SSM perspective
Background
Leak-before-break (LBB) piping assessments have shown that piping systems can withstand long through-wall cracks before pipe rupture is expected. This is somewhat because pipe ends are restrained. In LBB assessments analyses are often performed for a pipe segment uncoupled from the piping system, as pipe ends are assumed unrestrained. This assumption is conservative for crack assessment. However, predictions of the crack opening displacement will be overestimated and hence the calculated leakage rate.
The present study aims to investigate what impacts the assumption degree of restraint at the ends of an analysed piping segment have on leak-before-break assessments and leakage rate calculations.
Results
The study conclude that crack opening displacements were influenced by restraint length, relative crack length, degree of restraint and material constitutive properties in the same way as the J-values. However, the relative difference was generally smaller for the crack opening displacement. The crack opening displacement values and subsequently the leak rates may be overestimated in leak-before-break assessments if pipe ends restraints are not carefully considered.
It is recommended that the influence from restraint is studied in detail as a leak-before-break assessment of a selected case based on a real piping system analysis. The results from a detailed simulation of loadings and structural stiffness should be compared with the results obtained from common practice methods. Reliable conclusions can only be drawn taking the margin against fracture, leak detection as well as degradation mechanisms into account.
Relevance
The work has increased the understanding for how the degree of restraint at the ends of an analysed piping segment will affect leakbefore-break assessments and leakage rate calculations. The results can also lead to improved recommendations for how to perform analyses of cracked piping components.
Need for further research
SSM have not identified any need of further research at the moment.