Pipeline Monitoring Systems
This paper presents an overview of Pipeline Monitoring methods being used in the industry. Pipeline monitoring methods are broadly classified in terms of internal and external methods. Internal methods involve intrusive measurements to monitor fluid and pipe state/condition and while external methods are applied to the environmental condition of a pipe. Brief description of techniques used as well as their advantages and downsides are covered here.
Visual Inspection of Pipeline
The most widely used method for pipeline monitoring is visual inspection of the pipeline. Depending on terrain it may be a combination of on-the-ground walking, driving or an aerial monitoring.
Ground inspection is aimed at finding the evidence of vegetation color change near the pipeline or for a sound or visual evidence of the leak. To help surveillance crews in detecting a leak the odorant is often added to gaseous or fluid media being transported through the pipeline. The odorant can be detected by people or by special sniffing equipment. This method is quite economical to implement though it’s accuracy is hugely dependent on prevailing weather conditions. Winds and temperature extremes may disperse the smell. An atmospheric inversion may give an incorrect location of leak. The speed of ground inspection and routes accessibility often leaves much to be desired.
Aerial monitoring is helpful in detecting large scale leaks as well as third-party activity near the pipeline. Observation flights of the long scale pipeline systems are usually done on an irregular schedule. The frequency of the flights is on the order once per 30-45 days for most of the pipeline length. The probability of detection of the leak or a third-party unauthorized activity and security risk events is quite low due to infrequent coverage, human error in detection, limited data analysis capability, high mobilization costs and limited trend analysis.
Remote Sensing by Satellites
Disadvantages of the aerial and on-the-ground monitoring approaches can be partially addressed by means of satellite remote sensing. This technology is based on special hyper-spectral imaging tools set up on satellite platforms. Spatial resolution required for effective monitoring of the pipeline in terms of remote sensing is referred as high and very high – between 10 and 100 meters.
Imaging resolution and remote sensing specific requirements (invariability of light flux and inclination to monitored object) requires having remote sensing satellites operating on helio-synchronous orbit. It means that refresh time of a pipeline monitoring map is not less than 24 hours. Acquired data requires sophisticated post-processing performed by specialized companies. History of pipeline leaks shows that even 12-24 hours hydrocarbons leaks may result in a quite severe environmental damage and economical circumstances.
Approximate cost of a single pass over 120 km pipeline segment and further image processing with hydrocarbons leak analysis makes $30-50k. Thus annual budget associated with just a weekly satellite scans can make a figure comparable with the cost of permanently installed instrumentation systems which are discussed further in the paper.
Daily update intervals together with high cost of single scan of the pipeline makes satellite remote sensing unreasonable for prevention and detection of early leaks. However this technology is extremely useful in evaluation of the leak occurred as well as for monitoring large scale third party activities.
Flow Balance Monitoring
The Flow Balance method may identify presence of the leak within a segment of the pipeline. This basic method is required by industry standards in many countries. As it follows from the name the method it is based on the monitoring of amount of media (oil-gas products, etc.) pumped in pipeline segment and comparing it with the amount left the segment. A basic set of temperature, pressure and flow measurement system is required for the accurate monitoring. Despite an apparent simplicity there are few challenges with measurement of the flow. One of them is related to a change of the phase composition specially in case of pumping multiphase oil-gas products through the pipeline. Several factors including friction, pressure losses and variable environment temperature along the segment may drastically change the ratio between phases making it necessary to implement sophisticated instrumentation (at least multiphase flow meters) and models adjusted to a specific segment of the pipeline. Once again this method may tell operator that the leak present but cannot point out to exact location.
Pipeline Intervention
Pipeline intervention tools are not new and many proprietary systems exist. Intervention tools are distinguished by means of moving inside of the pipe. Self propelled systems as known as pipeline crawlers are usually tethered to the launch station and move by means of electrical motors. Tools without motors can be untethered however require steady pressure gradient in the flow to move inside if the pipe. This type of pipeline flow propelled intervention is called pipeline pigging.
Pipeline intervention tools are frequently used for pipeline commissioning – NDT, welding inspection, coating, etc., pipeline servicing - jetting, brushing, chemical dewaxing, and more recently for pipeline monitoring. This last type of intervention tools can carry a wide range of surveillance and monitoring equipment and can be used at regular intervals to check internal condition of the pipeline. The monitoring data acquired can be used to predict maintenance, cleaning or repairs required for a pipeline.
If a leak is detected by flow balance monitoring method the location can be found by using an intervention tool with acoustic receiving equipment on board. Every pipeline has a characteristic sound pattern determined by roughness, welds, pumping media, etc. Leak has very specific spectrum of acoustic noise which can be identified and distinguished from pipeline sound pattern. The location can be found from tracking information stored in untethered pigs or by cable length for the case of tethered crawlers.
The operating length of tethered crawlers is limited by cable length and thus may not be economical to have at every segment of the long pipeline. Untethered pigs do not have this drawback however they require retrieval at the end of inspected segment followed by download and analysis of acquired data during their trip. Segment of a 100 km length may be covered in 5-6 hours in the best case. During all this time one have to provide flow through the system thus creating a spill until pig is retrieved. Nevertheless intervention tools are helpful in monitoring condition of the pipeline, providing data for prediction models and detection of small leaks.
Pressure Wave Method
The acoustic methods are based on the recording and identifying of acoustic events associated with the pipe rupture and subsequent leak.
When pipe rupture occurs the negative pressure wave is produced. The pressure wave travels in pipeline fluid away from the point of rupture in both directions at acoustic velocity. The leak location can be identified by the arrival time of the wave at each pressure detection stations. The latter are installed on the pipeline and linked with a telemetry system. The pressure wave amplitude decays with a distance. It means that in order to maintain acceptable sensitivity levels, the pressure detection stations should be placed at regular intervals. The length of interval is determined by flow parameters and phase composition of the fluid since it greatly affects attenuation of the pressure wave (decay of the amplitude).
The Pressure Wave Monitoring method is simple, robust and relatively easy to implement given that the pipeline is equipped with telemetry system running along. Nevertheless it has few significant limitations. It can not detect slow leaks since it is required to have pronounced pressure wave usually occurred during sudden pipe wall rupture. Other limitation is related to a high attenuation and/or reflections of the pressure wave in a two-phase oil-gas mixtures.
Acoustic Emission
The fluid leaving a pipe rupture is accelerated by the pressure gradient between pressurized pipe and normal atmospheric pressure outside. The velocity of outflowing fluid is high enough to create turbulent flow in the vicinity of the leak. The acoustic noise generated by turbulent flow has a specific pattern which can be separated from a noise of the regular flow so the presence and location of the leak can be identified by “listening” to the pipeline noise.
The intensity of emitted acoustic noise is proportional to the turbulent flow velocity and increases with pipeline pressure, though the dependence is not linear. Leak noise is generally wideband ranging from hundreds of Hz to hundreds of kHz. Due to the lower attenuation of sound in a solid steel pipe than in a oil-gas fluids the sensors for Acoustic Emission would normally mounted on the outer surface of the pipe.
The Acoustic Emission method demonstrates good performance for detection of slow or minor leaks however there are several limiting factors for its application. The main problem is high rate of attenuation and low amplitude of acoustic emission signal. In order to locate a leak the system should have sensors spaced relatively close to each other (approx. every 100 meters). It means that the signal lines and communication to discreet acoustic sensors are going to be very complicated and expensive. Sensors are exposed to environment and should provide reliable operation at temperature and humidity extremes. The contact between sensor and pipe wall is subjected to deterioration with a time so the maintenance of the sensors array may be an issue. Despite advantages of the Acoustic Emission technique the factors mentioned above are limiting its use to a short pipelines segments.
Fibre Optic Distributed Sensing
The light scattering characteristics of optic fibres alter with temperature changes, mechanical stress and surface absorption of chemicals. These unique features allow designing various kind of sensors that can be efficiently applied in Pipeline Monitoring systems.
The fibre optic sensors systems can be categorized in two major classes – discrete sensor systems and continuous distributed sensing systems. The discrete fibre optic sensors should be placed with a regular intervals along the pipeline thus inheriting limitations of the acoustic emission or pressure wave systems.
The distributed sensing technique utilizes all length of the optical fibre cable as a sensor. The parameters of the optical fibre can be measured typically with 1-10 meters accuracy. There is no need for providing communication and power as in case of discrete sensors – the whole cable works as a measuring device. Recent fibre optics manufacturing technology makes available cables with extremely low attenuation of the light allowing reliable measurements over 50 km segments.
The basic pipeline leak detection system utilizing distributed fibre optic sensing technology relies on a concept of a changing of temperature distribution near the location of the leak. The temperature disturbance depends on the type of the pipeline and fluid being transported. For the buried pipelines the oil released is warmer than surrounding soil, while the gas produces local cooling effect. Sensitivity of the distributed temperature sensing systems is enough to track these temperature changes within minutes. The liquid hydrocarbons released form the pipeline also change thermal properties of the soil which influences day/night temperature cycles. It helps locating even minor leaks in the pipelines located on the ground.
Commercially available Distributed Vibration Sensing technology is effective in the prevention of oil spills and/or gas explosions due to its unique ability to detect, locate and classify vibrations caused by physical activity (such as third party interference), while simultaneously monitoring for early-stage leaks, along the entire length of the pipeline in real time. It is also capable of early detection and location of dynamic geological hazards, such as landslides, or excavating activity (by equipment or hand) anywhere within the vicinity of a pipeline, and in many cases prior to the excavating operation within the right-of-way.