Self-maintenance Dry Dock Design for USCG
The United States Coast Guard Yard selected JMS Naval Architects to provide engineering services to support the development of an acquisition plan for a floating dry dock to dock various cutters, in particular, the new 360’ Offshore Patrol Cutter. The Coast Guard Yard located on Curtis Bay in Baltimore, MD is the Coast Guard’s sole shipbuilding and major repair facility and has been in operation since 1899. The Yard’s only remaining floating dry dock was decommissioned in 2018. The dry dock, the ex-Navy USS OAKRIDGE (ARDM-1), was constructed in 1944 and served in the Pacific during WWII. After the Navy decommissioned the dock in 2001 it was transferred to the Coast Guard Yard with the intention of being operated for no longer than 5 years. OAKRIDGE was able to accommodate ships up to 437 feet in length and 6,000 long tons displacement.
JMS was tasked with developing a concept design to meet the Yard’s requirements for docking the 4,700 long ton Offshore Patrol Cutter and developing a rough order of magnitude cost estimate broken down by SWBS work groups. The concept phase defined the principal dimensions to provide sufficient lift capacity, stability, and access to the vessel for maintenance. A long service life and self-maintenance were design priorities for the customer. As a result, a pontoon-type of floating dry dock having continuous steel wings spanning a series of detachable pontoons was selected. The pontoon sections disconnect and float out from under the wing walls so they can be docked by the remaining sections allowing for re-coating and steel renewal of the underwater hull. Other design elements intended to increase the service life include the use of bulb flat stiffeners in ballast tanks to improve coating application, the use of double continuous welds to prevent crevices and corrosion, and a steel thickness allowance over the minimum scantling requirements.
The dry dock is 398’ long overall with 81’ clear width between the wing walls. The 380’ long pontoon deck is made up of eight individual sections and is rated for a uniformly distributed load of 1.3 long tons per square foot. JMS designed the scantlings to comply with ABS Rules for Steel Floating Dry Docks and determined the keel block loading limits based on blocking keel structure and longitudinal bending considerations. Stability limitations, including the maximum allowable vertical center of gravity, were based on ABS and U.S. Coast Guard stability standards. The ballast system uses one 10-inch submersible pump per ballast tank rated for 3,000 gallons per minute. Redundant deballasting is provided by cross-over valves in each pontoon section for reliability and flexibility. The pumps can easily be removed from the tank and replaced with a spare pump to reduce downtime associated with pump failures and maintenance. Flood valves will be controlled via electric actuators and reach rods at the safety deck. Wing wall crane alternatives for serving the dry dock were also examined in the concept design.