Hopper Dredge Recirculation System

Hopper Dredge Recirculation System

Background

The European community has developed a system to recirculate hopper

dredge overflow water to the draghead in a closed system to increase pro-

duction efficiency and reduce surface turbidity plumes resulting from con-

ventional hopper overflows. Instead of allowing the overflow (composed

of fine organics and transport water) to go directly back into the water,

the new system pumps the overflow along the dragarm to the draghead to

assist in suction operations. Reusing overflow water in this manner main-

tains a closed system so that only a minimal turbidity plume is produced

and potential contaminated sediment is not discharged. Both IHC, Hol-

land and Krupp Foerdertechnik of Germany have developed the recircula-

tion system technology.

Technology

The basic technology involves reuse of transport water overflow as both

pressure water and transport water in the suction head of a closed cycle

system. This new technique has three main benefits: (a) minimization of

sediment/turbidity clouds on surface water by avoiding transport water

overflow and a possible related water pollution problem, (b) increased

material flow in relation to transport water, which in turn increases dredge

efficiency by reducing loading time, and (c) decreased pressure drop

inside the draghead, which reduces dredge pulling force. This reduces

energy needed for propulsion and lowers fuel consumption.

The main purpose of traditional technology (i.e., pumping beyond over-

flow) is to increase dredge load density as much as possible by settling

out dredged materials and allowing transport water to flow back to the sur-

rounding waters. A simple rule of thumb to determine if the new recircu-

lation system will be effective on a particular job is to make sure that the

bottom materials have a density greater than 1,300 g/R. This allows mate-

rials to settle out and increases dredge load density. Materials with lower

density tend to remain in suspension longer and generally do not settle out

during the dredge's economic loading time.

Current usage

The hopper dredge recirculation system has been successfully affixed to

the dredge NAUTILUS. It has also been installed on Boskalis Westminster 's

new jumbo trailing suction hopper dredges (23,347-m 3 capacity), the WD

FAIRWAY, and QUEEN OF THE NETHERLANDS. These dredges have success-

fully met high expectations regarding their overflow systems.

Chapter 4

Detailed Evaluations