LOADS AND DYNAMICS IN LATTICE TOWER SUPPORT · PDF fileA lattice tower support structure with...
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Wind Power R&D seminar – Deep sea offshore wind power, 20.-21. January 2011, Trondheim, Norway LOADS AND DYNAMICS IN LATTICE TOWER SUPPORT STRUCTURES FOR OFFSHORE WIND TURBINES PhD candidate: Daniel Zwick Supervisor: Geir Moe Department of Civil and Transport Engineering BACKGROUND The extremely ambitious political goals concerning extensive use of offshore wind energy result in an in- tense demand of research and development in this field. As an example, round 3 in UK could mean a need to install several thousands of offshore wind turbines within the next ten years. To be able to fulfil this goal, components for offshore wind farms has to be produced by mass production techniques and within reasonably short fabrication time. New node concepts might be of interest for more au- tomated production of lattice towers. As a basis for such an investigation, loading and dynamic re- sponse by focusing on design of the nodes has been analysed with HAWC2 in this study. SUPPORT STRUCTURE CONCEPTS Where offshore wind turbines are planned to be in- stalled in the intermediate water depths of 30-70m, bottom-fixed support structures might be used. One promising concept is the lattice tower type, due to less material use compared to other concepts like monopile or tripod structures. A lattice topology could be used for the entire support structure be- tween sea bottom and turbine nacelle or for the lower part of the tower only. Bottom-fixed support structure concepts for the intermediate water depth of 30-40m LATTICE TOWERS Lattice towers are assembled from steel tubes, where legs and bracings are welded together in tubular joints. Legs and bracings are connected in K-joints, while bracings in the planes between the legs are connected in X-joints. Joint geometry of nodes in lattice towers NODE ANALYSIS WITH HAWC2 A lattice tower support structure with 84 beam ele- ments was modelled and analysed with HAWC2. Wind turbine and rotor configuration were taken from the NREL 5MW baseline turbine. NODE ELEMENT FORCES NODE ELEMENT MOMENTS FATIGUE ANALYSIS MEMBER DIMENSIONS The initial tower design of this study was analysed with constant leg and bracing dimensions over the tower height. As expected, results from the fatigue analysis show that dimensions for the legs has to be increased towards sea bottom, while bracing dimen- sions hast to be increased towards tower top. First calculations were based on a traditional node design with circular members intersecting each other. The shown load results will be used for the futher analy- sis of new node designs, suitable for mass produc- tion of lattice towers. Results from HAWC2 are obtained in time domain. The figure to the left shows an analysis of a com- plete K-joint in one leg at a specific node. The distri- bution of mean forces in one leg over the tower height is shown to the right, with standard deviation and min/max range. Absolute forces in z-direction are decreasing towards the tower top. Mean forces in the bracing X-joints are more or less stable over the tower height, but standard deviation and min/max range are increas- ing towards the tower top. From the same analysis, results for all member mo- ments were extracted. Mean values over the tower height were found to be close to zero in the legs. However, the range of min/max values is increasing strongly towards the tower top. Moments in the high- est tower nodes are dependent on the connection design of tower and nacelle. For the bracing members in X-joints, only small moments were found, varying around a zero mean. Bracing members are mainly loaded by axial forces. OBJECTIVES New node concepts for lattice towers will be devel- oped for the following purposes: - lower total production costs - faster production, towards mass production - more automated production - more reliable welding results - prefabrication of components If the complex fabrication of lattice towers can be solved in an effective way, this type might be a pre- ferred solution for support structures in the future. −15 −10 −5 0 5 10 15 0 20 40 60 80 100 120 Leg distance x [m] Lattice tower − 120m, 4 legs, 10 sections Tower height z [m] 0 100 200 300 400 500 −10 −5 0 5 10 X − legs − 0sti, c1, d2, 1120 Fx coo: local [kN] 0 100 200 300 400 500 −10 −5 0 5 X − bracings − 0sti, c1, d2, 1120 0 100 200 300 400 500 −15 −10 −5 0 5 10 Y − legs − 0sti, c1, d2, 1120 Fy coo: local [kN] 0 100 200 300 400 500 −4 −2 0 2 4 6 Y − bracings − 0sti, c1, d2, 1120 0 100 200 300 400 500 −6000 −4000 −2000 0 2000 Z − legs − 0sti, c1, d2, 1120 Fz coo: local [kN] Time [s] leg1 5 1 leg1 4 2 Node forces − time series in K−joints, node 5 0 100 200 300 400 500 −400 −200 0 200 400 Z − bracings − 0sti, c1, d2, 1120 Time [s] bra5−1 1 1 bra5−2 1 1 bra4−4 2 2 bra4−7 2 2 −8000 −6000 −4000 −2000 0 2000 0 50 100 Z − leg1 − 0sti, c1, d2, 1120 Tower height [m] Force [kN] −500 0 500 0 50 100 Z − braX−1 in 1−2−plane − 0sti, c1, d2, 1120 Tower height [m] Force [kN] −100 0 100 0 50 100 z − leg1 − 0sti, c1, d2, 1120 Tower height [m] Moment [kNm] −4 −2 0 2 4 0 50 100 z − braX−1 in 1−2−plane − 0sti, c1, d2, 1120 Tower height [m] Moment [kNm] 0 100 200 300 400 500 −50 0 50 100 X − legs, node 5 − 0sti, c1, d2, 1120 Mx coo: local [kNm] 0 100 200 300 400 500 −20 −10 0 10 20 X − bracings, node 5 − 0sti, c1, d2, 1120 0 100 200 300 400 500 −40 −20 0 20 40 60 Y − legs, node 5 − 0sti, c1, d2, 1120 My coo: local [kNm] 0 100 200 300 400 500 −15 −10 −5 0 5 10 Y − bracings, node 5 − 0sti, c1, d2, 1120 0 100 200 300 400 500 −50 0 50 100 Z − legs, node 5 − 0sti, c1, d2, 1120 Mz coo: local [kNm] Time [s] leg1 5 1 leg1 4 2 Node moments − time series in K−joints, node 5 0 100 200 300 400 500 −4 −2 0 2 4 Z − bracings, node 5 − 0sti, c1, d2, 1120 Time [s] bra5−1 1 1 bra5−2 1 1 bra4−4 2 2 bra4−7 2 2 initial tower design HAWC2 analysis results in time domain 0 50 100 150 200 250 300 350 400 450 500 −400 −200 0 200 400 Z − bracings − 0sti, c1, d2, 1120 Fz coo: local [kN] Time [s] bra5−1 1 2 bra5−4 1 2 bra5−1 2 1 bra5−4 2 1 rainflow counting Palmgren-Miner, S-N curve lifetime of the joint 0 5000 10000 0 50 100 150 Z − leg1 5 1 − 0sti, c1, d2, 1120 number of cycles FZ amplitude [kN] 101010101010101010S−N curve for tubular joints in seawater Number of cycles Stress range [MPa] 10 1 10 2 10 3 0 50 100 Z − legs − 0sti, c1, d2, 1120 Tower height [m] lifetime [years] leg1 X 1 leg1 X 2 20 years 10 1 10 2 0 50 100 Z − bracings − 0sti, c1, d2, 1120 Tower height [m] lifetime [years] braX−1 1 1 braX−2 1 1 braX−4 2 2 braX−7 2 2 20 years 1 k i i i n D N
Transcript of LOADS AND DYNAMICS IN LATTICE TOWER SUPPORT · PDF fileA lattice tower support structure with...
−15 −10 −5 0 5 10 15
−15
−10
−5
0
5
10
15
Leg distance x [m]
Lattice tower − 120m, 4 legs, 10 sections
Leg
dist
ance
y [m
]
Wind Power R&D seminar – Deep sea offshore wind power, 20.-21. January 2011, Trondheim, Norway
LOADS AND DYNAMICS IN LATTICETOWER SUPPORT STRUCTURESFOR OFFSHORE WIND TURBINES
PhD candidate: Daniel ZwickSupervisor: Geir Moe Department of Civil and Transport Engineering
BACKGROUND
The extremely ambitious political goals concerning extensive use of offshore wind energy result in an in-tense demand of research and development in this field. As an example, round 3 in UK could mean a need to install several thousands of offshore wind turbines within the next ten years. To be able to fulfil this goal, components for offshore wind farms has to be produced by mass production techniques and within reasonably short fabrication time.New node concepts might be of interest for more au-tomated production of lattice towers. As a basis for such an investigation, loading and dynamic re-sponse by focusing on design of the nodes has been analysed with HAWC2 in this study.
SUPPORT STRUCTURE CONCEPTS
Where offshore wind turbines are planned to be in-stalled in the intermediate water depths of 30-70m, bottom-fixed support structures might be used. One promising concept is the lattice tower type, due to less material use compared to other concepts like monopile or tripod structures. A lattice topology could be used for the entire support structure be-tween sea bottom and turbine nacelle or for the lower part of the tower only.
Bottom-fixed support structure concepts forthe intermediate water depth of 30-40m
LATTICE TOWERS
Lattice towers are assembled from steel tubes, where legs and bracings are welded together in tubular joints. Legs and bracings are connected in K-joints, while bracings in the planes between the legs are connected in X-joints.
Joint geometry of nodes in lattice towers
NODE ANALYSIS WITH HAWC2
A lattice tower support structure with 84 beam ele-ments was modelled and analysed with HAWC2. Wind turbine and rotor configuration were taken from the NREL 5MW baseline turbine.
NODE ELEMENT FORCES
NODE ELEMENT MOMENTS
FATIGUE ANALYSIS
MEMBER DIMENSIONS
The initial tower design of this study was analysed with constant leg and bracing dimensions over the tower height. As expected, results from the fatigue analysis show that dimensions for the legs has to be increased towards sea bottom, while bracing dimen-sions hast to be increased towards tower top. First calculations were based on a traditional node design with circular members intersecting each other. The shown load results will be used for the futher analy-sis of new node designs, suitable for mass produc-tion of lattice towers.
Results from HAWC2 are obtained in time domain. The figure to the left shows an analysis of a com-plete K-joint in one leg at a specific node. The distri-bution of mean forces in one leg over the tower height is shown to the right, with standard deviation and min/max range. Absolute forcesin z-direction are decreasing towards the tower top.
Mean forces in the bracing X-joints are more orless stable over the tower height, but standard deviation and min/max range are increas-ing towards the tower top.
From the same analysis, results for all member mo-ments were extracted. Mean values over the tower height were found to be close to zero in the legs.However, the range of min/max values is increasing strongly towards the tower top. Moments in the high-est tower nodes are dependent onthe connection design of tower and nacelle.
For the bracing members in X-joints, only smallmoments were found, varying around a zero mean. Bracing members are mainly loaded by axial forces.
OBJECTIVES
New node concepts for lattice towers will be devel-oped for the following purposes:
- lower total production costs - faster production, towards mass production - more automated production - more reliable welding results - prefabrication of components
If the complex fabrication of lattice towers can be solved in an effective way, this type might be a pre-ferred solution for support structures in the future.
−15 −10 −5 0 5 10 150
20
40
60
80
100
120
Leg distance x [m]
Lattice tower − 120m, 4 legs, 10 sections
Tow
er h
eigh
t z [m
]
0 100 200 300 400 500−10
−5
0
5
10X − legs − 0sti, c1, d2, 1120
Fx c
oo: l
ocal
[kN
]
0 100 200 300 400 500−10
−5
0
5X − bracings − 0sti, c1, d2, 1120
0 100 200 300 400 500−15
−10
−5
0
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10Y − legs − 0sti, c1, d2, 1120
Fy c
oo: l
ocal
[kN
]
0 100 200 300 400 500−4
−2
0
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6Y − bracings − 0sti, c1, d2, 1120
0 100 200 300 400 500−6000
−4000
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2000Z − legs − 0sti, c1, d2, 1120
Fz c
oo: l
ocal
[kN
]
Time [s]
leg1 5 1
leg1 4 2
Node forces − time series in K−joints, node 5
0 100 200 300 400 500−400
−200
0
200
400Z − bracings − 0sti, c1, d2, 1120
Time [s]
bra5−1 1 1bra5−2 1 1bra4−4 2 2bra4−7 2 2
−8000 −6000 −4000 −2000 0 20000
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Z − leg1 − 0sti, c1, d2, 1120
Tow
er h
eigh
t [m
]
Force [kN]
−500 0 5000
50
100
Z − braX−1 in 1−2−plane − 0sti, c1, d2, 1120
Tow
er h
eigh
t [m
]
Force [kN]
−100 0 1000
50
100
z − leg1 − 0sti, c1, d2, 1120
Tow
er h
eigh
t [m
]
Moment [kNm]
−4 −2 0 2 40
50
100
z − braX−1 in 1−2−plane − 0sti, c1, d2, 1120
Tow
er h
eigh
t [m
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Moment [kNm]
0 100 200 300 400 500−50
0
50
100X − legs, node 5 − 0sti, c1, d2, 1120
Mx
coo:
loca
l [kN
m]
0 100 200 300 400 500−20
−10
0
10
20X − bracings, node 5 − 0sti, c1, d2, 1120
0 100 200 300 400 500−40
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60Y − legs, node 5 − 0sti, c1, d2, 1120
My
coo:
loca
l [kN
m]
0 100 200 300 400 500−15
−10
−5
0
5
10Y − bracings, node 5 − 0sti, c1, d2, 1120
0 100 200 300 400 500−50
0
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100Z − legs, node 5 − 0sti, c1, d2, 1120
Mz
coo:
loca
l [kN
m]
Time [s]
leg1 5 1
leg1 4 2
Node moments − time series in K−joints, node 5
0 100 200 300 400 500−4
−2
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4Z − bracings, node 5 − 0sti, c1, d2, 1120
Time [s]
bra5−1 1 1bra5−2 1 1bra4−4 2 2bra4−7 2 2
initial tower design HAWC2 analysis results in time domain
0 50 100 150 200 250 300 350 400 450 500−400
−200
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400Z − bracings − 0sti, c1, d2, 1120
Fz c
oo: l
ocal
[kN
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Time [s]
bra5−1 1 2bra5−4 1 2bra5−1 2 1bra5−4 2 1
rainflow counting Palmgren-Miner, S-N curve lifetime of the joint
424 426 428 430 432 434 436 438 440
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Rainflow series of 185−Fz−bra10−7−2−2
Time [s]
Forc
e [k
N]
0 5000 100000
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150Z − leg1 5 1 − 0sti, c1, d2, 1120
num
ber o
f cyc
les
FZ
amplitude [kN]
104
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106
107
108
109
101
102
103
S−N curve for tubular joints in seawater
Number of cycles
Stre
ss r
ange
[MPa
]
101
102
103
0
50
100
Z − legs − 0sti, c1, d2, 1120
Tow
er h
eigh
t [m
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lifetime [years]
leg1 X 1
leg1 X 2
20 years
101
102
0
50
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Z − bracings − 0sti, c1, d2, 1120
Tow
er h
eigh
t [m
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lifetime [years]
braX−1 1 1braX−2 1 1braX−4 2 2braX−7 2 220 years
1
ki
i i
nDN
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