Technical and Economical Evaluation of Composite Riser Systems

Abstract

ABSTRACT: Use of composite materials in the deep sea oil production riser systems may allow a dramatic decrease in weight, as well as improved fatigue resistance to loads induced by environmental conditions. Many concepts have been developed by the industry and could be available in the next years. However, the cost of composite components will always be higher than the one of steel components, and only significant advantages for particular applications will justify their use by the industry. Up to now, cost comparisons have been made essentially for TLP or SPAR riser systems. This paper presents a study that has been carried out to compare steel and composite riser solutions for catenary risers, submerged export lines, and hybrid riser towers in ultra deep water. Specifications were first proposed, following which steel and composite solutions were designed and compared. This was done both from the feasibility point of view as well as from consideration of the operational advantages resulting from the lightweight and the fatigue resistance of the composite. Acceptable costs of composite risers were then deduced. The main conclusions are that in the mild conditions of the Gulf of Guinea or of Brazil, both steel and composite solutions are technically feasible, although steel solutions come close to their limits. Composite riser joints fabricated in moderately long lengths seem to be the most interesting solution for transportation and laying purposes. Large cost advantages may be obtained, particularly during the laying phase, which can justify using the composite solution. INTRODUCTION: During the nineties, the oil industry proceeded to exploit offshore fields in 1000 metres water depths and beyond. This has been done generally by simple extrapolation of existing architectural floating concepts, such as TLPs and FPSOs, or by using new ones such as SPAR systems. A significant new development has been the introduction of Steel Catenary Risers. Also the exploitation of new lightweight materials has begun with the introduction of the first mooring systems made out of polyester ropes. New problems have also arisen such as the necessity for significant thermal insulation of pipes. With increasing water depth, and particularly in Ultra Deep Water (between 2000 - 3000 m), the need to decrease the weight of the risers and mooring lines will become stronger. The introduction of lightweight materials may become the best economic option, or possibly the only technically available option. Among such materials, high strength composites have been the subject of intensive industrial research and may become commercially available for operational purposes in the present decade. Up to now attention has been generally focused on weight sensitive floating systems, such as TLPs, on which the balance of weight and the advantages induced can be easily deduced. The objectives of the present study were to evaluate the technical and economic interest of other pipe systems, such as catenary risers, export lines, or hybrid towers, where the balance of weight is only part of the interest, and where steel solutions have run into barriers including fatigue behaviour, stiffness, installation loads, thermal insulation, etc.