TRENCHING
FEATURE SPONSOR
PRE-LAY TRENCHING FOR CABLE INSTALLATION
Route clearance and integrity of the cable installation process continue to be an area of project risk for offshore wind export cable and subsea interconnector markets. The installation processes are maturing and as the cable runs become longer and in deeper water, the burial tools need to be smarter and more capable.
Windfarm boulder clearance often becomes a long and costly process with grabs and other dumb equipment used as a pre-cursor for bringing the cable lay and trenching equipment on site. Much of the existing subsea trenching assets such as multi-pass jet technology or single-pass power cable slot ploughs are not capable of handling surface or sub- surface boulders. Mechanical handling of a subsea power cable is also a risk for cable manufacturers and requires dexterity of equipment and operators with the increased number of export cable inherent faults evidenced in the press.
STRATEGIC PROGRAMME – IT’S ALL ABOUT THE CABLE
SMD has analysed the cable installation requirements, fault and failure criteria of power cables under its strategic programme ‘It’s all about the cable’ and developed a hybrid multifunctional pre-lay plough offering variable depth without recovery to service with a tried and tested plough share design suitable for a range of conditions. The plough will bring down the cost of installation by combining route clearance and pre-lay trenching on one platform, mobilised off line from the cable lay process and using cheaper and more readily available support vessels. The plough has three modes of operation; boulder clearance, single or multi-pass pre-lay trenching and post lay backfill. All of these operational modes are accommodated on a single, fully instrumented, steerable plough that does not touch or handle the product at any stage.
In boulder clearance mode the plough will move surface or sub-surface boulders and shed these to the side of the route. In trench mode, the plough share creates a ‘Y’ shaped engineered trench profile. The trench profile is suitable for laying cable into a wide enough target area to be viable and then centralised into a secure trench pocket. Using our knowledge about trenching in rock, we have used proven plough tip technology to allow operating in all ground conditions, from a soft silt or mud seabed to rippable rock.
POST-TRENCH CABLE LAYING PROCESS
The post-trench cable laying process can be used directly or in combination with a jet trencher for sediment removal, trench floor clearance and cable as-laid monitoring. The cable lay vessel is then independent of the towed plough or slowed down by the trenching process. Cable lay speed can be improved and cable assurance can be fed back to the vessel from the trencher. Progress rates can be transformed from the norm, 50 – 100m/hr, to the new norm, 200 – 300m/hr. In backfill mode, the plough can
be re-configured with pivoting mouldboards to recover the spoil into the open trench as required.
PROVEN EXPERIENCE
The plough which utilises the company’s proven pipeline and cable trenching experience, features triple mode front skids, for boulder clearing, trenching and backfill. Maximum cable protection is
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afforded by positioning the cable within a safe pocket at the base of a vertically sided trench base. The trench profile can accommodate cable lay bundles up to 600mm diameter and lower the product 1.5m below seabed. The plough design is scale-able, with options for 3.0m and 5.0m depths using share extensions and multi-pass arrangements. Vessel requirements increase to 250Te and 350Te bollard pull respectively. To ensure maximum efficiency of operations, the company can provide a turnkey solution with the pre-lay plough and launch and recovery system for safe operations up to sea state 6. This approach maximises asset availability for operators and reduces operational risk, by having all elements of the system integrated and tested prior to delivery. SMD’s turnkey approach also means reduced commercial complexity and simplified supplier management for the end-user.
ADVANTAGES
SMD envisage the combined cable protection improvement, lower vessel costs, minimal cable handling and the project risk mitigation from surface and subsurface boulders removal prior to lay, create an unrivalled project process suitable for a wide range of long distance cable projects.
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www.windenergynetwork.co.uk
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