This copy of the thesis has been supplied on condition that anyone who consults it is un

281 Pages·2015·36.66 MB·English

This copy of the thesis has been supplied on condition that anyone who consults it is un-
derstood to recognize that its copyright rests with its author and that no quotation from
the thesis and no information derived from it may be published without the author’s prior
consent.

Survivability of Wave Energy Converter and Mooring
Coupled System using CFD
Edward Jack Ransley
AthesissubmittedtotheUniversityofPlymouthinpartial
fulfillmentoftherequirementsforthedegreeof:
Doctor of Philosophy
SchoolofMarineScienceandEngineering
UniversityofPlymouth
July 2015

Abstract
SurvivabilityofWaveEnergyConverterandMooringCoupledSystemusingCFD
EdwardJackRansley
This thesis discusses the development of a Numerical Wave Tank (NWT) capable of de-
scribingthecoupledbehaviourofWaveEnergyConverters(WECs)andtheirmooringsun-
derextremewaveloading. TheNWTutilisestheopen-sourceComputationalFluidDynam-
ics (CFD) software OpenFOAM(cid:13)R to solve the fully nonlinear, incompressible, Reynolds-
AveragedNavier-Stokes(RANS)equationsforairandwaterusingtheFiniteVolumeMethod
(FVM) and a Volume of Fluid (VOF) treatment of the interface. A method for numerically
generating extreme waves is devised, based on the dispersively-focused NewWave theory
and using the additional toolbox waves2Foam. A parametric study of the required mesh
resolutionshowsthatsteeperwavesrequirefinergridsformeshindependence.
Surface elevation results for wave-only cases closely match those from experiments, al-
though an improved definition of the flow properties is required to generate very steep fo-
cusedwaves. Predictionsofextremewaverun-upandpressureonthefrontofafixedtrun-
cated cylinder compare well with physical measurements; the numerical solution success-
fully predicts the secondary loading cycle associated with the nonlinear ringing effect and
showsanonlinearrelationshipbetweenincidentcrestheightandhorizontalload. Withnear
perfectagreementduringanextremewaveevent,thereproductionofthesixdegreeoffree-
dom (6DOF) motion and load in the linearly-elastic mooring of a hemispherical-bottomed
buoysignificantlyimprovesonsimilarstudiesfromtheliterature.
Uniquely, this study compares simulations of two existing WEC designs with scale-model
tanktests. FortheWavestarmachine,apoint-absorberconstrainedtopitchmotiononly,re-
sultsshowgoodagreementwithphysicalmeasurementsofpressure,forceandfloatmotion
in regular waves, although the solution in the wake region requires improvement. Adding
bespoke functionality, a point-absorber designed by Seabased AB, consisting of a moored
float and Power Take-Off (PTO) with limited stroke length, translator and endstop, is mod-
elledinlargeregularwaves. Thisrepresentsalevelofcomplexitynotpreviouslyattempted
inCFDandthe6DOFfloatmotionandloadinthemooringcomparewellwithexperiments.
In conclusion, the computational tool developed here is capable of reliably predicting the
behaviour of WEC systems during extreme wave events and, with some additional param-
eterisation, could be used to assess the survivability of WEC systems at full-scale before
goingtotheexpenseofdeploymentatsea.
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Acknowledgements
I would like to thank my supervisors: Professor Deborah Greaves, Dr Alison Raby and
Dr Dave Simmonds, for their continued guidance, motivational messages and occasional
sarcasm. Ithasbeenapleasureworkingwithyou.
I would also like to acknowledge: Dr Joe Mellor for his ‘expert’ knowledge of ‘all-things-
computer’, Dr Captain Martyn Hann for putting up with me being "annoying", Dr Colin
"Jandal" Whittaker for not only listening but finding my constant stream of harebrained
lunacy interesting and both Thomas Vyzikas and Scott Brown for numerous OpenFOAM(cid:13)R
brain-meltingsessions.
Finally, I’d like to thank my parents for not putting me down at 30, Dr Keri Collins for all
thosedisapprovinglooks,AlastairReynoldsformakinggames-nighta‘challenge’andboth
PeterArberandConstantinos"Cos"Gitrowskiforsharingthe‘Ultra-Gnarl’withme.
ThisprojecthasbeenfundedbytheEngineeringandPhysicalSciencesResearchCouncil
(EPSRC) via the SuperGen UK Centre for Marine Energy Research (UKCMER) as part of
thethirdphaseintheconsortium’sprogrammeofwork(October2011-September2016).
EdwardRansley
March2015
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Author’s Declaration
AtnotimeduringtheregistrationforthedegreeofDoctorofPhilosophyhastheauthorbeen
registeredforanyotherUniversityaward.
Work submitted for this research degree has not formed part of any other degree either at
theUniversityofPlymouthoratanyotherestablishment.
An advanced doctoral training program supported by the SuperGen Marine UK Centre for
MarineEnergyResearchwasundertaken,whichincludedworkshopsontidalenergy,wave
energy,themarineenvironment,mooringsandreliability.
Relevant scientific seminars and conferences were regularly attended at which work was
oftenpresented. Asleadauthor,onepaperhasbeenacceptedforpublicationinarefereed
journal.
Signed:
Date:
Publicationsasleadauthor :
Ransley, E., Hann, M., Greaves, D., Raby, A. and Simmonds, D. (2013), ’Numerical
and physical modelling of extreme waves at Wave Hub’, in Conley, D.C., Masselink,
G., Russell, P.E. and O’Hare, T.J. eds, Journal of Coastal Research, Special Issue
65,1645-1650,ISSN0749-0208.
Conferenceproceedingsandposterpresentations :
Göteman,M.,Engström,J.,Eriksson,M.,Hann,M.,Ransley,E.,Greaves,D.,Leijon,
M.(2015),’WaveLoadsonaPoint-AbsorbingWaveEnergyDeviceinExtremeWave’,
inProceedingsofthe25thInternationalOffshoreandPolarEngineeringConference,
21-26June2015,Kona,HawaiiBigIsland,USA.
Ransley, E. (2014), ’Extreme Wave Impacts on a Mooring Buoy’, Poster Presenta-
tion: SuperGenMarineAnnualAssembly,28Nov. 2014,Edinburgh,UK.
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Vyzikas, T., Greaves, D., Simmonds, D., Ransley, E., Brown, S. (2013), ’Shape op-
timization of perforated caissons for wave energy harnessing’, in Proc. of the 6th
PanHellenicConferenceofMarineStructures,10-14Nov. 2013,Athens,Greece.
Ransley, E. (2013), ’Extreme Wave Impacts on a Fixed Truncated Circular Cylinder’,
Poster Presentation: SuperGen Marine Annual Assembly, 27-28 Nov. 2013, Edin-
burgh,UK.
Vyzikas, T., Ransley, E., Hann, M., Magagna, D., Greaves, D., Simmonds, D., Ma-
gar,V.,Conley,D.(2013),’IntegratedNumericalModellingSystemforExtremeWave
Events at the Wave Hub Site’, in ’Proceedings of Institute of Civil Engineers (ICE)
Coasts, Marine Structures and Breakwaters 2013: From Sea to Shore - Meeting the
ChallengesoftheSea’,18-20September2013,Edinburgh,UK.
Ransley, E., Hann, M., Greaves, D., Raby, A., Simmonds, D. (2013), ’Numerical
andPhysicalModellingofExtremeWaveImpactsonaStationaryTruncatedCircular
Cylinder’, in Proceedings of the 10th European Wave and Tidal Energy Conference
(EWTEC),2-6September2013,Aalborg,Denmark.
Ransley,E.(2013),’Numericalandphysicalmodellingofextremewaveimpactsona
fixed truncated circular cylinder’, Poster Presentation: International Network for Off-
shoreRenewableEnergySymposium,12-18May2013,Pembrokeshire,UK.
Ransley, E. (2012), ’Validation tests of a wave generation toolbox for OpenFOAM
CFD library’, Poster Presentation: SuperGen Marine Annual Assembly, 29 October
2012,Plymouth,UK.
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