Repower_Pros & Cons of Various Foundation Design & Installation Methodologies

7/29/2019 Repower_Pros & Cons of Various Foundation Design & Installation Methodologies

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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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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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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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Thorntonbank Phase 1


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Strabag Serial System


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Strabag Serial System


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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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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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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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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


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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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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Keystone Twisted jacket Met mast under tow on Kiel channel


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Keystone Twisted jacket Met mast under tow on Kiel channel


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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_Pros & Cons of Various Foundation Design & Installation Methodologies