Technical Justification to Extend Girth Weld Examination Intervals

The American Society of Mechanical Engineers (ASME) Power Piping Code B31.1 discusses operation and maintenance piping system program requirements in Chapter VII. These program requirements for covered piping systems (CPS) operating in the creep range include the process of piping system walkdowns and the assessment of piping system integrity. For CPS operating in the creep range, this paper provides a methodology to justify extending examination intervals for girth welds subject to low stresses and high remaining useful lives (RULs) considering the observed field anomalies. Piping system walkdowns frequently reveal piping support issues such as bottomed-out, topped-out, or broken spring hangers (Cohn, M.J., Gialdini, R.J., and Nye, O.B., 2021). These unintended field anomalies should be evaluated to determine the possible impact at the piping system girth welds. The author’s strategy assumes that the unexamined girth welds have no significant fabrication defects, that future operation is similar to the past, that there are no future malfunctioning supports, and that there is no future significant off-design event. Evaluation of the first set of nondestructive examination (NDE) results will provide higher confidence in subsequent RUL estimates. The assessment of the piping system integrity for malfunctioning supports includes calculating the highest ranked locations of consumed creep life and implementing applicable NDE at the selected high priority locations. The author’s strategy is to select locations with estimated RULs less than 20 years for NDE during the next scheduled outage. Locations with estimated RULs between 20 and 50 years are medium priority ranking and may be examined during the next scheduled outage, depending on the available resources. Locations with estimated RULs beyond 50 years can have much longer examination intervals. The process of 1) comprehensive piping system walkdowns, 2) simulation as-found stress analyses to the observed field displacements, 3) NDE at the minimum remaining creep life locations, and 4) determining the next set of minimum creep life locations has been used to evaluate the current piping system integrity and provide additional confidence in safely operating the piping system until the next scheduled outage. Several case studies are discussed to illustrate the piping system integrity evaluation process. This strategy typically identifies a few critical girth welds to be examined during the next scheduled outage and provides justification to extend examinations of girth welds with estimated remaining creep rupture lives beyond 50 years.