Figure 11. Fixed distance between draft and hopper level sensors

ERDC TN-DOER-I2

July 2000

Water Level Test. The fixed distance be-

tween the draft and ullage sensors allows a

quality assurance test to be conducted on

hopper dredges with bottom dump doors.

With the bottom dump doors open, the water

level inside the hopper will equal that of the

water surrounding the vessel. As shown in

Figure 11, addition of the draft and ullage

measurements should equal the fixed dis-

tance between the draft and hopper level

sensors. Although the Columbus does not

have dump doors, its discharge system al-

lows the hopper water elevation to equalize

with the surrounding water in the same man-

ner. On the Columbus, these ullage and

draft sensors have agreed within 30.5 mm

(0.1 ft). Results on the McFarland as well as

the Columbus are currently being analyzed Figure 11. Fixed distance between draft and hopper

to determine the effect of sensor geometries

level sensors

on relative accuracies.

ONGOING INVESTIGATIONS

Dry Sediment Density and Water Density. Sediment and water samples in the Mobile

Channel are currently being collected and analyzed to generate a database of specific density

variability for solids and in situ water. As this database is made more complete in conjunction with

more TDS measurements, analyses will be conducted to investigate such TDS issues as when and

how (e.g., time averaging) is the "best" way to measure the hopper load TDS value and the conditions

under which TDS is most justified for a dredging project.

System Performance. Sensor and TDS system performance on the McFarland will be

monitored during the course of regular dredging projects, and data collected will be analyzed with

regard to accuracy and precision and quality assurance aspects. Nearly a year of TDS data have

been collected on the Columbus and the contract-specified 90 percent data return has been exceeded.

The ullage sensors have had several failures, but they have been repaired within the 48-hr time

window specified in the contract.

CONTRACT SPECIFICATIONS: The Mobile District rental contract for the dredge Columbus

includes specifications that aid in implementing TDS and the SI system. For instance, the contractor

must develop a Dredge Plant Instrumentation Plan that shows how sensor data will be gathered,

quality control will be performed on those data, and calibration and repair of sensors/data reporting

equipment will be conducted when they fail. A standard interface is specified for connecting sensor

data and computed dredge-specific data to the ship server. The contractor is to keep a log of sensor

problems and repairs. Recalibration can be directed at any time during contract execution as deemed

necessary. No recalibration or adjustments to the calibration controls are to be performed in the

absence of the Authorized Representative of the Contracting Officer without prior written approval.