Dredging rock with a hopper dredger: the road to the ripper draghead
Type:
Presented during:
CEDA Dredging Days 2009 - Dredging Tools for the Future, Rotterdam
Authors:
Neelissen RFJ and Tanis A - Royal Boskalis Westminster, The Netherlands; van Gool VC - Boskalis Australia, Australia
Abstract: After an extensive selection process in 2004 Boskalis Australia Pty Ltd was selected by the Port of Melbourne Corporation to execute the Melbourne Channel Deepening Project. The aim of the project was to make the Port of Melbourne accessible for 14 meter draught vessels at all tidal conditions. One of the most challenging parts of the project was the deepening of the Entrance to Port Phillip Bay, which is located in an environment characterized by a rock bottom, strong tidal currents, a persistent and long swell, regular shipping traffic and a National Marine Park abundant in deep reef fauna nearby. The metocean conditions prohibited the deployment of a cutter suction dredger and the use of drilling and blasting. The latter method was also not preferred because of social and environmental reasons. Seeing the metocean constraints, a trailing suction hopper dredger remained as the preferred equipment for the project. 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.
Several parts of the dredging process were object of research. Literature and former tests were analyzed to derive the forces required for cutting the rock. A model was made to predict the cutting capabilities of ripper dragheads. Several types of pickpoints and cutting geometries were investigated during cutting tests with a testcart equipped with measuring and logging instruments in a quarry. The ripper draghead was engineered and constructed after having determined the optimal teeth configuration with respect to forces and dimensions of the cut rock. 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 dredging campaign 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. Extensive video monitoring showed that the dredging process had to be optimized with respect to the loose material left behind after dredging. Additional laboratory tests with a scale model of the ripper draghead were performed at the Delft Hydraulics Laboratory. The tests focused on the optimization of the suction process by investigating the effectiveness of the draghead's water jets and the influence of different draghead geometries. Based on the laboratory results, the existing ripper dragheads were modified and the work method was amended.
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: ripper draghead, rock, exposed working area, morphology