Monopile and Tripod/Jacket Foundations for Offshore Wind Foundations April 2014

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
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
Guyed Tower, "Twisted" Jacket, Suction Tripod, A-Framed Monopile

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

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.


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

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.

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.

Main Conclusions (2)
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.
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.
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.
Measurement, Monitoring and Mitigation for offshore wind structures is essential for long term design life O & M cost minimisation.
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.
Globally, early development of floating alternatives increasing, HYWIND [Statoil], Principle Power [WINDFLOAT], Wave Hub [Glosten], Blue H, Offshore Japan [Various], France [IDEOL, WINFLO, VERTIWIND].


Main Conclusions (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.
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.
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.
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")
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


Foundation Costs Comparisons

Source: UPWIND Project Final report
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



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

Foundation Concepts 2012 – 2020 [Roland Berger Study 2013]

Offshore Wind Foundation – Definition. The "Sub-Structure"
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.


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
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])

LCOE Ranges and Averages [IRENA, 2013]

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:
Early Refusals & Piling Noise
Vibro Installation & Scour
Grouted Connections
Monopile Resonance, Cyclic Friction Degradation & Long Term Tilt in Sands
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


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








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