Engineers are using simulation-based design to test the digital prototype of a floating military base that will be the largest structure ever to voyage on the high seas. Consisting of five separate modules joined by a series of hinged connectors, the proposed mobile offshore base (MOB) is planned to provide a mobile, sea-based alternative to fixed land bases outside the United States. The Navy's proposed mobile offshore base includes a 1-mile-long runway, 85 acres of interior space, and facilities for transferring containers and vehicles from cargo ships docked alongside. The self-propelled structure would serve as a semi-stationary base for military operations and humanitarian efforts. Feasibility studies are being performed at McDermott Technology Inc. in Lynchburg, Virginia, a supplier and operator of semisubmersible vessels for the offshore oil industry. Simulations, visualizations, and analyses for these studies are being conducted at the Simulation-Based Design Office of the Gulf Coast Regional Maritime Technology Center in Orange, Texas, which was established by the University of New Orleans in cooperation with the Office of Naval Research in Washington, DC.
Many highly advanced numerical techniques in use today were developed by engineers and scientists who needed to prove design concepts as early as possible for various space programs during the 1960s and 1970s. In the late 1990s, however, engineers working under contract to the U.S. Navy are pushing numerical techniques to the next level for the design of structures that will be far more complex than spacecraft—and will be deployed much closer to home. In this case, the structure is the largest floating structure ever envisioned: a gigantic self-propelled military base larger than 10 aircraft carriers.
Consisting of five separate modules joined by a series of hinged connectors, the proposed mobile offshore base (MOB) is planned to provide a mobile, sea-based alternative to fixed land bases outside the United States. Operating on the high seas, the MOB would partially submerge when on location, providing a stable platform for troop deployment and logistical support, command and control operations, and humanitarian efforts such as disaster relief. More than 1 mile long, the 500-foot-wide, 250-foot-high structure includes a runway long enough to land fully loaded C-130 and C-17 cargo planes plus interior quarters for up to 20,000 troops. Its 85 acres of storage space will be able to hold up to 150 aircraft, 5,000 cargo containers, and 3,500 tanks and other vehicles.
Because such a large, complex structure has never been attempted before, engineers are relying on computer simulations with a “virtual ocean” to determine how the MOB will operate in various sea states. Feasibility studies are being performed at McDermott Technology Inc. in Lynchburg, Va., a supplier and operator of semisubmersible vessels for the offshore oil industry. Simulations, visualizations, and analyses for these studies are being conducted at the Simulation-Based Design Office of the Gulf Coast Regional Maritime Technology Center in Orange, Tex., which was established by the University of New Orleans in cooperation with the Office of Naval Research in Washington, D.C.
The Navy’s proposed mobile offshore base includes a 1-mile-long runway, 85 acres of interior space, and facilities for transferring containers and vehicles from cargo ships docked alongside. The self-propelled structure would serve as a semistationary base for military operations and humanitarian efforts.
Given the MOB’s primary mission of logistical supply, operations of its cargo systems are of particular interest to engineers on the project, especially during the heavy wave action that could limit the ability to transfer cargo to and from adjacent supply ships. Cargo ships do not react to wave action in the same way as the comparatively stable MOB, so knowing their relative motion is critical to determine whether cranes on the MOB can lift cargo containers from ships without the containers swaying too much. Understanding relative motion is also essential to find out if vehicles can be driven safely over ramps between the vessels.
Using ADAMS mechanical simulation software from Mechanical Dynamics Inc. in Ann Arbor, Mich., engineers verified that these operations could be performed normally during sea state 4, corresponding to significant wave heights of 6 feet. Simulations also revealed that the cranes could continue to move cargo in the 9-foot waves of sea state 5, which was unexpected good news for program managers.
According to Dave Johnson, coordinator of digital media at the center, computer simulation was necessary because building a physical model of such an enormous structure is impractical, testing it on the open sea would be dangerous, and wave-tank scale models would not be sufficiently accurate. “Simulation-based design provides accurate predictions quickly, so we can run what-if scenarios and make corrections in the early stages of development,” Johnson said. “Lowering the number of late engineering changes reduces costs, shortens cycle time, and improves the overall design.” Physical scale models will still be used for a project of this size, he added, but only to validate the design before fabrication begins rather than to prove the design.
Cargo-transfer simulations were run on the center’s Onyx supercomputer from Silicon Graphics Inc. in Mountain View, Calif. With eight 200-megahertz processors, 4 gigabytes of RAM, and three graphics engines, the Onyx is used for its processing speed, graphics capabilities, and memory capacity in handling large engineering problems and displaying complex three-dimensional images.
CAD data defining the geometry of the MOB and cargo vessels are imported into the Onyx as DXF and IGES files from a naval design and FORAN, a modeling system from Sener Ingenieria y Sistemas, a software developer in Madrid, Spain. Simplified forms of the geometry are used to create response-amplitude operators in all six degrees of freedom (the ship’s roll, pitch, yaw, heave, surge, and sway) in WAMIT, a wave-body interaction program from the Massachusetts Institute of Technology in Cambridge.
The motions and geometries all come together in ADAMS, where cargo-transfer operations are analyzed for displacements, forces, accelerations, and loads on various components and subsystems. These data can be displayed as plots for evaluating the various parameters. Output from ADAMS is also exported to the Easy Scene visualization package from Coryphaeus Inc. in Los Gatos, Calif., to create a realistic animated motion picture that can be help researchers understand overall motion. Areas can be magnified or cross-sectioned for closer examination. The simulation is physics-driven and the action is based on mathematical calculations, so the animations very closely match actual sea-state motion.
“The key to successfully performing physics-based simulation with multiple software is coupling the programs so they exchange data accurately and efficiently,” said John Cardner, MOB project manager at the technology center. Integrating these high-end programs represents cutting-edge technology, requiring engineers to work with consultants at Mechanical Dynamics and at Dynamic Animation Systems in Fairfax, Va., to develop specialized routines. These routines will be used for exporting the geometry and motion data from FORAN and WAMIT into ADAMS for dynamic analysis, then feeding the resulting motion data into Easy Scene for visualization.
“Creating a virtual prototype of this scope and fidelity required us to leverage a variety of technologies,” said Patrick O’Heron, senior engineering analyst with the Mechanical Dynamics Consulting Services Group. “To accomplish this goal, we developed advanced mathematical models and extensive training courses tailored to the project. The result is a complete simulation-based design system that integrates CAD, simulation, and high-end animation.”
Studying Cargo Transfer
ADAMS was used to analyze the two primary operations for transferring cargo from adjacent ships to the MOB: vehicles driving over ramps between roll-on/ roll-off (RO/RO) ships and the MOB; and cranes lifting 30-ton cargo containers from vessels to automated transports on the MOB.
Mingli He, a mechanical engineer at the technology center who ran the ADAMS analysis, said the software determined not only the motion paths for various parts and subsystems but loads, forces, and displacements as well. Determining these dynamic parameters aided in evaluating details of cargo-transfer operations. According to He, simulation of the cargo-lifting crane to determine the magnitude, direction, and duration of the container swaying at the end of the 150-foot hoist cables had to account for momentum imparted to the container by the ship motion as well as the damping coefficient of restraining cables attached to the sides of the container.
In studying the RO/RO ramp from the ship to the MOB, ADAMS provided information on vehicle stability in making the run. The analysis will also prove useful to engineers in designing end rollers needed for the ramp to slide freely on the MOB platform. Otherwise, heavy vehicles could immobilize the end of the ramp; ship motions could then introduce buckling loads on the ramp and possibly overstress connecting joints.
“Rigorous dynamic analysis gave us the answers we needed quickly,” said Richard Currie, technical-program manager for the MOB project at McDermott. “We routinely use dynamic analysis to evaluate sea-going structures for stability, integrity, and mission performance. ADAMS provides a unique tool to examine critical mission systems in high-stress situations to ensure that we are adequately designing—but not overdesigning— these systems.”
ADAMS numerical output and data plots gave engineers detailed information on loads, forces, and displacements at specific times and locations to identify and correct potential problems anywhere on the structure. Moreover, Currie noted, coupling dynamic simulation with visualization was particularly important to the success of the project. Realistic animation of the models provided greater understanding of the overall motion and operation of the MOB than tabular printouts could.
“I can’t overemphasize the importance of the physics-based simulation that drives these visualizations,” he said. “I always point out to audiences who see these scenes that they are not cartoons but realistic representations of expected behavior, as accurate as we can make them. This is critical in selling a concept as far out as the MOB, particularly to managers overseeing the program and administrators coordinating scheduling and funding.”
Currie added that simulation-based design is critical for completing a project of this magnitude on time and within budget. “Analysis and visualization tools let us keep track of the program, evaluate various alternatives, and continually improve the design faster and more economically than would otherwise be possible.”