State-of-the-Art in Leak Detection and Localisation

Many fluids transported by pipelines are hazardous. It is therefore often necessary to install leak detection (and localization) systems (LDS), especially due to legal regulations like • the “Code for Federal Regulations (CFR) Title 49 Part 195” [1], API 1130 2 Ed. [2], both for the USA, or • the “Technische Regeln für Fernleitungen” (TRFL) (Technical Rules for Pipelines) in Germany [5]. This paper gives an overview of methodologies, methods and techniques for leak detection and localization; [6] and [7] are two other interesting sources giving an overview. Some remarks concerning (legal) regulations both for the USA and for Germany will be shown in chapter 1. Chapter 2 summarizes the requirement to LDS considering • reliability, • sensitivity, • accuracy, and • robustness. These terms had been defined with respect to LDS within API 1155 [3], and will be explained in some detail within this chapter 2. External systems. External based LDS (due to API 1130 2 Ed.) use local leak sensors to generate a leak alarm. Acoustic emission detectors, fiber optical sensing cable, vapor sensing cable and liquid sensing cable based systems are shortly presented in chapter 3. Internal systems. Internal based LDS (also due to API 1130 2 Ed.) use normal field sensors (e.g. flowmeters) for leak detection and sometimes leak localization. A significant part of the paper (chapter 4) deals with these internally based systems like • balancing systems (line balance, volume balance, compensated mass balance etc.), • Real Time Transient Model LDS (RTTM-LDS), • pressure/flow monitoring, and • statistical analysis LDS. Different methods for leak localization (gradient intersection method, wave propagation analysis etc.) will be shown in chapter 5. Pipeline Technology 2006 Conference 2 Extended RTTM. The presentation of an Extended RTTM approach (E-RTTM) combining advantages of conventional RTTM LDS and statistical systems follows in chapter 6, together with the demonstration of applicability by means of two examples, • a liquid multi-batch pipeline, and • a gas pipeline. PipePatrol. The University of Applied Sciences in Gelsenkirchen and KROHNE Oil & Gas B.V. from the Netherlands (KOG) closely work together on the field of leak detection. The outstanding properties of the E-RTTM technology therefore motivates KOG to choose ERTTM for PipePatrol, the KOG leak detection and localization system. 1 Regulatory Framework Companies operating pipelines transporting hazardous fluids (e.g. liquids or gases) often have to consider a dedicated regulatory framework. Examples are • Code for Federal Regulations (CFR) Title 49 Part 195 [1] (USA) • API 1130 2 Ed. [2] (USA), and • “Technische Regeln für Fernleitungen” (TRFL) (Technical Rules for Pipelines) [5] (Germany). 1.1 API 1130 2 Edition (USA) The 2 Edition of API 1130 “Computational Pipeline Monitoring for Liquid Pipelines” was published from the American Petroleum Institute in November, 2002, [6]. Other regulations like the “Code for Federal Regulations (CFR) Title 49 Part 195” [1] refer to API 1130. The API 1130 focuses on the design, implementation, testing and operation of CPM systems; it is limited to single-phase liquid pipelines. It defines a CPM-system as an “algorithmic approach to detect hydraulic anomalies in pipeline operating parameters”. The technical overview section introduces to methodologies of CPM-systems, classifying them into • externally based leak detection systems, and • internally based CPM systems. Externally based systems. Externally based systems use local sensors, generating a leak alarm. System costs and complexity of installation usually are high; applications therefore are limited to special high-risk areas, e.g. near rivers or nature protection areas. Examples for such a type of LDS are acoustic emission detectors monitoring noise levels and location and vapor sensing cables, sensing gas or hydrocarbon vapor near a leak. Internally based systems. Internally based systems utilize field sensors (e.g. for flow, pressure and fluid temperature) to monitor internal pipeline parameters. These field signals are used for inferring a leak. The classical line balance method balancing inlet and outlet volume flow is an example. Other sections of the API 1130 cover topics like • field instrumentation, SCADA/Communication, data presentation, Pipeline Technology 2006 Conference 3 • operation, maintenance and testing, and • descriptions of types of internally based CPM systems. 1.2 TRFL (Germany) TRFL is the abbreviation of “Technische Regel für Fernleitungsanlagen” (Technical Rule for Pipeline Systems); it is the successor of the TRbF 301, “Technische Richtlinie für brennbare Flüssigkeiten” (Technical Guideline for flammable Liquids). The TRFL summarizes requirements for pipelines being subject of official regulations. It covers • pipelines transporting flammable liquids, • pipelines transporting liquids being dangerous for water, and • many pipelines transporting gas. Figure 1: Requirements concerning LDS according to TRFL. The TRFL is divided into the two parts “Operation” and “Constitution” of pipeline systems. Only a small part is concerned with leak detection and localization, focusing to the specification of measures necessary in order to detect and locate leaks. Five different LDS and LDS functions are required, see Figure 1: • Two independent LDS for continuously operating 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 stand-still operation. • One LDS for creeping leakages. • One LDS for fast leak localization. 1 It is partially possible to combine different LDS functions into one LDS device. Pipeline Technology 2006 Conference 4 The TRFL is focused to general requirements necessary to detect and locate leaks; it gives not much information about technical aspects like design or implementation of LDS. 2 Requirements to LDS API 1155 [3] provides a common framework to evaluate the performance of LDS. This simplifies the selection of appropriate LDS meeting the customer requirements. API 1155 defines four performance metrics: 2.1 Reliability Reliability (due to API 1155) is defined as a measure of the ability of the LDS to render accurate decisions about the possible existence of a leak on the pipeline, while operating within an envelope established by the LDS design. It follows that reliability is directly related to the probability • to detect a leak, given that a leak in fact exists, and • to incorrectly declare a leak given that no leak has occurred. A system is considered to be reliable if it consistently detects actual leaks without generating incorrect declarations. 2.2 Sensitivity Sensitivity (due to API 1155) is defined as a composite measure of the size of leak that a LDS is capable to detect, and the time required for the system to issue an alarm. Minimum detectable leak rate and leak detection time depend on each other. Smaller minimum leak detection rates require longer leak detection times, and larger minimum leak detection rates permit smaller leak detection times. The performance of a LDS will best be described using an Operational Characteristic Plot. Figure 2: Evaluating sensitivity using the operational characteristic plot. 0 5 10 15 20 25 30 0 0.5 1 1.5 2 Operational Characteristics Leak Detection Time (min) M in im um L ea k D et ec tio n R at e (% ) There are some important things to note: • For very long leak detection times, for both LDS, the minimum leak detection rate conPipeline Technology 2006 Conference 5 verges asymptotically to a minimum limit value, the smallest possible leak detection rate. This value mainly depends from the accuracy of the flowmeters and therefore is nearly independent from the LDS used. • If detection time decreases, the minimum leak detection rate increases for both LDS. LDS #1 shows a much better performance (smaller minimum leak detection rates) than LDS #2. 2.3 Accuracy LDS may provide additional leak information like leak location and leak rate. The validity of these leak parameter estimates constitutes another measure of performance referred to as accuracy. 2.4 Robustness Robustness (due to API 1155) is defined as a measure of the LDS ability to continue to operate and provide useful information, even under changing conditions of pipeline operation, or in condition where data is lost or suspect. A LDS is considered to be robust if it continues to function under such less then ideal conditions. Robust LDS typically are able to tolerate sensor failures using some kind of redundancy evaluation. 3 Externally Based Systems There are many possibilities to classify externally and internally based systems; we follow the API classification scheme [6]. Local leak sensors of externally based systems generate a leak alarm which e.g. can be evaluated by SCADA-systems. This kind of LDS is characterized by a very good sensitivity to leaks and is very accurate with respect to the leak localization. On the other side, system costs and complexity of installation usually are high; applications therefore are limited to special high-risk areas, e.g. near rivers or nature protection areas. 3.1 Acoustic emission detectors Escaping liquids creates an acoustic signal as it passes through a perforation in the pipe. Acoustic sensors affixed to the outside of the pipe monitor internal noise levels and location, creating a baseline acoustic “fingerprint” of the line. When a leak occurs, the resulting low frequency acoustic signal is detected and analyzed. Deviation from the baseline “fingerprint” would signal an alarm [8]. The received signal is stronger near the leak site thus enabling leak localization, see below. 3.2 Fiber optic sensing cables The fiber optic sensing leak detection method involves the installation of a fiber optic cable along the entire length of the pipeline. The substances to be measured come into contact with the cable in case of a leak occurrence, changing the temperature of the cable. The distributed fiber optical temperature sensing technique offers the possibility to measure temperature along the pipeline. Figure 3: Leak detection and localization using optical fiber. ©GESO GmbH, Jena Pipeline Technology 2006 Conference