Performing a Reliability Centered Maintenance Workshop on the World’s Largest Prestressed Concrete Pipes

The Central Arizona Project (CAP) operates and maintains three (3) 21-foot diameter prestressed concrete non-cylinder pipes (PCP) as part of the aqueduct system that delivers almost 2 billion gallons of Colorado River water per day, to Maricopa, Pinal, and Pima Counties in central and southern Arizona. The water is delivered to municipalities, industries, Native American Communities, and agricultural users; over 5 million people (more than 80% of the population of Arizona) are provided water by the aqueduct system. The system traverses rugged mountains, desolate deserts, and highly populated urban areas as it conveys water from Lake Havasu at the western edge of the state to the southern boundary of the San Xavier Indian Reservation south of Tucson. The almost 350 mi long aqueduct system is composed of about 300 miles of open channel canals, about 10 mi of tunnels, and roughly 40 mi of pipelines; 14 pumping plants along the system provide the required lift to raise the water almost 3,000 ft from the lake to the final operating reservoir. The three large diameter pre-stressed concrete pipelines installed in the late 1970s have experienced pre-stressing wire breaks and have been repaired several times since the early 1990s; the wires have broken primarily due to defects in the wire. To better understand the current maintenance practices and determine if different or additional maintenance practices should be undertaken, a reliability centered maintenance workshop (RCM) was conducted on one of the pipelines, the Jackrabbit Wash Siphon. Items evaluated and discussed in the RCM included operating criteria of the pipeline, initial fabrication and construction of the pipeline, reviews of previous repairs and their condition, what triggered those repairs, current maintenance activities and condition assessment practices, documenting potential failure modes and the consequences, and what should CAP do going forward for maintenance of all three pipelines. The RCM concluded with a reliability centered design workshop (RCD) evaluating several repair options for the Jackrabbit Wash pipeline, should the need arise. This paper discusses the RCM/RCD Workshop conducted by the reliability engineering department at CAP and the subsequent results for the three large diameter prestressed concrete pipes, or inverted-siphons, which carry water beneath typically dry riverbeds: the Centennial Wash Siphon, the Jackrabbit Wash Siphon, and the Hassayampa River Siphon.