vibration-impact cutting of submerged frozen clay in an arctic trenching process

122 Pages·2016·9.86 MB·English

VIBRATION-IMPACT CUTTING OF
SUBMERGED FROZEN CLAY IN AN
ARCTIC TRENCHING PROCESS

SHAKING THINGS UP IN THE TRENCHING BUSINESS

In partial fulfillment of the requirements for MSc. Offshore & Dredging engineering – Delft
U niversity of Technology
By Robbin Thomas Motzheim
ii
PERCUSSIVE CUTTING OF PERMAFROST IN A
MARINE ENVIRONMENT
SHAKING THINGS UP IN THE FIELD OF ARCTIC TRENCHING
PERCUSSIVE CUTTING OF PERMAFROST IN A
MARINE ENVIRONMENT
SHAKING THINGS UP IN THE FIELD OF ARCTIC TRENCHING
Thesis
inpartialfulfillmentoftherequirementsforthedegreeof
MasterofScience
inOffshore&DredgingEngineering
attheDelftUniversityofTechnology,
tobedefendedpubliclyonWednesdayMay25,14:30.
by
RobbinThomas MOTZHEIM
MSc.Offshore&DredgingEngineering
borninGroningen,TheNetherlands
Thesiscommittee:
Supervisor: Prof.Dr.Ir.C.vanRhee TUDelft
Thesiscommittee: Dr.Ir.S.A.Miedema TUDelft
Dr.Ir.M.AlvarezGrima RoyalIHC
Dr.Ing.R.Plat RoyalIHC
Dr.Ir.P.R.Wellens Deltares
Keywords: Arctic trenching; Percussive cutting; Vibration cutting; Vibration-
impactcutting;Permafrost;Frozenclay;Marineplough
Printedby: RoyalIHC
Front: IHCEngineeringBusinessPL3PipelinePlough
Copyright©2016byR.T.Motzheim
ThisthesisisconfidentialandcannotbemadepublicuntilDecember31,2016
Anelectronicversionofthisdissertationisavailableat
http://repository.tudelft.nl/.
v

ABSTRACT
ToprotectpipelinesinArcticregionstheyareoftenburiedintrenches,effectivelyprevent-
ingdamagebyicegouging.WhentrenchingisperformedinArcticsoil,subseapermafrost
maybeencounteredatwaterdepthsupto100m,mainlywhileexcavatingclaysoil.Frozen
clayisananisotropicinhomogeneousmaterialwithatensilestrengthandbehaviorcom-
parabletosoftrock.
Previousobservationsofcuttingexperimentsonsubmergedfrozenclayindicatedthatthe
dominantfailuremechanismistensilebrittle. Withanincreasingthicknessofthelayer
cut,theincreaseofcuttingforcesseemstobelessthanproportional. Anexplanationfor
this trend is the size effect, which describes the probabilistic weakening of the material
whenthethicknessofthelayercutincreases. Anoptimalthicknessofthelayercutcould
resultinlowerspecificenergyconsumption,howeverthismayleadtohighercuttingforces.
Inordertoexcelinthedesignandconstructionofefficientsubseaexcavationequipment
(e.g. subsea trenchers) thorough knowledge of physical characteristics of permafrost is
mandatory. These characteristics include the failure mechanisms, the required cutting
forcesandpowerconsumption. Duetothebrittlebehaviorofthefrozenclaytheuseofa
vibration-impactmechanismhasbeeninvestigated.Aliteraturereviewidentifiedthatthe
knowledgeonvibration-impactcuttinginpermafrostisverylimited.ThereforeRoyalIHC
andDelftUniversityofTechnologyjoinedforcestofillthisknowledgegap.
Themaingoalofthisresearchistoidentifythegoverningparametersin(vibration-impact)
cutting of frozen clay. Knowledge of the effect of vibration-impact cutting leads to the
abilitytocalculatecuttingforcesinsimilarconditionsandtodeterminetheparameters
requiredfordesigningeffectivesubseaexcavationequipment.
Inthisstudyapredictionmodelofthevibration-impactcuttingtoolandsoilisdeveloped.
A specialized large scale vibration-impact cutting setup was designed and constructed.
Thecuttingexperimentswereperformedonstiffclayhavingsimilarpropertiesofthesoils
foundintheArctic. Theresultsfromtheexperimentswereusedtovalidatetheprediction
model. Arelationshipbetweendrawbarforcereduction,velocityratioandforceratiohas
beenfoundandiselaboratedtodesignparameters.
vii

CONTENTS
Abstract vii
Nomenclature xiii
ListofFigures xvii
ListofTables xix
1 Introduction 1
1.1 Contextoftheresearch . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Problemdescription . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.3 Problemdefinition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.4 Researchflowdiagram . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2 TrenchingintheArctic 3
2.1 Arcticoil&gasreserves . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Arcticpipelinerisks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.1 Seabed/icegouging . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.2 Mitigationmethods . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.3 Strudel/icescour. . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3 Currentoffshoretrenchingpractice. . . . . . . . . . . . . . . . . . . . . 5
2.3.1 Trenchshapes&sizes . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3.2 Subseaploughing . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3.3 Specificcuttingenergy. . . . . . . . . . . . . . . . . . . . . . . . 6
2.3.4 Towingvesselefficiency . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.5 Specificenergyofcuttingprocessvs.totalsystem . . . . . . . . . . 8
2.4 Permafrost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.1 Formationofsubeapermafrost . . . . . . . . . . . . . . . . . . . 8
2.4.2 LocationofsubseapermafrostintheArctic . . . . . . . . . . . . . 9
2.4.3 DepthoftheArcticsubseapermafrost. . . . . . . . . . . . . . . . 9
2.4.4 Typesofsubseapermafrost . . . . . . . . . . . . . . . . . . . . . 10
2.5 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 Clay,frozenclayandice 11
3.1 Mechanicalpropertiesofclay. . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.1 Unfrozenwaterinfrozenclay . . . . . . . . . . . . . . . . . . . . 11
3.1.2 Freezingprocessofclay . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.3 Conclusionsonclayandthefreezingprocessofclay. . . . . . . . . 13
3.2 Mechanicalpropertiesoffrozenclay . . . . . . . . . . . . . . . . . . . . 13
3.2.1 Compressivestrengthoffrozenclay . . . . . . . . . . . . . . . . . 13
3.2.2 Tensilestrengthoffrozenclay . . . . . . . . . . . . . . . . . . . . 15
3.2.3 Internal&externalfrictionangleoffrozenclay . . . . . . . . . . . 16
ix
x CONTENTS
3.2.4 Salinityeffect . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2.5 Conclusionsonmechanicalpropertiesoffrozenclay . . . . . . . . 16
3.3 Mechanicalpropertiesofice . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3.1 ISO19906 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.3.2 Tensilefailureandtensilestrength. . . . . . . . . . . . . . . . . . 17
3.3.3 Compressivefailureandcompressivestrength. . . . . . . . . . . . 17
3.3.4 Conclusionsonmechanicalbehaviourofice . . . . . . . . . . . . 18
3.4 Suitabilityoffrozenremoldedclay . . . . . . . . . . . . . . . . . . . . . 18
3.5 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4 Vibro-impactcutting 19
4.1 Vibro-impactcuttingasanimprovement. . . . . . . . . . . . . . . . . . 19
4.1.1 Vibro-impactcuttingexplained . . . . . . . . . . . . . . . . . . . 20
4.2 Previousresearch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2.1 Landbasedploughing . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2.2 Drilling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.2.3 Cuttingofrockysoils. . . . . . . . . . . . . . . . . . . . . . . . . 23
4.2.4 Cuttingoffrozensoil. . . . . . . . . . . . . . . . . . . . . . . . . 24
4.3 Vibrationtypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3.1 Harmonic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.3.2 ImpactingI&II . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.4 Parametersofinterest. . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.4.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
5 Rockcuttingtheories 27
5.1 Cuttingtheories. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5.1.1 Miedema’scuttingtheories . . . . . . . . . . . . . . . . . . . . . 27
5.1.2 Teartype. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
5.1.3 Materialdiscontinuitieseffectonthecuttingprocess . . . . . . . . 33
5.2 Highstrainrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.3 Permafrostcuttingtheories. . . . . . . . . . . . . . . . . . . . . . . . . 34
5.3.1 Liefferink’sWeibulltheorem . . . . . . . . . . . . . . . . . . . . . 34
5.3.2 Miedema’sBetatheorem. . . . . . . . . . . . . . . . . . . . . . . 35
5.4 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
6 Fracturemechanics 37
6.1 Fracturetoughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
6.1.1 Fracturetoughnessofice . . . . . . . . . . . . . . . . . . . . . . 37
6.1.2 Fracturetoughnessoffrozensoil. . . . . . . . . . . . . . . . . . . 38
6.1.3 Comparisonoffrozenandunfrozenclayeysoils. . . . . . . . . . . 39
6.2 Crackstability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
6.2.1 Crackstabilityofice . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.3 Crackpropagationspeed . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.4 Impactstrength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.5 Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42