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  1. ROSEN Group
21 April 2008

New Inspection tool combining Magnetic Flux Leakage and Ultrasonic Testing

Magnetic Flux Leakage (MFL) and Ultrasonic Testing (UT) are established technologies in the In-line Inspection (ILI) industry. Though an important technology used to detect many different types of defects in pipelines, MFL is not the ideal method for identifying certain geometries, notably large uniformly corroded areas and two-dimensional mid-wall discontinuities such as laminations. Similarly, UT tends not to reveal small corrosion pits and internal defects covered by wax and other deposits.

In response to these limitations, ROSEN has developed RoCorr UT™, a combo tool in which the two inspection methods ideally complement each other. As a result, highly reliable information is provided on a number of different aspects, notably metal loss as well as weld, geometry and pipeline wall features.

In the presence of internal or external metal loss, the magnetic flux created as part of the MFL method leaks out of the pipeline. Since this leakage is recorded by hall-effect sensors, the location of defects as well as the depth, length and width of the recorded metal loss can be accurately inferred.

The MFL pattern registered by the inspection tool is larger than the anomaly itself. MFL technology is therefore especially suitable for detecting even very small pitting defects. This strength of MFL technology is at the same time the weakness of the UT inspection method which cannot, as a general rule, detect anomalies smaller than 10–20mm. Since debris divert the ultrasonic beam, information on minor defects is often missing altogether.

The strength of the UT method is direct measurement of two-dimensional features, notably pipe wall thickness. Taking into account relevant factors such as the velocity of ultrasound in the coupling fluid and in the pipe wall, UT inspections measure the time of flight of ultrasonic signals reflected from the internal and external surfaces of the pipe wall as a basis for determining both metal loss and wall thickness.

In contrast, measurements of two-dimensional features using MFL are less reliable, since they must be calculated indirectly by means of a comparison of the MFL data from calibration joints.

Capable of compensating for the weaknesses of each technology used on its own, the combination of MFL and UT provides high accuracy in length, depth and width measurements for exact defect shapes and precise information on general wall thinning.

The obtained defect shapes allow a more detailed profile determination supporting assessment codes like RSTRENG. Inspection tools combining both methods furthermore show excellent detection performance for numerous metal loss flaws, mid-wall features, weld features, geometry features and other features (e.g. valves, fittings, bends).

As a part of an integrity management, typically in place for the operation of pipeline systems, an adequate ILI method is chosen based on the prospective thread derived from a pre-inspection risk analysis for the pipeline. Exemplary Microbiological Corrosion or Top of Line corrosion shall be mentioned.

To ensure that even small changes in corrosion depth occurring over many years can be reliably identified, corrosion growth analysis requires extremely accurate inspection results. Since the combination of the two different inspection methods MFL and UT leads to significantly higher measurement accuracy, the combo tool RoCorr UT makes an invaluable contribution to corrosion growth analysis as part of integrity management.

However, combined ILI technologies also have more general benefits for integrity management: with their wider application range, they reduce the need for future threat analysis and minimize the risk of choosing the wrong inspection technology for a given pipeline situation. Apart from its benefits for asset integrity management, the RoCorr UT tool also lowers operational risk. Since all data is collected in a single run, it cuts the risks and costs of repeated launching, receiving and tool tracking processes associated with successive inspections.

Amalgamating the advantages of the two NDT methods MFL and UT, ROSEN’s RoCorr UT tool is capable of traversing pipelines between 6In and 40In, negotiating a minimum bend radius of 1.5D, and conducting uninterrupted inspections of up to 255km.

This combined tool not only detects common defects such as corrosion pitting and irregular general corrosion, but also accurately sizes large uniformly corroded areas and laminations. Moreover, it measures pipeline wall thickness to an accuracy of approximately 0.2mm. RoCorr UT therefore not only makes an invaluable contribution to integrity management, but also significantly reduces both inspection costs and operational risk, since all data is gathered on the strength of a single run.

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