Large metal structures in offshore environments or other tough or hazardous environments are subject to heavy wear and corrosion and need to be monitored during their lifetime in order to maintain safety for personnel and security for operation. Examples in this respect are large vessels and offshore petroleum production structures.
Various measuring principles of such monitoring systems have been suggested, i.e. methods based on vibration analysis, acoustic emission, ultrasonic systems, registration of magnetic fields as well as visual inspection. In general, these methods have not been adequate, mainly because of the great strains to which the monitoring equipment can be exposed, in particular under water. In addition to being resistant to such strains, it is also important that the monitoring systems do not entail prohibitive installation and maintenance costs, since the components or areas which are to be monitored often have very large dimensions. Systems that, for example, are based on the use of a number of transducers, e.g. for detection of vibration, acoustic emission or ultrasonic signals, may involve the mounting of a large number of transducers, each one representing a comparatively high expense and a risk of failure.
Another previously disclosed method for examination of cracks in structural parts or components is based on the measurement of the electric field that is produced in the structure. This so-called potential drop method is used for detailed examination of a crack that has been localized beforehand. The size or depth of the crack is determined by means of contact points on each side of the crack, between which the voltage is measured. The supplied current is either DC or AC with a low frequency. Different versions of the potential drop method have been published in German Patent Application No. 25 57 658 and in United Kingdom Patent Specification No. 804323. In the former publication, a high frequency current supply is used, and the examination comprises measurements of the drop of potential as a function of the frequency. In the latter case, which in particular has been reported to concern surface cracks, a radio frequency potential is measured, which potential occurs between two separate electrodes that are moved on the surface of the structure while an oscillating electric current is supplied thereto from a source of radio frequency. Such an arrangement with movable electrodes which are to be guided all over the monitoring area cannot, however, be used for the purpose mentioned above.
In the 1980's a refined version of the potential drop method was developed, by the inventor denoted “the fingerprint method”, as described in U.S. Pat. No. 4,656,595 (Hognestad).
According to this method electric current is supplied to a steel structure which is equipped with contact points between which are measured voltage drops caused by the impressed current. A relatively large number of fixed contact points are used all over the area which is to be monitored. The voltage drops are measured between selected pairs of contact points and these voltage drops are compared with corresponding voltage drops that have been measured previously in the same manner when the structure was in an initial condition, preferably without any defects. The monitoring can thus be performed by means of robust and simple devices which are relatively impervious to rough environments. Though this method provided a substantial improvement over the earlier techniques, it still did not provide an efficient means for early detection of spot damages to the structure or defects that occurred as mainly linear cracks in the direction of the current passing therethrough.