Subsea Well Control

A Subsea well  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 Tree(s) including the Downhole Safety Valve, which is installed below the surface in the Well. The main difference is that the Christmas Tree will stand on the seabed, below the surface and out of reach. And the hydraulic connections will be via an umbilical, which could be a bundle of stick tubing or hoses. In case of hoses we also have to take care of the ballooning, which will mean that the reservoir and accumulator capacity will have to increase and pumps should be capable to have a much larger flow rate.

Controlled valves could be as follows:

• SSSV = Subsurface Safety Valve / DHSV = Downhole Safety Valve
• SSV = Surface Safety Valve
• UMV = Upper Master Valve
• WV = Wing Valve

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.

WHCPs may have multiple applications, which include, but are not limited to

• Safe and Sequential Operation of Wellhead Valves (SCSSV/SSV/Wing Etc.)
• Emergency (ESD station) and Fire Shutdown (Fusible Loop)
• Safe Operation of Riser Valves
• Flow line Pressure control (High/Low Pilot)
• Well Test Operation (Wireline Bypass)
• Hydraulic supply to SDV (HIPPS) /ESD/ Choke Valve
• Partial Stroking of field installed actuators
• Depending upon the pressure requirement for valves, HP, MP, and LP headers are envisaged.

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.

Sizing of the hydraulic lines between the WHCP and the Safety Valves, swept volume of each actuator and ballooning of the umbilical are the most important for overall WHCP design. Opening times are usually less important. And if quick closure is required a solution would be to used biodegradable oil that can be dumped.