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Designing backhoe dredgers for dynamic loads in severe conditions

Presented during:

CEDA Dredging Days 2012 - Virtue, Venture and Vision in the Coastal Zone

Authors:

de Vries G and Jacobse GW - Vuyk Engineering Rotterdam, the Netherlands

Abstract: The development of backhoe dredgers started by placing an excavator with tracks on a simple barge or pontoon positioned on wires and anchors or spuds. The operation area was mainly confined to inland waters. This idea proved to be a success and gradually the operation area and the dredging depth increased. So there was a demand for bigger (backhoe) dredgers with greater capacity, larger pontoons and improved workability in currents and wind. By lifting the pontoon out of the water and driving the spuds into the bottom, a stable work platform was created. This platform was also used for mooring purposes of the barges in which the dredged soil is discharged.

Because dredging contractors have to undertake larger waterworks even in coastal areas, their equipment had to be adapted for these conditions. So we have noticed that the dimensions of backhoe dredgers and the digging forces of the excavators increase; the weight of the excavators grows from 100 to more than 450 tons. And while working in more seagoing conditions and with bigger sized barges the total design of the dredgers became more complicated. This complexity has often been and still is underestimated and leads to a number of problems, especially regarding stability of the dredger and strength of major parts like spud systems and excavator foundations.

In the past few years, research has been done to evaluate the stability and strength of backhoe dredgers. Stability research focusses mainly on the stability of the backhoe after spud failure. Besides static stability, the dynamic heeling angle is determined to ensure the dredger will not capsize and no flood points will be submerged. Most of the strength issues are related to the wave dynamics. The backhoe dredger is excited by the waves and has to resist both 1st order and 2nd order wave forces. Because a barge is often moored to the side of the backhoe dredger, all loads acting on the barge may have to be absorbed by the spud systems and the pontoon.