Solving cabling issues is essential for success

WORLDWIDE: Problems linking offshore wind farms to the shore are resulting in damaged subsea cables and costly insurance claims. The scale of the problem is considerable.

At a recent Windpower Monthly offshore forum, Darren Patel, cables director of BPP-Cables said: "The level of insurance claims we are seeing at the moment is shocking and cannot be sustained. The industry can and must take steps to address cable failures." Tomas Boeme, principal engineer at DNV Clean Energy, who was also at the forum, added: "The subsea cabling industry seems to have gone through a cycle of denial, depression and acceptance."

So what is going wrong? Boeme picks out some examples in European seas, which include one project where poor installation and currents caused an unprotected cable linking two turbines to be cut through. In another, a loop had to be created in an export cable route to allow dredging of the sailing channel - a five-day weather window caused considerable delay. One cable-laying project almost doubled in time when bedrock was hit and burial requirements changed. And more complex seabed preparations in another project pushed it several months into the winter season. Then, to make matters worse, contractors carrying out erosion protection damaged the cables three times despite deploying simple, well-proven operations.

Improvements are clearly required. Effective survey work is key, says RES Offshore project manager Mark Lawrence. "Whether you do any or not, you'll pay for the cost of a site survey in the end," he says. And Boume would like to see a thorough written code of best practice, including installation and maintenance.

Dedicated technology is the answer, according to Stephen Wilson, marine development manager at CTC Marine, who says that the past few years have seen the offshore cable sector in a period of transition. "We have been using the experience of the offshore oil and gas and telecoms sectors. We now need to look at dedicated technology applied solely to offshore wind - purpose-built ploughs, for example."

We asked cable experts from across the industry which technologies have made a difference, and which future solutions might make a difference.


Any length, and growing in power

Pelle Andersson, service and after sales manager at Swiss-Swedish power company ABB points to its HVDC Light as a significant technological advance in cables. Its high voltage direct current (HVDC) cable, used to bring power from offshore wind turbine sub-stations to shore, has been in service for more than a decade and, like other similar models on the market, has cut costs and, crucially, offered unlimited transmission lengths.

The HVDC Light cables are lighter and smaller in dimension than their predecessor, the HVAC (high voltage alternating current) cables, producing a higher power density. Where the HVAC cable system needs three cables, HVDC cable systems only need two. And, before the introduction of HVDC Light cables, the market struggled to produce effective cabling beyond a length of around 100km.

When the HVDC Light polymer cable system was brought into service it offered a voltage of 150kV and power up to 400MW, explains ABB senior sales engineer Kenneth Johannesson. More powerful HVDC cables - 320kV and 1,100MW - were introduced in 2010, and are being used at the Borkum West II wind farm off northern Germany, due to complete in 2013, will be the first commercial installation of this version. More than 50% higher than the previous highest HVDC Light cable voltage of 200kV, they improve availability and reduce losses across the entire system, says ABB.

Since the first commercial HVDC Light cable system was installed in 1999, connecting a wind farm on the island of Gotland in Sweden, several other projects have been realised. Today almost 2,600km of HVDC Light cables have been installed.


Marine trenching firm CTC Marine Projects says that its new trencher, which completed its first job in the Middle East last October, has simplified the installation of export cables in a range of seabed conditions, including sands, clay and rock. The SWT-1 is capable of shallow water trenching and burial of cable in on wind farms with onerous soil conditions, managing the cable safely away from the path of the cutting tools, and placing it at the bottom of the trench.

It operates independently from the main cable installation vessel, cutting the amount of time the main vessel is required. Once the shallow water section of the cable has been laid, the main installation vessel is free to work on the deeper sections of the cable route, while the trencher buries the cable in shallow water, and on the beach if required. It can be supported by a small versatile catamaran, a cheaper option than a dynamic positioning vessel, previously needed.

The SWT-1 can be adapted to operate in either wheel-cutting or jetting mode, to provide deeper burial depths in various soils. Its integrated depressor removes the need for a separate post-trench survey to establish the lowering depth required, thus saving time and money. Additionally, its small size means that it can be transported around the world at a relatively low cost.


Dynamic subsea HV cabling

In the other offshore cabling category, inter-array cables that connect turbines to substations, BPP-Cables - a firm with a background in the oil and gas industry - has developed mediumand high-voltage cables for dynamic subsea use. "The design, installation and operation of high-power static cables are well-understood, mature technology areas. However, one missing element is the dynamic section of cable that will connect floating turbines in round 3 wind farms," says Darren Patel, director at BPP-Cables.

Dynamic subsea cables need to operate in rough seas and endure continual motions in the water column. BPP subsea dynamic cable designs are lighter in weight, have smaller diameters and are more flexible than current cables, Darren explains.

The Company uses specially engineered metallic component materials developed for dynamic applications including copper and aluminium alloys as well as high-strength steels. BPP-Cables subsea dynamic cable designs contain no lead. Product development began in 2004, and in October this year a new 132kV 100MW AC dynamic design power cable, suitable for lengths of up to around 100km, came onto the market.


Inter-array installations

There are several fundamental changes that must be made if inter-array cable installation is to become a reliable activity with an acceptable, manageable level of associated risk. That is the view of Scott Macknocher, general manager of offshore technical consultancy Enventi, which has recently conducted in-depth analysis into cable installation in offshore wind.

Macknocher would like the interface between inter-array cabling and the turbines to be redesigned so that the cable pull-in operation is eliminated or radically altered to remove the need for personnel and equipment to be transferred from the lay-vessel to the turbine foundation. Such activities, he says, are highly weather sensitive, potentially dangerous and often result in excessive non-productive time for the installation vessel. This can result in a direct additional cost of over £100,000/day (EUR115,500/day) to the developer for the vessel and when knock-on delays to overall construction schedules occur, the indirect costs can be far higher.

Cable burial requirements are another area that needs to be re-thought, says Macknocher, with sensible and achievable targets based on the type of seabed involved - and not based on arbitrary depths, as is usually the case today. Well-accepted burial standards exist in other industries, he points out, and there is no reason why the same cannot be developed for the offshore wind industry.

Enventi's studies show that cable burial equipment currently being used is not always correct for the activity being undertaken, something that Macknocher puts down to most of the equipment having been developed for adjacent industries such as offshore oil and gas. While there have been recent improvements in such equipment, these tend to have been limited to incremental adaptations to existing equipment and methodologies rather than the development of innovative new technologies. Enventi believes that a strong potential exists for the development of bespoke equipment, targeted specifically at inter-array cable burial and employing radically different approaches to those being used at present.

The study identifies other key areas for improvement such as developer understanding of cable installation, balanced contracting strategies, better project planning and higher levels of contractor competence. These, concludes Macknocher, must be tackled if inter-array cable installation is to grow out of its present position as the problem-child of offshore development.

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