The two innovative Carousel combi-tugs being built in Malaysia for Dutch salvage and towage company Multraship could revolutionize tug design, without the need for expensive technology, according to the Carousel system designer, Novatug.
The tugs ordered in July this year from Malaysian shipbuilder Nautica Nova Shipbuilding & Engineering (NNSE) incorporate the unique Carousel towing system and will address directly the needs of big ships requiring escort facilities into harbours and rivers.
Rolf Kievits, general manager of Rotterdam-based Novatug, says, “The beauty of the Carousel construction is that the solution it offers is so logical, and relatively simple. Of course it requires engineering, design work and stress calculations to ensure that everything is fail-safe. But the Carousel construction is in fact nothing other than a very clever and logical mechanical system which makes use of basic laws of physics and requires no expensive technology.”
Comparing the time taken to build the Carousel tugs, as opposed to conventional tugs, Kievits says, “The Carousel system is an integral part of the design. There is no extra steelwork involved. The Carousel and winch, as elements of the towing equipment, will be built off-site and installed on the vessels at the yard. So there is no extra construction time involved.”
In terms of cost, Kievits said, “It is not possible to equate safety with pure financial cost, but our aim is to design and build the safest tug afloat at an affordable price. Of course the great advantage of the Carousel system is that it incorporates just one winch, as opposed to the two which other, conventional vessels would need to carry out the same task. Carousel also incorporates a more cost-effective propulsion lay-out which will result in long-term economies of maintenance and fuel consumption.”
The Carousel design incorporates an integrated winch platform, together with appropriate finite element calculations to ensure that the entire structure is able to take its prescribed design load, fully independent of the ship, on the freely rotating Carousel and winch platform.
The system for the tugs being built in Malaysia has been engineered to a 300-tonne design load. This includes the whole spectrum of horizontal and vertical loads the winch has to work with. For practical purposes, however, and to ensure safe operations when assisting other ships, the system as a whole is restricted to a 160-tonne working load.
In order to power the winch on a freely rotating Carousel, Novatug has designed a twin-diesel-engine power-pack to power the hydraulic system. Where maximum power is not required, this system can be operated at fifty per cent capacity by using only one of the two engines.
The winch has been specifically designed for harbour towage, escort work and coastal applications. It is a sate-of-the-art hydraulic winch, manufactured by Rapp Hydeema of Norway, with full active escort capabilities. Kievits explains, “We had to find the right balance between the winch dimensions and the rope capacity, where we opted to use 72 mm Dyneema line
of 200 m length. In the event that the tugs were required to undertake deep-sea work, however, a winch with additional wire capacity could be fitted to the Multraship tugs.
“In terms of maintenance requirements, the usual manufacturer service intervals are recommended. For the Carousel, we recommend that the bearings are checked at regular service intervals, which will probably mean an annual inspection of the load-runners.”
Kievits is excited about the marketing opportunities for the Carousel system. He says, “So far, we have fitted the system to a conventional tug which originally had a bollard pull of twenty tonnes, and we have the escort designs building in Malaysia for Multraship. I think it fair to say that these two examples will be at opposite ends of the spectrum in terms of what we are able to offer owners and operators.”
Although doubts have been expressed about the impact on crew safety of operating on board a vessel fitted with the Carousel system, Kievits says, “Given current restrictions on crew working on deck during manoeuvres, there is no real difference in this respect between a Carousel tug and a conventional tug.”
The first of the two eighty-ton static bollard pull Carousel Combi-Tugs is due for delivery at the end of 2007, and the second, about three months later. The vessels will be among the most robust harbour and escort tugs in the world. They will be highly stable both statically and dynamically, and will therefore be of particular significance for large vessels making approaches to rivers and harbours. They are also especially fuel-effective, producing potential savings in this regard of between thirty and fifty per cent.