This article discusses some of the existing and developing technologies that will help owner/operators manage and maintain the integrity of their ASTs. What all of these innovations have in common is that they can be done without removing the tanks from service.
This not only allows for safer determination of tank integrity, keeping people out of hazardous situations, but also means that these inspections are carried out more quickly and in a more environmentally friendly manner. In most cases, these inspections also result in lower overall costs when considering cleaning, emissions, and loss of tank usage, among other factors.
Since ASTs have been used to store products for over 100 years, owner/operators have spent time, money and effort trying to balance operations, contractual commitments, regulatory requirements and integrity issues.
When I started in the industry over 35 years ago, it was common practice to operate ASTs until they failed; that is, to wait for the tank to leak before taking it out of service. Today, that is no longer a viable option for a responsible owner/operator. In fact, in many cases, owners must not only prove that their tank is not leaking, but also that it meets the minimum allowable bottom thickness as described in API 653.
Until relatively recently, the only way to determine the integrity of the bottom of a tank, the cause of leaks in 98-99% of cases, was to remove it from service. The introduction of robots more than 25 years ago allowed some fuel (diesel and jet fuel) and water tanks to be inspected while in service. More recent technological advances have allowed flammable liquid tanks, such as gasoline, to be inspected by more advanced robots, such as Square Robot.
Most tanks are regulated at the federal and/or state level in the United States, and in many other countries, at the national level, compliance with API 653 is not optional, but mandatory. Therefore, the ability to determine the bottom thickness of an AST without the time, effort and costs associated with removing it from service is highly desirable from an owner/operator perspective.
However, robotics is not the only innovation in leak detection and bottom-plate evaluation technologies. In the early 1990s, to provide owners with a cost-effective, non-invasive AST assessment tool, the American Petroleum Institute (API) funded extensive research into the effectiveness of external acoustic monitoring to assess tank bottom plates and detect leaks. I was privileged to serve on the committee that oversaw these investigations a few years later in the early 2000s.
The committee’s work during these years addressed both the development and implementation of new technologies. The research resulted in publications such as API Publication 307 An Engineering Assessment of Acoustic Methods of Leak Detection in Aboveground Storage Tanks (1992), API Publication 322 An Engineering Evaluation of Acoustic Methods of Leak Detection in Aboveground Storage Tanks (1993), API Publication 325 An Evaluation of a Methodology for the Detection of Leaks in Aboveground Storage Tanks (1993) and API Publication 334 A Guide to Leak Detection for Aboveground Storage Tanks (1996).
These investigations defined specific instrumentation, test procedures and data analysis requirements. The conclusion was that acoustic monitoring was a feasible and practical technique with great potential to allow non-invasive detection of leaks in AST bottom plates. However, in the 1990s and even in the early 2000s, the available electronics were too expensive, computational capacity was limited, and leak detection sensitivity was insufficient.
In addition, signal analysis limitations lead to noise problems and internal reflections that generate erroneous leakage locations, while the high cost of implementing all API recommendations represented a barrier to the adoption of this passive acoustic technology.
Today, Antech Systems, Inc. is picking up where API research and development left off. Antech Systems, led by Dr. Steve Ziola (who kindly allowed me to “borrow” significant portions of an article he previously published), is applying state-of-the-art electronics, advanced piezoelectric sensors and modern digital image processing to tank bottom inspections, bringing API research into the 21st century.
The first improvement made through currently available commercial off-the-shelf (COTS) low noise electronics. The use of these cost-effective components has reduced hardware noise levels from the millivolts common in API research in the 1990s to nanovolts today, an increase in sensitivity of approximately 100,000 times. Thus, leak signals that were previously lost in conventional detection systems can now be recorded.
The next improvement is the use of low-cost, high-strength piezoelectric films, which can be installed as individual sensors, in arrays or even stacked to further improve sensitivity. Finally, software-based signal analysis is state-of-the-art and takes advantage of digital image analysis techniques, similar to those used in facial recognition and MRI scans.
When a leak occurs at the bottom of an AST, it generates a wave that propagates through the liquid product to the tank wall. Sensors placed around the circumference of the tank detect the displacement of the wall caused by that wave. Noise generated by other sources, such as sand or rain hitting the tank, pumps in operation or valves operating, does not create a pattern that matches that of an actual leak.
A library of computational images of leaks, obtained from a grid of locations on the bottom of the tank, can be correlated with the recorded image to determine the location of the leak, i.e., a facial recognition of the waves. This analysis approach also eliminates the traditional leak location problems that arose from mechanical noise and internal reflections in the initial API investigation. These new advances also allow the tank to remain in service while the leak inspection is being performed.
While finding leaks is relatively uncommon (as, in most cases, the tank is removed from service immediately once a leak is suspected or discovered), a major concern for owners and operators of atmospheric storage tanks (ASTs) is not knowing the physical condition of the tank bottoms, a requirement set forth by the API 653 and other tank inspection standards.
The most common inspections today still require draining and cleaning the tank in order to perform them. Another option, in some ASTs, is to insert a robot to determine the thickness of the tank bottom. However, in certain cases, tank features such as penetrations, roof supports, piping and the presence of sludge often limit the effectiveness of the robot.
A more practical evaluation tool for tank bottom inspection is guided wave ultrasound (GWU). In this method, ultrasonic signals are sent through the bottom of the tank, and changes in thickness caused by pitting and corrosion affect the wave propagation velocity. The main advantages of this technique are that it is noninvasive and, in most cases, can be applied while the tank is in service. A significant limitation is the inherent design of the bottom welds, which tend to significantly attenuate ultrasonic signals as they propagate.
Again, Antech Systems’ hardware and sensor developments overcome this signal loss in two ways:
- Increasing the sensitivity of the system to detect weaker signals.
- Using a higher energy ultrasonic excitation source.
Conventional ultrasonic systems employ expensive and delicate electronics and amplifiers to generate high-voltage, single-frequency shaped pulses that excite the emitting transducer. This approach is ineffective for tank bottom inspections because it fails to generate a signal with sufficient energy to overcome attenuation caused by welds. Antech Systems signal processing techniques allow the use of broadband frequency signals generated by mechanical sources, such as hammer blows, to excite the waves used in the analysis, resulting in high resolution measurements of the tank bottom.
As anyone who has ever hit a steel structure with a hammer knows, a large impact is not required to generate a pulse that propagates easily over long distances. This is a simple and inexpensive approach to ultrasonic pulse excitation, easily implemented in the field. In this application, hammer blows around the tank brow generate waves that propagate along the bottom, and sensors, attached to the same brow, detect the waves after they pass through the bottom of the tank. Subsequently, using tomography software and measurements of wave propagation velocities, an image of the tank bottom is obtained.
In addition to leak detection and bottom plate assessment, improvements in hardware and the development of imaging software could prove beneficial in determining sludge levels. Having a fast, cost-effective and non-invasive method to accurately profile the various levels of sludge in an AST before opening and cleaning it would be advantageous to both the tank owner/operator and the cleaning company, especially in crude oil tanks, by representing significant time and cost savings.
Improvements in acoustic monitoring and GWU (guided wave ultrasound) technology are currently being developed through ongoing collaborations with NASA and the Department of the Navy. Antech Systems has initiated field tests on small tanks and plans to conduct tests on larger tanks soon. I look forward to attending these tests and hope to update the industry with results in the near future.
This article was developed by specialist Earl Crochet and published as part of the sixth edition of the Inspenet Brief September 2025, dedicated to technical content in the energy and industrial sector.