A seabed tractor belongs to the remotely operated underwater vehicle class, although their operations and the handling of these units are quite different. They can be put into use for submarine cable laying or completion of a bury operation. A typical operation involves the seabed tractor being lowered onto the seabed over the pipeline when the location has been confirmed. The weight of the tractor remains mostly on the gantry, being controlled by heave-compensation gear on the gantry. Only about 40 tonnes of the weight should rest on the seabed. The position reference of the vessel is now transferred to the trim-cube sensors on the seabed tractor support wires, which remain vertical. The position of the vessel is now being controlled by the movements of the tractor, with the trim¬cube feeding back wire angle data to the DP system. The DP, in turn, is correcting the position of the vessel in order to keep the tractor wires vertical. The DP system would be configured with the centre of rotation located on the trencher. Reading can thus be adjusted according to the environment or any other constraints.
This type of vehicle consists of a tracked Crawler excavator device, built and configured to carry out the job required. It is controlled tended from the vessel by an umbilical cable. Operating seabed tractors show great similarity with the operation of Remotely operated underwater vehicles. The control of the seabed tractor will be carried out from its control location, with the operator driving the unit as if they were on board. This can be achieved by using camera’s attached to the unit. These are also operated from the control location. There are a lot of factors to be taken into account. For example, when laying a cable, the cable itself forms a hazards as it can get caught in the umbilical cable. Seabed tractors are often compared to a remotely operated underwater vehicle, but they usually move somewhat slower, being less agile. They can be used together with a remotely operated underwater vehicle to enhance the overview and to survey the progress and performance.
- Advances in Underwater Technology, Ocean Science and Offshore Engineering Volume 33, 1994