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Remotely operated vehicles (ROVs)
Remotely operated vehicles (ROVs) have facilitated the development of oil and gas resources in deeper water. By enabling access to areas that divers could not safely reach, they have extended capabilities for handling more complex situations and operations in deeper water.
In the 1980s, divers used saturated and pressurized systems to do almost all well and subsea equipment intervention, inspection, and repair. If the divers could not complete the repair task and/or inspection, the blowout preventer (BOP) stack or other items had to be pulled out of the water for repair. Even with the most sophisticated equipment, divers had limited capabilities, because of variables such as:
- Water depth
- Bottom downtime
- Sometimes, questionable safety standards
Subsea television systems were, and still are, used to inspect and monitor hulls and subsea equipment by use of running down guidelines, but they can only view (not do) repairs or other physical tasks.
Starting in the 1990s, coinciding with the increase in subsea completions, well intervention with highly capable remotely operated vehicles (ROVs) has developed into a common third-party addition to a floating mobile offshore drilling unit (MODU).
Modern ROVs have the ability to “fly” by means of an umbilical that is attached to the transport cage (or “garage”). Once the ROV leaves its cage, it may traverse for approximately 100 ft. The operator, or pilot, controls the flight pattern and position of the ROV, so it will not become entangled in its own umbilical or other items.
Most ROVs have visual and recording capabilities, in addition to manipulator arms with various degrees of strength, feedback, and lifting capability. ROV technology has far exceeded water depth ratings of MODUs; the capabilities and reliability of these units have improved considerably. Common tasks include:
- Changing of wellhead sealing ring gaskets
- Control of some functions on the BOP stack in an emergency
- Retrieval/installation of items on the wellhead or production hardware
- Inspection, as well as inspections with the subsea television system
With the increase in the use of subsea completions to develop whole fields, ROVs have become an integral part of deepwater development. With subsea development, MODUs do the drilling and most of the completion, including:
- Setting trees
- Flying leads
- Jumper hoses and pipelines
- Production risers
- Production skids
All of the above require ROV intervention. When wells need to be worked over, ROVs are necessary and are usually launched off MODUs or intervention vessels working in conjunction with a MODU.
As ROVs have become more important in floating MODU operations, the size and space requirements have increased dramatically. For intervention and completions, it is not uncommon to have:
- Two ROV systems, requiring the storage and operating porch to be used as a work platform
- Structural reinforcement for the deployment winch
- Fendering to prevent the ROV from hitting the MODU columns/lower hull
- Electrical power to support the unit (can be > 200 to 300 KVA)
- Variable deck load (VDL)
- Deck space
This can amount to a considerable support system that the MODU must accommodate, so planning ahead is important. Not every MODU can accommodate the larger ROV systems from a weight (some times over 40 tons), space (2,500 square ft. or more), or power standpoint.
Noteworthy papers in One Petro
Giuseppe Conte and Andrea Serrani 2001. Sensor Fusion Approach in Remotely Operated Vehicle Control, Offshore Technology Conference, 30 April-3 May 2001. 13252-MS. http://dx.doi.org/10.4043/13252-MS.
Simpson, J. 1984. Remotely Operated Vehicles as Underwater Inspection and Maintenance Tools: An Operator's Viewpoint, European Petroleum Conference , 22-25 October 1984. 12976-MS. http://dx.doi.org/10.2118/12976-MS