Modal analysis of bottom founded offshore wind structures via the creation of a finite element model

Abstract

One way to implement a wind turbine in the sea is to use lattice structures. Since these structures are placed offshore, they must be able to withstand all kinds of loads. It is essential that the eigenfrequencies of the support structure do not correspond to the passing frequencies of the blades and any other dynamic actions. To avoid this the natural frequencies of the structure should be estimated during the design and the objective of this MSc study is the development of an easy to use finite element model to perform the modal analysis of an offshore wind support structure. It was built based on relevant inputs, defining the possible design of the lattice structure. Based on these parameters, the model can be adjusted and a high number of designs can be tested easily.
The model is built in Matlab and consists of several modules, each one representing a different part of the design. The user has access to a main script to enter all necessary inputs. Then the program starts and a second function takes over. This second function is divided into five parts. The first, the definition of the geometry, creates the nodes and elements composing the structure. The second part concerns the creation of matrices characterizing the model. Each element is associated with two matrices: a local element mass matrix and a local element stiffness matrix. Each is computed in a local frame of reference, then rotated and assembled into two global matrices, representing the complete structure. Then, the equivalent stiffness characterizing the soil-piles interaction is calculated in the third part. Depending on the pile size and the soil properties, the equivalent stiffness is determined. Once the matrices completely describe the model, eigenvalues and mode shapes are calculated in the fourth part. The fifth part of the function is the plotting of the structure.
The functionalities of the Matlab tool have been validated and thoroughly checked. Firstly, by comparing the analytical and numerical results of a simplified structure (a clamped beam), and secondly using a complete structural model by comparing the outputs of the program with the outputs of the professional software “Bladed”. All key results are verified. From these verifications, it can be concluded that the characterizing matrices of the structure are correctly defined and that the model correctly represents the modal behaviour of a lattice structure.
The tool is ready to be used for sensitivity studies to verify which parameters most affect the natural frequencies. The model can also be used as a pre-design tool to quickly obtain and test different scenarios for an offshore wind support structure.
Nevertheless, the tool has some limitations such as the restricted number of possible designs/configurations, the assumption that the transition piece is a rigid body, the use of the p-y curves that can overestimate the stiffness of the soil, the non-linearity of the system that does not take into account the variation in time. Recommended future work may address these issues.