|On the sedimentation process in a trailing suction hopper dredger|
Van Rhee, C. (2002). On the sedimentation process in a trailing suction hopper dredger. PhD Thesis. Technische Universiteit Delft: Delft. xvii, 244 pp.
The Trailing Suction Hopper Dredge (TSHD) was originally developed for maintenance dredging. However, during the last 10 years the TSHD (or hopper dredger) has become more involved in Capital Dredging. The large land reclamation projects in Hong Kong and Singapore were and are being realised by a large fleet of hopper dredgers. During dredging the TSHD lowers one or two suction pipes to the seabed. From the bed a sand-water mixture is sucked up and discharged into a large cargo hold, the so-called hopper. Provided the sediment in the mixture is not too fine a separation process takes place in the hopper under gravity. The sediment settles in the hopper and the excess water flows overboard. A part of the inflowing sand may not settle during loading into the hopper, but flows back overboard with the excess water. Depending on the particle size distribution (PSD) of the sediment, the hopper geometry and other process parameters this overflow loss can reach values up to 30-40 % of total volume dredged. It is important to quantify these losses both for the quality (PSD of sand loaded) and quantity (loading production).
Therefore, the VBKO (Vereniging van waterbouwers in Bagger-, Kust- en Oeverwerken) initiated a research program to improve knowledge on the subject of hopper sedimentation.
In the past different models were developed to estimate the amount of material lost overboard, the particle size distribution (PSD) of the sediment in the hopper and the PSD in the overflow mixture. These models were based on relative simple models developed for the field of sewage water treatment (Camp, 1946) and all have idealised or simplified inflow and outflow configurations and a prescribed velocity distribution in the hopper. The objective of the current investigation was to develop a new model to simulate the hopper sedimentation process because the existing models have too many restrictions.
The research program started in 1997 and can be divided into three parts: Laboratory experiments; numerical modelling; and prototype validation of the models. The laboratory experiments can be divided into model hopper sedimentation tests and more fundamental tests on one-dimensional sedimentation and the influence of bottom shear stress and air bubbles on sedimentation.
The research program started with a literature survey and formulation of scaling rules for laboratory tests. Hereafter laboratory tests were executed with large dimensions to minimise scale effects. During these tests the in- and outflow quantities were monitored and much effort was concentrated on taking measurements inside the hopper (velocity and concentration) to get a better understanding of the physics involved. This intensive test program resulted in a better perception of the flowfield and sedimentation process in the hopper.
The laboratory test revealed that buoyancy effects dominated the flow inside the hopper. The flow velocity close to the settled bed was relative high due to the presence of a density current. In the remaining part of the hopper the flow was in a vertical direction rather than horizontal. This finding motivated the development of a one-dimensional model in vertical direction. In the IDV model the supply of the sediment-water mixture is simplified. The mixture is equally distributed over the length and width of the hopper. The model includes the important effect of the vertical velocity component and the mutual interaction of the different grain sizes of the particle size distribution. This latter effect is totally absent in the above-mentioned simple models for the sewage water treatment, where every fraction is calculated independently. The IDV model was validated using the results of one-dimensional sedimentation tests and proved also capable to simulate the (three-dimensional) laboratory hopper sedimentation tests. It was however uncertain if the model would simulate the process on prototype scale equally well since the horizontal transport in the hopper was not included owing to the one-dimensional character. It was therefore decided to extend the one-dimensional model to two dimensions. The hydrodynamic 2DV model developed is based on the Reynolds Averaged Navier-Stokes equations with a k-epsilon turbulence model. The model includes the influence of the overflow level of the hopper (moving water surface) and a moving sand bed due to the filling of the hopper. The sediment transport equations are coupled to the momentum equations to enable the development of buoyancy-driven flows. The influence of the particle size distribution (PSD) is included in the sediment transport equations.
A boundary condition at the interface between the settled sediment and the mixture above must be formulated for the numerical model. It was recognised that quantitative information on the influence of the bed shear stress on sedimentation was missing for the situation present in a hopper (large concentration and relative low flow velocities). Therefore unique sedimentation tests were carried out in closed flumes. In the first instance pilot tests were carried out at WL | Delft Hydraulics. Later these tests were followed by a test program in the Dredging Technology Research Laboratory of the Delft University of Technology with larger dimensions and extensive instrumentation. An empirical relation based on these experiments was built in the two-dimensional model. The model was validated using laboratory sedimentation tests and prototype measurements. The agreement between the measurements and model is encouraging.