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Making connections in gas pipelines that may not be taken out of service requires special techniques. Hot tapping is often applied. Split tees are welded to pipelines that are and remain completely operational. The split tees are placed on the pipeline and then welded together using longitudinal welds to make one tee piece. After inspection, circumferential welds are made to create the connection between the tee piece and the pipeline. This same welding technique is also used to repair damage to pipelines. This can be in the form of a buttering up (surfacing) repair as part of a pipeline rehabilitation program or installing full-encirclement repair sleeves. Installing repair sleeves can also be a follow-up to an in-line inspection that has established that there is damage to the pipeline or to an activity that has been reported as doing damage as a result of external interference (e.g., excavation, trenchless techniques for installation of cables or other pipelines, plowing). N.V. Nederlandse Gasunie has had research conducted since 1978 by, among others, Netherlands Organization for Applied Scientific Research (TNO) in order to obtain a sound basis for a reliable and safe way to weld on live gas pipelines. This article covers over 20 years of operational experience with welding on live gas transmission pipelines. The article addresses regulations, critical aspects of welding on operational gas pipelines, and the control of these critical aspects. The metallurgical background issues are also discussed. [ABSTRACT FROM AUTHOR]