Once it has been released from its mother vessel, the escape of a free-fall lifeboat relies solely on gravity. This very feature contributes to make free-fall lifeboats a fast and reliable evacuation system. However, in rough sea conditions, acceleration loads on the passengers during water impact can cause severe injuries. MARINTEK has been studying this safety issue for several years, using a combination of experimental and theoretical methods.
A complex slamming situation
Under a contract with the Norwegian Oil Industry Association, MARINTEK carried out an extensive series of experiments on free-fall lifeboats, on both model and full scale. The experiments aimed to characterize acceleration levels on lifeboat occupants during water entry, pressure loads on the hull, and forward speed after water exit. Acceleration loads during impact in waves were found to be much higher than in calm water, and often beyond the limits imposed by current IMO regulations. Accelerations were also found to depend on a wide range of parameters: hull shape, mass distribution, wave heading relative to the lifeboat, and the point of impact on the wave surface.
Development and validation of a theoretical model
Under these conditions, assessing the operational limits of a free-fall lifeboat in terms of significant wave height would have been costly and time-consuming if it had been carried out by model testing alone. In collaboration with NTNU, therefore, MARINTEK developed a theoretical method of predicting three-dimensional motions of a body impacting a wave surface. The description of “slamming” is based on conservation of momentum, combined with a von Karman approach, meaning that the water rising close to the hull is neglected. Body motions are described in terms of six degrees of freedom and wave kinematics is described in three dimensions. This allows any type of impact in waves to be described. This theoretical model has been implemented in a free-fall lifeboat simulator and validated by comparing predicted accelerations at impact to experimental data. About 300 cases with different types of lifeboats in various wave conditions were used in this validation phase.
Reassessing operational limits of 16 types of lifeboats
The simulator has then been used to re-evaluate the operational limits of 16 different types of free-fall lifeboats installed on the Norwegian continental shelf. The efficiency of the software and the use of a high-performance computer cluster made it possible to carry out the whole project in less than seven months. Statistical estimates of the severity of impacts were obtained for a wide range of sea states. Selected impact situations were studied in detail by specialists in injury-biomechanics. The risk of injury was assessed here by means of numerical simulations, which were validated by full-scale and laboratory trials using instrumented dummies.
Towards new regulations and new lifeboats
MARINTEK experts have recently been involved in the development of recommended practice for the design of free-fall lifeboats. New contracts have also been awarded for the verification of novel designs. A similar approach that combines experiments and numerical simulations will be used.
Published in MARINTEK Review No 1 - April 2009.