Water Journal : Water Journal May 2012
pipeline cleaning & maintenance refereed paper 68 MAY 2012 water technical features Abstract When condition assessment of a water transmission main is needed, implementing a cost-effective program means that not all methods need to be applied to all segments of the pipe; rather, the most appropriate inspection technology is applied when and where it is needed. Introduction Buried pipelines often operate in a state of anonymity with respect to their condition until deterioration is severe enough to cause failure. A condition assessment program will provide information to better plan for repair, rehabilitation or replacement, and avoid service outages. Typically, for large-diameter water transmission mains, we do not have the ability to take the pipes offline to conduct internal inspections. The more we can do externally to assess the condition of these pipes and keep them in service, the more cost effective condition assessment will be. Failure modes for the large-diameter pipe are not the same as for smaller distribution mains. Internal or external corrosion can affect both pipes, but when a small-diameter pipe develops a leak at joints or from pinhole corrosion, the consequence of failure is not as high as for a larger, deeper pipeline. Distribution mains typically carry less water, operate under lower pressures, and are located at shallower depths than transmission mains; they also serve a smaller service area. Leak detection systems are readily available to identify leak locations for these small-diameter systems. The approach for large-diameter transmission mains is more complex and is focused on an approach that starts with a desktop assessment, then external assessment, direct assessment and, finally, structural assessment. The progression from one level to the next is based on an analysis of the data collected at each step to determine if the more specialised technology needs to be implemented. Beyond the desktop assessment, the approach will vary according to the material and access to the pipe. Desktop Study A desktop study uses all existing information to determine if the pipe has potential exposure to internal or external corrosion. Some of the types of data to be analysed include: • GIS Data GIS layers and GIS maps including attribute data such as asset ID, coordinates, length/ dimensions, diameter/size, street location/address, age, testing and maintenance data, repair/rehabilitation data and elevation contours; • WAE Drawings Construction details such as section lengths, material specification, details of joints or cathodic protection (CP), slope, plan view showing location, valve/structure details, and specifics of coatings and lining systems; • Capacity/Hydraulic Data Needed to determine if the pipe is meeting design capacity requirements (if not, then it can be recommended for replacement at a larger size and condition data may not be required depending on when the upsize is needed); • Operating Pressures If available, operating and surge/transient pressure data determines which sections of pipe have a higher risk of failure; • Maintenance Records Provides understanding of how pipe has performed over time and for prioritising assets for assessment; • Cathodic Protection WAE drawings and details, specifications and testing histories to determine design and performance effectiveness; • Previous Test Results Any previous testing results such as potentials, closed-circuit television (CCTV), electromagnetic, leakage, pressure or others, used to assess corrosion rates; • Geotechnical Data For any location at or near the pipe alignment to determine soil type, groundwater elevations and provide resistivity data if performed; • Groundwater Data Existing data taken in conjunction with geotechnical borings or separate piezometer readings; • Repair/Rehabilitation/Replacement Records Provides a history of asset performance or portions not requiring inspection; and • Water Quality Data Parameters needed include temperature, pH, total alkalinity, conductivity and total hardness in order to construct scaling indices. Water quality data is used to evaluate internal corrosion potential from current and historic water quality using the Puckorius Scaling Index (PSI) or the Langelier Saturation Index (LSI). PSI is used as a predictive tool for calcium carbonate (cement mortar lining) scaling/corrosion and may only be used accurately for untreated water or with treatments (phosphonates and acrylates) that "solubilise" calcium carbonate. It is not as accurate for estimating calcium phosphate, calcium sulphate, silica or magnesium silicate scale or calcium carbonate scale creation in waters already treated with "crystal modifiers". Such treatments (polymaleates, sulphonated styrene/maleic anhydride, or terpolymers) cause foulants to precipitate, but do not result in a true scale. The Langelier Saturation Index (LSI) is used when the PSI cannot be used. AL Ratliff A review of current technology BURIED PIPELINES -- WE CAn’t IGnoRe tHem Figure 1. Emag testing.
Water Journal July 2012
Water Journal April 2012