| Land falling oil and gas resources, ocean oil and gas become a developmentfocus. Increasing transport capacity, transport distance farther, the security of thepipeline is considered. Oil and gas pipeline steel with high strength, high toughness,large diameter, high pressure has become the development direction of pipelineconstruction.UOE X70steel has good mechanical properties, high strength, low temperaturetoughness, corrosion resistance, anti fatigue properties, mainly for the construction oflarge ships and offshore oil drilling platform, however, welding processing will causethe joint local uneven thermal plastic deformation and phase transformation, resultingin the welding residual stress. The welding residual stress will lead to deformation, inthe process of using the hull structure early cracking, brittle fracture problems,therefore, to carry out the research on welding residual stress of X70steel, it is veryimportant for the research of using security of oil and gas pipeline。The safety analysis of the cracked UOE X70steel and welded joint is technicalproblems of the pipeline in use, crack length critical value is an important index ofstructural safety evaluation, when the detected structures containing crack lengthvalue is less than the critical value to determine crack structure for safe state, however,conventional methods for determining the critical values of the crack length is limitedto the linear elastic stage and is not applicable to welding joint mechanicalheterogeneity. According to the new CTOD design curve derived BS7910specification, ASME specification of X70steel in the design of the stress crack lengthis determined through the lower critical value. This method is suitable for plasticwelding joint and non-uniform mechanical properties, and the calculation method ofcrack length threshold which can consider the effect of welding residual stress andfracture toughness.In this paper, the main work includes1) this paper adopts a new computing through the crack length threshold method, this method is based on BS7910-2005(Guide to methods for assessing theacceptability of flaws in metallic structures) specifications of2class a rating curve isdeduced a new CTOD design curve, named "BS79102A-CTOD design curve; TheCTOD design curve is suitable for elastoplastic range and mechanical properties ofheterogeneous welded joint, based on the curve of CTOD design type I through thecrack length threshold formula are derived, this method breaks through the traditionalcomputing through the crack length threshold is limited to the shortage of the elasticstage, but also considers the welding and the residual stress of the material fracturetoughness on the influence of crack structure safety performance.2) by adopting the technology of cell death, the combination of CTOD definition,direct calculating the critical CTOD value materials. And will test the materialfracture toughness (the critical value of CTOD) compared with the results of ANSYSnumerical analysis.3) using the ANSYS software, the establishment of X70pipeline steel V grooveANSYS model of temperature field of X70steel plate and joint and welding residualstress field calculation and analysis in turn.4) analysis BS79102A-CTOD design curve derived the calculation of mode Icrack length formula of the critical value, the formula to the welding residual stress,fracture toughness and type I through crack length to the relations between the criticalvalue, using the MATLAB software to map the surface of relationship between them,in order to direct observation.5) using ANSYS to calculate the X70steel after welding residual stressdistribution, under the condition of the containing welding residual stress is obtained,through the mode I crack length threshold numerical size distribution along the lengthdirection component.Using The BS7910-CTOD design curve to determing the the critical value ofcrack length of X70steel and welded is scientific and serious,it is can be used forreference of pressure vessels and piping production and research enterprise. |
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