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Tensile and fracture tests were performed on nickel-based alloy clad pipelines girth welds using customised rigs to elevate the temperature. Fracture tests were performed using single edged notched bend (SENB) and single edged notched tension (SENT) specimens, both with notch depth ratio of 0.2. The tensile test results showed reduction in strength at elevated temperatures for base metal, weld metal and clad metal while the fracture resistance reduced by up to 0.6 times depending on the notch location and the specimen geometry. The SENT specimens exhibited a higher fracture resistance than the SENB specimens, with an increase by about 1.3 times. [ABSTRACT FROM AUTHOR]

Finite element method was used to document the effects of temperature, environment, crack location and weld strength mis-match on the fracture capacity of modern pipeline girth welds. The results demonstrated a marginal reduction (>0.83 times) in the crack driving force for pipelines operating in a sour environment or at elevated temperatures or for cracks located in the weld centerline than at the fusion line. The crack driving force is lower for under-matching than over-matching welds. Furthermore, the numerical method was compared with analytical method as per BS7910 and the results showed that the latter was more conservative for deeper cracks ( a / t > 0.4). [ABSTRACT FROM AUTHOR]

Tensile and fracture specimens (pre-charged with H 2 S) were tested in solution using customised rigs. When comparing with uncharged specimens, the tensile test results showed significant reduction in ductility for base and weld metal. The fracture resistance test results showed similar reduction of ∼0.3 times in base and weld metals and heat affected zone. There was noticeable increase in yield and tensile strength in both base and weld metals, due to the effect of hydrogen. When using partially coated specimens, higher fracture resistance was obtained, with reduction of ∼0.4 times only. This was mainly contributed by crack tip hydrogen and less so by the bulk hydrogen. [ABSTRACT FROM AUTHOR]

A theoretical model is developed to investigate the fracture behavior for an elliptical inhomogeneity embedded in an elastic-plastic matrix. It contains an arbitrarily oriented crack under a uniform stress field at infinity. In the model, the generalized Irwin approach is applied for plastic zone correction for crack-tip yielding. The distributed dislocation technique is utilized to formulate the present problem. The effective stress intensity factors, the plastic zone size and the crack tip opening displacement are evaluated by solving the formulated singular integral equations. The solution takes into account the plastic zone size at the crack tips and thus could provide accurate stress intensity factors and crack tip opening displacements for the assessment of crack growth. In the numerical examples, the case under a y -direction uniaxial tension is considered. The effects of the ellipse's aspect ratio, the shear moduli ratio and the normalized crack-inhomogeneity distance are investigated. The results show that the shapes of the inhomogeneity and the material combinations have significant influences. Crack propagation may be hindered if the inhomogeneity is close to circular or a hole and the crack inclination angle is close to 90°. [ABSTRACT FROM AUTHOR]

Abstract: Steel pipelines used for oil and gas transportation are joined by girth welding and welds may contain weld imperfections/defects such as lack of fusion, under-cuts, porosity, slag inclusion etc. of certain height and length at certain location along the longitudinal direction of the weld. During the installation and operation, these pipelines are exposed to large plastic deformation. Fracture assessment procedures: BS7910:2005 [1] or R6 [2], are not suitable for such plastic strains (3%). The evolution of crack tip opening displacement (CTOD) of the pipeline with elliptical embedded crack in a girth weld is investigated under biaxial loading conditions using a three dimensional elastic–plastic finite element method. Based on the results, empirical formulas depicting relationship between the CTOD and the global strain are proposed. [Copyright &y& Elsevier]

Abstract: Offshore pipelines installed by reeling method are subjected to large plastic strains. When the steel pipes are joined by girth welding, both surface and embedded cracks are inevitably initiated in welding zone. The pipe lines should have adequate resistance against both crack extension by tearing and unstable fracture during installation as well as during operation. However, common flaw assessment procedures, e.g. BS 7910:2005 , are not explicitly developed for such situations with large plastic strains. The main objective of the current paper is to find a way to assess fracture capability of a practical pipeline subject to large plastic deformation. In our study, the evolution of crack tip opening displacement (CTOD) of the pipeline with semi-eliptical surface crack in weld girth is investigated under biaxial loading conditions (uniaxial tensile and internal pressure) using three dimensional elastic–plastic finite element analysis. The effects of crack depth, ratio of semi-major axis to semi-minor axis and internal pressure are examined. The results show that at moderate levels of global strain, the variation of CTOD with global strain can be well approximated by a simple linear relationship under tensile loading as well as biaxial loading conditions. Comparing the fracture assessment for the welded pipe made by BS 7910:2005 with that made by our current study, it is found that the assessment from BS7910:2005 is over conservative. A CTOD-estimation method for strain -based fracture is suggested for the pipelines when the global strain is 3%. [Copyright &y& Elsevier]