The function of the wellhead control system is to monitor each well’s flowline pressure, interface with the facility Emergency Shutdown System (ESD), and control the surface safety valve (SSV).
Like other shutdown systems described in Section 1300, a wellhead control system should be used to prevent the risk of injury or damage to personnel, the environment, or equipment. Wellhead control systems are designed to be “fail-safe.”
When required, the wellhead control system also monitors the surface controlled subsurface safety valve (SCSSV) for the well. SCSSVs are required for offshore platforms. Regulations, such as API RP 14 C and the Minerals Management Service OCS Order No. 5, govern the requirements for these safety systems.
SCSSVs have been installed on land wells in some facilities that are located near population centers or where the wellhead may be subject to physical damage. When hydraulically operated SCSSVs are required, the wellhead shutdown system must include a hydraulic reservoir and pump system to maintain pressure on the subsurface valves during normal operation.
Almost all wellhead control systems are pneumatic for sensing and control of the surface safety valve (SSV). Pneumatics have been widely used for a long time and are accepted by the users. Pneumatics work well in the vicinity of wellheads, where they are subject to vibrations and fluids from drilling activities.
On land, a separate wellhead control system is usually provided for wells operating under pressure flow conditions, when damage or injury to the environment, personnel, or equipment could occur.
In temperate climates the controls may be mounted individually out-of-doors. In colder climates the controls are often mounted in a small panel, which may be housed in a building or shelter.
On offshore platforms, the wellhead control systems are grouped on one or more panels. The control logic for each well is kept separate from the other wells so that wells may be easily added to or deleted as required. SCSSVs are hydraulically controlled and operated from these panels. A separate hydraulic system is usually furnished for every panel. Shutdown controls for each pneumatically operated SSV and its SCSSV are arranged together on the same control panel.
When instrument air is available, as it is on many offshore platforms, it is the best source of gas pressure to operate these safety control systems. When instrument air is not available, process gas can be used. The source of gas pressure for the control system must be dry and filtered and free of contaminants.
Nitrogen is often used on land as the source of gas pressure in the following circumstances:
• When only sour gas is available
• When hydrocarbons cannot be vented to the atmosphere
• When a source of clean, dry gas is unavailable
In extreme cold conditions, hydraulics have been used for the surface control systems.