Month: August 2021

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.

 

 

 

How the anchor-last oceanographic mooring deployment process works

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.