DFW’s need to secure water supplies for future regional growth is ongoing, and increasingly distant water sources are being sought. With predictions that the region’s water demand could double in 50 years, two of DFW’s largest water providers, Dallas Water Utilities and Tarrant Regional Water District, collaborated to develop a new water source at Lake Palestine—a location over 140 miles to the southeast of the DFW Metroplex.
A Collaborative Approach to Water Supply Development
Called the Integrated Pipeline Project, or IPL, the 350 MGD raw water transmission system will comprise nearly 150 miles of mostly 108-inch diameter pipe, six pump stations, a balancing reservoir, ancillary facilities and interconnections to existing water supply infrastructure. IPL is expected to cost $2B with an estimated completion date of 2035. Initial design started in 2010.
Steel Pipe Proves to be a Competitive Option
The first pipeline construction contract, Segment 15.1, was awarded in early February 2014 using a competitive sealed proposal process. While an option for Prestressed Concrete Cylinder Pipe (PCCP) was allowed, steel pipe furnished by NWP was selected by the installation contractor, Garney Construction. NWP has since furnished steel pipe for three additional IPL segments awarded to date—now totaling over 49 miles of 108-inch diameter pipe.
PCCP had been used for decades in the region for large-diameter water transmission pipelines, but long-standing performance issues with PCCP were also a concern. In more recent years, large-diameter steel pipe, up to 96-inch diameter, had been used successfully on several Tarrant Regional projects. However, industry experience with furnishing 108-inch diameter steel pipe with shop applied cement mortar lining was limited.
The IPL design team undertook an extensive vetting process which included constructing actual 108-inch diameter steel pipe demonstration sections. This proved AWWA C200 steel pipe with factory-applied AWWA C205 cement mortar lining and AWWA C222 polyurethane coating was a feasible addition to the IPL program specification.
NPW’s ability to furnish steel pipe in 50-foot lay lengths, compared to 24 foot lengths for PCCP, provided a significant installation cost advantage. The Weld-after-Backfill construction method also was incorporated with the use of bell and spigot lap welded joints, providing further installation savings.
Performance-based Design Provides Steel Pipe Savings
A 100-year design life was a stated goal of IPL but the program also recognized that pipe material would be a major cost component of the project. A performance based design, as set forth by AWWA Manual M11, proved cost effective. To provide sufficient cylinder stiffness for handling and transport, the minimum pipe wall thickness was based on a diameter to thickness (D/t) ratio of 230, a common limit for cement mortar lined steel pipe.
Using steel with 42,000 psi design yield strength, the D/t ratio provided a minimum pipe working pressure of 182 psi. However, many reaches of the project entail working pressures above 200 psi which would require an increase in pipe wall thickness above the handling minimum. In such situations, a higher yield strength steel of 46,000 psi was incorporated into the pipe wall design, partially offsetting the wall thickness increase that would be necessary with lower yield strength steel.
This performance based design approach is estimated to have provided a cost savings in steel of nearly 10% for the higher pressure segments of the pipeline.