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Anti-Corrosion Methods and MaterialsFSM — non-intrusive monitoring of internal corrosion, erosion and cracking
Roe D. Strømmen Harald Horn Gaute Moldestad John Kristian Ramsvik Kjell R. Wold
Ar ticle information:To cite this document:Roe D. Strømmen Harald Horn Gaute Moldestad John Kristian Ramsvik Kjell R. Wold, (1995),"FSM — non-intrusivemonitoring of internal corrosion, erosion and cracking", Anti-Corrosion Methods and Materials, Vol. 42 Iss 6 pp. 3 - 6Permanent link to this document:http://dx.doi.org/10.1108/eb007373
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C O N T R I B U T E D P P E R S
FSM - non-intrusive monitoring
of internal corrosion erosion and
cracking
Roe D.
Str mmen
Harald Horn Gaute
Moldestad
John Kristian Ram svik and
Kjell R. Wold
Corrosion m onitoring and inspection is
commonly done in industry, for a wide
range of applications:
inspection of wall thickness and
cracking of pipework for safety
and maintenance planning;
monitoring for control of
corrosion mitigation programmes,
e.g. injection of corrosion
inhibitors;
research, e.g. inhibitor testing and
materials evaluation.
Mostly, corrosion monitoring has been
carried out utilizing various intrusive
moni tor ing techniques. Such tech
niques provide high sensit ivity and
accord ingly ear ly in format ion ;
however, the validity of the
information may often be questioned,
since the methods use foreign samples
installed into the pipe as the m onitored
objec t . The methods a lso have
theoretical l imitations and practical
disadvantages such as requirements for
space for retrieval operations, possible
complica t ions dur ing re t r ieval and
possible leaks along the probe.
Inspection is normally based on
various NDT techniques and by the use
of intell igent pigs for assessment of
pipeline corrosion along its entire
length. Such inspection is costly, and
the sensitivity is low compared with
the corrosion monitoring techniques.
Hence, inspection has a limited value
for corrosion control and mitigation.
FSM - the field signature method -
is a non-intrusive method for
monitoring internal corrosion, erosion
and cracking. This means that the
sensing electrodes and all other
equipment are placed on the outside of
the pipes, tanks or vessels to be
moni tored . In compar ison wi th
t rad i t ional cor rosion moni tor ing
methods (probes ) , the opera t ional
advantages of the FSM are:
There are no components being
exposed to the corrosive, abrasive,
high temperature and high pres
sure environment often found in
process piping, or the corrosive
and often hostile environment of a
subsea or buried pipeline.
There is no danger of introducing
foreign objects into the piping.
There are no consumables . The
monitoring system can be
designed for a service l ife
comparable with the piping, tank
or vessel
itself.
There is no danger of leaks in
access fittings or of unsuccessful
retrieval operations.
Measurements are done on the
wall of the pipe, tank or vessel
itself,
not on a small probe or test
piece.
The sensitivity and reliability are
better than those of NDT
techniques.
The FSM method was developed and
patented by the Center for Industrial
Research (SI) in 1985/1986[1]. Corr-
Ocean has acquired all rights from the
SI for worldwide commercial exploita
tion of the FSM technique. The first
f ield installation of the FSM was
comm issioned in 1991, and the FSM
has since been installed in a wide range
of applications, mostly in the oil and
gas industry, but also for R&D
purposes and in the nuclear power
industry. The first FSM subsea systems
were installed on a 12-inch pipeline for
Elf Petroleum Norway and a ten-inch
flowline for Shell Exploration and
Production UK in 1994[2].
Principle
The FSM is based on feeding direct
current through the selected section of
the structure to be monitored, and
sensing the pattern of the electr ical
f ield by measuring small potential
differences set up on the surface of the
monitored object. By proper interpreta
tion of these potential differences, or
rather changes in potential differences,
conclusions can be drawn, e.g. pertain
ing to general wall thickness reduction.
Local phenomena can be monitored
and located by performing a suitable
number of potential measurements on a
given area.
The FSM is unique in that all
measured electr ic potentials are
compared with initial values measured
when monitoring of the object started.
These values represent the init ial
geometry of the object, and may be
regarded as the object 's f ingerprint.
Hence the name of the method. Plate 1
shows an FSM-inst rumented p ipe
section with transformer for feeding
electric current to the pipe section and
sensing electrodes distributed over the
pipe surface.
The monitored area is located
between tw o electrodes for feeding the
exci ta t ion cur ren t . Any poten t ia l
measurements between two selected
electrodes in the matrix are compared
with a measurement be tween two
reference electrodes and to the
A n t i - C o r r o s i o n M e t h o d s
a n d M a t e r i a ls ,
V o l .
4 2 N o . 6 ,
1 9 9 5 ,
p p . 3 -6 , © M C B
U n i v e r s i ty
P r e s s ,
0 0 0 3 -5 5 9 9 A C M M V o l .
4 2 N o . 6 , 1 9 95
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cor responding in i t ia l va lues when
monitoring started, i.e. the fingerprint.
The so-called fingerprint coefficient
(
FC
) is calculated according to the
expression as follows for each set of
measurements:
FC
Ai
= (B
s
/A
s
*
A
i
/B
i
- 1) * 1000
(ppt).
FC
Ai
= fingerprint coefficient for
electrode pair
A
at time
i
.
s
=
voltage for electrode pair
A
at start-up.
B
s
= voltage for reference pair
at start-up.
Aj -
voltage for electrode pair
A
at time
i
.
B
i
= voltage for reference pair
at time
i
.
The
FC
is the parameter which is used
for analysing corrosion rates and
accumula ted cor rosion . The
FC
is
expressed in parts per thousand (ppt)
and corresponds to reduction in wall
thickness in ppt when monitoring
general corrosion or erosion. When
monitoring star ts, the
FC
values are
always zero.
The reference pair of electrodes is
located in the vicinity of the
monitoring electrode matrix, but in an
area where corrosion will not occur.
This is necessary in order to achieve
ef fec t ive compensat ion for ( smal l )
f luctuations in temperature and
excitation current. Different practical
arrangements have been developed for
securing sufficiently small temperature
gradients between reference and
monitoring electrodes.
A very high sensitivity is achieved
by statistical filtering techniques. This
facilitates high-resolution monitoring
even when measurements are
performed on the opposite side of the
corroding wall as will be the case for
in ternal cor rosion moni tor ing .
Resolution figures obtained in practice
represent less than 0.05 per cent (0.5
ppt) of the wall thickness.
pplications and experience
Topside/land-based applications
The first field installation of the FSM
was on Shel l Expro ' s Dunl in A
platform in August 1991. (We refer to
earlier publications where results from
this first FSM installation at Shell's
Dunlin platform are presented[3,4].)
Since this installation, the FSM has
been installed for a wide range of
appl ica t ions for c l ien ts wor ldwide .
Typical FSM applications have been:
corrosion and erosion monitoring
of topside piping in oil and gas
production;
moni tor ing of land-based p ipe
lines (including buried pipelines);
cooling water pipes in nuclear
power plants;
laboratory use, e.g. at flow loops
for corrosion testing.
The FSM is available with various
configurations and features, and can
therefore be optimized for each
individual application. Plate 2 shows a
"c lamp-on device" , which is very
convenient e.g. for laboratory use and
cooling water pipes in hazardous areas.
The FSM has become particularly
popular for pipeline monitoring,
especially where access to the
monitoring location is difficult (buried
pipelines) . The fact that the FSM
requires virtually no maintenance and
replacement of consumeables (except
replacement of batteries) and that the
service life of the FSM equals that of
the pipeline are important factors in
this respect.
A n t i C o r r o s i o n M e t h o d s a n d M a t e r i a ls
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Experience example
-
pipeline
monitoring
Figures 1 and 2 show data from
existing FSM monitoring stations on
two offshore pipelines in the Gulf of
Mexico. Corrosion inhibitor is injected
into both pipelines at a short distance
upstream from the two FSM stations,
and the corrosion rates are accordingly
expected to be low. This has also
proved to be the case as documented by
the FSM measurements.
FSM data obtained from two
stations fitted to two pipelines onshore
(Figures 3 and 4) indicate even lower
corrosion rates. The difference
between the offshore and onshore rates
seems to be due to low mixing of
inhibitor in the fluids offshore, which
results in inadequate inhibition of the
pipe walls at such short distances from
the injection points. Onshore the
inhibitor is well distributed in the fluid
and a higher inhibition efficiency is
obtained. The similarity of the
monitoring results of the two parallel
pipelines demonstrates an excellent
reliability and successful hits of
adequate location of the FSM
installations.
An additional FSM system has been
installed in a produced water system
where no inhibitor is applied, and the
corrosion rate is accordingly expected
to be high. The accumulated metal loss
plot (Figure 5) confirms significant
corrosion rates for this pipeline.
The experience with the FSM
system onshore and offshore has
proved its reliability in monitoring
pipeline conditions and optimizing the
effectiveness of corrosion inhibition.
The FSM system provides an effective
tool for internal pipeline condition
monitoring and accordingly for
managing a corrosion mitigation
programme.
Subsea installations
The main features of FSM subsea are:
instrumented pipe section of the
pipeline, with FSM electrodes and
current feeding arrangement
permanently fitted to the surface;
retrievable instrument unit (RIU)
that can be installed/retrieved by
divers or ROVs - the RIU contains
all electronic components and a
battery package with an
operational life of five to ten years,
and communication options are
direct via cable or hydroacoustics;
topside data collection and
presentation system.
The first subsea FSM was installed for
Elf Petroleum Norway's Fr y to Frigg
12-inch pipeline at a depth of 119
metres. The system was installed in
summer 1994, and has been functioning
without problems and provides high
quality data. Data communication from
the FSM to the Fr y platform is
provided by hydroacoustics. This first
FSM installation was a test project ,
where Norsk Agip, Saga Petroleum and
the Norwegian Research Council
participated in addition to Elf and
CorrOcean. Plate 3 shows the
Fr
y
FSM section prior to subsea
installation.
A C M M
V o l . 42 N o . 6 ,
1 9 9 5
5
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The second FSM subsea system has
been installed on a 10-inch subsea
flowline for Shell UK's Brent field. The
system has been performing success
fully since November 1994. Shell
considered subsea corrosion monitoring
a requisite for the option of carbon steel
pipeline, and expected FSM to be a
valuable tool for monitoring pipeline
condition, optimizing the effectiveness
of corrosion inhibition and reducing
future inspection costs[2].
Two more FSM subsea systems are
currently on order for A/S Norske
Shell's Troll to Kollsnes 36-inch wet
gas pipeline.
eferences
1.
Hognestad, H., A series of in-house
reports on the FSM, Center for
Industrial Research, Norway.
2 .
CorrOcean News January 1995.
3 .
StrÆmmen, R.D., Horn, H. and
Wold, K.R.,
Corrosion
Paper N o. 7,
National Association of Corrosion
Engineers, 1992.
4 . Str mmen, R.D., Horn, H. and
Wold, K.R., "Monitoring corrosion,
erosion and cracking using FSM -
the electric fingerprint method",
Proceedings
of
th e Sixth Middle East
Corrosion Conference
Bahrain,
1 9 9 4 .
Roe D.
S tr
mmen,
Harald Horn, Gaute
Moldestad, John Kristian Ramsvik and
Kjell R. Wold are all employed by
CorrOcean A . S . , Trondheim, Norway.
A n t i - C o rr o s io n M e t h o d s
a n d
M a t e r i a l s
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This article has been cited by:
1. Fangji Gan, Guiyun Tian, Zhengjun Wan, Junbi Liao, Wenqiang Li. 2016. Investigation of pitting corrosion monitoring using field signature method. Measurement 82, 46-54. [CrossRef ]
2. Fangji Gan, Zhengjun Wan, Yuting Li, Junbi Liao, Wenqiang Li. 2015. Improved formula for localized corrosion using fieldsignature method. Measurement 63, 137-142. [CrossRef ]
3. Giuseppe Sposito, Peter Cawley, Peter B. Nagy. 2010. Potential drop mapping for the monitoring of corrosion or erosion.NDT & E International 43:5, 394-402. [CrossRef ]
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