For some time now shipping has hidden gratefully in the shadow of the aviation industry when the subject of greenhouse gas emissions has been in the news, taking comfort from the knowledge that it remains the most environmentally friendly means of transporting freight around the world.

But recently this has changed. There have been damaging headlines such as 'Ships' CO₂ twice that of planes' and a widely-publicised report stating that particulate matter from ships causes approximately 60,000 deaths a year from respiratory illnesses.

Today, when we talk about commercial marine transportation, we are essentially discussing diesel-driven motor ships. When we talk about the environmental impact of shipping, many of the issues are those associated with the diesel engine.

And while engine-driven ships do not have to emit sulphur compounds, if they are using any form of hydrocarbon as fuel then chemistry dictates that around three tonnes of CO₂ will be emitted for every tonne of fuel burned. This applies as much to clean fuel, such as natural gas, as it does to heavy fuel oil.

The best marine diesels currently achieve a thermal efficiency of roughly 50 per cent; that is, 50 per cent of the available energy is converted into motive power. The thermodynamic nature of the diesel cycle prevents it from being possible to reach 100 per cent efficiency. Even if this could be done, continuing the recent five per cent annual increase in the volume of shipping will double energy requirements in less than 15 years; and so converting all existing and new ships to hypothetically perfect heat engines would only maintain current emission levels.

Of course, not all ships currently use their engines at peak efficiency, not all hulls are in good condition, and many of these and the propellers, can be made more efficient by 'bolt-on' measures, or simply by optimising operational draft and trim.

BMT Group has been assisting several major ship owners to improve their fuel consumption by combining real-time monitoring with operational and maintenance measures.

These are projects for which no regulation or governmental support is required; economic self-interest is quite sufficient and could become more so if, as expected, CO₂ emission trading is introduced to the industry in 2011. Similar initiatives are underway elsewhere, such as Canada's SmartShip programme which is looking at the role IT, intelligent computation and robotics can play in improving the management of ships.

Some other rather more dramatic, and certainly more visually arresting, measures to reduce fuel usage have been proposed. A decade ago, MIT researchers built a propulsion system that utilised two oscillating blades that produced thrust by sweeping back and forth in opposite directions. A 12-ft scale model fitted with the blades, (named the 'penguin boat') showed 87 per cent propulsive efficiency compared with 70 per cent for a conventionally propelled ship. Based on laboratory results, MIT's penguin was capable of moving as fast as conventional propeller driven craft — and was possibly easier to manoeuvre.

Another efficiency breakthrough is 'Shark Skin', an environmentally friendly coating that mimics shark skin. Existing antifouling paints such as tributyltin, or TBT, kill algae and barnacles when they latch on but this is being banned by the International Maritime Organisation (IMO). Copper-based paints are less harmful than TBT, but are still toxic. 'Sharklets' use a combination of billions of plastic-and-rubber raised diamond-shaped patterns to replicate shark skin. In lab tests the coating reduced by 85 per cent the settlement of spores from Ulva, a common algae often seen on the sides of ships.

A company called Skysails is proposing the use of a kite sail as an auxiliary means of propulsion. This has an area of up to 3,000 sq m depending on the ship's dimensions and is manoeuvred automatically by routing software coupled with the control system of the kite. The company claims fuel reduction between 10 and 35 per cent depending on wind and sea conditions, and ship characteristics. The first commercial ship is scheduled to be fitted this year.

The use of air bubbles to reduce friction through the water has also been the subject of experimentation, and the DK Group has successfully developed and model tested an air cavity system. The cavity is formed by introducing a recess into the hull which is fed with air from a compressor. Test results show the carpet of air reduces resistance through the water to such an extent that claimed fuel savings of up to 15 per cent are possible. Full-scale tests are in progress and the first orders have been placed for a series of 200,000dwt (deadweight tonnage) bulk carriers.

Perhaps the most interesting concept is the E/S (environmentally-sound) Orcelle unveiled by Wallenius Wilhelmsen Logistics. This zero-emission, pentamaran car carrier draws on the sun, wind and waves for its power. Equipped with three photovoltaic-covered wingsails, 12 power-generating fins and fuel cells, it can carry 10,000 cars and is expected to cruise at about 15 knots. Its environmental credentials are further enhanced by the elimination of the need for ballast water. Wallenius suggest that a ship with some of Orcelle's features could be launched within five years — but a complete version might not be seen before 2025.

However, the greatest potential for marine transportation to actually reduce global greenhouse gas emissions lies not in small improvements in fuel efficiency but in promoting the shift from road, rail and air to marine, wherever practical.

Studies of relative emissions show that a tonne-kilometre of oceangoing marine transport uses around one-fiftieth of trucking fuel; and the true total costs are probably in a similar ratio. Over shorter distances, the energy advantage falls, but sea transport remains vastly better than road and about twice as efficient as rail. Air freight comes out significantly worse than any other mode.

Source : The Engineer Online