Geotechnical Design of Bottom- Fixed OWT Foundations

Geotechnical Design of Bottom-Fixed OWT FoundationsGeotechnical Engineering for Offshore Wind InfrastructureWorkshop organized by HDEC and NGIShanghai, China, 31 May, 2018

Dr.-Ing. EMMgt Hendrik SturmTechnical Lead Offshore Renewables, NGI

AgendaDefinition and overview of bottom-fixed OWT foundationsDesign aspects and general design approachRequired input to the design Design methodology for Monopiles, Suction Caissons and GBSsOther aspects─ Assessment of cyclic soil behaviour using UDCAM, PDCAM and other soil

models─ Monitoring and instrumentation─ Installation

Overview Bottom Fixed OWT FoundationsDefinitions

Source: IEC 61400-3

Overview Bottom Fixed OWT FoundationsType of Foundations

Design aspects and general design approach

Design aspects and general design approachFour main design aspects

ULS design, is the foundation capacity analysis typically done using extreme load events combined with cyclic load histories

SLS design, is the assessment of peak and accumulated deformations and rotations due to extreme load events and cyclic load histories

FLS design, is the assessment of the foundation stiffness and damping used in structural analysis

Installation design, is the assessment of the penetration resistance and soil-structure-interaction for structural analysis

Design aspects and general design approachDesign approach

The design is govern by the structural OWT design:─ OWTs are dynamically sensitive structures─ Fatigue is often a design-driver─ OWTs are designed performing integrated analysis

The soil-foundation response is an input to the integrated analysis

Two main approaches for foundation sizing:─ Load-stiffness iteration (e.g. caissons)─ Soil represented by Winkler springs (e.g. GBS, MPs)

Soil-pile interaction

Unsteady aerodynamic

effects

Turbulent wind

Irregular waves

Current


0 0.2 0.4 0.6 0.8 1f (Hz)

0

4

8

12

Rat

ed p

ower

(MW

)

Blade load frequencies (1P and 3P)Wind spectrum (Kaimal)Wave spectrum (JONSWAP, Hs = 2.4 m)

Turbines: DTU 10 MWVestas V164 (8MW)Siemens SWT-6.0-154 (6MW)Siemens SWT-3.6-107 (3.6 MW)Vestas V90 and V91 (3MW)

Soft-soft Soft-stiff Stiff-stiff

Source: Arany, L., Bhattacharya, S., Macdonald, J. H. G. and Hogan, S. J. (2016)


Most OWT are designed to have the first eigen-frequency in the soft-stiff range

Both, the structure and the foundation are often adjusted to meet the stiffness requirement

Outlook: Need to apply improved design models / methodologies in the design (see also Youhu Zhang’s presentation on R&D) Source: Kallehave, D., Byrne, B. W.,

LeBlanc Thilsted, C. andMikkelsen, K. K. (2015)

0

0.04

0.08

0.12

Den

sity

0.9 0.95 1 1.05 1.1 1.15 1.2 1.25fn,measured / fn,design

Larger than 1

Required input to the design

Required input to the designSoil data

From field investigations:─ CPT soundings (used for classification and assessing soil layering)─ Boreholes (used for retrieving soil samples and assessing soil layering)

From laboratory testing:─ Index data (used for classification and identifying soil units)─ Soil strength e.g. su, ϕ (used for capacity and installation)─ Stress-strain behaviour, e.g. p’-q, τ-γ (used for stiffness and serviceability)─ Description of cyclic soil behaviour (used for all design aspects)─ Other, such as interface tests, resonant column tests, etc.

References: Andersen et al. (2013) and Sturm (2017)

Required input to the designLoad data

Minimum required loads (for a serious design)─ Weights, water depths, scour protection, etc. (used for all design aspects)─ Extreme load cases, storm histories, etc. (used for capacity, stiffness and serviceability

design)─ Cyclic load histories, both severe and operational (used for capacity and serviceability

design)─ Representative operational and idling load cases (used for stiffness design)

Challenge: most standards and guidelines do not include recommendations for geotechnical loads, but focus on structural analysis and required Design Load Cases (DLC)

Qualified communication between geotechnical and load / structural engineer is important to identify the relevant load cases

Required input to the designStructural data and design guidelines

Structural data:─ Material and properties (important for stiffness and installation)─ Any relevant constraints, such as max. weight, dimensions, time, etc. (important

for sizing)

Design guidelines (examples):─ Certifiers:

─ DNVGL: DNVGL-ST-0126, DNV-CN 30.6, DNVGL-RP-C212, ...─ ABS: Pub. 176, Pub. 231, …

─ National standards:─ IEC: e.g. IEC 61400-3, …─ BSH: BSH No. 7005, Eurocode, DIN 1054, …─ API: API RP 2A-WSD, API RP 2GEO, …

Design methodology: Monopiles

Design methodology: MonopileOverview

Most often used OWT foundation

Design may be expected to be most mature, which is, however, not the case as OWTs are structural design driven

Soil behaviour is represented in a simplified manner

Sizing is typically done by structural designer (with some exceptions)

Soil-pile interaction

Unsteady aerodynamic

effects

Turbulent wind

Irregular waves

Current

Design methodology: MonopileGeneral design approach

Soil reactions are replaced with (non-linear Winkler) springs

Originally API p-y curves were used, later modification to these were suggested

Recently developments have results in new springs:

─ PISA Winkler springs─ RedWin macro-model


PISA curvesDeveloped in a Carbon-Trust research projectSets of distributed lateral and rotational Winkler-springsIntended for monotonic loadingWell suited for homogenous soil layering, but also expected to be applicable in layered soilsNot yet published, hence only accessible to few developers and consultancies (including NGI)

RedWin macro-modelDeveloped in Joint research project lead by NGIReplaces the soil-pile system with a macro-element at mudlineIntended for monotonic and cyclic loadingWell suited for all type of soils and soil layeringModel is only partially published, but will be made available in near future


Both model were validated against field tests

─ The PISA model describes the monotonic response well and can be used for capacity analysis

─ The RedWin model describes the monotonic and cyclic response including damping well, but is less suited for capacity

Calibration of the models:─ Rule-based or,─ Numerical analysis


Monopile response tocyclic loading plottedat mudline using themacro-element anddistributed monotonicWinkler springs withouthysteretic “feature”


Foundation sizing is done by either:─ The structural engineer assuming an initial geometry─ A geotechnical designer assuming some and run “quasi-static” push-over

analyses

Often – depending on the limiting criteria and soil profile – the─ Pile length and diameter depends on the foundation capacity and

serviceability, respectively,─ Wall thickness on the structural fatigue analysis

The process of foundation sizing is done iteratively

Design methodology: MonopileOutlook

The design process needs to be optimised by streamlining the workflow: Going from one turbine to a complete wind farm

All models assume implicitly the design case relevant soil state, an iteration procedure on the soil state is not (yet) implemented

Less attention was given so far on the assessment of the cyclic soil behaviour for the considered design load case: current approach is rather “one-soil-state-fits-all”

Serviceability is often (always ?) considered in a too simplistic way

Design methodology: Suction Caissons

Design methodology: Suction CaissonsOverview

Relatively new concept for OW

Long experience from the O&G industry for bottom fixed and subsea structures, including anchors (see also Hongjie Zhou’s presentation on FOWT foundations)

Main reference on the design of caissons for OWT foundations is Sturm (2017), which comprises many further references Source: Ørsted (formerly DONG Energy)

Design methodology: Suction CaissonsGeneral design approach

Design methodology: Suction CaissonsInterface between disciplines

Traditionally, point 1 is used for the geotechnical-structural interface

Point 2 is used in more recent projects enabling an optimization of the lid design

Point 3 has the advantage of Point 2 but in addition uses a less complex stiffness matrix

Design methodology: Suction CaissonsAssessment of cyclic soil properties

Two approaches:─ Empirical─ Analytical / Numerical

NGI method:─ Semi-empirical approach─ Using cyclic contour diagrams─ Implemented in FE (PDCAM / UDCAM)─ Probably the only practically used

method for cyclic loading

Design methodology: Suction CaissonsFoundation capacity

Distinguish between:─ Short term (i.e. essentially undrained) and─ Long term (i.e. essentially drained) conditions

Can be design driving:─ In sand (long term tension loading)─ Soft clay (short term compression)

Difficult to find / identify the governing load case(s) and corresponding soil strength (average and/or cyclic):─ Not defined in standards─ Not obvious which H-V-M load combination is governing as it depends also on

the corresponding cyclic load history

Design methodology: Suction CaissonsInstallation

Probably one of the most challenging aspects in design due to shallow waters and layered soils

Often design driving in stiff clay and layered soil

Three main situations:─ Undrained penetration─ Drained penetration─ Partially drained penetration


Existing calculation methods have limitations

Number of conditions which are not covered (see illustration)

Requires experience, engineering judgment, and planning for mitigation methods


In order to cope with the uncertainties, mitigation methods need to be considered

Two types of mitigation strategies:─ Pre-emptive, e.g.:

─ Water injection─ Stepped skirt

─ Re-active, e.g.:─ Cycling suction pressure─ Ballasting

(figure removed due to confidentiality)

Design methodology: Suction CaissonsStiffness and soil reactions

Foundation stiffness and soil reactions are output of the geotechnical design

Sizing shall not be done based on stiffness (or soil reactions)

Load-stiffness-iteration represents the outer loop in the design approach


Stiffness values need to be assessed for the relevant load cases:

─ ULS (used for structural utilization analysis)

─ FLS (used for the structural fatigue analysis)

─ Small-strain stiffness, which is often equal to the FLS stiffness values (used for the structural eigenmode analysis)

Caisson lid stiffness is very importance, particular for FLS

Soil reactions are assessed for installation and in-place performance

Most critical is the installation due to buckling

Soil reactions can be included in the structural design using Winkler-springs or continuum model(s)

Design methodology: Suction CaissonsSoil reactions and soil reactions

Design methodology: Suction CaissonsOther aspects

Grouting─ Used to fill the void between lid an soil after installation─ Main reason of using grout for suction caissons of bottom fixed OWTs is to reduce or avoid

potential (differential) settlements, and pumping-effects

Wind Farm design─ Cluster is typically based on the water depth (i.e. loading conditions) ─ Sizing can be based on capacity and installation analysis─ Load-Stiffness iteration is done using the softest and stiffest location within the cluster

Observational method─ Suction caissons for OWT is a new concept─ Thus the observational method may be considered in current and new projects

Design methodology: Suction CaissonsOutlook

The methods for geotechnical design are known from O&G

Particular attention need the─ Identification of the correct load cases─ Establishing correct foundation stiffness

values

The use of macro-elements can ease the design and reduce the number of iterations (e.g. RedWin)

Design methodology: GBSs

Design methodology: GBSOverview

The design is very similar to that of Suction Caissons, but somewhat easier

The overall design approach and foundation behaviour is understood and known from the Oil & Gas industry

Additional / particular design aspects for GBSs:─ Shape, type and arrangement of skirts─ Design and arrangement of Ribs and Dowels─ Underbase grouting─ Scour protection and / or seabed preparation─ Long-term settlements and tilt (applies in fact to all OWT foundations)

Other aspects of OWT foundation design:Assessment of cyclic soil behaviour

Other aspectsAssessment of cyclic soil behaviourBackground:

Change of soil state depending on load case and timeAccumulation and dissipation of pore pressureAccumulated strains, loosening and re-compactionNGI method: Cyclic contour diagrams, relating─ Average and Cyclic shear stress─ Average and Cyclic shear strain─ Pore pressure─ Damping (!)─ Number of cycles

Other aspectsAssessment of cyclic soil behaviour


NGI approach for assessing cyclic properties─ Developed during the last 4-5 decades─ For clay and sands (undrained and partially drained)─ Verified method used in many design

Cyclic contour diagrams describe the cyclic soil behaviour in a single point of the soil body

Application of these requires :─ To assume a representative point in the soil body and assumptions of the corresponding

stresses derived from the cyclic loads─ To assume a failure mechanism─ Perform FE analysis, where the cyclic contour diagrams are used as lookup-tables in each

integration point

Other aspectsAssessment of cyclic soil behaviourUnDrained Cyclic Accumulation Model (UDCAM)

Undrained behaviour under both average and cyclic loads (clay)Non-linear cyclic and average stress-strain relationshipsCyclic degradation of stiffness and strength (Neq)Accumulated shear deformation Anisotropic behaviour / stress path dependent (ADP)Based on input of laboratory results (interpolation and extrapolation between test results, contour diagrams), instead of based on a mathematical framework Verified by model and field testsImplemented as a UDSM (DLL) in PLAXIS 3D

Other aspectsAssessment of cyclic soil behaviourPartly Drained Cyclic Accumulation Model (PDCAM)

Conceptually similar to UDCAM but allowing for pore pressure dissipationPerfectly undrained during, at least, one single cycleEffect of varying cyclic shear stress level based on a pore pressure accumulation procedure Non-linear bulk stiffness for loading, unloading and reloadingCoaxiality between 3D principal strains and stressesVerified by model and field testsImplemented as a UDSM (DLL) in PLAXIS 3D


Pore pressure field at the end of 50-year design storm event

Other aspects of OWT foundation design:Monitoring and instrumentation

Other aspectsMonitoring and Instrumentation

Instrumentation is essential for project success and often mandatory

Measurements show the performance and proof the validity of the design method

Sometimes the actual in-situ conditions is different than assumed in the design, which may require to re-assess the design

Other aspects of OWT foundation design:Installation

Other aspectsInstallation – Suction Caissons

1995 Nkossa

1997 North Sea 1997-99 Brazil

1999 GOM Diana

2001-2003 APL2005 Trent 1994 &1996 Draupner & Sleipner T


-60 -40 -20 0 20 40 60Elevation difference

between each bucket (mm)

2

2

3

3

4

4

5

.0

.5

.0

.5

.0

.5

.0

Suction pressure (kPa)

Pene

trat

ion

dept

h (m

)

0 20 40 60 80 100

Suction penetration phaseSleipner T jacket

Ø15m “Buckets”


Testing the effect of cycling and water injectionin order to reduce the penetration resistance

Other aspectsInstallation – Piles JacketsNGI’s pre-piling metrology

Alpha Ventus - 6 off OWEC Quattropods installed by Norwind/GeoSeaOrmonde - 42 off OWEC Quattropods installed by GeoSeaThornton - 49 off OWEC Quattropods installed by GeoSeaBorkum West - 41 off OWT Tripods installed by GeoSeaBaltic II - 41 off Tripod jackets JV Hochtief/GeoSeaWikinger pile test – Bilfinger

NGI have been responsible for pile driving monitoring and “As installed” metrology. Today all monitoring systems are remote operated without any ROV/Diver subsea intervention required.

Other aspectsInstallation – Piles Jackets

MetrologyInstruments Sonars Cameras


Driving the piles to the same depth

0 5 10 15 20 251600

1700

1800

1900

2000

2100

2200

2300

2400

2500

2600

Pile

stic

k up

(mm

)

Elapsed time (min)

Target driving elevation

80 Blows

50 Blows

10 Blows4 Blows

Driving North Pile

100 Blows

15 Blows

5 Blows

50 Blows

30 Blows

9 Blows

2 Blows

80 Blows

30 Blows8 Blows

2 Blows

Driving South PileDriving East PileDriving West Pile

Summary

SummaryFoundation for bottom-fixed offshore wind turbines

Design of OWT foundations is a very exciting subject

Many aspects known from the O&G industry can be applied

However, there are a some “new” challenges:─ OWTs are dynamical sensitive─ High focus on stiffness and damping (and serviceability?!)─ Streamlined design: from prototype to mass-production

Monitoring is essential to verify and improve the design methodologies further

References on how to apply the methods presented can be provided on request. A first starting point is Sturm (2017), which comprises many additional references (not only for suction caisson design)

Sturm H. (2017) Design Aspects of Suction Caissons for Offshore Wind Turbine Foundations.Proceedings of TC 209 Workshop (Foundation design of offshore wind structures)

19th International Conference on Soil Mechanics and Geotechnical Engineering, 45-63

Thanks for your attention!

NORWEGIAN GEOTECHNICAL INSTITUTENGI.NO

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Geotechnical Design of Bottom- Fixed OWT Foundations

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