FATIGUE CRACK GROWTH RATE TESTING FOR CLAD AND LINED PIPE GIRTH WELD

The Direct Current Potential Drop (DCPD) method is a common method used to detect the initiation of cracks, measure crack growth rates and monitor crack propagation. The method records total measurement of crack propagation, and can be used without visual accessibility, being suitable for special environments like high temperature, high pressure and sour service. Due to the discontinuity represented by the presence of a crack, when electric current flows through the component the crack size can be measured indirectly by the change of the electrical field. The potential difference can be related to crack size through experimental, analytical or numerical calibration curves. Analytical solutions are only limited to a small number of simpler geometries and homogeneous material. For clad and lined pipe, the clad/liner is stainless steel or nickel base alloys but the base pipe is carbon steel. The conductivities of the different materials are different. For lined pipe a small gap exists between liner and base pipe, this may change the electrical field around the root of girth weld. In this paper, finite element analyses are performed to generate the numerical calibration curves for the fatigue crack growth rate testing in sour service environment for clad and lined pipe girth welds. The method developed and described in this paper measures the fatigue crack growth rate for the crack located at the weld root which is in direct contact with the operating environment.