rate, should be examined, especially considering the distance of pumping. Generally, a plugged 

 line can only be corrected by cutting open the line to physically clean out the sediment that has 

 stopped flow. Cutting and cleaning out a plugged pipe is not uncommon, but it is not a minor 

 task. A complicating factor is identifying exactly where the pipe is plugged. Given the semi- 

 fixed alternative discussed here, without pressure gauges along the 2,700 m of pipe, the closest a 

 plug could be identified would be between two booster pumps (i.e., within the 640-m section of 

 pipe between booster pumps). Trial and error efforts to locate the plug location within a 640-m 

 section of pipe could be significant. 1 



A plug in the pipe where the pipe is inaccessible (i.e., under the inlet) would render the entire 

 system inoperable and may require specialized equipment or installation of another pipe crossing 

 the inlet. A plug in the pipe under the inlet is of particular concern because even if solids 

 production is low enough to minimize plugging from a system shutdown, the inlet crossing is the 

 low point in the entire system. If flow ceased for any reason, there would be a tendency for 

 slurry to travel "downhill" to the low point, which may create a plug under the inlet. Mr. Green 

 has suggested that to minimize the impact of a plug under the inlet, two or three pipelines should 

 be installed during initial construction with valves at the up- and downdrift side. Therefore, in 

 the event a plug occurs under the inlet, the system can still be functional by switching to another 

 pipe. Because the relative cost of additional pipe versus installation costs is much less, installing 

 redundant pipes under the inlet will save future pipe installation costs by providing a safety 

 fallback in the event of plugging, and will also extend the life of the pipe by allowing alternating 

 between the pipes. 



Water hammer is a phenomenon that could result from a near-instantaneous pipe blockage 

 resulting from a booster shutdown or blocking off of the suction intake. When the flow is 

 suddenly stopped at one location, a pressure wave traveling at the speed of sound travels in the 

 direction opposite of initial flow until it reaches the previous booster pump, elbow, valve, etc. 

 This pressure can be so great that pipes or even a pump casing can burst. Proper system design 

 and pressure release valves will reduce the possibility of this occurring. 



The line plugging and water hammer issues should be considered in all of the cases involving 

 hydraulic pumping— semi-fixed, Crawldog, Punaise, and mechanical fillet mining. Neither of 

 these problems are insurmountable, and in fact sand slurries are often hydraulically pumped 

 along comparable distances in the mineral processing and phosphate industries. Much could be 

 learned by the operators and designers of a bypassing project by examining pumping and pipeline 

 systems in these industries. 1 



The Crawldog is the second-most-expensive of the alternatives on an annual basis (the most 

 expensive being the 1-year cycle floating plant alternative with onshore placement), primarily 

 because of the requirement for new design and construction (i.e., different from the Crawlcai). 



'Personal communication, Mr. Johnny Green, Standard Gravel Company, Franklinton, LA, 1996. 



70 Chapter 5 Other Bypassing Studies 



