Introduction
Fig 1:
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Modern maintenance concepts, such as
Total Productive Maintenance (TPM) and
reliability centred maintenance (RCM),
demand maximum use of installations at
minimum costs, without compromising on
safety. The role of non-destructive
inspections becomes more and more
important for condition assessment to
optimise maintenance management. NDT
is required during and after construction as
well as in the user phase of installations in
oil and gas production, petrochemical
industry, power generation, food
processing industry or other types of
production plants.
Condition assessment
Instead of corrective breakdown
maintenance or time based preventive
programs, many owners of installation turn
to condition based approaches [1].
Maintenance effort should be aimed at
those parts, where it is required most.
Condition assessment of the actual piece of
equipment provides adequate data to
optimise maintenance planning. Action can
be taken immediately where needed or
postponed where possible.
Inspection planning
Fig 2: off shore production platform
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Most inspection regimes are not in line
with the maintenance requirements, but
based on fixed inspection intervals
prescribed by governmental regulations.
Duration of these intervals is mostly
determined historically or based on generic
rules for certain types of equipment. These
criteria may not always be optimal for a
specific installation. Operational integrity
of installations should be judged on more
individual basis.
"On stream" inspections can be conducted
in service to provide information about the
actual condition of the object.
Off stream inspection can be based on the
actual condition of the moment and
interval duration may be made flexible.
In view of condition monitoring programs,
it is necessary to establish the baseline
condition of any installation. This may be
done at any moment in time, but should
preferably be implemented directly after
construction. In an early stage, one should
consider using a type of inspection, which
may be applied in an equal fashion in a
later stage during service time. Periodic
condition assessment allows trending and
prediction of the remaining time-to-failure
(MTTF). Operational lifetime extension
may be agreed upon with authorities and
increased plant availability is the result.
Risk Based Inspection
Inspection planning itself may be
optimised to obtain maximum result from
as little effort as possible. A study of
applied construction and process materials,
degradation mechanisms, inspection
history and operational conditions allows
risk assessment for each individual piece
of equipment. Inspection activity is then
prioritised based on a number of
operational parameters, e.g. operation
pressure and temperature, consequential
loss, personnel safety, environmental
damage etc. Risk assessment for each
individual asset leads to a risk based
inspection (RBI) approach.
In Service Inspection(ISI)
Preferably, NDT shall be performed before
a planned shutdown, while the equipment
is in service. Ample time is then available
to plan the necessary maintenance work
long before the actual shutdown takes
place. Since the inspection work has been
done before shutdown and the maintenance
work is well planned, total downtime for
the installation is reduced.
NDT requirements
Lifetime calculation models may predict
the remaining service time of any asset.
These calculations require input data
provided by the assets' history and
available NDT data. The outcome can only
be as reliable as the input. High probability
of detection (POD) is required to ensure
reliable operation until the next shutdown
and low false call rate (FCR) is desirable to
avoid unnecessary maintenance work.
Accurate and highly reliable data result in
a reliable prediction of the remaining
service time.
On stream inspection techniques
The number and methods of inspection are
always a trade-off between the minimum
requirements for safe operation and the
amount of information needed for optimum
maintenance management.
In a baseline inspection, it is important to
establish very accurately the zero condition
of a piece of equipment. Therefore, precise
methods should be applied, mostly in an
off line situation. Dedicated techniques are
available for full off stream inspection.
On the other side, ISI are faced with
limitations such as access restrictions, high
temperatures etc. Most on stream methods
are considered screening tools, which
assess general condition rather than exact
defect locations and dimensions.
Qualitative techniques indicate trends in
condition deterioration but cannot deliver
exact quantitative information.
Documentation
In signalling (monitoring) inspection,
reproducibility is of high importance.
Many conventional ultrasonic methods are
performed with hand held probes and
results are presented in hand written
reports. The influence of the particular
inspector on the outcome is significant and
hardly reproducible. Mechanised scanning
and automated reporting minimise this
'human factor'.
Access restrictions
Many inspection problems are associated
with restricted access. Conventional
techniques fail at complex objects or
locations that can hardly be accessed such
as inspection of nozzles on vessels, tapered
pipes, pipework under insulation or on
sleepers, under reinforcement or repair
patches, etc. New inspection techniques
have been developed to fill in the gap in
ancient problem areas.
New materials
Higher operational requirements demand
higher performance of construction
materials. Nowadays, high pressure vessels
are constructed in austenic or duplex steels
with internal cladding, special weld
materials and buffer layers which are often
very difficult to inspect.
Another trend in pipeline construction is
the use of glass fibre reinforced plastics
(GRP). Conventional inspection methods
are restricted by the material structure and
alternative techniques should be applied.
High temperature measurements
On stream measurements are often faced
with high process temperatures. Elevated
temperature measurement imposes
additional requirements on the inspection
equipment. Special ultrasonic probes and
couplants are applied for thickness
measurements up to 350ºC. Non - contact
methods may even go up to higher
temperatures.
During service time of installations many
known degradation mechanisms threaten
the condition of the equipment and
eventually cause failures. Several
phenomena are known to cause problems
in the service phase of equipment.
Depending on type of equipment,
materials, process parameters, service time
and external factors, certain types of
degradation may occur. It is desirable to
understand these mechanisms and
anticipate in an early stage on possible
consequences.
Degradation phenomena may be classified
in two groups: mechanical wear and
chemical corrosion. Mechanical wear
occurs in equipment under cyclic
mechanical or thermal loads. Depending
on its manifestation and the equipment's
function, mechanical wear may be more or
less threatening to the operational
reliability. Corrosion manifests itself in
several forms [2], which occur in a certain
environment in a single form or in a
combination of corrosion forms. Many
forms may have a severe impact on the
integrity of (parts of) process installations.
General corrosion
The most common manifestation of
corrosion is a uniform attack, caused by a
chemical or electro-chemical reaction
uniformly distributed over the exposed
surface. A combination of a corrosive
product and an oxygen containing
environment may start corrosion.
Environmental factors such as temperature,
electrochemical potential etc determine the
corrosion rate. Generally, this type of
corrosion is of no great concern, since a
slow, gradual loss of material is well
predictable and adequate measures may be
taken.
Pitting corrosion
Fig 3: pitting corrosion in a storage tank floor
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Localised corrosion may be a greater threat
to installations, because it may form small
pinholes that perforate the material rapidly.
Pitting is a result of an anodic reaction
process. At an initiation location, the
surface is attacked by a corrosive product
(e.g. chloride). Metallic atoms are
dissolved at the surface of the starting pit.
The dissolution causes excessive positive
charge in the surface area, which attracts
negative chloride ions to restore
electrochemical balance. The chloride ions,
again, dissolve new metal atoms and the
reaction becomes self propagating. Within
a short time the pit may penetrate the
complete wall thickness. The localised
nature of pitting makes it extremely
difficult to detect pits in an early stage.
Weld root erosion/corrosion
Fig 4: weld root erosion in a flow line
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Root erosion is a degradation phenomenon,
which is often encountered in flow lines. It
is difficult to grind the weld penetration on
the inside of pipelines. The excessive
penetration causes a discontinuity on the
surface, which disturbs the flow pattern.
On some metals, the wall is passivated by
an oxide film, which protects the steel
from corrosion processes. Turbulence and
cavitation affect the region adjacent to the
welds and hamper formation of this
passivation layer. Local wash out of wall
material at the weld side is the result.
Another form of weld root corrosion is
caused by selective corrosion. In many
corrosion resistant alloys or special
welding materials, selective leaching may
occur. Removal of the least noble metals
results in deterioration of the lattice
structure in alloys (e.g. dezincification in
brass components). If the weld material is
more susceptible to corrosion than the base
material, wash out of the weld causes root
corrosion and degradation of structural
integrity.
Fatigue cracking
Under cyclic mechanical or thermal loads,
a component may be subject to fatigue.
This process is caused by repeated stresses
just below the yield point. However, due to
stress peaks, microscopic plastic
deformations of material structure occur.
Under continuing stresses, these
deformations result in crack initiations.
Mechanical fatigue cracking manifests
itself as cracks with preferential orientation
perpendicular to the predominant stress
directions. Thermal fatigue cracking results
in a random web-like crack structure.
In combination with a corrosive medium,
the fatigue resistance of materials is
reduced. Corroded spots act as initiator of
fatigue cracks, which on their turn corrode
fastest at the crack tip. This combined
mechanical and chemical process is called
corrosion fatigue.
Stress Corrosion Cracking
Crack formation occurs at location where
tensile stresses act on the component in a
specific corrosive environment. High
pressure equipment, mechanical stresses,
thermal stresses, remaining stresses from
welding processes etc. may initiate stress
cracking. After a while, corrosion products
in cracks act as wedging forces. A
preferably hot, aqueous environment in the
presence of oxidizers is an ideal situation
where SCC grows rank. Failing cathodic
protection may accelerate the process.
In heavy duty austenitic materials, stress
corrosion cracking occurs mainly along the
grain boundaries. This phenomenon,
known as intergranular stress corrosion
cracking (IGSCC), manifests itself in the
heat affected zone of nuclear reactor
vessels and piping. With increased use of
high definition materials in petrochemical
plant work IGSCC may cause problems in
process plants as well.
Hydrogen Induced Cracking (HIC)
Chemical reactors containing hydrogen
may suffer from hydrogen induced
cracking (HIC). In contrast with molecular
H2 gas, Hydrogen atoms penetrate steel
reactor walls. Recombination of atoms
forms H2 gas, which piles up in voids in
the metal structure. Internal pressure
increases until blisters build up and
cracking occurs. Propagation of cracks
occurs often by transitional cracks between
two parallel cracks. This process is known
as stepwise cracking.
Hot Hydrogen Attack
At high temperature, hydrogen atoms react
with the metallic atoms to intermetallic
hydrides. This phenomenon is referred to
as Hot Hydrogen Attack, to discriminate
from HIC, which merely occurs at lower
temperatures. Formation of methane gas
(CH4) is caused by Hydrogen reaction with
carbides from the metal structure. Besides
expansion of the volume and crack
initiation, decarburisation of the steel
structure results in hydrogen
embrittlement, which leads to fast
deterioration of structural integrity.
With the upcoming of computer
technology and miniaturisation of
equipment, NDT techniques have
developed rapidly into modern highly
reliable inspection tools [3]. One of the most
important factors is the increased accuracy
and reproducibility of data. Further
mechanisation of inspection techniques
improved reliability greatly. One of the
main reasons for this is that full coverage
is assured by performing the inspection in
a mechanised fashion. Secondly, the
availability of a complete inspection record
greatly improves condition monitoring
facilities.
Special UT probes
Fig 5: various types of ultrasonic probes
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In the past, specific probes have been
designed to solve known inspection
problems. Special focused angle beam
compression wave probes have been
developed to inspect specific depth regions
in a weld. The compression wave concept
was introduced by BAM in the early
eighties [4]. Dual crystal probe construction
and beam focussing techniques enabled
high sensitivity ultrasonic examination of
complex materials, which could not be
achieved with standard shear wave
techniques.
In austenitic or duplex materials, a well
known phenomenon is intergranular stress
corrosion cracking (IGSCC). Especially in
nuclear component inspection, many
studies have been undertaken to find
appropriate methods for IGSCC detection.
For near surface cracks, special creeping
wave probes have been developed.
Creeping waves travel just below the
surface rather than in it, therefore they are
not influenced by the presence of coupling
liquids, and the influence of surface
irregularities. Moreover, since the creeping
wave is a compression wave type, they
suffer less from a coarse material structure
than shear waves. Another outstanding
example is the development of probes
focussed under cladding crack detection
(UCC). The focal range is calculated in the
parent material - cladding transition zone.
Cracks initiating from the clad layer into
the parent material may be readily detected
by UCC probes.
Mode conversion techniques and
combination of functions lead to
minimization of the number of probes
required for complete inspection coverage.
Tandem transducers with multiple crystals
can be build in one housing such as the
Round Trip Tandem (RTT) probe or
special functions such as Long-Long-Trans
(LTT) probes have been developed. Multi
crystal transducers combine a number of
tasks in one housing to save space in the
scanner setup and construction costs.
In all cases, it appeared to be of utmost
importance that the transducer parameters
are optimized for specific jobs. Once
having gained experience with a certain
weld type however, it is possible to
establish a "standard series" of dedicated
transducers, with which the inspections can
be performed without excessive lead times.
Mechanization of the inspection improved
inspection accuracy and reproducibility.
TOFD
The Time-Of-Flight Diffraction (TOFD)
technique is an advanced ultrasonic
inspection technique that fulfils a need for
reliable inspections. It is a powerful
technique because it can simultaneously
detect and size defects.
TOFD provides highly reproducible
fingerprints of installation, which makes
TOFD extremely suitable for condition
monitoring[5] .
TOFD Weld inspection
Fig 6: TOFD inspection of process pipework in a petrochemical plant
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The preparation required for a TOFD scan
is minimal, which makes the technique
attractive even when only a small number
of welds have to be inspected. TOFD may
be applied during construction, where time
constraints exist. TOFD allows
examination directly after welding (up to
200 °C) without hold up of production
speed. The acceptance result is directly
available. There is practically no
interference in continuation of nearby
construction works for safety reasons (no
radiation shielding etc).
Over the past years, the system has been
used in a great variety of applications,
ranging from circumferential welds in
pipelines (including joints of different wall
thickness and tapered pipes), weld
inspection of heavy wall pressure vessels
(up to 300 mm wall thickness). Also, the
TOFD technique was successfully applied
for inspection of partially filled welds,
which are hardly inspectable by any other
technique. Nozzle and flange welds
(complex geometry) can be inspected with
prior computer simulation modelling to aid
inspection planning and result evaluation.
On stream inspection with TOFD
In contrast with radiography, for TOFD
examination only external access to the
object is required. In the service phase of
process installations and pipework, TOFD
may be applied to detect and monitor
service induced defects (stress or fatigue
cracks etc). 'Fingerprints' of the object are
recorded during acceptance inspection of
welds directly after construction and
periodically every number of years. Initial
acceptable defects are monitored and
service induced defects are revealed and
progressively monitored.
Critical reactor vessels with heavy wall
constructions can only be adequately
inspected by means of TOFD. Other
techniques such as high energy
radiography with Cobalt-60 sources or
portable betatrons, are faced with high
safety requirements and extremely long
examination times. Ultrasonic meander
scanning is often too cumbersome and time
consuming. Spherical gas tanks and steam
generator headers may be surveyed for
cracks.
Root erosion in flow lines
Selective erosion/corrosion in flow lines
may be detected and sized by regular
TOFD inspection. Discrimination between
single or two side wash out is easily
achieved from the TOFD images. Long
stretches of pipeline may be inspected
rapidly with minimum preparation needed.
Hot Hydrogen Damage detection
Hydrogen embrittlement starts from the
internal surface and propagates slowly
through the wall material. It is often
difficult to detect due to minor changes in
structure. A TOFD image can display an
increased noise level at the affected
location and estimate the degree of attack.
Mapscan
The demand for mechanised ultrasonic
wall thickness measurement is fulfilled
with the introduction of the Mapscan.
Mapscan applies a mechanical link
between transducer and computer to record
the thickness data for each predetermined
measurement position. The transducer is
scanned manually over the surface and the
thickness readings are stored on disk. After
the scanning is finished, the data are
plotted in a wall thickness map. Each
thickness level is colour coded and wall
thinning by corrosion or erosion is readily
recognised. High reproducibility (within
0.3 mm wall loss) enables accurate
monitoring and calculation of corrosion
rates. This input is very important to
estimate time-to-failure.
Mapscan is applied on vessels, pipework
(bends), tank walls etc, for exact
documentation of corroded regions. It can
be applied in service at temperatures up to
250°C. Corrosion phenomena such as
general wall thinning, pitting corrosion,
flow accelerated corrosion (FAC),
hydrogen induced corrosion (HIC) and hot
hydrogen attack have been revealed
successfully by Mapscan.
For its high accuracy, Mapscan is accepted
au lieu internal visual inspection of vessels.
Based on the on stream Mapscan results,
off stream inspection interval extension is
often accomplished.
P-scan/Bandscan
P-scan
Fig 9: P-scan inspection of a nuclear reactor
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Ultrasonic mapping techniques prove to be
very useful in wall thickness measurement.
The same advantages apply for weld
inspections. However, ultrasonic weld
inspection is executed with angle beam
transducers and mapping algorithms
become more complex. This problem was
solved with the development of projection
scanning (in short 'P-scan'). Dedicated
scanners enable 3-dimensional presentation
of the inspection object. This technique
enabled extreme high reliability inspection
for monitoring of nuclear reactor vessels.
Meander scanning was implemented to
comply with existing ultrasonic inspection
standards e.g. ASME, AINSI, etc.
Regression in construction of new nuclear
power plants, meant a reduction in
inspection work. Increased inspection
requirements in process plants have
accelerated a technology transfer to
petrochemical equipment inspections.
Especially for high pressure vessels and
stainless steel reactors higher inspection
demands arose.
Enhanced scanning equipment has lead to
further automation of ultrasonic
inspections. A spin off of this development
is fast mechanised wall thickness mapping,
which in spite of the expensive equipment
is economically attractive due to its high
inspection speed.
Bandscan
Fig 10: Bandscan inspection on a spherical tank
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Mechanised weld inspection greatly
improved accuracy and reproducibility.
The development of special purpose probes
and focussing techniques opened new ways
of inspection. Instead of meander scanning
with standard shear wave angle beam
probes, the concept of line scanning was
implemented in the Bandscan. A
transducer frame on a guided vehicle is
moved parallel to the weld along a band.
Any number of probes can be mounted
with different functions.
For weld inspection, the cross section of
the weld is subdivided in several depth
zones, each of which is addressed with a
combination of probes. Using focusing
techniques, the ultrasonic beam is so
narrow at the focal position, that accurate
defect sizing is possible, even though only
a single line scan is performed. Improved
inspection speed and direct sizing
capability are great advantages for
application of Bandscan rather than
meander scanning (P-scan). Especially for
large structures, such as spherical gas
tanks, Bandscan is the preferred method.
Later on this concept was motorized for
high inspection speed on pipeline girth
welds. Inspection cycles of several minutes
in (off shore) pipeline construction can
now be achieved with the widely used
"Rotoscan" systems.
For non-routine weld inspection, such as
dissimilar metal welds (DMWs), Bandscan
may be equipped with special probes.
Joints of carbon steel to austenitic, duplex
or high nickel alloy steel materials can be
examined using optimised probes. Due to
the coarse structure of these materials, they
can hardly be inspected using conventional
shear wave methods. Compression wave
angle beam probes are capable to penetrate
these ultrasonic unfriendly materials. In
applications, where TOFD or shear wave
ultrasonic techniques fail, Bandscan may
do the job. In this field, once again,
Bandscan has proven its merits
LORUS (Long Range Ultrasonics)
Fig 11: LORUS scanner on the external lid of a
storage tank floor.
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Service induced defects such as corrosion
and cracking tend to favour locations
where access for inspection is limited.
Hidden corrosion is often found with great
difficulty or in a late stage when damage is
already done. LORUS has been developed
as a tool for fast screening of regions with
limited access [6]. From a single access point,
a range up to one metre may be examined
for corrosion or cracks. Inspection may be
carried out on stream without dismantling,
lifting or opening components, which
means substantial savings in shutdown,
process interruptions and/or cleaning time
and costs.
Special design, high sensitivity ultrasonic
probes are applied to achieve a
considerable inspection range. LORUS
measures reflection signals and composes
coherent projection images. Examination
results are documented in easy to
understand, colour coded, 2D top view
corrosion maps. Corrosion extent is readily
obtained and corrosion growth may be
monitored in recurrent inspections
Reflection amplitudes provide qualitative
information on corrosion severity but can
not present actual corrosion depth.
Storage Tank Inspection
An upcoming trend is the use of on stream
screening techniques to establish general
tank condition. LORUS may be part of a
carefully composed on stream tank
inspection package, containing Acoustic
Emission corrosion activity measurement,
and mechanised ultrasonic wall thickness
mapping. These complementary
techniques provide a firm basis for
decision making in maintenance planning
and service time extension for storage
tank operation.
LORUS focuses specifically on the high
risk zone of the annular plate, supporting
the tank shell. This region is considered
critical, due to high stresses and failures
may lead to large product spills or
endanger personnel and environment. The
ultrasonic beam is emitted under the proper
angle to propagate underneath the shell and
cover a range up to 1 metre. Projection
images show the area of the annular plate
region in top view and form a permanent
document for recurrent inspections.
General corrosion as well as localised
pitting corrosion is easily recognised.
Large area screening
Fig 12: A small strip needs to be prepared for LORUS inspection of a 2 m wide area.
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Fig 13: 100% area screening of a large tower
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Fig 14: LORUS inspection of pipe supports
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Mapping of large objects for corrosion or
cracks may be performed rapidly with
projection mapping techniques. Complete
volumetric coverage is achieved by
performing line scanning only. Substantial
time saving is achieved compared to full
area scanning methods (e.g. Mapscan).
In many cases, defect orientation may be
such that angle beam inspection is
required. Detection of cracks in through
thickness direction demands application of
angle beam probes. Projection images
present colour coded defect maps, with
exact defect location and extent. Depth
sizing may be by more quantitative
techniques e.g. TOFD, at those locations
indicated on the LORUS maps.
Even locations that cannot be inspected
directly because of access restrictions of
pipework, repair scales, bandages etc, can
be examined by means of LORUS.
Minimum surface preparation combined
with high inspection speed, essentially
provide a rapid and cost effective mapping
technique. This method is extremely suited
for mapping of cracking, such as fatigue
cracking with a preferential orientation or
randomly orientated SCC, but also suited
for small corrosion pits.
pipe supports
Corrosion under pipe supports or saddles is
a major problem area for inspection. The
region between outer pipe surface and
support is susceptible to corrosion due to
water ingress. Radiographic or ultrasonic
wall thickness techniques are not
applicable because of access limitation.
The only option is lifting the pipe from the
support to gain access. However, the
condition of the pipe is the unknown factor
and lifting may be a riskful enterprise.
With LORUS, the support region is
inspected from the free top surface of the
pipe without the need for lifting. Fast
screening of large numbers of supports is
achieved in a minimum of time.
Both pulse-echo and transmission
techniques are applied in circumferential
directions to obtain maximum information.
In a single scan over the top surface of the
pipe, two probes measure reflection and
transmission signals simultaneously.
Reflection signals are used to calculate
projection images, while transmission
signals are used to estimate corrosion
severity in several depth classes.
Guided wave pipeline inspection
Screening tools for fast assessment of large
parts of installations seem to have a
growing inspection potential. In stead of
spot checks, plant users demand complete
100% inspection coverage of their
installations. Where conventional
ultrasonic techniques, based on bulk wave
propagation, have a limited range up to one
meter, Lamb waves have the potential of
propagating over much longer distances. In
a confined geometry such as a pipe, guided
waves build up, which can travel over tens
of meters.
As a screening tool, this technique
provides on line information of long
lengths of pipework. Guided waves travel
across straight stretches of pipes, bends,
supports, T-joints, etc but cannot pass
across flange joints, end pieces, etc.
Fig 15: Guided wave inspection system
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Ring transducers have been developed [7],
which can generate waves in a specific
mode, optimal in range and sensitivity.
An extremely long inspection range is
achieved for screening of on and offshore
pipework, detection of corrosion under
insulation without removing lagging other
than for application of the probes, road
crossings and other hidden penetrations,
lined pipework, etc.
Since very low frequencies are applied, the
defect sensitivity is limited to larger areas
of (corrosion) wall loss. Welds cause
reflection signals at regular distance,
providing reference for sensitivity settings.
Internal features in the weld such as weld
root erosion may be discriminated in the
reflection signal by advanced signal
processing techniques. In a similar way,
guided wave inspection could discriminate
between corroded and unaffected pipes at
locations of supports. The full potential of
the technique will become evident when it
is applied more widely.
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