Offshore wind energy converters (OWECs) offer a huge potential for the expansion of renewable energy in Germany. In water depths of more than 30 m, tripod or jacket foundation structures will probably be used for the support of the tower. For these structures, driven steel piles are used as foundation elements. Design methods for axial loading of piles are given and discussed with respect to the soil conditions in the North and Baltic Seas and the loading conditions of OWECs. Also, the effect of interaction of axial and lateral forces is investigated and quantified by means of numerical modeling. This paper summarizes the state of knowledge for the design of offshore piles and gives hints for its application to the special loading conditions of piles for offshore wind energy foundation structures in the German offshore regions.
The offshore wind parks planned in the German part of the North Sea and the Baltic Sea will be constructed in water depths varying from approximately 15 to 40 m. By means of suitable foundation constructions, the large horizontal forces and bending moments resulting from wind and wave loadings must be economically and safely transferred to the sea soil. For moderate water depths up to about 25 m, monopiles can be used - and have already been applied for several existing wind parks in the North Sea and the Baltic Sea - as foundation structures. However, for larger water depths tripods or jackets are assumed to be more suitable. These consist of a steel structure, which is anchored to the sea bed by vertical or moderately inclined steel pipe piles with diameters of about 1 to 2.5 m applied in the edges. A schematic sketch of monopile, tripod and jacket foundations is given in Fig. 1. For OWECs the ratio of horizontal to vertical loading is much higher than for usual offshore structures.
