The influence of working fluid physical properties on weld qualification for in-service pipelines

Welding on in-service pipelines (hot tapping) has been practiced for many years on pipes containing both liquids and gases. The increasing frequency of sour conditions within gas and oil pipelines increases the risks involved in welding on pressure-containing lines and imposes additional restraints in the form of maximum tolerable hardness, especially in the heat-affected zone (HAZ). While there are existing procedures to deal with the cooling rate effects of the liquid or gas contained within a pipeline, little is known of the effects of the physical properties of the liquid or gas on the cooling rates to be expected. Most experimental work on procedure verification has been done on still or circulating water as the working fluid. In this study, a pressurized test coupon of X-52-type pipe was filled with liquids of differing physical properties - water, ethylene glycol, methanol, water-glycol mixtures and water-methanol mixtures. Thermocouples monitored inside wall-temperature changes caused by mechanized GTAW autogenous fusion runs on the pipe surface, and optical pyrometer measurements were used to assess outside wall temperature cycles. The results show both the working fluid physical properties and the pipe pressure conditions affect the weld zone cooling rate to a significant degree. it is possible to control cooling rates on the inside and outside walls of the pipe by adjusting working fluid properties and internal pressure conditions. Low-hydrogen SMAW deposits were compared to the autogenous GTAW runs for limited but practical repair procedural conditions. This showed the expected heat input increase due to more efficient SMAW are conditions is about 25%,This difference is somewhat lower than the usual GTAW/SMAW are efficiency comparisons in the literature. It indicates the necessity for more trials incorporating the range of welding process parameters to be considered in evaluating the working fluid cooling characteristics. Once the heat input relationships for various processes are established, the cooling rate control possible with various working fluid compositions can be utilized to assess procedure qualification in a controlled engineering environment. A procedure qualification involving the modified water-ethylene glycol working fluid has been used to make a nozzle to pipe weld on an active sour gas pipeline during the Canadian winter.