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Human divers are limited in the depth they can work, whilst breathing a mixture of helium, oxygen and hydrogen extreme depths of 500m have been achieved (Seedhouse, 2011). However, hydrocarbon extraction is being performed at deeper and deeper sea depths, with oil wells being operated at sea water depths of down to 2,400m (Reuters, 2010). The need to operate at extreme depths is just one reason for the use and development of remotely operated underwater vehicles (ROVs).
What is an ROV?
An ROV is a tethered underwater robot that allows the operating crew to remain in a safe environment whilst the ROV performs tasks in the sea below. ROVs are operated in a control cabin, which is typically on board a vessel or offshore structure such as an oil rig. The main components of an ROV system are the (Remotely Operated Vehicle Comittee of the Marine Technology Society, 2015);
- Vehicle itself, to perform the necessary work.
- Control cabin, the command point for the ROV crew.
- Umbilical or tether, to carry electrical power, video and other data signals between the operator and vehicle.
- Handling system to control the cable dynamics.
- Launch system.
- Associated power supplies.
ROVs as a minimum will be fitted with a video camera and lights. Examples of additional tooling are magnetometers, sonars, still cameras, manipulators, cutting arms, water samplers, dredgers, cutting tools and cleaning tools (Oceaneering, 2016).
Types of ROV
ROVs are grouped by size and weight, which in turn are defined by the required function of the ROV:
Micro - very small in size and weight, typically less than 3kg. Micro ROVs are used as an alternative as a diver in places where a diver might not be able to physically enter such as a sewer, pipeline or small cavity (Remotely Operated Vehicle Comittee of the Marine Technology Society, 2015).
Mini - typically weigh around 15 kg. Similar to micro class, mini ROVs are used as a diver alternative. Both can be transported, set up and operated with one operator. Mini and micro are grouped as ‘eyeball’ ROVs, because they are unable to perform any intervention work (can only observe). Eyeball ROVs are powered solely via electric motors (Remotely Operated Vehicle Comittee of the Marine Technology Society, 2015).
General - typically less than 5 HP (propulsion), occasionally utilize small three finger manipulators and can carry a sonar unit for light survey applications. Typically the maximum working depth is less than 1,000 meters, these ROVs are normally used for survey and inspection work (Remotely Operated Vehicle Comittee of the Marine Technology Society, 2015)
Work-class - light work-class have less than 50 HP of propulsion power and a maximum depth of around 2000m, heavy work-class are defined as between 50 and 220 HP typically with a maximum depth of 3500m. These ROVs consist of an aluminum chassis with a large buoyancy pack on the upper side. A tooling skid is fitted to the bottom of the ROV to accommodate necessary sensors and tooling packages. The effect of this is a large separation between the Centre of gravity and buoyancy, giving stability to do work underwater. Work-class ROVs utilize hydraulic power units (HPUs) on board the vehicle, which are powered by an electric input to meet the needs of the high power tooling and propulsion. (University, 2016).
Figure 1. Worker recovering robotics Remote Operated Vehicle (ROV) after entering sea surface during oil and gas pipeline inspection in the middle of South China Sea.
Trenching/Burial - Usually between 200 and 500 HP, can carry a cable laying sled and work at depths up to 6000 m.
Untethered autonomous underwater vehicle (AUV) - A robot that travels without any input from an operator. Commercially they can be used to build detailed maps of the seafloor prior to installation of subsea assets. In scientific research they can utilize a variety of sensors to determine concentration of various elements and compounds, or presence of microscopic life. (Remotely Operated Vehicle Comittee of the Marine Technology Society, 2015). Military applications intelligence surveillance and reconnaissance, payload delivery, anti0submarine warfare and mine disposal (Department of the Navy, United States of America, 2004)
ROV vs Diver
Basic inspection work can be completed much quicker by flying an ROV over the structure of interest, such as a pipeline, much quicker than a diver can swim or walk over it. ROVs can be deployed at almost unlimited depth, with no need for decompression and can navigate using sonar/acoustic tracking.
On the other hand, although ROV technology is rapidly evolving, some subsea work requires the dexterity of a human hand and then binocular vision a human can offer.
ROVs and divers are often used in conjunction, whilst working in waters saturation divers are able to operate an eyeball or general class ROV will be used to monitor the divers and increase safety.
Furthermore, ROVs are invaluable for surveying the work-place on arrival known as an “as-found survey”, the diver is not wasting underwater time locating and identifying problems, searching for damage, or determining a course of action to resolve the issue. Deploying an ROV before a dive allows stakeholders to see exactly what needs to be done and where, ensuring that all of the appropriate equipment is assembled and prepared before diving in (Vallier, 2016).
Sources and Further Reading
- Oceaneering, 2016. Intervention Tooling. [Online] Available at: http://www.oceaneering.com/tooling/ [Accessed 7 July 2016].
- Department of the Navy, United States of America, 2004. The Navy Unmanned Undersea Vehicle (UUV) Master Plan, Unite, Washington DC: Department of the Navy, United States of America.
- Reuters, 2010. UPDATE 1-Shell starts production at Perdido. [Online]
- Available at: http://www.reuters.com/article/shell-perdido-idUSN3123683920100331
- Seedhouse, E., 2011. Ocean Outpost - The Future of Humans Living Underwater. 1st ed. New York City: Springer.
- University, N. S. Y.-s., 2016. The Basic Components of an ROV, Taiwan: National Sun Yat-sen University.
- Vallier, K., 2016. Diver or ROV? It’s Not One or the Other. [Online] Available at: https://www.deeptrekker.com/diver-or-rov/. [Accessed 8 July 2016].