Piping integrity is always a major concern in the chemical process industries (CPI), but the level of concern elevates when piping is buried underground. Soil conditions not only exacerbate general external corrosion, but also increase the difficulty and cost of inspection — a necessary step for ensuring safe and reliable operation. Making matters worse, many underground piping systems are incompatible with so-called pipeline pigs, designed in short lengths and come in the most undesirable configurations known to man.
While removing a line from service to perform an inline inspection or hydrotest is common and feasible with aboveground piping, it does not prove sustainable or cost effective for underground piping. The necessary time and costs for excavation are simply too great (Figure 1 and 2). Fortunately, advancements in technology have led to numerous methods that help minimize the amount of excavation that is necessary, but their use is not well known in the CPI. While the full description of these technologies is beyond the scope of this article, this brief synopsis sheds light on the methods and tools that are appropriate and available for CPI use.
In order to prevent catastrophic failures or unplanned downtime, an assessment program and a sound inspection plan are the key steps. Within this program, the proper inspection technique can facilitate the location, examination and quantification of damage mechanisms, such as internal and external corrosion, cracking, third-party damage and manufacturing flaws, so the appropriate intervention or remediation can be taken.
During inspection implementation, numerous details must be taken into consideration, such as location, size, length to be tested, accessibility, existence of cathodic protection (CP) and the potential for corrosion, to mention a few.
The primary costs when evaluating buried piping is attributed to the time spent on preparing excavations to access questionable piping. So, achieving accurate and repeatable data is vital for minimizing the amount of excavations and minimizing costs.
Direct assessment. The form of inspection known as DA (direct assessment) was originally developed for natural-gas transmission pipelines to detect and analyze different types of integrity threats in non-piggable pipelines. By using several of these applications, such as external corrosion direct assessment (ECDA) and internal corrosion direct assessment (ICDA), end users can first identify areas of probable concern and high consequence within the facility before embarking on excavation. ECDA uses location, soil conditions, coating conditions, CP and so on, to determine the potential for external corrosion. ICDA analyzes product, pressure, flow and other parameters to determine the potential for internal corrosion. Beginning with the platform of pre-assessment, followed by indirect or direct inspection practices and post assessment, the integration of the data acquired by both ECDA and ICDA can help develop a maintenance and inspection program for future monitoring.
Indirect inspection. Aboveground methods such as close interval potential survey (CIPS), direct-current voltage gradient (DCVG) and alternating-current voltage gradient (ACVG) are all indirect methods of inspecting buried piping from above grade for the identification of active corrosion or coating faults. CIPS examines the pipe-to-soil potential. Cathodic protected piping propagates a current that flows through the soil onto the pipeline, measuring the level of this current and noting the contact interface changes that can determine the level of protection being provided by the system. DCVG and ACVG, in comparison to each other, are similar types of surveys. The foremost difference between the two inspection methods is the power source that sends current to the pipeline for measurement. Both systems measure the voltage gradient along the buried segment. The gradient measurements are generally viewed as “the larger the voltage gradient, the larger the coating defect”. These methods can locate areas of concern for subsequent excavation and verification using other complementary techniques.
Direct inspection. Direct inspection methods have been proven for numerous years and often only require a small area of excavation. Guided wave ultrasonics, for example, only require 4–8 ft of exposed pipe for inspection. In general, guided wave ultrasonics provide a way to inspect lengths of piping from a single test position by generating low frequency guided waves and transmitting these down the length of the piping. Within this diagnostic test length, 100% of the circumference is inspected. Although both internal and external metal losses are detected, this method cannot distinguish between the two. Systems use a pulse echo operation that provides the inspector with the ability to quickly identify problem areas and metal loss. Sensitivity of guided waves typically is in the range of 4 to 6% loss of cross section, but this greatly varies pending the condition of the system being evaluated.
When using guided wave ultrasonics on buried piping segments, many variables, such as coating and soil conditions, can affect the results. Considering the fact that most buried piping systems are coated with some type of coal tar, fusion bonded epoxy (FBE) or bitumen wrap, excessive testing lengths (> 15m) are not practical, and shorter lengths with limited access are ideal. Using this method as a screening tool, which is the intended use by manufacturers, the end user can obtain quantitative views of the segment tested and qualify the findings with other non-destructive examination (NDE) methods. The use of this application is considered an advanced technique, and only technicians with a high level of training and experience should be used for such applications.
Another direct inspection method, automated ultrasonic testing (AUT), is a qualitative method for obtaining exact wall thickness measurements on piping in the areas of pitting or general corrosion. With numerous systems and users within the industry, this technology uses digitally controlled ultrasonic scanners for data acquisition and specialized software for data processing and display. The use of automated systems is driven to ensure complete coverage of the test specimen with greater accuracy. The scanners often used are either raster type or encoded linear scanners, all automated and programmed for a particular grid span or coverage plot, designed to eliminate human error. Depending on which system is applied, different ultrasonic displays can be given; the basic displays in AUT will typically include A-Scan, B-Scan and C-scan.
Automated ultrasonics can be used as part of the overall process for determining mechanical integrity. Accuracy of the systems varies, most are in the range of ±0.0030.010-in. variance in thickness calibration, which provides very accurate data for service calculations. In most cases, the areas to be tested should be reasonably clean of any coating, with the external condition of the piping smooth with minimal external corrosion. This application is ideal for verifying anomalies discovered by use of other quantitative methods, such as guided wave inspection.
There are many different types of cracking found in the various industries; one of the more prevalent is stress-corrosion cracking. Various environmental conditions can attribute to cracking from temperature and soil conditions to pressure and stresses. Locating potential areas based on ideal conditions for environmental cracking and validating the existence and sizes are an important part of inspecting buried piping. Alternating-current field measurement (ACFM) provides a method for crack detection and sizing. The technology uses an alternating current field, which flows on the surface of the material being tested. When a surface breaking crack is present, the field is disturbed, thus revealing the crack location. Special techniques are used to induce these currents, which allow the disturbances to be measured and quantified in multiple dimensions. The use of these data eliminates unnecessary grinding out of indications and can provide accurate sizing information for integrity assessments. ACFM can be performed through most coatings with minimal surface preparation.
There is no one tool that can provide all the necessary data required when inspecting buried plant piping. Complementary techniques form a comprehensive testing strategy for discovering what lies beneath.
Edited by Rebekkah Marshall
Brant Shields is the technical manager of advanced inspection services for PetroChem Inspection Services, Inc., a Subsidiary of TÜV SÜD America Inc. (1475 East Sam Houston Parkway South Suite #100, Pasadena, TX 77503; Phone: 281-884-5100; Fax: 281-884-5199; Mobile: 225-802-5591; Email: bshields @petrochemintl.com). He has over 20 years in the inspection and NDE field and has managed numerous projects involving ECDA/ICDA on natural gas as well as liquid pipelines. He currently holds the following credentials: API 510 Inspection of Pressure Vessels and API 653 Inspection of Above Ground Storage Tanks; Guided Ultrasonics Level I, CSWIP Level II; Certified Level II in ultrasonics, penetrant testing, magnetic particle and visual inspection. He has also completed training in direct assessment of buried gas and liquid piping, RB assessment, and GW assessment of cased road crossings.
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