Study On Safety Assessment Of Over-matching Welded Joint

In the welded structure of offshore pipeline used to deliver oil andgas, the welded joints are the weakness location. This is mainly becausethere are inevitable welding defects, residual stresses and inhomogeneousmicrostructures and poor mechanical properties in the welded joint zone.In the production process of petroleum and chemical industry, plasticfailure of pressure pipelines is not avoidable. The appearance and rapiddevelopment of Fracture Mechanics provides a theoretical foundation forthe safety assessment of pressure pipelines containing defects. This papermainly introduce safety assessment of over-matching welded joint of X52offshore pipeline.In this study, through the Crack Tip Opening Displacement (CTOD) testsconducted at 0 degree and performed according to the requirements of thestandard of BS7448, Part1: Method for determination of KIc, critical CTODand critical J values of metallic materials and Part2: Method fordetermination of KIc, critical CTOD and critical J values of welds inmetallic materials, in which a standard TPB (three point bending) shapewas accepted, the values of CTOD were obtained. In terms of the CTOD resultsof various zones in welded joint, the weld metal is the weakest location.According to the CTOD test result and using the BS7910 produced by theBritish Standards Institution and the SINTAP (Structural IntegrityAssessment Procedure) sponsored by the European Commission recently whichwere international and more influential, given crack size and load,appling the advanced Failure Assessment Diagram (FAD) technology in theworld, the safety assessment was carried out for welded joints of X52pipeline steel with surface flaw at the weld toe. Based on the tensiletest results of base metal and weld metal, the failure assessmentcurves(FAC) of respective levels were created on different Standards. Theassessment results based on BS7910 indicate that all the assessment points(Lr, Kr) are located out of the regions defined on the failure assessmentdiagrams, so the structure is not safe. While the assessment results basedon SINTAP display that all the assessment points are located out of theregions defined on the failure assessment diagrams. so the structure cannot be accepted. This study laid the foundation of application of SINTAPto pipeline structure assessment.We can see from the safety assessment results and FADs that each levelhas more detailed require for the materials,tensile properties than theone before, so the corresponding more accurate and less conservativeresults can be obtained. This paper describes the defect assessment methodfor strength mismatched welded structures, resulting from SINTAP projectbased upon the ETM and R6 procedures. The SINTAP procedure provides twosituations in detail, which depend on the yield strengths of the weld andthe base materials differ by whether more than 10% or not. So comparingwith BS7910, SINTAP need more specific and detailed input data for thesafety assessment of the weld metal and base metal under strengthmismatched condition.BS7910 provides a safety assessment method for engineeringapplication which shows the influence between Lr and Sr. However, the FACsbuilded on Lr are different from those builded on Sr. The paper alsodisplay that Lr used as abscissa is superior to Sr.Through the comparison and analysis between the BS7910 and SINTAP topipeline circumferential surface flaws, we can achieve creative thoughtsfor the safety assessment of pipeline circumferential surface flaws whichis fit for china. The study is of great significance in terms of engineeringand method effectiveness.