Presentation 2009oil and Gas

Oil and Gas ProcessingModule B41OA2Processing SchemesGravity SeparatorsSizing MethodsOffshore ApplicationsFacilities Design

Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Processing SchemesObjectivesProvide stable environment for processing equipment to operate

Separate well head fluid - gas, oil, water and solids

Meter marketable productsProcess gas for disposal/export Crude stabilisation

Dispose of non marketable productsTreat water for injection/disposalClean solids

Oil and Gas ProcessingG.White EPS Chemical Engineering

Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Processing SchemesCrude stabalisation, export and storageRemove volatile gas components

Meet pipeline export specs - for water, gas, H2S etc

Onshore -Pumping stations for pipeline deliveryBuffer tanks

OffshoreExport to pipeline to land, sub-sea line to other platformsPumping facilities for pipeline deliveryIntermediate storage and buffer tanksOffshore - loading to shuttle tankers


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Platform Layout: Alba - Level 1M.Christensen, Chevron UK, IBC Tech. Conf. Nov 1991


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Processing Facilities


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Processing Facilities1995 Conoco installed the worlds 1st concrete Tension Legged Platform (TLP) in the Heidrun field in the Norwegian sector.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *TERN Topside Scheme


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Processing Scheme for Chevrons ALBA


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Generalised Processing SchemeChemical addition toReduce corrosionPrevent scaleReduce emulsionsRemoveGasWater from oilSolidsCompressionRemove waterRemove oilDe-gasRe-injectionChoke ValveTemperature conditioning


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Types of SeparatorsSeparators are classified byPressure ratingLow pressureIntermediate pressureHigh Pressure

Types of operationsBulk TreatersRemoving free water - Free Water Knock Out DrumSkim TanksGas scrubbers - for high gas to liquid ratios


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Functions of a SeparatorRequirements of a two phase oil-gas separatorProduce oil free gas - 10 ppm liquid in gasProduce gas free oil Maintain pressure for separation - pressure on gas outletMaintain pressure inside separator - liquid level controlProvision for water separation

Requirements for a three phase gas/oil/water separatorProduce gas free oil, oil free gasMaintain pressure.Produce oil free from water - typically allow 10% by volumeProduce water free from oilMaintain liquid levels for residence timesProvide for surges in flow.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Factors Affecting SeparationGas and liquid flowratesflowrates vary during the field life. Normally peak, minimum and average rates used for design purposes.Operating Pressures and Temperaturesaffect the density and viscositySlugging of feed streamsupsets in flow causing transient increases/decreases in flowratesPhysical propertiescompressibilitydensityDegree of separation specified for the designremoving 10 micron liquid dropsImpuritiessolids, sand, waxesTendency for crude to foamCorrosive tendencies.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Two Phase Gas-Liquid Separation PrinciplesDensity difference provides the least effort


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Three Phase Gas-Liquid Separation PrinciplesPrinciple is that the three fluids are left for sufficient time

Gas bubbles rising in heavy and light liquid phaseWater droplets settling in the lighter bulk oil layerOil droplets rising through the heavier bulk water layer.Coalescence of droplets within the dispersion band and with the respective bulk layers

The interface between oil and water may not be clear due to a dispersion band


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Separator SectionsZones inside a horizontal 3 phase separator

1. Inlet2. Liquid Profile3. Liquid from Gas5. Water from Oil4. Oil from water


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Description of zonesGas Disengagement - Inlet DistributorFluid changes direction, liquid forces down.Gas breaks free from liquidLiquid Profile - Solids DepositionLiquid profile establishedSolids separate outGas-Oil SeparationLiquid (oil) drops settle by gravityMist eliminator removes down to 100mm drops.Oil from Water SeparationOil drops rise due to density differenceCoalescence increases drop sizeSufficient time allows for processWater from Oil SeparationWater drops fall to o/w interface


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Separator Types and Selection GuideTwo common configurationsHorizontalVerticalSpherical vessels are also an alternativeBasic Criterion How much solid (sand) is produced:Buildup of solid material can lead to corrosion, reduction in performance. Regular cleaning by jet-wash system or manual removal.How steady the flow isSlugging and surges cause increases in feed rates causing levels to increase. Control system needs constant adjusting.How much water is producedIs any emulsion (dispersion) formedLong residence time unsuited for primary separators. May need to add chemical de-emulsifier or reduce water quality.Is foaming a problem


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Horizontal SeparatorsEasy to mount in modular systemBetter for foams and emulsionsLarger mounting areaPoor solids removalLower surge capacity


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Horizontal Separators


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Horizontal Separator ModuleKvaerner Separator module


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Vertical SeparatorsLiquid level control not so critical for operation

Good for surging flows

Work well for high GOR applications

Difficult for modular systems - transport & installation

Tend to be larger

Access for relief and control valves difficult


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Separator Sizing - The PrinciplesOil must be kept inside the vessel to allowAny water drops in oil pad to sink and coalesce with bulk water layerAny oil drops to rise and coalesceAny liquid drops in gas phase to fallGasOilWater


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Separator Sizing - The PrinciplesTime for water in oil drop to settle = time to travel effective lengthUgUsUnUnGasOilWaterEffective LengthOil Pad


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Theoretical Background - Drop Settling Equation Resolving forces give the acceleration of the particle as :If the external force is represented by some acceleration term ae say, then:


External force causing motion : Fe

Drag force opposing motion : Fd

Buoyancy : Fb

Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Settling/Rising VelocityUnder the influence of gravity, particle acceleration is then:Solving for 0 acceleration gives :This equation cannot be solved directly unless we have an expression for CdLaminar Stokes RegimeLiquid drops in gas phaseapplies for Re

Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Theoretical Background - Drop Settling EquationDrag coefficient varies with relative particle velocity for rigid spheres, we have


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Settling/Rising VelocityFor spherical drops of diameter in the laminar (Stokes) settling region,the drop settling velocity becomes:For liquids, viscosity correction is used :If Cd is given as a function of Re, then resort to an iterative process to find Up and Cd:Set Cd to a value (assume Cd=24/Re)Calculate UpCalculate ReCalculate Cd from correlationCalculate UpRepeat if required Where subscript d= dispersed phase, c= continuous phase


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Gas Phase Settling EquationTo avoid issues with drag equations. for liquid drops in gas phase, maximum gas velocity is given by :Souders-Brown Equation

Allowable VelocitiesK is a constant depending if the separator is fitted with a mist eliminator or not:


Situation(3)

K m/s

General Value

0.1000

In General

With a mist eliminator

0.12 - 0.185

Without

0.075 - 0.15

Knock-Out Drum

Vertical

Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Retention TimesRetention times give the minimum time liquid must be held in the separator.Accounting for the geometry, fill depth and shape of the separator, equations using retention time have a form similar to :Retention equations can apply to2 phase vessels - hold up the oil until gas is removed or liquid is removed from gas.3 phase - separate oil from water, water from oil, liquid from gas.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Estimating size of droplets of liquid-in-gas or liquid-in-liquid is difficult. Measurement techniques are difficult to implement on-line.

Gas-Liquid SeparationIn general, field experience suggests that section should be designed to remove 100m drops. This will prevent flooding of the mist eliminator. Mist eliminators can remove 99% of liquid-in-gas drops between 10 m to 100 m.

Special CasesGas-Scrubbers (Vertical 2 phase separators used in gas compression trains), are typically sized for 500 m drops.

Flare or Vent Scrubbers (used to prevent slugs of liquid reaching the flare stack), are designed to remove drops between 300 m to 500 m. Note - mist eliminators are not used here for fear of blockage.

Drop sizes


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Liquid-Liquid SeparationSize of water drops inside production separators is difficult to predict.

Water in Oil DropsExperience suggests equations should be applied for 500 m water-in-oil drops. If separator is sized for 500 m, oil from separator will contain less than 5%-10% water.

Oil in Water DropsSeparation of oil drops from water is easier than water from oil due to higher oil viscosity. Should separator be sized for water removal from oil, water from 3 phase separators can be expected to contain 2000 mg/l oil-in-water.Drop sizes


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Retention TimesField and laboratory tests can determine retention times easier than measuring drop sizes.

Gas-Liquid SeparationRetention (or residence time) for most 2 phase operations is between 30 seconds and 3 minutes. For foaming crudes, residence times are increased by factor of 4.

Liquid-Liquid SeparationFor both water-in-oil and oil-in-water, typical retention times vary between 3 minutes to 30 minutes. For design purposes used:Onshore 10 minutesOffshore 3-4 minutes

The longer the retention timeThe larger the vesselThe heavier the moduleGreater the cost


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Residence Time DistributionPerfect plug flow will not occur inside separators and a RTD (Residence Time Distribution) profile will be obtained


RTDPLOT1 PLOT

0000

0.0004793609000

0.00511318240.000107296100

0.01017310250.00724248930.00012636010

0.01496671110.0116952790.00108108110

0.0141145140.01190987120.00318708320.000029036

0.01102529960.01110515020.00428220430.0006097561

0.00953395470.00885193130.00456300460.0008130081

0.00750998670.00734978540.00506844510.0011614402

0.00687083890.00686695280.00564408560.0015389082

0.00543275630.00509656650.00613548610.0020034843

0.00431424770.00456008580.00631800630.0022648084

0.00378162450.0039163090.00651456650.0023809524

0.00271637820.00327253220.00641628640.0026132404

0.00239680430.00321888410.00621972620.0030778165

0.00122503330.0031115880.00589680590.0033681765

0.00031957390.00295064380.00515268520.0037166086

0.00005326230.00262875540.00424008420.0040069686

0.00203862660.00377676380.0050232288

0.00139484980.00321516320.0052845528

0.00112660940.00287820290.0051393728

0.00107296140.00282204280.0051684088

0.00037553650.00263952260.0047038328

0.00010729610.00235872240.0043263647

00.00214812210.0041521487

0.00181116180.0038617886

0.00157248160.0034552846

0.00126360130.0033681765

0.0010389610.0033391405

0.00088452090.0031068525

0.00082836080.0025842044

0.00064584060.0024680604

0.00049140050.0021777003

0.00036504040.0020615563

0.00022464020.0018292683

0.00012636010.0016550523

0.00005616010.0014227642

0.000014040.0014518002

0.0012195122

0.0011324042

0.0009581882

0.0007549361

0.0005807201

0.0004936121

0.0003193961

0.000232288

0.000116144

0.000029036

0

0

48

32

20

15

Time (sec)

Normalised Distribution "E" Curve

Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Useful Expressions & TermsStandard ConditionsFor oil use, this is 60F, 14.7 psia (15.5C, 1 atm) - best to confirm this is true.

DensityLiquid density is quoted as sg (specific gravity with reference to water), or commonly API

Gas specific gravity is with reference to air at standard conditions

for different temperatures and pressures, use compressibility factors


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Useful Expressions & TermsTypical densities of common crude oils

Gas Flowrates Gas volumetric flowrates are quoted at standard conditions:

Conversion to separator conditions requires the density


Type of Oil

s.g.

oAPI

Bitumen

> 1

< 10

Heavy Oil

1 to 0.93

10 to 20

Intermediate Oil

0.93 to 0.83

20 to 40

Light Oil

< 0.83

> 40

Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Useful Expressions & TermsLiquid Flowratebpd 1 US barrel of oil = 0.15899 m3A STB (Stock Tank Barrel) volume measure is also used - volume of liquid at standard conditions

Producing Gas-Oil Ratio (GOR)GOR is the volume of gas produced per unit volume of oil produced at standard conditions. The units for GOR are scf/STB or sm3gas/sm3oil.

BS&W - Base Sediment and Water - the non oil fraction of liquid found in oil from separation stages

Water Cut - Represents water content of well head fluid. Typically 10-20% but can rise to 80% as production life increases.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Area ChartQuick method for areas of segments


Chart1

0000.5

00.00906737570.00288623530.4971137647

00.0255351680.00812809640.4918719036

00.04670566150.01486687380.4851331262

00.0715901020.02278783720.4772121628

00.09960345140.03170476330.4682952367

00.13034166920.04148904190.4585109581

00.16350110880.05204401930.4479559807

00.19884086730.06329301380.4367069862

00.23616242450.07517283450.4248271655

00.27529750740.08762991830.4123700817

00.31610033150.10061786050.3993821395

00.35844237030.11409575010.3859042499

00.40220867840.12802699870.3719730013

00.4472952180.14237848990.3576215101

00.49360685840.15711994290.3428800571

00.54105584460.17222342430.3277765757

00.58956060.18766296750.3123370325

00.63904477320.2034142690.296585731

00.68943646770.21945444360.2805455564

00.740667610.23576182270.2642381773

0.79267342510.25231578770.2476842123

0.84539199470.26909662960.2309033704

0.89876388220.28608542910.2139145709

0.95273181080.30326395430.1967360457

1.00724038320.32061457170.1793854283

1.06223583660.33812016820.1618798318

1.11766582520.35576408160.1442359184

1.17347922660.37353003910.1264699609

1.22962596570.39140210120.1085978988

1.28605685520.40936461120.0906353888

1.34272344810.42740214790.0725978521

1.39957789980.4454994820.054500518

1.45657283930.46364153470.0363584653

1.51366124510.48181333870.0181866613

1.57079632680.50

&LDepartment of Mechanical & Chemical EngineeringHeriot-Watt university&RLiquid Fill Depth Chart

&LRevised &D at &TDr G.White&RPage &P of &N

2xArea of Segment/Area of circle

2xArea of Triangular sector/ Area of circle

Depth H

Segment Area

Sector

Radius R

Area of segment/Radius2

#REF!

Depth H / Radius R

Segment Area/R2

2xArea of Segment/Circle Area

Geometry of a Circle for areas of segments and sectors

Chart2

0

0.0090673757

0.025535168

0.0467056615

0.071590102

0.0996034514

0.1303416692

0.1635011088

0.1988408673

0.2361624245

0.2752975074

0.3161003315

0.3584423703

0.4022086784

0.447295218

0.4936068584

0.5410558446

0.5895606

0.6390447732

0.6894364677

0.74066761

0.7926734251

0.8453919947

0.8987638822

0.9527318108

1.0072403832

1.0622358366

1.1176658252

1.1734792266

1.2296259657

1.2860568552

1.3427234481

1.3995778998

1.4565728393

1.5136612451

1.5707963268

&LDepartment of Mechanical & Chemical EngineeringHeriot-Watt university&RLiquid Fill Depth Chart

&LRevised &D at &TDr G.White&RPage &P of &N

Depth H

Segment Area

Radius R

Depth H / Radius R

Segment Area/R2

Geometry of a Circle for areas of segments and sectors

Geometry

Radius =2.5Step =0.0714285714Dia=5C Area19.6349540849

HAsegH/RA/R2dThetalAtriAseg/AcirAtri/Acir

0.000.000000.000.002.5000.009.8200.5

0.070.056670.030.012.430.47923713731.199.760.00288623530.4971137647

0.140.159590.060.032.360.67938515231.679.660.00812809640.4918719036

0.210.291910.090.052.290.83411058862.039.530.01486687380.4851331262

0.290.447440.110.072.210.96553184672.329.370.02278783720.4772121628

0.360.622520.140.102.141.08219905192.589.190.03170476330.4682952367

0.430.814640.170.132.071.18849006542.809.000.04148904190.4585109581

0.501.021880.200.162.001.28700221763.008.800.05204401930.4479559807

0.571.242760.230.201.931.37942630793.188.570.06329301380.4367069862

0.641.476020.260.241.861.46693623643.358.340.07517283450.4248271655

0.711.720610.290.281.791.55038674663.508.100.08762991830.4123700817

0.791.975630.310.321.711.63042349033.647.840.10061786050.3993821395

0.862.240260.340.361.641.70754869823.777.580.11409575010.3859042499

0.932.513800.370.401.571.78216255513.897.300.12802699870.3719730013

1.002.795600.400.451.501.8545904364.007.020.14237848990.3576215101

1.073.085040.430.491.431.92510149584.106.730.15711994290.3428800571

1.143.381600.460.541.361.99392174854.206.440.17222342430.3277765757

1.213.684750.490.591.292.0612435134.296.130.18766296750.3123370325

1.293.994030.510.641.212.12723238784.375.820.2034142690.296585731

1.364.308980.540.691.142.19203250654.455.510.21945444360.2805455564

1.434.629170.570.741.072.25577056544.525.190.23576182270.2642381773

1.504.954210.600.791.002.31855896154.584.860.25231578770.2476842123

1.575.283700.630.850.932.38049827024.644.530.26909662960.2309033704

1.645.617270.660.900.862.44167923114.704.200.28608542910.2139145709

1.715.954570.690.950.792.50218435634.753.860.30326395430.1967360457

1.796.295250.711.010.712.56208925074.793.520.32061457170.1793854283

1.866.638970.741.060.642.62146370824.833.180.33812016820.1618798318

1.936.985410.771.120.572.6803726314.872.830.35576408160.1442359184

2.007.334250.801.170.502.7388768124.902.480.37353003910.1264699609

2.077.685160.831.230.432.79703360614.932.130.39140210120.1085978988

2.148.037860.861.290.362.85489751584.951.780.40936461120.0906353888

2.218.392020.891.340.292.91252070714.971.430.42740214790.0725978521

2.298.747360.911.400.212.96995347264.981.070.4454994820.054500518

2.369.103580.941.460.143.02724465144.990.710.46364153470.0363584653

2.439.460380.971.510.073.08444201915.000.360.48181333870.0181866613

2.509.817481.001.570.003.14159265365.000.000.50

&LDepartment of Mechanical & Chemical Engineering&ROffshore Processing&A

&LRevised &D at &TDr.G.White&RPage &P of &N

Depth

Segment Area

Geometry (2)

Radius =2.5Step =0.0714285714Dia=5C Area19.6349540849

HAsegH/RA/R2dThetalAtriAseg/AcirAtri/AcirA/R2H/R

0.000.000000.000.002.5000.009.8200.500

0.070.056670.030.012.430.47923713731.199.760.00288623530.49711376470.00906737570.0285714286

0.140.159590.060.032.360.67938515231.679.660.00812809640.49187190360.0255351680.0571428571

0.210.291910.090.052.290.83411058862.039.530.01486687380.48513312620.04670566150.0857142857

0.290.447440.110.072.210.96553184672.329.370.02278783720.47721216280.0715901020.1142857143

0.360.622520.140.102.141.08219905192.589.190.03170476330.46829523670.09960345140.1428571429

0.430.814640.170.132.071.18849006542.809.000.04148904190.45851095810.13034166920.1714285714

0.501.021880.200.162.001.28700221763.008.800.05204401930.44795598070.16350110880.2

0.571.242760.230.201.931.37942630793.188.570.06329301380.43670698620.19884086730.2285714286

0.641.476020.260.241.861.46693623643.358.340.07517283450.42482716550.23616242450.2571428571

0.711.720610.290.281.791.55038674663.508.100.08762991830.41237008170.27529750740.2857142857

0.791.975630.310.321.711.63042349033.647.840.10061786050.39938213950.31610033150.3142857143

0.862.240260.340.361.641.70754869823.777.580.11409575010.38590424990.35844237030.3428571429

0.932.513800.370.401.571.78216255513.897.300.12802699870.37197300130.40220867840.3714285714

1.002.795600.400.451.501.8545904364.007.020.14237848990.35762151010.4472952180.4

1.073.085040.430.491.431.92510149584.106.730.15711994290.34288005710.49360685840.4285714286

1.143.381600.460.541.361.99392174854.206.440.17222342430.32777657570.54105584460.4571428571

1.213.684750.490.591.292.0612435134.296.130.18766296750.31233703250.58956060.4857142857

1.293.994030.510.641.212.12723238784.375.820.2034142690.2965857310.63904477320.5142857143

1.364.308980.540.691.142.19203250654.455.510.21945444360.28054555640.68943646770.5428571429

1.434.629170.570.741.072.25577056544.525.190.23576182270.26423817730.740667610.5714285714

1.504.954210.600.791.002.31855896154.584.860.25231578770.24768421230.79267342510.6

1.575.283700.630.850.932.38049827024.644.530.26909662960.23090337040.84539199470.6285714286

1.645.617270.660.900.862.44167923114.704.200.28608542910.21391457090.89876388220.6571428571

1.715.954570.690.950.792.50218435634.753.860.30326395430.19673604570.95273181080.6857142857

1.796.295250.711.010.712.56208925074.793.520.32061457170.17938542831.00724038320.7142857143

1.866.638970.741.060.642.62146370824.833.180.33812016820.16187983181.06223583660.7428571429

1.936.985410.771.120.572.6803726314.872.830.35576408160.14423591841.11766582520.7714285714

2.007.334250.801.170.502.7388768124.902.480.37353003910.12646996091.17347922660.8

2.077.685160.831.230.432.79703360614.932.130.39140210120.10859789881.22962596570.8285714286

2.148.037860.861.290.362.85489751584.951.780.40936461120.09063538881.28605685520.8571428571

2.218.392020.891.340.292.91252070714.971.430.42740214790.07259785211.34272344810.8857142857

2.298.747360.911.400.212.96995347264.981.070.4454994820.0545005181.39957789980.9142857143

2.369.103580.941.460.143.02724465144.990.710.46364153470.03635846531.45657283930.9428571429

2.439.460380.971.510.073.08444201915.000.360.48181333870.01818666131.51366124510.9714285714

2.509.817481.001.570.003.14159265365.000.000.501.57079632681

&LDepartment of Mechanical & Chemical Engineering&ROffshore Processing&A

&LRevised &D at &TDr.G.White&RPage &P of &N

Depth

Segment Area

Geometry (2)

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

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Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Other ExpressionsOther features which may occur include:Drop Size DistributionDrop formation due to shearing is based on a critical Weber number

Which is used to predict mean drop diameters

Dispersed Phase (for liquid/liquid decanters)Dispersed phase is a function of density


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Other ExpressionsDispersion Band ThicknessDepth of the dispersion band is a function of flow rate and interfacial area between settling phases:

Qc = Continuous phase flowrateAl = surface area for contactn=2.5 to 7Perforated Plate DistributorsHorizontal decanters use two close mounted perforated platesUpstream plate - open flow area between 3-10% of separator cross-sectional areaDownstream plate - 20 to 50% of separator cross-section


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Other ExpressionsOutlet/Inlet Pipe VelocitiesFor sizing inlet and outlet pipes, a momentum or velocity limit is used

Example

Bend should be more than 5 pipe diameters before separator

Restrictions on velocities may be due to potential erosion problems


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Level Control SettingsLevel control positions give a degree of flexibility to separator designResponse times from control room or outside operationSlugging volumesResidence timesOilWaterGas


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Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Level Control Settings

Internal liquid level sensors are used to control separator liquid level measures hold up volume.Usually 4 recognised level locations


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Level Control Settings

For vertical vessels, there are additional considerations which give the distance from the liquid surface to the inlet deflector and to the mist eliminator pad.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing calculations to find the physical dimensions follow the same pathSpecify the inlet velocity using a momentum limit or some maximum value. This gives the size of the inlet pipe.Set the outlet velocity and hence the outlet pipe sizesCalculate the maximum allowable gas velocity at peak flowrates from Souders Brown equation or recommended limitsCalculate minimum area for gas flow. Actual vessel may have larger gas area e.g. 50% full of liquidCalculate liquid capacity based on drop settling or residence timeCalculate separator dimensions till size matches L/D constraints (3.5 to 5)Remember Production profiles vary over field life Oversizing can lead to extra costsSeparator Sizing Methods


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing Methods - Horizontal 2 Phase1. Gas Capacity - gives the maximum possible velocity for gas stream

2. Minimum area for flow

3. Liquid Capacity - for surging allow 2x hold up volume

4. Fix gas/liquid interface - Either assume vessel is half full, or write expression for DL in terms of slenderness ratio and liquid fill factor

Q=V/tr


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing Methods - Horizontal 2 PhaseUseful Procedure is to :Set % area occupied by liquid - Typically 50% or even 75%. Minimum area for gas flow is 12% of total csaCalculate length and diameterCheck Slenderness ratio L/D=3.5Set new % area for liquid and repeat.Note :Allow for control levels - e.g. High-High trip, High AlarmEffective length = 75% of seam-seam length


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing Methods - Horizontal 3 PhaseMore difficult to find since theres several constraints. Method follows that for 2 phase, with added complication of water from oil separation.1. Gas Capacity - Souders Brown equation2. Minimum area for gas flow

3. Water drop settling velocity

4. Set axial velocity of oil and water layers based on drop settling velocity

5. Check this should be

Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing Methods - Horizontal 3 Phase6. Calculate areas occupied by oil and water: Use velocities un. Assume un in oil=un in water ( this ensures residence times in both phases is the same.7. Set the % area occupied by gas: Base this on either calculated minimum area. Better still, use calculated area for liquid flow since liquid is usually a constraint.8. Find the overall diameter. Calculate total area from 7, then the diameter9. Locate the gas/oil and oil/water interfaces using geometric considerations: Use geometric chart or equations in Perry to match occupied area with distance from circumference.10. Using the oil layer thickness, calculate the overall length

11 Check the slenderness ratio: L/D range typically 3.5, maximum 6

12. Check the residence times. Typically 3 minutes, may be more depending on crude


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing Methods - Horizontal 3 Phase (Cont)Special ConsiderationsTurbulenceReduce settling velocity by some factor to account for turbulenceAxial VelocityA low settling velocity implies length to oil pad thickness is in ratio of 15:1Who sets the 15xUp limits ?SluggingAllow for increased hold up volumes - could increase size/costsInternals can decrease length through enhanced separation.Momentum limits for inlets and offtakes


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing Methods Vertical 2 PhaseCalculate Gas CapacitySouders Brown

Find minimum gas/vessel areaSet liquid hold up volume from residence timeCalculate liquid depthSet control levels.Set location of :Inlet from mist eliminatorInlet to liquid surfaceDisengagement height after demister

A=Qg/us


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Other Sizing MethodsMethods due to Arnold (Surface Production Operations)These appear different from others as they :Have built in unit conversions - e.g. Oil and gas unitsAllow for gas compressibilityUse a 500mm water in oil drop sizeSet the liquid level at 50% of vessel diameter


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Other Sizing MethodsExample: Horizontal 2 phase separatorGas capacity equation

or

Liquid capacity

There is therefore a trade off between the diameter and length of the separator.

Seam to seam length


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Other Sizing MethodsCompany methods - Best Practice ManualsThese are based on collective experience and should be followed by designers employed by the company, or by contractors working for them. Advantages includeApplication of standards across company and contractorsRegular updates ensure field experience is built into design rules.

Standards AuthoritiesAmerican Petroleum Institute StandardsAPI Spec 12JNORSOK - Norwegian Oil Industry Association, Federation of Norwegian Engineering Industries

Sizing procedures can be eased through use of spreadsheets


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing Methods SummarySouders Brown for maximum gas flow that is possibleDrop settling velocity for water-in-oil dropResidence time equations gives the axial velocity of each phase Geometry where hold up volume = length x areaResidence time in oil phase=residence time in water phaseGas density from compressibility factorsGeometry for filling levels


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sizing Methods - ProblemsSizing methods take no account ofDrop coalescenceDrop breakup due to internalsHindered settlingRetention times which vary between lab and full scaleActual velocity profile inside separatorEffect of decreased/increased water cut on dispersion - more water may actually help oil separationIf project allows, have the design verified by independent test


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Velocity ProfilesIdeal velocity profile is plug flow, but CFD predictions and now LDA measurements show flow is anything but plug flow in separators without proper internals

Expected flow pattern might be :


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Velocity ProfilesExample: Hansen et al Gullfaks A Separator CFD modelPredicts the velocity profile for normal and peak flows through a representation of the separator

CFD model consists of porous plates to simulate packing sectionsNote the separator is 3 phase, but the simulation is only able to work with a single liquid phase.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Velocity ProfilesLooking top down

Inlet Predicted circulation Are verified in laboratory experiment and suggest residence time will be longer than expected. It also shows flow is NOT plug flow a deviation from the theory used in finding the size.

Notice relatively fast streamline along base of separator!


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Velocity ProfilesInlet Clear signs of fast streamline along base of separator resulting in short circuiting

There will be a stream leaving the separator with LOWER residence time than expected.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Velocity ProfilesUsing CFDAlthough CFD has matured, simulating three distinct phases is non-trivial. Examples can be found in literatureVelocity profiles in three phases predicted by CFD

Difficult to predict droplets within each phase but sufficient to give details of flow patterns and influence of internals


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Velocity ProfilesUsing CFDAlthough CFD has matured, simulating three distinct phases is non-trivial. Examples can be found in literature


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Vessel Internals

Inlet Diverter/Momentum BreakerThis makes sure feed is directed towards one end of vessel, maximising distance liquid can use

Basic Dished Plate or Half Pipe



Inlet Diverter/Momentum BreakerProprietary designs have been developed by several operators and vendors to improve initial separation of oil from gas and to reduce tendency for system to foam.

Natcos Porta-Test Issues:Distribution of flow into each cycloneVelocity to achieve cyclonic flowPressure dropOperating range



Inlet Diverter/Momentum BreakerProprietary designs have been developed by several operators and vendors to improve initial separation of oil from gas and to reduce tendency for system to foam.

Kvaerner Process Systems



High Efficiency designsTo reduce the size of conventional separators, and to combat the tendency to reduce foam, several vendors employ cyclonic inlet devices.Natco Systems



Vortex BreakersRate of liquid withdrawn from separator may lead to formation of a vortex

A simple plate above the outlet is used to prevent the vortex from forming


4D

D/2

D

Vortex Breaker


Vortex BreakersTwo general types plates and gratings -



Pipe DiametersFlanges on the inlet and outlet nozzles allow pipe work to be joined to vessel. Diameters of these pipelines and hence nozzles depend on kinetic head/pressure drop limits.

Recommended limits are:



Pipe DiametersRun lengths are also recommended to ensure flow is relatively streamlined before and after the separator.



Mist EliminatorsSmall liquid droplets present in the gas stream can be carried along with the gas stream and contaminate the gas processing system. Mechanical devices to capture droplets are used close to gas outlet.Droplets are removed by impingement on solid surface when liquid collects and eventually drips off from unit.



Vane PacksThese change the direction of the gas flow, relying on the momentum of liquid droplets to carry drops towards solid surface. Liquid collects and is drained by special channels, or down inside of vanes.



Vane PacksSome vane packs disruptthe direction of flow

Operational issues with vane packs include:Flooding too much liquid or where liquid is unable to run free from packDeposition of solids waxes Pressure drop across pack



Fibre & Wire Mesh PadsSimilar construction but different pressure drop characteristics


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Vessel InternalsGeneral comparison between mist eliminator types

Pressure drop across vane packs will depend on spacing


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Vessel InternalsPlates are usually single sheets with perforated holes or squares on triangular or square pitch. Some double sheet systems give better performance.Square holes on triangular pitchDouble plate arrangement, with larger holes either before or after a plate with smaller holes


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Vessel InternalsPlates are usually perpendicular to direction of flow, but can be parallel to reduce re-circulation patterns developing


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Vessel InternalsWhere foam on the liquid surface is a problem, defoaming packs can be used to break foam downFoam is caused by gas bubbles trying to break through liquid surfacePacks are similar to vane type mist eliminators, usually submerged 10 to 20 cm below surface


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Alternative Separator Layouts


Oil and Gas ProcessingG.White EPS Chemical EngineeringAlternative Separator LayoutsKey Points

A single vessel may not be able to cope with all production conditions

Using novel internals helps to increase capacityIncreased production rates -> lower residence timesHeavier oil fractions -> more difficult separation.More difficult to access before installation


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Floating Production SystemsFloating production systems are typically used for MARGINAL field developments:Limited recoverable reservesDeep waterOffloading difficulties

FPSO - Floating Production Storage and Offloading


Oil and Gas ProcessingG.White EPS Chemical EngineeringMotion Effects on Offshore EquipmentTypical topside unit operations

Two phase gas/liquid separatorsThree phase gas/oil/water separatorsTrayed Distillation Columns - fractionationPacked Columns de-oxygenation, acid gas removalCondensers tube bundle or tube and fin gas liquefaction(Re)boilers Gasification of LNGSingle-phase heat exchangersStorage tanksPumps & CompressorsFlare systems


Oil and Gas ProcessingG.White EPS Chemical EngineeringMotion Effects on Offshore EquipmentGas/Liquid or Gas/Liquid/Liquid Separators


Oil and Gas ProcessingG.White EPS Chemical EngineeringMotion Effects on Offshore EquipmentPlatform motion affects :Separation Equipment Columns


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Equipment Affected by Sea MotionGravity Separation SystemsSpills over weir plateLevel control

ColumnsDistillation - uneven liquid distribution on trays, preferential gas flow on one side of plateAbsorption - preferential liquid flow to one side, reduction in contact between gas & liquidUneven distribution of liquid from distributor

Heat ExchangersLiquefaction & re-gassification - where distribution of liquid is affected by sustained angles of tilt .


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Liquid Response to MotionTwo Effects

At non-resonant conditionsSpirit level effects - reduce gas area through motion cyclePossible flow over weir plateProblems in level control

Towards resonanceOil/water interface can break-up causing mixing and further dispersionsLarge oil/water interface amplitudeIncrease in liquid velocity reducing separationPossible jetting of liquid through internalsPossible spillage of water over weir plate


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Reducing Motion EffectsResonant effects depend on natural period hence the fill depth/length ratio. Cutting down the vessel length might help -Making the vessel shorterInserting perforated bafflesUsing structured Packing

Moving vessel to location where amplitude of motion is reduced e.g. center of gravity

Using a vessel with better sea keeping abilities - converted drilling rigs and TLPs respond to all 6 degrees of freedom. Ship based structures are more susceptible to roll than pitch, although actual motion is more severe.

Using different designs, which respond better to motion.


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Common internals with gas/oil separators includeVortex breakersMist eliminatorsDefoaming packsBaffle Plates.

Reducing Motion Effects


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Recent innovations have been to use structured packing which promotes coalescence

Reducing Motion Effects


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Packing is similar to corrugated paper but allows flow through the pack while droplets of dispersed phase have a chance to collide and coalesce.

Reducing Motion EffectsPerformax PackingStructured Packing


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Baffle ReplacementsSome operators have replaced baffled with structured packing

Reducing Motion EffectsCONOCOs designs for Hutton


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sand Wash SystemsCertain fields produce solids - sand of small particle size. This can build up along the base of separators and increase erosion damage downstream.

Solids buildup reduces liquid area, increasing bulk velocity. Stagnant areas lead to higher corrosion


Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *Sand Wash SystemsRemoving OptionsShut down & manual removal

Automatic removal is via jet wash system:Series of jets directed to push sand towards central sand troughJets angled to prevent excessive erosion of sidesJet wash system used by Conoco(Courtesy Soc. Pet. Engineers)


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Oil and Gas ProcessingG.White EPS Chemical EngineeringSlide *SummarySeparation theoryDrop settlingRetention timesSizing constraintsTypes of equipmentHorizontal & vertical pressure vesselsInternalsMist padsBaffles, Coalescing packingVortex breakersOperational ProblemsFoams, slugging, level controlVelocity profilesUse on FPSOsReducing effects of motion


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