Combined numerical-experimental framework for strain based design and flaw assessment of girth welds

Abstract

An increasing number of transmission pipelines have to be installed and operated in harsh conditions, due to the diminishing reserves of exploited fossil fuel sources. Under certain environment related circumstances, longitudinal plastic strains are imposed. When tensile, these may induce failure in girth welds in conjunction with the inevitable presence of weld defects. A large number of factors related to environment, material, geometry and operating conditions influence the tensile strain capacity and, hence, the acceptability of girth weld flaws. Therefore, a project specific development of guidelines in this strain based context is recommended. This paper provides systematic frameworks on (a) how to design pipelines under strain based conditions, and (b) how to assess girth weld flaws that were detected using non destructive testing. Attention is given to several technical and economical aspects related to the selection and qualification of pipe and weld metals, and to the evaluation of flaw acceptability. Both frameworks comprise a combined experimental-numerical approach, collecting project development reports from literature with research carried out at Soete Laboratory. Concretely, four in-house realizations are adopted: the UGent equation for strain capacity, the curved wide plate tension test, the UGent stress-strain equation and a finite element model of full scale pressurized pipe tension testing. The proposed frameworks aim to facilitate in performing thorough and economically justifiable strain based design and assessment processes.Keywords: girth weld; weld flaw; strain based design; strain based assessment; project approach INTRODUCTIONThe availability of fossil fuel reserves decreases [1]. As a consequence, transport pipelines are installed and operated in increasingly challenging circumstances. Such pipelines may suffer from plastic strains that are being imposed under (partially) displacement controlled loading. Their structural integrity is to be judged in a strain based context, which considers strain as an evaluation quantity rather than stress. In this respect, the tensile strain capacity of the structure (i.e. the tensile strain level corresponding with failure of the structure) should exceed the strain demand (i.e. the strain level resulting from the imposed conditions).The strain based evaluation of pipelines under displacement controlled loading can be considered at two stages. First, devoted choices are to be taken during the design stage, prior to the construction of the pipeline. This aspect is referred to as a 'strain based design'. Second, the girth welds that connect pipe sections potentially contain weld flaws. A fracture mechanics based evaluation of their acceptability requires a method for strain based flaw assessment.To date, there are no standardized procedures for the strain based design of pipelines and their girth welds. Further, strain based assessment procedures for girth weld flaws have been developed during the last decade, but all of them show limitations...

Curved-Wide Plate Testing of X100 Girth Welds

“…Most aspects of the recommended practices were followed in this research, with additional techniques and measurements made to validate test procedures as well as elucidate potential issues that arise while conducting CWP tests. Numerous papers have been written with regard to specific test parameters, measurement methods, data analysis techniques, and finite element analysis (FEA) [4,9,[19][20][21][22][23][24][25][26][27].…”