The Entrance Structure

Portsmouth Naval Shipyard (PNS) is tasked with the overhaul, repair, and modernization of the US Navy’s fleet of fast attack submarines. At Dry Dock 1, PNS is challenged with executing this mission in an 80-year-old graving dock, originally built for submarines drawing half the water as today’s modern submarines. To support these availabilities, PNS would install artificial buoyancy to facilitate submarine docking and undocking—a system that was inefficient and high-risk. In 2017, the Navy endeavored to upgrade and modernize this facility through the design and construction of a first-in-class super flood basin in front of Dry Dock 1. This $160-million contract, known as MILCON P310 Dry Dock 1 Super Flood Basin, installed a lock-style system to artificially raise the water level above the adjacent river so that submarines can transit into and out of the dock without additional buoyancy. Challenged by site constraints, construction tolerances, and project schedule, the idea of the “entrance structure” was born—a pre-cast concrete seat and sill, floated into position and lowered onto a prepared foundation. This U-shaped entrance structure measures 110-ft long, 60-ft wide, by 60-ft tall, and was designed to be fabricated offsite, towed/floated into position, and lowered onto a deep foundation. The design, construction, and installation of the entrance structure represent a creative solution to challenging site constraints utilizing conventional construction techniques scaled to an unconventional size.