How is dredging done?

The primary dredging technique used for the project will be Trailer Suction Hopper dredging - a technique likened to under water vacuuming.

A trailing suction hopper dredge is one of the world’s most capable and technologically advanced dredging vessels. It meets all the requirements of latest best practice procedures and comprehensive safeguards and an experienced, professional crew will help ensure operational safety during dredging.

Trailing suction hopper dredging

The vessel is designed for both shallow and deep water operations and the large hopper capacity, one of the largest in the world, gives significant economies of scale. It is highly manoeuvrable and with powerful pumps and advanced technology, dredging is fast and accurate

These vessels have twin suction pipes - on either side - each pipe with a drag head on the end. The drag heads use water jets, teeth, rippers or blades to break up the seabed material for easier removal.

When arriving on location the dredgers are set in position using state of the art global positioning systems. The suction pipes are lowered overboard until the drag heads reach the seabed. The depth of the drag heads is controlled continuously to ensure correct depths are achieved.

While the vessel moves forward, the loosened seabed material is drawn up the pipes. The material is pumped into the dredger’s hopper where it settles to the bottom, while the excess water drains through a variable overflow system. The area is dredged in consecutive layers until the correct depth is achieved.

When the hopper is full, the dredger sails to one of two identified dredged material grounds where material is placed in a planned and managed operation.

Reducing the level of turbidity

At specific times during construction the Trailer Suction Hopper dredge will be required to dredge with minimal turbidity. As a commitment to world’s best practice the vessel is equipped with an environmental, or green, valve which reduces the amount of turbidity from the dredging process.

The valve is located in the overflow system of the dredger. Overflowing is a part of the dredging process and happens when the hopper is full. To reduce turbidity from the overflow, the green valve is adjusted to stop air being trapped within the water in the hopper.

If air is trapped in the water it carries the sediment particles towards the surface. But, when the green valve is used, the suspended sediment starts to settle towards the seabed immediately, staying closer to the dredging area and reducing the amount of turbidity.