Case Study: Bulls Bridge Penstock Replacement

Bulls Bridge Penstock & Gate Replacement

FirstLight Power Resources (Kent and New Milford, CT)

 

In early 2014 Gomez and Sullivan Engineers, D.P.C. was selected by FirstLight Power Resources (FLPR) to analyze replacement penstock alternatives and design the selected option for the Bulls Bridge Development. The site contains a 13-foot diameter steel penstock and an 8-foot-diameter steel penstock that were constructed in 1903 and 1912
respectively.

After more than a hundred years of use the two penstocks were exhibiting deformation and corrosion at the saddle supports particularly along the 13-foot diameter penstock and both penstocks required regular repairs of leakage. A 2014 alternatives analysis evaluated four options to rehabilitate the penstocks: replace the existing 13-foot diameter penstock with a new 13-foot diameter steel penstock; replace both penstocks with either a 14-foot by 14-foot concrete penstock or a 15-foot diameter steel penstock; replace both penstocks with two 10-foot diameter Fiberglass Reinforced Polymer (FRP) pipes; or rehabilitate the existing penstocks.

Based on the Alternatives Analysis findings and experience from previous penstock rehabilitations, FLPR selected the two 10-ft diameter FRP penstock alternative due to its low construction cost and minimal loss in energy generation. The design involved aligning the new penstocks to join with the existing manifold, designing three thrust blocks, and designing connections from the FRP pipe to the existing manifold. Centrifugally cast FRP pipe manufactured by HOBAS was selected for the site.

Construction of the new penstocks began in September 2015 and completed in January 2016. The new penstocks consist of two sections of FRP pipe that transition to the steel manifold at the powerhouse; the FRP penstocks are partially buried and do not have any cradle supports. The construction also included replacement of the penstock intake gates and repairs to the surge risers. Installation on a slope and under a Connecticut DOT bridge created difficulties during construction. The penstock was watered up and generation resumed by the end of January 2016.