UK: Osiris Projects Has Cable Route Survey Solutions

Accounting for around 16% of total project cost, submarine cabling procurement and installation for offshore wind developments is a significant commercial investment. Unfortunately for developers submarine cables do not exist in a static environment, and are subjected to a number of potential threats both natural and manmade.

Manmade hostile seabed intervention can include fishing, anchoring and dredging activities, with environmental characteristics and processes such as seismic events, submarine landslides, geology and sediment mobility profile also contributing to the dynamicity of the site.

Round 3 presents new challenges across the supply chain and cabling is no exception. While clearly, distance to shore becomes an issue, the cable also needs to cross an increasingly dynamic range of offshore environments; seabed types, deep channels, man-made obstructions and so on. These environments are often inhospitable for cable burial ploughs, sleds and tracked burial ROVs and similarly the deployment of heavily armoured cables can be difficult. Cabling can be left susceptible to damage and requiring additional monitoring, protection and inspection.

Despite these difficulties, potential damage from offshore vessel anchoring could mean target burial depths are increased. Dynamically positioned vessels capable of accommodating the necessary cable length are equally essential and scarce, and some suggest a similar situation applies to the sourcing of experienced personnel.

At Osiris Projects a large aspect of the work involves both inter-array and consented export cable route corridor surveys. Following desktop studies of existing available data, it undertakes 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 pipelines, and assist in the identification of sensitive benthic habitats.

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 in-tact. This level of detail assists in reducing uncertainty, or ‘de-risking’ the challenging installation phase.

Following installation, Osiris Projects conducts surveys 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.

The company´s newly procured Surgeon Tracked Inspection ROV is capable of performing such surveys. 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. This versatile platform can perform inspection operations in the surf zone and is capable of being loaded with an impressive array of subsea and monitoring tools. It can be operated from a variety of support platforms or may be operated from the beach.

The track-driven ROV is mobilised with a Teledyne TSS 350 for cable tracking, which utilises advanced pulse induction and digital signal processing technology to accurately determine ranges of buried cables. 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.

It has been suggested that the majority of difficulties encountered during the installation phase could be solved through adopting a collaborative approach. For Osiris Projects, a part of its success within offshore wind is derived from the experience serving the UK Oil and Gas industry working with clients such as Shell and BHP Billiton.

[mappress]

Offshore WIND Staff, July 16, 2012; Image: Osiris Projects