A PLC controlled Wellhead Control Panel can be either a single, multi or subsea-well operated control panel, but in this case controls will be arranged by a PLC (Programmable Logic Controller) and the hydraulic will be medium/high pressure and no control pressure. With no hydraulic/pneumatic controls the PLC must be powered at all time and must therefore get power from an Uninterruptible Power Supply (UPS).
The type of PLC can standard industrial or for harsh environment, but also certified to a certain Safety Integrity Level (SIL) in order to increase the reliability/availability of the system.
The PLC controlled Wellhead Control Panel is a cabinet that is a free-standing type and it is used to operate all safety valves that are attached to one or more Christmas Trees including the Downhole Safety Valve, which is installed below the surface in the Well.
The built up of this panel can be split in several section, namely the hydraulic power unit and the control section.
The hydraulic power unit consist out of a hydraulic reservoir, a pump that can be either air/gas or electric driven. Accumulators to store the generated hydraulic energy for later use and to maintain pressure.
The HPU will have various pressure levels required to operate the system, the various levels can be achieved by use of pressure regulators or various pumps with different outputs.
The control section is usually operated at a low pressure, which can be pneumatic or hydraulic. It consist out of various possible sequences in operation and various shutdown levels, but in the end will be open/close the safety valves in the field in the correct way.
The control section can be a drawer type of system, which means that the controls for every well can be isolated separately and then removed from the WHCP for testing, overhaul or replacement.
Nowadays most WHCP have only high pressure solenoid valves for opening/closing the Safety Valves and having these in a drawer is not preferred, due to a lot of cables that should be disconnected at time of replacement.
WHCPs may have multiple applications, which include, but are not limited to
The WHCP internal hydraulic circuit has tubing (small bore pipe), tube fittings (twin ferrule or threaded), check valves, isolation valves, quick exhaust valves, etc. as per requirement.
A fusible loop with fusible plugs are used to hold hydraulic pressure and installed around the wellhead. In case of heat (fire), the fusible plug melts and oil pressure will be released and will cause a shutdown in the panel which will ensure that the Safety Valves will be closed.
Sizing of the hydraulic lines between the WHCP and the Safety Valves, swept volume of each actuator and required closing times are important for overall WHCP design.