Tag: mooring

Why it’s hard to distinguish between the effects of buoy and line drag on mooring knockdown

The southern border between the Canadian provinces of British Columbia and Alberta is, in places, very irregular. It might even seem to be random at first glance. Yet, there is some logical reasoning behind what is there. This border was measured and established over one hundred years ago – far before GPS or other advanced survey techniques were around to help out. So how was the border established?

It has to do with hydrology. In other words, at the highest peak, the direction of rainfall flow establishes the border. Where rainfall flows to the west, it’s British Columbia, and when it flows to the east, it’s Alberta. It’s this clear reasoning that helps to distinguish things.

But we’re not always so lucky to have something to help distinguish things. At first glance, you might be looking at a seemingly random result without any clue how you landed there. When it comes to oceanographic mooring deflection, there are two primary sources of drag: the buoy and the mooring. However, distinguishing between what is more critical to mooring deflection can be a real challenge.

The southern portion of the border between BC and Alberta highlighted in red is highly irregular, but it’s not random. The direction of the flow of rainfall is used to define the border and distinguish between provinces. Picture credit: Google Maps

The vertical deflection of a mooring is crucial to understand

This vertical deflection, also called knockdown, is an increase in depth of all the mooring components from steady drag loads. Because it’s an increase in depth, you need to watch any parts approaching their depth rating. Flotation that exceeds depth rating may fail and result in the mooring collapsing to the seabed. But knockdown may have detrimental effects on instrumentation data quality, too. But regardless, you can’t get a good idea of mooring knockdown without considering the drag loads on the system.

Why is drag significant in calculating knockdown?

Without any currents and the resulting drag load, subsurface moorings are perfectly vertical in the water column. Knockdown results when a subsurface mooring leans over from the effects of steady loads like current drag. All components on the mooring will have some drag. But often, there are two primary sources of drag that tend to dominate mooring deflection and knockdown. The first is the drag on the primary buoy.

The primary buoy is a significant source of drag for a few reasons

The primary buoy is at or near the top of the mooring and often does the bulk of the work holding everything up in the water column. This means it tends to be relatively large compared to other components. On top of this, generally, but not always, currents tend to be larger higher up in the water column. This means the primary buoy is likely in higher flows than the rest of the mooring. Drag is proportional to structure size and flow speed, which explains why the primary buoy is a significant source of total drag on the mooring. But it’s not the only source to consider. The other source of drag to consider is the mooring line itself.

The NOAA CO-OPS DWEAB mooring uses a low-profile Mooring Systems Inc ellipsoid buoy. Picture credit: Laura Fiorentino

At first glance, a mooring line may seem minuscule compared to the primary buoy

Often the diameter of the mooring wire is only a tiny fraction of the size of the primary buoy. But the problem is that the drag area of the mooring is a function of both the diameter and the mooring length. When you consider the entire length of the mooring, the drag area can often be comparable to the top float. There is often at least some kind of current profile through the water column, and this means there can be a substantial amount of drag that accumulates over the whole system.

However, there are more details to the drag on the mooring than the drag area. The actual current profile is vital to get right. The inclination angle the mooring makes in the flow is a factor, too. The effect of inclination on drag is not something you can estimate as quickly by hand as by comparing drag areas. But suffice to say, it’s important enough that you shouldn’t ignore it, and it may even surprise you how important it can be.

How are these effects calculated?

Mooring deflection is typically calculated using well-established numerical techniques that factor in all mooring components, dimensions, and forces in a specific current profile. Purpose-built software like ProteusDS Oceanographic uses geometry, weight and buoyancy of the parts, and typical drag parameters to resolve the mooring deflection. The mooring deflection is a calculation of all these effects together.

So what’s more important: mooring or buoy drag?

It’s not easy to know, in general, what is more important. But we can use a validated example to show mooring and buoy drag contributions. NOAA CO-OPS uses the Deep Water Elliptical ADCP Buoy (DWEAB) design as a standardized design to measure the top portion of the current profile in various locations in the coastal United States. In a recent deployment in a high current region of the Gulf Stream, mooring knockdown field measurements show 48m in 2m/s flow. Mooring knockdown calculated by ProteusDS Oceanographic reached 52m in a 2m/s flow, reaching reasonably close to the field data results. The primary buoy is a large 58″ Mooring Systems Inc Elliptical float, while the mooring is only a 1/4″ in diameter. At first glance, it may seem like the primary buoy would dominate the mooring deflection because of its larger drag area. So how can we get an idea of how much buoy and mooring drag contribute to the knockdown?

To conceptually illustrate this, we looked at two additional load cases. Mooring deflection was calculated with drag on the primary buoy turned off and then again with the drag on the mooring line turned off.

The resulting numerically calculated knockdown with only primary buoy drag was 25m, while the knockdown with only mooring drag was 20m. Note that the total expected knockdown is not calculated by adding these deflections together. This particular example aims to give a rough idea about their contributions to the mooring knockdown for this system. In this specific example, the mooring drag is almost as significant as the buoy drag.

The 200m long DWEAB mooring deflection to scale in 325m water depth in 2m/s current

What if you don’t know the current profile ahead of a deployment?

A knockdown calculation is only as good as the information you have. Of course, before deployment, you may not have much information about the water flow in a region. In this case, it helps to think more about the boundaries of the problem rather than finding the most accurate or probably current profile. The most conservative approach is a uniform current profile. But that may only be appropriate in areas of intense flow. A linear shear, or power profile, may be more realistic. Still, it’s up to the designer to use their judgement and experience for what works best.

Summary

Mooring knockdown is crucial to understand ahead of deployment. This deflection can compromise data quality and damage equipment if depth ratings are exceeded. Knockdown happens in subsurface moorings as it leans over from steady current drag. The primary buoy is often a sizable source of drag in the system. But it’s a mistake to ignore the drag from the mooring line because it can make a surprising contribution to deflection. Unlike considering rainfall to determine a border between two Canadian provinces, there isn’t a way to easily distinguish the contribution of deflection from the primary buoy and mooring. Instead, the best approach is to make sure your analysis considers drag from all components at some point in the design process.

Next step

Download and explore the ProteusDS Oceanographic DWEAB 325m mooring file in the collection of designs available for download here.

Thanks

Thanks to Laura Fiorentino and Robert Heitsenrether from NOAA CO-OPS for providing data and pictures for the DWEAB mooring deployment used in the example.

When you need to fine tune surface buoy damping (and when you don’t)

Professional athletes make artistic synchronized swimming look effortless. Yet it is an incredibly demanding sport: it requires tremendous muscle power, endurance, and control to tread water and perform an elaborate dance simultaneously. On top of that, swimmers need to simultaneously keep in perfect sequence with a group in the water. So how do they keep synchronized while many of them are entirely underwater?

It’s essential to use special underwater speakers. The swimmers can each hear the music used for their performance loud and clear both above and below the water at the same time. Without this, there’s no way to keep the routine synchronized. Underwater speakers are crucial to fine tune the details.

Sometimes you have no choice but to fine tune the details to succeed in what you are doing. But sometimes a quick approximation might be all you need to get to the next step. This is certainly the case in surface buoy hydrodynamic damping: you can spend a lot of time and energy fine tuning hydrodynamic details, but there are workarounds. What we’re going to cover is:

  • Ignoring buoy damping altogether
  • Using a simple approximation when buoy motion may be important
  • Dialing in the details when buoy motion is crucial

First, we will cover the first point on ignoring damping altogether.

Special underwater speakers help artistic synchronized swimmers keep coordinated even when their ears are below the water surface

When can you ignore damping altogether?

Buoy damping comes into play when there is lots of relative motion between the buoy relative to the water surface. Suppose a surface buoy is light enough and has enough flotation to track the water surface, even in extreme storm conditions. In that case, buoy damping won’t have much of an effect on the system response. This kind of scenario often happens in deepwater oceanographic moorings.

In many deepwater oceanographic systems, the surface buoy may have a primary role in supporting the weight of the mooring itself. Full ocean depth moorings may have 5 kilometers of mooring line or more, bristling with dozens of instruments along most of the span. Surface buoys of a mooring like this are often very light, with a lot of extra flotation to support the weight of the mooring line and instruments. These buoys may be so large and light in the water they need a substantial mooring weight to stay stable and upright in the water!

In deepwater mooring, buoy heaving, or up and down motion, drives the dynamic mooring loads. Many surface buoys closely follow the water surface in moderate and extreme storm conditions. In this case, there isn’t much need to resolve the damping effects of the buoy itself when computing mooring loads. But not all surface buoys follow the wave surface perfectly, which can affect dynamic mooring loads. Also, buoy motion and acceleration may introduce constraints and limits on equipment and needs to be better understood. In these cases, ignoring buoy damping may not be good enough. This brings us to the second point about approximating buoy damping.

Some moored systems have instrumentation only in the buoy

The surface buoy may also be much larger and heavier to accommodate an extensive suite of devices to measure waves, wind, and current conditions. If the buoy is larger and heavier, it is less likely to merely follow the water surface in heave in most sea states. On top of this, if the system is in moderate or shallow depth water, the more detailed motion of the buoy – beyond only heave motion – may significantly influence the dynamic mooring loads. In these cases, it may be good to look into more detailed buoy motion, which means you need a rough approximation of buoy damping. But how can you make this rough approximation?

It doesn’t take much to make a rough approximation of buoy damping

It takes relevant experience or knowledge of the particular buoy you’re using. The main idea is you need to know roughly how the buoy responds in calm water to heave and tilt. This is sometimes referred to as the decay response. After a slight disturbance in heave or tilt, are there many motion oscillations before it calms down, or does the motion damp out and settle down right away?

You don’t need to be exact. Nevertheless, the expected decay response combined with the hull geometry, mass, and inertia defines the linear buoy damping. The buoy damping becomes an additional input into a dynamic analysis tool to resolve the mooring loads and buoy motion in various sea states.

While it’s a rough approximation, it’s still an excellent way to make progress on a mooring design problem. It’s an improvement on ignoring damping completely, too. However, more detail in buoy damping is warranted when accurate buoy motion is crucial. This brings us to the third point on dialing in the details on buoy damping.

In some cases, buoy motion has a critical impact on sensors or safety systems

Buoys using current or LIDAR wind profilers are particularly sensitive to surface buoy tilt motion. Visibility of Aid to Navigation buoys, and therefore safety at sea, is directly affected by the amount of tilt. If you’re uncertain about buoy damping, this directly translates into uncertainty about the buoy’s motion affecting data quality and equipment performance. So how do you dial in the detail of buoy damping?

The source of linear damping for floating systems is wave radiation

Wave radiation effects are often solved using potential flow software tools. These hydrodynamic software tools resolve how a moving hull shape creates radiating wave patterns at a range of motion frequencies. Dynamic analysis tools can then use the resulting forces to refine buoy motion.

An example of a potential flow software tool like this is ShipMo3D. The wave radiation forces are computed for a range of motion frequencies and all degrees of motion for a particular surface buoy hull shape. The dynamic analysis tool ProteusDS uses hydrodynamic data from ShipMo3D. Using these tools, mooring designers working with ProteusDS can incorporate more detailed buoy damping effects.

Do all buoys need to resolve wave radiation forces?

Not necessarily. Wave radiation effects are often only significant in larger and heavier buoy hulls that tend to displace a lot of water. These larger and heavier buoys may not simply follow the water surface in most conditions, and a more detailed look is warranted. Experience with a particular buoy form factor may provide the data and expertise to work with a rough approximation only without the need to get into the details of a tool like ShipMo3D. But the decision to use a tool like ShipMo3D should not be taken lightly: it takes time to collect necessary information, set up and compute the system hydrodynamics and validate it. A simple approximation may take a small fraction of the time and still get reasonable results.

Let’s look at an example

In this example, we are going to illustrate what happens to buoy motion when linear damping is ignored altogether. In a partnership between Nortek, AXYS Technologies, Caribbean Wind LLC, and NOAA CO-OPS, a shallow water surface mooring was deployed to explore the influence of buoy tilt on ADCP measurements. The buoy was self-stable with heavy ballast plates to keep it upright, while a low tension mooring helped keep it on station. This is an ideal system to compare with simulated motion predictions from ProteusDS because the buoy is self-stable and the mooring should not have a dominating effect on tilt.

Surface buoy configuration with ballast plates and Nortek Signature 1000 ADCP

A nearby bottom-mounted AWAC was used to record and verify the sea state condition independent from any measurements on the buoy itself. The Nortek Signature 1000 ADCP mounted on the buoy recorded the tilt angle of the buoy in a range of sea state conditions.

A ProteusDS model of the mooring and buoy was constructed to compare with measured results. The buoy was modelled as a rigid body with a cylindrical hull with viscous drag coefficients and no additional linear damping. The resulting buoy tilt in a 3m significant wave height sea state was measured at 13 deg standard deviation with maximums around 50 degrees. The ProteusDS simulated buoy tilt response showed a 12 degree standard deviation with extremes around 60 degrees tilt.

Surface buoy moored profile in 19m water depth

It’s summary time

All surface buoys will have some damping that will affect their motion. Some of this damping can be from wave radiation effects. But the critical question is how much does this damping affect the function of the buoy and mooring? You may be able to ignore it entirely if the focus is on mooring loads. But larger buoys used for measurements that are sensitive to motion or navigation safety may need to take a closer look at evaluating the buoy motion. A simple approximation based on experience can help. Still, there are also advanced hydrodynamic tools like ShipMo3D that can shed light on the problem, too.

A synchronized swimming performance may look like magic in how the athletes keep their timing. Swimmers need to be precise to get through their routine. When it comes to mooring design, you may or may not need this kind of precision to get through a design process. At least you don’t have to hold your breath the whole time!

Thanks

Thanks to David Velasco from Nortek for providing data and insight into the shallow water buoy used for the example, and for AXYS Technologies, Caribbean Wind LLC, and NOAA CO-OPS for publishing their work on the collaboration. Read more on their work published here.

How to take the guesswork out of wave radiation forces

In 1830, the Swedish Navy was reeling after a major continental war. Their fleet was decimated, with many of their wooden sailing ships lost or badly needing repair. Securing wood to rebuild and maintain their fleet in the future was a top priority. The Navy estimated they would eventually need 300,000 oak trees and immediately set to work planting them. It turned out they needed precisely zero.

Planting so many seemed like a reasonable assumption to help with supply in the future. Yet the particular oak trees needed are slow to grow and mature, and they weren’t ready to be used until 1975. But by then, the Navy didn’t need trees anymore because of technological transitions to steel ships. Still, nobody in 1830 knew what the future would hold, and with such long timelines involved, there was no getting around the guesswork involved.

Guesswork may come up when there’s lots of uncertainty. You try to make reasonable assumptions to help find a way forward and solve your problem. When it comes to floating systems, there’s often uncertainty on some aspects of hydrodynamics. The amount of motion damping caused by wave radiation can be an important question. However, with the right software tools, you can eliminate the guesswork using a straightforward process, and take a closer look at the effects of wave radiation.

The forest of oak trees on Visingsö in Sweden were planted years ago to use for wooden naval ships

What are wave radiation forces?

When floating structures move in the water, they create waves at the water surface. These waves radiate away just like the ripples you see after dropping a stone in a pond. Wave radiation forces are the loads that act on the floating structure as these new radiating waves are created.

Wave radiation can cause substantial damping effects

The effects from these radiated waves can be substantial, even dominating, on the motion of floating systems. The momentum and energy of these radiated waves come directly from the floating structure. One crucial effect they can have is damping. This damping can significantly slow down the motion of the floating system in specific ways. But damping is not the only thing affected. The inertia of the floating structure can be affected, too.

The ripples you see in a pond are waves radiating away from the splash. All floating structures radiate waves when they move in the water. But the important question is to figure out how much it affects floating system motions

Wave radiation can also change added mass

Generally, added mass tends to be constant for completely submerged hulls far from the water surface. But near the water surface, the effect of radiating waves causes shifts in the magnitude of added mass. Further complicating things is that wave radiation may increase or decrease the added mass depending on how fast the structure moves around. This effect is sometimes called motion frequency-dependent added mass.

It’s not always easy to tell when wave radiation is important

Generally, one sign wave radiation is critical is if a floating structure looks like it would be a good wavemaker. Good wavemakers displace a lot of water when they move around – and that causes a lot of wave radiation. For example, a typical barge hull is like a large flat paddle sitting on the water surface. It’s hard to think of a better wave maker! Wave radiation forces are significant in almost all degrees of freedom of motion of a barge. But it also depends on how the hull moves in the water. For example, barge yaw may not cause a lot of wave radiation.

What about a more typical ship-shaped hull with a rounded bottom? Typically, in heave and pitch, they are excellent wavemakers, too. Like a barge, there’s often a lot of water displacement, and powerful wave radiation effects are possible. Essentially all the damping of ship and barge motion in heave and pitch will be from wave radiation. But the rounded hull produces an entirely different effect in roll. With very little water displaced as the ship rolls in the water, there aren’t many significant waves generated.

How are wave radiation forces calculated?

There’s no back-of-the-envelope calculation to estimate these effects. Suppose you expect a damped motion response of your floating system. In that case, it’s possible to model wave radiation with a linear damping effect. But the problem with this is that you need to know the nature of the damped motion response from experience and data. You will miss the frequency-dependent added mass effects, too. Without enough information, it might introduce a lot of guesswork to fill the gaps.

But instead of using experience and guesswork, there is a myriad of numerical tools available. One of the most common ways to resolve wave radiation effects is with a potential flow calculation that determines the interaction of a hull form with the water surface. These numerical techniques developed and used for many decades are helpful to resolve all aspects of wave radiation forces – including the damping and added mass effects – for many types of hull shapes. One example of a program that does this is ShipMo3D. ShipMo3D uses potential flow techniques to resolve the interaction of the hull with ocean waves, including radiated waves.

It’s time for an example

The Generic Frigate is a sizeable ship. At 120m in length, there is a lot of displaced water when the entire hull heaves or pitches in the water. So what do the wave radiation forces look like? Rather than focus on the forces themselves, let’s look at a heave decay response of the ship in calm water. This is a simple test that shows the heave motion of the ship after some initial displacement in heave. By far, the dominating damping effects in heave motion are wave radiation.

At 120m long, the Generic Frigate has strong wave radiation effects in heave and pitch motion

The heave decay response was calculated in ShipMo3D. The Frigate was offset by 1m out of the water and dropped. Even though 1m may not seem like much, in this configuration, the total ship displacement is 1000 tonnes, so it will take a lot to stop the oscillations. Yet a little more than 10 seconds later, wave radiation has damped out the heaving motion to a bit of a ripple. For this kind of hull in this particular motion, wave radiation forces are substantial.

After only little more than 10 seconds, the multi-thousand-tonne Generic Frigate heave motion settles out from wave radiation forces. This shows the major influence wave radiation has on this particular hull shape in heave.

It’s a mistake to use potential flow tools for every problem

Though wave radiation forces may be present in a floating system, they are not necessarily a dominating factor. For example, in the problem of oceanographic mooring design, the focus tends to be on loads and deflection of the mooring line itself. Many oceanographic buoys are relatively small and generally follow the water surface in extreme conditions with large ocean waves that drive extreme mooring loads. In specific circumstances, wave radiation may be key in oceanographic mooring problems. Still, most oceanographic mooring design problems do not necessarily require this level of detail.

Summary

Floating systems moving in the water create waves. These waves radiate away from the hull and carry momentum and energy away. When this happens, the result is wave radiation forces that appear on the hull. Radiated waves may have strong damping effects and can also shift the added mass of a system. Generally, you can get an intuitive idea if wave radiation forces are significant if you expect the hull to be a good wavemaker. In other words, if a hull displaces a lot of water when in motion, much like a typical barge would be in heave or pitch. Wave radiation forces can be resolved in specialized software tools like ShipMo3D, but it doesn’t mean they need to be used for every problem. Still, it’s good to have options to reduce the amount of guesswork involved. But guesswork that goes sideways isn’t all bad. At least the Swedish Government now has a forest of 300,000 trees they can use as a national park rather than building Navy ships!

Next Step

Learning about the concept of wave radiation is one thing, but applying it in a specific application is something else. Learn more about what you can do about the effects of wave radiation in the specific application of surface buoys for oceanographic systems here.

PS

The oak trees on Visingso island in Sweden were cultivated to be particularly straight to make them better to use for shipbuilding. Take a closer look at Visingso forest in Sweden on Google Street View here.

Why the added mass force can be confusing

Being invisible makes a lot of sense if you want to avoid predators. Many animals that live deep in the sea use this strategy. One example is the Glass Squid, which is almost entirely transparent. Almost. The big problem is that it’s impossible to be completely invisible.

Eyes absorb light to work, so they will always cast a shadow. And what about food? Food generally isn’t see-through either, so the Glass Squid’s stomach also makes shadows. Yet the Glass Squid has a fantastic ability to help with these problems: it creates an effect of counter-illumination around its eyes and body using light-generating cells. This counter-illumination creates the illusion of faint sparkling sunlight from the water surface. Predators may be looking right at the Glass Squid but will be confused and see something else entirely.

Similarly, you may see something else than what’s really there when looking at a complex problem. There’s certainly no shortage of complex and confusing problems in the realm of hydrodynamics. One facet that can be confusing is the concept of added mass.

The Glass Squid is almost invisible to predators – but not quite. But it creates an illusion with the use of twinkling lights.

Your hands can teach you a lot about added mass

Things feel a lot different when you wave your hand around in the air than in water: waving your hand around in water takes a lot more work! Hydrodynamically speaking, there’s more than one thing going on when you wave your hand around under the water. But one notable effect is added mass. This effect of added mass is something that directly affects accelerations. Whenever anything accelerates in the water, some of the water accelerates around it, too. You don’t feel this effect much waving your hand around in the air because air is so thin compared to water. So how can the concept of added mass be confusing?

The name itself is a little confusing

Make no mistake: added mass is a force – a force that is proportional to acceleration. The name added mass is common because it creates an intuitive link to the effect on a system. When a structure accelerates in a fluid, it acts as if it has an additional amount of mass – this so-called added mass. But the name itself is not the only confusing aspect of added mass.

Mass and weight are intricately linked

But they are two completely separate things. Weight is another example of a force from a gravitational field (usually from Earth in our ocean engineering work!). Mass is not a force and instead is a physical property. So when we talk about how water can “add mass” to a body, another part of the confusion is that this in no way contributes to adding to the structure’s weight. Far from it – it is an effect that is entirely separate from any gravitational field. Added mass does not affect the weight of a body in the water at all.

Ultimately, it is a force proportional to acceleration and it arises from the acceleration of some amount of water around an accelerating structure.

How much is “some amount of water”?

How much water is accelerated is not apparent or intuitive and needs to be measured somehow for every structure. For simple shapes, you might expect an added mass around the same amount of displaced water as the hull. This is very simply because the structure and water can’t occupy the same space at the same time, so water needs to accelerate out of the way. But this is not universally true, and the more complex the system, the more complex the resulting acceleration of the water around it. Nevertheless, there are ways of resolving what the effect of added mass is.

A structure and water can’t occupy the same space at the same time. When a shape like a sphere accelerates to the right, the water around it must accelerate around it to the left. This produces a force on the sphere proportional to acceleration: the added mass force.

Historically, scientists and engineers experimentally measured and recorded the characteristics of added mass for a wide range of structural shapes in the literature. This data is often in lookup tables of the dimensionless added mass coefficient. Engineers and designers can use lookup tables of added mass coefficients to account for this effect fairly easily. But new and complex structures may require new physical experiments or advanced software tools to evaluate the added mass.

Added mass does not always come up first when considering a design problem

Yet it can have a critical influence on dynamic systems. The added mass can significantly change how systems accelerate in the water. Because of this, it can shift natural periods of vibration of specific systems. For example, a subsurface oceanographic mooring may have a natural pendulum period that is significantly affected by the added mass of the top float. When the forces from ocean waves line up or even gets close to this pendulum period, the subsurface mooring can have substantial motions in the water, disrupting instrument measurements. If you don’t understand and account for added mass, you can’t anticipate when this effect will happen in reality.

But where do I find lookup tables for added mass coefficients?

Added mass coefficient lookup tables are harder to find than drag coefficient tables. However, there are still many resources around. Fortunately, common shapes like spheres used in oceanographic moorings are well known and even incorporated into software tools like ProteusDS Oceanographic.

Three dimensional added mass coefficients for several shapes. Added mass is proportional to the displaced volume and the added mass coefficient. Adapted from DNVGL RP-C205: Environmental Conditions and Environmental Loads.

A glass squid relies on invisibility to get by and survive in the wild. The counter-illumination trick helps cover the times it can’t be fully invisible, tricking predators into seeing something that isn’t there. Added mass is one of the more complicated effects in hydrodynamics that, without careful consideration, can make you see something that isn’t there, too.

Next step

For fully submerged hulls, added mass is usually a constant. But when a hull is approaching or floating at the water surface, added mass can change dramatically. Learn more on how added mass can change by the effect of wave radiation here.

How to control downweight seabed clearance in wave powered profiler mooring design

Aerobraking can work well on paper in spaceflight dynamics, but if you’re not careful, it can end in disaster. Aerobraking is when a spacecraft deliberately flies through a planet’s atmosphere to adjust its trajectory. It’s not science fiction, either – the Venus Express spacecraft completed experimental aerobraking maneuvers in 2014. Aerobraking is a handy maneuver as it can save a tremendous amount of fuel when making significant orbital adjustments. But it requires careful planning.

The trick is to get close enough to the planet so there’s enough drag to cause an effect. Close, but not too close: without enough clearance from the planet, there is too much drag, damaging the spacecraft, or worse yet, causing a crash. Aerobraking requires careful control of this clearance.

In most circumstances, clearance is a buffer from disaster. Yet often, we are in circumstances where there’s no way around getting too close for comfort, so controlling clearance is crucial for success. Likewise, controlling clearance comes up with profilers on oceanographic moorings, too. In relatively shallow water areas, it’s a common goal to profile as much of the water column as possible. But wave-powered profilers won’t work at all if the downweight on the mooring comes in contact with the seabed. In this article, we’re going to cover several points controlling downweight clearance from the seabed:

  1. water depth accuracy
  2. tidal effects
  3. wave effects

First, we’re going to cover confirming water depth on site.

Simulated Venus Express aerobraking maneuver: orbit adjustment completed with hundreds of passes. Picture Credit: Ryan’s co-op work term report from 20 years ago

Water depth is a crucial input to any mooring design

The more shallow the site, the more critical it is to know the depth accurately. It is double critical for wave-powered profilers in these conditions. If you want to profile a decent fraction of the water column, the downweight has to keep clear of the seabed, or the profiler will stop working.

It’s OK to start sizing out a mooring for a project with only a rough idea of the water depth, like chart data or measurements from a nearby site. But the closer that downweight is to the seabed, the more critical it is to measure the water depth at the deployment site itself accurately. Checking the depth ahead of deployment is good. Still, it’s also worth confirming the water depth at the moment of actual deployment. There’s no reason to deploy the mooring if something has changed at the site, and you can’t be sure the mooring downweight will be clear of the seabed. In most places, the water depth doesn’t change too much, but that’s not the only thing affecting the downweight clearance. This brings us to the second point affecting downweight seabed clearance on tidal effects.

The tidal effects are very site-specific

There may not be much tidal variation in water depth in many places. In this case, tidal variation might be accounted for with a meter or two of downweight clearance. But surprising things can happen, especially in coastal areas. Tidal cycles are many hours in length. Suppose the water level drops enough that the downweight is sitting on the seabed. In that case, you might end up with similarly many hours of gaps in your profiler data record.

Typically, there’s enough regional information on tidal height variation that you can gauge how much change there might be. Tidal changes take place over many hours, but they aren’t the only environmental factor to consider. This brings us to the last point affecting downweight seabed clearance on the effect of ocean surface waves.

Wave-powered profiler mooring: A) Profiling range B) Downweight C) Downweight clearance from seabed. The downweight clearance from the seabed is crucial to ensure the wave-powered profiler continues to work. Water depth alone is not enough to ensure this clearance: the effects of tides and ocean waves should be accounted for, too

Like tidal effects, ocean waves will be very site-specific

Protected coastal areas may not have much in the way of wave action. On the other hand, fully exposed offshore locations may regularly see extreme sea states. The specific location, seasonal variation, and length of the mooring deployment factor into the size of sea state to expect at the profiler deployment location. Typically, a sea state is characterized by spectrum or at least a significant wave height. A quick way to assess the downweight clearance is to use a factor of the significant wave height.

A crude rule of thumb for a maximum wave expect in a particular sea state is two times the significant wave height. Since many profiler mooring buoys are lightweight and track the water surface closely in most sea state conditions, this can translate directly to downweight motion and the resulting effect on downweight clearance. Of course, this is a rough rule of thumb. Though a good starting point, you can always incorporate a more detailed check on the downweight clearance in a dynamic analysis program like ProteusDS Oceanographic.

In summary

Water depth, tidal effects, and wave effects must be accounted for when sizing up the downweight clearance from the seabed. The easy and conservative approach is to add up the impact of wave and tidal action on the water depth to ensure you have enough downweight clearance from the seabed. While this gives you a guideline to get started, it is the minimum clearance to consider. The consequence of losing data or damaging components has to be factored into the risk assessment for each project, too!

Next step

Explore a typical Del Mar Oceanographic WireWalker mooring using the free ProteusDS Oceanographic sample case here. Evaluate the impact on downweight in different wave conditions and investigate clearance yourself using ProteusDS Oceanographic.

PS

Read more on the final stages of the Venus Express mission, including the results of the aerobraking maneuver and new data produced from it here.