Duplex stainless steels (DSSs) are widely used in oil and gas subsea applications, such as pipelines, and manifolds and risers in production systems, due to their superior properties. In these environments, DSS components are often connected to ferritic steel components, which necessitate the application of cathodic protection (CP) as a means of corrosion prevention. Whilst successful in preventing degradation of the ferritic parts, CP can generate hydrogen at the bare surface of the subsea structures, whereupon, it can be absorbed into the alloy and cause embrittlement. Cracking of this embrittled material is known as hydrogen-induced stress cracking (HISC) and is recognised as a major cause of catastrophic failures in service, which are of significant fatal and financial losses. However, there are currently no established/standardised guidelines for evaluating resistance to cracking of DSSs in such environments, in a quantitative manner and against any established acceptance criterion.
The main objectives of this PhD project are to develop an in-depth insight to the cracking behaviour of DSSs once exposed to hydrogen, and provide new, quantitative methods for assessing and designing against HISC, as well as providing data to enable production of more reliable assists. This also helps risks of HISC to be managed, and therefore increase safety and reduce the operation costs and issues associated with, potentially fatal, catastrophic failures. A series of experimental and modelling tasks (but not limited to the following items), will be undertaken:
- Advanced microstructural characterisation of a few DDS product forms (and weldments);
- Modelling of test specimen geometries, and numerical calculation of strain/stress distributions along the stress raiser to be incorporated in those specimens;
- Mechanical and environmental testing of a number of specimens with different geometries (size, notch acuity);
- Post characterisation of tested specimens using metallography and fractography ;
- Data analysis and interpretation of the findings obtained.
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NSIRC is a state-of-the-art postgraduate engineering facility established and managed by structural integrity specialist TWI, working closely with a number of academic partners and universities and a number of leading industrial partners. NSIRC aims to deliver cutting edge research and highly qualified personnel to its key industrial partners.