Development of modeling procedures for branch welds. Final report

One major difficulty in developing fitness-for-purpose base flaw acceptance criteria for pipeline branch connections is the calculation of the stress distributions in the vicinity of the welds. Even with the latest computer-aided modeling technologies, finite element modeling of branch connections using three-dimensional (3-D) solid elements is the only way to accurately determine the stress distributions local to the weld toes of branch connections. This report describes work to access the applicability of linear elastic fracture mechanics (LEFM) principles to a saddle-pad reinforced branch connection modeled using 3-D elements. The effort concentrated on determining the amount and location of plastic zones resulting from different combinations of service loadings. Loadings considered include pressurization to 72% of the specified minimum yield strength with the addition of main pipe end tension or compression. The effect of contact and different material models were also quantified. In the end, none of the modeling strategies investigated were found to be suitable for LEFM evaluations due to excessive plasticity in the weld toe crack planes for almost all loadings. The only loading found to be acceptable for LEFM evaluation was that of main pipe compression, which is not of particular interest for a pressure containing geometry. Hence, formore » this geometry and loadings, LEFM is deemed inappropriate.« less