221-year-old Portsmouth Naval Shipyard Gets Modernized Entrance Structure. Meet the Super Flood Basin!


At 5,000 tons, this Super Flood Basin arrived at the Portsmouth Naval Shipyard (PNSY) in support of the Navy’s future fleet of submarines. As part of a $21 billion effort to upgrade and modernize the Nation’s four public shipyards – each of them more than a century old, the creation of the super flood basin means PNSY can now lift submarines without the use of buoyancy assist tanks allowing for safe, direct entry to the dry dock. Akin to the how the Panama Canal works for super tankers, these structures, and supporting expanded waterfront and landside infrastructure, means Virginia-class submarines that could previously only safely access the shipyard’s Dry Dock No.1 with the use of buoyancy assist tanks—an old-fashioned and risky operation, now have an entrance to bring access up to modern standards. This will help enable the 221-year-old Portsmouth Naval Shipyard, the Nation’s oldest continuously operated public shipyard to speed maintenance and repairs at a time when all the Navy and it shipyards are facing growing threats from China and Russia.

Sebago Technics played a key role in supporting the construction team with onsite survey geometry control to ensure tight construction tolerances were met and maintained for the success of all the teams involved. Sebago’s Survey-Geomatics Team member, Justin Brown, PLS, spent 9 months onsite daily at the Cianbro facility, located on the waterfront in Portland supporting the construction of concrete formwork and maintaining the onsite survey control located directly on the Super Flood structure itself, which was floating on a barge moored at the facility’s dock.

This work played an important role in the project by providing quality control and assurance documentation of the embedded locations and alignments, as well as casting flatness of the concrete to .005’, ensuring the required watertight fit with the Caisson Gate which will nest into it at the completion of the project.

More information on this project can be found here.

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