Coffman 0609

7/30/2019 Coffman 0609

1/6

28 MAY/JUNE | 2009

Vibration-based Wind toWer

Foundation design

this is a case study oF multiplewind turbinetowers located in dierent villages in Alaska wheresevere arctic weather conditions exist. Initially, a re-inorced concrete (RC) mat oundation was utilizedto provide vertical and lateral support. Where soilconditions required it, a pile oundation solutionwas devised utilizing a 30 thick RC mat contain-

ing an embedded steel grillage o W18 beams sup-ported by 20-24 grouted or un-grouted piles. Temixing and casting o concrete in-situ has becomethe major source o cost and diculty o construc-tion at these remote Alaskan sites. An all-steeloundation was proposed or aster installation andlower cost, but it was ound to impact the naturalrequencies o the structural system by signicantlysotening the oundation system. Te tower-oun-

dation support structure thus became near-reso-nant with the operational requencies o the windturbine leading to a likelihood o structural instabil-ity or even collapse. A detailed 3D Finite-Elementmodel o the original tower-oundation-pile systemwith RC oundation was created using SAP2000.Soil springs were included in the model based on

soil properties rom the geotechnical consultant.Te natural requency rom the model was veriedagainst the tower manuacturers analytical and theexperimental values. Where piles were used, nu-merous iterations were carried out to eliminate theneed or the RC and optimize the design. An opti-mized design was achieved with enough separationbetween the natural and operational requencies toprevent damage to the structural system, eliminat-

Wind towers must sustain continuous

vibration-induced forces throughout theiroperational life, so how can you engineer

a cost-effective design for structural

integrity? Coffman Engineers provides

one answer.

By Sai Hussain, S.E., and Mohamed Al Satari, Ph.D., P.E.

sf m. h, s.e., w e, cf, ff f cff e, i.

m a s, p.d., p.e., , f w . V www.ff.

. t w k k fw ff f j: sv ck,

l h, b mw, s t, p V b, W V. F - J W, p.e., leed ap.

7/30/2019 Coffman 0609

2/6

windsystemsmag.com 29

ing the need or any RC encasement o the steel orgrouting o the piles.

introductionWind towers have to sustain continuous vibration-induced orces throughout their operational lie. Teoperating requency o the three-blade turbine could

potentially cause dynamic amplication o these orc-es, signicantly posing a threat to the overall structuralintegrity. Sucient separation o the structural sys-tems natural requency rom the turbine operationalrequencies is key to avoiding potentially catastrophicailures. Te turbine operating requency is typicallylower than the structural system natural requency, butcould approach it as higher turbine output is obtained.Idealized assumptions o xity at the base o the tower

are un-conservative; a more realistic analysis account-ing or oundation fexibility yields lower estimates othe natural requency or the system. In such cases,soil-oundation-structure interaction needs to be con-sidered.

structural description

Te owner, Alaska Village Electric Cooperative(AVEC), desired to purchase new or used towers toprovide wind generated energy in concert with itsdiesel systems in the Alaska villages o Hooper Bay,Chevak, Gambell, Savoonga, Mekoryuk, Kasigluk,and oksook Bay, as well as others. Te tower modelsdier in height (23-46m), weight, and wall thickness.Furthermore, the three-bladed turbines vary in weight(7812-7909kg), blade diameter (19-27m), and poweroutput (100-225kW). owers were rom a variety omanuacturers and suppliers. Te turbine and tower

packages ultimately utilized were supplied by North-ern Power Systems o Barre, Vermont. Figure 1 showssome o the installed towers.

operational issuesAs the wind turbine blades start to rotate rom rest,their circular speed increases, and the induced vibra-tion requency increases. Depending on its power out-put capacity, the turbine blades rotate at maximum ro-tational (circular) speeds that range rom 45 to 60 rpm,corresponding to 0.75 to 1.00 Hz. Tese operational

requencies are very close to the range o natural re-quencies o the entire soil-oundation-tower-turbinesystem.

I more output power is desired, higher rotationalspeeds have to be accommodated. A poor design deci-sion would involve a maximum rotational speed thatis very close to the natural requency o the structuralsystem, resulting in a high likelihood o resonant am-plication causing structural instability. Another poordesign would have a rotational speed not very closeto yet higher than the natural requency o the struc-

tural system. In such cases the structure would haveto endure violent near-resonance vibrations as the op-erational requency approaches the natural requencywhile speeding up to and down rom the maximumspeed. Tis situation would result in very high dynam-ic orces, which could cause immediate damage to thestructure. Even i these dynamic orces do not exceedthe structures strength capacity, atigue-induced ail-ures could also be encountered.

A sound design would avoid allowing the opera-tional requency to approach the vicinity o the natural

requency by a certain saety actor. A saety actor o15 percent o the natural requency was recommend-ed by the turbine vendor and adopted by the authorsor this project.

design obJectiVeIn order to develop a sound overall structural systemthat meets the structural perormance requirementso the wind towers, the dynamic interaction o the sup-

7/30/2019 Coffman 0609

3/6

30 MAY/JUNE | 2009

porting soil, oundation, and superstructure needs tobe considered. Since the tower and turbine are pre-

abricated and manuactured, once selected or a cer-tain installation location only the oundation can bedesigned and ne-tuned in accordance with the sitesoil conditions and desired system requency.

Depending on the soil conditions, the optimumoundation system needs to be selected (spread oot-ing, deep piles, micro-piles, etc.). Additionally, theoundation must have adequate stiness in order tomaximize the systems natural requency within prac-tical limits. A suitably sti soil-oundation-structuresystem will allow or higher power output generated

by the turbines.

Foundation designBased on the geotechnical conditions at the dierentsites, two types o oundations were selected; largespread oundation, and deep piles. A 5 deep, 12x12reinorced concrete (RC) spread ooting was utilizedto provide the system with vertical and lateral sup-port, as well as damping and stiness. Where soilconditions necessitated it, a pile oundation solutionwas devised utilizing a 30 thick mat o RC ounda-

tion embedded with a steel grillage o W18 beamsounded on 20 grouted piles.

Ater some installations were made, it was deter-mined that the mixing and casting o concrete in-situis the major source o cost and diculty o construc-tion. An all-steel oundation was proposed or asterinstallation and lower cost, but such a oundation sys-tem impacted the natural requency and signicantlysotened the system. Consequently, the oundationdesign was driven by the systems natural requency.Multiple solutions combining dierent pile sizes,

grouted and un-grouted, and dierent beam sizeswere devised. Te optimum design was selected oreach location based on the highest practically obtain-able natural requency and cost eectiveness o thedesign.

modeling and analysisA detailed 3D Finite Element Analysis (FEA) model othe tower-oundation-pile system was created using

SAP2000. Te tower was modeled using a ne mesh

o thin shell elements, while the steel grillage and pileswere assigned the appropriate cross-sectional prop-erties. Tick plate elements were utilized to model theRC oundation. In order to capture the soil-ounda-tion-structure interaction, compression-only springswere devised to mimic the soil around the piles. Soildamping properties were conservatively neglectedand the turbine mass was lumped at the hub heightabove the top o the tower. Te natural requency

F. 1: o w w.

F. 2: m () f ().

7/30/2019 Coffman 0609

4/6


rom the model was veried against the tower manu-acturers analytical and the experimental values.

Discretization o FEA elements into sub-elementsis not as straightorward a task as some may believe.

Unavorable discretization can give rise to subsequentnumerical diculties. In vibration analysis, or exam-ple, abrupt changes in element size should be avoided,as such changes tend to produce spurious wave refec-

F. 3: t f rc f () - f 3d f ().

F. 4: s w f .

fastener solutions from the ground up

hededeneinc.co800.345.4976

A394TowerBolts

A325

StructuralBolts

mnucued

ancho Bolto mee You

specifcion:

1554, a449

& a307 gr C

7/30/2019 Coffman 0609

5/6

7/30/2019 Coffman 0609

6/6


using a set o typical grillage beam and pile sizes. Aseries o urther variations to the beam/pile sizes anddierent combinations yielded an optimized oun-dation design or each site. Te optimized designswere achieved with enough separation (15 percent)between the natural and operational requencies to

prevent damage to the structural system. Te optimi-zation eliminated the need or any RC encasement tothe steel oundation or grouting to the piles, in manycases.

In most cases, an optimized oundation systemdesign or a particular site was also ound to be sat-isactory or other locations. Tus, a small library ouniversally applicable standard designs was compiledin an eort to keep the abrication cost low. able 1summarizes the nal design or two o the tower lo-

cations and demonstrates how one optimized designis adequate in two locations with dierent geotechni-cal conditions. Figure 7 shows one o the optimizedall-steel tower support oundations.

conclusionsTe oundation system design was controlled by the

natural requency o the soil-oundation-structuresystem rather than by strength or serviceability con-siderations. aking into account the soil-oundation-structure interaction yielded a more realistic estimateo the natural requency. Had a xed-base-tower as-sumption been adopted, signicantly under-designedsystems would have been incorporated.

reFerences:1) American Concrete Institute. (2004), ACI 351-04

Foundations or Dynamic Equipment, Farmington

Hills, Michigan.2) J.P. Saylor & Associates, Consultants Ltd. VestasV27-225kW Specications and echnical Data,Des Moines, Iowa.

3) Golder Associates, Inc. Geotechnical Reports, An-chorage, Alaska.

4) R.D. Cook, D. S. Malkus and M. E. Plesha. (1989),Concepts and Applications o Finite ElementAnalysis, John Wiley & Sons, New York, NewYork, pp. 553-582.

F. 7: s w .

Documents

Coffman 0609