DESIGN CHALLENGES FOR LARGE OWTSWITH FOCUS ON SUPPORT STRUCTURES
Science Meets Industry Bergen
By Jørgen R. Krokstad
(with contributions from Loup Suja Thauvin and
Lene Eliassen – and others)
Content
Design trends – turbines and support structures
Standards
Design loads and design basis
Integrated analysis and design
Soil stiffness and capacity
2
The bottom fixed offshore wind turbineAN INTEGRATED structure
3
The “cut”
Outlook OWF Foundations
Gravity-based
foundations
Monopiles
(incl. XL)
Tripod/Tripile
Jacket
Floating
< 20
10 - 40
25-50
35-60
> 50
21%
75%
2%
2%
< 1%
• XL pipes will pot. replace
Jackets < 40 m depth
• Known technology
• High fabrication costs
• Too heavy
• Expensive logistics
• Stiffer structure/less steel
• Higher installation efforts
• Higher fabrication costs
• Commercial realisation
long term only
• Logistical challenges
• Environmental restrictions
• Complicated logistics
• Suitable only for lower
water depth
Source: Roland Berger Presentation: Offshore Wind towards 2020
Monopiles remain the dominant foundation concept, but trend
toward deeper water is shifting to jacket foundations
Foundation Depth (m) Trend 2020 CommentsCum 2012
Development of MP‘s
Source: A2Sea News - Winter 2013 and EEW SPC
20022008
2012 20142015
2018
Horns Rev 1
2.0 MW
Water depth up to 14 m
Lynn
3.6 MW
Water depth up to 18 m
London Array
3.6 MW
Water depth up to 25 m
Baltic II
3.6 MW
Water depth up to 27 m
Gode Wind II
6 MW
Water depth up to 35 m
Future MP‘s
8+ MW
Water depth up to 40 m
L 34 m
Ø 4 m
160 tL 45 m
Ø 4.7 m
350 tL 68 m
Ø 5.7 m
650 tL 73.5 m
Ø 6.5 m
930 tL 80 m
Ø 8.5 m
1050 t L >80 m
Ø >9 m
>1050 t
EEW SPC/Bladt EEW SPCSIF MT Hojgaard EEW SPC/Bladt
Jacket development from pre-piled to suction bucket.
6
Design trends on bottom fixed turbines
Large turbines (6-10 MW, 150 - 200 meter diameter)
Simple substructures – mono-columns, jackets
Possible integrated installation (foundation, tower, nacell and rotor in one
piece) but has not shown to be economical so far
INTEGRATED design – optimize tower and foundation design
7
8
Some important considereations DNV-OS-J101
9
Combination of wind and wave loads a huge challenge for the offshore wind industry.
Why? Consequence?
10
11
Example of use of load cases
12
Note! A large number
of simulations with only
10 min duration. Consequence
on extreme values?
Design basis – metocean requirements
Wave growth by the action of wind
Nonlinear wave-wave interaction
Dissipation due to white-capping, bottom friction and depth-induced wave
breaking.
Refraction and shoaling due to depth variations.
If deemed relevant, wave-current interactions
13
Design basis – metocean requirements
hub height wind velocity
dependence
misalignment dependence
Joint probabilities of Water Level
and Hs
Spectral shape and short term
directional distribution
Time duration – 1 hour (not 10 min
or 3 hours)
14
Inconsistent wave theories due to shallow water – affect strongly statistical method
15
Hmax given as deterministic input –
destroy statistic
Challenges – Dynamic Analysis
Offshore wind turbines – highly dynamic – integrated
How to cut a dynamic structure in two parts – contractual issues
How to optimize foundation (different designs for a selection of
park locations) with tower (wants to keep one design)
The troublesome top mass (nacelle and rotor)
Waves and wave loads are non-linear in extreme weather and at
shallow water
Design consequences (dimension selection)
1. Ensure no or limited frequency interaction between turbine and structural frequencies
2. Document sufficient design life (FLS)
3. Ensure sufficient structural capacity or integrity in storm conditions (ULS)
16
Eigenmodes and eigenfrequencies
where K is stiffness matrix, M is structural mass and is the
added mass (zero in air – significant in water).
17
𝐊 − ω2 𝐌+𝐦𝐚 𝚽 = 0
𝐦𝐚
The “cut”
Qualification of Design Assumed 5 MW NREL
turbine(External and Internal use of Fedem Windpower Software package)
Status integrated analysis – real ongoing projects
Operator is defining load cases together with turbine supplier
Research institute is producing wave load data
Turbine contractor is running integrated aerodynamic loads (including
controller) and calculates global load responses for tower and foundation
Foundation engineer do independent calculations for their foundation contract
based on input from turbine contractor – sequential approach
The design situation is truly not INTEGRATED – plausible cause LACK OF
ACCESS TO CONTROLLER – and INDUSTRY ESTABLISHED PRACTICE
Aerodynamics loads (nP – frequencies) illustrated on a blade. Changing distance
20
By Lene Eliassen
NTNU
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7
Pow
er
spe
ctr
al de
nsity
Frequency [Hz]
Wavespectrum
5MW
10 MW
By Loup/Lene
Statkraft/NTNU
Design challenge of support structure with increasing rotor diameter
21
Lower rotational speed of
large turbines give lower
1P and 3P regions
Reflection on integrated design
Future designs – in what frequency range do we want to design our support
structure to reduce cost? Consequence?
22
www.statkraft.com
THANK YOU
Support structure types
Offshore Oil&Gas versus Offshore WindSupport structure challenges
The soil stiffness models
2
6
Simplified: p-y curves
and springs
More complicated:
FEA models.
Design considerations
Foundation Design & Soil Conditions
• Foundation design and selection influenced
by chalk, or the «absence» of it (within
foundation depth), i.e. when chalk is:
• Shallow -> MPs
• Deep -> Jacket piles (in Swarte Bank)
• Uncertainty relates to Swarte Bank
dominated infill (blue and green areas)
Park effects on aerodynamic loading –Interaction between turbines
29
Standard IEC 61400-1 says:
“The increase in loading generally assumed to result from wake effects may be
accounted for by the use of an effective turbulence intensity, which shall include
adequate representation of the effect on loading of ambient turbulence and
discrete and turbulent wake effects.
For fatigue calculations, the effective turbulence intensity, Ieff, may be derived
according to Annex D.
For ultimate loads, Ieff, may be assumed to be the maximum of the wake
turbulence intensity from neighbouring wind turbines as defined in Annex D.”
•This refers to the model of Frandsen (2005)
Park effects on aerodynamic loading –Interaction between turbines
30
Is dist. > 5*D sufficient?
Is turb class C sufficient?
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