Some Remarks on the Strain Based Assessment of Pipeline Dents

Abstract

In order to use the method to estimate strain in dents proposed in Appendix R of ASME B31.8 – Gas Transmission and Distribution Piping Systems – 2003 Edition, in a previous paper the authors used fourth-order B-spline curves to approximate the dent profile in both longitudinal and circumferential directions. This approach proved to be effective to estimate bending strains when the coordinates of the deepest point of the dent are known. This paper studies the impact of the distance between the closest ILI tool sensor and the dent apex on the measured dent depth and on the circumferential bending strain. The results obtained were tested against a set of nonlinear finite element analyses of dented pipelines carried out at PETROBRAS R&D Center. It was concluded that the number of sensors of the ILI tool, and consequently the maximum possible angular distance of the closest sensor in relation to the deepest point of the dent, plays a decisive role on the exactness of the results achieved. It is also mentioned that the equations to estimate the total strain in the inner and outer surface of the pipe wall presented in Appendix R contains a conceptual error and therefore shall be revised. Furthermore, longitudinal membrane strains obtained using the formula proposed in Appendix R were compared to finite element results. The comparison showed that directions on how to measure the dent length should be incorporated into the ASME B31.8 Code.