Water & Wastewater Treatment

14 | january 2015 | WWT | www.wwtonline.co.uk Project focus: Pipes and pipelines assessments were carried out in each tunnel giving an inspection frequency of approximately 1 in 30, so we were able to gain a detailed picture of the current pipe condition. Having determined the wall thickness, it was a straightforward step to calculating the current pressure resistance capability. Getting access to the pipes in order to conduct such comprehensive assessments proved tricky. Complete circumferential access to the service tunnel pipes was hampered by the fact that they were mounted some 1.5m above the ground on concrete plinths, the twin pipes being accessed along a narrow walkway suspended between the supporting plinths. MWH devised an ingenious method of getting to the complete circumference by using a custom-made saddle that could be wrapped around the pipe and rotated by 50mm increments using a reference line at the springer of the pipe. The saddle incorporated a Velcro fastened pocket which housed the sensor and held it securely against the wall of the pipe during the sensor readings. Pipeline Analysis Having obtained a comprehensive set of wall thickness readings from pipes in the draw-off towers, the service tun- nels and the control houses (in total over 50 locations were assessed), the intended methodology was to use the wall thickness at installation, the cur- rent measured wall thickness, and the age of the pipelines, to determine the corrosion rate of the pipe walls. This required finding out the age of the pipelines and the standard minimum wall thickness at installation. The cast iron pipes were all stamped, the socket/spigot pipes as class B, and the flanged pipes as Class D. The pipe material was known (vertically pit cast iron) and date stamped 1938 and 1942 respectively. Reference to the applicable standard (BS78:1938) gave minimum installation wall thicknesses of 25.5mm and 34.9mm respectively. The pressure ratings of the Class B and D pipes were significantly in excess of the maximum static head of the reservoir, and as the pipes were under no external loading, the primary task was to assess the current wall thickness and confirm that sufficient thickness remained for pressure containment. It was then possible to estimate the remaining time to minimum allowable wall thickness. As the internal pressure was the crucial parameter in the remaining life methodology, we also undertook a comprehensive surge pressure analysis of the dam pipework, recommending the addition of air valves to guard against negative pressures in the service tunnel pipelines. A visual assessment of the 200 spigot/socket joints, and an analysis of remote manual methods for activating the bottom level guard valves in the event of an electrical failure completed the 'health check' of the pipework in the dam. The results of the assessment were generally comforting, but most importantly, Severn Trent were able to gain an accurate and complete picture of the condition of the asset. STW's Dams and Reservoirs Manager, Ian Hope concluded: "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 catastrophic pipeline failure." Similar assessments were carried out on the flanged stub pipes that extended through the draw tower walls just upstream of the guard valves, as these were seen as the points of critical failure. Any burst here might not be contained until the water level in the reservoir had dropped below the level of the draw-off pipe. The mid and high level draw-off pipes were accessed by narrow platforms which could only be got to by descending steep ladders from the top of the draw off towers. This required a full confined space risk assessment and meant that at all times our operatives were in full rescue harnesses, with a rescue team and top man in attendance at all times. It also required the BEM equipment to be winched down to the platforms and set up at each location prior to sensor readings being taken. Despite the difficulties in conducting the assessment it was crucial to assess these pipes so as to provide sufficient information for the client to be confident that there was still significant remaining life in these assets. www.teekaycouplings.com tel: +44 (0)1494 679500

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