Experimental and Numerical Investigation of Residual Stress for Structural Integrity Assessment of Proximity Girth Welds

Abstract

Over the last decade, there has been a growing interest in the life extension of existing welded structures in offshore and renewable industry. During this phase of extension, repair of welded structures is a common practise which requires careful estimation of residual stresses as per the defect assessment procedures of structural integrity. Repair of welds requires modification or fabrication of new welds in proximity to existing girth welds in highly constrained geometry of welded structures. Weld placement is a subject often debated by contractors and inspection engineers when deciding how to maintain a ‘minimum distance’ between repair and existing welds. International fabrication codes and standards are inconclusive in recommending minimum distance criteria between newly fabricated welds; however, it is completely lacking a description for this criterion for repair weld placement. This research investigates the challenges that are encountered due to proximity of welds on structural integrity of aging welded structures. Case study-based research method is used to perform this large-scale experimental study and its validation by use of non-destructive and numerical techniques for estimation of residual stresses in proximity girth welds. Firstly, weld placement criteria for maintaining ‘minimum distance between welds’ was investigated across various domains of fabrication and repair codes in offshore structures, pipeline s & piping’s. A clear lack of consensus was found when it comes to defining the criteria after assessment of various repair and fabrication codes and no reported technical justification was found in open literature. A detailed literature review for residual stress profile estimations at a distance away from the weld toe in defect assessment procedures of API 579, BS 7910, R6, etc., was conducted and which were found to be practically non-existent for girth weldments. Key parameters like pipe geometry (radius to diameter ratio), heat input per unit volume of weld area (J/mm3), etc. have been identified as important parameters in determining full field residual stress profiles at a distance away from the weld toe. Secondly, welding procedure qualification records and residual stresses profiles were established for proximity girth weld on S355 grade pipe weldments in a large-scale experimental study. The welding procedures qualification record (WPQR) for proximity joints was performed as per the EN 15614-1 & NORSOK M-101 code, which is a widely accepted welding qualification code. Sufficient mechanical test-based evidence was gathered to further investigate the effect of welding-induced residual stress profiles (beneficial or deleterious) developed between proximity welds. A non-destructive method of neutron diffraction (ND) was used to measure full field residual stresses between adjacent proximity welds at different depths, fabricated with two different welding processes. Furthermore, a numerical base thermo-mechanical simulation was performed to validate experimental results (XRD) as recommended in level three, defect assessment procedures of structural integrity, for assessing critical weld joints. The findings from this research study proposes thickness and transverse distribution region of harmful tensile residual stresses which are developed at weld root toe for proximity welds. These findings enable practitioners to qualify welding procedures during pipe replacement procedures and consider the effect of residual stresses in defect assessment procedures for critical welds placed in proximity.