tur In

in M ediu m-Scal e Ice-Structure Int eractions

by

@KurtPatr ickKennedy,B.Eng.

Athesissubmitt edto theSchool ofGraduateStudies inpartialtulfillment oftherequiremen ts (or thedegree of

Muter of Engineering

Faculty ofEngineeringand Appl ied Science MemorialUnivenity ofNewfoundla nd

October,1990

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Experimentalprocedures and resultsfromice indentat ion programscon- ductedbyMobil Oil CanadaatPondInlet(1984)andby Memorial Uni- versity of Newfoundlandon Hobson's Choice IceIsland (1989) aredetail ed.

Bothfield programsutilizeda hydraulicallypowered servo-controlledinden- tationsystemwith aseries ofspherica l and flatindenters.Nominal cont act areasinthe experimentsrangedfrom 0.02 m' to 3.0 m'.Themostsignifi- cant observationin thetesting program swasthe developmentof a dynamic load-ti me trace,Le.,ice-induced vibrati ons,withfrequenciesin thearea of 30 Hz. Alternate pulverizationofice and clearing(extrusion) ofcrushed ice productsfrom the impactzoneissuggested asthe primarymechanismfor the observeddynamics, consiste nt with previousresearch.

The Pond Inletand lee Island dat asets are examinedwithemphasison the behavior ofcrushed iceduringtheextrusion cycleand its importance totheoverall dynamicprocev,including crushedicelayerthickness, impact areareductionby spalling, and dynamicresponsespectra .Crushedicelayer thicknessisobservedto varyacross the impact zone and, togetherwithhighly variant localpressures,suggests thatcrushing and extrusion takes place in localized areas.Thesehighpressure zones, or"hotspots",vary both spat ia lly and temporallyandmakeit difficulttoinfer a pressuredistri butionII.CroS9the entireimpact zone.Rather, the impact area maybe thought ofasacollect ion of small hot-spots(high pressure zones)linked bya matrix ofhighly damaged orpulverizediceunderlow pressure.

Loads and pressuresobservedin theexperimentsaresimulated bytreati ng crushed ice as an incompressible linearviscousfluid duringtheext rusion cycle.'Simulation ofload and centralpressurefor aspherical patchloading caseis discussedin greatest detail and a bodyof data isdevelopedfor an indentationtest which exhibited dynamicbehavior in bothload andcentral pressure.ExamplesoCatwo-dimensional incrementalanalysisCor irregular crushed icelayerthicknessprofilesandextrusion overa three-dimensional ellipt ical contact zone are alsopresented.

Acknowledgements

The author is greatlyindebtedtoDr.Ian Jordaan,NSERC/MobilIndust rial ResearchProfessor in Ocean Engineering. Memori al Universi tyof Newfound - land.forhis helpfu lsuggestions and guidance throughoutthe courseofthis work.A special thanksalso toDr.Richar dMcKennafor hisgenerouseeeie- ranee and numeroushelpfuldiscussionsin variousaspect! ofice engineering . Resultsofthe Pond Inletexperimentswere generouslymade availableby Mobil Oil CanadaProperti es. Theassistanc eof Mr.Wes Abel andMr. Doug Goodridgein thisregar d is gratefully acknowledged. Thecontinuingencour- agement and interestin this and related projectsbyD:-.Robert Frederking, NationalResearchCouncilCanada.andDr.DerekMu&.~eridge.Chairman, OceanEngineeri ngResearch Centre, Memorial University,is greatlyappre- ciated. Thanksalso toMr. Barry Stone, Memorial Univenity,forhishelpful comments and suggestions.

Development andapplicationof viscousmodelsfor crushed ice extrusion owes greatlytothework ofDr MarcA.Maes, Queen'sUniversity. His assistance in allaspects of the modelspresentedher ein is greatly Appreciated.

Financialsupportfor this workwasprovided by the NaturalSciences and Enginee ringResearchCouncilof Canada(NSERC),Pet re-CanadaResour ces Limited , and the Centre forColdOcean ResourcesEngineering(C-CORE).

Tilecourtesyandgeneros ityoftheseorganizations is gratefu llyacknowl- edged.

Contents

Listof Figures List ofTables Nomenclature I Introductionand Scope

iv vii viii

2 Ice-Ind uced VibrationofStructures 6

2.1 FieldObservations ..••....• •. 7

2.1.1 BridgePiers . ., .. . .•. ., . . 7

2.1.2 Lightp iersand Navigati onalAids . . 8

2.1.3 OffshoreSt ructu res. ,.. ... . . 10

2.2 Damageand theIce Cru!hingProcess.. 14

2.3 Crushed lee Beha viour . .. 17

2.4 Indentation and Local Pressure 19 3 Med iu m -Scal eIceInden t at ion 21 3,1 Indentation Appa.ratu s ... .. ... . .. 22

3.2 ThePondInletExperiment!(1984) . 26 3.2.1 TestPlan ... ... .. 26

3.2.2 Main Results 32 3.3 Ice IslandFieldProgram .... ... 37

3.3.1 EquipmentDescription. ..• . . 37

3.3.2 SiteSelection ... ... ... 39

3.3.3 Test Preparation ... ...•. 43

3.3,4 Test Plan •.. . . . .. ..•. 46

3.3.5 Main Results ... . . ... •. 48

3.3.6 Ultr asonic Measurements.•.. 49

3.4 Equipment Performance 53

4 Indent ationAnalysi s 55

4.1 CrushedLayer Thickness. 55

4.1.1 UltrasonicMeasure men ts .. . . .. 62

4.2 Sieve Analysis..•. 63

4.3 SpallingandImpact Area Reduction . .. 66

4.4 DynamicCharacteristics .. 71

4.4.1 TestStart-Time• . . . . 72

4..1.2 Response Spectra . 74

4.5 PressureDistributions .... 78

5 Extrusion Simulat ion 80

5.1 SphericalPatch Model: ViscousFlow... 82 5.2 Variable LayerThickness. • . ...•... . ... 89 5.2.1 Increment al Analysis of IrregularLayers 89

5.3 Three-DimensionalExtrusion ... 93

9 .

97 97 99 .100 . .102 ..109 .... .1l3 .•.. .117 .. .119 ... 123 6 Simulation Resul ts and Disc ussion

6.1 Spherical Patch Loading .

6.1.1 Layer ThicknessEstimates . 6.1.2 CrushedIce Viscosity . 6.1.3 Contact Area Reduction . 6.1.4 Simulat ionResults . 6.1.5 Discussion. .. ... c.2 Variable Layer Thickness..•. •. . .

6.2.1 Discussion .

6.3 3-D Extrusion.

6.3.1 Discussion. ... ... . ..

7 Conclusionsand Recommendations 126

References 128

AppendixA:Pond Inlet(1984) Data Summary 133 AppendixB:lee bland(1989) Data Summary 14.5 App e nd ix C:Approximationfor Small8f 149

iii

List of Figures

2.1 Load-time Records for Hondo and PembridgeBridge Pier Ice-

StructureInteractions 9

2.2 TypicalOffshoreNavigational Structure . . .. 11 2.3 Idealizationof Shear Damage(in Plan) for FreshwaterInden-

tationTest. 15

2.4 Typical"Sawtoo t h" DynamicResp onsefor a Rigid Indent er . 17

3.1 Four-Ac tua torIndent erSystem 25

3.2 Artist'sConception of the PondInlet(1984) IcebergIndenra- tion Experiments .. ... .. .... . . . 2i 3.3 PressureCell Locationsfor Pond Inlet (1984) 0.5m¹and3.0

m²Indenters. . . • . .. .,... . . .. .. 31 3.4 Normalized Pond Inlet(1984) Pressure-AreaRelationship. 33 3.5 TypicalLoad-TimeRecordsfrom Pond Inlet(1984)Indenta-

tion Experiments T4Tl (1.0 m') and T4T3 (3.0 ml ). 35 3.6 Ice Island(1989) IndenterGeometryand Layout of the Pres-

sure Cells. . ... ... . 40 3.7 Test Location and Other Relevant Centresfor the 1989 Ice

IslandIndentation Test Program. . . 42 3.8 Schema ticofHobson'sChoice IceIsland(1989) . . . . 45 3.9 Shaped Multiyear IceTrench Walls (in Plan)Requiredfor Ice

Island Field Program (1989).•.•••... ., 47 3.10 TypicalLoad-TimeRecords for Hobson'sChoice feeIsland

(1989)Indentation TestProgram. ·50

3.11ConceptualArrangementfor Ultrasonic Detection of Oeushed Ice and Intact IceInterface, IceIsland(1989). .. , ., 51 3.12 Hypothe ticalResults of Ultrasonic Measurements... .52 4.1 Typical Indenter Ice Imprint •. , .• . . . . •.•.., 56 4.1 (continued) •. . ... •. . • . . ... ... . . •.. 57

4.2 DigitizedCrushed :ce Layer ThicknessProfiles of the PonJ Inlet{1984} Indentations. . . .. . ..•... 59 4.3 Results of the Ultrasonic Crus hed IceLayerDetectionTrial.

IceIsland (1989)Test NumberNRC01... 64 4.4 CompositeGradation Curveof Crushed IcePart icles from the

IceIsla nd(1989) IndentationTests ... 65 4.5 TheoreticalReductionof ContactAreabySpalling... 68 4,6 Hypot hetical Size Effect ResultingfromSpalling. 69 4.7 ObservedSize EffectfromPondInlet (1984) Data Set. • . . 69 4.8 CenterandProrated AveragePressure:Pond Inle t(1984)1.0

m'TestNumber T2T6.., . 70

4.9 Synchron izatio nofDat a Tracesfor Analysis . . . . .. 73 4.10TypicalResponseSpectra for Medium-Scale Indentation . 75 4.10(continued). .. ... ... ... ... 76 4,11Frequencyve.Indenter Velocity for Pond Inlet (1984)1.0m'

and3.0 m' Tests. . ,.. 78

5.1 Spherical Patch Indentation of anIceMass. . 83 5.2 Incremental Analysisof Irregular CrushedIceLayer. 91 5.3 Two-DimensionalExtrusionBetween FlatPar al lel Plates.,. 91 6.1 SectionofLcad-DleplecementTrace:Pond InletT2T6 (1.0m').99 6.2 Sensit ivityofLoadve.CrushedIceLayerThickness to Crushed

Ice Viscosity. .•. " " .. , . ... ,. ,101 6.3 Sensitiv ityofLoadandPressurevs.Area Reductionto Crushed

Ice Viscosity. " ,103

6.4 Sensit ivit yofLoadandPressurevs.CrushedIceLayerThick- nessto AreaReduction Factor, .. .• •" • • . ..• ,. , .104 6.5 SimulationResults(O.Sm'): Pond Inlet TestNumber 'I'2Tl.•106 6.6 Simula tionResults(1.0m'):Pond Inlet Test Number T2T6 ..107 6.7 Pond InletT2T6 (1.0m'):Tot al LoadandCenterPressure

Traces,,.. ... . .. .. . . .. . .. . ..•108 6.8 ContactAreavs. Indenter Velocity for theSpherical Patch

Simulationof Pond Inlet TestNumberT2T6 (1.0m'). ..112 6.9 Two-Di mensionalIncrementalAnalysis Examples .• • . .. •114 6,9 (continued) ." ., " " . . ,•.• •. . . •,. . .115 6.9 (conti nued) " .... . ....•,.,.••...•116 6,10 Incremental AnalysisSimulatio nResults for IceIslandTest

NRCOB..,,•., .•., •. .. . .•.•.•• . , . .. ,, .118 6.11 Typical Pressu re Distributionsfor 3·nExtrusion Simulation..120

PressureCellsPIThroughP:S. .. . 122 6.13Relevant Load-T imeTrace,PondInlet TestNu-nbcrTIT5

(3.0m~).. ... 123

6.14 3·D ViscousExtrusionSimula.tion Results forPondInlet Test

Number TIT5(3.0m¹) . 12-t

List of Tables

3.1 PondInlet ImpactParameters.... .. 28

3.2 PondInletIndentat ion Test Matrix. .. 30

3.3 Pond InletIndenterInstru menta tion.,• •, . . .. 32 3.4 PondInlet PressureCell Records..•,•. . 36 3.5 IceIslandInden terInstrumenta tion... ...• . . .. .. 39 3.6 TestMatrix:IceIsla ndIndentation Program(1989). ... 48 4.1 Crus hed IceLayer ThicknessDbse rvaticns Derivedfrom Pho-

tographsofthe PondInlet (1984) Indenta tions. 61 4.2 Area. Red uction by SpallingDerived from Photographsofthe

Pond Inlet (1984)Indenta tionTests. . . .,. . ... . . 67 4.3 ResponseSpectraSummaryforthe PondInlet(1984) and Ice

Island (1989)Load-Time Records ,.... . . •... 77 6.1 Pond InletT2T6: Extrusion SimulationResults 110 A.I PondInletPressureCellData... . .. .•..•. . ... .•. 144

vii

Nomenclature

p density ofcrushed ice(kg/ m³) ii velocity field (m/ s) R radius of curvature (m)

p pressure(MPa)

p. dynamicviscosity (MPa·s)

t time (s)

w frequency(Hz)

V gradient vector

,,2

^{theLaplacianopera tor}

Vr radial componentof velocity

v ,

^tangentialcom ponent of velocity

e

crushed layer thickness (mm) Va indenter velocity(m/s) F total load or force (MN) h viscous layer tuickness(mm)

v, indenter velocity

p average pressur e(MPa)

u

averagevelocity in x-dlrection(m m/s )

n number of nodes

o

angle (radians)

0, angle to edge of contact zone e eccentricityof ellipse [( velocityprofile constan t a one-halfellipse majoraxis length b one-half ellipseminor axis length .., crushed icelayer profile parameter h" layerthicknessat center of ellipse R ellipticalarea parameter

viii

· Chapter 1

Introduction and Scope

Economical developm ent oftheoffshoreresources of arctic and sub-arctic regionsisoneofthegreatestengineering challengesofrecenttimes. Frigid tempe raturescoupledwithiceinitsmanyform e makes thearcticoneof the harshestenvironmen t sfor offshore act ivity.Impact of leehas beenrecog- nized as an imp ortantloadingconditi on on arcticoffshore facilitiessince the firstsuch structurewascomm issioned inCookInlet, Alaska inthe 1960's.

Sincethattime,offsh orepetro leum discoveries in the Canadianarct icand east coasthaveledto an increasedinterest intheloa ds genere'edbythe intera ct ion ofice andstructure.

Icetoadsonanoffshorestructuremake take numerousforme. Anice sheetmay failin pure crushing,flexure, bucklin g, splitting,ora combinatio n of these modes. Highest iceleedsaregenerally ma nifested through pure crus hingorpulverizationof the ice sheet intoamassofverysma ll discrete par ticlesnear the contactzone. Theactualfailure mode experienced in anice-st ru ct ureinter action is directl y related to thepropertie softhe ice

an icefeature whichimpacts a vertical circular cylindricalpier at highspeed typicallyfailsinthecrushing mode,whereasaslopingorCOnicalindenter instigatesbending failure(result ingin comparatively lowerloads).

Arct icengineering experiencehas shownthatinteractionsof icefeat ures wit hoffshore structuresareoften accompaniedby regularstructuralvi- brations.Known as "ice-induced vibrat ions",this dynamic behaviourwas prevalentdur ingexte ndedperiodsof crushing failure andwas notconsid- eredthreateni ng to overallstructuralstability.The actua lextent andeffects of full-scaleice-induced vibra tions were clearlydemonstrated in practice in 1986,when theMolikp aqMobileArctic Caisson (MAC)experiencedsevere vibrat ions under theimpact of alarge ice floe Crushingfailure againstthe near-vertical sidesof the MAC createdanextendedperiodofseverevibra- tions and causeda portion ofthe sandcore oftheMolik paqtoliquefy. Struc- turalstability was compromisedby a correspondingloss oflat eral strengt h.

TheMotikpa qincide ntIllustrated theimport anceofice-inducedvibra':~ns inthedesignand operationofarctic offshore facilities.Dynamic,notstatic, loads were recognized aspotent ially critical in ice-struct ureinteractions and may evenrepresentthedesignloading condition.New experiment alprograms were designed andexistingdata setsreassessed inorder to shed light on the ice crushing process and itsroleinthe observed dynam ics.Although small-scale laborator y investigationsareveryimporta nt inthisregard,the tendencyforice pressuresand loads to decreasewith increasing contact area,

known as thescale effect,hampersextrapolationof these results to full- scale interactlcns,Ontheot he rhand, full-scale measu rementprogram s are expensiveanddifficult toco-ord inatefrom aresearchperspecti ve.

In 1984,anambit iousfield progra m wascommissioned by the Hib ern ia JointVentu reParticipants ledbyMobilOilCana daas operator.A seriesof high-speediceindentationtestswereperformed under controlledconditions ona grounded icebergnear thevillage ofPond Inlet ,BaffinIslandinthe NorthwestTeeritoeies,Canada. Theexperiments were originally designed to specifydesign crite ria. for potentia liceberg impacts withproductionetruc- turesinthe Hiberniafieldand to investigatethephenomenonof scale effect.

Contactareas inthetest seriesranged from 0.02 m²to 3.0 m²,such that the experimentsmaybeconsidered"medium-scale'lindent a t lcn tests , asopp osed tolaboratory (ice tank)experim ents or full-scale(Molikpaq)studies.One of the mostinteresting results fromthe PondInletfield programwasthe pres- ence of veryregularvibration sin the high-speedice impacts,accompanied by thepulverizationof iceinto a peatymassof crushe dparticles.Inlight ofthe dynami c activityobser ved ontheMofikpaq,these results represen t an excellentdataset for theanalysisof ice-structur einteraction.Anotherseries of indentation tests was performedbyMobil OilCanada in1986 on multiyear iceinthe Canadian Arctic,butthe resultsarepropriet ary.

In 1987,MobilOilCanad a Prop erties,on behalf ofthe HiberniaJoint Venture Participants, donatedthe ice indenta tionsystemusedin the Pond Inletexperiments to Memorial Un iversity ofNewfoundland. At the same

EngineeringResearchCentre,Memor ial Universityof Newfoundland,on a confidential basis.Direct accessto boththe indentation equipmentandthe resultsof the Pond Inlet test series enabledMemorialUniversityto further investigate ice-induced vibrations. A new ice indentation fieldprogram,held on"Hobson' s Choice"IceIslan d in theCanadian ArcticOceanin the spring of 1989,wasconceivedtocomplementthe Pond Inlet dataset.Thebehaviour of crushedice andlt e role in thedevelopmentof structuralvibrations was identifiedas the thrustoC theresearch program.

Detailsofthe medium-scaleice indentation experimentscond ucted on the IceIsland (1989)andin Pond Inlet (1984)arepresented herein.Ernphesis i!l placed onMemorial Univers ity'sIce Islandprogram Crom a planningand logistical viewpoint, thoughthe PondInletdata set is morecomplete and bettersuited to modelling.Crushedice,which plays a criticalroleinthe observeddynamicsof thehigh-speed teste,is investigatedin detail.Salient featuresof the indemstion zone and observationsofloadand pressureare utili zedin modellingthe thinlayerofcrushediceas a viscousmaterial.

Specifically,the scopeofthis work may be categorizedas follows:

1.Literaturesurveyof fieldobservationsofice-ind ucedvibrations, includ- ing recenticefailure theoriesand thebehaviourofcrushediceproducts.

2.Descr iption ofmedium -scaleiceindentation experimentsperformedin PondInlet (Mobil Oil Canada.,1984) and on Hobson'sChoice Ice b- land (Memoria lUniversityand others, 1989);presentaHo nofthemajor

findings of both testing programs.

3. Generalanalysis oftheinde ntationexperiments withanemphasison observatioosofcrushed icebehaviour, impactareared uct ionby spalling, and dynamiccharacteristics.

4. Simulationof observed loads and pressures based on the treatment of crushed ice as alinear visCQUSmaterial.

5. Discussion ormajor findingsand recommendationsfor furtherresearch and analysis.

Ultimately,thisworkisintended to contributeto the developmentof practical,manageableiceload models fordC!lignpur poses.

Chapter 2

Ice-Induced V ibration of Structures

A general consensus exists in the ice engineering community regarding the dynamic activitythat oftenaccompanies ice-structure interactio ns.Cyclic variations in iceload are attributableto crushingof ice against a structure followedbyextrusionof the crushed ice productsfrom the impact zone8.l:I the indenter(or the ice) advances. This process has been observedin both fieldand laboratory programs and is the subjectof a considerable body of research.

The mechanics of ice-structure interaction are undeniablycomplex and the actual process of ice crushing has yet to be resolved. Extensivemicro- cracking of ice underload is an importantfactor in complete pulverization.

while the behaviour of the crushed ice products governs, in part,the observed dynamics.

2.1 F ield Observations

2. 1. 1 BridgePier s

Incidents of ice damageto bridgepiers inRussia have been documented since the latenineteenth century(Korzhavin, 1962)while the first Canadian reports of such damage involveda.bridge in Quebec (Leonard,1898).Since then, numerous researchprograms have generateda considerable amount of data.on theinteraction of ice with both rigid and flexiblebridge piers. A general trend resulted from large-scale bridgepier observations:actual ice loads were much less than those prescribedby relevantdesigncodes and standards.

An indepth program of ice force data. collection from rivers in Alberta was initiated in 1967 bythe Transportationand Surface Water Engineering De part men t of the AlbertaResearch.Council and representsone of the most complete data sets of thistype. Although data. collection was sparseon most ofthese bridges,recorded ice pressures weremuch legs than designcode specifications. Two bridges monito red during the program,atHondo onthe AthabascaRiverandPembridgeon the PembinaRiver, yielded detailed and importantinformationon the dynamic nature of ice failure by crushing.

Vibrat ions induced by ice crushing were evident throughout theHondo Bridge field program,althoughmost ice failureswere by bending, splitting orsome combination of theseprocesses.Ithas been clearlydemonstrated thatthe dynamicsobservedat theHondo site are not relatedto the natura!

ofthe ice-induced oscillations were in the range of 15-20 Hz, while the nat ural frequency of the pier-loadcellsystemwas initial lyreportedas 100 Hz (Sanden and Neill, 1968)andre-evaluated as 57Hz (Gordonand Montgomery, 1981).

Either estimate precludes resonant effects. The massive bridge piers werealso very stiff,such that theobserved vibration frequencies may be attributed to thefailure of the ice alone.

The Pembridge measurementsystem consisted of a specially constructed vertical steelpile with afundamentalfrequency in the range of12-14Hz (Gordon and Montgomery,1981).It has been demonstratedthat ice-induced oscillations at Pembridgemay havesignificant frequencycomponentsin the 5-30Hzrange, resultingin magnificationofthose frequenciesin the 12-14 Hz range (Michel.1978). Typical load-time traces foreventsatbothHondo and Pembridgeare presented in Figure2.1andthe dynamic featuresareclearly evident.

2.1.2 Lightpiersand NavigationalAids

The vibratoryeffectsof ice interactionhave been well-documented from the experience ofnavigationalaids such as lighthousesand channel markers in Nort hAmericaand, to a greater extent, in Northern Europe.These str uc- turesmay be thought of as vertical indentingpiers cantileveredfromthe seabed.Globalwave and ice loads are minimized byemploying a small di- ameterat the wate r-plane,butthe narrowdiametergenerally results in the crus hingfailure of ice.The cantileverdesignof many of the struct ures fecil .

ZO.6

:;

-; 0.4

t£

~0.2

5 10 15

Time (s)

PEMBRIDGE PIERICE-STRUCTUREINTERACTION (!TOmGordon sndMontgomery,1981)

02~

~."

Q) 0.1

t£

~0.05

o

o

5 10 15 20

Time (s)

Figure2.1:Load-timeRecords forHondo and Pembridge BridgePierIce- Struc tureInteractions(GordonandMontgomtry,1981).

itatesresonant vibrationsunder the continuousattack of ala rge ice sheet or the impact shockof a. discreteice feature.Accelerationsasso ciatedwith the vibrationsareusuallygreatestnear the top of thestructureandmayaffect the perf ormanc eoflantern equ ipmentorcreate an uncomfortableworking environmentonmannedlightpiers.

Numerous navigational aidshave beeninstalledinthe northernBaltic. includinglighthouses,lightpiera(somewhatsmallerthanlighthou ses), and channelmarkers. Atyp icalstructureis illustratedin Figure2.2. Extreme vibrationswere observedby servicepersonnel on the Ncrstrcrnsgrund light · housein the GulfofBothniaunder the act ion ofdrifting ice. A struct ural defectWMnrstsuggested as thecause ofthe severeshaking,butsubsequent analysisindicated that the fa.cility wasresonati ng as itcutthroughthe ice sheet (Engetbre khon,1977).Similarly, severevibrationswere observedinthe Kemi I lightho use,a tubularsteelpile driven intothe seabedinthe northe r n Gulf ofBothni a,undertheattackofeven verythinice sheets.The struc- tureeve ntually collapsedin the springof 1974 during its first operational season ,mostlikely theresult ofan impa ct with a pressureridge (Miittinen, 1977and1975 ). TheoriginalKemi I was replacedwithabottom-founded concrete-caisson tower equippedwitha varietyof instr umentstofurther in- vestigate ice-str ucture interacti ons.

2.1.3 Offshore Structures

Petroleu m exp lorationinarctic and sub-arcticenvironments is a relatively recent endeavour.Examples ofice-inducedvibrationonfull-scaleoffshore

Figure2.2:TypicalOffshore NavigationalStructure (Danys,1977) . 11

structuresare therefore limitedhut demonstratetheim portance of dynamic iceloadsin design.Peyton (1966, 1968 )reportedvibrationsassociatedwith thecrushing failureofice againstthecircula r legsof drillingplatforms in CookInlet,Alaska. Vibrationfrequencywas intherange of 1·10Hz. Sub- seq uentana lys isof CookInletice force data. by Blenkarn (l!J70)con fir med thisso-calle d"rat cheving"behaviourbut disputedPeyton'sclaim thatice exhibitsa.clearly-defined failurefrequency ofapproximatel y1 Hz.

Fieldmeasurementsof dynamicice loadingOila.six-legged,steeljacket platformin theBe- hal Gulf.northern China.,were reported byJizu (1981).

Theplatform,usedfor accommodations and miscellaneousequipment ,Wa.'I

"..•shakenviolently" by driftingice in the area.A representative ice-in duced vibrationfrequencyof approxima tely2Hzwa.aobserved.During the event, anearbyBarejacketcollepeed, laterattribut edtoa piling failure below the mudline(J a nbu et3.1.,1983).

A seriesof indenta.tiontestswere performed onfreshwaterlakeice in the late 1970's/1..9reportedby Kryet801.(1978 ).A1.2meter diameter semi- cylindricalindenter was used to penetrate250 rom thick iceshee ts onEagle Lake,Alberte,In thehighervelocitytest s(penetrationretes approaching 10 mm/ s),a dynamicresponsewas observedduringfailure modestermed

"d uctile flaking"and "brittleBaking".The brittleflakingprocessyielded smallerfragmentsof iceandlowerpeak stressesthan ductile flaking and

occurred exclusivelyin the high-speedtests.

Pilkingtonet al. (1983 ) describedan iceinstrumentationsystemern- ployedonthe vertical-sidedTarsiutcaisson-retainedartificial islandsinthe landlastice zone ofthe Canadian BeaufortSea. Significantloadtranemis- sionsthroug hthe ice rubblefieldsurroundingthe structurewere observed.

High -speed crushingoliceagainstthe structu reorsignificant dynamic activ- itywerenotreported.Full-scaleimpactexperimentswere conductedat Hans Island in theKen nedyChannelwestof Gree nlandin 1980,1981 and1983.

Decelerati onofice floesimpactin gthesmallisland we re recorded and used to determine the globaliccforcesand pressures(Metgeetel.1981; Danielewicz andBlanchet,1988).Althoughforce variationswereobserved, theinterac- tions were complexand exhi bitedseveralmodesoffailure.TheHamIsland testseries was ala ndmark in the developmentof new design methodologies for arctic structures,illustrating thatfull-scaleglobal ice loads weregenerally less than 1MPa.

The most dramaticexample ofice-inducedvibration is thatoftheMo- likpaqMobile ArcticCaisson (MAC),deployedatthe Amauligak 1-65 site inthe CanadianBeaufortSea.While cut tingthrough a 2km! ice floe on April 12,1986, crus hing fa ilure agains,thenear-vertical sidesofthe MAC createdanextendedperiod ofsevere vibranons.Aportion of the sand core of theMotikpaqlique fiedand the lateral stabilityof thestructurewas seriously comprom ised.A completedescript ionof theMolikp aq and adetailed account ofthe ice loadingevent is given byJefferies andWright (Hl88). Ofpar t icular

13

of non-simultaneous failure events which synchronizeintime to producevery regular cyclicload variationsacrossthe entire contact width.

2.2 Damage and the Ice Crushing Process

It is evident fromthe previousdiscussionthat the dynamicsassociatedwith ice-structureinteru:tions are most pronouncedwhen ice failsin the crush- ing mode. The ice crushingprocess is complexandmuchtheoreticalanc' experimentalworkhas been undertakento study the processesthatcause crushing failure.Progressive degradation of ice under loadmay be termed

"damage", andanalysis of the physical behaviour oftheresultingdamaged material is known as "continuum damage mechanics",a relativelynew area of research.Damage mechanics has been recognizedas one of the mosten- couragingmethodeof analysisofthe icecrushingprocess,

Rheologically,ice may be consideredanelastic,creeping solid,yet its pro- nounced britt.leeessresults in a propensityto fracture under load.This be- haviour tends to dominatetheever-presentcree p,particularlyduring raoid indentation (high strain rates) .Triaxialcompression testingis one ofthe most important laboratory tests for iceand has yieldedinteresting andim- portant resultsas tothe behaviour oficeunder load,e.g.,Jones (1982).This approach was furtheredhy Stone et al.(198B) inthe analysisof a seriesof triaxialcompr essionexperimentswhich utilized the degradationof Young's

CrushedIceLayer

~~.,?----

15

Cracked ice Partly Damaged

Undamaged Ice

~

Figure 2.3: Idealization of Shear Damage (in Plan) for Freshwater Iudenta- tlon Test(Jordaan and Timeo,1988).

Modulusas an assessment of dvmege.

Microcracksas damage wereanalyzedbyJordaan and Tirnco(1988) based on ice sheet inden tat ionexperiments reportedby'I'imcc(1986).Under com- pressivestates ofstresswithmoderateconfiningpressures,damage results in a zone of crushedice nearthe indenter and a network

or

very fine micro- cracksfurther back withinthe relativelyintactice. Anidealizationofthe process presentedby Jordaan and Timeo(1988) is reproducedin Figure 2.3 andsuggests thatthe cracksfollow the linesof maximum shear.Jordaanand Timeo (1988)adoptedadefinit ionof <....magewherebypulverization results from a degradation of the shearmodulus.

As theload progreeeeeduringindent at ion, microcrackscoalesce to form discreteice particles which may then movewithrespectto each other.The

productof this progressive degradation iscrushed ice and behaves in a man- nerdist inctly different from thatof the virginmaterial. Crushed teeis squeezed out or extrudedfrom the contact zone as the indenter(or icesheet) advances. Theload buildsuponce again untila new zoneof damagedice pulverizes. In general, a dynamicforce cycle in a high-speed indentationis characterizedby a gradualincrease inloadfollowed by localizedfailure ofthe ice mass,resulting in a sudden drop in load.The load graduallyincreases again duringclearing of the crushedice productsuntil the load is sufficient to pulverize3.new zone,and thecycle repeats .Thisactivity ischaracterized by the classic sawtooth response of Figure 2.4. The very steep load drop is typicalof rigid indenters,i.e.,elastic rebound of both indenterand iceis insignificantand clearing of crushed ice particlesoccurs as the toad increases (Jordaan andTimco, 1988).Ifcontact areaincreases duringthe indentation, a steady increase in loadis also observed.

Itisimportant to notethat crushed ice may not remain a mass ofdiscrete particles during the interaction .Ithas been recentlysuggest ed that sintering or solidificationmay occur withinthecrushed ice zones, yieldinga material capableofsup por ting very high loads. Under highpressures developed in indentation,this material may be considered as a fluidwith very highvis- cosity.Anunderst andingofthebehaviourof the crushed icecontinuum is important in any analysis of high-speedindentation.

~tion RebO~& ^Extrusion

~·~n

IncreasingContact Area

~~

ConstantContaC1Area

Time

Figure2.4:Typi cGl "Sa.wt ooth" Dynamic Response for a RigidInden ter.

2.3 CrushedIce Behaviour

Crushedice asa viscousmateri ..iwasprop osedbyKheisinand Cherepanov (1970) intheanalysis ofthe damagedzone developed in drop-balliceimpact experiments.Hemispherical indenter swithmassupto300 kg were impact ed on afresh watericecover at velocities up to 6 m/s.TLeresult ingshattered icematerial,descri bedas pasty or powdery,developed inathinint ermediat e layerbet ween the indentorandthe intactice mass.A distinctint erfaceex- istedbetweenthis zoneofcrushed iceand theless damaged intacticebelow.

These result s suggestedthathigh-speedimpactsgeneratea distinct ,locaJi zed zoneofpulverized icewit h submicroe-oplcice particl esactingas alubricer-t for the matrixof larger crushedice particles.Followingthe forma.tionof the crushedice layer,the advancing indentercausesfur ther damageandtheex- trusionofthiscrushed ice layer. Kheisinand Cherepanov(1970)suggested

17

thisapproachwaslater adopted by Kurdyumovand Khelsln (1976)inall analysis of Kheisinand Cherepanov's experime nts.Both viscousand plast ic terms were includedin an axisymmetricanalysisof theobserved crushed ice layer,though the plasticity termwaslaterfoundtobe negligibleincompar- ison tothe viscous effects.Inertial forces werenot considered andelastic de- formations were deemed insignificant.Results ofthe model utilized crushed layer viscosities inthe 10 kPa·sto 100 kPa·srange andcompared well to the originaldrop ball experiments. The modelwas notconsideredapplicableto allcases ofice-struct ure interactionand in areview of ship-iceinteractions, Tunik(1989)concludesthat practkalapplicatio nof theKurdyum ov-Kheieln model tofull-scaleinteractions remains limited by a lackofexperimental data.

Assumingcrushing failur e,Nevel(1986)developed anicebergimpact force model basedonviscous theory.The Navier-Stokesequat ionsgoverning the behaviourof crus hed ice as a viscousmaterialwereappliedforthe case of a thin layerbet weenanindentor and intactice mess.Themodellingof crus hed ice as abTanularma terialhasbeen proposedby Hallam,.ndPickering (1988) andutilizes the well knownMohr-Coulombfailure criterion originally de veloped forsoils. The behaviourof crushedice was also studiedinaseries ofexperimentsreported byFinnet al. (1989).Laborat orypreparedcrushed ice wasplaced betweentwo fiatinstrumented platesand theice extr uded or squeezedout from betweenthe plates atvariousvelocities. Asimilar experimentalseries00crushediceextrusion was carriedout by Geotechni-

cal ResourcesLimited(1986). Result s of bot hexperi ment alprogra ms are proprietary,

2.4 Indentation an d Lo cal P r es sure

The design ofstructu ralmembers forarcticoffshorefacilitiesis based on localiceloads experiencedbysmallcontact areas. Remarkablyhigh ice pres- sures(upto 40MPa)developed infull-sca leiceb reakertrials were reported byGlenand Comfort(1983)withthe observationthat thesepressureswere unevenacrossthe contactsurface.Veryhighlocal pressures have also been repor tedduringice inte ractionswit hexploratio nandproductio nstructures intheBeaufort Sea.Theselocalpressures are considerablyhigherthan the laboratory uniaxial compressivestreng thof ice, dueinpart to highconfine- mentin the contact zone. Iyer (1989)refers to theseareas of highlocal pressu reas "hardspots",zones within the global nominal contact area in whicha maximum pressure may occur.The developm ent ofthesezonesof inte nse local pressure makes it unlikely thata uniformpressur e distribution existsacrossthenomi nalcontactarea. Riska (1987) , for exam ple, recog- nized thevaria bilityand irregu larityofice cont actinthedevelopmentofan interaction modelbetween sea ice andan icebreaker hull.

Iyer (1989)makes theconnectionbet ween full-scale observationsof local ice pressuresandcontr olledind enta t ion experime nts .The pressure devel- opedinthe nominal contac tzone ofanindentation test is analogousto the globalice loading condit ionexperie nced in arctic struct ures. Pressure max-

19

im4arisethrou~houtthenominalcontactarea~ultingin anon-uniform pressuredistrib utionacroa. thefaceoftheindente r.Controlled,high-speed ice-structureinteract io nlmaybeth«eforetypifiedbyirregul..r pressuredis- tribut ions and very higb localmexime,Progressive damage.pulverization.

andext rusion ofcrushedice in localizedzonesplay. significantroleinthe obser vedphenomenon.

Chapter 3

Medium-Scale Ice Indentation

Laboratoryinvestigations such as triax ia lcompressiontests or ice tank inden- tations may be generallyclassifiedMsmall-scale experiments.Ext rapolation ofthese data setsto full-scalearcticstructures has provendifficultsince con- tact areas may be hundreds oftimes greate r.Loads derived fromsmall-scale studies have considera blyoverestim atedactualglobal ice loadsobserv edin thefield.Full-scale experimentsonprototypest ructu res would be ideal,but theCO!!tinvolved is often prohi biti ve.Theice indentationsystem commie- sionedby Mobil OilCanadain 1984 and donatedtotheOcean Engineering Research Centre,MemorialUniversity in1987,appears to fillthegap between laboratorytests andfull-scaleprototypestudies. Contactareasranged from 0.02m²to3.0m²and maythereforebeconsidered"medium-scale"indents- tiona.The3.0 m'indenta tion s may alsoapproach contac tareas developed inCull-scaleice-struc tur e interactions.

21

Details of the indentationexperimentsconductedby Mobil Oil Canada at PondInlet,N.W.T. (1084) andbyMemorialUniversityonHobson',Choice Ice Island (1989)are presented hereafter .Theindentationequipmentre- mained essentially thesame forbothexperimentalprograms,althoughin the Pond Inlettestserieslarger indente rs andthe full capacityof thehy- draulic system were used.Interms of contactareaand indenterpower,the PondInletexperiments are more complete than that of theIceIslandtest- ing program.Localpressure measurementsin thelatter testseries arevery valuable, however.Astheauthorwas directlyinvolvedin the planningand logisticsof the 1989lee Island fieldprogram,thistest series is described in greater detail.

3.1 IndentationApparatus

In 1983,GeotechnicalResourcesLimited ofCalgary,Alberta(Geote ch)WM contractedbythe Hibernia Joint Ventu reParticipants(Mobil Oil Canada Limited,operator)to designandconstruct a controlledindentationsystem whichsimulatedthe impact betweena massive icefeature and arigidstruc- ture.Developmentofa systemwhichcouldproducethe requiredloads in a controlledmanner was a formidab letask: the system mustbe powerful yetmanageable. Geotechrecognized thispriorityin theirdesign philosophy for the indenter and concluded thatthe completeSjstem must withstandice impact, haveminimum weight and dimensionsfor mobility purposes,require a minimumofassembly in the field,and be versatilewithfuturetesting

programs in mind(GeotechArctic Services,1985).

The indentationsyst em consistsof a spherical indentingsurface ,actu- ator,and flexible base plate combination mountedon asteelskidofbeam andstrut constr uction.Skidswereconstructedforboththesingle andquad- actuatorarrangement s{or ease of movementin the field. A 1.0 m! back platebehind each actu atordistributedthe load fromthat actua torto a fiat icesurface, ensuringthat ice would not failin that region.Hydraulicpower was provided tothe actuatorsbya massive accumulator bankcapable of deliveringapproximately18,500MN at full capacity[four-actuatorsystem) . Five sphericalindenters with differing radiiofcurvature weremachinedwith nominalsurfaceareee rangingfrom 0.02 m²to3.00 m²•Theepberical ehepe wuselectedforits symme tryand to ensure that icecrushing failurewould be inst igated during indentation.Design ice pressures for the indenterswere establishedbasedon Geotecb'sprevious experienceand pressure-are arela- tionshipsavailable inthe literature .All indentersweremachined from alu- minum block and pressedto the desired radius of curvature, though the large 3.0 m²indenter was constructedfrom welded steelelements .The indenters wereinstru mentedwitha variety of load and pressure cells,the number and positioningof which depended on indenter dimensions.Complete detailsof thedesign of the indenters,actuators,accumulators, skids, andvariousre- lated items are detailed in the data reportprepared for the Hibernia Joint Venture Participants(Geotech ArcticServices, 1985).Thefinal configura- tion of the indentation system is illustratedin Figure 3.1and includes four hydrauli cactuators for the 3.0m'and 1.0 m²indenters ,anda single actuator

23

for the 0.5m',0.1 m'and0.02m'indenter!!.

Indente rmovementwu cont rolled hy closed-loop(feedback)met hodsin whicha test variable (output) iscont inuouslymonitoredand compared to its targetvalue(input ).Closed-loop servo-controlledhydraulic systemsare commonly used inmany aspectsofengineering materialstest ing.Any devi- ation from theinput (command) signal isinterpreted bytheservo-contr oller asacueto drivethe actuator insucha waysoa.sto eliminatethe error betweeninputand output signals.When a correctionis called for,the servo- controllerinstruct!theservo-valve topumpoilin orout ortheact uator to compensate.Indenterdisplacementwasselectedasthe controlvariab le for indentermovement,l.e.,indenter displacement wouldbemonitoredby pots mountedtotheindenterandafixedsurface, andthisoutputsignal usedas thefee<iba.ck parameter.All aspects ortheindentationt.estsweredigita lly contro lledanddatareeeededbycomputer.

25

Figure3.1:Four-Actua torIndenterSystem(Geotech ArcticStroi~$,1985).

3.2 The Pond Inlet Experiments (1984)

Suitable test sites for the proposed indentation experimentsrequiredII.corn- binat ion of large quantitiesof ice and available logisticalsupport.1'0this end,a grounded iceberg near PondInleton easternBaffin Island, North west Territories,Canada was identifiedas theidealsitefor theexperiments. A seriesof horizontaltunnels wereexcavated intothe icemassandthe indenter syst empositionedin atunnel with thespherical indenterand backingplates facing a smoothvertical wall.The complete operationisbest illustratedby the schematicof Figure 3.2,wherethe large 3.0 m²indenter mounted onthe quad-actuator skid hasbeenplaced inatunnel in preparationforatest .The accumulatorskids and data acquisitionhutare inthe foreground.

3.2 .1 Test

Plan

In order to simulate a potentia!icebergimpactwith an offshore struct ure, indentervelocitywas assumedto vary from some initialmaximum to zero, i.e.,to representthe~slowingdown"of aniceberg afterin't.act.Forsafety andotherreasons,displacementand notvelocity was selectedasthe Ieed.

backparam eter,althougha specific velocityprofile was desiredforthe tests.

Thevelocity profileoftheindent er {or alltests at PondInletfollowed the relationship :

v{t)=v,,'cos( wl) wherev: indenter velocity (mm/ s);

v,,:initial velocity (100mm/s{oralltests);

(3.1)

Figure3.2:Artist's Conception

o r

thePondInlet(1984) Iceberg Indentation Experiments (Geolech ArcticServices,1985).

27

Nominal Indenter R z. w T~t ContactArea (mm) (O.l R) (cycles/ s) Lengt h

(m'l (mm) (.J

3.0 2300 230 0.-135 3.61

1.0 1280 128 0.7111 2.01

0.' 900 90 1.11 1.4 1

0.10 '00 '0 ^{:2.5 0.63}

0.02 200 20 ;.0 0.31

Table 3.1:PondInletImpac t Parameters(Geo!ech ArcticServices,1985).

w:freq uency (cycles/.); and f:time(s).

Integra tionof this velocity profile yielded the displacementcommand signa l:

z(~)

=

z. · .sin(wt) (3.2)

where:r.,themaximum penetrationof theindenterinto the ice,wufixedat 10%ofindenlerradius ofcurvature(R).This resultedinasinusoid frequenc y, w. of(v"Iz . )or w=v.{(O.IR) .Relevant impact paramet ers forthetestsare sum m ari zedin Table3.1.

Inde nt at ionleah wereperformed in each of fourtunnel.carved out ofthe grounded iceberg.Table 3,2summarizes thetestin gschedule andinclu de.

the iden tifica t ion lag foreach testutilized througho ut thisdocument. The spherical indentersused at Po-dInlet were instrum ented to varyingdegrees aspresented in Table3.3.Smallpreeeurecellswere incorporated intothe face of the O.S m²and 3.0m²indenters at theloca t ions noted inFigu re3.3.

Other indentershad a singlepressu recell in the centerofthe indenter face.

Thegeneraltest ing procedureas reportedby GeotechnicalResources per- sonnel may besummarizedMfollows{Gectec h Arctic Services, 1985):

L Indentermounted011actuatorand instrumentationinst alled;

2.Indenterskid posit ionedin tunnel,aligned, andhosesco nnect ed;

3. Displacementpotsinstalledand aligned;

4.All cablesconnected,dataacquisition systemsw-rmedUP.andsystems calibrated;

5.Indenter broughtforwardto tunnel wall hydraulically and displacemen t com m and signalprogrammed;

6. Data acquisition started and hydraulicsapplied;

7.When full hydraulicpower attained, command signal triggered and test started.

Followingthe test,the indenter was relocatedand the impact surface exam- ined.Most ofthe impact areas from the Pond Inlet test series were pho- tographedin planwitbaroughscaledrawnonthe adjacent flattu nnel wall.

A cr-Is-sectionofeach impact zonewas cut out of theim pact zone with chainsaws and thisprofilephotographedaswell.

29

NominalIndenter R Tunnel Test Test Contact Area (mm) Number Number 1.0.

3.0m¹ 2300 1 5 TlT5

3 2 T3T2

4 2 T4T2

4 3 T4T3

1.0m¹ 1280 1 4 TlT4

2 5 T2T5

2 6 T2T 6

3 1 T3TI

4 1 T4Tl

0.5m¹ 900 2 1 T2Tl

2 2 T2T2

3 3 T3T3

3 4 'I'3T 4

0.10 m! 400 1 2 TIT2

1 3 TIT3

2 3 T2T3

4 4 T4T4

4 5 T4T5

0.02m¹ 200 1 1 TIT l

2 4 T2T 4

3 5 T3T5

4 6 T4T6

Table3.2: PondInletIndent a tion Test Matrix.

IpOND INLET (1984) PRESSURE CELLLOCATIONS

I

31

O.5m'

FRONTFACE

3.0m'

FRONT FACE ALL DIMENSIONS IN MM

Figure 3.3: Pressure CellLocationsfor Pond Inlet (1984) 0.5 m! and 3.0 m² Indenters (GeotechArctic Services,1985).

Instrument

Loadcell Pressurecell Displ acement pot

Table3.3:PondInletIndenterInstrumentation.

3.2.2 Main Results

As previouslymentioned , the impetus forthe PondInlet fieldprogram was theinvestigat ion oficebe rgcrus hing strengthsfordesignpurpos es and the confirmationof a pressure- arearelationship. Scale effectswereanalyzed in thisregardas reportedbyJohn son and Benoit(1987)and a pressure-area relationshipin which icepressuredecreases withincreasing cont ac t areaWall

recognized (F igure3A ). CompleteIe5ulhfrom the PondInletfieldprogr am suggest much more than theeffects ofscale iniUIice-st ru cture interaction . Themostsign ificant finding was the dyna micnatureofhigh-spe edice im- pact.

Ty~icalfeaturesand major findingsofthe PondInletindentation ex- periment !arepresentedin thissection, As therewereover 20complete indentation testsperform edwith upto 18 datachannelsmonitoredper test, presentationof thewholedata eetwouldquickly fill upseveral volumes.In- stead,only the Iced-time trace s forthe 0.5m",1.0 m'and 3.0m²test sare included in AppendixA.

NORMALIZEOPRESSURE AREARELATIONSHIP PONDINLE:T(1984) 3,0rnaINDENTATIONS

33

1 ....

J" .

~

.

~O~ ~ .,

Z

i

T""."._Te!rtSett TUn".et_Te~t23Tunnel 4_18:2 Tunne l_{Te~ 3}

4 .

_I

°0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Dimensionless Area

Figure 3.4:Normalized PondInlet(1984) Pressure-Area Relationship(J ohn.

son and Be.noit,1987).

[0general, load-timerecor ds (and occasionally pressure-tim e records) fromPondInlet exhi bitdynamicactivitywhich isspectacularin itsregu- lari ty and amplitude.The force-timehistoriesfortypical1.0m³and3.0m² indentationtests are presentedin Figure3.5 and illustratethephenomenon of ice-inducedvibration. In theseexamples,the classic sawtoot hresponseis evidentthroughoutthe entire test length except for tbe initialst ages,where crushingfailure has not yetbeen instigated.Asthe test windsdown andthe indentervelocityapproacheszerofro mits initialvalueof100 mm/s,the fre- quencyofvibrationdecreases. Forexample,vibrationfrequencyin the early stagesofthe1.0m²test T4TI(when indentervelocityis high) is 50Hz.

Near theend ofthesame test, the vibrationfrequencyis 15Hz. Dynamic

response is analyzedin detail in Section 4.4.

Videotaperecords of the Pond Inlettestsshowthe dramatic ejection of crushed ice fromthe impact zoneas the indenteradvances. Photographsof theimpactzonesafterthe testsshow clearly thatadist inct layerofpulverized iceformsduringimpact . Developmentand extrusionofcrushediceis an importantfactor inthe overallinte ract ionprocess.

Pressuresdeveloped in theindentations were generallyhigh.Peak and mean valuesfor both a centrally-locatedpressurecell and the computed averagepressure(i.e., total load dividedby area), are summarized in Table 3.4.Note thatthe0.5 m²and 3.0m²tests utilizeda series ofpressure cells incorporatedalongtheface of the indenter.All other testsutilized a single pressurecell at the geometricalcenter .,f theindenterplate.Onlyone of the four 3.0 m'l testshad an active pressure~11at thecente r oftheindenter .Peak and mean values for all pressurecells for eachPondInlettestare tabulated in Appendix A.

PONDINLET(1984) TES T T4Tl (1.0

m2)

I

^3S

0.8 1 1.2 1.4 1.6 Time(s)

I POND INLET (1984)TESTT4T3(3.0 m

²)

I

3.5

1.5 2 2.5

Time(s) 8

7

Z e

:::!!

:;;' 5 .9

⁴

~ :

1

ooe--"-="=-- -,-- ----,-=-- --::-- -=-=---::----z'

Figure3.5:TypicalLead-TimeRecords fromPond Inlet (19S4)Indentation ExperimentsT4T l (1.0r.t2) andT4T3 (3.0rol).

CenterPressure AveragePressure

Size

'1,,,'

Peak Mean Peak Mean

1m') \.D. (MP.) (MP.) (MP.) (MP.)

0.02 TITl 35 13

T2T4 44 6 9 2

T3T5 50 26 7 4

T4T6 48 13 8 4

0.1 TIT2 37 17 11 6

TIT3 56 27 12 7

T2T3 55 30 23 6

T4T4 56 25 10 3

T4T5 37 11 8 2

0.5 T2Tl 58 19 27 10

T2T2 54 11 7 2

T3T3 56 10 5 2

T3T4 '0 11 25 5

\.0 TIT4 31 6 11 3

T2T5 42 9 7 3

T4Tl 25

,

5 1

T2T6 48 15 10 4

3.0 TITS 60 10 12

,

T3T2 9 3

T'T2 11 3

T4T3 10 2

Table 3.4: Pond InletPressureCell Records.

3.3 Ice Island Field Program

In1987,Mobil Oil Canada Properties,on behalf of the HiberniaJoint Venture Participants.donatedthe ice indentationsystemused in PondInletto the OceanEngineeri ngResearch Centre,MemorialUniversityof Newfoundland.

PrevioustestsatPond Inlet (1984) andin multiyear ice inthe Canadian Arctic(1985) haddemonstratedthe efficiency andthe researchpotential of the system (t he multiyearicedataset from 1985 remainsproprietary). An experimentalprogramwas plannedbyMemorial University ofNewfoundland forthe spring of1989 followingintensiverefurbishingand additional indenter cons t r uction by GeotechnicalResourcesArctic Services,now a divisionof Sand well-Swa n-wocsterEngineering, Calgary,Alberta.

Like previous fieldindent at ion experiments,the emphasisof the 1989 Ice Island indent eticnexperimentswas da.ta acquisition, primarily intendedto investigate recenticefailure theories and irnproveestimates of design ice pres- sures and loads.Fundingfor the program wasprovidedby TransportCanada and CanadianCoast Guard Northern to the level of $620,000. Research groups,governmentagenciesand industrywere involved inthe planning of thetestseriesto reflectthe interestsof allinvolved in arctic operations.

3.3.1 Equipm ent Descrlptfon

The 1989 Ice Island fieldprogramutilized theexisting indentation equip.

ment with minimum modifications, butbudgetaryconstraints restdct ed the system to that of a singleactuator.This precluded the use ofthe large 3.0

37

m²indenterineheproposed testingseries.Asprevio uslydetailed ,theinden- ter systemconsist s essentiallyof one or morehydraulic actuatorsmounted upon a mobile skid.Thehydr aulic system,data acquisitionsystem andtest procedureswere essentiall ythe same asthePond Inletexperiments. For thesingle actuatorsystem, an indenterof specifiedsize, shape, andstiffness wouldbe mountedtothe frontoftheact uator and a fiat back plat eatta ched totherear.Based onthe specificationsof theprojectteam,threesepar at e indenterswereutilized in the field program:

1.0.8m:lspherical indent er: developedfrom the existingl.0 m:l,1.28 mradius of curvature,aluminum indenterbyreducingthe diameter to1.0m, The size reduction wea necessarytocompensate forthe reducedpower of thesingleactuatorsyst em andto limitthepossibility of excessive eccentric loading.

2.0.8 m²circular,fiat,compliant indenter:newlyconstructed basedon Canadian CoastGuardNorthe rn formulae fortheimpact facethick- nessof ship hullplate. This icebreakerhull simulationwas designedto investigatethe pressure distribution behind thecontact zone and pos- sible"bridging effects"resultingfrom ncn-continucue support ofthe indenterface.

3. 0.375m' rectangular, flat , rigidindenter:pre-existingwith dimensions of500 mmx750mm.Noreconditioning was requiredfor thisindenter , commonly referredto as Meganewt. Extrusiontests on crushedice performedby GeotechnicalResources Limited and Memorial University

Indenter Type

39

Instrument Loa d cell Pres sure cell Displacementnot

Tab le 3.5: Ice Island Indent er Instru menta tion.

of Newfoundlandmade use of this indenteras well.

Asumma ryofindenterinst rumentation for the Icebland testseries is present ed inTable 3.5.The spherical inde nter was suppliedwith100 mm diameter pressurecells,while theflat inde nte rsutilized muchsmaller (12.1 mm diameter)cells. Thelayout of thecellsforeachindenter,illust rated in Figure 3.6,was selected forinvestigatio nof thepressure regimeacrossthe indenter face.

3.3.2 Site Selection

Siteselection(orthe mediumscaletests was basedprimar ily on the Avail- abilityof largequa ntit ies

o r

ice andthe degree oflogistic supportthatcould be proc ured forthat site.Withthe involvementof Transpo rtCanadaand CanadianCout GuardNortherninthe proj ect , itwas decided thatmulti- year icewouldbe bette rsuited to the testing program(multiyearice floe s andridges present one ofthe greatesthazards to arctic shipping).Aprelim- inarysurveywas conductedandthe followingidentified as potentialsites for the fieldprogram(see Figure 3.7).

Comp lianl lndenler Area-a.8m3

8C911s @4\ -13

-!~ - ~- 1!. - 1

~

~§

- - - IT

~

:

,.. - -=!.J

- - - - 762- - - -

:---361- --

c-- - -+---.,---,

Meganewl Indenter Area·0.375rna

ALL DIMENSIONS IN MM

Figur e3.6:IceIslan d (1989) Indenter Geomet ry andLa.yout oftbe Pres sure Cells.

1.Hobson'sChoiceIceIsland:a 2.5 kilometerwide,B.Okilometerlong, 45meterthick floating block of ice thatbrokeaway fromthe Ward HuntIce Shel f.northern Ellesmere Island,in 1982.Forthe proposed seaso n ,theIceIslandwaa eitua t ed northwestofEllerRin gnesIsland at approx imate lySO"N,12{)" W.

2. Eureka,Ellesmere Island:historically,an areawiththe potentialfor both icebergandmultiyearice.

3.MouldBay,PrincePat rickIsland: aregion withconsistentlythick multiyear ice nearby.

Availabili tyof multiyearice and iceberg-like ice(in theform

or

^freshwater

shelfice)was assured atHobson'sChoiceIce Island butnotat the other potential sites. Theoffices oftheIceClimatologyDivision of Environment Canada,Ottawa,were visited in early1989toassesstheice conditionsfor Eureka and Mould Bay. Multiyearice was notforecast for Eureka for the prcpcaedtesting periodofMarch-April,1989.Ice conditionswerefavourable near Mou ldBay, but ata. considerab ledistance away fromthe base camp.

Att ention thereforeturned tothe Ice Islandasa.potentialtest site.

AlthoughHobson'sChoicelee Island is primarily composed of freshwater shelfice, significa nt amounts ofthick (up to 10 meters)multiyearice are at- ta.ched to theshelfice core.Since196~,the PolarContinentalShelf Project of Energy,Minesand ResourcesCanada, or"Polar Shelf" ,has maintaineda basecam p onthe Ice Island.ScientificexpeditionstotheIce Island are ccor-

41

Indentatio n TestProgram.

dinatedthrough the Polar Shell basein Resolute,Northwest Territories,and extensivelogistical support is made available to researchparties,including accommodations,meals,airtravel arrangements,and useofheavy equipment (snowmobiles,bulldozers,etc).With the involvement of Polar Shelf,itwas evident tha t theIce Island offeredthebestcombina tionofice availability andlogist ic al support.

Traveland accommodationof field personn elwere quicklyandeasilyer- ranged through Polar Shelfin Resolute. The indenter was to be shippedto the Ice Islandvia Resoluteon several HerculesCt3S Rights, thoughavail-

ability of Herculesaircraftfortheproposedtesting period waslimitedand schedulingwascritica l.A specialairstrip alsohadtobe constructed onthe island to accommodat e theHercules aircraft. PolarShelf agreedto provide theairstrip atno cost to theprojectand workbegan inearly Mar ch of 1989.Personnel from San dwell-Swan-Woostertravelledto the Ice Island in advance oftheindenter to prepare a test siteand aid in preparatio n of the airship.Grounddrifting kept theairstripsnowedin forseveraldaysand theHercule s flights werepost poned pendingsnowdearingoperationsand suitableweatherconditions. Theindente rand supportingequipmentwas eventua lly transported totheislandbytwoHerculesflightson April9, 1989, Siteprepar ation and syst em assembly comm enced immediat ely, aswell as thorough checkson thecomp uterdataacquisition system.

The original test plan foresaw thea.rrivalofmost research personnel just prior to the first indenta tiontest. The delays caused bywea therconditions andsnowclearingoperat lonawarrantedre-scheduling oftravelarrangements for these personnel,butthisprovedimpossible.The limitednumber of com- mercial flights toResolute andthedifficulty in changingreser vations onshort noticesaw mostresearchpersonnel onsiteseveral days beforethe arrivalof the indenter. This additional manpower wasputto usein refiningthe test plan,icecharacterization,and restockingthe IceIslandfueldump.

3.3.3 TestPr ep arat ion

indent ation experimentswereplannedin multiyearnear theedgeof theice island withthe possibility of someadditional experimentsinthecentr al fresh-

43

tiyea rice was ofpriority and provedtobe the most time consuming and labour intensive operation of the project.A single trenchwas Iavou rcdon the notion that theindenter systemcouldbe loweredinto one end of the trench and, followinga test,toweda short distancealong thetrenchtothe next testface.

A suitabletest site inmultiyearicewaaloca tedabout one kilomete rsouth- east of the beeecamp, clearedofsnowand levelledwitha.bulldozer prior to the arrivalof the indenter and support ingequipment. Alargeheated tent was erectedat the test siteto allow preparat ion oftheindenter and other equipmentin a sheltered, com fortabl e environment.A smaller heated data acquisitiontent weeplaced on skidsfor ease of movementa.8the tests progressedalongthelengthofthetrench.A heated enclosure wasalso con- structed forthe accumulatorskid. Locationofthe site andother feat ures of the IceIsland are illustrate d in Figure 3.8.

Numerousice coresweretaken onthe proposedtestingsite andtemper- aturemeasurementsindica teda proj ectedice thick nessof approxima tely10 to 11 met ers,leaving6 to7metersofice below the final projecteddepth of the trench .A modified"DitchWitch"trenchingunitwasutilized to cuttwo par aHel slots 1.5 meters deep, 100 meters long,and4 meters apart. The area between theseslots was rakedwith a rackof ice rippingtongsattachedto the backofa bulldozer, and the broken icepiecesremoved {rom thetrench witba small front-endloade r.Atadepth of approximately1.2 meters,slots

MYll:MultiyearLandfastSeaIce

~;~~fnU,'~tt~~~~et~\~~:~~"~~~~~~hetf^and MYPI:MultiyearPackIce

Anachad andconsolidatedtothe she"Icecore sincecalving01theleeIsland.

Figure 3.8:Schematicof Hobson'sChoiceIce Island (1989)

for thesecond lift of the trench were cut by the Dit chWitch and chain saws, followedbyfurth erripp ing and clearing. The 100me ter long trenchwas excavatedtoanaveragedepth ofapproximately 4meters in 5 days.Plans for a smaller test seriesin th*"freshwat ershelfice were puton bolddueto timeandlabourconst raints.

Trenchwalls were roughly squared and smoothe d withchainsaws and ice chisels. Specific test areas were outlinedon the wallsinregions free from crackingandthoseareas weremachinedto a verysmoothfinish with a verticallymountedcircularsaw.The walloppositetheteet facewas also machinedand parallelism ofthetwofacesensured. Duetothe limited load capacity ofthe singleactuatorsystem,it wasnecessaryto shape the impact

45

iceface forthe flat indent er tests(see Figure 3.9).Failurebycrushingand a progressiverise in load with incre asing contactarea were thenachieved.

3.3 .4 TestPlan

Thefollowingwererecognizedas areasof prim ar y research int erestin the indenta.tion test program:

1.Velocityof Impact: constantvelocitiesfrom 1rom/sto100 mmj swe re adoptedto explorereteeffects and the transitionfrombrit tle to ductile failure.

2.Indenter Size and Local StructuralStiffness:indentersof varyingsize (up to0.8m')and stiffnesscharacteristicsemployed,togethe rwith smellpressuresensors, to investigatelocal ice pressure variatiolls.

3. Characterizationof Ice and FailureZone: includingcrushedicelayer anddam age zonethickness, failure modes,crackdensities,sieve anal- ysis,video records, and ice physicalproperties.

Based on the requirementsof the participantsandthe primary researchin- terestsnoted above, the test matrix of Table3.6was developed.Thismatrix has been updated to reflectthe actualindentervelocitiesobtained inthe field.Thetestin g procedurefor the Ice Island testseries was essentiallythe same as thatfollowed in the PondInlet experiments.As the systemhad not beenutilizedsince 1985,thereweresomest art- up problemsand minor delays inthefield. A feedbackproblem delayedthe testingbyseveraldays but oncerectified ,testingproceeded as planned.

4.

8~ ... ^{T ren c h 8} 1

^{120 ,}

^NRC06

a:z _{wa,, ~}

Zt;

~~ _e:O

^{';// /~}

^;.:;

Tre~;~ ~07

,g~

g2

^/.//

N ~ ^,

^/~

^{/ ICE}

^//~/.«

Tr.~~ , 500~

^{200, 500}

~ ^NRCOB

~~ ^ICE

^%

NRC09

Trench

W''' \1

²⁵⁰

1

⁴⁰⁰¹

2:,1- .r,

~ _ICE

_%

U>

Tra~;~ r

³

°01

²⁰⁰

r

³

°°1 ^{NRC1 0}

~~ ICE / ALL DIMENSIONS IN MM

Figure 3.9:ShapedMultiyear IceTrenchWalls(inPlan)Required forIce Island FieldProgram (1989).

47

Test Test Indenter

7~~C;~)

^IceWall

Number LD. Type Shape

,

ABCOI spherical 0.24 flat

,

NRCO'

s.s

""

3 NRCO' 120 'al

4.1 NRC03 6.' fial

4.2 NRCO·' 17 'al

s

NRCO' 9 'al

6 NRC06 flexible 19.3 1:3 slope

7 NRC07 78.9 1:3 slope

8 NRC08 meganewt 76.9 1:5 slope

9 NRC09 10 1:5 slope

10 NRClO 36.7 1:5 slope

Table 3.6: Test Matrix: Ice Island Indentation Program (1989).

3.3.5 MainResults

Likethe PondInlet experiments,the volume of data collectedduringthe IceIsland fieldprogram is immense. Again, onlythe major findings and some generalfeatures of thedat a setwillbepresented in this section.Com- pleteload-timedata tracesare included as Appendix B for those tests which exhibited dynamicact ivity.

As observed in thePond Inlet experiment s,the IceIslandindentation testsexhibitedregular dynamic(sawtooth) behaviour,particularlyin the high-speed tests. Severalexamples ofthe load-timerecords are presentedin Figure 3.10and have vibrationfrequencieson the order of 20Hz to 40 Hz.Of particular interestin the IceIsland data setwas the pressur evariationacross

theface ofthe indent erduring thetest.Numeroussmallpressurecells were incorporated into the indenter plat e forthis purpose. The resultsindicate that localpressuresmay he ashigh as 80MPa andarehighlyvariableacross the indenterface, I.e.,asimplepressuredistributionor one ofconsistent pattern is not evidentfor the entireimpact zone.

Theuse of asingleactuator in theIce Islandexperiments raises quest ions on thedegree ofeccentric loading inthe tests. Considerablecare was taken to ensureproperalignmentof theindente r inthe trenchand parallelismof thetrenchwalls.Still, late ral movementof the indenterwas observed in several tests, most likely theresult of spalling near the edge of theimpact zone.As well,targetvelocitieswere not alwaysachieved dueto stallingof u,...;~,,:Ieactuatorsystem.

'\.1,1', Ultrasonic Measurements

To-e variation ofcrushedicelayer thicknesswith timeandthe actual thickness of thelayerduringindent ationarevery import ant in ice-stru ctu re interaction.

In an attemptto acquiredataon thesevariations,theIce Islandexperlrr-mta utilized an ultrasonic measurementsystem on a trialbasis. Theoriginal conceptualizationisillustrated inFigure 3.11. The ultrasound probe,with transmitterandreceiverin a single unit, was placedin a. small borehole at alevel equalto thatofthecente rof the indente r. Ethyleneglycolwould act as a couplingagent betwee n the probeand theice. The boreholehad to bepositionedfarenoughbacksothe probe wouldnot be damaged bythe advancing indenter.Theoreti cally, the ultrasonicpulseswouldbereflect ed

Time(s)

0.2 0.4 0,6 0.8 1 1,2 1.4 1.6 1.8 Time(s)

Figure 3.10:TypicalLoad-TimeRecordsfor Hobson's ChoiceIceIsland (1989)Indentation TestProgram.

ToD.A. System

Figure3.11: ConceptualArrangement forUltrasonicDetectionof Crushed Ice andI-..ctIceInterface,Ice Island (1989).

backto the probe by the interface existingbetweenthe highly damagedice layerneartheindenterand the int act ice further back.Thedistancefrom the probeto the near surface of the crushed zonewould then be known in time.With themovement of the indenterrelative to the probealsoknown, the thicknessof the crushed ice layer at any time duringthe test could be approximated.An idealizedsituation isiIIustra.ted inFigure3.12.

Initialtestswith a porta bleultrasoundunit wereeondccted on laboratory grownice atMemorial Universityof Newfoundlandto establisl, the feasibility of the proposedmeasurements. The unit, designed for non-destructivetesting of engineeringmaterials, was rented from Eastern TechnicalServicesLimited, St. John's.NF and arepresentat ive waspresent forthe tab trials. The

51