Tag: oceanographic

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 MCS 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 MCS 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.

 

When wave steepness pinpoints worst case sea states for mooring design (and not wave height)

Vintage wind-up toys may seem simple, but they are mechanical marvels. Many intricate details turn a compressed spring into something like a walking robot. But while these designs are clever, they have their limits. For example, nothing stops them from tumbling off the edge of a table or down the stairs – they wander entirely aimlessly.

Wandering aimlessly is not exclusive to wind-up toys. In the early stage of any design, you may find yourself wandering between the many factors to consider. Finding ways to zero in on key design conditions saves a lot of time. In the case of mooring design, there may be a wide range of sea states the system needs to withstand. In this article, we’ll talk about how wave steepness helps pinpoint the worst-case sea states you may want to check first in a mooring design process.

Wind-up toys wander aimlessly, even off the edge of a table.

When we talk about sea states, the wave height quickly comes up

It’s no wonder, either, as typically it’s what we think of when we look at pictures of the sea. After all, the wave height is the vertical distance from the trough to the crest, so it is often a visual cue that hints at how severe a sea state actually is. It is one of the most essential and fundamental parameters of ocean waves.

Another critical parameter is the wavelength. The wavelength is the horizontal spacing between successive wave crests. Now, both wavelength and height are a part of wave steepness, which is the ratio of wave height to the wavelength. So a high wave with a short wavelength means a really steep wave. So why is wave steepness useful?

Irregular ocean waves

The ratio of the wave height to wavelength – the steepness – is often what can give you a hint at how really severe a sea state is for design and analysis

Wave steepness gives hints about what the resulting forces may be like

Waves with low steepness are often not very exciting: they don’t typically cause rapid changes in loads like buoyancy, drag, or wave excitation forces on a floating system.

On the other hand, waves with high steepness can cause all kinds of problems. High steepness often means rapid changes in buoyancy, drag, and wave excitation. These can mean large accelerations and forces in a floating system. If there are large forces, it can mean significant stresses in the hull or mooring system and the risk of structural failures.

Wave steepness can act as a filter

When there are so many sea states to consider for a specific project, it’s helpful to pinpoint conditions that may cause a problem in the design. You can save a lot of time in the design phase by considering the harshest sea states first – because the mooring will surely survive more benign conditions. The sea states with the steepest waves are likely to be the most problematic conditions. In this way, considering steepness then acts as a filtering mechanism. You can spend less time looking through a wide range of conditions that are possible and zero in on what specific sea states may drive your mooring design.

Checking wave steepness is also important because you might miss the worst-case scenario. It’s definitely a mistake to zero in on the maximum wave height without checking wave steepness. Often, sea states with the maximum wave height can cause the biggest loads in a mooring system. But this is not always the case.

It’s possible at a certain location that the maximum wave steepness comes up in lower wave heights that happen to have much shorter wavelengths. Without considering steepness, you might miss this by initially zeroing in on only the maximum wave height.

You also need to be thorough when looking at wave steepness. Remember that wave steepness changes with both wave height and wavelength. Beware that there may be more than one set of sea state conditions that give large wave steepness depending on the wave climate at a specific location.

But wave steepness is not a perfect filter

Remember that wave steepness is a ratio – and a ratio that may mislead you in some cases. For example, you may have relatively small wave heights from wind chop – but if the wavelengths are small too, it might result in a large wave steepness. But most often, wave chop isn’t going to drive a mooring design. In this way, it still makes sense to do a reality check on the absolute wave heights along with wave steepness – be sure to check both wave steepness and total wave height together.

Ultimately, you need to check your floating system’s response to environmental conditions. Many floating systems have one or more natural periods of motion – these are conditions in which resonance may be possible. The resulting large motions in resonance (and loads in the moorings) may result from relatively small environmental forces. Environmental conditions that may excite the system in these conditions often need to be carefully considered regardless of the wave steepness in those conditions.

It’s time for an example

The SOFS mooring, deployed and maintained by CSIRO and IMOS, is located in almost 5000m deep water in the southern ocean. This full ocean depth mooring has provided a valuable time-series of measurements for many years at that location. The single largest wave measured there topped 22m. So does this kind of extreme wave height drive the maximum loads in the mooring?

Not necessarily. In all locations in the ocean, there is a wide range of sea states and wave heights. The largest wave heights occur in sea states with a 17m significant wave height and 19 second wave spectrum peak period. But the maximum loads tend to occur in conditions with lower significant wave height, around 10m and 12 second wave spectrum peak period.

A rough estimate of the steepness is possible from the wavelength of a 19 second wave and 12 second deepwater wave and the significant wave height. In the 17m sea, the steepness is only 0.03, while in the 10m sea, the steepness is 0.044.

In this particular case, the larger wave steepness in the 10m sea corresponds with more rapid loading and motions in the mooring, creating larger loads. This is also seen in field measurements and ProteusDS Oceanographic dynamic analysis modelling of the SOFS mooring tension.

SOFS buoy in ocean waves

SOFS buoy riding through ocean waves. Picture credit: Eric Schulz from Australia Bureau of Meteorology / IMOS

In summary

There’s often a range of environmental conditions to consider when designing a floating system and its mooring. Finding a way to narrow down the driving environmental load cases helps save time. Wave steepness, which is the ratio of wave height to wavelength, can help filter down critical load cases. It’s a mistake to assume the maximum wave height will be your worst-case scenario because it isn’t always the steepest wave condition. But steepness alone is not a perfect filtering mechanism, so you still need to do a reality check on the maximum wave height.

At the start of a design project, you may feel like you are a clockwork toy, zinging around aimlessly when digging through mounds of environmental data. But if you consider wave steepness, you may find you can quickly identify crucial sea states to start a mooring design and get to the next step faster.

Next step

We have a few ProteusDS Oceanographic sample moorings on our website. Check out a mooring layout for SOFS in the downloads area here.

Thanks to CSIRO

Thanks to Pete Jansen from CSIRO Marine National Facility / IMOS for sharing technical pointers, sharing data, and helping assess the SOFS mooring system and buoy dynamics.

 

 

Why you see modular oceanographic mooring design everywhere

Something strange happens to you when you start thinking about buying a new car or bicycle. These are relatively big purchases for most people, so you spend a bit of time thinking about a few choices. When you spend time thinking about different options, the subtle design details and shape of the vehicle become more familiar. This is when something weird starts to happen. The lines and shape of a specific car or bicycle you are considering will stand out when you out around town – you start seeing them everywhere.

This effect is actually quite common and can happen with anything – not just a new bicycle or car. In oceanographic mooring design, you will find a lot of subtle design details. For example, many oceanographic moorings use are modular in nature. You may not pick up on these kinds of design details at first glance. Once you realize they are there, you might start seeing them everywhere. In this article, we’re going to cover a few of these modular design details:

1) working on a limited deployment ship deck space

2) adjusting sensors to reach target instrument depth

3) adapting moorings to different water depths

First, we’re going to talk about how moorings are assembled for deployment on the deck of a ship.

My trusty Toyota Yaris: I still see them everywhere!

It’s easy to get lost in the details of the mooring design

After all, there are often dozens of components to keep track of. But ultimately, there’s going to be a time when you need to assemble the whole kit and get it ready to deploy in the ocean. There’s no way around this part of the operation.

You have to assemble the mooring on the deck of a ship. It’s rare for longer moorings to build the entire mooring line in one go. They usually need to be laid out in finite lengths, with all the components and clamp-ons added before connecting to the next segment. In this way, a notable limiting factor is the amount of deck space you have available to you.

The mooring segments laid out to prep for assembly are often some factor of the ship length itself. Quite literally, the ship’s deck size can be a factor in the mooring design. If the mooring segments are too long, you won’t be able to lay them out properly on the ship deck to assemble them sequentially. This is why particularly long moorings can look like they are made of modular segments of rope with connectors instead of one long string. But these modular segments also help in another critical facet of oceanographic mooring design. This brings us to the next point in how the mooring design is adjusted to reach target instrument depths.

The ultimate goal of oceanographic moorings is to measure something in the ocean

This might be temperature, salinity, or pressure, but it can be many other things, too. But these measurements can’t just be made anywhere the sensor ends up in the water column. There is a plan for where these measurements need to be placed: this is the target instrument depth.

But there might be dozens of sensors over the entire mooring line. As you go through the design cycle, adding components here, adjusting flotation there, the instruments’ position in the water column will shift around. The final design needs to show how each sensor is reaching its target instrument depth. So, do you go through and adjust every single sensor’s clamp-on position on their segment? Not necessarily.

It’s usually faster and easier to adjust the lengths of the modular segments above or below

In this way, groups of instruments on a mooring segment move together up or down in the water column. Adjusting the segment length is a much easier way to quickly and easily reach the required target instrument depths. But this also comes into play when changing the mooring for new locations and water depths. This brings us to the third point on adapting a mooring design for different water depths.

In any design problem, it helps to know about other designs that have worked in the past

It’s the same for oceanographic moorings, too. It’s possible an identical mooring with the same target instrument depth needs to be deployed at another location – with a major change being water depth.

Adopting the design to a different water depth is easy to deal with by adjusting the mooring segments’ lengths – either removing and shortening some or adding even more segments as needed. This modular nature of mooring designs helps old designs provide a lot of helpful insight for new designs.

What if I don’t have a mooring design to start from?

Everyone has to start somewhere. It can be hard to find details on existing moorings. Mooring diagrams you aren’t intimately familiar with might also have missing design details you need to be wary of. But ultimately, the oceanographic community is very open and collaborative and asking others for help can be a way to get started. On top of this, we are building a library of sample moorings, largely based on actually deployed moorings where possible, that work with Proteus Oceanographic.

Let’s look at a specific example

CSIRO maintains an array of subsurface moorings that measure flow conditions in the East Australia Current (EAC). This array of moorings covers several hundreds of kilometers along the continental shelf. The array of moorings covers a water depth ranging from 200m to full ocean depth at 5000m. Each mooring measures salinity, temperature, and also flow speed over the water column.

Wire segments and fibre rope segments are used to fine tune instrument position and accomodate different water depths

The moorings in the array have a similar design in spite of the significantly different water depths. You can see a picture of the subsurface mooring used in 4200m of water depth below. The length of segments of wire in between the ADCP buoys and clusters of flotation were fine-tuned to reach target instrument depths. The longer fibre rope segments at the bottom of the mooring were adjusted to accommodate the specific water depth at this location. In this way, a similar modular mooring design was used across all locations in the EAC array.

Summary

It’s no accident that oceanographic moorings are naturally modular. It arises from constraints from available ship length. But it also makes adjusting the design to reach target instrument depths, or adapting old designs to new water depths, much easier.

This aspect of modular mooring design is fairly common, especially in long moorings. Now you might start seeing it elsewhere in your design work – just like when you are thinking of a new car or bicycle!

Next Step

We’ve added the EAC 4200m mooring mentioned in the example above as another sample mooring to the collection you can check out with ProteusDS Oceanographic. Read more and download the free version of ProteusDS Oceanographic here.

Thanks to CSIRO and IMOS

Thanks to mooring engineer Pete Jansen from CSIRO Marine National Facility / IMOS for sharing technical pointers, sharing data, and helping to assess the EAC mooring system.