repower_pros & cons of various foundation design & installation methodologies
TRANSCRIPT
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Conference
Deeper Water Offshore Wind
Pros & Cons of various
foundation design &
installation methodologies
Dr.-Ing. Marc SeidelLeading Expert
REpower Systems SE
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Introduction and overview
Topics for this conference
Conference Programme:
Is there an optimum type of foundation for deeper, more hostile waters?
Which are the best technologies and is standardisation possible?
Investigating the most cost-effective foundation installation methodologiesavailable and being developed today new technologies, speed installation,
project optimisation, cost reductions
Analysing the design and installation constraints and structural integrity
issues related to foundation loads, dynamic loads and the increased
weights of larger turbines in deeper waters
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Substructures for deeper waters
Substructures Some options
Currently employed substructure types:
Jackets
Gravity Base Structures (GBS)
(Tripods, Tri-Piles)
Promising novel types:
Keystone Twisted jacket
Universal foundation suction bucket
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Project experience with jackets
Beatrice Windfarm
Demonstrator (2)
Thorntonbank
Phase 1, 2 & 3(48 jackets)
alpha ventus (6)
Ormonde (30)
6 Projects with jackets:
- 63 jackets installed
- additional 72 in fabrication
Nordsee Ost (48)
Bremerhaven (1)
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Advantages of jacket substructures
Advantages of jacket substructures for offshore wind turbines
High structural stiffness: The turbine behaves nearly like an onshore turbine,
virtually no wave-induced vibrations.
Light-weight: Compared to other structures, jackets are the lightest. This is
beneficial for material cost and installation.
Potential for large supplier base: Jackets are not complicated and dont need
large wall thicknesses. Potentially many suppliers (with sufficient space) can
build them.
Water depth: Jackets can be used for a large range of water depths from
about 20m to (at least) 60m.
Site conditions: Jackets can be used in nearly all conditions. Waves can be
very high and soil conditions are much less relevant as for a monopile.
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Project experience with jackets
Ormonde Pre-piling
Source: http://www.gaga1.be/EN/Projects_post.html?postId=61
Methodology significantly
improved compared to
alpha ventus
Time required for pre-
piling: 1,67 day perlocation, ex weather
Maximum speed: One
location in 24h
In practice all piles weredriven during 2,5 months
Further improvements for
Thorntonbank
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Project experience with jackets
Ormonde Jacket installation
Photo: http://www.foundocean.com/webpac_content/global/documents/more/Case%20Studies/Case%20Study%20-%20Ormonde%20Offshore%20Wind%20Farm.pdf
Nr. of days jacket lifting 31
Nr. of days topside lifting 2
Nr. of days additional work 11
Nr. of days wait for others 20
Nr. of days WOW 26
Nr. of days total 90
Jacket installation: Less than one day
per jacket lifting operation for HLV
Rambiz!
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Bremerhaven prototype
Bremerhaven Prototype
REpower-owned design
All structural calculationsperformed in-house no externalconsultants
Newly developed Transition Node Very light and slender
construction:
Transition node: 48tJacket tubulars: 206t
Castings: 61t
Total: 315t
(Weights are without internals, secondary items,etc.; weights for tubulars will increase for offshoreapplications)
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Bremerhaven prototype
Advantages of cast design
Only simple circumferential welds
Quality control can be automated and ismore reliable
Cast elements have a high fatigue and
ultimate capacity Nodal angles can be varied on a broader
scale
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Gravity Base Structures
Pro
Concrete is cheap
No noise emission
Offered by a number ofexperienced European
construction companies Well known proven concrete
technology
Highly resistant to damage by saltwater
Maintenance costs are low
Fully removable
Contra
New approach for deep water
Complex permission - legal issues
Applicable to limited water depthdepending on location
Detailed soil investigation isrequired
Applicability depending on seabed / soil conditions; suitable forsoils with high bearing capacityonly (dredging to some extent
possible)
Seabed preparation normallyrequired
Scour protection is normallyneeded
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Gravity Base Structures
Thorntonbank Phase 1
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Gravity Base Structures
Strabag Serial System
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Gravity Base Structures
Strabag Serial System
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Gravity Base Structures
Gifford/BMT/Freyssinet concept (http://gbf.eu.com/)
Installation of pre-assembled, pre-commissionedWT with foundation:- Lower weather risk?- Early revenue? What about cable installation??
Purpose-built transport and installation barge
Seabed preparation?
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Gravity Base Structures
Seatower design (http://www.seatower.com)
Combined steel / concrete solution
Steel parts prefabricated and transported toconstruction site
Lower part consists of steel skirt and concrete body
constructed and casted at the construction site Installation up to HS = 2.0m with standard tugs
Concrete injected in void below bottom slab andstructure ballasted with sand
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Gravity Base Structures
Gravitas (Arup, Hochtief, Costain) design (http://www.gravitasoffshore.com)
Claims to minimise seabed preparation byaccommodating existing seabed slopes and surfacesediments
Skirt variants to suit seabed soil conditions
Self-buoyant, installed with standard tugs
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Gravity Base Structures
Where are the limits?
Main influencing factors:
Soil conditions
Wave climate (heights, periods, directionality)
Allowable weight & size for chosen installation method
Logistics and capability to produce, store and install
Guesstimates:
North Sea, typical soil with dense sands:- app. 45-55m
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Promising novel concepts
Keystone Twisted jacket
Advantages:
Few members, few welds
Small guide structure
No under-water pile driving
Low weight
Most of the weight is in the cheap piles
Disadvantages:
Pile splices required
Several grouted connections
Two different pile sizes
Inclined pile driving
Noise mitigation difficult due to complex structure
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Promising novel concepts
Keystone Twisted jacket
Design exercise for a German North Sea project:
REpower 6M turbine, 126m rotor diameter
40m water depth
Extreme wave 20.8m
Sandy soils
Model built in ANSYS ASAS(NL), based on Keystone SACS model
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Promising novel concepts
Keystone Twisted jacket Met mast under tow on Kiel channel
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Promising novel concepts
Keystone Twisted jacket Met mast under tow on Kiel channel
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Promising novel concepts
Universal foundation
Advantages:
No piling noise issues
Leveling can be achieved during installationprocess
Quick installation process Simple decommissioning
Disadvantages:
Relatively complex steel structure at seabed(difficult to inspect)
Fabrication cost and weight?
Large wave loading due to large diameter structure
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Standardisation
Is standardisation possible?
Jackets:
Standardisation only possible for construction principles
Difficulties for more general standardisation are variations in water depths,differences in loading and pile capacities (footprint)
GBS:
Standardisation of bottom part possible if ground conditions sufficientlyhomogenous
Variation in water depth can be easily accommodated
Universal foundation / Twisted jacket:
Standardisation probably similarly difficult as for standard jacket
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Critical points in the design and design process
Load simulations wind turbine and substructure
Flex5
ANSYS
ASA
S(NL)
Responsibility of the external designer
FE model of the jacket can contain wide range ofstandard finite elements
Input format is precisely defined by REpower, suchthat external models can be directly used within ourenvironment without any further modification
This has worked very well for four projects already
REpowers tool for aeroelastic simulation
28 Degrees of Freedom, 6 for the substructure
Turbulent wind field
Controller behaviour, electrical system
Very fast and efficient tool
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Critical points in the design and design process
GBS / Keystone / Universal
Excitation ofglobal vibrations by
waves in fundamental mode
significant
Misaligned waves may cause large
fatigue loads in support structure
Detailed consideration of wind-wave-
misalignment is required
Soil data most important parameter
for load simulations (stiffness and
damping contribution)
Conventional jacket
Stiff jacket structure prevents global
vibrations to be excited
Wind-wave-misalignment completely
meaningless!
Only local (quasi-static) wave loads
on jacket and appurtenances must
be considered
Soil properties do not have
significant influence on the design
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Summary
Summary
Currently jackets are the most mature option for deeper water
Several GBS options are offered to the market all of them are specific to
one supplier
Key factors are fabrication and logistics may be attractive depending on
the project specific conditions
Promising new concepts are the Keystone jacket and the Universal
foundation
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REpower Offshore Engineering
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