Post Rehabilitation Assessment of System Integrity and Effectiveness of Retro Fitted Cathodic Protection Using Long Term Acoustic Monitoring Data

摘要

The Great man Made River Project is one of the largest water projects in the world. It consists of 4000 Km (2485 miles) of mainly four meter diameter (158-inch) prestressed concrete cylinder pipe (PCCP) and can convey over 6 million cubic meters of water per day from well fields deep in the Sahara Desert to the populated areas on the coast. After experiencing failures on their pipeline between 1999 and 2001, the Great Man-Made River Authority (GMRA) conducted an aggressive rehabilitation program and implemented various technologies to assess the condition of remaining pipe sections. These failures were caused by chloride induced corrosion of the prestressing wire and affected only some of the pipes that were not coated with coal tar epoxy in Phase I of the project. These constitute only a small proportion of the projects pipes but affect an important section of the Phase I supply line. In parallel to the rehabilitation program, the Authority retrofitted 1000Km (620 miles) of the pipeline with a Cathodic Protection system to prevent future corrosion of the prestressing wire. Initially the pipeline was inspected using electromagnetic techniques to determine the extent of the problem. Once the rehabilitation program was underway acoustic monitoring was introduced to monitor sections of pipe that were under operation while other sections were being replaced or repaired. Site excavations of distressed pipes revealed that many of the adjacent pipes were also affected by corrosion that had still not resulted in broken wires. This was confirmed by acoustic monitoring which accurately detects wire breaks as they occur under operation. Increased demand also meant that the necessary dewatering of the pipeline to undertake electromagnetic inspections was no longer an option. Both these factors have led the Authority to rely primarily on acoustic monitoring during the operation of the pipeline. Developments in this technology has enabled the Authority to increase the scope of monitoring from 3Km (1.8 miles) in 2000 to current 800Km (500 miles) in 2010 with a projected 1200Km (746 miles) in 2012. Over the past ten years a wealth of monitoring data has been collected which has been an essential element of GMRA's Pipe Risk Management System. Initial analysis of this data in 2007 revealed there was a marked decrease in the rates of deterioration after the installation of cathodic protection. However it was not clear at that time if this reduction in deterioration rates would be maintained over a longer time span. Long term monitoring of a much larger section of the pipeline between early 2007 to the end of 2010 have enabled the Authority to make a more accurate assessment of the pipeline integrity. This paper will concentrate on the analysis of data over a ten year period to evaluate the effectiveness of the following; the initial rehabilitation program, the retrofitted cathodic protection system and the inspection and monitoring technologies used. Ultimately it will provide conclusions regarding the overall integrity of the system.