Shortage of sand
The past decades have seen an ever increasing demand for raw materials. These are not only used for infrastructural projects and the building industry, but also for land reclamation and beach replenishment. The existing sources on land are not adequate, and environmental regulations prevent nowadays the use of sand, mined on land, in sea-related infrastructural projects. As a result, sand is mined at sea. Yet there is a shortage of sand near the coast lines. Hence the mining concessions granted are located further and further away from harbours and coast lines. This has resulted in sharply risen sailing times – and in more important dredging depths. These elements have resulted in the development by Damen Dredging Equipment of high tech offshore mining equipment.
Conventional mining by hopper dredgers
Up to now deep sea mining has been done using Trailing Suction Hopper Dredgers (TSHD). These dredgers are fitted out with a fixed steel trailing pipe, reaching from deck level to the required dredging depth, usually -30 meters. The mining jobs of jumbo TSHD’s go to a maximum dredging depth of 155 meters. Yet, as the trailing pipe is fixed and works under 45°, the trailing pipe length is 1.5 times the dredging depth – which has to be stowed on board. This results in extremely long vessels. Moreover, the TSHD is equipped with a cargo hold, the hopper. When the hopper is filled with sand, the TSHD sails to its discharging location. During this sailing time the heavy trailing pipe gear is dead cargo and therefore limits the THSD’s carrying capacity. The jumbo hopper dredgers involve a huge investment and are operated at considerable running costs, yet its vital parts cannot operate continuously. To optimize profitability the operating time of dredging equipment has to be maximized – hence the new RoRo Deep Dredge concept.
The RoRo Deep Dredge
In the RoRo Deep Dredge installation the dredging equipment has been separated from the storage and transport facility of the mined sand. Moreover, due to a flexible suction pipe, the system is very compact. Hence the dredging depth can vary and has become independent from the vessels length. The installation consists of a submersed excavation unit, which includes a drag head and a dredge pump. 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. On deck the dredge piping connects the reel to discharge spreaders. Barges sailing alongside are filled continuously . As a result the dredging process is a continuous one, boosting the efficiency of the dredging gear. No sailing time is required for the dredging gear, thus idle time is sharply decreased. As the operating time of all equipment involved has been maximized, the profitability of investments is increased. Another important advantage is that the modular dredging system can be temporarily mounted on a commonly available vessel, resulting in a relatively minor investment and a shortened return-on-investment time.
Submersed excavation unit
The submersed excavation unit has been designed to work at a maximum dredging depth of -200 m. The unit consists of a drag head, connected to an electrically driven dredge pump by a short steel suction pipe. The DAMEN dredge pump, which has a capacity of 6.000 m3/h, is powered by a pressure-compensated 2.500 kW electric motor. The dredge pump, type OBP6055HD, has a 600 mm suction and discharge diameter.
To boost the mixture concentration the unit has been fitted out with a jetwater pump, doing some 3.000 m3/h at 8 bar. The jetwater pump, which is a Damen dredge pump type OBP3530MD-HS, is driven by a 1.000 kW electric motor.
The dredge pump discharge is connected to a 600 mm internal diameter flexible hose running to the reel on deck. In this connection a dump valve is constructed; should the flexible hose be blocked it can thus be emptied.
The entire unit is suspended by steel cables and lowered and hoisted by hoisting gear placed on deck. The hoisting time of the unit from -200 meters to deck level is 8 minutes.
On deck various modular elements are placed. All of these elements are containerized and can be mounted effortlessly due to for instance their twist lock mounts. The deck equipment consists of the reel, the discharge system, the hoisting gear as well as the power and control units.
The hose reel accommodates the dedicated Trelleborg Subsealine®, which is continuously rolled on and off following the required dredging depth. The reel is 8 meters in diameter and has a capacity of 225 meters of Subsealine®. The reel is connected by deck mounted discharge piping to the two discharge spreaders.
These two spreader arms, mounted at portside, are continuously fed by the submerged dredge pump. The spreaders, which have a maximum outreach of 15 m, transfer the mined sand to a barge sailing alongside. In order to maximise efficiency, the system is fitted out with a Light Mixture Overboard feature.
The hoisting system consists of two gantries and three winches, designed for lowering the submersed excavation unit to the required dredging depth and for hoisting the unit in board. The steel cables are fitted out with a swell compensator system with a 2 m stroke for 4 m swell compensation. The umbilical cable, transferring power and control data, has its dedicated cable reel.
The three containerised 40’ power units generate the power required for the dredge pump and the jetwater pump. Apart from the 5.000 kW power units, a containerised control and monitoring unit is placed on deck. The footprint of the entire system on deck is some 600 m2, and it’s weight is some 700 tonnes.
Monitoring and control
The monitoring and control system itself is a modular unit located at the bridge. Its data are supplied by a remote system from the containerised control and monitoring unit on deck where system data are gathered. At the bridge an automatic control and monitoring system supplies all required information to the dredge master. Available are for instance the highly accurate real time absolute position of the submerged excavation unit making use of wide band mini transponders. All essential process control data such as mixture flow and mixture density are monitored as well.
As the RoRo Deep Dredge system remains close to the TSHD concept the operation of the system is obvious for the dredge master and therefore requires minimal extra skills.
Due to the highly economic separation of functions in the RoRo Deep Dredge system, transport of the mined material is done by other vessels. For this transport, smaller standard cargo vessels can be used and barges can be brought in continuously making use of tugs. The barges sail alongside, and when a barge is nearly full the next one is already waiting its turn. All these relatively simple vessels are cheaper than TSHD’s and normally readily available at any location, where they can be sourced temporarily from the local market. These barges have a high useful cargo factor as they do not have idle dredging gear as additional steel weight. Moreover local barges usually have enhanced accessibility to local ports.
The RoRo Deep Dredge system is not just a half-baked idea. Elaborate tests have been done to ascertain the feasibility of the concept. In cooperation with various research partners DDE has investigated a number of key elements to the system, such as the behaviour of the submersed excavation unit while dredging and during hoisting manoeuvres, the construction and the behaviour of the Subsealine®, the behaviour of the mixture in the hose reel and the barge sailing alongside the main vessel.
One of the recognised technology institutes Damen Dredging Equipment cooperated with was MARIN, the Wageningen-based tank model research station. In 2 series of feasibility model tests various elements of the new dredging concept were examined. A scale model of a Platform Supply Vessel (PSV) was fitted out with all deck equipment and the submersed excavation unit miniaturised to scale. The tests included the study of the behaviour of the submersed excavation unit as well as of the Subsealine® in flowing water while dredging, the line loads in the steel hoisting cables and the determination of limiting conditions on mooring the barge to the main vessel. All tests were carried out in various harsh operational conditions and proved that the new dredging concept will perform perfectly at over 95% of the world’s top 10 mining locations.
The behaviour of the main vessel was studied in several numerical ship motion simulations. The operability was analysed at 7 different wave situations simulating various representative realistic and harsh ambient conditions as found at different locations around the globe. For these simulations scientifically gathered wave statistics and prevailing wave climates were used. The tests provided insight in the limiting sea states. The limits on the system are derived from the maximum tolerable behavior of the vessel, i.e. the limits of human comfort and overall safety. The most demanding areas of operation proved to be around Scotland and the Bay of Biscay area, yet in most other areas the RoRo Deep Dredge system was nearly 100% of the time operable.
The model tests included dredging and loading simulations. Not only the behaviour of the sand excavation unit was analysed, also manoeuvring test were done with the barge moored at Port Side during the dredging operation. The model test proved that the system will perform perfectly at trailing speeds between 2 and 4 knots, and a significant wave height up to 3 to 4 metres. During the in total 42 manoeuvring tests the leading vessel was the barge, moored alongside the PSV in loaded condition. From these test the preferred relative wave direction was established at placing the PSV at the leeward side of the barge. Also the turning circle of the combined vessel was tested and its manoeuvrability was concluded to be satisfactorily.
Infinite loop tests
At another highly regarded technology institute, Delft University of Technology, research was carried out on the transport of sediment in an infinite loop. Using the University’s test facilities such as a dredge pump test circuit, the behaviour of the dredged mixture in the hose stored on the reel was studied. Various elements were determined such as the pressure at different process locations, the critical velocity of the mixture, the process of clogging and the methods of getting settled sand in motion. The test proved that the discharge hose on the reel does not get clogged yet should this be the case the settled sand can easily be set going.
For the test a scale model of the reel at 1:3 was made which accommodated a hose with a 200 mm internal diameter. During the various tests the pressure losses while transporting sand were determined. The conclusion was drawn that the mixture resistance in a reel-wound hose is less than the resistance in a straight [flexible] pipe line. Moreover, the resistance of a reel with a horizontal rotation axis is lower, than that of a reel having a vertical alignment.
Another detailed study were the properties of clogging, i.e. a completely blocked reel-wound hose. The sand was left for over 5 hours to settle. After settlement, the sand was rather easily put in motion again by just starting the dredge pump. Not required for starting the blocked system, yet as extra possibilities, the same tests were carried out rotating the reel 180º as well as rolling off the hose. In all cases the mixture is put in motion again by the dredge pump.
On the open market no flexible discharge hose was available being able to withstand both the pressure at -200 m depth yet being able to be wound up the deck-based reel. Trelleborg Velp, a leading supplier of flexible rubber hoses to the dredging industry, participated in the project. Their research department used their years of experience and knowledge to make several test lengths. Various tests determined the physical hose properties such as its bending stiffness and its mass moment of inertia, and the test outcome has resulted in the Subsealine®. In this special hose the internal layering was adapted in such a way that reel storage and a dredge pump pressure of 10 bars were no problem, and the container-sized hoses have a relatively limited weight yet cannot be flattened by outside pressure. Moreover, should the hoisting wires fail, the submersed excavation unit can be hauled to the surface using the Subsealine®.
Not only practical or logistical issues had to be addressed. In depth analysis had to be made on a phenomenon called Vortex-Induced Vibrations. When a cylinder, such as the flexible hose connecting the submersed excavation unit to the reel, is placed in a cross flow, an oscillating vortex might develop.. The vortex-shedding causes the pressure on the surface to oscillate. This oscillating pressure is translated by the structure in vibrations – called Vortex-Induced Vibrations. These vibrations can cause extreme fatigue damage.
VIVARRAY software was used to perform a three-dimensional analysis of the dredging system at 3 different trailing dredging speeds, i.e. 2, 3 and 4 knots. The analysis have shown that fatigue resistance is not the most relevant design issue. As more often than not in dredging, the wear of the Subsealine® is much more relevant.
Another partner in the project is IMOTEC, a company specialised in the modelling of the dynamic behaviour of a load suspended by wires in sea conditions. The study IMOTEC carried out has resulted in a special control system specifically made to control the submersed excavation unit during hoisting – even at high speeds, plus the recovery of the unit and the stowage of the unit on deck.
The software which IMOTEC developed predicts the behavior of the system, also when certain elements have altered. These alterations can be the dredging depth, the hose diameter, the hoisting points etc. As a result, for designs varying from the tested model no further tank tests will be required. The software will allow for accurate winch control, always being able to keep the submersed excavation unit in control.
The final result of all test and studies done on the new flexible dredging concept is, that the system will work as reliable and profitable modular piece of dredging equipment and that it will be an asset to the changing demands in the dredging market.
The dredging market
The possibilities for the use of the RoRo Deep Dredge system are not limited to the sand mining industry only. Other uses for the RoRo Deep Dredge system are mineral mining, such as phosphate mining and winning iron sands which can for instance be found off Florida, off the Western African coast as well as near Australia and New Zealand. Another field in which the RoRo Deep Dredge system can be used, is trenching. Trenching is a preparation of the sea bottom for oil and gas pipe lines or communication and power cables. The sea bottom is to be made level very accurately. This accuracy can be obtained by the submersed excavation unit as MARIN tests proved. Therefore not only mining but also the construction of off shore wind parks and oil and gas platforms can be done differently in future.
The main vessel – the PSV
The flexible dredging system has been designed with some very strict basic principles in mind. One of these is, that the modular dredging gear is only placed temporarily on the vessel. This vessel can be any vessel with a sufficiently dimensioned flat deck to accommodate the containerized modules. Within the Damen group, the Platform Supply Vessel (PSV) suits this purpose perfectly. Yet any other type of vessel or even a pontoon can be used. Most important is that the required type of vessel has plenty of deck space and cargo carrying capacity, is commonly used and therefore widely available. This is the reason why the dredging gear is independent from for instance the vessels power supply. All the vessels requires, is its propulsion to ensure trailing dredging at a speed of between 2 and 4 knots.
The conclusion on the flexible solution
The RoRo Deep Dredge is a flexible dredging concept which revolutionizes dredging in the 21st century. With its optimum use of invested capital and therefore its lower cost per m3 it will challenge current projects and dredge
fleets. Its modular approach will break the dredging market open by enabling other vessels or contenders to enter the deep sea mining market. The new RoRo Deep Dredge will cause mutiny in dredging to the extreme.