NDT methods for oil pipe inspec3ons
Abstract
This paper has been wri-en to analyze the different non destruc7ve tes7ng techniques efficiency and effect for
oil pipe inspec7on .Pipeline inspec7on is important to determine flaws , weakness ,corrosion or suscep7bili7es
that could lead to pipeline failures . The techniques commonly being used are ultrasonic tes7ng , magne7c flux
leakage tes7ng , eddy current tes7ng , Pipeline Inspec7on Gauge (PIG) Systems, Un-Piggable Robo7c Inspec7on
Systems, Electromagne7c Acous7c Transducer (EMAT) etc. Working principles, advantages,
and disadvantages are analysed; emphasis is on whether the methods can detect the cri7cal defects with
performance under challenging field condi7ons. With a growing interest in recent advances including robo7cs
and ar7ficial intelligence – an electronics acceptance examina7on and automated inspec7ons have gained
popularity . Despite tremendous progress, challenges con7nue to exist, including more reliable defect
characteriza7on in complex geometries. Future work would include improvement in the accuracy and efficiency
of NDT methods besides overcoming poten7al prac7cal constraints in the field applica7ons.
Introduc3on
Oil pipelines are essen7al infrastructure systems for conveying energy resources, yet they suffer from significant
degrada7on through corrosion, cracking, and mechanical damage. Ensuring the integrity of pipelines requires
preven7ng hazards from accidents in the opera7on of such lines, including environmental disasters, financial
loss, and safety hazards. Nondestruc7ve tes7ng is an important technique that provides inspec7on of defects
without structural damage during pipeline inspec7on.
During the years, many NDT methods have been developed, from ultrasonic and radiographic tes7ng to magne7c
flux leakage and eddy current techniques. They provide unique advantages, matched for specific defects and
inspec7on scenarios, although issues like proper characteriza7on of a defect and field deployment in complex
environments are s7ll among such challenges. Some of the techniques as men7oned in the papers are as follows
– Microwave based tes7ng , Here non-metallic components are evaluated by exposing the material to high
frequency electromagne7c waves. These waves are emi-ed from a specialized microwave probe, designed to
enhance system security , without requiring direct contact with the surface being inspected . When the wave
interacts with the material, changes in the material’s proper7es alter the wave’s propaga7on part of the wave
penetrates the structure, while the rest is reflected back. The interac7on between the wave and the material
depends on the geometric and electromagne7c proper7es of the specimen ; In magne7c flux leakage inspec7on
when a ferromagne7c object is subjected to a strong magne7c field, various geometric discon7nui7es in the
test specimen permit the flux to leak out from the material into the surrounding air. This is referred to as
magne7c flux leakage (MFL). Sensors placed around the circumference of the
object catch the phenomenon of MFL and thus assist in the measurement of the size and nature
of defects found in the material; Ultrasonic Inspec7on is a key conven7onal NDT method used to detect defects
in materials or on their surfaces by u7lizing high-frequency sound waves. As these ultrasonic waves propagate
through a material, they lose energy and reflect off surfaces. By analysing the reflected signals, it is possible to
iden7fy the presence and loca7on of discon7nui7es or defects. UT is par7cularly effec7ve in detec7ng cracks,
crevices, metal loss, and other irregulari7es at various depths within a sample, thanks to the reflec7ve,
diffrac7ve, and transmissive proper7es of ultrasonic waves. Eddy current tes7ng is widely used for crack
detec7on since it is sensi7ve to conduc7vity, permeability and thickness varia7on of materials;
Radiographic tes7ng is widely u7lized in the industry, offering a clear internal view of the inspected specimen
while providing permanent image records. One of its key advantages is that it does not require surface
prepara7on or the removal of insula7on, making it highly efficient for various applica7ons.
This review aims to give a comprehensive analysis of key NDT methods for oil pipeline inspec7ons. Topics
discussed include the applica7on principles and their limita7ons, with addi7onal recent developments
concerning ar7ficial intelligence and robo7c systems. The present challenges and the research priori7es outlined
in the conclusion promise greater effec7veness in the applica7on of NDT in oil pipeline management.
Methods
1.Inspec3on using radiographic test:
Radiographic tes7ng finds extensive applica7on in industry because of its facility to produce internal views of
specimens under inspec7on and permanent record of images . The biggest advantage of radiographic tes7ng is
that it does not require treatment of any surface , removal of insula7ons , or cleaning of external deposits
naturally present on pipeline surfaces . It is o\en used in welding inspec7ons, and research is s7ll carried out to
improve automa7c radiographic analysis.
The radiographic tests have been carried out using gamma rays (source:Co-60) and X-rays geometries: double
wall-single exposure and single wall-single exposure. In case of gamma-ray exposure, four films per weldment
are used while in case of X-rays exposure eight films per joint is required. The primary reason for digi7zing the
radiographic films is to improve image quality . Digital filters were used to eliminate noise from the digi7zed
radiographic films for be-er visualiza7on of defects . However , care needs to be taken while filtra7on so as to
ensure the filtra7on process does not down grade defect sizes .
Recent trends in RT tend to emphasize automa7c image processing and computer-aided detec7on to overcome
human interpreta7on shortcomings. For example, Yazdani et al[1]. developed an automa7c image-processing
technology intended for acquiring high-quality X-ray images with be-er contrast and a reduc7on in noise for
sizeable measurements of defects for accurate iden7fica7on .
2.Inspec3on using manual and automa3c ultrasonic tes3ng
As discussed in the study by Ma et al. [2], pipeline in-line inspec7on methods have advanced significantly in
recent years , Three qualified inspectors tested the specimen as part of a reliability test of manual ultrasonic
pulse-echo inspec7on methods for defect length detec7on and sizing. The 6 dB method was employed in sizing,
acceptance criteria using the distance amplitude curve (DAC). A reference block has been manufactured and
detailed in the inspec7on procedure PR-011 and calibra7on of the DAC curve will be performed and each
inspector will trace his or her own DAC curve for the tes7ng purposes.
On the contrary, the intelligent inspec7on u7lized a dedicated hardware configura7on that consisted of an
inspec7on vehicle, magne7c wheels, a ruler having transducers for scanning along the weld, a conven7onal
ultrasonic device, a mul7plexer to interface eight transducers, and a computer system for handling control and
data storage. The ultrasonic device analog output was connected to the computer through an AD converter
board, which operated at a 20 Ms/s digi7za7on rate and allowed the digi7zed ultrasonic signals to be stored. The
computer managed the control of the system using transducer selec7on through the mul7plexer, movement of
the vehicle through the serial port, and signal storage.
A tailored program and MATLAB so\ware were applied to process the data generated from the Time of Flight
Diffrac7on technique. The processed A-scan data were then applied to form the D-scan images for discon7nuity
detec7on and characteriza7on along the weld bead. Perimeter lengths around the pipeline wall were measured,
and heights within the wall thickness were established for the exis7ng defects. A calibra7on step ensured that
all recorded defect sizes were properly calibrated.
3.Eddy Current Techniques
The Eddy Current Tes7ng is highly sensi7ve for crack detec7on since it depends on conduc7ng material
proper7es; it is the sensing of varia7ons in conduc7vity, permeability, and thickness which cause a change in the
distribu7on and intensity of eddy currents upon interac7on of an alterna7ng magne7c field with a conduc7ve
material. Such varia7ons are sensed due to altera7ons in the magne7c field that result in the genera7on of a
voltage in detector coils to measure conduc7vity and defect features emana7ng from material characteris7cs,
defect size, and shape. EC is a non-contact inspec7on technique with high speed capabili7es but is mainly limited
for the surface or near-surface inspec7on of conduc7ve materials with all the disadvantages of the skin effect. In
addi7on, performance is extremely sensi7ve to the so-called li\-off distance—the space between the probe and
the surface under test.
Other advancements in EC are RFEC, ECA, and PEC. RFEC employs low-frequency currents, which traverse tube
walls twice, so defects are detectable internally and externally or a reduc7on in thickness. The detector coil of
RFEC is o\en situated almost two 7mes the pipeline diameter from the exciter coil. It is very useful in detec7ng
wall defects and thinning in metal tubes.
ECA works through the par7cipa7on of several elements arranged in a pipe to prevent the complexity of scanning
paths and provides greater reliability than the conven7onal single-probe EC systems. PEC uses pulse excita7on
signals to cause transient currents through the material under inspec7on, thereby offering richer spectral
content for the analysis of defects and material proper7es at varying depths. The method also does not have the
depth limita7ons present in the standard EC because of the skin effect.
The innova7ons in applied research further strengthen these techniques. For instance, Denis et al. op7mized PEC
probes for improved sensi7vi7es at higher li\-off distances. Li et al. have proposed pulse-modula7on eddy
current for improved accuracies in external corrosion detec7on and imaging at long distances. Chen et al[3]. have
developed a magne7c force transmission ECA probe for long-distance pipeline inspec7ons. In a like manner,
advances in RFEC applica7ons regarding pipeline corrosion detec7on and defect differen7a7on have been
achieved. For example, theore7cal evalua7on of RFEC proposed by Sun et al. was applied for SCC detec7on
pipeline tes7ng, and Kim et al. proposed an RFEC-based prototype pig featuring a mul7stage rota7ng magne7c
field. Fukutomi et al[4]. op7mized probes of RFEC with advanced electromagne7c field analysis to improve the
sensi7vity for micro-defects detec7on in non-magne7c steam generator tubes.
These achievements, therefore, unveil the increasing prowess of EC methods in overcoming some of the complex
inspec7on challenges and enhancing defect detec7on accuracy.
4.Accous3c Emission
AE is a physical phenomenon origina7ng from the sudden release of elas7c energy due to deforma7on or
applica7on of external forces, thus crea7ng a transient stress wave. It is a dynamic NDT approach that can be
used to monitor internal structural changes, defects, or poten7al defects as they begin to develop. The structural
integrity of the material under considera7on and its performance can therefore be evaluated by capturing and
analyzing AE signals origina7ng from crack propaga7on, plas7c deforma7on, or phase transforma7on.
AE is highly promising for pipeline monitoring with significant contribu7ons in the research field. For example,
Quy et al[5]. u7lized AE signals and the Time Difference of Arrival technique to determine the loca7on of emission
sources in pipelines where high-pressure liquid flow occurred, enabling the detec7on and localiza7on of irregular
structural changes such as crack-forma7on. Other similar work that draws a-en7on includes the research study
by Paton et al. on the acous7c proper7es of steam pipes in industrial applica7ons with a focus on con7nuous
monitoring and fracture load predic7on. However, AE signals are very material-geometry-sensor-dependent;
hence, it is problema7c to directly relate AE data from fracture events and material condi7ons. Advanced signal
processing techniques need to be applied on clustering AE ac7vity and fracture-related features indica7on.
5. Magne3c Barkhausen Noise (MBN)
Magne7c Barkhausen Noise (MBN) detec7on technology is applied increasingly widely to assess residual stress,
fa7gue, and aging in ferromagne7c materials. The technique assesses signal characteris7c analysis of magne7c
or acous7c emissions developed during domain reversal in the magne7za7on process. These signals reflect the
microstructure of a material and its distribu7on of stresses.
Jiles et al[6]. have reported the studies on the influence of residual stress, elas7c and plas7c deforma7on, and
the Babbi- effect on steel, and how the microstructure of the steel can influence MBN. This technique may,
based on the mechanism of MBN dis7nguish not only stresses but also microstructural features like fa7gue life
and small cracks. In addi7on, Jancarik et al.[7] inves7gated the li\-off effect in carbon-stainless steel samples:
the slope of the MBN distribu7on amplitude did not change, but the sensi7vity decreased.
In summary, the reproducibility of MBN under changing magne7za7on condi7ons and environmental and
material dependent effects are s7ll open issues that need further inves7ga7on. Nevertheless, stresses are
appropriately determined by means of MBN, and also all material states of health can be es7mated.
6.Eddy Current Pulsed Thermography (ECPT)
As discussed in the study by Ma et al. [1], pipeline in-line inspec7on methods have advanced significantly in
recent years Eddy current pulsed thermography (ECPT) is based on the principles of eddy currents and Joule
hea7ng in electromagne7sm. It employs an infrared thermal imager to capture the temperature distribu7on and
heat conduc7on in conduc7ve materials when subjected to pulsed eddy currents. By analysing mul7ple thermal
images, defects can be detected. Compared to other infrared thermal imaging methods, ECPT uses pulsed
electromagne7c excita7on to heat the volume, offering advantages such as electrical, magne7c, and thermal
characteris7cs, along with rich transient data, high spa7al resolu7on, and high sensi7vity for detec7ng near-
surface defects. The induc7on hea7ng is concentrated at the defect site, which enhances the temperature
contrast between the defect and non-defect areas. This improves the signal-to-noise ra7o and detec7on
sensi7vity, par7cularly for small defects or mul7ple defects in complex geometries. The method is effec7ve for
detec7ng cracks with minimal depth, and studies have explored its applica7on for surface defect detec7on in
metals, as well as the impact of defect size and orienta7on on thermal distribu7on. Addi7onally, a leading
experimental plaporm for ECPT has been developed for weld inspec7ons and external pipeline maintenance.
7.Microwave based tes3ng
Microwave NDT has been effec7vely used for inspec7ng non-metallic pipes [8]. Microwave tes7ng for
nonmetallic components inves7gates a sample structure using high frequency electromagne7c waves. These are
radiated from a microwave probe, which is applied in order to maximize system sensi7vity, and, amazingly, there
is no need to have a direct contact between the probe and the structure. When the wave interacts with the
structure, changes in the proper7es of the material of the medium result in a part of it gerng into the structure,
while another part bounces back. The type of interac7on between the wave and the inner structure is
characterized by both geometric and electromagne7c proper7es of the wave. The geometric as well as material
property anomalies like cracks, voids, delamina7on and disbonds which are brought by these anomalies,
microwaves are very sensi7ve to. Therefore, any such anomaly causes measurable varia7on in the reflected and
transmi-ed waves. The system admits one-sided, non-contact inspec7on by measuring the reflected wave from
the same probe that was used to send out the signal. The design of the probe and the detec7ng circuitry affects
the sensi7vity and resolu7on of the microwave system.
As discussed earlier, the basis of microwave near-field NDT involves using an open-ended probe to illuminate the
SUT while simultaneously receiving the reflected signal. The combina7on of the transmi-ed and reflected signals
produces a standing wave. There will be a DC voltage propor7onal to the power of this standing wave which is
obtained through a diode detector. The probe stays sta7onary and the SUT moves in fixed increments with a
custom-made scanner. The standoff distance between the probe and the SUT are kept constant. At each scanning
step, the a-ained voltage reading is taken and transmi-ed through a USB cable to a computer. There, the
readings are processed to create an image of the SUT.
Table 1: Key NDT Methods and Their Characteris8cs
NDT Method Key Concepts Advantages Disadvantages
Effec3ve for deep Requires surface contact;
Ultrasonic Tes:ng U3lizes high-frequency sound defects; provides operator skill
(UT) waves to detect defects. precise loca3on. dependent.
Detects defects in
ferromagne3c materials by Good for surface
Magne:c Flux measuring magne3c field and near-surface Limited to ferromagne3c
Leakage (MFL) leakage. defects. materials.
Measures varia3ons Limited to conduc3ve
Eddy Current Tes:ng in electrical conduc3vity to Non-contact; materials; surface
(ECT) iden3fy defects. fast inspec3on speed. sensi3vity.
No surface Safety concerns with
Radiographic Tes:ng Uses gamma rays or X-rays to prepara3on needed; radia3on; requires film
(RT) visualize internal structures. permanent records. processing.
Monitors stress waves Real-3me monitoring; Highly dependent on
Acous:c Emission generated by crack forma3on or can detect growing material and sensor
(AE) deforma3on. defects. configura3on.
Employs high-frequency No contact required; Limited applica3on
Microwave electromagne3c waves for non- sensi3ve to material scope; complex
Tes:ng metallic components. proper3es. interpreta3on.
Table 2: Applica8ons and Results of NDT Techniques
NDT Method Applica:ons Results Obtained
Ultrasonic Tes:ng Pipeline weld inspec3ons, Accurate defect sizing and loca3on
(UT) thickness measurements. iden3fica3on.
Magne:c Flux Pipeline integrity assessments, Detec3on of cracks and corrosion at weld
Leakage (MFL) corrosion detec3on. joints.
Eddy Current Surface crack detec3on in High sensi3vity to small surface defects and
Tes:ng (ECT) pipelines, thickness gauging. material proper3es.
Radiographic Inspec3on of welds, joints, and Clear internal images allowing for defect
Tes:ng (RT) internal structures. iden3fica3on and assessment.
Acous:c Emission Monitoring structural integrity in Iden3fica3on of crack propaga3on rates
(AE) real-3me during opera3on. and loca3ons.
Microwave Evalua3on of non-metallic pipes Detec3on of internal flaws without direct
Tes:ng for integrity assessment. contact, enhancing safety in inspec3ons.
Table 3: Recent Advances in NDT Techniques
Technique Recent Developments Impact on Applica:ons
Development of intelligent inspec3on
Automated Ultrasonic systems with automated data Increased efficiency and accuracy in
Tes:ng processing. defect detec3on.
Integra3on of thermal imaging with
Eddy Current Pulsed eddy current techniques for enhanced Improved detec3on of near-surface
Thermography (ECPT) sensi3vity. defects in complex geometries.
Machine Learning in Use of AI algorithms for defect Enhanced reliability and speed in data
NDT classifica3on and characteriza3on. interpreta3on, reducing human error.
These tables encapsulate the cri3cal aspects of various NDT methods used in oil pipeline
inspec3ons, highligh3ng their opera3onal principles, advantages, limita3ons, applica3ons,
results obtained, and recent advancements that enhance their effec3veness in detec3ng
pipeline flaws and ensuring safety in energy resource transporta3on.
Future Scope and Gaps in Non-Destruc4ve Tes4ng (NDT)
for Oil Pipeline Inspec4ons
The field of non-destruc3ve tes3ng (NDT) for oil pipeline inspec3ons is evolving rapidly,
driven by technological advancements and the increasing need for effec3ve maintenance of
pipeline infrastructure. As outlined in the review of various NDT methods, several promising
areas for future research and development exist, alongside notable gaps that require further
explora3on.
Future Scope
Integra(on of Advanced Technologies: The incorpora3on of robo3cs and ar3ficial
intelligence into NDT prac3ces holds significant poten3al. Automated inspec3on systems can
enhance efficiency and reduce human error. Future research should focus on developing
more sophis3cated algorithms that leverage machine learning to improve defect detec3on
and characteriza3on.
Enhanced Sensi(vity and Accuracy: Current NDT methods face challenges in detec3ng small
or complex defects, par3cularly in intricate geometries. Advancements in techniques such as
eddy current pulsed thermography (ECPT) and microwave tes3ng could lead to beaer
sensi3vity and accuracy. Research should explore op3mizing these methods to enhance their
effec3veness in iden3fying subtle defects.
Real-Time Monitoring Systems: The development of real-3me monitoring systems using
acous3c emission (AE) techniques can provide con3nuous assessment of pipeline integrity.
Future studies should inves3gate the feasibility of integra3ng AE with other NDT methods to
create a comprehensive monitoring solu3on that can detect issues as they arise.
Material-Specific Approaches: Different materials exhibit unique responses to NDT
techniques. Future work should focus on tailoring NDT methods to specific material types,
par3cularly non-metallic components, which are becoming increasingly common in pipeline
construc3on. This could involve refining exis3ng methods or developing new ones that are
beaer suited to these materials.
Standardiza(on and Protocol Development: As NDT technologies evolve, there is a need for
standardized protocols that ensure consistency and reliability across various inspec3on
methods. Future research should aim to establish universally accepted guidelines that can
be applied across the industry.
Gaps to be Studied
Characteriza8on of Complex Defects: One of the significant challenges iden3fied is the
reliable characteriza3on of defects in complex geometries. Research is needed to develop
advanced imaging techniques and analy3cal methods that can provide more accurate
assessments of defect sizes, shapes, and loca3ons.
Field Deployment Challenges: Many NDT techniques face prac3cal constraints when
deployed in the field, par3cularly under challenging environmental condi3ons. Further
studies should inves3gate how to improve the robustness and adaptability of these methods
for real-world applica3ons.
Data Interpreta8on and Analysis: The interpreta3on of data generated from NDT
inspec3ons ocen relies heavily on human exper3se, which can introduce variability and
subjec3vity. Research into automated data analysis tools that u3lize ar3ficial intelligence
could help mi3gate this issue by providing consistent interpreta3ons based on large
datasets.
Long-Term Performance Studies: There is a lack of long-term studies examining the
performance of various NDT techniques over 3me, par3cularly regarding their effec3veness
in detec3ng evolving defects. Future research should focus on longitudinal studies that
assess how these methods perform under varying opera3onal condi3ons.
Environmental Impact Studies: As the oil industry increasingly focuses on sustainability,
understanding the environmental impact of different NDT methods is crucial. Research
should explore eco-friendly alterna3ves to tradi3onal NDT techniques while maintaining
their effec3veness.
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