Session: MF-12-01 Leak Before Break
Paper Number: 129960
129960 - Leak-Before-Break Benchmark Analyses by Kiwa
Leak-before-break (LBB) piping assessments of detected and postulated flaws evaluate the relationship between leakage rates and limiting factors such as fracture and plastic collapse.
Static or dynamic piping system analyses with globally and locally declared loads along with rigid and flexible restraints are usually available, but the LBB analyses are often performed on limited piping segments judged representative of the particular location within the actual piping system.
It is expected that the results of an LBB assessment is influenced, to an unknown degree, by how accurately the structural stresses, forces and moments as well as the stiffness of the piping system are transferred from the preceding piping system analysis to the typically uncoupled piping segment analysis.
Uncoupled piping segment analyses are often based on standard geometries, which in the case of a pipe segment likely involves unrestrained pipe ends. The use of idealized geometries is a common and established approximation but may potentially deviate from the actual behavior of the piping system.
The present work has examined the effect on the J-integral and crack opening displacement (COD) with three-dimensional finite element analyses for different circumferential through-wall crack lengths and positions, different restraints and normal operating as well as normal plus safe shutdown earthquake as specified by the benchmark.
The study shows that the margin against fracture and plastic collapse is conservative when unrestrained pipe ends are assumed. However, COD and subsequently the leak rates may be overestimated if pipe end restraints are not carefully considered.
The linear elastic as well as elastic-plastic J-values in this work vary significantly along the analyzed crack fronts, which may question the applicability and relevance of idealized circumferential through-wall cracks. Furthermore, the elastic-plastic J-values increase primarily along the interior part of the crack front after the elastic phase implying that the margin against fracture based on an elastic-plastic analysis is determined by the state along the interior part of the crack front.
Significant parts of the loading, e.g. global thermal expansion and global dead weight, originate from the actual piping system at a global level. These loads are transmitted into the particular location across sections corresponding to the ends of the limited piping segment. The challenge of applying loads and different end restraints further adds to the discussion regarding modelling of uncoupled piping segments.
Presenting Author: Daniel Mångård Kiwa Technical Consulting AB
Presenting Author Biography: Daniel Mångård joined Inspecta Technology (now Kiwa Technical Consulting AB) in 2011. His work covers the areas of structural integrity, fracture mechanics and weld residual stress calculation towards the nuclear, power, off-shore and conventional industries.
He is also a PhD candidate at the Royal Institute of Technology within the field of 3-D mixed-mode crack propagation simulations - a project that was started during his M.Sc. thesis work at MTU Aero Engines in Munich, Germany.
Authors:
Daniel Mångård Kiwa Technical Consulting ABAndrey Shipsha Kiwa Technical Consulting AB
Petter Von Unge Kiwa Technical Consulting AB
Peter Dillström Kiwa Technical Consulting AB
Leak-Before-Break Benchmark Analyses by Kiwa
Paper Type
Technical Presentation Only