Engineering Critical Assessment of Clad Pipeline Installed by S-lay for the Operation Phase

Abstract

Pipeline material with internal cladding or liner of corrosion resistant material has become more customary in the later years as a replacement for carbon steel with high corrosion allowance due to the high corrosivity of unprocessed hydrocarbons. Clad material however, offer new challenges in the fracture assessment methodology due to inhomogeneous material, where the backing steel usually is low alloy carbon steel of 415 or 450MPa yield stress and the clad layer basically has a significantly lower yield stress. In addition, the weld metal mechanical properties may only be partly overmatching compared to the base material. For operational conditions where the local material utilization exceeds the elastic limit, commonly used assessment methods based on analytical fracture models such as BS7910 might be inadequate for clad pipelines, and more detailed finite element modeling must be performed.This paper addresses the relevant load scenarios for a clad steel pipeline installed by S-lay. A dedicated finite element program, LINKpipe has been used for the analyses to verify the acceptance criteria for defects in the girth welds. Input data for the analyses are taken from the detailed pipeline design calculations and material testing of the actual welding procedures of girth welds.