Theoretical Study of the Water Entry of a Body in Waves: Application to Safety of Occupants in Free-Fall Lifeboats

The safety of occupants in free-fall lifeboats (FFL) during water impact is addressed. The first part of the paper describes a theoretical method developed to predict the trajectory in six degrees of freedom of a body entering water waves. Slamming forces and moments are computed, based on momentum conservation, long wave approximation and a von Karman type of approach. The added mass matrix of the body is evaluated for impact conditions by a boundary element method. The second part of the paper focuses on the application of the method to free-fall lifeboats, which are used for emergency evacuation of oil platforms or ships. Acceleration loads on FFL occupants during water impact are dependent on numerous parameters, especially the hull shape, the mass distribution, the wave heading relative to the lifeboat, and the impact point on the wave surface. Assessing operational limits of FFL by means of model tests only has therefore been costly and time consuming. This issue is addressed here by applying the theoretical method described in the first part. The model has been validated for FFL through extensive model testing in calm water and regular waves, and statistical estimates of acceleration levels for lifeboat occupants, as well as acceleration time series were obtained that can be used as inputs to numerical human response models.