Developments In Sub-Sea Leak and Condition Monitoring


As sub-sea exploration and production becomes increasingly important globally, engineers and technicians are faced with challenges in the marinization of evermore complex machinery and in sub-sea monitoring of pipelines and plant. These challenges have led to development of new solutions and even new technologies and this article looks at some recent advances in sub-sea leak detection and plant condition monitoring.

Introduction
 
Oil and gas exploration and production relies on the use of miles of pipeline and on associated plant, ranging from pumps and valves to compressors and turbines. Leak detection of surface pipelines is often merely a matter of visual inspection, although different technologies are applied to locate leakage points. This is especially the case with buried pipelines.  Typical techniques are those that rely on the measurement of flow and pressure, say at pipe inlet and outlet, while success has been gained with infra-red leak detection and fibre optic technology.

Plant condition monitoring is also well-established for surface- or platform- located machinery and equipment. Condition monitoring of rotating equipment predominantly uses sensors which detect out-of-alignment running of shafts, by means of eddy current or seismic/vibration technology.

Unfortunately, those techniques which perform so admirably on land or on platform do not readily transfer to the sub-sea environment. Fortunately, there is one technology which does.

Acoustic Emission

Acoustic emission is a non-destructive testing (NDT) technique which is successfully applied throughout industry for condition monitoring of rotating plant.  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, typically, be the initiation of cracks, misalignment of shafts or fluids escaping through holes.

Acoustic emission monitoring relies on applying sensors to detect changes in acoustic emissions and then correlating the signals received with known failure or activity characteristics. Traditionally, the signals to be processed 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 analyse the waveforms received.

Acoustic Emission Underwater

Acoustic emission is a communication and navigation technique which is employed by marine mammals, such as whales and dolphins. It was identified in the 15th Century by Isaac Newton and has been used to great effect by man ever since the invention of submarines. 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 sub-sea vessels.

Fig 1. An acoustic emission monitoring system is lowered into the sea

Naval military needs were largely responsible for the development of passive emission monitoring, as systems and techniques were created which purely listened for sub-sea sounds. These were the sounds emanating from propellers and other equipment, as well as sub-sea fluid releases. During the Cold War, passive acoustic emission techniques achieved great sophistication, as the signature sound of submarine classes and even individual submarines could be identified. This technology has now been transferred to the commercial sector and is being used for sub-sea leak detection and condition monitoring of sub-sea machinery.

Passive Acoustic Emission Monitoring in the Oil & Gas Sector

Essentially, a sub-sea leak detection system using passive acoustic emission consist of a sensor assembly, which is located on a designated location on the sub-sea structure, a data processing system and a sub-sea electronics module which contains all the electronics required for power and communication. The sensor system is installed on the required location, which can be template, a manifold, a satellite etc), by means of a Remotely Operated Vehicle (ROV).  Data processing can be done either sub-sea or topside, depending on the communication and all data, including raw data is available for the lifetime of the system, which can be around 25 years with no maintenance, when the best sensors are used. Data is made available to the operator topside via a graphical user interface. The operator also has the ability to up-load raw data, configure the system and download any new software.

Fig. 2 A graphical display of wide area leak detection

The sensor assembly is an array of hydrophones, configured to discriminate the noise of a leak from other sources of sound. 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 will have a detection range of up to 500m from the sensor location.

Multi-Domain Condition Monitoring

By introducing additional hydrophones, specially designed for detecting sound from rotating machinery, sub-sea processes, structural integrity etc, it is possible to combine acoustic condition monitoring with acoustic leak detection. This works in much the same way as surface-based acoustic emission monitoring and can be used to monitor rotating machinery to check speed tracking and resonant frequencies. It can also monitor rotational defects of sub-sea plant such as bearing damage, out-of-balance, mechanical breakdown and reduced efficiency. It can identify structural vibration and detect irregularities  and defects in valves and chokes.

Multi-domain condition monitoring can be further extended by employing sensors that measure underwater electric potential or stray alternating electric field, emanating from sub-sea electronics equipment, machinery and cables. When used in combination with acoustic condition monitoring, these can be used to determine the electrical condition of sub-sea machinery and plant in detecting electrical faults, establishing electrical signatures for sub-sea machinery, monitoring electrical power and obtaining very precise measurements of conditions such as pump slip ratios.

Fig. 3 A graphical user interface for pump condition monitoring

Signal Processing and Data Display

Today’s signal processing software is specially designed to meet specific purposes, ranging from acoustic leak detection to acoustic pump monitoring. Data processing itself can be performed sub-sea inside special purpose electronics canisters, manufactured from high grade titanium and comprehensively sealed against sea-water ingress. Alternatively, data processing is carried out topside on a dedicated computer.

Data display is facilitated and supported by user-friendly software. For example, in the case of a leakage, an alarm is issued if a set of leakage parameters is fulfilled.  The leakage status is then communicated to the electronics module and then routed to the topside computer for display. Trend value plots enable the monitoring of leakage development. A range of software solutions provide graphical user interfaces for all monitored situations.

The Importance of Initial Configuration To Minimise False Calls

The initial configuration of any leak and/or condition monitoring system is absolutely vital to the system’s success. Sub-sea installations typically have a range of different noise levels and structural obstructions can cause reflections and attenuations. If a system is not configured correctly, then false alarms can occur, leading to unnecessary maintenance activity and perhaps even shutdown. Fortunately, today’s monitoring software features a number of parameters which are adjusted during commissioning to meet the requirements of specific sub-sea installations. These adjustments also need to be carried out in the event of major operational changes to the sub-sea installation.

Conclusions

Passive acoustic emission has now proved itself to be a robust technology for sub-sea leak detection and condition monitoring. With numerous installations globally, multi-domain condition monitoring is helping to meet the challenges of ensuring the safety and continuing productivity of ageing assets, as well as providing an essential ancillary tool as sub-sea drilling explores ever greater depths.