Pipeline leak detection using in-situ soil temperature and strain measurements

Abstract

This project investigated whether by measuring temperature and strain changes in the ground around a pipeline a leak can be detected using fibre optic instrumentation. The concept entails installation of an optic fibre along the length of the pipeline in the pipe trench when a new pipe is installed. Alternatively, the possibility also exists to retrofit such a leakage detection system by burying it near (above) existing pipes, although this has not been investigated in this project. The success of such a leakage detection system is based on the hypothesis that the temperature differential between the water in a pipeline and the ground around the pipe will result in a detectable temperature change in the ground when a leak occurs. Softening of the pipe support due to leaking water should result in strain changes in the soil immediately around the pipe and in the pipe itself, both of which should be detectable. All three of these quantities, i.e. ground temperature, ground strain and pipe strain can be measured using fibre optic technology. This study is based on detecting temperature and strain changes using fibre Bragg grating sensors (FBGS) at discrete locations along the length of a pipe. The success of a system based on temperature measurement implies that temperature changes caused by a leak should be distinguishable from naturally occurring temperature cycles. Installations were therefore conducted to measure ground temperature changes to a depth of 3m over the course of a year and comparing those to temperature changes measured in active water mains over the same period in Pretoria. A temperature differential that always exceeded 2°C was recorded, indicating that the system has potential to provide a means of leak detection. A laboratory study was carried out to observe temperature changes associated with an advancing wetting plume caused by a simulated leak using thermistors buried in fine sand. Depending on the magnitude of the soil-water temperature differential, a rapid drop in temperature was observed at monitoring locations during the progression of the wetting plume (the water temperature is typically lower than the soil temperature). However, an initial shortduration increase (spike) in temperature was consistently observed at measurement locations upon first passage of the wetting front. It is hypothesised that this spike is caused by the release of free surface energy upon wetting of the soil. As expected, immediately following this spike after the passage of the wetting front, a significant and rapid change in temperature was noted during the tests. The temperature reduction is dependent on the temperature differential between the water and the surrounding soil and illustrated the potential of the proposed method of leak detection (i.e. detecting leaks by observing a reduction in ground temperature). After the laboratory phase, a field study was conducted during which a 110 mm diameter 12 m long uPVC pipe was installed with sensor arrays consisting of both temperature and total strain FBGS. Thermistors were used as temperature sensors as in the case of the laboratory investigation. Strain sensors used were discrete optical strain gauges or fibre Bragg grating sensors (FBGS). Fifty percent of the FBGS were epoxied to the pipe to measure pipe strains, while the remaining 50% were free-floating, situated in a thin oil-filled plastic tube buried in the corner of the pipe trench. The purpose of the epoxied FBGS was to measure pipe strain changes, while the purpose of the free-floating FGBS was to detect temperature-induce strain changes. During a leak, strain changes were recorded in the free-floating FBGS several times exceeding the values expected based on the temperature changes measured by the thermistors. In addition, very significant pipe strain changes were observed. These observations indicate that significant ground and pipe strains occur due to wetting and subsequent softening of the soil caused by the leak. The change in total strain and temperature observed during a leakage event provided strong evidence that both parameters can be used to effectively indicate the presence of a leakage event. A complication identified in the study is that network pressure fluctuations result in significant pipe strains which would complicate leak identification. It is therefore recommended that the leakage detection system should comprise of an optic fibre separated from, but in close proximity to the pipe. This study should be followed up to investigate the performance of a leakage detection system based on distributed strain measurement and the potential of retrofitting the proposed detection system to existing pipelines.