Table of Contents
The wellhead is the top part of the well leading down to the reservoir. Oil from the reservoir comes out through the Well hole with casing. The oil or gas flow from the well should be sufficient to make it commercially feasible. Then only the wellhead is installed at the top of the well. It consists of valves and equipment that control the wellhead pressure and flow. That equipment and its control are the most crucial in oil fields. They avoid hazardous conditions in oil and gas plants. The hazard is caused by the flow of flammable materials out of the well and the high pressure inside the well. Thus, the flow through the wellhead must be controlled and made fail-safe. That’s why the Well Head Control Panels (WHCP) are employed.
The wellhead is also known as the Christmas tree (because of its structure). The Christmas tree consists of the Subsurface Controlled Safety Valves (SCSSV), Surface Safety Valves (SSV), and other wellhead safety valves (Choke, ESD, HIPPS). These valves are used to close the well when needed. At the top of the tree structure, a pressure gauge will indicate the pressure in the tubing.
The processes that should happen in the wellhead are manipulated through these three valves with the help of gauges and choke.
2 What is a Well Head Control Panel (WHCP)?
The purpose of a WHCP is to monitor the Subsurface Controlled Safety Valves (SCSSV), Surface Safety Valves (SSV), and other wellhead safety valves (Choke, ESD, HIPPS) for the safety of the well. A WHCP should prevent the risk of injury or damage to personnel, the environment, or equipment. Wellhead control systems are designed to be “fail-safe.” The wellhead will be programmed and controlled by PLC or SCADA systems.
The WHCP receives input signals from various gauges, including pressure, temperature, and flow gauges in the wellhead. In addition, the main inputs are from emergency shutdown systems (ESD), emergency pushbuttons, and fusible plugs. The output is generated by reading these signals, which is usually a command to shut down the valves on the wellhead to ensure the plant’s safety.
The WHCP employs both hydraulic and pneumatic components. SCSSVs are mostly installed on land wells that are operated by hydraulic power. In that case, the WHCP must include a hydraulic reservoir and pump system to maintain pressure on the subsurface valves during normal operation. On the other hand, WHCP uses pneumatics for sensing and controlling surface safety valves (SSV).
Hence, WHCP is a collection of diverse control systems that enable the controlled execution of plans and emergency shutdowns. On land wells, a separate wellhead control system is used for each well working under high pressure. The wellhead control systems for offshore platforms are grouped onto one or more panels. Each well’s control logic is kept distinct from the other wells to add or delete new wells as needed.
3 Principle of WHCP
There are different types of WHCP categorized based on the source of operation. These are electric control panels, manual, solar-powered, and pneumatic controlled panels. The selection of type will be chosen according to the application requirements. Also, the control panel works in two different configurations; a single wellhead and a multi-wellhead control panel. We will talk about these types in the coming sections.
WHCP controls the oil and gas wells. WHCP accomplishes this task by controlling valves installed at the wellhead, which are SCSSV (Surface Controlled Sub-surface Safety Valves) and SSV (Surface Safety Valves), which consist of Master Valves (MV) and Wing Valves (WV). The sequential operation of SCSSV and SSV facilitates the Emergency Shutdown (ESD) process.
Other than the basic components of the control panel, WHCP consists of a hydraulic reservoir, strainer, hydraulic pumps, accumulator, wellhead control module, and hydraulic lines, which supply and return to the wellhead control module. The hydraulic fluid inside the hydraulic reservoir operates the wellhead. The reservoir’s size is determined by the amount of fluid necessary for valve functioning and the distance between the reservoir and the well.
The SCSSV and SSV operate at hydraulic pressure, SCSSV at high pressure, and SSV at medium pressure. A hydraulic power pack or a hydraulic power unit is employed to meet the pressure requirements, which consists of a reservoir, hydraulic pumps, and an accumulator. The reservoir stores hydraulic fluid, and there will be two hydraulic heads and pumps in each wellhead. Then each pump will have a strainer to filter the hydraulic fluid from any particulate so that the hydraulic fluid going to SCSSV or SSV is clean. Depending on the operational environment, hydraulics can be pneumatic driven or electric driven. An accumulator will be employed downstream of the pump to achieve the desired pressure for SCSSV or SSV.
The accumulated hydraulic fluid at the header is supplied through hydraulic supply lines to the wellhead control module when it is required. The sequence and logic for operating the SCSSV and SSV are established at this wellhead control module. To control the flow or execute the ESD procedure, signals from the panel go to this wellhead control module. After receiving the control signal, the wellhead control module will open the SCSSV and SSV by sending high pressure and medium pressure, respectively. The hydraulic fluid will return to the hydraulic line by activating a three-way valve to close the valves.
This is how the wellhead control panel functions to control the wellhead flow and emergency shutdown procedure.
4 Main functions of WHCP
- Shut down the well in the event of hazardous situations
- Controls the critical security parameters
- Sequential start-up and down procedure of the wellhead
- Closely monitor and control SCSSV.
- WHCP employs separate hydraulic power units (HPUs) that facilitate hydraulic pumps, accumulators, reservoirs, etc.
- Special pushbuttons are used in the panel to stop the wing valve, master valve, and SCSSV.
5 Components of WHCP
WHCP is installed near the wellhead, but the main controlling part, PLC or SCADA, will be installed in a safe zone. All logic signals sent to the WHCP are from the control system (PLC or SCADA). The WHCP near the wellhead will do the controlling action directly. The main part of a WHCP is the Hydraulic Power Unit (HPU) that consists of a hydraulic pump, reservoir, and accumulator. The components of the WHCP are described in the below sections:
5.1 Hydraulic pump
Hydraulic pumps distribute hydraulic fluid at the necessary pressure to headers. The necessary pressure determines the sizing and capacity of the pump to the wellhead, the distance from the panel to the wellhead, and the delivery time. The process control system, which is either a PLC or a SCADA, controls the motor.
5.2 Oil reservoir
Oil tanks provide the required oil supply for the hydraulic pump. Strainers are utilized at the outlet of the tank to ensure clean oil is passed to the pump. Also, a return line from the valves is connected to the top of the tank.
An accumulator is an energy-saving device used to satisfy the high-pressure demand for SCSSV (Surface Controlled Subsea Safety Valves) or SSV (Surface Safety Valves). The accumulator is installed downstream of the hydraulic pump. It should be carefully sized to meet the pressure demands.
The accumulator has two sections: the gas section and the hydraulic section. The gas section is usually charged with dry nitrogen gas, which is filled first. Then the hydraulic oil is filled into the accumulator, compressing the gas section. When there is a demand for hydraulic, the accumulator releases the oil, allowing the gas section to expand. This mechanism will allow the fast discharge of hydraulic oil.
For each supply header, downstream oil headers are used, and regulators give them regulated pressure. High-pressure ranges are handled by regulators, which are accompanied by relief valves.
5.5 Hydraulic circuit
The hydraulic circuit consists of the control valves (SCSSV and SSV), hydraulic tubing, and hydraulic supply headers. The hydraulic tubing equipment is selected according to pressure needs, i.e., high, medium, and low-pressure requirements. The circuits contain a check valve, an isolation valve, tubes, tube fittings, etc.
A fusible plug is employed in the hydraulic line for fire safety. In the case of a fire, the fuse plug will melt to cease the hydraulic pressure, closing the wellhead valves. The well will automatically shut down.
5.6 Instrumentation interface
Measuring instruments are essential when it comes to controlling and safeguarding the wellhead’s activities. On the hydraulic headers, pressure transmitters are installed to monitor valve operations, and the live status is relayed to the PLC controllers. Additionally, level gauges are used to check the reservoir level continually. The WHCP is linked to the PLC/SCADA system for central monitoring and control of the wellhead. For ease of maintenance, troubleshooting, and redesigning, a separate junction box should be used.
6 Types of WHCP
6.1 Single wellhead control panel
A single wellhead control panel can control only one wellhead. It manages all wellhead equipment and allows for both automated and manual shutdowns. This WHCP is utilized for monotomous or remote wells that are far from other wells. The single wellhead control panel is further divided into the following categories:
- Manual control system
- Electrical control system
- Pneumatic control system
- Solar-powered control system
6.2 Manual control system
Employs manual hydraulic pump to supply pressure to SSV. ITS MAIN FUNCTIONS ARE remote ESD, Fusible plugs, low pressure / high-pressure detection, and manual shutdown at the panel.
6.3 Electrical control system
An electrical motor controls hydraulic flow to SSV. Electrical control systems have been developed and are reliable in harsh climates or when dealing with hazardous substances in the flowline. Electric shutdown systems are more flexible, easier to deploy, and less expensive than pneumatic shutdown systems, where signalling the shutdown sensor is essential. Electric systems are considerably easier to connect with a Supervisory Control And Data Acquisition (SCADA) system for remote monitoring and control.
6.4 Pneumatic control system
It does all the main functions that other systems do. But the controlling of the hydraulic pump is through pneumatic energy. This type uses for high-power operations.
6.5 Solar-powered control system
This type of control system is most suitable for wells at remote locations. It offers energy conservation, reduces operation expense, dual power supply, the standard control feature, etc.
7 Designing a WHCP
Proper sizing of the components that are the accumulator, tube, and reservoir must be done for the controlled and sequential operation of the wellhead. Designers can use the help of different software for the precise sizing of components. This software will do the sizing for the given requirements.
7.2 Flowline safety
For safety, each end of the flowlines should use a choke to reduce inline pressure. The factors that must be considered for flowline safety:
- Check whether the first choke device in the initial flowline segment is less than 10 feet from the wellhead. When the distance between the choke and the first segment of the flow line upstream of the choke is less than 10 feet, pressure sensors in the first flow line upstream of the bottleneck are not needed. When the distance is more than 10 feet, a low-pressure sensor is all that is needed to detect leaks and ruptures.
- Both high and low-pressure sensors are required to detect a blocked line or flow control failure and a leak or rupture when the shut-in tubing pressure (SITP) is greater than the maximum permissible working pressure (MAWP) of the last section of the flowline after the choke.
- When the MAWP of the end flowline section is less than the SITP, a pressure relief valve and high and low-pressure sensors are required.
While sensors are being tested, calibrated, or replaced, a three-way valve on the panel can be fitted to bypass the high and low-pressure pilots, as well as the sand probes. The usage of panel-mounted indicators should indicate from a distance that the bypass valve has been switched for safety reasons.
A three-way valve on the panel can be installed to bypass the high and low-pressure pilots, as well as the sand probes, while sensors are being checked, calibrated, or replaced. The use of panel-mounted indicators should clearly show that the bypass valve has been turned off for safety reasons from a distance.
Needle valves should be fitted in the supply gas and hydraulic oil tube lines to enable component replacement without shutting down a single wellhead or all of them.
Although the WHCP may be powered by pneumatic, hydraulic, electric, or solar energy, the system is hydraulic due to its hydraulic output. In oil and gas facilities, pumping oil or gas from the reservoir is a major task. A WHCP should regulate the wellhead more precisely and efficiently, allowing for continuous oil and gas production.