The Central Arizona Project (CAP) aqueduct stretches 336 miles across the state of Arizona, predominantly with open channel canals. When the system needs to cross a water course, such as a river or wash, it transitions to closed conduit pipelines. Depending on the location in the aqueduct, the pipelines range from 13- to 21-feet in diameter, and are an assortment of steel, prestressed concrete and reinforced (monolithic cast-in-place) concrete. The pipelines are critical for delivering almost two billion gallons of Colorado River water per day to over five million people for municipal, industrial and agricultural use, and are considered “forever” assets.
The pipelines are critical for delivering almost two billion gallons of Colorado River water per day to over five million people for municipal, industrial and agricultural use, and are considered “forever” assets
When installed in the mid-1990s, two of the pipelines, which are 21-feet diameter steel, were lined with a coal tar epoxy coating system to provide corrosion protection to the internal steel surfaces. However, both pipelines are experiencing deterioration of the liner and, consequently, there are areas of corrosion throughout their interior surfaces, predominantly along the invert. The worst conditions are at the joints, at locations where the pipes change direction, and at the welds. In addition, there are multiple areas in each pipeline where the lining is failing around the complete circumference of the pipe. Both pipelines have pitting in the steel, though, to date, no structural damage has been observed. Left unchecked, pitting and steel deterioration will continue to occur, resulting in leakage and eventual failure of the pipelines.
The pipelines, one about 8,500 feet long, and the other, about 9,000 feet long, are referred to by the rivers under which they cross, the Salt River Siphon and the Agua Fria River Siphon. The steel pipelines were installed to replace prestressed concrete pipelines installed in the late 1970s that were experiencing premature distress of the prestressing wires. The prestressed concrete pipelines were abandoned in place and are adjacent to the steel pipelines at each river crossing.
Maintenance practices on smaller diameter steel pipelines have been to repair or replace the liner, depending on the overall condition. To replace the interior coating, the pipeline must be removed from service for an extended period. A recent relining project on a 2,500-foot long, 12-foot diameter pipeline required the pipeline be out of service for 90 days. Due to the location in the aqueduct system, neither the Salt River Siphon nor the Agua Fria River Siphon can be out of service more than six weeks.
A recent relining project on a 2,500-foot long, 12-foot diameter pipeline required the pipeline be out of service for 90 days.
A 2018 inspection of the Agua Fria River Siphon estimated about 20 percent of the lining was worn and corroded steel exposed, resulting in the recommendation to completely replace the lining. A 2018 inspection of the Salt River Siphon estimated about 15 percent of the lining was worn and corroded steel exposed, also resulting in the recommendation to completely replace the lining. In late 2019, a partial relining project was undertaken for the Salt River Siphon. During the six week outage, less than 10 percent of the interior lining was able to be replaced. It became apparent a different maintenance strategy was required.
CAP decided to perform a lifecycle cost analysis (LCCA), considering a 100-year period to evaluate several maintenance strategy alternatives to ensure the pipelines’ reliability and the reliability of the aqueduct system. A consultant was hired to assist and was provided three alternatives to evaluate:
- Continue to repair/replace the interior liner during a six-week outage;
- Construct a second, smaller capacity pipeline to allow the main steel pipeline to be removed from service for longer than six weeks (allows more of the lining to be repaired during an outage);
- Abandon the steel pipeline and construct an equal capacity pipeline of a material that requires less maintenance.
The problem with the first alternative is the operational constraint that only allows the pipeline to be removed from service every five years. Based on production rates from previous pipeline relining projects, it would take 15 outages to reline the pipe. By only removing the pipeline from service every five years, it would take 75 years to reline the pipeline – the pipeline would fail from corrosion before it could be maintained. This alternative was deemed unsustainable, but was the base alternative to which the others were compared.
The second alternative initially looked at constructing multiple, smaller diameter pipelines to provide some bypass capacity so the steel pipelines could be removed from service for longer than six weeks. However, this alternative morphed into rehabilitating the abandoned prestressed concrete pipelines by lining them with a structural concrete liner able to withstand the operating pressure, and then lining the steel pipelines with a cementitious mortar.
The third alternative considered building a new pipeline of the same capacity as the steel pipeline, but from a material requiring significantly less maintenance, such as monolithic concrete. In such a large diameter, concrete is the only feasible choice; CAP has three, 21-foot diameter monolithic concrete pipes that are virtually maintenance free.
For each of the three alternatives, design costs, construction costs, and operations and maintenance costs were considered over a 100-year period. Table 1 provides the estimated total cost of ownership over the next 100-years in 2022 dollars for each alternative.
As part of the alternatives analysis, a multicriteria analysis was developed for considering factors that are critical, but often more difficult to quantify than costs, such as project planning, constructability, operations and resiliency (see Figure 1 matrix). Weighting factors were applied to each criterion, regardless of the alternative, and scores were given to each criterion for each alternative based on how important the criterion was. A lower number indicated the criteria was not as important to CAP in the evaluation, higher numbers were more favorable.
From the multicriteria matrix and the ownership costs, it is clear Alternative 2 is the preferred alternative.