Pipe Coating Inspection Guide

Pipeline distribution networks for oil and gas extraction, transportation, and distribution are a critical part of the day-to-day operations in explosive environments and must be managed to exceptionally high levels of consistency and safety. In order to achieve exceptional levels of operational efficiency, the industry relies on expert inspections to ensure that the networks are able to

These pipelines are normally buried underground and are made up of carbon steel mainly X65 or X70. While carbon steel encompasses quite a few pluses, i.e., low cost, easy availability, and suitable mechanical strength. However, they have low corrosion resistance when buried under soil or exposed to an explosive environment.

In an explosive environment, hazardous substances are in high concentration in the form of mist, vapors, and fumes under an atmospheric condition in the air. These substances are highly flammable and can ignite instantly in the presence of a spark.

Therefore, the pipeline in such regions must be coated properly and in compliance with various international standards such as ISO, ASTM, NACE, DIN, and CAN.

In this article, we highlight the complexity of working in explosive environments, the importance of pipeline coatings, the pipeline safety inspection process, and a wide variety of coating inspection kits. Also, the job responsibility of a NACE inspector, the functional aspects of Elcometers, and preparation of a good surface profile for pipeline networks and oil field tanks.

2.0 Explosive Environments

An explosive environment can be defined as the mixture of flammable liquids, vapors, gases, combustible dust that are expected to be present in significant concentration and sufficient to trigger a fire explosion. 

When the concentration of an explosive substance in the air is contained in the lower and upper explosive limits, the mixture can be ignited. The temperature and pressure needed for ignition are 200 to 400 degrees Fahrenheit and 0.8 to 1.1 bar, respectively.

Explosions can be a cause of loss of life in severe cases, damage to property, and significant injuries and catastrophe in the surroundings.

Explosive Atmosphere Classification

Explosive environments can be categorized into three zones,

Zone 0

An area in an explosive environment where the mixture of air and flammable substance in the form of air, gases, and volatile vapors are available in a sufficient concentration.

Zone 1

An area in an explosive environment where the mixture of air and flammable substance in the form of air, gases, and volatile vapors are available in normal concentration.

Zone 2

An area in an explosive environment where the mixture of air and combustible substances in the form of air, gases, and volatile vapors are available for a brief period.

Explosive environment or hazardous area is present in a large number of industries and unforeseen locations, i.e.,

• Petroleum refinery
• Oil and gas processing plants
• Oil rigs
• Automobile refueling stations and pumps
• Chemical processing plants
• Aircraft refueling stations
• Surface coating industries
• Underground coal mines

Coating Inspection

A pipeline’s infrastructure in an explosive environment is the most valuable and significant structure and hence, its inspection and monitoring should be consummated at regular intervals to assure the extended working life and best possible underlying pipeline operation. To counteract and inspect the failure, operators and regulators inspect on a timely basis. The purpose of an inspection is to figure out the vulnerabilities in the target pipelines in the following:

• Corrosions
• Deformations
• Cracking

Additionally, the coating inspection involves a chloride test, surface profile measurement, and wet and dry film thickness test.

Chloride Testing

Chloride testing is one of the essential tests of coating inspection and it must be performed before sandblasting. The soluble salts stick to the metal surface even after a sandblasting process. The removal of salts is not an easy task and will cause paint failure due to osmotic action and moisture retention. Further, these contaminants also lead to corrosion of metal beneath the coating.

Job responsibilities of NACE pipeline inspector

In 1943 the National Association of Corrosion Engineers (NACE) was developed to build an organization for professionals and engineers that concentrated on the methods of corrosion control. The NACE Institute offers courses and certifications for those professionals who are willing to pursue their careers in the inspection of protective coatings and corrosion control structures.

Coating inspectors are employed in a wide range of industries and specialized in protective coatings inspection. These coating inspectors have the expertise to inspect coatings, paint coatings, and assessment of metal structures from corrosion. Also, NACE corrosion control program equips professionals with the ability to comprehend the concept of multiple corrosion control methods and technologies. Further, NACE certified professionals solely work on oil and gas pipelines. These professionals further acquire the certifications as Internal Corrosion Technologist.

Use of Elcometer

An Elcometer is a coating thickness gauge for rapid and precise measurement of the thickness of the coating on a metal substrate. This gauge is accessible in three forms: basic, standard, and top. Following are the features of an Elcometer:

• It can be calibrated to a wide range of smooth and rough surfaces.
• Backlight for clear visibility of graphic user interface.
• Interchangeable probe
• Simple statistics
• RS 232 interface

Elcometer formally follows the following international standards:

• Ferrous (F)
  • ASTM B 499
  • ASTM D 1186-B
  • ASTM G 12
• Non-Ferrous (NF)
  • ASTM D 1400
  • BS 3900 (C5)
  • BS 5411 (3)
• Ferrous and Non-Ferrous (FNF)
  • AS 2331.1.4
  • AS 3894.3-B
  • AS/NZS 1580.108.1
  • ASTM D 7091

Surface profile for explosive environment

The surface preparation or surface profile is the technique of modifying the surface of the substrate preceding the coating (coating, painting, and lining, etc). The surface preparation is the most significant stage for the treatment of steel-substrate before the application of a coating.

The performance of a coating is considerably inspired by its ability to adhere to the metal substrate. In the case of steel surfaces, residual mill scale is unacceptable, so usually, the option will be to apply an abrasive blast cleaning process that would improve the surface profile. Further, some additional contaminants on rolled steel surfaces such as stains of oil and grease must be removed before the blast cleaning process. Moreover, conventional impurities on the surface of the steel surface, i.e., stains of oil and grease, corrosion products, dirt, moisture content should be removed during surface preparation.

After the removal of all sorts of contaminants, the surface must acquire the level of spotlessness and smoothness appropriate for the intended coating and enable good adhesion on the metal substrate.

The cleaning method as per the requirement of improving the surface profile:

• Blast cleaning
• Acid pickling
• Flame cleaning
• Manual cleaning

Ideal Tools to be Used in Explosive Environments

For potentially explosive and hazardous environments, TFT Pneumatic provides a wide variety of tools for surface preparation, grinding, and cutting tools certified by DNV. 

These tools are termed as Cold work tools and are safe to be used in refineries, process plants, chemical plants, offshore platforms, onshore drilling rigs and mines where the volatile organic contents are present in high concentration. 

These tools are ideal for:

• Surface preparation in pipe repair
• Beveling
• Coating maintenance
• Steel Grinding
• Composite wrap pipeline repair
• Weld seam removal
• Pipe cutting
• Drilling deck projects
• Grating removal
• Cutting stuck bolt heads

Inspection tools

Most of the current inspection process encompasses the use of manual labor and human involvement. The major drawback is that it requires plenty of time to accomplish the entire valuation process and even human error is also expected during the evaluation process. This scenario motivates engineers and professionals to devise innovative and state of the art methods for the inspection of pipeline networks. As the length of the pipelines extends up to hundreds of miles and the majority of the pipelines are buried underground. Therefore, inspection methods for pipelines should be automatic and free from human involvement and errors.

• Close circuit television method (CCTV)
• Laser scanning
• Ultrasonic intelligent pigging
• Magnetic flux leakage technique (MFLT)
• Radiographic testing (RT)
• Acoustic detection

Closed-circuit television method (CCTV)

It is the most common method of detecting defects and examining the internal condition of the pipeline. Also, it is a cost-effective technique and clearly identifies the physically damaged pipeline’s internal walls, pits, cracks, or deformations due to corrosion.

In the CCTV inspection method, a remote-controlled car is introduced inside the pipeline and it serves as a visual inspection tool to observe inside a pipeline without unearthing it. The front camera on the CCTV car can be rotated at an angle of 3600 inside the pipeline and detects even minor defects with accuracy and precision. After the identification of defects, these flaws can be treated in the following ways:

• Cleaning
• Repairing
• Replacing      

The visual data in the form of images and videos are transmitted to the operator via electrical cable in real-time and recorded for further analysis. However, CCTV data includes error percentages up to 25-30%. Further, the process of CCTV inspection involves a human operator that can move the camera forward or backward instantaneously to determine the region of interest (ROI). ROI can be defined as, “areas where the probability of potential defects is high”. After determining the potential defects, the operators must decide whether further examination is required for the region of interest or else. In the image below, a summary of defects and all the attributes of CCTV inspection is explained in a precise manner.

Laser scanning

The working principle of laser scanning inspection method is almost like CCTV inspection process. But laser scanning provides even better-detailed evaluation, which is advantageous and enhances the efficacy of successfully identifying the defects in the pipeline networks. With the help of laser scanning, segmentation, and classification of some of the defects can be identified such as cracks, joints, and holes. Laser scanning is not widely implemented because of its high operating cost, time it takes to evaluate the pipeline network, and various other implementation limitations.

In laser scanning, the CCTV can be mounted on a laser base profiler and an optical diffuser is backed by a defect classification algorithm. This algorithm is based on the image intensity values quantification of the anticipated rings perceived by the camera where the artificial intelligence and artificial neural network techniques are adopted. This strategy creates a database for several defect types for future automatic identification.  Furthermore, the suggested method has no substantial prerequisite to have the laser ring indicating directly in the pipe center and does not need an additional smooth motion.

Ultrasonic Intelligent Pigging 

Ultrasonic intelligent pigging permits a pipeline to be inspected for faults, cracks, corrosion defects, and deformations. It employs non-destructive testing NDT, i.e., ultrasonic testing and magnetic leakage testing enables the pipeline to be cleaned.

The inspection inside of the pipeline can be done by utilizing the application of magnetic fields with the help of permanent magnets. An anomaly in the magnetic field due to fluctuating wall thickness will be spotted by the sensors and therefore, can be employed to evaluate quantitative data and the presence of defects in the pipeline networks.

Moreover, the ultrasonic detection method is capable of acquiring quantitative data relating to the depth of defects with accuracy up to milimeters. This process relies on the transmission delay time of an acoustic wave radiated from ultrasonic transducers and travels in a liquid medium and reflected with the internal walls of the pipeline system to detect the corrosion defects. The identification of the defects depends on the following factors:

• Properties of the ultrasonic transducers employed for emitting radiations.
• Accuracy of time measurement of the propagation of acoustic waves.
• Harmonization of the transducer’s trigger with the measurement of the pig’s movement.

The Pig generally has a mechanical part, containing a cylindrical capsule encapsulated by a rubber disc. The capsule revolves easily around its longitudinal axis to direct the ultrasonic transducers to the lower half of the pipe.

Identification of defects inside the pipeline network involves the following three phases. In the first phase, the robot moves at a normal speed for the detection of defects and anomalies inside of the pipeline. In case of finding potential defects, the robot restricts its movement and records better quality visual images and data to verify if it is either a genuine defect or just a false alarm.

The second phase involves the complete stopping of the robot after the detection of the defect. Further, a complete examination will be performed of the identified defects and distinguish each defect among other defects with greater accuracy and precision. 

In the third phase, the classification of defects can be finalized either during the inspection or after the completion of the inspection process.

Magnetic flux leakage technique

Magnetic flux leakage technique is one of the most common techniques for the identification of defects on internal and external surfaces of pipelines. This technique employs a non-destructive testing approach, i.e., the use of magnetic sensitive sensors to detect the magnetic leakage fields of defects both internal and external to the surface of the pipeline. It generates higher accuracy and precision in detecting the defects inside and outside the pipeline.

This in-line inspection technique magnetizes the pipe wall as it moves parallel to the structure and utilizes coil sensors to determine magnetic flux leakage intensity along the pipe wall. MFL further assists by the presence of the sensors along the inside of the pipeline wall, which exposes the anomalies or variations in the magnetic fields. These irregularities are those where the potential defects are present and can be assessed to determine the defects in the pipeline. The seriousness of the defects is revealed by estimated defect depth and resultant safety of the pipe by calculating the maximum allowable operating pressure of oil and gas flowing through the pipeline. Mostly the defects determined by this technique are corrosion pits & scars, deformations, fatigue, hairline fractures, cracks, and improper welds.    

Radiographic testing

Radiographic testing is performed by recording degrees of absorption of penetrating radiation throughout the pipe wall. This results in generating a latent image of the object being under consideration or examined on a film that is later on chemically treated to transform the latent image into a permanent shadow image of internal and external defects in the pipeline. High intensity of radiation passes through the defected area when compared to regions without defects. Finally, these radiographer images can be evaluated by either professionals or automated computer vision systems.

Acoustic detection

 Acoustic detection uses an array of microphones and speakers generally placed in a single position to identify the defects in the whole length of the pipeline system. Further, the interpretation of the data can be accomplished using the software.

Acoustic detection has the following advantages:

• Detect all types of defects.
• Employ rapid measurement guided waves.
• Guided waves differentiate between materials.
• Simultaneous use of methods allows for more precise prediction of defect severity.

Inspection of coating and its tools

The finished coating on the surface of the pipeline can be inspected by the following methods and any deficiency will be addressed accordingly.

Visual inspection: In the first step visual inspection of the coating is carried out for identifying the following defects, i.e., blisters, swelling, air traps, and lumps, etc. These defects must be inspected otherwise if neglected then these are harmful to the performance of the coating.

Coating thickness: For determining the thickness of the coating, non-destructive testing will be performed.

Holiday inspection: Every pipe is tested at 5V per micron of coating thickness, or 25 kV as specified for identifying any incoherence or anomaly in coatings by suitable DC-based holiday detectors.

Peel or bond strength test: Peel testing is being performed on random samples of pipes for verifying the bond strength. The coating is peeled at a defined rate of 50-65 mm and the strength of the coating is recorded.

Impact test: The coated pipe is tested for impact energy at 7 Joules/mm. The tested area is reverified with Holiday testers for any cracks.

Hardness test: The Shore-D hardness is tested for coated pipes for certifying values should be greater than 55 Shore D. 

Coating inspection kits

Inspection of pipeline coating can be best executed by coating inspection coating kits offered by, “Elcometer”. Elcometer has attained a privileged in providing a wide variety of inspection coating instruments. It includes kits for onsite coating inspection, evaluation of surface profile, determining dew point, relative humidity, adhesion, and measurement of both dry and wet film thickness. Following are the most common coating inspection tools available in the market:

• Hardness testing kit
• Environmental parameter evaluator
• Surface profile preparation equipment
• Gauges test strips
• Wet film measurement gauge
• Dry film thickness measurement gauge
• Ultrasonic thickness gauge
• Adhesion tester
• Holiday detection
• Microscope mirrors and lightning
• Equipment cases and bags.

Oilfield tank coating inspection

The vertical cylindrical steel tanks will be used for the storage of oil and oil products. The st0rage process is predominantly a long-term process and several factors must be in account for its maintenance, safety, and security. In case of any catastrophic event at an oil storage tank, it will put a considerable impact on human life, the surrounding environment, and material loss.

Therefore, to mitigate the risk of complete or partial destruction of the oilfield tank, a regular inspection should be scheduled after regular intervals of time. The goal of the technical inspection is to acquire the data for their present condition. This information is beneficial in the following way:

• Likelihood of further reliable use of the storage facility. 
• Indication of the areas of elements that need to be repaired or replaced.
• Mode and the terms for the next inspection and estimate of the remaining resource of the facility.

The inspection of a tank is carried out by the following process:

• Non-destructive testing (NDT) inspection
• Low-frequency electromagnetic testing (LFET)
• Balanced field electromagnetic testing
• Longitudinal ultrasonic testing (UT)
• Shear wave ultrasonic testing (SWUT)

Non-destructive testing (NDT) inspection

This inspection focuses on 100% testing of the floor, lower dome, upper dome, barrel, and extension. The inspection can be performed by utilizing TS-200 NDT multi-channel system based on the principle of low-frequency electromagnetic technique. The purpose of an inspection is to figure out the surface, sub-surface fissure and pinholes in the entire structure.

Low-frequency electromagnetic testing (LFET)

In this process, the inspection process is accomplished by placing two ends of the electromagnetic driver on the metal surface, and a sensor is placed in between the two ends of the driver. The driver is a source of emitting low frequency (3-40Hz) alternating current signal and the sensors detect the magnetic field develops between the two poles of the driver. The principle of this testing is, “flaws in the steel structure alter the magnetic field”, this alteration is recorded by the sensors in the form of amplitudes and variation in phase. If more flaws are found, the deviation in the magnetic signal will become wider and the sensors will record more shifts in the magnetic signal. Finally, the recorded signals transformed into a percentage of material loss by using numerical calculations.  

Balanced field electromagnetic testing

This process the inspection of cylindrical storage tanks can be consummated by placing an electromagnetic probe near a metallic body. The divergence in the magnetic field is recorded; the vertical and horizontal sections of the signal are phase-shifted to reduce the noise in determining the magnetic field.

Longitudinal ultrasonic testing  

In ultrasonic testing, a transducer emits high-frequency ultrasonic waves and propagates them across the material under investigation. Further, the transducers record the time lapse between the waves released and when the waves’ echoes are received back to the transducers. In case of any flaws the time duration between the released and echoed waves condensed (comparing the absence of any flaws in the material). In ultrasonic testing, the particles in the material under consideration oscillate in response to the energy present in the ultrasonic waves propagated through it. The direction of the oscillation of particles is in the longitudinal direction.

Shear wave ultrasonic testing 

The working principle of shear wave ultrasonic testing is the same to that of longitudinal ultrasonic testing. The only difference is the movement of particles is perpendicular to the direction of sound/ultrasonic waves. SWUT is also called angle beam testing, as it is employed to find out the flaws’ dimensions and their depth in the material under consideration.

Conclusion

Pipeline network systems are responsible for the transports of oil, natural gas, and various other fluids from one end to another safely and reliably. Therefore, a sophisticated process for the repair, maintenance, and inspection of entire pipeline networks must be followed to avoid any interruptions. This study investigated in detail various explosive environments, their zone, coating inspection procedure for pipeline network systems in explosive environments, various coating inspection kits, and methodology of preparing a good surface profile of steel surfaces to combat the fumes of hazardous substances present in the explosive atmosphere. Also, a detailed study of oilfield tanks and various techniques for their inspection also encompassed in this study.