Why the sensitivity of ship roll damping makes naval architects sweat
Originally published on dsaocean.com
Dolphins might act like they have x-ray vision. But it’s not their eyes that help them see through the ground. Their fine-tuned sonar capabilities have made them a valuable part of maritime security and the protection of coasts and harbours for decades. They’re used to living and working in all ranges of conditions, including murky water and acoustically noisy environments – where there are many other animals and lots of marine traffic. Trained Dolphins can easily find intruders or even mines in places and conditions where our own sonar technology completely falls flat. In fact, Dolphins can differentiate between entirely distinct types of metal, like brass, aluminum, and steel, buried almost a meter under the seabed because their natural sonar is so sensitive.
But a high level of sensitivity can be both a blessing and a curse. It can be a curse because seemingly minor changes can cause a completely different response. When it comes to ship motions, roll is extremely sensitive. Small changes in the ship design can result in significant changes in motion characteristics. In this article, we’re going to talk about why naval architects sweat the details to really understand just what will happen to a ship in roll. So why are ship roll motion so sensitive?
Not all ships have sensitive roll
Barges and multi-hull ships like catamarans and trimarans don’t tend to have sensitive roll. The kind of ships we are talking about are smooth monohull systems. The key is that there is a single primary hull that is relatively slender in terms of length to beam ratio. Often, monohull ships are quite smooth with a round form, which is one crucial reason roll motion is so sensitive. The rounded shape means there is very little damping resistance when it rolls.
What do you mean by resistance?
By resistance, I mean either ocean wave radiation or viscous damping. Barges are large and flat – almost like floating paddles – and their particular shape is ideal for generating waves when rolling. These radiated ocean waves absorb massive energy, making an enormous amount of resistance that slows down barge motion in roll. A multi-hull vessel like a catamaran also does an excellent job of radiating waves when rolling. This is because of the way the flotation hulls are offset from the centerline. But a rounded monohull makes almost no waves when it rolls and, as a result, relies heavily on viscous drag for resistance.
But viscous drag damping doesn’t work very well in oscillation
Even if the ship roll velocities are high, because of the back-and-forth motion, drag loads tend to be weak. Because of the low resistance, even relatively mild ocean wave conditions can lead to significant roll motions and accelerations. This can limit ship operations or, worse, cause injuries. Equipment, cargo, or the ship itself can be damaged, too.
The result is that naval architects spend a lot of time and energy thinking about ship roll
A lot of time and energy goes into the design details of the shape of the hull and appendages that help amplify viscous drag resistance. Features like bilge keels are often added specifically to create viscous resistance to help reduce roll motion. Other technologies can help, including passive and active systems like antiroll tanks and stabilizers, but simple appendages are the most straightforward place to start. But with hull appendages, it’s vital to understand the viscous drag effects reasonably well because of how they can substantially impact roll motion performance. So how is viscous roll damping evaluated?
One way to evaluate roll motion is from physical scale model testing
But this approach is by far the most costly and time-consuming. A scale model of a ship in a wave basin will verify the ship’s expected roll response based on the design details used. But because of the costs involved, it’s not an economically viable approach to iterate on the design of the hull and appendages – it serves more as a confirmation of the expected roll performance. Yet there are alternative numerical methods that work well in evaluating roll.
Roll damping can be computed with Computational Fluid Dynamics tools
These software tools resolve the physics and details of the water flow interacting with the hull and appendages. While they provide a great deal of useful information and are helpful for a degree of design iteration, it can be a computationally costly and time-consuming approach. A tight project timeline won’t leave room for investigating many different loading conditions or design iterations on hull and appendage configurations. But there is more than one numerical approach to resolve ship motion.
Some numerical models use empirical relationships to resolve the viscous forces
Empirical equations describing viscous roll resistance are helpful but have more significant uncertainty than the other approaches. Yet ship designers can use them across various conditions and appendage configurations. They are also very computationally efficient and can help screen out different designs quickly. Dynamic analysis tools like ProteusDS are designed to set up typical hull design, including appendages and resolve ship motion in a broad range of sea and ship conditions much faster than CFD tools.
It’s example time
So how much difference can something like bilge keels make to damp roll motion on a smooth-bottomed monohull vessel? Here is a comparison of ship roll response of the Generic Frigate computed by the ProteusDS ShipMo3D toolset with and without bilge keels.
A roll decay comparison starting the vessels at 10 degrees shows a significant difference. While bilge keels don’t eliminate roll oscillations entirely, they are substantially better than nothing at reducing roll motion. But this is only the roll decay response. What happens to the ship motion response across a range of wave frequencies? The RAO can give some insight.
The RAO gives a broad spectrum idea of how the ship will respond, including at resonance. Often, for slender monohull ships, one of the worst-case loading conditions is in a beam sea. The roll RAO of the Generic Frigate with and without bilge keels in a beam sea condition is illustrated below. The RAO shows that maximum roll motion is reduced by a factor of two! There is also much lower roll motion in the region around the peak as well. This implies that the bilge keels help significantly control roll motion, especially around the natural frequency in roll.
It’s time to summarize
We covered a few details on ship roll response, and now it’s time to review. Roll response is something to get right because the stakes are high – large roll motions can make the ship inoperable, cause injuries, or even damage. Not all ships have sensitive roll response, and usually, it’s an essential issue for monohull vessels. The slender form of the vessel with a rounded bottom means there is little resistance in roll.
These vessels don’t radiate ocean waves very well compared to other vessel forms like barges or multi-hulls like catamarans and trimarans. So monohulls rely almost totally on viscous damping to control roll motions. Viscous forces are weak in the back-and-forth oscillatory motion in roll. So ship designers and naval architects spend a lot of time characterizing the viscous roll effects however they can. Like dolphins measuring different materials through seabed soil and mud layers, ship designers will use whatever tools they can to measure ship roll response.
Bilge keels are one way to help control roll motion, but there are other systems that help. Anti-roll tanks are another example that can make a big difference on roll activity. Read more on what anti-roll tanks are, how they work, and how to evaluate them in the next article here.