Bdredging Rock With a Hopper Dredger: the Road to the Ripper Draghead
Type:
Presented during:
WODCON XIX: "Dredging makes the World a better place" - 2010, Beijing, China
Authors:
R.F.J. Neelissen, A. Tanis, V.C. van Gool
Abstract: In 2004 Boskalis Australia Pty Ltd was selected by the Port of Melbourne Corporation to execute the Melbourne Channel Deepening Project. Aim of the project was to make the port accessible for 14 meter draught vessels. One of the most challenging parts of the project was the deepening of the Entrance to Port Phillip Bay, which is characterized by a rock bottom, strong tidal currents, a long swell, regular shipping traffic and a National Marine Park abundant in deep reef fauna nearby. Metocean and environmental conditions prohibited the deployment of a cutter suction dredger and the use of drilling and blasting. Seeing the metocean constraints, a trailing suction hopper dredger remained as the preferred equipment. However, the layered, cemented limestone was too strong to be dredged with conventional dragheads. This paper describes the development of a ripper draghead, capable of dredging rock.
First, literature and former tests were analyzed to derive the forces required for cutting the rock. Cutting tests were conducted with a test-cart in a quarry. Based hereon the ripper draghead was engineered and constructed. In addition, vessel motion and vessel maneuvering studies were undertaken to investigate the operational limits of the dredger. The vessel crew was trained on a dredging vessel simulator whereby the actual currents and the predicted cutting forces were used as inputs.
A full scale trial was undertaken with a trailing suction hopper dredger, the Queen of the Netherlands, in 2005. The trial demonstrated that the rock at the Entrance could be dredged with the ripper draghead. Video monitoring showed that the dredging process had to be optimized with respect to the loose material left behind after dredging. Laboratory tests with a scale model of the draghead were performed at the Delft Hydraulics Laboratory. The tests focused on the optimization of the suction process. Based on the results, the existing ripper dragheads were modified.
The entire Entrance was successfully dredged from April to September 2008. The realized productions accorded with the estimated productions and video surveys proved that the quantity of loose material left behind was well within expectations.
Keywords: cutting tests, rock, exposed working area, morphology, environment.