Virtually real hopper dredger for economic training


Back to overview simulators.

Introduction
When in the late 1990's the size of trailing suction hopper dredgers exploded, the complexity of systems to guide and manage them had to follow suit. IHC Systems'brief was to further develop sensors and instruments on one hand and make the mass of their signals intelligible for dredger masters on the other.Thus came into being IHC Systems' fully integrated bridge, crowning the comprehensive range of dredging instruments and computer systems that have made jumbotrailers governable. Which still leaves them very big beasts, a fair bit too complicated for 'plug and play'. They are also too expensive to double act as schoolship and their impact is far too big to keep your educative composure while a trainee is blundering around. So, Belgian contractor Jan de Nul decided he needed a realistic simulator to train future dredger masters and IHC Systems provided just that.

Integrated simulator bridge
Basically, a simulator must be able to do what the real thing can, without the damage that a hopper dredger can inflict on shipping, its own dredging installation, the environment -including loss of human life- and the owner's balance sheet. In other words: all the physical processes that take place in a real-life hopper dredger must virtually take place in the simulator in a controlled, instructive and harmless way, but the trainee must feel as if he is in the thick of it.

The integrated simulator bridge, similar to the one on board Jan de Nul’s newest hopper dredger JUAN SEBASTIAN D’ELCANO
The simulator looks like a genuine integrated bridge with all the accoutrements one would expect, mainly four touch-screen monitors and the hardware to move a dredger's system, such as operating controls for the suction tubes, yield indicators, potentiometers for controlling the diesel engines' revs and various emergency operating facilities for valves and pumps. The input and output is handled by a PLC. A survey system provides bathymetric data, and an instructor's panel is linked to a projector. A number of participants' positions can be created. The simulated dredger is operated with the controls on the bridge and through SCADA interfaces, just like a real trailer. The interfaces make it possible to operate components such as pumps and valves separately. The dredge automation can also "setup" these valves and pumps automatically, depending on the selected operation mode. During the 'dredging process' the instructor can save specific situations. He can also influence specific processes or instrument readings, for instance by freezing a process value or entering an unusual sensor value. The trainee dredger master is supposed to spot the trouble and act adequately.

A la carte
The dredging process is a sequence of interacting sub-processes. For a realistic imitation, the sub-processes as well as the interactions between these processes are simulated with dynamic models, concerning mainly: the suction process; the sedimentation of the mixture in the hopper; and the discharge process. Dredging is all about soil; the interaction between suction head and soil is of decisive importance for the entire dredging process. Hence the possibility to select any of eight different soil types, from slurry to gravel, and to enter crucial process dependent parameters. The simulator subsequently takes into account this particular soil type's influence on the dredging process, from sea bottom to discharge pipeline. Humans may be created equal, hopper dredgers definitely are not, so since the simulator is to be practical for more than just one particular dredger, the vessel's configuration can be selected. The choice of parameters includes electrically or directly diesel driven dredge pumps; two pipe sections on the suction tube or extending it with a third and a submerged dredgepump; and active or passive draghead.

The integrated simulator bridge also has a projection screen for training purposes

Figure 1: interface of the simulator for the mixture transport

Four basic models
The simulation embraces the entire system for mixture transport: draghead parameters, actual pipeline lengths, level differences and running times. In order to cut the dredging process to imitable size, four main basic components are distinguished: a (centrifugal) pump model, a valve model, a pipeline model and a nozzle model. Models for jet water, hydraulics and gland water are derived from these same main components. Additional models, for the draghead (the suction process), for sedimentation in the hopper and for suction tube movements, have been developed alongside the four models for the basic components. The simulator interface for the mixture transport is shown in figure 1.

The model for the draghead is based on the fact that energy is needed to excavate the soil and accelerate the mixture. With sufficient underpressure, the mixture will be drawn up, the quantity of solids depending on the excavation energy, speed of the draghead, jet water discharge and pressure, and the position of the visor. The influence of these process parameters can be adapted in the simulator for each soil type. The suction tube movements are realistically simulated and presented on the Suction Tube Position Monitor. At the start of the exercise, the suction tubes rest in their saddles. As soon as sufficient hydraulics are available, the gantries can be put overboard and the suction tubes lowered. The behaviour of the suction tube is a function of the length of cable paid out, speed of the vessel and crab angle, depth and tide, the friction of the draghead over the soil, and the seabed's slope. For instance, the suction tubes may end up under the vessel because of the tide, drift angle (rotation) of the vessel, or the draghead's sliding down slopes. The system's survey computer is the source of information on the type of soil and the depth.

Inside the hopper
For the sedimentation model the hopper is divided into seven parts. Volume or concentration differences create flows between these parts. If a hopper part contains a fluid mixture with a given concentration, sedimentation of this material will lower the concentration in the liquid above it. Because flows in the hopper depend on the kind of material, the sedimentation will likewise be characteristic for the soil type. The sedimentation rate depends on the grain size of the material and the concentration in the hopper. The smaller the grain size, the easier the material will flush away even though it may have deposited already. For instance, a sediment of fine sand will be flushed away readily and re-enter the liquid mixture to settle elsewhere in the hopper, whereas gravel will be immovable as soon as it has settled, immediately after entering the hopper. The hopper has two overflow ducts of adjustable height, which drain excess water and affect the hopper loading process. The flow through the overflow ducts depends on the water layer above them and the position of the environmental valve. From the weights in the hopper the simulator calculates the levels of their water layers as well as the vessel's trim and draught. As on a real dredger, the IHC Systems Draught and Loading Monitor (DLM) calculates the loading from these process values.

Figure 2: simulator interface for the hopper.

The seven hopper parts can be seen, with an overflow tube in parts one and six Figure 2 shows the seven bottom doors, two pre-dumping doors, and the trim and draught of the vessel. Also shown are the seven hopper parts, each with the levels of the sediment and the liquid material. The current positions (height) of the overflow ducts are shown in the first and one but last part.

A screen, developed in cooperation with the client for the simulator, depicting the resistance curve of the shore discharge line and the pump curves. It clearly shows the dredge pump's working point and the critical speed in the shore discharge line. It gives the dredger master better insight in the dredging process and reduces the risk of the shore discharge line clogging up.

Graph showing the resistance curve of the shore discharge line and the pump curves.

Soon on stage: virtually real
In the future, the simulator will be extended with a navigation simulator, incorporating IHC Systems' Dynamic Positioning and Dynamic Tracking (DP/DT) system. Pulling forces generated by the draghead's drag, and the effects of load variations on draught and mass of the vessel, will then realistically induce the dredger's behaviour. The system will also be linked to IHC System's Dredge Track Presentation System, a survey system that accurately displays the effects of the dredging on the seabed. Linking up with the ECDIS system allows to plan the tracks with an eye on avoiding interference with shipping lanes and show these on the screen. With today's computer generated virtual reality capabilities, much can be done to make the simulator even more attractive by, for instance, adding a realistic outside view of the ship's and dredging installation's movements projected in the seascape. It will be even better than the real thing; a pity that it wouldn't dredge anything other than for the trainees' hidden talents.

Epilogue
The simulator has been put through its paces by seasoned dredger masters of IHC and dredging company Jan de Nul. All parties were very pleased indeed with the new training tool's realistic performance and conformity of the test results. The Jan de Nul Group will organise a comprehensive training programme for some 100 employees a year. Based on thorough comprehension of the dredging process and specialist knowledge from IHC Holland's R&D-institute MTI, IHC Systems has created a cost-effective tool that may prove vital for the dredging companies to stay ahead in their global market.

(Source : P&D nr. 158 from 2002)