Design and Qualification of Fatigue Resistant Heavy Wall Threaded and Coupled Premium Connectors for Drilling and Production Riser Applications in Deepwater HPHT Dry Tree Systems

In the late 2006, as a part of its GoM HPHT technology program, BP initiated with V&M the design and qualification of a 15k dual barrier dry tree production riser system capable of being installed in water depths up to 10,000 ft. This was driven by the fact that X-80 riser systems became so heavy that they were not considered workable, whereas switching to higher strength steel and T&C connector designs made the systems much lighter and became enabling technologies for dry trees. Developments concern an 11 3/4^″ OD with 1.1^″ wall thickness C-110 sour service resistant material as the inner riser rated to 15k, a 16^″ OD with 1^″ wall thickness Q-125 material as the outer riser rated to 10k, and a 21^″ OD with 1^″ wall thickness Q-125 material as the drilling riser rated to 10k. The specification called for all three connections to be designed with an external environmental metal to metal seal, and superior fatigue performance. Based on existing know-how on fatigue resistant T&C connectors, the development work started with the 11 3/4^″ inner riser, with a need to accommodate existing design rules for such a heavy pipe. A comprehensive evaluation program was built, with a total of 39 samples including 5 design variations, and full scale tests combining Make & Break, sealability, fatigue, and thermal cycles. The qualification was completed with a connector achieving over 25 M&B and a stress factor less than 1.5 compared to DNV-B1 curve, that is a stress factor matching DNV-C. With success on the 11 3/4^″ and some lessons learned along the way, focus was then shifted to the 16^″ outer riser. Key issues were identified here concerning the surface treatment quality of the metal to metal seals when combining large sliding distance and high contact pressure. The 21^″ drilling riser development is on-going. As we near full product qualification, we focus on final optimizations to ensure easy field running. This paper will prove the robustness of the design process elaborated to develop heavy wall riser T&C premium connectors, leading to outstanding fatigue performance and fatigue compliant internal and external environmental metal to metal seals. Such results prove the applicability of T&C for dry tree HPHT systems, and further establish the potential of T&C to be used for SCR and Flowline applications.