ship design tools. The simplest model of postural stability is a rigid body model similar to the size and shape of the human body. Such a model has been suggested by Graham et al. [11] for predicting the incidence of loss of balance ('motion-induced interruptions') as a function of lateral acceleration. The definition of a motion-induced interruption (Mil) is an incident where ship motions become sufficiently large to cause a person to slide or lose balance unless they temporarily abandon their allotted task to pay attention to keeping upright. This model has become widely used in both the naval and commercial ship design community, for example Crossland [6] and Crossland and Johnson [7]. In general terms, the Mil model predicts that a person will lose balance during a simple gross motor task when the accelerations experienced by that person exceed a threshold. In ship design tools the results are usually expressed as the number of Mils per minute [ 10]. This Mil rate can then be compared with pre-defined Mil criteria to assess the effect of the motion on the individual. The Mil model treats the person as a passive rigid body, unable to react to the ship motion environment. The full formulation of the equation for Mils is given by Graham [10]. However, Mil can be derived from two LFE (Lateral Force Estimator, Baitis et al. [2]) functions (representing falls to left and falls to the right) given below. Using the notation and sign convention adopted in [10] , a tip will occur if:

l " h

1 " ) - D 2 - gry4 ( -hij4 3

l h

-D3 > - g

(I)

or

(-

.. 3lh'r/4 +

D

2

+

)

g'r]4

-

hl D.. 3 > hl g

(2)

P. Crossland eta!. I Motion-induced interruptions aboard ship

185

0

,.'"'\ )~ ~

I d F ~c u:,e

All:

Fig. I. Mil model.

Where is the roll angle i74 is the roll acceleration is the ratio of half stance width over height of COG g is the acceleration due to gravity D3 is the vertical acceleration at the subject's COG b 2 is the lateral acceleration at the subject's COG In effect, the acceleration threshold, above which postural control problems will manifest themselves, is detennined by the ratio of half stance width over vertical height of the centre of gravity (so called tipping coefficients). The experiments described in more detail here, were used to validate the concept of the model and moreover to derive empirical tipping coefficients to reflect the ability of a person to resist ship motion. As mentioned the tipping coefficient is a function of COG height and half the stance or shoe width (i.e. half the length of the base of support). For example, an individual facing athwartships, adopting a natural stance with feet apart but in-line would be less able to resist the lateral accelerations caused by roll motions along their sagittal plane (falling forwards or backwards) than if that individual faced longitudinally and experienced the same accelerations along their coronal plane (falling left or right). Also, for equal stance width, a short individual would be more stable than a taller one. In ship design tools the tipping coefficients are taken from the geometric consideration of the model described in Fig. 1. This means, for example, that for the average person, that the tipping coefficient for falling left or right is 0.25 and falling forwards or backwards is 0.17. Simulator experiments by Crossland and Rich [8] have indicated that the model can predict Mils, but that Mils do not always occur when predicted by the model. Human subjects are not rigid and can in some cases anticipate the motion and utilise both ankle and knee strategies to resist de-stabilising forces, Pykko et al. [13]. In real shipboard tasks, the height of the body COG will depend on the working posture. In the case of dynamic tasks involving gross body movement, the location of the COG will not even be fixed. Together with the aforementioned problem of active postural control, it can be seen that the simple mechanical model will tend to overestimate the number of Mils that will occur in many situations. Consistent with all postural instability models, regardless of their complexity, is the fact that the probability of falling will depend, to a very large extent, on the ratio of lateral and vertical accelerations 7]4

t

186

P Crossland eta!. I Motion-induced interruptions aboard ship

Fig. 2. The RV Triton.

at the subject's centre of mass. This suggests that for ship design purposes, a pragmatic solution is possible. By measuring deck accelerations when Mils occur in the performance of different tasks, threshold of acceleration or empirical tipping coefficients can be developed. This approach has the attraction of dealing with the added complication of changing posture, oscillations in body COG and active postural control. Thus, instead of attempting to derive tipping coefficients for complex tasks using mathematical models, Mils are counted under known conditions of deck motion and the values of the tipping coefficients are estimated from deck acceleration data. In order to calculate the coefficients, deck accelerations have to be measured at the exact time the Mil occurred. These empirical values of tipping coefficients can then be implemented into design tools that predict the motion of a ship in a sea way and from these predicted motions, estimates of Mil incidence can be derived. Further simulator work by Crossland and Rich [9] demonstrated that empirical tipping coefficients give better predicitions of Mil incidence. The overall aim of the trial reported in this paper was to assess the effects of ship motion on postural stability during task performance. This was done by quantifying Mil incidence at sea over a period of several days on board a fully instrumented vessel (Fig. 2).

2. Method 2.1. Trial vessel and ship motion data capture All motion and vibration data relevant to the trial were recorded on the U.S. Trials Instrumentation System (TIS). The TIS was a ship wide computer network that recorded all of the data (environmental conditions, ship condition, motions, local hull strains) onto optical disk. There was a series of tri-axial accelerometers mounted around the ship, Fig. 3 shows their locations. The motions were recorded at 20 Hz, local strains at 200 Hz and slam events logged at 2 kHz. The motion data acquired from the centre of gravity (CG) were used in this study, as this point was in proximity to the areas of the vessel in which the subjects undertook their tasks. The location of the trials laboratory is marked on Fig. 3.

P Crossland eta!. I Motion-induced interruptions aboard ship

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Table 1 Summary of the subjects' anthropometric data Mean Height (em) Mass (kg) Shoe Length (em) Shoe width (em)

179.2 71.4 31.13 11 .00

Standard deviation 11.1 11.3 2.6 0.8

Fi g. 3. Accelerometer positions and location of the trials laboratory on the RV Triton.

The trial took place on board RV Triton in January 2001 in slight to moderate seas and again in March 200 I in rough to severe seas. The trial was carried out in the Western Approaches to the English Channel. Due to the limited accommodation on board ship, there were only 5 subjects per voyage.

2.2. Subjects Ten Royal Naval subjects took part in the study, each volunteering and giving their informed, written consent. The study complied at all times with the Declaration of Helsinki, as adopted at the 52 nd WMA General Assembly, Edinburgh, October 2000. Subjects were screened at their parent establishments for fitness to participate, that is P2 Fit for Full Duties. Following this they completed a medical questionnaire and underwent a full medical examination (including 12-lead ECG) before the trial. The medical questionnaire included items related to middle or inner ear problems, vertigo or motion illness susceptibility. It must be noted that any subjects who suffered regularly from motion illness were not considered for the trials. This basic screening and self-reporting of medical problems were intended to exclude subjects with locomotor or neurological dysfunction, low back pain, orthopaedic pathology, middle ear or labyrinthine disease, or recent abdominal surgery. As an additional control an on-board Independent Medical Officer was responsible for ensuring that subjects well-being throughout the trial. Motion illness medication was only permitted in severe cases. The consumption of alcohol during the trial was not permitted. The IMO was present during all trials and the subjects were free to withdraw from the experiment and to return to their bunks for any reason, at any time without penalty. Table 1 gives the statistics of the subject's anthropometry.

188

P Crossland et a/. I Motion-induced interruptions aboard ship

Ship Centreline

Starboard

Port

Fig. 4. Layout of trials lab.

2.3. Tasks Subjects performed a series of tasks (each of 4 minutes duration) based on those used by Crossland and Rich [8]. Task 1. Standing facing aft (SFA). The subject was required to stand along the roll axis of rotation of the ship with the feet spaced approximately shoulder-width apart and not offset. Task 2. Walking a designated track (WA). This task was chosen to investigate the effects of ship motions on walking. Each subject was required to walk athwartships (port/starboard) along a track approximately 4 m long marked out in the trial laboratory. The task was 'self paced' and the subject instructed to walk as smoothly as possible, at their usual walking speed. Task 3. Carrying out a simulated weapon loading task (WL). This task represented the chaff missile-loading task undertaken at sea. Each subject was required to shift two 11.1 kg cylinders from the bottom of the missile rack to the top and then return them to the bottom again. Subjects worked at their own pace. The task was designed to force gross positive repositioning on the subject, in order to stimulate Mils. Task 4. Standing facing athwartships (SFX). This task was chosen to validate the longitudinal or body fore-and-aft Mils with the subject facing the pitch axis of rotation . The feet were placed slightly apart (approximately 480 mm, varying slightly according to the subject's preference within a range of 450- 510 mm) and not offset.

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Table 2 Experiment boundaries Motion induced interruptions (/min) Motion sickness incidence (%) RMS lateral acceleration (m/s/s) RMS vertical acceleration (m/s/s)

Insufficient

Excessive

6 > 10

< 0.10 g < 0.05 g

> 0.20 g > 0.15 g

Task 5. Simulated fire fighting task (FF). Each subject was required to stand holding a fire hose and simulate boundary cooling. The simulation required the subject to move the hose nozzle in a figure of eight pattern, as if covering the bulkhead with a spray of water. The experimenter gave instructions for the method of holding the hose and manoeuvring it in the figure of eight. The weight to be supported was approximately 10 kg. There is negligible load to be resisted when operating a hose used for boundary cooling; consequently the trial did not incorporate a method of simulating the force of expelled water. All subjects performed Task 1, one after another. When this task was completed by all, Task 2 was carried out in the same order of subjects. This design has the advantage of maximising the chance that all the subjects carried out the same task in the same motion condition. About 30 minutes were needed for all subjects to complete each task. Interaction effects between tasks were avoided because of the 30 minutes separation between tasks for each subject. The routine was conducted twice a day on a daily basis for the duration of the trial except on day 1, when subjects were given a practice session in calm water. The task duration was four minutes to allow time for a sufficient number of MUs to occur.

2.4. Subjective measures Perceived physical work load, perceived postural instability, and performance and motion sickness incidence symptomatology scales were completed at the beginning of the task sequence to give a baseline for comparison with reports post-exposure. Physical workload was assessed using Borg's Scale of Perceived Exertion, see Borg [3]. Perceived postural sway and instability was assessed using the scale developed by Chiou [4] Motion Sickness Incidence symptomatology and performance was assessed using the Performance Assessment Questionnaire (PAQ) developed by Colwell [5].

2.5. Trial conditions The trials took place in the Trials Instrumentation Laboratory in RV Triton. The severity of the motion responses of Triton was of course dependent on the prevailing weather conditions at the time of the trials. However, a certain amount of control was applied by suitable choice of ship speed/heading combinations and with the use of the roll stabiliser fins. To obtain protocol approval, operational boundaries for trials were set from Triton model test data, previous Mil simulator trials, the Mil predictive model and experience of postural control in a moving environment. These boundaries defined speed/heading and sea state combinations where there are sufficient Mils to be able to undertake statistical analysis feas ible yet not too many to invalidate it or give rise to an unacceptably high risk of injury to personnel. The boundaries are listed below in Table 2. To maintain safety, and to ensure that the tasks could be perfonned to a reasonable level, it was intended to operate within the above boundaries.

190

P Crossland eta/. I Mo tion-induced interruptions aboard ship

Fig. 5. Simulated weapon loading task.

2.6. Apparatus The trials took place in the Trials Instrumentation Laboratory of RV Triton shown in Fig. 3 along with the locations of the tri-axial accelerometer packages. Figure 4 shows the location of the tasks within the trials lab. The static tasks and weapon loading task positions are shown by the cross whilst the walking path is shown with an arrow. The weapon loading equipment position is shown as a rectangle (the weapon loading task is shown in Fig. 5). The rectangle area on the starboard side shows an area closed off with a curtain where one of the experimenters manually recorded the Mils as they occurred using a specially adapted joystick. This joystick allowed the experimenting to mark the occurrence of the Mil, its direction relative to the subject and the duration. The experimenter observed the subject's performance from a monitor connected to a video recorder and camera (the positions of the camera are also indicated in Fig. 3). On viewing an Mil, the experimenter would push the joystick in the pertinent directrion and hold that position until the subject regained their posture. This simple electronic means of capturing the Mil allowed the occurrence to be exactly correlated to the accelerations The same experimenter recorded Mils throughout the entire trial. The box used for the missile loading (see Fig. 5) task was securely fitted to the floor in the trials laboratory. Each subject entered the lab at the start of the session and completed the appropriate questionnaires. This was repeated for all tasks, twice a day.

3. Analysis methods The analysis consisted of an assessment of all Mils in each task and to correlate those Mils with the accelerations that caused them. The Mil information was categorised into two groups per task, i.e. those

191

P Crossland et a/. I Motion-induced interntplions aboard ship Identification of Mil and Source 15

10

5

20

-5

i

i

! !

-10

First Mil

Second Mil

I

Mil Fwd

~ ..... M.II .Le.ft ......~ 2 sees -15

i

i

2 sees

.

Time (sees)

Fig. 6. Identification of Mll and source.

occurring forward/backwards and left/right always relative to the subject. Within these groups the mean values of Mil duration and the tipping coefficients where calculated. For comparison against the earlier simulator experiments these data were summarised per subject for each task. Because of the difficulties in exactly identifying the accelerations that caused the Mil (delays in marking the Mil mainly) the occurrence of Mils was attributed to the maximum acceleration within 2 seconds prior to the event. Furthermore, in order to allow time for the subject to recover from an Mil, Mils occurring within a finite time of each other we regarded as one. To make sure that only valid Mils were identified the following assumptions were made: - The 2 second period after an Mil was considered as the recovery period. - If another Mil occured within the recovery period it was regarded as an extension to the recovery. - This recovery Mil was then be included as part of the initial Mil, hence extending its duration. To illustrate this, Fig. 6, shows by way of an example a sample of Mil gathered data for one subject, the first Mil is quite clear as being in the left direction with a duration of 4 seconds. In the 2 sees following no more Mils occur hence the Mil has ended. The second Mil (which was forward) lasted for 6.5 sees during which multiple Mils occur. On closer inspection it can be seen that an Mil to the right occurs within the 2 sees recovery period of the initial Mil forward and is therefore consider to be part of the recovery period hence not included in the analysis of tipping coefficient. As such the Mil forward is extended to take into account the next two Mils and so the duration of recovery is lengthened. In order to use the acceleration data from the accelerometers on the ship, the data had to be transformed from the measurement position to the ship induced accelerations experienced at the CG (centre of gravity) of the subject in the trials lab. Figure 1 shows that the accelerometer at the CG of the ship is the closest

192

P. Crossland eta/. I Motion-induced interruptions aboard ship

accelerometer to the trials lab, hence represents the most suitable source of data. To obtain the vertical CG of a subject, measurements where taken of their heights. Using this it was found that I m was a good approximation of the vertical distance of the subjects CG to the floor on which they where standing. The accelerations from which the tipping coefficients are derived are dependent upon the subjects' orientation to the ship. Equations (1) and (2) represented acceleration thresholds for task performance, from these tipping coefficients are derived. Due to the nature of the ship motion environment, i.e. forces acting in essentially two directions parallel to the plane of the deck, two tipping coefficients can be derived, a lateral tipping coefficient representing Mils to the left or right and longitudinal tipping coefficients representing Mils forward and backwards. Both these tipping coefficients can be derived from Eqs (1) and (2) and can indeed be calculated as continuous time histories directly from the ship motion data by: Longitudinal tipping coefficient

( ~hc9 B- X cg ) ( Zcg

+g)

Lateral tipping coefficient

( -~hc9 ¢ ( Zcg

- Ycg )

+g)

Where g = acceleration due to gravity in rn/s/s hc9 = height of subject's centre of gravity above the floor in m ~cg = longitudinal acceleration at subjects CG in plane of the deck in rn/s/s ~cg = lateral acceleration at subjects CG in plane of the deck in rn/s/s ~cg = vertical acceleration at subjects CG in rn/s/s ¢ = roll acceleration in rads/s/s = pitch acceleration in rads/s/s

e

4. Results and discussion 4.1. Environmental conditions The wave conditions (and hence the ship motion conditions) varied over the duration of the two trials. Table 3 summarises the conditions encountered during each trial. The World Meteorological Organisation (WMO) agreed the standard sea state code that links significant wave height (defined as the mean of the highest third waves) with sea state and a description of the wave environment. This is presented in Table 4. Table 5 summarises the root mean square (RMS) accelerations determined at the COG of the subject during their tasks, where: RMSLFE = root mean square lateral acceleration in a plane parallel to the deck across the ship (port to starboard) for the period when Mils were recorded.

P Crossland et a/. I Motion-induced interruptions aboard ship

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Table 3 Wave and ship motion conditions for both trials Parameter Significant wave height (m) Wave period (sees) Heading to waves Ship speed (knots) RMS roll (deg) RMS pitch (deg) RMS heave acceleration (m.s- 2 ) RMS surge acceleration (m.s- 2 ) RMS sway acceleration (m.s- 2 )

Trial! 1.5-2.8 10 Beam 5- 8 4 0.8 0.03 0.006 0.07

Trial2 2-7 10-12 Beam, bow 4--16 6 2.5 0.09 0.018 0.115

Table 4 World Meteorological Organisation sea state code Sea state

0 I 2 3 4

Mean significant wave height (metres) 0 0.05 0.3 0.875 1.875

5 6

3.25 5.0

7

7.5

8

11.5

9

over 14.0

Description Sea surface like a mirror Ripples Wavelets - foam of glassy appearance Small waves, becoming longer; fairly frequent white horses. Moderate waves, taking longer form; many white horses and chance of spray. Large waves, extensive white foam crests and probably spray Moderately high waves, crests begin to break. Foam is blown along in streaks Very high waves with long overhanging crests. The surface takes on a white appearance. Visibility affected Exceptionally high waves. Sea is completely covered in long white patches of foam. The wave crests are blown into froth The air is filled with foam and spray. Sea completely white with driving spray. Visibility seriously affected.

RMSLOFE = root mean square longitudinal acceleration in a plane parallel to the deck along the ship (stem to bow) for the period when Mils were recorded. RMSVFE = root mean square vertical acceleration in a plane perpendicular to the deck for the period when Mils were recorded. Due to calm conditions in Jan, there were only 4 experimental sessions in which Mils were counted. In March, conditions were rougher and more variable, enabling Mils to be counted over 7 experimental sessions.

4.2. Tipping coefficients The average tipping coefficient for each subject and each task is shown in Table 6 along with the estimated standard error. For ease of interpretation the lower the tipping coefficient, the lower the threshold of accelerations which can be tolerated for that task, i.e. the task that are more susceptible to motion will have a lower tipping coefficient. In the first instance the tipping coefficients compared very well with those previously by Crossland and Rich [8]. For the tasks, that were replicated from these experiments i.e, the standing facing aft and athwartships tasks, the similarity between tipping coefficient was expected. The general frequency content of the motions from this trial and from the simulator studies are similar - the differences in tipping coefficient in the trials and in the simulator experiments is attributable to the subject differences.

194

P Crossland et a!. I Motion-induced interruptions aboard ship Table 5 Mean (and range) RMS Accelerations for Jan 01 (Group 1) and March 01 (Group 2) Voyages on RV Triton Group I* Group 2**

RMSLFE (g) 0.072 (0.057-0.082) 0.092 (0.041 - 0. 138)

RMSLOFE (g) 0.0023 (0.00 16--0.0033) 0.0076 (0.0035-0.0 187)

RMSVFE(g) 0.029 (0.025- 0.034) 0.064 (0.023- 0.1 04)

*Recorded over 4 experimental sessions. **Recorded over 7 experimental sessions. Table 6 Tipping coefficients- Mean values (and ESE) Task number I 2 3 4 5

I 2 3 4 5

2 0.212 0.152 (0.011) 0.124 (0.011) 0.137 (0.005) 0. 124 (0.005)

0.292 0.155 0.165 0.134 0.165

I (0.016) (0.009) (0.061) (0.007) (0.021)

Subject number (trial I) 3 4

0.149 (0.007)

0.173 (0.008)

0.150 (0.007) 0.197 (0.031)

0.147 (0.005) 0.206 (0.003)

2 0.264 (0.023) 0.190 (0.010) 0.166 (0.0 13) 0.133 (0.005) 0.159 (0.0 I0)

0.156 (0.022) 0.101 0.146 (0.010) 0.166 (0.010)

Subject number (trial 2) 4 3 0.314 (0.0 19) 0.326 (0.020) 0.171 (0.0 I 0) 0.166 (0.010) 0.176 (0.014) 0.169 (0.011) 0.151 (0.008) 0.146 (0.006) 0.182 (0.021) 0.219 (0.033)

5 0.234 (0.0 II) 0.156 (0.016) 0.146 (0.006) 0.168 (0.0 12)

Mean 0.223 0.157 0.113 0.145 0.172

5 (0.046) (0.013) (0.0 17) (0.006) (0.009)

Mean 0.294 0.172 0.168 0.138 0.175

0.274 0.178 0.163 0.128 0.151

For any individual on any one task, the above mean tipping coefficient is an average value taken from all of the Mils that were experienced during the trials.

4.3. Mil duration Within this trial the maximum Mil duration was as long as 22.5 seconds. This occurred during the second trial whilst the subject was undertaking the fire fighting task. Given the severity of the weather in this trial and the unbalancing weight of the fire hose the subjects sometimes had great difficulty recovering. The shortest Mil recorded was for 0.1 seconds represented a very short momentary loss of balance which the subject quickly recovered from. Table 7 shows the mean duration of Mils for each subject and task. These values are generally lower than the times derived previous data taken from previous experiments Crossland and Rich [9]. However, the analysis methods employed were different. Mil durations derived from the simulator trial were timed using data from a set of force plates - the assumption being that an Mil duration is the time taken when the subject takes at least one foot off the force plate until it is returned. In these Triton trials, in addition to using the joystick to record the incidence of the Mils, it was used to record the duration also. The use of an Mil observer meant that Mil could be accounted for the full range of tasks considered which would not be practicable with force plates.

4.4. Mil rate The mean Mil rate can be calculated by taking the number of Mils experienced and the time of exposure for each subject and task combinations. These results are shown in Table 8. The average

P. Crossland et a!. I Motion-induced interruptions aboard ship

195

Table 7 Mean Mil duration (sees) Task I 2 3 4 5

I 2 3 4 5

1.80 0.59 0.89 3.12 3.23

2.05 1.87 0.65 3.96 3.54

number 2 1.30 0.68 1.05 4.28 2.13

Subject number 4 3 1.55 0.64 0.77 0.97 4.34 3.81 1.73 3.31

(trial I) Mean 5 0.61

0.66

3.24 1.93

3.76 2.47

Subject number (trial 2) 4 5 2 3 1.80 2.13 4.14 2.61 1.48 1.55 1.40 1.50 0.86 0.67 0.52 0.72 2.34 2.42 2.24 3.36 1.45 1.19 2.46 2.64

Mean 2.55 1.56 0.68 2.86 2.26

Table 8 Mean Mil rate (MIUmin) Task number I 2 3 4 5

I 2 3 4 5

0.250 0.625 1.875 1.625 4.500

Subject number (trial I) 4 5 3 0.500 0.500 0.625 0.250 0.250 0.250 1.438 1.438 1.500 2.563 0.333 0.625 0.813 0.500

Mean 0.375 0.450 1.063 1.713 1.354

0.917 0.875 0.625 4.150 1.100

Subject number (trial 2 4 3 0.438 0.250 0.375 0.667 0.375 0.850 1.9 17 1.000 0.375 4.250 2.750 3.536 1.714 0.786 0.417

Mean 0.571 0.628 1.033 3.766 1.332

I

2

2) 5 0.875 0.375 1.250 4.143 2.643

number of Mils per minute for each task differs greatly between trial 1 and trial 2. However, this result was expected as the sea environment was far more provocative during in trial 2 than trial 1. Of course, each trial had different subjects (albeit all Royal Navy Personnel of similar age and fitness) but this is believed to have a minor effect on the Mils per minute compared to the differences associated with the rougher seas. In the case of the weapon loading and fire fighting tasks the Mils per minute are similar over both trials. This is as the subjects where allowed to adopt a solid stance to counter act the weight of the shells or hose. As mentioned before an Mil recovery from such a (task) stance tends to take quite a long time. In addition subjects were allowed to adapt their pace to the ship motions for these two tasks.

5. Discussion 5.1. Choice of tipping coefficient For illustrative purposes, Fig. 7 shows how the tipping coefficient affects the predicted Mils at the flight deck of a ship in beam seas at 10 knots. The horizontal line indicates typical Mil criteria of 1

196

P. Crossland et a/. I Motion-induced interruptions aboard ship

10 knots, Sea state 5, Beam seas Cl

!l1~---.-~----~~=---,-~ 0.1

0.15

0.2

0.25

0.3

0.35

Tipping coefficient

Fig. 7. Variation in Mils with tipping coefficient.

Mil per minute. There is a clear exponential decrease in Mils as the tipping coefficient increases, which reflects the Mil formulation . This emphasises how important it is to have the tipping coefficient correct in the Mil model. For general ship design, Crossland and Rich [8] recommended that only transverse tipping (side to side) should be considered because people, where possible, will naturally adopt this most resistant stance by standing sideways to the predominant motion. In some ship design scenarios a more detailed task analysis maybe desired especially if the person is not free to adopt the best stance. In this case a more complex approach is required using a selection of the tipping coefficients listed in Table 6. For example, if a task consists largely of manual lifting, this could be represented by the weapon-loading task and the appropriate value for the tipping coefficient is chosen. If a complex operation, such as attaching the helicopter haul-down cable, consists of a collection of simpler tasks, say, walking, standing, and a manual type task, then the choice of tipping coefficient is not so clear. The problem was approached with a time domain simulation by Baitis et al. [2] where the lateral accelerations were compared with a threshold (or tipping coefficient) that varied with time depending on which part of the operation was being undertake at the time. This approach is not conducive to standard ship design tools, which are generally frequency domain analysis programs. So, in this case a weighted tipping coefficient could be used where the weighting function is derived from the proportion of time that each of the individual tasks form the make up of the complex operation. In some circumstances a shipboard operation may not be represented as a function of the simple tasks tested in these simulator experiments. In this case, careful consideration should be given to the possibility of performing further Mil experiments using more appropriate tasks.

5.2. Task effectiveness It is conjectured that effectiveness can be derived by calculating the Mils for a specific task. In this context task effectiveness ETASK can be defined as the ratio of the time to complete the task under calm conditions T CALM with the time for task completion during rough weather TwAVES That is

TcALM TwAvEs Now, a person would experience Mil RATE per minute when undertaking a specific task in a moving environment with each Mil taking D M I I seconds to recover from . A relationship between task effectiveness, Mil rate and Mil duration derived by Crossland and Rich [9] is given as:

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Table 9 Mil risk level Risk Level

Mils per minute

I. Possible 2. Probable Typical Mil criterion 3. Serious 4. Severe 5. Extreme

0. 1 0.5 1.0 1.5 3.0 5.0

if M lhATE D MII 1 60 < The task will not be completed if

M JIBAlo ED M u

~ 1, i.e. ETASK = 0

Or alternatively for a given level of acceptable task effectiveness an Mil criterion can be derived. M llc RIT =

(

1

- ETASK ) 60 D MII

5.3. Mil criteria The Mil model allows that shipboard operations criteria can be expressed as a limit on the number of Mils that can be tolerated during a specific task. Mils per minute were presented by Graham [10] as a convenient unit to express performance criteria for deck operations. The reference presented preliminary values as shown in Table 9. Also shown in Table 9 is a value of 1 Mil per minute which represents a typical Mil criterion for ship design use. These risk levels were derived from a detailed analysis of the initial hook-up of the helicopter message line to the arresting system on the deck. This task was considered to represent a potential crew safety problem due to the lack of balance produced by the moving deck and wind. Simulated ship motions were used with a computer program simulating members of the crew performing this task. Changes in stance position on the flight deck and the shifting of the crews' centre of gravity were modelled in an imprecise way. This combined model was used to determine the relationship between ship motions and the incidence and severity of Mils during this 50-second helicopter recovery task. Mil limits were applied for a number of levels of risk. The first level corresponds to the occasional, or possible, occurrence of an Mil; the second corresponds to a probable occurrence of an Mil. Subsequent levels are a "serious Mil problem", "severe limitations" and "extremely hazardous" respectively. The last three levels represent conditions of ever increasing impact on safety to the crew. Crossland and Rich [9] related Mil rates to task effectiveness in using the equations derived above. To give some indication of the severity of the motion experience during the Triton trials (especially trial 2) the measured Mil rates (Table 8) can be compared with the levels and descriptors in Table 9. It can be seen in the 2 nd trial that the Mil rates associated with standing facing aft (task 1), walking (task 2) and fire fighting (task 5) tasks suggest that there is a probable risk of an Mil (one every two minutes). This means there would be no serious degradation of performance of these tasks due to the ship motion. The weapon loading task (task 3) has about 1 Mil per minute in both trials which is the criterion level for helicopter operations. For such a task this could present performance related problems. The only task to be seriously affected is standing facing athwartships (task 4) which incurred a severe risk level. As such, tasks on board Triton requiring this stance will suffer a large performance degradation in these sea conditions.

P. Crossland et a/. I Motion-induced interruptions aboard ship

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5.4. Subjective measures Neither the Borg or postural stability scales showed significant variation with motion conditions. This was because the data were range limited. All the task were rated as 'light' and motion had little effect on these ratings. This accords with the findings by Wertheim [14] that although working in a motion environment is more physically demanding than working in stable environment, the increase in energy expenditure is not large. Due to the prevailing sea conditions at the time of the trial, ship motions did not vary greatly during either voyage.

6. Conclusions The results reported here compare well with the motion simulator studies lending confidence that the Mil prediction methods used in this study are valid for application to ship design. The paper presents further tipping coefficients that can be used in traditional ship design tools. These tipping coefficients vary from 0.113 to 0.294 and give very different predictions of Mil incidence than coefficients derived from models. This paper has presented an approach for determining the lateral accelerations required to cause an Mil that has lead to the derivation of tipping coefficients for real people. Further analysis has led to the ability to quantify the length of time it takes to recover from an Mil. Both these techniques enable the derivation of task effectiveness and the development of rational Mil criteria which, given a validated Mil model, are key to use in ship design tools.

Acknowledgements The authors acknowledge the UK Defence Procurement Agency and the US Department of Defense for their financial support in undertaking these trials. The authors would like to acknowledge the assistance provided by the officers and crew ofRV TRITON. The highly dedicated Trials Instrumentation System team assisting the authors in their efforts to collate the relevant data. Moreover, thanks goes to the Royal Naval volunteers for their enthusiastic participation in sometimes difficult conditions.

References [I]

[2) [3)

[4) [5) [6)

ABCD Working Goup, Generating and Using Human Performance Simulation Data to Guide Designers and Operators ofNavy Ships: Two Large Multinational Programs, RINA International Conference on Seakeeping and Weather. March 1995, London. A.E. Baitis, T.R. Applebee and T.M. McNamara, Human factors considerations applied to operations of the FFG8 and LAMPS MKIII, Naval Engineers Journa/97(4) (May 1984). G.A.V. Borg, A category scale with ratio properties for intermodal and inter-individual comparisons, in: Psychophysical Judgement and the Process of Perception, H.G. Geissler and P. Petzold, eds, 1983, Berlin: VEB Deutscher Verlag Der Wissenschaften. S. Chiou, Effects of environmental and task risk factors on worker's perceived sense of postural sway and instability, Occupational Ergonomics 1 (1998), 81 - 93. J.L. Colwell, NATO questionnaire: Correlation Between Ship Motions, Fatigue, Sea Sickness and Naval Task Pelformance, RINA conference on Human Factors in Ship Design and Operation. London, 2000. P. Crossland and K.J.N.C. Rich, A Method for Deriving Mil Criteria, RINA conference on Human Factors in Ship Design and Operation. London, 2000.

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P. Crossland and M.C. Johnson, Developing Seakeeping Criteria fora Helicopter Training Vessel, 2001 Eight International Symposium on Practical Design of Ships and other Floating Structures. Shanghai, China. P. Crossland and K.J.N.C. Rich, Validating a Model of the Effects of Ship Motion on Postural Stability, 1998, The 8th International Conference on Environmental Ergonomics, San Diego, USA. P. Crossland, The influence of ship motion induced lateral acceleration on walking speed. Proceedings of the 2nd International Conference on Pedestrian and Evacuation Dynamics 2003, E.R. Galea, ed., Published by CMS Press. ISBN 1-904521-08-8 . R. Graham, Motion-induced interruptions as ship operability criteria, Naval Engineers Journal, March 1990. R. Graham, A.E. Baitis and W.G. Meyers, On the development of seakeeping criteria, Naval Engineers Journal (May 1992). NG/6 on Ship Design, Specialist Team on Seakeeping. Compiled by P Crossland. A rational approach to specifying seakeeping performance in the ship design process. RINA international conference WARSHIP '98. June 1998, London. I. Pykko, P. Jantii and H. Altto, Postural Control in Elderly Subjects, Age and Ageing 19 (1990), 215-22 1. A.H. Wertheim, Working in a moving environment, Ergonomics, 1998. © QinetiQ Ltd,© Crown Copyright 2007.