This paper provides an overview of how design and qualification testing of rigid composite risers can be accomplished using performance-based requirements. The successful qualification of composite pressure vessels for use as energy accumulators on riser tensioning systems is used as a model. Technique discussed applies to design and qualification of composite tubulars used as production risers, workover and intervention risers, drilling risers, auxiliary lines (choke, kill, booster, and hydraulic), drill pipe, casing and production tubulars. Performance-based qualification requires design analysis, manufacturing development and qualification testing. Determination of minimum Factor of Safety for composite structures should be based on standard reliability analysis that considers the stress rupture behavior of the selected reinforcement.
With offshore structures becoming more and more weight-critical as drilling and production activities are extended to increasing water depths, the use of advanced composite materials in primary structural applications is a logical and sensible technological evolution. The same process has occurred and continues to occur in the aircraft, aerospace and automotive industries. Rigid composite risers offer significant overall weight and cost savings for TLP's and other deep water floating production platforms (Ref. 1). However, there is significant cost pressure on composite riser products (Ref. 2) which demands structural efficiency. The higher cost of high-performance composite materials relative to steel does not allow the composite riser designer the luxury of excessive design margins. The structural capabilities of the composite riser must address the performance requirements of the application, with sufficient margins to ensure safe and reliable service. This is best accomplished through qualification to performance-based specifications.
Historically, the efficient usage of composite materials in all industries has been facilitated by the utilization of performance-based rather than prescriptive specifications. This paper describes how a typical composite product progresses through design and qualification and on to production. The discussion is extended to the performance-based design and qualification of a rigid composite riser, based on experience gained from successful development projects. The role of regulatory agencies and third-party inspectors in assuring that the necessary quality control systems exist and are maintained is included in the discussion. Finally, we will discuss the importance of stress rupture to composite tubular structure design and how this be addressed in a straightforward manner by composite riser design guidelines or codes.
The qualification of Class I vessels per ASME BPVC Section X (Ref. 3) is a familiar example of a performance-based specification for composites. Composite Accumulator Bottles (CAB's) qualified to Class I of Section X are currently used as energy accumulators on riser tensioning systems on five TLP's in the Gulf of Mexico. Figure 1 is a flow chart showing the steps required to successfully qualify a Class I vessel.
The Section X requirements for Class I vessels are performance-based, with the suitability of the vessel design demonstrated by the destructive test of a qualification unit.