Abnormal Wave-Induced Load Effects in Ship Structures

Abstract

The paper presents an approach to determine the global load effects induced on shif structures by abnormal, freak, or episodic waves. It refers to the present procedure of determining extreme values of wave-induced responses, including the recent advances of adopting time series of wave elevation as reference design conditions t calculate the wave-induced structural loads on ships in heavy weather. It is show how this procedure can be extended to account for abnormal or episodic waves Reference is made to what is presently known about abnormal or freak waves showing that although it is possible to determine the loads induced by these wave in floating and fixed structures, the present knowledge about the probability of occurrence of these waves is not enough to allow a wave design criterion to be defined in a way consistent with the present probabilistic approaches. However, it is suggested that at the present stage of knowledge it is possible to determine the load induced by abnormal waves similar to ones that have been measured at various ocean locations and that are thus realistic; a method is described to perform such calculations. Although this information cannot replace the wave-induced loads calculated with the presently established procedures, it can serve as guidance for th design. An application example is presented of a containership subjected to a wav trace that includes an episodic wave that was measured during a severe storm in Central North Sea. The measured wave time history is modified in order to investigate the influence of the wave steepness on the induced vertical motions and loads. Th loads induced by the abnormal wave are compared for the first time with extreme values from long-term distributions.