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Irregular and Breaking Waves
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Presenter: I. Prowell, Ph.D., P.E. Contributors: A. Horvat, D. Dolan, R.L. Nichols, P.T. Gayes, L. Pietrafesa,
T. Yan, S. Bao, E. Hackett, R. Gurka, F. Driscoll, and W. Musial
State
NC SC VA GA MA NJ NY MD ME DE CT RI NH
Offs
hore
Win
d R
esou
rce
at 9
0Mw
ithin
50n
m o
f sho
re (G
W)
0
50
100
150
200
250
300
3500-30 m H2O30-60 m H2O>60 m H2O
Outer Continental Shelf Wind Energy Resource
4
Conceptual Reliability
5
WRF ROMS
SWAN
Wind
Wind, Heat Flux
SST
Current
Breaking Wave
Wave induced stress
Roughness length
Macro Wind-Wave Modeling
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• Analyze Hurricane Hugo and Hurricane Irene • Breaking wave hazard
• Map % of waves that meet BWI during storm • Histograms, statistics • Map slam force on a cylinder • Breaking wave index graphs
Breaking wave
Non-breaking wave
Hazard Definition
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Met-ocean Modeling (Preliminary Results) • Waves begin breaking offshore as waves come over OCS • Type of breaking varies with water depth • Energy released varies with breaking wave type
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Metocean Modeling (Preliminary Results)
• Spatial and temporal variability
A
B
ADCP
9
Verification Against Observation
50 100 150 200 2500
2
4
6
8
10
12Significant Wave Height (Observation vs Simulation)
Met
er
50 100 150 200 2500
2
4
6
8
10
Met
er
Significant Wave Height (Observation vs Simulation)
Time since 8/20/2011 (unit: 30 minutes)
Station ASimulation
Station BSimulation
Model initial time
Model initial time
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Translating Waves to Forces (IEC)
( )
CR
321' here
CR
3212'
323for
'64
'1Arctan3
'8'6
42
−=≤≤
−−=
ttwtCR
RCt
RCt
RCt
CtRRCtF b πρλη
( )
8CR0for
4
1Arctan222
≤≤
−−=
t
RCt
RCtRCtF b πρλη
0
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
0 0.02 0.04 0.06 0.08 0.1
Impa
ct F
orce
(kN
)
Time (sec)
λ.ηb ηb Hb
SWL Rmp
C
area of impact
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Challenges • Primarily based on small diameter members in shallow waters where
breaking conditions are frequent • e.g. wharf piers
• Primarily based on experimental fitting • Applicability for large diameter monopiles in question • Applicability to infrequent breakers in deeper water in question • Structural response highly dependent on force time history • Broadband excitation unlike many turbine loads
-200,000
-150,000
-100,000
-50,000
0
50,000
100,000
150,000
200,000
250,000
0 2 4 6 8 10OTM
(kN
-m)
Time (sec)
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CFD Modeling of Breaking Waves
• Initial 2D model to understand computational parameters • Resulted numerical model with predictable periodic breaking
waves that agreed with theory and experimental observation
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Three Dimensional Model • 3D model complete • Preliminary results with monopile introduced
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Improvements • Conduct “numerical experiments” • Improved resolution
• Spatial distribution as a time varying pressure • Effects for large diameter members explicitly considered • Force variation for changes in breaking location
• Breaking initiated before, at, and after monopile • Water depth effects
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Outcomes • Framework for hazard mapping
• Allows definition of hazard • Size • Period/Celerity • Return period
• Validations of macro model from observation • Slam loads with consideration of water depth and member diameter
• Compare to existing approach • Advise on updates to existing approach
