Development of Induction Heating Technology for HPHT Pipeline Fabrication

Pipeline design for high pressure/high temperature applications (HPHT) requires the use of heavy walled pipe up to 2" thickness. This leads to enhanced pipe chemistry & complexity for girth weld fabrication, particularly when sour service requirements are specified. Subsea 7 executed recent HPHT projects consisting of heavy wall SCRs and Flowlines laid in up to 2000m/6600ft water depth; technical knowledge has been advanced in the areas of pipe weldability, hardness control & heating technology. A critical parameter for welding qualifications is to ensure accurate control of the pipe preheating & interpass temperatures. High preheats of up to 250°C (482°F) are required for hardness compliance to NACE MR0175/ISO 15156 (2020), while the interpass temperature cannot exceed 300°C (572°F) to ensure adequate weld strength overmatching for reeling. Using experience gained across 2 deepwater HPHT projects, Subsea 7 developed new induction heating equipment and procedures to successfully manage the heating specification. A fixed ring induction system was employed at the front end of the production spoolbase for preheating prior to fit-up; fixed rings were selected to provide uniform heating with a target variation of <15°C around the circumference. Uniform heating is critical for SCRs to ensure consistency of the weld root profile for fatigue sensitive design; the flaw acceptance criteria within this part of the weld is typically no planar indications permitted. The remainder of the production line utilised open-type induction coils set in between the welding stations for interpass temperature maintenance. Both types of equipment were constructed with 4x integrated thermocouples for monitoring & regulation of the heating cycle. This work will present the technical solution developed including an overview of the production layout & its benefit for HPHT pipeline fabrication.