In the offshore industry, once a new oil or gas pipeline is completed it has to go through a check known as a "hydro-test" prior to commissioning. The pipeline is sealed and filled with seawater, and the internal pressure is raised up to a test value and then subsequently monitored. If pressure is observed to be decreasing, it indicates that the pipeline has a leak somewhere along its length. This can prove incredibly costly for the pipeline construction company, and, in general, clients will not accept pipelines until a hydro-test has been successfully completed. If hydro-testing determines that leaks are present, it is then important to locate the leaks as soon as possible in order to start repairs. Problems arise when trying to locate holes that can be only a few millimetres in diameter somewhere along possibly hundreds of kilometres of pipeline. This is particularly difficult if, as is often the case, the pipeline is either partially or totally buried in sediment. Work of this nature may take a long time, and time is expensive- especially if one has barges, vessels and personnel offshore. Traditional techniques for leak inspection consist of filling the pipeline with a solution of water and a chemically or optically detectable compound. The pipeline is then followed by an ROV equipped with either a dedicated sensor or a video camera, as appropriate. During numerous offshore surveys (in which our equipment was often used in parallel with these conventional techniques), we observed a number of limitations with traditional leak detection systems. The efficiency of these systems is a function of the concentration of the detectable component in the seawater in the area of a leak. Even in the case of a large leak, this concentration can be dramatically reduced by ambient conditions such as current flow, pipeline burial or water visibility. Furthermore, in the case of very small leaks, the flow rate is often too low to reach the threshold concentration of the systems, even when used in suitable ambient conditions. Several years ago, Co.L.Mar. was requested by a major oil company to research an alternative solution for underwater pipeline leak detection. In order to avoid the limits of the optical and chemical systems, an acoustic solution was investigated. Acoustic propagation is not dependent on current or turbidity, and the acoustic signal generated by a leak can be very strong, even if the source is a leak of relatively small dimensions. Acoustic signals generated by pipeline leaks are detected by a system comprised of a hydrophone array, a preamplifier and a cable driver. The underwater unit may be used in a variety of ways, such as on a vessel-towed fish, an ROV installation or in handheld mode by a diver. This last method of operation is particularly suited to inspection around flanges, valves, etc. The signal is brought onboard via cable where it is initially preconditioned prior to acquisition by a PC. Data is immediately processed, displayed and digitally recorded. The software visualises in various ways the results of the analysis of the signal along the track, allowing evaluation in real time of the evolution of the acoustic content and, therefore, the rising of components related to the leakage presence.


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    Title :

    Acoustic leak detection for underwater oil and gas pipelines


    Contributors:

    Published in:

    Sea Technology ; 43 , 11 ; 39-44


    Publication date :

    2002


    Size :

    6 Seiten



    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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