Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/891178
18 AGGREGATES MANAGER / November 2017 W hen it comes to successful deepwater dredging, the devil is in the details. The cost and size of vessels have grown exponen- tially, making it imperative to work out the process conditions and requirements – such as how far material will be pumped – before proceeding, says Jos Clement, director of technical services for CEDA Dredging & Fluid Management. Just about 15 years ago, dredging 25 to 30 feet deep was sufficient, Clement explains. Gradually, the dredging depth requirement grew to 50 to 60 feet. Now, dredging at the 70- to 100-foot deep mark isn't extraordinary. "With the increase in water caps in tailings ponds, we are having to build equipment to get down below 100 feet," Clement says, adding that his operation began dredging at deeper depths about a year and a half ago. "Some of the tailings are getting very deep. With tailings and reclamation work, you can't expand the geographic footprint in a horizontal frame anymore, because getting permits to expand is diffi- cult. Deposits are deep so you have to go deep." Before digging deeper, Clement says producers need to understand what they are working with and how deep they need to go. A dredge designed for extracting material at 40 feet isn't adequate for dredging at 60 or 70 feet. "The deeper you go, the more conditions change," Clement says. "Once you go beyond 40 feet deep, Mother Nature doesn't give much assistance into a dredge pump. Below the 50-foot mark, you need to have some form of 'jet assist' or a submerged ladder-mounted pump." Clement cautions that a jet assist may provide additional material pick up capability, but it will dilute the slurry and actually reduce the overall dredge efficiency. A ladder-mounted submerged pump is more efficient. "When you have a ladder pump and you are excavating below 40 feet, the primary item of concern will be the actual ability of the cutter and suction arrangement on the dredge to acquire the desired material. "At deeper depths, the ability to get the material into the pump becomes the critical factor for dredge production," Clement says. D esigning a dredge to mine material in deepwater is both "challenging and fun" – especially when intended to get to depths of 150 to 200 feet, says Bob Wetta, president and CEO of DSC Dredge. From a design aspect, safety is very important. This can be challenging, because the dredge needs to accommodate several things without compromising the stability of the dredge." Achieving this can be difficult, because dredges are designed where the dredge ladder – a.k.a. the boom – experiences a big shift in the center of gravity because the weight is moving the entire time it is going up or down. "On the long boom, you have deflection in the ladder, so it will sag and bend," Wetta says. "You have to design around these structural members bending and make sure not to fatigue the beam and jeopardize its useful life. If you start noticing a difference in stability, your flotation tank may have taken on water," Wetta points out. "You shouldn't have to watch for that on a properly built dredge." Finite element analysis software can be used with the dredge structures to run through worst-case scenarios to tweak as needed during the design process. However, it doesn't mean the barge can be put in the water and forgotten about. "There is vibration when mining underwater," Wetta says. "If you think you'll run it for years without inspecting bolts, etc., you'll start seeing things fall off." Understanding what to inspect on a dredge is all part of the process – a big part – of successful dredging, especially in deep water applications. "Whether dredge operators are new or pros, mining deeper is different, and dredges are custom pieces of equipment," Wetta says. "Auto- mation can take a lot out of the hands of operators, but you still need to know how to check and operate the equipment and make sure bad habits aren't developed." T he quality of material being dredged can change as the depth increases, making it essential to conduct test boring and look at the history of an area, explains Bob Woodington, owner and manager of Phoenix Pinelands Corp. "You want to keep going if you are getting valuable material," he says. "However, you can go from dredging at 90 feet to 125 feet, but it may not be the material you want, so you have wasted money doing it. Digging deeper takes more horsepower to produce material." Although initial testing may have been done, Woodington suggests that producers continually sample the material as it is being pro- duced when beginning to dredge at a new site. "You should be sampling the product all day long as it's produced," he says. "If the product is staying consistent in the first couple days, you don't have to sample the material every three to four hours." Plotting at a deeper depth helps determine whether dredging deeper makes sense. Depending on what the producer is looking for, such as gravel or coarse sand, the material has to justify digging that deep. Woodington also notes that having a computer-generated screen that identifies where and how deep dredging has occurred is very useful to monitoring dredge operations, especially in a deepwater environment. Sometimes, Woodington says he will see something on the monitoring system while in his office that he didn't notice in the field because he was busy focusing on something else. "If you're unsure if you dredged an area, you can look back on the computer," Woodington says. "You want to go to one area to dredge, be efficient, and be done with it. In this business, sand in the water is revenue. If you leave sand behind, you're leaving money behind." Voices of Experience J. (Jos) Clement Bob Wetta Bob Woodington