How antiroll tanks work to passively reduce ship roll
Originally published on dsaocean.com
The Hippopotamus has four legs for walking on land, yet it is closely related to whales and dolphins. This shows particularly when it comes to sleep: Hippos can do so underwater. But they don’t have gills like fish – so how do they pull this off without drowning? It turns out they have a built-in reflex that helps them out. A Hippo will automatically gently rise, breathe, and submerge again without waking up. It is all done entirely passively.
When a system works passively, you don’t have to intervene, control, or monitor things closely. It frees you up to do something else (even if it’s sleep!). When it comes to controlling ship motion, a few systems work entirely passively. One example is antiroll tanks. So what are antiroll tanks, and how do they work?
What are antiroll tanks?
Antiroll tanks are large chambers partially filled with water on a ship. Typically, these structures take up space across the entire beam of the vessel. Channels may connect more than one chamber to facilitate air and water flow between the chambers. These tanks need to be significant to contain enough water mass that it can influence the motion of the entire ship.
Antiroll tanks can minimize the effects of roll motion on the ship
When designed properly, antiroll tanks can significantly reduce roll motion and acceleration. Reducing these severe motions and accelerations can substantially improve the vessel’s safe operation in unfavourable weather. It makes it easy to do work on the ship in all conditions, and minimizes sea sickness and the potential for damage to equipment, too.
The nature of these tanks is truly all about dynamics.
The significant mass of water in the antiroll tank can shift from one side of the ship to the other. But it takes a certain amount of time to move. As the ship rolls, this out-of-phase motion of water shifting from one side of the ship to the other creates resistance to roll motion.
When the antiroll tanks are designed well, there is both a beneficial static and dynamic effect. The static effect is from the weight of water on one side of the ship, which creates a counter-balancing roll moment that reduces roll motion. The dynamic effect is inertial: when the ship is accelerating in roll in one direction, the mass of water in the tanks is accelerating the other way, absorbing energy and resisting greater roll accelerations.
When do you use antiroll tanks?
It is not always obvious whether you should use antiroll tanks specifically or not. They are one of several options that a ship designer can consider to help improve the performance of a ship. There are always trade-offs in any design process. After all, these tanks use up a large volume on the vessel, so there is less space for cargo or other equipment. So the ship’s performance may be better, but it comes at a cost. There are additional costs as well, as these systems can create large forces within the ship and affect the structural design.
But despite these trade-offs, the safety and performance increase may be worth it. Often, they are well worth considering as an option for narrow ships with a relatively smooth and rounded hull that otherwise little roll damping. One common type of antiroll tank is the U-tube tank.
U-tube antiroll tanks typically have two significant chambers at either side of the ship
A sizable channel connects them at the bottom of the tanks to allow water to flow from one side to the other. An additional connection at the top also provides air pressure balance between the tanks. Without this air connection, the expansion and compression of air create pressures that resist the water flow and can prevent the tanks from working as expected.
These are the most simple antiroll tank design. U-tube tanks can be cost-effective as they are the most straightforward design. However, their antiroll capability depends on the tanks’ size and spacing, which means there is little room for any adjustment once installed. These may work best on ships that keep a relatively consistent displacement through their operation – such as military, coast guard, and pleasure craft. But there are other types of antiroll tanks, too.
A free surface tank is like one large container
Unlike a U-tube tank, a free surface tank is like one large tank. It might sound simple, but often there can be a bit of complexity. Often, there are baffles throughout the chamber to help adjust how much time it takes for water to slosh from one end of the tank to the other. This may be important to ensure the tank works as intended but also to help decrease the chance of sizeable internal slamming loads from the sloshing water. The dynamic response can also be adjusted based on the water level in the tank. This makes it more useful for ship designs that may have a broader range of displacement throughout their operational life. But this comes at a cost – more support and control systems are needed to add, remove, and measure the tank’s water level.
How can you properly size an antiroll tank?
Sizing an antiroll tank is not always obvious. Experience with existing vessels is always helpful, but not all designs are identical. Each new ship design comes with specific requirements. One possibility to size an antiroll tank is with seakeeping analysis software. Many seakeeping analysis software tools can compute ship motions in a seaway with and without antiroll tanks. This gives a way to establish the required size of the tank that will have a meaningful impact on ship motion. The ProteusDS ShipMo3D toolset is seakeeping analysis software with antiroll tank capabilities like U-tube and slosh tanks.
Let’s look at an example
We can add a U-tube tank to the Generic Frigate model in the ShipMo3D toolset to see the effect on motion response. A U-tube tank configuration can be seen in the ShipMo3D toolset validation report in section 10. Once the U-tube tank is configured in the software, it’s a relatively rapid calculation to evaluate the resulting effect on ship motion.
When the right dimensions are set up for the U-tube tank, the resulting impact on ship motion can be dramatic. The plot shows the change in peak roll response for the Generic Frigate with and without the U-tube tank. There’s almost a 50% reduction in roll motion during motion at the roll natural period.
You can see how to set up this U-tube tank with the Generic Frigate in this video tutorial from our YouTube channel. The U-tube tank is based on the same parameters as in the ShipMo3D toolset validation report.
It’s time to summarize
Roll motion in heavy seas can often hobble a ship. The result can be Interrupted work, seasickness, and even damaged equipment. Antiroll tanks are one option for ship designers to control roll motion and increase the ship’s safety and performance. The movement of water across the tanks creates resistive weight and inertia that reduces the overall roll of the ship. But they come at a cost – they take up valuable space on the ship and have their structural design requirements, too. Ship designers must carefully evaluate antiroll tanks to ensure they work as expected at sea. Fortunately, many seakeeping analysis programs, such as ProteusDS, have these capabilities, making evaluating and designing antiroll tanks cost-effective. Well-designed systems that work passively allow you to sleep easily – just like a Hippo who can rest easily under water!
This particular U-tube tank configuration is detailed in the ShipMo3D validation report. Read more about verifications of functionality like the U-tube tank and comparisons of ship motion predictions to tank scale and full scale ship motion data here.