Inspection of buried process piping is different from above ground process piping because significant external deterioration can be caused by corrosive soil conditions and the inspection is much more difficult due to the inaccessibility of the piping. However, as difficult as it may be, buried piping MUST be inspected to determine its external surface condition. The facility has to develop an inspection plan(s) that takes into consideration the effectiveness of the cathodic protection, if any exists, whether the pipe was coated, and on inspection information obtained from one or more of the following methods:
- during maintenance activity on connecting piping of similar materials of construction
- from representative portions of the actual piping;
- from buried piping in similar circumstances (temperature changes within the pipe, depth of pipe, soil moisture, soil pH, etc.)
- from permanently installed thickness monitoring devices;
- from inspections conducted with remote visual equipment; or
- from the results of cathodic protection surveys, or from guided wave examination used to locate areas of interest for follow-up inspection using more quantitative thickness measurement techniques.
One way we should be able to establish leaks in underground piping is what API calls “Above-grade visual surveillance indicators” in buried piping. These indicators could include:
- a change in the surface contour of the ground,
- discoloration of the soil,
- softening of paving asphalt,
- pool formation,
- bubbling water puddles, or
- noticeable odor.
Surveying the route of buried piping for any of these signs is one method of identifying problem areas in the piping.
A close-interval potential survey performed at ground level over the buried pipe can be used to locate areas where the cathodic protection systems may not be effective and active corrosion on the pipe’s surface is present or may occur. However, it may not be a reliable method for corrosion wall loss inspection, since it can only infer wall loss from cathodic protection potential but not directly detect the presence of wall loss.
A pipe coating “holiday survey” [e.g. direct current voltage gradient (DCVG)] can be used to locate coating defects on buried coated pipes, and it can be used on newly constructed pipe systems to ensure that the coating is intact and “holiday-free”. More often it is used to evaluate coating serviceability for buried piping that has been in-service for an extended period of time. From survey data, the coating effectiveness and rate of coating deterioration can be determined.
Corrosion of bare or poorly coated piping is often caused by a mixture of different soils in contact with the pipe surface. The corrosiveness of the soils can be determined by a measurement of the soil resistivity. Lower levels of resistivity are relatively more corrosive than higher levels, especially in areas where the pipe is exposed to significant changes in soil resistivity. The depth of the piping shall be considered in selecting the soil testing method to be used and the location of samples. The testing and evaluation of results should be performed by personnel trained and experienced in soil resistivity testing.
Cathodic Protection Monitoring
Cathodically protected buried process piping shall be monitored regularly to assure adequate levels of protection. Monitoring shall include periodic measurement and analysis of pipe-to-soil potentials by personnel trained and experienced in cathodic protection system operation. More frequent monitoring of critical cathodic protection components, such as impressed current rectifiers, may be needed to ensure reliable system operation. Owner/users should keep appropriate records of Cathodic Protection monitoring and maintenance performed as a result of system monitoring. Refer to NACE SP0169 and Section 11 of API 651 for guidance applicable to inspecting and maintaining cathodic protection systems for buried piping.
Inspection Methods
A number of direct examination techniques methods are available that may be applied to buried piping and a more extensive guide to these can be found in API RP 574. Some methods can indicate the external or wall condition of the piping, whereas other methods indicate only the internal condition. Additionally, some methods are able to simultaneously detect and quantify both wall loss and deformation damage such as denting, ovality, bulging, swelling, etc. An array of technologies are now available that can be externally applied to buried piping at a location and screen select areas from that position. These techniques may require some excavation but considerably less than a full access.
An example of these techniques is guided wave examination, previously known as long-range ultrasonics (LRUT) or guided wave ultrasonic testing (GWUT). These technologies may allow 15 ft or longer distances to be screened from one installation and provide a screening assessment of the pipe. Distance traveled and the degree of detection/accuracy is a function of the applied technology and pipe conditions including degree of corrosions, external and internal coatings and soil conditions, transported product and type and number of accessories in the path of the signal. Other technologies employing ultrasound may be used to screen several feet from one location and are useful for assessing damage in locations such as soil-to-air interfaces. Reference API 574 for examples of other technologies.
Frequency and Extent of Inspection
Above-grade Visual
The owner/user should, at approximately six (6) month intervals survey the surface conditions on and adjacent to each buried piping path
Pipe-to-soil Potential Survey
A close-interval potential survey on a cathodically protected line may be used to verify that the buried piping has a protective potential throughout its length. For poorly coated pipes where cathodic protection potentials are inconsistent, the survey may be conducted at 3-5 year intervals for verification of continuous corrosion control.
For piping with NO cathodic protection or in areas where leaks have occurred due to external corrosion, a pipe-to-soil potential survey may be conducted along the pipe route. The pipe should be excavated for inspection or inspected with appropriate NDE at sites where possibilities of active corrosion cells have been located to determine the extent of corrosion damage. A continuous potential profile or a close-interval survey may be required to better locate active corrosion cells.
Pipe Coating Holiday Survey
The frequency of pipe coating holiday surveys is usually based on indications that other forms of corrosion control are ineffective. For example, on a coated pipe where there is a gradual loss of cathodic protection potentials or an external corrosion leak occurs at a coating defect, a pipe coating holiday survey may be used to evaluate the coating.
Soil Corrosivity
For piping buried in lengths GREATER THAN 100 ft (30 m) and NOT cathodically protected, evaluations of soil corrosivity should be performed at appropriate intervals based on the likelihood of change. Soil resistivity measurements may be used for relative classification of the soil corrosivity. Additional factors that may warrant consideration are changes in soil chemistry and analyses of the polarization resistance of the soil and piping interface.
External and Internal Inspection
Intervals If internal corrosion of buried piping is expected as a result of inspection on the above-grade portion of the line, inspection intervals and methods for the buried portion should be adjusted accordingly. The inspector should be aware of and consider the possibility of accelerated internal corrosion in dead legs. The external condition of buried piping that is not cathodically protected should be determined by either pigging, which can measure wall thickness, or by excavating according to the frequency given in Table 4.
Significant external corrosion detected by pigging or by other means may require excavation and evaluation even if the piping is cathodically protected. Piping inspected periodically by excavation shall be inspected in lengths of 6 ft to 8 ft (2.0 m to 2.5 m) at one or more locations judged to be most susceptible to corrosion. Excavated piping should be inspected full circumference for the type and extent of corrosion (pitting or general) and the condition of the coating. If inspection reveals damaged coating or corroded piping, additional piping shall be excavated until the extent of the condition is identified. If the average wall thickness is at or below the minimum required thickness, it shall be repaired or replaced. If the piping is contained inside a casing pipe, the condition of the casing should be inspected to determine if water and/or soil has entered the casing. The inspector should verify the following: a) both ends of the casing extend beyond the soil surface, b) the ends of the casing are sealed if the casing is not self-draining, c) the pressure-carrying pipe is properly coated and wrapped, and d) there is no metallic or electrolytic contact between the casing and the pressure carrying pipe. 9.8.6 Leak Testing Intervals An alternative or supplement to inspection is leak testing with liquid at a pressure at least 10 % greater than maximum operating pressure at intervals one-half the length of those shown in Table 4 for piping not cathodically protected and at the same intervals as shown in Table 4 for cathodically protected piping. The leak test should be maintained for a period of eight (8) hours. Four hours after the initial pressurization of the piping system, the pressure should be noted and, if necessary, the line repressurized to original test pressure and isolated from the pressure source. If, during the remainder of the test period, the pressure decreases more than 5 %, the piping should be visually inspected externally and/or inspected internally to find the leak and assess the extent of corrosion. Sonic measurements may be helpful in locating leaks during leak testing. Buried piping also may be surveyed for integrity by using temperature-corrected volumetric or pressure test methods. Other alternative leak test methods involve acoustic emission examination and the addition of a tracer fluid to the pressurized line (such as helium or sulfur hexafloride). If the tracer is added to the service fluid, the owner/user shall confirm suitability for process and product.
Table 4—Frequency of Inspection for Buried Piping Without Effective Cathodic Protection