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Page Title: MONITORING SYSTEM COMPONENTS
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ERDC TN-DOER-I3
July 2000
of material removed. In the past, these conditions have resulted in legitimate claims by contractors.
The contracting officer needs a source of additional dredge process data beyond post-dredging
surveys and the typical dredge logs provided by the contractor to evaluate the productivity and
efficiency of the dredge.
MONITORING SYSTEM COMPONENTS: Dredge monitoring systems contain three compo-
nents: an active data gathering component consisting of sensors for measuring dredge process
parameters; a data acquisition system for acquiring and storing raw data; and a post-processing
component for reducing the data into a useable form for interpretation. Monitoring systems and
their associated components are described for both pipeline and hopper dredges.
Pipeline Dredge. The productivity of a pipeline dredge, such as a cutterhead dredge, is dependent
on variables such as sediment characteristics, suction line losses, cutface limitations, dredge
advance, and discharge line losses. Variables such as digging depth and friction losses in the suction
line limit the ability of the dredge to pick up sediment, whereas the cutface limits the amount of
material available to the dredge. In this sense, the cutterhead dredge is suction line limited, i.e., in
that suction line losses and cutface limitations will dictate production. In addition, cutterhead
dredges must be capable of generating high discharge heads for pumping long distances. Therefore,
the cutterhead dredge is also discharge limited, in that it must have the horsepower available to
transport the solids picked up by the suction line. To properly evaluate the productivity and
efficiency of a cutterhead dredge, it is therefore necessary to acquire dredge process data, which
provides information on both the suction and discharge performance of the dredge.
Dredge process monitoring for quality assurance involves two stages. The pre-dredge stage consists
of analyzing the capability of the proposed dredge plant during the bidding process. This is a
theoretical analysis that evaluates the suction and discharge limitations of the dredge given project
site constraints. The second stage involves acquiring real-time dredge process data while the dredge
is working to monitor dredge productivity and efficiency.
Pre-Dredge Analysis of Dredge Capability. When contractors submit bids for the dredging
job, the dredge plant design should be evaluated to insure that it can nominally meet the performance
requirements for the project. This involves a theoretical evaluation of its capabilities. The contract
administrator can utilize dredge computer models to evaluate the theoretical performance of the
dredge. Project pre-dredge survey data will provide an estimate of the cutface available to the
dredge. The efficiency of the dredge is dependent on the method used for advancing the dredge
through the cut. A walking spud arrangement has an approximate cycle efficiency of 50 percent
while a spud carriage arrangement has an approximate efficiency of 75 percent (Turner 1984).
Based on this information, and the design and setup of the dredge plant, the dredge capability can
be approximated (Scott 1998). Inputs to the model include digging depth, suction line diameter and
length, discharge line diameter and length, in situ sediment density, water density, pump efficiency,
bank height factor, and cycle efficiency among other dredge parameters. A number of these models
exist in government and industry that are capable of performing these calculations. These models
can tell the user the approximate average slurry density that the dredge should be capable of
maintaining over the project when the cutterhead is engaged in the material, and also over the entire
cycle which includes the inefficiencies caused by the advance of the dredge. The contract
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