Use of risk analysis to evaluate the seaworthiness of offshore structures

The evaluation of ·risk’ is based on long-term cumulative distributions of such design parameters which influence the seaworthiness of offshore structures. It is shown how risk distribution functions can be derived from cumulative distribution functions and how safe design values are defined with regard to specified risk levels. The examples given include two most modern designs which have already obtained a quality assurance at Germanischer Lloyd: a semi-submersible LNG-liquefaction platform with a built-in concrete LNG-tank and a semi-submersible drilling platform. Special attention is given to the question of reasonable platform clearance above still water level as specified by the Classification Societies, as well as to the definition of safe design values of loads which occur independently but have to be combined in a structural analysis. Finally, safety factors are critically examined in light of risk analysis and a basic design principle is suggested.