Trenching Considerations


Pipeline trenching is much more difficult than cable or flexible pipe trenching because of the relative inflexibility of the pipeline.

The pipeline touchdown length to the bottom of the trench, which is dependent on the stiffness of the pipeline and the depth of the trench, can typically be 50-150 metres long.

This means that the soils underneath the pipeline have to be cut up & removed from the trench, and the newly cut trench must remain relatively clear until the pipeline touches down in it. Depending on the trenching speed, the time taken for the pipeline to touch down in the trench can be around 0.5-2 hours.


Cables & Flexible Pipelines

Cables and flexible pipelines (eg. high density polyethylene (HDPE) pipelines or Coflexip-type) are usually much easier to trench than rigid pipelines. This is because they can often be deflected to the bottom of the trench within the trenching device. This means that problems with the trench infilling before the pipeline/cable touches down are eliminated and a suction device on the machine is not required thus reducing the trenching power requirements and the pull force.

Trenching of telecommunication cables is usually accomplished by simultaneous ploughing and laying. The plough is pulled behind the cable lay barge and the cable catenary touches down just behind the plough. This method of trenching is very fast and is suitable for soft cohesive soils and sands with very flexible pipelines or small diameter cables.

Simultaneous ploughing and laying is not feasible for hard clay soils, for cables/flexible pipelines greater than 50 mm OD or if there is an existing cable present nearby. This is because ploughs are not suitable in hard clays, as the diameter of the cable/pipeline increases the bending radius increases thus requiring a larger plough, and if there is an existing cable nearby the risk of damage is considered to be too high when using a plough.


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