Power Engineering International

Offshore wind farms: Floating solutions for deep-water cables

Flotation technology developed by Trelleborg Offshore for the deep-water oil and gas industry has opened up a promising route for resolving the cabling challenge presented by Statoil’s innovative Hywind project, currently exploring floating wind turbine technology at a site 10 km from the coast of Norway.

David Somerville, Trelleborg Offshore, UK

While coastal locations across the globe have no shortage of wind, effectively capturing that wind for power generation can be harder than you might think.

Offshore wind power generation has traditionally been restricted to shallow water locations as wind farm technology has, to-date, concentrated on seabed fixed platforms. These tend to be monopile or tubular jacket platforms, which are only suitable for shallower waters.

Norway, for instance, has comparatively little shallow water so traditional wind turbine technology cannot be utilized, despite the abundance of wind in the North Atlantic.

However, in locations where coastal offshore wind farms are not appropriate, such as in coastal marine reserves, deep-water installations can offer an alternative option. Statoil’s Hywind offshore wind turbine, 10 km off southwest Norway, provides a glimpse into what may be the future of wind power generation.

Exploiting offshore wind in deep water

Although the increasing need for renewable energy sources makes it vital to exploit offshore wind power in deep-water locations, the technological demands for inshore and offshore wind turbines are quite different.

The degree of buoyancy for the cable, for example, was key for this offshore wind turbine, while less relevant for shallow-water projects. In this case, offshore wind power generation applications such as the Hywind project can apply a wide range of proven technologies developed for deep-water oil and gas extraction – such as Trelleborg Offshore’s syntactic foam-based distributed buoyancy modules (DBMs), which offer a means to manage the power offtake cable connecting the offshore generator with Norway’s national grid.

Deep-water turbine technology

The construction of the Hywind floating wind turbine is based on a proven deep-water floating platform design. This comprises a tall cylinder, containing flotation tanks partly submerged 100 metres (330 feet) below the surface. It is ballasted at one end to ensure it remains upright in the water, and secured to the seabed creating a comparatively small footprint. Located on top of this is the wind turbine generation unit.

A floating Hywind pilot turbine in Åmøyfjorden, Norway Source: Hild Bjelland Vik/Statoil

The Hywind design is suitable for deployment where the sea is between 120 and 700 metres deep. While the heavy structure, which weighs over 5000 tons, is incredibly stable, it operates in a very dynamic ocean environment. The buoy itself may be virtually static but the movement of ocean currents at different depths can impose heavy loads on, for example, the power cable – even though the depth at which it is deployed from the cylinder is below the influence of the waves.

Cable buoyancy protection

Hywind is currently moored in 220 metres of water with over 100 metres of power cable weighing about a ton per ten metres. The cable must be able to accommodate all movement and loading from the ocean in relation to the static platform, as well as its own weight, and therefore has a different performance specification from that for the cable used in static, shallow-water wind farms.

A proven method of reducing load on the cable from fixed and floating offshore structures is to provide midwater suspension of the cable by means of controlled buoyancy. In this way, the full weight of the cable poses no threat to the security of the connection with the structure above.

Trelleborg Offshore’s syntactic foam distributed buoyancy modules (BDMs) reduce load on the cable connecting the floating Hywind turbine with Norway’s national grid Source: Trelleborg Offshore

Nexans, a global cable company with extensive experience in supplying cable and fittings for oil and gas platforms, supplied the almost 14 km of power cable reaching back to shore. Trelleborg Offshore worked closely with Nexans to develop a solution to support the 10.6 tons of cable at a depth of around 150 metres.

The proven Trelleborg Offshore syntactic foam DBMs provided an ideal solution to support the cable. The DBMs were carefully positioned to create what is known as a ‘lazy wave’ formation, where a section of cable between the platform and ocean floor floats horizontally mid-water to accommodate any changes in tension on the cable.

Custom-built buoyancy

Trelleborg Offshore designed and manufactured 45 DBMs for the Hywind project. Made from syntactic foam, the DBMs have a final buoyancy weight of 235 kg (518 pounds). The final buoyancy weight provides the rating for the end of design life of the DBMs and cable; it allows for some compression of the foam and the very small amount of water infiltration over the 30-year design lifetime of the solution.

The DBMs consist of two half-rings with a syntactic foam core rated to a depth of 220 metres, encapsulated within a tough protective polyethylene shell. A three-part clamp is located at the centre of the ring to fix the two halves to the cable, with a restraining band around the exterior.

Syntactic foam is an essential component, as it is designed to resist the crushing pressure, which would reduce normal flexible or rigid foams to a non-buoyant thickness. Trelleborg Offshore used its widely specified Evasyn grade of foam, which consists of a mixture of gas filled macrospheres and microspheres held within a foam resin matrix. The formulation is precisely calculated to deliver the required buoyancy, as a mixture with a high compression resistance and low density.

The use of macrospheres and microspheres, distributed throughout the resin, imparts the desired compression resistance. The glass-walled microspheres, with an average diameter of 0.05–0.1 mm (0.00195–0.0039 inches), are used as a buoyant filler around the thermoplastic resin macrospheres.

The high-grade macrospheres, unique to Trelleborg, are injection-moulded to a range of diameters to meet the buoyancy and compressive resistance requirements of the application.

Design for a marine lifetime

With more than 50 years’ experience in developing and manufacturing syntactic foams to meet different physical requirements, Trelleborg Offshore provides bespoke solutions to meet marine, offshore, aeronautical and naval requirements. The foam core is light and dimensionally stable, but very rigid, while the use of different materials for the macrospheres and microspheres enables a wide range of precise specifications to be met.

The polyethylene shell provides a protective and highly visible surface that helps resist abrasion, impact damage, and marine growth. The moulded shape of the shell also enables easy handling, which ensures the on-ship deployment of the cable can be done quickly and effectively. As the DBMs are fastened into position on the cable while at sea, easy handling is vital.

An essential part of the DBM is the clamp, which allows each module to be fixed accurately in position on the cable for a precise distribution of buoyancy and optimum control of the cable configuration in the water. The clamp includes machined nylon segments lined with compressible rubber fingers, which produce a secure grip on the cable surface.

The clamp segments are designed to bolt together, until they contact each other, giving a simple and clear installation method. The clamping pressure is achieved by the compression of the rubber fingers, which also provide flexibility to accommodate the effects of diametric variations and bending of the power cable. All components are resistant to corrosion in sea water ensuring structural integrity for the whole service life.

Qualification and verification testing

For deep-water applications such as DBMs, the foam and PE shell are hydrostatically batch-tested to ensure they meet the requirements for buoyancy and compression resistance, using Trelleborg Offshore’s own test facilities. The results of the tests enable verification of the materials for the task required. Before the DBMs are shipped out geometric tests are also carried out to ensure all components fit together. A clamp functionality test verifies that the clamp will withstand the design axial loads on the cable without slipping or damaging the cable.

The DBM is designed to be easily attached to the cable as it is lowered into the sea from the afterdeck of a cable laying vessel. As well as having a short ‘safe’ weather window to perform these tasks, it is also important that the DBMs can be fastened securely under motion at low temperatures and while wearing gloves. The entire installation is carried out from on board the ship, avoiding the cost and risk associated with the use of divers or ROVs (remotely operated vehicles).

Other product applications developed initially for offshore oil and gas work but now also relevant to both shallow- and deep-water wind turbine installations include bend restrictors and stiffeners, protective ducting to minimize the effects of impact and abrasion, and vortex induced vibration suppression systems that reduce current induced cyclic stress on flexible cables and pipelines.

Offshore wind: power generation for the future

The world of wind turbine energy generation is entering an exciting new era with the deployment of the Hywind turbine. It provides an additional dimension in the race to create lower-cost energy resources, to complement existing land and inshore technologies.

Although it is a new field, it is a source of reassurance that a large proportion of the technology that it requires has previously been well tried and tested in other demanding offshore environments, such as in the oil and gas industry.

It is essential that all available avenues of renewable energy are explored and utilized. The Hywind project demonstrates that deep offshore wind power generation is a genuinely viable source of power generation for the future and that tried-and-tested methods are available to combat the issues presented by deeper water locations.

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