Standardizing Second-Order Roll Analyses for FPSOs in Brazilian Deep and Ultra-Deep Waters: Approximations and Damping Evaluation

Abstract Evolution in the design of FPSOs for Brazilian deep and ultra-deep waters has resulted in larger platforms with increased weight and footprint. Consequently, the dynamics of FPSOs, particularly in terms of rolling motions, have been affected, leading to an observed increase in the natural periods of rolling motion in recent years. While the increase in the natural period of roll motion significantly reduces the observed first-order motion, platforms are still subject to second-order excitations, resulting in responses at their resonance periods. The analysis of second-order motions is more complex and typically requires substantial computational power for accurate assessment. Approximations are often used to evaluate second-order loads and motions, with the level of approximation complexity depending on the natural period being assessed. To meet design schedules, a tradeoff between computational cost and the quality of the approximation may be necessary since second-order motions are input for riser and structural analyses. Assessment of roll damping in second-order motions is also crucial due to its impact on roll motion levels. The use of roll damping devices remains important in controlling and reducing second-order platform response. This paper discusses different approaches to assess second-order loads and responses, aiming to establish the minimum requirements for a reliable assessment of second-order roll motion.