Extreme mooring cable tensions due to wave-frequency excitations

Abstract

The mooring cables of floating platforms respond non-linearly to the fairlead motions. Even though the wave-frequency excitation can be assumed to be a Gaussian process, the cable tension generally is not. Industry regulations prescribe calculation of maximum wave-frequency cable tension to obtain the total design value. However, time-domain calculations show that differently 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 perform statistical analysis on tension–time history. In this paper, the 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 that only one time-domain cable dynamic analysis plus statistical treatment are necessary to obtain a proper design value, instead of several simulations with differently realised excitation samples. The extreme values predicted in this way are also compared with those from the Hermite moment-based method. Major parameters such as cable slackness, current, excitation amplitudes etc. are varied to study their effects on the asymptotic distribution model and extreme values. The proposed procedures could be applied to other similar problems. Research on the extreme values for combined wave- and low-frequency excitations are continued.