CASE STUDY: OFFSHORE PIPELINE
Client: BP Exploration
Project: 24″ North Sea export gas line
Task: Checking valve status
Figure 1: Comparison of acoustic response when SSIV was Open and Closed. The figure shows that the signals recorded with the valve open and then closed are similar until approximately 2.56 seconds, when they begin to differ considerably. This corresponds to the time it has taken for the acoustic pulse to travel from the platform to the closed valve and back.
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 suggested 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 analysing 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.