Alternative Alloys for Mechanically Lined Pipes: Material Performance, Weldability and Inspection Requirements for Subsea Systems

Abstract Petrobras has been utilizing Alloy 625 in the Mechanically Lined Pipes (MLPs) of Brazilian pre-salt environments. Due to the high and volatile cost of alloying elements commodities, alternative CRA alloys (such as nickel Alloy 825, and 904L and 6Mo stainless steels) are being considered. To demonstrate the integrity of the alternative CRA liner, key concerns include corrosion, corrosion fatigue, metallurgical properties, mechanical behavior, fracture behavior, weldability, and inspection of such alloys. Special attention is required at the intersection of the weld overlay at the pipe end, the liner, and the backing steel, commonly referred to as the “triple point” (TP). Testing these alloys in the welded areas of MLPs is challenging, particularly for the liner seam weld and the TP area. Additionally, inspection techniques need to be adapted, especially for the triple point zone (TPZ). An Engineering Critical Assessment (ECA) is essential to evaluate the presence of circumferential planar flaws at the TP and at the weld overlay area. While fatigue performance data in aggressive environments are available for Alloy 625, similar information is currently still scarce for most alternative CRA. Corrosion fatigue performance significantly influences the resulting Non-Destructive Testing (NDT) flaw acceptance criteria. Primary inspection of MLP relies on Digital Radiography Testing (DRT), which employs a go/no-go criterion that may lead to unnecessary rejection of inspected pipes. Advanced ultrasonic techniques have been successfully implemented to measure liner thickness near the TP area. Ongoing research efforts are focused on developing ultrasonic inspection procedures for flaws in the TP. Additionally, the paper highlights recent research and development initiatives undertaken in collaboration with partners, which focus on evaluating the performance of materials and specialized inspection techniques for alternative CRA options. Emphasizing structural integrity and cost efficiency in subsea systems, the study underscores the necessity of conducting a comprehensive set of tests and manufacturing control to ensure the suitability of alternative alloys for use in MLP liners.