Pipeline Ovality Assessment at Low Internal Pressures

While pipe ovalization is a common occurrence during the manufacturing of pipe sections, ovalization can also occur in pipelines during and after installation and construction. To determine the acceptability of newly constructed sections of pipeline, inline inspection (ILI) tools, or pigs, are often run to measure ovality. However, the ILI tool often flags numerous locations that do not satisfy the maximum allowable ovality criteria of 5% required by CSA Z662 (3%–6% if limit states design was used). This is possibly due to external forces (e.g., soil cover) and the low internal pipeline pressure used to run the ILI tool. It is postulated that these non-satisfactory ovality readings would not be present at higher internal pressures (i.e., operating pressures). The major objective of this study is to better understand the relationship between internal pipeline pressure and other variables with the ovality measurement. Based on results from elastic finite element analyses (FEA) of different pipe sections and loading conditions, equations are proposed to estimate pipeline ovality based on depth of cover, pipe geometric and material properties, and internal pressure (represented by a “deflection parameter”, α). A quadratic equation, as well as a simplified linear equation, were developed in this study. These equations, based on 68 different FEA load cases, can be used as a screening tool to estimate the ovality of a pipe section. Whichever equation (i.e., linear or quadratic) is used to predict the ovality in a pipe section, the deflection parameter, α, provides a means of summarizing overall trends. For example, if the pipe stiffness is increased, α (and thus ovality) decreases. Conversely, if overburden pressure is increased, α increases. Finally, if the internal pressure stiffening is increased, α decreases. The last observation is the most critical for this study as internal pressure is the most practical mitigation for reducing the number of ovalities flagged by an ILI tool post-construction. Increasing internal pressure has the additional benefit of improving the quality of the measurement data by reducing/eliminating “surging” of the ILI tool (i.e., erratic velocity changes due to sticking and slipping of the tool).