Examination of residual stress state in pipeline girth welds subject to high plastic deformation
Welding residual stresses can have a profound influence on the fracture and fatigue behaviour of welded joints. Consequently, structural integrity assessment procedures (such as British Standard BS7910 and UK nuclear industry’s structure integrity assessment procedure R6) include compendia of residual stress distributions for a range of joint types, plus methods for incorporating the effects of residual stress into analyses. They also include advice on circumstances in which residual stresses can be relieved, for example through the use of post-weld heat-treatment or mechanical stress relief.
However, the structural integrity assessment codes and standards adopted by the industry are fundamentally stress-based procedures, ie they assume that the gross section stress on the component being analysed is below yield. There is, however, considerable interest in extending the use of these procedures to situations of high applied strain. For example, in the oil and gas industry, small-diameter offshore pipelines are sometimes installed by a process known as reeling. In this process, seamless pipes are girth-welded and inspected onshore, wound onto a reel for transport to the site and subsequently wound off. Both the ’reeling on’ and ’reeling off’ processes introduce high plastic strain, with no subsequent inspection step. During operation, the subsea pipelines may experience biaxial loading due to lateral buckling. Land pipelines may experience axial plastic straining combined with internal pressure for example due to frost heave or seismic ground movement.
Substantial research has been carried out over the past 20 years to develop improved design tools for such pipelines and pressurised applications including, in particular, fracture mechanics based engineering critical assessment (ECA) methods for assessing the significance of girth weld flaws under longitudinal tensile plastic straining. Some of these projects focussed on onshore pipelines and led to detailed methods for estimating tensile strain capacity (maximum tensile strain that can be sustained for a given combination of a girth weld flaw and pipe geometry, tensile and fracture toughness properties, and pipeline loading). Other projects led to the development of ECA methods for ductile tearing assessment of girth weld flaws in subsea pipelines, which experience plastic straining during installation. All these methods were developed and validated based on results from finite element analysis (FEA) and/or test data. However, there is still very limited
There is still very limited knowledge on the residual stress state of the girth welded pipelines subject to high plastic deformations and also the general residual stress state of narrow gap welded pipe spools is still subject of a number of academic and company internal research activities. Therefore, it is aimed at filling a technological gap via this PhD by examining the behaviour of weld residual stresses in a pipeline girth weld (of API 5L X65 ferritic steel) manufactured using a narrow gap welding procedure with and without circumferentially oriented cracks under conditions of high plastic deformation and quantify the level of relaxation if applicable. The examination of the state of residual stress will commence with finite element analyses which will form the core of this study and then will be supported with experimental studies if required.
A fully-funded PhD scholarship is available for a suitable candidate with a strong interest in fundamental and applied research in the area of structural integrity. Scholarships cover an amount to £16,000 per annum for 3 years, Home/EU tuition fees and support for research. Overseas applicants are welcomed, with total funding capped at £24k/year.
Candidates should have a relevant degree at 2.1 minimum, or an equivalent overseas degree in mechanical 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 is 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