Girth Weld Defects Research Articles

Reducing Pipeline Construction Costs: New Technologies

The Oil and Gas industry is constantly looking for ways to reduce capital investment for new projects while maintaining performance and quality standards. As the demand for oil and natural gas increases, so does the need for an expanded network of onshore and offshore transmission pipelines. However, the capital investment associated with new pipelines is enormous. There are a number of materials joining technologies that are being investigated to reduce pipeline construction costs. This document presents three technologies showing particular promise: laser pipeline welding, the use of high strength pipeline materials, and advanced automatic inspection. Test have shown that the HLAW process, which combines the LBW and GMAW processes, is felt to offer a number of benefits in terms of higher potential productivity, ability to produce welds with excellent material properties and a reasonable tolerance for fit-up variations. For existing laser power levels, a viable scenario for implementation of lasers for pipeline welding applications is the use of HLAW for root bead welding, followed by joint filling with conventional mechanised GMAW-P. Concerning the use of high strength pipeline materials, results of a project are presented. The primary objective of this study was to evaluate the mechanical properties of weld metals deposited using a number of commercially available welding consumables and welding processes. This work included characterization of weld metal microstructure, HAC susceptibility of the GMA weld metal and X100 base metal, and solidification cracking of selected welding consumables. The project evaluated commercially available consumables using several welding processes and shielding gases. The third part of the document covers results of a project aiming to collect third-party independent data and statistically characterise the systematic and random errors in girth weld defect sizing, as measured by mechanised ultrasonic testing P/E, TOFD methods and PA ultrasonic technology, in support of pipeline reliability assessments. The defect sizing accuracy of the automated ultrasonic testing (AUT) of girth welds was evaluated using zonal discrimination with focused and non-focused search units arranged for P/E and TOFD methods and PA technology. The objectives were achieved by conducting “open” and “blind” UT trials. NDE flaw verification and destructive testing for flaw size validation were performed. AUT data were collected and statistically analysed.

Very low cycle fatigue properties of butt welded joints containing weld defects. Study of acceptable size of defects in girth welds of gas pipelines

Summary This paper describes an investigation of the very low cycle fatigue strength of HT570 steel welded joints containing weld defects to determine the acceptable size of weld defects in girth welds of underground gas pipelines subjected to cyclic ground displacements due to earthquakes. Butt welded joints containing incomplete penetration (IP), blowholes (BH), lack of fusion in theintermedi‐ate pass (LF), and cracks in the penetration bead (CR) were prepared and tested under strain‐controlled conditions. All specimens tested in the present research study satisfy the fatigue design curve for girth welds of underground gas pipelines. The fatigue strength of specimens containing weld defects generally decreases with an increasing equivalent defect size. The shape of a crack initiated from a defect is affected by the reinforcement, with the surface crack propagating rapidly along the weld toe of the penetration weld over the width of the specimen. The relationship between the J‐integral range and the crack propagation rate under a very low cycle fatigue load is virtually the same as the extension of the relationship in the low J‐integral range. A crack propagation analysis based on the defect being regarded as a crack is performed to determine the relationship between the defect size and the number of cycles over the plate thickness. The critical size of defects in welds of gas pipelines under cyclic ground displacements is proposed by the analysis. The critical crack size of surface defects is applicable to buried defects, because surface defects give a more conservative evaluation than buried defects. The analytical results provide more conservative estimations than the experimental ones.

溶接きずを有する継手の極低サイクル疲労特性 ガス導管円周溶接部の許容きず寸法の検討

Very low cycle fatigue strength of welded joints of HT570 steel with weld defects was investigated to study the acceptable size of weld defects in girth welds of gas pipelines under cyclic displacement of land by earthquake. Butt welded joints containing incomplete penetration, porosities, lack of fusion in the intermediate pass, and crack in penetration bead were manufactured and tested under strain controlled conditions.All specimens tested in this study satisfied the fatigue design curve for girth welds of gas pipelines. The fatigue strength of specimens with weld defects generally decreases with the increase of equivalent defect size. The shape of cracks initiated from defects is affected by the reinforcement, and the surface crack propagates rapidly along the weld toe of the penetration welds and passes through the width of the specimen. The relationship between the J-integral range and the crack propagation rate under very low cycle fatigue load is almost the same as the extension of the relationship in low J-integral range. The crack propagation analysis which regards defects as cracks leads to the relationship between the size of defects and the number of cycles passing through the thickness. The critical size of defects in welds of gas pipeline under cyclic displacement is proposed by the analysis. The critical crack size of the surface defect could be applicable to the buried defect because the surface defects give a more conservative evaluation than buried defects. The analytical results give conservative estimations compared to experimental results.

Comparison of toughness in bending and in tension of a pipeline steel

Acceptance of defects in a structure can be evaluated from a failure assessment diagram (FAD), knowing the fracture toughness of the material and the limit load of the structure. Evaluation of these quantities is discussed here for plate material loaded in tension and bending. Fracture toughness has been shown to depend significantly on the mode of loading. The values obtained with specimens loaded in tension may be larger than those obtained in bending. In the current work, the J- integral has been evaluated with centre-cracked and double-edge-notched specimens loaded in tension; J- resistance curves are compared to previous results obtained with specimens loaded in bending. The tension geometry simulates short, through-wall, girth-weld defects in a pipe loaded in bending. Crack extension during tearing was obtained from compliance measurement, allowing the determination of the J- resistance curves. It is shown that for centre-cracked specimens, which present the lowest value of constraint (as measured by the T-stress), the measured toughness is much larger than for other types of specimen. No tearing of the crack was observed for these specimens, even after maximum load. The maximum load attained in the tests may be related to an average constraint applied to the ligament for each kind of specimen. Using these constraints, the predictions from the FAD are in excellent agreement with the experimental results. These constraints correspond to a state of plane stress for the centre-cracked specimen and almost to a state of plane strain-plane stress for the double-edge-notched specimen. For this pipeline steel (and for other modern steels with similar strain hardening characteristics) the toughness of the steel parent material at room temperature and a thickness of 6 mm was high enough that the net-section stress for the centre-cracked specimen exceeded the tensile strength before the onset of tearing.

Report on Welding Metallurgy of Girth Weld Defects in Mechanised GMA Field-Welded Pipe Lines

The terms semi-automatic arc welding and fully automatic arc welding are wel-known, yet they need to be cleanly demarcated. In semi-automatic welding the wire feeder maintains the arc and feeds the wire, the operator's job is to manipulate the arc along the joint. In fully automatic welding all the welding parameters are oresent and the entire operation from start to finish is accomplished by the equipment. The operator's role is nill.

Modelling ultrasonic response to defects in girth welds

Characterization of tissue microstructure from the backscattered ultrasound signal using the spectral autocorrelation (SAC) function provides information about the scatterer distribution in biological tissue. This paper demonstrates SAC capabilities in characterizing periodicities in A-scans due to regularity in the scatterer distribution. The A-scan is modelled as a cyclostationary signal, where the statistical parameters of the signal vary in time with single or multiple periodicities. This periodicity manifests itself as spectral peaks both in the power spectral density (PSD) and in the SAC. Periodicity in the PSD will produce a well defined dominant peak in the cepstrum, which has been used to determine the scatterer spacing. The relationship between the scatterer spacing and the spacing of the spectral peaks is established using a stochastic model of the echo-formation process from biological tissue. The distribution of the scatterers within the microstructure is modelled using a Gamma function, which offers a flexible method of simulating parametric regularity in the scatterer spacing. Simulations of the tissue microstructure for lower orders of regularity indicate that the SAC components reveal information about the scatterer spacing that are not seen in the PSD and the cepstrum. The echo-formation process is tested by simulating microstructure of varying regularity and analyzing their effect on the SAC, PSD and cepstrum. Experimental validation of the simulation results are provided using in vivo scans of the breast and liver tissue that show the presence of significant spectral correlation components in the SAC.

Evaluation of Tensile Failure of Girth Weld Repair Grooves in Pipe Subjected to Offshore Laying Stresses

The repair procedure for removal of a girth weld defect in offshore pipelines involves arc-gouging a groove into the defective area. Since high bending stresses in the pipes exist at the repair station, the concern was whether the weld repair groove may cause failure of the pipes. An experimental research program was conducted to determine the critical length and depth of repair grooves. Experiments were conducted on 100, 150, and 762-mm-dia pipes. A plastic instability failure criteria was developed and verified to predict the failure stresses for carbon steel pipes. The failure criterion is given as a function of three nondimensional parameters. These parameters are groove depth to pipe thickness ratio, groove length to pipe circumference ratio, and longitudinal stress to flow stress ratio. Stable versus unstable (i.e., leak versus break) behavior is also predicted from the failure criteria.