Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | january 2015 | 13 on either side of the dam. From here the water was pumped into regional aqueducts. All pipelines had double isolation in the form of twin inline valves within the draw-off towers. It was this arrangement of pipelines within the dam that presented the key areas for assessment. Tunnel flooding fear STW's principal concern was that a catastrophic pipe failure of any one of the four low-level pipelines within the below ground service tunnels, or at the base of the draw-off towers, would result in flooding of one of the two service tunnels. Whilst, as a contin- gency, the actuated low-level valves could be operated from a dual source located in the adjoining tunnel, the concern was that a serious leak would cause significant damage and could compromise the electricity supply in the locality of the leak. The obvious consequence of such an event would initially be an uncontrolled release from the reservoir. In order to deter- mine the likelihood of such an event occurring, MWH was asked to conduct a condition assessment survey of the pipework within the dam. Access constraints The two key constraints were how to obtain a comprehensive and robust assessment of the current pipe condi- tion, and how to get the necessary access to the pipelines in order to achieve this. MWH elected to use a broadband electro-magnetic (BEM) non- destructive testing (NDT) technique to determine the current condition of the pipework within the dam. BEM assessment involves coupling a sensor array to the side of the pipe and holding it there whilst an alternating current is passed through a transmitter coil at the surface of the pipe which generates an alternating magnetic field. This induces a voltage across the pipe, which creates eddy currents in the pipe wall, creating a secondary magnetic field. The wall thickness of the pipe is measured by detecting the attenuation and phase delay of an electromagnetic signal passing through the pipe wall. The BEM output gives an average wall thickness over a 50mm by 50mm square sensor. The total assessment area was the entire pipe circumference over an axial distance of 900mm. This meant that localized areas of wall thinning could be detected as well as maximum and minimum thicknesses. In all, 10 such • Perspectives Severn Trent Water Dams and reservoirs Manager, Ian Hope: "Although access to many of the sites was both difficult and hazardous, careful risk assessment, planning, and preparation allowed a large amount of data to be collected safely. This meant MWH were able to provide us with a robust estimate of remaining asset life and identify an action plan to militate against cata- strophic pipeline failure." MWH Pipelines Engineer adrian Davies-jordan: "The results of MWH's recent survey of the pipe work within STW's Lady- bower dam were gener- ally comforting. However, whatever the outcome, it is now accepted that such assessments will inevitably form a neces- sary part of managing one of the most critical assets types within the UK water industry." accessing the pipeline beneath the draw-off towers of the 70-year-old Ladybower Dam presented an unusual challenge • Innovations ● Access to the pipelines in the draw towers and the below floor areas of the control houses was the most challenging aspect of the project. ● Designing of a custom made 'saddle' to provide sensor access to the whole pipe circumference. ● Investigation of a remote, hydraulic valve closure method not sus- ceptible to power failure.

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