The TidalStream project was originally funded by the Department of Trade & Industry along with private enterprise to investigate second generation tidal power generators. Existing generators, which operate in maximum depths of 35m, are difficult and costly to install and maintain.
The aim of the project was to reduce risks and expense, allowing efficient energy harvesting at 60m depths. Ramboll acquired the advanced engineering department appointed to develop the preliminary platform design and became a partner in TidalStream Limited.
Initially, a feasibility study was conducted into the costs and risks of prototype development, and potential harvesting sites identified in the Pentland Firth, between Scotland and the Orkneys. Foundation designs were produced and scale models tested before commencement in preparation for the design of of a full-size prototype for the new Semi-Submersible Turbine (SST).
The twin-turbine SST can generate up to 2MW. It consists of a steel monocoque with two 20m rotors and two ballast 'legs'. The structure is semi-submersible, showing 10-15m above water when the ballast tanks are filled. It operates in depths up to 50m. When water is pumped out of the tanks the structure swings up horizontally, providing a stable platform for maintenance. The SST has to withstand high hydrostatic pressures in an upright position and has lifting surfaces that must resist hydrodynamic loads. It also has to deal with wave impacts, sagging and hogging.
Under water, the tether arm of the SST has a pivot that connects to a piled or gravity base foundation, allowing the rotors to change direction with the tide. Foundations are pre-installed with electrical cables and guide wires so installation and maintenance can be conducted from the surface. Three and six rotor versions are under development — the latter will provide 10MW output from a single installation at a depth of 60m.