Hybrid Method for Soil-Structure Interaction for Offshore Wind Turbine Monopile Foundations

Abstract

The Offshore wind industry is constantly expanding and one of the key cost drivers is the offshore wind turbine support structure. The foundation accounts for up to 35 % of the total installation cost of the wind turbine. The monopile foundations are the most commonly used support structure with diameters up to 6m and Embedded length/depth (L/D)<7. Proper modeling of the soil-structure interaction (SSI) is important for safe and efficient design. With the introduction of large diameter rigid monopile foundations, the applicability of the current design standard is in question.

The current industry standard model for SSI for laterally loaded monopiles is the Winkler foundation model. In this model, the soil is idealized as a row of one dimensional discrete springs spanning along the beam. The stiffness of these springs is obtained using empirically derived p-y curves, which gives the relation between the soil-pressure (p) and deflection (y). The p-y method describes the construction of the p-y curves for any arbitrary set of soil parameters. This method has been applied in the oil and gas industry and is based on field tests conducted on slender piles with diameter 2m.

Various studies on the applicability of the model have shown to give erroneous results for large diameter monopiles. Since the soil is idealized as a row of discrete (local) springs, it neglects the continuous (non-local) nature of soil. Additionally, monopiles with L/D <7 exhibit a more rigid behavior and hence industry standard assumptions for laterally loaded piles with respect to vertical tangent criteria and 'zero-toe-kick' do not hold on.

A plane strain semi-analytical model is developed which considers the continuous (non-local) nature of soil as well as the soil below the pile toe/tip, which could be significant to the dynamic behavior of the monopile foundations. In this approach, the pile is modeled as an elastic rectangular plate which is discretized using modified Euler-Bernoulli beam elements. and the soil is modeled as an elastic continuum waveguide. The two entities are combined via the discretized interface. Assuming wave motion in soil, semi-analytical solutions satisfying homogeneous and inhomogeneous boundary conditions are obtained for the waveguide, which are used to derive the frequency dependent soil stiffness matrices at the interface. These are then coupled to the plate.

For the current study, the L/D ratio of the plate is restricted to the flexible regime, due to the availability of a comparable reference solution in this regime. It is shown that, with L/D ratio of 12.5 already, the dynamic behavior of the plate is sensitive to the soil below the tip. Therefore, with smaller L/D ratios, it is expected that the tip will have a significant influence on the dynamic behavior of the pile.