DESIGN OF THE FULL SCALE EXPERIMENTS FOR THE TESTING OF THE TENSILE STRAIN CAPACITY OF X52 PIPES WITH GIRTH WELD FLAWS UNDER INTERNAL PRESSURE AND TENSILE DISPLACEMENT

摘要

Pipelines can be subjected to significant amounts of tensile forces due to geotechnical movements like slope instabilities and seismic activities as well as due to frost heave and thaw cycles in arctic regions. The tensile strain capacity (ε_t~(crit)) of pipelines is crucial in the prediction of rupture and loss of containment capability in these load cases. Currently the Oil and Gas Pipeline Systems code CSA Z662-11 0 contains equations for the prediction of ε_t~(crit) as a function of geometry and material properties of the pipeline. These equations resulted from extensive experimental and numerical studies carried out by Wang et al using curved wide plate tests on pipes having grades X65 and higher. Verstraete et al 0 conducted curved wide plate tests at the University of Ghent which also resulted in tensile strain capacity prediction methods and girth weld flaw acceptability criteria. These criteria are included in the European Pipeline Research Group (EPRG) Tier 2 guidelines. Furthermore Verstrate et al 0 introduced a pressure correction factor of 0.5 in order to include the effect of internal pressure in the tensile strain capacity predictions in a conservative way. Further research by Wang et al with full scale pipes having an internal pressure factor of 0.72 also showed that ε_t~(crit) decreases in the presence of internal pressure. In their work, Wang et al presented a clear methodology for the design of full scale experiments and numerical simulations to study the effect of internal pressure on the tensile strain capacity of pipes with girth weld flaws. However, there has been limited testing to enable a precise understanding of the tensile strain capacity of pipes with grades less than X65 as a function of girth weld flaw sizes and the internal pressure. In this paper the experimental setup for the testing of grade X52 full scale specimens with 12" diameter and ¼" wall thickness is demonstrated. In the scope of this research 8 full scale specimens will be tested and the results will be used to formulate the tensile strain capacity of X52 pipes under internal pressure. The specimens are designed for the simultaneous application of displacement controlled tensile loading and the internal pressure. Finite element analysis is applied in the optimization process for the sizes of end plates and connection elements. Also the lengths of the full scale specimens are determined based on the results from finite element analysis. The appropriate lengths are chosen in such a way that between the location of the girth weld flaw and the end plates uniform strain zones could be obtained. The internal pressure in these experiments is ranging between pressure values causing 80% SMYS and 30% SMYS hoop stress. The end plates and connection elements of the specimens are designed in such a way that the tensile displacement load is applied with an eccentricity of 10% of the pipe diameter with the purpose of increasing the magnitude of tensile strains at the girth weld flaw location. The results of two full scale experiments of this research program are presented. The structural response from the experiments is compared to the finite element simulation. The remote strain values of the experiment are found to be higher than the ε_t~(crit) values predicted by the equations in 0.