"INTRODUCTION Appurtenances@ such as full-encirclement sleeves@ patches@ stopples@ half-soles@ and branch connections@ are frequently welded to pipelines while the carrier pipes remain pressurized in service. Though there are certain obvious potential hazards associated with such welding@ it has been done for many years with few known hazardous incidents. It is generally believed that the good safety record associated with such welding is due to the conservative practices of pipeline operators. Recently@ however@ isolated instances of failures originating at appurtenance welds have drawn attention to a possible problem related to hydrogen-induced cracking. This phenomenon appears to be associated with the rapid cooling effect of a liquid product being in the pipeline during welding. To address this problem@ the project described herein was undertaken by Battelle and Edison Welding Institute (EWI) under the sponsorship of American Petroleum Institute (API). Three areas of concern regarding the safety of welding on pressurized pipelines are (1) A local blowout or ""burn-through"" at the time of welding caused by the heat of welding reducing the capacity of the pipe wall to support the stress imposed by the pressure in the pipe. A burn-through will permit the pipe to leak@ which usually means that a full shutdown of the pipeline is required for repair. (2) Cracking of the carrier pipe or the branch pipe caused by hydrogen-induced underbead cracking in the heat-affected zone of carbon steel pipe. Under the influence of subsequent stresses@ such cracking may cause the pipeline to rupture. (3) Thermal degradation of the contents of the pipe resulting in undesirable chemical or physical reactions. For example@ ethylene has been know to undergo violent decomposition when improper welding procedures have been used. These problems can be avoided through careful selection and control of welding procedures@ processes@ and materials. The results of this project are intended to help pipeline operators make the proper selections of these parameters."
API D12750-1991 history
1991API D12750-1991 INVESTIGATION AND PREDICTION OF COOLING RATES DURING PIPELINE MAINTENANCE WELDING