Performance of a subsurface buoy for directional wave measurements
Don Conlee, NortekUSA
Torstein Pedersen, Nortek-AS
Eric Siegel, NortekUSA
Long term directional wave measurements in deep water environments (> 50 m depth) are
intrinsically difficult to achieve. Surface wave buoys may be damaged by storms, ships and
vandalism. The ability to mount an acoustic Doppler current profiler and wave gauge on a
subsurface buoy would permit the instrument to be close enough to the surface for high quality
wave measurements yet be removed from the dangers of exposure at the surface.
In 2005 Nortek introduced the SUV method for measuring ocean waves from an acoustic wave
and current profiler (AWAC) mounted on a subsurface buoy. The SUV method differs from the
traditional maximum likelihood method (MLM) array approach of measuring waves from a
Doppler profiler because it permits the instrument to rotate during the wave burst.
In 2006 NortekUSA initiated a collaborative experiment with Bedford Institute of Oceanography,
Dalhousie University, Open Seas Instrumentation, and Mooring Systems, Inc to validate the SUV
method and learn how the shape and buoyancy of subsurface buoys affects the measurements.
Two Nortek AWACs were deployed on two subsurface buoys (spherical and asymmetric shapes)
next to a Datawell DWR wave buoy for independent reference. The deployment location was
offshore Lunenburg Bay, on the eastern side of Nova Scotia, Canada, with open exposure to
Atlantic Ocean waves coming from the South and East.
Several large wave events occurred during the 2 month deployment from September to
November 2006, with a significant wave height reaching up to 5 m. The non-directional wave
estimates (such as wave height and period) agree very well with the wave buoy over all wave
energy and frequency bands.
Directional data from both subsurface buoys agreed well with the independent DWR
measurements. Most importantly, and perhaps contrary to common expectation, the directional
estimates were accurate across all frequency bands during times of increased wave energy.
However, it was found that each buoy had a specific frequency band of increased directional
uncertainty. The spherical buoy showed directional uncertainty in a band centered on 0.11 Hz (9
seconds) and the asymmetric buoy showed uncertainty in a band centered on 0.05 Hz (20
seconds).
A mooring analysis taking into account subsurface buoy shape, buoyancy, mass and mooring line
length was conducted to estimate the natural frequency of each subsurface mooring. This study
suggests that excessive buoy motion at the natural frequency accounts for the directional
uncertainty in each particular wave band for each buoy. A comparison between two independent
methods of estimating the non-directional wave energy (AST and near-surface orbital velocity)
corroborates that buoy motion is the likely cause of directional uncertainty for each buoy.
We address both mechanical and processing methods to reduce the amount of directional
uncertainty and achieve high quality directional wave measurements using commercial off-theshelf equipment for open ocean wave measurements in water depths deeper than 50 m.