SPONSORS OF THE OPERATIONS AND MAINTENANCE FEATURE
UNDERSTANDING SUBSEA CABLE CHALLENGES
Subsea cabling procurement and installation is estimated to account for around 16% of total capital expenditure for offshore wind projects, representing a significant investment. For developers, cabling brings a multitude of challenges and risks, and cabling issues are thought to account for around 80% of all offshore wind insurance claims.
Seabed intervention of both manmade and natural origin creates a hostile, dynamic environment, and protection methods such as burial, mattressing, and armouring are employed to help ensure the integrity of the cable and ultimately, energy stability (For further reference, see page 78 of the previous issue of this publication, where Wood Group Kenny discuss cable protection systems in detail).
pipelines, and assist in the identification of sensitive benthic habitats.
DETAILED INFORMATION A clear understanding of the conditions the cable is likely to be subjected to allows the final route and burial depth to be selected with greater confidence that the integrity of the cable will remain intact. The detailed information received by the client reduces uncertainty, and helps in ‘de- risking’ the challenging installation phase.
POST INSTALLATION Following cable installation, surveys are conducted to assess horizontal positioning, areas of exposure or movement and depth of burial. While undoubtedly interested in overall cable integrity, developers are particularly concerned with the depth of burial achieved.
Forming part of their consenting conditions developers must often conform to a blanket depth of burial; commonly around 2m. Unless a detailed, independent survey is conducted the only information available to developers is that which is provided by their trenching and burial contractor. This can be a laborious process, and survey contractors are continually exploring ways to increase efficiency in this area.
This has led Osiris Projects to investigate remotely operated
PRE-CONSTRUCTION SURVEYS Pre-construction cable route surveys comprising both acoustic and intrusive methods are performed to determine the safest and most economically viable route, and can provide a baseline for post-construction inspection activity. This information also informs decisions regarding protection measures required to meet the conditions of the route.
At Osiris Projects, a large element of their work within offshore wind involves inter-array and consented export cable route corridor surveys. Following desktop studies of existing available data, they undertake reconnaissance and detailed surveys comprising geophysical, acoustic, geotechnical and ground-truthing methods to assess site characteristics, potential hazards such as unexploded ordnance or wrecks, the location of existing cables and
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www.windenergynetwork.co.uk
‘seabed crawlers’ for such inspection and monitoring work.
SURGEON TRACKED INSPECTION ROV In 2012 they procured a Surgeon Tracked Inspection ROV from Global Marine Systems specifically for cable inspection and depth of burial operations. With a weight in air of 3 tonnes, the system can reach sea water depths of 100m and benefits from a full surveillance suite featuring 3 cameras, an obstacle avoidance sonar and a survey quality depth sensor as well as a various depth of burial profilers.
The overriding benefit of the track-driven system is the ability to function within a wider environmental window in the challenging UK coastal environment, when compared with free-flying ROVs. The track- driven ROV is mobilised with a Teledyne TSS 350 for cable tracking, which utilises
This is the outcome of a wider on-going R&D project involving a similar ROV concept. Osiris Projects are currently finalising the development of their own compact, state of the art Crawler ROV specifically for depth of burial analysis surveys.
The system will provide a unique opportunity to undertake large-scale pipe/ cable depth of burial analysis extremely cost effectively, when mobilised to their DP1 catamaran, ‘Bibby Tethra’.
NEW CHALLENGES As offshore wind moves further offshore, it is no secret subsea cabling contractors across the supply chain will face new challenges. Notably, distance to shore clearly becomes an issue, and current installation methods such as burial ploughs, sleds and tracked burial ROVs are confronted with an increasingly dynamic and inhospitable seabed environment. Knowledge and experience can continue to be drawn from the oil and gas industry, with subsea contractors already driving this discussion.
This will inevitably be a demanding learning curve, but the UK is well- positioned to lead and take ownership of this process.
Osiris Projects
www.osirisprojects.co.uk
advanced pulse induction and digital signal processing technology to accurately determine ranges of buried cables.
DETAILED DATA Resultant data is supplied to the support vessel via the ROV’s umbilical where all navigation and depth of burial data is logged by the survey computer. These results are logged and displayed in real time, with an option to be overlaid on the ROV video footage.
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