Hopper Dredge

ERDC TN-DOER-I3

July 2000

consolidated that specified may require an alteration in cutterhead ladder positioning and swing

rate, thus reducing the cycle efficiency. As more water is pumped in relation to sediment, the

production will go down.

Dredge configuration problems can also impact the solids transported. 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 losses 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. As the effective pipe diameter is reduced

from the deposits, the flow rate is reduced even further to compensate for the increased resistance.

If the pump is running at the maximum speed and cannot provide the additional head required,

plugging of the pipe will occur resulting in extensive downtime.

To illustrate how dredge process data can be used along with predicted performance data, the

following example is presented for a cutterhead dredge. The dredge has a 0.71-m- (28-in.-) diam

suction line, 30.49 m (100 ft) in length, and a 0.61-m- (24-in.-) diam discharge line, approximately

991 m (3,250 ft) in length, with an 2.44-m- (8-ft-) lift to the disposal site. The digging depth in the

channel was approximately 9.15 m (30 ft), and the sediments were composed of a medium sand

with an in situ density of approximately 2.0 g/cm3 (125 lb/ft3). This dredge has a spud carriage for

advancing the dredge with an estimated cycle efficiency of 75 percent.

The dredge model was run for the worst case scenario of a minimum bank height to cut, which

results in an approximate overall dredge efficiency of 20 percent. By comparing the actual

monitoring data to the worst case model data, the user can determine the minimum acceptable

performance allowed by the contractor. The data for the six cutterhead cycles indicates that the

dredge is performing at a higher efficiency than the minimum estimated efficiency of 20 percent.

The computer model indicates for the worst case condition an average density while engaging the

material of about 1.12 g/cm3 (69.89 lb/ft3), and an overall average cycle density of about 1.09 g/cm3

(68.02 lb/ft3) at a 20 percent overall dredge efficiency. The average of the monitoring data for all

six cycles indicates an average density of 1.18 g/cm3 (73.63 lb/ft3) while engaging the material, and

an overall cycle density of about 1.14 g/cm3 (71.14 lb/ft3). The average dredge model production

is 643 m3/hr (840 yd3/hr) at the minimum efficiency, while the average actual dredge production

was 1,000 m3/hr (1,307 yd3/hr). The average cycle efficiency for the dredge data is 77 percent,

reflecting the use of the spud carriage method of advancement. For this example, the dredge

productivity is substantially higher than predicted.

Hopper Dredge. The productivity of a hopper dredge is based on a number of factors. Hopper

dredges are generally used to dredge free-flowing sediments such as sand or uncompacted fines,

unless teeth are attached to the draghead to break up more consolidated materials. Therefore, a

change in sediment characteristics can substantially reduce dredge production. The speed that a

hopper dredge operates at can also influence productivity. When hopper dredges are operating in

fine to coarse sand environments, they frequently overflow the hopper to maximize the load. The

productivity of the overflow operation can be high if the discharge into the hopper is low enough

to allow settling. If the discharge is too high, the turbulence in the hopper will maintain the solids