Free fall fish: collecting data while in motion

Paul Boughton

The ODIM MVP (Moving Vessel Profiler) is a computer controlled winch system that deploys a free fall fish that carries water column measuring instruments.

In the case of the Geosund ODIM MVP200 system, the instruments include a conductivity temperature and depth (CTD) sensor and a direct reading sound velocity and pressure (SV&P) sensor.

The water column data profile collected is used to calibrate a multibeam sea floor mapping system or other vessel specific acoustic systems.

The ODIM MVP200 can collect the profile to a depth of 200 metres while the vessel travels at a speed of 12 knots. If the vessel speed is reduced, the depth of the profile can be extended. The ODIM MVP system can enhance the accuracy of data collection and therefore the quality of the multibeam acoustic data being collected.

This results in reduced post processing time for the customer. “When the fish is deployed, it measures sound velocity in the water column,” continued Darrell.

The rate at which sound travels in water is dependent on factors such as temperature and salinity. In order to obtain accurate data for charting of the sea floor, it is vital to have precisely calibrated equip-ment on board and in this instance, the ODIM MVP system is of invaluable help. Besides CTD, SV&P and/or SVP&T, the ODIM MVP fish can be extended to a multi-sensor function whereby other oceanographic instrumentation such as a laser optical plankton counter (LOPC) can also be deployed. The LOPC is another product manufactured by ODIM Brooke Ocean.

Increased safety

The alternative for the Geosund and other vessels engaged in survey work or seismic research is to stop the vessel and have  scientists on deck deploy a CTD instrument or an SV&P instrument while the vessel is stationary to collect the necessary sound speed water column data.

This is cumbersome, takes time and requires personnel to be on deck in rough weather. With an ODIM MVP, only one person is required on deck when the fish is being deployed.

Once deployed, speed of sound data collection occurs with the ODIM MVP being fully automated from a computer while the vessel is underway.

ODIM has supplied around 55 MVP winch systems. Most installations are on a variety of diverse vessels servicing the oil industry but many research and Navy vessels have also purchased our equipment. Norwegian Geoconsult has been a key customer throughout the development of the MVP winch system with the first commercial installation of an MVP on board the survey vessel Geograph in 1998.

Examining seabed sediment

There is also a plan to add the Free Fall Cone Penetrometer (FFCPT) capability to the ODIM MVP200 system. The FFCPT instrument can  determine sediment type for seabed classifi-cation and sediment geotechnical strength. Again, while the vessel is underway.

“I have seen the practicality of the ODIM MVP technology in many workplaces such as on oceanographic and hydrographic survey vessels as well as the benefits it could provide on seismic exploration vessels. The major advantages are with respect to the time saved during survey operations and the added value of obtaining more accurate acoustic data during for example, sea floor charting operations. Of course there is always the value of safety. The ODIM MVP means less time required manhandling deck equipment in rough waters.”

This comment was made by Darrell Groom, an electrical engineer at ODIM Brooke Ocean. The Canadian was recently aboard the seismic vessel Geo Atlantic testing the operation of the MVP with the seismic streamers deployed.

He was also recently in Kristiansund, Norway, and completed a successful installation of an ODIM MVP200 system on board the vessel Geosund. This is a vessel that, among other func-tions, carries out charting of the sea floor in connection with laying oil and gas related equipment on the sea floor and the place-ment of other subsea installations. ODIM Brooke Ocean has recently received multiple orders for delivery, installation and training of the Moving Vessel Profiler MVP300-3400 model. Recent orders have arrived from Fugro-Geoteam, Acergy and Geoconsult. All companies are conducting various ocean mapping and seismic survey-ing activities on a worldwide basis.

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