Cost optimization in testing welds of duplex stainless steel for umbilicals and implementation in a new DNV recommended practice for umbilicals

摘要

The requirement of "no intermetallic phases" in the microstructure of duplex stainless steels (DSS) has given rise to dispute for several years, and the problem is highly relevant when welding steel tubes for umbilicals. Det Norske Veritas (DNV) has previously, in co-operation with Nexans Norway, carried out a project that addressed the methods used when examining the microstructure of duplex stainless steels. DNV has now launched a new Joint Industry Project (JIP) that represents a new approach to the problem. Participants in this latter JIP are Nexans Norway, Aker Kvaerner Oilfield Products and AB Sandvik Materials Technology. During the former of the two above projects a reliable method to measure the content of intermetallic phases, based on electron microscopy and digital image analysis, was developed. The results from this project were presented at the 6th World Duplex Conference in Venice, Italy, 2000 /1/. The latter work has formed the basis for a new JIP aiming to develop simpler and more efficient test methods to verify the welds' fitness for service in umbilicals. This represents a new approach since focus is drawn away from examination of the microstructure towards testing of the relevant weld properties. Existing standardised mechanical testing is in many cases not relevant to umbilical steel tubes, due to the small dimensions often held by such tubes. The project therefore aims to establish new test methods that are specially designed for the umbilical application. The outline and preliminary results from this project will be presented. Further, DNV, in co-operation with the Norwegian umbilical industry, has initiated a work to identify further needs for development in standardisation related to umbilical manufacture. Highlights from this work will also be presented.