Tag: oceanographic

Why you need breathing space between glass floats on oceanographic moorings

When you’re in a rush, finding a place to park your car can be a relief. But relief can turn to dread when you eyeball the spot and start to think it might be a squeeze. Trying to park a car in a place that doesn’t quite have enough space is stressful. When it’s tight, it’s more likely to damage things, leaving a gash in a wall or another car.

You know you need to get the angle right, but without much space, you might be frustrated going backward and forward in little bits to adjust. And when you’re next to a pillar or wall, somehow, you have to squeeze yourself in and out awkwardly through a narrow gap in your door. A little extra breathing space makes all the difference.

Similarly, breathing space makes all the difference when it comes to groups of glass floats on oceanographic moorings. There’s a good reason you need more than a narrow gap between them: it’s more likely to damage things when it’s tight. But the damage we’re talking about is more severe than bumps or scrapes.

Parking a car in a tight spot is stressful!

Why is breathing space between glass floats important?

Each glass float creates an uplift force because they’re hollow. They’re often made with particularly thick walls that make them incredibly strong and useful for all ocean depth ranges. But the problem is that although they’re incredibly strong, they’re not invincible. Glass is also very brittle – when it breaks, it tends to shatter catastrophically all at once. So when a glass float fails at some water depth, under immense hydrostatic pressure, there is an instantaneous and catastrophic implosion – an implosion so violent it creates a shockwave.

Glass floats in protective hard hats in a bundle on a mooring line. Picture credit: James Potemra U. Hawaii SOEST

You want a buffer from the implosion shockwave

If other glass floats are too close by, the shockwave can be enough to trigger even more failures – an event referred to as sympathetic implosion. But what exactly do we mean by breathing space?

It means spacing things out

It’s common to see clusters of glass floats on moorings. Because glass floats are small, they’re often used in bundles to provide enough flotation to support a mooring in the water column. Typically, these bundles are made up of a few pairs of glass floats spaced out on a short length of chain – perhaps a total of four on a 4m chain. The connecting hardware required to string the lengths of chain together introduces even more spacing, too. This spacing helps spread out the impact from the shock wave when any single float might fail. Spacing is helpful, but there’s another way to guard against the effects of implosion.

Implosion of a single float means an instant loss in uplift.

Too much loss in net uplift can mean the mooring may collapse to the seabed. Fortunately, redundancy is a direct way to counteract this. With some redundancy, the mooring can remain upright and recovered even if some floats implode.

A destroyed hard hat wtih imploded glass float. Picture credit: Dan Kot WHOI

But how many more floats should you add? It depends on the level of risk involved and how important the data and equipment on the mooring are. Understanding design measures are practical, but it’s not all you can do. What about reducing the chances of implosion from happening?

But just how common is implosion before rated depth?

Modern glass float manufacturing techniques have steadily improved quality and reliability over the years. Glass floats are always pressure tested to prove some capacity of depth rating when manufactured. Through years of designing moorings, it may be rare to see any implosions before hitting the depth rating.

Yet there are some cases when failures can be more common, though the underlying causes are not always obvious. Nevertheless, glass floats are usually reused over many deployments, and the risk of implosion increases as they age. Fortunately, this is where there is more control over the risk of implosion.

It’s not a snowball! Remains of an imploded glass float after implosion. Picture credit: James Potemra U. Hawaii SOEST

What can you do to reduce the chances of implosion?

Inspection of each of the glass floats used before and after deployment is helpful. Cumulative damage can show up in the form of spalling that looks like small chips off the surface of the float. Glass dust will start to appear inside the float from the load cycling as it is deployed at working depth and recovered. Too much of this dust, or glass chips above a specific size, means it’s time to retire the float. In addition to inspection, glass floats have their own particular handling needs.

Because glass is brittle, it doesn’t have the same capacity to absorb bumps and scrapes while handling them. Bumping a glass float on a bulkhead might seem like a minor accident, but it can cause a microscopic flaw in the glass. But under the incredible hydrostatic pressures at working depths, these flaws can be enough to trigger an implosion. Yet this is easily addressed by specific training in properly handling glass floats and is readily available from manufacturers like Nautilus.

Are there any alternatives to glass floats?

Syntactic foam is one alternative. These solid floats aren’t brittle and don’t have the same kind of handling requirements as glass floats need. They can also be engineered to withstand the immense pressures at most water depths. But there’s a downside. The larger the depth rating, the denser the foam must be for increased strength. This makes the float heavier and less effective at creating lift for the same volume as glass floats at more significant depth ratings.

Let’s look at examples of spacing

The CSIRO EAC mooring and SOFS moorings both have large groups of glass floats to support the mooring, particularly near the acoustic release assembly. Both use multiple groups of glass floats on short sections of chain, with varying levels of spacing. There are examples of a range of four to six floats spaced evenly on 4m chain section.

It’s time to summarize

Glass floats are commonly used, especially in applications reaching the deepest places in the ocean. They are cost-effective and extremely strong. But they are also brittle and can spectacularly fail when they implode at depth. An implosion shockwave can lead to a cascading failure through sympathetic implosion when used in clusters on a mooring. But you can minimize the problem by inspecting them before and after use, handling them properly, and using redundancy. Just don’t forget some space between them, too. A little breathing room makes everything easier – just you wish you had some when you’re stuck in a tight parking spot!

Next step

Check out some of the sample mooring layout files in ProteusDS Oceanographic Designer to see more detailed examples of glass float spacing. The CSIRO EAC and SOFS moorings have many glass floats near the acoustic release assembly. Download ProteusDS Oceanographic Designer and the sample files here.

Thanks to Nautilus GmbH

Thanks to Steffen Pausch at Nautilus GmbH for the helpful discussion and information on their Vitrovex glass flotation systems.

Join the interactive Oceanographic Mooring Design Seminar at the Buoy Workshop September 19, 2022

When designing moorings, it’s easy to get lost in the details of all the parts you need

Purpose-built software is needed to make it easy to find parts, arrange them, and adjust them into a design. ProteusDS Oceanographic Designer is a free purpose-built software tool for oceanographic mooring design. But learning any new software takes time.

Quickly learn the ins and outs of ProteusDS Oceanographic Designer in a training session during the Buoy Workshop. We’ll cover the design process and create basic mooring designs with the software. Use the interactive training session to ask questions on software use and collaborate with others in the oceanographic mooring sector.

The Design Process is not always a straight shot. The Mooring Design Seminar covers these points, but stays focused on Detailed Design with ProteusDS Oceanographic Designer.

Attend the training session virtually or face to face

In partnership with MTS and the Buoy Workshop, a room at the Cape Fear Community College will be used for the training session. Just like the Buoy Workshop has virtual or face-to-face attendance, the training is delivered virtually, so you can meet with the group or through a weblink from any location.

Details on the Mooring Design Seminar session:

who: DSA Ocean CTO Ryan Nicoll will host the Mooring Design Seminar via Zoom

 

when:  Monday, September 19, from 14:00-16:00 Eastern Time

 

where: via Zoom and also via meeting room at the Cape Fear Community College, Wilmington, NC. Zoom link and meeting room details on the meeting room are provided after you register for the Buoy Workshop.

 

what will be covered: an overview of the design process and how to assemble simple mooring designs with ProteusDS Oceanographic Designer

 

how much does it cost: It is included with your Buoy Workshop registration

 

what you need to attend: You’ll need a Windows PC laptop that can run Zoom and the freely available ProteusDS Oceanographic Designer. Download Designer from this direct link:
https://dsaocean.com/downloads/oceanographic/ProteusDS%20Oceanographic%20Setup.exe

Read more on ProteusDS Oceanographic here:
dsaocean.com/oceanographic

Register for the Buoy Workshop to join the interactive Mooring Design Seminar with ProteusDS Oceanographic Designer

Read more and register for the Buoy Workshop here:

 

Why it’s hard to distinguish between the effects of buoy and line drag on mooring knockdown

The southern border between the Canadian provinces of British Columbia and Alberta is, in places, very irregular. It might even seem to be random at first glance. Yet, there is some logical reasoning behind what is there. This border was measured and established over one hundred years ago – far before GPS or other advanced survey techniques were around to help out. So how was the border established?

It has to do with hydrology. In other words, at the highest peak, the direction of rainfall flow establishes the border. Where rainfall flows to the west, it’s British Columbia, and when it flows to the east, it’s Alberta. It’s this clear reasoning that helps to distinguish things.

But we’re not always so lucky to have something to help distinguish things. At first glance, you might be looking at a seemingly random result without any clue how you landed there. When it comes to oceanographic mooring deflection, there are two primary sources of drag: the buoy and the mooring. However, distinguishing between what is more critical to mooring deflection can be a real challenge.

The southern portion of the border between BC and Alberta highlighted in red is highly irregular, but it’s not random. The direction of the flow of rainfall is used to define the border and distinguish between provinces. Picture credit: Google Maps

The vertical deflection of a mooring is crucial to understand

This vertical deflection, also called knockdown, is an increase in depth of all the mooring components from steady drag loads. Because it’s an increase in depth, you need to watch any parts approaching their depth rating. Flotation that exceeds depth rating may fail and result in the mooring collapsing to the seabed. But knockdown may have detrimental effects on instrumentation data quality, too. But regardless, you can’t get a good idea of mooring knockdown without considering the drag loads on the system.

Why is drag significant in calculating knockdown?

Without any currents and the resulting drag load, subsurface moorings are perfectly vertical in the water column. Knockdown results when a subsurface mooring leans over from the effects of steady loads like current drag. All components on the mooring will have some drag. But often, there are two primary sources of drag that tend to dominate mooring deflection and knockdown. The first is the drag on the primary buoy.

The primary buoy is a significant source of drag for a few reasons

The primary buoy is at or near the top of the mooring and often does the bulk of the work holding everything up in the water column. This means it tends to be relatively large compared to other components. On top of this, generally, but not always, currents tend to be larger higher up in the water column. This means the primary buoy is likely in higher flows than the rest of the mooring. Drag is proportional to structure size and flow speed, which explains why the primary buoy is a significant source of total drag on the mooring. But it’s not the only source to consider. The other source of drag to consider is the mooring line itself.

The NOAA CO-OPS DWEAB mooring uses a low-profile Mooring Systems Inc ellipsoid buoy. Picture credit: Laura Fiorentino

At first glance, a mooring line may seem minuscule compared to the primary buoy

Often the diameter of the mooring wire is only a tiny fraction of the size of the primary buoy. But the problem is that the drag area of the mooring is a function of both the diameter and the mooring length. When you consider the entire length of the mooring, the drag area can often be comparable to the top float. There is often at least some kind of current profile through the water column, and this means there can be a substantial amount of drag that accumulates over the whole system.

However, there are more details to the drag on the mooring than the drag area. The actual current profile is vital to get right. The inclination angle the mooring makes in the flow is a factor, too. The effect of inclination on drag is not something you can estimate as quickly by hand as by comparing drag areas. But suffice to say, it’s important enough that you shouldn’t ignore it, and it may even surprise you how important it can be.

How are these effects calculated?

Mooring deflection is typically calculated using well-established numerical techniques that factor in all mooring components, dimensions, and forces in a specific current profile. Purpose-built software like ProteusDS Oceanographic uses geometry, weight and buoyancy of the parts, and typical drag parameters to resolve the mooring deflection. The mooring deflection is a calculation of all these effects together.

So what’s more important: mooring or buoy drag?

It’s not easy to know, in general, what is more important. But we can use a validated example to show mooring and buoy drag contributions. NOAA CO-OPS uses the Deep Water Elliptical ADCP Buoy (DWEAB) design as a standardized design to measure the top portion of the current profile in various locations in the coastal United States. In a recent deployment in a high current region of the Gulf Stream, mooring knockdown field measurements show 48m in 2m/s flow. Mooring knockdown calculated by ProteusDS Oceanographic reached 52m in a 2m/s flow, reaching reasonably close to the field data results. The primary buoy is a large 58″ Mooring Systems Inc Elliptical float, while the mooring is only a 1/4″ in diameter. At first glance, it may seem like the primary buoy would dominate the mooring deflection because of its larger drag area. So how can we get an idea of how much buoy and mooring drag contribute to the knockdown?

To conceptually illustrate this, we looked at two additional load cases. Mooring deflection was calculated with drag on the primary buoy turned off and then again with the drag on the mooring line turned off.

The resulting numerically calculated knockdown with only primary buoy drag was 25m, while the knockdown with only mooring drag was 20m. Note that the total expected knockdown is not calculated by adding these deflections together. This particular example aims to give a rough idea about their contributions to the mooring knockdown for this system. In this specific example, the mooring drag is almost as significant as the buoy drag.

The 200m long DWEAB mooring deflection to scale in 325m water depth in 2m/s current

What if you don’t know the current profile ahead of a deployment?

A knockdown calculation is only as good as the information you have. Of course, before deployment, you may not have much information about the water flow in a region. In this case, it helps to think more about the boundaries of the problem rather than finding the most accurate or probably current profile. The most conservative approach is a uniform current profile. But that may only be appropriate in areas of intense flow. A linear shear, or power profile, may be more realistic. Still, it’s up to the designer to use their judgement and experience for what works best.

Summary

Mooring knockdown is crucial to understand ahead of deployment. This deflection can compromise data quality and damage equipment if depth ratings are exceeded. Knockdown happens in subsurface moorings as it leans over from steady current drag. The primary buoy is often a sizable source of drag in the system. But it’s a mistake to ignore the drag from the mooring line because it can make a surprising contribution to deflection. Unlike considering rainfall to determine a border between two Canadian provinces, there isn’t a way to easily distinguish the contribution of deflection from the primary buoy and mooring. Instead, the best approach is to make sure your analysis considers drag from all components at some point in the design process.

Next step

As we saw with the DWEAB mooring in the high flow Gulf Stream current, drag loads can be a dominating, even overwhelming, force. But there are times when drag doesn’t do much at all to floating systems, even if there is a lot of motion. Read the next article here about why viscous drag forces can be underwhelming at damping floating system motion.

Thanks

Thanks to Laura Fiorentino and Robert Heitsenrether from NOAA CO-OPS for providing data and pictures for the DWEAB mooring deployment used in the example.

PS

Download and explore the ProteusDS Oceanographic DWEAB 325m mooring file in the collection of designs available for download here.

When you need to fine tune surface buoy damping (and when you don’t)

Professional athletes make artistic synchronized swimming look effortless. Yet it is an incredibly demanding sport: it requires tremendous muscle power, endurance, and control to tread water and perform an elaborate dance simultaneously. On top of that, swimmers need to simultaneously keep in perfect sequence with a group in the water. So how do they keep synchronized while many of them are entirely underwater?

It’s essential to use special underwater speakers. The swimmers can each hear the music used for their performance loud and clear both above and below the water at the same time. Without this, there’s no way to keep the routine synchronized. Underwater speakers are crucial to fine tune the details.

Sometimes you have no choice but to fine tune the details to succeed in what you are doing. But sometimes a quick approximation might be all you need to get to the next step. This is certainly the case in surface buoy hydrodynamic damping: you can spend a lot of time and energy fine tuning hydrodynamic details, but there are workarounds. What we’re going to cover is:

  • Ignoring buoy damping altogether
  • Using a simple approximation when buoy motion may be important
  • Dialing in the details when buoy motion is crucial

First, we will cover the first point on ignoring damping altogether.

Special underwater speakers help artistic synchronized swimmers keep coordinated even when their ears are below the water surface

When can you ignore damping altogether?

Buoy damping comes into play when there is lots of relative motion between the buoy relative to the water surface. Suppose a surface buoy is light enough and has enough flotation to track the water surface, even in extreme storm conditions. In that case, buoy damping won’t have much of an effect on the system response. This kind of scenario often happens in deepwater oceanographic moorings.

In many deepwater oceanographic systems, the surface buoy may have a primary role in supporting the weight of the mooring itself. Full ocean depth moorings may have 5 kilometers of mooring line or more, bristling with dozens of instruments along most of the span. Surface buoys of a mooring like this are often very light, with a lot of extra flotation to support the weight of the mooring line and instruments. These buoys may be so large and light in the water they need a substantial mooring weight to stay stable and upright in the water!

In deepwater mooring, buoy heaving, or up and down motion, drives the dynamic mooring loads. Many surface buoys closely follow the water surface in moderate and extreme storm conditions. In this case, there isn’t much need to resolve the damping effects of the buoy itself when computing mooring loads. But not all surface buoys follow the wave surface perfectly, which can affect dynamic mooring loads. Also, buoy motion and acceleration may introduce constraints and limits on equipment and needs to be better understood. In these cases, ignoring buoy damping may not be good enough. This brings us to the second point about approximating buoy damping.

Some moored systems have instrumentation only in the buoy

The surface buoy may also be much larger and heavier to accommodate an extensive suite of devices to measure waves, wind, and current conditions. If the buoy is larger and heavier, it is less likely to merely follow the water surface in heave in most sea states. On top of this, if the system is in moderate or shallow depth water, the more detailed motion of the buoy – beyond only heave motion – may significantly influence the dynamic mooring loads. In these cases, it may be good to look into more detailed buoy motion, which means you need a rough approximation of buoy damping. But how can you make this rough approximation?

It doesn’t take much to make a rough approximation of buoy damping

It takes relevant experience or knowledge of the particular buoy you’re using. The main idea is you need to know roughly how the buoy responds in calm water to heave and tilt. This is sometimes referred to as the decay response. After a slight disturbance in heave or tilt, are there many motion oscillations before it calms down, or does the motion damp out and settle down right away?

You don’t need to be exact. Nevertheless, the expected decay response combined with the hull geometry, mass, and inertia defines the linear buoy damping. The buoy damping becomes an additional input into a dynamic analysis tool to resolve the mooring loads and buoy motion in various sea states.

While it’s a rough approximation, it’s still an excellent way to make progress on a mooring design problem. It’s an improvement on ignoring damping completely, too. However, more detail in buoy damping is warranted when accurate buoy motion is crucial. This brings us to the third point on dialing in the details on buoy damping.

In some cases, buoy motion has a critical impact on sensors or safety systems

Buoys using current or LIDAR wind profilers are particularly sensitive to surface buoy tilt motion. Visibility of Aid to Navigation buoys, and therefore safety at sea, is directly affected by the amount of tilt. If you’re uncertain about buoy damping, this directly translates into uncertainty about the buoy’s motion affecting data quality and equipment performance. So how do you dial in the detail of buoy damping?

The source of linear damping for floating systems is wave radiation

Wave radiation effects are often solved using potential flow software tools. These hydrodynamic software tools resolve how a moving hull shape creates radiating wave patterns at a range of motion frequencies. Dynamic analysis tools can then use the resulting forces to refine buoy motion.

An example of a potential flow software tool like this is ShipMo3D. The wave radiation forces are computed for a range of motion frequencies and all degrees of motion for a particular surface buoy hull shape. The dynamic analysis tool ProteusDS uses hydrodynamic data from ShipMo3D. Using these tools, mooring designers working with ProteusDS can incorporate more detailed buoy damping effects.

Do all buoys need to resolve wave radiation forces?

Not necessarily. Wave radiation effects are often only significant in larger and heavier buoy hulls that tend to displace a lot of water. These larger and heavier buoys may not simply follow the water surface in most conditions, and a more detailed look is warranted. Experience with a particular buoy form factor may provide the data and expertise to work with a rough approximation only without the need to get into the details of a tool like ShipMo3D. But the decision to use a tool like ShipMo3D should not be taken lightly: it takes time to collect necessary information, set up and compute the system hydrodynamics and validate it. A simple approximation may take a small fraction of the time and still get reasonable results.

Let’s look at an example

In this example, we are going to illustrate what happens to buoy motion when linear damping is ignored altogether. In a partnership between Nortek, AXYS Technologies, Caribbean Wind LLC, and NOAA CO-OPS, a shallow water surface mooring was deployed to explore the influence of buoy tilt on ADCP measurements. The buoy was self-stable with heavy ballast plates to keep it upright, while a low tension mooring helped keep it on station. This is an ideal system to compare with simulated motion predictions from ProteusDS because the buoy is self-stable and the mooring should not have a dominating effect on tilt.

Surface buoy configuration with ballast plates and Nortek Signature 1000 ADCP

A nearby bottom-mounted AWAC was used to record and verify the sea state condition independent from any measurements on the buoy itself. The Nortek Signature 1000 ADCP mounted on the buoy recorded the tilt angle of the buoy in a range of sea state conditions.

A ProteusDS model of the mooring and buoy was constructed to compare with measured results. The buoy was modelled as a rigid body with a cylindrical hull with viscous drag coefficients and no additional linear damping. The resulting buoy tilt in a 3m significant wave height sea state was measured at 13 deg standard deviation with maximums around 50 degrees. The ProteusDS simulated buoy tilt response showed a 12 degree standard deviation with extremes around 60 degrees tilt.

Surface buoy moored profile in 19m water depth

It’s summary time

All surface buoys will have some damping that will affect their motion. Some of this damping can be from wave radiation effects. But the critical question is how much does this damping affect the function of the buoy and mooring? You may be able to ignore it entirely if the focus is on mooring loads. But larger buoys used for measurements that are sensitive to motion or navigation safety may need to take a closer look at evaluating the buoy motion. A simple approximation based on experience can help. Still, there are also advanced hydrodynamic tools like ShipMo3D that can shed light on the problem, too.

A synchronized swimming performance may look like magic in how the athletes keep their timing. Swimmers need to be precise to get through their routine. When it comes to mooring design, you may or may not need this kind of precision to get through a design process. At least you don’t have to hold your breath the whole time!

Thanks

Thanks to David Velasco from Nortek for providing data and insight into the shallow water buoy used for the example, and for AXYS Technologies, Caribbean Wind LLC, and NOAA CO-OPS for publishing their work on the collaboration. Read more on their work published here.

What the playground can teach us about resonance in dynamics

Kids are always happy to visit a playground. When my son turned three, the swings became one of his favourites. He always wanted to go higher and higher. At that age, he hadn’t quite figured out how to swing by himself yet, though, and needed a push to keep going. Fortunately for me, swings only need a little effort and get a significant response. In this way, swings can teach us a lot about dynamics, and in particular, resonance.

The key to resonance is that a little effort can mean a big response. Knowing how resonance works is essential because it can make or break your system. So what is resonance?

Small kids need a push to get going on the swings. Fortunately, resonance helps out here, as small pushes over time lead to large motions. And happy children!

Resonance is a large response to a small disturbance

In mechanical systems, a large response might mean large amplitudes of motion. The thing about resonance is that it is often inherently a vibration. So these large responses are in some way an oscillation – and that means the external disturbances also need to be an oscillation as well.

So how does resonance work? Resonance can only occur when a system has some form of inertia as well as a restoring effect. This means a physical mass to provide inertia. The restoring effect is any kind of force that acts to bring this mass back into an equilibrium position. The specific combination of inertia and a restoring force produces a natural frequency. This natural frequency appears when the mechanical system is in motion without any dominating external force. It’s when external forces, even tiny ones, come into play at a rate around the natural frequency that you get resonance.

The swings are a perfect example of resonance

In this case, my son provides most of the inertia. Gravity provides the restoring effect that always tries to bring the swing back into its center position. Now all I need to do is give a little push at the right moment, and with this bit of effort, after a little while, he is soaring up high into the sky (and typically demanding to go higher).

Another example of resonance is ship motion response

Often, the roll response of a ship can be a problem. All ships have a certain amount of inertia to them. Depending on the loadout and shape of the hull, the ship will have a certain amount of restoring effect in roll, too. The problem with resonance, in this case, is when the frequency of ocean waves line up with the natural frequency of a ship in roll – and then you get roll resonance.

This can create extensive roll motions or large roll accelerations – causing people to get seasick, fall over, get hurt, or damage equipment on the ship. The MSC Zoe lost 350 shipping containers in a rare storm that was partially attributed to roll resonance. So keeping an eye on ship motions and how big these motions get is a big concern in ship seakeeping analysis.

The MSC Zoe lost 350 shipping containers during a rare storm that resulted in roll resonance. Picture credit – Hummelhummel, Wikipedia Commons, License CC-BY-SA 3.0

Is resonance always a problem?

Resonance can be good and bad. A lot of engineering systems rely on resonance to work correctly. But resonance can also spell disaster. If minor disturbances create significant effects, there will be countless opportunities to make large forces and motions and damage equipment or get someone hurt.

Damping can drastically reduce the resonant response. Back to the swing set at the playground, there is only a bit of air drag slowing things down. So it tends to be an excellent example of how little effort can lead to a big response. That little effort, such as a helpful push from a parent, needs to be periodic and applied at just the right time, though.

Back in the marine world, there are examples of significant damping in ship motion, too. For many ships, wave radiation considerably damps pitch motion. As a result, resonance is not always a big concern for ship motions in pitch. Regardless, carefully understanding when and how a system might reach resonance is essential.

Can you always figure out resonance?

The more complex the system, the more difficult it is to figure out how resonance works and whether it is a problem. In ship seakeeping analysis, it helps to have a specific software tool that takes all the details of a ship, including the hull shape and inertia, to establish just how the system will move – and possibly resonate – in different sea conditions.

Summarizing

Resonance is when small disturbances lead to a large response. In mechanical systems, it’s a vibration effect, and so you can’t get resonance without some kind of inertia and a restoring force. Resonance is a good thing in the playground as it helps me keep my son happy without a lot of effort. But it can lead to disaster and damaged equipment if you don’t keep an eye on it.

Next step

In one of the examples, we covered how resonance is a dangerous condition that can show up in ship motions. A seakeeping analysis is what helps understand just what kind of ship motion will occur in different sea states, and if resonance is a concern too. Read more on what seakeeping analysis is all about here.