Effects of Fully Coupled and Quasi-Static Semi-Submersible Vessel Motions on Steel Catenary Riser's Wave Loading Fatigue
Authors: Ye W. , Shanks J. , Fang J. 
Published: October 2008

Abstract
A Semi-submersible Floating Production Facility (Semi) combined with Steel Catenary Risers (SCRs) is a costeffective solution for deepwater floating production systems. However such a solution presents significant challenges for the SCR design mainly due to the relatively larger motions of a typical Semi as compared to other deep-water concepts such as Tensioned Leg Platforms (TLPs) and Spars. This paper addresses one of the most critical aspects of this process, namely the effect of fully coupled vessel/mooring/riser global motion analysis on SCR fatigue.
The dynamic effects of mooring lines and risers on vessel motions are studied by means of a comparison between conventional quasi-static and fully coupled global motion analysis for a deepwater Semi in 1,700 m (approx 5,600 ft) water depth. The impact of inertia and drag forces from the mooring lines and risers on vessel motions is demonstrated and the associated effects on SCR fatigue assessed.

Introduction
A Semi-submersible Floating Production Facility combined with SCRs represents one of the most appealing solutions for production and export in deepwater developments. Compared to other floating structures, the Semi's advantages include better economics than TLPs and Spars in terms of payload per unit cost, plus better motion performance than ship-shaped Floating Production Storage and Offloading Systems (FPSOs). The motivation to utilize SCRs lies in the plain fact that they are much simple and cheaper to construct compared to other riser systems.
Worldwide the offshore industry has already observed SCR applications with TLPs and Spars in the Gulf of Mexico and with Semi-submersible floating production units offshore Brazil. However, SCRs are yet to be seen for more onerous applications such as Semi-submersibles in the Gulf of Mexico and FPSOs offshore West Africa. The recent oil and gas discoveries in deep and ultra-deep waters in the Gulf of Mexico has pushed more SCRs into service. In the coming years this trend is expected to continue and the use of SCRs with Semi-submersible floating production system is anticipated in several geographical regions including the Gulf of Mexico.
For SCRs connected to a typical deepwater Semisubmersible floating platform, fatigue is one of the key design aspects. The main sources of SCR fatigue damage include wave loading induced by vessel motions and wave kinematics, current-induced VIV and heave-induced VIV. Of these, vessel motion is the major contributor to SCR fatigue damage due to the Semi's relatively larger motions as noted above. It is important that this effect be considered at an early stage in the vessel layout and design in order that the Semi's response is well optimized with respect to riser dynamics. In addition, the accurate prediction of Semi-submersible motions is critical to the SCR fatigue design.
Currently there are two approaches to predict Semisubmersible vessel motions: quasi-static and fully coupled. The main difference between these two methods is that whereas the fully coupled analysis considers the dynamic interactions between the vessel and mooring lines, risers and umbilicals by accounting for the inertia and drag forces of these lines, such effects are ignored in the conventional quasistatic analysis approach.