About the Author/WAYNE WALLACE

Wayne Wallace is the president of Applied Bolting Technology Products.The company provides bolting consulting services and manufactures directtension indicating washers from its base in Rockingham, Vermont. Wallaceis a member of the Research Council on Structural Connections, owner ofseveral patents, and author of numerous papers on the practical aspects ofquality assurance in structural bolting. He can be reached by phone at 800-552-1999, by fax at 802-460-3104, or e-mail at wwllace@sover.net.

Applied Bolting & Germanischer Lloyd WorkTogether For Wind Turbine Tower Flange

Bolt Squirter® DTIs

T wo thousand-eight saw 7500 MW of wind energycapacity added in North America. This was 4,500

wind turbines of various sizes, consisting of 13,500tower sections, and 2.25 million bolts used in the towerflange splices. Furthermore, between now and 2011, itis forecast that 1/5th of the worlds wind power capac-ity will be installed in the USA. By 2011 the number oftower flange splice bolts required annually will likelyreach 5 million. And the typical size is M42 diameterby 330 mm long – a substantial bolt indeed.

In a recent visit to Cardinal Fastener in Cleveland,even the new American President seemed to recog-nize the importance of having good bolts for thesesplices (Figure 1).

But just obtaining large diam-eter metric high strength bolts inNorth America isn’t the entirestory. Getting them correctly in-stalled in the wind tower splices,that is, to the correct tension, orpreload, is the other part of thestory. That’s where AppliedBolting’s newly designed Squirter®

DTIs will come in handy in fillingout the Quality Management (QM)system that all the wind turbinemanufacturers must design andmaintain.

Applied Bolting, knows thatcorrect tensioning of the wind tur-bine tower flange splice bolts is critical to any thirdparty QM system. We also know of the difficulties cur-rently being experienced in the bolt tensioning opera-tion as the towers are erected. These observationsprompted us a few years ago to design a set of largediameter Squirter DTIs which could be used on theM36, M42, and M48 property class 10.9 bolts typicallyused in the tower flange splices. Prototype parts, manu-factured and tested by Applied Bolting, confirmed thatSquirter DTIs could allow more accurate and fastertightening of the flange bolts, and demonstrations of

the product to several prominent wind turbine manu-facturers verified their interest.

However, since these Squirter DTI products areoutside the size and compression load range of con-ventional DTI specifications we decided that an out-side certification agency such as Germanischer Lloyd(GL) of Hamburg, Germany should certify their per-formance.

In 2007, Applied Bolting contacted GL with the in-tention of obtaining their input and advice on certifica-tion of our Squirter DTIs. Up to M36 grade 10.9, thestrength/bump compression properties are controlledby ASTM F959M. But the wind industry uses mostly

splice bolts of diameter M42 andM48, so no recognized worldstandard existed against whichGL could audit our M42 and M48Squirter products. We did, how-ever, know the strength proper-ties and design preload for theM42 and M48 grade 10.9 boltsthat were typically used for towersplices (see Figure 2), so wewrote a performance specificationfor the Squirter DTIs againstwhich GL could perform an audit.

Why submit parts to an out-side auditor like GL?Germanischer Lloyd is the lead-ing third party certification body

in the wind-energy sector, offering project and typecertifications in the field of renewable-energy equip-ment for manufacturers, banks and insurers of windturbines and components, and they’ve been doing itfor 30 years.

The rapid growth of the wind energy industry andthe growing size of wind farms force financing banks,insurance companies, and jurisdictional authorities torequire reliability and safety assessments of theseprojects. Assessments are carried out by means of

Figure 1: President Obama at Cardinal Fastener

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WIND TURBINE, from page 40

certifying components, turbines, and the wind farms intotal, by a third party.The certification process empha-sizes reliability, safety, strength and fatigue in an at-tempt to guarantee safe operation.

Before third party manufacturing surveillance maybegin, certain Quality Management (QM) requirementshas to be met by each manufacturer or componentsupplier. As a rule, the QM system should be certifiedto comply with ISO 9001, otherwise the third party cer-tification body has to take part in devising and approv-ing an applicable QM system. The extent of the sur-veillance during production depends on the level thatthe QM system demands. In general, actions and ap-provals such as inspection and testing of materials andcomponents, scrutiny of QM records (test certificates,reports), surveillance of production, inspection of thecorrosion protection and of the electrical power sys-tem will take place.

European designers of wind turbines specify thatcertain torque values be used to tighten the flange splicebolts, but in North America this is not allowed by theResearch Council on Structural Connections. UnderRCSC regulations, since the tower itself is clearly a

structure, the bolts MUST be preloaded by one of onlythree allowable methods – none of which involves atorque value specified by a remote manufacturer. TheRCSC points out that using a prescribed torque valueto tighten bolts is too inaccurate.

Does it matter? You bet. Tower flanges MUST bebrought together and the bolts used MUST be preloadedcarefully in order to resist the external loading. Windturbines have sufferred many highly visible failures ofentire machines and of components, including failurescaused by incorrectly tightened bolts. All the turbinemanufacturers are very interested in getting their boltsinstalled, that is, preloaded, correctly, but until nowthey only had torque to use.

The minimum bolt preload as designed by the windturbine tower makers is as follows:

M36 Three load levels 458 kN, 510 kN, and 572 kNM42 One load level 710 kNM48 One load level 930 kN

Applied Bolting designed Squirter DTIs to suit thesepreloads. For example, the M42 product we designed

Bolt dia.

M36

M36

M42

M48

PropertyClass

8.8

10.9

10.9

10.9

StressArea

mm2

817

817

1121

1473

NominalUltimateStressN/mm2Note 1

800

1000

1000

1000

ActualUltimateStressN/mm2

Note 1

830

1040

1040

1040

ActualTensileStrength

N

678,110

849,680

1,165,840

1,531,920

.2% OffsetYield Stress

N/mm2

Note 1

660

940

940

940

.2% OffsetYield

StrengthN

539,220

767,980

1,053,740

1,384,620

Note 1: From ISO 898.1 (M36) or DAST Richtlinie 021 (M42, M48)

Figure 2: Bolt Properties

Diameter,Grade

ofAssembly

M36 8.8M36 M 10.9M36 10.9

M42 10.9

M48 10.9

MinimumPreloadNeeded

kN

458510572

710

930

YieldStrengthof Bolt

kN

539768768

1054

1384

MinimumUltimateStrengthof Bolt

kN

678849849

1166

1532

TargetPreload

kN

507550632

780

1024

AchievablePreloadMin/Max97.7 %

ConfidencekN

495-526556-604598-638

753-821

983-1042

TorqueMin

Nm

170019002100

3200

4500

Preload

kN

315352390

510

625

TorqueMax

Nm

320035004000

6000

8000

Preload

kN

592650740

950

1110

Using Squirter DTI Using Torque Values

Figure 3: Preload Accuracy

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to show a reproducible squirt at preloads between 750and 820 kN. The mean expected would be half waybetween these two limits, around775 kN, safely above the mini-mum of 710 kN, but not too high.Furthermore Applied claimedthat this preload would beachieved in BLIND tests, that is,after Squirter calibration, onlyusing the squirt feature as thedeterminant of correct preload –that is, no feeler gage. Also, GLrequired that we demonstrate tothem that Squirters can controlpreload accuracy to a 97.7%confidence level.

Figure 3 shows the PreloadAccuracy of Squirters vs. the ex-isting “Torque Control” procedure.As an example, for the M42 Prop-erty Class 10.9 bolt, using aSquirter DTI, the preload can becontrolled to between 753 and 821kN, whereas using the torqueingprocedure, preloads as low as510 kN and as high as 950 kNcan be expected. These values

are to be compared to the710 kN (minimum) needed,and not too much more than 800 or 850 kN to avoid

“overtensioning” and “shake-down” problems during the re-tightening of the bolts that is doneafter some months of service.

Figure 4 shows the SquirterDTI compression test setup andwhat the calibrated squirt ap-pearance looks like at the in-tended preload. The field install-ers doing the bolting will continueto use the torque values giventhem by the turbine tower manu-facturers, but at the same timethey will aim to get the squirtappearance shown. If they donot, or if the squirt appearanceoccurs before the torque valueis fully applied, they can modifytheir procedure accordingly: alittle less torque, a little moretorque. Once Squirter DTIs arepart of the QM plan, a superiorbolting result will be obtained,every time.

Figure 4

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