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Ocean Renewable Energy

Preliminary Design of Bottom-Fixed Lattice Offshore Wind Turbine Towers in the Fatigue Limit State by the Frequency Domain Method

[+] Author and Article Information
Haiyan Long

Gamle Drammensvei 149B, NO-1363 Høvik, Bærum, Norwayhaiyan@ntnu.no

Geir Moe

Department of Civil and Transport Engineering,  Norwegian University of Science and Technology, NO-7491 Trondheim, Norwaygeir.moe@ntnu.no

J. Offshore Mech. Arct. Eng 134(3), 031902 (Feb 02, 2012) (10 pages) doi:10.1115/1.4005200 History: Received August 17, 2010; Revised September 12, 2011; Published February 02, 2012; Online February 02, 2012

The fatigue assessment of support structures is one of the most significant challenges in the design of offshore wind turbines (OWT). Fatigue analysis can be conducted in either the time-domain or the frequency-domain. The advantage of frequency-domain analysis is its time efficiency. This paper shows how the frequency domain method can be used to dimension lattice-type OWT towers such that they meet the fatigue criteria in the preliminary design stage. Two types of lattice towers, a three-legged and four-legged version, were redesigned in the fatigue limit state for the NREL 5 MW baseline wind turbine sited at a water depth of 35 m. The wall thickness of the members was chosen as the only variable and varied during the design process until the towers could survive for at least 20 years. In comparison with designs based upon ultimate strength, the mass of both types of towers increased no more than 30% when the fatigue limit state was considered. It is concluded that the lattice type structure requires only half as much material as its tubular counterpart. The three-legged tower is promising because of its simple geometry, even though it displayed a lower torsional stiffness than the four-legged tower. All the analyses in this paper were performed by an in-house FE code, intended for the early design stage of lattice towers. Once the optimum configuration is found in the early design stage, integrated time-domain analyses for the entire OWT system might be required to refine the design, taking all the nonlinear parameters into account.

Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Flow chart of the hybrid methodology for the fatigue assessment of OWTs. The cross symbol denotes the superposition of the structural responses to wind and waves by a proper method. RFC stands for the rainflow counting method.

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Figure 2

Lattice towers for offshore wind turbines with three-legged and four-legged geometries

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Figure 3

Flow chart of the fatigue analysis

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Figure 4

Sketch of the K joint with definitions of the terms

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Figure 5

SCF according to load types: (a) axial force F, (b) in-plane bending moment MIP, and (c) out of plane bending moment MOP

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Figure 6

Relation of a brace SCF with D/t

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Figure 7

Definition of the possible hot spots and the relative stress (Ref. [11], Figs. 3–7)

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Figure 8

Flow chart of the process to determine the transfer function (PSD stands for the spectral density)

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Figure 9

Linearization of the drag term for the transfer function

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Figure 10

PM and Jonswap wave spectrum shapes

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Figure 11

Sketch of SDOF system for wind turbines

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Figure 12

Clamped beam model for calculating local wave loads

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Figure 13

Typical S-N curve for a structural detail with slopes 3 and 5 on log-log scales

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Figure 14

Thrust spectra with and without structural peak at V = 12 m/s

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Figure 15

Mode shapes and eigenvalues of the four-legged tower with the leg bottom distance of 20 m

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Figure 16

Tower mass redesigned under the combined loading of thrust and waves in FLS: (a) three-legged and (b) four-legged

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Figure 17

Details of the simple joint [23]

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