An Accurate Estimation of the Eigenfrequency of an Offshore Wind Turbine Considered as the Stepped Euler-Bernoulli Beam in Three-Spring Flexible Foundation Using the Power Series Method

The present study employs the power series method (PSM) to accurately predict the natural frequencies of eleven offshore wind turbines (OWT). This prediction is very important as it helps in the quick verification of experimental or finite element results. This study idealizes the OWT as a stepped Euler-Bernoulli beam carrying a top mass and connected at its bottom to a flexible foundation. The first part of the beam represents a monopile and the transition piece while its second part is a tower. The foundation is modeled using three springs (lateral, rotational, and cross-coupling springs). This work’s aim is at improving therefore the previous researches, in which the whole wind turbine was taken as a single beam, with a tower being tapered and its wall thickness being negligible compared to its diameter. In order to be closer to real-life OWT, three profiles of the tapered tower are explored: case 1 considers a tower with constant thickness along its height. Case 2 assumes a tower’s thickness being negligible compared to its mean diameter, while case 3 describes the tower as a tapered beam with varying thickness along its height. Next, the calculated natural frequencies are compared to those obtained from measurements. Results reveal that case 2, used by previous researches, was only accurate for OWT with tower wall thickness lower than 15 mm. Frequencies produced in case 3 are the most accurate as the relative error is up to 0.01%, especially for the OWT with thicknesses higher or equal to 15 mm. This case appears to be more realistic as, practically, wall thickness of a wind tower varies with its height. The tower-to-pile thickness ratio is an important design parameter as it highly has impact on the natural frequency of OWT, and must therefore be taken into account during the design as well as lateral and rotational coupling springs.

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The effect of the TMD on the vibration of an offshore wind turbine considering three soil-pile-interaction models

Advances in Structural Engineering ◽

10.1177/13694332211008316 ◽

2021 ◽

pp. 136943322110083

Author(s):

Mouafo Teifouet Armand Robinson ◽

Zhenyu Wang

Keyword(s):

Power Series ◽

Wind Turbine ◽

Offshore Wind ◽

Mode Shapes ◽

Offshore Wind Turbine ◽

Series Method ◽

Power Series Method ◽

Mass Damper ◽

Pile Interaction ◽

Homogeneous Soil

In this paper we propose the use of the power series method and the Newmark-Beta algorithm to study the mitigation by the tuned mass damper (TMD) of an offshore wind turbine(OWT). The monopile of the OWT is taken as slender beam buried in a homogeneous soil while the tower is considered as tapered slender beam. Mathematically, both monopile and tower are modeled as elastic Euler-Bernoulli beams, with a point mass at the tower top representing the rotor nacelle assembly (RNA). First of all, the power series method is utilized to calculate the first natural frequencies of AF and CS models. The obtained results are compared with the first natural frequency of DS model obtained from FEM-Abaqus with good satisfaction. Next, the obtained mode shapes are used to establish the system of ordinary differential equations (ODE) governing the dynamic of OWT subjected to a TMD. Afterwards, the Newmark-Beta algorithm is employed to solve the ODE. Accuracy of the introduced approach is verified by setting a comparison between our results with those obtained using FEM-Abaqus. Finally, the influence of several parameters on the performance of TMD is shown in some plots.

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