OPERATIONS & MAINTENANCE
An example of a theft event generating 0.4 bar pressure drop only
methods is extremely beneficial for theft detection, with the use of offline analysis and further instrumentation. As every pipeline is different, a ‘one size fits all’ approach is not suitable, however: each technology has its advantages and can often be combined to provide an integrated approach. Furthermore, non-intrusive pressure sensors with remote radio and cellular communications and battery-powered data logging can provide additional accuracy to support GPS location and offline analysis.
THEFT DETECTION SOLUTIONS Avoiding detection is a key target when thieves are going to commence an operation to extract the product from a pipeline. Tis approach differs from pipeline leaks in several ways: a small amount of product is stolen (ranging from 10 to 3,000 litres); theft flow rate can be less than 0.1%; theft events last for less than one hour usually although occasionally a theft continues unchecked; and the changes in pressure are very small when the tapping point is opened/closed at the end of a long hosepipe. With these unique characteristics the
main requirements of theft detection are: sensitivity (detecting the small product withdrawal); accuracy (locating the tapping point as accurately as possible); and response time (detecting the product withdrawals as quickly as possible). Different leak detection technologies can be adapted to meet the above requirements. Te main theft detection
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options are negative pressure wave, statistical volume balance and theft service approach.
NEGATIVE PRESSURE WAVE Tis technology relies on high-speed analogue pressure sensor readings to identify whether a leak/theft has occurred on the pipeline. Te system acquires and analyses the pressure data at a frequency much higher than the typical five-second SCADA rate, capturing data at 60 samples a second. Specialised equipment is thus needed to acquire data at such high frequency. Te main advantages of this system
are: accurate leak location within metres of the actual location; short detection time for all leak sizes; and high sensitivity provided through the 60hz sample rate. Tese are the key features in effectively detecting theft events in all operational conditions.
STATISTICAL VOLUME BALANCE Tis type of leak detection technology relies on the pressure and flow measurements taken from a pipeline. It uses the existing instrumentation and connects via existing SCADA, PLC or remote terminal unit (RTU) systems. Tis system monitors the difference between the inlet and outlet flow corrected by the inventory change. Tis is also referred to as the “corrected flow difference” to determine whether the pipeline is in a leak condition. Te statistical hypothesis testing method is known as the sequential
probability ratio test (SPRT). It is applied to the corrected flow difference to decide if the probability of a leak has increased. Te main advantages of this system
are a low false alarm rate, the ability to detect leaks under steady-state, transient and shut-in conditions, and accurate leak size estimate. Leak location accuracy is improved through higher data sample rates. Since the theft rate is usually
below the flow meter accuracy and repeatability level, it is difficult for this technology to detect small thefts under running conditions unless false alarms are accepted. Te system includes an additional theft module for detecting thefts during shut-in conditions to maintain reliability for both leak and theft detection. Te figure on the following page shows an example of it working. Tefts are not a constant and can
fluctuate. Tapping points are often left for years. In the UK a recent tapping point was located that was likely installed as far back as 2015 and left dormant until earlier this year. When the volume of thefts along a pipeline reduce, it becomes necessary to lower the minimum leak size to be detected. However, in doing this, it can result in increased false alarms as the identified flow and pressure are mostly below the instrument repeatability and process noise level. An offline service can be offered to pipeline firms. Combining technology with an offline service can provide
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