1. SMARTER FOUNDATION AND FLOATING CONCEPTS ON NORTH
AMERICAN COASTAL HARD SEABEDS: EUROPEAN LESSONS
Dr.C.R. Golightly, BSc, MSc, PhD, MICE, FGS.
Geotechnical and Engineering Geology Consultant, Belgium
Abstract
The majority of European offshore wind [OW] fixed structure foundations to date have been steel monopiles ( WD
< 35 m) or piled steel tripods/jackets (WD 30 to 55 m). Average CAPEX costs rose steadily between 2005 and
2010 and there is recognition that these must probably halve for the industry to be subsidy independent. There is
considerable research now on how to do this.
Several UK, German Baltic Sea and French sites consist of "shallow bedrock", with soil cover of < 5 to 10 m.
Monopiles or “pin” piles are driven/drilled then redriven or drilled and grouted. This is expensive, requiring
specialist equipment spreads and weather dependent vessels. Large UK projects have been cancelled primarily due
to bedrock, with questions over the viability of Baltic and French projects. In the US, high costs have cast doubt
upon the feasibility of some of the seven offshore wind DoE demonstrator sites originally funded.
Turbine sizes of 5 to 8 MW suggest piled foundations will only be cost effective for WD < 50 m if seabed shallow
geology allows non-drilled piling. Adoption of non-piled foundations has been slow due to risk aversion and
conservatism in a growing but incentive driven industry. Beyond 50 m WD, bedrock anchored floating units are
likely, possibly supporting vertical axis turbines [VAWT].
For OW costs to be halved, substructure and foundation costs must be drastically reduced if the industry is to grow
without subsidies. Current fixed structure designs will struggle to achieve this and major "gamechangers“ designs
are needed. Simplified construction/installation operations with reduced use of expensive installation vessels are
necessary. Structures and foundations should be fabricated onshore and floated out to minimise weather downtime.
For fixed and floating structures, installation of either single or multiple suction caissons is possible if soil
overburden cover is deeper than the caisson diameter. Concrete gravity bases (GBS) may be used if seabed
conditions are hard and/or there are low settlement requirements
For future floating wind sites for WD > 50 m, technology is proposed consisting of groups of skirted seabed
templates [SAFT] including suction caissons and pressure grouted tendon rock anchors, a well known technique in
onshore civil engineering. This subsea anchoring technology does not exist in tried and tested form yet.
It is essential the nascent North American offshore wind and marine hydrokinetics (wave and tidal) industries
develops and encourages capabilities in the design, construction and installation of foundations for hard soil/rock
sites which do not involve piling. This will permit cost reductions, financial independence from government
subsidies, and eventual cost-effectiveness against baseline fossil fuels.
Alternative to Piles [1] Mono and Multiple Suction Caissons [SC]
• SCs are suitable for all sand densities and intermediate strength clays.
• Installation is relatively simple & there is extensive oil & gas experience from GoM, North Sea, W.Africa.
• Installation/capacity prediction design analyses are well developed. Scour protection usually necessary.
• Highest quality geotechnical data and analyses necessary for stability and critical cyclic loading assessments.
• Monopod SCs were installed successfully for the Horns Rev (DK) and Dogger Bank (UK) Met Masts in 2009 &
2012 (Universal Foundation Monopod). Six SCs planned for LEEDCo Lake Eerie turbines in 2016.
• Suction Pile Technologies in the Netherlands are developing a tripod SC solution funded by the UK Carbon
Trust. Dudgeon full field SC jackets are planned for 2016.
Dr.C.R. Golightly, BSc, MSc, PhD, MICE, FGS .Geotechnical and Engineering Geology Consultant, Rue Marc Brison 10G, 1300 Limal, Belgium, Tel: +32 10 419525 Mobile: +32 478 086394 e-mail: chris.golightly@hotmail.com
skype:chrisgolightly Linked In: www.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. EU Technology Wind Platform WG4 Membership & Submission: http://www.windplatform.eu/structure/working-group-4-offshore/
European Offshore Wind – Rising Costs
To achieve emissions targets, over 8,000 OWT may be installed on North, Irish and Baltic Sea continental shelves, in
water depths [WD] of up to 70 m. Fixed support structures include monopiles, tripods (Germany), 4 leg jackets
(mainly UK) and increasingly, concrete gravity bases [GBS] and single and multiple suction caissons. Numerous
moored floating concepts are also being developed, particularly in France and Japan.
Design loads for OWT are different to those for heavier oil and gas industry platforms, with high moments and lateral
loads and low deadweights. Shallow geological conditions consist predominantly of dense sands or stiff glacial
boulder clays and glacial tills and moraine deposits over shallow bedrocks of varying type and strength, such that
piled OWT for the prevailing load combinations have ultimately been found to be increasingly unjustifiable.
Between 2005 and 2010 construction & installation costs increased from 2.5 to 5 US$[2011] /MW, mainly due to:
• Bottlenecks in supply chain and limited numbers of WTG suppliers
• Rising commodity prices and FX rate volatility
• Complex geology and morphology of sites distance to shore, greater water depths
A number of innovative foundations have been developed to address the issue of complex geological conditions in
WD < 100 m, where shallow bedrocks of different types and strengths are frequently encountered on northern
hemisphere coasts and soils are often highly variable, including dense/hard glacial clays and gravelly moraine and
washout deposits. These innovations include single/multiple suction caissons, GBS, lightweight space frames,
concrete tripods and guyed towers. There has been considerable reluctance to adopt due to inherent conservatism and
risk aversion. The increased risks which have now resulted due to continued reliance upon piling include:
• Risk of driving refusal and/or buckling of steel piles driven into hard glacial soils, chalk or layered bedrocks.
• Excessive underwater piling noise affecting marine life adversely, requiring a suite of mitigation methods.
• Larger costly vessel and installation equipment spreads, operating in difficult marine and weather conditions.
• Over 65% failed and cracked monopile grouted connections, requiring extensive remedial repairs.
• Piles installed out-of vertical leading to shakedown settlement and tilt due to long term cyclic lateral loading.
• Long term changes in natural structure resonant frequencies leading to vibration and fatigue.
Alternative to Piles [2] Concrete and Hybrid Gravity Base Structures
Simplicity: Certainty of delivery, increased programme opportunities with fewer constraints.
Minimal Seabed Preparation: Installed directly onto seabed 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 to ensure stability.
• Generally impractical for OWT in relatively shallow (< 15 m) water.
• Bad publicity: German Strabag BSH rejection & over-designed Thornton Bank GBS.
Conclusions
1. The relatively new northern European offshore wind industry has adopted mainly conservative monopile, piled
tripod (Germany) & 4-leg jacket (UK) solutions to date. CAPEX investment levels remain limited compared to
other energy industries.
2. Several less conventional foundation solutions have been developed, including steel /concrete, monopiles, piled
tripods, tripiles, triple & 4-leg jackets, truss towers, twisted jackets, guyed & A-frame monopiles, a monopod
suction caisson, triple/quad suction caissons.
3. The main foundation risks and excess costs have been related to: Grouted connections, piling noise mitigation,
over-conservative long stiff, heavy pile design, pile refusals and potential steel tip buckling, unplanned drilling
out /re-driving in bedrocks, tilt /settlement and excessive steel corrosion.
4. A number of UK projects have been cancelled due to “challenging” bedrock conditions (notably the Atlantic
and Argyll Array projects). There are several other projects in the UK, France and the Baltic Sea with similar
issues related to excessive piling costs and drilling and grouting. Difficult rocky, irregular sites in deeper water,
innovative require creative thinking at an earlier stage.
5. Approximately 70% of UK monopile grouted connections have failed, so the use of bolted flanges or other
direct connections has increased. The use of shear keys & robust grout seals is essential for valid fatigue design
life Non shear keyed conical [1o-3o] sections and/or elastomeric spring bearings have also been adopted.
6. The OW industry must be realistic about turbine tilt design criteria, which appears to be a hangover from
onshore wind (Ref. 1). Over stringent values (0.5 degrees permanent tilt) have had a big impact upon sub-
structure and pile foundation costs. Development of more tilt-tolerant direct drive turbines with less stringent
lateral movement criteria at nacelle level is essential.
7. New foundation solutions have only been slowly adopted (Met. Masts) in UK/Germany. Concrete GBS,
twisted jackets & suction caissons can be more suitable and cheaper for some sites. Design principles and
installation techniques are available from offshore oil & gas experiences, despite the loading regime differences
between Oil & Gas platforms and offshore wind turbine structures.
8. For foundation costs to reduce [at least halved in LCOE terms], innovative solutions are needed,
selected/tailored to specific site conditions. Conservative risk averse attitudes in a relatively new industry
should change as experience is gained. Current industry plans to move to ~10 m dia., 1200 Tonne, 60 m +
length monopiles in ~40 m WD may be questionable & should be challenged.
9. Development of floating wind alternatives is increasing, such as such as HYWIND [Statoil], Principle Power
[WINDFLOAT], Wave Hub [Glosten], France [IDEOL, WINFLO, VERTIWIND]. And particularly in Japan
[Various], Gyro-stabilised fully submerged units, using tension tethered damped synthetic mooring lines,
template anchoring supporting vertical axis turbines [VAWT] in WD > 50 m hold out most promise. Hybrid
wind/tidal and wind /wave floating solutions should develop further., as well as the following:
Wind Fixed Structures [WFS] Wind Floating Units [WFU]
Tidal Fixed Structures [TFS] Tidal Floating Units [TFU]
Wave Energy Convertors [WEC] Ocean Current Energy [OCE]
Ocean Thermal Energy [OTE] Offshore Subsea Energy Storage [OSES]
Moored Floating Structures [MFS] Floating Production Storage [FPSO]
10. An ROV drilling system capable of installing tension anchors in bedrock via preinstalled seabed templates is
presented. This type of solution is likely to be required in the future for installation of fixe structures and
floating units, particularly off the eastern and north-western coasts of the USA.
Seabed Anchored Foundation Template [SAFT]
A buoyant float-out hybrid structure concept has been developed , which may be designed as a foundation base or
mooring point template. The Seabed Anchored Foundation Template [SAFT] is a GRP /reinforced concrete base
configured to support tripods, jackets or GBS or as a pre-installed templates for inclined or vertical (TLP) taut or
slack catenary mooring lines. Steel or concrete edge skirts and suction caissons [SC] , anchor piles or helical screw
are options for different soil cover. Type and thickness
Tension resistance is provided via a pressure grouted rock anchor installed below upper support casing for deeper
pressure grouted single or multi strand rock anchors installed from an ROV operated marinised drilling unit.
External GRP, concrete or steel mudmats and/or integral plastic anti-scour frond mats/mattresses may also be
included. This configuration of skirted anchored base/template has considerable in-built lateral seabed resistance
and tension uplift capacity. Design is preceded by high quality shallow geophysical investigation of the seabed and
upper layering plus confirmatory soil and rock coring by the same ROV drilling unit used to install the anchors,
which are then fully proof-loaded. To twice the working load.
References
1. 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.
2. Madsen, J., Ponte, A. and Cribb, C. (2015), “Role of Geological Models and Geotechnical Characteristics in
Reducing Costs and Uncertainties and Assessing Risks in the Development of Offshore Wind Projects”, Paper
No. PO200, EWEA 2015, Copenhagen.
3. National Renewable Energy Laboratory: Energy Analysis of Offshore Systems
[www.nrel.gov/wind/offshore_analysis.html]