L51385 Evaluation of the Inherent Safety Factors in the Tentative API Girth Weld Defect Tolerance Approach

During the last few years, considerable attention has been given to the defect tolerance of girth welds in line pipes.The most notable application of a defect tolerance analysis involved the Alyeska Pipeline Company girth welds. The Alyeska work provided an expansion of the API 1104 workmanship acceptance limits through the use of fracture mechanics analyses, but the safety factors were believed to be overly conservative. The NBS-DOT efforts showed that the Alyeska analysis was conservative, but noted that the degree of conservatism was unknown since a proven analysis or applicable experimental data did not exist.

The objective of this study is to investigate the inherent safety factor in the proposed API optional approach for allowable girth weld discontinuities. This was to be accomplished by conducting full-scale experiments on pipes containing intentionally flawed girth welds to evaluate the safety factor on flaw size at a toughness level corresponding to those cited in the proposed API criteria.

Three experiments were performed on intentionally flawed girth welds in 30-inchdiameter by 0.625-inch thick X60 pipe were conducted by bending the unpressurized pipe. The defects were surface flaws extending from the inside of the pipe and were located in the center of the weld metal. The test welds were made using a manual metal arc technique with Phoenix Cel 90 electrodes. This weld metal was used to insure the weld yield strength overmatched the yield strength of the base metal.