Designing green ships, from sails to micro-bubbles


Maritime engineering is no exception in worldwide effort to save energy and protect the environment. In 2008 the International Maritime Organization, a UN agency, set up its Marine Environmental Protection Committee (MEPC) to draw up and implement regulations to reduce greenhouse gases from shipping.

Generally, the energy efficiency of ships is very high compared to other forms of transport. The energy required to carry one tonne of cargo one kilometre is less than 1.5% of that required for an aeroplane, and 15% that of road haulage. But while shipping is the most effective transportation in terms of CO2 emissions, without regulations to limit them these are projected to triple in the next three decades.

The reduction of greenhouse gases and other air pollutants from shipping has been approached in three ways: designing ships to be energy efficient, optimising ships’ capacity and shipping routes, and regulating the shipping business. The IMO will require ships to reduce their energy use, based on an efficiency index known as EEDI, by at least 30% by 2025, for example.

Several methods to reduce air pollutants are under consideration. First of all most of the harmful gases, such as 95% of sulfur oxides (SOx) and nitrogen oxides (NOx), can be collected by equipping proper filters devices in exhaust gas system. Reducing CO2 emissions is more difficult, the most promising method is to adopt a liquid natural gas-fueled engine. LNG is more environmentally friendly than oil-powered ships, and entirely electric engines are not mature enough to be used for very large commercial ships.

Don’t be a drag

The development of energy-saving devices is also underway in order to increase the ship’s propulsion efficiency. For example, specially designed fins on ship’s hull or propeller blades, waste-heat recovery systems, or creating micro bubbles around the ship’s hull to reduce drag in the water.

Recently container ships fitted with kites large enough to pull the ship have been tested. With the cost of fuel oil being a major expense, this has led to a surge of interest in fitting the square-rigged sails of clipper ships that plied the Atlantic trade winds only a century ago. More energy saving can be achieved from designing hulls with lower resistance, which would also use less fuel. While some techniques have already been trialed, others are still at the testing stage, but it’s a quickly developing area.

A ship’s speed makes a very large impact on fuel consumption. In general, speeding up a ship requires a dramatic use of fuel. In the case of large container ships, oil consumption increases to the power of three (3n) with ship speed. Increasing speed by 10% increases oil consumption by more than 33% for example, while if speed decreases by 10%, fuel use is reduced by 27%. These days slow steaming is a general trend in the shipping business, which shipping firms make up for by building and operating larger and larger ships to carry more cargo.

Bigger is better

On June 14, 2013, the world’s largest container ship was launched, built by the Korean company Daewoo Shipbuilding and Marine Engineering for Maersk, the world’s largest shipping firm. The only ship in the 18,000 TEU class (Twenty-foot Equivalent Unit, the size of the universal shipping container), the Danish firm has ordered 20 of these giant ships, which when fully loaded are larger than aircraft carriers or ocean liners like the Queen Mary 2. At 399m long and 59m wide it is significantly larger than any other and can carry 18,270 TEU containers, which if lined up would stretch more than 111Km. Ship size has been determined mostly by the need to fit the width of the Panama or Suez Canals, or to fit particular container ports. However these huge vessels signify the start of a new generation, and it will have a major impact on port management.

More importantly than their huge size, Maersk’s newest container ships are the most environmentally friendly vessel in the business. Known as the Triple-E series, this stands for Economy of scale, Energy efficient, and Environmentally improved. Compared with smaller, conventional 8,000 TEU container ships, the Triple-E series requires 50% less fuel to move the same amount of cargo, produces 20% less CO2 emissions than 15,500 TEU container ships, and will cut 100,000 tonnes of CO2 every year. A waste-heat-recovery system is also installed, with an expected 10% increase of engine efficiency. The shape of the ship’s hull has been designed based on systematic hydrodynamic research to minimise resistance, and the build includes other environmentally friendly machinery and materials. Taken together, all these design improvements mean these huge ships use 80,700 horsepower - using only 10% more power than 10,000 TEU container ships but carrying 80% more cargo.

Today the demand for more environmentally friendly ships is growing. The construction of the Triple-E will be the trigger to build that demand and kick-start further “greenship” technologies that will deliver even bigger, more efficient means to trade across the world, without costing the earth.