The hurricane hit three days ago. Huge, it both flooded and littered debris on roads, creating a state of emergency. The good news: half your wells are still in production. The bad news: your SCADA system is still down and you think three tank batteries might be full but your pumper and the tanker can’t get there. There is a berm, but it might be flooded and any spill is bad for business.
A long-range (3+ miles) wireless telemetry remote shutdown system with failsafe logic is ideal to monitor inputs and generate control commands autonomously to shut down wells when tank levels get above a set level.
Increasingly, oil and gas operators are implementing remote shutdown systems to prevent spills or other adverse events from impacting well pad operations. In a typical oilfield layout, a tank may collect oil from a series of wells located several hundred yards to several miles away from the tank battery. When a tank collecting oil from several wells becomes full, wells must be shut down (turned off or a valve closed) to prevent overflow. Wireless shut down systems are becoming more prevalent as they offer substantial advantages over wired systems.
Wireless Vs. Wired Remote Shutdown Systems
The simplest version of a remote wireless shut down system mimics wired systems. In a wired system, a set of high level switches, attached to tanks, are wired in series with pumps. If any tank level reaches the high point, the circuit opens and the pump turns off, simply and effectively.
Wireless systems can mimic this topology by monitoring a contact closure on the tank and tying it to a relay at the pump power source. Even a simple wireless system configuration can be very cost effective while reducing the risks of lighting-induced failures. Wired systems, on the other hand, can be very expensive or even impossible if wells are thousands of feet away. Trenching may also be required. The drawback to both systems, however, is that operators do not have visibility to operational status.
Adding Logic to Wireless Systems
For monitoring and control of critical systems, a long-range (3+ miles) wireless telemetry remote shutdown system with failsafe logic is ideal to monitor inputs and generate control commands autonomously to shut down wells when tank levels get above a set level and turn them back when the levels come down.
Placing remote shutdown logic within the context of a SCADA system provides substantial benefits if done properly. Control can now be expanded. For example, monitoring other critical parameters such as tubing and casing pressures can be added to shut down logic. Even more importantly, when a shutdown event happens, notification can be sent so possible problems can be addressed and production resumed. The old adage in the oil field that lost production can never be made up drives this requirement.
Remote shutdown as part of a SCADA system can also facilitate refinements such as the implementation of hysteresis conditions to create a separation between the value that causes a shutdown and the value that permits restarting. This prevents short cycling of controlled devices, which is particularly important with large electric motors.
Regardless of features, one key attribute of a remote shutdown system is that logic remains local to the devices being shut down. As more communications links are added in a system, it reduces its reliability. In most cases, companies implementing remote shutdown systems have logic run on the wellpad, either in an RTU, flow computer, or within the wireless network. This architecture is advantageous as it is not reliant on a backhaul network to a remote server. That type of network could have multiple links, which in the event of a disaster, might fail. It is important for the system to work when it should – but only when it should.
The health of a communications network also needs to be monitored to ensure its ongoing reliability. At a minimum, the system should provide the option to monitor each leg of the communications link. If any portion of the link goes down, the system should be able to put the controlled device, in this case a well, in a “safe” state.
Designing systems with these capabilities requires distributed intelligence. The intelligence has to be stationed in at least two places. The device monitoring the network, either the RTU or the wireless gateway, needs to regularly determine if some portion of the critical communications links failed and take an action that can be recognized by the device under control.
That is where the other part of the distributed intelligence comes into play. The local control (typically a relay) must be actuated upon indication of communications failure either positively by command or negatively by the absence of command. A wireless gateway with a key control point and monitor sensor, RTU, and actuator communications is one type of configuration that can provide failsafe operations.
In the accompanying diagram, a gateway in a stick is configured to update the remote shutdown system nodes based on the status of tank levels as received from tank sensors. For example, a radar-level sensor in the tank would send data wirelessly to the gateway in a stick, which would be configured to update the remote nodes appropriately. A basic logic command can include: “If tank level from sensor A is greater than X.X feet, set relay B on remote node C to be de-energized”. Following this logic, the gateway would send control commands via a wireless link to a Remote Shutdown/Modbus in a Stick that automates this action without the need for long, costly conduit runs.
Properly implemented, a wireless remote shutdown system can provide substantial benefits to an operator over a similarly wired system. In addition to integrating assets at much greater distances, a wireless remote shutdown system can be less susceptible to lightning, provide robust communications and operate more cost effectively. The key to implementing a wireless system in the oilfield is to choose a system that has been designed from the ground up to operate in this difficult environment and to provide the required robust and reliable communications.