Tag: ProteusDS Oceanographic

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.

Why adapt shallow water geometric compliant moorings with a midwater float

Even though the Sargassum fish lives in the middle of the sea, it spends a lot of time crawling around. This is possible because its habitat is the tangled morass of Sargassum seaweed that floats in large patches in the ocean. These fish are highly adapted to live in this environment. They have camouflage – both colour and appendages – that help them blend in with Sargassum seaweed. But what’s even more curious is how their fins are different than most fish. Their pectoral fins can grip onto the Sargassum weed itself to help them crawl around rather than swim. In this limiting environment, Sargassum fish have many adaptations.

In a limiting environment, you need adaptations to survive. When it comes to mooring design, shallow water conditions are an example of an extremely limiting environment. A key adaptation for geometric compliant mooring designs is using a mid-water float.

Sargassum fish have many adaptations, including claw-like fin-hands to work their way through tangled seaweed. Picture credit: Izuzuki Wikimedia Commons, License CC-BY-SA-3.0

What is a midwater float?

A midwater float means any flotation attached to a mooring line that is not at the water surface. It can be near the water surface or seabed, or anywhere in the water column. A designer can choose from a dizzying range of sizes and shapes of float to use. In oceanographic moorings, it’s common to use something as small as a trawl float, a few inches in diameter, to giant syntactic foam floats that are more than a meter in diameter.

A midwater float helps absorb ocean wave forces and motion

A geometric compliant mooring absorbs motion and energy from ocean waves by deflecting the mooring in the water column. Adding a midwater float can increase the capacity of geometric compliance. Sometimes midwater floats are referred to as a spring buoy – that’s simply because the float’s buoyancy acts like a spring, reeling the mooring back into a central position as ocean waves try to push the mooring away. The larger the buoy, the more force is needed to move the mooring and stretch it out.

A mooring in extreme wave conditions may stretch out without a midwater float and suffer snap loading. These snap loads mean massive dynamic shocks to the system – potentially damaging components, breaking the mooring free, or causing the anchor to hop to a new position. But how do you find the size of midwater float needed?

Sizing the midwater float is not a problem with an obvious answer

Very generally, the greater the size of waves at a specific location, the larger the float size is needed. Advanced dynamic analysis software calculates mooring motion and deflection in different ocean wave conditions. For oceanographic systems, designers can use dynamic analysis software like ProteusDS Oceanographic to see how different placement and sizes of floats affect mooring loads.

Should we always use a midwater float?

In deep water, the mooring deflection from ocean waves is often not very large compared to the mooring length. Geometric compliant moorings in deep water are often possible using a combination of sinking and floating line material without the need for midwater float.

There are other considerations for using a midwater float as well. Enormous midwater floats are costly. They are also heavy and can be awkward to handle when actually deploying the mooring in the water. Depending on the mooring design, large midwater floats may be pulling directly upward on the anchor, too. This may lead to a larger anchor design, again increasing mooring component costs and adding vessel equipment and capabilities for mooring deployment.

 

The Sofar Spotter buoy is a small and portable wave measurement buoy, but even it can produce substantial loading on a mooring line. Picture credit: Sofar Ocean

It’s example time

The Sofar Spotter is a small and portable wave measurement buoy. The PacWave Marine Energy Testing Center uses Spotters to keep track of ocean conditions on their offshore marine renewable energy test sites. While the test sites are offshore, they are in relatively shallow coastal waters and fully exposed to Pacific Ocean storms. The combination of these giant waves with relatively shallow water makes for a challenging mooring design problem.

All wave measurement buoys need to move as freely as possible to accurately track and measure wave heights, and the Spotter buoy is no exception. It might be tempting to use a single length of light rope to keep such a small buoy in place. But without appropriate mooring compliance, even small Spotter buoys generate loads large enough to damage components or lift and drag their small anchors in storm conditions. The resulting mooring loads would also disrupt the Spotter’s measurement capability, seriously degrading data quality in moderate and extreme conditions. On top of that, if the moorings are shifting location from hopping or dragging their anchors, tracking historical weather conditions is a challenge.

In this example, a Spotter buoy is moored in 50m water with a design condition for 6m significant wave height. The mooring consists of a single span of fibre rope. A 12 inch midwater float is necessary to keep the mooring loads under control in these conditions. Without this midwater float, the dynamic mooring loads are over ten times as high, increasing the risk of hopping and dragging the anchor, damaging components, and reducing the quality of wave measurements.

Even a small midwater float can have a huge impact on dynamic mooring forces, reducing damage to components or anchor lift and drag

Summary

Not all geometric compliant moorings need a midwater float. But in certain conditions, especially in limiting shallow water environments, a midwater float may be a key design adaptation for survival. Having adaptations to survive is essential – just like the Sargassum fish with its hand-claw fins that it uses to crawl around seaweed.

Next steps:

Check out a ProteusDS Oceanographic sample layout mooring based on the PacWave Spotter example here. Use this as a starting point to investigate the use of a midwater float for your own Spotter buoy mooring. Read more technical guidance on mooring Spotter buoys in Sofar’s online documentation here.

Thanks to PacWave and Sofar

A special thank you to Brett Hembrough at PacWave and Zack Johnson at Sofar for providing data and technical collaboration on the Spotter buoy mooring used in the example.

How to avoid wuzzles when recovering oceanographic moorings

It was Christmas time, but I didn’t expect to get a haircut as a present. On this particular holiday, one of my young son’s presents was a small automatic flying drone. It was a seemingly harmless small toy that merely hovers and slowly drifts around indoors automatically, without the need for a pilot. But sometimes, it would change direction quickly and catch us off guard. It bumped into my head within less than 5 minutes, and the propellers snarled into my hair. It took a lot of working on getting it unwound and even a few snips to get it free. It was a tangled mess!

Nobody likes to deal with a tangled mess, so if you can avoid one entirely, so much the better. When it comes to oceanographic mooring recovery, there is a considerable risk for the entanglement of the line. When the line forms one of these giant tangles (also known as wuzzles!), it can take a lot of time to sort out and even damage equipment. But with the right precautions, it’s possible to avoid a wuzzle altogether.

Getting a small flying drone tangled in your hair is a serious tangled mess. It pays to avoid problems like this!

So what exactly is a wuzzle?

Strange as it might seem, wuzzle is an actual term to describe a tangled mess in a line. Why does this happen? If a mooring line loses tension and has a chance to bunch up in an area, it may drift into a convoluted mess. When there is low line tension, the residual twist can easily throw the system into a tangle.

During mooring recovery, there’s ample risk for a wuzzle

This is because there is both low line tension and the opportunity for the mooring line to bunch up. There’s a strong chance of this if you’re working with a mooring recovered using an acoustic release.

A wuzzle in an oceanographic mooring is also a serious tangled mess. Picture credit: Danny McLaughlan, courtesy of CSIRO/IMOS

An acoustic release facilitates mooring recovery

An acoustic release is fired when a unique coded signal is transmitted. The result is the device opens a mechanical connection on the mooring line. Acoustic releases are usually placed near the anchor along with a significant amount of flotation.

It’s this significant flotation that brings the tail end of the mooring up to the water surface. The problem is when the tail end catches up to the top float or surface buoy. In the worst-case scenario, the entire mooring can be bunched up in one location in a low tension state. It’s a wuzzle in waiting, and unless you take action, it’s going to be the biggest headache of your life to untangle.

How can you avoid a wuzzle on oceanographic mooring recovery?

You can reduce the risk of a wuzzle with careful observation and operations management. The recovery vessel needs to have a rough idea of the ocean currents at the time, so they expect where the mooring and ship will drift through the process. Typically, the recovery crew can get a location fix of the acoustic release, so they know a safe distance to wait.

However, the recovery vessel may attach a towline for a surface buoy and start pulling gently on the buoy laterally before even firing the release. For subsurface moorings, the recovery crew is on the lookout for the top buoy to reach the surface first. Once they see it, they’ll steam over in the recovery vessel and attach a tow line. Either way, the key is for the recovery vessel to pull the surface buoy or top float away from where the tail end of the mooring is expected to come up. It keeps the mooring from pooling up and reduces the chances of a tangled mess.

How do you know how much time you have before the acoustic release reaches the surface?

There are a few ways to anticipate this. The easiest way is to keep acoustically pinging the release as it rises through the water column. A fix on the release tells you how fast it is rising in real-time. But it’s also possible to complete a recovery analysis in a software tool like ProteusDS Oceanographic. Depending on how much flotation and the line properties you use in your mooring, you can get an idea of how much time you might expect to see it reach the surface.

Snapshot from mid-simulation of mooring recovery. The acoustic release has already been fired and the flotation pack is bringing that end of the mooring to the surface at A. The primary buoy is already at the surface at B.

Summary

When it comes to entanglement, the best approach is prevention. When my son wants to bring out his automatic drone, I now keep a safe distance. I am also considering wearing a hat, and possibly just going into another room altogether. When it comes to oceanographic mooring recovery, prevention is key as well through ship operations.

Next step

Read more on tools for oceanographic mooring design with ProteusDS Oceanographic here.

Thanks to WHOI and CSIRO

Thanks to engineer Don Peters at WHOI for the discussion and feedback on the article topic and also to CSIRO/IMOS for the use of the image.

What to keep an eye on during oceanographic mooring deployment

The longest continuously running experiment started in 1930 and is still going right now. The Pitch Drop Experiment looks like an hourglass filled with a thick, black substance called pitch. Pitch is a resin that is so thick it looks and acts like a solid. It is so much like a solid that it can even shatter into small bits. But despite all this, it will indeed flow over time as a liquid. In fact, it flows so painfully slow that it takes about ten years to form and separate a drop! In the Pitch Drop Experiment, because things move so slowly, there isn’t a need to constantly keep a close eye on things.

On the other hand, you need to keep a close eye on things when there are rapid changes in a system. Rapid changes mean it is easy to miss important information that may affect a design. When it comes to deploying oceanographic moorings, it may seem painfully slow for a mooring to settle in deep water. But don’t be fooled by the time it takes to settle because several rapid changes happen to the mooring and its components that can affect the design. This article will cover an anchor-last deployment’s characteristics that you may want to look at closely.

What we’re going to cover is:

1) peak load during deployment

2) lateral deflection

3) acoustic release effects

First, we’re going to cover the peak load during deployment.

Professor John Mainstone was the custodian of the Pitch Drop Experiment for 52 years. Picture credit: University of Queensland

You might expect a mooring’s maximum loads to appear during an extreme storm

But that’s not always necessarily the case. There can be significant loads during deployment, too. Many moorings are deployed anchor-last, with the top float and mooring strung out along the water surface and the anchor released in the final step for deployment.

Because the anchors tend to be so heavy and with nothing to support it the moment they are dropped from the ship, there can be a sizeable local load on the mooring. This instant of time can be when mooring tension may reach a local maximum. At the very least, the mooring anchor assembly, including acoustic release, may see a load close to the anchor’s full weight at that moment.

The anchor-end of the mooring then accelerates down into the water

As the mooring is pulled into the water, drag relieves the abrupt initial accelerations and resulting loads from the anchor’s free fall. Drag forces on the mooring mean the anchor will descend steadily to the seabed. As mooring flotation is submerged, the descent rate may further slow.

Once the anchor lands on the seabed, the soil will then support the anchor’s weight and further reduce this significant load on the lower portion of the mooring. Either way, the peak deployment load may be between dropping the anchor from the ship and the anchor landing on the seabed. But checking loads during the anchor’s initial descent is not the only thing to take a closer look at during a mooring deployment. This brings us to the next point on the lateral deflection of the mooring.

This snapshot from ProteusDS Oceanographic shows the SOFS5 mooring during anchor-last deployment. After the anchor (A) is dropped, the mooring is pulled down toward the seabed. The buoy (B) stays at the surface for some time and travels horizontally during the transition

Anchors can be so heavy that it seems unlikely they would drift sideways in free fall

There is a lot of concentrated weight in the anchor, and it is hard to imagine it budging much while in free fall. But don’t underestimate the relentless pull from the mooring on the anchor. While the anchor descends and more of the line submerges, there is a reaction load from the mooring. This builds with time. The greater the descent time, the more this acts on the anchor and steers it laterally.

By the time the anchor rests on the seabed, the anchor’s lateral fallback could be ten percent of the water depth for a surface mooring (but less for a subsurface mooring). This is assuming no ocean currents are present. But if there are currents, it can add an interesting wrinkle to the problem. Any prevailing current profile can introduce additional drag loads on the mooring and cause even more lateral movement. You have to be aware of these lateral deflections because it will tell you how far off your intended placement you may be. It all depends on the specifics of the mooring, of course.

However, once the anchor hits the seabed, there’s ideally enough holding capacity to resist lateral loads on the mooring system from any prevailing currents. You may think deployment dynamics are over once the anchor lands, but there’s one more critical stage to check. This brings us to the last point on the effects of the acoustic release.

Everything that moves has momentum

The entire mooring has built up momentum as it descends through the water. After the anchor impacts the seabed, it will take time for the mooring to slow down. The time it takes to slow down and the distance the mooring covers in this time are critical to the acoustic release. It is not a lot of distance, but it’s not zero.

The acoustic release must not impact the anchor assembly or the seabed

If the acoustic release hits the seabed, there’s a chance it can get damaged. If it’s damaged, the release may not work, making mooring recovery extremely difficult or impossible. But clearing the seabed is only half the battle.

When the mooring has slowed down and stopped, and ideally, the acoustic release is still clear of the bottom, the flotation will pull up the lower portion of the mooring back up. This snapback is also not a very large distance, but it’s not the distance that’s important.

The snapback load can cause a problem, too

There’s typically only a short line between the acoustic release and the anchor. If this line is a stiff material like chain, there isn’t a lot of compliance. This means the snapback forces can be very high. If the snapback load is high, it means there’s a chance it can damage components, including the acoustic release – again risking the ability to release and recover the mooring.

What’s vital here is introducing a line with a bit of elastic compliance in the material. It doesn’t need to be a bungee rope – but something a bit softer than chain will reduce the snapback loads significantly and protect the acoustic release. The SOFS5 mooring uses 20 meters of Nystron rope between the acoustic release and the anchor to absorb the snap back load that results from 40 glass floats just about the acoustic release.

Detail on the anchor assembly of the SOFS5 mooring that includes Nystron rope to absorb deployment snapback load

But how can you quantify these loads?

Experience, rules of thumb, and design practices can help a lot to refine mooring designs. This kind of experience is valuable and always plays a vital role in mooring design. But complementing this kind of experience are dynamic analysis software tools like ProteusDS Oceanographic. It’s often worth ensuring there are no surprises or failures in components due to these loads as the system goes through the deployment process.

How to set up anchor-last deployment analysis in ProteusDS Oceanographic

It’s possible to set up an anchor-last deployment scenario in ProteusDS Oceanographic. This can help provide insight into some of the dynamic effects you might see during deployment. Check out the video tutorial below that shows how to set up the scenario.

It’s summary time

There are crucial operational and design aspects introduced by mooring installation and anchor descent to the seabed. You may see considerable tensions during initial anchor release and from snapback after the anchor lands on the seabed. Operationally, the lateral drift of the mooring is critical to understand where the anchor will actually land. You should check the acoustic release clearance from the seabed, too, and be aware of snapback loads because you’re in for big headaches on mooring recovery if it gets damaged.

A full ocean depth mooring might take a while to descend to the seabed fully. But at least it will take a lot less than ten years like it does for a drop to fall in the Pitch Drop experiment!

Thanks

Thanks to Brian Hogue at the WHOI FIXIT Lab for providing feedback and discussion on the article.

PS

The Pitch Drop Experiment has a live webcam feed. Keep a close eye on the experiment in progress here.