Summary The inspection of ferromagnetic and non-ferromagnetic material components is an attractive and efficient method due to its fast screening of large areas with low surface preparations. Already qualified in various requests within the oil & gas industry and successfully applied in the field, the technique has been proven as a fast corrosion screening technique with high defect detection sensitivity and reliability for the inspection of pipes, vessels, storage tank floors and other type of steel constructions. Because of its ability to inspect through various wall thicknesses, coating types and thicknesses as well as under high temperatures, this technique has been highly recommended for risk-based as well as nonintrusive inspection strategies. This paper describes the principle of the technique, technical details, applications and field examples of this technique as a corrosion screening tool for the NDT industry. Technique The technique can be regarded as a readily accepted NDT technique in an increasing number of industries. The Oil & Gas industry recognised the advantages of this technique for the inspection of ferromagnetic and non-ferromagnetic steel components such as storage tank floors, piping components and pressure vessels. Several inspection projects and qualification tests in the Oil and Gas Industry have shown the technical and economical advantages of this technique in terms of high defect detection sensitivity, ability to inspect thick-walled components and through different coating types, high inspection speeds and coloured defect mapping capability. These features are leading to further applications and developments. It should be noted that in direct contrast to ultrasonic inspection, no acoustic coupling is required between the sensor system and the object to be inspected which enables the Saturation Low Frequency Eddy Current technique to be utilised as a fast and reliable method in the detection of local corrosion. The first impression of the technique reveals similarities with the traditional Magnetic Flux Leakage (MFL) technique. However, the sensitive Saturation Low Frequency Eddy Current technique allows the inspection of steel components with wall thickness of up to 30mm. This extended thickness capability not only makes this corrosion screening method suitable for the inspection of thick-walled components but also for thinner walled components with thick non-metallic protection layers such as glass fibre reinforced epoxy coating on storage tank floors. Moreover, the ability to differentiate the external from the internal defects is a unique and valuable feature.
The technique has shown advantages in the detection of the following different defect profiles: Single pits Groups of corrosion attack Small shallow and narrow pits in their early stage of development Shallow rounded pits with different diameters that are particularly difficult to detect with conventional manual Ultrasonic The ability of this technique to execute non-invasive inspection on carbon steel piping components directly through the aluminium coatings on their surface without the need for removal as well as on components with surface temperatures up to 100 C results in significant time and cost savings. Introduction of the Technique The inspection technique uses the Eddy Current principle in combination with a magnetic field. Utilising superimposed DC-magnetisation, the depth of penetration is increased to such an extent that the sub-surface (internal) corrosion attack (metal loss) can be detected from the external surface. This technology is an inspection method for the detection of surface and sub-surface corrosion in thin- and thick-walled plates and piping components. Due to the signal phase separation, internal and external defects can be clearly distinguished from each other. Plates and piping components with non-magnetic and non-conductive coatings such as GRP, rubber, paint etc. with thickness up to 10mm can be inspected with this technique. Direct surface coupling is not necessary for this technique due to its electromagnetic principle. Picture 1 Principle of
Technical Information After the successful utilisation of the technique for corrosion detection in a 32mm thick cooling water pipe, further features of the technique were tested due to the similarity with the MFL method. A direct comparison of the sensitivity in relation to wall thickness was evaluated and correlated in the table below. Picture 2 - Relative Sensitivity Curve MFL in comparison to Successful laboratory and field qualification tests have been conducted by various Oil & Gas organisations. The qualification tests have been performed for storage tank floor and pipeline inspections. Very high detection sensitivity of even the smallest pits has been documented from these tests. Inspection through coatings on tank floors and piping components is a typical challenge. It has been verified in qualification tests that the method enables the inspection through coatings of up to 10mm in thickness. For example, a typical tank floor has a 4-6mm coating of glass fibre reinforced epoxy. The Saturation Low Frequency Eddy Current method is able to execute the inspection without the need for removing the coating or any other special preparations. These qualification tests have further verified the method s unique ability to distinguish between internal and external defects, its impressive inspection speed and the significant reduction in the required amount of defect verification by Ultrasonic inspection. The immediate and direct signal phase separation offers possibility of mapping the external and internal defects independently.
Picture 3 Signal phase separation between external and internal defects Inspection Applications The inspection of different objects is usually accompanied by different geometries. Any scanning geometry is readily adapted by the basic MEC-Scanners and the connected computerised Eddy Current equipment. The Eddy Current unit is operated with signal analysis and coloured scan reporting software. For storage tank floor inspection, the specially designed and developed MEC-Floorscanners are capable of handling scanning width up to 400mm, material wall thickness up to 30mm and coating thickness up to 10mm at speed much faster than other methods. The MEC-Floorscanners are capable of scanning annular plates right up to the edge of adjoining walls in order to detect typical corrosion which is most often located at the interface of these annular plates. The MEC-Pipescanners are designed to facilitate the inspection of different wall thickness ranges and surface coverage requirements. The MEC-Pipescanners are externally driven along the surface of the pipes. The parameter settings of the method allow inspection speed of up to 25m/min. Specially adapted scanners enable the further applications for tubular tanks, loading lines, boiler pipes, large cooling/water pipes and pressure vessels. Both ferromagnetic and stainless steel materials can be readily inspected. Field inspections have been successfully executed on piping components with surface temperatures of approximately 70 C without any problem.
Pipe Length Fast Corrosion Screening Technique Picture 4 MEC-Pipescanner Picture 5 MEC-Floorscanner The Reporting The scanned areas are directly transferred to a colour coded report. The condition of an inspected component is displayed and reported in different colours which represent different percentages of wall loss. A signal phase analysis window caters for colour mapping, filters signal directions for internal and external defects as well as displaying the condition of other detected indications. 2.5 2.0 1.0 36 27 18 9 Pipe Circumference 0 0 Picture 6 Coloured C-Scan Report
Conclusion The inspection technology is now a readily more accepted technique in NDT industry with a constantly increasing and widespread range of applications. Its unique features make it readily adaptable to new inspection strategies which demand a fast, sensitive and reliable condition screening technique. Its coloured reporting capability for corrosion mapping provides an overview of the condition of the inspected component. This allows the evaluation and determination of corrective actions and modifications for future maintenance and inspection strategies. The summarised features of technique include: Fast screening method for detection of local metal loss Inspection of thick walled components (up to 32mm) Inspection through thick coatings (up to 10mm) Higher defect detection sensitivity than MFL Distinction between internal and external defects Non-intrusive and/or low object preparation prior to the inspection Coloured condition mapping capability Inspection of Ferromagnetic and non-ferromagnetic materials Inspection at high temperatures (up 120ºC) High potential for additional applications