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

h Background

h Choice of monitoring location

h Review of current techniques

h Future trends

Talk structure

h Oil and gas infrastructure is ageing

h Increasingly aggressive fields (high T, high P, H2S, sand)

h Repairs and replacements are costly

h Negative publicity from environmental damage

h BP perspectiveh Corrosion accounts for ~10% of lifting costs per barrelh 60,000 km of pipeline (50% are unpiggable)h 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

h Improved safetyh Reduced environmental impacth Lower operating costs

h Reduces maintenance/inspection costsh Minimises unscheduled shutdowns

h Optimised process efficiencyh Inhibitor injection ratesh Oxygen concentrationsh Flow rates

h Assessment of effect of operational changes/upsets

Why Monitoring?

h Purpose of corrosion monitoring is to optimise balance between corrosion control and replacement costs

h Each monitoring technique has inherent random error minimised by increasing number of techniques / monitoring points (with associated cost)

h 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)

V

a

l

u

e

Benefit/Cost of Monitoring

Cost of Monitoring Programme

Example from inhibitor monitoring

h Inappropriate selection of location or technique is worse than no selection

h Quality of data is often never questionedh Physical access important but should not dictate

monitoring locationh Specification of monitoring locations should be intrinsic

part of design stage (not afterthought)h Review of historical experience should influence

selection

Monitoring Location

h Location of corrosive phaseh Top of line: water condensationh Bottom of line: water drop out

h Corrosion mechanismh General vs localisedh Process upset detection

h Effect of flowh Target areas with enhanced water drop out and water hold-uph Sited away from turbulence, e.g. bends, reducers, valvesh Elevation changes often affect corrosivity

h Process stream changesh Third party entrantsh Chemical injection points

Monitoring Location

Monitoring at riser where slug flowmay cause erosioncorrosion

Monitoring Location

h Access fittings exist for low and high pressure systems

h On-line retrieval at up to 400 bar (6000 psi) possible

h Safe use is very important

High Pressure Retrievable Fitting

On-line retrieval tool

Access Fittings

Access fittings

h Orientation important for multi-phase systems

h Retrofitting can be costlyh Bottom of line fittings can be

fouled by debrish Galling of threads an issueh Material compatibilityh Water traps may be used but often

act as corrosion initiation sites

h Two basic types of measurement which provide necessary information are:h Inspection Data

h which are related to changes in wall thickness or structural change (wastage, cracks and pits), i.e. non-destructive evaluation and inspection

h Monitoring Datah where 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

h Main In-Line Monitoring Techniquesh Mass Loss Couponsh Electrical Resistance (ER)h Linear Polarisation Resistance (LPR)h Zero Resistance Ammetry (ZRA)h Process Stream Monitoring

h Other Techniquesh Ultrasonic Thickness Measurementh Electrochemical Noise (ECN)h Hydrogen Permeation h Electrochemical Impedance Spectroscopy (EIS)

Monitoring Techniques

Monitoring Devices

Coupons Probes

Mass Loss Coupons

h Simplest form of corrosion monitoring, can be used in any environmenth Mass loss measured over a period of several weeks/months (NACE RP0775-99)h Gives a visual indication of corrosion type as well as rateh Can 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)

h The change in the electrical resistance of an element (wire, tube or strip) is measured using Wheatstone Bridge arrangement

h This is then related to the change in cross-sectional area and hence provides indication of metal loss

h Will not work if corrosion is localised and gives poor performance in thermally noisy systems

h Trade-off between sensitivity and probe lifetime

ER Monitoring Options

h ER Probe and Portable Instrumentation

h On-line (permanent) instrumentation / data logger

Electrical Resistance

h Direct measurement of material lossh Can be used in any environment (conducting and non-conducting)

and does not require continuous aqueous phaseh Different types of probe elements available to cover different

requirements, i.e.h High sensitivity h Long lifeh Flush or protruding shapes

h Show time evolution of corrosion rateh Intermittent (single readings taken daily/weekly/monthly)h Continuous (readings taken at regular intervals, typically hourly)

h Can be used to monitor erosion (e.g. by sand) as well as corrosion

h New generation of high sensitivity ER systems now available

h These provide combination of longer probe life, increased sensitivity and better temperature compensation

h Systems include:h Cormon CEIONh Rohrbach-Cosasco MicroCorh CorrOcean HSER

h These systems combine special probes with new instrumentation and hence are NOT interchangeable with previous ER applications

High Sensitivity ER

h Non-intrusive variation on ER method

h Pipe wall is used as active electrode area

h Electric current fed through two contact pins

h Voltage drop proportional to wall thickness

h Sensitivity ~ 1/1000 of original wall thickness

Field Signature Method (FSM)

h Probes can be either protruding or flush mountedh Probes can be 2- or 3- elementh A small dc current is passed between electrodes to

polarise them approx 10-20 mVh V-I slope is directly proportional to the corrosion rate

Linear Polarisation Resistance (LPR)

Linear Polarisation Resistance

h The current - potential relationship is linear close to the corrosion potential

h Polarisation Resistance (Rp) :

h Corrosion rate is inversely proportional to Rp

-20

20

-20 20

U

J

[mV]

J [mA/cm2]

JURp

=

Linear Polarisation Resistance

h Advantagesh Gives instantaneous corrosion rateh Sensitive to any process changes or upsets

h Disadvantagesh General corrosion rates indicative of trend rather than absoluteh Can only be used in conductive media, need continuous

aqueous phaseh Results need to be corrected for IR drop in low conductivity

solutionh If electrodes become fouled can give erroneous resultsh Generally 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

m

p

y

Typical LPR Output

h Measures galvanic current flowing between two dissimilar metal electrodes (typically copper & steel)

h Current is proportional to the oxygen content of the water

h As sensitive as an oxygen probe but more robust

Zero Resistance Ammetry (ZRA)

h Demonstrates that process control activities are functioning correctly

h May be used to predict corrosion ratesh Helps with troubleshooting when corrosion is detected

h Online monitoring reduces manpower costsh Database management is critical

Process Stream Monitoring

h Iron Countsh Measures dissolved iron (Fe2+) in solutionh Based on knowledge of contact surface area and contact time

can give indication of corrosion rateh Precipitation/complexation of iron will affect results

h Chemical Analysish pH (for control of glycol corrosivity)h O2 content (efficacy of de-aeration system for water injection)h Chlorine residuals (efficacy of chlorination system for water injection)h Inhibitor residuals (confirms presence / effectiveness of inhibitor)

h Bacterial Enumerationh Sample tested to determine presence and extent of bacterial

contamination

Process Stream Monitoring

h Corrosion monitoring can be used to determine the extent of inhibitor residuals in a line after a batch treatment

h With increase in inhibitor concentration, corrosion rate will droph As inhibitor concentration decreases with time, corrosion rate increasesh Need to measure corrosion rate regularly to see effects over a period of

days/weeks

Inhibitor Concentration

Corrosion Rate

Mean Corrosion Rate

Time

I

n

h

i

b

i

t

c

o

n

c

/

C

o

r

r

r

a

t

e

Inhibitor Monitoring

h Wall thickness measurement using reflected ultrasonic wave

h One of the most important non-destructive test methodsh Sensitivity typically ~ 1/200 of original wall thicknessh Automatic/manual scanning

h Probe is scanned over area of interest to produce maph Flexible Ultrasonic Transducer Mat

h Designed for permanent installationh Mounted on printed circuit boardh Can be installed in areas of restricted access or under lagging

Ultrasonic Thickness Measurement

h Impedance spectroscopyh Limited applicability other than measurement of solution

resistance

h Electrochemical noise (ECN)h Data interpretation difficulth Currently seen as a useful supplement to other methods

h Hydrogen permeationh Monitors flux of hydrogen through defined area of pipeh Both pressure-based and electrochemical sensors availableh May 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]

C

o

r

r

o

s

i

o

n

R

a

t

e

[

m

p

y

]

CEION F10 microCOR CEION F40 CeionF80 Traditional ER10

Traditional ER20 traditional ER40 Ceion Spool FSM 20" 10mmWT Fleximat20" 10 mm WT

L

P

R

c

o

r

r

o

s

i

o

n

r

a

t

e

[

m

m

/

y

r

]

Ma s

s lo

s s c

oupo

ns

Response Time

h Wherever possible at least two different monitoring systems should be used

h Selection will be based on:h Media conductivity/water cut

h LPR only applicable in aqueous systems (> 10-20% water cut)h Speed of response

h Corrosion coupons only provide data over periods of monthsh ER can provide data in days / weeks if corrosion rate is rapidh LPR / HS-ER can provide data within minutes / hours if correctly

applied and interrogated on-lineh In sour systems need to consider effect of conductive FeS scale on

probes

Technique Selection

h Techniques for monitoring localised corrosionh Pitting corrosionh Crevice corrosionh Underdeposit corrosion

h Techniques for monitoring corrosion in inaccessible locationsh Corrosion under insulationh Subseah Unpiggable lines

h Intelligent crawlersh Cleaning processesh Tethered tools

Future Trends

h Improved modelling of corrosion mechanismsh CO2 corrosion, e.g. effect of films, sour environmenth Localised corrosionh Erosion corrosion (sand)h Hydrogen assisted cracking

h Improved data visualisation/integration processesh Remote monitoringh Database managementh Analytical toolsh Intelligent traffic light systems

Future Trends

h Corrosion monitoring is importanth For problem diagnosis/troubleshootingh To assess plant condition for improved maintenance/replacement

strategiesh To demonstrate process control/adequate inhibitor dosageh To support inspection programmes

h Innovative monitoring techniques are still requiredh For localised corrosionh In inaccessible locationsh Faster response times without compromising probe lifetimeh To facilitate data handling and visualisation

Summary

Corrosion Monitoringin the Oil & Gas IndustryAcknowledgementsBP IRF FlagshipTalk structureBackgroundCorrosion ManagementWhy Monitoring?EconomicsEconomicsMonitoring LocationMonitoring LocationMonitoring LocationAccess FittingsAccess fittingsTypes of MeasurementMonitoring TechniquesMonitoring DevicesMass Loss CouponsMass Loss CouponsMass Loss CouponsMass Loss CouponsElectrical Resistance(ER) ER Monitoring OptionsElectrical ResistanceHigh Sensitivity ERField Signature Method (FSM)Linear PolarisationResistance (LPR)Linear Polarisation ResistanceLinear Polarisation ResistanceTypical LPR Output Zero Resistance Ammetry(ZRA)Process Stream MonitoringProcess Stream MonitoringInhibitor MonitoringUltrasonic Thickness MeasurementOther techniquesResponse TimeTechnique SelectionFuture TrendsFuture TrendsSummary