Abstract

In the coming years, there will be a growing demand for Floating Production and Storage Units (FPSOs) for ultra deep waters (greater than 2000 m [6,000 feet]), as evidenced by the large number of deep water FPSOs being considered for West Africa, Brazil and possibly the Gulf of Mexico. Several standard solutions for FPSO systems exist - turret moored FPSOs, turret moored FPSOs with thruster-assistance, and spread-moored FPSOs for benign, directional environments. However, beyond certain water depths, the technical and economical constraints associated with the use of passive mooring systems may favour a fully dynamically positioned FPSO. This system can either be utilized as an early production system designed to operate on many fields, or as a full-field development solution with service life up to 25 years. The areas most suited for this application are offshore West Africa, Brazil and the Gulf of Mexico.

The paper describes a joint study undertaken by the various companies represented by the authors to develop designs for fully dynamically positioned FPSOs for ultra deep waters. Two systems have been developed - a full-field development solution for the Gulf of Mexico (DP-FPSO), and an early production system for West Africa (DP-EFPSO). The various technical challenges for both systems have been identified and solutions to them provided. The differences in environment and operation of the two systems are discussed. Results from computer simulations and model tests are also provided to illustrate system performance in mild and harsh environments, and to study the performance as a function of control strategy.

Introduction

There is a growing demand for cost-effective and reliable floating production system concepts for ultra-deep water depths (greater than 2,000 meters). Floating, Production, Storage and Offloading (FPSO) systems are a mature floating production technology that is readily adaptable to deep water. There are a number of discoveries in ultra deepwater worldwide that will require floating production units, and a need for the industry to provide technical and economical solutions to face this challenge.

One of the critical issues in the design of FPSOs for ultra deep waters is the design of the most cost-efficient station keeping system for the specified operational requirements. The capital cost of the station keeping system including its installation can increase dramatically with an increase in water depth. This is even more important when a system is used as an early production system and may be relocated to several fields over its life. In addition, seafloor congestion, poor geotechnical conditions, or short field life may result in the traditional mooring system not being an optimum solution. Thus beyond certain water depths and for certain other conditions and applications, the technical and economical constraints associated with mooring systems may favour the fully dynamically positioned FPSO.

The development of the dynamically positioned FPSO concepts described in this paper builds from the experience obtained with the BP SWOPS Vessel (MV Seillean), disconnectable turret technology for the Terra Nova field, and the latest generation of dynamically positioned drill ships specifically designed for water depths up to 3,000 meters.

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