INVESTIGATING THE EFFECT OF CRACK GEOMETRIES AND WELD MISMATCHING IN ORDER TO OPTIMIZE ECA ANALYSIS OF GIRTH WELDED OFFSHORE PIPELINES

Offshore pipelines are an effective tool for transportation of oil and gas which are usually assembled by the use of girth welds. Since flaws may naturally exist at such welds, fracture assessment of girth welded offshore pipelines is substantial. Current fracture assessment procedures like BS ۷۹۱۰ consider identical material properties for the weld and the base metals. However the strength difference between weld and base materials has significant effect on fracture assessment results. This effect is magnified greatly for pipelines which are operated in deep waters and are subjected to large plastic loads. In this paper ۳D nonlinear elastic-plastic finite element analyses using the ABAQUS software are performed in order to investigate the effect of weld mismatching at various crack geometries on fracture assessment of pipeline’s girth weld. It is noteworthy that such a quantitative study on the effect of weld mismatching condition at different crack geometries on ECA analysis has not been performed so far. Based on simulation performed, a new optimized formula is proposed for fracture analysis of girth welded pipeline with surface cracks considering the effect of weld mismatching conditions at plastic strains. The results show that comparison of proposed formula results with those available experimental data reveals a great agreement. Furthermore, it is observed that the effect of strength difference between the base and the weld materials is insignificant for short cracks whereas mismatching plays a more dominating role in long cracks. Also, with increasing the crack heights the effect of weld mismatching raises meaningfully. In addition, ECA analysis results with and without weld mismatching effect are compared.

Keywords:

Engineering critical assessment (ECA) , offshore pipeline girth weld , surface cracks , weld mismatching , optimized CTOD formula.