Condition Assessment of 30-Inch Steel Force Mains

King County, WA, recently completed a condition assessment of two parallel 30-in. diameter steel force mains that were constructed in 1998 after a pipeline failure was observed early in the pipeline’s service life. The condition assessment revealed internal corrosion. The epoxy lined, cement mortar coated pipes are approximately 25,000 lineal feet long and have a steel cylinder thickness of 0.14 in. This paper describes the methods selected for assessing the thin walled steel pipes, the assessment conducted, and conclusions drawn from the information collected. The assessment included construction of access points as no access was provided with the original installation. Assessment methods included external investigations (soil corrosivity, close interval potential survey, and direct assessment) and internal investigations (CCTV inspection and ultrasonic wall thickness measurements). The condition assessment concluded that although most of the joints have some corrosion and there are many locations of liner failure and pitting, the corrosion visible in CCTV video is mostly on the steel surface. Localized external corrosion was observed but the rate of corrosion is slow. Wall thickness measurements at points of corrosion indicate the remaining wall is adequate for continued operation at current operating conditions. Recommendations included spot repair of the lining, installation of passive cathodic protection for long welded sections, and periodic inspection to determine the probable rate of corrosion in order to plan for future service.