Ballast-free ships

While this blog is about ballast-free ships, it is important to understand why cargo ships require ballast in the first place. What is ballast, and what role does it play in keeping a cargo ship stable?

Cargo ships are used to transport cargo, which is typically loaded at a port, transported across the world, and then unloaded. A major issue with unloading cargo, and therefore weight, from a ship is that it causes the ship to sit higher in the water (the ship becomes less dense, which makes it more buoyant). This is a problem because it can make the ship unstable on its return trip (or on a trip to a further port). 

Keeping things balanced

This is where ballast comes in. The ballast replaces the weight of the unloaded cargo, making the ship less buoyant so it sits in the water at a desirable (i.e. stable) position. In the distant past, ballast would be in the form of rocks and sand – shovelled onto the ship via manual labour. As an aside, the use of rock as ballast actually influenced architecture in some cities. Montreal is one such example – many of its buildings were made of Scottish sandstone which arrived as ballast on returning ships and had to be removed (to be replaced with cargo). So it was used in construction. 

The use of solid ballast decreased in the 1900s due to improvements in pumping technology which meant water could be pumped from the ocean to be used as ballast. Water remains the most common choice for cargo ships today. It is far more efficient than manually loading and unloading rock and sand and, using water, a ship can change its ballast while out at sea. 

Environmentally unfriendly 

As with many long-existing technologies, however, it has become apparent in more recent times that pumping water onto and then from a ship is not necessarily the perfect solution it was once thought to be. That is because seawater contains “bacteria, microbes, small invertebrates, eggs, cysts and larvae” (according to the International Maritime Organisation) which are taken onto a ship along with the seawater ballast and then transported by the ship to a new location where they are discharged into the sea. In other words, in addition to transporting your Amazon goods across the world, ships can also transport a whole host of other less desirable things in their ballast. 

This is an issue because foreign organisms can become invasive and out-compete native species. Bacteria can also cause disease – in one example, an outbreak of Cholera in Peru in 1991, which killed 12,000 people, was believed to have originated from ballast water shipped across the world. 

What’s the solution?

So, water as ballast clearly has some issues. What is being done to solve them? One way to minimise the environmental impact of ballast is to release the water some distance from the coast, where harmful organisms are less likely to cause problems. Another option is to include a ballast water management system on ships to actively treat the ballast water (e.g. using filters, chemicals, light, etc.) so that when it is later released into the sea it does not contain those harmful organisms. While these solutions can be successful in minimising environmental impact, they are also typically very costly to implement. This is why some engineers have been seeking an alternative. 

One proposed alternative is so-called ballast-free ships. The origin of the ballast-free ship concept appears to be research from the University of Michigan (in fact, they have a granted US patent for the concept). In general, the idea is to provide a continuous flow of water through the ship rather than holding (and thus transporting) a quantity of water. To allow for this, the proposed design modifies the ship hull to include three “trunks” (or passages) that extend from the bow (front) of the ship to the stern (rear), and which can be opened or closed as required. When opened, water flows through the passages, decreasing the “watertight” volume of the ship which decreases buoyancy. When closed and filled with air, the trunks become part of the watertight volume of the ship which increases the buoyancy of the ship.  

A pressure difference between the bow and stern of the ship ensures water moves along the trunks so that a given volume of water is only held in each trunk for about an hour while the ship is moving. The benefit of this is that any harmful organisms in the water are discharged within an hour’s sailing distance from where they were taken on, rather being than transported to entirely different waters. 

While the UoM design appears to be the first, it is not the only ballast-free design. Saudi Aramco came up with a similar patented arrangement in which water flows upwardly to an outlet at the waterline. Yokohama National University also proposed a (yet again patented) ballast-free system in which water flows into the underside of a ship, mixes, and then is discharged back through the bottom (see below). 

Can the ballast be removed?

Another somewhat simpler option is to avoid exchanging ballast water at all. Some solutions propose a V-shaped hull which naturally ensures the ship has a higher draught (i.e. ensuring the ship sits lower in the water in both the loaded and unloaded state). Gaztransport’s more recent (patent-pending) solution modifies the V-shape slightly by providing a trapezium-shaped hull that also provides this increased draught. Gaztransport’s ship also includes a “permanent” ballast system formed by a series of fluidly connected tanks filled with water that always remains on the ship but can be transferred between the tanks. The water and tanks are used to adjust the trim of the ship (front-to-back tilt) when it is unloaded. Uneven trim is a particular problem with ships (such as LNG transport ships) where the removable merchandise (i.e. the goods being shipped) is towards the bow of the ship and the permanent structures (such as the engines and superstructure) are at the rear of the ship. When the merchandise is removed, the ship naturally tilts towards the heavier stern due to the remaining weight of the permanent structures. Being able to move water to the front of the ship using the “permanent” on board ballast system can correct this, while avoiding the need to take any seawater onto the ship.

Gaztransport says that its system is cheaper to manufacture because it doesn’t require expensive ballast pumps, and is cheaper to run because it doesn’t require the maintenance associated with taking seawater onto a ship (such as dealing with sediment that inevitably ends up distributed throughout the ballast system). This therefore presents a great example of innovation at least partly driven by sustainability leading to a solution that is ultimately also more cost effective. 

While ballast-free systems aren’t going to be suitable in every situation (and they certainly aren’t going to be the norm any time soon), it’s great to see examples like these in which the world of shipping is spending serious effort innovating to provide more sustainable marine transport. Along with advances such as better fuel, and the use of big data for fuel efficiency, there is no doubt maritime transport is heading in the right direction in terms of a sustainable future.  

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