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Two Platforms, Two Shore Approaches, Two Lines And A Hurricane
The 'other factors' Prout refers to include tidal surges, shipping lanes, very soft riverbed sediments, a relatively narrow S-shaped right-of-way (ROW) with two bends of 45 and 32 degrees, a main road that couldn't be blocked, the possibility of historic shipwrecks and time constraints. To make matters worse, an unexpected massive storm (Hurricane Irene, sixth costliest in United States history) swept through the area mid-project.
Project design took six months. Working with side-scan sonar surveys and geotechnical information provided by DVP and factoring in tidal flows, designers worked out drill paths that avoided buried structures and stayed within the ROW established by DVP. This ROW had been established years previously, based on a design for a conventional dredged crossing and was relatively narrow for HDD. Mears designers also worked out construction schedules and staging area requirements and created contingency plans for evacuations due to hurricanes.
The length of the river crossing dictated that intersect drilling be used on the platform-to-platform and north shore-to-platform crossings. The platforms were stable structures in 20-25 feet of water, built to support 150 tons of equipment and supplies, and to withstand the pull loads of a 7,300-foot crossing. Barges, which are more usual in HDD shore approaches and water-to-water crossings, were ruled out due to the York River's variable currents and 5 to 7-foot tidal surges. Corman Marine Construction provided platform construction and general marine support including tugs, cranes and divers.
Corman built 50-foot by 150-foot platforms on 90-foot pilings, and loaded and unloaded equipment and supplies, with some individual loads weighing 85,000 pounds. The platforms were built in three weeks and Corman also installed 'goalposts' – twin pilings, with a cross member, that supported the drill string and held it in place from the drill rigs to the river bottom.
A water source for drilling operations had to be found before work commenced. "We tested the river water," explains Mears Project Manager Brent Curtiss, "But that water was too brackish." On the north shore, Mears dug two temporary wells that provided enough fresh water for drilling use. South shore water was taken from a hydrant. After the shore-to-platform crossings were completed, water was pumped through the three-inch HDPE to large holding tanks that were staged on the platforms.
With the platforms in place and water supply assured, HDD operations began in February 2011.