Sensor Deployment Methods
There are many different ways
to deploy buoys and their moorings. The methods used in deployment
can vary widely, but are based upon the buoy’s size, the conditions
of the water that is being monitored, the depth of the water the buoy
will be moored at, the delicacy of the instruments, and the resources
of the organization deploying the buoys. In general, small buoys less
than 300 kg such as weather buoys or drifter buoys can be deployed by
a small craft and one or two people. There is also an option to
attach buoys with particular instruments to an already-present moored
“mother” buoy. Finally, especially rough waters may warrant the
use of helicopter deployment. They tend to be used more often for
event-triggered observations, rather than long-term buoy placements.
DART-style buoys are very large, displacing over 4000 kg water
and thus require the use of larger boats that have A-frame structures
and cranes. Helicopters are impractical due to the distance the buoys
are deployed from shore. This type of deployment requires up to ten
crew members actively working on the boat deck (Dever), and
deployment sites are many hours from shore. Weather conditions must
be carefully watched to ensure success. Buoys must be serviced every
Deployment of a tsunami DART buoy.
In a DART-style
mooring for the main buoy, the anchor is 6850 pounds (http://www.ndbc.noaa.gov/Dart/dart.shtml). 15 meters of chain
with the appropriate shackles connects the anchor to the main length of
plaited nylon rope. The total length of the ropes and chains is 25
percent longer than the site depth to allow for error in sea depth and
variations in the weather. (Gates 22). To ensure the best safety, an
anchor-first deployment sequence should be used along the depth contour
of the ocean floor where the buoy is to be moored (Gates 24).
If possible, the buoy or the satellite transmitter should be turned on and tested at the dock before it is transported to the mooring site. Water current direction should be determined near the mooring site, and the ship should move down-current of the site. Drop locations are programmed into GPS. The buoy is released at a distance of two-thirds of the mooring length. The method for releasing the buoy will be described shortly. The boat will then steam upstream past the deployment site until it runs out of rope; this is done slowly and without any pressure on the rope so that the buoy is not towed behind the boat. The boat will be at a distance of one-third of the mooring length from the intended deployment site and the boat will release the anchor. The anchor will descend through the water with a pendulum-like motion until it reaches the bottom, which will take approximately fifteen minutes. The tension increases to a maximum just prior to contact with the sea floor (Gates 25-27, 32).
The sequence of components on the boat deck should be carefully mapped out to prevent tangling and other snares. The mooring rope should not pass over sharp edges, and everything should be arranged so that no person must ever be positioned between the components and the deployment point (usually the stern of the boat). The rope, chains, anchor, and buoy should each be arranged and secured on the deck prior to leaving the dock if possible. Assembly of any parts should take place either on dock or on the boat deck if tools are available on the boat deck (Gates 28-29).
A crane, in conjunction with an A-frame, which is a large, u-shaped structure that extends up and over the stern of the boat, and winches, machines that help regulate the raising and lowering of rope, is used to get the buoy and anchor off the boat deck. The crane will pick up the buoy and move along the A-frame until it is over the water, then slowly lower and release the buoy. Once the buoy has been placed in the water, the chain is slowly let out for the anchor as the boat moves upstream, but no load is allowed on the chain. The heavy anchor is deployed in a similar fashion to the buoy, using the A-frame and a crane (Dever).
To recover a buoy system, the buoy is caught using a hook and lifted from the water onto the boat deck using the A-frame and a crane. Once removed from the water, the load is taken off the anchor and a bull line is attached. The anchor is then slowly winched out of the water and brought upon deck, using the A-frame and a crane (Dever).
The bottom package, consisting of the seismometer and a BPR and tsunameter, is deployed in a similar way at a short distance from the buoy mooring. It will be slowly lowered to the water’s surface, and then the anchor and the bottom package will be allowed to free-fall to the ocean floor. They are designed to work for 24 months. Units are recovered by a triggered release from the anchor. The anchor system is not recovered for this portion (http://www.ndbc.noaa.gov/Dart/dart.shtml).
The boat should then check satellite connections to the buoy and ensure that the bottom package has reached the ocean floor before moving back to dock.
Deployment of a buoy using an A-frame at the stern
of a ship and a crane.
“Deep Ocean Assessment and
Reporting of Tsunamis.” National Data Buoy Center. Retrieved
10/11/2005 from http://www.ndbc.noaa.gov/Dart/dart.shtml.
Ed and Harvey, Paul (2001). “CoOP/WEST Buoy Deployment and Recovery
Procedures.” Retrieved 10/11/2005 from http://shipsked.ucsd.edu/schedules/2003/nh_2003/dever/coopdep.pdf.
Gates, P. D., Preston, G. L., Chapman, L. B. Preston. (1998). Roberts, M. (Ed.) (1999). “Fish Aggregating Device (FAD) Manual: Volume III: Deploying and Maintaining FAD Systems.” Noumea, New Caledonia: Government of Taiwan/ROC. Retrieved 10/11/2005 from http://www.spc.int/coastfish/Fishing/FAD3_E/fad3_e.htm.
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