Failure assessment on offshore girth welded pipelines due to corrosion defects

Abstract

A B S T R A C T Corrosion is an electrochemical process in offshore pipelines where the material strength begins to decrease as corrosion advances. Numerous studies have been performed to determine the remaining strengths (failure pressure) of corroded pipelines. Currently the axial corrosions of the girth welded pipelines still leave much to be understood. This study attempted to simulate girth welded pipeline with various corroded depths and lengths in order to compare with offshore pipeline design manuals. Based on the numerical results, the influence of corrosion defects parameters on remaining strengths were investigated for girth welded pipelines. The investigation on the effect of strength mismatch revealed that in the cases of under-matched, higher failure pressures are obtained.Comparisons of current results with B31G-2012 and DNV-RP-F101 demonstrated that both codes may produce somewhat conservative predictions on the failure pressure. Furthermore, an equation was proposed to evaluate the corrosion progress across girth welded pipelines.B = a coefficient determined by the ratio of the corrosion depth to wall thickness d = depth of corrosion defect D = outer diameter of girth welded pipeline E = Young's modulus K = strength coefficient for weld metal L = length of corrosion defect M = a coefficient relevant to length of corrosion defect n = strain hardening coefficient P F = failure pressure calculated by B31G or DNV-RP-F101 P Failure = predicted failure pressure in this study Q = a coefficient related to length of corrosion defect S flow = flow stress S F = failure stress calculated by B31G or DNV-RP-F101 t = average wall thickness of girth welded pipeline α = an intermediate parameter α U = material strength factor γ d = partial safety factor for corrosion depth γ m = partial safety factor for offshore pipelines η = an intermediate parameter η d = the sensitivity factor for the corroded depth Correspondence: Z. M. Xiao.

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