Qualification of a Flexible Riser for Deepwater Dynamic Applications

Abstract

Abstract As the offshore oil industry continues to move into deeper waters, there is an increasing requirement for the development and qualification of technologies to enable this expansion into deep water. Most of these developments are currently based on semi-sub or tanker based floating production systems, which use flexible risers for production, export and injection services. The continuing qualification of flexible risers for higher pressures/temperatures, deeper water depths and more severe load conditions is critical to the future success of deep water field developments. This paper describes the qualification of a 10-inch internal diameter, 4000 psi flexible riser for dynamic applications in the severe Northern North Sea environmental conditions. Results from static and dynamic prototype tests are presented and compared with analytical results. The tests performed on the riser include collapse, axial tension, burst and dynamic bending fatigue tests. The philosophies and methodologies used in the qualification process are discussed and compared with the requirements and guidelines in the most recent standards for flexible pipe, namely API Spec 17J [I] and API RP 17B [2]. Introduction In recent years floating production systems (FPS/FPSO) have being increasingly used for the development of offshore oil and gas fields, in particular for small reservoir fields in shallow waters and fields in deep waters from 300m water depth and greater. A key component of FPS technology is the riser systems, which have been predominantly unbonded flexible risers. Flexible pipe is also used widely for flowline systems, in particular wellhead and manifold tie-backs to the FPS systems. To meet the increased demand for this project the flexible pipe industry is expanding manufacturing capacity. In 1996, NKT commenced production of flexible pipe from their new factory in Kalundborg, Denmark. The NKT pipe design is largely based on the designs developed and qualified by Furakawa [3, 4, 5], which uses the unique Cshaped wires for the pressure armour layer. Furakawa has previously performed extensive qualification tests on this pipe design, including both static and dynamic tests. In 1997, NKT commenced a qualification programme to verify their pipe. The qualification programme is intended to verify the pipe design, design methodologies and manufacturing procedures. This paper describes the' qualification programme, in particular related to a high pressure 10-inch ID pipe for deep water severe environment applications. Qualification Programme Objectives The overall objective of conducting a qualification programme for flexible risers is to verify and document quality, reliability and safety of the pipe products to prove that they are suitable for use according to the specified design requirements. To fully qualify a dynamic riser, due attention must be given to the applied design philosophy, long term performance characteristics of the materials (in particular polymers), fabrication means and methods, as well as the conduction of relevant material and performance tests. The present riser qualification programme is part of a Type Approval Certification carried out by Bureau Veritas according to API Spec 17J [I], including verification of calculation' tools, review of design rules, investigation of material dossier and fabrication methods, acceptance of QA/QC procedures, as well as undertaking performance tests to correlate with predictions.