Uncertainty Analysis Applied to Dredge Production Calculation

ERDC TN-DOER-I5

August 2000

Uncertainty Analysis Applied

to Dredge Production Calculation

INTRODUCTION: The accurate measurement of dredge production is essential for maintaining

the maximum efficiency and cost effectiveness of the dredging process. The use of production

measurement systems on pipeline and hopper dredges provides dredging personnel with tools for

measuring and monitoring production quantities. The accuracy of these production monitoring

systems varies according to the instrumentation used and the knowledge of the sediment and water

properties associated with the dredging activity.

Each measurement and physical quantity associated with the calculation of dredging quantities has

some error or uncertainty associated with it. The equations for calculating production are functions

of multiple variables (measurements and physical quantities), each contributing some error. These

errors propagate through the data reduction equation to the final calculation. It is essential that the

error associated with each variable is accounted for, and that the individual error contribution to the

total error is recognized.

Equations are introduced that describe production for both pipeline and hopper dredges. An

uncertainty analysis expression is derived for each equation (Scott 1993). The general uncertainty

analysis technique is applied in a step-by-step manner to one of the equations to show the derivation

of the uncertainty analysis expression. Example dredging situations are introduced to demonstrate

the uncertainty analysis application. Numerical solutions are obtained which show the error

contribution of each variable, and the effect of uncalibrated instruments and unmeasured sediment

and water properties on the accuracy of production calculations. Uncertainty analysis calculations

indicate that with properly calibrated instruments and measured sediment and water properties, the

error associated with production measurements can be 10 percent or less. If the instruments are not

calibrated and the sediment and water properties are not measured, an error potential of 25 to 50

percent is possible in production calculations.

BACKGROUND: The overall efficiency of dredging operations is directly related to the produc-

tion rate of dredged materials. The dredging process is optimized when there is maximum

production at the lowest operating cost. It is essential that dredging personnel are informed of the

optimum operating conditions of the dredge plant, as well as the capability of the production

monitoring system. The use of production monitoring instrumentation has provided dredging

personnel with a useful tool for monitoring dredge production and overall dredge operation.

Production monitoring equipment is primarily utilized on pipeline dredges which dispose of the

dredged materials through a pipeline, or by hopper dredges, which store the dredged material in

onboard hoppers for later disposal.

Two types of production monitoring systems are commonly used on either pipeline or hopper

dredges. An in-line production meter system directly measures the density and flow velocity of the

material in the dredge pipe. This system utilizes a nuclear density gauge for determining the density