Ocean Space Utilization

Lattice Towers for Bottom-Fixed Offshore Wind Turbines in the Ultimate Limit State: Variation of Some Geometric Parameters

[+] Author and Article Information
Haiyan Long

 Department of Civil and Transport Engineering NTNU, Høgskoleringen 7A 7491 Trondheim, Norwayhaiyan@ntnu.no

Geir Moe

 Department of Civil and Transport Engineering NTNU, Høgskoleringen 7A 7491 Trondheim, Norwaygeir.moe@ntnu.no

Tim Fischer

 Research Assistant Stuttgart University Allmandring 5B, D-70550 Stuttgart Germanytim.fischer@ifb.uni-stuttgart.de

J. Offshore Mech. Arct. Eng 134(2), 021202 (Dec 06, 2011) (13 pages) doi:10.1115/1.4004634 History: Received July 28, 2010; Revised February 24, 2011; Published December 06, 2011; Online December 06, 2011

Optimal solutions for offshore wind turbines (OWTs) are expected to vary from those of their onshore counterparts because of the harsh offshore climate, and differences in loadings, transportation, access, etc. This definitely includes the support structures required for service in the sea. Lattice towers might be a competitive solution for OWTs due to less physical impact from waves and less concern for visual impact. This paper addresses the design methodology of lattice towers for OWTs in the ultimate limit state and presents a FEM code that has been developed to implement this methodology. The structural topologies are specified in terms of tower cross-section geometry, the inclination of bracings, and the number of segments along the tower height. For each topology a series of towers is designed in which the bottom distance between the legs has been varied; the resulting tower mass is evaluated as a major parameter for the cost assessment. The study was conducted using the NREL 5-MW baseline wind turbine for an offshore site at a water depth of 35 m. The optimal design is searched for according to tower mass and fabrication complexity. The most economical tower geometry appears to have a constant inclination of bracing owing to its simplicity of fabrication and strong antitorsion capacity. Three-legged and four-legged alternatives have different advantages, the former having simpler geometry and the latter offering better torsion resistance. As a design driver for offshore steel structures, the fatigue life of the towers designed in the ultimate limit state should be assessed and the structures are consequently modified, if necessary. However, fatigue assessment is out of the scope of this paper and will be done in a later work.

Copyright © 2012 by American Society of Mechanical Engineers
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Figure 1

OWT support structures in moderate water depth

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Figure 2

160 m lattice tower accommodating a EL2500 turbine produced by Fuhrlaander in Germany (copy from [2])

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Figure 3

Lattice towers for offshore wind turbines with three-legged (a) and four-legged (b) geometries

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Figure 4

Flow chart of the lattice tower design code

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Figure 5

Two optional topologies for lattice towers

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Figure 6

The relationship of buckling reduction factor χy to the Euler critical force Ncr

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Figure 7

Global and local coordination system showing two segments

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Figure 8

Buckling deformation of lattice towers

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Figure 9

Wave- and current-induced loads on a clamped beam

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Figure 10

Sketch of multiplanar KT joints with rectangular hollow sections

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Figure 11

Sketch of the application of the thrust force: (a) for four-legged towers and (b) for three-legged towers

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Figure 12

Three-legged tower mass economically designed under wind and wave: (a) individually and (b) simultaneously (in the top title, TopDis. denotes the distance between legs at the top; SegNo. means the number of segment; ConstantH denotes the constant segment height)

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Figure 13

Four-legged tower mass economically designed under wind and wave: (a) individually and (b) simultaneously

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Figure 14

Percentage of the mass difference in two alternatives relative to the three-legged mass under the wave only case

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Figure 15

Members’ sizes designed under the wave only case

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Figure 16

Axial force distribution in the load case of torsion (the linewidth represents the magnitude of the axial force)

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Figure 17

Member diameters of the three-legged towers under the different load cases

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Figure 18

Tower mass modified by varying the braces: (a) three-legged and (b) four-legged

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Figure 19

Tower mass at 8 m top width for (a) three-legged tower and (b) four-legged tower

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Figure 20

Four-legged tower mass with 16 isometric segments: (a) three-legged tower and (b) four-legged tower

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Figure 21

Tower mass having a constant brace angle relative to legs: (a) three-legged and (b) four-legged (Constant A denotes the constant inclination of the braces relative to the legs)

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Figure 22

Tower mass of the three-legged type having a constant brace angle relative to legs and 6 m leg top distance




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