Integrity Issues for Buried Piping Subjected to Overburden and Cyclic Traffic Loads

Abstract

Buried medium and small diameter piping at road crossings and under access roads within plant or oil terminal boundaries that is not protected by any sort of casing presents inspection difficulties because access normally calls for a selective or total excavation of such piping. Often, only when an oil or gas leak is suspected to originate in the area of a road crossing, is such piping excavated and inspected. Sometimes such a finding will stimulate inspections at other selected road crossings as well as failure analysis of the pipe where the oil or gas leak was found. One of the root causes of piping failure at road crossings and under access roads may be the overburden (the soil cover above the buried pipe) and cyclic traffic load stresses that are increased in the carrier pipe because of local areas of reduced wall thickness, mechanical damage such as a dent or surface cracking. In-service corrosion or damage of the uncased pipe at road crossings can lead to integrity problems since the original design assumptions or safety margin for such piping may no longer hold. This paper reviews a typical approach to analyze buried pipe design at road crossings and adds selected API RP 579 procedures to illustrate the analysis of fatigue loadings. A brief note on common inspection practices and techniques for buried medium and small diameter piping at road crossings and within plant limits is also included.