STRAIN CAPACITY OF LARGE DIAMETER PIPES: FULL SCALE INVESTIGATION WITH INFLUENCE OF GIRTH WELD, STRIP END WELD AND AGEING EFFECTS

The strain capacity of pipes under combined loading is a significant research topic if the pipes are provided for Strain Based Design scenarios. Displacement controlled scenarios such as ground movements may significantly affect transmission pipelines by inducing large amounts of plastic axial strains, which need to be considered in the design process. For these combined loading cases with internal pressure combined with pronounced longitudinal strains from environmental conditions it is essential to evaluate critical deformations on the one hand and to conclude the required structural performance and material parameters, on the other hand. Also pipe laying procedures introduce axial strains in pipes and pipe strings, e.g. cold bending of pipes for onshore pipelines, or S-Laying of offshore pipelines in combination with external pressure. For these cases also the strain capacity of the pipes and pipe connections must be guaranteed. In any case, the structural behaviour needs to be checked via full-scale tests to confirm and validate engineering approaches and computational models. This paper presents a full-scale test series of UOE pipe X70 (OD = 914 mm, WT = 14.1 mm) and Spiral welded pipe X70 (OD = 1016 mm, WT = 20 mm) subject to internal pressure and bending load. Full-scale 4-point-bending tests on pipe joints subject to internal pressure were performed. The test series included the influence of girth weld, strip end weld for spiral pipe, and ageing effects of thermal treatment from coating process. The local bending strains measured via strain gauges and via optical strain measurements in the bending zone are evaluated for the tensile and compressive zone and discussed with respect to existing buckling models. The results of the full-scale test program confirmed that the weld connections of the pipe joints are capable of withstanding bending load. The effects of the girth weld and strip end weld during bending test are analyzed and discussed. The test results are extended by finite element simulations that widen the experimental parameter range.