The Navy’s newest Littoral Combat Ship is set to leave a Virginia naval base this week following a December engine casualty that left the ship sidelined for two months.
USS Milwaukee (LCS-5) is set to leave Joint Expeditionary Base Little Creek-Fort Story as early as Wednesday under propulsion restrictions for shock trials in Mayport, Fla., several sources told USNI News.
Sources told USNI News the ship would proceed underway under its own power with a set of restrictions on its propulsion plant designed to prevent stress on the gearing system that ties the ships diesel engines together with its gas turbines and routes the output to the ship’s waterjets.
Under the restrictions that will limit Milwaukee’s speed, the ship will arrive at Mayport by the end of the month to conduct shock trials to prove the ship’s systems can operate in combat conditions. The remaining work to make the ship fully operational will be completed in Florida, USNI News understands.
When reached by USNI News on Tuesday, U.S. Surface Forces Pacific (SURFPAC) would not confirm the ship’s status or its impending departure from Virginia.
Milwaukee has been sidelined since Dec. 11 after the ship suffered a propulsion failure and was towed into port.
A subsequent investigation found that the high-speed clutch on both of the ship’s combining gears had been damaged following an emergency stop on Dec. 7, according to a February Naval Sea Systems Command memo on the incident obtained by USNI News.
“During this transit, the ship was running in CODAG (both gas turbines and main propulsion diesel engines with all four waterjets in operation) at full power,” read the memo.
“This is referred to as Propulsion State 9.”
During the full-power run a fuel valve failure led both of the Rolls Royce MT30 gas turbines to shutdown at the same time, according to a report last week in Defense News and confirmed to USNI News by several sources. If operating correctly, the clutches should have disengaged the gearing from the MT30s and allowed the ship to proceed on its twin Colt-Pielstick diesel engines.
The following is a description from an American Gear Manufactures Association technical paper on Freedom-class propulsion system.
Four water jets are driven by two symmetrically arranged [Combined Diesel and Gas (CODAG)] systems, where the high speed combining gear and the low speed splitter gear are separately installed, linked by long intermediate shafts each. Gas turbines and diesel engines can be engaged separately in cruise or higher speed propulsion modes, or jointly drive the water jets achieving a top speed in excess of 40 knots in this CODAG mode. To match with different required water jet speeds, the diesel engine inputs are equipped with two gear stages where the adequate gear ratio is selected via a multi disc clutch engagement.
Instead the high speed clutches were ground severely for approximately two-and-a-half seconds spraying chunks of the clutch plates and contaminating the lube oil system of the propulsion plant, sources told USNI News.
Four days after the emergency stop, the lube oil systems in both Milwaukee’s combining gears lost pressure and the ship was unable to proceed on its own power.
The Navy and shipbuilder Lockheed Martin consider a flaw in the ship’s software as the prime culprit for why the clutches didn’t immediately disengage from the MT30s, sources told USNI News.
In January, Lockheed Martin’s vice president of Littoral Ships & Systems Joe North told reporters the company and the Navy were focusing on the logic statements from the ship’s systems to tease out what the software told the combining gear hardware to do and how those can be prepared.
Following the Milwaukee incident, NAVSEA’s engineering directorate recommended last week the other USS Freedom (LCS-1) and USS Fort Worth (LCS-3) not to use Propulsion State 9 until it evaluated the control systems and combining gear designs but later took back the recommendation based on the differences between the two ships and Milwaukee.
Fort Worth is currently sidelined in Singapore following a casualty to its own combining gears due to operator error during a propulsion system test in January.
The Navy said it was “unlikely” the cause of the combining gear damage in Milwaukee and Fort Worth were related.