Tag: mooring

Why adapt shallow water geometric compliant moorings with a midwater float

Posted on December 2, 2021
Ryan Nicoll

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.

Why shallow water mooring design takes longer than expected (especially compared to deep water)

Posted on November 19, 2021
Ryan Nicoll

IKEA makes furniture. Not a little bit, either. They’ve been the world’s largest furniture retailer since 2008. What do you think their best-selling item is? At over 1 billion units sold, it’s head and shoulders above anything else they make. Here’s a hint: you eat it.

Ikea sells more meatballs than anything else. Does this make Ikea a furniture shop or restaurant??

That’s right: their best selling item is not furniture at all. It’s meatballs. Ikea sells 22 times more than their most popular furniture item (the Billy bookcase). Of course, Ikea found out long ago that keeping customers fed and happy keeps them around in the shop longer. If customers are in the shop longer, they’re more likely to purchase furniture. When you have all the details, it makes sense. But at first glance, a furniture shop selling massive amounts of food doesn’t match expectations.

Similarly, when it comes to mooring design, you may have your own particular expectations. One example could be that mooring design is inherently more troublesome in deep water when compared to shallow water. At first glance, this can make sense – deep water often means an exposed open ocean location, with towering waves and immense forces to contend with. But in some cases, mooring design in shallow water can take a lot longer, even if the waves aren’t as high.

What do we mean by shallow water?

How shallow is shallow, or how deep is deep when we talk about mooring design? It’s rare to find absolutes in design work: a comparison to something else helps provide context. Often, this context comes from the expected size of the ocean waves in combination with water depth.

In the context of mooring design, deep water can mean that the height of the worst-case ocean waves is a small fraction of the water depth. Shallow water then means that the size of the worst-case ocean waves is a sizeable fraction of the water depth. But why does this matter?

Moorings are typically around at least the length of the water depth

If the ocean waves are a small fraction of the depth, it means that the moorings don’t need to deflect very much to absorb the motion induced by the effect of ocean waves. But in water where the waves are a sizable fraction of depth, it means there may be significant mooring deflections required. If there are substantial deflections are necessary, then substantial mooring compliance is needed, too. So what does substantial mooring compliance look like in shallow water conditions?

Many moorings use some form of geometric compliance

Mooring compliance is the give-and-take action that absorbs motion and forces caused by wind, waves, and currents. Geometric mooring compliance comes mainly from the mooring curve deflection through the water column and not from the material stretch. Geometric compliance may come in the form of extra mooring chain lifting up and down on the seabed, or the use of a mid-water float spring buoy, or a combination of both. In deep water, there’s lots of room for the mooring to move and absorb wave motion. But there may not be enough space in the water column for enough mooring deflection in shallow water conditions.

In shallow water, the only way to get enough mooring compliance may be a long length of chain

 

Even with long mooring lines, it doesn’t take much movement to reach a taut mooring line and immense mooring loads. Shallow water mooring often requires a lot of design iteration and careful examination of motions and loads

The search for a viable design that works in shallow water can be time-consuming

To find a viable solution with enough geometric compliance, a designer often uses a trial-and-error approach, using different line configurations, float sizes, and float positions. This kind of trial-and-error approach is often used in ProteusDS to check the resulting loads in mooring components in different wind, wave, and current conditions. But regardless of the analysis tools used, searching for the right combination of materials, float sizes, chain length and checking to see what works or not can take a lot of time. The less room there is for the mooring to deflect, the more challenging the loads tend to be in extreme conditions, often requiring even more trial and error. In deep water, there isn’t always a need for so much design iteration to find something that works, because there’s lots of space for geometric compliance – for the mooring to move in the water.

What about a simple all-chain mooring in shallow water?

Keeping the design simple with an all-chain mooring in shallow water might seem like a good idea, but there can be pitfalls. A long chain means additional cost and weight, complicating deployment. For smaller applications such as wave measurement buoys, the static weight from the chain may be too heavy, too. But another critical problem is that chain is exceptionally stiff under axial load. This means if the mooring is nearing full extension during extreme conditions, the line tension can abruptly spike. The result is a lot of design iteration on chain size, chain length on the seabed, number of lines to use, and so on.

Let’s look at an example of a shallow water mooring

Principle Power develops floating wind power technology and projects. Their prototype, WindFloat-1, was successfully deployed from 2011-2016 in a water depth of 50m. WindFloat-1 used a 2MW wind turbine with a platform displacement of 2300 cubic meters. The moorings were a combination of a portion of wire rope at the top with chain in the lower portion and along the seabed. Because of the shallow water, much of the mooring compliance came from chain lifting up and down on the seabed. The mooring design process required a lot of careful examination of the forces on the floating system and exploring different chain sizes and lengths to see what worked best.

The result was that the ocean-facing storm mooring lines reached 1 km in length. The design proved reliable, too, as WindFloat-1 rode through storms with waves 17m in height and 60 kt winds during its deployment.

 

WindFloat-1 was successfully deployed from 2011-2016 in 50m of water. The ocean-facing storm mooring lines were 1km in length to get the right amount of compliance for these shallow water conditions. Picture credit: Principle Power

What about elastic compliant mooring materials?

Elastic compliant tethers can stretch several times their length before failure and often can easily absorb significant motions and loads from ocean waves. However, commercially available elastic tethers are relatively new compared to traditional mooring components. Traditional mooring components typically have a greater range of breaking strength than elastic tethers. Nevertheless, every mooring has a purpose, and the effect of the mooring on the floating system needs careful consideration.

It’s summary time

In the context of mooring design, shallow water means that the worst-case ocean waves expected are a sizeable fraction of the water depth. In deep water, geometric compliant moorings are often straightforward to design as there is so much space in the water column for deflection. But geometric compliant moorings may be cramped in shallow water conditions with little room for mooring deflection to absorb ocean wave effects. The result may be a time-consuming process to go through many different mooring design combinations to see what works for shallow water conditions.

Like finding out Ikea’s top-selling item isn’t furniture at all, but food, it may come as a surprise at how challenging shallow water mooring is. But careful evaluation with tools like ProteusDS can make it possible to find solutions.

Next step

Something that can improve mooring design in shallow water are midwater floats. Learn more about why they make such a big difference in the next article here.

Thanks

Thanks to Antoine Peiffer at Principle Power for information on WindFloat-1 for the example in the article.

How to avoid wuzzles when recovering oceanographic moorings

Posted on October 15, 2021
Ryan Nicoll

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

Posted on August 26, 2021
Ryan Nicoll

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.

 

 

 

How the anchor-last oceanographic mooring deployment process works

Posted on August 6, 2021
Ryan Nicoll

The first time I started running for exercise, it was a disaster. It was a disaster because there are pitfalls that aren’t obvious to people who are new to running. My problem was I just jumped right in without a detailed plan. I had a general idea that I needed to build up distance slowly. But that wasn’t specific enough, and I ended up with painful shin splints that stopped me in my tracks.

It was over a year before I started training again. Before starting the second time around, I learned a lot more about preventing injuries like this and, more specifically, how to increase mileage slowly. Before starting the second time around, I had a much more detailed process in place.

A detailed process lays out the steps you need to take to move forward. Whether you realize it or not, it helps you avoid problems that may or may not be obvious. When it comes to deploying oceanographic moorings, there is no shortage of challenges, especially when working with longer systems. The longer the mooring, the greater the risk of entanglement and damage to the components. We’re going to talk about the key steps in a process involved in a common way to deploy long oceanographic moorings: anchor-last.

You need a process when running, especially when starting for the first time, to avoid injuries like shin splints

What is anchor-last deployment?

Anchor-last deployment is a process in which the assembled mooring is laid out on the water surface, starting with the top float or surface buoy. The anchor assembly is then connected and dropped as the final step in the deployment of the mooring.

Anchor-last is by far the most common way to deploy moorings for a few key reasons. Following this process makes it possible to keep the mooring aligned and prevent any loops or snags from forming that can cause severe damage before the mooring is even in place. It also provides some easy control over the deployment location – that is to say, where the anchor lands on the seabed. Finally, the cost of a mooring deployment is affected by the size of the vessel you need to deploy the mooring. The nice thing about anchor-last deployments is that vessel requirements are typically the minimum possible.

How are moorings actually deployed anchor-last?

We’ve talked about anchor-last deployment in broad strokes, but now it’s time to dig into the details a little for clarification. Before anything is even put in the water, the ship needs a proper starting position.

The ship needs to advance slowly toward the deployment location. There needs to be enough distance such that at the ship’s forward speed, there is enough time to comfortably assemble the mooring and string it out on the water behind the vessel. Depending on the water depth and length of the mooring involved, the ship may start several kilometers from the deployment location!

Next, we get to the first stages of putting equipment in the water

The top float or surface buoy is prepared and connected to a short section of the upper mooring. What’s vital here is that the lower portion of the mooring is also tied off on the ship’s deck. Mooring wire may be spooled on a winch or laid out in segments on the deck. Segments may be tied off on the deck with the integrated sling or pear links in the mooring.

A crane or A-frame is then used to hoist the buoy off the deck and into the water. The buoy may be one of the heaviest components in the mooring, and the crane or A-frame needed to hoist the buoy may be one of the essential requirements for vessel size.

Now the assembly of the mooring begins

As the ship continues forward at a slow pace, the buoy is towed behind the vessel. Because the mooring is tied off on the deck with a sling link, additional modular segments of the mooring can be easily laid out on the deck, along with assembled instruments and connected when ready.

The ship’s deck length limits how long these mooring segments can be, which may be an essential factor in the mooring design. Nevertheless, as each section of the mooring is attached, it is then deployed into the water. Gradually, the mooring is assembled and strung out and towed behind the ship.

The final stages involve the anchor assembly. For deepwater moorings, it’s common to use a glass float cluster and acoustic release along with the anchor. Once these are in place, the anchor assembly is prepared. Either a crane drops the anchor or a skid plate slides the anchor assembly into the water. Once the anchor assembly is in the water, the final stage of deployment begins.

1) Buoy is deployed 2) mooring is assembled and spooled out behind the ship 3) Anchor assembly prepared 4) anchor released. Image courtesy of J. Doucette © WHOI

What happens after the anchor is dropped?

Even though oceanographic moorings can be completely different, there are still a few typical dynamic stages the mooring goes through once the anchor is dropped.

After the anchor is dropped, it begins its descent to the seabed. A subsurface mooring will often reach a constant descent rate – its terminal velocity – once the mooring is totally submerged. But a surface mooring will always have a varying descent rate because of the effect of drag on the mooring.

Eventually, the anchor will land on the seabed. There is some settling, including a bit of overshoot, as the entire mooring slows down and the flotation pulls it taut to its expected static tension.

This snapshot from a ProteusDS Oceanographic analysis shows a surface mooring in the middle of an anchor-last deployment. The anchor (A) moves vertically downward to the seabed, while the buoy (B) stays at the surface and travels horizontally during this transition.

Why isn’t anchor-last deployment used everywhere?

There aren’t many different ways to deploy moorings. But in some cases, anchor last deployments just can’t be used. For moorings deployed in ice-covered seas, an anchor-first process has to be used. In an anchor-first process, the ship crane supports the entire anchor and mooring weight as the system is deployed through a small hole cut in the ice. As you can imagine, there’s no room to lay out the mooring on the water and steam an icebreaker to the deployment location in these circumstances!

In summary

The anchor-last process is used widespread for longer moorings. Typically, a ship will slowly advance toward the final deployment location. This allows the mooring to be assembled and strung out behind the deployment vessel to prevent any snags. The last step is dropping the anchor near its installation point.

Many new runners may jump right into the sport and end up with injuries without training that follows a tried and true process. Likewise, anchor-last deployment is a tried and true process for oceanographic moorings. There are always many details and specifics that arise with different mooring designs and lengths, but many fundamental steps follow this anchor-last deployment process.

Next step

We covered what the anchor-last process looks like. But what about specific details to look closely at during the deployment process? Read more to learn about specific effects to look out for during deployment here.

Thanks to WHOI

Thanks to Rick Trask at WHOI for sharing technical pointers and details on the anchor-last deployment process.