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.