Damen Dredging has developed a new dredging concept; challenging current technical limitations, and existing dredging practises.
The new development is an answer to the increasing demand for raw materials (sand and minerals); while mining locations are located further from shore at increased dredging depths; at the edge of the continental shelf.
At first sight; this is a "Frankenstein"-dredger: with parts of a J-lay cable-vessel, half a suction pipe, and a barge loading system. In this conecpt, the whole thing is mounted on an offshore supply vessel.
The modular system consists of 4 containerized power and control units, the hoisting gear - with the gantries, winches and swell compensator, plus the mixture transport pipes. These are the Subsealine® hose including the reel (with a 10 m diameter), and the discharge spreaders. The submersible excavation unit is equipped with a dredge pump and jetwater- pump, both electrically driven.
Two aspects of the RoRo Deep Dredge installation are key to its flexibility.
1. The dredging equipment is placed on a offshore supply vessel (or another vessel of choice) – thus the storage and transport function of the dredged sand has been separated from the dredging function. Transport is done by a number of barges.
2. The steel suction pipe of trailer dredgers have been replaced by a flexible hose. Hence the dredging depth can vary and is not limited because of vessels' length. The "RoRo Deep Dredge" can work up to -200 m dredging depth.
The Subsealine® was designed, manufactured and tested at Velp; Trelleborg. The picture shows the production process of the hose, specifically designed for the RoRoDeep Dredge system.
The installation consists of a draghead and a submersible dredgepump.
A hose is connected to the unit and runs to a storage reel on deck. The flexibility of the system is evident as the hose can be rolled on and off the reel to adapt the dredging depth – hence the name "RoRo" Deep Dredge.
The behavior of the submersed excavation unit was studied at MARIN. During the tests weights were placed on the drag head to simulate the suction force. The movement of the unit was monitored by an underwater camera.
On deck the dredge piping connects the reel to a barge loading system. Barges moored alongside are filled almost continuously. Barges are equipped with constant tensioning winches.
An alternative would be, to have a floating pipeline streaming from the dredger aft, and to load barges through this line. This would increase capital cost, but would also increase the weather envelope for safe operations in open sea.
At MARIN a series of tank model tests were carried out, to test the behaviour of the whole spread in waves. A scale model of the supply vessel was fitted out with the dredging gear and the deck equipment. During the tests dredging and loading simulations were done, with the supply vessel coupled to the tug and barge.
As a result the dredging process is nearly continuous. No sailing time is required for the dredging equipment, thus effiiciency of the dredge equipment is boosted. As the operating time of all equipment involved has been maximized, the profitability of investments is increased.
An advantage is that the modular dredging system can be mounted on suitable vessel of choice, resulting in a relatively minor investment and a shorter return-on-investment time.
With such a short-term approach to invested capital; and initial lower cost, this concept challenges current dredging practices. Its modular approach may give upstart companies a change to break in the offshore mining market.
Marc Van de Velde