Design for bigger and faster ro-ros

Larger, more efficient and faster – these adjectives describe the main design trends within short-sea
ro-ro-shipping in a nutshell. Every detail in the hull lines as well as the deck and ramp arrangement must be carefully thought of, when
creating the ultimate ro-ros of the 21st century.

The ever increasing pressure upon the ship owners to make profit in an environment characterised by fierce competition from both older and newer vessels has lead to a continous need for reducing costs and improve the efficiency of the operations. In a sea transport system, the vessels form the most expensive components. The vessels are also in focus both when it comes to cutting costs and increasing revenues.
The recent development of the short-sea ro-ro-vessels has been all about improving an already well proven concept. The true inventions in this field were made back in the 1960’s. Since then it has all been about refinements.
An excellent example is the transition from cargo lifts to ramps in cargo handling on board. In the beginning the lifts had a length just enough to carry a rolltrailer. A substantial improvement was introducing longer lifts with greater capacity. Now hardly any ro-ro-operator would even think about employing a newbuilding not equipped with internal ramps. By refining and developing the layout of the cargo holds and cargo handling equipment it has been possible to build ramps in considerably smaller vessels than before. The loading and discharging is much smoother and faster, enabling shorter stays in port and a more efficient use of the vessel. A well known truth is, that it is at sea where a vessel earns money.
When looking at the development of the short-sea ro-ro-vessel, there are some general “trends” in the design. These several small but important improvements would hardly have been possible to accomplish without the intense research and development work made by shipbuilding engineers using state of the art designing tools. The design process has become more and more important, and surely this is an essential reason why an average ro-ro-newbuilding of today is more efficient than ten years ago.

Increase in speed
One of the most striking features about most of the recent newbuildings in the short sea ro-ro-fleet is the increase of service speed. During the last decade the average speed of the short-sea ro-ro-fleet has increased from 17.0 knots to 19.5 knots according to information from ShipPax Information.
This also tells us a lot about the engine output installed. According to information from the consulting engineers Deltamarin, an increase in speed from 19 knots to 20 knots for a 12,000 DWT ro-ro-vessel means an increase in power of 2,300 kW or 23 per cent. To increase the service speed from 19 to 21 knots, an addition of more than half the installed power is needed. At this stage every gained knot becomes extremely expensive. This is of course also affecting the fuel consumption in the same way. From this it is possible to draw the conclusion, that an optimal service speed is considered more important to the owner or operator than lower building and operational costs.
The importance of an optimised hull form is increasing drastically when higher service speeds are included in the design.
– We have several examples where considerable savings have been made on existing vessels by redesigning the bulb and making small alterations to the hull lines aft, Mr Markku Kanerva, head of marketing and development at Deltamarin, explains.
In an actual case, just the modification of the bulb on a ferry, led to savings of seven per cent. By adapting a four-metre ducktail and an optimised trim wedge a further 13 per cent were saved.
Mr Kanerva says that the hull form aft is of great importance, especially in vessels with a service speed exceeding 20 knots. Instead of using typical conventional ro-ro-hull forms with twin
skeg arrangement, Deltamarin has been concentrating on developing an efficient pram type hull form for open shaft lines, suitable for pod-propulsion as well.
The pram type or buttock flow stern first became the typical form for twin screw vessels, and later also for single screw vessels.
– The hydrodynamic qualities are much better with a pram type stern and therefore we have put emphasis on developing this type, Mr Kanerva says.
A recent development is the application of trim wedge combined with a ducktail. Successful design of trim wedge gives better transom wave pattern as well as reduction of dynamic stern trim and sinkage into even keel, leading to propulsion power reductions of several per cent depending on the hull form.

Minimising bow flare
The hull form is not just essential for the vessel’s performance regarding speed and the installed power. It also has a great impact on seakeeping performance, manoeuvrability and stability.
– It is possible to squeeze some more lane metres in a vessel by adapting an extreme deck shape in the fore part, but it also leads to an extreme type of fore ship hull form and bow flare, Mr Kanerva says.
Excessive bow flare means high wave inducted impact loads, high accelerations, noise, whipping vibrations, involuntary speed loss and also voluntary speed loss for safety reasons.
– A good rule of thumb is to avoid bow flare angle against waterline below
50 degrees in unlimited service, Mr Kanerva explains.
Minimising the bow flare is a question of both economy and safety. Experience has shown that the greed for the last deck square metres or some extra centimetres in the width of the bow ramp during operational conditions may lead to poor or even unacceptable performance already in moderate head and bow quartering seas. The master has to make drastic reductions in speed and perhaps alterations of course not to jeopardise the safety of the vessel and its crew and inflicting heavy cargo damages. The sea margin is lost and the schedule cannot be kept. In the long run the last lane metres may cost more than they earn.
Mr Kanerva stresses, that a well designed fore ship is also of great importance for the comfort on board, especially when many of the new designs have the superstructure and accommodation forward. A main advantage with this solution is that it offers shelter for the cargo on weather deck.
– A vessel with a good bow design makes the ride much smoother in heavy seas for the crew as well, he says.

Capacity
There has been a clear trend towards larger and larger ro-ro-vessels in the short-sea segment. The average capacity has doubled from some 1,000 lane metres at the beginning of the 1970’s to close to 2,000 lane metres.
– Today we are going towards an average capacity of 3,000 lane metres in short-sea ro-ro-vessels designed for trailer transports, Mr Kanerva says.
Despite an increase in size, the efficiency of the vessels has improved too. The layout of the ro-ro-decks is a key issue. All lane metres should be available and easily reachable during the short port calls. The era of cargo lifts was passed years ago, now efficient arrangement and location of fixed and hoistable ramps and wide stern doors provide simultaneous loading of all ro-ro-decks.
Also the access to lower hold is important. In designs with both stern and bow doors, it is possible to arrange a drive-through principle for lower hold.
The ro-ro-vessel of today is more efficient in port and more efficient on route than its predecessors. So far there is no end in sight for this development, even if the giant leaps already have
been taken. Still there is much to be done in the details, together forming the whole.
The ship designers still focus on improving both the layout of cargo holds and equipment and the vessels hull forms. New propulsion systems, such as pods, enable new possibilities to a more efficient usage of the space on board. They also improve the manoeuvring capabilities dramatically.
– But the time has not quite come for introducing the somewhat more expensive pod-propulsion on standard ro-ro-vessels, Mr Kanerva thinks.
However, investments are also needed in ports, because insufficient infrastructure may ruin the earning potential of the most efficient ro-ro-vessel.
Tailor-made transport systems with a limited number of fixed ports also put pressure on the port operators to reach optimum total efficiency.

Two paths to go
The short-sea ro-ro-vessels being built can roughly be divided into two main groups. There are allround vessels, mainly designed for shipments of vehicles, rolltrailers and other cargoes that may be moved by wheels. The owners of these vessels seldom have a charter already at the building stage. The vessels are also planned for being as attractive as possible on the second hand market.
And then there are different kinds of vessels built for a specific trade or cargo, such as forest products. Usually the owner orders the vessel after having received a long-term charter.
An interesting project in the latter field is a special ro-ro-carrier for the aircraft manufacturer Airbus. Deltamarin is carrying out the project design for the vessel, which is intended for carrying parts of the so far largest Airbus airliner between the factories in Germany and France.
– Despite being designed for one specific task and one specific type of cargo, the vessel will also have a potential on other ro-ro-trades, which is typical for most of the purpose-built special vessels of today, Mr Kanerva says.
// Pär-Henrik Sjöström

Back to SSG 3, March 8

Latest update 18-10-2006 8:49

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