FACTORS THAT AFFECT DREDGE PRODUCTION
This is a must know for all Dredge owners, manufacturers, specialist, consultants, masters, project planners, managers, etc
In one of my recent article I talked about the ‘importance of obtaining proper information before carrying out a dredging project’. Unanticipated subsurface conditions encountered during dredging are by far the largest source of dredging related claims for additional payment by contractors and cost overruns. It is pertinent to state that many dredge contractors have really made a mistake of under/over quoting for dredge projects and later find it difficult to meet up with ‘milestones’.
These inability to meet up milestones are sometimes due to improper site investigation to ascertain the basic factors that enhance dredge production. The inability of reading dredge pump curves as also been a major issue; a client once contacted me that his dredge pump was rated 4,000m3/hr but they could barely pump 500m3/hr . . . hence the need to hire dredge specialist. (http://bit.ly/2hG26b4). The impeller must be properly adjusted to reduce the gap between the impeller and the front liner. If the gap is too great, the lack of pressure generating capability of the pump causes the slurry to recirculate. Production falls and pump wear increases. By adjusting the pump impeller to reduce the gap to 1/16-in. at a maximum, pressure is restored, recirculation decreases and production increases.
In this article, I will briefly discuss the parameters that affect dredge solid production. It is assumed here that the dredge is in a good state and all component are working efficiently. The five (5) major factors that affect dredge production are;
- Material characteristics
- Discharge pipeline length
- Terminal elevation
- Digging depth
- Dredge pump location on the dredge
- Material characteristics
Dredge performance is a function of the slurry specific gravity. The higher the specific gravity, the higher the production. sand is commonly divided into five sub-categories based on size: very fine sand ( 1⁄16–1⁄8 mm diameter), fine sand ( 1⁄8 mm — 1⁄4 mm), medium sand ( 1⁄4 mm — 1⁄2 mm), coarse sand ( 1⁄2 mm — 1 mm), and very coarse sand (1 mm — 2 mm).
- Discharge pipeline length
As a dredge moves farther away from the discharge point, the pump speed is increased to maintain a proper slurry velocity within the pipeline. Once maximum pump speed has been achieved, further increasing the discharge distance causes the cost-per-ton of material moved to rise, while production declines
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Turbulence within a pipeline is the product of velocity and friction. And turbulence is the key — because that is what keeps material suspended so that it can travel in the pipeline. If a pipeline is too long, the increased friction causes the slurry’s velocity to decrease. Because of this decrease in velocity, turbulence also decreases, meaning the maximum percentage of solids that can be safely transported in the pipeline also decreases. Since the slurry now has a greater chance of becoming too dense, it increases the possibility of pipeline plugging and, at the same time, causes a higher wear-rate on the pump because of poor pump efficiency and increased recirculation of solids within the pump itself. At the same time, the dredge is producing less material to pay for wear parts.
- Digging depth
At deeper depths it takes more energy to overcome two things; one, lifting the material from the deeper depth up to the pump, and two, the increased friction due to a longer suction pipe. As long as this energy — expressed in feet for suction head losses — is less than about 30 ft (9 m) (1 atmosphere), the negative impact on production is very minimal.
- Dredge pump location on the dredge
In general, a dredge with a hull mounted pump will produce less material as the digging depth increases. At deeper depths it takes more energy to overcome two things; one, lifting the material from the deeper depth up to the pump, and two, the increased friction due to a longer suction pipe. As long as this energy — expressed in feet for suction head losses — is less than about 30 ft (9 m) (1 atmosphere), the negative impact on production is very minimal. When the digging depth increases to where the suction head losses exceed 30 ft, then the negative impact on production becomes more pronounced; i.e. less production with every added foot of digging depth.
The available energy/pressure for a hull pump is only one atmosphere. For a submerged pump the available energy/pressure is the one atmosphere plus the pressure provided by the depth of water above the pump inlet. This is basically why submersible pumps are better.
Other factors that may affect dredge production include atmospheric pressure, design and condition of the dredge and supporting equipment, skill of management and operation staff.
Udemezue C. Daniel is an energetic and result-driven individual. He holds BSc. in Geology, a Certificate in Environmental Impact Assessment, Diploma in Computer Application Packages, a Certified International Project Manager ((License No. IPMP/2012/BAC3110)), Certified Project Director and Certified Quality Health, Safety and Environment (HSE Level 1–3). His experience covers both mining operations and also reclamation, river maintenance, sweeping, channeling and stockpiling type of dredging. He has good troubleshooting ability on dredges. He has acquired various skills;Technical, Dredge Operations management, Educational, Skilled, Entrepreneurial, and Administrative skills, with an excellent proficiency in the use of Map-Info, Hydrography, Global Mapper, GPS, Microsoft Word, Excel, Power-Point, Internet and E-mail.
Daniel can be contacted via
mobile:+2347069471585
email: dredgedanielcu@gmail.com
