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Why compliance is at the heart of resilient oceanographic surface mooring design

Originally published on dsaocean.com

Deep in the Andes Mountains, straddling jagged peaks, sits the 15th century Inca citadel Machu Picchu. It’s a harsh and remote location. To add to this, the ancient site sits on top of fault lines that are prone to earthquakes. But these earthquakes were no match for Incan engineering and architecture. So how did their designs survive earthquakes?

Their structures used many-faced blocks in different sizes and shapes. They fit together so precisely that no mortar was needed. When the shaking starts in an earthquake, these blocks can shift and move. But when the shaking stops, they settle right back into place. Lasting hundreds of years, it’s a design that is truly resilient.

Similarly, this resilience is needed in oceanographic moorings. When the shaking starts in an extreme ocean storm with wind, waves and currents, surface moorings have to survive severe forces and motions. It’s in these storm conditions that oceanographic mooring survival is possible only through mooring compliance. This mooring compliance must be carefully understood for the design to be successful and last through its deployment. So what is mooring compliance?

It’s the give-and-take that prevents things from blowing up

There are dynamic and steady forces from the environment. Ocean waves produce dynamic forces and water currents produce steady drag forces. Mooring compliance is the give-and-take in an oceanographic mooring that allows it to absorb the effects and forces of the ocean. So what happens when there’s not enough compliance?

A broken mooring line can mean a lot of headaches in lost equipment and data

Mooring compliance allows the instruments to ride out the massive waves in severe ocean storms. If there’s not enough mooring compliance, the mooring can break, or it may “walk” as the anchor is overloaded by forces. A resilient mooring design is one that has enough compliance.

Oceanographic moorings have many expensive sensors that make various measurements in the ocean. Sometimes, these measurements may go on for years and years at a time. Often, the data from these sensors can only be retrieved if the mooring is recovered.

That data may be gone forever somewhere at the bottom of the sea if the mooring fails. The risk of losing valuable equipment and data is reduced with careful design and understanding of compliance. Usually, moorings may have geometric compliance or elastic compliance, or some combination of the two. Moorings with elastic or geometric compliance can look very different.

Geometric compliance is the ability of the mooring to change shape

The shape of the mooring might look like a zig-zag through the water column, formed with careful placement of floats and weights along the mooring span. The mooring zig-zag then acts like a giant geometric spring that can extend or contract to absorb the dynamic effects of the environment. In comparison, elastic compliance doesn’t rely on the shape of the mooring.

Elastic compliance is when the mooring relies directly on the material stretch

The shape of the mooring may be as simple as a straight line from anchor to surface. Some materials can stretch several times their length, much like a rubber band. While geometric compliance relies on the mooring moving through the water, elastic compliance relies on the mooring line to directly stretch out. So when do should use geometric or elastic compliance?

Shallow water moorings are a challenge

Shallow doesn’t always mean near the shoreline. It can be anywhere where the ocean waves are large relative to the water depth. Because the wave effects are so severe, there may not be enough space in the water column to make a geometric shape. This means it’s more likely for the mooring line to be pulled straight and taut, leading to massive tension spikes that can break the mooring. So in shallow water conditions prone to more massive waves, elastic moorings can really shine.

How does elastic compliant mooring transmit power and data?

It’s true, elastic compliant materials are often highly stretchy synthetic materials. These elastic materials don’t transmit data through electrical or optical signals. So what can be done to provide power and send data through the mooring? One way to do this is with a coil of data and power lines helixed inside the elastic member. They are used in a variety of applications, but one example is in passive acoustic whale monitoring.

Let’s look at an example of elastic compliance

Off the east coast of the United States near Cape Cod and Nantucket Island are important Northern Right Whale feeding grounds. It’s at these feeding grounds that at various times of the year these whales concentrate. This concentration increases the chance of vessel strike because of the marine traffic in the area.

To guide local marine traffic, warnings are issued by a system that detects the presence of whales by their acoustic signature in the region. The system uses an array of special moored sensors, developed by WHOI and EOM Offshore.

Faced with the job of monitoring these whales on the continental shelf, the buoys need to be moored in the range of 30-150m water depth. They rely on elastic compliance to survive in the severe Atlantic storms in the region and have been successfully operating for several years. Power from the buoy is transmitted through the conductors helixed within the EOM stretch hoses to the hydrophone that listens for whales. Those same conductors pass the acoustic data from the hydrophone back up to the buoy for transmission to the shore.

Summary

We’ve covered a few details in understanding mooring compliance, so let’s take a quick review. Only resilient oceanographic mooring designs will survive ocean storms using compliance. Mooring compliance is the give-and-take in the mooring that allows it to absorb the dynamic effects of the environment, such as from ocean waves. Ocean waves can easily break a mooring, resulting in substantial losses in years’ worth of data and expensive sensors to the bottom of the sea, if there isn’t enough compliance.This compliance can be in the form of geometric or elastic compliance. Geometric compliance is the change in shape of the mooring in the water column. In comparison, elastic compliance is a direct stretch of the mooring materials. Often, elastic compliance is better in shallow water. But only detailed dynamic analysis will point out the correct design specifics.

The Inca engineers had the right idea

Structure design allowing movement in the stones is really also a form of compliance, too. The motion of the structural blocks in extreme earthquakes was the give-and-take that prevented catastrophic structural failure. But Inca engineers relied on experience and trial and error in their designs. So how can mooring compliance be tested without a trial and error approach?

Next step: how to evaluate mooring compliance

Mooring designers use dynamic analysis software to evaluate mooring compliance ahead of deployments. We talked about why mooring compliance is important in this article, but not how to check your own design. Check out this article that outlines a systematic process to do so.

Thanks to EOM Offshore

Thanks to EOM Offshore’s David Aubrey and Beth Unger for sharing technical pointers and information on the Whale Monitoring project.