WASHINGTON — Repaired — again — with parts from a sistership, the high-tech destroyer Zumwalt left Panama on Wednesday night bound for California, where a more thorough investigation will try and determine why several rather prosaic pieces of equipment have failed several times in the ship's short life.

The Zumwalt broke down Nov. 21 while in the Miraflores locks of the Panama Canal during a transit from the Atlantic to Pacific oceans. Tugs towed the ship to the former Rodman US Naval Station, where it remained for several days before being moved across a waterway to Balboa, Panama, on the Pacific side of the canal.

At issue are coolers for the main shaft bearing lubrication oil. Each of the ship's two propeller shafts has two sets of bearings, each with a lube oil system and cooler. The coolers have proven problematic ever since the ship sailed away on Sept. 7 from its builder's yard at General Dynamics Bath Iron Works in Bath, Maine.

It is not clear who makes the coolers, which are a common feature on virtually all powered ships with a propeller shaft. "There are several vendors involved," said a Navy official, who noted that the Zumwalt has 32 coolers throughout its engineering plant — four on the shaft bearings, 16 on the ship's two advanced induction motors (AIM) and 12 more on dynamic brake resisters that manage drastic voltage changes.

General Electric's Power Conversion business is the prime contractor for the power plant. GE, according to the Power Conversion website, supplies "the integrated power systems, including advanced induction propulsion motors, switchboards, propulsion converters, power management, power quality equipment and system engineering and integration services — in essence, the essential equipment to make the power system operate."

The company noted that "these systems are a first on a US Naval destroyer."

The coolers are not in an exotic installation, although the Zumwalt is packed throughout with new technologies and configurations. Essentially, rotating propeller shafts sit on bearings that also rotate at high speed, kept rotating smoothly by lubrication oil. The oil gets hot and is cooled by recirculating through tubes, which pass through coolers containing seawater.

The leaks appear to be from seawater in the coolers getting into the lube oil, which is routinely monitored for quality. Leak problems have been discovered from inspection of the oil and from noticing seawater gathered in sumps — basins that collect excess fluids.

Two of the main shaft bearing coolers are located immediately inside the AIMs.

Each of the Zumwalt’s two AIMs is in its own machinery space, along with a shaft and a gas turbine. The shafts run directly into the AIM, without the reduction gearing familiar to most large-ship propulsion plants. The coolers are independent systems not connected to each other.

On Sept. 19, while getting underway from Norfolk, Virginia, a seawater leak was discovered in the lube oil system for one of the ship’s shafts, and a systemic problem was suspected. Navy officials ordered all four shaft bearing coolers to be replaced — two taken from the third ship in the class, the Lyndon B. Johnson — still under construction at Bath — and two from a land-based test facility in Philadelphia that replicates much of the Zumwalt’s power plant. The work was completed before the ship left Norfolk on Oct. 7 for her commissioning ceremony in Baltimore, Maryland, which was carried out Oct. 15.

The Navy official noted that one of the AIM coolers failed on Oct. 24 while the ship was at sea, and two of the dynamic brake resister coolers also have failed. Pit corrosion was found in one of the coolers, the Navy official said, but it remains unclear what is causing the failures.

"They have not figured out why they are failing," the Navy official added.

The coolers that failed Nov. 21 were two of those installed in September in Norfolk.

"There was a loss of propulsion on the port shaft," the Navy official said. "Then the starboard shaft was shut down after water was found." Again, the problem was traced to seawater getting into the main shaft bearing lube oil from the coolers.

In Panama, the Naval Sea Systems Command replaced three coolers — the two that failed and, according to the Navy official, another on the starboard shaft "as a precautionary measure." The latest replacement coolers came from the other Zumwalt-class sistership, the Michael Monsoor, christened at Bath in June.

As a precaution, the Navy official said, engineers have hooked up a temporary potable-water feed to the Zumwalt’s coolers rather than using seawater as the cooling agent, an effort to avoid damage should the leaks recur.

While the investigation continues into the cause for the leaks, the Navy official noted no mishap or criminal investigation is being carried out.

"There is no indication of noncompliance or procedural problems," the Navy official said. "The crew responded appropriately. There is no other damage to the ship."

The US Pacific Fleet on Dec. 1 issued a more complete statement about the latest cooler problems.

"On Nov. 21, USS Zumwalt (DDG 1000) experienced two engineering problems while transiting the Panama Canal, requiring it to be towed to port for repairs," said Cmdr. Clay Doss, a spokesman for the Pacific Fleet in Pearl Harbor. "Both problems involved failed lube oil coolers for the ship's advanced induction motors (AIM) that propel the port and starboard shafts.

"In response to a loss of propulsion in the port shaft, the crew found water in an AIM bearing sump, and after inspecting all four bearings, found water in a bearing sump on the starboard shaft as well. Each AIM has two shaft bearings, forward and aft, each of which is supported by a lube oil cooler. Water in the bearing sumps meant that the lube oil cooler for that bearing was leaking.

"Over the next ten days while the ship was moored in ex-Rodman naval station and in Balboa, Panama, the crew worked with technicians from Naval Sea Systems command and the engineering plant's prime contractor, General Electric, to inspect the propulsion system, replace the failed lube oil coolers and conduct dock trials. Apart from the failed coolers, additional damage was not found in the bearings or in other parts of the drivetrain. [On Nov. 30] the ship got underway and resumed transit to San Diego during which the bearings will be monitored constantly.  Upon arrival in San Diego, the ship will prepare for a post-delivery availability and combat systems activation.

"The technical community is working with the equipment manufacturers to determine why the failures occurred and to prevent recurrences," Doss said. "At this point it is unclear if the problems were caused by a design flaw. In addition to reviewing the design, the technical community is looking at ways to operate the lube oil coolers more effectively."

The cooler failures are the latest embarrassment for the Zumwalt — which has attracted enormous interest because of its unique design and new technologies. Defense News reported Nov. 6 on a Navy decision to end procurement of shells for the ship’s 155mm guns due to excessive costs — upwards of $800,000 per round. The Navy is seeking alternative shells to arm the ship’s guns.

The Zumwalt is expected to arrive in San Diego early in December.

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