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June 2009 – ROV "Subtrench Two " makes trenches for new 400 kV Cable

On our first mission with SUBTRENCH TWO, it trenched a distance of 6300 m. in the Great Belt.

Introduction to "SUBTRENCH TWO"

Based on our experiences from the development of our first Sub sea wheel trencher, JD-Contractor A/S has developed a new heavy wheel trencher ROV for pre-trenching in hard soils, such as clay, chalk and stone reefs.

SUBTRENCH TWO is mainly suited for making trenches for larger power cables and pipes in depths up to 100 meters. The unit is built for trenching before laying the cable, but it can also be fitted with tracking devices for following an existing cable, which can then be moved into the trench. It grinds a narrow trench, which protects the cable well against dragged objects such as trawls and anchors.

On our first mission with SUBTRENCH TWO, it trenched a distance of 6300 m. in the Great Belt.

Method of trenching

The trenching unit is placed on the seabed, and grinds a trench 60 cm wide, and 80, 100 or 115 cm deep depending on the customer's requirements.

The material is pressed into the wheel, and taken up over the seabed, where it is taken away by the suction system, and scattered over the seabed besides the trench.

The machine operator can follow the process on TV screens showing pictures from cameras placed at several key positions on the machine.

SUBTRENCH TWO digs trenches with very even and vertical sides. They become half filled with light materials that act as the first protection for the cable when the cable is jetted down. The cable will not be covered further, but light material will over time settle in the trench and protect the cable.

Environmental effects

The machine makes a narrow trench compared to digging from a barge on the surface.

The material is scattered in a thin layer, and hereby does not have a heavy influence on the environment near the trench.

On the multibeam below, the difference between making trenches with a traditional surface controlled excavator and the trenching wheel can be seen. On the right side of the scan the trench has been made with a mechanical excavator placed on an excavator vessel. Here you can see the trench is very wide, and the debris from the excavation is visible next to the trench. On the left side the trench has been made with the trenching wheel, and consequently the trench is much narrower. There is very little debris on the side of the trench.

Summary

The wheel trencher, SUBTRENCH TWO:

  • is very efficient where the seabed consists of clay, chalk and other hard materials.
  • makes a narrow deep trench that protects the cable well.
  • is friendly to the environment due to a narrow trench.
  • is little influenced by weather.
  • operates at large depths.
  • is not hindered by large stones.