Sea Technology

NOV 2017

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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18 st / November 2017 by GD-APS. The first is a specialized marine X-band Dop- pler radar capable of sensing inbound ocean wave motions out to a maximum range of 5,000 m. The radar's azimuthal sweep rate and range resolution are set to adequately sam- ple the local wave field in space and time. Doppler scans are fed into a suite of real-time filtering and processing algo- rithms that translate the radar data into forecasts of surface wave elevation maps and ship hull forces. The radar-to-wave processing scheme uses an innovative least squares inver- sion approach to determine the underlying propagating components of the wave field, and these waves are evolved forward in time according to ocean wave propagation phys- ics. A precalculated database of wave interaction forces with the ship is used to rapidly produce a forecast of the ship's response to predicted waves. These continuously up- dating ship motion predictions can then be used to inform the timing of ship operations. Although maximum forecast time varies with sea condition and vessel speed, the Future- Waves system has been demonstrated to provide accurate, phase-resolved wave and ship motion forecasts to several minutes into the future. FutureWaves also produces a directional power spec- trum of the ocean waves—a map of where the wave energy is coming from and at what wave period—similar to what a wave buoy can provide, but instantaneously and at multiple locations within the radar field of view. In the FutureWaves system, the wave spectrum data are fed to the ship motion model to produce a plot of the statistical representation of the ship motions as a function of ship heading and speed, allowing selection of ship orientation to minimize particu- lar modes of motion. This planning tool is continuously up- dated from the radar-calculated directional power spectrum and can be fed wave spectrum forecast products (e.g., from NOAA) to inform longer-term planning. "Doing it in real time was the real challenge," explained Dr. Ben Connell, one of the key architects of ESMF and FutureWaves. Connell has authored several papers on the subject of real-time ship motion forecasting. "Our approach of a reduced-order model, or ROM, using precalculated co- efficients was the key to a time-domain model that could provide useful results in real time." The necessary added inertia and hydrostatic restoring terms are precalculated from a representation of the ship ge- ometry and input mass specifications. The impulse-response functions and force response amplitude operator functions U.S. Marine Corps vehicles transfer from USNS Dahl, a large, medium-speed roll-on/roll-off, onto USNS Montford Point and are loaded onto LCACs for beach deployment. (Photo Credit: U.S. Navy)

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