Van Buren, Eric_NTNU

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Wind Power R&D Seminar Dee Sea Offshore Wind

Royal Garden Hotel, Trondheim, NorwayJanuary 21, 2011

Effect of Foundation ModelingEffect of Foundation ModelingMethodology on the Dynamic Response ofMethodology on the Dynamic Response ofOffshore Wind Turbine Su ort StructuresOffshore Wind Turbine Su ort Structures

Eric Van Buren, PhD Offshore WindEffects of Foundation Modeling Methodology


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AgendaAgenda

Motivations for research

Research Questions

Project details and methods

esu s

Conclusions

Further work



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To reduce the costs while increasing the performance and reliability ofoffshore wind energy through advancements in foundation modelingtechniques and design methods


Courtesy: GL-Garrad Hassan 2009


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offshore support structuresoffshore support structures

Support structures make up a much higher percentage of the total costs offshore This trend is likel to continue as water de th increases at wind farm sites

Contribution to Total Cost

Com onent Onshore Offshore

OFFSHORE Turbines (excludingworks)

68-84% 49%

Su ort Structure 1-9% 21%

Grid Connection 2-10% 16%

Consultancy 2-8% 9%

-

Other 2-10% 1%

Courtesy: EWEA 2010



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Installation DifficultiesInstallation Difficulties ery arge an expens ve ns a a on vesse s are requ re

Foundations and tower must be installed to very precise tolerances

Many components must be installed in calm weather to avoid damage Bad weather can lead to large amounts of downtime , running up costs

Foundation installation is the most time consuming part of the process Extremely large diameter piles or immensely heavy gravity based must be installed

Preparation of the seabed and scour protection may be required Offshore foundations cost 2.5x more than for a similar land-based wind turbine



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Reducing the costs...Reducing the costs...

Efficiently designed support structures and foundations Specifically engineer foundations for loads and site conditions at each offshore wind

turbine

Develop computer software tools specifically produced for offshore wind turbine

foundation design

Towers and foundations must be designed in a way that is economical to mass-produce

Efficient use of materials, manufacturing facilities, and manpower

Pur ose built offshore wind su ort structure manufacturin facilities will be needed

Improved installation techniques and equipment New foundation technology which is easier and quicker to install

Pur ose-built installation vessels to install wind turbines in a cost effective manner



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Pile Foundations ModelsPile Foundations Models

Fully coupled finite element model simulation Most comprehensive modeling technique, includes many additional non linear effects

Includes interactions between soil layers (vertical) and between adjacent piles (horizontal) Very time consuming and expensive, requires extensive soil lab testing

S

I

MP

C

E

Sequential analysis with finite element simulations Combines the capabilities of the multiple non-linear spring model with finite element simulations

Allows for dynamic FE simulations of the foundation without the need for a fully coupled model

Multiple non-linear spring representation (p-y curves)

L

I

C

I

T

A

I

N

Dependant on accurate soil profile and characteristic parameters Single non-linear spring representation Entire foundation modeled with single springs at mudline for each DOF

Does not account for pile flexibility or soil profile non-homogeneity

T

Y

&

T

Y

& o e w an equ va en x y ep pparen x y eng

Very simple and fast in computations, more representative than fixed condition

Does not capture any soil-structure interaction

Assume fixed boundary conditions Extremely simple, fast computations

S

P

E

C

O

S Gross misrepresentation of stiffness of the foundation

D

Eric Van Buren, PhD Offshore Wind


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efficiency?efficiency? Separately design each pile to give the minimum installation time and maximum

structural efficiency Each foundation designed to only the minimum required length and diameter

Less overall material use, reduced fabrication effort, less time and effort for installation

More time, man-hours, and money spent during the testing and design phase

Develop a single pile design that can be utilized for all structures in the entire wind park ,

Higher overall material use, increased fabrication effort, more difficult installation Less time, effort and money spent in the testing and design phase

VSVS



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Research QuestionsResearch Questions

1. Do the most sim le modelin techni ues rovide and accurate enou hdescription of the dynamic characteristics to be used for preliminarydesign and analysis?

2. Does the added accuracy and certainty in analysis and design of anoffshore wind turbine foundation when using more advanced modeling



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Fixed boundary conditions

Apparent Fixity Length (AFL)

Distributed non-linear spring model using force-displacement (p-y) curves



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certain depth (AFL) below the seabed

AFL chosen to match the stiffness of the pile with distributed spring model - -

Can also be determined based on soil properties

~



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,

uncoupled spring stiffnesses

Can be determined using two different approaches pp e orce omen me o

Forced displacement/rotation method

Can be modeled with linear or non-linear springs

Borrowed from Zaaijer (2002)



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Distributed NL spring modelsDistributed NL spring models Force displacement (p-y) curves found in the design standard are used

for horizontal and vertical displacements ISO 19902:2007(E) Petroleum and natural gas industries fixed steel offshore structures (Ch 17)

Dependant on undrained shear strength profile, friction angle, unit weight of soil, and pile diameter

Not really suitable to extremely large diameter piles (such as those used on monopile wind turbines)

Hyperbolic force displacement relationship used for torsional stiffness -

Dependant on undrained shear strength profile, unit weight of soil, pile stiffness, pile diameter

1200p-y curve for clay at depth z=16

12000t-z curve for clay at depth z=16

800

1000

tion(kN/m

2)

8000

10000

tion

(kN/m2)

200

400

600

Subgrade

Reac

2000

4000

6000

SubgradeRea

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0

Horizontal Displacement (meters)


0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

Vertical Displacement (meters)

Effects of Foundation Modeling Methodology


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Monotower

Chose a generic design, representative of currently producing turbines

120m height, 35mm wall thickness, diameter tapering from 5.5m to 3m

Full-height lattice tower Designed by former NTNU PhD student Haiyan Long

120m height, 4 legs, 10 sections. 21 meters wide at base, 4 meters at nacelle



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Monotower ComparisonMonotower Comparison



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Lattice tower comparisonLattice tower comparison



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ConclusionsConclusions

Significant discrepancies noted between the different foundation models

Not immediately clear which is most accurate, but worth investigating further

Response is very sensitive to changes in the selected soil parameters More detailed soil descriptions and response models are needed

No interaction between soil layers or between adjacent piles Future models must include 3-D soil interaction effects



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Se uential Anal sis FE Method Method used to investigate the response of a piled foundation to the loads experienced on an offshore

wind turbine structure using the finite element method

An iterative process of finite element simulations of the soil-pile structure and the wind turbine structure

Static FEMNL Soil Springs HAWC2 Simulation

oes no a ow a a o ee n o e aero-e as c co e a eac me s ep, on y as n a con ons


Dynamic FEM

Time Series Force Data


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Full -Cou led FE Model

Foundation, or Geo module to be develop using open source FEM foundation code (such as

OpenSees, Code Aster, etc.)

Geo Module then fully coupled with an Aero-Servo-Hydro-Elastic code (FAST, FLEX5, ADAMS, etc.) ng an ana ys s oo or e oun a on sys em s e as p ece nee e o prov e a proper ana ys s o

the entire wind turbine system



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Further WorkFurther Work

Develop a FEM code for foundation response which can be coupled to a

Aero-Servo-Hydro-Elastic simulation (Aero-Servo-Hydro-Geo-Elastic) Can be implemented and coupled with FAST/ADAMS or other open source code

Allows for a time domain analysis of the entire wind turbine system

Investigate dynamic processes of scour and the impacts on soil stiffnessand damping Changes in soil properties can have significant impacts on the fatigue life of the structure

Impact will be more significant with shallow foundations such as suction caissons

Extend investigations to suction caissons and other foundation solutions Potential foundation concepts can be used in conjunction with a number of different

tower concepts

a a e numer ca mo e s w e a a



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Questions?Questions?

an you or your a en on

Contact:

Eric Van Buren, Ph.D. Candidate, NTNU

[email protected]


Van Buren, Eric_NTNU

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