Study On Fracture Toughness Of Offshore Pipeline And Safety Assessment

In the welded structure of offshore pipeline used to deliver oil and gas, the welded joints are the weakness location. This is mainly because there are inevitable welding defects, residual stresses and inhomogeneous microstructures and poor mechanical properties in the welded joint zone.In this study, the Crack Tip Opening Displacement (CTOD) tests were carried out to investigate the crack resistance behavior of weld metal, HAZ and base metal of X65 pipeline steel. The Drop Weight Tearing test (DWTT) was carried out to investigate the crack arrest property of X65 base metal. CTOD tests were conducted at 0 degree and performed according to the requirements of the standard of BS7448, Part1: Method for determination of KIc, critical CTOD and critical J values of metallic materials and Part2: Method for determination of KIc, critical CTOD and critical J values of welds in metallic materials, in which a standard TPB (three point bending) shape was accepted. For the heat affected zone (HAZ) specimens, the microstructure observation was performed to insure that the tip of the crack was located in the coarse grain zone in order to obtain valid and reliable results. The result explains the dispersity of the test values, and provide reliable evidence for ECA (engineering critical assessment), validate the BS7448 and embody its superiority. The test results show that the impact and fracture toughness of weld metal are much lower than that of base metal, which is mainly because the microstructure of the weld is coarse and non-uniform, although the weld has the same microstructure which is ferrite and pearlite as the base metal. The results of crack tip opening displacement tests show that the crack resistance of X65 base steel is much better than that of the weld. The temperature is –35℃ for X65 pipeline steel when the section of shear is 40 percent of the whole fracture surface, which prove that the X65 steel has good crack arrest property.In this investigation, Structure Integrity Assessment Procedure (SINTAP) which is internationally and the most influential safety assessment standard was applied to assessment for welded joints of the API 5L X65 pipeline steel with embedded flaw and surface flaw at the weld toe. The failure lines of level 1 and 3 of the weld were derived from the tensile test results. The results of the assessment showed that all assessment points (Lr, Kr) are located within the failure lines of analysis level 1 and 3. So welded joint of the pipeline is safe. Critical flaw size was calculated and fatigue life was estimated according to the above assessment results. The study is of great significance in terms of engineering and economic effectiveness. This study laid the foundation of application of SINTAP to pipeline structure assessment.