Feature 4 | SUBSEA SUPPORT VESSELS Linked in


Laying interconnecting power cables for offshore wind turbine farms can be difficult, given the restricted space and heavy cables required. Now, one trenching technology specialist has turned to robotics to assist the process.


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ubsea cable laying is an intensely precise art, and one made trickier by the proliferation of new offshore wind


turbine farms. Securely laying a cable is not just a question of digging a deep trench on the seabed; any successful operation has to factor in variants such as soil type and the presence of rocks, for example, and must ensure that the cable is positioned in such a way that the trench walls and backfill are subjected to as little soil disturbance as possible. Aſter all, a cable buried to a depth of 1.5m in soſt soil stands far more chance of becoming dislodged than one buried to just 0.5m in a bed of hard soil. Traditionally, this latter scenario has been


realised by conducting investigations into seabed condition prior to any installation plans, and by taking special precautions when the cable route is planned to run beneath or near busy shipping lanes and active fishing grounds. However, offshore wind farms, dependent on a network of interconnecting power cables, present an additional challenge for cable laying projects, given the confined nature of these facilities. Te scope for disturbance to these cables,


and for a subsequent accident, is heightened by a number of design factors. For instance, for both monopile and jacket structures, the power cable typically exits the foundation through a J-tube, located beyond the scour protection. Obviously, should a large item be accidentally dropped on the cable at this point, or should the cable be damaged by an installation vessel’s jack-up leg, the result could be catastrophic. Even if installation crew members have


the fortune not to suffer injury or fatality in such an event, the cost for an accident on this scale is staggering, given that average hire rates for offshore wind turbine installation vessels can sometimes top US$60,000 per day. Offshore solutions provider Pharos Offshore Group summarises the potential financial fallout that companies can expect to weather from delayed installation projects:


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“An interconnecting or burial delay of just one hour at each end of an inter-turbine array cable connection…could extrapolate into more than 200 hours of additional project time. Even with operations running around the clock, this could add over eight days to the installation time.” With eight days of downtime equating


to nearly half a million dollars of additional costs, at the very least, it is essential that once the cable is entrenched, it remains safe from external sources of disturbance.


More farms, more complexities Tis is particularly important as offshore wind turbine farms look set to increase notably in size over the course of the decade. In Europe alone, the average number of turbines for each offshore wind farm under construction and planned until 2012 is 58; this figure increases to 106 turbines per wind farm when extending the outlook to 2019. And, while the average length of export cable required by the majority of UK Round 3 wind farms currently stands at 50km, this length is predicted to expand to 100km at future sites. And, even with the above factors accounted for, one needs to also consider random


The car-sized ITAT 800 will generate approximately 588kW and use water jetting


technology to bury subsea power cables at depths of up to 3m below the seabed.


causes of trench disturbance. A distressed vessel, entering the wind farm array and dragging its anchor across the seabed, could just as easily affect operations, while a general increase in offshore oil and gas exploratory and drilling activities over the next few years, plus an upsurge in the commissioning of new offshore wind farms in line with European Union (EU) targets (see page 13), will put space and location at a premium over the course of the next 10-20 years. Tis scenario has inspired Pharos Offshore


to design and commence development of a new remotely operated vehicle (ROV), specifically to handle subsea cable burial on offshore wind farm sites. Te forthcoming ROV, monikered the Inter Turbine Array Trencher (ITAT) 800, is intended to function as an alternative to the process of cable laying by underwater plough. Clifford McDougall, director of renewables


operations at Pharos Offshore, tells Offshore Marine Technology: “For many cable-laying operations, the ship will tow a metal plough which creates the trenches on the seabed. It’s a good method, and the most economical way of doing so – provided you are covering a long distance. But for wind farms with limited space – say, 1km between each monopile –


Offshore Marine Technology 4th Quarter 2011


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