Effect of Coupling of Mooring and Riser Systems and FPSO Roll motions on the Design of Flexible Risers

摘要

For the development of the new frontiers in Brazil, spread moored FPSOs with a large number of flexible risers connected to one board of the unit are considered. Due to the large water depth and severe environmental conditions, the design of the flexible risers is challenging and the roll motions of the FPSO are a key parameter. The roll motions may induce large vertical displacements at the top of the risers which affect the ultimate strength of the flexible pipe as well as fatigue of end- fittings. Moreover, the vertical motions and accelerations may induce compression of the riser which could lead to exceedance of allowable curvature limit. The common practice for estimation of FPSO roll motions is to consider the unit in the free floating condition, without the presence of mooring lines and risers. The resulting motions are then imposed at the top of the risers in a decoupled way. Ideally, a fully coupled analysis should be performed in time domain in order to take into account the dynamics of the floater and lines systems simultaneously as well as the nonlinearity involved. However, as this type of analysis is very time consuming, an alternative methodology is proposed, which enables to consider the effects of coupling at early design phase at a reasonable timeframe. This methodology consists in performing preliminary numerical tests (forced motions) in order to obtain the effects of additional stiffness, added inertia and damping originated by the mooring lines and risers to be included in a decoupled motion analysis in the frequency domain. In this study we consider the example of a spread moored system in deep water depth designed for operating in Brazilian conditions. In this case it is observed that the coupling effects are very important, mainly for intermediate and shallow drafts of the unit. In particular, the lines (mooring and risers) may significantly contribute to damp the roll motions. As a consequence, the top tensions in the flexible pipes would be lowered comparing to the results of usual decoupled analysis.