Elastic tether technology for shallow water moorings in harsh environments: results from Georges Bank

In order to meet our needs for moored observations on Georges Bank, compliant elastic elements are being used in the mooring line. As many coastal and continental shelf regions, the harsh environment at the crest of Georges Bank puts severe stress on moorings, and often significantly shortens mooring life with subsequent loss of data. Compliant elastic tether technology holds the mooring hardware off the bottom, allows the buoy to move easily with the waves, and significantly reduces mooring component wear and fatigue. The Georges Bank crest site in 42 meters of water, has 1.5 m/sec tidal, plus wind driven currents and 15 meter waves. The Crest mooring was deployed in October 1994, serviced in March 1995 and recovered in September 1995. To better understand the mooring dynamics, a load cell was mounted on the buoy at the mooring attachment point. The mooring tension was sampled at 1 Hz and hourly statistics calculated, recorded and telemetered back to shore. Two months of data which included several storms were collected. During that time the maximum mooring tension did not exceed 600 kg. The mean tension was close to the 300 kg design tension. The major fluctuation in the hourly averages was due to the semidiurnal tidal currents. Also, weather forced currents caused hundred kg lower frequency fluctuations with periods of 5 to 10 days. The minimum tension in the mooring did not dip below 50 kg, showing that the mooring never went fully slack. Finally, the standard deviation of the hourly averages showed strong correlation with the significant wave height from the nearby NDBC buoy. Therefore, the compliant element allowed the buoy to move with the tides and low frequency currents to reach an equilibrium state, and still be able to move with the waves and stretch the elastic element to compensate. This reduced the wear on the mooring components and allowed the moorings to remain in harsh environments longer than conventional mooring technology. Another benefit of the elastic element is that buoy motion is smoother and should make a better platform for scientific observations.