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Buoyant platforms provide solutions to cutting tidal costs

Steep installation and maintenance costs are threatening the growth of tidal technology. But buoyant platform mooring systems could help save money

Buoyant designs can be easily floated into place, removing the need for craneage
Buoyant designs can be easily floated into place, removing the need for craneage and barges

As with any industry, once mass production and installation of marine energy conversion devices is possible the cost per unit will decrease significantly.

According to firm Sustainable Marine Technologies (SMT), initial estimates for the tidal sector, somewhat, optimistically, expect installation costs to be around two per cent of build and setup costs, with maintenance expenditure accounting for around 29 per cent of the price to run a wind farm. However, the reality so far is nowhere near that, the company notes in its paper Techno-economic analysis of tidal turbine installation and access, presented to the European Wave and Tidal Energy Conference.

“Currently installation is perhaps closer to 100 per cent or more of the prototype unit manufacture cost,” the report says. “Furthermore, the methods employed for installation are not financially scalable and each subsequent recovery and reinstallation could incur a similar or higher cost.”

One reason for this is fluctuations in the international vessel market, on which most installations currently rely, are primarily driven by the demands of the oil and gas sector. According to Jason Hayman, SMT’s managing director, on average, installation and maintenance represents around 70 per cent of total tidal project costs.

For the long-term financial viability of the tidal energy industry, a substantial reduction in both the costs for installation and maintenance access is therefore required. SMT says that “target costs of 18 pence per kWh or less could be achieved by designing and building a dedicated tidal farm Dynamic Positioning (DP) heavy left vessel, capable of installing and servicing a 300MW tidal farm”. An alternative to achieve the same ends would be to develop a buoyant platform mooring system for arrays of 5MW or larger.

SMT’s research indicates that for shallow water a dedicated DP vessel could achieve a levelised cost of energy (LCoE) of around GBP 140 per MWh, while, for deeper water, a buoyant solution could result in a LCoE of GBP 152 per MWh.

“Even with a discount rate of 10 per cent we find both solutions are able to generate electricity at a commercially viable price,” the company says.

With DP vessels being a high cost per operation solution, SMT (which is working with ocean energy firm Voith Hydro on a 1MW tidal project in the Orkneys, having worked with it on a tidal project inKorea) is working on a potential buoyant mooring solution to slash costs. Known as PLAT-O, it is a “common platform” to carry a wide range of horizontal axis tidal energy devices and should  “dramatically cut the costs of deployment and maintenance access”.

The basis of the PLAT-O technology is an array which can mount up to five devices providing a total capacity of up to 5MW. A single cable is used to export the power. As the platform is towable heavy lift vessels are not required, which also “removes the need to compete with oil, gas and offshore wind for the limited number of these costly vessels available”.

In the meantime, integrated buoyancy enables PLAT-O to succeed its position and achieve descent and ascent in the water, without the requirement for surface lifting. Individual devices can also be easily retrieved to the surface for access, removed from site and swapped if required.

"We believe that not only will it create a real step change in the costs of deployment and maintenance access, but also it has the flexibility and usability to become the industry standard as it will enable project developers to use the same platform for whichever device technology they choose," says Hayman.

In being a subsea buoyant mooring platform, it can be placed “very efficiently” in the water column.

“We will be able to access deep water sites which are not available to piled or gravity-based devices. It will handle uneven seabeds and any type of seabed condition," he adds. The company hopes to deploy a large scale prototype of the Plat-O platform in 2014.

SMT is not the only company firmly backing the concept of buoyant mooring solutions for horizontal axis tidal turbines.

TidalStream, developers of the Triton tidal turbine platform system, says: “The advantage of buoyant designs is that they can be floated into place, removing the need for craneage and barges which are impractical in the tidal flows found in waters such as thePentland Firth.

“With the Triton platform no cranes, barges, jack-up rigs or divers are required for any part of the operation,” the company adds. “In fact, the only thing required is “a workboat with moderate lift capabilities”.

John Armstrong, the company director, says another advantage is very large capacity per installation, with up to 10MW per platform at a good site.

Having announced a major investment in the company by Schottel GmbH, a marine propulsion company based inSpay,Germany, TidalStream hopes to move to full scale development and commercialisation of its floating system.

Gerhard Jensen, Schottel’s CEO, says: “We strongly believe that a platform technology that enables efficient installation of large power arrays and easy maintenance access is of key importance for the economic harvest of energy from tidal currents.”

Triton features a semi-submerged turbine-carrying catamaran structure. It includes a rigid swing-arm tether which anchors it to the seabed. The platform can be towed to site in an assembled state. Water ballasting is used to deploy it into its operating position, while de-ballasting will bring the system to the surface when maintenance or repair is required.

“The Triton concept is adaptable and can accommodate turbines provided by different developers,” says Armstrong. “The versatility of the platform approach enables the adoption of units, ranging from open or shrouded rotors to geared or direct drive transmissions.”

Various versions of Triton are being developed including Triton 3, which is designed for mid-depth tidal flow sites and can generate up to 3MW from a single installation. With three 20m diameter rotors, it is likely to be the first of the company’s systems built at full scale.

Meanwhile, Triton 6 is designed for deeper waters and can accommodate turbines of up to 10MW capacity. Alternatively, Triton S is designed to operate totally submerged for non-surface piercing applications.

The company says: “Unlike with most other tidal turbine systems, there are no large overturning moments at the seabed with the Triton, only shear, so the required anchorage will be smaller, lighter and cheaper to install.”

Depending on the water depth, currents and seabed condition, a number of anchoring options are possible, including gravity base, pinned frame, or single pile.

The system provides on-board access for scheduled maintenance and unplanned repairs, with minimal downtime, while the catamaran sea-going stability ensures its suitability for the roughest conditions. Significantly, TidalStream says the system will enable “cost of energy at or below that achieved by offshore wind”.