MEASUREMENT IN THE PIPELINE (cont.)

ERDC TN-DOER-T6

September 2004

Sand slurry flow results were more consistent and accurate than those for gravel.

The Doppler meters produced higher values than the control meter at low slurry velocities

but fell off significantly at higher slurry velocities, producing much lower values than the

control.

The most critical element in the use of production meters is the calibration of the individual

components.

A "pipeline" production meter system was temporarily installed on the dustpan dredge Jadwin to

evaluate the reliability and repeatability of the system and components (Pankow 1989) where the

data proved consistent and reasonable. Scott (1992) summarizes the testing and evaluation of a

production meter (electromagnetic flowmeter and nuclear density meter) installed on the hopper

dredge Wheeler while dredging in fine-grained sediment. This study indicated that, provided the

density and flow meters are calibrated and maintained, reliable, accurate density determination in

the pipeline is possible when dredging silts. The study concluded that use of production meters

for calculating production on a load-to-load basis may be limited due to overflow and leakage

through the hopper doors, but for calculating the total production of dredged material through the

pipeline for a given project, production meters can provide a reliable and accurate measurement

of dredge production (Scott 1992).

Rullens (1993) states that an accuracy of 2 to 3 percent can be obtained under ideal conditions

with calibrated instruments (electromagnetic flowmeter and nuclear density meter) and an

inverted u-tube pipeline configuration. However, space restrictions onboard hopper dredges

usually preclude the use of this optimum pipeline configuration. Additional error components

include:

Presence of gas in the slurry.

Flow that falls below critical velocity value for stationary deposition.

Use of an integration method over a period of time that multiplies measurement errors.

Difficulty in calibrating systems.

Very rapid variations in density and velocity in the pipe.

Presence of debris.

Wear and tear.

Rullens concludes that the velocity and density measurement method should only be used to

ensure optimum production in the dredging cycle of a hopper dredge under two conditions:

When in situ sediment density is very difficult to establish.

When overflow is used, but overflow losses are not measured.

Accurate measurement of in situ sediment density is essential for the accurate calculation of

volumetric production (Scott 2000). When the overflow method is used to optimize dredge

production it is impossible to measure absolute production of the dredge; the measurement of

velocity and density only gives relative information about the quantity and quality of the dredged

material (Rullens 1993).