Reson SeaBat 7125 ROV, Operator'S Manual

Page: 115 / 200

ACTIVE SONAR USAGE
SeaBat 7125 ROV Operator‟s Manual
Page 99 May 30, 2011
Version 3
shipboard equipment should be conducted to determine if there is a potential for
machinery interference prior to conducting at-sea testing. For means of compensating
for this, see subsections in
6.2 above .
6.6.3 Speed
Hydrodynamic flow noise can be a concern when operating at higher speeds. Externally
mounted acoustic sensors should have fairings that are as flush (or near-flush) mounted
to the ship hull as is practically possible. This way the hydrodynamic flow-induced
vibration noise should be minimal. For means of compensating for this, see subsections
in
6.2 above .
NOTE
A poorly designed sonar fairing can degrade the system performance by a factor of 4
for speeds above 8-10 knots. Typically, this noise source is the most common problem
on all sonar installations.
6.6.4 Air Bubbles
Bubble sweep-down can be another potential noise problem. The significance of this
problem is associated with the noise generated as the bubbles cascade along the hull
after being ingested in the bow wave of the ship and, more importantly, with the baffling
produced by the entrained air layer between the face of the sonar sensor and the water
environment.
If air bubbles are present between the sonar and the water, the system will not function
properly. This phenomenon is transient in nature occurring only for a few seconds per
wave period. It is typically most prevalent on shallow draft ships and increases in
intensity at higher sea states and ship speeds.
Modeling of this problem only indicates that air bubble noise may be present by
determining the transmission vehicle of potential bubbles. Actual at-sea measurements
are usually required to fully determine the presence and severity of bubble sweep-down.
At-sea diver viewing has been very successful in the past to capture the bubble sweep-
down characteristics of a particular ship hull on film.
Air bubbles mask some or all of the view of the sonar where the affected area will
completely blank out. Common causes for bubble masking include:
The vessel props when thrusting into reverse and pours a stream of bubbles
over the sonar.
The sonar has not been deployed deeply enough and there is bubble wash
from the sea surface.
Bubble ingestion as the vessel pitches into the sea.