It is rare in this day and age when something new comes along. But a recent collaboration between a number of Dutch companies seems to have come up with just that. The companies, Shipkits, Vuyk Engineering Groningen, Alewijnse, Vacon and the shipping company Amasus, are all well-established names in the maritime industry. The project is a 6,000 tons deadweight, open top, special diesel- electric powered multi-purpose cargo, bulk and container vessel, called MV Jaguar.
The general concept
The initial idea for the vessel was conceived in 2009 by Vuyk Engineering Groningen following discussions they had been conducting with a group of shipping companies. These shipping companies led by Amasus included Capt. Gorter and Capt. Klos from Jaguar Shipping. The contract between Shipkits and Jaguar Shipping for this vessel, to be named MV Jaguar, was signed in 2010. Jaguar is the first of three vessels of this type awarded to date, but more will probably be built in time.
To deal with Jaguar’s outward appearance first, the vessel is the latest and largest of a family of vessels of heavy cargo Dynamic Positioning (DP) equipped vessels designed to support the offshore market and other special project cargos. The aim with these vessels is to provide a highly versatile means of transport for a wide and diverse range of special cargoes. So a great deal of thought has gone into the cargo hold areas. Every effort has been made to exploit the space available to provide extremely large holds in relation to the overall size of the vessel.
Versatile hatch cover arrangement
The vessel, which has its superstructure toward the bow to permit the largest possible deck space has, amongst its cargo-carrying armory reinforced tank tops, reinforced and removable tween decks and hatch covers. All the hatch covers can be removed and stowed at one end of the cargo hold or deck to allow the vessel to be completely open. The upper hold has an impressive 80 metres length making the vessel suitable for carrying parts for the most recent of wind farm designs. Moreover the tween deck hatches can be positioned and stored at every position, thus making the areas as versatile as could possibly be imagined.
Manipulating the hatch covers is achieved by means of a single gantry crane that runs the length of the cargo deck. Some smart thinking has gone into the lifting arrangements and the crane has two spreader frames for manoeuvring the hatches. One frame sits inside the other and the two frames together lift the hatch covers. However, by disengaging the inner (smaller) spreader frame it is small enough to fit inside the hold and lift the ‘tween’ deck hatches covers by means of container style twist locks, which can be engaged in special deck recesses before the spreader frame is lowered into position. Coops and Nieborg supplied both the crane and all the hatches and tween decks.
With the cargo holds full, equipment can be transported above decks courtesy of the reinforced hatch covers. The forward located superstructure provides an element of protection to the cargo carried there. Another example of inventive thinking, is the ability of the vessels to carry large drums of cable or coils of steel vertically in the hold with the hatch covers stowed.
A closed circuit television system by Orlaco is installed for monitoring the decks and hold, so cargo loading and unloading operations can be seen when necessary. The data network can also be monitored both onboard as well as onshore. As good crews are increasingly difficult to source, efforts have been made to achieve the highest levels of comfort inside the vessel. The finish of the interior, choice of furniture, selection of fabrics and quality of carpentry are all to a very high standard. This is supported by the installation of a sophisticated state-of-the- art internet and tv system fitted in all cabins.
Third generation ‘green’ diesel-electric propulsion
This series of vessels has been designed to very tight operational criteria, which is a closely guarded secret. However other requirements were environmental (green) considerations, and a desire to maximise the operational life of the vessels by being ahead of both programmed and anticipated legislation and regulations.
It is this last consideration that has made Jaguar even more special. A determination to think ‘outside of the box’ by the team lead them to enlist the services of several specialists in diesel- electric propulsion. The attractive aspects of electrical propulsion for a cargo vessel is that several small engines can be used in place of one or two large diesel main engines. Moreover the large engines have to be aligned with shafts and propellers, so they have to be in a particular place in the hull. Of course this place is exactly where the operators wish to put cargo. Electric propulsion ‘alignment’ issues are only the cables, which are considerably more flexible in comparison to propeller shafts. As a result the diesel generators can be positioned wherever it is convenient and the propeller shafts can be very short with electric motor coupled to them. In the case of Jaguar the generators are located at the aft end of the hull optimizing the hull cargo spaces and the shafts are replaced with steerable Z drive thrusters. This allows for one or more generators to be running within a relatively narrow speed band for maximum fuel efficiency whereas a large diesel is very fuel inefficient at slower speeds. If there is a need to go faster another small generator diesel is started up and shut down when no longer needed.
Whilst all the state-of-the-art options were laid out on the table nothing seemed to satisfy the requirements completely so in typical fashion they went looking for something new. One of those approached was Alewijnse Marine Systems, who is the installer of the propulsion system. Working with Vacon they have produced what they call the third generation of diesel-electric propulsion. Ironically the ideal solution came from one of the fields that the ship is designed to support: that of wind generation.
The DC-philosophy: advantages over AC driven propulsion systems
So what is so special about wind generators? Quite simply: the wind cannot be guaranteed to blow consistently in either strength or speed for any period of time. But the output has to meet the requirements and tolerances of a power grid and so direct current electricity is often used.
One of the disadvantages of AC generators (alternators as they are technically known) working together is that if they are to operate together they have to run synchronously, that is to say they have to run at exactly the same speed. If one runs faster than its partner it will take more than its share of the work (load) causing undue wear on that machine. If the speed difference becomes large enough the working machine will drive the other machine as an electric motor. This is known as a ‘reverse’ power situation and can severely damage a machine. As a result complicated control gear is required to both prevent reverse power situations and ensure that the machines are properly synchronized before they are put together on the same power supply.
Direct Current (DC) machines – the true generators – do not have this constraint on their operation, they can quite happily run at different speeds without any risk of damage. This makes them ideal for wind power where consistency is not possible. It also makes it ideal for powering this sort of vessel when in transit it will be travelling quite fast whilst in DP mode the power requirement to ‘stay on station’ will be quite small.
Another problem with AC systems is that all motors will run at a fixed speed dependent upon the frequency (Hz) supplied to them. To achieve the speed control necessary for propulsion it is necessary to vary the supply frequency. Traditionally this is done by using a variable speed drive, the problem with these is that they cause harmonics, which can wreck havoc in an electric distribution system. With large motors like those required for propulsion and thrusters the energy in these harmonics is correspondingly high. Vacon’s solution to this is to employ an Active Front End (AFE) AC/DC rectifier that in turn powers a 750Volt DC bus.
The engine rooms: a power plant MV Jaguar has six Scania diesel generators to power the DC bus via the AFE Rectifier mentioned above. The DC bus feeds variable frequency DC/AC inverters to drive the propulsion system consisting of two Azimuthing Z Drive thrusters in the stern and two bow thrusters all of which are AC motors. The remaining motors for pumps and winches etc. are fed by either variable speed or fixed speed inverters as their operational requirements dictate. Fixed frequency inverters fed all other consumers because they do not have the requirement to change speed.
In DP mode alongside a wind turbine or platform only one or two generators will be required. But when itis time to set off to the next location and push the vessel through the water at speed then all six generators can be called upon to push the water out of the way.
The selection of the correct number of generators for any given scenario is the job of the power management system, which is both less complex than its AC counterpart and more responsive. The reason for this is that all it has to do is start the generator whereas an AC system has to start an alternator and then synchronise it with those already running before it can be switched in.
The concept in a nutshell
The result of all this smart thinking is a unique twin-screw diesel-electric concept based on Active Front End and DC-bus technology. In summary the VG6000-E is an extremely versatile multipurpose special cargo vessel of 6,000 dwt designed for open top and with a limited draught. Though the vessel is classed for worldwide trade and unrestricted services, it will initially be used for short sea services and offshore wind market support. While the first vessel of this design has only just been handed over to its new owners Jaguar Shipping, the concept already is a marketing success. Its performance when in service will be keenly watched and within a short time Jaguar will have at least two sisters assisting her in the same market. It will not be long before other varieties to this concept are added to the fleet.
Tom Oomkens and Andy Rudgley