Corrosion Monitoring

Corrosion Monitoring
in the Oil & Gas Industry
Dr Gareth Hinds
National Physical Laboratory
CED Working Day, Warrington, 29th April 2010
Acknowledgements
Alan Crossland
Don Harrop
John Martin
Simon Webster
Richard Woollam
BP IRF Flagship
Talk structure
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Background
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Choice of monitoring location
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Review of current techniques
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Future trends
Background
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Oil and gas infrastructure is ageing
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Increasingly aggressive fields (high T, high P, H2S, sand)
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Repairs and replacements are costly
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Negative publicity from environmental damage
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BP perspective
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Corrosion accounts for ~10% of lifting costs per barrel
60,000 km of pipeline (50% are unpiggable)
Localised corrosion is greatest threat to integrity
Corrosion Management
Integrity
NDE Technicians
Repair/Replace
Engineering
Inspection
(Remaining wall)
Corr Mitigation
and Control
Corr Mechanism
and Rate
Life
Management
Raw Chemical
Risk
Crew
Chemicals
Transportation
Deployment
Corrosion Monit
Processing
Prod Chemistry
Corrosion
Engineering
Chemical Crew
Warehouse
Inventory
Coupons/probes
Why Monitoring?
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Improved safety
Reduced environmental impact
Lower operating costs
Reduces maintenance/inspection costs
h Minimises unscheduled shutdowns
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Optimised process efficiency
Inhibitor injection rates
h Oxygen concentrations
h Flow rates
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Assessment of effect of operational changes/upsets
Economics
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Purpose of corrosion monitoring is to optimise balance
between corrosion control and replacement costs
Each 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
Example from inhibitor monitoring
Cost of Monitoring Programme
Value
Benefit/Cost of Monitoring
0
0.5
1
1.5
2
2.5
Log (number of locations)
3
3.5
4
Monitoring Location
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Inappropriate selection of location or technique is worse
than no selection
Quality of data is often never questioned
Physical access important but should not dictate
monitoring location
Specification of monitoring locations should be intrinsic
part of design stage (not afterthought)
Review of historical experience should influence
selection
Monitoring Location
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Location of corrosive phase
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Corrosion mechanism
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General vs localised
Process upset detection
Effect of flow
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Top of line: water condensation
Bottom of line: water drop out
Target areas with enhanced water drop out and water hold-up
Sited away from turbulence, e.g. bends, reducers, valves
Elevation changes often affect corrosivity
Process stream changes
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Third party entrants
Chemical injection points
Monitoring Location
Monitoring at riser
where slug flow
may cause erosion
corrosion
Access Fittings
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Access fittings exist for low and high
pressure systems
On-line retrieval at up to 400 bar
(6000 psi) possible
Safe use is very important
High Pressure
Retrievable Fitting
On-line retrieval tool
Access fittings
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Orientation important for multiphase systems
Retrofitting can be costly
Bottom of line fittings can be
fouled by debris
Galling of threads an issue
Material compatibility
Water traps may be used but often
act as corrosion initiation sites
Types of Measurement
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Two basic types of measurement which
provide necessary information are:
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Inspection Data
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which are related to changes in wall thickness or
structural change (wastage, cracks and pits), i.e. nondestructive evaluation and inspection
Monitoring Data
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where measurements from insert probes and chemical
analysis of process streams are used to monitor
changes in the corrosivity of the process environment
Monitoring Techniques
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Main In-Line Monitoring Techniques
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Mass Loss Coupons
Electrical Resistance (ER)
Linear Polarisation Resistance (LPR)
Zero Resistance Ammetry (ZRA)
Process Stream Monitoring
Other Techniques
Ultrasonic Thickness Measurement
h Electrochemical Noise (ECN)
h Hydrogen Permeation
h Electrochemical Impedance Spectroscopy (EIS)
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Monitoring Devices
Mass Loss Coupons
Coupons
Probes
Mass Loss Coupons
Strip
Disc (flush-mounted)
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Simplest form of corrosion monitoring, can be used in any environment
Mass loss measured over a period of several weeks/months (NACE RP0775-99)
Gives a visual indication of corrosion type as well as rate
Can provide pitting rate data
Mass Loss Coupons
Coupons made from pipe material where possible
Mass Loss Coupons
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Strip Coupon
Dimensions
l =—75
3" mm
l
w w=—
12.5
0.5"oror251"mm
t
= 1.5 or 3 mm
t —0.0625" or 0.125"
t
l
Strip coupo n
Flush-mounted disc
Rod Co upon
3.65 ×10 m
CR =
A ⋅ d ⋅T
5
(assumes general attack)
CR
m
A
d
T
=
=
=
=
=
corrosion rate (mm/yr)
mass loss (g)
coupon surface area (mm2)
metal density (g/cm3)
exposure time (days)
Electrical Resistance
(ER)
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
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ER Monitoring Options
ER Probe and Portable
Instrumentation
h On-line (permanent) instrumentation
/ data logger
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Electrical Resistance
Direct measurement of material loss
h Can be used in any environment (conducting and non-conducting)
and does not require continuous aqueous phase
h Different types of probe elements available to cover different
requirements, i.e.
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Show time evolution of corrosion rate
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High sensitivity
Long life
Flush or protruding shapes
Intermittent (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
High Sensitivity ER
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New generation of high sensitivity ER systems now
available
These provide combination of longer probe life,
increased sensitivity and better temperature
compensation
Systems include:
Cormon
h Rohrbach-Cosasco
h CorrOcean
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– CEION™
– MicroCor™
– HSER™
These systems combine special probes with new
instrumentation and hence are NOT interchangeable
with previous ER applications
Field Signature Method
(FSM)
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Non-intrusive variation on ER
method
Pipe wall is used as active
electrode area
Electric current fed through
two contact pins
Voltage drop proportional to
wall thickness
Sensitivity ~ 1/1000 of
original wall thickness
Linear Polarisation
Resistance (LPR)
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Probes can be either protruding or flush mounted
Probes can be 2- or 3- element
A small dc current is passed between electrodes to
polarise them approx 10-20 mV
V-I slope is directly proportional to the corrosion rate
Linear Polarisation
Resistance
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The current - potential
relationship is linear close
to the corrosion potential
Polarisation Resistance
(Rp) :
ΔU
Rp =
ΔJ
Corrosion rate is inversely
proportional to Rp
20
η
[mV]
ΔU
J [mA/cm2]
ΔJ
-20
-20
20
Linear Polarisation
Resistance
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Advantages
Gives instantaneous corrosion rate
h Sensitive to any process changes or upsets
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Disadvantages
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General corrosion rates indicative of trend rather than absolute
Can only be used in conductive media, need continuous
aqueous phase
Results need to be corrected for IR drop in low conductivity
solution
If electrodes become fouled can give erroneous results
Generally gives little information on localised corrosion
Typical LPR Output
mpy
LPR Corrosion Rate
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
Oct 1
Thu 8
Thu 15
Thu 22
Zero Resistance Ammetry
(ZRA)
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Measures galvanic current
flowing between two
dissimilar metal electrodes
(typically copper & steel)
Current is proportional to the
oxygen content of the water
As sensitive as an oxygen
probe but more robust
Process Stream
Monitoring
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Demonstrates that process control activities are
functioning correctly
May be used to predict corrosion rates
Helps with troubleshooting when corrosion is detected
Online monitoring reduces manpower costs
Database management is critical
Process Stream
Monitoring
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Iron Counts
Measures dissolved iron (Fe2+) in solution
h Based on knowledge of contact surface area and contact time
can give indication of corrosion rate
h Precipitation/complexation of iron will affect results
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Chemical Analysis
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)
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Bacterial Enumeration
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Sample tested to determine presence and extent of bacterial
contamination
Inhibitor Concentration
Corrosion Rate
Mean Corrosion Rate
Inhibit conc / Corr rate
Inhibitor Monitoring
Time
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 drop
h As inhibitor concentration decreases with time, corrosion rate increases
h Need to measure corrosion rate regularly to see effects over a period of
days/weeks
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Ultrasonic Thickness
Measurement
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Wall thickness measurement using reflected ultrasonic
wave
One of the most important non-destructive test methods
Sensitivity typically ~ 1/200 of original wall thickness
Automatic/manual scanning
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Probe is scanned over area of interest to produce map
Flexible Ultrasonic Transducer Mat
Designed for permanent installation
h Mounted on printed circuit board
h Can be installed in areas of restricted access or under lagging
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Other techniques
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Impedance spectroscopy
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Limited applicability other than measurement of solution
resistance
Electrochemical noise (ECN)
Data interpretation difficult
h Currently seen as a useful supplement to other methods
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Hydrogen permeation
Monitors flux of hydrogen through defined area of pipe
h Both pressure-based and electrochemical sensors available
h May be inserted in access fitting or simply a patch on exterior of
pipe
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Response Time
Minimum Response Time for Different Monitoring Techniques
1000
Mass loss coupons
LPR
Corrosion Rate [mpy]
100
10
1
0.1
corrosion rate [mm/yr]
10
1
0.01
0.1
0.1
1
10
100
1000
Time [hrs]
10000
CEION F10
microCOR
CEION F40
CeionF80
Traditional ER10
Traditional ER20
traditional ER40
Ceion Spool
FSM 20" 10mmWT
Fleximat20" 10 mm WT
Technique Selection
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Wherever possible at least two different monitoring
systems should be used
Selection will be based on:
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Media conductivity/water cut
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LPR only applicable in aqueous systems (> 10-20% water cut)
Speed of response
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Corrosion coupons only provide data over periods of months
ER can provide data in days / weeks if corrosion rate is rapid
LPR / HS-ER can provide data within minutes / hours if correctly
applied and interrogated on-line
In sour systems need to consider effect of conductive FeS scale on
probes
Future Trends
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Techniques for monitoring localised corrosion
Pitting corrosion
h Crevice corrosion
h Underdeposit corrosion
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Techniques for monitoring corrosion in inaccessible locations
Corrosion under insulation
h Subsea
h Unpiggable lines
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Intelligent crawlers
Cleaning processes
Tethered tools
Future Trends
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Improved modelling of corrosion mechanisms
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CO2 corrosion, e.g. effect of films, sour environment
Localised corrosion
Erosion corrosion (sand)
Hydrogen assisted cracking
Improved data visualisation/integration processes
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Remote monitoring
Database management
Analytical tools
Intelligent traffic light systems
Summary
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Corrosion monitoring is important
For problem diagnosis/troubleshooting
h To assess plant condition for improved maintenance/replacement
strategies
h To demonstrate process control/adequate inhibitor dosage
h To support inspection programmes
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Innovative monitoring techniques are still required
For localised corrosion
h In inaccessible locations
h Faster response times without compromising probe lifetime
h To facilitate data handling and visualisation
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