Cable protection and remedial burial

How does Cable protection and remedial burial work, step by step?

1. 1

   Step 1: Establish the burial intent and the protection strategy

   The designer sets out, for the whole route, how the cable is to be protected along each length and why. That assessment weighs the hazards present in each area, which vary with water depth, seabed type, fishing activity, anchoring practice and shipping density, against what the ground will allow. The output is a strategy that identifies where burial is expected to work, where it is not, and what will be used instead. It is agreed with the regulator and with the owners of any assets nearby, because placing material on the seabed is itself a consented activity with its own limits.

2. 2

   Step 2: Record what the burial campaign actually achieved

   During trenching, progress is recorded continuously along the route rather than sampled. That record is what identifies the lengths where the tool did not achieve what was intended, and it is produced as the campaign runs so that remedial planning can start immediately rather than after everything is finished. The record is verified independently by survey after the trenching spread has moved on, because a trenching tool reports what it did and a survey reports what is there. Discrepancies between the two are investigated rather than averaged.

3. 3

   Step 3: Plan the protection scope from the survey

   The verified survey produces a list of lengths needing protection, with position, extent and the reason. Each is assigned a method: rock, mattress or a manufactured system, depending on the exposure, the water depth, the proximity to structures and what else is on the seabed nearby. Quantities are calculated and materials ordered, and for rock placement that means arranging supply from a licensed source and a vessel to carry it. Because the scope is not known until the burial campaign is done, projects carry a provisional allowance and a standby arrangement rather than waiting to procure from scratch.

4. 4

   Step 4: Place rock where a berm is the answer

   Rock placement is carried out from a purpose-built vessel that lowers graded rock through a pipe to the seabed, building a berm along the cable to a designed profile. The vessel holds position and tracks along the cable while the placement is monitored, because the objective is a berm centred on the cable with the designed cover and slope, not simply rock in the area. Rock is used for long lengths, for crossings, for scour repair around structures and for anywhere a durable, high-volume solution is wanted. It is effective and permanent, but it consumes a great deal of material and it is a significant cost line.

5. 5

   Step 5: Lay mattresses where precision matters

   Concrete mattresses are articulated units lowered on a frame and placed over the cable, typically by a construction support vessel with a crane and a remotely operated vehicle to guide and confirm the placement. They suit shorter lengths, tight positions, crossings where a low profile is wanted and places where a rock berm would obstruct something else. Placement is a repetitive lifting operation with each unit positioned, released and checked before the next. They are quicker to mobilise than a rock vessel for a small scope, but they do not suit long runs.

6. 6

   Step 6: Fit protection systems at the structures

   Where the cable leaves the seabed and rises into a foundation or platform it is at its most exposed, and manufactured protection is fitted around it: shells, half-pipes or bend-restricting arrangements that clamp around the cable and are secured to the structure or the seabed. Fitting is done by divers or by a remotely operated vehicle with purpose-made tooling. This is close-quarters work at the base of a structure and it is planned like any other subsea operation, with its own procedure and its own weather limits.

7. 7

   Step 7: Survey the protection and register the result

   Everything placed is surveyed afterwards to confirm the position, extent and profile, and the result is entered into the cable protection register alongside the burial record. That register is the definitive statement of how every metre of the cable is protected, and it is what the operator, the regulator, the insurer and any future works will refer to. It also becomes the baseline against which later surveys are compared. Without it, every subsequent inspection is guesswork.

8. 8

   Step 8: Run periodic surveys and mobilise remedial campaigns

   Through the operating life the cables are surveyed on a cycle set by the exposure and by how mobile the seabed is, with the areas known to be active surveyed more often. Results are compared against the baseline to find lengths that have become exposed, berms that have moved and scour that has developed. Where action is needed a remedial campaign is mobilised, using the same methods as the original work, and the register is updated. Projects usually bundle several remedial scopes into one mobilisation, because the cost is dominated by getting a vessel on site rather than by the work itself.