Ultrasonic Based Technique to Measure Residual Stresses in Offshore Structures

The offshore construction environment can be very difficult to be accurately predicted, due to various uncertainties in the geotechnical elements, and highly probabilistic events at which ocean conditions govern the design. The integrity assessment and life extension activities towards end of field life is becoming very common in older fields surrounding Malaysia, and other areas such as the Arabian Gulf and the North Sea. The in-line measurement of the existing residual stresses on these assets is very critical for the remaining life assessments, and also to categorise assets for shutdown to undergo rehabilitation work. The novel idea of using ultrasonic waves is developed and explained in detail in this paper. This method is based on the measurement of the time of propagation of the critically refracted longitudinal wave components travelling along the subsurface region of the specimen. Numerical validation is backed up by experimental work to verify this method to be fit-for-purpose for the intended application, through the development of an acoustoelastic calibration curve for several sectional shape/sizes of carbon steel material. These tests show good correlation between the numerical and experimental work, and this has the potential to be a reliable tool for use in the offshore/marine engineering sector where shutdowns are expensive.