Tensile and fracture toughness properties are important materials parameters used in fracture mechanics based Engineering Critical Assessment (ECA) to define alternative flaw acceptance criteria or evaluate Fitness-For-Service (FFS) of pipeline girth welds. In the oil and gas industry, pipelines are commonly made of carbon manganese steel and, if operating under a sour environment, their mechanical properties may be degraded and compromise their integrity. The extent of deterioration of toughness depends upon a number of factors e.g. the severity of the environment, type of material, temperature and pressure. All these factors control the amount of hydrogen absorbed in the steel. Furthermore, if a crack exists in the material subjected to service loading, hydrogen will accumulate in the plastic zone at the crack tip due to locally enhanced solubility, thus enhancing the hydrogen embrittlement. Pipelines are subjected to different levels of cyclic plastic strain during reeling and un-reeling operations prior to installation. Pipelines containing corrosive/sour environment may have different levels of H₂S and thus the severity of the environment can vary broadly. The extent of environmental deterioration is dependent upon the accumulated plastic strain introduced during reeling and unreeling operations along with many other factors. This PhD work will focus upon girth welds in C-Mn steel pipes made of API X65 and API X80. Tensile and fracture mechanics tests will be performed on virgin and pre-strained welds (pre-strained to different levels) in air and environments with a range of severity (i.e. pH of 3 to 5, covering mild to severe environments). The data generated will be used to obtain empirical models relating plastic strain, loading rate and materials properties (fracture toughness and tensile strength) for specific environments. The study will be extended to evaluate the effect of plastic zone size at crack tip on hydrogen embrittlement for specific materials and environments. The outcome of this research will help in developing knowledge of mechanical behaviour of specified materials operating in a range of less corrosive to severely corrosive environments and thus minimise uncertainness associated with materials parameters (tensile properties and toughness) used in pipeline assessments e.g. defining flaw acceptance criteria or evaluating FFS.
A number of fully-funded PhD scholarships are available for suitable candidates with a strong interest in fundamental and applied research in the area of structural integrity.
Candidates should have a relevant degree at 2.1 minimum, or an equivalent overseas degree in mechanical, Electrical/Electronics or Civil/Structural Engineering, Material Science, Metallurgy or Physics. Candidates with suitable work experience and strong capacity in numerical modelling and experimental skills are particularly welcome to apply. Overseas applicants should also submit IELTS results (minimum 6.5) if applicable.
NSIRC will be a state-of-the-art postgraduate engineering facility established and managed by structural integrity specialist TWI, working closely with lead academic partner Brunel University, the universities of Cambridge, Manchester, Loughborough, Birmingham, Leicester and a number of leading industrial partners. NSIRC aims to deliver cutting edge research and highly qualified personnel to its key industrial partners.
For more information about The National Structural Integrity Research Centre, visit www.nsirc.co.uk
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