DESIGN OF IN-SERVICE REPAIR WELDING PROCEDURES FOR OPERATING PIPELINES: CRITICAL ASSESSMENT OF VARIABLES AFFECTING RESTRAINT LEVEL AND HEAT-AFFECTED ZONE MICROSTRUCTURES OF VINTAGE PIPELINES

摘要

In order to maintain pipeline operation during repair and maintenance work, operators typically install branch (i.e. hot-tap) and repair fittings (i.e. sleeves) onto flowing pipelines. In-service welding procedures must be designed for these installations per code requirement. Welding induced cracking during the installation of pressure containing repair fittings is a major concern when welding onto flowing pipelines. Repair fitting dimensions influence cooling rates and restraint conditions. A combination of high stress and brittle microstructures formed during the rapid cooling of high carbon equivalent vintage pipeline steel can create conditions that promote the formation of cracks. CSA Z662 and API 1104 specify essential variables (requirements) that aim to mitigate risk of cracking by qualifying the weld procedure to equal or more severe conditions than expected in the field. These essential variables can include material carbon equivalent, cooling rate, and level of restraint limitations to be applied during qualification of the weld procedure. This paper will focus on the creation of a safe welding procedure by pre-welding assessment of the phase transformations that occur during welding on liquid product vintage pipelines and modelling the influence of readily quantifiable variables on the level of restraint induced by repair fittings. Finite element analysis (FEA) was utilized to study the thermal history of simulated in-service weld heat affected zones to approximate the stress and strain magnitudes (level of restraint) at the fillet weld toe of simulated sleeve repairs. Thermal analysis was conducted on various weld bead geometries to simulate the effects of cooling rates and tempering. To aid in the design of a safe weld procedure, two continuous cooling transformation (CCT) diagrams were constructed from a vintage 1960s API 5L X52 pipe with a carbon equivalent of 0.51 % (CEN and IIW). This enabled the selection of optimal welding parameters that produced desirable HAZ microstructures. The modeling of restraint level accounted for the thermal expansion and contraction of a multipass fillet weld sequence on various pipe and sleeve thicknesses. The sleeve-on-pipe configuration was compared to the plate-on-plate configuration. Sleeve wall thickness was varied from 1 to 7 times the pipe wall thickness to account for any possible instances where a very thick fitting, such as emergency fittings (e.g. STOPPLE®), may be installed on a thin pipeline. Test welds were completed on the 1960s vintage pipeline steel with a high volume water flow loop to simulate operating conditions. The heat affected zone hardness values correlated well with those predicted by the FEA and CCT results.