Inspection Methods for Oil and Gas Storage Tanks

Summary

Regular inspections are critical in ensuring the integrity and safety of oil and gas storage tanks. Neglecting inspections can lead to significant risks and consequences. Visual inspections are essential for assessing the overall condition of the tanks, covering both external and internal components and accessories. Following recommended procedures and best practices is crucial to ensure accurate and thorough assessments. In this regard, non-destructive testing techniques are often employed to conduct detailed inspections. Common examples include ultrasonic testing, magnetic particle testing, radiographic testing, and eddy current testing. On the other hand, corrosion monitoring methods help understand and manage the risks associated with corrosion. The standard corrosion monitoring methods include corrosion coupons, electrical resistance probes, and ultrasonic thickness measurements. Special attention is often given to inspecting tank bottoms through manual floor scanning, automated floor scanning, and settlement monitoring to detect any issues promptly. Environmental testing is another inspection method that is crucial for identifying potential risks related to environmental factors. These include soil testing, water testing, and cathodic protection assessment. Likewise, the transparency and accountability of tank inspections often require accurate documentation, comprehensive inspection reports, and compliance with regulatory requirements. This enables identifying maintenance needs, prioritizing tasks, and implementing proper repair techniques to maintain tank integrity. In recent years, emerging technologies (drones, artificial intelligence, internet of Things) have contributed significantly to inspection procedures’ speed, accuracy, and reliability. The long-term integrity and safety of oil and gas storage tanks can be upheld by conducting regular inspections, following best practices, and ensuring proper maintenance. This article comprehensively reviews the different inspection methods and techniques to identify potential issues and problems with oil and gas storage tanks.

Introduction

Oil and gas storage tanks are a significant part of the industry, requiring proper inspection after a specific time interval. Such scheduled maintenance ensures safety, structural integrity, reliability, and environmental compliance. These tanks contain volatile and potentially hazardous products. Their regular inspection enables the detection of potential issues such as corrosion, pitting, erosion, leaks, structural defects, and other abnormalities. It helps beforehand prevention of severe consequences such as environmental contamination, equipment failure, financial losses, or health and safety hazards [1].

Therefore, understanding and employing appropriate inspection methods for oil and gas storage tanks are critical for protecting equipment, personnel, the environment, and surrounding communities. In addition, proper inspection also reduces the downtime and maintenance costs of storage tanks.

The American Petroleum Institute (API) provides guidelines such as API 650 and API 653 for designing, constructing, inspecting, maintaining, altering, and repairing storage tanks. API 650, Welded Tanks for Oil Storage, is for designing a new storage tank. On the other hand, API 653 covers the inspection, maintenance, and repair of in-service API 650 storage tanks [2].

Inspectors evaluate the performance of storage tanks by following the API 653 guidelines to check whether the tank is suitable for continued service. This evaluation includes explicitly the tank’s general information, settlement evaluation, and measurement of corrosion and thickness level of the shells, bottoms, welds, and other parts of the tanks [3]. The different inspection methods are discussed in the subsequent sections.

Visual Inspection

Visual inspection is the fundamental method for inspecting oil and gas storage tanks which is simple, cost-effective, and the easiest technique for regular inspection. This method involves the direct surface examination of different parts of the storage tanks using the visual aid of an experienced inspector. Historically, this visual aid included the human eye, requiring proper light to perform the direct visual inspection. Nevertheless, now cameras are employed for remote visual inspection using specific lights for improved images or video of the problem areas [4].

This technique provides information about the condition of the storage tanks, such as corrosion, pitting, coating degradation, or any other visible abnormalities. Visual inspection of storage tanks includes the inspection of the following components:

External inspection

External inspection involves visually examining the external surface to detect coating damages, reduce wall thickness, pitting, corrosion, and holes on the exterior shell, bottoms, weld, joints, or tank roof.

Internal inspection

Internal visual inspections require the cleaning of the tank before the inspection. An experienced inspector accesses the tank’s interior through maintenance holes or other openings. He examines the interior surface of the tank’s shell, bottom, joints, welds, or roof to identify the potential areas of corrosion or any other damages [5].

Inspection of accessories and fittings

Visual inspection also includes the inspection of the tank’s accessories, such as valves, gauges, nozzle, flanges, vents, and other tank parts that are installed to monitor the tank performance. The trained inspector carefully examines the leaks, wall thinning, or overall condition of these accessories to ensure the optimized performance of the tank [5].

To ensure effective visual inspections, the following procedures and best practices should be considered:

• Develop a detailed inspection plan that outlines the scope, frequency, and methods to be employed.
• Train inspectors on visual inspection techniques, including the identification of corrosion, damage, and abnormalities.
• Follow safety protocols, including proper personal protective equipment (PPE) and adherence to confined space entry procedures for internal inspections.
• Document inspection findings thoroughly, including photographs, descriptions of observed issues, and their locations.
• Regularly review and update inspection procedures to incorporate lessons learned and industry best practices.
• Ensure compliance with applicable regulations, standards, and codes governing tank inspections.

By adhering to these recommended procedures and best practices, visual inspections can effectively identify potential issues and contribute to maintaining the integrity and safety of oil and gas storage tanks.

Non-Destructive Testing (Ndt) Techniques

Non-destructive testing (NDT) techniques are used to examine the material characteristics, surface defects, and other discontinuities in oil and gas storage tanks. These methods allow for thoroughly inspecting tank integrity without causing any damage [6].

Several commonly employed NDT methods for inspecting oil and gas storage tanks include:

Ultrasonic testing (UT)

Ultrasonic testing is an NDT method to inspect oil and gas storage tanks [7]. An inspector introduces high-frequency sound waves generated by ultrasonic transducers into the tank material during this technique. The reflected signals are then analyzed to identify structural imperfections such as cracks, holes, corrosion, wall thickness changes, or other tank anomalies. UT is particularly effective in detecting internal defects and determining the thickness of the tank’s walls (Figure 1).

Figure 1: Ultrasonic Testing

Magnetic particle testing (MPT)

Magnetic particle testing is a non-destructive testing method used to inspect the surface and subsurface imperfections in oil and gas storage tanks made of ferromagnetic materials. This technique involves inducing a magnetic field in the tank being inspected using a magnetic current.

Iron particles are then applied or spread over the surface of the tank. Any cracks or flaws in the material interrupt the flow of the magnetic current that, leads to a change in the magnetic field. This change causes the accumulation of iron particles at the locations of the defects, making them visible to the inspector, as shown in Figure 2. MPT effectively detects surface cracks and discontinuities in ferromagnetic tanks, aiding in assessing their structural integrity [7].

Figure 2:  Magnetic Particle Testing

Radiographic testing (RT)

Radiographic testing is a non-destructive testing technique that utilizes gamma rays or X-rays to penetrate through the material of oil and gas storage tanks during inspection (Figure 3). These rays interact with the tank material and produce an image on a radiographic detector. This image is then analyzed to identify flaws or defects within the material, such as internal damages not visible to the naked eye.

RT can reveal hidden cracks, voids, corrosion, or other structural imperfections within the tank. It provides valuable insights into its integrity and ensures the safety of oil and gas storage operations [8].

Figure 3: Radiographic Testing [9],[10]

Eddy current testing (ECT)

Eddy current testing (ECT) is a non-destructive testing technique used to identify flaws or imperfections on oil and gas storage tank surfaces (Figure 4). It involves using a specially designed probe that induces eddy currents in the material being inspected. The probe utilizes alternating current to generate a changing magnetic field, which interacts with the tank material.

Figure 4: Eddy Current Testing [11]

During the inspection, the probe is moved along the tank’s surface, and the response of the eddy currents is monitored. Any variations in the electrical conductivity or magnetic permeability of the material caused by flaws, such as corrosion, wall thickness variations, or welding cracks, will alter the eddy current behavior.

The changes in the eddy currents are detected and analyzed. It enables the collection of valuable information about the presence and characteristics of surface flaws. Eddy current testing is particularly effective for detecting localized corrosion, thinning of the material, and other surface defects [8].

Liquid penetrating testing (PT)

Figure 5: Liquid Penetrant Testing [12]

Liquid penetrant testing (LPT) is a cost-effective non-destructive testing technique used to identify surface discontinuities in oil and gas storage tanks. In this method, the surface of the tank material is coated with a low-viscosity liquid that contains regular or fluorescent dye [7].

The liquid penetrates any surface cracks, pores, or other openings in the material. The excess liquid is removed after a specified penetration time. This way, only the dye trapped within the discontinuities is left behind. A developer, such as a white or ultraviolet light-sensitive powder, is then applied to the surface. The developer draws the trapped dye to the surface, making the discontinuities visible.

Inspectors perform a visual inspection under appropriate lighting conditions. They use either white or ultraviolet light, depending on the type of dye used (Figure 5). This inspection allows for detecting and identifying surface discontinuities such as cracks, porosity, or other flaws that may compromise the tank’s integrity.

Magnetic Flux Leakage (MFL)

Magnetic flux leakage (MFL) is a technique utilized to detect corrosion, pitting, or erosion within oil and gas storage tanks constructed with stainless steel [13]. This method involves the application of a magnetic field to the tank material using a powerful magnet. This magnetic field saturates the tank structure.

Figure 6: Magnetic Flux Leakage Testing [14]

If there are any imperfections or flaws (e.g., corrosion or pitting) in the material, it disrupts the uniformity of the magnetic field. This causes a change in the magnetic field density, detected by sensors positioned near the tank’s surface.

The sensors pick up the magnetic flux leakage signals resulting from the variations in the magnetic field caused by the presence of defects. By analyzing these signals, inspectors can identify and assess the severity of the tank’s corrosion, pitting, or erosion (Figure 6).

Leak Test (LT)

Leak testing technology is employed by inspectors to detect and identify leaks in oil and gas storage tanks [8]. Various methods [15] are utilized for this purpose, including:

Pressure change testing: This method involves pressurizing the tank and monitoring for any pressure drop over a specific period. A significant pressure drop indicates the presence of a leak.

Bubble leak testing: In this technique, the surface of the tank or suspected leak areas are coated with a solution or gas that produces bubbles when it comes into contact with escaping gas or liquid. The formation of bubbles indicates the presence of a leak or similar issue.

Mass spectrometer testing: Mass spectrometers are used to detect and analyze gas compositions. This method involves introducing a tracer gas into the tank and using a mass spectrometer to detect any trace amounts of the gas leaking from the tank.

Halogen diode testing: This method uses a detector to identify leaks. A halogen-based gas is introduced into the tank, and the diode detector senses any leakage of the halogen gas, indicating the presence of a leak.

Corrosion Monitoring

Corrosion poses a significant challenge for storage tanks because of several factors, such as corrosive chemicals, moisture, oxygen, temperature fluctuations, and bacterial activity. It leads to the degradation of tank metal surfaces through electrochemical reactions involving anodes and cathodes in aqueous environments.

Various types of corrosion can compromise the structural integrity and lifespan of storage tanks. Common ones include uniform corrosion, pitting corrosion, galvanic corrosion, erosion-corrosion, and microbiologically influenced corrosion [16]. To address these concerns, it is crucial to understand corrosion phenomena and implement effective monitoring strategies comprehensively.

Common methods for corrosion monitoring include:

Corrosion coupons

One standard method for corrosion monitoring is the use of corrosion coupons. These coupons are typically made of carbon steel and are exposed to the tank environment for a specific duration. The corrosion rate can be determined by measuring the weight loss of the coupons before and after exposure. It enables the collection of valuable data on the corrosive behavior of the tank environment over time [17].

Electrical resistance probes

Electrical resistance probes are an effective tool for monitoring the corrosion rate in oil and gas storage tanks. These probes consist of a steel component exposed to the tank environment. As corrosion occurs on the surface of the steel, the resulting metal loss is detected by measuring the changes in electrical resistance. This method enables the rapid detection of corrosion because any fluctuations in the electrical resistance provide valuable insights into the corrosion rate [18]

Ultrasonic thickness measurement

The ultrasonic thickness measurement technique is widely used to identify corrosion, erosion, and other defects in tank walls. Ultrasonic gauges measure the thickness of the tank walls over time. It enables the identification of areas susceptible to corrosion. Regular wall thickness monitoring helps detect potential weaknesses or thinning areas, facilitating timely maintenance and repair interventions [19].

It can be concluded that understanding corrosion mechanisms, implementing preventive measures, and conducting regular inspections are crucial for ensuring tank integrity, safety, and longevity. Companies can minimize environmental impacts, protect personnel, and maintain reliable storage operations by effectively managing corrosion risks.

Tank Bottom Inspection

Tank bottom evaluations are essential to prevent oil or gas leakage, which can lead to environmental hazards and significant economic losses. Tank bottoms are particularly susceptible to corrosion, pitting, uneven settlement, and cracking due to various factors. Common ones include the presence of water or hazardous chemicals, the pressure exerted by stored oil or gas, and the properties of the surrounding soil [20, 21].

Different techniques, such as manual and automated floor scanning, inspect the tank bottom. They enable the detection of corrosion, pitting, settlement, cracking, or any other forms of deterioration to reduce the effect of these potential issues. Settlement monitoring is also performed to identify tank position or alignment changes. These techniques are elaborated as follows.

Manual Floor Scanning

The manual floor scanning technique is a traditional method for inspecting tank bottoms and involves personnel utilizing visual or non-destructive testing (NDT) methods [22]. Its purpose is to detect corrosion, cracking, or damage in the tank’s bottom plates, annular plates, and weld joints.

Before conducting manual floor scanning, it is imperative to empty and thoroughly clean the tank to ensure safe access. Inspectors carefully examine the tank’s bottom and search for indications of corrosion or signs of cracking deformations that could compromise the tank’s structural integrity.

Automated Floor Scanning

Automated floor scanning employs robotic systems with sensors and cameras to inspect tank bottoms. These devices navigate the tank bottom surface, collecting data for subsequent analysis. This technique reduces the need for human entry, mitigates safety risks, and improves overall efficiency [22].

Tank Bottom Settlement

Tank bottom settlement results from uneven support, inadequate base design, and variations in soil properties beneath the tank [23]. Various monitoring techniques are employed to evaluate tank bottom settlement, including laser scanning and total station surveys. These methods assist in identifying and monitoring any settlement or unevenness in the tank floor.

The results obtained from tank bottom inspection are analyzed and compared against established industry standards. If any signs of damage are detected, appropriate measures must be taken. Tank bottom inspections are essential for upholding the structural integrity of oil and gas storage tanks and mitigating the risk of damage.

Environmental Testing

Environmental aspects include the characteristics of the surrounding soil and the quality of water. They significantly impact the maintenance and integrity of oil and gas storage tanks. It is crucial to monitor these factors to identify any changes affecting the tanks’ condition and prevent environmental contamination. Environmental testing, which includes soil testing, water testing, and the assessment of the cathodic protection system, is essential in this regard.

Soil testing

Soil testing involves the collection of soil samples from the area surrounding the tank to examine their properties, such as pH levels, corrosiveness, and any leakage [24]. Inspectors can assess the risk of corrosion or structural damage caused by soil conditions by analyzing the soil samples.

Water testing

Water samples collected from the tank are examined to evaluate their quality and to detect the presence of any corrosive chemicals or contaminants. Water testing helps identify potential risks to the tank’s integrity and can inform appropriate maintenance measures [25].

Cathodic protection system

The cathodic protection system is vital to mitigating corrosion in oil and gas storage tanks [26]. It works by controlling the electrical potential to prevent the corrosion of the tank structure. Regular inspection and assessment of the cathodic protection system ensure its proper functioning and effectiveness in preventing corrosion.

The results obtained from environmental testing are compared with standard guidelines and regulatory requirements. If any risks or deviations are identified, appropriate measures are taken to address them promptly. This may include adjusting the cathodic protection system, implementing corrosion control measures, or improving the tank’s surrounding environment.

By conducting thorough environmental testing and monitoring, potential risks to tank integrity can be identified early, allowing for proactive maintenance and minimizing the risk of environmental contamination. Compliance with environmental regulations and adherence to best practices in environmental testing contribute to the overall safety, reliability, and environmental stewardship of oil and gas storage tanks.

Documentation And Reporting

Accurate and detailed documentation of inspection activities is crucial for ensuring compliance, traceability, and effective maintenance of oil and gas storage tanks. The documented inspection records facilitate decision-making for maintenance/ repair actions.

A number of reasons underline the importance of appropriate documentation for the inspection activities of oil and gas storage tanks. For example, it helps to ensure compliance with regulatory requirements and industry standards. It also provides traceability of inspection findings, which can help identify trends and make informed decisions about maintenance and repair.

In addition, it facilitates decision-making for maintenance/ repair actions. In this way, inspectors can make informed recommendations about the need for repairs or maintenance. Generally, the documentation should include the following information:

• The date and time of the inspection
• The name of the inspector
• The type of tank being inspected
• The location of the tank
• The results of the inspection, including any defects or damage
• Any recommendations for maintenance or repair
• How often should the tanks be inspected?

The inspection frequency depends on the tank type, environment, and severity of potential hazards. As a general practice, a majority of the tanks should be inspected at least annually [27, 28].

Maintenance And Repair

The results of tank inspections are invaluable in identifying maintenance and repair requirements. These inspections provide crucial information about the presence of corrosion, structural imperfections, or any other issues that may affect the tanks’ integrity. Proper analysis and understanding of these inspection results are essential for determining the urgency of maintenance and repair activities.

By thoroughly analyzing the inspection data, operators can assess the severity and extent of the identified issues. This allows them to prioritize maintenance actions based on the urgency of addressing critical issues. Critical issues, like severe corrosion or structural defects, should be promptly addressed to prevent further degradation of the tank material or potential failure. Urgent repairs are essential to maintain the safety and reliability of the tank.

Non-critical issues, on the other hand, can be scheduled for repair based on the severity of the issue and its potential impact on tank performance. Prioritizing repairs ensures that available resources are allocated efficiently and that maintenance activities are carried out logically [1].

The repair process for tanks involves various techniques depending on the nature of the issue. These include welding, tank bottom or floor replacement, or coating application. The selection of the appropriate repair technique depends on factors such as the type and extent of the issue, the tank’s design and material, and industry standards or regulations.

After the repair work is completed, it is essential to conduct a follow-up inspection to confirm the success of the repair and ensure that the tank meets the required specifications. This post-repair inspection verifies that the tank’s integrity has been restored and is again fit for its intended purpose [1].

Emerging Technologies And Trends

Emerging Technologies

Researchers continuously work to develop innovative technologies that enhance safety precision and efficiency in oil and gas tank inspection. Their persistent efforts aim to overcome existing limitations, including human error, data collection, and the need for improved safety measures [29]. In recent years, technological advancement simplified the inspection process. Such technologies include robotic inspection systems, drones, and advanced imaging technologies [30].

Robotic inspection system

Robotic inspection systems have emerged as an innovative solution for remotely and automatically inspecting oil and gas storage tanks. These systems utilize advanced technologies such as cameras, detection systems, and artificial intelligence to conduct inspections with minimal human intervention [31].

By employing robotic systems, the need for human personnel to physically enter the tanks is significantly reduced. This not only improves safety but also reduces potential risks. The robots have high-resolution cameras and sensors that capture detailed images to collect data on the tank’s interior and exterior surfaces.

Artificial intelligence algorithms analyze the collected data and identify potential issues or anomalies. These algorithms can detect corrosion, cracks, leaks, or other structural defects by comparing the captured images with predefined patterns or known damage indicators.

Drones

Drones, also known as unmanned aerial vehicles (UAVs), provide a cost-effective and efficient solution for covering large areas and capturing high-resolution images of oil & gas storage tanks [32]. Their small size and flying ability enables access to confined surfaces and inspect considerable heights without scaffolding or an extensive workforce. This capability enables them to reach areas that may be challenging or hazardous for human inspectors.

The high-resolution cameras equipped on drones allow for capturing detailed images and videos of the inspected sections. These images provide valuable visual data that can be analyzed to detect defects, corrosion, leaks, or structural abnormalities. Inspectors can zoom in on specific areas of interest and make informed decisions based on the visual evidence provided by the drone’s imaging capabilities.

Moreover, the use of drones in tank inspections offers efficiency and time savings. Drones can quickly and efficiently cover large areas, reducing inspection time and associated costs. They can fly over the tank and capture images or videos in a fraction of the time it would take for traditional inspection methods.

Advanced imaging techniques

Advanced technologies, such as 3D laser scanning and thermographic imaging, have emerged as valuable tools in oil and gas tank inspection over the past few decades. Laser technology offers a cost-effective and time-efficient approach, enabling the collection of high-quality data compared to previous inspection technologies. 3D images of the tank bottoms, floors, and inside walls can be generated using laser inspection. These images can then be compared to identify any defects or imperfections in the tank structure [33].

On the other hand, thermal imaging technology contributes to a safer workplace, environmental friendliness, and cost-effectiveness. It enables the visualization of invisible gases in the images, enhancing the ability to detect potential leaks or anomalies. Thermal imaging produces a 2D temperature profile that can be compared with standard images to identify any tank deviations or imperfections [34, 35].

Adoption of digitalization and automation

The integration of digital inspection platforms like Internet of Things (IoT) connectivity and artificial intelligence (AI) has transformed the field of tank inspection. It has incorporated significant improvements in data quality, analysis efficiency, and overall inspection outcomes. These technologies offer numerous benefits, enhancing control, increasing productivity, and more informed decision-making.

Digital inspection platforms have revolutionized tank inspections by employing integrated software solutions that streamline data collection, analysis, and reporting processes. These platforms have significantly reduced inspection time while improving the accuracy and reliability of collected data.

Automating workflows and standardization of methodologies enable digital inspection platforms to ensure consistent data collection, analysis, and reporting.  These practices enable more informed decision-making and the development of effective maintenance strategies. Inspectors now have better control over complex systems, leading to increased productivity and streamlined operations [36].

IoT connectivity has revolutionized tank inspection practices by enabling real-time, accurate, and continuous monitoring of crucial parameters. Common ones include temperature, pressure, and corrosion rates within oil and gas tanks. IoT technologies utilize sensors and connected devices to access data from multiple locations remotely. This scheme enhances safety, accuracy, and ease of tank operations.

The availability of up-to-date information empowers operators to make informed decisions and promptly identify anomalies or deviations. With IoT connectivity, tank systems can be effectively monitored, ensuring optimal performance, minimizing downtime, and maximizing overall integrity and efficiency [37].

Integrating AI and machine learning models in tank inspection processes has also been transformative. AI and ML algorithms can analyze large volumes of inspection data, detecting patterns, trends, and potential maintenance requirements. Predictive maintenance strategies can be developed by leveraging AI and ML models. It allows for proactive interventions and optimized inspection schedules.

These technologies enable operators to make informed decisions regarding maintenance activities, allocate resources effectively, and ensure storage tanks’ long-term reliability and safety. Proactive maintenance also helps minimize the risk of unexpected failures and extend the lifespan of tank assets [38].

By employing these innovative technologies, researchers are striving to revolutionize the inspection process within the oil and gas industry. The objective is to mitigate risks, enhance collected data quality, and optimize inspection outcomes.

Conclusion

In conclusion, implementing proper inspection methods is crucial for maintaining oil and gas storage tanks’ integrity, safety, and reliability. Regular inspections allow potential issues such as corrosion, defects, settlement, or other abnormalities to be identified and addressed promptly, reducing the risk of catastrophic failures and environmental hazards. Adhering to best practices, industry standards, and regulatory requirements is essential in consistently obtaining reliable inspection results. A comprehensive approach includes visual inspections, non-destructive testing techniques, corrosion monitoring, tank bottom inspections, environmental testing, documentation, and proactive maintenance. Such an approach ensures that necessary steps are carried out to safeguard the integrity of tank assets. Embracing emerging technologies and trends further enhances inspection capabilities and contributes to the overall sustainability and success of the oil and gas industry. By prioritizing inspection methods, utilizing advanced technologies, and fostering a culture of proactive maintenance, the industry can ensure the long-term reliability and safe operation of oil and gas storage tanks, ultimately benefiting the industry and the environment.

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