Rather than replacing plant assets, condition assessment using MFL and spot repair can often extend the life of a storage tank or pipeline. MFL inspection technology allows asset managers to proactively manage a tank or pipeline by repairing problematic locations. MFL Inspection reduces the risk of tank or pipeline failure.

The following information outlines some of the major considerations that need to be addressed in order to achieve reliable, fast and economical inspections of above ground storage tank floors.

Sensor Types

MFL inspection tools typically use one of two types of sensors: Coils and Hall Effect Sensors. They are both capable of detecting the magnetic flux leakage fields caused by corrosion on tank floors. There is a fundamental difference, however, in the way that they respond to leakage fields.

1. Coils are passive devices and follow Faraday’s Law in the presence of a magnetic field. As a coil is passed through a magnetic field, a voltage is generated in the coil and the level of this voltage is dependent on the number of turns in the coil and the rate of change of the flux leakage. From this, it is clear that speed will have some influence on the signals obtained from this type of sensor.
2. Hall Effect Sensors are solid state devices which form part of an electrical circuit and, when passed through a magnetic field, the value of the voltage in the circuit varies dependent on the absolute value of the flux density. It is necessary to carry out some cross referencing and canceling with this type of sensor in order to separate true signals from other causes of large variations in voltage levels generated by the MFL inspection process.

MFL Technique Application Considerations

Coverage Limitations

It is virtually impossible to achieve 100% coverage using this technique due to the physical access limitations. The MFL inspection equipment should be designed so that it can scan as close as possible to the lap joint and shell. There are obviously compromises to be made as the wheel base of the scanner is an important consideration on tank floors that are not perfectly flat. Smaller scanning heads or supplemental visual and ultrasonic inspections can be used in confined spaces to increase coverage.

MFL Limiting Factors

Adjustment of Equipment

It is vital that the equipment is adjusted in accordance with the manufacturer’s recommendations. These adjustments will vary in relation to both thickness of the plate to be inspected and the thickness of any coating. In order to create leakage fields from corrosion it is necessary to achieve near saturation of the magnetic flux in the material. If near saturation does not exist then no leakage field will be generated and therefore there will be nothing for the sensors to detect.

Surface Condition

The top surface condition of the plate to be inspected absolutely dictates the achievable level of sensitivity of the inspection. The rougher and dirtier the surface, the more noise that will be generated from the inspection process and the lower the achievable sensitivity. Real signals may be masked by the level of noise generated.

Plate Curvature

Tank floors are seldom flat. The degree of curvature of the plates may have a significant impact on the reliability of the inspection. When the curvature of the plate causes an increase in lift off by either the magnetic bridge or sensor the sensitivity may suffer dramatically. This can often be overcome by scanning in more than one direction. Curvature of plates close to lap joints and/or the shell can make it virtually impossible to scan these areas reliably. In some cases smaller scanning heads can be used to overcome this problem.

Material Properties

The material itself can also have a significant impact on the inspection. Most floor plates are manufactured using low carbon steel, which lends itself very well to this technique. Higher grade materials require a much higher magnetizing force to achieve the same levels of saturation. This also applies to materials whose properties have been degraded by the service. i.e. Sour Crude Storage. In this case the material suffers from hydrogen attack and it may not be possible to achieve the necessary flux saturation of the material to achieve the required sensitivity. This can also be a major contributor to noise levels. The inspection direction can also have an impact on the amplitude of signals from the same sized corrosion pit. It has also been noted that noise levels can vary even on clean smooth surfaces due to either variations in the material properties or method of manufacture.

Accessibility

It has often been claimed that MFL can achieve 100% coverage of the floor area. In reality this is virtually impossible to achieve due to the overlapping nature of the plates and the number of obstructions in most floors. It is also difficult to inspect adjacent to the shell because of the curvature due to edge settlement. There are often many areas of the floor that are inaccessible to the regular scanners. Smaller scanning heads can be used to alleviate some of this problem but it is rarely possible to cover the entire floor using MFL alone and supplementing with visual and ultrasonic inspections.

Product Side & Underside Discrimination

Magnetic Flux Leakage is incapable of reliably discriminating between product side and underside corrosion. In most cases this is not a problem as it is possible to discriminate signals visually. In the case where there is a known topside problem that has been documented and passivated (by coating) after an earlier inspection and removal of the coating is not necessary then it is helpful to the inspection agency to have access to the earlier inspection information. This can speed the inspection process significantly. The very nature of MFL means that in some cases product side corrosion cannot be detected by the floor scanner. As a general rule the response from a product side pit with the same volume loss as an equivalent underside pit will be significantly less. This is due in part to the higher flux leakage levels being retained within the depth of the pit itself whereas on the other side (underside) of the ligament the leakage field will be above the surface and therefore in the direct path of the sensors.

Coated Floors

In addition to the above limitation and in the case of thicker coatings, there is another factor that affects achievable sensitivity. While it is often possible to overcome the additional lift off of the magnetic bridge by lowering it within the carriage, the same cannot be done with the sensor array to place it as close to the surface as possible because the coating is in the way. This obviously means that it will no longer be possible to achieve the same levels of sensitivity that could be demonstrated on the uncoated plate of the same thickness. In all such cases, it is necessary to run a simulation prior to the inspection to see what can be achieved.

Operator

As with any equipment the operator is responsible for its correct use and the validity of any inspection is absolutely dependent upon the operator’s integrity.  It is probably better to have a good operator with the worst equipment rather than a bad operator with the best equipment.  Of course, the best situation is to have a certified operator, equipment, and procedure.  All operators should, as a minimum, be able to show proof of training in the use of the equipment they are operating along with a reasonable understanding of the technique.  Ranger Inspection provides training according to API 653 Appendix G. Practical test plate training and 40 hours of training based on API 653.  NO TWO PIECES OF EQUIPMENT ARE THE SAME. This is not the type of equipment or technique that can be made to be independent of the operator. All MFE/MFL equipment and most NDT equipment operation are to some degree “operator dependent”.

No two tank floor inspections are the same and can range from smooth, clean, flat, defect free, rough, dirty, buckled and corroded throughout. It is obvious that a good inspection of the latter is going to take significantly longer and require more effort than the former.

Magnetic Flux Leakage may provide speed and reliability provided that the technique is properly applied and that all the limitations are fully understood.