"Rough Water," Feature Article, January 2004

rough water

The Navy designed a virtual ship—but was it tough enough?

by Harry Hutchinson,
Executive Editor

When the U.S. Navy officially named the San Antonio this past summer, it was honoring the city of the Alamo and introducing the first in a new class of amphibious transport dock ships. The Navy said that the San Antonio, also known as LPD 17, was its first surface ship to be completely modeled on computers before construction began.

The "LPD" stands for "landing platform dock," a former name for ships of the San Antonio class. They are 648 feet long and 105 wide, and designed to carry tilt-rotor Ospreys or helicopters on deck and amphibious attack craft below, as well as a crew of more than 350 sailors and a force of 800 Marines.

Among the innovations on the new LPDs is an Advanced Enclosed Mast/Sensor system, a composite material structure that protects radar and communications antennas from weather, and helps reduce the ship's vulnerability to detection by hostile radar.

Northrop Grumman Ship Systems built the LPD 17 at its Avondale shipyard in New Orleans. Sen. Kay Bailey Hutchison of Texas broke a bottle of sparkling wine on the hull to christen the ship in July. (The wine came from Texas, too—Millennium Cuvée, from Cap Rock Winery in Lubbock.)

The San Antonio, which was christened in July, is the first in a new class of U.S. Navy ships that are designed to carry men and advanced air and landing craft to wage amphibious warfare.

The hull can accommodate two landing craft air cushion and more than a dozen advanced amphibious assault vehicles. Each landing craft can carry 60 tons, which might include an Abrams tank, at speeds of up to 40 knots. The assault vehicles can travel on the water at about 25 knots and can keep up with the tanks on land.

The Osprey and the advanced assault vehicle have not been deployed yet. The Osprey, or V-22, flies as a turboprop plane, but can rotate its nacelles 90 degrees to take off and land like a helicopter. The airplane was grounded in December 2000 after two accidents in one year killed more than 20 Marines. Testing resumed in May 2002.

The advanced amphibious assault vehicle is expected to enter service later in this decade. It is faster, and more heavily armed and armored than the current amphibious vehicle that it will replace.

The San Antonio has hydraulically controlled gates at the stern, where the landing craft and assault vehicles leave the ship and return. The hydraulic system underwent several design changes in virtual prototype before it ever rocked in the water.

The hydraulic power unit, shown here in Huber's fabrication shop, controls the San Antonio's sterngate, where landing craft are to leave the ship.

Huber Inc., a manufacturer of fluid power systems in Jefferson, La., supplied the hydraulics, as well as the upper and lower sterngates.

According to Dominic Lorino, a Huber mechanical engineer, one of the biggest challenges was in the hydraulic power units. Like all mission-critical parts of the ship, they had to be able to withstand powerful shocks. He estimated that hull-mounted equipment could undergo accelerations as great as 190 gs.

"The manifold for the upper gate is about 900 pounds and the one for the lower gate is about 1,500 pounds," Lorino said. "Controlling and supporting these massive weights during shock loading was a challenge."

An underwater blast at a Hi-Test Laboratories site in Virginia puts real-world stress on a design.

Lorino analyzed a finite element model of the sterngate hydraulic power unit using software from Algor Inc. of Pittsburgh.

"The manifolds were modeled as lumped masses," he said, "and connected to the skid with beam elements to simulate the weight of the objects. Another lumped mass was placed inside the tank to simulate the weight of the fluid, which is about 2,800 pounds. The rest of the structure was modeled with plate elements. The base of the model was fully constrained to simulate the bolts that fix the sterngate HPU to the skid."

Lorino, an ASME member, performed natural frequency analysis and simulated the Navy's tests, which subject prototypes to shocks that are equivalent to those from a torpedo, missile, or depth charge. During separate runs, he analyzed the model's response to various shocks, including fore and aft, athwartships, and vertically.

Analysis led Lorino to revise the design. "The first design went way past the yield point," he said. "I redesigned the whole unit, which included using a stronger material, changing the location of equipment, changing the orientation of supports, and increasing the number of bolts."

A simulation using Algor software calculated the stress contours resulting from a blast and predicted that Huber's design would pass the test.

After the unit was fabricated and assembled, it was trucked to a lakeside facility operated by Hi-Test Laboratories Inc. in Arvonia, Va., for shock testing. There, mounted on a barge, the system took the brunt of five underwater explosions that shocked the ship from various directions.

According to Lorino, problems involved crushed lock washers and sheared bolt heads, which were corrected on the spot, between the first three explosions.

Northrop Grumman is building three more San Antonio-class vessels, LPD 18 through 20. They have been named, in order, the New Orleans, Mesa Verde, and Green Bay. According to a spokesman for the Naval Sea Systems Command in Washington, the contract for a fifth ship, which will be named the New York, is still pending.

The Navy is planning to incorporate into the New York some of the steel that was recovered from the World Trade Center as a memorial to victims of the attack on Sept. 11, 2001.

The Navy spokesman, Chief Petty Officer David Nagle, said that the San Antonio is scheduled for commissioning early in 2005.