Floating, rotating platforms guarantee returns for offshore wind operators at any water depth. Hexicon explains how its multi-turbine concept could be the key to competitive deep-sea energy production.

Hexicon conceived the idea of a floating, multi-turbine wind energy platform that would make it possible to build wind energy parks on the oceans, regardless of water depth, in 2009. Making whatever lies beneath largely irrelevant makes it possible to establish a park wherever the best winds are, instead of having to remain in the shallows. The basic idea was a mooring in a central turret that enabled the whole platform to turn in order to make optimal use of the wind.

Hexicon’s founder’s backgrounds were in shipbuilding, and today the Stockholm-based firm designs, engineers and optimises floating energy parks, as well as marketing its ideas to the oil and gas industry.

Companies attempting to establish floating facilities must do a lot of pioneering work in order to convince developers, utility companies, maritime organisations, investors and so on of the potential of this new market segment.

Proven tech, new applications
Hexicon’s guiding design principle is to use competence and components that are proven in harsh marine environments. Floating platforms for oil and gas production have been in operation for almost 30 years and more than 160 are currently in use around the world. The first such facility was designed and built in Gothenburg in 1986 and is still in operation.

Hexicon’s platforms can be used with any manufacturer’s turbines and scaled to any size of rotor. The large, modern wind turbines of 5MW and upwards they support are designed for offshore use and the electrical installations used are well proven in this environment. The steel structure, mooring systems, turrets, thrusters, winch systems, power back-up, control systems and so on used by Hexicon are also all standard components in offshore oil and gas work, but the combination of them is new.

Design principles
There are a few initiatives to build floating bases for wind energy facilities around the world, but only Hexicon’s has several turbines per platform, making it economically compelling in every aspect. The firm evaluated several conceptual designs and calculated revenues, capex, opex, depreciations and LCOE (levelised cost of energy). This work resulted in different designs, but with recurring design principles. This work has generated many innovations and resulted in several patents being granted and more pending.

The platform is semisubmersible. It is configured to pick up a minimum amount of energy from the waves in order to be as stable as possible. The advantages with this design are:

  • large-scale deployment far offshore
  • minimum of individual site engineering
  • minimum offshore operations/installation
  • turbines with proven track records
  • every turbines stands in free wind
  • proven mooring system
  • one power cable per platform, reducing theamount of array cabling
  • depth-independent
  • seabed condition is not critical
  • minimum environmental footprint
  • possible integration of complementary technologies
  • the platform can be relocated
  • the platform can be renovated in harbour
  • the platform can be towed for decommissioning.

Optimised for production, transportationand operation
The latest addition in Hexicon’s portfolio is a rectangular platform that is optimised for construction in dry docks, or on quayside, and designed to be towed easily. More than 30 dry docks around the world are large enough to build this platform, the wet-tow cost is negligible and numerous advantages in terms of the installation maintenance and scrapping make the concept very appealing in comparison with traditional, bottom-mounted turbines. The platform’s turning capability is made possible by a winch system that combines with the standard yawing system used by wind turbines to cover 360° and still have every rotor in free wind.

Reference projects in the pipeline
Hexicon is developing a number of reference projects it aims to put into production within the next five years. The Swedish reference project is a floating 18MW platform to be located in the southern Baltic sea, in a concession owned by Blekinge Offshore. This project is scheduled for 2016-2018 and Hexicon has been allocated a corner in the concession, where the depth is about 45m.

Another reference project with the similar spec will be based in northern Scotland, where the government runs a special incentive programme for floating wind energy demonstrators. The platform will be designed specifically for the harsh environment of the North Sea.

Hexicon also has a facility in mind for Gran Canaria (Canary Islands), where the water depth is around 250m, and has offered a wind energy platform combined with a desalination plant in the Black Sea to supply the city of Istanbul with fresh water.
This latter idea could be attractive for islands and nations with water shortage; producing water with wind energy also solves the problem of storing power, since fresh water can easily be stored.

Other markets being pursued by Hexicon include Taiwan, China, USA, France, India, Germany and Japan.

Smart operation and maintenance
The concept-wide design and abundant use of steel may seem expensive, but this is compensated for by the platform’s many operational and maintenance features. It can be equipped with a quayside, a helipad, work boats, davit cranes, self-hoist cranes, accommodation and workshops. Some of these optional features can be pooled on one or two platforms in a large park. A substation can also be placed on board platforms to make use of the large structure.

The challenges ahead
The Hexicon concept is still at the beginning of its lifecycle and faces several challenges. The platform must be optimised to produce electricity at a competitive cost. LCOE will go down as lessons are learned and improvements to the idea are made, and the cost development forecast indicates that the LCOE will be lower than from traditional offshore wind energy, due to all the advantages mentioned.

Different locations bring a variety of local challenges including weather, wave patterns, sea depth and other environmental parameters. The Swedish firm has also had to convince its stakeholders that the platform’s performance is as good in reality as it has been on the drawing board. This is, however, made easier now by the sophisticated aero and hydrodynamic calculation tools that exist today.

Complementary technologies
Finally, it is worth mentioning that there are other applications that can be co-hosted on a Hexicon platform. Here are a few technologies that are currently being evaluated to coexist with wind energy production:

  • wave, tidal and solar energy
  • desalination and storage of fresh water
  • oxygenating of dead sea beds
  • hydrogen production and storage
  • fish farms.

Only imagination will limit the number of applications that can be hosted on a large structure at sea.