MONITORING SYSTEM COMPONENTS

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