D352408X012 - Best Practices in Leak and Theft Detection.indd

In the Oil & Gas pipeline leak detection market today, there are several software based methodologies available – ranging from simplistic pressure wave methods, to real-time model-based volume balance techniques. All approaches have inherent advantages and disadvantages, defi nable in terms of simplicity of implementation/maintenance needs, reliability, cost, robustness, availability, and accuracy (minimum leak size and location). It is a fact that no one methodology can lay claim to be able to satisfy all the advantages over all operating regimes and so the choice of a leak or theft detection system must be based on a clear user defi ned set of objectives. This white paper aims to bring some clarity to the various leak and/or theft detection methodologies available today. It specifi cally highlights the fundamentals for achieving the best leak detection system and for each method; it considers the advantages, the disadvantages, and the applicability of that method for particular pipeline characteristics with particular hydraulic behaviour. Introduction The ability to detect leaks – accidental or planned (theft) – on liquid and gas pipelines continues to be a subject for debate and the focus on leak detection has recently intensifi ed. This is a result of high profi le litigation, the increasing frequency of product theft and the continual drive for improved safety and environmental protection. This paper focuses on software based leak and theft detection as one of the detection technologies with the best cost-benefi t ratio when based on instrumentation and data acquisition already required for operation of the pipeline. Best Practices in Leak and Theft Detection Midstream Oil and Gas Solutions 2 www.EmersonProcess.com/Remote April 2016 Emerson have been working in the area of leak and theft detection and location since the 1970’s and have built an extensive level of experience through the implementation of over 300 of its live installations and resulting in a combination of good and bad experiences. It is fi rmly believed by the company that practical experience of the real life implementation of leak and theft detection systems is the only true qualifi cation for being able to provide an accurate assessment for what can be achieved with the technologies available today. There is a wealth of material available in today’s market that claims an independent consideration of available technology but all too often this material culminates in the form of biased hypothetical reports supported only with theoretical assessment or tests under ideal conditions. The fundamental requirement for a leak or theft detection system is that it is able to detect leaks with limited false alarms. Too many false alarms and the system will be rendered practically useless. The system must also be able to cope with all operating conditions with minimal desensitization of the leak detection capability. Minimal desensitization is actually the diffi cult part. Unfortunately, many leak detection system vendors will claim that their particular solution can cope with all operating conditions without any loss of performance and this ultimately leads to disillusionment with the industry when this can not be maintained. Users frequently want a system installed with minimal interference and without the need to provide signifi cant amounts of data, they want high sensitivity, no false alarms, operation without attention and no continuing maintenance costs. This should also be across all operating conditions. The industry has been too willing to promise all the good features and avoid mentioning the pitfalls, to the extent that there is skepticism about the existence of practical leak detection systems in the market. Emerson are acutely aware of this problem and will continue to actively take steps to correct it. The following describes the step-by-step process for ensuring that a leak detection system has its greatest chance of achieving satisfactory performance. The Challenge of Leak and Theft Detection and Location Let’s start by outlining some typical relations between leak size and location – whether accidental or intentional (theft). A typical expectation of a leak detection system is to detect a leak of 1% of the normal fl ow rate and locate it within say one hundred meters but what does that actually mean. In approximate terms, a leak of 1% of the current fl ow rate changes the pressure drop in the pipeline by anything between 0 and 2% depending on the location of the leak, and creates a fl ow imbalance of approximately 1% of the fl ow rate. Compared to typically specifi ed measurement accuracy’s of 0.1%, it seems to be a fairly simple task to detect the changes due to the leak effect. However, the stated changes shall now be compared to the following typical operational variations and uncertainties: a. a fl ow change of 0.5% creates a change in pressure drop change of 1% b. a density uncertainty of 1% also creates a pressure drop uncertainty of 1% c. a pump easily generates pressure oscillations in the order of 3-5% of the pressure drop d. a tank change easily creates a pressure change of the same magnitude In terms of leak location it shall be remembered that moving the 1% leak 10% of the distance between pressure measurements, only creates a change in pressure drop of less than 0.1%. In other words, even if the impact of the leak is signifi cant compared to the meter accuracy it is often very small compared to even small operational changes and uncertainties. For leak location it is especially critical as even very small measurement errors or other uncertainties can destroy the leak location capability if not managed properly. Even under ideal conditions, it can be extremely diffi cult to locate small leaks within a 100 meters, as this will require a maximum distance between pressure instruments of 5 -10 km and a sampling rate and a maximum time skew of 0.1 second.