MONITORING TECHNOLOGY FOR EARLY DETECTION OF INTERNAL CORROSION FOR PIPELINE INTEGRITY

Abstract

Transmission gas pipelines are an important part of energy-transportation infrastructure vital to the national economy. The prevention of failures and continued safe operation of these pipelines are therefore of national interest. These lines, mostly buried, are protected and maintained by protective coating and cathodic protection systems, supplemented by periodic inspection equipped with sensors for inspection. The primary method for inspection is ''smart pigging'' with an internal inspection device that traverses the pipeline. However, some transmission lines are however not suitable for ''pigging'' operation. Because inspection of these ''unpiggable'' lines requires excavation, it is cost-prohibitive, and the development of a methodology for cost-effectively assessing the structural integrity of ''unpiggable'' lines is needed. This report describes the laboratory and field evaluation of a technology called ''magnetostrictive sensor (MsS)'' for monitoring and early detection of internal corrosion in known susceptible sections of transmission pipelines. With the MsS technology, developed by Southwest Research Institute{reg_sign} (SwRI{reg_sign}), a pulse of a relatively low frequency (typically under 100-kHz) mechanical wave (called guided wave) is launched along the pipeline and signals reflected from defects or welds are detected at the launch location in the pulse-echo mode. This technology can quickly examine a long length of piping for defects, such as corrosion wastage and cracking in circumferential direction, from a single test location, and has been in commercial use for inspection of above-ground piping in refineries and chemical plants. The MsS technology is operated primarily in torsional guided waves using a probe consisting of a thin ferromagnetic strip (typically nickel) bonded to a pipe and a number of coil-turns (typically twenty or so turns) wound over the strip. The MsS probe is relatively inexpensive compared to other guided wave approaches, and can be permanently mounted and buried on a pipe at a modest cost to allow long-term periodic data collection and comparison for accurate tracking of condition changes for cost-effective assessment of the integrity of the susceptible sections of pipeline. The results of work conducted in this project, with the collaboration from Clock Spring{reg_sign} and cooperation with El Paso Corporation, showed that the MsS probe indeed can be permanently installed on a pipe and buried for long-term monitoring of pipe condition changes. It was found however that the application of the MsS to monitoring of bitumen-coated pipelines is presently limited because of very high wave attenuation caused by the bitumen-coating and surrounding soil and resulting loss in defect detection sensitivity and reduction in monitoring range. Based on these results, it is recommended that the MsS monitoring methodology be used in benign, relatively low-attenuation sections of pipelines (for example, sleeved sections of pipeline frequently found at road crossings and pipelines with fusion epoxy coating). For bitumen-coated pipeline applications, the MsS methodology needs to increase its power to overcome the high wave attenuation problem and to achieve reasonable inspection and monitoring capability.