Floating offshore structures such as e.g. oil platforms, offshore wind turbines, and offshore meteorological towers are known.
Several configurations have been proposed for the floating or buoyancy structures: many of these employ floater elements in the form of substantially hollow floater tanks that in use are arranged substantially below the mean sea level and provide a buoyancy force to support the structure, e.g. a wind turbine. Ballast and/or mooring lines anchored to the seabed are provided for achieving stability.
In some of these floating structures, the buoyancy structure is designed to provide an excess buoyancy force and is maintained floating under the mean sea level by taut mooring lines tensioned by the excess buoyancy force.
For example, concepts have been developed such as the “Taught Leg Buoy” (TLB) floating wind turbine, with a slender cylindrical buoy and two sets of tensioned mooring lines, inclined relative to the seabed and connected to gravity anchors and to the buoy; or such as the “Tension Leg Platform” (TLP) floating wind turbine, in which the tensioned mooring lines are substantially vertical and are connected between gravity anchors on the seabed and arms or braces extending radially outwards with respect to the vertical axis of the wind turbine. The TLP arms may be part of the buoyancy structure, for example in the form of hollow spokes that extend radially outward from a hollow central hub, or may be arranged above the sea level, in which case the buoy may be a slender cylindrical tank like in the TLB concept.
TLP structures are also used for offshore oil platforms. Other configurations for offshore structures are not based on providing an excess buoyancy force and tensioned mooring lines for their stability, but instead on having a ballast at a relatively point of the structure. The resulting centre of gravity may thus be located at a relatively low point of the structure as well. As such, the resulting structure may be inherently stable and does not depend on the tensioned mooring lines for stability.
The buoyancy structures of offshore structures are subject to several loads, such as for example the weight of the wind turbine itself, impacts, forces exerted by waves, currents and tides, and, in case of a wind turbine, also aerodynamic forces associated to the wind, rotor rotation, etc.
As mentioned before, the offshore structures that rely on excess buoyancy for stability have tensioned mooring lines or cables. In order for these structures to be stable, the mooring lines must always be under tension, otherwise the offshore structure could become unstable and could fall over. Since the loads on the offshore structures, and particularly on a floating wind turbine may vary considerably both in magnitude and direction, a high amount of excess buoyancy and high tension in the cables may be required.
Having a high tension in the mooring lines or cables means that the cross-sectional area of the cables needs to be increased as well. The resulting cables may thus be heavy and expensive. Additionally, having such a high tension in the cables may result in the rest of the structure to be dimensioned accordingly. Also the other parts of the structure may become heavier and more expensive.
An alternative solution would be to have a high number of lines for providing stability and distributed in such a way that even if some of them could in certain circumstances enter in compression, the other lines would still be able to maintain the stability of the offshore structure. However, also this solution can become very expensive.
There thus exists a need for a floating offshore structure that reduces or resolves at least some of the afore-mentioned problems.