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MARIN Predicts Dredge Downtime

The maritime research institute MARIN in Wageningen, the Netherlands, has developed a numerical model and computer program, now in prototype stage, for use by dredging contractors to predict downtime due to weather when dredging in the open sea. The weather is simulated by a wind-wave model. The model combines wind and wave patterns with wave proliferation parameters such as bottom features, currents and blockages, to produce an accurate prediction of wave action. Using information on the forces that working cutterhead dredges and trailing suction hopper dredges can withstand, the program predicts the amount of time a dredge will be unable to work because of heavy weather.



The program is expected to be available to contractors in about one year.



The study is being carried out by Johan Wichers, who has a doctorate in civil engineering from Delft University of Technology, and Ellen Claessens, a meteorologist with a masters of Science degree from Wageningen Agricultural University. Wichers is vice president of MARIN USA Inc. in Houston, and Claessens, is based at MARIN in Wageningen.



The study concerns a long-term wave simulation (multi-times of the net project duration), that uses wind speed and direction to hind cast significant wave height, mean wave period and mean wave direction. A highly accurate prediction is produced when the wave conditions are calculated at the exact location of the dredge. The wind-wave model contains wave growth and other processes that act as sinks for wave energy, taking wave propagation processes into account. In this way, sufficient information was obtained to derive reliable statistical information to predict the average real project duration and the standard deviation of the project duration.



The team used data from the wind-wave model SWAN, developed at the Delft University of Technology. This model has the following parameters: the local geometry of the sea bottom, bottom friction, water level variations that allow tides to be simulated, currents, wind speed and direction at a height of 10 meters, and incoming swell at the boundary of the study area. Wichers and Claessens carried out a validation of this model using North Sea measurements from 1996. They show the comparison in the time domain of long-term significant wave heights, zero-up-crossing periods and wave directions at two buoy locations in the North Sea. It was reported that though the computed significant wave height compares well with the buoy measurements, SWAN sometimes underestimates significant wave height and zero-up-crossing periods. To correct for this, MARIN is working on a grid translation so finer wind input grids can be used. The wave directions correspond well with the measurements.



This program will be available to all dredging contractors, and will be useful in large bodies of water such as the Great Lakes in North America, as well as the open sea.



Johan Wichers can be contacted at MARIN USA Inc., 2500 City West Blvd Suite 300, Houston, TX 77042, phone 713-267-2234, fax 713-267-2267, e-mail jewwichers@earthlink.net; Ellen Claessens, can be contacted at MARIN, 2 Haagsteeg, Wageningen, The Netherlands, phone 31-317 493466, fax 31-317-493245, e-mail: e.claessens@marin.nl


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