How to control uncertainty in water current profile when designing oceanographic moorings

Image of the Rockland Nemo uncertainty in water current profile

Rockland Scientific’s Nemo MicroRider turbulence measurement platform (photo courtesy of Rockland Scientific)

When you get something different than you expect, it’s a sign that uncertainty has reared its head

“Just make sure that it’s good!” It was the last thing I heard from my wife as I bolted out the front door. On a twilight dash to the supermarket down the road, I’d still handily get there before closing time for critical essentials – coffee, milk, and ice cream.

Making quick work of the aisles, I stopped short and hesitated in front of the freezer. Some of the cartons had frost on the labels, but I could tell there was some strange choices: cinnamon dust, citrus tea, and chocolate chip; who doesn’t like chocolate chip?

I was back home with time to spare. “You realize of course,” my wife said, brushing frost off the label of the carton, “you bought the only ice cream that tastes like toothpaste?” It was mint chocolate chip. Frost had covered up part of the name.

I ended up with something I didn’t expect

When deploying oceanographic moorings, there’s a lot to do on a tight schedule. There are many factors that can affect results. Uncertainty can cause something that’s different than expected.

Image of Ice Cream Freezers for uncertainty in water current profil blog

Mint Chocolate Chip and Chocolate Chip are definitely not the same

Uncertainty and risk will cost you

Oceanographic sensors ($) collect data from the ocean and are specifically designed to survive for long periods of time in harsh conditions. With the ocean covering 70% of the planet at an average depth of 4000m, these sensors are often submerged, under high pressure, in very remote locations and need ships and crew ($$) to deploy, and hopefully, recover them.

These devices collect measurements of pressure, salinity, current, and any other number of parameters, at precise locations in the ocean and potentially for many years at a time. This data is worth far more than the cost of the equipment and its deployment ($+$$ = $$$), not to mention the loss of years worth of data when sensors are invalid or lost. So how does uncertainty in the water current profile cost you?

It’s a serious drag

The force from water drag grows with the square of the current speed. This force can be a severe challenge in designing a mooring because of how quickly the forces increase and deflect the mooring. More deflection means more sensor tilt and greater knockdown.

With too much tilt, some sensors stop working, and every bit of knockdown is an error from the intended position. Much like the frost covering the ice cream label, it means you might not know what you’re going to end up with. But is there a way to control the risk?

Rockland Scientific’s Nemo MicroRider platform as seen in ProteusDS

Bounds can control uncertainty

Understand the problem accurately at the boundaries, and you know the answer is somewhere in the middle. Using a dynamic analysis program like ProteusDS, a mooring design can be checked with both a conservative and optimistic current profile.

Using the bounds allows adjustment of the design to understand the forces, and their effect on tilt, and knock down to protect the data that is acquired at great cost.

But how does dynamic analysis complement field experience and intuition?

There’s always new combinations of equipment and conditions in the environment. Field experience has value and is based on real equipment in specific configurations in a set of environmental conditions.

Dynamic analysis software like ProteusDS helps systematically evaluate both new and existing combinations of equipment, confirming a hypothesis and trends in design behaviour, and broadens a mooring designer’s intuition and knowledge.

Example: 30m Nemo mooring

Rockland Scientific produces the MicroRider sensor package that continuously measures turbulence in the water at a specific depth. To measure at the desired depth in the water, the turbulence sensor is mounted on a streamlined float hull, called Nemo, that minimizes drag forces, so it is less affected by the wide range of flow speeds.

Mooring analysis in ProteusDS shows the effect of conservative and optimistic bounds of the water current profile on knockdown, which highlights the potential error in the data from the target measurement depth. Mooring designers can plan for effects at the optimistic bound and assess implications at the conservative bound.

In this example, the Nemo float is moored in 100m water depth on a 30m mooring. A power law current profile with 1.5m/s and 2.5m/s surface current was used to find the steady-state deflection in ProteusDS. The excursion was 4.6m and 11m in low and high speed flows, respectively, and knock down was 0.4m and 2.5m in low and high speed flows, respectively.

Depending on the level of risk for the specific application, a larger current speed or more conservative current profile could be used, and the mooring design can be adjusted to reduce excursion and knock down.

A ProteusDS PDS Image of the Rockland Nemo.

Bounding the Nemo mooring deflection A) in 1.5m/s current (optimistic estimate) and B) 2.5m/s current (pessimistic estimate)

Designers design, tools help

The costs of oceanographic data are very high because of the specialized equipment and remote access to sites of interest. Uncertainty from factors like water current profile can introduce serious risks, like loss of equipment, gaps in data, or erroneous results from too much sensor movement. However, mooring designers can control risk by bounding the problem with a tool like ProteusDS, and then everybody gets the ice cream flavour they expected.

Next step

At DSA, we have created ProteusDS to help oceanographic mooring designers. License the software to check for yourself how current profile assumptions affect your designs and oceanographic data.

Check out this video tutorial on how to quickly assess an oceanographic mooring deflection to a current profile in ProteusDS.

Thanks to Rockland Scientific

Thanks to Rockland Scientific‘s Fabian Wolk, Jeremy Hancyk, and Peter Stern for sharing technical pointers, information, and pictures of the Nemo MicroRider.