Lightweight Materials For Deepwater Offshore Structures

Abstract

ABSTRACT Advanced lightweight fiber reinforced materials have been identified as an attractive class of materials for the development of deep water compliant structures. The application of these materials for the fabrication of process vessels, drilling derricks, tendons, and risers is discussed. The advantages of these materials in terms of weight saving and cost benefits are addressed, and an evaluation program to ensure their successful application is described. INTRODUCTION The oil industry is continuing its effort to develop oil and gas reservoirs in deeper waters. Recently, exploration in water depths of 7,000 feet (2,134 meters) was performed, and the design of offshore structures for 1,800 feet (549 meters) is currently in progress for Conoco's Green Canyon Block 184 in the Gulf of Mexico. It is projected that by the year 2000, production facilities for 8,000 feet (2,438 meters) of water may be required (Figure 1). There are three basic structural concepts that have been proposed for deep water developments. These are fixed structures, guyed towers, and tension leg platforms (TLPs). Hybrids of these systems are also being considered. Recent studies1,2 on the relative cost of these three basic concepts demonstrated that the TLP is the most cost effective for deepwater, as shown in Figure 2. These relative costs do not take into account annual maintenance cost, which includes inspection, and structural removal cost. If these costs were considered, the TLP concept would become even more attractive. The TLP is a compliant structure which is very sensitive to weight. The ability to economically develop deep water TLPs depends on the success in reducing its weight. The weight sensitivity of a TLP can be easily demonstrated by examining Conoco's Hutton TLP design (Table 1). The design requires that for every pound added to the deck, 1.3 pounds of fabricated steel are needed in the hull to provide the necessary buoyancy at a cost of about $6.50. In addition, for the same I-pound weight on the deck, an 0.65-pound load of mooring pretension is required, thus impacting the cost of both the mooring system and the foundation.3 Similar effects of weight on other TLPs can be illustrated by examining other TLP designs such as Conoco's TLWP design for the Green Canyon project. The penalty of weight on the cost of structures is not new. The aerospace industry has always faced this problem and one of their solutions, i.e., the use of advanced lightweight fiber composites, represents an attractive option to the offshore industry. This paper discusses the potential application of advanced fiber composites as a substitute for steel in the fabrication of several components in a TLP and demonstrates the Viability and cost effectiveness of composite materials. The paper also proposes an evaluation program to develop the engineering data needed for the incorporation of these materials in future designs.