In offshore technology driven steel pipes are often used as foundation elements for offshore wind energy converters like monopile, tripod and jacket systems. For a pile foundation, the scour caused by wave and current may decrease the lateral resistance. However, the effect of scour on the lateral resistance of piles has not yet been investigated intensively. This paper presents a study on the effect of scour on the deformation response of these foundation elements embedded in sandy soil. A three dimensional finite element model is used to investigate the lateral deformation of a pile with a scour under monotonic and cyclic loading. To account for cyclic loading, a special approach termed the degradation stiffness model is applied, which makes use of the results of cyclic triaxial tests. With this method, the increase of lateral displacement and rotation of the pile head due to scour can be quantified. The effects of scour depth on the lateral deformation and in particular on the rotation of piles with different lengths and diameters are investigated and evaluated.
Pile foundations have been and will be used in many existing and planned offshore wind farms in the North Sea and the Baltic Sea in Europe. In particular, different pile foundation types like monopile, tripod and jacket are used. In recent projects with monopile foundations, usually a maximum permanent rotation of the pile at seabed Φ of 0.5° (including installation inaccuracies) was required to ensure the long-term serviceability of the wind turbine. An important aspect in the design of offshore foundation elements is the potential development of scour. For an offshore pile foundation, the scour caused by waves and currents may decrease the lateral resistance substantially, inducing excess lateral deformations under monotonic design loads as well as large permanent deformations under cyclic lateral loads.
