Dimensioning of the monopile

Offshore wind power plant with monopile foundation

The design of a monopile requires special calculation tools to take into account the main effects due to the dynamic stress caused by wind and waves and the nonlinear soil properties (e.g. scour) can.

In addition to the known methods of the API py curves comparative calculations are carried out often with FE methods to capture the nonlinear effects in the cyclic pile-soil interaction for larger monopile diameter. The monopile is generated in a first approximation as a beam model with continuous elastic bedding. Therefore, the realistic detection of the soil properties is of great importance. An idealization with 6-degree of freedom single spring must not be done because of the strong field side diagonal of the stiffness matrix.

The required length of the monopile can be determined using the "no-tip displacement" criterion. With this criterion, the horizontal displacements of the monopile are evaluated under different combinations of actions. Border changes rule on the embedment of the pile. Ideally, the pile tip is to not move horizontally, for which, however large embedment depths are necessary, which often would not be economical. 

The monopile foundation is greatly influenced in the structure calculation on the ground conditions. The changes in soil stiffness and the related natural frequency shifts affect on

  • the gust response factors and hence the wind loads in the ultimate limit state
  • the law applicable to the limit state of fatigue wave period and frequency
  • the required pile length to fulfill the "no-tip displacement" criterion (no foot displacements of the pile head). 

The following figure shows the calculated horizontal displacements for embedment depths 25, 30 and 35m are shown as examples. It can be seen that the insertion depth of 25m is too small, because the horizontal displacement at the base point is relatively large compared to the on the ocean floor. Further, since the displacement curve has no horizontal tangent to the foot, can not be assumed that the clamping in this case. In contrast, is a embedment depth of approximately 30m in front of a horizontal tangent at the base. The version with 35m embedment has a horizontal tangent at about 26m and the horizontal displacements at the base of the monopile go back to zero. However, the displacements on the ocean floor are not significantly reduced compared to the variant with 30m embedment depth, so for further design the 30m embedment would be recommended.

Modeling of monopile

Modeling of a monopile with bar elements and elastic bedding

Parametric study for the embedment of a monopile

Horizontal displacements of a monopile in the seabed