Abstract

This paper presents an overview of the sensitivity of free-fall lifeboat simulations during free-fall conditions. Using MARIN's time domain XMF framework, simulations are conducted for the VIKING NORSAFE GES50MK3 free-fall lifeboat, which is released from a stationary host. Results are presented for free-fall lifeboat drops in calm water, which are compared to basin test experiments. A good agreement is achieved, indicating that the proposed simulation method is robust. Varying the skid pitch angle shows that pitch angles at water entry exceeding 70° results in lifeboat motions with a negative forward velocity. Lastly, simulations are conducted including wind shielding from a generic FPSO. These results indicate that the wind shielding has a pronounced influence on lifeboat trajectory.

INTRODUCTION
Background

In general two methods can be distinguished for lifeboat deployment: a davit launch with fall wire deployment or a free-fall deployment from an inclined skid. In the last decade extensive research has been executed to predict the hydrodynamic forces and moments on the lifeboat during the impact phase of a skid launch. Model basin tests, Computational Fluid Dynamics (CFD) computations and instrumented full-scale free-fall drops contributed to a better understanding of the physics during the landing. This knowledge forms the basis of the recently developed freefall time domain lifeboat models in MARIN's Extensible Modelling Framework (XMF). This C++ framework is the basis for MARIN's deskand simulator studies.

The developed models benefit from an attractive real-time factor to allow simulation of a large number of events. The derived large set of time signals and statistics is needed to account for a reliable prediction from a statistical point of view. To allow the generation of this dataset, simplifications must be made to obtain the required computational performance compared to CFD-type computations.

During the free-fall phase of a lifeboat the body is subjected to wind loads. This is the only external force acting on the body in air. The magnitude of these wind forces and moments depend on the state of the lifeboat velocity vector and the spatial-temporal wind field in which the lifeboat falls. Up to now the modelling of the environment in the timedomain considers an undisturbed wave and wind field.

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