This paper presents a new technology for inspection and monitoring of metallic objects, pipes and vessels. The system is based on a field proven, non-intrusive, internal corrosion monitoring technique, which is designed to detect and quantify general metal loss, cracking, or pitting due to corrosion or erosion.
The inspection tool allows inspection in areas where more conventional inspection techniques (ultrasonic or intrusive probes) are either costly or difficult to apply.
The significant advantages include the ability to operate in a wide temperature range (- 40 ° to + 400°C); non-intrusive, no operator dependency; measures changes in the actual pipe wall, allows remote intervention; can be applied to any geometry or wall thickness and maintains a high sensitivity of one part per thousand of wall thickness for general corrosion.
This paper describes the inspection system and results from a validation test performed to prove the quality of the measured data for weld root corrosion.
The techniques and technologies that are currently applied have been utilised in the market for many years and their potential with respect to sensitivity and inspection life cycle costs would appear to have been fully exploited. The electric mapping is a sensitive monitoring technique, which has been utilised since 1991 for monitoring of corrosion/cracking/erosion in vessels and pipework, at more than 90 locations world-wide, both top-side and subsea. Systems are being operated under various environmental conditions from extreme heat in the desert to extreme cold. Furthermore, systems have been installed in different hazardous areas: in nuclear power plants and for oil companies in explosive atmospheres, classified as Ex Zone 1 and 2.
Most the systems delivered are fixed installations where the sensing pins and instrumentation are permanently attached to the monitored object. The remaining 10% are based on a ~portable instrument' with clamp on pin holders which have mostly been delivered for laboratory applications. This design has the same functional specification and has shown that the electrical mapping technique could be applied for portable NDT instrumentation.
In offshore operations, process plants and refineries, an extensive amount of inspection work is undertaken, often in the form of 'key point' inspection. Various techniques are used, such as ultrasonics, radiography and eddy current. It was felt that the mapping technology could be developed to be a significantly more sensitive, versatile and user friendly inspection tool than the traditional techniques, and at a competitive cost level. Modification to the technology was however, required for such a development.
THE ELECTRICAL FIELD SIGNATURE MAPPING PRINCIPLE
The Field Signature Method (FSM) ~ is based on feeding a current through a selected section of the structure to be monitored and sensing the electric field patten1 by measuring small potential differences set up on the surface of the monitored object. The first measurement (signature) is unique to the geometry of the object. When general or local corrosion occurs the pattern of the electric field will change and can be compared with the signature. By proper interpretation of the changes in the potential differences, conclusions can be drawn, e.g. regarding general wall thickness reduction or [ocalised corrosion.
Figure 1 gives an illustration of the principle: The induced electric current in a pipe, will create a pattern determined by the geometry of the structure and the conductivity of the metal. This pattern is represented by current flow lines and equi-potential lines which are normal to the current flo
