Trenching Considerations - Cables & Flexible Pipelines

General Methods

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.

OES Fluidised Bed Cable/Flexible Pipeline Trenching Sled

OES have developed a "diverless" cable/flexible pipeline trenching sled that uses jets to fluidise sands and clays up to 15 kPa undrained shear strength. Depending on the soils, the sled should achieve a trenching speed of 2-4 m/min. The cable/flexible pipeline is guided by a shroud that deflects it to depth in the trench. As the sled moves forward, the soils settle back into the trench. The current design has a shroud with a 2 metre bending radius suitable to make a 1.5 metre deep trench for 6"-8" NS OD cables/flexible pipelines.

The sled has pressure compensated buoyancy skid tanks that can adjust the submerged weight from 0-1.5 tonnes. Also, the skids allow the sled to be floated up to the shoreline. There are two submersible pumps mounted on the unit which can be operated in series or parallel (i.e. the pressure or the flow rate can be doubled) depending on the soils requirements. The current design requires a pull force of about 0.7 tonnes. The sled can be pulled form a DP vessel with a clump weight arrangement, or from the vessel bow anchor wire.

The umbilical for the OES sled includes two 2 hydraulic hoses which are used to supply hydraulic power to the submersible pumps and the controls. On the deck, the umbilical is spooled onto an hydraulically operated reel and has a split ring commutator for continuous operation, even during winching. The sled is deployed over the cable with the assistance of an ROV. The sled is lowered onto the seabed and the cable is picked up and placed into the roller guide by an ROV or other surface support diverless techniques.

When using a DP vessel with a clump weight arrangement, the vessel will be programmed to follow the as-laid cable route at a specified speed. If the soils become stiffer than anticipated, the sled will slow and the clump weight will move astern. The positions of the clump weight and the sled will be monitored by pingers, so the speed of the vessel can be controlled to maintain the sled within a specified region. The orientation of the sled to the cable will be monitored using a rheostat device and controlled with hydraulic steering rams.

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