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Research Papers

Dynamic Models for Load Calculation Procedures of Offshore Wind Turbine Support Structures: Overview, Assessment, and Outlook

[+] Author and Article Information
Paul L. C. van der Valk

Section of Engineering Dynamics,
Delft University of Technology,
Delft 2628CD, The Netherlands
e-mail: p.l.c.vandervalk@tudelft.nl

Sven N. Voormeeren

Offshore Center of Competence,
Siemens Wind Power,
The Hague 2595BN, The Netherlands
e-mail: sven.voormeeren@siemens.com

Pauline C. de Valk

Section of Engineering Dynamics,
Delft University of Technology,
Delft 2628CD, The Netherlands
e-mail: P.C.deValk-1@student.tudelft.nl

Daniel J. Rixen

Chair of Applied Mechanics,
Faculty of Mechanical Engineering,
Technische Universität München,
Munich D - 85748, Germany
e-mail: rixen@tum.de

Usually, a limited number of different designs are used at various similar locations in the wind farm.

Note that the current discussion focusses on reduction methods for linear structures. For a selective overview of reduction methods for nonlinear structures, the reader is referred to Refs. [22] and [23].

Also note that one must take care of potential aliasing effects arising from directly down-sampling the interface signals.

Contributed by the Design Engineering Division of ASME for publication in the JOURNAL OF COMPUTATIONAL AND NONLINEAR DYNAMICS. Manuscript received March 3, 2014; final manuscript received July 29, 2014; published online April 2, 2015. Assoc. Editor: Carlo L. Bottasso.

J. Comput. Nonlinear Dynam 10(4), 041013 (Jul 01, 2015) (15 pages) Paper No: CND-14-1064; doi: 10.1115/1.4028136 History: Received March 03, 2014; Revised July 29, 2014; Online April 02, 2015

One of the main mechanisms for driving down the cost of offshore wind energy is to install ever larger wind turbines in larger wind farms. At the same time, these turbines are placed further offshore in deeper waters. As a result, traditional monopile foundations are not always feasible and multimembered foundations, such as jackets and tripods are required. Typically, thousands of load cases need to be simulated for the design and certification of offshore wind turbines (OWTs). As models of such foundations are significantly larger than their monopile counterparts, model reduction is often applied to limit the computational costs. Additionally, the foundation design is generally done by a specialized company, which bases its design on the results of the load simulations. Hence, an accurate estimation of the stresses in load simulation is essential to predict the integrity and the lifetime of different designs. The effect on the load accuracy of both the model reduction as well as the postprocessing method used by foundation designers (FDs) are investigated in this paper. A case study is performed on a jacket-based wind turbine model to verify and quantify the findings. First, it is observed that the effect of the reduced foundation model on the wind turbine loads is negligible. However, both the reduction method and the postprocessing method applied by the FD have a large influence on the fatigue loading in the jacket. It is shown that the popular Guyan reduction results in significant errors on the fatigue damage and that a static postprocessing analysis leads to serious underestimations of the fatigue loads. Finally, an outlook is given into future developments in the field of load calculations for OWT foundation design.

Copyright © 2015 by ASME
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Figures

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Fig. 1

Schematic overview of the design cycle of an OWT support structure [11]

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Fig. 2

Steps required for obtaining foundation loads from reduced foundation models

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Fig. 3

Overview of the different postprocessing options available to reconstruct the full foundation response

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Fig. 4

Visualization of the DC method as presented in Sec. 4.2.1

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Fig. 5

Visualization of the FC method as presented in Sec. 4.2.2

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Fig. 6

Model of the OC4 jacket structure

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Fig. 7

Relative error on the eigenfrequencies for the modes up to a normalized frequency of 3.5

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Fig. 8

Error on the damage equivalent loads for different elements. Result from DLC 1.2 with va =  13 m/s and Hs =  3.75 m. (a) Tower bottom element, (b) tower top element, and (c) blade 1 root element.

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Fig. 9

Relative fatigue damage errors from a direct expansion for DLC 1.2 with va =  13 m/s and Hs =  3.75 m. (a) Box-and-whisker plot of elements with an error between −100% and 250%. (b) Error versus normalized damage for elements with Dnorel > 0.01.

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Fig. 10

Relative fatigue damage errors from direct expansion for DLC 1.2 with va =  25 m/s and Hs =  9.4 m. (a) Box-and-whisker plot of elements with an error between −100% and 250%. (b) Error versus normalized damage for elements with Dnorel > 0.01.

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Fig. 11

Relative fatigue damage errors from FC and DC analysis for DLC 1.2 with va = 13 m/s and Hs = 3.75 m. The rows show the effect of quasi-static or dynamic postprocessing, the columns represent FC or DC approaches.

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Fig. 12

Normalized fatigue damages versus the relative errors from FC and DC analysis for DLC 1.2 with va = 13 m/s and Hs = 3.75 m for elements with Dnorel > 0.01. The rows show the effect of quasi-static or dynamic postprocessing, the columns represent FC or DC approaches.

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Fig. 13

Relative fatigue damage errors from FC and DC analysis for DLC 1.2 with va = 25 m/s and Hs = 9.4 m. The rows show the effect of quasi-static or dynamic postprocessing, the columns represent FC or DC approaches.

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Fig. 14

Normalized fatigue damages versus the relative errors from FC and DC analysis for DLC 1.2 with va = 25 m/s and Hs = 9.4 m for elements with Dnorel > 0.01. The rows show the effect of quasi-static or dynamic postprocessing, the columns represent FC or DC approaches.

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Fig. 15

Spectra of the detrended resultant interface forces and moments for the different models from DLC 1.2 with va =  13 m/s and Hs =  3.75 m

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