Hughes, Gregory; (1992) The interaction of ultrasonic waves with surface defects beneath layers of marine deposit. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The detection and characterisation of fatigue cracks in offshore structures is a primary concern of offshore operators. These structures accumulate various layers of marine deposit. Soft marine growth and marine organisms are relatively easy to clean from submerged tubulars. Cathodic protection systems are often used to prevent the corrosion of steel structures in seawater. The deposition of a cathodic scale by a cathodic protection system obscures any surface defects (especially fatigue cracks), and is very difficult and costly to remove prior to either visual inspection or sensor based inspection. Ultrasonic leaky Rayleigh waves propagating on a fluid loaded solid surface are one of the major contrast mechanisms found in acoustic microscopy, where such investigations are typically carried out using high frequency ultrasound (50 MHz and above). This study considers low frequency (0.5 to 5 MHz) ultrasonic characterisation of surface defects beneath layers, where the defect depth is similar in size to the wavelength of the interrogating ultrasound. The interaction of leaky Rayleigh waves with surface defects in steel in the presence of an obscuring calcareous layer is examined both experimentally and using a mathematical model. A finite difference mathematical model is used to examine the influence of different types of layer, depths of layer and inspection scenarios on detection and characterisation of defects. It was necessary to produce a substitute for calcareous deposit for use in experiments to overcome the difficulty of obtaining real samples. Experimental work using the substitute layer clearly indicates that a surface defect (a slot) can be reliably detected when inspecting from a cleaned strip adjacent to the region of the defect. This novel approach could greatly reduce the cost of fatigue crack inspection offshore.

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