Monopiles and tripod or jacket structures are favorized foundation solutions for offshore wind energy converters. The piles of these structures are subject to intensive cyclic loading by wind and wave actions. Degradation of pile capacity as well as pile stiffness and accumulation of deflections can occur and has to be considered in the design according to German guidelines. The state of knowledge regarding the behavior of piles under cyclic horizontal loading is discussed and methods to account for cyclic load effects in the design are presented.
In the North Sea and the Baltic Sea in Europe a vast number of offshore wind farms are being planned and several have already been installed in recent years. In the beginning, in most cases wind farms were erected in moderate water depths (less than 20m) and monopile foundations have been built as support structures for the wind tower and the turbine. Diameters of up to 5m have been realized recently, and monopiles with diameters of 6m or even more are under planning. A monopile consists of a single open steel pipe pile of large diameter which is driven into the seabed. The tower is connected to the monopile by a transition piece located above the water level (Fig. 1, left). This type of foundation transfers the loads from wind and waves mainly by horizontal stresses into the ground and is believed to be suitable for water depths of up to 25m. For larger water depths, steel frame structures (jackets with four legs or tripods with three legs) can be used, which are supported by four or three piles located in the edges of the construction (Fig. 1, right). Although the axial loads are usually design-driving regarding the required lengths of these piles, also for these piles cyclic horizontal loading has to be considered in the design.
