Tag: CSIRO

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.

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.

How to effectively model oceanographic surface buoy dynamics

Flying sheep may help you get to sleep, but they can save your life, too. In 1934, the Italian army had a huge logistics problem: crossing one of the most inhospitable deserts in the world. Resupply for the trip was vital, and the army had to consider their options carefully.

Flying Sheep

One option was for soldiers to carry their supplies. But weighing down soldiers wasn’t possible: they would move too slowly under the oppressive sun and wouldn’t survive heat stroke.

Another option was trucking in supplies. But that wasn’t going to work either, as the stifling desert heat was already spoiling their food.

The army was left with only one viable option: using aircraft. Soldiers were able to travel quickly through the desert on foot, and supplies were sent by airdrop. Everything, including live sheep, was sent in by parachute! It was the only effective option.

In contrast, there’s often more than one effective option in modelling surface buoys. Surface buoys are a critical component of oceanographic mooring analysis and need to be considered by mooring designers carefully. Without careful consideration of dynamics caused by harsh ocean waves, the buoy and mooring won’t survive. In this article, we’re going to cover a few useful approaches for modelling surface buoys:

  1. point mass
  2. simple rigid body
  3. complex rigid body

First, we’re going to look into point mass approach.

The point mass model is one of the most straightforward approaches to use

They’re straightforward because they ignore all rotational effects of the buoy: a single point represents the hull. It can still account for the linear buoy motions: heave, surge, and sway. But it also means only the most basic shapes can be used to represent the buoy hull, like a sphere or in some exceptional cases, a cylinder.

It’s natural to underestimate simple models

While simple, point mass models can be very extremely useful. Often, a point mass model is more than capable enough to analyze an oceanographic mooring response in currents and waves. This level of detail may be all that’s needed to finalize a robust mooring design.

There are only a few critical parameters needed to get started with a point mass buoy model. Fundamentals like buoy mass, net flotation, and basic hull shape are required and sufficient for many simple systems. But in certain circumstances, buoy rotational effects can be significant. This takes us to the second point on modelling surface buoys: using a simple rigid body approach.

Like a point mass model, a rigid body model accounts for all linear motions

These are heave, surge, and sway. But the difference is that rigid body models also account for rotational effects: roll, pitch, and yaw. These rotation motions may be vital because they can have a significant impact on the equipment used on the buoy. Of course, this means more information is needed to get the model set up correctly.

SOFS buoy in ocean waves

SOFS buoy riding through ocean waves while on station in 4km deep water near Tasmania. Picture credit: Eric Schulz from Australia Bureau of Meteorology / IMOS

 

But some of this information is not easy to find. For example, most buoy spec sheets provide mass, but usually don’t contain information about what the buoy rotational inertia is. This lack of information is not a surprise because the rotational inertia can change depending on the specific buoy loadout.

This is where the simple rigid body model really shines

With a few assumptions and approximations, mooring designers can estimate many of the additional missing parameters. The first thing is to approximate the hull shape with something simple like a sphere or cylinder. The rotational inertia is easily calculated from these basic shapes and the mass of the buoy.

This is a significant improvement on the point mass model because there is some basis for the rotational effects. A rigid body model can also handle more detail like an offset mooring connection point below the hull. However, it’s still an approximation to the actual buoy shape.

Regardless, it’s certainly a starting point to understand rotational motion. Like the point mass approach, it doesn’t need a lot of information to set up the model. But the simple rigid body model relies on a lot of assumptions to help fill the gaps in knowledge. So what can you do if you have a lot more details on the buoy on hand and want to make use of it? This brings us to the third and final section on modelling surface buoys: using the complex rigid body models approach.

There are a lot of assumptions used in the simple rigid body approach

Complex rigid body models are about reducing some of those assumptions. Using a complex rigid body approach will undoubtedly require a lot more information. In some circumstances, a detailed CAD software model of the buoy may be available. This CAD model can help provide the specific buoy hull geometry. The CAD software can also offer more accurate details like rotational inertia and the location of the centre of mass.

However, it can take a lot of time to set up a detailed CAD model. On top of this, generating a custom mesh of a buoy hull can be a challenging task in its own right. Nevertheless, it is the most flexible and powerful approach to modelling surface buoys.

What about the computational cost of these approaches?

It’s reasonable to expect the computational cost to increase if the model complexity increases. This is always on the top of my mind because more computational cost means more time needed to compute a mooring design. However, in this particular case, it’s not so obvious.

An oceanographic mooring simulation tends to require a fair number of elements in the mooring line. The extra equations introduced between a rigid body and point mass model does not make much of a difference in the computational time. That said, some buoy hulls can have intricate shapes. These intricate shapes mean a lot of geometric detail needs to be used in hydrodynamics calculations to resolve the hull forces. In practice, a complex rigid body model with a detailed hull might take something like twice as long to compute a mooring solution when compared to the other models. But it all depends on how much detail is in the hull.

Let’s look at an example

We’ve looked at the Southern Ocean Flux Station (SOFS) system in previous articles. In 4km deep water off the coast of Tasmania, it uses a 3m diameter buoy at the surface. We set up a detailed configuration of the entire mooring and surface buoy in ProteusDS. The ProteusDS model allowed us to examine what happens to the mooring loads and buoy motions when using the three different model approaches.

The general environmental conditions used for this example were 0.5m/s surface current dropping to zero after a few hundred meters. The wave conditions were 3m significant wave height and 10 second spectrum peak period.

We set up separate ProteusDS projects using the three different surface buoy models configured as a point mass, simple rigid body, and complex rigid body models.

A ProteusDS ExtMassCylinder was used to represent the simplest approach – the point mass surface buoy. The diameter and length of the hull were set to encompass the flotation volume of the SOFS buoy. The mass and maximum wet weight were set equal to the SOFS buoy mass and maximum reserve buoyancy, respectively.

Surface buoy in waves

SOFS surface buoy represented by a point mass approach

 

The simple rigid body approach was represented in ProteusDS by the Rigid Body model along with a Cylinder Mesh hull. Just like the simplest model, the hull geometry was set to encompass the flotation volume of the SOFS buoy. The mass was set equal to the SOFS buoy mass. The rigid body inertia was approximated using a solid homogenous cylinder with the centre of mass right in the middle of the flotation volume.

SOFS surface buoy represented by the simple rigid body approach

 

The complex rigid body model was represented using a ProteusDS Rigid Body model along with a Custom Mesh hull. The hull mesh was formed based on the shape of the SOFS buoy hull. The rigid body inertia and centre of mass location were computed by a detailed CAD model provided by CSIRO mooring engineer Pete Jansen.

SOFS buoy represented by the complex rigid body approach

 

Each model approach produced 3 hour peak mooring tensions of 23kN, 24kN, and 25kN, respectively, in the set environmental conditions. This shows how well the simplest model did at driving the peak loads in this storm condition. But what about buoy motions?

The simple point mass model tells us nothing about the rotational motion of the buoy. But there are results from the simple and complex rigid body models. In the 3m significant wave height conditions, we compared standard deviation and maximum buoy tilt. Each rigid body model produced 5.8deg and 5.7deg standard deviation. The maximum tilts were 37deg and 39deg, respectively. Since they were so close together, it shows for this kind of buoy and mooring combination, the simple rigid body approximation does a reasonably good job.

Summary

The Italian army only had one option when crossing a desert to succeed: pass as quickly as possible and airdrop supplies (live sheep included). Fortunately, you have more than one option when analyzing buoys in your oceanographic mooring. Now it’s time to review them.

The first approach is the most direct and straightforward way using a point mass. This approach is the fastest to set up and easiest to use. It often does the job for mooring design. But it does not account for any buoy rotational effects.

When these rotational effects may be significant, it’s time to look at the second approach: the simple rigid body model. The simple rigid body approach is excellent when there’s limited information on the buoy available. It uses underlying approximations and assumptions to fill in gaps in details like those for rotational inertia.

If you have a lot more information on hand, and likely a CAD model to help out, the third approach can be useful: the complex rigid body model. Not for the faint of heart, it can take a lot more time to set up. But you can use more detailed buoy hull geometry, and the rotational inertia computed from a CAD tool for much more accuracy.

Next step: check out this video tutorial comparing point mass and rigid body models

DSA develops ProteusDS as a software tool to help evaluate oceanographic mooring designs. We post a variety of materials online, including video tutorials on our YouTube channel. Check out this video tutorial below showing more detail on a comparison of point mass and rigid body models in the software.

 

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.

How to systematically evaluate oceanographic mooring compliance

People that live in northeastern India use tangled foliage to speed their way through the jungle. But it takes a lot of patience. They can do this with certain kinds of plants, like the rubber tree. As you might expect from a rubber tree, the roots are quite flexible as they grow. It is this flexibility that is used to cultivate and direct the roots so they grow across ravines and streams in the jungle and take root on the other side.

The result? A living bridge that can last for a long time. By tending and adjusting the roots, the problem of getting around is solved with a systematic approach.

Similarly, a systematic approach is needed when evaluating oceanographic mooring compliance. This mooring compliance is what ensures survival in the face of dynamic forces from ocean waves, wind, and currents. In this harsh ocean environment, success for a designer means an oceanographic mooring that can last a long time.

Jungle bridge

A living root bridge in northeastern India

There are a lot of details in designing an oceanographic mooring. It can be easy to get lost in what often will be an iterative process. A roadmap is always helpful when you get lost. That’s what we’re going to talk about in this article: three steps in a systematic approach in assessing oceanographic mooring compliance using:

  1. steady state
  2. regular ocean waves
  3. irregular ocean waves

We will begin by addressing steady state in the first step.

Ocean waves are usually not the only source of forcing from the environment

The first step is to understand steady state effects on the oceanographic mooring. These steady state effects are often from drag forces caused by constant wind and ocean currents. The critical effect of interest here is how these constant forces deflect the oceanographic mooring.

This is particularly important for very long ocean moorings that can deflect a very long distance. It is this deflection that can have a drastic effect on reducing mooring compliance. A mooring that is almost stretched taut from these mean steady state forces will be at risk of breaking when ocean waves come along, which brings us to the next step. In the second step, we check what regular ocean waves will do to the mooring.

In this stage, the analysis picks up from where the first stage left off

The analysis starts with the steady state current and wind and deflected mooring profile. Now is when things get interesting: it’s time to bring in some ocean waves to see what happens to the mooring. But ocean waves also have a lot of parameters. Before we get lost in a vast number of load cases and long computation times, it’s a good idea to start simple. The simplest way to start is with regular ocean waves.

These regular waves are just sinusoidal ocean waves. They are a very simple starting point, but don’t let that simplicity fool you: they can still break your mooring very easily. This simple starting point is ideal because if there’s a problem with the mooring design, you can make adjustments, quickly re-evaluate step 1 again, and be right back to test with regular waves once more.

Regular waves do a great job of adding a dynamic component to test the response of the mooring

If the mooring is near the breaking point from the deflection produced from step 1, often just a short dynamic analysis with regular waves will show whether breaking loads are reached very quickly. But regular waves are very simple and don’t always accurately represent the kinds of ocean wave conditions at sea. This takes us to the third step in the mooring compliance assessment. In the final step, we look at with a wave spectrum, or irregular ocean waves, do to the mooring.

Ususally, when you look at the sea, the water surface doesn’t look very sinusoidal

In reality, it’s most common to see many of these waves of different sizes and wavelengths at the same time: in other words, a spectrum of ocean waves. It is this seemingly random water surface that makes what we call irregular ocean waves. But if they seem random, how can we use irregular waves to design a mooring?

Irregular ocean waves

Irregular ocean waves in a storm

It is the wave spectrum that truly governs this seemingly random nature

While the water surface at any instant might appear to have random waves, over a long period of time, a pattern emerges of typical wave heights and lengths.

However, this pattern is statistical. It isn’t something that can be so easily applied directly to a mooring design to test the mooring compliance. The solution to this is simple but the most time consuming step of all: you have to check what the mooring does in irregular waves over a long period of time.

You don’t need a long time when working with regular waves

This is because regular waves are sinusoidal, and you may see a pattern in the dynamic response of the mooring motion and forces emerge after only a few wave periods. In an irregular sea state, you may never see a specific pattern emerge. However, you will start to see statistical patterns emerge over time.

These patterns may be in a typical envelope of motion or tension of the mooring. But one of the most interesting patterns is in the peak tensions. It is these peak tensions that so often drive the design of the mooring. These are the loads that happen when a single big wave appears and really pummels the mooring.

Mooring designers look for these kinds of events

They’re essential because how high the tensions are provides a more realistic idea of how robust the mooring compliance is and if the mooring will survive or not. And if it doesn’t survive in the analysis? Then the design has to be adjusted, perhaps with stronger or longer components or a different float configuration, and you go back to step 1 and keep going through the systematic process.

But aren’t you going to be looking at irregular wave load cases forever?

It’s true, the final step is not a fully deterministic design step to checking the mooring compliance. It doesn’t mean that you need to look at an infinitely long storm load case. But it does depend on a few factors like the magnitude of the storm, the spectrum type, how long the mooring is, and how it responds to the waves.

The most important thing is that a mooring designer has a way to run load cases that are long enough to see a statistical pattern of the peak loads emerge.

Let’s look at an example

The Southern Ocean Flux Station (SOFS) is an oceanographic mooring deployed in about 4km deep water off the coast of Tasmania. The mooring uses a 3m diameter surface buoy and over 6km of mooring line to provide the compliance needed to ride massive waves in the southern ocean. We configured the entire mooring in ProteusDS and ran through the three steps to determine maximum loads at the top of the mooring, next to the buoy. The general environmental conditions used for this example were 0.5m/s surface current dropping to zero after a few hundred meters. The wave conditions were 3m significant wave height and 10 second spectrum peak period.

SOFS buoy on station in 4km deep water near Tasmania. Picture credit: Pete Jansen from CSIRO MNF/IMOS

In step 1, the steady state profile of the mooring was resolved from the current forces. The top tension of the line was 11kN in this condition. This was only a little higher than the static weight of the system without any current. The deflection of the mooring is significant but expected because of the very long mooring line.

SOFS mooring deflection in steady current profile. This represents 6km of mooring line in 4km of water depth so the buoy and instruments are not visible

In step 2, we used the steady state profile of the mooring as the starting point. It’s a very crude approximation but we used 3m regular waves to represent the sea state condition. These loads were resolved after only a few wave periods so we had the results very quickly. The maximum tension was 13kN so you can see how the loads are increasing from the dynamics. But while it is dynamic, it doesn’t necessarily capture the peak loads caused by a more realistic irregular sea state. This takes us to step 3.

In step 3, we used a JONSWAP sea state with 3m significant wave height and 10 sec spectrum peak period. Running a few separate realizations of this spectrum produced an average 3 hour storm peak tension of 26kN. Note this was just an example to illustrate going through the steps with an existing mooring design and to show how the loads change in each step. In reality, the design may need to be adjusted depending on the needs of the design and strength of components in an iterative fashion.

Because the design process is iterative, it’s easy to get lost in the details

In the steps discussed here, complexity increases with each step. If there’s not enough mooring compliance in any step, a design change is needed and you must go back to step 1. Let’s review the three systematic steps discussed in checking oceanographic mooring compliance:

  1. start with the steady state response: deflection of the mooring to constant currents is the starting point to step 2
  2. rough out the dynamic response to regular ocean waves: it’s a simple but quick check with a dynamic effect
  3. polish with a dynamic response to irregular ocean waves: it’s the most realistic, but most computationally costly.

The systematic process is a roadmap

People living in jungles can use a systematic approach to cultivate living root bridges. It takes a certain amount of time before the roots reach the other side of a river or gulley, and only after the process is completed can the bridge be used. In a similar way, mooring designers need to follow a systematic process to evaluate their designs to ensure they’re safe and will survive.

Next step: evaluate your own mooring designs

DSA develops ProteusDS as a software tool to help evaluate oceanographic mooring designs. Mooring designers use this kind of systematic approach with a tool like ProteusDS to check their work and improve designs. Try the software and judge the results of your own designs. Apply for a free ProteusDS demo 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.