Reduced Analysis Run Times Speed Up Doubling of Dive Suit Depth

Robert Hughes, a mechanical engineer for the Naval Coastal Systems Center in Panama City, FL, is using his finite element modeling, meshing and analysis expertise to increase the depth rating of an atmospheric dive suit. The goal is to develop a design to withstand the ocean's pressure at twice the depth of the original design. By utilizing Mesh Enhancement Technology to optimize the number of elements, Mr. Hughes reduced analysis run times with no loss of accuracy.

Overcoming Marine Exploration Challenges

Marine explorers and rescuers face many challenges. To withstand the cold, dark ocean depths, human beings must use life support equipment which provides them with breathable air and other life support. Equipment must also operate at the ocean's pressure, which increases at greater depths. At 1,000 feet deep, the pressure is approximately 445 PSI.

Saturation diving equipment, the most common type used for deep diving, uses a helium-oxygen air mixture. Helium moves quickly out of the blood stream, avoiding the potentially deadly "bends," which occurs when nitrogen, found in regular air, builds up in the blood due to the pressure. If the pressure is released too quickly, potentially fatal bubbles form in the blood. Generally, this equipment cannot be used below 600-700 feet without water pressure posing long-term health risks. In addition, divers must spend from a few days to several weeks decompressing in hyperbaric chambers on the support vessel.

The dive suit being optimized at the Naval Coastal Systems Center has many advantages over saturation diving. It is almost like a one-man, human-shaped submarine. Completely self-contained, it is composed of 25 pressure vessels that keep the diver encased in a one atmosphere environment that completely eliminates pressure-related ailments and the need to spend time in hyperbaric chambers. The current design is able to withstand the pressure of the ocean at 1,000 feet. Mr. Hughes and his team are working to double that depth.

Life support is provided by an air scrubber which removes exhaled carbon dioxide and adds oxygen from an "on-board" supply that can last up to one day. No unique air mixture is required.

Unique pressure balancing mechanisms at the joints permit the diver to move the arms and legs of the suit to walk on the ocean floor or perform undersea tasks. Thrusters can be used to propel the diver through the water. The suit is generally "tethered" to the support vessel with a power supply cable. On-board weights can be shed to let the diver float to the surface in the event of an emergency.

The atmospheric dive suit requires minimal support equipment and personnel. This saves money, and it also means that an atmospheric dive suit and its support equipment can be airlifted into a rescue situation. Diving can begin immediately because the diver needs no time to adjust to the ocean's ambient pressure.

Detailed Modeling for Accuracy

Mr. Hughes and fellow project members began by modeling the dive suit in Intergraph and exporting the model into Algor via a Patran neutral mesh file. He then used Merlin for surface mesh enhancement and Hexagen for solid mesh generation. Approximately 25 types of main pressure vessels comprise the system, and must be accurately modeled and appropriately meshed to obtain highly accurate finite element analysis results and, thus, meet stringent safety requirements.

While the suit must be safe, it must also be light enough to float when the weights are shed. Because of the weight concern, material could not simply be added to boost the safety factor of the vessel. Mr. Hughes' linear and nonlinear stress analyses of each component had to be highly accurate. That required a carefully optimized finite element mesh.

Careful Meshing Decreases Analysis Run Times

Geometric complexity tends to increase the number of finite elements comprising the model, which leads to increased analysis run times. Because of the dive suit's extreme complexity, Mr. Hughes used Merlin to reduce the number of elements without sacrificing accuracy. The surface mesh had to be dense around small features to facilitate accurate analysis, but could be larger in other areas to cut down the number of elements.

"I have used just about every individual mesh control and meshing option available on this project in order to reduce the number of elements," said Mr. Hughes. "By reducing the number of elements, run times dropped drastically, allowing more design iterations to be accomplished under a tight schedule."

"Using these techniques, we have reduced element counts in the solid models by one-half to one-third, compared to the raw form, with no loss of accuracy," concluded Mr. Hughes.

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