Underwater Remotely Operated Vehicles—Motion Solutions

Motion solutions from Elmo Motion Control Ltd are perfect for the ever-growing field of underwater remotely operated vehicles (ROVs). Apart from physical benefits, Elmo also offers ultra-high voltage drives to meet high-speed underwater thrusting, advanced servo algorithms to fulfill thruster start-up against water currents, and obligatory sturdiness for deepwater shocks and pressures.

Background

An ROV is an unmanned underwater robot linked to an operator through a series of cables. Underwater robotic technology serves an extensive range of applications that range from servicing of deep-water oil rigs to scientific and environmental expedition.

ROVs are fitted with different sensors, robotic manipulators, and cameras. They are used to perform operations like monitoring, taking samples, welding, repairing, and many more. ROVs are highly dynamic vehicles that have been demonstrated to be robust, reliable, and comparatively simple to control. They are the perfect solution for areas that cannot be reached by humans (at times several kilometers below the sea level).

ROVs are subjected to extreme underwater pressures and extended subsea temperature (they are operated in the arctic at times). Autonomous underwater vehicle (AUV) is another type of underwater robot that is generally used for mapping and survey missions. In contrast to the ROVs, they do not essentially have an umbilical cord. They are also fitted with thrusters.

underwater robots

Mechanical Construction and Limitation

A number of different multi-directional thrusters work to self-propel ROVs, maneuver them, and stabilize them in water. Although some ROVs are merely used for monitoring or capturing images, others use robotics manipulators also (usually 1, 2, or 3 per ROV based on their intended use) for a range of applications.

In general, these manipulators are used for interaction with the environment with tasks like gripping, closing/opening of valves, sample taking, cleaning, cutting, screwing, clearing, welding, and much more. As in the case of a majority of the marine applications, the space within a vessel is usually restricted. Therefore, it is necessary for each component onboard the ROV to be as compact as possible.

Power

Since the ROVs are equipped with extremely high-speed thrusters to maintain stability against high water currents, they consume more power. A connecting umbilical cord (which is 4–6 km long in certain cases), sends power, commands, and other data to and from the ROV. The core of the cable is constructed by integrating copper conductors for power and signaling, as well as fiber-optic elements for communication.

The umbilical cord that connects the ROV to the ship can often be very long, measuring up to several kilometers. Hence, it is susceptible to considerable voltage drops, which is a huge challenge for the industry since power transmissions should be increased (while minimizing weight) as ROVs dive deeper. In general, the strain on the cables and the ROV is reduced by installing a Tethering Management System (TMS) between the boat surface and the ROV.

The TMS is connected just a few tens or hundreds of meters from the ROV, though it could be connected to the boat by a few kilometers of heavy cords. This considerably minimizes the stress on the ROV and is an industry apparatus commonly used on the intermediate TMS station. This is precisely where a step-down transformer decreases and rectifies the high AC voltage from the surface into a lower and usable DC voltage required by the ROV.

Industry Trend: ROVs Going Electric

Conventionally, the pumps of the manipulators and thrusters are hydraulic. However, there is an industry-wide trend to replace these hydraulic pumps with electrical and electromechanical components. Regulations mandate any leak in the robot’s hydraulic mechanisms to be reported and evaluated for environmental reasons.

The fines levied because of these leaks are incredibly high and thus raise the operation and insurance cost of these robots in the hydrocarbon exploration and exploitation fields. This fact, together with the low efficiency of hydraulic systems, is what drives the demand for a switch to a more efficient and eco-friendly ROV technology.

D(r)ive into the Deep End

High Speed/Power Thrusting

The extensive range of drives from Elmo can offer ultra-high currents and voltages to ROVs to adapt to any thruster speed or acceleration. Thrusters with a power ranging from a few Watts or several Kilowatts can be accommodated without any difficulty.

Compact and Light ROV

Although Elmo drives are compact, they are also packed with power, thereby contributing more to the miniaturization of ROVs. Exceptional power density is critical for weight reduction and overall footprint.

Thruster Start-Up Against Water Currents

State-of-the-art servo algorithms like Hot Plugging enable ROV thrusters to start rotating irrespective of any thruster motion that exists due to water current.

Handle the Pressure—Ultimate Deep Sea Robustness

Elmo drives are developed such that they can function under intense environmental conditions such as high pressures, extended temperatures (−40 °C to 70 °C), vibrations, or shocks.

Umbilical Power Supply—No Problem

Elmo drives are a versatile class of drives with the potential to handle any supply scenario such as high voltage or stepped down from AC supply, TMS, or already rectified DC.

Ideal Solution for the All-Electric ROV

Elmo’s unrivaled efficiency and exceptional power density make it a perfect partner for the all-electric ROV, whether to smoothly rotate thrusters or to accurately control the manipulators.

Other Features

Elmo’s High Voltage Gold and SimplIQ drives are usually used in ROV applications across the globe. In general, no sophisticated positioning is used, but instead, accurate speed control is necessary. Hot Plugging is available with the Gold (as well as with SimplIQ to a certain extent).

Since ROVs are becoming completely electric, manipulators will be in need of multi-axis control with comprehensive kinematic support, and considerably more accurate operation than what is needed for thrusters.

Extreme temperatures? It should be remembered that ROV manufacturers usually position drives against surfaces cooled by surrounding water.

Extreme underwater pressure? Definitely, there could be scenarios in which drives go down several kilometers below the water surface where pressure could be beyond 600 bars. However, in such scenarios, they are placed in pressurized vessels and not really exposed to such pressure.

This information has been sourced, reviewed and adapted from materials provided by Elmo Motion Control Ltd.

For more information on this source, please visit Elmo Motion Control Ltd.

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