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DYNA

In progress

Analysis of the dynamic behavior of an offshore wind turbine, taking into account the interaction between the ground, monopile, and structure

Coordinateurs du projet

Context

Monopiles are the most common foundation type for offshore wind turbines. These monopiles support a tower-shaped structure topped by a rotor-nacelle assembly. The tower structure is relatively flexible and must withstand hydrodynamic and aerodynamic forces caused by waves and wind, respectively. In order to reduce their cost, wind turbines must have a powerful generator and minimal overall weight. This makes the wind turbine structure sensitive to dynamic loads even at low frequencies.

Scientific breakthroughs and innovation

The state of the art in analyzing the behavior of offshore wind turbine foundations is based primarily on methods that involve soil-foundation interaction (in this case, soil-monopile interaction for wind turbines mounted on large-diameter monopiles) and neglect interaction with the tower (see the PISA project, among others). Furthermore, analyses are generally performed statically. However, wind turbines are subject to hydrodynamic and aerodynamic forces due to waves and wind, respectively, which are dynamic in nature.

The originality of this project lies in performing a dynamic analysis of the wind turbine that takes into account the forces of waves and wind and also considers the various interactions between the ground, the foundation, and the superstructure, as these interactions can have a significant influence on the desired response (e.g., displacement and rotation induced at the top of the monopile and at the top of the tower; natural frequency of the wind turbine).

Expected technical and economic impact

The DYNA project focuses on the dynamic analysis of the soil-monopile-structure system, taking into account wave and wind loads with their dynamic and random nature and integrating the various interactions between the soil, the foundation, and the superstructure. Indeed, the interaction between the superstructure and the soil-foundation system is often represented in a simplified manner in the methods used in practice. The calculation methods developed will enable more reliable and economical dimensioning, thereby reducing the conservatism of current approaches. The software developed will be available for use by companies in the MRE sector.

Demonstrator

Development of digital tools for analyzing offshore wind turbine foundations that can be used by companies and design offices working in the MRE sector.

Results

Our work consisted of developing a complete three-dimensional model of the OWT for a 10 MW DTU offshore wind turbine using the commercially available finite element (FE) software ABAQUS/Standard. The geometric properties of the various components of the wind turbine (blades, tower, transition piece, and monopile) are explicitly taken into account, and the ground is modeled as a 3D continuum. Figure (1) shows the 3D model developed.

A modal analysis based on the developed 3D model is performed to calculate the natural frequencies of the offshore wind turbine when stationary. Figure (2) shows the first ten vibration modes of the OWT, taking into account soil-structure interaction. The results obtained showed that soil-structure interaction has a significant effect on the natural frequencies of the tower, but a negligible effect on the natural frequencies of the blades.

The adequacy of the various simplified models commonly used in the literature for the superstructure (Figure 3) and the soil-foundation system (Figure 4) was studied and discussed. In addition, the effect of several parameters related to the soil-foundation system (such as soil stiffness, geometric properties of the monopile, and height of the transition piece) on the first natural frequency of the OWT was presented and discussed.

 

Based on the numerical results obtained, the following conclusions can be drawn:

    • The first bending vibration modes of the tower in the fore-aft and side-side directions are the main modes of a monopile-based OWT;
    • Detailed modeling of the transition piece connecting the tower and the monopile has no significant effect on the first natural frequency of the system.
    • The first vibration frequency of the tower was significantly reduced (by 11.1%) when soil-structure interaction was taken into account.
    • Among the various foundation models used in practice, the ‘improved apparent fixity’ approach by Løken and Kaynia (2019), which considers two successive cylinders and is calibrated on the basis of the 3D model developed in this work, resulted in the best estimate of the first natural frequency with a deviation of approximately 2.5%. The distributed spring model, based on the foundation reaction modulus, came in second place. It gave the best estimate compared to the other distributed spring models, with a deviation of approximately 5%.
    •  The calibration of the various simplified foundation models (coupled springs, improved apparent fixity, and distributed spring model) based on the developed 3D model proved to provide a very good estimate of the first natural frequency with a maximum deviation of approximately 2.5%.
    • The results of the simplified superstructure models showed that representing the RNA as a localized mass with the corresponding mass and moment of inertia properties (see Figure 3, model 1) gives an estimate of the first natural frequency with a deviation of only 2.5%. Furthermore, the assumption of modeling the tower with 3D shell elements (see Figure 3, model 1) does not significantly improve the results. Thus, the use of a beam with variable sections (see Figure 3, model 2) having geometric and mass properties similar to those of the tower is sufficient when calculating the first natural frequency. Finally, model 3 in Figure 3 with a cylindrical section underestimates the value of the OWT’s first natural frequency by 11.4%.
    • The natural frequency of the OWT increases with the increase in the outer diameter of the monopile and the thickness of the wall. This can be explained by the increase in the rigidity of the foundation. The increase in relative soil density results in a very slight increase in the first natural frequency (approximately 1.2% when the soil type changes from loose sand to very dense sand).
    • The first natural frequency of the OWT increases with the increase in the buried length of the monopile, then becomes constant beyond a critical embedment depth (see Figure 5). The critical embedment depth decreases as the monopile diameter and relative density of the sand increase. The finding related to critical embedment depth is important in the design process in order to avoid unnecessary excess length of the monopile embedment depth.

 

PRESENTATION OF THE PROJECT AT THE WEAMEC SEMINAR “GEOTECHNICAL & GEOPHYSICAL FOR MARINE RENEWABLE ENERGY APPLICATIONS”

Perspectives

  • Development of a TMD (Tuned Mass Damper) to reduce excessive vibrations;
  • Development of a simplified mechanical model of the ground-monopile system that will be useful for fatigue calculations for offshore wind turbines.