FPSO Second-Order Roll Motions and the Impacts on SLWR Riser Design

Abstract

Abstract The dynamic behavior of risers is highly influenced by the FPU motions, due to the effects of the environmental conditions (waves, wind and current) on the hull. Representing the wave numerically by a velocity potential, we will assume that it can be expanded as a power series with respect to a parameter ε. The first order effects generate motions with the same frequency of the incident waves and are directly proportional to their amplitude. Second order effects generate motions with frequency equal to the difference (low frequency) or sum (high frequency) of the frequencies of the incident waves, treated as bichromatic in the numerical model, and are proportional to the product of the amplitude of each incident wave in this pair. Significant second order motions are commonly observed in the horizontal plane (surge, sway and yaw), due to the low mooring stiffness. Recently, however, this phenomenon was also observed for roll motions in several new FPSOs that will operate in the Pre-Salt of Santos Basin. The phenomenon had already been detected in a deep draft semi-submersible from Campos Basin, but only more recently in FPSOs. The new units have high roll natural periods, above the typical wave period range on the pre-salt region, but the roll response occurs with periods close to the natural period of the system and can be significant. In this way, sea states with incident waves in the Santos Basin typical period range (4 to 20 s) generate additional roll excitations and motions on the floater outside the incident range (above 20 s). This phenomenon was detected in model tests and confirmed by numerical studies. This new FPSO behavior requires some care with how the risers and their interfaces will be studied. While 2nd order motions of surge, sway and yaw can impose a slow motion on the riser system due to their periods being typically well above 150 s, 2nd order roll motions are only slightly above 20 s, imposing displacements and accelerations that can directly affect the behavior of the riser top region. This paper aims to present the joint effort of Petrobras’ Naval Architecture and Subsea Risers teams to properly represent this phenomenon in FEM time-domain riser analyses, as well as the most important issues observed and incorporated into the risers’ projects in terms of extreme combined load effects and wave&motion fatigue.