Motions and associated transfer functions of a wave-following buoy with a single-point slack mooring are studied by using two buoy/cable numerical models (one time domain model and one frequency domain model) and field-measured data. Field data were measured by a National Data Buoy Center (NDBC) 30m discus buoy that has an all-chain slack mooring with a significant amount of excess chain lying on the sea floor. Motion transfer functions obtained from the two numerical models and field data are compared. This study shows that the buoy responses are weakly nonlinear within the meaningful wave frequency range (f < 0.4 Hz). The time domain model compares more favorably to the measured data than the frequency domain model.
Ocean buoys are used for various purposes that include navigational aides, data collection, and communication relays. A buoy needs to be designed correctly to properly perform the desired functions. Some design considerations for ocean buoys were discussed by Timpe and Teng (1993). It is important to know the buoy characteristics and performance through analysis and testing to properly design and use a buoy. Teng and Timpe (1994) reviewed and discussed various tools used for analysis and testing of ocean buoys. These include mathematical and numerical analysis, physical model testing, and prototype testing. NDBC, an element of the National Weather Service (NWS) of the National Oceanographic and Atmospheric Administration (NOAA), maintains a network of data buoys that are deployed off the coast of the United States to measure environmental information and report data in real time. Currently, NDBC has 73 moored data buoys at the U.S. east and west coasts, Gulf of Mexico, Great Lakes, and Hawaiian Islands. Depending on missions, locations, and environmental conditions expected at the buoy location, a variety of buoy types are used, including 2.4-, 3-, 10-, and 12-m discus and 6-m boat-shaped NOMAD buoys (see NDBC, 1994).
