Continuous monitoring guided wave encoded sensor for oil rig flooded member detection

Structural flooded member detection of offshore oil platforms involves the detection of seawater in their normally hollow steel crossbeam members. NDT methods such as ultrasound have been used to inspect for the presence of water in these applications, often in conjunction with remote operating vehicles. Alternatively, a guided wave sensor system is now being developed which can be permanently attached to a sub-sea installation and that can be powered by the action of the seawater. Upon activation, the transducer is able to transmit encoded information to monitoring systems at deck level. Present work has focused on the assessment of guided waves and their attenuation not only due to steel joints and distance but also due to leaky waves coupled into surrounded water. Low-frequency excitation exhibits lower attenuation depending on the selected mode, however this can also be exposed to acoustic interference leading to a poor SNR. A compromise between these factors has been considered in this application for the successful transmission and reception of encoded information. Low-power narrow-bandwidth chirp signals were employed with 21 kHz PZT transducers to excite axi-symmetric modes in tubes. Experiments have been carried out using excitations of 0.5 V and 5 V respectively for a water-loaded steel k-joints scale model and a cylindrical bridge steel structure with several k-joints. Important results have been attained, moreover in order to reduce acoustic noise a real-time digital signal processing board has been used to perform digital filtering and signal identification.