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nonstop 02/2009

Introduction: Evacuation

L ifeboats form one of the key safety ele- ments for the passengers and crew on ships and offshore platforms. To ensure reliable evacuation, lifeboats must not suf- fer any damage when entering the water; they must also leave the danger zone quickly. In addition, the accelerations acting on the oc- cupants should not exceed a defined level over a certain period of time.

Until recently, lifeboats were analysed and optimized solely on the basis of experi- ments. The pressure was measured at a cer- tain number of points distributed over the hull. At the same time, the velocity and ac- celeration was recorded. However, because of the many different sizes of lifeboats and the great diversity of operational conditions that are possible, it is not feasible to analyse all of the possible hull types and situations in this manner. There are several other reasons why the experimental approach is inadequate for optimizing lifeboats:

  • 1.

    Model trials cannot provide complete emu- lation of the real-world situations, since the sizes of the models (approx. 1 :10) and the wave heights (normally less than a metre) are limited. Extrapolating to full scale and design conditions introduces uncertainty into the re- sults.

  • 2.

    Full-scale experiments can only be performed under good weather conditions. In contrast, wave heights of up to 15 m and very strong winds may be used as the basis for design conditions. This discrepancy makes it difficult to evaluate the test data.

  • 3.

    In respect of the falling height, the facilities at model basins are subject to the limitations imposed by the ceiling height of the labora- tory. Likewise, dropping a full-scale lifeboat from a height corresponding to that of the usual launch point is often impracticable or too costly.

  • 4.

    Experiments are suitable for determining the actual loads acting on the structure and the crew, but they do not yield enough informa- tion to allow for an improvement in the de- sign. Consequently, it is important to ascertain the pressure and velocity distribution around the hull during entry into the water and the subsequent immersion period.

Advances in flow simulation (computa- tional fluid dynamics – CFD) now permit sim- ulations of lifeboat launches at full scale and under realistic initial and boundary condi-

Overlapping Meshes

STRESS. Free surface and pressure on lifeboat wall during water entry (50 ms between frames).

tions. Moreover, simulation makes it possible to examine the effects of hull design chang- es without having to produce physical mod- els. This approach allows designers to investi- gate a larger number of design parameters for a whole spectrum of operational conditions and to find a design that is optimized for the expected purpose.

Overlapping Meshes

The method described here uses the latest CFD software from CD-adapco. It is linked to a CAD tool and uses a solver for calculat- ing the boat movements with six degrees of freedom. For computation of the flow, a finite volume method is used that employs unstruc- tured meshes with arbitrary polyhedral con- trol volumes. The water flow as well as the

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