Since the early 1970s a new industry, consisting of the design, manufacture and operation of remotely operated vehicles (ROVs), has emerged Most of the ROVs which have been developed are in the tethered free-swimming category and have been designed as multi-functional vehicles capable of carrying out tasks which previously had been undertaken by divers

Whilst this market was expanding, a few companies instead concentrated their efforts in the development of seabed contact vehicles able to accomplish tasks which were outwith the capability of the diver Vehicles in this category are primarily task specific in their design The main function for which these vehicles have been developed is in the trenching/backfilling of pipelines and in the installation and burial of flexible flowlines and umbilical cables Unlike the free-swimming ROVs most seabed contact vehicles are operated by the same company responsible for their construction

There is no general configuration to which seabed contact vehicles tend to follow, however, the one aspect common to all vehicles is that they are large and require a highly skilled crew and an extensive range of support equipment Launch, recovery, and maintenance of a vehicle of this type require a surface support vessel which has to satisfy a wide range of criteria

Most operators of these vehicles therefore do not operate on the principle of using a vessel of opportunity, instead they try to operate from a single dedicated vessel throughout the working season With such high overheads and falling day rates due to the downturn in the industry, vehicle operators have been compelled to be more innovative in their approach in an effort to increase their share of the declining market

PRINCIPLE OF OPERATION

Whilst the different trenching techniques used by the few vehicles on the market vary dramatically, the principal mode of operation is the same

The vehicle, as illustrated in Figure 1, usually consists of a rigid framework on to which is mounted a pair of hydraulically driven caterpillar tracks These tracks are similar to those found on any land-based construction vehicle or military tank

Seabed contact vehicles in general require more accurate positioning than their land-based counterpart and therefore usually have independent drive to each pressure compensated track

Hydraulic power to the tracks is provided either through a series of umbilicals from the support vessel or more commonly by means of electrohydraulic pumps mounted on the vehicle chassis In this case power is transmitted to the vehicle using a single umbilical cable UDI have found that the use of a single umbilical, not only to transmit power, telemetry data, TV and sonar signals, but also to launch and recover the vehicle is advantageous1

Because of the vehicle's size and complexity of tooling, a wide selection of video and sonar systems is necessary to provide adequate visual coverage of the vehicle and its surroundings A variety of control functions and monitoring equipment provides the operator on the support vessel with sufficient data to monitor the vehicle's progress and performance

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