An Empirical Method for the Estimation of Resistance of a Life Raft in Various Sea States

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TypeTechnical Report
Series titleTechnical Report; no. TR-2006-01
SubjectLife raft; tow; force; waves; wind
AbstractCurrent IMO regulations require life rafts to be tow tested only in calm water. In real evacuation situations, life rafts are deployed in the prevailing environmental conditions, with wind and waves. Added wave resistance is small at low wave heights but increases non-linearly with increased wave height. If life rafts are to be towed in moderate seas (up to 4 m significant wave height), tow force estimates based only on calm water tow resistance become less reliable. Tow patches, towline, towing craft etc. also need to be designed to withstand dynamic wave loading in addition to mean load. Therefore, mean tow force, tow force variation and maximum tow force are important. A full-scale 16-person, commercially available, SOLAS approved life raft was towed in the tank, in upwind, head seas with significant wave height of 0.5 m. The measured tow force showed that it could be treated as a linear system with wave amplitude, by demonstrating that tow force is mainly inertial and follows a Rayleigh distribution. Therefore, extreme-value statistics used for waves can be applied to developing equations for predicting tow force. A methodology is proposed to predict life raft tow force at different tow speeds and in various sea states, with waves and wind. The methodology involved using tank experiments to obtain tow force response for one sea state. The information can then be used to predict life raft tow force in wind and waves for different sea states. Three equations are proposed to demonstrate that a simple tank experiment could provide valuable information necessary to empirically estimate the mean tow force, tow force variation and maximum tow force for a specific life raft in different sea states. The equations are developed for upwind, head seas. These equations were extensively validated using tow force measured in the tank. They were partially validated with limited sea trial data, by towing the same 16-person life raft and a 42-person life raft in upwind, head seas with significant wave height of 1.3 m. The equations were able to predict maximum tow forces to within 15% of the measured.
Publication date
PublisherNational Research Council Canada. Institute for Ocean Technology
PlaceSt. John's, NL
AffiliationNRC Institute for Ocean Technology; National Research Council Canada
Peer reviewedNo
NRC number6309
NPARC number8894990
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Record identifieraf1c6aec-410b-4bc0-8b9c-72f4e80cfdda
Record created2009-04-22
Record modified2016-10-03
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