Thermal protection in liferafts : assessment of occupant heat balance and development of performance criteria

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TypeTechnical Report
Series titleTechnical Report; no. TR-2009-06
Physical description77 p.
SubjectInflatable life raft; Engineering human factors; Thermal; Protection
AbstractInflatable liferafts are currently used worldwide as a means of evacuation and survival from almost all ocean-going vessels, regardless of their size and purpose. This ranges from fishing and other commercial vessels with small crew sizes to offshore oil installations and passenger ships with thousands of persons onboard. While International Maritime Organization (IMO) standards currently require inflatable life raft components to “provide insulation” or “be sufficiently insulated”, no performance criteria accompany these requirements (IMO, 1996). This report outlines the methodology and results from an engineering and human factors research project which investigated the gaps in knowledge surrounding inflatable liferaft performance with respect to occupant thermal protection in cold environmental conditions. The research objectives of this project were to: 1. Develop thermal protection criteria for inflatable life rafts assuming otherwise unprotected occupants. 2. Propose an objective methodology for testing inflatable life raft thermal protection performance. 3. Develop tools for Search and Rescue (SAR) planners to predict survival times of liferaft occupants. 4. Provide guidance to training authorities and manufacturers. The research was conducted at the National Research Council’s Institute for Ocean Technology in St. John’s, Newfoundland and Labrador, Canada. This world-class facility allowed an interdisciplinary team of researchers to systematically examine the environmental, human and technological factors related to heat exchange within a liferaft-occupant system. This report details experimental protocols, empirical research findings and physical modelling approaches considered over a three-year period to address the project objectives. The project was undertaken in a multi-phased approach. A phase-by-phase summary of the project findings is provided below, followed by overall project findings and recommendations. Phase 1 Phase 1 was designed as a one-week pilot experiment with human subjects, aimed at better understanding the effects and sensitivity of the variables, to observe the rate of occupant heat loss, to validate the proper functioning of equipment and to collect data for preliminary investigation. The primary focus was to assess heat loss from direct contact with the raft floor through conduction. The air temperature and water temperature were 19ºC and 16ºC respectively. Phase 1 results informed the design of Phase 2 and Phase 3 experiments. Phase 1 results summary: · The effect of wave height is less important with leeway and may be ignored as a first approximation to reduce the number of environmental variables. Heat conductance increases non-linearly with leeway but appears to level off above 0.5 m/s leeway speed. Therefore, waves can be eliminated as an independent variable and leeway speed set to 0.5 m/s for Phases 2 and 3. · It was observed during an 11-person test that the CO2 concentration inside the liferaft reached an uncomfortable level (over 5000 ppm) in less than an hour when the canopy was closed and no active ventilation system used. Phase 2 Phase 2 was designed to assess occupant heat loss and life raft thermal protection in mild conditions (19ºC air temperature and 16ºC water temperature) based on the findings of Phase 1, using human subjects. Tests were designed to assess floor insulation (inflated or uninflated) and clothing wetness (dry or wet) in four baseline cases (Phase 2 and again for Phase 3): 1. Inflated raft floor; dry clothing (Idry) 2. Inflated raft floor; wet clothing (Iwet) 3. Uninflated raft floor; dry clothing (Udry) 4. Uninflated raft floor; wet clothing (Uwet) Phase 2 results summary: · During exposure in mild conditions inside an enclosed life raft, the wetness of the clothing worn by the occupants and the absence of floor insulation would both significantly decrease the occupant mean skin temperature. However, only the clothing wetness will increase the heat loss from the subjects. (Uwet = 67.4±11.2 W/m2; Iwet = 64.9±11.0 W/m2 compared to Udry = 55.8±8.9 W/m2; Idry = 52.4±5.1 W/m2). · The thermal stress induced by the different test conditions was not sufficient to significantly and consistently increase the metabolic rate of the occupants through shivering. Despite the mild responses to cold during the exposures (skin temperature above 30°C, Heat loss increased by < 15%; no definitive shivering), rectal temperature significantly decreased for all conditions tested by as much as 1.1°C. · Since there were no noticeable differences in the thermal responses between the conditions involving 2 versus 6 occupants, it was not apparent that heat generated from multiple occupants helped to reduce heat loss. The effect of multiple occupants was further assessed in Phase 3. · To maintain CO2 concentration at a safe level during the trials (< 1000 ppm), a constant flow of fresh air was added to the liferaft. The flow rate of fresh air was 19 l/sec and 38 l/sec for two and six human subjects respectively. Following the analysis of Phase 2 data, it was determined that the ventilation rate produced by the wind fans alone was sufficient to prevent significant CO2 build-up. As a result, no active ventilation was used for Phase 3. · It was necessary to assess if the rectal temperature is a true indicator of the core body temperature when localized cooling is taking place around the buttocks. To this end, a secondary experiment was carried out which confirmed that prolonged lower body surface cooling resulted in a localized cooling effect that compromises the validity of the rectal temperature as a core temperature index. There was no difference in measurement between tympanic and esophageal probes throughout the different phases of the experiment. The use of tympanic probes in the liferaft setting was verified and tympanic probes were added to Phase 3 human subject testing. · It was determined to be desirable to have test conditions that can induce shivering, so as to assess if the heat loss can be offset by the heat produced. In addition, longer test duration would help to determine if the body heat storage decreased over time. All of these issues were addressed in Phase 3. Phase 3 Phase 3 was designed to assess occupant heat loss and life raft thermal protection in cold conditions (5ºC air temperature and 5ºC water temperature). In addition to assessing the liferaft system thermal protection, the data collected was used to develop an occupant heat loss model which interfaced with Cold Exposure Survival Model (CESM) to predict survival time. In Phase 3 testing, both human subjects and a thermal manikin were employed. Phase 3 results summary: · Manikin measurements of the thermal insulation of a combined system of clothing and liferaft give good agreement with measurements on humans. · Two repeatability tests conducted demonstrated that the thermal manikin results are repeatable: 0.177 (m2°C)/W versus 0.171 (m2°C)/W in the Udry baseline case; and 0.101 (m2°C)/W versus 0.104 (m2°C)/W in the Uwet baseline case. · Closed cell foam floor insulation provides a comparable level of insulation to that provided by liferafts with an inflatable floor. The Idry baseline case (inflatable floor) has an insulation value of 0.236 (m2°C)/W compared to 0.221 (m2°C)/W and 0.236 (m2°C)/W for closed cell foam floors provided by two different liferaft manufacturers. · A thermal protective aid (TPA) provides considerable additional insulation compared to all baseline cases. The insulation increases most considerably in wet clothing cases (61% and 54% in Iwet and Uwet cases respectively). · TPA provides more insulation than a wet suit in all cases except Udry. · For Idry, the best scenario, the insulation obtained by sitting on an inflatable pillow (0.243 (m2°C)/W) or a lifejacket (0.241 (m2°C)/W) is comparable to sitting directly on the inflated floor (0.236 (m2°C)/W) or closed cell foam floor (0.236 (m2°C)/W). · For Uwet, the worst scenario, the insulation obtained by sitting on a lifejacket (0.149 (m2°C)/W) is less than wearing a TPA (0.158 (m2°C)/W). However, both are better than sitting directly on an uninflated floor (0.104 (m2°C)/W) or a closed cell foam floor (0.129 (m2°C)/W). · There is a significant decrease in insulation value sitting in 10 cm of water (0.05 (m2°C)/W). · A special case test comparing one instrumented primary subject in 2-person and 12-person test situations show similar heat loss, indicating that the number of occupants has limited effect in keeping each other warm under the conditions tested. Overview This study addressed the four projects objectives by: 1. Summarizing, in a single graphic, the minimum system insulation required for a survival time or functional time of 36 hours at various temperatures using various raft and personal clothing configurations. 2. Proposing a general method to assess the system thermal insulation, with the liferaft afloat in a pool with turbulent water, typical seating arrangement and realistic amount of water on the raft floor and in the clothing. 3. Demonstrating that the raft occupant heat loss model developed can be used to generate a set of curves showing the calculated survival and functional times for various scenarios. 4. Confirming the importance for raft occupants to stay dry, to provide insulation in the liferaft floor, to control ventilation rate that is adequate for breathing and still allow liferaft internal temperature to rise, and to provide TPA for everyone. Conclusions and Recommendations The main conclusions of the study are: 1. Manikin measurements of the thermal insulation of a combined system of clothing and liferaft give good agreement with measurements on humans. 2. System insulation values coupled with a Cold Exposure Survival Model can be expected to give search and rescue planners reasonable predictions of survival time in liferafts where hypothermia is a limitation. 3. Factors which substantially affect the survival time are: a. Wearing of a TPA b. Clothing wetness c. Liferaft floor insulation d. Liferaft ventilation rate The recommendations for liferaft standards or design are: 1. Liferafts should include a TPA for every occupant 2. Liferafts should include a system to keep the floor dry or enable every occupant to sit above the level of water on the floor. 3. Liferaft floors should be insulated or every occupant should be able to sit on an insulated surface. 4. Liferafts should have a mechanism for controlling ventilation to a level which is adequate for breathing but which will allow the internal temperature to rise.
Publication date
PublisherNational Research Council Canada
PlaceSt. John's NL
AffiliationNRC Institute for Ocean Technology; National Research Council Canada
Peer reviewedNo
NPARC number18227279
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Record identifierdbd6856d-b5f0-4342-9f6e-515ef1ffe86e
Record created2011-07-07
Record modified2016-10-03
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