Shaking grid
Dredging in soils containing UXO's happens a lot and is obviously dangerous. To prevent UXO's entering the ship, the draghead needs to be fitted with a UXO grid. The aperture size of this grid will vary according to the UXO's that are expected but, in most cases, this will be 100 *100 mm or less.
A standard UXO grid consists of steelbars that are welded together. Because of the small aperture size of a UXO grid, the excavation rate will be significantly lower than without a grid. Another obstacle with a standard grid is that the UXO's get stuck between the bars of the grid. Removing the UXO's out of the grid has to be done by hand on deck which means interrupting the dredging process and bringing the head on deck. Because the grid is fixed, the force required to remove the UXO's can be high, increasing the risk.
To remedy this, the shaking grid visor was developed. This design drastically reduces the holding forces on the UXO's, increasing the safety as well as the production for an important amount of UXO's can be removed under water by simply stopping the dredgepump.
Design
The basic idea of a visor that makes dredging in UXO-soils safer and more productive is to form a grid that contains a number of loose elements. This will reduce the holding forces of the grid drastically: when the dredgepump is stopped, or the draghead lifted off the bottom, the loose parts of the grid will move from their original position and the UXO's will drop out of the grid.
The shaking grid is a combination of a fixed structure and loose steel bars. One additional advantage is that the loose elements can easily be replaced when damaged, without welding. Aperture size can also easily be adapted to the circumstances.