Flood member detection for real-time structural health monitoring of sub-sea structures of offshore steel oilrigs

A structural flood detection system for real-time health monitoring in the hollow sub-sea members of new offshore steel oilrigs is presented. Field-proven flood member detection techniques, integrated within the concept of health monitoring, offer an alternative to underwater nondestructive testing methods based on ultrasound and x-rays, which have been used to detect the presence of seawater in these applications, often with diverse or remote operating vehicles. The system employs a single piezoelectric transducer which can be permanently attached to the inner wall of every sub-sea structure and which is powered by a normally inert seawater battery. Upon activation, the sensor transmits ultrasonic chirp or tone encoded pulses, in the range of 21-42kHz, to a monitoring receiver system at deck level for decoding and identifying flooded members. Experiments have been carried out using a jointed steel pipe structure, 7m in length, 0.5m in diameter and 16 mm in thickness. This structure was flooded and completely immersed in seawater. Two approaches to the system were considered during the investigation, depending on the communication channel exploited; the former utilized guided waves, on the basis of exploiting the steel structure as a wave-guide; the latter employed underwater ultrasound, based on using the seawater as a propagation medium. Although severe losses were encountered in both approaches, the system effectively identified the signals above the background noise. This work forms the foundation for the future development of a system that can be used with large, commercial offshore platforms.