CODED comes of age

A number of Finnish companies are working together to produce vessel concepts that combine diesel-electric with diesel-mechanical machinery. Wärtsilä, the engine manufacturer, Kvaerner Masa-Yards, the shipbuilder, and ABB, the electrical systems developer, are among those companies studying alternative machinery solutions for fast ropax vessels under the joint Finnish shipbuilding research projects SeaTech 2000+ and EMPRO.

The propulsion concept they are all working with is known as CODED (combined diesel electric and diesel mechanical). It involves installing diesel-electric machinery to power fixed pitch azimuthing pod propellers and conventional diesel-mechanical machinery to drive a controllable pitch propeller.

The hybrid solution, it is claimed, combines the best characteristics of a diesel-electric power plant with those of the proven diesel-mechanical propulsion arrangement. Among the claimed advantages of the CODED plant system is the greater flexibility it gives in machinery location, thereby making it possible to design a narrow beam at tank top level.

Although they are working together, Wärtsilä and Kvaerner Masa-Yards have proposed different designs based on the CODED machinery concept. Wärtsilä?s solution involves exploring the full potential of the propulsion machinery by employing an electrically powered contra-rotating pod (CRP) behind a directly driven CP propeller. ABB is conducting tests in this area to support the viability of what it calls the ?power rudder? concept. KMY is somewhat more conservative in its latest design, recognising that many ferry operators see the advantages of pods but are reluctant to look at a CRP solution while question marks still hang over the reliability of conventional pod solutions.

CODED with CRP

The solution put forward by Wärtsilä consists of a conventional mechanically driven CP propeller powered by two diesel engines. In addition, three smaller diesels are employed to generate electrical power for a single CRP FP propeller, located directly behind the main propeller, and the hotel load.

Wärtsilä?s rationale behind the development is the considerable interest shown in fast full-displacement ropax vessels in recent years, a number of which have entered service in both the Mediterranean and north European waters. These ropax vessels usually have conventional diesel mechanical propulsion with twin CP propellers mounted on open shaft lines with brackets. Wärtsilä?s experience, however, is that a single screw configuration is hydrodynamically more efficient and gives a relatively better performance compared to twin propeller drives.

The design Wärtsilä has produced is of a fairly large ship compared to other high-speed vessels, having a gross tonnage of about 37,000g. It is designed for a service speed of 30 knots. It features a full-displacement hull with a very long and slender form and a single centerline skeg to minimise resistance. It can be outfitted for overnight trips or with only day passenger facilities for shorter routes.

The proposed CODED machinery has the latest in environmentally friendly technology, such as EnviroEngines equipped with common rail and Compact SCR units. It is envisaged that the ship will be fitted with two Wärtsilä 16V46C engines for the mechanical propulsion, two Wärtsilä 12V46C engines driving one generator each, and one small Wärtsilä 6L32 generating set giving a total installed power of 61.5 MW. All engines run on HFO to offer low fuel costs.

Locating a CRP FP propeller directly behind a conventional CP propeller installed on the centerline skeg is claimed to offer several advantages over a twin screw hull with open shaft lines and rudders. Among them is better hydrodynamic efficiency.

The aft propeller takes advantage of the rotative energy left in the slipstream of the forward propeller when it turns in the opposite direction. The single skeg hull form offers a more favourable wake than an open shaft line. Additionally, the resistance of the single skeg hull form with a single pod is lower. The new concept reduces the total delivered power demand at the propeller by more than 10% compared to a conventional solution, says Wärtsilä.

However, because of the higher transmission losses and much higher capital cost of the electrical propulsion, it was decided that more power should be on the mechanical propeller for the best economical performance. The pod power, however, should not be too low, says Wärtsilä, otherwise the full benefits of the CRP concept will not be achieved. A power split of 60/40 in favour of the mechanical propeller was adopted.

Although only part of the power is electrical, the proposed CODED machinery still offers most of the benefits of a fully electrical power plant, says Wärtsilä, since both the pod and the entire hotel load are supplied by the same power plant. The electrical capacity is designed to cover the load at full speed at sea and, since full pod power is not needed while manoeuvring, no extra generator capacity needs to be installed for the bow thrusters.

The pod is used for steering the vessel and can be turned 360 deg. This enhances possibilities for quick port turnarounds and obviates the need for separate stern thrusters, says Wärtsilä. The proposed machinery arrangement also offers a certain degree of redundancy since the machinery is located in two separate compartments. If the mechanical engines are damaged, then the diesel-electric power plant and the pod will provide the hotel and propulsion power needed to operate the ship. On the other hand, if the electric power plant is damaged, then the ship can be propelled back to port with the mechanical engines while the emergency generator can be used to steer the vessel by turning the pod, which in such a scenario would function like a conventional rudder.

Model testing CRP

On paper, use of a pod in a contra-rotating application raises questions about cavitation, mechanical engineering and course stability. However ABB believes that the ability of a CRP Azipod to absorb rotational energy from the main propeller and the relative simplicity of gears and bearings compared to conventional contra-rotating propeller solutions give the CRP Azipod good potential, particularly in high power applications.

Aside from working with Wärtsilä on the fast single-skeg ropax ferry outlined, ABB has presented ideas in combination with Korean shipbuilder Samsung Heavy Industries for ultra large container vessels. It further believes there are applications for the CRP Azipod on certain types of tankers and car carriers.

The company says it has conducted extensive investigations and model tests to support its views. The most recent were cavitation tests performed earlier this year at Krylov, Poland (see separate boxed story). “We have been quite convinced [from the result of these investigations] that from a cavitation point of view there will be no problem,” explains Thomas Hackman, marketing manager for ABB Marine.

He explains that course stability is an issue of ship design, which hasn?t been investigated with CRP yet. “But it is not seen as something that could create problems.”

ABB confesses that aspects of the mechanical engineering – including sealing systems, bearing arrangements and steering system characteristics – remain unsettled. This can best be done by taking the concept forward from tank tests to larger-scale applications, says the company.

Despite the unresolved questions, ABB is confident of the technical feasibility. “It is a good indication that many yards are interested in it,” says Hackman.

CODED without CRP

Kvaerner Masa-Yards (KMY) next generation ferry features CODED propulsion machinery with a single CP propeller in a central shaft-line and two azimuthing pods positioned either side. The yard says the configuration gives improved propulsion efficiency with lower fuel costs and safe operation, through redundancy and excellent manoeuvrability. Its design has an innovative wave damping transom stern resulting in low wave amplitude to lessen the environmental impact of shore wave erosion. It adds that passenger comfort is improved through lower noise and vibration levels.

The main criteria of the project was to produce an efficient ferry with a speed of up to 28 knots using the single propeller configuration as opposed to the less efficient twin propeller concept. However, the yard recognised that on short ferry routes good manoeuvrability and a degree of propulsion redundancy was essential for operational purposes. It also, as stated earlier, recognised ferry operators suspicions on podded solutions reliability and therefore their reluctance to go for more advanced CRP solutions.

The shipbuilder conducted hydrodynamic tests which it says showed that the single screw in conjunction with podded propulsors produced an overall improvement in efficiency of up to 20%.

In KMY?s design, the diesel electric machinery is positioned just below the funnel thereby freeing extra space for roro decks. Although the proposed design is based on medium-speed main engines, KMY is also examining the possibility of installing slow speed two stroke engines in order to offer greater fuel efficiency.

KMY has also teamed up with its Piikkio works to incorporate pre-fabricated cabins, including full picture window, within the design. This would reduce the logistical complexity of constructing such a vessel by taking away the need to build and outfit the cabins in situ.