Blog

Why adapt shallow water geometric compliant moorings with a midwater float

Originally posted on dsaocean.com

Even though the Sargassum fish lives in the middle of the sea, it spends a lot of time crawling around. This is possible because its habitat is the tangled morass of Sargassum seaweed that floats in large patches in the ocean. These fish are highly adapted to live in this environment. They have camouflage – both colour and appendages – that help them blend in with Sargassum seaweed. But what’s even more curious is how their fins are different than most fish. Their pectoral fins can grip onto the Sargassum weed itself to help them crawl around rather than swim. In this limiting environment, Sargassum fish have many adaptations.

In a limiting environment, you need adaptations to survive. When it comes to mooring design, shallow water conditions are an example of an extremely limiting environment. A key adaptation for geometric compliant mooring designs is using a mid-water float.

What is a midwater float?

A midwater float means any flotation attached to a mooring line that is not at the water surface. It can be near the water surface or seabed, or anywhere in the water column. A designer can choose from a dizzying range of sizes and shapes of float to use. In oceanographic moorings, it’s common to use something as small as a trawl float, a few inches in diameter, to giant syntactic foam floats that are more than a meter in diameter.

A midwater float helps absorb ocean wave forces and motion

A geometric compliant mooring absorbs motion and energy from ocean waves by deflecting the mooring in the water column. Adding a midwater float can increase the capacity of geometric compliance. Sometimes midwater floats are referred to as a spring buoy – that’s simply because the float’s buoyancy acts like a spring, reeling the mooring back into a central position as ocean waves try to push the mooring away. The larger the buoy, the more force is needed to move the mooring and stretch it out.

A mooring in extreme wave conditions may stretch out without a midwater float and suffer snap loading. These snap loads mean massive dynamic shocks to the system – potentially damaging components, breaking the mooring free, or causing the anchor to hop to a new position. But how do you find the size of midwater float needed?

Sizing the midwater float is not a problem with an obvious answer

Very generally, the greater the size of waves at a specific location, the larger the float size is needed. Advanced dynamic analysis software calculates mooring motion and deflection in different ocean wave conditions. For oceanographic systems, designers can use dynamic analysis software like ProteusDS Oceanographic to see how different placement and sizes of floats affect mooring loads.

Should we always use a midwater float?

In deep water, the mooring deflection from ocean waves is often not very large compared to the mooring length. Geometric compliant moorings in deep water are often possible using a combination of sinking and floating line material without the need for midwater float.

There are other considerations for using a midwater float as well. Enormous midwater floats are costly. They are also heavy and can be awkward to handle when actually deploying the mooring in the water. Depending on the mooring design, large midwater floats may be pulling directly upward on the anchor, too. This may lead to a larger anchor design, again increasing mooring component costs and adding vessel equipment and capabilities for mooring deployment.

It’s example time

The Sofar Spotter is a small and portable wave measurement buoy. The PacWave Marine Energy Testing Center uses Spotters to keep track of ocean conditions on their offshore marine renewable energy test sites. While the test sites are offshore, they are in relatively shallow coastal waters and fully exposed to Pacific Ocean storms. The combination of these giant waves with relatively shallow water makes for a challenging mooring design problem.

All wave measurement buoys need to move as freely as possible to accurately track and measure wave heights, and the Spotter buoy is no exception. It might be tempting to use a single length of light rope to keep such a small buoy in place. But without appropriate mooring compliance, even small Spotter buoys generate loads large enough to damage components or lift and drag their small anchors in storm conditions. The resulting mooring loads would also disrupt the Spotter’s measurement capability, seriously degrading data quality in moderate and extreme conditions. On top of that, if the moorings are shifting location from hopping or dragging their anchors, tracking historical weather conditions is a challenge.

In this example, a Spotter buoy is moored in 50m water with a design condition for 6m significant wave height. The mooring consists of a single span of fibre rope. A 12 inch midwater float is necessary to keep the mooring loads under control in these conditions. Without this midwater float, the dynamic mooring loads are over ten times as high, increasing the risk of hopping and dragging the anchor, damaging components, and reducing the quality of wave measurements.

Summary

Not all geometric compliant moorings need a midwater float. But in certain conditions, especially in limiting shallow water environments, a midwater float may be a key design adaptation for survival. Having adaptations to survive is essential – just like the Sargassum fish with its hand-claw fins that it uses to crawl around seaweed.

Next steps:

Check out a sample Sofar Spotter mooring based on the PacWave Spotter in the example in the collection of sample mooring files here. You can also read more technical guidance on mooring Spotter buoys in Sofar’s online documentation here.

Thanks to PacWave and Sofar

A special thank you to Brett Hembrough at PacWave and Zack Johnson at Sofar for providing data and technical collaboration on the Spotter buoy mooring used in the example.