Posted on Aug 22, 2023
Ryan Nicoll

Why anchor size causes a chain reaction on operations costs (and how to keep it under control)

Despite the name, hermit crabs are surprisingly social, and for good reason. These animals don’t grow their own shell and need to make do with what they find in the environment – usually abandoned shells from other species. But finding a new home is an ongoing effort because their bodies grow slowly over time. Yet hermit crabs have a curious way they work together on this problem. When a crab finds a new larger empty shell, it’s often not the right fit for them. If it’s too big, they can’t just take it because it can be too heavy to carry around, and they’re slow and vulnerable. But they don’t just walk away: instead, they stay nearby. As more and more hermit crabs approach and check out the new shell but find it won’t work, they even make an ordered line from largest to smallest, waiting patiently.

Then something incredible happens. Finally, when the abandoned shell is the right fit for one animal, they leave their old, slightly smaller shell behind. And lo and behold, it’s just the right size for the next hermit crab in line. The result is a rapid cascade of shell-swapping as they all hop out one after the other from the old into the next size up, in one big chain reaction.

This kind of chain reaction is a useful thing with many benefits that come out of it. But not all chain reactions are good. They can, in fact, go the opposite direction and set you up for serious problems. This is something that can happen when sizing an anchor for a mooring system. In this case, the chain reaction is on operations costs. In this article, we will talk about how anchor size affects operations costs and what you can do to keep things under control.

“The shell fits perfectly, but I don’t like the colour…”

Picking an anchor size is a crucial step in the mooring design process

It’s crucial because anchors are the foundation of any mooring and they have one job: keep the entire system rooted on the seabed. In oceanographic systems, it’s common to see anchors in the form of clump steel or chain or stacks of retired railway wheels. Its size and corresponding force of the weight of these anchors that has to resist the upward and lateral tension from the mooring that comes from flotation, drag, and dynamic effects from ocean waves. But there can often be uncertainty in figuring out the right size of anchor to use: the seabed condition may be unknown, or no limited information on the expected anchor loading can result in a tendency to use a larger anchor size to increase the chances that the system will stay put.

Modular oceanographic mooring anchors are often made from stacks of retired steel railway wheels. Picture credit: Joe Cacciola at Naushon Engineering Ltd.

Using a larger anchor is where the chain reaction can come in on operations costs

Deploying a mooring means deploying the anchor, too, no matter how heavy it is. Most moorings are deployed anchor-last, meaning the top buoy and mooring is laid out on the water, with a crane to lift the anchor off the ship deck to deployment position over the side. But the heavier the anchor is, the larger the crane must be to lift it safely.

But cranes are not something you can plug and play in different ships

You’re stuck with the cranes already set up on ships available for your project. A more massive anchor may mean you have to use a much larger vessel than you planned. And larger ships mean an increase in day rates: fuel cost and crewing requirements for bigger vessels can mean a substantial jump in operations costs. This factors in multiple times through a project – sea time during deployment, maintenance, and recovery and redeployment are opportunities for recurring ship time costs. Regardless of the impact on the budget, if the project is in a particularly remote location, there may not even be any vessels with suitable crane capacity to carry out the project. But an under-sized anchor isn’t the solution, either.

Some oceanographic systems such as the Wirewalker are small and light enough to be deployed by hand in very small craft, with minimal operations costs. Picture credit: Chris Kontoes at Del Mar Oceanographic

An under-sized anchor can cost you, too

If the anchor weight isn’t enough to resist the lateral loads from current drag on the entire mooring or dynamic loads from surging ocean waves, the result can be dragging or hopping to a new location. It may not move much, but any movement will increasingly affect data quality. Yet if it goes too far, it may require an intervention – which means more ship time and increased costs. So what can you do to find the right anchor size?

At the other end of the scale for vessel size, the 94m long CSIRO/Marine National Facility RV Investigator main crane and A-frame has a lifting capacity of 20 tonnes and it is capable of deploying the largest anchors used by full ocean depth moorings. Picture credit: Owen Foley from Above & Beyond Photography

Using mooring design software as part of a standard design process helps dial in anchor size

What’s needed is software that calculates mooring loads and deflections in wind, waves, and currents. Accounting for the environmental loads on the mooring, including current drag, weight of components and buoyancy of floats, and dynamic effects from ocean waves, reduces uncertainty about the loads on the anchor and allows you to establish a reasonable anchor size. The mooring design software ProteusDS has these features and is particularly well suited to set up detailed oceanographic moorings for sizing components and anchors.

The SOFS full ocean depth mooring anchor assembly in preparation for deployment from the RV Investigator A-frame. Picture credit: Pete Jansen at CSIRO / IMOS

But environmental conditions drive mooring loads, so what if these are uncertain at the project site?

All projects are highly site-specific because the environmental conditions and the uncertainty can be challenging to address. Yet preliminary desktop studies are possible with numerical modelling of ocean conditions to help reduce this uncertainty. If the project’s scope is big enough, it can also justify some preliminary metocean measurements with smaller-scale instrumentation to verify the metocean conditions. It may be well worth it if it avoids major chain reaction increases in costs resulting from a larger vessel. In any case, following a systematic mooring design process leaves you with a much more certain outcome when compared to waiting in line for a new shell to arrive like a hermit crab!

Next step

Finding the loads from the mooring on the anchor in a specific wind, waves, and current condition is key to finding the right anchor size. The ProteusDS post-processing tools help isolate the loads on the anchor and provide input on sizing the right anchor you need. Watch the process unfold in resolving an oceanographic mooring motions and loads with mooring and anchor loads, computed using the Report Tool in one of the ProteusDS YouTube video tutorials here.