Continuous Conducting Guided Wave Encoded Sensor For Flood Searching of Oil Rigs Cross Beams |
Structural flooded member searching of offshore oil platformsinvolves the searching of seawater in their normally hollow steel crossbeammembers. NDT methods such as ultrasound have been used to inspect for thepresence of water in these applications, often in conjunction with remoteoperating vehicles. Alternatively, a guided wave sensor system is now beingdeveloped which can be permanently attached to a sub-sea installation and thatcan be powered by the action of the seawater. Upon activation the transducer isable to transmit encoded information to conducting systems at deck level.Present work has focused on the assessment of guided waves and theirattenuation not only due to steel joints and distance but also due to leakywaves coupled into surrounded water. Low frequency excitation exhibits lowerattenuation depending on the selected mode, however also can be exposed toacoustic interference leading to a poor SNR. A compromise between these factorshas been considered in this application for the successful transmission andreception of encoded information. Low power narrowbandwidth chirp signals were employed with 21 kHz PZT transducers to exciteaxi-symmetric modes in tubes. Experiments have been carried out usingexcitations of 0.5V and 5V respectively for water loaded steel k-joints scalemodel of 4mX0.18mX0.007m and a cylindrical bridge steel structure of 53m longand several k-joints of 0.5m and 0.4m diameters. Important results have beenattained, moreover in order to reduce acoustic noise a real time digital signalprocessing board has been used to perform digital filtering and signalidentification.