Abstract
This article presents the superelement formulation newly implemented in OpenFAST to simulate fixed-bottom substructures with a reduced-order model similar to a common industry practice. The Guyan and Craig-Bampton methods are used to reduce the number of degrees of freedom and generate a so-called “superelement”. The formulation allows manufacturers to exchange such superelements to perform load calculations without revealing sensitive information about the support structure (e.g., foundation, substructure, and/or tower) or turbine. The source code is made publicly available in the OpenFAST repository. Test cases with varying degrees of complexity are presented to validate the technique, and accuracy issues are discussed.
Highlights
It is common practice in the offshore wind industry to perform sequentially coupled load analyses for fixed-bottom wind turbines [1, 2]
A closer look at the time series reveals that the amplitude of higher frequencies from the fully coupled OpenFAST results are not completely matched by the sequentially coupled ExtPtfm response; the mean relative error between the two time series is 3.7%
Specificities related to OpenFAST modeling To increase the frequency content of the model, we suggest using the BeamDyn module of OpenFAST to represent the blade when accurate blade modeling is required
Summary
It is common practice in the offshore wind industry to perform sequentially coupled load analyses for fixed-bottom wind turbines [1, 2]. This is referred to as a sequential approach because the substructure and turbine designers perform time simulations separately and exchange interface loads and displacements iteratively. The main difference with the new implementation of ExtPtfm is that in SubDyn the hydrodynamics are computed with an external module and provided at each time step to SubDyn. Full wind turbine simulations with a fixed-bottom substructure may be performed in three different ways in OpenFAST (see Figure 2): (1) using the module ExtPtfm (superelement approach), (2) using the module SubDyn without CB reduction (fully integrated), or (3) using the SubDyn module with CB reduction (fully integrated with reduction).
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