ErosionGuidelinesRevision 2.1(1999)

J W Martin

Main CDContents

Sunbury Report No. S/UTG/102/99dated October 1999

EROSION GUIDELINES REVISION 2.1 (1999)

ByJ W Martin

Summary

Erosion can be defined as the mechanical loss of material by the impact of liquid dropletsand/or solid particles.

Under aggressive operating conditions velocity limits, and hence production limits, are setto avoid erosion. If these limits are overly conservative then BP AMOCO losesproduction; if they are too optimistic then BP AMOCO risks erosion damage and the lossof system integrity.

This document updates the knowledge on the erosion of piping and tubing in productionand injection service (Ref. 1). The two 'Flow Charts' for the assessment of erosion riskhave also been updated:

The 'Velocity Limits for Avoiding Erosion' flow chart lays down rule-of-thumbvelocity limits for the avoidance of erosion damage in non solids-containingenvironments, i.e. ‘totally solids free’ or ‘nominally solids free’ conditions.‘Nominally solids free conditions’ are defined as up to 1 pound of solids perthousand barrels of liquid for liquid systems or up to 0.1 pounds of solids permillion standard cubic feet of gas for gas systems.

For solids-containing environments it is necessary to first establish the likely rate oferosion by referring to the ‘Calculation of Erosion Rates’ flow chart. The velocitylimit flow chart can then be used to determine whether erosion-corrosion is likelyand to evaluate the possible rate of erosion-corrosion.

The 'Calculation of Erosion Rates' flow chart makes recommendations forevaluating the erosion rate for solids-containing duty, or where greater precision isrequired than afforded by a simple velocity limit for ‘nominally solids-free’conditions in the 'Velocity Limits for Avoiding Erosion' flow chart.

Different velocity limits will apply in different situations, depending on the flow (gas,liquid or multiphase gas/liquid), the environment (corrosive or non-corrosive) and whetheror not solids are present.

The models used for the calculation of erosion wastage rates are based, in the main, onlaboratory test programmes. Hence they are likely to be at their most reliable for simpleflow conditions in non-corrosive environments. There is less confidence in the models for

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multiphase solids erosion and guidance for erosion-corrosion (solids plus corrosiveenvironment), as these are based on a very limited data set.

All of the predictive models suffer from limited comparison with field experience.

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Contents

Erosion Guidelines....................................................................................................... 1Summary.......................................................................................................... 1Contents .......................................................................................................... 2

Summary Guidelines - Flow Charts and General Comments ......................................... 3Figure 1 - First Pass Velocity Limits................................................................. 4Figure 2 - Calculation of Erosion Rates ............................................................ 5Notes on Flow Charts. ..................................................................................... 6

Figure 1 - First Pass Velocity Limits ..................................................... 6Figure 2 - Calculation of Erosion Rates................................................. 8

General Comments and Conclusions................................................................. 10Erosion Guidelines - Discussions ................................................................................. 12

Introduction ..................................................................................................... 12Discussion of the Guidelines............................................................................. 17

1. Non-corrosive fluid flow, no solid particles ...................................... 172. Corrosive fluid flow, no solid particles ............................................. 173. Non-corrosive fluid, with solid particles ........................................... 19

References: ...................................................................................................... 30

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Summary Guidelines - Flow Charts and General Comments

A flow chart for determining 'first pass' erosional velocity limits (entitled "Velocity Limitsfor Avoiding Erosion") is given in Figure 1. The recommendations in this flow chart aregenerally based on an allowable erosion rate of 0.1 mm/yr.

For ‘totally solids free’ or ‘nominally solids free’ conditions, if production is requiredoutside these limits then advice can be sought from the relevant specialists in the UpstreamTechnology Group (UTG).

For ‘solids containing’ conditions, reference should first be made to Figure 2 ("Calculationof Erosion Rates") for evaluating the possible erosion rate and then to Figure 1 ("VelocityLimits for Avoiding Erosion”) to assess whether erosion-corrosion is likely to be an issueand to evaluate the possible erosion-corrosion wastage rate. This should be used toestablish whether the predicted wastage rates are acceptable. This approach, in allowingfor bends in pipework and constrictions in tubing, is likely to be conservative for straightpiping and tubing.

A flow chart for the assessment of erosion rates (entitled "Calculation of ErosionRates") is given in Figure 2. It is recommended that this is used with care. There aremany areas of uncertainty and the models recommended in the flow chart are relativelyunproven and many are still being developed. Under conditions of erosion-corrosion theguidelines in Figure 1 are applicable for estimating the erosion-corrosion rates.

Yes

No No

Yes

No

No

Yes

YesNo≥80°C <80°C

Yes

Yes

No

Yes

No

No

Yes

No

Yes No

Yes

No

Yes

No

Yes

No

No

Yes

Yes

No

Yes

No

Yes

No

Yes

No

Data Collection

Solidspresent?

Gas, no liquid?

If estimatederosion rate

acceptable nofurther action

required

Carbon steel? 13 Cr SS? Duplex SS?Seek

furtheradvice

Evaluate erosionrate

(refer to'calculation oferosion rate'

chart)

WR = ER

WR = ER +UCRc/s WR = ER + 2 * UCRc/s

WR = ER +CR13cr WR = ER + UCRc/s

IronCarbonateScaling?

OperatingTemperature?

Solids present Totally solids free

Estimatederosion rate>0.1mm/yr

Gas, no liquid?

Liquid, no gas?

Vmax = 250/√ρm

(Carbon Steel)Vmax = 300/√ρm

(13% Cr Steel)Vmax = 450/√ρm

(duplex stainless steel)

Nominallysolids free

Evaluate erosionrate

(refer to'calculation oferosion rate'

chart)

Are theconditions

non-corrosive?

Vmax = 135/√ρm

Assumemulti-phaseCarbon steel?13 Cr SS?Duplex SS?

Vmax = 300/√ρmVmax = 350/√ρm

Seekfurtheradvice

Is thesystemcarbonsteel?

Gas, no liquid?

No velocitylimits for theavoidance of

erosion

No velocitylimits for theavoidance of

erosion

Seekfurtheradvice

Liquid, no gas?

Assume wet gasor multi-phase

Assumemulti-phase

Limit velocityto 70 m/s

(230 ft/sec)

Liquid, no gas?

No velocitylimits for theavoidance of

erosion

Are corrosioninhibitors

being used?

Vmax = 200/√ρm

or 20 m/swhichever is lower

No velocitylimits for theavoidance of

erosion

Note 2: Solids Present?"Totally solids free" - the flow stream are such that there is no risk of solids beingtransported in the fluids. It should be noted that even very low levels of solids can causesignificant wastage (erosion or erosion/corrosion) rates. Hence it is very important for theuser of these guidelines to be sure that there is no risk of solids entrainment before usingthese limits."Nominally solids free" - less than 1 pptb for liquid systems, less than 0.1 lb/mmscf forgas systems; no solids detectable."Solids Present" - solids detectable in system. In this case the levels of solids will need tobe known, or appropriate assumptions made on their likely level.

Note 1: Data CollectionGas/Liquid ratio. Production rates. Tubing or piping internal bore. Solids present orabsent. Gas and liquid densities at temperature and pressure (if these are not knownthen a rough assessment can be made on the basis of an oil density of 800 kg/m3, awater density of 1000 kg/m3 and a gas density of 1 kg/m3 at STP and then adjusting thedensity for pressure and temperature.)

Note 3:Gas, No Liquid?Pure dry gas streams.No significant liquid loading.

Note 3:Gas, No Liquid?Pure dry gas streams.No significant liquid loading.

Note 9: Evaluate Erosion Rate (refer to 'Calculation of Erosion Rate' chart)

For pure dry gas streams with solids present it is not possible to define a rational

flow velocity for all possible conditions below which erosion will not occur. In this

case it will be necessary to undertake an assessment of the likely erosion rate

using the models outlined on the 'Calculation of Erosion Rate' flow chart. Account

will also need to be taken of the likelihood of the sand becoming entrained in the

gas such that it will be transported at/near the gas velocity or whether the solids

will 'settle' out of the flow stream creating a stationary bed or more slowly moving

bed of solids.Note 3:Gas, No Liquid?Pure dry gas streams.No significant liquid loading.

VELOCITY LIMITS FOR AVOIDING EROSION

GQS38294/2

∗ ∗

∗ ∗

Note 10: Erosion-CorrosionSynergy between erosion and corrosion assumed for carbon steel withan iron carbonate scale (doubling of 'unfilmed' corrosion rate) and 13%Cr stainless steel up to 80°C (corrosion rate equal to that expected for'unfilmed' carbon steel in non-erosive environment). No synergyexpected for duplex stainless steel or for 13%Cr steel above 80°C.

Note 4: Evaluate Erosion Rate (refer to'Calculat ion of Erosion Rate' char t)For pure gas streams with any solids present itis not possible to define a rational flow velocityfor all possible conditions below which erosionwill not occur. In this case it will be necessary toundertake an assessment of the likely erosionrate using the models outl ined on the'Calculation of Erosion Rate' flow chart. For'nominal ly sol ids free' condit ions i t isrecommended that it is assumed that the levelsof solids are 0.1 lb/mmscf. Account will alsoneed to be taken of the likelihood of the sandbecoming entrained in the gas such that it willbe transported at/near the gas velocity orwhether the solids will 'settle' out of the flowstream creating a stationary bed or more slowlymoving bed of solids.

General Comments:Velocities refer to net mixed velocities (nominal gas velocity plus nominal liquid velocity).Units are in ft/s (1 m/s = 3.281 ft/s).ρm refers to mixed fluid density in lbs/ft3 (1 kg/m3 = 0.06242 lbs/ft3)C factors relating Vmax to √ρm are in ft/s(lbs/ft3)1/2. Multiply by 1.22 to convert to C factorsin m/s(kg/m3)1/2

pptb - pounds of solids per thousand barrels of liquid.lb/mmscf - pounds of solids per million standard cubic feet of gas.Advice on erosion-corrosion is best available at time of publication. The situation isuncertain and the guidelines are subject to change.Fur ther advice can be obtained from the relevant special ists in UTG.

Note 5: Liquid/no gas: Vmax=250/ √ρm (carbon steel);Vmax=300/ √ρm (13 Cr steel); Vmax=450/√ρm (duplexstainless steel)Vmax=250/√ρm for carbon steel based on strength ofprotective scale on carbon steel in sea water injectionservice.Vmax=300/√ρm for 13Cr steel based on the criteria used formulti-phase conditions.Vmax=450/√ρm for duplex stainless steel based on tests forsea water injection service undertaken on behalf of BPA byDNV, Norway.

Note 6: Estimated Erosion Rate > 0.1mm/yrFor liquid and multi-phase flow streams with solids present it is not possibleto define a rational flow velocity for all possible conditions below whicherosion will not occur. In this case it will be necessary to undertake anassessment of the likely erosion rate using the models outlined on the'Calculation of Erosion Rate' flow chart. If the calculated erosion rate is lessthan 0.1mm/yr then the erosion/erosion-corrosion rate is likely to beacceptable. If the calculated erosion rate is greater than 0.1mm/yr then forcarbon steel and 13Cr steel (where the operating temperature is less than80°C) the possibility of erosion-corrosion needs to be considered and thepotential erosion-corrosion rate calculated.

Note 7:Vmax=300/ √ρm(for 13 Cr stainless steel)If higher production ratesrequired seek further advice.

Note 8:Vmax=350/ √ρm(for duplex stainless steel)If higher production ratesrequired seek further advice.

Note 11: Nomenclature for Erosion-Corrosion EquationsWR - Wastage RateER - Erosion RateUCRCS - 'Unfilmed' corrosion rate for carbon steelFCRCS - 'Filmed' corrosion rate for carbon steelCR13Cr -Corrosion rate for 13%Cr steel

Note 12: Totally Solids FreeThis guidance is only applicable to 'totally solids free' conditions,i.e. where there is no risk of solids particles being transported inthe flowstream. It should be recognised that even very low levelsof solids (below the detection levels of even 'state of the art' solidsmonitoring techniques) can cause significant wastage (erosion orerosion/corrosion) rates. Hence it is encumbent on the user of thisflow chart to ensure that there is no risk of solids entrainmentbefore using the guidance for 'totally solids free' flow.

Note 13: Are the Conditions Non-corrosive?For the purpose of these Guidelines 'non corrosive' isdefined as either:• A system where there are no corrodents (i.e. the system is totally dry or there are no corrosive species, such as H2S, CO2, O2, acids). or• A system where the materials of construction are fully corrosion resistant to the anticipated conditions.

Note 14: Non-corrosive; Gas noliquid; No Velocity Limits for theAvoidance of ErosionThere are other flow related phenomenathat need to be considered for highvelocities, e.g. noise and vibration.

* see Note 5

Note 15: Non-corrosive;Liquid no gas; No VelocityLimits for the Avoidance ofErosionIt is important to take necessarysteps (including possibly limitingthe fluid velocity) to avoid otherpossible problems, such ascavitation; plant noise/vibration;water hammer; etc.

* see Note 5

Note 16: Non-corrosive; multiphase;limit velocity to 70m/s (230ft/sec)This is the maximum velocity limit definedto avoid the possibility of droplet erosionfor gas-condensate wells in the DNVRecommended Practice ('Assessment ofErosive Wear in Piping Systems')

Note 17: Corrosive; Liquid nogas; No Velocity Limits for theAvoidance of ErosionConsideration may need to begiven to the possibility of flow-enhanced corrosion, which isoutside the scope of theseGuidelines. It is important to takenecessary steps (includingpossibly limiting the fluid velocity)to avoid other possible problems,such as cav i ta t ion; p lantnoise/vibration; water hammer;etc.

Note 17: Corrosive; Liquid no gas; No Velocity Limits for theAvoidance of ErosionConsideration may need to be given to the possibility of flow-enhancedcorrosion, which is outside the scope of these Guidelines. It is importantto take necessary steps (including possibly limiting the fluid velocity) toavoid other possible problems, such as cavitation; plant noise/vibration;water hammer; etc.

Note 18: Vmax=200/ √rm or 20m/s whicheveris lessCorrosion inhibition selection will need to takeaccount of the fact that the inhibitor will have to'work' under flowing conditions and it may bepossible to select an inhibitor that will 'work' atvelocities above the limits defined here.

Note 5:

1st Pass: Salama, RCS and/or API model, 2nd Pass: Full Tulsa model.In liquid systems particle impact velocities are reduced by the flow regime and the presence of a liquid buffer layer at the metal surface. TheRCS and API models are based on empirical tests in liquid piping and bends and have built-in allowances for such effects. This does mean,however, that there can be scaling problems in different geometries or with different solid particle sizes. The Salama model is still a 'simplified'model, but will take some account of solid particle sizes.

The Salama Model is: E = (5x10−4 W x V2 x D)/(d2 x ρm)

where E is the erosion rate in mm/yr, W is the sand flow rate in kg/day, V is the mixture velocity in m/s, D is the sand size in microns, d is thepipe internal diameter in mm, ρm is the fluid mixture density in kg/m3.

From the assessment of the Salama Model undertaken within UTG, it best equates to a 5D bend situation in comparison with the 'full'Tulsa/Harwell models. It is therefore recommended that it is not used for systems where geometrical features other than 5D bends may bepresent (e.g. 1.5D elbows, tees, severe constrictions). The model is most probably suitable for application to downhole completions, althoughin this instance care needs to be taken regards regions of significant flow constriction (e.g. insert valves).

Simplified versions of the RCS and the API models, applicable to carbon steel bends, are:

RCS: E = 4.1 x MV2.5/d2

API: E = 5.33 x MV2/d2

where E is the erosion rate in mm/yr, M is the solids production rate in g/s, V the mixed velocity in m/s and d the pipe diameter in mm.

Note 3:

Salama (Salama and Venkatesh) or Full Tulsa.The Salama model is best used for single phase (gas or liquid) systems and can be used for a 'first pass' assessment. The full Tulsa modelshould be used where the Salama model indicates an unacceptably high wastage rate, to 'optimise' the prediction (NB the Full Tulsa Modelwill often give a lower wastage rate than the Salama model).

The Salama Model is: E = (5x10-4 W x V2 x D)/(d2 x ρm)

where E is the erosion rate in mm/yr, W is the sand flow rate in kg/day, V is the mixture velocity in m/s, D is the sand size in microns, d is thepipe internal diameter in mm, ρm is the fluid mixture density in kg/m3.

From the assessment of the Salama Model undertaken within UTG, it best equates to a 5D bend situation in comparison with the 'full'Tulsa/Harwell models. It is therefore recommended that it is not used for systems where geometrical features other than 5D bends may bepresent (e.g. 1.5D elbows, tees, severe constrictions). The model is most probably suitable for application to downhole completions, althoughin this instance care needs to be taken regards regions of significant flow constriction (e.g. insert valves).

A very simplified version of the Salama model (developed by Salama & Venkatesh), applicable to gas systems with carbon steel bends (including1.5D elbows, tees, etc.) is:

E = 604 x MV2/d2

where E is the erosion rate in mm/yr, M is the solids production rate in g/s, V the mixed velocity in m/s and d the pipe diameter in mm.

Yes

No

No

Note 2:

Gas, No Liquid?Pure gas streams. No significant liquid loading.

Note 4:

Liquid, No GasSingle phase liquid streams. No gas bubbles.

Note 6:

Slug Flow?The Harwell model for multiphase erosion isbased on vertical flow. Under such conditionsslug flow, which leads to liquid being throwndown onto the bottom of a pipe, is not produced.Thus the standard Harwell models for annularmist, churn and bubble flow are not applicable. In slug flow the 'liquid slug' will be thrown againstthe pipe wall at velocities approaching the netmixed velocity. In addition, at the slug front therewill be considerable mixing and hence entrainedgas, such that the slug front will approach thehomogenous mixture. Therefore, it isrecommended that the pure liquid models beused (see Note 5) but that the mixed fluid velocityand mixture properties should be used ratherthan the liquid velocity and density.

No

Yes

Yes

No

Yes

General Comments:

Advice is best available at time of publication.Most of the models used assume sharp sand particles with a diameter of 150 µm. The Salama model (usedfor single phase gas or liquid conditions only) and more detailed Tulsa and Harwell models can make allowancesfor solids particle size (all three models), plus density and shape (Tulsa model only).The erosion calculations are generally for bends and conditions of turbulence (e.g. constrictions) only. Theexception to this is the Tulsa model that has a (as yet untested) module for evaluating the erosion rate in straightpipe. In general, erosion in straight sections is at least an order of magnitude less than at bends. The onlyexception to this will be horizontal slug flow where liquid is thrown against the pipe wall.The Full Tulsa Model is available as a computer software package ('Sand Production Pipe Saver'; SPPS v. 4.1.)The Harwell Model is available as a computer software package ('Design Procedure for Erosion-Corrosion inMulti-phase Flow'; Sandman v. 3.9.).Further advice can be obtained the relevant specialists in UTG.

No

Yes Yes

Note 1 :

Data Collection.For the simpler models:Production Rate (i.e. liquid and gas flow rates [orGOR]). Pressure and Temperature. Liquid densityand gas density (under operating conditions). Tubingor piping size. Solids content and particle size.

For the 'full' Harwell and Tulsa Models:The data indicated above plus; Gas Viscosity (underoperating conditions). Liquid Viscosity (under operatingconditions). Solids density and 'shape' (e.g. sharp,semi-rounded). CO2 and H2S partial pressures. Tubingor piping geometry and configuration. Steel hardness(if material of construction is a carbon/low alloy steel).

Data Collection

Salama,(Salama and Venkatesh)

orFull Tulsa

Stratified flow?

1st Pass:Salama,RCS

and/or API Model

2nd Pass:Full Tulsa Model

2nd Pass:Full Tulsa Model

Full Tulsa Model

Bubble/ChurnFlow?

Annular flow?

Harwelland/or

Full Tulsa Model

HarwellModel

CALCULATION OF EROSION RATES

GQS38294/1

Gas, no liquid?

Liquid, no gas?

Slug flow?

Note10:

Bubble/Churn Flow? Harwelland/or Full Tulsa ModelDo not use the Tulsa Model forChurn flow. For bubbly flow withthe Full Tulsa Model use the mixed(averaged) velocity and liquidproperties

Note 11:

Annular Flow? Harwell Model.For comparison, check using the FullTulsa Model with the mixed velocity andwith:(i) Mixed (averaged) fluid properties(ii) Liquid propertiesThe actual erosion rate should besomewhere between the two values.

Note 7:

Stratified Flow? Full Tulsa ModelUse the liquid velocity calculated forthe hydraulic diameter

Note 9:

Slug Flow? 2nd Pass, Full Tulsa ModelUse the mixed (averaged) fluid properties (densityand viscosity) and velocity

Note 8:

Slug Flow? 1st Pass Salama,RCS and/or API ModelUse the mixed (averaged) fluid density andvelocity

1st Pass:Salama, RCS

and/or API Model

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Notes on Flow Charts.

Figure 1 - Velocity Limits for Avoiding ErosionGeneral guidance on velocity limits for corrosive or non-corrosive fluids in injection andproduction service. This guidance does not take into account velocity limits for corrosionalone (e.g. allowable flow rates for effective corrosion inhibition) or the effect of flow oncorrosion (i.e. flow-enhanced corrosion).

General Comments:Velocities refer to net mixed velocities (nominal gas velocity plus nominal liquid velocity).Units are in ft/s (1 m/s = 3.281 ft/s).ρm refers to mixed fluid density in lbs/ft3 (1 kg/m3 = 0.06242 lbs/ft3)C factors relating Vmax to √ρm are in ft/s(lbs/ft3)1/2. Multiply by 1.22 to convert to Cfactors in m/s(kg/m3)1/2

pptb - pounds of solids per thousand barrels of liquid.lb/mmscf - pounds of solids per million standard cubic feet of gas.Advice on erosion-corrosion is best available at time of publication. The situation isuncertain and the guidelines are subject to change.Further advice can be obtained from the relevant specialists in UTG.

Note 1: Data CollectionGas/Liquid ratio. Production rates. Tubing or piping internal bore. Solids present orabsent. Gas and liquid densities at temperature and pressure (if these are not known then arough assessment can be made on the basis of an oil density of 800 kg/m3, a water densityof 1000 kg/m3 and a gas density of 1 kg/m3 at STP and then adjusting the density forpressure and temperature.)

Note 2: Solids Present?“Totally solids free” - the flow stream are such that there is no risk of solids beingtransported in the fluids. It should be noted that even very low levels of solids can causesignificant wastage (erosion or erosion/corrosion) rates. Hence it is very important for theuser of these guidelines to be sure that there is no risk of solids entrainment before usingthese limits."Nominally solids free" - less than 1 pptb for liquid systems, less than 0.1 lb/mmscf for gassystems; no solids detectable."Solids Present" - solids detectable in system. In this case the levels of solids will need tobe known, or appropriate assumptions made on their likely level.

Note 3:Gas, No Liquid?Pure dry gas streams. No significant liquid loading.

Note 4: Evaluate Erosion Rate (refer to ‘Calculation of Erosion Rate’ chart)For pure gas streams with any solids present it is not possible to define a rational flowvelocity for all possible conditions below which erosion will not occur. In this case it willbe necessary to undertake an assessment of the likely erosion rate using the models

8

outlined on the ‘Calculation of Erosion Rate’ flow chart. For ‘nominally solids free’conditions it is recommended that it is assumed that the levels of solids are 0.1 lb/mmscf.Account will also need to be taken of the likelihood of the sand becoming entrained in thegas such that it will be transported at/near the gas velocity or whether the solids will‘settle’ out of the flow stream creating a stationary bed or more slowly moving bed ofsolids.

Note 5: Liquid/no gas: Vmax=250/√√ρρm (carbon steel); Vmax=300/√√ρρm (13 Cr steel);Vmax=450/√√ρρm (duplex stainless steel)Vmax=250/√ρm for carbon steel based on strength of protective scale on carbon steel in seawater injection service.Vmax=300/√ρm for 13Cr steel based on the criteria used for multi-phase conditions.Vmax=450/√ρm for duplex stainless steel based on tests for sea water injection serviceundertaken on behalf of BPA by DNV, Norway.

Note 6: Estimated Erosion Rate > 0.1mm/yrFor liquid and multi-phase flow streams with solids present it is not possible to define arational flow velocity for all possible conditions below which erosion will not occur. Inthis case it will be necessary to undertake an assessment of the likely erosion rate using themodels outlined on the ‘Calculation of Erosion Rate’ flow chart. If the calculated erosionrate is less than 0.1mm/yr then the erosion/erosion-corrosion rate is likely to beacceptable. If the calculated erosion rate is greater than 0.1mm/yr then for carbon steeland 13Cr steel (where the operating temperature is less than 80°C) the possibility oferosion-corrosion needs to be considered and the potential erosion-corrosion ratecalculated.

Note 7:Vmax=300/√√ρρm (for 13 Cr stainless steel)If higher production rates required seek further advice.

Note 8:Vmax=350/√√ρρm (for duplex stainless steel)If higher production rates required seek further advice.

Note 9: Evaluate Erosion Rate (refer to ‘Calculation of Erosion Rate’ chart)For pure dry gas streams with solids present it is not possible to define a rational flowvelocity for all possible conditions below which erosion will not occur. In this case it willbe necessary to undertake an assessment of the likely erosion rate using the modelsoutlined on the ‘Calculation of Erosion Rate’ flow chart. Account will also need to betaken of the likelihood of the sand becoming entrained in the gas such that it will betransported at/near the gas velocity or whether the solids will ‘settle’ out of the flowstream creating a stationary bed or more slowly moving bed of solids.

9

Note 10: Erosion-CorrosionSynergy between erosion and corrosion assumed for carbon steel with an iron carbonatescale (doubling of ‘unfilmed’ corrosion rate) and 13 % Cr stainless steel up to 80°c(corrosion rate equal to that expected for ‘unfilmed’ carbon steel in non-erosiveenvironment). No synergy expected for duplex stainless steel or for 13%Cr steel above 80°c.

Note 11: Nomenclature for Erosion-Corrosion EquationsWR - Wastage RateER - Erosion RateUCRCS - ‘Unfilmed’ corrosion rate for carbon steelFCRCS - ‘Filmed’ corrosion rate for carbon steelCR13Cr -Corrosion rate for 13%Cr steel

Note 12: Totally Solids FreeThis guidance is only applicable to ‘totally solids free’ conditions, i.e. where there is norisk of solids particles being transported in the flowstream. It should be recognised thateven very low levels of solids (below the detection levels of even ‘state of the art’ solidsmonitoring techniques) can cause significant wastage (erosion or erosion/corrosion) rates.Hence it is encumbent on the user of this flow chart to ensure that there is no risk of solidsentrainment before using the guidance for ‘totally solids free’ flow.

Note 13: Are the Conditions Non-corrosive?For the purpose of these Guidelines ‘non corrosive’ is defined as either:• A system where there are no corrodents (i.e. the system is totally dry or there are no

corrosive species, such as H2S, CO2, O2, acids). or • A system where the materials of construction are fully corrosion resistant to the

anticipated conditions.

Note 14: Non-corrosive; Gas no liquid; No Velocity Limits for the Avoidance ofErosionThere are other flow related phenomena that need to be considered for high velocities, e.g.noise and vibration.

Note 15: Non-corrosive; Liquid no gas; No Velocity Limits for the Avoidance ofErosion

It is important to take necessary steps (including possibly limiting the fluid velocity) toavoid other possible problems, such as cavitation; plant noise/vibration; water hammer;etc.

10

Note 16: Non-corrosive; multiphase; limit velocity to 70m/s (230ft/sec)This is the maximum velocity limit defined to avoid the possibility of droplet erosion forgas-condensate wells in the DNV Recommended Practice (‘Assessment of Erosive Wearin Piping Systems’)

Note 17: Corrosive; Liquid no gas; No Velocity Limits for the Avoidance of Erosion

Consideration may need to be given to the possibility of flow-enhanced corrosion, which isoutside the scope of these Guidelines. It is important to take necessary steps (includingpossibly limiting the fluid velocity) to avoid other possible problems, such as cavitation;plant noise/vibration; water hammer; etc.

Note 18: Vmax=200/√√ρρm or 20m/s whichever is lessCorrosion inhibition selection will need to take account of the fact that the inhibitor willhave to ‘work’ under flowing conditions and it may be possible to select an inhibitor thatwill ‘work’ at velocities above the limits defined here.

11

Figure 2 - Calculation of Erosion RatesGeneral Comments:Advice is best available at time of publication.Most of the models used assume sharp sand particles with a diameter of 150 µm. TheSalama model (used for single phase gas or liquid conditions only) and more detailedTulsa and Harwell models can make allowances for solids particle size (all three models),plus density and shape (Tulsa model only).The erosion calculations are generally for bends and conditions of turbulence (e.g.constrictions) only. The exception to this is the Tulsa model that has a (as yet untested)module for evaluating the erosion rate in straight pipe. In general, erosion in straightsections is at least an order of magnitude less than at bends. The only exception to thiswill be horizontal slug flow where liquid is thrown against the pipe wall.The Full Tulsa Model is available as a computer software package (‘Sand Production PipeSaver’; SPPS v. 4.1.)The Harwell Model is available as a computer software package (‘Design Procedure forErosion-Corrosion in Multi-phase Flow’; Sandman v. 3.9.).Further advice can be obtained the relevant specialists in UTG.

Note 1: Data Collection.For the simpler models:Production Rate (i.e. liquid and gas flow rates [or GOR]). Pressure and Temperature.Liquid density and gas density (under operating conditions). Tubing or piping size. Solidscontent and particle size.For the ‘full’ Harwell and Tulsa Models:The data indicated above plus; Gas Viscosity (under operating conditions). LiquidViscosity (under operating conditions). Solids density and ‘shape’ (e.g. sharp, semi-rounded). CO2 and H2S partial pressures. Tubing or piping geometry and configuration.Steel hardness (if material of construction is a carbon/low alloy steel).

Note 2: Gas, No Liquid?Pure gas streams. No significant liquid loading.

Note 3: Salama (Salama and Venkatesh) or Full Tulsa.The Salama model is best used for single phase (gas or liquid) systems and can be used fora ‘first pass’ assessment. The full Tulsa model should be used where the Salama modelindicates an unacceptably high wastage rate, to ‘optimise’ the prediction (NB the FullTulsa Model will often give a lower wastage rate than the Salama model).

The Salama Model is:

E = (5x10-4 W x V2 x D)/(d2 x ρm)

where E is the erosion rate in mm/yr, W is the sand flow rate in kg/day, V is the mixturevelocity in m/s, D is the sand size in microns, d is the pipe internal diameter in mm, ρm isthe fluid mixture density in kg/m3.

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From the assessment of the Salama Model undertaken within UTG, it best equates to a 5Dbend situation in comparison with the ‘full’ Tulsa/Harwell models. It is thereforerecommended that it is not used for systems where geometrical features other than 5Dbends may be present (e.g. 1.5D elbows, tees, severe constrictions). The model is mostprobably suitable for application to downhole completions, although in this instance careneeds to be taken regards regions of significant flow constriction (e.g. insert valves).

A very simplified version of the Salama model (developed by Salama & Venkatesh),applicable to gas systems with carbon steel bends (including 1.5D elbows, tees, etc.) is:

E = 604 x MV2/d2

where E is the erosion rate in mm/yr, M is the solids production rate in g/s, V the mixedvelocity in m/s and d the pipe diameter in mm.

Note 4: Liquid, No GasSingle phase liquid streams. No gas bubbles.

Note 5: 1st Pass: Salama, RCS and/or API model, 2nd Pass: Full Tulsa model.In liquid systems particle impact velocities are reduced by the flow regime and thepresence of a liquid buffer layer at the metal surface. The RCS and API models are basedon empirical tests in liquid piping and bends and have built-in allowances for such effects.This does mean, however, that there can be scaling problems in different geometries orwith different solid particle sizes. The Salama model is still a ‘simplified’ model, but willtake some account of solid particle sizes.

The Salama Model is:

E = (5x10-4 W x V2 x D)/(d2 x ρm)

where E is the erosion rate in mm/yr, W is the sand flow rate in kg/day, V is the mixturevelocity in m/s, D is the sand size in microns, d is the pipe internal diameter in mm, ρm isthe fluid mixture density in kg/m3.

From the assessment of the Salama Model undertaken within UTG, it best equates to a 5Dbend situation in comparison with the ‘full’ Tulsa/Harwell models. It is thereforerecommended that it is not used for systems where geometrical features other than 5Dbends may be present (e.g. 1.5D elbows, tees, severe constrictions). The model is mostprobably suitable for application to downhole completions, although in this instance careneeds to be taken regards regions of significant flow constriction (e.g. insert valves).

Simplified versions of the RCS and the API models, applicable to carbon steel bends, are:

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RCS:E = 4.1 x MV2.5/d2

API:

E = 5.33 x MV2/d2

where E is the erosion rate in mm/yr, M is the solids production rate in g/s, V the mixedvelocity in m/s and d the pipe diameter in mm.

Note 6: Slug Flow?The Harwell model for multiphase erosion is based on vertical flow. Under suchconditions slug flow, which leads to liquid being thrown down onto the bottom of a pipe,is not produced. Thus the standard Harwell models for annular mist, churn and bubbleflow are not applicable. In slug flow the ‘liquid slug’ will be thrown against the pipe wallat velocities approaching the net mixed velocity. In addition, at the slug front there will beconsiderable mixing and hence entrained gas, such that the slug front will approach thehomogenous mixture. Therefore, it is recommended that the pure liquid models be used(see Note 5) but that the mixed fluid velocity and mixture properties should be used ratherthan the liquid velocity and density.

Note 7: Stratified Flow? Full Tulsa ModelUse the liquid velocity calculated for the hydraulic diameter

Note 8: Slug Flow? 1st Pass Salama, RCS and/or API ModelUse the mixed (averaged) fluid density and velocity

Note 9: Slug Flow? 2nd Pass, Full Tulsa ModelUse the mixed (averaged) fluid properties (density and viscosity) and velocity

Note 10: Bubble/Churn Flow? Harwell and/or Full Tulsa ModelDo not use the Tulsa Model for Churn flow. For bubbly flow with the Full Tulsa Modeluse the mixed (averaged) velocity and liquid properties

Note 11: Annular Flow? Harwell Model.For comparison, check using the Full Tulsa Model with the mixed velocity and with:(i) Mixed (averaged) fluid properties(ii) Liquid propertiesThe actual erosion rate should be somewhere between the two values.

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General Comments and Conclusions

1. In the absence of any solids, erosion by non-corrosive fluids (e.g. droplet erosion)is not significant at velocities below 70 m/s (230 ft/s). However, totally solids-free, non-corrosive fluids at such high velocities are relatively uncommon in oil/gas field service,with the possible exception of flow through choke valves, which are covered in separateGuidelines.

2. Erosion by solids is generally proportional to MV2/d2 if all else (e.g. flow regime,gas-liquid ratio) remains constant, where M is the solids production rate (e.g. in g/s), V isthe net fluid velocity (e.g. in m/s) and d is the pipe or tubing internal diameter (e.g. inmm).

- It should be noted that this can be expressed as SV2/d2 where S is the solidsconcentration in the fluid (e.g. in pounds per thousand barrels of liquid, lbs/mmscf of gasor ppm).

- Thus if the production rate doubles then the pure erosion rate (ie ignoringcorrosion) will increase by a factor of 8.

- Given that increased production can often increase the solids concentration (orsolids "loading") then a rule-of-thumb would be that a two-fold increase in productiongives an order of magnitude increase in erosion if solids are present.

3. Erosion rates are proportional to the solids concentration in the fluid. It is unclearwhether there is a threshold solids concentration below which erosion cannot occur.However, 1 pound per thousand barrels (1 pptb) of liquid for oil/multiphase systems(equivalent to about 0.1 lb/mmscf of gas for gas systems) is at the level of detection ofcurrent solids (e.g. sand) monitoring techniques. Therefore, for the sake of theseGuidelines "nominally solids free" conditions are assumed to contain 1 pptb forliquid/multiphase systems and 0.1lb/mmscf for gas systems. “Totally solids free” indicatesduties where there is absolutely no risk of entrained solids in the flowstream under anycircumstances (e.g. some treated gas transport lines, some gas fields).

4. Erosion depends critically on the fluid flow regime. Solid particles carried in gasflow may hit pipe walls at the full gas velocity, although it is likely that in many cases thesolid particles will drop out of the gas stream and either form a static bed or a moving bed(moving dunes, scouring). Under full liquid flow, solid particles will frequently travel atthe liquid velocity, but will be significantly slowed by a liquid barrier layer on the pipe wallbefore striking the pipe material surface. Under multiphase flow, some solid particles maybe carried at/near the gas velocity (if the flow is annular mist) and may or may not beslowed down by a liquid barrier layer - depending on the thickness of the annular liquidfilm. Careful assessment and a knowledge of flow regimes is required in such cases.

5. Empirical and field data suggest that there is a threshold solid particle size belowwhich erosion will not occur. This threshold is unclear and probably relates to whether, atthe net fluid velocity, a given particle has the momentum to carry it through the barrierfluid at the pipe or tubing surface. It should be noted that such thresholds are only

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applicable to ‘normal flow’ conditions. For example, is has been found that for downholesand screens through which very small particles can ‘pass’ even these very small particlescan result in erosion due to the very high energy flow and high probability of impacting themetal surface. Most of the work reported is based on sand particles of 150 µm diameter.However, the full Tulsa model can make allowance for different particle sizes, densities,shapes and sharpness. The full Harwell model and the Salama model can make allowancefor different particle sizes.

6. Most erosion damage will occur at bends and flow disruptions and is likely to be atleast an order of magnitude greater than erosion in straight pipe or tubing. The possibleexception to this is slug flow where flow can impact on the pipe or tubing wall on straightsections. The full Tulsa model now contains a module (as yet not validated) for erosion instraight pipe. Presently this only covers single phase flow (e.g. slug flow is not covered).

7. Although different materials exhibit different solids erosion characteristics, thevariation is not large between the common materials, e.g. carbon steel, 13 Cr stainlesssteel and duplex stainless steel. As a first pass, it is sufficient to ignore differencesbetween the erosion resistance of such materials.

8. In many production and injection services there will be a significant corrosion riskfrom either CO2 or O2 corrosion. It should be noted that velocity can effect suchcorrosion in three ways:

- increase the mass transport of the corrosion species.- in the absence of solids, lead to flow that can damage the protective layers

normally formed in such service.- in the presence of solids, lead to erosion that can damage or remove protectivelayers as well as cause physical removal of metal.

All of the above are referred to at times as erosion or erosion-corrosion. In this report thefirst is referred to as flow-enhanced corrosion. The second and third are forms ofenhanced corrosion resulting from erosion-corrosion.

9. The severity of erosion-corrosion depends on whether there is a synergistic effectbetween erosion and corrosion or whether the erosion and corrosion are independent. Ifthe former then the total wastage will be greater than the sum of the independent erosionand corrosion wastage.

10. In environments containing CO2 and/or O2 corrosion is often controlled by thepresence of protective layers. In the case of carbon steel this is normally a precipitatedlayer of corrosion product; in the case of duplex and austenitic stainless steels it will be avery thin (around 10-9 m or 10's of Å) passive layer; in the case of 13 Cr stainless steel itwill be something intermediate between a precipitated layer and a passive film. Undersolids-free conditions these protective layers can be damaged or eroded by pure fluid flow.Droplet impact in multiphase flow is possible (e.g. in annular-mist flow) and the resultant

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damage can be significantly more severe than the damage caused by shear stress forces inpure liquid flow. Passive films on materials such as duplex stainless steel are the strongestand most adherent and reform very rapidly; precipitated films on carbon steel are theweakest and least adherent and reform relatively slowly.

11. In solids-containing environments, the situation for erosion-corrosion is unclear. Ifeither the expected erosion or expected corrosion are an order of magnitude less than theother then synergistic effects are likely to be small. Laboratory data suggests that solidserosion can lead to severe localised attack in carbon steel if the erosivity is below a certainvalue or totally destroy a region of protective layer at higher values (leading to generalcorrosion but not penetrating the wall so quickly). There is evidence to suggest that, inanaerobic CO2 containing environments, solids can damage protective layers on 13 % Crmaterials leading to erosion-corrosion at temperatures up to 80°c. Above this the 13%Crsteel has been found to re-film very quickly, i.e. no synergy between erosion and corrosionis expected. Results on duplex stainless steel suggest that there is no corrosion-erosionsynergy - implying that the wastage is only through erosion.

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Erosion Guidelines - Detailed Discussions

Introduction

Erosion can be defined in a variety of ways, but is essentially the wastage of material dueto the mechanical removal of material surfaces by flowing environments. Such wastage ismost extreme when solids are present in the environment.

Erosion is a problem to BP AMOCO when operating conditions lead to erosion andconsequent damage to equipment or, conversely, when velocity and hence productionlimits are set to avoid erosion. If these limits are overly conservative then BP AMOCOloses production; if these limits are overly optimistic then BP AMOCO risks erosiondamage, with consequential loss of production, increased maintenance costs and/orpossible loss of system integrity.

Erosion problems are likely to increase in BP AMOCO in the future because of:

• increased water cuts putting pressure on total fluid production rates to maintain oilproduction,

• increased use of multiphase flow in the transport of production fluids,• increased sand and solids production rates due to a number of factors, such as

increased water cut, use of proppant and reservoir fracturing techniques.

Many flow dependent wastage mechanisms are termed "erosion". For produced fluidsthere are four main mechanisms to be considered:

• erosion by non-corrosive fluids through liquid droplet impact• "pure" solids erosion by a non-corrosive fluid carrying solid particles• erosion-corrosion by a corrosive medium in the absence of solids• erosion-corrosion by a corrosive medium containing solids.

The third of these is sometimes confused with flow-enhanced corrosion, where the flowregime leads to enhanced mass transport of corrosion products and reactants. In theseGuidelines erosion-corrosion in the absence of solids is taken to refer to enhanced wastagedue to the physical rupture of the protective, corrosion-product layer by energetic fluidflow regimes and the consequential corrosion. The mechanical removal of inhibitor mightbe defined as a form of erosion-corrosion but is not discussed in detail in these Guidelines.

Erosion-corrosion occurs in environments which have the potential to be both erosive andcorrosive. The erosion and the corrosion can either be independent, in which case thetotal wastage is the sum of the wastage produced by each mechanism in isolation, orsynergistic, in which case the total wastage is greater than the sum of the independentprocesses of erosion and corrosion.

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Discussion of the Guidelines

1. NO solid particles

The Guidelines in this Section are only applicable to ‘totally solids free’ conditions, i.e.where there is no risk of solids particles being transported in the flowstream. It should berecognised that even very low levels of solids (below the detection levels of even ‘state ofthe art’ solids monitoring techniques) can cause significant wastage (erosion orerosion/corrosion) rates. Hence it is encumbent on the user of these Guidelines to ensurethat there is no risk of solids entrainment before using the guidance in this Section.

1.1. Non-corrosive fluid flowFor pure single phase non-corrosive gases in the total absence of solids or entrainedliquids there are no velocity limits to avoid erosion. However, there are other flow relatedphenomena that need to be considered for high velocities, e.g. noise and vibration.

For single phase non-corrosive liquid flow (i.e. totally solids free and with no entrainedgas bubbles) there are no velocity limit requirements to avoid erosion damage. However, itis important to take necessary steps (including possibly limiting the fluid velocity) to avoidother possible problems, such as cavitation1; plant noise/vibration; water hammer2; etc.

Liquid droplet erosion (e.g. in annular mist flow) of metals under non-corrosive conditionsin the total absence of solids will only be a concern at velocities above 70 m/s (230 ft/sec).This is the maximum velocity limit defined to avoid the possibility of droplet erosion forgas-condensate wells in the DNV Recommended Practice (Ref. 13). Totally solids-free,non-corrosive fluids at such high velocities are relatively uncommon in oil/gas fieldservice, with the possible exception of flow through choke valves, which are covered inseparate Guidelines.

1.2. Corrosive fluid flowIn the total absence of solids, erosive effects can be produced by the flow regimephysically damaging protective/semi-protective corrosion-product layers. However,corrosion will still occur in corrosive regimes even if this does not happen; ie if thevelocity or production rate is below a critical threshold for physical disruption of anyprotective layers. This corrosion will be fluid-flow dependent. For example, carbon steel

1 Where liquid pressures are at or near the vapour pressure/gas bubble point pressure then bubbles canform at regions of localised pressure drop these can then implode abruptly at points where the localpressure rises again above the saturation/bubble point pressure. These implosions can cause removal ofmaterial [cavitation] and/or noise problems.2 Water hammer results from the shock pressure due to the sudden stopping of a liquid (e.g. when closinga valve or where reciprocating pumps or compressors are used). The magnitude of this shock pressure is afunction of the fluid velocity, the stoppage time and the elasticity of the pipe. The accompanyingmechanical vibrations can result in fatigue failure if corrective actions are not taken.

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in CO2 and O2-containing environments (eg sea water injection) will generally sufferaccelerated attack as the flow rate increases, as a result of increased mass transport.

The situation is made much worse if the flow rate increases enough to cause erosive ormechanical breakdown of protective layers. There are two circumstances to be considered- multiphase gas-liquid flow and single phase liquid flow. The former is generally muchmore energetic than the latter and thus more likely to lead to mechanical disruption ofprotective product layers.

For single phase liquid flow (i.e. totally solids free and with no entrained gas bubbles)there are no velocity limit requirements to avoid erosion damage. However, as note above,it is important to take necessary steps (including possibly limiting the fluid velocity) toavoid other possible problems, such as enhanced corrosion under flowing conditions;cavitation1; plant noise/vibration; water hammer2; etc.

For wet (i.e. potentially corrosive) gas and multi-phase flow conditions, in the specificcase of inhibited carbon steel it is recommended that the maximum velocity for designconsiderations should be taken as C=200 or 20m/s (whichever is lower). However,corrosion inhibition selection will need to take account of the fact that the inhibitor willhave to ‘work’ under flowing conditions and it may be possible to select an inhibitor thatwill ‘work’ at velocities above the limits defined here. For other materials/conditions it isrecommended to consider the limits for ‘nominally sand-free’ conditions as an interimmeasure, as there is little/no information available on how the limits for thesematerials/conditions may differ for totally solids free conditions (i.e. where the onlyerosion damage mechanisms are the result of liquid droplet or gas bubble impingement).

2. Nominally solids free

For the purpose of these Guidelines ‘nominally solids-free’ conditions are defined as lessthan one pound of solids per thousand barrels of liquids (<1pptb) for ‘liquid’ (e.g.oil/water) systems and less than 0.1 pounds of solids per million standard cubic feet of gas(<0.1lb/mmscf) for gas systems.

The origin of the 1pptb limit is that this was determined to be the minimum level of solidsthat could be detected using ‘state of the art’ sand detection tools. The 0.1lb/mmscf wasdetermined to be the equivalent quantity of solids for a gas system. Therefore these limitsshould be applied to systems where there is the possibility of solids being present, butwhere these are likely to be (or actually are) below the limits of detection when using‘state of the art’ sand detection monitors3.

3 Note the limit of detection of less rigorous sand detection methods is significantly less than these limits.For example in the case of the ‘shake out’ centrifuge test the limit of detection is only 275pptb and thelimit of detection for the ‘Leutart Sampler’ is 5pptb. This must be taken into account when determiningwhether a system can be considered ‘nominally solids free’ or not.

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2.1. Non-corrosive fluid flow

There are no specific issues for ‘nominally solids free, non-corrosive fluid flow’conditions. This can best be dealt with in the same way as for ‘Solids containing - non-corrosive fluid flow’ (Section 3.1.) with the solids content being set to 1pptb or0.1lb/mmscf, as appropriate for the particular application.

2.2. Corrosive fluid flow

There are two circumstances to be considered - multiphase gas-liquid flow and singlephase liquid flow (NB guidance for wet gas is included under multi-phase gas-liquid flow,as liquid water is required for corrosive conditions). The former is generally much moreenergetic than the latter and thus more likely to lead to mechanical disruption of protectiveproduct layers.

2.2.1. Multiphase FlowFor nominally solids-free conditions C values of 135, 300 and 350 ft/s(lbs/ft3)0.5 arecurrently recommended for carbon steel, 13 % Cr and duplex stainless steels respectivelyunder conditions of CO2 corrosion. Damage, if it occurs, is most likely at bends andelbows between the 15o and 50o positions on the outer radius. The rate of attack isuncertain. For carbon steel the localised damage of any protective layers is liable to initiatea form of "mesa" attack (steep-sided pitting in CO2 service) and the rate of penetrationcould be up to twice the bare-surface “Cassandra” rate (see Ref. 2 for details onestimating CO2 corrosion rates using the BP Amoco ‘Cassandra’ software package). For13 % Cr steel localised pitting may result, but there is little service experience with suchattack. Under such circumstances the ability of the alloy to repair damage to theprotective film will be critical. There could be a significant delay in the reformation of theprotective film on 13 % Cr material at lower temperatures (below say 80°C). However, ithas been found that above this temperature film repair can be rapid in CO2 service. Forduplex stainless steel the protective film (passive layer) is very resilient, even if it isdamaged it reforms (repassivates) very rapidly. Therefore, little or no interaction betweenerosion and corrosion would be expected for duplex stainless steel. This has been borneout by laboratory experiments (Ref. 3).

In the case of 13%Cr steel the C-factor of 300 was determined from previous testing atAEA Harwell (Ref. 14) and field experience. Rather than defining a true 'velocity limit'above which unacceptable erosion/erosion-corrosion will occur, this represented themaximum C-factor for which data was available and for which there was no evidence ofunacceptable erosion/erosion-corrosion. Therefore, it represents a limit of understandingrather than an actual acceptance limit. A number of E&P Business Units have identified aneed to exceed the present maximum allowable velocity to maximise production. There istherefore a clear business driver to understand the maximum flow rates that could beallowed for 13%Cr steel. As a result if this a Project has been set up within the ‘NoCorrosion R&D’ programme for 1999/2000 to evaluate the maximum allowable velocityfor 13%Cr steel via ‘Field Tests’ on gas flowlines in the Tuscaloosa (Louisiana) Field.

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Similarly for duplex stainless steel, the C-factor of 350 was established by examining thelimits of data available from previous testing at AEA Harwell, published information andfield experience. The test work in 1999/2000 to evaluate the maximum allowable velocityfor 13%Cr steel may well be extended to duplex stainless steel to examine if this C-factorcan be increased, if there is sufficient Business Unit interest.

In stratified and annular mist flow direct impingement on the pipe wall will be most severeat bends. The situation with multiphase slug flow is more uncertain. In slug flow thechurning and breaking wave at the leading edge of a slug can give rise to perpendicularimpacts on the bottom of straight horizontal pipe as well as at bends. There is currentlyno well defined limit for the initiation of such damage, especially as the situation iscomplicated by the presence of significant mixing and entrained gas bubbles in the slugfront. If it is assumed that the liquid slug impacting on the wall needs to have the sameimpact velocity as above and that the liquid slug impact velocity is, at worst, equal to themixed fluid velocity, then the API limit with C=135 ft/s(lbs/ft3)0.5 could be applicable in thecase of carbon steel. Thus for carbon steel if slug flow is established and if the mixed fluidvelocity is above the API limit with C=135 ft/s(lbs/ft ft3)0.5 then pitting damage could beexpected at any location all along the bottom of a pipe. The situation might be mitigatedsomewhat if the protective layer on carbon steel can reform between slugs This is notpossible in continuous annular flow and not likely at bends in slug flow. (NB apply thesame principle but use C=300 and 350 ft/s(lbs/ft3)0.5 for 13 % Cr steel and duplex stainlesssteel respectively)

The situation is further complicated in multiphase annular mist flow and multiphase slugflow when corrosion inhibitors are added. There is some suggestion that corrosioninhibitors might be effective up to the same velocity as protective corrosion-product layers(Refs. 4 & 5). If this is the case, then once the thresholds for physical damage toprotective corrosion-product layers have been reached, corrosion inhibition is unlikely tobe effective. However, the strength of the bond between the corrosion inhibitor and themetal surface may be greater than that of the precipitated corrosion product layer. Thelatter is only physically bonded to the metal surface whereas the corrosion inhibitor will bechemically bonded and perhaps more able to resist displacement. If the corrosion inhibitoris bonded to the corrosion product layer then the layer/metal bond may be the weak link.In such a case the erosion may clean the surface of weakly bonded corrosion productlayers and the corrosion inhibitor can then bond directly to the bare metal surface,providing far greater resistance to corrosion even under erosive conditions.

Flowing sand particles do eventually remove a corrosion inhibitor film from a steel surfacein experiments using an impinging liquid jet containing sand. However, work at theUniversity of Tulsa showed that a suitable corrosion inhibitor chemical was still beneficial,by significantly increasing the safe operating velocity of the fluids by as much as a factorof 4 or 5. These are still preliminary findings for a particular product and set ofconditions. It is not yet possible to derive a semi-quantitative rule of thumb.

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For wet (i.e. potentially corrosive) gas and multi-phase flow conditions, in the specificcase of inhibited carbon steel it is recommended that the maximum velocity for designconsiderations should be taken as C=200 or 20m/s (whichever is lower). However,corrosion inhibition selection will need to take account of the fact that the inhibitor willhave to ‘work’ under flowing conditions and it may be possible to select an inhibitor thatwill ‘work’ at velocities above the limits defined here.

Loss of corrosion inhibitor from bulk fluids by adsorption onto the surface of sandparticles can be a significant effect under certain circumstances ,such as high inhibitorconcentrations (>150 ppm) and high sand concentrations (>35 pptb). The adsorptionlosses are normally insignificant for low corrosion inhibitor concentrations (<50 ppm) andlow sand concentrations (<35 pptb).

2.2.2. Single Phase Liquid FlowProvisionally, it is recommended that a C value 250 ft/s(lbs/ft3)0.5 should be used as thelimit for carbon steel under CO2 corrosion in the absence of corrosion inhibition.

However, the situation in the field is often aggressive enough to require the use ofcorrosion inhibitors. If this is the case, highly turbulent flow will increase corrosion ratesfurther. Some corrosion inhibitors perform poorly under highly turbulent flow conditionswhilst others can perform acceptably under extremely aggressive flow. In general, themore turbulent the flow regime, the higher concentration of inhibitor that will be requiredto achieve acceptable corrosion rates and therefore operating costs will increase. Undersuch circumstances corrosion inhibition selection (and dosage levels) will need to takeaccount of the fact that the inhibitor will have to ‘work’ under flowing conditions upto themaximum liquid velocity expected. In addition, flow velocities in excess of 10 m/s shouldbe viewed as high and extra thought given to corrosion control and monitoring. UTGhave issued guidelines on the prediction and monitoring of CO2 corrosion (Refs. 6 & 7).

For 13%Cr steel it is recommended that the C-factor developed for ‘multi-phase’ flow of300 is used in the absence of any better information (this is likely to err on theconservative side).

For duplex stainless steel a series of laboratory based flow loop tests were carried out onbehalf of BP Amoco by Det Norske Veritas Industry AS (DNV), Norway using treatedsea water. Interpretation of the test results demonstrated that for single phase liquid flow aC-factor of 450 ft/s(lbs/ft3)0.5 could be applied for the ‘nominally solids free’ condition ofup to 1pptb (Ref. 15).

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3. Solids-containing flow

3.1. Non-corrosive fluids

3.1.1. IntroductionAlthough the specific erosion models produced by the different R&D programmes are notalways in good agreement, there are several areas of general agreement.

The basic mechanism of erosion of most metals (i.e. ductile materials) is ductile ploughingof the surface by impacting solid particles. The material lost per impact is greatest atangles of impact between 15° and 60° and is proportional to m(Vi)

n where n is between 2and 2.5, m is the particle mass and Vi the actual particle impact velocity. The overallwastage rate is then the mass loss per impact times the impact rate. In the simplest case,the rate of impact is equal to the mass flow rate of the particles divided by the mass perparticle and if it is assumed that area of impact is the projection of the cross-sectional areaonto a bend (or a projected area in the path of the flow, such as a restriction) then theoverall wastage rate per unit area (i.e. the penetration rate) will be a function of m(Vi)

n

times M/m divided by the pipe cross-sectional area A, where M is the solids productionrate. However, M will be proportional to the product of the solids concentration, S, andthe mixed fluid velocity, V. Thus:

E = K x m(Vi)n x M/(m x A)

orE = K' x (Vi)

n x S x V/d2

where d is the pipe diameter, K and K' constants and E the erosion rate.

If a further simplification is made that the particle impact velocity, Vi, equals the mixedfluid velocity, V (or is a constant proportion of the mixed fluid velocity) and that n=2then:

E = K' x V3 x S/d2

orE = K' x V2 x M/d2

This, in essence, is the core form of all of the ‘simple’ erosion models produced by RCS,API, Tulsa, and Salama & Venkatesh (but not the Harwell model for multiphase flow),i.e.:

RCS:E = 4.1 x MV2.5/d2

API:E = 22.4 x MV2/d2

Salama & Venkatesh:E = 604 x MV2/d2

Tulsa:

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E = 4280 x MV1.73/d2

where M is the solids production rate in g/s, V the mixed velocity in m/s, d the pipediameter in mm and E the erosion rate in mm/yr.

As can be seen, the difference between these models lies in the different values of theconstant K' and some variation in the exponent of V.

Although the Salama & Venkatesh, simplified Tulsa, RCS and API approaches are simpleto use, a full understanding of the effect of various parameters such as flow regime, pipesize and fluid viscosity is only possible by utilising either the full Tulsa model (SPPS v.3.0) or the AEA Harwell Model (Sandman version 3.9).

3.1.2. Single phase flow

For single phase gas flow the Salama & Venkatesh approach can be used to give an ‘orderof magnitude’ indication of the likely wastage rate. This will give the worst case erosionrates in the absence of liquid buffering at the metal surface and assuming that the solidsremain within the gas stream, i.e. that they do not 'drop out'. Alternatively the more recentSalama model (Ref. 16) can be used to give an indication of the likely wastage rate.However, in this case it should be noted that an assessment of the Salama Modelundertaken within UTG indicated that it best equates to a 5D bend situation in comparisonwith the ‘full’ Tulsa/Harwell models. It is therefore recommended that it is not used forsystems where geometrical features than 5D bends may be present (e.g. 1.5D elbows, tees,severe constrictions). The model is most probably suitable for application to downholecompletions, although in this instance care needs to be taken regards regions of significantflow constriction (e.g. insert valves). For a more detailed consideration of the likelyerosion rate the full Tulsa model (SPPS v. 3.0) should be used.

For single phase liquid flow the full Tulsa Model (SPPS v. 3.0) should be used wherepossible. However, given that this is a computer software package that will not beuniversally available, the API and/or RCS models can be used for initial assessments (thelatter giving rapid assessment and the former a more accurate assessment based on bendgeometry). These models are based on simple slurry impingement tests and lab-scale flowloops and may suffer a problem with scale-up to field conditions. However, they shouldgive rates of the correct order of magnitude. Alternatively the more recent Salama model(Ref. 16) can be used to give an indication of the likely wastage rate. However, in thiscase it should be noted that an assessment of the Salama Model undertaken within UTGindicated that it best equates to a 5D bend situation in comparison with the ‘full’Tulsa/Harwell models. It is therefore recommended that it is not used for systems wheregeometrical features than 5D bends may be present (e.g. 1.5D elbows, tees, severeconstrictions). The model is most probably suitable for application to downholecompletions, although in this instance care needs to be taken regards regions of significantflow constriction (e.g. insert valves).

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3.1.3. Multiphase flow

For multiphase flow regimes the situation is more complicated. For regimes very close topure gas flow the Salama & Venkatesh or Salama models can be used for ‘order ofmagnitude’ estimates (see restrictions on use of the Salama Model in Section 3.1.2.).However, when there is any appreciable liquid present then this rate will be mitigated,although the degree of mitigation will depend very strongly on the flow regimecharacteristics. For regimes very close to pure liquid flow the API, RCS and/or Salamamodels can be used (see restrictions on use of the Salama Model in Section 3.1.2.).However, where there is any appreciable gas present this will not be appropriate as it islikely to be non-conservative. For multi-phase flow erosion rates below those for pure gasbut above those for pure liquid flow would normally be expected.

The Harwell programme complemented the Tulsa programme; the latter is based on fluidflow and modelling and has started with single phase flow conditions while the Harwellprogramme was an empirical programme based on multiphase flow conditions. The majorconcern with the Harwell programme is that it was based almost entirely on a 2" test loopand scale-up complications are likely to be present in multiphase flow.

The Harwell programme showed that, even for the same mixed velocities, the erosion ratedepends on the flow regime. The dependence was so strong that the proposed erosionmodel was a function of SxV rather than SxV3:

E = S x (C1 + C2 x V x √ρm)

where E is the erosion rate, C1 and C2 constants which depend on flow regime, S thesolids concentration, V the mixed fluid velocity and ρm the mixed phase density.

Harwell have developed a computer software program “Design Procedure for Erosion-Corrosion in Multi-phase Flow, Release 3”. As with the Tulsa software package thisprogram is not available commercially, but is only available to participants in the JointIndustry Programme (JIP). BPX was a member of this JIP. The Program enables the userto determine the flow regime, it then calculates the likely erosion wastage rate based onthe appropriate C1 and C2 values.

For ‘first pass’ assessments of the likely erosion wastage rate the following procedures canbe used:

It is recommended that the flow regime for the intended multiphase duty is firstly assessed.The following criteria can then be applied:

Annular Flow:

Use the Harwell Release 3 software package to assess the likely erosion wastage rate. An‘order of magnitude’ assessment can be achieved using the Tulsa SPPS v. 3.0 software

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package using the mixed velocity together with (i) the averaged fluid properties and (ii)the liquid properties. The actual erosion rate should then fall between these two values.

Bubble/Churn Flow:

Use the Harwell Release 3 software package to assess the likely erosion wastage rate.Additionally the Tulsa SPPS v. 3.0 software package with mixed velocity and liquidproperties can be used for comparison purposes.

Stratified:

Use the Tulsa SPPS v. 3.0 software package with the liquid velocity calculated for thehydraulic diameter and the liquid properties.

Horizontal Slug Flow:

One flow regime that has not been covered by either the AEA Harwell or Tulsa JIPs todate is horizontal slug flow. Slug flow is of interest to BP Amoco at a number oflocations, e.g. in Alaska slug flow in large diameter flow-lines is often encountered, wheresolids are often present and, indeed, failures have been experienced. Unfortunately, thereis no available data from either the JIP programmes or the literature in this area and BPAMOCO's own experience is complicated by CO2 corrosion. If erosion is a problem insuch regimes then there are two possible solid impingement mechanisms:

• solids on the bottom of a line are picked up and thrown down by a passingslug but do not get carried forward a significant distance.

• solids are entrained in the slug carried forward and thrown against the pipewall by the breaking wave at the slug front.

In both cases solids are unlikely to be carried at velocities exceeding the mixed fluidvelocity. The erosion may be mitigated to some extent as the pipe wall would be expectedto be protected by a significant liquid layer. However, the liquid slug front will be a zoneof considerable mixing and entrained gas, such that the liquid slug front may approach thehomogenous mixture. Therefore, as an interim measure until this type of flow has beenfully investigated, it is recommended that for such instances the Tulsa SPPS v. 4.0software package is used with the mixed fluid properties (density and viscosity) andvelocity. For an ‘order of magnitude’ assessment the API or RCS Models can be used,again employing the mixed fluid properties (density) and velocity. The use of these modelstogether with the mixture properties and velocity are likely to give a conservative estimateof the erosion under slug flow, as it assumes that any point on the pipe wall will besubjected to impingement by a liquid slug front continuously. Whilst this ‘in built’conservatism needs to be recognised, it is considered that this represents the best adviceavailable at this time.

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3.1.4. Effect of Material

All of the erosion rate models show a dependence of erosion rate on the substratematerial. The programmes that consider alloyed steels show a slight increase in erosionresistance going from carbon steel to the more highly alloyed materials. This effect is,however, not marked. Thus, for example, the Tulsa programme has a factor of 1.5between carbon steel and annealed 13 Cr steel, with 22 Cr duplex stainless steel beingapproximately the same as carbon steel.

The Salama & Venkatesh model has an inverse relation between erosion rate and carbonsteel hardness, and the Tulsa model has the erosion rate proportional to the hardness tothe power of -0.59. Given the range of hardness likely for carbon steel pipework andtubing neither correction will account for much more than a factor of two.

Thus, for pure solid particle erosion, the effect of substrate material (when comparingsteel alloys) on the erosion rate is a second order effect of much less importance than flowregime, mixed velocity or solids content. The effect can be quantified in different models,but a reasonable 'rule-of-thumb' would be that steel alloy composition does not have asignificant effect on erosion resistance.

3.1.6. Effect of Particle Size

In general, the erosion resulting from the impact of a single solid particle is a function ofthe momentum of that particle at impact and the total erosion is a function of the totalmomentum impacting on a surface. By this reasoning, there could be several impacts froma large number of small particles or one impact from a single large particle but, so long asthe total momentum was the same, the erosion would be the same.

However, the presence of a barrier layer of liquid at the surface and the bulk flow offluids round bends can mean that smaller particles are less likely to reach the surface thanlarge particles; or, at least, suffer a greater percentage loss of momentum. Thus, inpractice, erosion is likely to be less for smaller, less massive, particles than for largeparticles, even if the total solids mass production rate is the same. Only three of theModels, i.e. Tulsa’s SPPS v.4.0., Harwell’s Sandman v.3.9. and the Salama Model takeany account of the particle size in their calculation of the erosion wastage rae.

3.2. Corrosive liquids

3.2.1. Synergy between erosion and corrosionIf there is no synergism between corrosion and erosion for a given environment then thewastage will be the sum of the corrosion wastage and the erosion wastage. Guidelines areavailable for the prediction of likely corrosion rates (eg Ref. 2) and hence allowances forcorrosion can be calculated.

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In the Harwell project, broadly speaking for the conditions tested (2bara CO2, 30oC) theerosion-corrosion rate was found to be equal to erosion rate plus the 'unfilmed' corrosionrate. It is worth pointing out that under the conditions tested (2bara CO2, 30oC) theformation of iron carbonate films (often termed ‘scaling’ in CO2 corrosion) would not beexpected.

The Tulsa programme tested carbon steel in CO2 and sand-containing environments with50 psig CO2 at 200 oF (93.3 oC) and at pH 5.0, 5.5 or 6.0 (i.e. conditions under which theformation of iron carbonate films is likely). Three regimes in the erosion-corrosionwastage of carbon steel were identified. These were as follows:

(i) 'Scaling Regime'. In this regime the semi-protective corrosion product layer isretained on the metal surface, affording some protection. This is the normal situation forsolids free conditions, or more benign erosion-corrosion conditions.

(ii) 'General Wastage Regime'. In this regime any scales/surface films are removed fromthe metal surface by solids erosion and/or do not have the time to form. Hence metalwastage as a result of both erosion and corrosion can go on unabated. This is the normalsituation for very aggressive erosion conditions.

(iii) 'Pitting Regime'. In this regime the solid particles prevent scales/surface filmsforming at impingement points on the metal surface, whilst scale/surface films form on therest of the surface. This leads to pitting damage. Corrosion in the 'bare' impingement areascan be significantly more aggressive in terms of metal penetration rate than for generalwastage. Some scales/surface films can act as cathodic areas, significantly accelerating thecorrosion rate in the relatively small anodic 'bare' impingement areas. This occurs atconditions intermediate between 'scaling' or 'general wastage'. Corrosion rates up to twicethat anticipated for ‘un-filmed’ conditions have been observed.

ECRC have developed a software program (SPPS-EC), which can predict the thresholdvelocities for these three regimes. However, at present the model can not predict the likelywastage rate under erosion-corrosion conditions. The Tulsa work has also indicated thatsome corrosion inhibitors may be able to increase the threshold velocities for these threeregimes (Ref. 10). However, this effect is not yet sufficiently well established for use indesign. In any event, any such increase is likely to be corrosion inhibitor and systemdependant, meaning that to apply any increase in threshold velocity to the design wouldrequire specific testing of the candidate corrosion inhibitors under the anticipated systemconditions.

3.2.2. Carbon Steel

It is clear from the above that there is possible synergy between erosion and corrosion incarbon steel systems. However, the quantification of such effects is difficult. At this stageit is suggested that no clear velocity thresholds can be established for erosion-corrosion.As an interim measure the following philosophy is recommended:

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If the erosion rate is less than 0.1 mm/yr then there is no need to considererosion/corrosion interactions, i.e. the total wastage rate will be the predicted corrosionrate plus the predicted erosion rate.

If the predicted erosion rate is greater than 0.1 mm/yr, then use the CO2 model (Ref. 2) todetermine the likelihood of iron carbonate scale formation.

For the case where no iron carbonate scale is anticipated the total wastage rate can betaken as the erosion rate plus the un-filmed corrosion rate (i.e. in line with the conclusionsof the Harwell work).

For the case where iron carbonate scale is anticipated the total wastage rate can be takenas the erosion rate plus twice the un-filmed corrosion rate (i.e. to reflect the ‘pittingregime’ in the Tulsa work).

Alternatively, when it is available, the Tulsa SPPS-EC computer software programme canbe used to determine the ‘regime’ into which the service conditions fall, then the followingcriteria can be applied:

Scaling regime: wastage rate = erosion rate + ‘filmed’ corrosion ratePitting regime: wastage rate = erosion rate + twice the ‘un-filmed’ corrosion rateGeneral wastage regime: wastage rate = erosion rate + ‘un-filmed’ corrosion rate

3.2.3. 13%Cr Steel.

In the Harwell programme the 13%Cr steel was found not to corrode at lowertemperatures (30°C) under erosion-corrosion conditions until about 2 µm of material hadbeen removed by erosion. Thereafter the wastage rate increased to 1 - 2 mm/yr, remainingat this level even after the sand was removed. The 'corrosion resistant' properties wereonly restored once the material had been re-exposed to air. This observation is inagreement with studies in Sunbury (Ref. 11), which found that at 30°C in CO2-containingsolutions the protective layer never completely reformed. At higher temperatures (50°Cand 80°C) the results from Harwell indicated no synergy between erosion and corrosion.These results were again supported by data from the Sunbury experiments (Ref. 11),which found that the protective film reformed very rapidly after damage at temperatures of80°C and above (temperatures up to 150°C were tested) in a CO2-containing solution.

As a result of these observations, the following is recommended:

If the erosion rate is less than 0.1 mm/yr then there is no need to considererosion/corrosion interactions, as it is anticipated that the protective film will not bedestroyed, i.e. the re-filming processes will be faster than the wastage rate. Therefore, thetotal wastage rate will be the predicted corrosion rate (if any, see Ref. 12 for furtherdetails) plus the predicted erosion rate.

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If the erosion rate is greater than 0.1 mm/yr then the total wastage rate at temperatureslower than 80°C should be taken as the erosion rate plus the corrosion rate for ‘un-filmed’carbon steel in the given chemical environment. For temperatures above 80°C, the totalwastage rate should be taken as the erosion rate plus the corrosion rate expected on13%Cr steel (Ref. 12).

3.2.4. Duplex Stainless Steel

In the Harwell work the duplex stainless steel was found not to corrode under theconditions used, even in the presence of sand. Therefore, it is recommended that in thiscase the total wastage rate is taken to equal the erosion rate, i.e. that no allowance is madefor corrosion.

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References:

1. "Erosion Guidelines Revision 2.0 (1996)", J W Martin & J Pattinson, BP GRE ReportNo. ESR.97.ER.002, January 1997.

2. "A Corrosion Philosophy for the Transport of Wet Oil and Multiphase FluidsContaining CO2", J Pattinson, ID Parker & AS Green, BP GRE Report No.ESR.93.ER.013, March 1993.

3. "Erosional Velocity Limits for Duplex Stainless Steel", J Pattinson & J W Martin, BPGRE Report No. ESR.95.ER.058, July 1995

4. "A Review of Erosion Corrosion in Oil and Gas Production", JS Smart, Paper 10,NACE Corrosion Conference, 1990

5. "Materials Performance in Khuff Gas Service", R Duncan, Materials Performance, Vol19, No. 7, July 1980

6. “Corrosion Prediction Modelling”, A McMahon & D M E Paisley, ESR.96.ER.066

7. “Corrosion Monitoring Manual”, S Webster & R C Woollam, ESR.95.ER.053,November 1996

8. "Salt water velocities in pipes; for continuous flow", British Standard MA18, 1976

9. "The Wear Equation for Erosion of Metals by Abrasive Particles", E Rabinowicz, Proc.5th Int. Conf. on Erosion by Solid and Liquid Impact.

10. "Erosion/Corrosion Research Center: Advisory Board Report May 11, 1996", E FRybicki, University of Tulsa, USA

11. Report in Preparation, A McMahon, 1996

12. “Guidelines for the Use of 13%Cr Stainless Steels in Chloride Containing WatersUnder Non-Sour Conditions”, DME Paisley, BP GRE Report No. ESR.95.ER.040,April 1995.

13. “Assessment of Erosive Wear in Piping Systems”, DNV Recommended Practice DNV

RP O501, 1997. 14. “Erosion - Material Limitations (115-4277) 1995 End of Years Status”, J W Martin,

BP GRE Report No. ESR.96.ER.002. 15. “Erosion of Alloy 625 and 25%Cr Duplex Stainless Steel in Water Injection Service”,

memorandum by J W Martin to S Whitehead dated 22nd April 1997.

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16. “An Alternative to API RP14e Erosional Velocity Limits for Sand Laden Fluids”, M

M Salama, OTC Proceedings 1998, Paper 8898.