Offshore structures are exposed to random wave loading in the ocean environment and hence the probability distribution of the extreme values of their response to wave loading is required for their safe and economical design. Due to nonlinearity of the drag component of Morison's wave loading and also due to intermittency of wave loading on members in the splash zone, the response is often non-Gaussian; therefore, simple techniques for derivation of the probability distribution of extreme responses are not available. However, it has recently been shown that the short-term response of an offshore structure exposed to Morison wave loading can be approximated by the response of an equivalent finite-memory nonlinear system (FMNS). Previous investigation shows that the developed FMNS models reduce the computational effort but the predictions are not very good for low intensity sea states. Therefore, to overcome this deficiency, a modified version of FMNS models is referred to as MFMNS models is used to determine the extreme response values which improves the accuracy but is computationally less efficient than FMNS models. In this paper, the 100-year responses derived from the long-term probability distribution of the extreme responses from MFMNS and FMNS models are compared with corresponding distributions from the CTS method is investigated with the effect of current to establish their level of accuracy. The methodology for derivation of the long-term distribution of extreme responses (and the evaluation of 100-year responses) is discussed. The accuracy of the predictions of the 100-year responses from MFMNS and FMNS models will then be investigated.


For an offshore structure, wind, wave and gravitational forces are all important sources of loading. The dominant load, however, is normally due to wind-generated random waves. Probabilistic properties of the loading and the resulting responses are therefore required for risk-based design of these structures. The major obstacle in the probabilistic analysis of the response due to wave and current loading, is the nonlinearity of the drag component of Morison's wave loading (Morison, J.R. et al., 1950) which results in non-Gaussian probability distributions for both loading and response (Borgman, L.E., 1967; Tickell, R.G., 1977; Burrows, R., 1979 and Eatock Taylor, R. et al., 1981). The problem is further compounded by current and by intermittent loading on members in the splash zone, which have a significant effect on the statistical properties of response (Tung, C.C., 1995 and Liaw, C.Y. et al., 2003).

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