WASHINGTON — Gulf shipbuilders gained a new amphibious ship late Friday, and the US Navy has agreed to add a destroyer at a key Maine shipyard. The moves reflect the resolution of a long-standing "hull swap agreement" involving the ships and the Navy's two biggest shipbuilders.

Huntington Ingalls Industries received a $200 million not-to-exceed undefinitized contract action (or UCA) to order long-lead time material and perform design work on LPD 28, a yet-to-be-named 12th ship in the LPD 17 San Antonio class of amphibious transport dock ships. The ship, which the Navy did not request, was added by Congress to increase the fleet's amphibious fleet and better meet US Marine Corps requirements.

The total cost of a "fully-scoped" LPD 28 is expected to be around $2.023 billion, the Navy said last year.

As a result of the award, General Dynamics will receive another DDG 51 Arleigh Burke-class destroyer beyond the batch already on order.

By creating a 12th LPD 17, the government invoked a hull swap agreement dating back to from 2002, when a memorandum of understanding (MOU) between the Navy and its shipbuilders transferred the contracts of three LPDs that would have been built at the General Dynamics Bath Iron Works (BIW) yard in Bath, Maine, to what was then Northrop Grumman’s Ingalls and Avondale yards in Louisiana and Mississippi. Bath, in exchange, received three DDG 51s originally assigned to the Ingalls shipyard in Pascagoula, Mississippi.

Since then, the LPD program was cut from 12 to 10, then back up to 11 ships, ending with LPD 27. Northrop got out of the shipbuilding business and a new entity — HII — was formed, including Ingalls and the now-closed Avondale shipyards. The DDG 51 program was also scheduled to end at DDG 112, but subsequently was subsequently restarted by the Navy, awarding contracts to both Ingalls and BIW. As of now, all destroyer contracts through DDG 126 in fiscal 2017 have been awarded, split between Ingalls and Bath.

The MOU was reaffirmed by the Navy in 2009, when another swap agreement was signed between the Navy, BIW and Ingalls over work on DDG 1000 Zumwalt-class destroyers.

But the original MOU included a phrase that would come into effect should a twelfth LPD materialize: "A fourth DDG 51-class ship or equivalent workload would be awarded to [BIW] preceding, or concurrent with the award of LPD 28."

With Congress' action to add long-lead funding for LPD 28, the Navy and the shipbuilders worked to resolve the issue.

"Consistent with the 'swap agreement,' the Navy will award BIW a corresponding DDG 51 ship," said Capt. Thurraya Kent, spokesperson for the Navy's acquisition directorate. "This ship would be in addition to the currently contracted multiyear ships, subject to congressional authorization and appropriation."

Based on the current 10-ship multiyear buy, which runs through DDG 126, the next ship could wear hull number DDG 127, although that determination is not yet official. Since LPD 28 was not requested by the Navy and added by Congress, the extra destroyer for Bath might also be outside the Navy's existing multiyear destroyer buy.

Congress, however, still needs to authorize and approve funding for the destroyer, Kent cautioned.

Complicating the assignment of a specific hull number for the destroyer is the situation at Bath, where all destroyer building schedules — including DDG 51s and three DDG 1000 Zumwalt-class ships — are under review. It is not clear where the extra ship could fit into the yard's production schedule.

General Dynamics declined to comment on the award, but Steve Sloan, Ingalls' LPD 28 program manager, was enthusiastic about keeping the LPD 17 production line going and filling a gap until the new LX(R) amphibious ship program comes online in a few years.

"LPD 28 is a transition ship from LPD to LXR," Sloan said. "We'll have those shipbuilders rolling off LPD 26 and 27 and into 28 on a hot production line."

LPD 26, the John P. Murtha, is 95 percent complete and scheduled to begin sea trials in early 2016. LPD 27, the Portland, is 70 percent complete and will launch in January. About 2,000 employees typically work on an LPD, Sloan said.

Compared with previous ships, LPD 28 will have several modifications, Sloan said — most coming from Ingalls' proposed LX(R) design. The most visible changes will be elimination of the towering enclosed masts, which had been built of composite material at HII's now-closed Gulfport, Mississippi, facility.

"LPD will have an open mast, similar to DDG 51," Sloan said. "No more composite enclosures. And we borrowed the platform mast from [the assault ship America design] for the air search radar on the aft mast."

The schedule for the ship's construction is still being finalized, but Sloan expects the Navy to issue a request for proposal in early 2016.

"Our intent is to turn that proposal around pretty quickly," he said. "We expect a contract award late in 2016, and start-of-fabrication in December 2016."

Delivery, he added, would likely come in the second half of 2021.

The Ingalls shipyard builds more kinds of ships than any other yard doing major government work. Under construction now are the large amphibious assault ship Tripoli, four DDG 51-class destroyers, and three National Security Cutters for the US Coast Guard.