When welding onto an in-service pipeline, to facilitate a repair or to install a branch connection using the “hot tapping” technique, there are two risks that need to be considered. The first is the risk of burnthrough, where the welding arc causes the pipe wall to be penetrated allowing the contents to escape. The second is the risk of hydrogen cracking that arises from the fast cooling rates that tend to be produced by the ability of the flowing contents to remove heat from the pipe wall. Various methods exist for predicting safe welding parameters for welding onto in-service pipelines with regard to the risk of both burnthrough and hydrogen cracking. This paper reviews these methods and presents a comparison of the results for a typical in-service welding application.

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