Power constraints for the 10,000+TEU boxship.

Propulsion power plant dilemma for ultra large container ships .

Container transportation volume has been growing at an average annual rate of 9 % in the past fifteen years, making it the fastest growing sector of marine transportation. What is driving this growth is the shifting of wage-intensive manufacturing away from Europe, the US and Japan to the Far East and, in particular, China. Experts estimate that global economic growth of only 1% leads to a 4 to 5% increase in container traffic

The fastest growing segment of the container vessel market, large container vessels above 4000TEU, is expected to grow by more than 50 % during the next few years. The maximum container vessel size has also been increasing steadily over the past years to optimise the economy of scale benefits. The largest vessels under construction today measure over 8000TEU with Seaspan of Canada recently ordering a quartet of 10,200TEU ships from the South Korean shipbuilder Samsung. This size is considered to be the limit based on the current 90 mm hull plating and engine sizes. Breaking this barrier would require heavier scantlings which, because of its weight, would cancel out the increased cargo capacity.

From an economical viewpoint, the propulsion systems are regarded as a limiting factor for growth in the size of container ships. Until recently, the largest 12-cylinder engines available developed a maximum power of 69,000 kW and which provide adequate propulsion for a post-Panamax ship of some 8,000 TEU with a speed up to 25 knots. The manufacturers now offer even larger engines which provide adequate power for up to 10,000TEU ships.

As soon as stronger engines are available, the propeller becomes the limiting factor for ship size with optimum efficiency and tolerable cavitation being the decisive parameters. From today’s viewpoint, ships with a stowage capacity ranging up to 10,000 TEU will still be able to sail with a single propulsion plant.

The problem is the 10,000+TEU ultra large container ship (ULCS) where the main issue is the power plant and propulsion system. A number of machinery solutions for future ULCSs, are discussed below:

l Single-screw installation

Although the 80,000kW required to drive a 10,000TEU ship at 25 knots is now available from the recently introduced 14-cylinder in-line engines from Wärtsilä and MAN B&W, Wärtsilä does not envisage building engines with more 14 cylinders owing to limitations in technology, manufacturing and handling. Power output can be increased up to 90,000kW for the single 14 cylinder engines if combined with a total heat recovery plant.

This power plant will be able to drive a 11,000TEU vessel but the single screw option appears to have limited potential for much larger ULCSs.

l Twin-screw installation

The most obvious alternative to the traditional single screw plant is a twin-screw installation. This has a number of benefits of which the chief one would be the redundancy gained from having two fully independent main engines. It is estimated that, although the twin screw ship will be more expensive to build, it would actually need some 3% less power than the single-screw ship which will give savings in operating costs. The lower power requirement for the twin-skeg ship is the net result of a combination of factors, namely larger wetted surface area, lower hull efficiency and increased propeller efficiency. It is possible to achieve an 11% increase in propeller efficiency for the twin-screw vessel.

Other propulsion concepts

In addition to the conventional single- and twin-screw proposals, there is the option of the contra-rotating propellers (CRP). Although not a new concept, it has the potential to achieve a power saving of about 10% while higher savings are possible by good design.

Large CRP installations have been built in recent years using an epicyclical gear drive to split the power mechanically from a single engine between the two propellers.

However, for a large container vessel, it might make sense to feed additional power into the CRP drive by electric transmission. A substantial portion of the propulsion power would then be provided by diesel gensets particularly if combined with a total heat recovery system. A concentric shafting for such a powerful plant has not yet been built but operating cost savings and a good propulsion redundancy are possible with this propulsion system. Good ?slow-steaming? characteristics can also be achieved by operating the vessel with only the electric drive.

A variation on the CRP theme could also be achieved by arranging an electrically powered contra-rotating azimuthing podded propeller immediately aft of the main engine-driven propeller. In this case, the power sharing between the forward and aft propellers is limited by the available sizes of pod drives.

Although the CRP pod concept might offer less propulsion power savings than the conventional CRP with purely mechanical drive, it does have additional benefits since the pod unit is used to steer the vessel thereby eliminating the need for a rudder. The pod drive also provides good redundancy with a completely separate plant.

Perhaps the most interesting possibility for the CRP pod concept is to use it to achieve a substantial increase in installed propulsion power. For example, a 25MW pod could be combined with a forward propeller driven by a 14-cylinder engine to provide a total output of 105MW which would be more than sufficient for a 13,000TEU ship.

Another concept which can be employed to increase propulsion efficiency is the Lips Efficiency Rudder (LER). This features a fixed bulb attached to the rudder horn immediately aft of the propeller. The rudder blade can also be equipped with a flap at its trailing edge to increase the lift generated by the rudder. The LER offers gains in propulsion efficiency of 3 to 7% compared to single screw ships with FP propellers, and 2 to 5% for twin-screw ships. The LER also reduces vibration caused by pressure pulses from the propeller acting on the hull. This is of particular relevance for vessels with heavily loaded propellers, such as container ships.

The author wishes to acknowledge Wärtsilä”s assistance with this article.