EXTREME MOORING CABLE TENSIONS DUE TO WAVE-FREQUENCY EXCITATIONS

The mooring cables of floating platforms respond non-linearly to the f airlead motions. Even though the wave-frequency excitation can be assu med to be a Gaussian process, the cable tension generally is not. Indu stry regulations prescribe calculation of maximum wave-frequency cable tension to obtain the total design value. However, time-domain calcul ations show that various realised time records of excitation with the same spectrum can result in big differences in simulated maximum cable tensions. Therefore, it is necessary to make many simulations or to p erform statistical analysis on tension-time history. In this paper, th e cable tension-time histories are determined by nonlinear time-domain dynamic simulation, and then the order statistic theory is applied to determine the expectation of maximum cable tension (extreme tension) corresponding to a specified time period. By doing this, it is shown t hat only one time-domain cable dynamic analysis plus statistical treat ment are necessary to obtain a proper design value, instead of several simulations. The extreme values predicted in this way are also compar ed with those from the Hermite moment-based method. Major parameters s uch as cable slackness, current, excitation amplitudes etc. are varied to study their effects on the asymptotic distribution model and extre me values. The proposed procedures could be applied to other similar p roblems. Research on the extreme values for combined wave- and low-fre quency excitations are continued. (C) 1998 Elsevier Science Ltd. All r ights reserved.