Example Hopper Dredge Specifications

ERDC TN-DOER-I5

August 2000

that the dredged material consists of a homogeneous fine sediment mixture, therefore, the uncer-

tainty of a properly calibrated nuclear density gauge as stated by the manufacturer is 0.001 g/cm3

Uρs . The magnetic flow meter has a full scale value of 9.76 m/sec and an uncertainty of

0.25percent of full scale or 0.024 m/sec (UV).

Example Hopper Dredge Specifications. The hopper dredge is equipped with differential

pressure transducers for measuring the vessel draft, and ultrasonic water level sensors above the

hopper for measuring the slurry level in the hopper. The vessel draft measured by the pressure

transducers is used to determine the weight of material in the hopper by comparing the measurement

to a draft versus vessel weight diagram (Carene diagram). The hopper volume is calculated by

comparing the ultrasonic water level measurements of the hopper slurry level to data relating the

hopper depth to hopper volume. For this example hopper dredge, the hopper has an average volume

of 6,122 m3 (VOLh), with an average water load of 272,160 kg (Wh). The uncertainty of the sensors

for measuring hopper depth (volume) and load were calibrated using water in the hopper. Statistical

data on the water level transducers for measuring the volume as a function of depth in the hopper

indicate a measurement uncertainty of 61.22 m3 UVOLh , and the differential pressure transducers

used for measuring the draft of the vessel have a measurement uncertainty of 2,722 kg UWh .

Low Uncertainty Assumptions. Properly calibrated instruments will have accuracies as

previously stated in the dredge specifications. The measured density of the sediment (mineral) and

water will be assumed to have uncertainties of 0.03 g/cm3 and 0.01 g/cm3 respectively

Uρm and Uρw . The in situ density will be assumed to have an uncertainty of 0.036 g/cm3 Uρi .

High Uncertainty Assumptions. For poorly calibrated instruments and unmeasured sediment

and water properties, the following uncertainties are applicable. An uncertainty of 5 percent of full

scale will be assumed for poorly calibrated hopper monitoring instruments, or 13,608 kg UWh

and 306 m3 UVOLh . An uncertainty of 0.01 g/cm3 Uρs is assumed for a poorly calibrated

nuclear density probe, 0.098 m/sec (UV) for inaccurate magnetic flow meter calibration,

0.03 g/cm3 Uρw for estimated water

Table 1

density, 0.053 g/cm3 Uρm for inac- Pipeline Dredge Variables and Uncertainties

curate sediment mineral density, and

Variables

0.18 g/cm3 Uρi for estimated sedi-

ρs (g/cm3)

D (m)

V (m)

Values

ment in situ density. The variables and

Nominal

0.61

4.57

1.20

their uncertainties for both the pipeline

High Uncertainty (U)

0.00025

0.097

0.01

and hopper dredge examples are listed

Low Uncertainty (U)

0.00025

0.0024

0.001

in Tables 1, 2, and 3.

UNCERTAINTY ANALYSIS EXAMPLE CASES - RESULTS AND DISCUSSION: To

illustrate the error range possible when calculating production with the previously described

systems, a series of uncertainty calculations were performed for four example cases. For these