An axisymmetric guided wave encoded system for flood detection of oil rig cross-beams

A novel and completely autonomous guided wave system for flood detection in the hollow cross-beam members of offshore steel oil rigs is presented. Underwater non-destructive testing methods such as ultrasound have been used to inspect for the presence of seawater in these applications, often in conjunction with remote operating vehicles. Alternatively, a monolithic PZT guided wave transducer which can be permanently attached to a sub-sea installation and that can be powered by the action of the seawater is now being developed. Upon activation, the transducer transmits an ultrasound-encoded signal to a receiver, in the form of a real-time digital signal processing system at the surface level. Experiments have been carried out using a jointed steel pipe structure, 10 m in length, 0.5 m in diameter and 16 mm in thickness, completely immersed in seawater. The transmitter was attached to the inner wall of a spur pipe and configured to generate narrow bandwidth, low frequency ultrasonic chirp signals, coupled to the pipe as an axisymmetric mode. Results confirmed that although some attenuation occurs, the system signal processing system successfully identified the signals above the background noise.