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Environmental Fracture Mechanics Testing of Dissimilar Metal Welds

Project Code: 34252

Start date and planned duration: February 2021, 24 months

Objectives

  • Quantify how environmental parameters affect susceptibility to failure of dissimilar metal welds (DMWs).
  • Produce a set of guidelines on how to approach environmental fracture mechanics testing of DMWs.

Project Outline

This project replaces Project 32900, which was placed on hold in 2020 due to disruption caused by the COVID-19 pandemic. The new project has a reduced scope.

Dissimilar metal welds (DMWs) are commonly used in subsea applications, such as within manifold components, jumpers, flowlines, Christmas trees, etc. They are often deployed in deep, inaccessible locations, meaning that intervention is impossible, should the structural integrity become compromised. A number of high-profile incidents have taken place involving in-service failure of DMWs, which have raised the alarm regarding interfacial microstructures and the effect the operating environment has on hydrogen embrittlement of these zones. The two major environmental concerns are as follows:

 

  • The external surfaces of such components are subjected to cathodic protection (CP) while subsea in order to prevent corrosion of nearby ferritic materials. However, as a consequence of the applied CP, hydrogen is able to evolve at the surface of exposed (uncoated) metallic surfaces, whereupon it can diffuse into the material, and embrittle interfacial microstructures.
  • The internal surfaces of clad DMWs are often subjected to extremes of pressure and temperature, while carrying hazardous fluids containing H2S and/or CO2. Corrosion in such environments can generate hydrogen, which, if allowed to accumulate in critical concentrations, can also cause cracks to initiate (e.g. via sulphide stress cracking).

TWI has developed a methodology for ranking environmental performance and for testing joints at different temperatures, pressures, and in different fluids. These ranking methods aid materials selection, as they provide an economic means of shortlisting candidate materials. Currently, however, there are no guidelines on how to conduct a fracture mechanics assessment on a dissimilar joint, such that test data can be used directly during design. This is in-spite of new testing standards mandating that industry use such data in fitness for purpose assessments.

This project will investigate the effects of various fracture mechanics testing parameters on the susceptibility to failure of dissimilar metal welds. This will be used to inform the development of reliable testing procedures, and, ultimately, critical assessment of dissimilar joints. This research will develop best-practice guidelines for environmental fracture mechanics testing of DMWs.

 

Industry Sectors

Power

Oil and Gas

 

Benefits to Industry

Industry will benefit through improved operational safety and reduced costs of qualification of DMWs as a result of the inclusion of unified and dependable test techniques within a reliable qualification procedure.

 

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