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originai pipe they purchased. Their dredging jobs are relatively small $0.5 to $1 million! and consist of maintenance dredging of mud and fine sands. The dredge sizes ranged from 12 to 18 inches in diameter. Every respondent agreed that in the proper application, their HDPE pipe outlasted steel, for some by as much as five times. Some found that HDPE was slightly less expensive than steel, and its ease of handling made mobilization and demobilization faster and less expensive than steel, and its ease of handling made mobilization and demobilization faster and less expensive, Some felt that contracts have been won because of these reduced costs. All agreed that the lightweight fusible nature of HDPE pipe is an economic blessing. Working lengths of 500 to 1,500 feet of butt-fusion-joined HDPE are easily floated to the project site. As the pipe is filled with slurry, it sinks to the bottom and conforms to the contours of the terrain. Ease of repair was also cited as a favorable feature. Damaged sections of pipe can be removed with a chainsaw and repaired with a circle clamp or a butt-fusion joint. The need for elbows is reduced by the flexible property of the material which not only allows the pipe to conform to the terrain but allows bends of 20 to 40 pipe diameters, Fewer elbows and fused joints provide improved hydraulic efficiency which requires less pumping power. Many dredging practices, such as pipe rotation, are common to all situations; however, some practices are specific for HDPE pipe. None of the users employ HDPE pipe in the suction line of the dredge, and ail use steel immediately following thedredge pump. The rigidnature of steel andits higherworking pressure are important in these locations, and this practice is based on intuition, tradition, and experience. The length of steel used before switching to HDPE varied from as little as 40 feet to as much as 4,000 feet. Again this is based on individual preference. Depending on the job, it may be cost effective to rent rather than buy the butt fusion equipment, Pipe sections can also be joined using circle clamps, which should be 1.5 pipe diameters wide to distribute pressure and not deform the HDPE pipe. The material is rugged, and nicks and gouges on the outside have not necessarily resulted in pipe failure; however, there is a need to teach field personnel to be careful when handling HDPE pipe with heavy equipment. As was seen during the field test reported in this paper, it is helpful to know the type of material thatistobedredged.Sharpmaterialsuchasoystershellscanbedredgedfora limitedtimeh;owever,dredging of this material for more than 24 hours frequently results in tears and leaks. Coral and sharp volcanic sands are not suited to HOPE pipe. The light weight of the pipe can create problems when anchoring submerged HDPE pipe in water deeper thanabout15feetorwithcurrentsgreaterthanabout4 knots,Iftheslurrycanbemaintainedat30percent solids,the pipemayremainsafelysubmerged.AlthoughHDPEpipemanualsexplainproperanchoring procedures, several operators choose to use steel in these situations. Conversations with one HDPE pipe manufacturer's engineer revealed that overpressuring a pipe is acceptable in some instances, but the wear life will be significantly reduced. Manufacturer's tests of pipe under given pressure and slurry velocity conditions produce a predicted wear life of 50 years. Changing the pressure and/orslurryvelocitywillcorrespondinglychangethisvalue.Therearevalidapplicatonsof HDPEpipewhere wear life of the pipe will be reducedbut will still be sufficientand economicalfor the project requirements, Properlysizedpipefor theworkingpressuresandvelocitiesis alwayspreferred.OverpressuringHDPEpipe doesnotchangethematerialdensity;theshorterlifespanisa functionofthelong-termcreeppropertyofthe material.This long-termcreep relaxation!resultsin a slowthinningof the pipe wall, which lowersthe pipe pressurerating. HDPE materialhas the ability to "recover"from overpressuringif certain limits are not exceeded.A manufacturer'sengineerisalwaysavailabletoprovideguidanceinthisarea,andanswerany questionsconcerningtheuseof hisproduct.Ingeneral,if qualitypipeis purchased,andthe manufacturer's suggestionsandrecommendationfsor usearefollowed,theprojectshouldproceedwithoutHDPEproblems. No productor procedureis withoutsomefailures,andthe use of HDPEpipe in dredgingis no exception. Mostfailurescan be evaluatedandthe resultsaddedto the poolof HOPEpipe information.One maintenance dredgingprojectreportedverydisappointinrgesultsalmostimmediatelyafterInstallingHDPEpipe,The recommendehdigh-pressurpeipewasnotpurchaseda,nd12-inchnominals,izelowpressure0-psi!,40- foot-longsectionsflangedonbothendswereused.Themateriadlredgedconsistedofangular1/4-to 1/2-inch diametesrand,andgravewlithsome6-inch-diametreorcksA. workinpgressureof60psiwasmaintainewdith a pumpingvelocityof15fpsanda 12-percenstolidsconcentrationT.hefirstfailurewaslocated2 to3 feetfrom theflangedendof a pipesectionandoccurredwithin3 days.Atfirst,thedredgerwassurprisedatthe smoothnesosftheslurryflow;butwithin24hourshewasawareofa dramaticincreaseofpiperesistance. Whenthepipefailed,theinteriorwallswereexaminedandfoundtobebadlytomwithshredsof materialPDF Image | Dredging Applications of High Density Polyethylene Pipe
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