Figures 1-3

Introduction
This article discusses the emergence and acceptance of manual eddy current weld inspection as a high quality and cost saving NDT method for in-service inspections. In a recent American Welding Society journal article on NDT, eddy current testing was not mentioned as a basic weld inspection method (AWS, 1997). In fact, it has become common in the European sector, and American oil and gas companies are now beginning to use eddy current.

All NDT methods have advantages and limitations for detecting various types of weld indications, for example:

• Magnetic particle testing (MT) is used for detecting short length and shallow surface breaking indications. Its sensitivity, however, is severely reduced in detection of indications through coatings (generally 0.2-0.4 mm [0.006-0.012 in.]), and MT is difficult to use outside on wet surfaces.
• Eddy current is used for detecting surface breaking indications through coating thickness as great as 2 mm (0.08 in.), and it can be used on wet surfaces. Because, however, only the area under test is being inspected, several scans must be employed for complete coverage.
• Ultrasonic testing (UT) is used for detecting volumetric indications. It is generally not as sensitive as MT for detection of fine, surface-breaking indications, and UT requires a higher level of operator skill.
• Radiographic testing (RT) is used for volumetric detection of indications. It cannot, however, detect lamellar indications. RT also requires special safety precautions.

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All NDT methods have advantages and limitations for detecting various types of weld indications...

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Discussion of Eddy Current and MT
Consideration must be given to the type and size of indication requiring detection (during fabrication, while in service, or during repair inspections) including the type of component being inspected. For example, for in-service inspections on offshore structures, the predominant indications are surface breaking, of which the majority occur in the toe of the weld. As MT is ideally suited for detection of this type of indication, it has been the method most widely employed. Its main drawback, however, is its inability to "see" through certain coating thicknesses; nor can it be used on wet surfaces (e.g., on surfaces wet from rain). Eddy current has the ability to overcome both of these disadvantages. The following is a discussion of advantages and limitations of eddy current compared to MT (Sea Test, 1996).

• MT loses its sensitivity when applied through coating thicknesses greater than 200-300 µm (0.008-0.012 in.) (Electric Power Research Institute, 1988). Most painted structures have coatings greater than 300 micrometers (0.012 in.) thick. Thus, the coating must be removed and reapplied if MT is to be used. Coating removal and recoating (including "clean-up" from coating removal and access, usually scaffolding) comprises the majority of MT inspection cost. In comparison to MT, eddy current can be reliably performed through 2 mm (0.08 in.) of nonconductive coating.
• Both wet and dry MT methods are difficult or impossible to implement in wet or windy environments. In contrast to MT, portable eddy current instruments can be placed into lightweight, waterproof enclosures. Eddy current probes are inherently waterproof and can be used on wet surfaces.
• MT is a two-handed operation. While not a major constraint for most MT applications, projects with considerable overhead MT are difficult ergonomically. In contrast to MT, "scanning" with lightweight eddy current probes can be accomplished with one hand. Coupled with a lightweight instrument (approximately 3 kg [6 lb]), the eddy current method is ideal for "rope access" and for overhead applications.
• Eddy current can be used with minimal visibility in which MT would be difficult to perform, typically underwater inspection of jack-ups. In order to verify any eddy current indications, however, visibility to perform MT must return.
• MT produces a residue (particles) which is left in the environment. While particles (wet and dry) may be nontoxic, they may require workers to wear protective equipment to reduce airborne particle inhalation. This may be an important consideration for nuclear applications.
  
  Eddy Current has distinct limitations, though, compared to MT.
• Compared to other surface breaking indication detection methods (primarily MT), eddy current requires a higher inspector skill level for accurate interpretation of signals.
• Eddy current requires the probe to be close to the indication for detection. Specific scanning patterns must be used for the heat affected zone, the toe of the weld, and weld surface inspections. Careful attention must be given to geometry, access, and full inspection of the part. MT inspection with yokes covers base metal, heat affected zone, toe of weld, and weld face with one placement (one each for longitudinal and transverse indications). It also geometrically covers "rat hole" type details which might be hard to reach with the eddy current probe.
• Assuming equal surface preparation, normal access, and that the entire weld needs to be inspected (not just one weld toe), MT is faster per unit of weld length.
• Unlike MT, eddy current does not produce a visible "spot" on the part. The confidence level with MT is slightly higher because there is a direct "readout" on the part. Eddy current indications require verification with MT. Typically, the eddy current indication is cleaned to bare metal using hand tools or needle gunning and then inspected using American Petroleum Institute (API) RP 2X (American Petroleum Institute, 1996).
• On extremely corroded, rough surfaces, eddy current performance is degraded due to the low signal-to-noise ratio.
• Eddy current is not suitable for evaluation of indications by grinding because detection of extremely shallow (less than 0.5 mm [0.020 in.]) indications is unreliable. MT provides excellent sensitivity for shallow indications and a visual confirmation that cracks have been ground out.
  
  Sea Test Services has been involved in several eddy current weld inspection research and development projects and field implementations of eddy current. Before being allowed to perform insections, eddy current inspectors were independently qualified by performing practical demonstrations on test specimens having indications in the range of sizes and geometries of those to be found in the field. Eddy current field results were then spot checked with MT to confirm reliability of the eddy current test method. Using qualified recommended practices and personnel, results of eddy current were found to be in agreement with those results found by MT. As part of a joint industry project sponsored by Exxon, Shell, Mobil, and Sea Test Services, (Sea Test Services, 1996), a recommended practice using lightweight "off-the-shelf" eddy current equipment and weld inspection probes was developed.

 

Production
Production rates for eddy current and MT inspection using efficient practices and inspectors on bare metal favor MT. As stated above, however, operational, surface condition, and cost are important factors. Inspecting with MT works on the principle that the area between the yoke legs is fully inspected in one yoke placement (approximately (150 mm wide ´ 75 mm long [6 ´ 3 in.]). In order to inspect a weld completely, the yoke must be placed in two directions. The scanning rate for MT inspection for indications parallel with the weld (e.g. toe cracks, centerline cracks, cracks parallel in the base metal) is approximately 2 min per 0.3 m (2 min per ft) and approximately 1 min per 0.3 m (1 min per ft) for transverse indication scans.

Eddy current inspects only the area directly under the probe. Five eddy current scans are typically used for weld inspection: two for the base metal (parallel and transverse); one specifically targeted for the weld toes; and two for the weld face (parallel and transverse). Additionally, rough weld faces typically will decrease scan speed due to increased signal complexity. Thus, scanning rates for eddy current vary depending on the size and profile of the weld face. The production rate for eddy current is approximately twice that of MT.

There are two other factors that affect cost when choosing between MT and eddy current. Eddy current is easier to use from rope access. If the majority of the inspection is in the overhead position, the one-handed eddy current method is ergonomically easier than MT.

The discussion above requires qualification as there has been significant confusion on European studies which researched MT efficiency using permanent magnets or coils. In the United States, MT using AC yokes with white light visible particles is the only MT method used both underwater and topside for inspection of offshore structures. This practice (American Petroleum Institute, 1991) has been in place since 1981. MT using AC yokes for detection of longitudinal indications has a proven field production rate of 120 s per 0.3 m (120 s per ft). This is based on several thousand feet of MT weld inspections performed for several major oil companies. The reported rate for MT using coils and fluorescent particles for detection of longitudinal indications is 480 s per 0.3 m (480 s per ft).

 

Application and Conclusion
Of the two methods (MT and eddy current), MT has a slightly greater sensitivity for indication detection. API RP 2X (American Petroleum Institute, 1996) states the sensitivity of MT as 6 mm length ´ 1 mm (0.25 ´ 0.04 in.) depth. While there is no industry domain eddy current recommended practice, the majority of reports on eddy current weld inspection give the sensitivity of eddy current as 5 mm (0.2 in.) length by 1.5 mm (0.06 in.) depth. The slight difference in sensitivity between MT and eddy current may not, however, be critical in many applications. Given the above considerations, the overall sensitivity of MT is only marginally greater than that of eddy current. The benefits of eddy current compared to MT are significant, though, considering cost and application.

Eddy current then, should be considered for inspection applications of intact coatings:

• where the use of MT would require coating removal and reapplication,
• for wet or damp surfaces (bare metal or painted) when the surface would have to be dried to perform MT,
• for operations using rope access, and
• for underwater operations where visibility limits the use of MT.

A combination of eddy current and MT has been successfully used on a number of applications including the topside structural inspection of painted offshore oil rigs, large above ground storage tanks, and the inspection of painted ship details.

The most prudent approach is to give the customer knowledge of the merits of various NDT methods so that a "tool bag" approach can be taken to provide the customer with NDT methods that account for safety, required sensitivity, and consideration to cost. Given the success of eddy current ferritic weld inspection, when the above criteria are considered, eddy current can be used as a stand alone inspection method with MT for verification. Ideally, a specific recommended practice for eddy current weld inspection and qualification of inspectors, similar to the API RP 2X and RP 2MP documents, should be adopted so that the eddy current method is traceable to a recognized standard.

 

References
American Welding Society, Welding Journal, Oct 1997, NDT Supplement.

Sea Test Services, Ferritic Weld Inspection Using Eddy Current, 1996, Joint Industry Study.

Electric Power Research Institute, Reliability of Magnetic Particle Inspection Performed Through Coatings, Jul 1988, Palo Alto, CA, NP-5951.

American Petroleum Institute, Recommended Practice for Ultrasonic and Magnetic Examination of Offshore Structural Fabrication and Guidelines for Qualification of Technicians." Nov 1, 1996, Washington, DC, API RP 2X 3rd ed.

American Petroleum Institute, Recommended Practice for Underwater Magnetic Particle In-Service Weld Inspection of Offshore Fixed Platforms and Guidelines for Qualification of Inspector Divers, Jan 1991, Washington, DC, (draft).

 

* Sea Test Services, 1095 Shady Lane, Merritt Island, FL 32952; (407) 452-5619; e-mail seatest@aol.com.

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