Acoustek® has been used to solve a number of industrial problems and the technology is now commercially available through Circor for surveying pipelines in the upstream Oil & Gas industry and through Phoenix Inspection Systems for surveying shell and tube heat exchangers. Two case studies involving the deployment of the Acoustek technology are described below:
To date, Acoustek® has been successfully deployed for a number of the world's leading energy companies across three continents. Each of these tests have progressed the technology to where we are today.
In this study, Acoustek® was used to check the status of a subsea isolation valve (SSIV) on a 24" export gas line in the North Sea area. The pipeline, operated by BP Exploration, spanned 24 km between two platforms and was pressurized to 70 bar during the survey. At the time of the survey, it could not be verified whether the SSIV was opening and closing properly and so platform operations had been suspended. To determine the status of the valve, the acoustic response of the pipeline was measured when the SSIV was believed to be closed and then after it had been given the instruction to open.
Figure 1 compares the acoustic response of the pipeline when the valve was nominally in the open and closed positions. Although only a single acoustic pulse was injected into the pipeline, this pulse will reverberate around the local pipe network. Consequently, the signal which propagates along the pipeline will inevitably contain a broad range of frequency components. When the SSIV is open, this signal gradually attenuates but, when the SSIV is closed, significant changes can be seen, especially after a time lapse of approximately 2.56 seconds. Using an approximate speed of sound, obtained using industrial standards, 2.56 seconds corresponds to the acoustic signal travelling a total distance of 1006 m. This indicated distance is in good agreement with the distance to the SSIV (and back), which was estimated to be 990 m. Further examination of the two acoustic responses suggests that, when the valve was in its closed position, the acoustic reflection was consistent with that from a complete blockage of the pipeline.
There are several reasons for the discrepancy between distances to the valve in this study. Firstly, the standards used to estimate the speed of sound require precise knowledge of the average temperature in the pipeline and the composition of the gas, both of which were only known approximately. In practice, the accuracy can be improved by analyzing the reflection from a known source in the pipeline to determine the actual speed of sound in the gas. This has been shown to improve the accuracy and enables blockages to be located with an accuracy of ±1m over a distance of 1 km.
In the example described above, the raw data collected from the microphone gave clear information regarding the pipeline. However, the high levels of noise associated with pumping and compression equipment means that the raw data may not always provide clear information when there is flow in the pipe. In a second test, again undertaken on a subsea pipeline, Acoustek® was used to locate an obstruction in a pipeline. In this particular study, it was necessary to use Acoustek® because parts of the pipeline were buried in a rock dump, meaning access to the pipeline was restricted, such that no alternative techniques could be used to survey the pipeline with the detail required. In this study, the field was still producing at the time of the survey and there was considerable background noise.
The upper graph in Figure 2 shows the signal recorded by the microphone for one acoustic pulse test. The high levels of noise in this signal make it very difficult to identify any subtle reflections emanating from the obstruction in the pipeline. The lower graph in Figure 2, however, shows the response after it has been filtered using digital filter technology. This filter is able to reject the high and low frequency noise that is present in the signal and enables the reflections from an obstruction located approximately 450 m away from the platform to be clearly identified. 1st, 2nd and 3rd reflections can be seen from the obstruction as the acoustic signal passes back and forth between the platform and obstruction. Excavation of the pipeline after these tests identified the obstruction in the precise location determined by Acoustek® .
Here we present the results of a validation test performed at the Petrofac Montrose Training facility. The pipeline was 10" in diameter and was 1km in length.
The figure below shows a schematic of the pipeline. The pipe is a loop that consists of two pig traps (labelled “T1” and “T2” respectively) and two motorized valves (“M1” and “M2”) with 1 km of pipe in between them. There are also two drainage valves (“X” and “Y”), each separated from the main pipe by short 2" diameter branches.
The resulting signal from firing the gas-gun at location T2 is shown in the figure below. Each feature identified in the signal has been annotated. The locations of each of the features are as follows: 133m (A), 162m (B), 210m (C), 280m (D), 414m (E), 545m (F), 721m (G) and 920m (H). These locations are also marked on the pipe schematic above. The minor blockages (A-G) detected are thought to be pools of water left in the undulations of the pipeline when it was drained. The large feature at H shows a complete blockage, caused by a slug of water.