Recirculation at a Trailing Suction Hopper Dredger an Experimental Study, Comprising Laboratory and Full-Scale Trials at a Hopper Dredger to Minimize Turbidit
WODCON XXIII - Dredging is changing - The Practice. The Science. The Business.
R.F.J. Neelissen, M. Biesheuvel, J. Pennekamp
"In order to reduce the environmental impact of dredging operations in sensitive areas overflow of Trailing Suction Hopper Dredges (TSHD) is often limited or even forbidden. The consequence is that a TSHD stops loading when the overflow is reached, resulting in a reduction of the payload and cycle production. For this reason, Boskalis initiated a R and D project to develop a recirculation system with the goal to increase the hopper load without increasing turbidity. This system uses the low concentrated mixture from the surface of the hopper content as process water, by leading it back to the draghead. In this way the inflow of ambient water at the draghead is reduced or even cancelled. Pumping water back to the draghead decreases the net inflow of water into the hopper and therefore postpones the moment of overflow and, consequently, the creation of a turbidity plume. This results in an increase of dredging time and with that the hopper payload and the cycle production compared to when overflowing is restricted or prohibited. To investigate the optimal way of inflow of the recirculation mixture at the draghead, laboratory tests were conducted. A transparent scale model of a draghead was made, and several configurations of the draghead have been tested. The laboratory tests showed a high potential for some of the tested recirculation concepts. To get more confidence under field circumstances, a full scale trial was conducted on a small TSHD. A recirculation system was installed at the TSHD Sospan Dau to research the efficiency of the draghead configurations, established at the laboratory tests. For detection purposes a high concentrated shot of salt water was injected during the dredging process as a tracer in the recirculation line or in the jet line. By measuring the salt content of the dredged mixture in the dredge pipe, the efficiency of the draghead and the effect on the suction production was monitored in an accurate way. The results from these trials showed that it is possible to apply the recirculation concept and entrain almost all of the recirculation flow by the draghead, without reducing the production. Deltares, the Dutch institute for applied research in the field of water, soil and subsurface, contributed to this research with their expertise, attended these trials and acknowledged the measurement methods and results. The tests have proven that the developed dragheads perform very well. The recirculation concept on a TSHD will be very beneficial at dredging projects where restrictions apply to overflowing. The load and with that the cycle-production can be significantly increased without overflowing, so the additional burden on the environment (dispersal of fine sediment) is significantly reduced."
Keywords: Recirculation, Dredging Technology, Turbidity, Innovation, Draghead