ABSTRACT

A Trailing Suction Hopper Dredger (TSHD) is a sea-going vessel that is developed for dredging and reclamation. Sedimentation of incoming material in TSHD is a going concern during loading process. This study employed a hopper model based on an actual TSHD to carry out experiments to observe sedimentation phenomenon of fine sediments. 12 experiments were selected for further analysis in group S1, S4 and S5. The measured velocity and calculated turbulence demonstrated that the inlet slurry via diffusers play a dominant role in flow field and frequency and amplitude of velocity variation. Fine sediment with low concentration increased the average velocity but restrains turbulent intensity, changing vertical flow structure. No significant bed level was seen with inlet sediment of fine silt, but there was a bimodal pattern of sedimentation with coarser silt or fine sand, presenting higher crest under the first diffuser due to the imbalance of bypass. Even during the entire overflow process, sediment less than 45 μm was difficult to deposit in hopper. Additionally, finer sediment had a larger dimensionless overflow rate S* and cumulative overflow loss. There was a power function fitting between S* and hindered settling velocity. S* showed a significant increase with the decrease of settling velocity when it was less than 5 mm/s. Both S* and Tons of Dry Solids Mass Ratio (TMR) indicated that inlet particle size and inlet concentration play an essential role in loading efficiency and overflow loss

INTRODUCTION

The trailing suction hopper dredger (TSHD) is a vessel equipped with suction pipes for maintenance dredging and reclamation. The dredging cycle of a TSHD can be divided into three main stages: (1) sucking up a mixture of sediment and water from the seabed, (2)loading and overflowing process in the hopper, and (3) pumping out, rainbowing, or bottom-dumping the hopper load (ERDC, 2020). This research focuses on the second process, i.e. loading and overflowing process in the hopper. Before overflow, the sediment-water mixture is fully retained inside the hopper, and the sediment segregates into coarser particles with higher settling velocity and finer particles with lower settling velocity. Fine sediment is an essential factor of production efficiency (Baldock, 2003), and increases total overflow loss (Miedema & Vlasblom, 1996; Smith & Friedrichs, 2011). Production efficiency depends on factors such as particle size distribution (PSD), sediment concentration, hydrodynamic conditions, and hopper parameters (Braaksma, 2007; deWit, 2014; Vlasblom, 2007; Rhee, 2002a). A number of models have been proposed and modified to estimate the sedimentation and overflow loss (Miedema & Rhee, 2007). The Camp model (Camp, 1936), developed for wastewater treatment, is an ideal basin with steady and uniform flow. Vlasblom and Miedema (Miedema & Vlasblom, 1996; Vlasblom & Miedema, 1995) modified this model by incorporating PSD and bed rise effect. Furthermore, unsteady state of flow and sediment concentration were introduced into the sedimentation model due to the development of monitoring sensors (Braaksma, 2009; Stano, 2013) and numerical methods (Boone, 2016; Sloof, 2017).

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