The Otter is a self propelled underwater trenching vehicle capable of simultaneous lay and burial to a depth of up to 2 metres.

During the trenching operation there is no need for diving assistance and the surveillance is taken care of by TV-cameras.The Otter is designed for work down to 50m water depth, requiring diver assistance for positioning on the seabed and for docking onto the cable/pipeline.

Divers are available during the trenching operation if boulders, timber etc. have to be removed from the trench. The divers also carry out regular visual checks and adjustments for the cable depressors to ensure trench depth is maintained.

The trenching principle is based on fluidization and hydrodynamic transport of the fluidized materials. Water jets directed toward the seabed will gradually cause a suspension of the fluidized materials. The water pressure decides the size of particles that can float in the suspension. When the seabed is fluidized heavy objects, such as a cable or a pipeline, will sink gently down to the bottom of the trench by its own weight without being exposed to any hardware forces.

Downward directed water jets operating via depressors will fluidize the seabed to a predetermined depth. At this depth rear directed water jets start blowing the suspended soil particles backwards. These rear directed water jets cause the machine to move forwards along the subsea line and no other propulsion is required. The machine's cleverly designed front roller keeps the machine in the correct burial track.

Diagram of Otter - also available in specification

The water jets "cut" a narrow trench with almost perpendicular walls. This is due to the over-pressure in the fluidized trench. As the fluidizing process is diminished, the walls may collapse and together with the deposited particles, provide an overburden. The final overburden will depend on the seabed materials. Very small particles may drift away while suspended.

By experience there is usually no trace a few days after the operation when the seabed consists of sand or silty sand and only slight traces in a clayish seabed.

The Otter trenching system is characterised by short time mobilisation/demobilisation, minimal risk for damaging the cable and that adjustments can easily be carried out to meet the requirements of the customers' technical specification.

The Otter operates with water pumps installed onboard a vessel, barge or pontoon with flexible hoses connecting the Otter with the pumps. The machine performs burial on pre-laid subsea lines and because the lines are not mechanically manipulated during the operation, any possible risk for equipment induced damages is