Onshore Companies implements solution for crossing pipelines safely
Terrebone Parish, Louisiana
Onshore Companies was awarded the first federally funded Morganza to the Gulf Project in Terrebone Parish. This project is just a portion of the overall project consisting of 98 miles of earthen levee, 22 floodgates, 23 environmental water control structures, and a lock complex. While working on the Terrebonne Levee and Conservation District’s “Reach L” project, Onshore encountered four pipelines owned by Williams. Driving heavy earth moving equipment continuously over the buried pipelines was determined to be an unnecessary risk.
Williams, Onshore Companies, Delta Coast Consultants, and Robishaw Engineering determined the safest solution would be to flood an area over the pipelines and move the heavy equipment over the pipelines using a 220’ Flexifloat floating bridge assembly. The floating bridge assembly allowed the 117,000 lb. loaded dump trailers and tractors to cross the pipelines in approximately 2 ft. of water. An extra 1 to 1.5 ft. was added as a safety factor to make sure the floating bridge assembly did not touch the bottom. This approach allowed approximately 200 loads a day to cross the pipelines and keep the bearing pressure on the pipelines a constant 3 to 3.5 ft. of waterhead. This waterhead comes to approximately 200 psf or about the same as a man standing on top of the buried pipelines.
Robishaw Engineering utilized the Flexifloat hinge connection at the center of the 220’ floating bridge span to ensure the bridge did not become simply supported on the ends if the water level dropped. The width of the Flexifloat assembly was also sized to limit draft and distribute the ground bearing pressure if all water was lost in the basin during a worst-case scenario.
