Different Methods for Calculating the Productivity of Dredger
The cutter suction dredge is commonly used to excavate sediment in ship channels, rivers, bays and ports. It can work almost continuously while excavating and pumping the sediment (dredge material) through a pipeline to a placement area. Delays and loss of production are a consequence when the cutter encounters debris such as roots than can foul or clog the suction inlet.
1)Some researches have been done to show the effect of the roots on dredging compared with that of dredging in sand only to demonstrate the reduced production resulting from fouling the suction inlet located directly behind the cutter back ring . Results show that the fouling causes the suction pressure to increase, slurry velocity to decrease, and the instantaneous production to decrease. Results of laboratory tests also show that instantaneous production for sand only is approximately 50% more than the instantaneous production measured while dredging sand with implanted simulated roots. [1]
Figure-roots in suction mouth(left),cutter(middle),and being cleaned from cutter(right)
Severe blockage of the suction inlet can cause the dredge pump to cavitate and that also further reduces production.
The instantaneous production (IP) in cubic yards per hour (CY/hr) is calculating using:[1]
IP = D2 V (SGs – 1) * 0.661
Where D is the pipe inside diameter (inches), V is the average velocity (ft/s) and SGs is the specific gravity of the slurry.
2)Many factors contribute to limitations in the throughput of cutter suctions dredge. While most performance data from cutter suction dredge manufacturers outlines theoretical design throughput, others factors need to be considered when computing expected throughput. This is obviously important for estimating purposes but also to understand what factors cause limitations on the performance of the dredge .the actual production output of a cutter suction dredge is governed by numerous conditions including atmospheric conditions, the physical site, the design and condition of the dredge and support equipment and the skill of management and operation stuff. Estimating throughput requires knowledge about each of these. The factors that limit cutter suction dredge production can be broadly grouped into four categories: excavation, acquisition, transportation and operations. On any given job, one of the first three factors will set the theoretical throughput of the dredge, while the fourth factor operational efficiency, will lower the production to the level that is actually achieved. It is important to clearly define and understand and apply each of these factors to accurately estimate cutter suction dredge throughput.[2]
2.1)Excavation
The excavation limitation of a dredge encompasses all of the condition that are involved In controlling the maximum rate at which sediments can be sheared loose from the in-situ soil so that it can readily be acquired by the dredge pump. The most prevalent of these conditions are the soil geology and the dredge cutter, swing and mooring systems. [2]
The excavation rate for a cutter suction dredge can be defined by :
Ve = 60 * Ze * Le * Se
Ve = excavated volume in m3/hr
Ze = average depth of cut in (m)
Le =average length of cutter engaged in (m)
Se =average swing speed perpendicular to the channel in m/min
Figure- Cutter tooth path
2.2)Acquisition
The acquisition or suction, limitation of a cutter suction dredge controls the volume of water and solids that can be slurrified and made available to the dredge pump. The factors involved in limiting acquisition throughput are varied and include atmospheric conditions, site conditions and the design of the dredge pump. The acquisition rate of any hydraulic dredge can be defined by : [2]
Va = Qa * CVa
Va = acquired volume in m3/hr
Qa = acquisition flow rate in m3/hr
CVa = acquisition concentration of solids by volume
The values for flow and concentration that the acquisition system is capable of are dependent solely on conditions on the suction side of the first dredge pump . centrifugal pumps require a certain amount of positive suction head at their inlets in order to operate . The amount required is determined by the pump manufacturer and varies with the type of impeller inlet, impeller design , pump flow rate , pump rotational speed and the nature of the liquid . By considering all important factors the maximum acquisition concentration could be computed:
cva = (hatm – hvap – NPSHr – SGl * (Zp – Zw) – Ls * fs *Qa2 / (156849727 * Ds5) – SGl * 1.5 * Qa2 / (49926819 * Ds2)) / (Zp * (SGs – SGl) + 0.22 * Ls * (SGs – SGl) * (w * (8 / fs)0.5 * cosh (60 * d50 / Ds) / (2827 * Qa * Ds2))1.7 + (SGs SGl) * 1.5 * Qa2 / (49926819 * Ds2))
There are physical limitations to the concentration that can be carried in both the acquisition and transport systems , but to make it easier , a good practical maximum of 0.3 could be used if it’s computed as a higher value.
2.3)Transportation
The transportation limitation of a cutter suction dredge includes all factors that influence the discharge capability of the system , primarily head and power restrictions . for the discharge throughput we have :[2]
Vt = Qt * cvt
Vt = transportation production in m3/hr
Qt = transportation flow rate in m3/hr
cvt = transportation concentration of solids by volume
2.4)Operation
The lesser of the three production rates found in previous equations can be used , considering operation efficiency factor to estimate cutter suction dredge throughput . the efficiency factor is due to soil geometry , dredge design and finally dredge operation and site condition in production estimating , the soil geometry primarily refers to the angle of repose (rise over run) that the sediment bank will shear at .
High angles of repose are often desirable in mining applications because it allows the dredge to operate in a confined area without stepping forward for long periods of time.
The theoretical throughput of dredge in m3/hr would be the minimum of transportation throughput, excavation and acquisition throughput.
3)The productivity of a dredge is dependent on variables such as sediment characteristics , cutface limitations , dredge advance and discharge line losses. Variable such as digging depth and friction losses in the suction line limit the ability of the dredge to pick up sediment , whereas the cutface limits the amount of material available to the dredge .
The production rate for the dredge can be calculated by following equation :
If the dredge is configured to operate in a fine sand, and coarse sand deposits are encountered , the increased suction and discharge line losses can be greater than the pump capability ,thus significantly impacting production and efficiency . variation in cutface height has a significant impact on production . Areas where the cutface height is minimal will have a decreased production rate . large suction pipe diameters require higher flow rates to insure that stationary deposits do not occur on the bed of the pipeline , if the discharge pipe diameter is significantly less than the suction pipe diameter the high discharge velocities can generate head looses exceeding the pump capability , thus reducing solids transport.
The pump performance is a function of resistance in the pipeline. If the piping is not correctly sized for the job, the pump may operate at a flow rate that will not sustain the sediments in suspension with stationary deposits forming on the bed of the pipe. Also if the pump is running at the maximum speed and can not provide the additional head required , plugging of the pipe will occur resulting in extensive downtime .
[1] Effects on cutter suction dredge production while dredging simulated debris in the laboratory , R.Randal, M.Warwick, D.Young
[2] FACTORS USED IN ESTIMATING THROUGHPUT FOR CUTTER SUCTION DREDGES ,William J. Wetta II, PE1
[3] ERDC TN-DOER-I3 , July 2000 , Application of Dredge Monitoring Systems to Dredge Contract Administration Quality Assurance