Fatigue Testing of Shrink-Fit Couplings for Joining High-Strength-Steel Riser Pipe

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 24069, ’Fatigue Testing of Shrink-Fit Couplings for Joining High-Strength- Steel Riser Pipe,’ by J. Shield and J. Wightman, Subsea Riser Products; J. Pappas, RPSEA; and J. Bowman, Chevron, prepared for the 2013 Offshore Technology Conference, Houston, 6-9 May. The paper has not been peer reviewed. The shrink-fit connection is a method of joining pipes to couplings or flanges by thermally expanding the coupling or flange over the pipe; sealing and static capacity are achieved by the contact pressure and friction between the components once they have cooled. This method enables the manufacture of high-pressure deepwater riser systems when traditional pipe-join methods are not feasible. A program was devised to test the manufacturability of shrink-fit joints made from high-strength (greater than 100 ksi) mill pipe and forged couplings. Introduction The objective of the project was to qualify the shrink-fit technology to project-ready status—any further testing on the connector would be for project qualification rather than further development. This development project involved two stages: (1) proving the feasibility of machining and assembly from seamless pipe and (2) hydrostatic and resonant fatigue testing of shrink-fit connections. Six specimens were manufactured to demonstrate that pressure containment is maintained for maximum working pressures and hydrotest pressures, while target fatigue life is achievable without loss of pressure integrity in the connection. Before testing, a finite-element model was used to address residual shrink-fit stresses, safe hydrotest pressure, predicted connection-fatigue response, and the choice of internal pressure for fatigue testing. Current Technology The use of pipe for offshore oil and gas risers in various types and sizes requires intermediate connections because continuously cast pipe is not used for large-diameter offshore risers, and the pipe tends to come in limited lengths. For all drilling risers, the riser is usually required to be deployed and retrieved on a regular basis, so a coupling is employed on the joint extremities, which are designed for repeated assembly/disassembly. For shallow-water drilling risers, joint lengths of 40 ft are commonplace, but, for deepwater, larger joint lengths of up to 90 ft are increasingly desirable to reduce riser running time (and thus reduce drilling costs). These larger joints usually require one or more midjoint pipe connections. The shrink-fit connection has al-ready been delivered for a full 19.25-in.-bore drilling-riser project for the North Sea. The benefits of this shrink fitting include a connection demonstrably stronger than the pipe. Also, very-high-strength material grades can be used. The problems with the technology include a lack of resonant-bending-fatigue data and a need for pipe that is produced to reasonable dimensional tolerances.