Presenter: I. Prowell, Ph.D., P.E. Contributors: A. Horvat...

MMI Engineering www.mmiengineering.com

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

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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

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Questions? iprowell@mmiengineering.com ralph.nichols@srnl.doe.gov