In deepwater Gulf of Mexico, vacuum insulated tubing (VIT) has been successfully utilized for thermal isolation to prevent the transfer of heat from well fluids, thus improving production flow assurance issues such as paraffin deposition and hydrate plugging the problems. As wells become high pressure, recent projects have introduced a need for VIT requiring the combination of sour service tolerance, good fracture toughness, fatigue resistance and high strength. This transfer of the VIT concept to high strength/sour service has necessitated the consideration of a different group of materials. Candidate materials were screened based primarily on their corrosion resistance to environments; minimum yield strength requirements; minimum Charpy toughness; workability (welding and threading); and economy. After the first screening, a detail evaluation procedure was developed to select a material most suitable for the application. In order to assure mechanical integrity of the material, standard welding qualification tests (tensile, Charpy V-notch test), fracture and fatigue (CTOD fracture toughness, and fillet joint low cycle fatigue) were performed. A simplified fatigue test was developed to assess fatigue strength of fillet welded joint to reduce cyclic time and costs. Stress corrosion cracking susceptibility was also evaluated in a sour environment using C-rings and Slow Strain Rate Testing (SSRT). Based on the test results, an age hardened Ni-base alloy was selected for sour service applications.
Vacuum Insulated Tubing (VIT) consists of concentric inner and outer tubes, fillet welded at the ends, with vacuum the annulus between them. The assembly process for VIT includes drilling an evacuation port, grit blasting both tubes, wrapping the inner tube with alternating layers of reflector and barrier material, and vertically inserting the inner tube into the outer tube prior to welding the tube ends.
