Autonomous systems hold promise for reduced cost and more environmentally friendly subsea control systems. A Long Range Remotely Operated Vehicle is described which provides a simple control system option based on extensions of field proven technology
In order to reach full potential autonomous subsea well control systems must be developed for producing wells. Nevertheless consideration of a water injection well forms a useful starting point and the concepts described in this paper could form the basis of an autonomous control system for multiple well subsea developments. The incentive for autonomous operations are cost savings and environmental advantages stemming from elimination of the control umbilical. The limited telemetry utilized in the system described will be accomplished with acoustic signals transmitted along the flowline.
Although it may seem premature to be considering improvements in autonomous systems, when such systems have yet to reach the point of serious commercial application there is no better time to explore a wide range of possibilities than in the formative days of new technology. The concept described leads to major simplification and even lower cost autonomous systems by eliminating local power generation and many control system components. Although the concept may, at first glance, appear to be somewhat futuristic it is simple straightforward and based on technology well proven in underwater defence operations
A conventional subsea tree which could be equipped with standard valve actuators, is fitted with a local monitoring system For a water injection well this would include wellhead pressures and simple electronics to monitor a "minder" signal from the surface facility. Output of pressure transducers, and the input from the "minderv signal, are routed to a microprocessor coupled to a single battery powered, pilot for a single hydraulic vent valve.
As long as flowline and annulus pressures remain within limits and the "minder" signal is received the tree valves are held open. When these conditions are not met the pilot operated vent is released and all valves fail closed There, is no routine telemetry to the surface but a single signal is returned to confirm that the tree is shut in The microprocessor could also be coupled with a small logging device to periodically store readings of wellhead pressures
When the surface facility records a signal that the well has been shut in an action initiated from the surface facility in most instances, and a decision has been taken to re-open the well, a Long Range Remotely Operated Vehicle (LRROV) is prepared and launched the LRROV swims to the designated tree.
When the LRROV approaches the tree, the vehicle identifies the docking point and mechanically couples itself to establish electrical and hydraulic communication This is a totally remote, self controlled operation although the status of maneuvers will be monitored remotely. Once the LRROV is docked the initial activity is to interrogate the data logging device to confirm the reason for shut-in, and current pressure status, to ensure that it is safe to open the tree valves.