Tag: profiler

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

Posted on January 12, 2022
Ryan Nicoll

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.

When profilers are optimized for control

Posted on January 8, 2021
Ryan Nicoll

It’s impossible to see a Frogfish. It’s not because of their camouflage, even though it is incredibly intricate. It’s not because they swim quickly, either: they are not very streamlined, and many Frogfish will lumber around slowly and mainly try to stay in one spot. What I mean is that you literally won’t be able to see them. When it comes time for them to strike their prey, they can do so in as little as 6 milliseconds. How fast is this? It is well below normal human reaction time – and most other animals’ – of 200 milliseconds. But while the speed is impressive, it means nothing if they miss their target.

So how do Frogfish control their attack to make sure they don’t miss? Their jaws are extremely flexible: when they expand outward, they increase their mouth volume by a factor of 12. This helps them contain their prey more easily. In addition to this, they suck in a massive amount of water around their target to draw them in. The excess water is simply filtered out their gills while their meal gets sucked right into their stomachs. Enveloping their prey and sucking them in makes sure their attacks are well controlled.

Control is key to getting consistent and specific results. In the case of oceanography, there’s more than one way to measure profiles in the water column. Wave powered profilers are impressive, but they have limitations. Making consistent and specific profiles may require a powered profiler, and in this article, we’re going to talk about how these technologies work.

Frogfish envelop their prey and suck in water to control their attacks.
Picture credit: Betty Wills (Atsme), Wikimedia Commons, License CC-BY-SA 4.0

A powered profiler has an onboard energy supply

These powered profilers carry their own battery and electric motors and use them to move on the mooring. This is in contrast to wave powered profilers that ratchet along a mooring that moves from ocean waves.

The powered profiler drive mechanism is straightforward: it uses a traction wheel to crawl in either direction along the mooring line. The onboard batteries and electric motor ultimately drive this traction wheel.

Powered profilers may seem a bit overkill

After all, there’s extra complexity with carrying their own batteries and motors – particularly in contrast to a simpler alternative like wave powered profilers. However, wave powered profilers just won’t work if there are no waves or no surface buoy on the mooring. But there is another advantage to powered profiles, and that is in their control capabilities.

Recovery of a McLane Moored Profiler. Picture credit: Scripps MOD

Powered profilers have a lot of ways to make controlled profiles

There are lots of options to control when profiling. Powered profilers can make profiles on a regular schedule. This schedule can be adjusted over time to sample seasonal variations in the environment, too. Either way, regularly scheduled profiles will generate a consistent dataset with evenly spaced samples in time, reducing data post-processing later on.

When powered profilers are used in a real-time mooring with satellite communication, they can even profile on-demand. But when to profile is only half the story. The other half is where to profile.

There’s also a lot of control over the profile span

A basic approach is to use mechanical stops on a mooring line. But powered profilers have many programming options, too. They can run between specified depths. Different depth ranges can even be used in separate profiles, as well.

All these time and spatial measurement capabilities give a lot of control over profiling. While this is a significant advantage, there are also a few instances when they are the only choice for making profiles.

Mooring motion is vital for wave powered profilers

Wave powered profilers need the mooring motion from surface waves to work correctly. Subsurface moorings, by their nature, have no surface buoy. So if you want to make profilers on subsurface moorings, your only option will be a powered profiler.

But the longer moorings are, the less the lower portion of the line moves when there is wave action. If profiling is required in the lower part of the mooring that is isolated from surface waves, or even in particularly deep water, powered profilers may be the only option.

Like everything in life, there are advantages and limitations

There is only so much energy in the batteries. The traction motors can only exert so much force, too. If there are high current regimes or excessive wave action, it can slow down profiling or use more energy than expected. The drag forces can also exceed the traction motor’s ability to move on the mooring. So there are some limits for use in extreme environments. While wave powered profilers thrive on ocean waves and excel in profiling near the surface, powered profilers typically operate in deeper than 20m water depth to avoid disruption from wave forces.

Another limitation is that the mooring deployment length may be governed by how much energy the batteries can hold for profiling. So how easily you can get out to the mooring to service it is a factor for how they can be used effectively.

Let’s look at an example

Scripps Institute of Oceanography used powered profilers on subsurface moorings to measure internal ocean waves in the Tasman Sea. You can see a schematic that highlights the use of two McLane Moored Profilers below. Note the mooring schematic is not to scale: each powered profiler traversed over a kilometre of line independently. The profilers needed to cover this distance because the internal waves Scripps Oceanographers were looking for were hundreds of meters in size, though also slow-moving, as they propagated through the ocean.

Two McLane Moored Profilers were used on this subsurface mooring to track internal ocean waves propagating in the Tasman Sea. Picture credit: Gunnar Voet from Scripps MOD

It’s summary time

Powered profilers carry their batteries and an electric traction motor on board to work. This means they can also control the depth and rate of profiling. These parameters can vary specifically over time – such as to account for seasonal changes in the ocean. But their onboard energy supply may be a limiting factor for the mission duration. They also have a limit to how much force they can use to crawl along the mooring line – so be wary of extreme environments with large surface waves or currents.

Even though a Frogfish can strike in the blink of an eye, it still needs a way to control its attack so they don’t miss. Similarly, control may be a significant factor when you are planning profiles for your oceanographic project – and a powered profiler may be just what your project needs.

Next step

McLane Research Laboratories makes both powered and wave powered profilers. Read more on their powered profiler MMP here and the wave powered profiler Prawler here.

Thanks

Thanks to Tom Fougere at McLane Research Laboratories for the discussion and information on powered profilers. The helpful mooring example was also provided by Gunnar Voet at Scripps MOD.

PS

You can see a video of how fast a Frogfish can strike below. Don’t blink!

Why profilers stop massive data processing headaches

Posted on November 27, 2020
Ryan Nicoll

When the going gets tough, I like to think about the year 536, and it helps me feel better. The year 536 was a seriously difficult one: a strange fog enveloped most of Europe and large parts of Asia. The mysterious fog was so thick that daytime sunlight was reduced to mere twilight.

You’d think perhaps a stiff wind might clear it up after a few days. Unfortunately, the fog and daytime twilight lasted for 18 months! It turns out the fog was caused by a massive volcanic eruption in Iceland that spewed ash across half the planet. A team of researchers learned more about this by looking at ice core samples. These core samples were ancient, with accumulated layer upon layer of ice over thousands of years. These researchers used super-precise analysis of these ice cores, painstakingly scraping and scanning layer by layer, to examine the ice composition. The ice showed what atmosphere composition looked like through time. These samples revealed massive quantities of volcanic ash throughout the year 536. It took a lot of processing to unravel just what happened back then.

The ash from a massive volcanic eruption blotted out the sun for 18 months back in the year 536

Now most people working with data measured from the ocean are no strangers to lots of processing. While collecting the raw data in the first place can be a challenge in its own right, your headaches may just be beginning when it’s time to look at what you actually recorded. In this article, we will talk about how profilers can help cut through data processing headaches.

Moored profilers are the result of rethinking typical moored oceanographic instrumentation

Most typical oceanographic systems use sensors clamped at a fixed location on a mooring line. It’s a straightforward approach and a simple way to start. But profilers take an entirely different approach: a profiler is mobile on their mooring lines, crawling up and down to new positions in the water column. Each profiler consists of a frame that can be bristling with sensors. These sensors make measurements through the water column as the profiler inches back and forth along the mooring line.

Profilers actively move up and down a mooring line while on-board instruments take measurements

At first glance, there’s an obvious advantage

If you’re using a profiler, you certainly don’t need a whole batch of sensors individually clamped on the mooring line. Longer moorings may have dozens of sensors, and these sensors aren’t cheap! It also can take a lot of time assembling the mooring during deployment. So there is a cost savings factor. But there is a less obvious advantage. While less obvious, it can be significantly more important. Critical, even. This factor is in the data quality profilers can produce.

The problem is in the nature of the ocean itself

The ocean is so big, it may feel intuitive that properties like temperature, salinity, and velocity could only smoothly and slowly vary through the vast depths. But in certain circumstances, nothing can be farther than the truth. In fact, these properties can change extremely abruptly in the water column. There are thin and thick layers of water can form that don’t mix for hours, days, or even years. But these layers can still wobble and move around in the water column through time.

This brings us to the big problem

This big problem is spatial resolution. With fixed sensors, you can completely miss details in the gaps between measurements. This can lead to a severe disconnect between the real ocean and what you read in your data.

A profiler cuts straight through all that confusion: it slides along the mooring, making measurements along the mooring span as it moves. So you get the whole picture along a slice in the ocean.

Traditional moorings have several sensors clamped in place. They can miss a lot of detail as layers of salinity, temperature, and other factors often form in the water column

Profilers are a good idea when there is interest in high resolution measurements

Or perhaps when there’s uncertainty about just what’s going on in the ocean. If there’s some reason to expect complexity and variation in ocean properties, it makes sense to use something like a profiler.

But what about time resolution of the dataset?

It’s true, profilers are a great option, but they can’t do everything. The spatial resolution of the datasets captured by a profiler can be a big advantage. But these measurement profiles can’t be made instantly, or necessarily one right after the other, rapidly. Ultimately, the time resolution of a profile will be limited by how fast the profiler can move. There are other limitations on how often profilers can sample, but it is specific to each kind of profiler.

However, sometimes more than one profiler can be used on a mooring line to help. Sometimes hybrid approaches can also be used with fixed sensors on a mooring line in tandem with a mooring, or sometimes even a profiler mooring with a more standard instrumented mooring nearby as well.

It’s time for a summary

The ocean is a complicated place. There may be many complex layers of temperature, salinity, or other properties that traditional moorings iwth fixed sensors might miss. Profilers are a specialized tool that can help you better understand these complexities by making high resolution measurements as they move along the mooring line. Profilers can slice through the water column, giving you information layer by layer about what’s going on. Not entirely unlike scanning through ice cores to learn more about medieval volcanic disruptions!

Next step

There are a few different kinds of profiler design. But many of them have similar design elements. In all cases, the mooring line slides through the unit, and some kind of clamping mechanism is used. Read more on Del Mar Oceanographic’s WireWalker profiler here and McLane’s Moored profiler here.

Thanks

Thanks to Chris Kontoes and Drew Lucas from Del Mar Oceanographic for the discussion and information on profilers, as well as the inspiring mooring graphics!