Non destructive tests are used on steel offshore structures for underwater inspection. These mspectiol1s are carried out to guarantee the integrity of the structure, mainly for fatigue crack detection on welded tubular joints Despite its potential the use of ultrasonic equipment is not very common in this area and the detection and sizing of cracks is mainly performed by electromagnetic methods (Magnetic particle inspection, Eddy current, Alternative field measurement). Given this state of affairs and bearing in mind the poor capabilities of the electromagnetic methods for fatigue crack characterization, an evaluation of the capabilities of phased array ultrasonic equipment has been performed. This equipment allows, without specific motion of the ultrasound beam, by means of electronic scanning and focusing, the characterization of fatigue cracks on complex welded joints The inspection is performed with only one probe, this being placed on the external part of the welded tube. This paper presents the process of the evaluation of the technique, the choice of the probe (size, frequency, number of elements), the methods used (tip detection by rebomld, creeping waves, ….). The results are very promising and the system will now be tested for inspection and repair tasks.
It has been noted, by Dover (1995), that in the North Sea offshore infrastructure that the fatigue is the main cause of repair of the structures. The complex geometry of tubular joints results in high stress concentration located along the weld of the chord-brace connection. It is these areas where the fatigue cracks appear and grow. More than 90% are located at the chord toe. In order to guarantee the safety of the platform, inspection campaigns are scheduled and different Non destructive Techniques are used. These techniques have been evaluated within the ICON (InterCalibration of Offshore NDT) program by Ifremer (1996).