Analysis of wave induced loads on a FPSO due to abnormal waves

Thus various hydrodynamic methods were developed and calibrated with experiments and various statistical descriptions of climatic wave variability were developed. Methods for long term estimation of the design loads were developed in which the information about the probability of occurrence of the sea states was combined with the ship response to each of these excitations. The paper presents an analysis of structural design wave loads on a FPSO. The vertical bending moment at midship induced by rogue waves are compared with rule values. The loads induced by deterministic rogue waves were both measured in a seakeeping tank and calculated by an advanced time domain method. Analysis of accidents indicated that in several cases the existence of a specific wave with very extreme proportions was critical and often responsible for major damage or even loss of the structure. Suggestions have also been made that the approach to assess design loads should be revised and that design situations should be based on the survival to the so-called freak, rogue or abnormal waves (Faulkner, and Buckley, 1997). Two procedures are used to calculate the expected extreme vertical bending moment during the operational lifetime of the ship. The first one relies on a standard linear long term prediction method, which results from the summation of short term distribution of maxima weighted by their probability of occurrence. The short term stationary seastates are represented by energy spectra and the ship responses by linear transfer functions. This has led to queries about the nature of these waves and about their predictability, both in terms of probability and of the geometric characteristics of their profile (Guedes Soares et al., 2003). The second procedure to calculate the design wave loads is a generalisation of the former, however it accounts for the nonlinearity of the vertical bending moment. It uses nonlinear transfer functions of the bending moment sagging peaks which depend of the wave height. Techniques have also been improved to represent the large waves in model testing tanks and to include them in sequences of random waves (Clauss et al., 2004a). So, being possible to reproduce these situations in tank and being able to calculate them with numerical methods, it is natural to conduct models tests to validate the calculations against test data.

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