CONCLUSIONS - doeri50012

ERDC TN-DOER-I5

August 2000

when the instruments are operating properly. However, the error potential becomes very high

(40 percent) when the instruments are not operating correctly. For the solids content calculation,

the water density is the dominant variable when the instruments are calibrated. These instruments

typically operate in an unstable environment, subject to vessel motion and environmental changes

(wind, rain, humidity, temperature extremes). Significant error can occur in the calculations

(50 percent) if the instruments are not properly calibrated and the sediment properties are not known.

These examples show the utility of the uncertainty analysis procedure in identifying the variables

that have the most influence on the production calculation. The variables and their associated

uncertainties used in the above examples were chosen to illustrate the procedure. Not all production

systems use the same makes or types of instruments, therefore the uncertainties associated with the

instrumentation may be different for other applications. Also, the calculations were performed for

only one set of dredging conditions (flow velocity, pipe diameter, sediment and water densities).

The above examples should only serve as a guide for applying the uncertainty analysis method for

determining the accuracy of production system calculations. It should be apparent from the analysis

that accurate instrument calibration, along with a thorough knowledge of the properties of the

dredged sediments and water is necessary to insure the highest degree of production accuracy.

CONCLUSIONS: The following conclusions are based on the uncertainty analysis results:

Pipeline production meter systems are capable of measuring dredge production to within less

than 10 percent, given calibrated instruments and known sediment and water properties. The

percent error can be as high as 25 percent if steps are not taken to insure that the instruments

are calibrated and material properties known.

Because of the inherent accuracy of the nuclear density and flow meter instrumentation, the

in situ sediment density and the water density have the greatest influence on the accuracy of

the in situ volumetric and solids pipeline production calculations.

For the hopper production calculations, the error potential is greatest for the case of poorly

calibrated instruments (40 percent), because the average density measured in the hopper is

dependent on two measured variables.

For hopper production calculations, the in situ sediment density and water density contribute

significant error when the instruments are properly calibrated.

The error in hopper production calculations can range from a low of about 10 percent for

calibrated instruments and known sediment and water properties, to almost 50 percent for a

worst case of uncalibrated instruments and unknown material properties.

The general uncertainty analysis performed on the production equations reveals the need for

determining the correct application and calibration of production monitoring instrumentation, as

well as the knowledge of sediment and water properties. The purchase of instrumentation for

monitoring production should always be contingent on a thorough training program for dredging

personnel. The supplier of the instrumentation should reveal calibration techniques and mainte-

nance schedules necessary for attaining the highest degree of measurement accuracy.