Corrosion Monitoring in the Oil & Gas Industry Dr Gareth Hinds National Physical Laboratory CED Working Day, Warrington, 29 th April 2010
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Corrosion Monitoring in the Oil & Gas Industry
Dr Gareth HindsNational Physical Laboratory
CED Working Day, Warrington, 29th April 2010
Alan Crossland
Don Harrop
John Martin
Simon Webster
Richard Woollam
Acknowledgements
BP IRF Flagship
Background
Choice of monitoring location
Review of current techniques
Future trends
Talk structure
Oil and gas infrastructure is ageing
Increasingly aggressive fields (high T, high P, H2S, sand)
Repairs and replacements are costly
Negative publicity from environmental damage
BP perspectiveCorrosion accounts for ~10% of lifting costs per barrel60,000 km of pipeline (50% are unpiggable)Localised corrosion is greatest threat to integrity
Background
Life Management
Inspection(Remaining wall)
Corr Mechanism and Rate
Corr Mitigation and Control
Raw Chemical
Transportation
Deployment
Chemicals
Prod Chemistry
Chemicals
Prod Chemistry
Corrosion Monit
Corrosion Engineering
Crew
Coupons/probes
Processing
Integrity
NDE Technicians
Repair/Replace Engineering
Chemical Crew
WarehouseInventory
Risk
Corrosion Management
Improved safetyReduced environmental impactLower operating costs
Reduces maintenance/inspection costsMinimises unscheduled shutdowns
Optimised process efficiencyInhibitor injection ratesOxygen concentrationsFlow rates
Assessment of effect of operational changes/upsets
Why Monitoring?
Purpose of corrosion monitoring is to optimise balance between corrosion control and replacement costsEach monitoring technique has inherent random error –minimised by increasing number of techniques / monitoring points (with associated cost)Benefit of additional corrosion monitoring should outweigh incremental cost incurred
Economics
Economics
0 0.5 1 1.5 2 2.5 3 3.5 4Log (number of locations)
Valu
e Benefit/Cost of Monitoring
Cost of Monitoring Programme
Example from inhibitor monitoring
Inappropriate selection of location or technique is worse than no selectionQuality of data is often never questionedPhysical access important but should not dictate monitoring locationSpecification of monitoring locations should be intrinsic part of design stage (not afterthought)Review of historical experience should influence selection
Monitoring Location
Location of corrosive phaseTop of line: water condensationBottom of line: water drop out
Corrosion mechanismGeneral vs localisedProcess upset detection
Effect of flowTarget areas with enhanced water drop out and water hold-upSited away from turbulence, e.g. bends, reducers, valvesElevation changes often affect corrosivity
Process stream changesThird party entrantsChemical injection points
Monitoring Location
Monitoring at riser where slug flowmay cause erosioncorrosion
Monitoring Location
Access fittings exist for low and high pressure systemsOn-line retrieval at up to 400 bar (6000 psi) possibleSafe use is very important
High Pressure Retrievable Fitting
On-line retrieval tool
Access Fittings
Access fittings
Orientation important for multi-phase systemsRetrofitting can be costlyBottom of line fittings can be fouled by debrisGalling of threads an issueMaterial compatibilityWater traps may be used but often act as corrosion initiation sites
Two basic types of measurement which provide necessary information are:
Inspection Datawhich are related to changes in wall thickness or structural change (wastage, cracks and pits), i.e. non-destructive evaluation and inspection
Monitoring Datawhere measurements from insert probes and chemical analysis of process streams are used to monitor changes in the corrosivity of the process environment
Types of Measurement
Main In-Line Monitoring TechniquesMass Loss CouponsElectrical Resistance (ER)Linear Polarisation Resistance (LPR)Zero Resistance Ammetry (ZRA)Process Stream Monitoring
Other TechniquesUltrasonic Thickness MeasurementElectrochemical Noise (ECN)Hydrogen Permeation Electrochemical Impedance Spectroscopy (EIS)
Monitoring Techniques
Monitoring Devices
Coupons Probes
Mass Loss Coupons
Simplest form of corrosion monitoring, can be used in any environmentMass loss measured over a period of several weeks/months (NACE RP0775-99)Gives a visual indication of corrosion type as well as rateCan provide pitting rate data
Mass Loss Coupons
Disc (flush-mounted)
Strip
Mass Loss Coupons
Coupons made from pipe material where possible
CR = corrosion rate (mm/yr)m = mass loss (g)A = coupon surface area (mm2)d = metal density (g/cm3)T = exposure time (days)
TdAmCR
⋅⋅×
=51065.3
Strip coupon Rod Coupon
wtStrip Coupon
l —
w —
t —
0.5" or 1"
0.0625" or 0.125"
Dimensions
l
3"
Flush-mounted disc
l = 75 mmw = 12.5 or 25 mmt = 1.5 or 3 mm
Mass Loss Coupons
(assumes general attack)
Electrical Resistance (ER)
The change in the electrical resistance of an element (wire, tube or strip) is measured using Wheatstone Bridge arrangementThis is then related to the change in cross-sectional area and hence provides indication of metal lossWill not work if corrosion is localised and gives poor performance in thermally noisy systemsTrade-off between sensitivity and probe lifetime
ER Monitoring Options
ER Probe and Portable InstrumentationOn-line (permanent) instrumentation / data logger
Electrical Resistance
Direct measurement of material lossCan be used in any environment (conducting and non-conducting) and does not require continuous aqueous phaseDifferent types of probe elements available to cover different requirements, i.e.
High sensitivity Long lifeFlush or protruding shapes
Show time evolution of corrosion rateIntermittent (single readings taken daily/weekly/monthly)Continuous (readings taken at regular intervals, typically hourly)
Can be used to monitor erosion (e.g. by sand) as well as corrosion
New generation of high sensitivity ER systems now availableThese provide combination of longer probe life, increased sensitivity and better temperature compensationSystems include:
Cormon – CEION™Rohrbach-Cosasco – MicroCor™CorrOcean – HSER™
These systems combine special probes with new instrumentation and hence are NOT interchangeable with previous ER applications
High Sensitivity ER
Non-intrusive variation on ER methodPipe wall is used as active electrode areaElectric current fed through two contact pinsVoltage drop proportional to wall thicknessSensitivity ~ 1/1000 of original wall thickness
Field Signature Method (FSM)
Probes can be either protruding or flush mountedProbes can be 2- or 3- elementA small dc current is passed between electrodes to polarise them approx 10-20 mVV-I slope is directly proportional to the corrosion rate
Linear Polarisation Resistance (LPR)
Linear Polarisation Resistance
The current - potential relationship is linear close to the corrosion potentialPolarisation Resistance (Rp) :
Corrosion rate is inversely proportional to Rp
-20
20
-20 20
Δ U
Δ J
η [mV]
J [mA/cm2]
JURp Δ
Δ=
Linear Polarisation Resistance
AdvantagesGives instantaneous corrosion rateSensitive to any process changes or upsets
DisadvantagesGeneral corrosion rates indicative of trend rather than absoluteCan only be used in conductive media, need continuous aqueous phaseResults need to be corrected for IR drop in low conductivity solutionIf electrodes become fouled can give erroneous resultsGenerally gives little information on localised corrosion
0.51.01.52.02.53.03.54.04.55.0
Oct 1 Thu 8 Thu 15 Thu 22
LPR Corrosion Rate
mpy
Typical LPR Output
Measures galvanic current flowing between two dissimilar metal electrodes (typically copper & steel)Current is proportional to the oxygen content of the waterAs sensitive as an oxygen probe but more robust
Zero Resistance Ammetry (ZRA)
Demonstrates that process control activities are functioning correctlyMay be used to predict corrosion ratesHelps with troubleshooting when corrosion is detected
Online monitoring reduces manpower costsDatabase management is critical
Process Stream Monitoring
Iron CountsMeasures dissolved iron (Fe2+) in solutionBased on knowledge of contact surface area and contact time can give indication of corrosion ratePrecipitation/complexation of iron will affect results
Chemical AnalysispH (for control of glycol corrosivity)
O2 content (efficacy of de-aeration system for water injection)
Chlorine residuals (efficacy of chlorination system for water injection)
Inhibitor residuals (confirms presence / effectiveness of inhibitor)
Bacterial EnumerationSample tested to determine presence and extent of bacterial contamination
Process Stream Monitoring
Corrosion monitoring can be used to determine the extent of inhibitor residuals in a line after a batch treatmentWith increase in inhibitor concentration, corrosion rate will dropAs inhibitor concentration decreases with time, corrosion rate increasesNeed to measure corrosion rate regularly to see effects over a period of days/weeks
Inhibitor Concentration
Corrosion Rate
Mean Corrosion Rate
Time
Inhi
bit c
onc
/ Cor
r rat
e
Inhibitor Monitoring
Wall thickness measurement using reflected ultrasonic waveOne of the most important non-destructive test methodsSensitivity typically ~ 1/200 of original wall thicknessAutomatic/manual scanning
Probe is scanned over area of interest to produce map
Flexible Ultrasonic Transducer MatDesigned for permanent installationMounted on printed circuit boardCan be installed in areas of restricted access or under lagging
Ultrasonic Thickness Measurement
Impedance spectroscopyLimited applicability other than measurement of solution resistance
Electrochemical noise (ECN)Data interpretation difficultCurrently seen as a useful supplement to other methods
Hydrogen permeationMonitors flux of hydrogen through defined area of pipeBoth pressure-based and electrochemical sensors availableMay be inserted in access fitting or simply a patch on exterior of pipe
Other techniques
Minimum Response Time for Different Monitoring Techniques
10
1
0.1
0.01
0.1
1
10
100
1000
0.1 1 10 100 1000 10000Time [hrs]
Cor
rosi
on R
ate
[mpy
]
CEION F10 microCOR CEION F40 CeionF80 Traditional ER10
Traditional ER20 traditional ER40 Ceion Spool FSM 20" 10mmWT Fleximat20" 10 mm WT
LPR
corr
osio
n ra
te [m
m/y
r]
Ma s
s lo
s s c
oupo
ns
Response Time
Wherever possible at least two different monitoring systems should be usedSelection will be based on:
Media conductivity/water cutLPR only applicable in aqueous systems (> 10-20% water cut)
Speed of responseCorrosion coupons only provide data over periods of monthsER can provide data in days / weeks if corrosion rate is rapidLPR / HS-ER can provide data within minutes / hours if correctly applied and interrogated on-lineIn sour systems need to consider effect of conductive FeS scale on probes
Technique Selection
Techniques for monitoring localised corrosionPitting corrosionCrevice corrosionUnderdeposit corrosion
Techniques for monitoring corrosion in inaccessible locationsCorrosion under insulationSubseaUnpiggable lines
Intelligent crawlersCleaning processesTethered tools
Future Trends
Improved modelling of corrosion mechanismsCO2 corrosion, e.g. effect of films, sour environmentLocalised corrosionErosion corrosion (sand)Hydrogen assisted cracking
Improved data visualisation/integration processesRemote monitoringDatabase managementAnalytical toolsIntelligent traffic light systems
Future Trends
Corrosion monitoring is importantFor problem diagnosis/troubleshootingTo assess plant condition for improved maintenance/replacement strategiesTo demonstrate process control/adequate inhibitor dosageTo support inspection programmes
Innovative monitoring techniques are still requiredFor localised corrosionIn inaccessible locationsFaster response times without compromising probe lifetimeTo facilitate data handling and visualisation
Summary
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