Characterization of Through-Wall Cracks and Resulting Leakage Rates in Pipelines

Abstract

Major challenges are foreseen in quantitative risk assessment of ILI detected crack-related features for thin-wall pipelines due to disproportionate ILI sizing uncertainties relative to pipe wall thickness. Therefore, the likelihood of defects growing into through-wall cracks, leading to product leakage, even at relatively low operating pressures needs to be considered in thin-wall pipelines. To support quantifying the risk associated with operating such pipelines, leak rate simulations were conducted to help with the release consequence assessment and risk ranking of ILI reported crack features to design an appropriate mitigation plan. Finite element analysis (FEA) and computational fluid dynamics (CFD) methods were used to determine the physical characteristics of through-wall cracks and the resulting leakage rates. The study highlights that, for a given fluid, the threshold for leak occurrence and the leakage rate depend primarily on the crack geometry and the operational pressure. CFD simulation results for the sensitivity cases modelled in this study showed that the leak rate can become very significant as the crack opening and internal pressure increase. These CFD results were then compared with the results obtained from a closed-form analytical model. It was determined that the analytical model started to deviate from the CFD results as the internal pressure increased and the crack opening became larger. This was explained by the fact that the analytical model was intended to be used for single-phase flow under laminar, isothermal conditions. Since its applicability to the turbulent flow regime has not been established, the deviation between the CFD results and the analytical results suggests that the use of the analytical model in the turbulent flow regime could greatly underestimate the leak rate. In addition, the importance of the design of experiment, and proper modeling of turbulence and crack surface roughness in the leakage rate estimation was demonstrated.