Foinaven Super Duplex Materials Cracking Investigation

Abstract

Abstract Development of the BP Amoco Foinaven Field received a major setback when leaks were discovered during final commissioning of a subsea manifold. The problem was identified as hydrogen embrittlement, a phenomenon that can be created by the combined effects of the cathodic protection system (providing a source for hydrogen) and unacceptably high peak stresses applied to steels during the manufacturing, construction and installation phases. Although this mode of failure in conventional subsea materials was known, the limits of sensitivity of duplex stainless steels in a subsea environment were found not to be fully appreciated. A BP Amoco Technology Team was formed who investigated the problem and then delivered a way forward that offered a long-term solution to the integrity of the subsea facilities in order to maintain a viable project. The challenge was to use the skills and experience of the team to review and adapt known and new technologies to engineer a "fit-for purpose" solution that integrated with, and met the rapid pace of the project. Because a single solution to all of the scenarios generated by the cracking problem was not possible, it was necessary to identify and prove several options and then consider the merits of their application to every connector location. Sophisticated computer-modelling techniques, adapting known technology, laboratory testing of materials and full scale testing, were only part of the overall programme. Establishing an envelope of suitability for each option, and developing a justifiable case for each connector that accounted for all possible installation and operational start-up/shut-down cases, required the combined efforts of the multi-disciplined team with external specialist support. The investigative work carried out by the BP Amoco Technology Team has given the industry a fresh awareness regarding the use of these new materials for future subsea projects. Introduction The Foinaven Field development shown in Fig 1 utilizes dynamic Risers, Manifolds, Flowline Termination Assemblies (FTA), Pullheads, Subsea Xmas Tree Flowbases and a ipeline Umbilical Manifold (PLUM) which are installed on the seabed. These special items of equipment are all fitted with special forged steel connectors, similar to those fitted to the manifolds at the drill centres DC#1 and DC#2. In addition to the 18 hubs on the two manifolds, there are a further 163 super duplex hubs in a full 24 well development which connect the various items of seabed equipment. These large, complex forgings were manufactured from "Super Duplex" Zeron 100 stainless steel because of its superior resistance to corrosion from the produced fluids. During routine pressure testing of various piping/flowline circuits some 6 months after the DC#1 manifold had been placed on the seabed, leaks were discovered in two of these special forged connectors (see Fig 2). Fig 2 DMaC Super Duplex Hub Forging with Welded Pipe Connections (Available in full paper)