Fit for Purpose and Inspection of Subsea Equipment Dissimilar Welds

Abstract This paper addresses material challenges in Subsea System Projects involving dissimilar metal welds in well completion equipment, such as PABs and VCMs. Some components were supplied with welded joints made in low-alloy steel buttered with Alloy 625 consumable and post-weld heat treated, resulting in a dissimilar interface, metallurgically susceptible to Hydrogen Induced Stress Cracking in the presence of atomic hydrogen generated by the cathodic protection system. Catastrophic failures in dissimilar joints have been recorded in subsea equipment pipelines since early 2000’s. In response to these incidents, various design solutions were implemented. Recent subsea equipment supplied for Petrobras’ projects exhibited a structural dissimilar interface between 0,3%C low-alloy steel and nickel alloy, not according to specified requirements. Rather than opting for materials less susceptible to hydrogen embrittlement, the supplier chose to mitigate the risk of failure by limiting the stress levels at the dissimilar interface. This approach was deemed unsafe and insufficient, as it did not eliminate the risk of embrittlement of the dissimilar interface. Consequently, there remained a possibility for hydrogen assisted subcritical crack growth, contingent upon the presence and size of planar flaws close to the fusion line. There was not enough time to qualify new welded joint configurations. Different disciplines as metallurgy, welding, fracture mechanics, and inspection, were employed to assess the criticality of planar flaws, increasing the reliability of the equipment throughout its operational life. The assessment was based on the Failure Assessment Diagram method and API 579 guidelines. Finite Element Analysis was conducted to determine stress levels at the dissimilar interfaces of the lines between the production and annular connectors, with and without a safety factor of 1.2. A lower bound material threshold toughness value for HISC (KIH) was adopted in the analyses. All dissimilar joints involving AISI 4130 low-alloy steel components buttered with Alloy 625 and then heat treated were reassessed to fit their operating conditions, exposed to the effects of hydrogen from cathodic protection. The dissimilar interfaces of these joints were reexamined using advanced ultrasonic techniques. Some joints required reworking, either finishing or improving access, to facilitate ultrasonic reinspection, while one joint was repair welded. A comprehensive validation program for the detectability of the ultrasonic inspection procedure was not established. Instead, a simplified conservative approach was implemented, incorporating sensitivity calibration, inspection analyses and methodology of the evaluation criteria through two advanced ultrasonic techniques: TFM/FMC and Phased Array. This approach, along with other premises adopted for loads and fracture toughness, provided consistency in the analyses. Some weld configurations, as closure welds, leads to the use of nickel alloy consumables to join the components, along with a low carbon C-Mn buttering welding consumable to eliminate the critical dissimilar interface and there is no consensus yet on which low-alloy steel dissimilar interfaces are indeed not susceptible to HISC. Despite the challenges and uncertainties associated, the adopted technical approach and thorough assessments ensured the operational reliability of the subsea equipment. This work underscores the importance of meticulous evaluation and robust engineering practices in managing dissimilar welds submitted to cathodic protection in subsea systems.