In this study, the slow-drift motion response of a floating-production-storage-offloading (FPSO) is considered in the presence of irregular waves, particularly when the wave spectrum has double peaks. In order to observe the statistical characteristics of the slow-drift motion of FPSO in the ocean waves of double-peak spectrum, a direct time-domain coupled analysis is performed. The probability of peaks of response signals are observed, and their peak statistics are modeled using normal distribution and Rayleigh distribution, comparing the peak distributions directly sampled from response signals. It is found that averaging the values of normal distribution and Rayleigh distribution can be practically used, showing better correspondence with the peak values sampled from the response signals.


Most offshore structures operate in fixed locations for a long period of time. Therefore, it encounters various environmental conditions, and there are practical cases in which ocean waves have the both components of typical irregular waves and swell. Offshore structures and mooring systems must be designed to endure these various environmental conditions, and this requires an accurate estimation of the peak values of design parameters, such as the displacement of platforms and the mooring line tensions.

However, the analysis of floating offshore platforms moored at sea includes various sources of nonlinearities, such as the coupling effects between the interaction of platform and mooring lines, wave-drift damping and the nonlinear viscous drag. The geometric nonlinearity in mooring lines arising from the large displacement caused by slow-drift motion is also a source of complication. To analyze the nonlinear problems by using the coupled dynamic analysis in time-domain, Emmerhoff (1994) and Kim and Sclavounos (2001) developed their own numerical code for fully coupled dynamic simulations, later named as SML (SWIM-MOTION-LINES). Wichers and Devlin (2001) also conducted a fully coupled time-domain analysis for the dynamics of a turret-moored FPSO, and they concluded that coupling effects are significant especially when a loop current exists. Kim et al. (2005) developed a numerical tool for fully coupled time-domain analysis and validated the simulation results through comparison with the results of an experiment performed by Offshore Technology Research Center. Furthermore, studies have been conducted that approach statistically to nonlinear responses of offshore platforms. Lim and Kim (2017) conducted a comparative study on statistical models for nonlinear responses, and introduced a design wave method for the slow-drift motion of moored platforms by a nonlinear stochastic approach (Lim and Kim, 2018). Nam and Kim (2019) conducted a nonlinear stochastic prediction of deck slamming problem in irregular waves.

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