When BP completed a difficult underwater repair in a closure weld at a water depth of 182m on its Magnus platform in the North Sea, TWI staff from several disciplines provided support on various technical aspects of the repair.
During routine inspection of the platform ( Fig 1) a flawed member ( Fig 2) was found to be flooded. Diver inspection revealed a through-thickness crack approximately 0.5m long in the circumferential weld connecting the diagonal brace to a leg node stub. BP's investigation of fabrication records showed the weld to have been repaired several times during construction because of root flaws. The weld was adjacent to an internal thickness transition - 45mm in the brace to 63.5mm in the stub ( Fig 3). A mechanical clamp was placed around the joint to bridge the crack, and to prevent relative movement of the two ends should the crack propagate further. At this stage, BP consulted TWI on a number of aspects of repair welding, heat treatment, NDT interpretation and possible causes of cracking.
To definite the scope of the repair, BP took a single-wall gamma radiograph of the weld. To retard crack propagation 'crack-stop' holes were introduced at the crack tips by electrolytic trepanning; this technique had the added advantage that the trepanned core samples could be retained for examination. By locating an additional hole across the crack, a sample containing portions of the fracture faces was also obtained for examination at TWI to determine the failure mechanism.
Based on the findings, together with a structural and fracture mechanics assessment, BP elected to repair approximately 20% of the joint circumference. Abrasive water jetting was used to cut out the defective sections whole, leaving suitably bevelled openings for subsequent repair. A comprehensive failure investigation including metallographic sectioning and mechanical properties tests was then conducted at TWI on the sections removed.
Repair was effected by TIG and manual metal arc welding in a purpose-built hyperbaric habitat. TWI carried out a full fitness for purpose assessment, based on data supplied by BP. Finally, flaw blocks containing flaws close to the acceptable limits at different positions within the weld were used to qualify and approve NDT procedures for the finished repair.