Investigation on the Effect of Bimodal Sea States on Mooring Systems’ Extreme Response Offshore Brazil

Abstract Due to second-order effects, low frequency wave drift forces may excite resonant responses for moored Floating Production Storage and Offloading units (FPSOs) in horizontal motions, resulting in mooring responses in both low and wave-range frequencies. Time-domain analyses are usually performed since frequency-domain analyses fail to properly represent the system’s nonlinear effects. Furthermore, the most accurate approaches to determining extreme load effects are based on the long-term statistics of the response. Meanwhile, the typical sea state in offshore Southeast Brazil is comprised of wind sea and swell waves. The separate wave description is expected to be relevant in response calculations of turret-moored floating units, which are sensible to directional loading, and are expected to lead to less conservative tension estimates in spread-moored floating units. Despite this, it is common practice to combine wind and swell waves into a simplified unimodal model, where a total significant wave height that combines each wave energy is adopted. In this study, a sensitivity analysis is carried out to investigate the extreme response characteristics of a spread and a turret-moored FPSO when subjected to the bimodal sea state comprised of wind sea and swell waves and to the combined unimodal sea state. Extreme responses of the investigated mooring systems are obtained through an efficient full long-term integration approach, using Importance Sampling Monte Carlo Simulation (ISMCS).