Concrete Tension Leg Platforms for the Gulf of Mexico

Abstract

ABSTRACT Concrete-hulled tension leg platforms for the Gulf of Mexico are technically feasible and cost competitive. This paper discusses design criteria for concrete TLPs and presents results for four Gulf of Mexico design cases. Site selection criteria are described and construction methods proposed. Cost and schedule estimates support the economic viability of the concrete TLP in the Gulf of Mexico, indicating that additional work should be undertaken by industry to develop the concrete hull as a competitive option to the steel hull. INTRODUCTION Recent successful exploration of deep water fields in the Gulf of Mexico (GOM) has accentuated the need for feasible and cost effective structures for hydrocarbon production in water depths of 300 to 1500 m (1000 - 5000 ft). The tension leg platform (TLP) concept has gained considerable attention as a candidate for these applications. TLP technology has matured in a relatively shorttime. After six years' successful operation on the North Sea Hutton field in 148 m water depth (485 ft) [1] and the recent installation of the Jolliet TLWP in 536 m (1760 ft) [2] in the Gulf of Mexico, the technical feasibility and economic viability of the TLP concept is generally recognized. Fabrication has started on a TLP for the North Sea Snore field in 310 m (1020 ft) [3] of water and a TLP for the GOM Auger field in 872 m (2,860 ft) [4] of water is being designed. All of these TLPs have steel hulls. However, a concrete TLP hull has been proposed for the North Sea Heidrun field in 350 m (1150 ft) [5] of water. The basic layout of a concrete TLP is the same as a steel TLP except that, generally, the concrete hull has a greater displacement and a Dee-per draft. The construction of a concrete-hulled TLP in the North Sea utilizes the technology developed in Norway for the large offshore gravity-based platforms [6]. The present paper discusses the application of this technology in the Gulf of Mexico, and is based mainly on TLP studies carried out by Norwegian Contractors a. s and Exxon Production Research Company on behalf of Esso Norge a.s. The investigation included both GOM and North Sea designs to achieve the best possible basis for comparison of costs, construction methods and schedules. The study focused on a full production and drilling platform with 19,743 mt (metric ton) deck payload in 750 m (2460 ft) of water as the base case. Three sensitivity cases were also considered, including the base- case TLP in 1,500 m of water, a wellhead TLP with a payload of 9,830 mt in 750 m of water, and the wellhead TLP in 1,500 m of water. The key conclusions of the study are: concrete hulls are technically feasible for deep water GOM applications; the concrete-hulled rtP can either be built on the U.S. Gulf Coast or in Norway and towed to the Gulf of Mexico; and the concrete TLP appears to offer an economic incentive when compared to a steel-hulled TLP.