Numerous offshore wind farms in the German parts of the North and Baltic Sea with wind energy converters of a rated power up to 5.0MW have been approved up to now. Their special features are distances to shore of about 30km and more, water depths between 25 and 40m and a large number of wind energy converters up to several hundred plants per wind farm. These challenging conditions require an economic, sustainable and reliable foundation design. In general the choice of the foundation type depends on the loading characteristics and on the soil mechanical properties at the specific location. However, very often a monopile foundation has been found to be the best design solution. In the present paper, different design aspects of monopile foundations for offshore wind energy converters are evaluated. Special attention is paid on the characteristics of the monopile-soil interaction under cyclic lateral loading. A design concept is presented in which the results of cyclic triaxial element tests are combined with numerical calculations to predict the monopile behaviour.

Development of German Offshore Wind Energy

The German policy forces the extension of renewable energies to a level of 20% of the whole energy consumption by 2020. For this reason the currently installed wind power has to be expanded by utilisation of offshore wind energy resources. Due to a lot of restrictions in the German North and Baltic Sea, like existing nature reserves or shipping routes, most of the planned wind farms are located in the Exclusive Economic Zone in a distance to shore of 30km and more, where water depths range between 25 and 40m. Up to several hundred wind energy converters shall be installed per wind farm. For an economical operation, converters are needed with a rated power up to 5.0MW, which are still in test phase. Hence, major economic and technical problems exist and have to be solved, for example the grid connection, the maintenance and monitoring, but also the foundation design of the wind energy converters. Among the various foundation concepts, monopiles at present are a clear and consequential solution to carry the structural loads transferred by the tower of the wind energy converter into the subsoil1.

Monopile Behaviour under Cyclic Lateral Loading
Load characteristics

The loading of a monopile foundation results from wind, waves and currents. These loads are highly stochastic in nature. Although their effects on the structure can be more or less reliably estimated, it is not easy to quantify and assess the loading which is relevant for the foundation design. Especially the reduction of the complex loading to a series of simple cyclic load parcels, i.e. load collectives, is difficult. Regarding the wave loads it is relatively clear how to model a cyclic loading sequence characterised by wave height, wave period and ideally also wave direction. Available data exist which allow the configuration of a load collective, e.g. Hapel2. For this reason only wave loads are considered in the following. The transformation of wind and currents to load collectives requires more research efforts.

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