The detection principle is based on a new version of the potential drop method, which is a well established method for detailed examination of cracks in laboratory test specimens. In the application considered here, potential differences are measured between a number of fixed contact points distributed over the area to be monitored, i.e. at selected nodal points on an offshore steel structure.
A change in the potential difference reflects changes in the resistance distribution caused by the formation of a crack. Advanced measuring techniques and signal analysis make it possible to discover and locate cracks by measurements between a just few contact points. Tests have been performed in the laboratory and in sea water with good results. Typically a crack in a tubular weld, or in the heat affected zone, corresponding to a few percent loss of cross section area can be detected in joints of 25 mm wall thickness and diameters over one meter.
The current and voltage necessary to set up the field in a tubular joint of these dimensions are about 30 A and a few millivolts.
Further development and tests on full scale objects show interesting possibilities for monitoring localized areas on offshore structures.
Monitoring offshore steel structures for the detection of crack formation is important for safety as well as economic reasons. During several years considerable attention has been given to the development of improved methods.
This paper concerns a new method for crack detection based on electrical potential drop measurements, which involves sending an electric current through the test piece and measuring the potential difference between electrodes placed around the area to be monitored, ref. Figure 2. The same basic principle has previously been used for measuring the depth of known, existing cracks.(1)(2).
About 5 years ago, we began some simple laboratory experiments which showed that potential difference measurements could presumably also be used to detect unidentified cracks in large steelstructures.
Our experiments indicate unambiguously that cracks in a steel structure can be detected at an early stage. The measurements techniques have now been developed to the point where long term testing is undertaken on a steel structure in water.
This presentation is concerned mainly with the laboratory experiments, followed by some points of view concerning applications on offshore structures.
We have been working on potential difference measurements directed towards detecting cracks in a given area of a steel structure. The method we have arrived at has been named the Field Signature Method. This is to emphasize an important point. The electric field set up in the steel in its initial, undamaged condition is taken as a "fingerprint" or signature of the state of the structure in the area in question. Small deviations from this state, as result of cracks, or of material loss due to corrosion, lead to changes in the electric field pattern. These changes have to be measured and interpreted.
