Each time a black attack submarine travels the Thames River, Coast Guard boats are close by, protecting the valuable asset and its crew.

It has been this way since Sept. 11, 2001, when the Coast Guard offered to escort submarines because the Navy was busy preparing for combat in Afghanistan and later, in Iraq. Coast Guard crews were armed and trained to protect the submarines against attacks by terrorists or saboteurs.

Five years ago, the two services began discussing whether the Navy could resume escorting its own submarines nationwide so Coast Guard ships and personnel could be used for other missions.

Beginning this fall, instead of the distinctive metal boats with Coast Guard orange coloring, the Navy's Coastal Riverine Force will protect submarines on the Thames River. The Navy has already taken on most of the security missions for submarine transits in Alaska.

After Groton, the transition will begin in other ports along both coasts.

"Both services are working together to develop a long-term solution for transferring this mission back to the Navy," said Lt. Cmdr. Jamie Frederick, spokesman for the Coast Guard's Atlantic Area.

The Coastal Riverine Force is a new command that is responsible for conducting maritime security operations to defend high-value assets, critical maritime infrastructure and ports and harbors. The Navy's Riverine and Maritime Expeditionary Security Forces merged in 2012.

Coastal Riverine Squadron 8, headquartered in Newport, R.I., has a platoon that operates out of Groton. Twenty-two sailors will escort the submarines using four boats, said Lt. Cmdr. Charity Hardison, spokeswoman for the Navy Expeditionary Combat Command, which oversees the Coastal Riverine Force.

The Navy also is looking at using its forces for submarine escorts in Norfolk, Va., Bremerton and Bangor, Wash., Mayport and Cape Canaveral, Fla., Kings Bay, Ga., and San Diego, Calif., said Lt. Cmdr. Brian Badura, spokesman for U.S. Fleet Forces Command.

The services have not yet decided on the schedule, or on how the responsibilities for routine escort missions in each port will be divided. The Coast Guard would support the Navy during any contingency or heightened security environment, Badura said.

"This re-leveling of effort is part of that normal process, which is necessary, especially in this financially-constrained environment," he said.

Coast Guard Station New London also is responsible for patrolling local waterways, conducting law enforcement boardings, ensuring boaters are safe, responding to search-and-rescue calls and maintaining safety zones. About 200 "high-value units" needing escorts travel through the area each year.

It costs about $2 million annually to escort the Groton submarines, Badura added. The Navy does not reimburse the Coast Guard. He did not have an estimate for how much it will cost the Navy to escort submarines in ports nationwide.

The threat to submarines has not diminished, Badura said, and while the duties are shifting from one service to the other, "the requirement remains the same."

"Submarines are one of our most capable platforms," he said. "We want to make sure when they're coming in or out of port, we give them the escort they need."

j.mcdermott@theday.com