Multi-Stage Weld Repair of Offshore Pipelines
Proceedings Publication Date
During pipelay operations offshore, pipe joints are welded together on the lay-barge firing line. On completion of the girth weld, Non-Destructive Testing (NDT) is performed to locate the existence of any welding flaws. If an unacceptable flaw is detected, a weld repair operation is carried out: The weld is opened up to the required depth, the material containing the flaw is removed and re-welding takes place. The maximum acceptable groove dimensions for a single cut repair can be determined by means of fracture mechanics. If a particular flaw is longer than the allowable gouging length for a particular depth and position however, a repair cut in stages is carried out (multi-stage repair). The first excavation is made to the calculated allowable length and subsequently re-welded. One or more further excavations follow, adjacent to the first repaired section. Each cut is equal or less than the allowable length, until the repair is completed. The integrity and safety of performing a multi-stage repair operation offshore has been evaluated using fracture mechanics concepts and the results are presented in this paper. The approach involves numerical modelling undertaken to justify and demonstrate that multi-stage repairs can be performed safely and reliably. The results were compared with actual fracture toughness tearing resistance data obtained by Saipem for the pipelines/ girth welds under consideration and satisfactory reserve factors were estimated. Multi-stage weld repair procedures for Saipem welding materials and consumables have not lead to limiting failure conditions during welding. This indicates minimal risk of unstable fracture/ tearing during the operation. Saipem has safely implemented the multi-stage repair method for many years. It is therefore anticipated to be safe for current and future projects with similar welding/repair methods, materials and consumables. It is recommended that further testing should be undertaken for different processes and materials.
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