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

Abstract

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.

Effects of the asymmetric riser and bilge keel arrangements on FPSO green water assessment

“…It can be seen that the added mass of the riser lines has the largest contribution, while the effects of damping and stiffness of the riser lines are smaller. This conclusion contradicts the finding by Rezende et al [9], who concluded that the damping effect of the riser was the most significant and the inertia term might be negligible. The FPSO model used in this study operates in a shallow water environment while the model studied by Rezende et al [9] operates in the deep sea environment.…”

“…This conclusion contradicts the finding by Rezende et al [9], who concluded that the damping effect of the riser was the most significant and the inertia term might be negligible. The FPSO model used in this study operates in a shallow water environment while the model studied by Rezende et al [9] operates in the deep sea environment. As the damping effect of risers has a nature of quadratic drag, and also closely related to the dynamic line behavior, the increase in water depth and riser length may rapidly increase the damping level, thus making the effect of added mass relatively small.…”

“…Studies by Tom et al [6], Seah et al [7] and Bigot et al [8] suggested that the effect of asymmetric bilge keel is mainly due to quadratic damping. Rezende et al [9] claimed that the inertia effect of asymmetric risers is insignificant in their study, and simplified the riser effect using Morrison element. In such case, it is possible that the asymmetric effect of bilge keel and risers at opposite side will cancel with each other, leading to a reduced overall asymmetric effect dominated by the stronger side; in the condition of perfect cancellation, the FPSO may even have a symmetric response to waves from both sides.…”

“…The KC-dependent drag coefficient was also implemented by Bigot et al [8] in the frequency domain simulation, and results comparison indicated that the constant drag coefficient is already performing very well. Rezende et al [9] illustrated significant heave-roll coupling effect on FPSO caused by the damping effect from asymmetric risers, by using a simplified frequency-domain approach following the method of Bigot et.al [8]. By conducting the numerical analysis of the forced oscillation, they concluded that the inertial effect of the riser is negligible.…”