What Powers a Cruise Ship?

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A cruise ship is like a self-contained city. Just like a city, we need to keep the lights on, but we also run air conditioning and ventilation as well as actually sail through the water – no mean feat when you weigh over 40,000 tonnes! Every operation on the ship requires electricity, and at the moment this requires us to burn fuel.

Cruise Ship Engines

Most cruise ships sailing these days use a diesel-electric configuration. This means that each engine is connected directly to a generator, providing electricity for propulsion as well as the rest of the ship’s operation. Most ships have conventional piston engines, as they are versatile in what fuel they can burn. Some ships will use gas turbines (jet engines) powering a generator – they have a huge amount of power for their size and weight, but will only burn the lighter fuels such as MGO and so are relatively expensive to run.

One benefit of a diesel-electric setup is its ability to be scaled from the low power demands (5-10 megawatts) when in port, right up to the full power draw (25-40 megawatts) of a cruise ship at maximum sea speed. This is due to the use of several smaller engines. Some ships will have five or six separate engines, allowing them to start or stop each one as the power needs change. This contrasts with many cargo ships which use a single larger engine to provide all the propulsion power for the ship. By having such a scalable system, it allows our engines to be run at their most efficient range, reducing our fuel consumption and emissions.

A ship's diesel engine may stand 'only' 2-3 decks high. (CC0)
A cruise ship’s diesel engine may stand ‘only’ 2-3 decks high. (CC0)

Another benefit of having this engine setup is the flexibility it allows ship designers. If we had one big engine, designers are very limited in where they can place it, and it would extend up through the ship for several decks. Having several smaller engines to provide the power allows them to sit lower in the ship without reaching up to the guest decks. As we only need to route electrical cables from the engines to the propulsion, they don’t need to be aligned with propeller shafts as one large engine would. Finally using electrical propulsion gives many more options for what propulsion system we actually use – but that is another whole topic.

Cruise Ship Fuel

From Heavy Fuel Oil (HFO) to Marine Diesel Gas Oil (MDO or MGO), each type of fuel has its unique properties – and prices. The sulphur content of fuel is a key indicator of our pollution potential, and therefore many countries impose limitations on fuel types that may be burned in their waters. These limitations may be even more restrictive when in port, or when sailing in ‘Emission Control Areas’ such as those found in Europe or North America. Cruise ship operations are planned to incorporate these limitations, often having different tanks assigned to the various grades of fuel. Plenty of time is allowed to change between the fuel types, ensuring that we are well within the limitations as these areas are reached.

The majority of fuel we use is HFO with a limit of 3.5% sulphur – the highest sulphur content allowed to be used globally. The International Maritime Organisation will likely reduce this limit to 0.5% by 2020. This fuel is the cheapest, as it requires the least refinement, costing around USD$400 per tonne. We order fuel with different sulphur content depending on where we sail, and the limits imposed by local governments. Common limits are 2% and 0.5%, with the price increasing to cover the additional cost of producing these fuels.

Did you know: HFO is so thick you can scoop it with a shovel – we need to heat it up to be able to pump it around. To do this, the HFO tanks have pipes filled with steam running through them.

A barge or small ship is used to carry enough fuel to fill a cruise ship's tanks. (Jesse)
A barge or small ship is used to carry enough fuel to fill a cruise ship’s tanks. (Jesse)

MDO or MGO is almost the grade used in cars and trucks on land. This fuel has varying sulphur contents, right down to 0.1%, and we’re required to burn only this fuel in many ports and in special ‘Emission Control Areas’ such as in Europe or North America. This fuel is the most refined, and the most expensive, costing around USD$550 per tonne.

We load or ‘bunker’ fuel usually every 7-14 days, often over 1000 tonnes at a time. If we had to, we could keep the ship running for a month or more when we’re fully bunkered – but that would mean not sailing the ship anywhere! These bunkering operations can produce flammable vapours, so we take extra precautions to prevent a fire on board, and constant communication between the supplier and the ship’s Engine Control Room ensures everything goes smoothly.

Air Emissions Of Cruise Ships

To help reduce the air pollution from ships, some companies have invested in exhaust gas cleaning technologies. While these have been in use for many years in factories, power stations etc. for something as large as a cruise ship, there hasn’t been a lot of development done until recently. In their simplest form, these scrubbers spray sea water through the exhaust gas, capturing the sulphur into the water. This is then heavily diluted before being returned to the ocean. When functioning correctly these scrubber systems allow us to use cheaper fuel.

These systems need to be approved for us to use them in ‘Emission Control Areas’. This approval requires careful monitoring of both the air quality coming out, and the conditions of the sea water used in the system. If anything happens to this monitoring system, or any of the limits are exceeded, we need to change to a cleaner fuel immediately to ensure we still comply with local emission regulations.

Unfortunately scrubber systems such as this sea water design only move the problem from the air to the ocean. Other systems can use a ‘closed loop’ where the products can be captured and disposed of ashore, but we are also looking at other ways to power a ship without burning fuels at all.

Shore Power For Ships

In some ports around the world, the dock we tie up to is equipped with an electricity connection. These connections allow us to use electricity from the city supply to power our entire operation, allowing us to shut off all our generators. By doing so, we bring our air emissions to zero for our time in the port, resulting in a cleaner city! As more cities see the potential these connections have to improve the air quality for their residents, we can connect more often and cut down our overall emissions. However it isn’t an easy task for a city to put one in place!

Did you know: a ship totally shutting down it’s engines and generators is referred to as ‘cold ironing’ – a term from the days of steam ships who would allow their iron boilers to cool down when in port.

A mid-size cruise can use between 5 and 10 megawatts when sitting in port. This is the same as around 6000 homes! Being able to add this sort of power demand to a city supply takes an investment in infrastructure to provide the cables and transformers. Some cities such as Vancouver and Amsterdam have made their air quality a priority and have invested in these connections.

Ships also need to be properly equipped. Safely receiving this amount of power, and being able to transfer the electricity load from the ships generators to the shore power and back again, requires delicate control by the ship’s Technical Department. Should anything go wrong in this transfer of power, it can cause systems on the ship to shut down, or result in a complete power cut or ‘blackout’. The equipment required to manage this connection adds roughly a million dollars to a new build, but many companies see this as a priority to reduce their emissions.

Fuelling Future Cruise Ships

The shipping industry is working to develop cleaner technology all the time. Some being developed right now include natural gas or LPG, hydrogen and even battery power. Unfortunately the infrastructure to support these technologies isn’t widespread enough for the global operation cruise ships take part in.

Trials are in place for most of these using ferry trips between two specially equipped ports. This allows the supply chain to be set up, and the ferries to have the necessary equipment installed. Such installations may be found in parts of Europe, and are slowly being rolled out in other parts of the world. As trials prove successful, the supporting infrastructure may be scaled up to provide for coastal shipping, and eventually global trade.

In the meantime, companies do their best to reduce their fuel consumption both to save money, and their environmental impact.

How can you help save fuel?

  • Reduce your fresh water use. Most of our fresh water on board is created from sea water. To remove the salt by reverse osmosis or evaporators takes a lot of power, so we use more fuel.
  • Close doors to the outside decks and balconies. One of the major uses of energy on board is to support the ventilation and air conditioning. By keeping doors to the outside closed, the air conditioning doesn’t need to work as hard.
  • Turn off your lights when you leave your cabin. Just like at home, leaving the lights on uses more electricity. Though we use energy efficient lighting, some ships have over 3000 cabins, each with multiple lights.

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