WASHINGTON — For the second time in a month, a US Navy littoral combat ship (LCS) has been sidelined due to machinery problems.

The Fort Worth, a Freedom-class LCS that has been operating for more than a year in the western Pacific, "experienced a casualty to the ship's combining gears during an in-port period in Singapore Jan. 12," according to Lt. Cmdr. Matt Knight, a spokesman for the US Pacific Fleet.

So far, according to Knight, "the casualty appears to be caused by a failure to follow established procedures during maintenance."

The ship remains at the Changi Vaval Base in Singapore while an investigation continues. "A maintenance team consisting of technical representatives and shipyard personnel is on board to evaluate the gears and to make the necessary repairs," Knight said in an email.

"There is no estimated date of completion at this time," he added. "The investigation into this casualty is ongoing."

The Fort Worth's deployment had been a major success for the LCS program up to now. The ship left its home port of San Diego on Nov. 17, 2014 for a scheduled 16-month Western Pacific deployment, and was recently extended due to its usefulness in the region. Since arriving at Singapore at the end of December 2014, the ship has taken part in numerous operations throughout the region, including the search for the missing Indonesia AirAsia Flight 8501, and visits and exercises with Korean, Vietnamese, Indonesian, Indian and Thai naval forces.

During the deployment the ship has made several crew swaps, part of a plan to rotate three crews each serving about four months on board.

The news about the Fort Worth's mishap comes after the next ship in the class, the Milwaukee, suffered a breakdown at sea about 40 miles off the Virginia coast on Dec. 11 and had to be towed in to Little Creek, Virginia during its delivery voyage from the shipyard at Fincantieri Marinette Marine in Marinette, Wisconsin.

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The Milwaukee also suffered problems with its combining gear, caused by shavings and debris getting into the lubrication system. That incident remains under investigation, and the ship is still at Little Creek awaiting a determination of the problem and a fix.

While the problems sound similar, Navy officials are claiming they are not due to the same causes.

The Fort Worth's problem "is similar, but not related to what happened with the Milwaukee," said one source familiar with the situation. The Milwaukee's problem could have a physical cause, while the Fort Worth's problem seems to have come from personnel not following proper procedures.

During sea trials last summer, the Milwaukee also experienced a problem when an engine was improperly turned on while the ship was in port, and resulting repairs caused the trials to be delayed.

A Navy source provided more details of the problem experienced with the Fort Worth in Singapore.

"During startup of the main propulsion diesel engines, lub[rication] oil was not supplied to the ship's combining gears due to an apparent failure to follow standard procedures," the Navy source said.

"The insufficient flow of lube oil resulted in high temperature alarms on the port and starboard combining gears. An investigation is underway to examine the issue in depth and determine the corrective action required to prevent such actions in the future."

Knight noted that "casualties involving watch standing procedures are rare. Our LCS crews are well-trained and familiar with this LCS variant."

The Fort Worth and Milwaukee belong to the Freedom class, the single-hull version of the LCS built by Lockheed Martin and Marinette Marine.

The other LCS type, the all-aluminum Independence class, has an entirely different machinery installation.