Need: The welding of attachments onto pressurized pipelines and storage vessels (referred to as hot tapping) is a necessary and common operation in the transmission industry. Care must be exercised to choose a welding procedure that can reduce the risk of burn through and the formation of very high hardness levels in the heat-affected zone (HAZ) that can lead to a higher than normal risk of cracking.
Benefit: This report summarizes a state-of-the-art review of current procedures for hot tap welding on pressurized pipelines. All open literature related to hot tap welding were examined and catalogued, and areas requiring further research were identified. Although a large volume of literature exists on this subject, the majority gives accounts of successful repair work on specific items, rather than describing generalized safe procedures. The methods and controls vary widely; no national or industry standards exist.
Result: The literature review revealed that process parameters need to be developed for welding on thin-wall pipe, sour service pipelines, and duplex stainless steel. In addition, further research should be done on the delay time for hydrogen-induced cracking.
