Dr. Chris Golightly GO-ELS Ltd. Geotechnical & Engineering ... · Offshore Wind Foundations 10th...

Monopile and Tripod/Jacket Foundations for Offshore Wind Foundations 10th April 2014 Braemar Adjusting, London

Dr. C. R. Golightly GO-ELS Ltd. - Monopile and Tripod/Jacket Foundations for Offshore Wind Foundations 10th April 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 Foundations

LCOE Ranges and Averages [IRENA, 2013]

Differences; Oil & Gas Platforms – Wind Turbines

Types of Foundation for Offshore Wind Turbines [OWT]

Codes and Standards; DNV, GL; IEC, US

Foundation Concepts 2012 – 2020 [Roland Berger Study 2013]

Maps UK Round 3, French & German North Sea/Baltic Sites

Environmental, Geophysical & Geotechnical Site Investigations

Monopiles – Design & Installation

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

Tripods – Weserwind Alpha Ventus & OGN-Aquind

Gravity Base Structures [GBS] – Arup/Hochtief, Vici Ventus, Vinci, Seatower, Etc.

Suction Caisson UF Monopod, Tripods, Quadrapods

Guyed Tower, “Twisted” Jacket, Suction Tripod, A-Framed Monopile

TITAN 200 Jack-up Foundation

Foundation Issues & Problems:

1. Early Refusals & Piling Noise

2. Vibro Installation & Scour

3. Grouted Connections

4. Monopile Resonance, Cyclic Friction Degradation & Long Term Tilt in Sands

5. Steel Corrosion

Foundation Costs – Comparisons

Offshore Wind Cost Trends – Need for Reduction

Fabrication Costs (Early 2010)

Offshore Wind; Measurement, Monitoring, Mitigation 1: BELWIND; 2: Bucket Jacket Foundation

Offshore Floating Solutions – Huge Potential Offshore Wind Resource

Conclusions, References, Contact Details


LCOE Ranges and Averages [IRENA, 2013]


Differences; Oil & Gas Platforms – Wind Turbines

Oil & Gas Platforms

Relatively stiff structures, usually founded on long driven piles and mudmats

Axial loads dominate due to high structure weights

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

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

Straightforward Force – Response relationship

Each design is one-off “Prototype” at a single location

Offshore Wind Turbines

Relatively flexible towers on variety of foundation types, monopiles 4 to 9 m diameter, tripods/4 leg jackets, GBS.

Structural dynamics always critical. 3P Eigenvalue resonance

Bending moment and lateral response more important than axial load

Wind and wave loads both very important

Complex uncorrelated/uncoupled loading

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) predictions

Smarter solutions available (suction caissons, GBS, lighter jackets/trusses, hybrids, seabed anchored templates)

“Foundations” (or sub-structure) 30 to 40% of CAPEX & rising. Cost reductions essential

“Smarter” 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 Standards

Det Norske Veritas DNV Offshore Standard DNV-OS-J101, Design for Offshore Wind Turbine Structures, Norway, 2004.

Germanischer Lloyd Rules and Guidelines, IV – Industrial Services, Part 2 – Guideline for the certification of offshore wind turbines, Germanischer Lloyd Windenergie GmbH Hamburg, 2005.

BSH Standard: 2007-06, Design of Offshore Wind Turbines

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.

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

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

Norsok Standard N-003 Marine Actions, 2007.


Foundation Concepts 2012 – 2020 [Roland Berger Study 2013]


Offshore Wind Foundation – Definition. The “Sub-Structure”

Maps: UK Round 3, French & German North Sea/Baltic Sites


Environmental, Geophysical & Geotechnical Site Investigations

Environmental Surveys

Biogenic reefs & Benthic communities

Marine archaeology, wrecks and seabed obstructions

Grab and gravity core sampling of seabed surface sediments, for scour, plumes and cable burial

Seabed mobility, sand waves and shoals

Geophysical and Geotechnical Surveys

Swath bathymetry

Side scan sonar imagery

Seismic reflection profiling for geological shallow stratigraphy and shallow gas presence

Magnetometer for pipelines, cables, metal objects, seabed “junk” & unexploded ordnance [UXO]

Boreholes, vibrocores, cone penetration testing for geotechnical design 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 Caisson 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 monopile with brittle high strength cement ~ very strong granite > problems

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

• Structure frequency limits, 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. Variable diameter, seabed penetrations and wall thickness permitted on same project

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

• Flexible & adaptable to:

- different/varying soil conditions

- water depth up to ~ 45 m

- scour conditions (no protection vs protection/mitigation

• Acts in tension & compression (“push-pull”)

• Flexibility in installation methods & vessels (pre-piling templates Vs through jacket sleeves).

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

• Move to suction caissons for tripods & jackets? (DONG, Statoil, Dudgeon trials)


BIFAB Jacket Beatrice. Source: SSE Renewables

Source: OWEC Tower

Tripods – Alpha Ventus, OGN-Aquind, BARD Tripile

Weserwind - ALPHA VENTUS

• German federal funding 2001 – 2007

• 6 OWEC jackets/6 OWT tripods

• EPCI Contract value EUR 32m

• Vattenfall, EoN & Federal EWE (DOTI)

• 1st seabed template pre-piling (IHC)

• Adopted by Borkum West 2, Globaltech 1

OGN-Aquind

• Newcastle UK based Oil & Gas fabricator

• TRITON 3 leg truss jacket for use in WDs of 30 to 80 m

• Major UK Govt. funding in 2012 for development/design of prototype jacket

• Steel savings allow fabrication of 150 jackets/year at Hadrian’s Yard Wallsend

BARD Tripile

• “One-off German project (so far)

• 400 Te+ 3.35 m Dia. Steel piles & transition triangle. WD 25 to 40 m

• Clever but costly – 100 units/year.

• Installed WINDLIFT1 – 2600 te lift.


Gravity Base Structures [GBS] – Arup/Hochtief, Vici Ventus, Vinci, Seatower, Etc.

Simplicity: Certainty of delivery, increased programme opportunities with fewer constraints

Minimal Seabed Preparation: Installed directly onto seabed whenever possible avoiding need to remove or disturb surface sediments

Self-Floating: No heavy lift or specialist towing or installation vessels required. Reduced supply chain & weather constraints. Improved cost certainty, increased supplier base & lower costs

Flexibility: 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, “Twisted” Jacket, Suction Tripod, A-Framed Monopile


Source: WA Design Ltd.

Source: Bunce and Carey EWEA 2001

TITAN 200 Jack-up Sub-Structure

• Jack-up platform designed from well understood offshore O&G technology, designed to ABS/SNAME standards

• Float out with turbine, self-installing

• Lifting jacks are recovered and reused on other platforms after installation

• Current design 20 – 80 meters WD

• Total structure weight ~1800 tonnes

• Scalable for up to 10MW turbine

• Interchangeable spudcans designed for various soil types . Exposed bedrock, micro-pile anchored from inside the legs. Minimal/zero seabed preparation

• Transition piece integrated into the structure – no grouting; no heavy lifts

• Adjustable air gap beneath structure reduces wave and current loads

• Structure resonance (natural) eigen-frequency can be adjusted


Based on O&G Technology

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 (2/1); Vibro Installation & Scour

Vibro-Installation

Tripods levelled using seabed vibro-installation to ~8 – 15 m using vibro hammers 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 (2/2); Vibro Installation & Scour


Source: Fugro-EMU Ltd. Spudcan Footprints & Scour Pits @ Monopiles

Source: NORTEK UK – Sonar Scour Monitoring

Pile Foundation Issues & Problems (3/1); Grouted Connections

www.academia.edu/6616972/Offshore Wind Monopile Grouted Connections December 2011

Monopile [MP] - Transition piece [TP] joint transmits high bending moments. Brittle rock-like grout [cement] connections mistakenly used on European projects - speed & cost savings.

All but 2 excluded reinforcing shear keys due to DNV design code omission. Settlement, cracking, failure on 70% UK monopiles. Systemic design fault. Extensive/costly repairs.

Heavy oil & gas platforms - API leg-pile connections used for decades, always in compression. OWTs are light, with cyclic, complex vertical + bending force coupling & tensile stresses.

Ability to transfer large moments still not fully understood & design theories have limitations & shortfalls. Use of conical TP sections [“controlled failure”] uncertain in the long term.

Industry best practice and code guidelines under review & DNV guidelines revised 2011 (new Code 2014). Still anomalies in behaviour. Research ongoing on size & fatigue effects.

Many developers reverting to bolted flanges (Scroby Sands, North Hoyle and Blyth 12 years ago), with some considering pile swaging or even slip joints as reliable long term solution. Requires verticality, very careful driving.

Many projects have adopted/are adopting Trelleborg spring bearings (BELWIND, Robin Rigg, Sheringham Shoal). Long term uncertainties for non shear keyed connections?

LONG TERM MEASUREMENT & MONITORING ESSENTIAL


Source: Harding et al 2012

Source: Lotsberg 2012

Source: Billington 2014

Source: Billington 2014

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)

Pile Foundation Issues & Problems (5/1); Steel Corrosion


Pile Foundation Issues & Problems (5/2); Steel Corrosion


Foundation Costs Comparisons


Source: UPWIND Project Final report

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

Fabrication Costs (early 2010) – Why Not Concrete GBS?


Source: Ballast Nedam, 2010.

Offshore Wind; Measurement, Monitoring, Mitigation (1: BELWIND) Poster Paper EWEA 2014 Barcelona – Vrij Universiteit Brussel – De Sitter et al


Offshore Wind; Measurement, Monitoring, Mitigation (2: Bucket Jacket Foundation) Presentation Oceanology International 2014 – Norwegian Geotechnical Institute – Per Sparrevik


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. UK 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? M-M-M


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. Measurement, Monitoring and Mitigation for offshore wind structures is essential for long term design life O & M cost minimisation.

10. 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.

11. Globally, early development of floating alternatives increasing, HYWIND [Statoil], Principle Power [WINDFLOAT], Wave Hub [Glosten], Blue H, Offshore Japan [Various], France [IDEOL, WINFLO, VERTIWIND].


Major Monopile Project Conclusions – Owner/Developer


The demand for WTGs is HIGH – There are few incentives for innovation by established Suppliers. Innovation may have to be initiated and driven by the Owners.

Present WTG and foundation designs are not entirely suited for offshore use and not suitable for offshore installation. A dedicated offshore designed WTG incl. foundation must be developed by this industry.

To save cost and time you have to spend money in the early project phase in order to safeguard so as to to… do it right the first time…

At present, lump sum installation contracts are just not achievable – no incentive for good performance and counter-productive. Availability of suitable installation vessels must be increased.

The Future: Offshore Floaters – Huge Potential Offshore Wind Resource


Source: The Offshore Valuation, 2010.

The Future: Offshore Floaters – Japanese plus European HiPR Wind


Source: Maine Int. Consulting, 2013.

References & Links

References Douglas-Westwood (2013), “World Offshore Wind Market Forecast 2013 -2022”, 5th Edition.

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.

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.

The Offshore Evaluation Group (2010), “The Offshore Valuation Report; A Valuation of the UK’s Offshore Renewable Energy Resource”, Public Interest Research Centre, p. 108.

Maine International Consulting (2013), “Floating Offshore Wind Foundations; Industry Consortia and Projects in the United States, Europe and Japan; An Overview, May 2013, p. 45

Roland Berger (2013), “Offshore Wind Toward 2020; On The Pathway to Cost Competitiveness”, April 2013, p. 25.

Recommended Links

EWEA Offshore Statistics 2013 ewea.org/fileadmin/files/library/publications/statistics/EWEA_OffshoreStats_July2013.pdf

EC Marine Knowledge 2020 Database ec.europa.eu/maritimeaffairs/policy/marine_knowledge_2020

Global Wind Energy Council Country & Global Reports gwec.net/publications/country-reports

IRENA Costs Database; irena.org/costs

UK Govt. Offshore Wind Industrial Strategy gov.uk/government/uploads/system/uploads

USA Offshore Wind Database: offshorewind.net

4C Offshore Wind Database: 4coffshore.com

UPWIND EWEA Project Final Report: upwind.eu

UK Floating Wind: thecrownestate.co.uk/media/428739/uk-floating-offshore-wind-power-report.pdf


Contact Details

Dr. C.R. Golightly, BSc, MSc, PhD, MICE, FGS.

Geotechnical and Engineering Geology Consultant

Rue Marc Brison 10G, 1300 Limal, Belgium

Tel. +32 10 41 95 25

Mobile: +44 755 4612888

Email: [email protected]

skype: chrisgolightly

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

Twitter: @CRGolightly Academia.edu: https://independent.academia.edu/ChristopherGolightly

“You Pay for a Site Investigation - Whether You do One or Not” – Cole et al, 1991.

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


Dr. Chris Golightly GO-ELS Ltd. Geotechnical & Engineering ... · Offshore Wind Foundations 10th...

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