St Helena Marine Outfall Project No. 176130 Technical Note

WaylandEngineeringcc

LuckyStar:StHelenaBay

TechnicalNote:MarinePipelineStability&HydraulicAssessment

DocNo:WAYLAND/176130/TN/001 Revision:0

Date:29/11/2017

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StHelenaMarineOutfallProjectNo.176130

TechnicalNote

StabilityDesignandHydraulicAssessmentofnewMarinePipeline

LuckyStar

Rev.No. Date ReasonforIssue Preparedby Verifiedby Approvedby

0 29/11/2017 PreliminaryDesign DP MlR DP





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TableofContents

1 INTRODUCTION.......................................................................................................................3

2 EXTREMEWAVE&CURRENTCLIMATE.....................................................................................4

2.1 EXTREMEWAVES........................................................................................................................4

2.2 EXTREMECURRENTS....................................................................................................................7

3 PIPELINECONFIGURATION......................................................................................................8

4 PIPELINESTABILITYDESIGN...................................................................................................10

5 HYDRAULICASSESSMENT......................................................................................................11

6 REFERENCES..........................................................................................................................15

ANNEXUREA:GENERALLAYOUT–MARINEPIPELINE...................................................................16





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1 IntroductionOn 24May 2017 theWestern Capewas declared a provincial state of disaster as a result of the

magnitudeandseverityofdroughtaffectingtheprovince.Thedrasticmeasurewasinresponsetothe

currentdroughtcrisis,theworstsince1904.

OceanaGroupLimitedhastwolargecanningandfishmealprocessingfacilitiesontheWestCoast.The

operationsarehighlydependentonwaterforsteamgeneratingwhichisrequiredforpilchardsand

industrial fishprocessing.Thetwofacilitiesemployatotalof2000employeesfor its landandsea

basedoperations.LuckyStarOperationshasimplementedseveralinterventionstocomplywiththe

municipalities request for a 30% reduction in consumption. Engagements with the Saldanha Bay

Municipality and Berg River Municipality have led us to conclude that the municipal emergency

intervention plans will not be completed in time to avoid DAY ZERO (anticipated May 2018)

consequences.

Duetotheabove,OceanaGroupproposestoimplementthefollowing:

• Constructa seawaterdesalinationplantsatStHelena factory,ofwhich thebrine (effluent

stream from desalination plant) will be co-discharge with the existing effluent from the

factory.

• Installanewmarinepipelinefromtheexistingjettytoapproximately10mwaterdepthin

ordertodischargethecombinedeffluentstream

AnchorEnvironmentalhaspreviouslyconductedastudywhichmodelledthedispersionoftheeffluent

andassesstheenvironmentalimpactofadischargepointat10mwaterdepthWaylandEngineering

cchasbeenappointedbyOceanatodesigntheweightingrequirementsofthenewmarinepipeline

toensurestabilityandconductahydraulicassessmentofthenewpipeline.Wayland’smethodology

toconducttheabovecomprisedthefollowing:

1. Determinetheextremewaveandcurrentclimatebasedonexistinginformation;

2. Recommendthemost suitablemarinepipelinediameterandmaterial, taking intoaccount

existing infrastructure, hydraulic and operational performance of the system, installation

loadsandmethodology;

3. Determinerequiredpipelineweightingtoensurestability(permanentdesign);and

4. Desktophydraulicassessmentofnewpipeline.

Thistechnicalnotesummarisesthefindingsoftheabove.





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2 ExtremeWave&CurrentClimateEstimatesofextremewavesandcurrentsarerequiredforthecalculationofpipelinestabilityonthe

seabed. The scope of the present study excluded any numerical modelling or measurement

campaigns,thereforeexistingdatasourceshavebeenadopted.Duetothelimitationsofthestudy,

cautionhasbeentakentoerronthesideofconservatism.

2.1 Extremewaves

TheproposedpipelinewillbesituatedinthewaveshelteredembaymentofStHelenaBay.Thewave

climate for theBayhasbeenproducedusing thenumerical codeSWAN (SimulatingWAves in theNearshore)aspartofaCoastalVulnerabilityAssessmentforSouthAfrica(Theronetal.,2014).Sixteen

yearsoftheoffshorewaveclimatewererefractedintothenearshoreviaanestednumericalgridding

approach.Theoutputresolutiononthe7mcontour,indicatedinFigure1,was500m.Thereanalysed

wavemodelproduct,providedbytheNationalCentreforEnvironmentalPrediction(NCEP)wasused

forboththeoffshoreswellwaveandtheuniformwindfieldforcing.

InFigure1theextractionpointutilisedforthepresentanalysisisindicated.Thispointwaschosenas

aconservativeestimateasitisslightlylessshelteredfromtheincomingwaveclimatethantheother

modeloutputlocationsincloserproximitytothepipeline.Thisconservativeapproachwasadopted

astheclosestmodeloutputlocationstothelocationofinterestareinfactmoreshelteredthanthe

locationofinterest.





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Figure 1: Location of pipeline and themost appropriate availablewave climate extraction point on the 7mcontour.

InFigure2thedirectionalwaveroseoftheextractionpointisprovided.Thisdatasetwasalsousedfor

theExtremeValueAnalysis(EVA)presentedinFigure3andError!Referencesourcenotfound..

Figure2:Waveroseindicatingthepredominatewavedirectionandintensities.Thecoloursscaleissignificantwaveheight(Hm0)inmeters.





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Figure3:ExtremeValueAnalysisfortheextractionpointwaveclimateindicatedinFigure1.Indicatedaretheextremesignificantwaveheight(Hm0)returnperiods.

Table1:ExtremeValueAnalysisfortheextractionpointwaveclimateindicatedinFigure1.Indicatedaretheextreme significantwave height (Hm0) return periods. The return periods are also provided as a function ofdirectionalsectors.

EXTREMESTORMRETURNPERIOD

DIRECTION Events Events/

Year

1:1 1:5 1:10 1:25 1:30 1:40 1:50 1:100

ALL 684 40.691 2.38 2.77 2.92 3.12 3.15 3.21 3.26 3.4

N 636 37.836 2.35 2.72 2.87 3.06 3.09 3.15 3.19 3.33

NE 14 0.833 1.15 1.72 1.97 2.31 2.38 2.48 2.57 2.82

NW 19 1.130 1.16 1.60 1.74 1.9 1.93 1.98 2.01 2.12

InFigure4ascatterplotoftheentirewaverecordofsignificantwaveheightsvspeakperiodsare

provided. The 1:100, 1:30 and 1:5-year significantwave heights are also indicated. In a thorough

analysis,theEVAofthewindseasandswellshouldbetreatedseparately,especiallybecauseboth

componentscontributedtotheextremewaveheightanalysispresentedabove.Aconservativerange

ofextremepeakperiodsarethussuggested.Largerperiodwaveswillproducehighernear-bottom

currentsactingonthepipeline.Itisthussuggestedthatavalueof18sbeusedasaconservativepeak





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period estimate in the calculation of orbital velocities for the pipeline stability. The directions

associatedwiththeseextremewaveconditionsarepredominantlyfromtheNNW(refertoFigure2).

Figure4:Scatterplotofextractedwaveparametervaluesindicatingbothwindseasandswell.

2.2 Extremecurrents

Theestimationofextremecurrentsat the study site is complicatedby the fact thehydrodynamic

modellingfallsoutsidethescopeofthisproject,aswellasduetothelackofcurrentmeasurements

near the locationof interest. Typical currentswithin StHelenaBay canhoweverbe inferred from

variousmeasurementcampaignstothenorthofthestudyareae.g.Fawcettetal.(2008),Lucasetal.

(2014).Fromthesedataonecandeducethatmaximumsurfacecurrentsaretypically<0.7m/s,where

maximumnear-seabedcurrentsaretypically<0.3m/s.Near-seabedinthiscasecanbeassumedto

bearoughestimateforcurrentsinthebottom3mofthewatercolumn.Itisanticipatedthatallowing

fora0.4m/scurrentinconjunctionwiththevelocitiesduetoextremewavespresentedaboveshould

allow for sufficient conservatism in the pipeline stability calculations. Note that this high-level

allowanceexcludesanyinfluenceduetowavedrivencurrents,whichwouldbecomeimportantinthe

surfzone.





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3 PipelineConfigurationPresentlytheeffluentfromthefactoryisdischargeviaapipelinewhichextendsfromasumponland

totheendofanexistingjetty.Thetotallengthoftheexistingpipeline(measuredfromthesumpto

theendofjetty)is135m.TheexistingpipelineconsistofuPVCwithaninternaldiameterof300mm.

AschematicoftheexistingpipelineisillustratedintheFigurebelow.

Figure5:Schematicofexistingoutfallpipeline

ItisproposedtoreplacetheexistingpipelinewithanewpipelineconsistingofanewuPVCpipeline

onshoreandanewoffshorepipelinewhichwillconsistofHDPEmaterialandapproximately1258m

inlengthmeasuredfromtheconnectionattheendofthejettytoapproximately10mwaterdepthas

illustratedinFigure6.Thetotallengthofthepipelineisanticipatedtobe1393mlong.

Themarinepipelinewillbelaiddirectlyontheseabed(fromtheconnectionattheexistingjetty)and

weighteddownbymeansofconcretecollars,spaced4mc/c.RefertoSection4ofthisdocumentfor

amoredetaileddescriptionoftheconcretecollars.

Anoutletstructure,consistingofaT-piecesection(2xports)willbeconnectedtotheoffshoreendof

themarinepipeline. Theportsaretobemanufacturedfroma315mmnominaldiameterteeand

each port will have an internal diameter of 276 mm. The tee will be orientated to discharge

horizontally.TheT-pieceoutletwillbeprotectedbyrockorgravelbagstopreventrotationdueto

localscourandminimizetheriskofvesselanchorscatchingtheoutletstructure.Aschematicofthe

T-pieceoutletstructureisillustratedinFigure7below.





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Figure6:Orientationofnewmarinepipeline

Figure7:Schematicoftypicaloutletstructure





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4 PipelineStabilityDesignOnbottomstabilityofthepipelineisensuredthroughtheadditionofconcreteweightcollarstothe

pipeline.TheweightofeachconcretecollarisdeterminebasedonguidelinespublishedbyDetNorsk

Veritas,DNVRP F109On-BottomDesign of Submarine Pipelines. The intentionof thedesign is toachieveabsolutestabilityandisbasedona2-dimensionalresolutionofforces.Belowisaschematic

oftheforcesactingonthepipeline.

Figure8:2-DimensionalStabilityAnalysisMethod.

Therequiredsubmergedweightisdeterminedbasedonthefollowingequation:

!" =$% +$( + )$(

) *"+

Where

Ws=Submergedweight

FH=HorizontalForce(consistingofbothdragandinertialcomponents)

FL=LiftForce

gsc=SafetyFactor

µ=CoulombFrictionFactorforpipeinsand(0.6)

Thehorizontalandliftforcesarecalculatedbasedonthehydrodynamicloadonthepipelineattributed

tobothwaveandcurrentloading.AiryWaveTheoryisusedtocalculatethewaterparticlevelocityat

the pipeline resting on the seabed. Due to the water depth and wave length, in this case the





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approximationfortransitionalwaveswasusedtocalculatethevelocity.Thecomponentofthewater

particle motions perpendicular to the pipeline were calculated and used to determine the

hydrodynamichorizontalandverticalforces.

Belowisanimageofatypicalconcreteweightcollar.

Figure9:TypicalConcreteWeightCollar.

Thecollarsaretobespacedat4mcentretocentre,andwillhaveanin-airweightof770kgtoachieve

anin-waterweightof106kg/maveragedoverthelengthofthepipeline.

Theconcretecollarsaretobeheldinpositionwithstainlesssteelboltsforcorrosionresistance.

5 HydraulicAssessmentThehydraulicperformanceofthesystemwasassessedforanumberofscenarios.Duetotheageand

highfrictionallossesthroughtheexisting300mdiameteruPVCpipelineitisrecommendedthatthis

shouldbereplacedaspartoftheprojectworks.

Thepipelinevelocityandfrictionlosseswereassessedbasedonanew400mmnominaldiameterand

comparedwitha450mmnominaldiameterpipeline.

Theconfigurationwhichwasassessed(i.e.pipelinelength,material,etc.)isdescribedinSection3of

thisdocument.

Thefollowingtabledetailsthescenariosassessed.





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Table5-1:Listofscenarios(variousflowandpipelinediameter)assessed

Scenario FlowRate(m3/h)

OnshorePipe OffshorePipe

1 800 350mlonguPVC400Class9

(ID=372mm)

1258mlongHDPE400diaPN10

(ID=350mm)


(ID=372mm)


(ID=350mm)


(ID=419mmmm)


(ID=394mm)


(ID=419mmmm)


(ID=394mmmm)

Thehydraulicstudyisbasedonthehydraulicenergybalanceforthecompletesystembycomparing

thespecificenergybetweenanytwopointsinthesystemandtakingintoaccountallfrictionandfitting

lossesbetweentwoadjoiningpoints.Thisbalanceensurescontinuityofflow.

TheenergyprincipleisrepresentedbytheBernoulliequationforpipeflow:

v11/2g+p1/ρg+z1=v2

2/2g+p2/ρg+z2+hf+hL

where:

v=Q/Aaveragevelocity

p=intensityofpressureatcentre-line

ρg=specificweight

z=centrelineelevation

hf=frictionalheadloss(DarcyWeisbach-ColebrookWhite)

hL = local head loss (losses occurring where the flow velocity changes in magnitude or

direction)

Thecontinuityequationforsteadyincompressibleflowisofuseforeffluentdischarge:

ΣQIN=ΣQOUT

Thefrictionlossesinpipesdependonthepipediameter,meanflowvelocity,wallroughnessandthe

densityandviscosityofthefluid.TheDarcy-Weisbach,Colebrook-Whiteistheappropriate(generally

acceptedmostaccurate)formulatobeusedtodeterminefrictionlossesinpipes.

Darcy-Weisbach:





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hf=(λLv2)/(2gD)

where:

hf=headlossduetofriction

λ=frictionfactor

v=effluentvelocity

D=diameterofpipe

L=lengthofpipe

g=accelerationduetogravity

λiscalledthepipefrictionfactorandcanbedeterminedbyapplyingtheColebrook-Whiteformula:

Colebrook-Whiteformula:

λ=0.25{log10{ks/3.7D+2.51/(Reλ1/2)}]-2

where:

kS=roughnessheight(mm)

Re=Reynoldsnumber

Reynoldsnumber

Re=rvD/µ

where:

r =Fluiddensity(kg/m3)

v=Velocity(m/s)

D=Diameter(m)

µ=Viscosity(kg/m.s)

ThemainoutfallpipevelocityandfrictionlossesforallscenariosarelistedinTable5-2below:





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Table5-2:Pipevelocityandheadloss

Scenario FlowRate(m3/h)

Velocity-OnshorePipe(m/s)

Velocity-OffshorePipe(m/s)

FrictionLoss-NewPipe(m)

FrictionLoss-AgedPipe(m)

PumpShaftPower(kW)

1 800 2.0 2.3 14.2 15.3 56

2 1350 3.5 3.9 38.5 42.1 258

3 800 1.6 1.8 7.9 8.4 31

4 1350 2.7 3.1 21.2 23.0 141

It shouldbenotedthatno local losses (i.e.bends,contractions,etc.)weretaken intoaccountand

assumedtobeminimalcomparedtothefrictionlosses.Thefrictionlossesestimatedandpresented

intheabovetableassumedthetypicalfrictioncoefficientsofanewandoldpipeline.However,the

actualfrictionlossesofanoldpipewillmostlikelyvaryslightlyfromthetheoreticalestimates.

Theresultsindicatedthefollowing:

• Self-scourvelocitiesareachievedforallcases;

• Airbubblesandpocketswillmovedownstreamwiththeflowandnotresultintheformation

ofairpocketsinthepipeline;

• There is a significant reduction in the friction head losses when the pipeline diameter is

increasedfroma400nominaldiameterpipelinetoa450nominaldiameterpipeline;

• There isasignificantreductioninthepowerconsumptionofthepumpswhenthepipeline

diameter is increased from a 400 nominal diameter pipeline to a 450 nominal diameter

pipelineandthesavinginprocurementofasmallerpumpwouldjustifytheadditionalmaterial

exepditure.

Itisrecommendedtoreplacetheexistingonshorepipelinewithanew450mmnominaldiameter

pipelineforboththeonshoreandoffshoreportions.





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6 ReferencesDean,R.(1965).”Streamfunctionrepresentationofnonlinearoceanwaves.JournalofGeophysical

Research,70,4561-4572.

Fawcett,A.,Pitcher,G.,&Shillington,F.(2008).NearshorecurrentsonthesouthernNamaquashelf

oftheBenguelaupwellingsystem.ContinentalShelfResearch,1026-1039.

Lucas,A.,Pitcher,G.,Probyn,T.,&Kudela,R.(2014).Theinfluenceofdiurnalwindson

phytoplanktondynamicsinacoastalupwellingsystemoffsouthwesternAfrica.Deep-SeaResearchII,50-62.

Theron,A.K.,Rossouw,M.,Rautenbach,C.,Luck-Vogel,M.,&VanNiekerk,L.(2014).SouthAfricanCoastalVulnerabilityAssessment-CSIRContract:Phase2.Stellenbosch:CSIR/DEA.

DNVRPF109(2010)OnBottomStabilityDesignforSubmarinePipelines

HenryT.Falvey(1980)Air-WaterFlowinHydraulicStructures

USArmyCorpofEngineers(2006)CoastalEngineeringManual





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AnnexureA:GeneralLayout–MarinePipeline

NOTES :

1. CO-ORDINATE SYSTEM : UTM - Zone 332. CO-ORDINATE DATUM : WGS843. LEVEL DATUM : METERS CHART DATUM (mCD)

AREV.

1 : 5 000SCALE

A3SIZE

PROJECT :

GENERAL LAYOUT

DO NOT SCALE FROM DRAWING

DRAWING TITLE :

DRAWN : W. M

CHECKED:

DESIGNED :

APPROVED :

DATE :

D. Pitt

M. Le Roux

D. Pitt

2017-11-27

LUCKY STAR MARINE OUTFALL PIPELINE ROUTE

REV DATE DRN DES APP REVISIONS DESCRIPTION

REVISIONS

- - - - - -

Registration No. 2008/172141/23

2 Heath StreetNewlands7700Cape TownSouth Africa

Tel : +27 (0)83 648 8942

[email protected]

www.wayland.co.za

171121 PE 001- 01- -PROJECT DISCIPL. ELEMENT SHEET

DRAWING NUMBER

ConsultantClient

C COPYRIGHT RESERVED

2m

2m

3m

3m

2m

2m

4m

4m

6m

6m

7m

7m

8m

8m

9m

9m

2m

3m

3m

4m

4m

4m

4m

6m

7m

6m

6m

8m

7m

7m

4m

4m

4m

4m

4m

9m

9m

9m

9m

9m

5m

5m

10m

10m

5m

5m

Y 780 000

X 6 375 000

Y 780 500

X 6 375 000

Y 779 500

X 6 375 000

Y 779 000

X 6 375 000

Y 780 000

X 6 375 500

Y 780 500

X 6 375 500

Y 779 500

X 6 375 500

Y 779 000

X 6 375 500

Y 780 000

Y 780 500

X 6 376 000

X 6 376 000

Y 779 500

X 6 376 000

X 6 376 000

Y 779 000

uPVC CLASS 9, 135m LONG PIPELINE EXTENTS FROM SUMP ON LAND TO CONNECTION WITH MARINE PIPELINE

LUCKY STAR FISH FACTORY

NOTES :

1.

AREV.

1 : 5 000SCALE

A3SIZE

PROJECT :

TYPICAL WEIGHT COLLAR DETAIL

DO NOT SCALE FROM DRAWING

DRAWING TITLE :

DRAWN : W. M

CHECKED:

DESIGNED :

APPROVED :

DATE :

D. Pitt

M. Le Roux

D. Pitt

2017-11-27

LUCKY STAR MARINE OUTFALL PIPELINE ROUTE

REV DATE DRN DES APP REVISIONS DESCRIPTION

REVISIONS

- - - - - -

Registration No. 2008/172141/23

2 Heath StreetNewlands7700Cape TownSouth Africa

Tel : +27 (0)83 648 8942

[email protected]

www.wayland.co.za

171121 PE 001- 02- -PROJECT DISCIPL. ELEMENT SHEET

DRAWING NUMBER

ConsultantClient

C COPYRIGHT RESERVED

r=200mm PIPE

r=215mm

15mmTHK NEOPRENE LINER

WEIGHT COLLAR DETAIL

SCALE 1 : 10

St Helena Marine Outfall Project No. 176130 Technical Note

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