Chris Golightly February 2014; Offshore Wind Foundation Capabilities and Limitations

Foundation Capabilities and LimitationsOffshore Wind Turbine Optimisation Seminar3rd - 4th February 2014 Dexter House, London

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

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

Source: Univ. Mass. 1974

Source: WINDFLOAT Website

Sources from top left clockwise: Arup, BIFAB, COWI, RAVE Alpha Ventus

Source: BELWIND Website

Summary - Offshore Wind Turbine Foundationsy Introduction Global Offshore Wind Energyy Differences; Oil & Gas Platforms Wind Turbinesy Types of Foundation for Offshore Wind Turbines [OWT]y Codes and Standards; DNV, GL IEC, USy Environmental, Geophysical & Geotechnical Site Investigationsy Monopiles Design & Installationy 4 Leg Piled Jackets OWEC, BIFAB, Truss Towers, Twisted Jackety Tripods Weserwind Alpha Ventus & OGN-Aquindy BARD Tripiley Gravity Base Structures [GBS] Gravitas, Vici Ventus, Gifford-Vinci, Seatowery Suction Caisson UF Monopod, Tripods, Quadrapodsy Others: Guyed Tower - A-Framed Monopile - TITAN Jack Upy Foundation Costs - Comparisonsy Foundation Issues & Problems (1); Early Refusals & Piling Noisey Pile Foundation Issues & Problems (2); Vibro Installation & Scoury Pile Foundation Issues & Problems (3); Grouted Connectionsy Pile Foundation Issues & Problems (4); Monopile Resonance, Cyclic Friction Degradation & Long Term Tilt in Sandsy Offshore Floating Solutions Huge Potential Offshore Wind Resourcey Fabrication Costs (Early 2010)y Maps: UK Round 3 & German North Sea Sitesy Offshore Wind Cost Trends Need for Reductiony Seabed Anchored Foundation Templates [SAFT]y Conclusions, References, Contact Details


Introduction Global Offshore Wind Energy

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

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

But: as OW industry goes large scale, developers & lenders are conservative and risk averse. Stated liking for Creative innovation 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 be twin 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 and TLP floaters [40-60 m +WD]

In UK, offshore wind developers registered interest in deploying 46 GW of capacity & 10 GW has been progressed to consent determination, construction and operation.

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


Source: Moustafaeipour, 2009

LCOE Ranges and Averages [IRENA, 2013]


Differences; Oil & Gas Platforms Wind Turbines

Oil & Gas Platformsy Relatively stiff structures, usually

founded on long driven piles and mudmats

y Axial loads dominate due to high structure weights

y Structural dynamics are not critical with weight >>> bending moments

y Wave loads tend to dominate design in high energy areas such as North Sea

y Straightforward Force Response relationship

y Each design is one-off Prototype at a single location

Offshore Wind Turbinesy Relatively flexible towers on variety of

foundation types, monopiles 4 to 9 m diameter, tripods/4 leg jackets, GBS.

y Structural dynamics always critical. 3P Eigenvalue resonance

y Bending moment and lateral response more important than axial load

y Wind and wave loads both very important

y Complex uncorrelated/uncoupled loading

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


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) predictionsSmarter solutions available (suction caissons, GBS, lighter jackets/trusses, hybrids, seabed anchored templates)Foundations 30 to 40% of overall CAPEX & rising. Cost reductions essentialSmarter lighter hybrid foundations needed & move away from riskier costly conventional driven tubular steel piling.


Source: UPWIND Project Final Report 2011

Source: NREL

Codes and Standards; DNV, GL IEC, US

Codes and Standards Hierarchy Offshore German Windfarms

A. Bundesamt fur Seeschifffahrt und Hydrographie [BSH, Federal Regulator]

B1. Germanischer Lloyd [GL]

B2. Det Norsk Veritas [DNV]

B3. IEC

B4. DIN (German National Standards)

C1. API-RP2A (Oil & Gas Offshore Structures)

C2. DIBt

C3. Norsok (Norwegian Offshore)

C4. DASt Richtlinie

D. Other Specific Standards where above do not cover technical design in sufficient detail

Most Relevant Codes and Standardsy Det Norske Veritas DNV Offshore Standard DNV-OS-J101,

Design for Offshore Wind Turbine Structures, Norway, 2004.y Germanischer Lloyd Rules and Guidelines, IV Industrial

Services, Part 2 Guideline for the certification of offshore wind turbines, Germanischer Lloyd Windenergie GmbH Hamburg, 2005.

y BSH Standard: 2007-06, Design of Offshore Wind Turbinesy API RP 2A Recommended Practice for Planning, Designing

and Constructing Fixed Offshore Platforms LRFD Load and Resistance Factor Design, First Edition, July 1993.- WSD Working stress design, 21st edition, December2000.

y EN 1997-1:2009-09: Eurocode 7: Geotechnical Deisgn; Parts 1, 2 and 3.

y RECOFF Recommendations for Design of Offshore wind turbines (RECOFF), European Energy, Environment and Sustainable Development Programme

y Norsok Standard N-003 Marine Actions, 2007.


Foundation Concepts 2012 2020 [Roland Berger Study 2013]


Environmental, Geophysical & Geotechnical Site InvestigationsEnvironmental Surveys

y Biogenic reefs & Benthic communitiesy Marine archaeology, wrecks and seabed obstructionsy Grab and gravity core sampling of Seabed surface sediments, for scour, plumes and cable burialy Seabed mobility, sand waves and shoals

Geophysical and Geotechnical Surveys

y Swath bathymetry, side scan sonar imageryy Seismic reflection profiling for geological shallow stratigraphy and shallow gas presencey Magnetometer for pipelines, cables, metal objects and seabed junk & unexploded ordnance [UXO]y Boreholes, vibrocores and cone penetration testing for geotechnical engineering parameters and soil

layering

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 Offshore Windfarms. BSH Standard No. 7004, p. 40.


Monopiles Design & Installation

Not a Pile but Driven Tubular Steel Thin Walled Shell.

Typically 4.5 - 9 m diameter, sometimes tapered

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

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

Weights up to 900 tonnes, limited by float out & crane capacities

Driven or drive-drill-drive (UK) or even drilled and grouted (France)

Transition piece glued onto monopilewith brittle high strength cement ~ very strong granite > problems

Simple, quick, suited to shallow water: problems - driving refusals & weight.

Structure frequency limitations & fabrication, handling and installation constraints.


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

Usually driven tubular steel piles up to 2.5 m Dia.

Reasonably well understood design and drivability methods with offshore track record / experience

Flexible & adaptable to:- different/varying soil conditions- water depth- scour conditions (no protection vs protection/mitigation

Variable diameter and wall thickness permitted on same project

Acts in tension & compression Different penetrations and number Flexibility in installation methods

vessels (pre-piling Vs through sleeve).

Allows for drilling out and redriving if necessary (but expensive & to be avoided)

Move to SCs for jackets (DONG, Statoil, Dudgeon trials)


BIFAB Jacket Beatrice. Source: SSE Renewables

Source: OWEC Tower

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 to fabricate 150 jackets per year at Hadrians Yard in Wallsend


BARD Tripile


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

Simplicity: Certainty of delivery, increased programme opportunities with fewer constraintsMinimal Seabed Preparation: Installed directly onto seabed whenever possible avoiding need to remove or disturb surface sedimentsSelf-Floating: No heavy lift or specialist towing or installation vessels required. Reduced supply chain & weather constraints. Improved cost certainty, increased supplier base & lower costsFlexibility: Can be relocated, repowered and removed 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 loading required to ensure stability

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

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


Suction Caisson UF Monopod, Tripods, Quadrapods

Suitable for all sand densities and intermediate strength clay

Installation relatively simple & extensive oil & gas experience from GoM, North Sea, W.Africa

Installation/capacity prediction analyses well developed. Scour protection design essential

Highest quality geotechnical data and analyses necessary for stability assessment. Cyclic loading assessment critical

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

SPT in NL developing tripod SC solution funded by Carbon Trust. Dudgeon full field SC jackets planned for 2016.


Source: Oxford University Civil Engineering

Source: SLP Engineering

Source: DONG

Source: DONG

Guyed Tower and A-Framed Monopile


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

TITAN 200 FWSS Jack Up Concept


Source: http://offshorewindpowersystemsoftexas.com/titan_200_deep_offshore_platform

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

Piling Refusals

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

Pile Tip Buckling

(cf. Valhall Norwegian Aker/BP problems in 2004, Oil & Gas platform expensive repair and claim). Over driving in very dense and /or cemented glacial materials in S. North Sea may lead to buckling failures if the industry continues to 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 observations required. Helical piles considered in Scotland. Germany Air Bubble Curtains [ABC] & Hydro Sound Dampers [HSD] London Array, Baltic Sea tests.


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

Vibro-Installation

Tripods levelled using seabed vibro-installation to ~8 15 m using vibrohammers to reduce conventional hammer noise, allowing sequential levelling. Newish technique used on several large projects.

Accepted commercially viable offshore Germany for partial pile installations through pile sleeves or pre-installed groups or monopiles.

Scour Prediction & Mitigation

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

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

Alternatives include frond mats (plastic seaweed), rock mats, pile eddy breaking fins or diversion berms and fences

Accurate and cheap acoustic direct scour monitoring now possible (e.g. Alpha Ventus). Available commercially.


Source: Thyssen-Krupp.Source: SLP Engineering

Source: CEFAS Travelling Sand Waves @ Monopiles

Pile Foundation Issues & Problems (3); Grouted Connections

For OWT monopiles, the transition piece [TP] transmits high bending moments. Brittle rock-like grouted connections were adopted for most European projects for speed & cost savings.Most excluded reinforcing shear keys due to design code omission. These have settled, cracked and failed on 70% UK monopiles. Systemic design fault. Variety of extensive and costly repairs have been required on many European projects.Oil & gas platform jackets used API designed grouted connections for decades, but grout connection in jackets hold a 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 of conical TP sections as a solution [controlled failure] is uncertain in the long term.Industry best practice and code guidelines review on reliability of 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), with some considering pile swaging or even slip joints as a more reliable long term solution. Requires verticality, careful driving.


Source: Harding et al 2012

Source: Lotsberg 2012

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

Monopile Resonance

Selection of dynamic properties essential for cost effective/reliable design. Affects rotor and support structure interaction & soil-foundation dynamic response.

Design solutions depend upon ratio between fundamental structure eigenfrequency fo, rotor frequency fR and blade passing frequency fb = Nb* fR choice between soft-soft [fo< fR], soft-stiff [fR < fo < fb] and stiff-stiff [fB < fo].

Cyclic Friction Degradation

Substantial reductions in axial pile friction and lateral P-Y response may occur due to the cyclic long term loading experienced by monopiles supporting large heavy 3-bladed 5 MW + HAWT turbines

Long Term OWT Tower Tilt in Sands

Settling of towers/monopiles embedded in sands but not keyed into bedrock may be large, leading to excessive tilt and shutdown & resetting for gearbox turbines.

Tilt of 0.5 deg is usual for OWT. Permanent tilt due to Construction tolerance permanent tilt is subtracted, with typical values 0.20 to 0.25 deg. Allowable operational rotational stiffness is typically 25 to 30 GNm/radians.


Cyclic Displacement Accumulation in Sands. Source: Achmus, Abdel-Rahman & Kuo (2007)

Foundation Costs Comparisons


Source: UPWIND Project Final report

Offshore Floating Solutions Huge Potential Offshore Wind resource


Source: The Offshore Valuation, 2010.

Future Offshore Wind Tethered Floating Structures 2 Examples


Source: Maine Int. Consulting, 2013.

Fabrication Costs (early 2010)


Source: Ballast Nedam, 2010.

Maps: UK Round 3 & German North Sea Sites


Offshore Wind Cost Trends Need for Reductions

Cost increases since 2005 due to commodity price rises (mainly steel) and installation

Monopile costs per kW flat-lining 1991 2008

Deeper waters:- heavier and longer over-designed monopiles- more extensive and expensive equipment and vessel spreads- higher downtime and weather standby costs

Insistence on known technology leading to lack of innovation, conservatism, risk aversion on the part of developers and lenders.

Lack of experience in developer organisations; general skills shortage.


Source: van der Zwaan et al, 2011

Source: The Offshore Valuation, 2010

Main Conclusions (1)

1. Initially this new offshore industry has understandably used conservative monopile and piled tripod (Germany) & 4-leg jacket (UK) solutions. CAPEX and investment still limited compared to other energy industries.

2. European Offshore Wind Industry has developed several foundation solutions, 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, unplanned drilling/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. Atlantic and 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 shear keys & robust grout seals. Are non shear keyed conical [1o-3o] sections and/or elastomeric spring bearings valid for fatigue design life?


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, influencing business 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 & suction caissons more suited to some sites. Solutions extensive in offshore oil & gas.

8. For foundation costs to reduce [halved acc. US DoE], innovative solutions needed, selected/tailored to specific site conditions. Conservative risk averse attitudes 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 + length monopiles 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, tension tethered damped synthetic mooring line, FPSO template, vertical axis turbines [VAWT] in WD > 50 m hold out most promise. Hybrid wind/tidal?


References & Links

Referencesy Douglas-Westwood (2013), World Offshore Wind Market Forecast 2013 -2022, 5th Edition.y Golightly, C.R. (2014), Tilting of Monopiles; Long, Heavy and Stiff; Pushed Beyond Their Limits, Ground

Engineering; 2014, vol 47, No. 1, pp 20-23.y van der Zwaan, R., Rivera-Tinoco, R., Lensink, S. & van den Oosterkamp, P., (2010) Evolving Economics of Offshore

Wind Power: Cost Reductions from Scaling and Learning , Amsterdam 2010, p. 9.y The Offshore Evaluation Group (2010), The Offshore Valuation Report; A Valuation of the UKs Offshore Renewable

Energy Resource, Public Interest Research Centre, p. 108.y Maine International Consulting (2013), Floating Offshore Wind Foundations; Industry Consortia and Projects in the

United States, Europe and Japan; An Overview, May 2013, p. 45y Roland Berger (2013), Offshore Wind Toward 2020; On The Pathway to Cost Competitiveness, April 2013, p. 25.

Linksy EWEA Offshore Statistics 2013 ewea.org/fileadmin/files/library/publications/statistics/EWEA_OffshoreStats_July2013.pdfy EC Marine Knowledge 2020 Database

ec.europa.eu/maritimeaffairs/policy/marine_knowledge_2020y Global Wind Energy Council Country & Global Reports

www.gwec.net/publications/country-reportsy IRENA Costs Database; irena.org/costsy UK Govt. Offshore Wind Industrial Strategy

https://www.gov.uk/government/uploads/system/uploadsy USA Offshore Wind Database: offshorewind.nety 4C Offshore Wind Database: 4coffshore.comy UPWIND EWEA Project Final Report: upwind.eu


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

YouPayforaSiteInvestigationWhetherYoudoOneorNot Coleetal,1991.

IgnoreTheGeologyatYourPeril Prof.JohnBurland,ImperialCollege.


Chris Golightly February 2014; Offshore Wind Foundation Capabilities and Limitations

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