When welding onto an in-service pipeline, to facilitate a repair or to install a branch connection using the “hot tapping” technique, two risks 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. To prevent hydrogen cracking, at least one of the three conditions necessary for its occurrence must be eliminated. Beyond the use of low-hydrogen electrodes to minimize hydrogen levels, it is prudent to develop and use procedures that minimize the formation of crack susceptible microstructures. This paper reviews existing methods for selecting parameters and qualifying procedures for welding onto in-service pipelines. HAZ hardness is an indicator of the susceptibility of a microstructure to cracking. A widely-used value below which it is generally agreed that hydrogen cracking is not expected is 350 HV. Unfortunately, there is no one hardness level above which the risk of hydrogen cracking becomes unacceptable. This paper also describes the development of a hardness evaluation criterion that can be used to quantify the trade-offs that can be made between HAZ hardness, hydrogen level, and chemical composition. Finally, the results of a recently-completed group-sponsored project, where procedures for welding onto in-service pipelines were developed over a wide range of conditions, are also reviewed. The results can be used to select an appropriate procedure that is resistant to hydrogen cracking for a particular application. The use of these results allows in-service welding to be carried out in a safe, cost-effective manner, allowing both economic and environmental benefits to be realized by avoiding pipeline shutdown and interruption of service.

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