Use of phased array ultrasonic equipment for fatigue crack characterization for underwater inspection of offshore structures

Non destructive tests are used on steel offshore structures for underwater inspection. These inspections 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 characterisation 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 rebound, creeping waves,....). The results are very promising and the system will now be tested for inspection and repair tasks.