Tag: mooring

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.

How to correct subsurface mooring knockdown

Manatees are sophisticated, high-performance swimmers. You might look at one of these cuddly blubberous hulks and think otherwise, but it’s true. By high performance, I don’t mean they are incredibly fast. Instead, it’s how they keep their ungainly bulk in control. Because they are so large, it would be a big challenge to rely on muscle power alone to navigate around. This is where Manatees have a trick up their sleeve.

They rely on the careful use of digestive gasses to keep the right balance in the water. It’s not all about flotation, either. Critically important for them is they use this gas to control their tilt, too. They have special pockets that hold these digestive gases through their intestines that enable this control.

But it requires precision: too little or too much gas and they are out of alignment in the water. If they have too much gas, Manatees end up swimming awkwardly around with their tails way up over their heads. So what do they do if they have too much gas? Well, it turns out they just need to fart. With judicious use of farts, their swimming tilt can be corrected.

Similarly, you will be faced with making corrections when you work on a design problem. Often, a design will get out of balance, and you need to make some adjustments. When it comes to subsurface mooring design, any amount of steady current can lead to significant mooring knockdown. In this article, we’re going to talk about correcting subsurface mooring deflection by adjusting:

  1. flotation
  2. line size
  3. sensor location

All these parameters can affect knockdown. But they each affect knockdown differently, and it pays to consider them separately. First, we’re going to talk about adjusting flotation.

As a general rule, consider not swimming behind Manatees

You might think adding flotation is a brute force approach

It certainly makes sense: the larger the flotation used, the greater the mooring’s static tension. The larger the static tension, the more it will resist deflection from an ocean current.

But there are limits to the amount of flotation you can add. After all, working with larger and larger floats starts to get awkward and heavy to handle on a ship during mooring deployment. There is a cost factor too: larger floats are heavier and will be more expensive to manufacture and transport. So while it is a simple approach to use more flotation, there are certain practical limits. You may find that you need as much flotation as you can handle to reduce knockdown in your mooring deployment. So it’s good to be aware of what your practical limits for flotation are.

You might wonder about the drag force on the float itself, too. While this can be an important factor, there is a lot of nuances to float drag. Before considering the float’s drag, it is far more essential to consider the drag on the mooring line itself. This brings us to the second point: adjusting line size.

Mooring knockdown is proportional to the total amount of drag

And drag force is always proportional to the amount of structure area in the flow. Simply put, for mooring lines, this area is a factor of line length and diameter. You can’t do much about line length if you need to get sensors in a specific position in the water column. So that leaves line diameter to play with.

A few millimeters in line diameter can make a massive difference to knockdown. While a few millimeters in line diameter may seem insignificant, the effect adds up over the entire span of the mooring line. The longer the mooring line, the more significant impact it has.

So what does it mean to adjust the line diameter? In a subsurface mooring, the line tension is often driven by how much flotation you’ve added to the mooring. If you are using a smaller line diameter, the same rope material will have a smaller minimum breaking load. Of course, you’ve got to make sure the line’s minimum breaking load is sufficient given the tensions you expect.

If you want to try a smaller diameter, you may need a different line material

There are lots of sophisticated fibre ropes that have incredible minimum breaking loads at smaller diameters. Higher performance ropes will come with a higher cost. But the cost of the line should be considered in the context of the entire project. Again, only a few millimeters in line diameter can significantly change knockdown, so it is worth investigating in the design stage to see what happens.

Knockdown is proportional to the drag on the mooring. But it also makes a big difference if certain portions of the mooring line are in areas of higher current than the rest. It’s rare to have a completely uniform current profile, especially in full ocean depth conditions. This brings us to the last point on sensor location in the water column.

Sometimes you can work around the problem

Every mooring is driven by requirements to make measurements in the water column. In some cases, there is some flexibility on where sensors can be located to make these measurements. If you know something about the current profile for where the mooring will be deployed, you can use this to your advantage about where to place sensors.

For instance, there are many different kinds of ADCP, each with their own capability to measure ocean currents over various distances. If you’re not constrained to a specific ADCP, you can choose an alternate instrument and make adjustments to where these sensors are located in the water column to measure current profiles.

Knockdown is proportional to the amount of drag, and drag is driven by current speed. If you can make measurements of currents from a distance out of the main flow, so much the better. Because there are so many options for ADCP, the trick is evaluating which is the right one that will work well enough for your mooring. However, there aren’t ways of measuring things like salinity or temperature from a distance, in which case there isn’t a way to adjust sensor location.

So how do I evaluate all these options on flotation, line diameter, and sensor location?

Calculating the drag on the mooring line from each design is a process. Drag is not an obvious calculation you can do in your head or on paper for most typical moorings. You can use a software tool like ProteusDS Oceanographic to lay out flotation size, line diameter, and sensor types in different current profiles and calculate the resulting knockdown. It’s a straightforward process to adjust each parameter and see the effect on mooring knockdown.

It’s time for an example

We set up three subsurface mooring configurations in ProteusDS Oceanographic to highlight the effect of making current measurements out of the main flow speed. We assumed a water depth of 3000m. A current profile representative of extreme open ocean conditions was used with a current profile of 1m/s at the surface, tapering to 0.5m/s at 1000m, and further to zero at the seabed.

The objective is to measure the top 1000m of the water column. We used three typical subsurface mooring configurations to compare the effect on mooring knockdown and tilt by measuring flow speed away from the main ocean current. The three mooring configurations were:

  • Configuration A: uses one Teledyne 45kHz Pinnacle unit – a newer ADCP with double the measurement range from older models – in a single float, located at 1000m nominal depth
  • Configuration B: uses two Teledyne 75kHz Workhorse units – an older ADCP technology with a moderate measurement range, located at 500m nominal depth
  • Configuration C: uses two floats, each with a single Teledyne 75kHz Workhorse unit, located at 500m and 1000m nominal depth

The goal was to use ProteusDS Oceanographic to determine the float sizes such that the knockdown was less than 50m, and allowable tilt around 5 degrees. For simplicity, only the floats, 1/4″ mooring wire, and wet weight of the ADCP units were accounted for in the mooring layout. We selected float sizes based on the spherical ADCP models listed on the DeepWater Buoyancy website.

Three typical subsurface mooring configurations are used to compare the effect on mooring knockdown and tilt by measuring flow speed away from the main ocean current

The results for each configuration were:

  • Configuration A: 56″ sphere 1500m depth rating: knockdown 22m and 1deg tilt
  • Configuration B: 62″ sphere 1500m depth rating: knockdown 87m and tilt 3 degrees (we would have needed a bigger float or a second float to bring the knockdown below 87m!)
  • Configuration C: Two 56″ spheres 1500m depth rating: knockdown 32m, tilt 5.6deg

The results highlight how an ADCP with a longer measurement range, like the Teledyne Pinnacle, allows a mooring design with float and mooring out of larger currents. This reduces the total amount of drag on the mooring and the resulting knockdown. The other configurations needed more flotation or much larger floats to achieve the same knockdown requirements.

Summary time

We covered a few aspects of correcting knockdown in a subsurface mooring design, and now it’s time to review. The first thing to work with is the amount of flotation. Increasing this should directly help reduce knockdown, but there are practical limits in cost and logistics, too. The next thing to review is line diameter. Even a few millimeters reduction of line diameter can make a huge difference, especially on longer moorings. It may mean using a more expensive line material, but it may be a small additional cost compared to the entire project. Finally, consider sensor placement and what you actually need to measure. If you can measure high-speed currents without the mooring sitting in the middle of it, you could avoid significant drag loading on the mooring and the resulting knockdown. But it depends on what sensor options you have available, and in some cases, there might not be a workaround.

Manatees can get into trouble when they are out of alignment in the water. But they can always make some corrections to keep things running smoothly. Similarly, when there’s an ocean current around, your mooring might get into trouble and get out of alignment in the water. But now you have a roadmap for making corrections to the design to help reduce knockdown and keep things running smoothly, too.

Next Step

Try out the free version of ProteusDS Oceanographic. The Official Parts Library includes a host of DeepWater Buoyancy floats to make trials of different equipment easy to do. Read more and download the software here.

Thanks

A huge thank you to Paul Devine at Teledyne Marine and David Capotosto and Dan Cote from Deepwater Buoyancy for collaborating on the example configurations. Also a big thanks to Chris Beck at DSA Ocean for running the analysis through ProteusDS Oceanographic!

When to check instrument depth in oceanographic mooring design

In the late 19th century, the Hawaiian sugar industry was sweating, but it wasn’t from the heat. It was because of rats. The rat population was eating up way too much of their sugar crop. And the problem was getting worse. So what could be done to help keep things under control? One idea was to bring in Mongoose to help out. There was a lot to like about Mongoose: they’re relatively small animals, they’re carnivores, and they’re great hunters. But the best part? They like to go after rodents.

The plan looked good on paper. So Mongoose was then imported to Hawaii. But there wasn’t any impact on the rat population. Everyone came to realize the big problem: a rat is nocturnal, a Mongoose isn’t. The plan was a total failure – and chiefly from this crucial detail.

Many plans can look good on paper. But without carefully checking the details, things can go sideways very quickly. In oceanographic mooring design, a key detail to check is instrument depth. There are a few steps to do this through the mooring design process to make sure they are not far off target depth. What we’re going to cover in this article is calculating instrument depth from:

  1. static mooring stretch from weight and buoyancy
  2. static mooring deflection in mean current
  3. dynamic mooring deflection in current and waves

These form steps of a plan to check how the instrument depth might have strayed from a target location as you go through the mooring design. It’s also an opportunity to make adjustments to the mooring to get sensors where you want them in the water column. First, we will look at static mooring stretch from weight and buoyancy.

Mongoose: wily and capable hunters, but since they aren’t nocturnal, aren’t effective against rats

It may seem a bit anticlimactic

The ocean has intense forces from wind, waves, and current, but all we’re looking at, to begin with, is the mooring sitting by itself in calm water. But intense forces can come from the mooring itself, too. Flotation can come in all sizes and pull on the mooring with tons of static load. All materials will stretch some amount. But in certain circumstances with software materials or very long moorings, this stretch can be significant, indeed.

It’s the effect of this stretch on instrument depth you need to get a handle on first

Maybe the error from target instrument depth isn’t much, but you should still check it as a first step. Besides, many locations in the ocean spend a lot of time in low current condition, so this is an excellent time to make sure you’ve dialed in your instrument depths to where you want them to be on the mooring.

At this point, adjustments you might need to make in the mooring design might look like shifting the instruments’ location clamped on the mooring line, or more likely, adjusting mooring component lengths.

While many moorings may spend a lot of time in a calm environment, it doesn’t take much ocean current to cause problems. This brings us to the next step, calculating static mooring deflection to a mean current.

You might think you need massive current speeds to cause any mooring deflection

But that’s not necessarily the case. The longer the mooring, the more it acts like a giant lever, where small currents through the water column can cause significant mooring deflection. The more the mooring deflects, the more there can be an error from target depth. So it’s this deflection in a mean current we need to tease out and properly understand. It’s often the knockdown in subsurface moorings that really throws off instrument depth.

Evaluating this deflection and the resulting instrument depth error gives designers another stage to adjust their mooring. Fortunately, a calculation of static mooring deflection is a rapid calculation. The mooring response to ocean current can often be the most significant influence on instrument depth. But it’s not necessarily the last thing to check.

If there are parts of the mooring at or even near the surface, it’s a good idea to check the effect of ocean waves. This brings us to the final section, calculating the dynamic mooring deflection in mean current and ocean waves.

Evaluating the effects of ocean waves is the most complex stage

It also takes the longest to evaluate. There may be a variety of wave states at any location. It’s not always obvious which waves will have the most significant influence on the mooring. This means there’s often no way around systematically checking a range of different wave heights and periods to see what happens to the mooring, how it deflects, and what this does to the instruments. It may be that ocean waves do not significantly impact the instrument location, but it’s still useful to rule it out all the same.

Nevertheless, this process reveals a new mooring profile. On top of this mean profile is a dynamic variation caused by ocean waves. It’s this dynamic variation you want to get an understanding of. Position in the water isn’t the only thing affected by ocean waves. The mooring motion caused by ocean waves can introduce errors in the measurement of ocean current velocity. You have to be comfortable with the amount the sensors are moving about the target depth.

But do I always need to check instrument depth in current and waves?

Not necessarily. If you’re redeploying a mooring and the site conditions are expected to be the same as a previous deployment, you should expect similar performance. But if the mooring configuration has changed, it’s a good idea to check what will happen to the instrument depths.

You might be tempted to skip a detailed check for very simple moorings that are either short or only have one or two sensors. But if you skip this step, you have to be comfortable with the additional headaches it can make for you in post-processing the data you get from your mooring. Regardless, instrument depth becomes more complicated with longer and more complex moorings, so extra care is needed.

Let’s look at an example of a real mooring’s deflection

CSIRO and IMOS maintain an array of subsurface moorings that monitor the East Australia Current. This massive and complex ocean current has a significant impact on the environment. This impact can only be understood if the current is measured and understood. The array consists of a series of subsurface moorings in a region stretching along the continental shelf to full ocean depth 200km away from Brisbane, Australia.

 

The East Australia Current is a massive and complex flow along the coast of Australia. CSIRO/IMOS maintains a subsurface mooring array to measure the complexity and magnitude of this flow in the yellow box indicated off the coast of Brisbane. These currents can cause a significant deflection of the moorings.

 

One of the subsurface moorings in the array is located in 4200m water depth. We compared the deflection of a ProteusDS Oceanographic model of the mooring with measurements from the real deployment in the 95th percentile ocean current measurement. The maximum knockdown calculated was 151m. This compares well with the measured maximum knockdown of 160m depth. The corresponding maximum tilt of the primary floats was 15 degrees.

The knockdown has to be carefully accounted for to make sure the ADCP devices near the top of the mooring can still measure the ocean current all the way to the water surface even when the mooring is deflected.

A) schematic of the 4200m depth EAC subsurface mooring, showing an array of temperature, pressure, and current sensors (ADCP) B) Deflection of the 4000m+ mooring in 95th percentile current profile with 150m of vertical knockdown at the top of the mooring

Summary

We covered a few aspects of checking target instrument depth error, and it’s time to review. Once you’ve laid out your mooring, the first thing to check is the effect of static stretch on the mooring from weight and buoyancy in calm conditions. The next step is to consider environmental effects – first, steady current and then after that both current and waves together – particularly for surface moorings or subsurface moorings near the ocean surface. Each step along the way increases analysis complexity and offers you a chance to adjust the mooring as you go along.

Importing Mongoose to stop a rat problem for the 19th century Hawaiian Sugar Industry seemed like a good idea at the time. But they missed a critical detail and their plan failed. Using a systematic approach to check and adjust an oceanographic mooring is key to checking the crucial details like instrument depth.

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.

PS

Check out a numerical visualization of the East Australia Current on Windy.com here.