FPSO Mooring: Design and Regulatory Considerations
Authors: Lee Ming-Yao 
Published: October 2008

Recent advances in mooring technology have pushed Floating Production, Storage, and Offloading (FPSO) systems from merely marginal field applications in relatively shallow waters to some full field developments in water depths of several thousand feet.

In view of the rising demand for FPSO systems, an assessment of the design criteria and requirements for the FPSO mooring is presented in this paper. Current trends and future needs related to FPSO mooring design practice are identified. Based on a comprehensive data base of ABS classed FPSOs and several new mooring designs currently underway, some acceptable design approaches and criteria are described. Both conventional spread type and weathervaning single point mooring (SPM) systems are addressed. Weathervaning mooring with and without thruster assisted systems are particularly emphasized. Design methods using both model tests and analytical simulations are included. Some discussions on new mooring concepts such as taut leg mooring systems, vertically loaded anchors (VLA), and the use of synthetic mooring lines are also provided.

This paper focuses on the environmental conditions, mooring analysis methods, safety factors and fatigue aspects of the mooring design requirements. It is hoped that this paper will provide some guidelines for the engineer/designer to design a safe, reliable and cost-effective FPSO mooring system in a more consistent manner.

Introduction

Demand for hydrocarbon products has grown steadily for the last few years. This trend is expected to continue for several more years. Geographically speaking, new developments are happening on almost all continents. Activities in the Gulf of Mexico, Offshore Brazil, the North Sea, South-East Asia, Offshore Australia and West Africa are showing very strong signs of increasing.

To keep up with production demands, both operators and designers are aggressively seeking exhaust ways to accelerate development plans. The use of a Floating Production, Storage, and Offloading (FPSO) systems is considered to be one of the most attractive development options in terms of technology acceptance and project economics. For the development of a remote field, use of FPSO systems is almost always a definitive choice due to the lack of existing infrastructure.

Whether an FPSO system is based on a converted tanker or a purpose-built vessel, some form of stationkeeping capability is required if the unit is to remain on station. A major criterion for the selection of a mooring system for a weathervaning FPSO is the use of a swivel for transfer of the produced fluids onto the vessel.

With advances in mooring technology, FPSOs are no longer limited to marginal field applications in shallow waters. In terms of water depth, Petrobras recently installed the Barracuda FPSO in 2,700 feet water depth (Reference 1). The latest FPSO design (P-37) by Petrobras aims at a water depth approaching 3,000 feet. In terms of field size, the Zafiro Producer FPSO is so successful that its operator is moving ahead with Phase I1 of the development plan which will double the production rate (Reference 2). An artist's rendering of a typical FPSO field development layout is shown in Figure 1.