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Research Papers: Ocean Renewable Energy

Nonlinear Pitch Decay Motion of a Floating Offshore Wind Turbine Structure

[+] Author and Article Information
K. P. Thiagarajan

Department of Mechanical Engineering,
The University of Maine,
Orono, ME 04469
e-mail: krish.thiagarajan@maine.edu

R. Urbina, W. Hsu

Department of Mechanical Engineering,
The University of Maine,
Orono, ME 04469

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received November 14, 2013; final manuscript received September 29, 2014; published online October 23, 2014. Assoc. Editor: António Falcão.

J. Offshore Mech. Arct. Eng 137(1), 011902 (Oct 23, 2014) (7 pages) Paper No: OMAE-13-1108; doi: 10.1115/1.4028744 History: Received November 14, 2013; Revised September 29, 2014

Model tests were conducted on three generic floating wind turbine systems in 2011 and reported in a series of papers at the 31st Ocean, Offshore, and Arctic Engineering Conference in 2012. These tests were conducted at the MARIN facility in The Netherlands, by a consortium of universities, government research organizations, and industry. As part of the testing program, decay tests in platform pitch were conducted with and without wind forcing. It was found that for spar and semisubmersible type structures, resonant pitch motion was damped due to wind in storm sea conditions. The nonlinear decay motion of a floating wind turbine platform is modeled using a one degree-of-freedom nonlinear oscillation equation about a mean offset angle. Attention is paid to the turbine thrust coefficient and its variability with respect to oncoming flow speed, which in turn is affected by the structure pitch motion. The equation of motion reveals that the mean offset position has an important role in the stiffness, damping, and consequently the natural period of pitch motion. Several important dimensionless parameters are introduced. The paper discusses a simple thrust model for an offshore wind turbine (OWT) based on rudiments of blade element theory. Using the simplified thrust coefficient formulation, the increase in platform pitch damping due to wind is formulated. Experimental data reported from prior tests described above show good agreement with the theoretical model.

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References

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Koo, B., Lambrakos, K., Goupee, A., and Kimball, R., 2012, “Model Tests for a Floating Wind Turbine on Three Different Floaters,” 31st International Conference on Ocean, Offshore, and Arctic Engineering, Rio de Janeiro, Brazil, July 1–6, Paper No. 83642.
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Figures

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Fig. 1

Prototype semisubmersible modeled in experiments reported in Refs. [1-3]

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Fig. 2

Prototype spar modeled in experiments reported in Refs. [1-3]

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Fig. 3

Schematic of OWT and substructure showing coordinate system

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Fig. 4

Displacement versus blade swept area for various turbine substructure concepts

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Fig. 5

Wind speed versus g∇1/3 for various turbine substructure concepts

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Fig. 6

Pitch decay curves for the semi-submersible: (a) no wind and (b) with wind of 11.2 m/s [2]

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Fig. 7

Damping ratio versus pitch magnitude for the semisubmersible. Experimental results with and without wind, compared with the theoretical results.

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Fig. 8

Damping ratio versus pitch magnitude for the spar. Experimental results with and without wind, compared with theoretical results.

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Fig. 9

Pitch decay curves for the semisubmersible from theory and experiments versus time

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Fig. 10

Pitch decay curves for the spar from theory and experiments versus time

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Fig. 11

Damping ratio versus pitch amplitude for (a) semisubmersible and (b) spar for prototype and model conditions

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Fig. 12

Definition of angles in the plane of the rotor

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