Tag: oceanographic

This is part 3 of an article series on maximum dynamic loads in moorings. If you haven’t seen it yet, start from part 1 here first.

Mooring design is often driven by the maximum load in a severe sea state. It’s common to use a dynamic analysis process and a numerical model of a mooring system to resolve the extreme peak loads. In a time-domain analysis, the mooring loads are calculated in a specific realization of a sea state. A critical mistake to avoid is a repeating sea surface in the sea state realization. But what does this mean exactly and how does a repeating sea surface happen?

Southern Ocean Flux Station (SOFS) full ocean depth buoy recovered after over a year at sea in 4km deep water near Tasmania. With an average sea state of 4m significant wave height, often reaching 10m significant wave height and higher, peak loads are important to understand and anticipate. Picture credit: Peter Jansen from CSIRO / IMOS / Marine National Facility

A repeating sea surface what happens when there isn’t enough detail in the sea state realization

In a time domain analysis, this looks like a pattern of water surface that repeats itself over time. How often it repeats depends on how much detail is in the sea state realization. A sea state realization is formed by superimposing a finite number of sinusoidal ocean waves. Each sinusoidal wave segment has its own period with an associated height governed by the representative wave spectrum. The problem with a repeating wave surface is when there aren’t enough wave segments to represent the sea surface. Consider, in the absolute worst case, a sea state realization of a wave spectrum using only a single ocean wave segment: it will have a repeating sea surface after one wave period! The effect of this repetition can mean a disaster for mooring analysis.

The problem is that a repeating sea surface fools you into a false sense of security

A repeating sea surface causes a repeating mooring load response. No matter how long you run the analysis, you still get the same repeating tension. A consequence is that you are missing an accurate representation of the extreme values of the mooring response. You might think you’ve captured a reasonable maximum dynamic mooring history by checking it with a long simulation time and multiple sea state realizations. But in reality, you’ve just been looking at the results of a repeating pattern over and over. You can miss a lot of detail in the system’s dynamics, and there’s a risk you can underestimate the maximum loads. In this way, the repeating sea surface hides the extreme values from you.

How can you tell you have a repeating sea surface?

It’s difficult to tell in a systematic way, but often looking at a time history of the sea surface and the mooring tensions, it can become evident with some experience. If you see a repeating pattern of groups of peaks, there’s a good chance that you may have a repeating sea surface. A repeating pattern may be evident in the motion or mooring tension, too. But how can you avoid the problem of a repeating sea surface?

The key is in having enough detail in your sea state realization. You need enough individual wave segments to have a healthy representation of the sea state spectrum without repetition. Building up experience and visually checking your sea surface and mooring response is essential to look for telltale signs of a repeating signal.

But why not use thousands of wave segments in all sea state realizations?

The practical consideration is that the computational burden increases with wave segments. The sea state surface and water particle velocities and accelerations are all the result of contributions from every wave segment that’s included in the sea state. In turn, these physical parameters are all factors in the forcing of a floating system and its mooring. So with the number of wave segments used, the computational burden goes up, and the analysis speed goes down. A careful balance is needed to get enough detail for a realistic design process without slowing the entire process down too much.

It’s time for an example

Let’s look at more data generated by ProteusDS of the Southern Ocean Flux Station (SOFS) full ocean-depth mooring. This mooring response was calculated in a 9m, 13s wave spectrum. The sea state realization was constructed using a custom sea state with only 10 wave segments with an anticipated energy repetition of 36 seconds.

Looking at a time series of the sea state realization, you can see a repeating wave pattern, but it looks like it repeats around every 100 seconds.

However, the dynamic mooring tension shows something slightly different. The largest peaks show a repeating pattern and similar shape approximately every 36 seconds. While the sea surface looks like it takes longer to repeat, the most dominant energetic wave pattern is driving the mooring tensions with a repetitive tension at this rate.

The big problem is that these repetitive peaks are not representative of what to expect in a realistic sea state – just like you wouldn’t expect the peak loads from a single sinusoidal ocean wave to realistically reconstruct the dynamic loads of a mooring in an irregular sea state. You can’t rely on results from any statistical extrapolations to estimate maximum tensions over a time period longer than your simulation either. The consequence is that you may drastically overestimate or underestimate the maximum tensions in the mooring because of this uncertainty. This is an effect that can show up in any dynamic analysis tool that generates a time series of sea state realizations.

Note that ProteusDS uses a few tricks to avoid a repeating sea surface even when only a few wave segments are specified. While you may not see a repeating sea surface with only a few wave segments, there still may not be enough detail in the sea state realization to get meaningful maximum values – and again you may significantly under- or overestimate maximum tensions. When you’re looking at a new kind of mooring in a particular sea state, sensitivity studies on the detail in the sea state realization and the resulting mooring and motion peaks may help give you an idea of what works for future reference.

Summary

A sea state realization is formed from the superposition of many sinusoidal ocean waves. But beware that you can get a repeating sea surface if you don’t use enough of these wave segments. The problem is that once the sea surface starts repeating, you don’t get any new information from the dynamic response of a floating system and its mooring. You could even be significantly underpredicting the maximum loads. You might think you are capturing a lot of detail in a long simulation or in multiple sea state realizations, but you’re missing important information on the true extreme loads in the system.

Next step

Comparison of measured mooring tensions to dynamic analysis results is always an important validation exercise. The comparison of SOFS mooring measured tensions with ProteusDS results factors in multiple sea state realizations, long-duration simulations, and many wave segments to ensure reliable results. Read more on the detailed comparison of measured peak mooring loads on the SOFS mooring compared to ProteusDS calculations in the oceanographic validation report here.

This is part 2 of an article series on maximum dynamic loads in moorings. Start from part 1 here first.

The maximum load in a severe sea state usually drives a mooring design. It’s common to use a dynamic analysis process and a numerical model of a mooring system to resolve the extreme peak loads. In a time-domain analysis, the mooring loads are calculated in a specific realization of a sea state. But what exactly is a sea state realization?

It’s not always flat calm in the ocean

Most places in the ocean at any given time have many different sizes and frequencies of ocean waves. A compact way to express the sea state is with a wave spectrum. A wave spectrum is like a convenient short-hand that describes how much wave height there is at different frequencies. But a wave spectrum isn’t the same thing as an actual sea state. This is where a sea state realization comes in.

A sea state realization is a fully detailed description of the state of the ocean surface. This is in the form of a complete picture of the water surface height in space and time. Thousands of specific sea state realizations have the same wave spectrum, but each realization is unique in that it has its own differences in surface elevation through space and time. The subtle differences in the water surface are vital to revealing extremes in the analysis of floating systems, like the maximum peak load in a mooring system.

Sea state realizations all have the same spectrum, but they are individually different from each other.

How do sea state realizations reveal extreme mooring loads?

The key is in the subtle differences in the time variation in the water surface. A wave spectrum can give an idea of the general characteristics of the sea surface. But it’s not always clear what rare events in the realization will look like. For example, the maximum distance from a wave trough to the next wave crest is one indication of the worst-case wave height in a specific sea state realization. You can’t know what this worst-case wave will be for a sea state realization by looking at the wave spectrum alone. You also won’t know exactly how a floating system will respond to each worst-case wave. A systematic approach is needed to check how a floating system will react in specific sea state realizations and confirm the resulting dynamic mooring loads. But rather than just checking one very long sea state realization, there are good reasons to use several.

One reason to check multiple sea state realizations is because of variations in the intensity of the water surface

In theory, a well-constructed sea state realization will have a perfect match to the characteristics of its matching wave spectrum. Attributes like the significant wave height and peak period should be identical. But the practical reality is that sea state realizations tend to have some variation in these parameters. One sea state realization might have a slightly higher significant wave height, and another might have a slightly lower one.

Significant wave height is an essential driver of motions and mooring loads, so checking multiple sea state realizations ensures you are aware of a single sea state realization that is less severe than you intend. This is a critical facet in using multiple sea state realizations, but there’s also a practical reason to evaluate multiple sea state realizations.

Another key reason to check multiple sea state realizations is that calculations can be computed in parallel

Calculating one long time series of mooring dynamics can take a while, slowing down the design process while you need to wait for the results to get design feedback. Yet it’s trivial to use multicore computers to calculate the mooring response in different sea state realizations in parallel. Often, shorter simulations in various sea state realizations can be computed much faster than one extended analysis, giving more robust design feedback than a single simulation.

While each simulation may be shorter than a storm duration, it still needs to be long enough individually to capture a meaningful number of extreme peak loads. Comparing the extreme peak loads produced by a mooring in each sea state realization gives you confidence that you have a reasonable peak load to base the design on the mooring.

But how much variation can you have between extreme peak loads?

There isn’t a specific number to use and you need to rely on engineering judgment. The extreme loads from each sea state realization will be different every time. However, the spread between the extreme values should not be too severe. If they are too far apart in magnitude, it’s an indication that your simulations are too short or that there may be severe tension shocks in the mooring response, and some changes may be needed in the design. Typically, for oceanographic mooring design, it makes sense to start by looking at least three sea state realizations to get a good picture of the extreme peak loads.

Example time

Let’s look at more data generated by ProteusDS of the Southern Ocean Flux Station full ocean-depth mooring. This mooring response was calculated in a 9m, 13s wave spectrum. The first plot shows the mooring tension from the first sea state realization. The maximum tension is around 32kN.

Another simulation in ProteusDS with a new realization of the same sea state reveals a different time series of dynamic tension. The peak is just under 30kN.

The third realization shows a peak tension of about 28kN. These load cases were all calculated in parallel on a desktop computer much faster than one long individual simulation. It’s encouraging to see similar maximum peak values across the three realizations, building confidence that the maximum is likely in this range of 30kN. A shorter duration of 500 seconds may even be justified but this is driven by the characteristic peak values that have already come out of the analysis.

Summary

A sea state realization is a fully detailed representation of the ocean surface. It’s derived from a wave spectrum and includes a time series of the water surface elevation everywhere in space and time. Thousands of sea state realizations may have the same wave spectrum and statistical characteristics, like the significant wave height. But it’s the specific details and time variation that drive floating systems, and sometimes rare events like a sequence of wave troughs and crests, that affect extreme peak loads. While in theory, sea state realizations should be characteristically the same, they often are slightly more or slightly less intense in terms of significant wave height. With low-cost multicore computing readily available, it’s also easy to run numerical models of moorings in different sea state realizations in parallel. These are both good reasons to check the extreme loads in multiple sea states to find the extreme values faster.

Next step

This is the second article in a series of three on finding extremes in mooring analysis. Read the next and final article in the series that focuses on the effect of a repeating sea surface here.

In 1995, astronomers wanted to point the multi-billion dollar Hubble Space Telescope at a small portion of a completely empty sky. It was risky because they couldn’t tell in advance anything about what they might get: there was truly no evidence to expect anything but total emptiness. Yet they still wanted to follow a process and see what they got out of it. Because Hubble has limits of light sensitivity, it needed to point at precisely the same spot for 100 hours to collect enough light to have a chance at detecting something. The result was a huge success that showed the small portion of the sky wasn’t so empty. Thousands of new galaxies were discovered by following their process to reveal the data.

Similarly, the design of floating systems in a marine environment involves using a process to reveal data. The maximum load in a severe sea state often drives mooring design. But mooring systems can behave in a complex way, and you can’t tell in advance anything about what a mooring might do in a sea state just by looking at it. So how do you calculate the maximum loads? It’s common to use a process and see what you get out of it. In this case, the process consists of a dynamic analysis of a numerical model of the mooring in different sea states. The ISO mooring standard recommends a process that determines stable statistical values of mooring response. Following an approach based on this reveals data on the maximum loads. In this article, we’re going to cover three elements that are key to the process of finding these maximum loads:

1. Checking the simulation length
2. Investigating multiple sea state realizations
3. Avoiding a repeating sea surface

First, we’re going to cover the simulation length.

Without knowing what might happen, Hubble spent 100 hours starting at a single “empty” spot collecting light and found thousands of new galaxies. When you’re not sure what to expect, it helps to have a process to follow and check on the results, like finding maximums in dynamic analysis.

An ocean wave spectrum causes an irregular sea surface in space and time

The general characteristics of the sea surface can be predictable, but what’s not so predictable is the resulting mooring response in such conditions. A numerical model of the mooring that accounts for the components’ weight, buoyancy, drag, and elastic stretch is necessary, with the forcing from ocean waves moving the system around. A mooring will usually have some irregular dynamic tension proportional to the intensity of the sea state. However, what’s of most interest is the maximum load. In a well-designed mooring, these peak loads only occur sometimes and appear unpredictably. What’s crucial to understand is that to capture some reasonable expected value of the maximum tension, you need a time series of the calculated mooring tension that covers a suitable amount of time.

The ISO offshore mooring standard recommends running simulations in a storm duration scenario but leaves it to the mooring designer to determine a specific time to use. Another standard for offshore moorings by DNVGL suggests a typical storm duration is three hours. But ultimately, what’s important is finding a stable statistical indication of the maximum loads. Running longer simulations always has a chance of producing higher loads. But pay attention to the maximum values you find in a mooring tension – if you find multiple examples of peaks that are reasonably close to each other, it’s a good chance you have a simulation that’s long enough to capture an expected value of the maximums. Simulation length is crucial, but other aspects to the analysis process are essential to keep track of. This brings us to the second point on investigating multiple sea state realizations.

Not all sea states with the same spectrum are created equal

An ocean wave spectrum is one form of description of the state of the ocean. It shows you how much energy is present at different frequencies. A sea state realization is a physical manifestation of that wave spectrum. It’s a specific instance with a time history of water surface elevation as the waves propagate through the sea. What’s vital about sea state realizations is how they can drive the extreme values of a mooring analysis. It’s not simple to anticipate the characteristics of each sea state and how the mooring will respond to it in advance. Each sea state realization will have different attributes like the maximum trough-to-crest wave height. But even with a severe trough-to-crest wave event, the maximum mooring loads depend on the mooring’s position and orientation at the time. The result can be a significant difference in peak mooring loads between different sea state realizations. Even in a single sea state realization, a fairly long span of time may not show the same peak load as another sea state realization.

Both ISO and DNVGL offshore mooring standards recommend completing dynamic mooring analysis in multiple sea state realizations to check the characteristics of the mooring response. By studying several sea state realizations, you increase your chances of capturing extreme mooring responses to severe wave loading. Sea state realizations are an important concept to grasp and leverage in the mooring design process, but there’s also a crucial mistake to avoid when working with them. This is the hazard of the repeating sea surface and is the third and final point we’re going to cover.

A sea state realization is formed from a finite number of sinusoidal ocean waves

As the mooring analyst, it’s in your power to control how many sinusoidal ocean waves are used to construct the realization of the wave spectrum. The problem of a repeating sea surface comes up when there aren’t enough of these waves used to represent the spectrum. For example, in the worst-case scenario, using only a single sinusoidal wave to construct a wave spectrum produces a sea surface that repeats once every wave period! While a repeating sea surface does give an initial picture of the dynamics of a mooring system, it causes a serious problem in the analysis process because it hides an accurate representation of the extreme values.

It doesn’t matter how long your analysis runs or how many sea state realizations you check – the repeating sea surface will keep producing the same repeating mooring load response. The consequence is that you may be significantly underpredicting the maximum mooring loads.

Let’s look at an example

Maintained and designed by CSIRO, the Southern Ocean Flux Station (SOFS) includes a full ocean-depth mooring. Located in extremely harsh conditions, with an average sea state of 4m significant wave height, it often sees much higher and intense waves. Here’s a time series of the top tension of the SOFS mooring in a severe wave spectrum with 9m significant wave height and 13 second period calculated by ProteusDS. How long should you run an analysis to find the maximum tension? The answer depends on what the peaks look like in the results.

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

Here’s a plot of the tension history from the first 200 seconds. There’s quite a few peaks in the mooring tension around 22kN. But how do we know these are the maximums?

A longer simulation time to 500 seconds reveals a jump up in peak loading to 33kN.

Extending to 1000 seconds shows another peak at 34kN. Extending even further to 2000 seconds reveals another lower peak around 31kN.

This doesn’t mean we always need at least 2000-second simulations to find the peak loads for oceanographic moorings like the SOFS system. The takeaway is that if you don’t have an expectation of the mooring response, an analysis that is too short in time can miss critical information on the maximum load. If you miss these maximum peaks and select components with lower load ratings, there’s a higher chance the mooring may fail in a severe sea state. Your general analysis needs to have some kind of stable statistical representation of the values. Generally, from this one example, the standard deviation of the mooring tension stabilizes around 500-750 seconds, and there tend to be a few of these large peaks in the 500-750 second range.

It’s summary time

We covered a few topics on finding the hidden extreme in mooring design, and it’s time to summarize. The most important concept to understand is that you need to have stable statistical values of your dynamic mooring loads. You can do this by evaluating the dynamic mooring response in an ocean wave spectrum with a few careful considerations. The first consideration of the analysis process is the simulation length. The analysis should be long enough that you see a grouping of maximum peak values that are reasonably close to each other. The second element is checking multiple sea state realizations. This is because it’s not easy to anticipate uncommon characteristics of the water surface – like a maximum individual wave – and how it will interact with the floating system. Finally, watch for the hazard of a repeating sea surface. You need to avoid a repeating sea surface in your analysis because it will obscure the true extremes in the system – no matter how long your simulation or how many realizations you investigate. All of these concepts are part of a process in finding important information. Much like Hubble starting at a single spot for 100 hours, you also need to follow a process to find new information on maximums in dynamic analysis.

Next step

This is the first article in a series of three on finding extremes in mooring analysis. Check out the second article that focuses on sea state realizations and why they are important in finding maximums here.

PS

Check out the result of the Hubble Deep Field image here.

Losing an eye might seem like a critical point of failure, but not for a starfish. This is because starfish have multiple eyes on the tip of each arm. While their eyes aren’t very complex and consist only of a few photosensitive cells, they’re still simple and effective enough to navigate around their underwater habitat. However, they aren’t the fastest animal underwater. It’s common to lose a limb or two to predators from time to time. But even when things go wrong like this, it isn’t a big headache for them because they can keep going just fine with all the other arms and eye spots they have as backup.

Backup is one way to increase reliability. If something can take over when things go wrong, you still have a way forward. When it comes to recovering a mooring, the acoustic release is a critical point of failure. If the acoustic release doesn’t work, it can be a big headache to get your equipment and data back. In this article, we’re going to talk about when to use a simple and effective way to use a backup acoustic release.

Starfish have light-sensitive cells they use for vision at the end of each arm. Talk about backup!

What is a backup acoustic release?

An acoustic release consists of a mechanical release that opens when it receives a specific coded acoustic signal. In theory, only one is needed to recover a mooring. A backup acoustic release is when two devices are used in tandem in such a way that either unit can fully release the mooring. So what can this configuration look like more specifically?

A redundant configuration like this is simple and effective to set up. Two acoustic releases are attached to a single frame. Both release mechanisms are connected to a short line of chain that runs through a ring. The ring is then connected to the remainder of the mooring and anchor assembly. This way, when one of the acoustic releases is triggered, the chain will slip through the ring, allowing the rest of the mooring to float to the surface for recovery.

Tandem acoustic release configuration. Only one unit needs to trigger to release the mooring. Picture credit: Mooring Systems Inc.

Why is a backup acoustic release important?

When configured correctly, either acoustic release will disconnect the mooring from the anchor for recovery. This adds redundancy, so you can trigger the second one during recovery if there are any problems. Because of this redundancy, it significantly reduces the risk of losing the mooring. An acoustic release is unusual because it must function perfectly for a successful mooring recovery, so it needs to be reliable. So what affects acoustic release reliability?

Some reliability factors are in your control

A mooring design that keeps the acoustic release clear of the seabed during deployment and operation will help. Completing pre-deployment checks and standard battery life testing is crucial for a successful operation and recovery. But not all factors can be controlled, and one of these is battery reliability during a long-term deployment.

It isn’t easy to predict acoustic release battery life down to the minute

The longer the deployment, the more uncertainty there is. Many factors affect battery life: storage temperature, storage time, and operational subsea temperature, to name a few. How the energy is used affects battery life, too, and each time the acoustic release runs routines to listen for their specific release code, a little bit of battery energy is used up. Of course, some amount of energy is needed in reserve to trigger the mechanical release.

Yet modern acoustic release designs do an excellent job of managing their battery energy to maximize deployment life. Regardless of how sophisticated the energy management is, there’s going to be some uncertainty in energy levels, especially if you are pushing the limit of the deployment life of the acoustic release. Nevertheless, mooring designers can reduce the uncertainty and risk of totally depleting batteries on long deployments by using two separate acoustic releases in tandem. While this is a good technical reason for using a backup acoustic release, another good reason is the context of the project itself.

Consider the consequences if a single acoustic release doesn’t fire

Whether or not to use a backup release may depend on your backup plan to recover the mooring. A small surface mooring in shallow water could be retrieved by a diver or simply by pulling on the mooring line. But in deeper waters or for subsurface moorings, these are not practical options. Using a grappling line to recover the mooring is one possibility. But it can take a lot of time to drag the grappling hook back and forth in the hopes that you snag the mooring and pull it in. You also risk damaging instrumentation and equipment from the grapple!

Another factor to consider in the context of the project is the cost of the mooring equipment and ship time. Ship time alone can be tens of thousands of dollars per day, and the expenses of replacing lost mooring equipment and extra ship time to troubleshoot a recovery can be substantial. In this context, doubling up on an acoustic release makes sense!

It’s summary time

An acoustic release is a critical component for mooring recovery. It’s possible to use two acoustic release units together in a redundant configuration to increase the chances of a successful recovery. Though acoustic release units have excellent energy management, battery life can be uncertain, especially when pushing the limits of deployment life. A backup release helps increase the chances of recovery in a long term deployment. But the context of the project should be considered, too. The more complex the mooring and the higher cost involved in deployment and recovery make it worth having a redundant release assembly. Redundancy can pay big dividends in the long run – just like a starfish with eyes on each of its arms!

Next step

Check out a simple way to configure a redundant acoustic release assembly in ProteusDS Oceanographic Designer in this video tutorial from our YouTube channel:

Thanks to Teledyne Benthos

Thanks to Paul Devine and Summer Farrell at Teledyne Benthos for the information on acoustic releases.

Bumblebees have large bodies and tiny wings but are much better at flying than you might think. They’re found thriving on mountain ranges far beyond sea level. The challenge with altitude is the higher you go, the thinner the air gets, making it much harder to generate lift forces. For a Bumblee, which already doesn’t have much to work with such a bulky body and tiny wings, you’d think they’d stay near sea level where the air is the thickest. But Bumblebees have proved they can handle a wide range of conditions. Controlled lab tests prove air thinner than the top of Mt Everest doesn’t really bother them and they keep on flying just fine. They’re incredibly insensitive to these harsh conditions.

Being insensitive to harsh conditions can be a significant advantage. It can even be a design objective in some instances. When it comes to floating systems, spar hulls are very insensitive to disturbances from the ocean. Spar floaters can remain rock-solid-stable in a wide range of ocean wave conditions. Forces from extreme storms can be substantial and make many floating systems heave and tilt substantially, but they don’t really bother spar floaters.

Bumblebees are insensitive to harsh conditions: they’ve been found flying and thriving at thousands of meters of altitude on mountain ranges. Lab tests show they have no problem flying in air thinner than at the top of Mt Everest, too.

Why are spars so insensitive to ocean waves?

Though there’s no strict definition for what makes a spar, but you’ll know it when you see one: they are typically a slender vertical shape in the water. It’s most common to see a long vertical cylinder with a small diameter compared to the length. It’s their slenderness that makes them insensitive to the environmental effects of ocean waves.

Ocean waves can create immense forces. For a sizeable floating ship, the changing water surface from ocean waves is literally like the ground shaking beneath your feet – and it means the vessel will start heaving, pitching, and rolling. But the amount of motion depends on the nature of the floating hull that’s in the water, too. But on the other hand, a floater with a spar form factor will be very insensitive to ocean waves: it doesn’t move around much at all despite extreme ocean wave conditions.

What’s useful about this insensitivity is that it is a very stable and static platform

This lends itself to a lot of advantages depending on what the particular goal is. For example, an oceanographic buoy may have specific instruments whose measurements are disrupted by hull motion. In this case, minimizing the buoy’s motion is crucial to improve data quality.

But spars aren’t limited to small oceanographic buoys. They can also be a large platform, too, with people on board. They’re used as large working platforms in the oil and gas industry that make it easy and safe to do work in a wide range of ocean conditions.

The key to understanding their stability is in the link between buoyancy and the waterplane area

Spar floaters typically have a sizeable submerged volume to support the weight of the platform. In addition to this, because of how slender they are, there is always a tiny waterplane area. The waterplane area is merely the intersection of the water surface through the floater hull. When ocean waves roll past, this moves the intersection up and down and causes fluctuations in buoyancy. But as long as these fluctuations in buoyancy force are small, the disturbance is slight, and there’s not much discernable motion. Because a spar has such a large submerged volume relative to these changes at the waterline, there isn’t much change in buoyancy and resulting movement, even if ocean waves are large.

Spar stability comes from how slender they are. Long and thin, spar buoys have a lot of static buoyancy. As waves pass, the change to displaced volume is minimal, making very little change to buoyancy.

Are spar floaters always stable?

Though very stable, spar hulls can also resonate like all floating systems. Fortunately, the heave resonance period is very predictable for spar floaters. By carefully adjusting the design diameter and length, the resonance period can be adjusted far from typical ocean wave conditions. It’s normal for spar floaters to have very long heave resonance periods that coincide with extremely rare and long ocean swell. The heave natural period and motion in these conditions is still an important consideration, even though it always depends on the specifics of each spar floater.

What’s the downside to such insensitivity to ocean waves?

In the most towering waves, insensitivity can become a problem. If the platform isn’t moving in heave in spite of waves much taller than the platform, it can mean the entire system can submerge. However, this can be a significant advantage and survival strategy because it prevents an overload of the mooring system. But it does introduce new challenges to the system design, including hull strength or instrument depth rating. Submerging is also not an option for a working platform with people on board! But the solution to this is also considering how much reserve buoyancy the system has – in other words, building up the dry portion of the platform, so it’s likely to avoid submerging in the tallest wave conditions.

Let’s look at a few examples

JASCO Applied Sciences manufactures the ObserveBuoy Spar. This 6m long buoy is built for harsh environments with instruments carefully sealed within the hull. While a relatively small platform, other examples in the oceanographic space can be much larger.

The JASCO ObserveBuoy spar is 6m long. Picture credit: JASCO Applied Sciences

The National Research Council of Italy (CNR) maintains a 50m long data buoy in the Mediterranean. It has been in operation for decades as a stable platform that facilitates air-sea interaction studies and can collect data in very rough conditions. While spar floaters are useful for oceanographic applications, they also show up in aquaculture systems, too.

Mediterranean spar buoy used for air-sea interaction studies. Picture credit: Roberto Bozzano and Sara Pensieri from CNR Italy

Floating aid to navigation systems are crucial for marine safety. Yet many of them need to maintain upright and visible in fully exposed conditions. This 38m tall north cardinal spar buoy was installed off the west coast of Australia and was designed for survival in 11m significant wave height and an 18m maximum wave. Spar buoys also come in handy in helping aquaculture systems survive in extreme conditions, too.

North cardinal spar buoy off the west coast of Australia. Picture credit: Adrian Hannam courtesy of Great Pacific Consulting

TendOcean develops technology for the offshore seaweed aquaculture industry. Depending on the farm location, there can be substantial loads on the farm and mooring system from currents and waves. TendOcean uses 6m long spar floaters as a crucial element in the design of these systems. Their spar buoys keep the mooring system secure in operational conditions but have minimal mooring loads because they don’t try to fight to stay at the surface in extreme conditions. Spars are a key part of this resilient and stable system design. So far, these are relatively small examples compared to what can be found in offshore energy.

One of several TendOcean 6m spar buoys used in an offshore aquaculture seaweed farm. Picture credit: Luis R. Rodriguez, courtesy of TendOcean

Equinor develops floating offshore wind systems. The Hywind Scotland offshore wind project consists of an array of five turbine systems using spar floaters. Each of the hulls has a displacement of 78m and provides a very stable platform for generating power in extreme wind and wave conditions.

Hywind Scotland spar floaters provide a stable platform for generating power in extreme conditions. Picture credit: Equinor

It’s summary time

Spar floaters typically have long and slender form factors and sit vertically in the water. What’s notable about them is they are so insensitive to a wide range of ocean wave conditions. The form factor with a large submerged volume and small waterplane area means that ocean waves don’t create significant changes in buoyancy when they roll by. Spar floaters make a wonderfully stable platform for making measurements in the ocean or a stable working platform either for offshore wind or oil and gas systems. But each spar floater needs to be carefully designed as they do have a resonance period, and careful consideration needs to be made on what happens in the most extreme conditions when submergence is possible.

A spar hull form is only one of many types. But it offers a lot of advantages like insensitivity to extreme conditions. Much like the humble high-altitude bumble bee!

Next step

Most of the time, spar buoys are designed to be self-stable, meaning they stay upright on their own without any need for additional static mooring load. Any dynamic model of a self-stable buoy must account for linear and rotational motion. The RigidBody model in ProteusDS captures the effects of linear and rotational motion and dynamics and would be one way to model a spar buoy in an oceanographic mooring system. Check out this video tutorial on how to generate a self-stable buoy using the RigidBody model in ProteusDS on our YouTube channel: