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Foundation Capabilities and LimitationsOffshore Wind Turbine Optimisation Seminar

3rd

- 4th

February 2014 Dexter House, London

Dr. C. R. Golightly GO-ELS Ltd. - Offshore Wind Turbine Optimisation Seminar 3 rd - 4th February 2014

Dr. Chris Golightly GO-ELS Ltd.Geotechnical & Engineering Geology Consultant

Source: Univ. Mass. 1974

Source: WINDFLOAT Website

Sources from top leftclockwise: Arup, BIFAB,COWI, RAVE Alpha Ventus

Source: BELWIND Website

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Summary - Offshore Wind Turbine Foundations

Introduction Global Offshore Wind Energy

Differences; Oil & Gas Platforms Wind Turbines

Types of Foundation for Offshore Wind Turbines [OWT]

Codes and Standards; DNV, GL IEC, US

Environmental, Geophysical & Geotechnical Site Investigations

Monopiles Design & Installation

4 Leg Piled Jackets OWEC, BIFAB, Truss Towers, Twisted Jacket

Tripods Weserwind Alpha Ventus & OGN-Aquind

BARD Tripile

Gravity Base Structures [GBS] Gravitas, Vici Ventus, Gifford-Vinci, SeatowerSuction Caisson UF Monopod, Tripods, Quadrapods

Others: Guyed Tower - A-Framed Monopile - TITAN Jack Up

Foundation Costs - Comparisons

Foundation Issues & Problems (1); Early Refusals & Piling Noise

Pile Foundation Issues & Problems (2); Vibro Installation & Scour

Pile Foundation Issues & Problems (3); Grouted Connections

Pile Foundation Issues & Problems (4); Monopile Resonance, Cyclic Friction Degradation & Long Term Tilt in Sands

Offshore Floating Solutions Huge Potential Offshore Wind ResourceFabrication Costs (Early 2010)

Maps: UK Round 3 & German North Sea Sites

Offshore Wind Cost Trends Need for Reduction

Seabed A nchored Foundation Templates [SAFT]

Conclusions, References, Contact Details

Dr. C. R. Golightly GO-ELS Ltd. - Offshore Wind Turbine Optimisation Seminar 3rd

- 4th February 2014

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Introduction Global OffshoreWind Energy

Clean & abundant energy on global scale should accelerateas fossil fuel costs rise & renewables gain economies ofscale and innovation occurs The Crossover

First offshore windfarm Denmark 1991. Proportion of RE inseveral European countries is increasing.

But: as OW industry goes large scale, developers & lendersare conservative and risk averse. Stated liking for Creativeinnovation but also proven technnology .

European focus is on Germany, Denmark, Sweden, Belgium& UK. France, USA, China, Japan developing rapidly Meditteranean, India, Brazil, S. Africa & others in future.

Bigger, higher larger conventional 3 blade Siemens/VestasHAWT turbines dominant. Several 8 MW versions could betwin blade and VAWT in future (Sandia Labs. Studies).

Move offshore from monopiles [15 - 30 m WD] jackets(UK)& tripods (Germany) [30-45 m WD] eventually to spar andTLP floaters [40-60 m +WD]

In UK, offshore wind developers registered interest indeploying 46 GW of capacity & 10 GW has been progressedto consent determination, construction and operation.

UK governments Renewables Roadmap aims to cut cost ofwind power to 100 per megawatt hour (MWh), with 18GW capacity off UK coast by 2020.


Source: Moustafaeipour, 2009

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LCOE Ranges and Averages [IRENA, 2013]


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Differences; Oil & Gas Platforms Wind Turbines

Oil & Gas Platforms

Relatively stiff structures, usuallyfounded on long driven piles andmudmats

Axial loads dominate due to highstructure weights

Structural dynamics are not critical withweight >>> bending moments

Wave loads tend to dominate design inhigh energy areas such as North Sea

Straightforward Force Responserelationship

Each design is one- off Prototype at asingle location

Offshore Wind Turbines

Relatively flexible towers on variety offoundation types, monopiles 4 to 9 mdiameter, tripods/4 leg jackets, GBS.

Structural dynamics always critical. 3PEigenvalue resonance

Bending moment and lateral responsemore important than axial load

Wind and wave loads both veryimportant

Complex uncorrelated/uncoupledloading

Large Nos. of OWT in arrays (80 [German AVTripods] to 2000 [FOREWIND Statoil UK])


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Types of Foundation for Offshore Wind Turbines [OWT]

Choice of foundation solution influenced by: Water depth and seabed conditions,

especially depth to rockhead Environmental loading (wind, wave, tidal) Onshore fabrication, storage and

transportation requirements. Offshore vessel & equipment spread costs

& availability Installation & Construction methodology

available. Developer CAPEX investment appetite and

OPEX (Repair & Maintenance) predictions Smarter solutions available (suctioncaissons, GBS, lighter jackets/trusses,hybrids, seabed anchored templates)Foundations 30 to 40% of overall CAPEX &rising. Cost reductions essential

Smarter lighter hybrid foundations needed& move away from riskier costly conventionaldriven tubular steel piling.


Source: UPWIND Project Final Report 2011

Source: NREL

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Codes and Standards; DNV, GL IEC, US

Codes and Standards Hierarchy Offshore German Windfarms

A. Bundesamt fur Seeschifffahrt undHydrographie [BSH, Federal Regulator]

B1. Germanischer Lloyd [GL]

B2. Det Norsk Veritas [DNV]

B3. IEC

B4. DIN (German National Standards)

C1. API-RP2A (Oil & Gas OffshoreStructures)

C2. DIBt

C3. Norsok (Norwegian Offshore)

C4. DASt Richtlinie

D. Other Specific Standardswhere above do not covertechnical design in sufficientdetail

Most Relevant Codes and Standards Det Norske Veritas DNV Offshore Standard DNV-OS-J101,Design for Offshore Wind Turbine Structures, Norway, 2004.Germanischer Lloyd Rules and Guidelines, IV IndustrialServices, Part 2 Guideline for the certification of offshorewind turbines, Germanischer Lloyd Windenergie GmbHHamburg, 2005.BSH Standard: 2007-06, Design of Offshore Wind Turbines

API RP 2A Recommended Practice for Planning, Designingand Constructing Fixed Offshore Platforms

LRFD Load and Resistance Factor Design, First Edition, July1993.- WSD Working stress design, 21 st edition, December2000. EN 1997-1:2009-09: Eurocode 7: Geotechnical Deisgn;Parts 1, 2 and 3.RECOFF Recommendations for Design of Offshore windturbines (RECOFF), European Energy, Environment andSustainable Development ProgrammeNorsok Standard N-003 Marine Actions, 2007.


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Foundation Concepts 2012 2020 [Roland Berger Study 2013]


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Environmental, Geophysical & Geotechnical Site Investigations Environmental Surveys

Biogenic reefs & Benthic communitiesMarine archaeology, wrecks and seabed obstructionsGrab and gravity core sampling of Seabed surface sediments, for scour, plumes and cable burialSeabed mobility, sand waves and shoals

Geophysical and Geotechnical Surveys

Swath bathymetry, side scan sonar imagerySeismic reflection profiling for geological shallow stratigraphy and shallow gas presenceMagnetometer for pipelines, cables, metal objects and seabed junk & unexploded ordnance [UXO] Boreholes, vibrocores and cone penetration testing for geotechnical engineering parameters and soillayering

Guidance Notes

Society for Underwater Technology (SUT)/ Offshore Site Investigation and Geotechnics (OSIG) Committee(2005). Guidance Notes on Site Investigation for Offshore Renewable Projects, Rev. 02, March 2005.

Bundesamt fur Seeschifffahrt und Hydrographie [BSH], (2008). Ground Investigations for OffshoreWindfarms. BSH Standard No. 7004, p. 40.


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Monopiles Design &Installation

Not a Pile but Driven Tubular SteelThin Walled Shell.

Typically 4.5 - 9 m diameter,sometimes tapered

Wall thicknesses 30 - 80 mm. D/t ratiovery high ~ 80 120.

WD cut-off 20 to 35 m > pile lateral &seabed soil stiffnesses & layering.

Weights up to 900 tonnes, limited byfloat out & crane capacities

Driven or drive-drill-drive (UK) or evendrilled and grouted (France)

Transition piece glued onto monopile

with brittle high strength cement ~very strong granite > problems Simple, quick, suited to shallow water:

problems - driving refusals & weight. Structure frequency limitations &

fabrication, handling and installationconstraints.


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Tripods Alpha Ventus & OGN-Aquind

Weserwind - ALPHA VENTUS German federal funding 2001 2007 6 OWEC jackets/6 OWT tripods EPCI Contract value EUR 32m Client consortium: Vattenfall, Eon & EWE

(DOTI) 1st offshore us of seabed template pre-

piling (IHC) Adopted by Borkum West 2, Globaltech 1

OGN-Aquind Newcastle based Oil & Gas fabricator TRITON 3 leg truss jacket for use in WD

over 30 m & up to 80 m Major UK Govt. funding in 2012 for

development and design of prototype jacket

Steel savings, planning to be able tofabricate 150 jackets per year atHadrians Yard in Wallsend


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BARD Tripile


- 4th February 2014

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Gravity Base Structures [GBS] Gravitas, Vici Ventus, Gifford-Vinci, Seatower

Simplicity: Certainty of delivery, increasedprogramme opportunities with fewer constraintsMinimal Seabed Preparation: Installed directlyonto seabed whenever possible avoiding needto remove or disturb surface sedimentsSelf-Floating: No heavy lift or specialist towingor installation vessels required. Reduced supplychain & weather constraints. Improved costcertainty, increased supplier base & lower costs

Flexibility: Can be relocated, repowered andremoved at end of operational life. RC non-piled ballasted GBS with skirt option

best solution in WD up to 60 m Large OWT up to 8 MW & standardised

design Collar designs can accommodate ~ 2 deg

vertical alignment tolerance Loading situation different to piled

foundations & substantial vertical loadingrequired to ensure stability

But: Generally impractical for OWT inrelatively shallow (< 15 m) water

Bad publicity: German Strabag BSH rejection& over-designed Thornton Bank GBS.


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Suction Caisson UF Monopod, Tripods, Quadrapods

Suitable for all sand densities andintermediate strength clay Installation relatively simple &

extensive oil & gas experiencefrom GoM, North Sea, W.Africa

Installation/capacity predictionanalyses well developed. Scour

protection design essential Highest quality geotechnical data

and analyses necessary forstability assessment. Cyclicloading assessment critical

Monopods installed successfullyfor Horns Rev Met Masts in 2009& adopted in 2012 for UKForewind/Firth of Forth Met Masts(Universal Foundation Monopod).

SPT in NL developing tripod SCsolution funded by Carbon Trust.Dudgeon full field SC jacketsplanned for 2016.


Source: Oxford University Civil Engineering

Source: SLP Engineering

Source: DONG

Source: DONG

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Guyed Tower and A-Framed Monopile


Source: WA Design Ltd.Source: Bunce and Carey EWEA 2001

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TITAN 200 FWSS Jack Up Concept


Source: http://offshorewindpowersystemsoftexas.com/titan_200_deep_offshore_platform

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Pile Foundation Issues & Problems (1); Early Refusals & Piling Noise

Piling Refusals

Heavy long large diameter monopilesand jacket piles increasingly being over-driven and drilled out in glacial depositsand bedrocks: Expensive and risky.

Pile Tip Buckling

(cf. Valhall Norwegian Aker/BP problems

in 2004, Oil & Gas platform expensiverepair and claim). Over driving in verydense and /or cemented glacialmaterials in S. North Sea may lead tobuckling failures if the industry continuesto adopt conservatively long piles

Piling Noise

2011 rules in Germany 160 Dba @ 750

m. restricted working periods &expensive mitigation measures. In UK soft start up piling and observationsrequired. Helical piles considered inScotland. Germany Air BubbleCurtains [ABC] & Hydro Sound Dampers[HSD] London Array, Baltic Sea tests.


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Pile Foundation Issues & Problems (2); Vibro Installation & Scour

Vibro-InstallationTripods levelled using seabed vibro-installation to ~8 15 m using vibrohammers to reduce conventional hammernoise, allowing sequential levelling. Newishtechnique used on several large projects.

Accepted commercially viable offshoreGermany for partial pile installations

through pile sleeves or pre-installed groupsor monopiles.

Scour Prediction & Mitigation

Scour prediction according to DNV; S=1.3-1.6 * D. depends upon WD, soil type andgrading and seabed current.

May be allowed to develop (longer piles) orgravel and rock dump protection required (~500 -700 k Euros per monopile)

Alternatives include frond mats (plasticseaweed), rock mats, pile eddy breakingfins or diversion berms and fences

Accurate and cheap acoustic direct scourmonitoring now possible (e.g. Alpha Ventus).

Available commercially.


Source: Thyssen-Krupp.Source: SLP Engineering

Source: CEFAS Travelling Sand Waves @ Monopiles

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Pile Foundation Issues & Problems (3); Grouted Connections

For OWT monopiles, the transition piece [TP] transmits highbending moments. Brittle rock-like grouted connections wereadopted for most European projects for speed & cost savings.Most excluded reinforcing shear keys due to design codeomission. These have settled, cracked and failed on 70% UKmonopiles. Systemic design fault. Variety of extensive andcostly repairs have been required on many European projects.Oil & gas platform jackets used API designed grouted

connections for decades, but grout connection in jackets holda large mass so are always in compression. OWTs are light &subjected to long term cyclic bending, so complex vertical +bending force coupling & tensile stresses.

Ability to transfer large moment is not fully understood &design theories have limitations & shortfalls. The use ofconical TP sections as a solution [controlled failure] isuncertain in the long term.Industry best practice and code guidelines review on reliabilityof grouted connections. DNV guidelines were revised in 2011(new Code 2014), but still anomalies in predicting behaviour.Research ongoing to understand size and fatigue effects.Many developers reverting to bolted flange connections(Scroby Sands, North Hoyle and Blyth 12 years ago), withsome considering pile swaging or even slip joints as a morereliable long term solution. Requires verticality, careful driving.


Source: Harding et al 2012

Source: Lotsberg 2012

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Pile Foundation Issues & Problems (4); Monopile Resonance, CyclicFriction Degradation & Long Term Tilt in Sands

Monopile Resonance

Selection of dynamic properties essential for costeffective/reliable design. Affects rotor and supportstructure interaction & soil-foundation dynamic response.

Design solutions depend upon ratio between fundamentalstructure eigenfrequency f o, rotor frequency f R and bladepassing frequency f b = N b* f R choice between soft -soft [f o < f R ], soft -stiff [f R < f o < f b] and stiff -stiff [f B < f o].Cyclic Friction Degradation

Substantial reductions in axial pile friction and lateral P-Yresponse may occur due to the cyclic long term loadingexperienced by monopiles supporting large heavy 3-bladed5 MW + HAWT turbines

Long Term OWT Tower Tilt in Sands

Settling of towers/monopiles embedded in sands but notkeyed into bedrock may be large, leading to excessive tiltand shutdown & resetting for gearbox turbines.

Tilt of 0.5 deg is usual for OWT. Permanent tilt due toConstruction tolerance permanent tilt is subtracted, withtypical values 0.20 to 0.25 deg. Allowable operationalrotational stiffness is typically 25 to 30 GNm/radians.


CyclicDisplacement

Accumulation in

Sands. Source: Achmus, Abdel-Rahman & Kuo(2007)

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Foundation Costs Comparisons


Source: UPWIND Project Final report

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Offshore Floating Solutions Huge Potential Offshore Wind Resource


Source: The Offshore Valuation, 2010.

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Future Offshore Wind Tethered Floating Structures 2 Examples


- 4th February 2014

Source: Maine Int. Consulting, 2013.

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Fabrication Costs (early 2010)


Source: Ballast Nedam, 2010.

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Maps: UK Round 3 & German North Sea Sites


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Offshore Wind Cost Trends Need for Reductions

Cost increases since 2005 due tocommodity price rises (mainlysteel) and installation

Monopile costs per kW flat-lining1991 2008

Deeper waters:- heavier and longer over-designed monopiles- more extensive and expensiveequipment and vessel spreads- higher downtime and weatherstandby costs

Insistence on known technologyleading to lack of innovation,conservatism, risk aversion on thepart of developers and lenders.

Lack of experience in developerorganisations; general skillsshortage.


Source: van der Zwaan et al, 2011

Source: The Offshore Valuation, 2010

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Main Conclusions (1)

1. Initially this new offshore industry has understandably used conservative

monopile and piled tripod (Germany) & 4-leg jacket (UK) solutions. CAPEXand investment still limited compared to other energy industries.

2. European Offshore Wind Industry has developed several foundationsolutions, steel /concrete, monopiles, AV piled tripods, BARD tripiles, triple& 4-leg jackets, truss towers, twisted jacket, guyed & A-frame monopiles,monopod suction caisson, triple/quad suction caissons.

3. Main Foundation Risks: Grouted connections, piling noise mitigation, over-conservative long, stiff, heavy pile design, pile tip buckling, unplanneddrilling/re-driving, tilt and settlement.

4. As more difficult rocky, irregular sites are encountered in deeper water,innovative and creative thinking necessary at an earlier stage (c.f. Atlanticand Argyll Array cancellations due to challenging seabed conditions)

5. Grouted connections fiasco -70% UK MPs failed. To be avoided if possible.Use bolted flanges or other direct connections. If unavoidable use shearkeys & robust grout seals. Are non shear keyed conical [1 o-3o] sectionsand/or elastomeric spring bearings valid for fatigue design life?


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Main Conclusions (2)

6. Industry as a whole needs more realistic offshore turbine tilt criteria, based

upon sound engineering analysis. Big impact on structure costs, influencingbusiness cases. Development of tilt-tolerant DD turbines can reduce costs.

7. New foundation solutions [e.g. Carbon Trust] slowly & patchily embraced(Met. Masts) in UK/Germany. Concrete GBS, twisted jackets & suctioncaissons more suited to some sites. Solutions extensive in offshore oil & gas.

8. For foundation costs to reduce [halved acc. US DoE], innovative solutionsneeded, selected/tailored to specific site conditions. Conservative risk averseattitudes in a relatively new industry should change as experience is gained.

9. The current plans to move to ~10 m dia., 1200 Tonne, 60 m + lengthmonopiles in ~40 m WD may be questionable & should be challenged.

10. Globally, early development of floating alternatives increasing, HYWIND

[Statoil], Principle Power [WINDFLOAT], Wave Hub [Glosten], Blue H,Offshore Japan [Various], France [IDEOL, WINFLO, VERTIWIND].

11. Gyro-stabilised floaters, fully submerged concrete/composites, tensiontethered damped synthetic mooring line, FPSO template, vertical axisturbines [VAWT] in WD > 50 m hold out most promise. Hybrid wind/tidal?


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References & Links

References Douglas- Westwood (2013), World Offshore Wind Market Forecast 2013 -2022, 5 th Edition.Golightly, C.R. (2014), Tilting of Monopiles ; Long, Heavy and Stiff; Pushed Beyond Their Limits, GroundEngineering; 2014, vol 47, No. 1, pp 20-23.van der Zwaan, R., Rivera-Tinoco, R., Lensink, S. & van den Oosterkamp , P., (2010) Evolving Economics of OffshoreWind Power: Cost Reductions from Scaling and Learning , Amsterdam 2010, p. 9. The Offshore Evaluation Group (2010), The Offshore Valuation Report; A Valuation of the UKs Offshore RenewableEnergy Resource, Public Interest Research Centre, p. 108. Maine International Consulting (2013), Floating Offshore Wind Foundations; Industry Consortia and Projects in theUnited States, Europe and Japan; An Overview, May 2013, p. 45Roland Berger (2013), Offshore Wind Toward 2020; On The Pathway to Cost Competitiveness, April 2013, p. 25.

LinksEWEA Offshore Statistics 2013 ewea.org/fileadmin/files/library/publications/statistics/EWEA_OffshoreStats_July2013.pdfEC Marine Knowledge 2020 Databaseec.europa.eu/maritimeaffairs/policy/marine_knowledge_2020Global Wind Energy Council Country & Global Reportswww.gwec.net/publications/country-reportsIRENA Costs Database; irena.org/costsUK Govt. Offshore Wind Industrial Strategyhttps://www.gov.uk/government/uploads/system/uploadsUSA Offshore Wind Database: offshorewind.net4C Offshore Wind Database: 4coffshore.comUPWIND EWEA Project Final Report: upwind.eu


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Contact Details

Dr. C.R. Golightly, BSc, MSc, PhD, MICE, FGS . Geotechnical and Engineering Geology ConsultantRue Marc Brison 10G, 1300 Limal, BelgiumTel. +32 10 41 95 25Mobile: +44 755 4612888Email: [email protected]: chrisgolightly;

Linked In: www.linkedin.com/pub/5/4b5/469

You Pay for a Site Investigation -Whether You do One or Not Coleet al, 1991.

Ignore The Geology at Your Peril Prof. John Burland, Imperial College.

d
http://www.linkedin.com/pub/5/4b5/469http://www.linkedin.com/pub/5/4b5/469

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