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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.

Composite materials offer an interesting alternative to classical steel solutions in the deep offshore risers and export lines systems. They are able to bring lightness both with high mechanical and compliance resistance, and important developments have been made by the industry to qualify them for offshore use. Most applications planned up to now have been however for TLPs, SPARs or Drilling Risers, where saving weight has direct influence on the design of the floating supports and corresponding equipment. However new riser system concepts have appeared, such as catenary risers, hybrid towers, or “Lazy W” export lines, used for instance on the Girassol field. The objectives of the present study are, starting from operational hypothesis, to design and verify feasibility of steel and composite risers for these types of applications in the very deep offshore (down to 2500m), including laying conditions. Composite and steel solutions are then compared and the advantages induced by the composite version can be evaluated. An acceptable cost of the composite version can then be deduced. Main conclusions are that both steel and composite versions are technically feasible in the mild conditions of the Gulf of Guinea or of Brazil. Steel is however close to its limit and may not work in more severe conditions. The cost advantages of the composite version are found essentially during the laying phase, and may justify the use of such an application. An important issue will be however the industrial availability of long length composite joints.

Le projet d'extension du port de Roscoff-Bloscon comprend la création d'une digue à talus de 550 m de long. Ces travaux ont fait l'objet d'un appel d'offres public, sur la base d'une carapace de digue en blocs artificiels de type ACCROPODE TM de 6.3 m 3. L'entreprise Charier a proposé une variante fondée sur le principe d'une "digue à berme reprofilable". Ce type d'ouvrage est constitué d'une carapace en enrochements naturels présentant une berme susceptible d'être remaniée lors de fortes tempêtes pour acquérir un profil en S stabilisé sous l'action de la houle. Le profil de digue proposé a fait l'objet d'un pré-dimensionnement analytique puis d'une campagne d'essais menée par Océanide. Ces études ont permis de valider les performances de l'ouvrage en termes de stabilité et de franchissement. Elles ont permis au maître d'ouvrage (CCI de Morlaix) de se prononcer en faveur de cette solution performante d'un point de vue technique, économique et environnemental.

RésuméL'implantation d'éoliennes en mer nécessite de connaître les conditions de stabilité des supports de ces structures face aux forces exercées par la houle. Les résultats que nous présentons ici portent sur l'analyse de la condition de non basculement d'une structure support, de concept Seaflower (SAIPEM) avec caissons gravitaires. Or les formules classiques définies pour des cylindres horizontaux de section uniforme, ne conviennent pas à ce type de structures constituées d'éléments cylindriques verticaux. Dans notre analyse nous avons dissocié les efforts exercés par la houle sur différents types d'embases gravitaires à ceux exercés sur la structure complète. Nos premiers résultats ont permis de proposer des solutions d'optimisation des embases gravitaires et de quantifier l'influence du mât. AbstractInstallation of offshore windmill requires to understand stability conditions against wave load. The formulation of Morison is traditionally used to described the total load for fixed cylinder (Hald 2002). However this relation is recognized as not fit well to the study of vertical cylinder almost if this one is made up. In the analysis of the stability we studied the inversion of structures according to the Seaflower concept (Saipem) with gravity boxes. These first results to the measure of the torque reaction, enabled us to optimize the concept and to give a solution more stable for that kind of gravity structure.