Acoustics technique improves accuracy of subsea leak detection
Luigi Barbagelata
Co.L.Mar.
Co.L.Mar, based in La Spezia, specializes in underwater acoustics which, since 1998, have been applied to underwater leak detection for use on pipelines, risers, flanges and other structures. This has driven the company to conceive, realize and patent the ALD (Acoustic Leak Detector) technology.
The sudden expansion of a pressurized fluid (gas or liquid) passing through a leak and subsequent turbulence generates an acoustic signal. ALD is designed to acquire and process the acoustic data in order to extract this leakage signal from the ambient noise, even in a very noisy environment.
The system’s main components are:
- An underwater sensor to acquire the acoustic data along the pipeline – different sensors are provided according to the operational mode, whether towed fish, vertical deployment, diver- or ROV-installed
- An umbilical conveying the data back to the surface support vessel
- A receiving unit, in which data are pre-conditioned, acquired by a PC, processed, digitally displayed and recorded
- Proprietary software that visualizes the results of the signal analysis along the track. This allows evaluation in real time of the signal and its development, and therefore of the component(s) associated with the leakage. The software are used to analyze the data from a statistical, energetic and spectral viewpoint.
System refinements
Over the past 12 years, Co.L.Mar. has accumulated a wide range of experiences working in offshore environments including strong currents, zero visibility, shallow and ultra deep water, and buried pipelines.
The company has come to the conclusion that acoustics is the most reliable tool to detect leaks. Other techniques such as dye detection or hydrocarbon detection rely mainly on the chemical or optical detection of a dispersed substance. However, their effectiveness is often impacted by current, turbidity, and burial depth – this is not the case with ALD, where the acoustic signal propagates consistently well in seawater.
In some circumstances, however, it can be appropriate to use our equipment in parallel with other technologies, and for this reason parts of the ALD system are equipped with a free channel to host a non-acoustic sensor.
Over the years, Co.L.Mar. has developed and refined the system to increase its sensitivity and its capacity to detect small leaks even in a very noisy environment. Also, the company has optimized the technology to suit four inspection techniques: ROV mounted, towed, diver handheld and vertical.
For ROV inspection systems, the ALD sensor is installed on the vehicle which follows the pipeline or riser at a speed of around 1 knot. The sensor can also be towed in a “fish” configuration behind the vessel along the pipeline track at speeds of up to 6 knots. In a vertical deployment, the sensor is lowered from the vessel on a line which is maintained in a vertical position by ballasts – in this case, the sensor’s position is monitored by an underwater positioning system. The newest development is ALD-DIVER, a hand-held instrument which requires no connection to the surface.
To date, the ALD system has had a 100% success rate, leading to deployment for many of the bigger players in the offshore sector, including Eni, Exxonmobil, Total, Shell, BP, Fugro, Saipem, and J. Ray McDermott. Most of the campaigns have been on a very urgent base, typically related to the hydrotest failure of a new pipeline or an oil spill. Lately, Co.L.Mar. has performed more frequent inspections to verify the integrity of an operating pipeline.
Acoustic signature of a leak.
Routine pipeline inspections, normally performed by an ROV, could be improved by adding to the vehicle a leak detection system. The company believes this is a coming trend, and it is are working to optimize our service in this direction.
Leak monitoring is another important issue these days. Again, acoustics is the most appropriate technology, and some of the ALD signal processing techniques can be applied to permanent sensors monitoring the critical point of underwater installation.
Offshore Articles Archives
View Oil and Gas Articles on PennEnergy.com
