Teledyne WORKHORSE Operation Manual page 38

May 2015
 Deployment Depth
The ideal deployment depth is approximately 10m submergence angled slightly up-
1.
ward (2 to 5 degrees pitch) so that range cells at 100m are at about 3 to 5 meters sub-
mergence.
2. A large sea state will create wave troughs that block the beams if the deployment depth is too
shallow: 6 meter waves will be a problem for a 3 meter deployment depth.
3. A very deep deployment depth will limit the wave frequencies and wave height that can be
measured.
4. If a deep deployment is unavoidable, a greater tilt can be used but should not exceed about 15
degrees.
5. See Table 1, page 3 for the upper cutoff frequency.
 Motion (dynamics)
The best data is collected with the H-ADCP fixed in position: In many installations, such as when hanging
over the side of a FPSO or Oil Platform, the H-ADCP will have motion as the vessel moves. This motion is
typically coupled to the actual wave activity (and the vessel motion because of that): the result is that the
quality of the H-ADCP waves measurement will be reduced. The default thresholds in WavesMon are set
conservatively to avoid the biases from these motions. What follows are the motion limits that this con-
servative approach will work. Note that these motions assume that accelerations are minimal and the sys-
tem moves slowly to the tilts and yaw angles indicated. You must make sure that your system will not
move with high acceleration or tilts and yaw angles greater than what is specified below.
1. A fixed mounted system will perform better than a moving one. The system must pitch, roll,
and yaw with the waves less than ± 10 degrees. The performance with frequency and min-
imum wave height has been de-rated for these kinds of dynamics.
2. Pitch is the most damaging of the types of motion. Pitch causes the depth of the range
cells to move up and down in partially correlated fashion with the waves. The consequence of
dynamic pitching is that the highest usable frequency will be reduced (0.2 Hz rather than
0.5Hz at 10m deployment depth).
3. Roll is less of a problem than pitch but still de-rates the performance.
4. Yaw at wave frequencies causes the directional distribution to be smeared but is not a serious
problem to wave parameters. If the system is rotating (>45 degrees in 17 minutes) then expect
the wave direction to be biased accordingly.
 Platform Influence
1. Floating platforms like drill ships will influence waves near the platform.
Long period waves will move the platform (heave): the pressure sensor cannot be
o
used as a reference because it will not see the long period waves.
Shorter period waves will reflect off the platform causing wave energy at these fre-
o
quencies to be exaggerated.
It is best to select range cells that are distant from the platform (60-
o
100m) because the platform will reflect high frequency waves and move with low
frequency waves. The H-ADCP can accurately measure both long and short waves, in
the vicinity and far away from the platform. Other instrumentation (such as pressure
sensor, or surface tracker) attached to the platform, will not be able to accurately
measure the waves because the platform partially moves with the longer period waves
and reflects shorter period waves. Because the H-ADCP can profile a range of cells
distant from the platform and because it exclusively measures the horizontal compo-
nent of the waves, the H-ADCP can resolve the real wave environment.
Set the small wave screening frequency to 0.03 Hz if the platform is heav-
o
ing with the waves. This keeps the processing from using the pressure sensor data
Page 26
EAR-Controlled Technology Subject to Restrictions Contained on the Cover Page.
WorkHorse H-ADCP Operation Manual