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13 Tips for Operating an Underwater ROV in Strong Currents

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In an emergency, you may need to pilot your remotely operated vehicle (ROV) through strong currents. Without the proper thrust, tether or piloting technique, you risk damaging your vehicle and compromising the mission.

In this blog, we’ll give you 13 tips for operating your underwater ROV in strong currents. Plus, we’ll help you improve your tether-handling technique, enhance your underwater visibility and plan out a challenging mission.

 

1. Keep Your Tether Short to Avoid Tangles

The tether is vital—it connects your vehicle to the pilot, vessel and power source. If you place too much of the tether in the water, it acts as a sail and pulls on the vehicle. To avoid creating drag, release your tether slowly. This gradual release enables the vehicle to travel across the current without resistance. Remember that drag will increase if you work perpendicular to the current.

If you’re operating your vehicle from a ship, keep the tether away from the ship’s propellers, rudders and other obstacles. In rough seas, tethers with too much slack can become tangled and result in costly damage.

2. Use the Current to Your Advantage

The current can help you move your underwater robotic system to its destination faster. As you pilot your ROV, try using the tether like a fishing line to control its movements. If you fly from shore, it’s better to reposition your control box several times, rather than use too much tether in the water.

Natural current breaks, known as eddies, can protect your robot from the current. You can also shield your robot with structures like a ship’s hull, a bridge footer or debris on the seafloor.

Heading into the current will give you the best control over your robot. To reach your target, you will need to take a diagonal heading and “crab” across the current, rather than flying in a straight line. Remember that it’s best to start upstream and go downstream with a short “leash” of tether.

3. Improve Visibility With Sonar

Sonar is a helpful tool, especially in conditions with low visibility. With sonar, you can preview the area with a long-range sonar scan. This allows you to identify underwater hazards like tree branches and plan your inspection around them.

Some crews like to utilize localization and mapping (SLAM) methods to build a map of their target area using sonar. With SLAM software, the robot can pilot through an unfamiliar area while mapping its own location.

4. Reduce Drag by Using a Clump Weight

Tether drag makes piloting in strong currents unpredictable. To reduce drag, try using a clump weight on the tether. This weight will help stabilize the tether in surface and mid-water currents, serving as a counterbalance.

To add a clump weight, lash a carabiner or small weight bag to your tether behind the robot (safely spreading the load over at least six inches of tether) and leave a short leash. Add the necessary weight(s) to the carabiner or bag until the tether hangs straight down in the water column.

5. Optimize Your Vehicle for Its Environment

Not all underwater robotic systems are created equal. Before you conduct your mission in an environment with a strong current, make sure it can withstand harsh conditions.

Some key features to look for in a robotic system built for strong currents include:

Thrust

If your system doesn’t have powerful thrust, you’re probably not going to be able to operate in strong currents. Understand the thrust you have available and how to command it. A good rule of thumb is to have around twice the amount of thrust to the robot’s mass.

Lateral thrust combined with an auto heading feature can also make the vehicle more responsive and better able to maintain its heading than a vehicle without lateral thrust.

Auto Heading

Auto heading can be used to maintain an existing heading or turn the robot to a specific heading. The feature is designed so that you can pilot without having to engage and disengage it when alternating between holding a course and changing directions. ​​When auto heading is engaged, the robot will automatically respond to changes in heading (measured by the compass) by applying horizontal thrust to maintain the current heading.

Enhanced Power

Some systems, like VideoRay’s Mission Specialist Defender, can also use onboard batteries and a copper or fiber tether, which is much thinner than a typical tether. This provides maximum power to the vehicle as there is no loss of power going down the tether. Thinner tether also reduces tether drag, especially over long distances.

6. Plan Your Mission in Full

It’s always important to have a plan before starting your mission. However, rough waters can be unpredictable so having one or two backup plans can save the mission if things go awry.

7. Go With the Tides

Get to know the tidal windows for the area in which you’ll be working and time your operations for as close to slack tides as possible. Slack tides occur during the short time when the tide is neither rising nor falling. During this period currents are usually at their weakest, which can make maneuvering your submersible easier.

8. Plan Around Spring and Neap Tides

Pay attention to neap tides and spring tides. Spring tides have nothing to do with the season. Instead, they happen twice each lunar month throughout the year. Spring tides occur over the new moon and full moon when the Earth, sun and moon are in alignment causing an increase in gravitational pull. This results in higher-than-average high tides and lower-than-average low tides.

Seven days after the spring tide, the neap tide occurs. During the neap tide, the sun and moon are at right angles to each other. Because of this angle, high tides run lower than average and low tides run higher than average. Taking spring and neap tides into account can help you add time to your operational window.

9. Get the Timing Right

General tide times can be misleading depending on where you plan to execute your mission. Moving just a short distance from the port where the tide times are listed can change the slack water times by 30-60 minutes, so be ready to throw in the robot an hour before the quoted slack tide.

Depending on your environment, it may also be helpful to maneuver your robot to the target area before the tide changes so work can immediately begin. Sonar can also be used to find your target as the vehicle makes its way down to its destination.

10. Know When and How to Live Boat

Some missions may require you to deploy from a boat that’s not anchored, in a process called live boating. When live boating, run the system downstream, working it left and right, and letting tether out as needed.

If you’re working in a current greater than three knots, live boat with the engine facing upstream and deploy the robot from the bow downstream. Hold on to the tether and move the boat to position the system in the right spot.

Live boating can cause the vessel to move unpredictably, especially in rougher waters. Don’t forget to be mindful of your tether’s proximity to the rudder or propellers. As mentioned above, keep your tether short to avoid tangles.

11. Maintain Seamless Communication

Open communication is critical during any mission, but even more so when navigating heavy currents. It’s especially important to maintain seamless communication between the pilot and the tether handler.

The tether handler should communicate to the pilot as the tether is deployed into the water—usually every 5m. From there, the tether handler needs to be ready for the pilot to communicate what the handler needs to do. There is usually no time for delays.

12. Pay Attention to the Turns Counter

Pay careful attention to the number of turns in your tether. Tether has memory—for each turn you put in the tether, it will try and counter, which puts torque on the system. This situation makes it difficult to fly the ROV, but you can overcome it with awareness and skill.

13. Know Your Tether Types

You can select tethers with different lengths, strengths, and buoyancy. Choosing the right tether and managing it can have a significant impact on mission outcomes.

Here are the primary types of tether to help you make the right choice:

  • Performance / PPT: This tether has less drag but also smaller conductors and less power transmission capacity.
  • Negative tether: This type of tether sinks but has larger conductors, which means longer lengths can be used without affecting the power available to the system.
  • Neutral tether: Neutral tether is neutral in freshwater (slightly buoyant in salt water) but has thinner conductors.
  • Expeditionary copper or fiber tether: Expeditionary copper or fiber tether is thinner than most other tether types but requires an onboard battery to maintain maximum power to the system.

Get More Tips About Operating Your Underwater ROV

Sometimes the best you can do is to position yourself upstream of your target and fly the robot like a kite. Be soft in your wrist and tough in your flight. If you fly with fear or hesitation the current will win every time!

If you’re looking for more tips about operating your underwater ROV, visit our ROV Technology Video Hub. You’ll get in-depth training videos, interviews with expert operators and more.

Author

Luci Morreale

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