FOSA Installation Considerations
The main uses for distributed fiber optic sensing in monitoring pipelines are:
- Detecting unauthorized (or unexpected) third party interference in the vicinity of the pipeline
- Detecting pipeline product leaks (or valve operation), whether liquid, gas or a combination of liquid and gas
- Tracking the position of instrumentation and cleaning PIGs (Pipeline Inspection Gauges or Pipeline Intervention Gadgets)
- Detecting excessive strain being applied to the pipeline due to shifts in the soil caused by subsidence, landslides or other geotechnical reasons
Monitoring all four of these conditions is very important to pipeline operators. Third party interference, whether intentional or not, as well as excessive strain can lead to a potential pipeline leak, which needs to be reported to the pipeline operator as soon as possible. Tracking PIGs is important, as they can get stuck from time to time, and knowing the location of a stuck PIG can greatly speed up its remediation.
All three of the distributed fiber optic sensing technologies can be used in monitoring pipelines, as each provides unique insight into the operational characteristics and environmental conditions of the pipeline.
Distributed Acoustic Sensing (DAS) can sense the vibration characteristics of the pipeline, and can quickly detect unauthorized (or unexpected) 3rd party interference or intrusion by monitoring vibrations in the vicinity of the pipeline. DAS can go as far as to determine the potential cause of the vibrations, and therefor alert the pipeline operator of potential threats to the pipeline. DAS can also be used to detect the subtle vibrations very near to the pipeline that result from product escaping under pressure from the pipeline into surrounding soil (or into air if an above ground pipeline).
To improve leak detection performance, Distributed Temperature Sensing (DTS) is deployed to monitor the subtle temperature variations that occur from product escaping from the pipeline. Escaping high pressure gas will lower the surround temperature due to the sudden reduction in pressure as it leaves the pipeline, where oil typically increases the surrounding temperature, as it typically has a higher temperature than the surrounding soil.
Distributed Strain Sensing (DSS) can be deployed along or on the pipelines to monitor changes in strain that might be caused by shifts in the soil in the vicinity of the pipeline. If the strain from these soil shifts gets large enough, it could cause the pipeline to shift and possibly buckle and maybe even break, allowing product to escape. Where DAS can measure relative dynamic strain over shorter time period, DSS can measure progressive developing strain over long time periods.