The photo is sourced from Gazelle Wind Power
Tension leg platforms used at the water depths between 300 and 1,500 metres are floating structures moored to the seabed with rigid steel cables and a heavy weight attached to them. In their turn, semi-submersible platforms used at the depth of 60 to 3,700 metres outwardly resemble a catamaran. At least four legs with the pontoons attached from below support its deck. Both types of platforms were originally used in the oil industry: due to their high stability, tension leg platforms simplify preparation of wells for commercial production after drilling, and semi-submersible platforms are suitable for construction of wells with a subsea wellhead (top of the well).
However, these types of platforms also have disadvantages complicating their use in renewable energy. Semi-submersible platforms are quite bulky as their stability depends on the area they occupy, while tension legs require a high load on the seabed to maintain an upright position. The Gazelle Wind Power engineers tried to overcome these limitations by creating a modular platform with an underwater structure similar to letter “M”. The platform itself consists of two horizontal bases interconnected by six oblique piles, the outermost of which go beyond the deck boundaries and are directed upward at the 45-degree angle.
Each side pile is equipped with a swivel arm. A separate mooring cable runs along each of them, securing the platform to the seabed. One end of each cable is attached to the ballast on the seabed and the other – to the weight between the bottom and the platform. Two cables and three weights attached to them form the above-mentioned letter “M”. Along with the swivel arms, this design balances the platform: the movement of sea waves is transferred from the platform to the system of underwater weights, which allows minimising the pitch (angular movement relative to the horizontal axis) and eventually returning the platform to a horizontal position.
According to Gazelle Wind Power, the specific mooring length of the new platform will be 75% lower than that of semi-submersible platforms, and the specific weight of underwater weights used for mooring will be 50% less than that of tension leg platforms. This should reduce the construction costs of offshore wind turbines, which are still higher than the costs of the onshore ones. For example, in China, offshore wind power plants cost $1,160 per kW of capacity, while the onshore ones cost $2,860 per kW.
The new platform will be tested by the PLOCAN project near the Canary Islands, where a 2 MW wind power plant (WPP) is under construction, and by a 25 MW WPP project to be implemented by WAM Horizon in the coastal region of Agusadora, northern Portugal.
