oe

ic

Teblverall wind turbine is between 1P and 2P/3P), the ratiotanpsef /fveni

under the working loads.(b) The values of the spring (stiffness of the founda-

the whole structure as this is linear eigenvalue analysis.

l

Reference Article1st published in Month 2014Offshore wind turbines (OWTs), because of theirshape and form (i.e. a long slender column with aTypes and Nature of the Loads Acting onthe Foundations

Further details on the analysis required can be found in[2, 3].(c) The values of the springs will also dictate the overaldynamic stability of the system because of its non-linear nature. It must be mentioned that thesesystem either in use or proposed and will constitutethe main part of this article.

tion) is necessary to compute the natural period ofgrounded system (xed to the seabed) foundationFoundation selection plays an important role in theoverall concept design for offshore wind farms asthere are large nancial implications attached to thechoices made. Typically, foundation costs 25 to 34%of the overall costs. For the North Hoyle project thecost of foundation was 34%, [1] and it has beenreported that the development of the Atlantic Arraywind farm did not go ahead and one of the mainreasons is the expensive foundation. Foundations forwind turbines can be classied into two main types:xed (or grounded to the seabed) and oating.Although most of the currently installed or operatingturbines are supported on xed/grounded foundationsystem, research and development of oating founda-tions are underway. Fig. 1 shows the different types of

simple mechanical model of the whole systemshowing the different components and the designvariables. In the model, the foundation is replacedby four springs: KL (lateral spring), KR (rockingspring), KV (vertical spring) and KLR (cross-couplingspring). It is therefore clear that the stability anddeformation of the system is very much dependenton these four springs. A few things may be notedregarding these springs:

(a) The properties and shape of the springs (load-deformation characteristics) should be such that thewhole structure should not collapse under the actionof extreme loads and the deformation is acceptableProfessor Subhamoy Bhattacharya Chair in Geomechan

AbstractDesigning foundations for offshore wind turbines (OWstructures in the sense that natural frequencies of thesthe wind, wave and 1P (rotor frequency) and 2P/3P (widely used soft-stiff design (target frequency of the oof forcing frequency to natural frequency is very closeof responses such as deection/rotation which may enhdesign life. Therefore, a designer apart from accuratelyalso ensure that the overall natural frequency becautowards the forcing frequencies making the value of fcritical components of OWTs not only because of the ocial viability of the project. The article highlights techoffshore wind farm.

Importance of Foundation DesignChallenges in DesignOffshore Wind Turbinheavy mass as well as a rotating mass at the top)are dynamically sensitive because the natural fre-quency of these slender structures are very close tothe excitation frequencies imposed by the

Eng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.0041o 1 and as a result is prone to dynamic amplicationce the fatigue damage, thereby reducing the intendedredicting the natural frequency of the structure, mustof dynamic-soil-structure-interaction does not shiftn even closer to 1. Therefore, foundations are one ofrall stability of the structure but also because of nan-cal challenges associated with foundation design for

environmental and mechanical loads. Fig. 2 shows af Foundations fors

s, University of Surrey, UK

s) are challenging as these are dynamically sensitivestructures are very close to the forcing frequencies ofade shadowing frequency) loading. Typically for the

doi: 10.1049/etr.2014.0041ISSN 2056-4007

www.ietdl.orgsprings are not only frequency dependent but alsochange with cycles of loading because of dynamicsoil structure interaction. Further details on thedynamic interaction can be found in [4, 5].

1& The Institution of Engineering and Technology 2014

Loads acting on the foundationsThe loads acting on the wind turbine tower are ultim-ately transferred to the foundation and can be classi-ed into two types: static or dead load because ofthe selfweight of the components and the dynamic

turbulence in the wind. The magnitude of dynamiccomponent depends on the turbulent wind speed.(b) The load caused by waves crashing against the sub-structure very close to the foundation. The magnitudeof this load depends on thewave height andwave period.(c) The load caused by the vibration at the hub level

Fig. 1 Figure showing different types of foundations

IET Engineering & Technology Reference Subhamoy Bhattacharyaloads (or it some instances this can be cyclic).However, the challenging part is the dynamic loadsacting on the wind turbine which are discussed below:

(a) The lateral load acting at the hub level (top of thetower) from the rotating blades produced by theFig. 2 Simplied mechanical model of an offshore wind turbine

2& The Institution of Engineering and Technology 2014because of the mass and aerodynamic imbalances ofthe rotor. This load has a frequency equal to the rota-tional frequency of the rotor (referred to as 1P loadingin the literature). Since most of the industrial wind tur-bines are variable speed machines, 1P is not a singleEng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.0041

do

IET Engineering & Technology Reference Challenges in Design of Foundations for Offshore Wind Turbinesfrequency but a frequency band between the frequen-cies associated with the lowest and the highest rpm(revolutions per minute).(d) Loads in the tower because of the vibrationscaused by blade shadowing effects (referred to as2P/3P in the literature). The blades of the windturbine passing in front of the tower cause a shadow-ing effect and produce a loss of wind load on thetower and is shown in Fig. 3. This is a dynamic loadhaving frequency equal to three times the rotationalfrequency of the turbine (3P) for three bladed windturbines and two times (2P) the rotational frequency

Fig. 3 Blade shadowing load on the tower (3P load)

Left-hand side diagram shows the load on the tower when the bladesof the turbine for a two bladed turbine. The 2P/3Ploading is also a frequency band such as 1P and issimply obtained by multiplying the limits of the 1Pband by the number of the turbine blades.

The turbulent wind velocity and the wave height onsea are both variables and are best treated statisticallyusing power spectral density functions. In otherwords, instead of time domain analysis the producedloads are more effectively analysed in the frequencydomain whereby the contribution of each frequencyto the total power in wind turbulence and in oceanwaves is described. Representative wave and wind(turbulence) spectra can be constructed by adiscrete Fourier transform from site specic data.However, in absence of such data, theoreticalspectra can also be used. The DNV standard speciesthe Kaimal spectrum for wind and the Joint NorthSea Wave Project spectrum for waves in offshorewind turbine applications.

Eng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.0041Fig. 4 shows the main frequencies for a three-bladedNational Renewable Energy Laboratory standard 5MW wind turbine with an operational interval of 6.9to 12.1 rpm. The rotor frequency (often termed 1P)lies in the range 0.1150.2 Hz and the correspondingblade passing frequency for a three-bladed turbinelies in the range 0.3450.6 Hz. The gure alsoshows typical frequency distributions for wind andwave loading. The peak frequency of typical NorthSea offshore waves is about 0.1 Hz. Further detailson loading can be found in [12].

es not shadow the towerDesign optionsIt is clear from the frequency content of the appliedloads (see Fig. 4) that the designer has to select asystem frequency (the global frequency of the overallwind turbine including the foundation) which liesoutside these frequencies to avoid resonance andultimately increased fatigue damage. From the pointof view of the rst natural frequency ( f0) of the struc-ture, three types of designs are possible (see Fig. 4)

(1) Soft-Soft design where f0 is placed below the 1Pfrequency range which is a very exible structure andalmost impossible to design for a grounded system.(2) Soft-Stiff design where f0 is between 1P and 3P fre-quency ranges and this is the most common in thecurrent offshore development.(3) Stiff-Stiff designs where f0 have a higher naturalfrequency than the upper limit of the 3P band whichwill need a very stiff support structure.

It is of interest to review the codes of practice in thisregard. DNV [11] code suggests that rst natural

3& The Institution of Engineering and Technology 2014

ho

IET Engineering & Technology Reference Subhamoy Bhattacharyafrequency should not be within 10% of the 1P and 3P

Fig. 4 Frequency spectrum of the dynamic loads showing design cranges as indicated in Fig. 4. It is apparent from Fig. 4that for soft-stiff design, the rst natural frequency ofthe wind turbine needs to be tted in a very narrowband (in some cases the 1P and 3P ranges may evencoincide leaving no gap).

A few points to be noted:

(1) From the point of view of dynamics, OWT designs areonly conservative if the prediction of the rst naturalfrequency is accurate. Unlike in the case of some otheroffshore structures (such as the ones used in the oil andgas industry), under-prediction of f0 is unconservative.(2) The safest solution would seem to be to place thenatural frequency of the wind turbine well above the3P range. However, stiffer designs with highernatural frequency require massive support structuresand foundations involving higher costs of materials,transportation and installation. Thus from an econom-ic point of view, softer structures are desirable and it isnot surprising that almost all of the installed wind tur-bines are soft-stiff designs and this type is expectedto be used in the future as well.(3) It is clear from the above discussion that designingsoft-stiff wind turbine systems demands the consider-ation of dynamic amplication and also any potentialchange in system frequency because of the effects

4& The Institution of Engineering and Technology 2014of cyclic/dynamic loading on the system, that is,

icesdynamic-structure-foundation-soil-interaction.Typically, the rst modal frequency of the wind turbinesystem lies in the range of 75 to 120% of the excita-tion frequencies and as a result, dynamic amplica-tions of responses are expected.(4) Clearly, for soft-stiff design, any change in natural fre-quency over the design/operation period of the turbinewill enhance the dynamic amplications which will in-crease the vibration amplitudes and thus the stressesand fatigue damage on the structure. Therefore,fatigue is one of the design drivers for these structures.Predicting fatigue damage is undoubtedly a formidabletask because of the complexity associated with theuncertainty in the dynamic amplication (owing tochanges in system characteristics over time and numberof cycles), randomness of the environmental loadingand last but not the least, the impact of climate change.

Design Considerations for FoundationsOne of the main aims of the foundations is to transferall the loads from the wind turbine structure to theground within the allowable deformations. Guidedby limit state design philosophy, the design considera-tions are to satisfy:

1. Ultimate Limit State (ULS): This would require thecomputation of capacity of the foundation. For

Eng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.0041

states. Therefore, it is necessary to understand the strin-gent criteria of 0.25 for grounded wind turbinesystem. A value of 0.25 represents a horizontal deec-tion of 450 mm for a typical 80 m tower. Clearly, a lessstringent tilt criterion will save on the foundation costsand installation time and make wind energy cheaper.

Challenges in Analysis of DynamicSoil-Structure InteractionOWTs are new types of offshore structures and areunique in their features. The most important differ-ence with respect to oil and gas installation structuresis that they are dynamically sensitive (as explained in

IET Engineering & Technology Reference Challenges in Design of Foundations for Offshore Wind Turbinesmonopiles type of foundation, this would require com-putation of ultimate moment, lateral and axial loadcarrying capacity.2. Serviceability Limit State (SLS): This would requirethe prediction of tilt at the hub level over the lifetime of the wind turbine.3. Fatigue Limit State: This would require predictingthe fatigue life.4. Robustness and ease of installation: Can the foun-dation be installed and are there adequate redun-dancy in the system?

A note on SLS design criteriaServiceability criteria will be dened based on the tol-erance requirements for the operation of the windturbine and is often described as turbine manufactur-er requirements. Ideally, these should be turbine spe-cic, that is, size and the hub height, gear boxed ordirect drive. Typically, these tolerances are speciedin some codes of practice (eg DnV) or a design speci-cation supplied by the client which may be dictatedby the turbine manufacturer. Some of the specicrequirements are:

(a) Maximum allowable rotation at pile head after in-stallation. DnV code species 0.25 degree limit onTilt at the nacelle level.(b) Maximum accumulated permanent rotation result-ing from cyclic and dynamic loading over the design life.

Example methodology to predict the foundationstiffness required: For Walney farm (in the Irish Seahaving Siemens SWT-3.610 type turbine having anoperating wind speed range of 4 to 25 m/s), it has beenestimated that for 9 m/s wind speed, the maximummoment at the mudline level is about 60 MN and for20 m/s wind speed, the maximum value is 125 MNm.Assuming design over turning moment is 125 MNmand if the allowable tilt is 0.25 at the foundation level,one can therefore estimate the Rotational Foundationstiffness required and is given by (1)

KR = 125MNm4.36 103 rad = 28.6GNm/rad (1)

This is a very large number and would require largediameter monopile or equally alternative foundationmultiple pod foundations may be used.

In this context it may be mentioned that SLS criteriaimpacts the foundation design and thereby costs. It

has been reported that oating wind turbines areallowed to tilt by up to + / 5 in the worst sea

Eng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.0041Section 2) and moment-resisting. Fig. 5 shows atypical monopile supported wind turbine and a pilesupported xed offshore jacket structure. There are,however, obvious differences between those twotypes of foundations. Piles for offshore structures aretypically 60110 m long and 1.82.7 m diameter andmonopiles for OWTs commonly 3040 m long and3.56 m diameter. Degradation in the upper soillayers resulting from cyclic loading is less severe foroffshore piles which are signicantly restrained frompile head rotation, whereas monopiles are free-headed. The commonly used design method using abeam on non-linear Winkler springs (p-y method inAPI code or DNV code) may be used to obtain pilehead deection under cyclic loading, but its use islimited for wind turbines because:

(a) the widely used API model is calibrated against re-sponse to a small number of cycles (maximum 200cycles) for offshore xed platform applications. In con-trast, for a real offshore wind turbine 107108 cycles ofloading are expected over a lifetime of 2025 years.(b) under cyclic loading, the API or DNV model alwayspredicts degradation of foundation stiffness in sandyFig. 5 Offshore wind turbines and offshore oil and gas installations

5& The Institution of Engineering and Technology 2014

IET Engineering & Technology Reference Subhamoy Bhattacharyapriority. [4, 5, 8] carried out experimental testing of a1:100 scaled wind turbine to characterise the free dy-namics of the system and to study the long term behav-iour under the action of the dynamic loading.

The following conclusions could be reached from thestudysoil. However, recent work by [4, 6, 7,10] suggestedthat the foundation stiffness for a monopile in sandysoil will actually increase as a result of densicationof the soil next to the pile.(c) The ratio of horizontal load (P) to vertical load (V) is veryhigh inOWTswhen comparedwithxed jacket structures.

Although, offshore wind turbine structures aredesigned for an intended life of 25 to 30 years, butlittle is known about their long term dynamic behaviourunder million of cycles of loading. Although monitoringof existing offshore wind turbine installation is a possi-bility and can be achieved at a reasonable cost, fullscale testing is very expensive. An alternative methodis to carry out a carefully planned scaled dynamictesting to understand the scaling/similitude relation-ships which can be later used for interpretation of theexperimental data and also for scaling up the resultsto real prototypes. There are mainly two approachesto scale up the model test results to prototype conse-quences: rst is to use standard tables for scaling andmultiply the model observations by the scale factor topredict the prototype response and the alternative isto study the underlying mechanics/physics of theproblem based on the model tests recognising thatnot all the interaction can be scaled accurately in a par-ticular test. Once the mechanics/physics of the problemare understood, the prototype response can be pre-dicted through analytical and/or numerical modellingin which the physics/mechanics discovered will beimplemented in a suitable way. The second approachis particularly useful to study the dynamics of OWTsas it involves complex dynamic wind-wave-foundation-structure interaction and none of the physical modellingtechniques can simultaneously satisfy all the interactionsto the appropriate scale. Ideally, a wind tunnel com-bined with a wave tank on a geotechnical centrifugewould serve the purpose but this is unfortunately notfeasible. It is recognised that not all physical mechan-isms can be modelled adequately and therefore thoseneed special consideration while interpreting the testresults. As dynamic soil structure interaction of wind tur-bines are being studied, stiffness of the system is a top(a) The change in natural frequencies of the windturbine system may be affected by the choice of

6& The Institution of Engineering and Technology 2014foundation system, that is, deep foundation or mul-tiple pods (symmetric or asymmetric) on shallow foun-dations. Deep foundations such as monopiles willexhibit sway-bending mode, that is, the rst two vibra-tion modes are widely spaced typical ratio is 4 to5. However multiple pod foundations supported onshallow foundations (such as tetrapod or tripod onsuction caisson) will exhibit rocking modes in two prin-ciple planes (which are ofcourse orthogonal). Fig. 6shows the dynamic response of monopile supportedwind turbine and tetrapod foundation plotted in theloading spectrum diagram.(b) The natural frequencies of wind turbine systemschange with repeated cyclic/dynamic loading. In thecase of strain-hardening site (such as loose tomedium dense sandy site) the natural frequency isexpected to increase and for strain-softening site(such as normally consolidated clay) the natural fre-quency will decrease.(c) The results showed that the multipod foundations(symmetric or asymmetric) exhibit two closely spacednatural frequencies corresponding to the rockingmodes of vibration in two principle axes.Furthermore, the corresponding two spectral peakschange with repeated cycles of loading and they con-verge for symmetric tetrapods but not for asymmetrictripods. From the fatigue design point of view, the twospectral peaks for multipod foundations broaden therange of frequencies that can be excited by the broad-band nature of the environmental loading (wind andwave) thereby impacting the extent of motions. Thusthe system lifespan (number of cycles to failure) mayeffectively increase for symmetric foundations as thetwo peaks will tend to converge. However, for asym-metric foundations the system life may continue tobe affected adversely as the two peaks will not con-verge. In this sense, designers should prefer symmetricfoundations to asymmetric foundations.

Foundation DesignAlthough design guidelines are available for offshoreoil and gas installation foundations, its direct extrapo-lation/interpolation to offshore wind turbine founda-tion design is not always possible, the reasons ofwhich is explored in the earlier section. There aretwo reasons: (a) The foundations of these structuresare moment resisting, that is, large overturningmoments at the foundation which are disproportion-ately higher that the vertical load; (b) The structure isdynamically sensitive and therefore fatigue is a

design driver. This section of the article thereforeexplores a simplied foundation design methodology

Eng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.0041

ca

IET Engineering & Technology Reference Challenges in Design of Foundations for Offshore Wind Turbineswhich may be used during option engineering or pre-liminary design.

Fig. 6 Relationship between effect of natural frequency of suction1. Compute the maximum mudline bending moment,considering the different load combinations. The over-turning moments because of 1P (misalignment) and3P (blade shadowing) may be neglected in this step.2. Based on the allowable tilt criteria for the particularproject-determine the foundation stiffness required asshown in Equation 1. This is the minimum stiffnessthat is required to satisfy the SLS.3. It is then required to check the ULS criteria, that is,the foundation capacity. If the foundation is not ad-equate, the size must be increased.4. The soil surrounding the foundationswill be subjectedto tens of millions of cycles of cyclic and dynamic loadingof varying strain as well as varying frequency. It must beensured that the soil remains in the linear elastic rangeso as not to alter the dynamic stiffness of the foundation.For detailed design, Resonant Column testing is recom-mended to nd the threshold strains for the groundand further details on the use of threshold strainconcept in monopile design can be found in [9].5. Beam on non-linear Winkler model or niteelement analysis can be carried out and it must beensured that the p-y curves in soil are within thelinear elastic section at all depths. However, 3DFinite Element Analyses are recommended to under-stand the strains around the foundation.

Eng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.00416. It is now required to obtain stiffness of the founda-tion to calculate natural frequency of the wholesystem to check where the overall system is placed:

isson and monopile on the forcing frequenciessoft-soft, soft-stiff or stiff-stiff (see Fig. 4). If thenatural frequency is not acceptable, the design para-meters such as foundation stiffness, tower stiffnessand mass may be altered so that the desired frequencyis obtained. This is an iterative process.7. The foundation stiffness may change over the lifetime of the wind turbine because of soil-structure inter-action which will have an impact on the natural fre-quency of the system and tilt. If the ground is sandysite (shown as strain-hardening site in Fig. 4), thenatural frequency to expected to increase and if it isa clay site (shown as strain-softening site in Fig. 4),the natural frequency may decrease. If the site islayered, the change in natural frequency cannot beascertained a priory and depends on various factors in-cluding the geometry of layering. This is termed as un-certain site in Fig. 4. Engineers need to carry outcalculations to predict the change in frequency whichis also necessary to compute the fatigue loading.

Challenges in monopile foundation design andinstallationMonopiles have been predominantly used to supportwind turbine generators in water depths up to 30 m.However, there are discussions with regard to the

7& The Institution of Engineering and Technology 2014

having low natural frequency) and therefore sensitive

EN1, 2007, pp. 2129

IET Engineering & Technology Reference Subhamoy Bhattacharyause of monopiles in deeper water depths termed asXL monopile. Preliminary calculations suggests that10 m diameter monopiles weighing 1200 tonnesmay be suitable for 45 m water depth and of coursedependent of ground conditions. However, the useis uncertain because of the following: (a) no codiedcyclic design to predict long term tilt; (b) lack of redun-dancy in foundation system and therefore chance ofsingle-point failure; (c) installation costs and lack of ad-equate specialised vessels; (d) connection betweenfoundation, transition piece and the tower. Some ofthese aspects are described below in further details:

1. Lack of redundancy: Monopiles are overturningmoment resisting structures and there are two maincomponents: (a) overturning moment arising fromthe thrust acting at the hub level; (b) overturningmoment because of the wave loading. Also thesetwo moments can act in two different planes andwill vary constantly depending on the time of theday and time of the year. Monopiles are rigid pilesand the foundation collapse can occur if the soilaround the pile fails, that is, there would be rigidbody movement. If the foundation starts to tilt, it isvery expensive to rectify.2. Cyclic (rather dynamic) design of monopile: The re-sponse of monopiles under cyclic/dynamic load is notwell understood and there is a lack of guidance incodes of practice. If cyclic design is incorrect, monopilecan tilt in the long term. If the tilt is more than the al-lowable limit, the turbine may need a shutdown.Monopile design is usually (also wrongly) carried outusing API design procedure calibrated for exible piledesign where the pile is expected to fail by plastichinges.3. Issues related to installation of monopiles: Largemonopile installations require suitable vessel availabil-ity as well as specialised heavy lifting equipments.Other issues are noise refusals, buckling of the piletip, drilling out, grouted connections. If the site con-tains weak rock (siltstone/sandstone/mudstone) andwhere the local geology shows bedrock or hardglacial soils at shallow depths, drive-drill-drive techni-ques may be required, with subsequent increases incost and schedule. It must be mentioned here thatdriving reduces the fatigue life.

Jacket on exible pilesThere has been considerable interest in jacket typestructures for deeper water applications, but it is per-

ceived as being expensive because of the amount ofsteel required. However jackets supported on piles

8& The Institution of Engineering and Technology 2014[2] Adhikari, S., Bhattacharya, S.: Dynamic analysis of windturbine towers on exible foundations, Shock Vib., 2012,19, pp. 3756

[3] Bhattacharya, S., Adhikari, S.: Experimental validation ofsoilstructure interaction of offshore wind turbines, SoilDyn. Earthq. Eng., 2011, 31, (56), pp. 805816

[4] Bhattacharya, S., Cox, J., Lombardi, D., Muir Wood, D.:Dynamics of offshore wind turbines supported on two foun-dations, Geotech. Eng.: Proc. ICE, 2013, 166, (2),pp. 159169

[5] Bhattacharya, S., Nikitas, N., Garnsey, J., et al.: Observeddynamic soilstructure interaction in scale testing of offshorewind turbine foundations, Soil Dyn. Earthq. Eng., 2013, 54,pp. 4760

[6] Adhikari, S., Bhattacharya, S.: Vibrations of wind-turbinesconsidering soil-structure interaction, Wind Struct. Int. J.,2011, 14, pp. 85112

[7] Leblanc, C.: Design of offshore wind turbine support struc-turesSelected topics in the eld of geotechnical engineer-to the dynamic loading imposed upon them. Thearticle discusses the complexity involved in designingthe foundation of these structures. It has beenshown that design guidelines available for offshoreoil and gas installation foundations cannot be directextrapolated/interpolated to offshore wind turbinefoundation design.

REFERENCES

[1] Carter, J.: North hoyle offshore wind farm: design andbuild. Energy: Proc. of the Institution of Civil Engineerscan be considered as a safe solution because of excel-lent track record of good performance in the offshoreoil and gas industry. The offshore oil and gas industryhave been using long exible piles (diameters upto2.4 m) which are easy to drive, the necessary vesselsare readily available (relatively as opposed to vesselsto install monopiles). This aspect will drive down thetime in construction costs regarding piling and alsolarge vessels are not required for pile installation.However there are costs associated with jacket instal-lation. One of the requirements is the optimisation ofthe jacket so as to consume minimum steel. There aretwo types of jacket normal jacket or twisted jacket.The advantage of twisted jacket over normal jacketis fewer number of joints and therefore less of afatigue issue.

ConclusionOWTs are new types of offshore structure charac-terised by low stiffness (as a result exible anding (Aalborg University, Denmark, 2009)[8] Bhattacharya, S., Lombardi, D., Muir Wood, D.M.: Similitude

relationships for physical modelling of monopile- supported

Eng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.0041

offshore wind turbines, Int. J. Phys. Model. Geotech., 2011,11, (2), pp. 5868

[9] Lombardi, D., Bhattacharya, S., Muir Wood, D.: Dynamicsoil-structure interaction of monopile supported wind tur-bines in cohesive soil, Soil Dyn. Earthq. Eng, 2013, 49,pp. 165180

[10] Cullar, P., Georgi, S., Baeler, M., Rcker, W.: On thequasi-static granular convective ow and sand densication

around pile foundations under cyclic lateral loading,Granular Matter, 2012, 14, (1), pp. 1125

[11] DNV (Det Norske Veritas): Guidelines for design of wind tur-bines (DNV, London, UK, 2002, 2nd edn.)

[12] Arany, L., Bhattacharya, S., Macdonald, J., Hogan, S. J.:Simplied critical mudline bending moment spectra of off-shore wind turbine support structures, Wind Energ., 2014,doi:10.1002/we.1812.

IET Engineering & Technology Reference Challenges in Design of Foundations for Offshore Wind TurbinesEng. Technol. Ref., pp. 19doi: 10.1049/etr.2014.00419& The Institution of Engineering and Technology 2014

Importance of Foundation DesignTypes and Nature of the Loads Acting on the FoundationsDesign Considerations for FoundationsChallenges in Analysis of Dynamic Soil-Structure InteractionFoundation DesignConclusionREFERENCES