Design of Steel Lazy Wave Riser for Disconnectable FPSO
Authors: Cao Peimin , Cheng Jingyun 
Published: May 2013

The disconnectable Floating Production Storage and Offloading system (FPSO) is one of the preferred solutions for deepwater fields in harsh environments and far away from existing pipeline infrastructures. This paper presents a design of steel lazy wave riser (SLWR) system for an internal turret moored disconnectable FPSO in the Gulf of Mexico. The integrated systems of FPSO, disconnectable buoy, riser, and mooring are discussed while focusing on the design challenges of SLWR systems. Due to the complexity of SLWR geometry, a systematic configuration approach is introduced based on buoy payload and riser performance criteria. The study includes the strength and fatigue analysis of production, gas export and water injection risers for the connected, disconnecting, and disconnected conditions. It concludes that SLWR with disconnectable FPSO is a feasible and cost effective solution for deepwater field development in the Gulf of Mexico. The study demonstrates the importance of an integrated design approach, and provides guidance for configuring and design of future disconnectable systems with SLWRs.

Introduction

The disconnectable Floating Production Storage and Offloading system (FPSO) is one of the preferred solutions for remote fields in harsh environments in the Gulf of Mexico, South China Sea, offshore West Australia, and offshore arctic region from both technical and commercial aspects. The advantage of a disconnectable system compared to a permanent system is that the mooring system does not have to be designed to accommodate the economically penalizing severe loadings associated with hurricane and typhoon conditions. It allows rapid vessel removal for maintenance or upgrade. It also enables phased development due to production uncertainty, which reduces reservoir risk. During the past decades, the disconnectable FPSOs have been deployed successfully using flexible riser and free standing hybrid riser technology in shallow to deepwater developments [Refs.1, 2, 3].

Compared with flexible riser and especially with hybrid riser, steel catenary riser (SCR) is simple and cost effective for deepwater development, particularly under high pressure, high temperature and sour service conditions. It reduces the risks to delivery schedule and lifetime operation due to additional riser components. SCR has been the preferred riser solution for deepwater floating production facilities in the Gulf of Mexico. It has also been used in the permanent moored FPSOs for relatively benign environment offshore West Africa. However, SCR is very sensitive to vessel heave motion, and thus may not be feasible for FPSO in the harsh environment. Steel lazy wave riser (SLWR) has been recently used to decouple vessel motion under such conditions, e.g., offshore Brazil [Ref. 4]. With many lower tertiary fields discovered in the Gulf of Mexico far from the existing oil export pipeline infrastructures, it is important to investigate the feasibility of SLWR for disconnectable FPSO while using existing field proven SCR technology and maintaining commercial competiveness. Internal turret moored FPSO has been considered in this study. External turret system of the MoorSparTM type is a viable alternative to enable SCRs [Ref. 5].