Understanding Subsea Acoustic Leak Detection and Condition Monitoring – Part 1
As more and more equipment is placed on the seafloor, especially in deep waters but also in shallow waters, concerns grow over potential hydrocarbon leaks from trees, manifolds, pumps, pipelines, flowlines, risers and valves. It’s no small challenge to have a reliable subsea leak detection system that can monitor the large array of subsea systems used in modern deepwater fields and on top of that there is the need to monitor the working condition of all this equipment and others, such as ESP’s and other pumps, which are rotating equipment.
Acoustic emissions are the stress waves produced by the sudden internal stress redistribution of materials caused by changes in the internal structure. These changes can cause the formation of cracks, misalignment of shafts or fluids escaping through holes. Acoustic emission monitoring is a non-destructive testing (NDT) technique, which applies sensors to detect changes in acoustic emissions and then correlates the signals received with known failure or activity characteristics. The signals to be processed by the sensors can relate to certain features and patterns of the acoustic emission such as peak levels and energies, while some systems use complex machine learning algorithms to analyze the waveforms received.
Active acoustic emission was the first development in the form of sonar, where sound waves are used for depth measurement and also for the detection of other subsea vessels. Subsea leak detection systems using passive acoustic emission consist of a sensor assembly, which is installed by a ROV on a designated location on a subsea structure. The systems are composed of a data processing system and an electronics module which contains all the electronics required for power and communication. The sensor system is usually installed on a template, manifold, satellite or other selected subsea equipment.
Data processing can be done subsea or topside, depending on the communication method and depth. All data collected, including raw data is available for the lifetime of the system, which can be around 25 years with no maintenance, when top-of-the-line sensors, such as the Naxys A10 are used. Operators visualize the data topside through a graphical interface. This flexible control system allows the operator to up-load raw data, configure the system and download any new software.
The sensor is composed of an array of hydrophones, configured to discriminate the noise of a leak from other sources of sound, including common ocean sounds. A hydrophone is a microphone specifically designed for listening to underwater sound. Most hydrophones use piezo-electric transducers, which convert the acoustic pressure waves into electrical signals. Hydrophones are arranged in a carefully selected configuration in order to obtain directional information, both horizontally and vertically and the latest leak detection systems like the aforementioned Naxys A10 and others such as the Asel-Tech system may have a detection range of up to a 500m radius from the sensor location.