In my previous Edison’s Desk blog entry I spoke about the challenges of going into deep sea exploration and the implications of working at 4,000 meters depth for technology.
All subsea operations after the human diving limit depth (a few hundred meters) are achieved remotely. It is very expensive to send manned probes or submarines to those depths, put them in the right place, plug and unplug cables, switch valves on and off, or even just be there as observer, while all commands come from the surface, from a drilling rig or an exploration platform. Not to mention all risks involved in working at such depth: equipment that lasts a long time is heavy and very expensive.
Therefore, finding ways to accomplish tasks reliably from a distance (either from the water’s surface immediately above or from a comfortable control room onshore) has always been the key for subsea exploration.
The most used vehicle to perform such subsea operations is an ROV or Remote Operated Vehicle, which can be thought of as a very versatile, remote-controlled and tethered submarine. ROVs draw the power needed for thrust and to activate instrumentation from an umbilical cable that connects the vehicle to a ship on the surface where the ROV operator sits. That power (either electrical or hydraulic) can also be relayed to additional equipment carried, installed, or already in place on the subsea floor.
ROVs are designed to carry a certain amount of payload that ultimately reflects on the total size. They carry enough thrusters so they can move and roll to any desired direction. That, combined with one or more robotic arms, gives the ROV the ability to orient itself in the correct position.
Without a tether to feed the ROV with power, the only other way to keep the vehicle running is by using batteries. If the system also has means to operate autonomously, that is, without being steered by an operator, then it is named an AUV, or Autonomous Underwater Vehicle.
Compared to an ROV, an AUV can reach longer distances based on mission targets and battery capacity. However, because of the battery itself, AUVs are not the best choice if the mission requires applying physical forces, such as opening and closing valves; or if the mission requires hydraulic power, as there is none available. For that reason AUVs operate usually on surveillance of longer step-outs of cabling, pipes, umbilical or risers. Put another way, AUVs are equipped to monitor the cables, pipes, umbilicals and risers that connect subsea assets that are separated by great distances. Equipped with a camera, a light source and a variety of sensors, AUVs can be used to detect leakages or structural damages for example.
The problem of recharging can be mitigated with wireless charging stations. In that way, an AUV can stay in service longer, recharging in between performing multiple missions in the same way some vacuum cleaner robots operate. The data collected during a mission can be sent topside using acoustic modems that transmit information through water, or alternatively, a charging station also can have a faster link to the surface with wires or optical fibers.
There is still another type of underwater vehicle that works as a crawler on the sea bottom. This type of vehicle is mainly designed to lay down or even bury subsea communication cables.
Sensing technologies for subsea assets
Regardless of the type of vehicle chosen, a large field for research unfolds when one thinks about the many possible applications for these carriers. The Offshore & Subsea Systems CoE team at the Brazil Technology Center is also looking at a variety of sensing technologies that, once adapted for the marine environment, can be attached to underwater vehicles and used to acquire valuable information about subsea assets, which will increase their availability. Sensors like these would allow operators to perform maintenance on machinery only when it is really needed. Today, many countries mandate that subsea assets must be retrieved from the sea bottom to the topside (or even onshore) for periodic inspection. A reliable subsea inspection in situ would save millions by leaving the assets where they are and bringing the inspection technologies to them. If the sensing technology can be made resident as part of the asset, continuous monitoring is also possible. The question here whether it is more cost-effective to place one sensor for every asset or to have a single sensor attached to a vehicle that inspects all the assets, as this option would have to include the cost of a mission to take this sensor to subsea.
Lastly, robotic systems can be installed as a resident part of an asset to perform tasks that were once accomplished by ROVs. The challenge again is to come up with technologies that can beat the operational costs of using ROVs. Unlike an ROV, such a robotic system could operate in an autonomous way as the AUVs do, without relying on a specific training of how to steer robotic manipulators, enabling them to perform tasks automatically.