Detailed Elastic-Plastic Finite Element Analysis Study to Determine an Optimal Process for Type-B Repair Sleeve Welding

Abstract A typical type-B sleeve consists of two half cylinders fillet-welded to the carrier pipe and joined together with butt welds. The butt welds will not impact the carrier pipe, but the residual stress produced from the fillet weld installation will impact the stability of any existing crack-like flaws in or near the new weld. The orientation and severity of the residual stresses can be reduced depending on the installation method. For instance, the residual stresses may differ if the upstream and downstream fillet welds are installed simultaneously versus sequentially, as simultaneous welding can restrict thermal contraction as the welds cool. The sequence in which the fillet welds are installed can also impact the orientation and severity of residual stresses. Prior studies have demonstrated that sequential welding and more stopping points can decrease residual stresses. However, it is not clear whether this benefit will effectively offset the increased time needed to execute these procedures. Excess time required in the ditch presents commercial concerns and, more importantly, increased risk to safety and quality. It is increasingly difficult to maintain proper quality control requirements as more time is spent in the ditch. This, in turn, elevates the risk associated with environmental threats such as the formation of hydrogen cracking in the welds due to poor moisture control. Quest Integrity and Colonial Pipeline executed a study utilizing detailed elastic-plastic finite element analysis to demonstrate if sequentially welding type-B repair sleeve fillet welds is beneficial compared to a simultaneous welding approach. Sleeve lengths of 0.5, 1.0, 2.0, and 4.0 ft were modeled for both sequential and simultaneous approaches. The initial results from this study identified that for longer sleeve lengths, the post-welding residual stresses were nearly identical between the two welding techniques. However, for shorter sleeve lengths, a sequential welding sequence generated lower residual stresses than the simultaneous process. Additional analysis is in progress to further identify an optimal process for type-B sleeve repair welding to minimize residual stresses while also limiting the amount of time required in the ditch to execute the welding process. This additional work includes quantifying the impact of these welding procedures on the stability of potential crack-like flaws located in the repair welds, as well as the probability of failure associated with these potential flaws.