The first jumbo heavy duty cutter dredgers

In December 2003, IHC Holland, the Dutch shipbuilder specialising in dredgers, officially delivered the self-propelled cutter suction dredger (SCSD) JFJ de Nul to the Belgium-based Jan de Nul Group. With a total installed power of 27,190kW, it is the most powerful of its kind in the world, exceeding by 35% J. De Nul?s Leonardo da Vinci of 20,230kW, built in 1986. Subsequently, IHC Holland constructed the 22,669kW Mashhour and the stationary cutter Al Sadr with similar total power as the Leonardo da Vinci, but not for work under the same harsh conditions and with less power on the cutter.

Unlike the expansion in the suction hopper dredger market, few SCSD’s have been built since 1986, and those which entered service during the 70’s and 80’s are mostly still sound partly due to low occupation rates, but also because the dynamic loads are concentrated on renewable items such as cutters and teeth. With more offshore jobs in hostile waters becoming available, such as trenching pipelines in rocky soil, Jan de Nul wanted a SCSD even more powerful and more seaworthy than its Leonardo da Vinci, with the capability of dredging in harder soils and worse sea states than its predecessors.

Challenging design

The whole project combined experience in design, building and exploitation of dredging equipment with the latest insights in the field of dredging technology, naval architecture, marine engineering, electrical engineering, material technology, IT and control technology. The result is a multifunctional vessel which can be used for a great number of operations, in all kinds of weather conditions.

The design choices made for the well-proven configuration of Marco Polo and Leonardo da Vinci, also embraced for the JFJ de Nul, posed a big challenge for designers and engineers. The unhelpful hydrodynamic aspects of a heavy spud carriage forward, the vulnerability of the ship’s propellers and rudders alongside the ladder well aft, and the challenges of having a 2 x 40 tonnes hoisting capacity portal deck crane, travelling the entire length of the deck, had all been solved for the current relatively smaller ships. However, the sheer size of the newbuilding posed some tricky issues. For instance, the spuds weigh 230 tonnes each, the cutter ladder 1,550 tonnes, the anchor booms 31 tonnes each and the towering deck crane 215 tonnes. Add 270 tonnes for the large deckhouse and the complications for the dredger?s trimmed condition become apparent. Computational Fluid Dynamic calculations by MARIN established the best balance for the newbuilding?s dredging role with the weight of the ladder aft, and forward for countering the pitching effect of heavy spuds, and the clean run aft and fine entry that make for a slippery and sea-kindly hull.

For jobs such as dredging slopes in narrow channels a short hull is preferred, while length improves sailing efficiency and behaviour in waves. Model tests confirmed the effectiveness of the ensuing hydrodynamic design. MARIN also did hydro-mechanical simulation studies to predict the behaviour of the dredger in waves, especially with analysing the operational endurance of constructions in mind. MTI Holland tailored various dynamic simulations for the newbuilding, for instance the dynamic behaviour of the cutter, using the results as inputs for calculations of torsion, vibration, fatigue and stress. They also did the research for the spud system, where an intricate balance had to be found between spud weight and lowering system.

IHC also had to rethink the principles of spud handling which resulted in three spuds, each of 230 tonnes compared with 130 on the Leonardo da Vinci, with a completely redesigned spud handling system. The spuds can be tilted from horizontal to vertical and backwards and also have an emergency hoisting system. Much thought had also to be given to the construction of the 1550 tonnes cutter ladder, suitable for working at depths from 6.5 to 35 metres. With the suction pipe diameter increased to 1,000 mm, tough problems had to be solved to install it with the enlarged cutter, weighing 35 tonnes. A new tooth system with very heavy duty teeth was also designed for the cutter. The large forces on gears, bearings and shafts, made with hardened materials to cope with the large teeth stresses, touched, according to IHC Holland, the frontiers of physical possibilities for the suppliers.

Power plant

JFJ de Nul is the first fully diesel-electric SDSC. The maximum dredge pump power is 15,800kW, with one submerged dredge pump on the cutter ladder and two inboard dredge pumps. All this power is delivered by three medium speed MAN B&W 8L48/60 main diesel engines that drive three 8,400kW generators. In addition to delivering power for dredging, the diesel-electric power plant also drives the electric motors of the cutter, winches, hydraulic power packs and the two 3800kW propulsion drives. The latter power the two Wärtsilä Lips 4-bladed FP propellers giving the vessel a service speed of 12.5 knots. Additionally, there is a Deutz 1800kW auxiliary genset and a 190kW emergency genset to complete the power pack.

The diesel-electric system allows the engine room to be in almost any place; in this case it has ended up forward of the pump room. The other side of the coin is that highvoltage cables had to be accommodated all over the ship, through very large cable trays.

JFJ de Nul is the first cutter dredger boasting an automation system of similar sophistication as has become the norm in trailing hopper dredgers. The power management system is new for a CSD. All alarm systems are integrated into a single control system. Sub-systems can all be monitored and controlled through several workstations all over the vessel.

Like the engine room, the pump room is spacious, with wide hatches overhead, making access to all parts, maintenance and replacement of machinery straightforward.

Dredging can be carried out from a depth of about 6.5 metres to 35 metres, in calm as well as in more heavy sea states. For this purpose, the ladder has been designed with two pivotal points so that it can be shifted from high to low and vice versa while the vessel is on site. The cutter power of 6000kW, an increase of about 30% in comparison with previous cutters, is delivered by two electric motors through a 220 tonne gearbox. Cutter replacement is carried out with the aid of the 80 tonne travelling deck crane.

Production capabilities

Calculations indicate that the dredger’s pump output in relation to pipeline length will be between 140 and 150% of the Mashhour and Leonardo da Vinci at a relative discharge length of 50%. With increasing length its output will be even more impressive; at 70% output the JFJ de Nul will outperform the Mashhour with 180% and the Leonardo da Vinci with 200%. The ship is also equipped with a special barge-loading system which, by means of self-tensioning winches, can load barges moored alongside.

The vessel can also work in harder soils and worse sea states than its predecessors, thus further increasing productivity. Where stationary dredgers have to be brought to sheltered waters, a seaworthy, self-propelled one can ride out heavy weather at or near the job or proceed on its own power to a more sheltered place. With accommodation for 60 people, continous operation with two crews will be possible and, as a result, great importance was given to reducing vibrations to minimise the danger of crew fatigue and to make the vessel liveable during dredging. Noise levels will remain below 60 dB in the cabins and below 65 dB inside working areas during the dredging operation. Air conditioning, heating and ventilation further create pleasant living conditions with outside temperatures ranging from -20C to +45C.

All this has come at a price. Problems were experienced during construction because some design work had to be altered due to practical difficulties. As a result, IHC Holland lost money on the contract although the final result was to the customer?s satisfaction. Moreover, the yard is now well placed to profit from the expected surge in demand for SCSDs since the existing fleet, with an average age of 20 years, is due for replacement which may be accelerated because the older vessels will find it difficult to compete with the new generation of cutter dredgers.

This is evidenced by the fact that the Belgian group DEME (Dredging, Environmental and Marine Engineering) recently confirmed an order for the construction of a new self-propelled cutter suction dredger with IHC Holland. This SCDS will be similar to the J. de Nul newbuilding but will be fitted with the so-called ?Dracula? system, a technique developed by DEME that uses high-pressure waterjets. This will dramatically increase the efficiency of rock dredging. Including this system, the DEME vessel will have a total installed power of 28,500kW making it the most powerful dredger of this type in the world and delivery is scheduled for mid 2005. n