Riser System Selection and Design for a Deepwater FSO in the Gulf of Mexico
Authors: Petruska D.J. , Zimmermann C.A. , Krafft K.M. , Thurmond B.F. , Duggal A. 
Published: October 2008

The process used to select a riser concept for a deepwater Floating Storage and Offloading system for the Gulf of Mexico is presented. Numerous riser concepts were screened with three taken forward through the more rigorous concept selection process. The three riser configurations were the Steel Lazy Wave Riser, the Single Line Hybrid Riser and the Tension Leg Riser. A system approach was adopted where the turret location, the mooring system and the risers were designed together. The process involved rigorous evaluation to verify both technical and installation feasibility, along with engineering definition sufficient to establish cost estimates. Analysis results for the three risers are presented to demonstrate important design issues that must be investigated to make an informed riser selection.

Introduction

The recent increase in discoveries in deep and ultra-deep water, coupled with fast-paced development schedules for Floating Production Systems, has led to a rapid evolution in the design of risers in terms of their complexity as well as variety. In addition, ship-shaped systems, including both Floating Production, Storage and Offloading systems (FPSOs) and Floating Storage and Offloading systems (FSOs), are often the preferred option for various technical and commercial reasons. Equipped with turrets, they can be highly functional solutions even in harsh environments. However, the motions of ship-shaped systems are typically more severe than the motions of other types of floating facilities due to their heave, roll and pitch motions being in the same frequency range as the wave energy. This further complicates the design of the risers. Consequently, riser systems that may be feasible on a Tension Leg Platform, Spar or semi-submersible-based facility may not always work on an F(P)SO. Particularly in harsher environments, this requires the F(P)SO riser system to have either a more compliant or a de-coupled configuration compared to simple catenary or toptensioned vertical risers.

A study was performed to assess the technical feasibility and commercial viability of several riser options for a 2- million barrel new-build FSO studied for possible deployment in approximately 1,370 meters water depth in the Gulf of Mexico. Steel Lazy Wave Riser (SLWR), Single Line Hybrid Riser (SLHR) and Tension Leg Riser (TLR) concepts were investigated in detail. To establish technical feasibility, an integrated system approach was adopted where the FSO hull form, turret location, mooring system and riser system were investigated jointly to capture the important interactions between key components.

To assess technical feasibility of the risers, extreme hurricane conditions were investigated to check allowable stresses and compression in the riser and to determine top termination requirements in terms of tension and rotation. Fatigue analyses were also performed. Both wave frequency fatigue and slow drift fatigue were examined along with fatigue due to Vortex Induced Vibration (VIV). The technical assessment confirmed that the three riser concepts are feasible for the conditions specified in the study. Note, however, that the SLWR was close to the fatigue limit while the SLHR and TLR, which de-couple their steel riser segments from the motions of the vessel, exhibited good fatigue performance.