Stochastic extreme motion analysis of jack-up responses during wet towing

Abstract

This paper aims to perform stochastic motion analysis and the system probability estimation of exceedance in motion of a jack-up unit in wet tow mode to assess its safety in terms of its translational and rotational motions. When a jack-up unit is afloat and towed in seaways, it is important to determine the extreme motion response for the stability of the platform and to ensure the safety of equipment on the deck. This paper discusses the hydrodynamic characteristics and motion behavior of a jack-up unit with mat at a certain towing speed in various sea states with significant wave height ranging from 2m to 5m and wave directions ranging from 0° to 90°. The significant motion responses, such as heave, roll and pitch, are estimated by the frequency domain method. The probability of exceedance of the motion responses of the jack-up in random seas is estimated by means of system reliability analysis, considering wave overtopping, instability for roll, instability for pitch, and their series system. In order to efficiently determine the limit state exceedance probabilities due to time varying random variables, the Monte Carlo simulation technique and the subset simulation method have been employed in this study. The results from deterministic analysis and probabilistic analysis are comparably discussed, and the estimation of the extreme value statistics of motion responses in various sea states is provided. The approach developed in this paper can be applied to predict an unsafe condition of the jack-up and other platforms during wet towing in seaways supporting the officer׳s decisions on the action taken if excessive responses are foreseen for the present course and speed.