An Analysis of Tanker-Based Floating Production Systems for Small Offshore Fields

Abstract

Summary A historical review of floating production storage and offloading (FPSO) systems for marginal fields is presented. The areas in which such systems are presented. The areas in which such systems are operating, ranges of model tests, and current applicability of mooring systems are reviewed to present capabilities of these concepts. Limitations are reviewed to assess the applicability of these concepts to similar sites.Consideration of processing operations on floating vessels are identified. Particularly, the effects of vessel motion on the processing equipment, gas disposal, and crude offloading are addressed. It will be shown that operations are becoming well understood and routine. Future problem areas and solutions under development are identified. Specific examples are cited and design refinements are noted.While various floating production systems have been in operation for only a few years, their potential for solving the problems of producing small offshore fields and for achieving early production is impressive. Introduction Oil discoveries have been made in offshore areas the world over, which, even at today's crude prices, are considered uneconomical to develop with the conventional fixed platform approach. The type of discovery considered marginal varies according to location, water depth, and recoverable reserves. Several innovations to produce such marginal fields economically have been produce such marginal fields economically have been implemented. Among these are tankers moored on site, taking production through risers from seafloor wells and with processing equipment located on board the tanker. This paper discusses the applicability of such tanker-based floating production systems in two diverse areas-offshore Southeast Asia and the North Sea. The discussion makes use of the results of studies conducted by Marathon Oil Co. over the past 5 years. Historical Development of FPSO Systems Early floating production systems used converted semisubmersible drilling units. Argyll gained prominence as the first floating production system using prominence as the first floating production system using the Transworld 58. ENIEPSA, Spain's national oil company, moored a SEDCO 135 class semisubmersible over a three-well cluster as a production platform in 305 ft (93 m) of water in the Dorado field in the Spanish Mediterranean. Development of several Petrobras fields offshore Brazil, Chevron's Casablanca field offshore Spain, and BP's Buchan in the North Sea also used semisubmersibles.In some cases, the semisubmersibles were selected primarily because they were immediately available. But primarily because they were immediately available. But high day rates, conversion time and costs, and low availability of semisubmersibles have made tankers a more attractive component of floating production systems. A tanker surplus in recent years has made it possible for operators to purchase them at a relatively possible for operators to purchase them at a relatively low cost, thus reducing field development costs significantly by unifying the tanker with subsea production systems. production systems. The use of moored tankers as permanent floating storage at offshore oil fields started in the mid-1960's. However, the first FPSO system went into service in 1977, in Shell Oil Co.'s Castellon field. The development began with production from a single well, with provisions for an increase to three wells. The time from provisions for an increase to three wells. The time from go-ahead to production was only 11 months, primarily because the production system was a variation of several relatively simple systems already in use in the offshore industry. Even though the mooring system was new, design and fabrication were carried out within this limited time. The basic simplicity, combined with relatively low capital and operating costs, permits economical development of offshore fields with small recoverable reserves such as Castellon. The production equipment aboard the Castellon tanker is designed to handle 20,000 B/D (130 m3/h) oil. The associated gas is used as fuel in the ship's boilers, and the excess is disposed of in a pair of incinerators. JPT p. 1871