51315-5500-Failure of an 8630M Low Alloy Steel to C-Mn Steel Dissimilar Metal Weld Made Using Nickel Alloy 625

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Product Number: 51315-5500-SG

ISBN: 5500 2015 CP

Author: Kasra Sotoudeh

Publication Date: 2015

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Approximately one year after installation it was discovered that a failure at the joint between a water injection flowline and a forged connector used on a subsea water injection manifold had occurred in deep water. It was found that the line had separated at the welded joint between an 8630 low alloy steel connector and a C-Mn steel elbow made using nickel alloy 625 welding consumables. Although this type of joint is commonly employed subsea there have been a few failures and this combination of materials is no longer preferred for this type of service.Both halves of the fractured joint were retrieved from the seabed for failure investigation. A detailed investigation of the failed component was undertaken including extensive visual inspection metallography and fractography using scanning electron microscopy to characterize the fracture event fracture morphologies and corresponding microstructures. It was also attempted to estimate the loads which might have been responsible for fracturing the joint and displacing the flowline from the connector. In addition a range of supporting tests and examinations was carried out on the materials to assess the assembly against the original construction specifications. The paper will present the observations made and discuss the failure mechanism and its relationship with the environmental conditions encountered.