Leak detection
Pipeline leak detection is used to determine if a leak has occurred in systems which contain liquids and gases. Methods of detection include hydrostatic testing, tracer-gas leak testing, infrared, laser technology, and acoustic or sonar technologies. Some technologies are used only during initial pipeline installation and commissioning, while other technologies can be used for continuous monitoring during service.
Pipeline networks are a mode of transportation for oil, gases, and other fluid products. As a means of long-distance transport, pipelines have to fulfill high demands of safety, reliability and efficiency. If properly maintained, pipelines can last indefinitely without leaks. Some significant leaks that do occur are caused by damage from nearby excavation, but most leaks are caused by corrosion and equipment failure and incorrect operation. If a pipeline is not properly maintained, it can corrode, particularly at construction joints, low points where moisture collects, or locations with imperfections in the pipe. Other reasons for leaks include exterior force damage and natural forces.
Overview
The most common leak detection method for pipeline operators is called the Supervisory Control And Data Acquisition system. This system uses a series of sensors to track data such as pressure, flow rates, temperature, and whether valves are open or closed. The sensors relay the information to a control room where operators determine the legitimacy of the leak alarms. Some systems have added the Computational Pipeline Monitoring System, whose main task is to detect leaks. These systems have been reported by pipeline operators to the US Department of Transportation's Pipeline and Hazardous Materials Safety Administration to be inefficient in leak detection. Even with these in place, the SCADA system is reported to have detected only 19% of leaks, and the CPM system only detecting 10% of leaks.The primary purpose of leak detection systems is to help pipeline controllers to detect and localize leaks. LDS provide alarms and display other related data to the pipeline controllers to assist decision-making. Pipeline leak detection systems can also enhance productivity and system reliability thanks to reduced downtime and inspection time.
According to the API document "RP 1130", LDS are divided into internally based LDS and externally based LDS. Internally based systems use field instrumentation to monitor internal pipeline parameters. Externally based systems use a different, independent set of field instrumentation to monitor external pipeline parameters.
Rules and regulations
Some countries formally regulate pipeline operation.API RP 1130 "Computational Pipeline Monitoring for Liquids" (US)
This recommended practice focuses on the design, implementation, testing and operation of LDS that use an algorithmic approach. The purpose of this recommended practice is to assist the Pipeline Operator in identifying issues relevant to the selection, implementation, testing, and operation of an LDS.TRFL (Germany)
TRFL is the abbreviation for "Technische Regel für Fernleitungsanlagen". The TRFL summarizes requirements for pipelines being subject of official regulations. It covers pipelines transporting flammable liquids, pipelines transporting liquids that are dangerous for water, and most of the pipelines transporting gas. Five different kinds of LDS or LDS functions are required:- Two independent LDS for continuous leak detection during steady-state operation. One of these systems or an additional one must also be able to detect leaks during transient operation, e.g. during start-up of the pipeline
- One LDS for leak detection during shut-in operation
- One LDS for creeping leaks
- One LDS for fast leak location
Requirements
- Sensitivity: An LDS must ensure that the loss of fluid as a result of a leak is as small as possible. This places two requirements on the system: it must detect small leaks, and it must detect them quickly.
- Reliability: The user must be able to trust the LDS. This means that it must correctly report any real alarms, but it is equally important that it does not generate false alarms.
- Accuracy: Some LDS are able to calculate leak flow and leak location. This must be done accurately.
- Robustness: The LDS should continue to operate in non-ideal circumstances. For example, in case of a transducer failure, the system should detect the failure and continue to operate.
Steady-state and transient conditions
if the pressure at inlet or outlet changes, and when a batch changes, or when multiple products are in the pipeline. Gas pipelines are almost always in transient conditions, because gases are very compressible. Even in liquid pipelines, transient effects cannot be disregarded most of the time. LDS should allow for detection of leaks for both conditions to provide leak detection during the entire operating time of the pipeline.
Internally based LDS
Internally based systems use field instrumentation to monitor internal pipeline parameters which are used to detect possible leaks. System cost and complexity of internally based LDS are moderate because they use existing field instrumentation. This kind of LDS is used for standard safety requirements.Pressure/flow monitoring
A leak changes the hydraulics of the pipeline, and therefore changes the pressure or flow readings after some time. Local monitoring of pressure or flow at only one point can therefore provide simple leak detection. As it is done locally it requires in principle no telemetry. It is only useful in steady-state conditions, however, and its ability to deal with gas pipelines is limited.Acoustic pressure waves
The acoustic pressure wave method analyses the rarefaction waves produced when a leak occurs. When a pipeline wall breakdown occurs, fluid or gas escapes in the form of a high velocity jet. This produces negative pressure waves which propagate in both directions within the pipeline and can be detected and analyzed. The operating principles of the method are based on the very important characteristic of pressure waves to travel over long distances at the speed of sound guided by the pipeline walls. The amplitude of a pressure wave increases with the leak size. A complex mathematical algorithm analyzes data from pressure sensors and is able in a matter of seconds to point to the location of the leakage with accuracy less than 50 m. Experimental data has shown the method's ability to detect leaks less than 3mm in diameter and operate with the lowest false alarm rate in the industry – less than 1 false alarm per year.However, the method is unable to detect an ongoing leak after the initial event: after the pipeline wall breakdown, the initial pressure waves subside and no subsequent pressure waves are generated. Therefore, if the system fails to detect the leak, the system will not detect the ongoing leak.
Balancing methods
These methods base on the principle of conservation of mass. In the steady state, the mass flow entering a leak-free pipeline will balance the mass flow leaving it; any drop in mass leaving the pipeline indicates a leak. Balancing methods measure and using flowmeters and finally compute the imbalance which is an estimate of the unknown, true leak flow. Comparing this imbalance against a leak alarm threshold generates an alarm if this monitored imbalance. Enhanced balancing methods additionally take into account the change rate of the mass inventory of the pipeline. Names that are used for enhanced line balancing techniques are volume balance, modified volume balance, and compensated mass balance.State-observer-based methods
These methods are based on state observers which are designed from fluid mathematical models expressed in state-space representation.These methods can be classified into two types: infinite-dimensional observers and finite-dimensional observers. The first type is based on a couple of quasi-linear hyperbolic partial differential equations: a momentum and a continuity equations that represent the fluid dynamics in a pipeline. The finite-dimensional observers are constructed from a lumped version of the momentum and a continuity equations.
Several types of observers have been used for leak detection, for instance Kalman filters, high gain observers, sliding mode observers
and Luenberger-type observers.
Statistical methods
Statistical LDS use statistical methods to analyse pressure/flow at only one point or the imbalance in order to detect a leak. This leads to the opportunity to optimise the leak decision if some statistical assumptions hold. A common approach is the use of the hypothesis test procedureThis is a classical detection problem, and there are various solutions known from statistics.