Sabarna Roy, Senior Vice President, Business Development, Applications Technology, Engineering and Strategy Department, Electrosteel Group, Kolkata
Pipelines are widely used for the transportation of water over millions of miles all over the world. The structures of the pipelines are designed to withstand several environmental loading conditions to ensure safe and reliable distribution from point of production to the shore or distribution depot. However, leaks in pipeline networks are one of the major causes of innumerable losses in pipeline operators and nature. Incidents of pipeline failure can result in serious ecological disasters, human casualties and financial loss. In order to avoid such menace and maintain safe and reliable pipeline infrastructure, substantial research efforts have been devoted to implementing pipeline leak detection and localisation using different approaches. This paper discusses pipeline leakage detection technologies and summarises the state-of-the-art achievements.
The major causes of pipelines failure are pipeline corrosion, human negligence, defects during the process of installation and erection work, and flaws occurring during the manufacturing process and external factors.
Pipeline leakage detection methods
- Acoustic sensing
- Fibre optic sensing
- Vapour sampling
- Infrared thermography
- Ground penetration
- Electromechanical impedance
- Capacitive sensing
Interior/computational based methods
- Negative pressure
- Pressure point analysis
- Digital signal
- Dynamic modelling
- State estimators
Exterior based leak detection methods
Exterior methods mainly involve the use of specific sensing devices to monitor the external part of the pipelines. The operational principle, strengths and weaknesses of these methods are discussed in the subsequent sections.
Acoustic emission sensors
When a pipeline leak occurs, it generates elastic waves in the frequency range up to 1 MHz due to high-pressure fluid escaping from the perforated point that allows one to detect pipeline leakage incidents. The time lag between the acoustic signals sensed by two sensors is employed to identify the leakage position.
Accelerometers are another type of vibro-acoustic measuring device that are also useful to monitor low-frequency pipe-shell vibrations. The use of both accelerometers and hydrophones for monitoring pipelines was proposed.
Fibre optic method
This method involves installation of fibre optic sensors along the exterior of the pipeline. The sensors can be installed as a distributed or point sensor to extensively detect the variety of physical and chemical properties of hydrocarbon spillage along the pipelines. The operation principle of this method is that cable temperature will change when pipeline leakage occurs and hydrocarbon fluid engross into the coating cable. By measuring the temperature variations in fibre optic cable anomalies along the pipeline can be detected.
Vapour sampling method
Vapour sampling is generally used to determine the degree of hydrocarbon vapour in the pipeline environment. Though, it is applicable in gas storage tank systems, it is also suitable to determine gas discharges into the environment surrounding the pipeline.
Pipeline leakage detection systems based on the infrared thermography (IRT) mechanism are also applicable for the detection of pipelines leakages. IRT is an infrared image-based technique that can detect temperature changes in the pipeline environment using infrared cameras which shows the infrared range of 900-1400 nm. The image captured using an IR thermography camera is referred to as a thermogram.
Ground penetration radar
The emergence of ground penetration radar (GPR) is considered as an environmental tool which is valuable to detect and identify physical structures such as buried pipelines, water concentrations and landfill debris in the ground. GPR is a non-invasive high resolution instrument which utilises electromagnetic wave propagation and scattering techniques to detect alterations in the magnetic and electrical properties of soil in the pipeline surrounding.
Fluorescence methods for hydrocarbon spill detection employ light sources of a specific wavelength for molecule excitation in the targeted substance to a higher energy level. The detection of the spill is based upon the proportionality between the amount of hydrocarbon fluid discharged and rate of light emitted at a different wavelength which can then be picked up for detection of occurrence of hydrocarbon spillage.
In this technique the change in the dielectric constant of the medium surrounding the sensor is measured to identify existence of hydrocarbon spillage. The capacitive sensor is a local coverage point sensor which is generally employed in subsea pipelines. Capacitive sensor has been introduced to the market for environmental monitoring. Besides, buoyancy effects may carry the leaking medium away from the sensor vicinity which can be overcome by installing a collector for hydrocarbon spills over the monitoring structure.
Electromechanical impedance-based methods
In electromechanical impedance (EMI)-based techniques a variation in structural mechanical impedance instigated by the incidence of pipeline damage is monitored to detect the occurrence of pipeline failure. In the event of pipeline defects, the EMI employs high-frequency structure excitation (usually greater than 30 kHz) through a surface-bounded piezoelectric sensor to sense variations in structural point impedance.
Interior or computational methods utilise internal fluid measurement instruments to monitor parameters associated with fluid flow in pipelines. Details of each of these techniques are discussed in the subsequent sections.
The mass-volume balance approach for leak detection is straightforward. Its operation is based on the principle of mass conservation. The principle states that a fluid that enters the pipe section remains inside the pipe until it exits from the pipeline section. In a normal cylindrical pipeline network, the inflow and outflow fluid can be metered. In the absence of leakage, the assumption is that the inflow and outflow measured at the two ends of the pipeline section must be balanced, so a discrepancy between the measured mass-volume flows at the two ends of the pipeline indicates the presence of a leakage.
Negative pressure wave
(NPWs) are based on the principle that when a leakage occurs, it causes a pressure alteration as well as a decrease in flow speed which results in an instantaneous pressure drop and speed variation along the pipeline. As the instantaneous pressure drop occurs, it generates a negative pressure wave at the leak position and propagates the wave with a certain speed towards the upstream and downstream ends of the pipe. The wave contains leakage information which can be estimated through visual inspection and signal analysis.
Pressure point analysis
(PPA) method is a leak detection technique based upon the statistical properties of measured pressures at different points along the pipeline. The leakage is determined through the comparison of the measured values against the running statistical trend of the previous measurements.
Digital signal processing
In this process, the extracted information such as amplitudes, wavelet transform coefficients and others frequency response is employed to determine leakage events.
This process is gaining considerable attention as they appear to be a promising technique for the detection of anomalies in both surface and subsea pipeline networks. In this approach, mathematical models are formulated to represent the operation of a pipeline system based on physics principles.
State estimators/Observers method
State estimator or observer is a method that is based on dynamical modelling of flow process in the pipeline to estimate or observe variations in the variables associated with the fluid flow and indicate the occurrence of fault as a result of pipe damage.
Research gaps and open issues
Variations in physical parameters of the pipeline operation such as vibration, temperature, pressure etc. are expected to be detectable and communicated to reveal the incidences of anomalies. Leaks can only be accurately detected if the incident is within the vicinity of the monitoring sensor and thus the accuracy of leak detection systems becomes questionable if the leaks are not within the receptive fields of the sensors. However, the development of simple but realistic models for analysis and optimization still remains as a challenging research questions.
Based on the study, it can be concluded that each technique has some merits and drawbacks. Most of the interior methods are sensitive to small leakages, especially if the point of leakage is close to the sensing device, but they are more prone to false alarms as they can easily be affected by environmental noise. We observe that despite having invested a considerable amount of research effort in pipeline leak detection and localisation systems, various gaps must still be filled before a reliable real-time leakage detection in pipelines can be fully achieved. w