Day: 26 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.