The USS Ohio (SSGN-726), one of four Navy submarines converted from a ballistic missile to guided missile capability, has rejoined the fleet after undergoing nearly four months maintenance as well as upgrades to its radar and sonar systems.

The Ohio was in maintenance for 110 days that also included upgrades to navigation and communication systems as well as work on the diver air system, superstructure and depth control tanks, the Navy said last week.

The work was carried out by Puget Sound Naval Shipyard & Intermediate Maintenance Facility in Bremerton, Wash., and was completed on July 11.

The first four ships of the Ohio-class ballistic nuclear missile submarines were converted in the 2000s to guided missile submarines under nuclear weapons reduction pacts with Russia. Rather than retiring the vessels only halfway through their expected lifetime, they were retrofitted to carry Tomahawk Land Attack Missiles (TLAMS).

The SSGNs can carry up to 154 TLAMS–more conventional fire power than any other platform in the U.S. military.

The value of that firepower was demonstrated for the first time in combat in March 2011, when the USS Florida (SSGN-728) was deployed off the Libyan coast and accounted for about half of the Tomahawks fired during the initial stages of Operation Odyssey Dawn.

Meanwhile, the Navy has begun looking at options for replacing the firepower offered by the SSGNs once they are retired in the 2020s. Rear Adm. Barry Bruner, now the director of Undersea Warfare (N97), said in October one option would be stretching the hull used on the Virginia-class (SSN-774) to add four additional tubes.

The concept would effectively merge the SSN-SSGN mission and help offset the retirement of the four Ohio-class subs, he said.