OM-7986
Page 10 of 23
Issue A - Rev 8
NOTE
– Strong tides or currents acting on the instrument mooring or sub-surface floats can
significantly increase the in-line load due to drag. This has the effect of increasing the
release load on the LRT and therefore could affect the operation of the unit.
Breaking Load
Testing has shown the LRT to have a typical breaking load in excess of 500kg when new.
This is the load that causes structural failure in one or more parts of the assembly causing
the load to part from the release mechanism.
2.7
Deploying the LRT
Ensure that the LRTs transducer has a clear line-of-slight path to the surface. Do not
obstruct the transducer or the acoustic performance will be degraded.
Ensure that all shackles are ‘moused’ or wired to prevent them coming undone.
Ensure that there is a note of the LRTs acoustic identity and associated DGPS drop position.
Ensure that when the LRT is deployed, there is enough buoyancy to lift it and any other
attached instrumentation/mooring to the surface. The weight of the unit in water is
approximately 0.4kg or 0.75kg with rope canister. It is recommended that at least 8kg of
buoyancy is used.
Once deployed range to the LRT to check that it is in the correct position.
2.8
Ranging to the LRT
Deploy the remote transducer over the side of the vessel and ensure that it is well below the
keel and as far away from the propellers or other sources of acoustic noise as possible.
Select the correct acoustic identity on the surface unit. Press the TEST / RANGE button.
The range in meters to LRT will be displayed and a beep will be heard if a valid reply is
received.
NOTE -
if the LRT has the Tilt feature fitted, only every other interrogation will give a Range
reply if the unit is tilted beyond 30 degrees.
The maximum slant range from which the transponder can be ranged to depends on the
water depth and the local acoustic environment. If the water is shallow and/or if the local
noise from the vessel etc is great then the range could be significantly reduced.
By continuously ranging to the LRT, either from fixed points on the surface or from a moving
vessel with the remote transducer fixed to a pole over the side of the vessel, it is possible to
‘home’ into the transponder (ranging distance decreases) so that the vessel is as close as
possible. This aids recovery as it is easier to visually see the unit when it comes to the
surface. It can also make sending the release commands easier, if in a noisy environment.
The RS232 port on the surface unit can be used to automatically send continuous ranging
commands to the LRT on the seabed and log the range data returned.
This mode can also be used to accurately position the LRT on the seabed – dedicated
software can carry out a calibration on acoustic ranges and the associated dGPS positions
from a dynamic vessel.
2.9
Releasing the LRT in Water
Once you have ranged to the transponder to confirm its position and manoeuvred the vessel
into the best position then you can send the secure release sequence to recovery the LRT
and mooring etc.
