Tag: oceanographic mooring

Why parts libraries are essential to save time in mooring design

After a disastrous World War 2 naval battle, a lone allied warship was left surrounded behind enemy lines. Vastly outnumbered and outgunned, the situation seemed very grim. Still, the crew came up with a plan to sneak back home: disguise the warship as an island. The crew gathered as much foliage as they could from nearby islands to meticulously cover the entire ship.

A stranded World War 2 allied warship sneakily made its way back home by disguising itself as an island. Picture credit: Wikipedia

They patiently stayed at anchor by day, only travelling at night to avoid detection. To help their disguise, they picked a meandering route that would keep them near other islands on their way. They eventually made their way back to allied territory safely, but it was slow going and took a lot of time.

Now taking a lot of time might make sense in some circumstances, especially when safety is on the line. But in oceanographic mooring design, there’s often a time crunch. Without realizing it, you may be taking a meandering route in your design process. But a parts library can help speed up the process and get you where you need to be with a solid design in your hands much faster. What we’re going to cover is:

  1. finding part information
  2. avoiding input error
  3. starting with a finite pool of parts

First, we’re going to start with finding part information.

It may seem a little counterintuitive

But despite the massive amount of information on the Internet, it can still be hard to find what you need. Equipment suppliers do a great job putting part information on their websites and in catalogues for you to find. But the big problem is that sometimes not all the information you need is on a single technical specification sheet.

Sometimes, the information you need is in more than one spot

This might mean looking at more than one spec sheet just for one component. But complicating this is that sometimes the very layout of your go-to websites can change over time, too, shifting around where you previously found the information you needed.

All of this boils down to a lot of time spent crawling websites looking for key pieces of information. And when you don’t find what you need? You have to play phone or email tag with staff at the equipment supplier to fill the holes in your information.

See a sample snapshot of an equipment supplier website below: Mooring Systems Inc. has a lot of really useful information on their website. But you often need to find more information to make an entire mooring design.

Equipment supplier websites like Mooring Systems, Inc have a lot of useful information for their mooring components

A parts library ready to go saves the time needed to find this information

It saves a lot of time following up with equipment suppliers, trying to fill in the gaps in the information you need. This is definitely a significant aspect of how parts libraries save time in mooring design. But it’s not the only one. This brings us to the next point on reducing input errors.

What do we mean by avoiding input errors?

Avoiding input errors just means making sure you have the right understanding and the right numbers going into your design. When looking at breaking strength, did you confuse metric tons with short tons? Or did you perhaps make a mistake when converting another from one type of unit to another?

Input errors may happen if you are rushing to get a design done

Just a typo for a particular input can cause havoc with the design numbers. Did you put in 400kg for net flotation or 4,000kg? These input errors can be subtle, yet still have a significant impact on the mooring.

But how does reducing input error save time?

Often, with enough experience, you can get a sense if something seems wrong with the design. Tracking down just where that input error is can take a lot of time. It can leave you scratching your head, pouring over all the inputs in your entire mooring, trying to find what went wrong. Parts libraries significantly reduce these kinds of input errors. This is especially important when you are copying numbers from a technical specification into your mooring design tools.

Of course, nobody wants to make mistakes

While saving time by avoiding input errors is helpful, it’s not something substantial, especially when you are faced with an empty drawing board at the start of your design project. This brings us to the third and final point in how parts libraries help save time: working with a finite pool of parts.

When faced with thousands of choices, it can be easy to freeze up

Why do you freeze up? Your brain wants to evaluate every possible option: you can get analysis paralysis. You can face a similar kind of situation at the start of a mooring project. There are dozens of connector types and sizes, wire sizes, and hundreds of fibre rope materials.

It’s a situation ripe for analysis paralysis

But if you have a finite pool of parts to start with, it helps break you out of this paralysis. You don’t have nearly as many choices to make. You can start clicking together something resembling a mooring and then tweak the design as needed without infinite variation or refinement in the parts. In this way, working with the finite pool of parts in your library saves you time and gets you to the next step.

Will I need a ton of time to get a parts library up and running?

Not necessarily. The benefits start to grow as your parts library grows in size. You don’t need to make one up all at once in a single go. But collecting the information is only the first part. It’s crucial to save the information in a format that makes it easy for you to re-use over and over again. Better yet, if there’s a way you can share this with your team in your group, you all contribute and benefit, spreading the work and benefits around.

So what might a parts library look like?

It depends on what mooring analysis and design tools you’re working with. The most basic form might just be a spreadsheet, or better yet, a cloud spreadsheet that you can easily collaborate with your team for everyone to work with. A Google Sheet with a range of columns for different properties would work well. But there’s also another option that is even more effective.

Proteus Oceanographic comes with a Parts Library Editor

The free toolbox Proteus Oceanographic comes with a Parts Library Editor. It makes it easy to add new parts and makes a database file you can share with your colleagues.

Proteus Oceanographic Parts Library Editor

But even better is that there’s an Official Parts Library waiting for you to use

You can always make your own local parts libraries with the Proteus Oceanographic Parts Library Editor. But in the meantime, we spend a lot of time collecting useful parts information and adding it to the Official Parts Library for everyone to use in their Proteus Oceanographic mooring designs.

We went through a few details on how Parts Libraries help out, so now it’s time to review

Even simple mooring designs can be deceptive in the amount of detail that’s involved. This detail comes in the form of all the parts, connectors, and instruments that make up the mooring. Coordinating this information can leave you struggling to get the right information to check your mooring design. It may be obvious that parts libraries save you time in looking up spec sheets. But it also reduces the risk of making an input error when copying new information or incorporating the numbers into your calculations. And then there’s the benefit of having a finite pool of parts to eliminate the chance of analysis paralysis and overwhelm with too much to choose from.

An allied warship hiding out by blending in with other islands. Picture credit: Wikipedia

An allied World War 2 warship disguised as an island could afford to take their time on a meandering route. Now, if you’re working without a parts library, you may without realizing it be taking your time on your own meandering route through your mooring design. Parts libraries will help you click together a design and get to your project’s next step quickly and with confidence.

Next Step

Read more about and download the free design toolbox Proteus Oceanographic here.

There are several video tutorials that help show how to use Proteus Oceanographic for mooring design. Check out the video tutorial below to learn more on how to work with parts libraries with Proteus Oceanographic and the Parts Library editor.

Thanks to Mooring Systems, Inc

Thanks to Lucas and James Cappellini from Mooring Systems Inc. for sharing information on their equipment to incorporate into the Proteus Oceanographic Official Parts Library.

 

When to avoid a static solver in oceanographic moorings

It was when the traffic light turned green, and my car lurched forward from a stop when it happened. A warning light flickered on the dash, but then quickly turned off again. Fortunately for me, it wasn’t a sign of disaster with the engine – it was just a light indicating more window washer fluid was needed. But the warning light wasn’t on all the time – yet.

Dashboard with warning light

The to-do list just got a bit longer

Now, it’s right at this stage that I turn into Mr. Scrooge with my window washer fluid. The little bit left in the tank is sloshing around, and I know if I use one tiny spritz more to clean my windshield, the light will stay on. And if the light stays on? Well, it’s another problem I need to solve on my long to-do list! So my short term solution? Never clean the windshield. It may seem counterintuitive because that’s exactly what washer fluid is supposed to be for – cleaning my windshield. But instead, I completely avoid it.

Likewise, there’s some things I also completely avoid in mooring analysis. In particular, when we’re looking for a static mooring profile, it may seem counterintuitive to avoid using a static solver. But there’s good reason to avoid them in some circumstances. In this article, we’re going to focus on when to avoid using a static solver and what to do instead.

A static solver can mean many different things depending on the problem you’re trying to solve

In the case of mooring analysis, one problem we’re trying to solve is to find the static deflection of a mooring system to steady loads. Most often, these steady loads are from effects like ocean currents and wind. In the case of mooring design, this is what we mean by a static solver.

A static solver is a great tool to have

Ideally, they work quickly to give you a solution that shows what the mooring deflection looks like to specific steady loads. These static solvers compute the tensions in the mooring lines, too. This feedback is really helpful in the early stages of the mooring system’s design, so you can narrow down the materials and concepts to find something workable.

Often, you may need to screen mooring designs to make sure the deflection isn’t too high and check the lines are strong enough to handle the loads. But timelines are often tight in these projects. So getting to the next step in mooring design in short order is crucial.

Finding a static mooring deflection may sound like a straightforward problem

But it can be tricky depending on the complexity of the mooring system. Because of this complexity, there’s no single static solver algorithm that’s useful for all mooring systems. Ultimately though, there’s a common idea behind these static solvers. The idea is that they look for a mooring configuration in which the external forces, such as those from ocean currents and wind, are in balance with the internal forces, such as the mooring tensions.

As a static solver goes through a solution process, it can make large jumps to test what the mooring profile looks like. These jumps aren’t random. But they certainly do depend on how the forces are balanced between the environmental effects and the mooring loads. These jumps are a static solver’s greatest advantage: when they work well, they jump instantly to the right solution! But then sometimes they also struggle. When they struggle, it looks like they jump around and around a potential solution, but never quite land on one. Or, in some circumstances, there just may not be any solution at all.

It’s when static solvers struggle that they lose their advantage of speed

Suddenly, the static solver process is churning away, but just not getting anywhere. It can take a long time to process, whirring away on calculations, and never end up with anything useful. We call this a failure to converge to a solution. Suddenly, the trusty tool that quickly got you to the next step has betrayed you!

Now what do you do?

One alternative is dynamic relaxation. Dynamic relaxation is a very simple and direct approach, and there’s only a few steps. First, you start with a basic crude guess for the mooring system layout. This crude guess might look like diagonal straight lines from the anchor to the fairlead. Next, you simply run a dynamic analysis solver to let the system respond dynamically to steady forces from ocean currents and winds. The mooring system “relaxes” and deflects naturally to its steady state configuration.

The advantage here is that it’s a typically very robust approach: there’s no guessing about mooring profiles – it evolves a realistic deflection of the mooring over time and eventually to a steady state profile.

Why don’t we use dynamic relaxation all the time?

It all has to do with time. Unfortunately, dynamic relaxation can be quite slow! The longer the mooring lines and the deeper the water, and the lower the forces involved, the longer it takes to relax and settle to a steady state profile. A full ocean depth mooring might take over 24 hours to compute a steady state profile by dynamic relaxation – but only a few minutes for a static solver. So when do you want to use dynamic relaxation?

It’s hard to know when to use dynamic relaxation ahead of time

But there are a few guidelines you can keep in mind. Generally, what you need to keep your eyes peeled for are strong forces or abrupt changes in the environmental effects. This might look like a very severe shear profile in the ocean current, or perhaps an abruptly varying sea bottom. But one of the most common effects you’ll see is the effect of shallow water. In shallow water, small changes in mooring deflection can mean big changes in forces acting on the mooring – from buoyancy if the lines come out of the water, or ground contact if they touch the seabed.

This is where those jumps that static solvers make in guessing the mooring profile can run into problems. It can take a long time to settle and often you may find it fails to converge to a solution. But the good news is that dynamic relaxation can really shine in shallow water conditions. After all, often, mooring lines are shorter in shallow water. It doesn’t take much time for a mooring to deflect a short distance and reach a steady state.

Wait a minute. What do you mean by shallow water?

It’s one of those grey areas. There isn’t an exact answer. But fortunately, it’s often easy to try both techniques in parallel – static solver and dynamic relaxation – so really it’s up to you to learn what works. I think you will find that there is a range where one works better, and a range where they both work OK, too.

NOAA COOPS Currents Buoya (CURBY) mooring in 10m water is pretty shallow water. Picture credit: Laura Fiorentino

No really, can you give me a specific water depth?

Ok. Fine. For oceanographic moorings, I would suggest definitely trying dynamic relaxation if your mooring is in less than 100m of water depth. But again, it’s ultimately up to you to judge what works best for your specific problem.

Let’s look at an example

The CURBY (CURents BuoY) is a shallow water mooring deployed in the Delaware River. It’s certainly shallow at just over 10m depth. But the currents can rip through there fairly quick. The mooring is a simple design with a surface buoy and a few different sizes of chain along the span. In this example, the ProteusDS static solver struggles to find a solution in certain environmental conditions. But the shallow water is so short, that a dynamic relaxation finds a solution in only about 20 seconds of simulation in a strong current.

CURBY mooring static deflection calculated by ProteusDS

It’s time for a summary

We covered a fair bit on static mooring solutions, so now it’s time to review. There are many different techniques that may be used in static solver algorithms. It’s common for them to use an iterative approach that jumps to a solution, continually seeking a balance of internal tensions with external environmental loads from ocean currents and winds. But sometimes they just don’t work. When they don’t work, either they take way too long or fail entirely to reach a solution. You may find this when there are big discontinuities in the environment, like a very strong current shear, or a shallow water depth. In these cases, you may consider dynamic relaxation as an alternative.

Dynamic relaxation is a process that uses a dynamic solver that lets the system evolve through time from the effects of constant loads. While it’s often much slower than static solvers, in shallow water conditions, it should be plenty speedy – and at least there are no problems like failing to converge. Think about using dynamic relaxation in 100m water depth or less – but always be sure to judge for yourself what works best.

You may not be as annoyed as me when the window washer fluid light goes on. Of course, it seems counterintuitive to avoid using the window washer in your car. In a similar way, avoiding static solvers in mooring design may seem counterintuitive. But fortunately, there are only a few circumstances in which you need to use a workaround.

Next step

Learn more about how static solvers work with an article using an example with a deep water oceanographic mooring example here.

Thanks to NOAA CO-OPS

Thanks to Laura Fiorentino from NOAA CO-OPS for sharing technical pointers and data for the CURBY mooring for the example.

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.

How to control uncertainty in buoy drag coefficient when designing oceanographic moorings

It was the summer of 1991 and I stood in front of the first hedge maze I’d ever seen before. What could be more fun to a young boy that getting lost in a hedge maze and finding your way out?

I had a simple plan: get totally lost as quickly as possible by making random choices. Then I’d take my time to find my way out. So I ran straight in at full speed, turning left or right at each turn without thinking about it.

Hedge Maze

Ryan’s first strategy for solving hedge maze: run fast, think later!

 

The plan completely backfired. By a complete fluke, within a minute or two, I made it to the exit of the maze. I couldn’t believe that I got straight to the end of the maze by making random decisions.

Making random decisions may work once in a while in solving a hedge maze. But you can’t make a random decision for a drag coefficient for an oceanographic buoy. These buoys can be substantial structures. Because they’re substantial, the drag forces on them can also be substantial. If they have even remotely complex shapes, it’s difficult to really know what the drag coefficient is.

We’re going to look at three ways to evaluate oceanographic buoy drag coefficients. These ways increase in complexity but still give you something to start with. These three ways to determine the drag coefficient are using:

  1. Lookup tables
  2. Computational Fluid Dynamics (CFD)
  3. Field deployment data

First, we’re going to cover the use of lookup tables.

Lookup tables are a fantastic starting point

They provide drag coefficients for quite specific shapes and geometries. Helpfully, there are often tables with parameters that help you zero in on your particular shape. For example, a lookup table for a squat cylinder shows a few different values of drag coefficient based on the length to diameter ratio. Lookup tables are easy to use. You find the shape that is closest to what you’re working with, and then that’s the drag coefficient that you use in your calculations.

 

Cylinder drag coefficient lookup table

Cylinder drag coefficient lookup table from Applied Fluid Dynamics Handbook by Blevins

 

Where do these lookup tables come from?

These lookup tables are the results of decades of research and experiments. These experiments measured the total drag force on these shapes in different flow conditions. The resulting drag coefficient is computed from the data and then published in the lookup tables for future reference.

But there are also quite a few limitations

It’s rare to get an exact match to the shape you want. Even with basic shapes like cylinders, your particular form may be out of range of the lookup table. Or you may have only a few examples to work with from the lookup tables you have on hand.

So, while lookup tables give you a starting point, it still leaves some uncertainty. This takes us to the next approach that can be used to resolve the drag coefficient: computational fluid dynamics (CFD).

You can work directly with your specific buoy geometry

Previously, we learned that lookup tables were produced by decades of painstaking research and experiments that measured the total drag force on actual structures. CFD software calculates the dynamics of fluids flowing past structures. Essentially, you can use CFD to run your own virtual experiment on your specific structure directly on your computer.

Pre-processing StableMoor geometry in Altair HyperMesh in preparation for CFD analysis in Altair AcuSolve

 

Almost any kind of geometry can be used in a CFD tool. Software tools like Altair HyperMesh make it easy to work with 3D models generated from CAD software. This software prepares the geometry for use in a CFD program like Altair AcuSolve. Regardless of the CFD program used, once the geometry is in place, you can set the water flow conditions you want to check. Then, when you run the program, it calculates the corresponding drag coefficient for that geometry in those conditions. This is a significant improvement from lookup tables because you can use much more precise geometry. You aren’t left trying to guess which shape best fits your specific oceanographic buoy.

But there’s a different kind of uncertainty when working with CFD

As incredible as CFD tools are, the dynamics of fluid flows can be incredibly complex. There are also many inputs and settings for the CFD flow physics models. In some particular circumstances, some of these settings can make substantial changes in the output of the drag coefficient calculations. So how do we deal with this new kind of uncertainty? This brings us to the next and final section, validation with field deployment data.

Nothing is more real than reality

If you put an oceanographic buoy in a known water current and you can measure the total drag force, well, you’ve got the actual drag coefficient! Of course, it makes sense that this would eliminate those niggling uncertainties that remain with the previous two methods. We are indeed working with the exact shape, unlike the lookup tables. And we are working with real water flows, unlike an approximation of the water flow as calculated by CFD.

This creates a bit of a chicken and egg problem

We do need to know in advance what the drag coefficient is before we design and deploy the mooring. Otherwise, the mooring is at risk if it deflects far too much, or if it breaks. However, these risks can be controlled with a staged approach using a smaller mooring in lower flow speeds, or even scale model tests in a flow tank. These scale model tests are the kinds of tests done over decades of research to make lookup tables.

What about the cost of a field deployment?

Of course, making a field deployment is incredibly complex and expensive. You need a ship and crew to deploy the equipment. The equipment itself is costly as well. It can be a complicated job to just measure the flow at a site, never mind also some characteristic of the mooring response. But that only shows how unique and valuable the knowledge of the specific drag coefficient is for that particular structure. And that drag coefficient can be used again for new mooring designs and for different locations with different flow speeds in the future.

StableMoor deployment

StableMoor deployment for turbulence measurement by APL

 

Let’s look at a specific example

DeepWater Buoyancy’s StableMoor Buoy is a streamlined float designed to work in high flow conditions. It’s a fairly specific shape. When reviewing lookup tables, there are a few examples that are pretty close, but not exact. Is it more like a rounded rectangle, or cylinder? How can the tail ring be accounted for? There’s only so much we can answer using this approach. The rounded block has a range of 0.25 and 0.55, so we could try 0.55. It may seem a bit random, but that’s the best we can do at this stage. The next stage is using a CFD software tool.

DeepWater Buoyancy StableMoor ADCP buoy

DeepWater Buoyancy StableMoor ADCP Buoy

 

Lookup table values

Lookup table drag coefficient values for rounded block from the Applied Fluid Dynamics Handbook by Blevins

 

We used the CFD program Altair AcuSolve to compute the drag forces on the actual geometry of a StableMoor Buoy in a few flow speed conditions. Based on the projected area from the main hull, the CFD calculated drag coefficient is 1.0. This is higher than the rounded block lookup table values because the tail ring adds extra drag to the system. At this stage we have a good idea of the buoy drag coefficient to use. To improve on this, the final stage is a validation using field data.

CFD picture of flow speed

Altair AcuSolve CFD calculations show the flow structure surrounding the StableMoor Buoy

 

Oceanographers at the UW Applied Physics Laboratory deployed a short mooring with a StableMoor Buoy in a high flow tidal channel. The onboard sensors measured the flow velocity as well as the altitude of the StableMoor Buoy off the seabed. As the total drag forces on the mooring and StableMoor Buoy deflect the system, the altitude of the StableMoor Buoy decreases. This reduction in altitude is often called knockdown.

Mooring schematic

APL mooring schematic with StableMoor Buoy

 

We reconstructed the mooring in ProteusDS to compare the results to the measured the knockdown. Using a drag coefficient of 1.0 for the StableMoor Buoy showed knockdown within the measured range of values from the field deployment: at about 2m/s flow speed, the system shows about a 1m knockdown. So this looks like the CFD software tool did a pretty good job. This builds confidence to use the drag coefficient and the CFD analysis process again for other mooring designs.

We covered a lot of ground in our search for ways to find a drag coefficient

Now it’s time for a quick review. A starting point to resolve a drag coefficient is to use lookup tables. These represent decades of work from real experiments on various shapes. But often there’s not an exact fit to the oceanographic buoy geometry you’re working with.

The next step is to try using a CFD software tool. These software tools can use the specific buoy geometry you’re working with and provide you with the drag coefficient. While these software tools are powerful, they are still an approximation to potentially very complex fluid physics.

Indeed, there’s no replacement for reality, and so the final step would be some kind of real measurement of the buoy in actual flow conditions. While this can be a massive effort, it does provide a valuable validation of the drag coefficient for a particular buoy that can be used again in similar conditions.

Working with drag coefficients may make you feel like you are running around in a maze

You can’t just pick a drag coefficient randomly, rush on to the next step in the mooring design process, and expect success. You may be doing this if you have only a limited lookup table to work with.

Next step

Request a demo license for ProteusDS and explore how the knockdown of your specific mooring configuration can change with different drag coefficients. Use the parts library to get a good starting point in evaluating oceanographic mooring knockdown quickly.

Thanks to APL and DeepWater Buoyancy

Thanks to Jim Thomson and Alex de Klerk from APL and David Capotosto and Dan Cote from DeepWater Buoyancy for sharing technical pointers and information on the mooring deployment and StableMoor Buoy.