Design changes approved by the U.S. Navy for its new high-speed transports left them vulnerable to wave slam, resulting in bow damage to four ships that cost more than $2 million to repair.
In a report to Congress assessing the performance of the joint high speed vessels (JHSVs), the Pentagon’s chief weapons tester said the entire class “requires reinforcing structure” between the hulls. The aluminum catamarans, redesignated last year as expeditionary fast transports (EPFs), are built by Austal USA in Mobile, Ala.
“The Navy accepted compromises in the bow structure, presumably to save weight, during the building of these ships,” Michael Gilmore, director of Operational Test and Evaluation, wrote in a letter to top members of the House and Senate defense committees. “Multiple ships of the class have suffered damage to the bow structure, and repairs/reinforcements are in progress class-wide.”
The report, delivered to Congress in September and released publicly in February, states that the American Bureau of Shipping (ABS) originally recommended that the forepeak be built to withstand loading of 26.1 pounds per square inch (psi) caused by wave slam. Austal USA proposed a revised safe operating envelope (SOE) “to eliminate the likelihood of head seas,” recommending a design that could withstand pressures of only 17 psi. The ship design manager approved the proposal and the Navy’s program manager concurred.
“The decision to accept a limiting SOE and build the ships to withstand only 17-psi loading from wave slamming has resulted in a ship class that is easily damaged,” the report states.
Michelle Bowden, a spokeswoman for Austal USA, referred questions about the revised ship design to the Navy. Lt. Amber Lynn Daniel, a Navy spokeswoman, said the service accepted Austal’s recommendation because the shipbuilder’s analysis showed that the lighter-weight design met ABS and Pentagon criteria.
“(The report) validates that EPF meets, and in certain areas exceeds” key Defense Department performance parameters, Daniel told Professional Mariner. “Through operational testing, the Navy validated the ship’s structural design fulfills requirements.”
In his letter to lawmakers, Gilmore stated that to utilize the speed capability of the ships — 35 knots when loaded — waves must not exceed 1.25 meters, or about 4 feet. In waves up to 2.5 meters, or about 8 feet, the ships must slow to 15 knots. In waves up to 4 meters, or about 13 feet, they must slow to 5 knots. In higher seas, the ships can only hold position and wait.
USNS Fall River moored with USNS Choctaw County.
Photos courtesy U.S. Navy
“Operating the ship outside of the SOE or encountering a rogue wave that is outside of the current sea state limits can result in sea-slam events that cause structural damage,” Gilmore wrote. “The operational restriction of the SOE is a major limitation of the ship class that must be factored into all missions.”
Austal USA has delivered six of the 338-foot ships to the Navy, with five more under construction or under contract. Four have had their hulls repaired and reinforced: USNS Spearhead (EPF 1), repair cost $511,000; USNS Choctaw County (EPF 2), repair cost $310,000; USNS Millinocket (EPF 3), repair cost $366,000; and USNS Fall River (EPF 4), repair cost $1.2 million.
“EPF 4 repairs were emergent in nature and unscheduled, resulting in a higher cost than the other ships,” Daniel said. The Navy expects the cost to modify USNS Trenton (EPF 5) and other ships in the class to be consistent with Choctaw County and Millinocket.
The modifications add 1,736 pounds to each ship’s weight, displacing 250 gallons of fuel, but the report predicts minimal impact on range. Reinforcing the bows “should allow full use of the ships within the original SOE” without continued risk of damage, according to the Navy.
The report cited other problems with the new class of fast transports, including generators that failed “at a much greater rate than predicted” and waterjets that “suffered broken or failing reversing plates.” In a key test of EPF capability, USNS Millinocket failed to transfer a vehicle to a Navy mobile landing platform on the open ocean.
“By design (ramp limitation), it can conduct vehicle transfers (only) in sea states with significant wave heights of less than 0.1 meters, which are normally found in protected harbors,” Gilmore wrote in his letter to congressmen. “I do not consider vehicle transfers inside a harbor as operationally realistic.”
Despite the report, the Navy has expressed confidence in the transports and will continue to “update and evolve” operating guidance, Daniel said.
“EPFs are currently deployed to the Arabian Gulf, West Africa and the central and western Pacific, performing a number of missions above and beyond their original design as a fast troop transport,” she said. “The ship continues to not only meet, but to exceed the Navy’s expectations.”