Design of Two 108-in. Welded Steel Pipes on a Concrete Span Bridge

In 2016, a joint venture that includes Jacobs Engineering, Inc. was hired to complete the design-build of a new 360-MGD (expandable to 540-MGD) water treatment plant for the city of Houston (COH) and its partners. This new facility is known as the Northeast Water Purification Plant (NEWPP) and draws water from a new intake on Lake Houston and conveys it approximately 2 mi to the site of a new water treatment facility. Due to shallow depths of Lake Houston and raw water intake requirements of the Texas Commission on Environmental Quality (TCEQ), the new intake pump station is located approximately 1,000 ft offshore, requiring a bridge used for access and utilities. Conveyance of the raw water is through twin 108-in. diameter welded steel pipes that utilize a bridge to connect the offshore pump station to the land and ultimately the water treatment plant. Design of these 108-in. pipes was coordinated closely with the bridge design to ensure both objectives were met for loading and support spacing. Ultimately, a 50-ft spacing was selected for the bridge spans and pipe support lengths. Design of the support systems on the bridge required special consideration due to large differential in thermal expansion, while inside the pump station further consideration was given to stresses combined with thermal expansion forces. The design-utilized ring girder style supports commonly found in penstocks combined with fixed and dimensionally flexible supports to support and restrain the pipe while accommodating thermal movement without exceeding allowable pipe stresses. The design required special stress analyses to ensure all piping stresses are accommodated for the piping limits as well as the more important aspect of not placing damaging forces on the pumps through the discharge piping.