MISSISSIPPI SHIPBUILDER SAVES TIME AND MONEY PERFORMING DESIGN OPTIMIZATION AND SHOCK TESTING WITH FEA SOFTWARE

Ingalls Shipbuilding developed a new quick-acting, watertight door for military ships that is lighter, less detectable by enemy radar, requires less maintenance and has a more advanced sealing mechanism than existing doors. The company performed a virtual shock test using Mechanical Event Simulation from ALGOR, Inc. to ensure that the door withstands U.S. Navy shock requirements.

October 23, 1998, Pittsburgh, Pennsylvania -- The United States Navy requires ships that can endure dangerous conditions like severe weather, a missile attack or a mine explosion. Ship components are tested for water pressure and shock endurance prior to installation to ensure that they can withstand such threatening conditions.

Ingalls Shipbuilding division of Litton Industries in Pascagoula, Mississippi has designed and manufactured 76 of the U.S. Navy's 339 active ships and modernized dozens of its surface combatant ships. The company recently developed a new quick-acting, watertight door for military use and used linear stress analysis and Mechanical Event Simulation software from ALGOR, Inc. to optimize the design to meet the U.S. Navy's water pressure requirement. It then used ALGOR's Mechanical Event Simulation software to simulate a shock test of the door. The computer analyses avoided expensive and time-consuming prototype testing in a laboratory and ensured that the new watertight door withstands the military's water pressure and shock requirements.

Military Interest in More Efficient, Inconspicuous, Watertight Doors

Over one hundred watertight doors are located both within a ship and on a ship's exterior in direct contact with weather and wave splash. They must close securely and form a watertight seal to guard the ship's compartments from being consumed by water if damage causes a leak. They are also used to block the spread of noxious fumes and stall spreading fires. The staff of the Carnival Cruise ship that caught fire on July 20, 1998 as it set sail for Key West, Florida and Cozumel, Mexico closed the ship's rear watertight doors to seal the compartments and delay the fire from spreading forward from the stern.

Ingalls Shipbuilding's new quick-acting, watertight ship door has many advantages over current watertight ship doors that would be attractive to the military. It is lighter, requires less maintenance, has a more advanced sealing mechanism and is less detectable by enemy radar.

The door has a longer life than traditional carbon steel watertight doors because it has an organic e-glass/vinyl-ester surface that is corrosion and rust resistant. Its balsa wood and fiberglass composite door panel reduced its weight by nearly 50% because it is lighter than carbon steel. The door's weight was reduced because a lighter door is easier to control.

The new door has bulkhead support built into its frame. The bulkhead is the ship's interior wall that separates compartments. When turbulent waters cause the ship's decks to move in opposite directions, the bulkhead experiences shearing loads that concentrate around the doors. Existing doors distort and then fail prematurely as a result of shearing loads, causing the bulkhead to experience plastic deformation. Ingalls Shipbuilding's door now has a stiff ring of material around its frame that absorbs the shearing load path to reinforce the door. This bulkhead support is also intended to reduce the rework needed after the door is assembled in the ship and the ship is transported to the dry dock by rail car and lowered into the water. Shipbuilders do not design for the damaging effects of the door's assembly and transportation because it is difficult to define the loads in such conditions. It is also more costly to develop features not applicable to the door's intended use than to rework the doors after their assembly and transportation.

The door has a metal, corrosion-resistant sealing mechanism with six to eight latches that open and close the door, forming a watertight seal, with the single motion of a handle turned in a 180-degree arc. The innovative sealing mechanism functions easier, has a longer life, and requires less maintenance than existing watertight doors. Its seal is stronger because its soft, spongy, neoprene gasket at the edge of the door frame compresses against the bulkhead, like a refrigerator, rather than against a metal knife edge.

Furthermore, the door was designed without "top side clutter," raised structures or surfaces on exterior doors that could be easily detected by radar. It is possible for enemies to identify the geography of a ship by locating top side clutter. In an effort to make the new watertight door inconspicuous, Ingalls Shipbuilding placed the door's closing mechanism within its frame, rather than on the door's exterior like existing doors' closing mechanisms.

Linear and Nonlinear Stress Analysis Software Used to Optimize Door's Geometry

Ingalls Shipbuilding had to determine if the new door could withstand the U.S. Navy's water pressure endurance standard that ensures the door's functionality in intense water pressure conditions. In the laboratory water pressure test, or "hydrostatic" test, conducted at Ingalls Shipbuilding, the door is installed in a tank filled with enough water to produce the U.S. Navy's required water pressure level, then removed and mounted on a test wall to determine if it opens and closes as designed. Each laboratory hydrostatic test can cost $3,000. To reduce costs, Ingalls Shipbuilding used ALGOR's linear and nonlinear stress analysis software to test and optimize the door's design under pressure loading on the computer.

Engineers at Ingalls Shipbuilding designed a model of the door in AutoCAD and used ALGOR's Houdini™ program to convert it to a 3-D solid brick finite element model. Then they added a 3-D solid brick model of the sealing mechanism to the door model.

An AutoCAD image of the watertight door.

The sealing mechanism was created by Hartwell Corporation in Placentia, California who used ALGOR's linear static stress analysis software to test its strength and geometric design under loading conditions required by the U.S. Navy before presenting it to Ingalls Shipbuilding for analysis with the entire door. For example, its engineering team applied given force loads to the handle that were meant to represent the door being opened, closed and experiencing abuse when pushed or pulled farther than necessary.

After assembling the door and sealing mechanism models, Ingalls Shipbuilding used ALGOR's linear stress analysis composite processor to analyze the strength of the door's inner balsa wood and fiberglass layers under pressure that represented the U.S. Navy's standard for water pressure endurance. (Later in the nonlinear analyses, the collective properties of this sandwich composite were used for the door's material properties.) Unexpectedly, the linear hydrostatic analysis revealed excessive stress on the door's composite surface that would cause cracking and peeling.

To uncover specific deflections in the door under varying pressure loads over time, engineers turned to ALGOR's Accupak/VE Mechanical Event Simulation for Virtual Prototyping with Linear and Nonlinear Analysis. The nonlinear analysis indicated excessive stress and deflection in the core portion of the door panel. The panel required more flexibility so engineers reduced its thickness. Once the door was optimized, engineers turned again to ALGOR's Accupak/VE to perform a virtual shock test.

Mechanical Event Simulation Chosen to Perform Virtual Shock Test

Military ship doors, along with all ship components, must withstand shock, the intense vibration caused by the accelerated movement of a ship responding to a severe disturbance like a storm, mine explosion or missile launch. Ingalls Shipbuilding had to verify that the optimized watertight door would operate after experiencing shock levels specified by the U.S. Navy. Its sealing mechanism must function after shock in order for it to close properly. The door's shape must remain undistorted to guarantee a watertight seal.

Prior to using Mechanical Event Simulation software, Ingalls Shipbuilding sent military ship component prototypes to a laboratory to test them on a shock table before building final prototypes to test on a real ship. During the laboratory test, a large weight is suspended above the shock table on which the test component rests. It is dropped onto the shock table and the table's resulting vibration causes the component to bounce. The weight is dropped from a distance calculated to accelerate the shock table at a rate that will cause the component to vibrate at its required shock absorption level. Following this physical event, the component's remaining functionality is physically analyzed. Laboratory technicians mount the door on a test wall and determine if it opens and closes as designed.

Ingalls Shipbuilding performed a virtual shock table test using ALGOR's Mechanical Event Simulation software. In Image One, the weight (top right block) is dropped onto the table a distance corresponding to the component's required shock absorption. In Image Two, the weight has contacted the table (bottom L-shaped object) and the impact caused it to bounce. The resulting shock, or severe vibration, from the weight's initial impact reverberates to the component, causing it to bounce on the table. In Image Three, the weight has contacted the table a second time as the component continues its ascent. Ingalls Shipbuilding studies the component's stress results from the initial impact. If the component withstands the shock vibration of the initial impact, a force that represents the maximum acceleration force from a ship's thrusting movement, it inherently withstands subsequent shock vibrations.

Each laboratory shock test costs between $5,000 and $15,000, can take up to two weeks and often leads to further design improvements and additional laboratory shock tests of the component. To cut costs and save time, Ingalls Shipbuilding decided to replicate the shock table test on the computer with ALGOR's Accupak/VE Mechanical Event Simulation for Virtual Prototyping with Linear and Nonlinear Stress Analysis.

Mechanical Event Simulation expands upon traditional finite element analysis (FEA) because it includes physics and considers time, motion and impact. Mechanical Event Simulation provides a visual simulation of the entire real-world event. Further, in a shock test, it is difficult to define input loads required for traditional FEA. But Mechanical Event Simulation does not require engineers to define input loads because they are defined by the physics operating during the event.

In preparation for the virtual shock test, Ingalls Shipbuilding performed a modal analysis to determine the door's natural frequency. The Mechanical Event Simulation's timestep was set to one-tenth of the natural period to ensure that the shock vibrations would not occur at the door's natural frequency. A small timestep was needed to analyze all responses during the event's evolution.

Engineers inserted contact elements in the model between the table and the weight being dropped, and between the table and the door to analyze their interaction. They also placed contact elements between the floor and the table to prevent the floor's damping effect. These contact elements simulated springs that are placed between the floor and the table in the actual laboratory test.

Virtual Shock Test Results

The virtual shock table test performed with ALGOR's Accupak/VE software was identical to the laboratory test. In the computer analysis, engineers specified the direction of gravity and dropped a weight onto the table a distance that correlated to the U.S. Navy guideline for shock endurance.

The weight dropped onto the table, and the reverberating shock produced stress in the door. Mechanical Event Simulation calculated stresses in the door over time caused by the dropped weight. Engineers compared the stress values to the material properties' yield values. As in the physical test, the effect of the initial bounce was the only concern because it represents the maximum acceleration force from a ship's thrusting movement. If the component withstands the initial shock vibration, it theoretically could withstand any remaining vibrations.

Ingalls Shipbuilding simulated one laboratory shock table test using ALGOR's Accupak/VE software and detected no catastrophic deflection in the door or any of its components, indicating that the door would not fail when subjected to real-world conditions.

Watertight Door Prototypes to be Tested on Military Ships

A physical laboratory test to confirm ALGOR's results has not yet been conducted, but six prototypes of the final door are scheduled for installation on two military ships and will be tested for six to twelve months. If the U.S. Navy is satisfied with the door's performance, Ingalls Shipbuilding intends to negotiate the installation of the doors on new military ships as well as on ships already operating in the U.S. Navy's fleet.

Ingalls Shipbuilding determined that analyzing the door on the computer saved the company months of time and thousands of dollars it would have taken to test several prototype doors in a laboratory.

Six prototypes of the watertight door analyzed with ALGOR software are scheduled for installation on two military ships and will be tested for six to twelve months. If the U.S. Navy is satisfied with the door's performance, Ingalls Shipbuilding intends to negotiate the installation of the doors on military ships like this guided missile destroyer, the USS McFaul (DDG 74,) which the company built and the U.S. Navy commissioned in April 1998.