Precision Pressure Control Technical Services

Rotary blowout preventer

1. Purpose and Scope of Application
The rotary blowout preventer is installed atop the annular or ram-type blowout preventer and, during pressurized drilling operations such as underbalanced drilling, serves to seal the rotating drill string while also providing flow diversion. When integrated with hydraulic blowout preventers, drill‑string check valves, gas‑liquid separators, and snubbing pressure‑boosting equipment, it enables safe pressurized drilling and snubbing operations. It plays a critical role in specialized applications, including releasing low‑pressure oil and gas reservoirs, effectively preventing well kicks and losses, conducting gas‑drilling, performing snubbing‑type drilling and workover operations, and more.

2. Main Operating Principle
The drill string rotates through the rotary blowout preventer’s upper bearing assembly, causing the rubber sealing element to rotate in tandem with the drill pipe. The sealing element relies on its own elastic deformation and well pressure‑assisted sealing to create a seal around the drill string. The dynamic seal between the central tube and the bearing assembly is achieved by the upper and lower dynamic sealing assemblies.
The hydraulically controlled power unit is used to control the opening and closing of the clamp, while also providing lubrication and cooling for the bearing assembly to ensure its proper operation.

3. Technical Parameters

Casing diameters: 346 mm (13-5/8 in), 480 mm (18-3/4 in), 540 mm (21-1/4 in)
Bottom flange specifications: 13-5/8 in - 35 MPa, 18-3/4 in - 70 MPa, 21-1/4 in - 35 MPa, 21-1/4 in - 14 MPa
Side outlet connection: 7-1/16 in – 35 MPa, Type 6B steel ring; side inlet connection: 2-1/16 in – 35 MPa, Type 6B steel ring (users may choose a tubing nipple or a union).
Bearing assembly bore diameters: 178 mm, 192 mm, 197 mm, 203 mm, 220 mm
Operating pressure: maximum dynamic pressure 17.5 MPa, maximum static pressure 35 MPa
Maximum rotational speed: 130 r/min
Sealable drilling tools: 5-1/2 in drill pipe, 5 in drill pipe, 3-1/2 in drill pipe, 2-7/8 in drill pipe, etc.
Working medium: gases, foams, and various drilling fluids
Overall dimensions of the standard blowout preventer: casing height 1000 mm; overall height of the bearing assembly installed in the casing 1655 mm.
Environmental protection blowout preventer external dimensions: housing height 1,100 mm; overall height of the bearing assembly installed in the housing 1,755 mm.

4. Structural Composition

The rotary blowout preventer primarily consists of a housing, a bearing assembly, a hydraulically controlled power unit, control lines, a hydraulically operated ram valve, and auxiliary tools.

Applications of Managed Pressure Drilling

Core Objective: The essence of managed pressure drilling lies in precisely and in real time controlling the annular pressure profile within the wellbore—particularly the bottomhole pressure—keeping it as stable as possible within a predefined target window, typically the narrow density window between formation pore pressure and fracture pressure. This is akin to walking a tightrope at great heights, demanding exceptionally fine‑tuned control.

High-precision real-time data acquisition

Data source: The system employs a high-precision sensor network installed on surface equipment—sometimes integrated with downhole tools—to collect critical data in real time:

• Flow rate: inlet mud pump stroke rate/displacement, outlet mud flow rate (the most critical parameter).
• Pressure: Risers pressure, casing pressure, and choke manifold pressure (upstream/downstream of the choke valve).
• Liquid level: The liquid level in the reflux drum (separator) indirectly reflects the balance between fluid inflow and outflow.
• Density: Inlet mud density.
• Gas Content: The gas content in the fluid returned at the outlet (if applicable).
• Downhole pressure: (if a measurement-while-drilling tool/PWD/LWD is installed).

Function: Using this data, the system can “determine” the actual pressure conditions within the wellbore—particularly the bottomhole pressure, which is calculated via a hydraulic model and calibrated against measured data—and assess fluid flow behavior, including whether anomalies such as losses or kicks are occurring.

Intelligent control algorithm

Setpoint: The engineer inputs the target bottomhole pressure or the target equivalent circulating density in advance.
Models and Computation: The system incorporates or interfaces with advanced hydraulic models. These models leverage real-time sensor data—such as pump displacement and mud‑property measurements—along with wellbore geometry to compute the current pressure distribution within the wellbore, with particular emphasis on determining bottomhole pressure.
Algorithm Core – Closed-Loop Control:

• The core control variable is the throttle valve opening. The throttle valve functions as a precision “pressure‑regulating valve”; its degree of opening directly affects the wellhead backpressure, which in turn influences the pressure throughout the entire wellbore.
• The system performs a real-time comparison between the calculated bottomhole pressure/key-point pressure (or equivalent density) and the target value (setpoint).
• Based on pre-defined advanced control algorithms (such as PID control—proportional–integral–derivative control—or more sophisticated methods like model predictive control and robust control, among others)
• PID: The required change in throttle valve opening is calculated based on the deviation magnitude (P term), the accumulated deviation (I term), and the rate of change of the deviation (D term). The proportional term corrects the current deviation, the integral term eliminates accumulated historical deviations, and the derivative term predicts future deviation trends to provide damping.
• More advanced algorithms: They can better handle complex system characteristics such as time delays and strong coupling, thereby improving response speed and stability.
• Interference Compensation: The system can detect and mitigate major sources of interference in real time.
• Displacement variations: Drilling‑string movements (such as making up or pulling a drill pipe joint) and the start‑up or shutdown of the mud pump can cause instantaneous changes in displacement, representing the primary source of disturbance. The system either anticipates these changes—via pump‑stroke counting—or detects them, then adjusts the choke valve opening in advance or promptly to counteract the pressure impact of the displacement variation, thereby maintaining stable bottomhole pressure. This is the essential automatic continuous circulation compensation.
• Mud density/performance changes: If the injected mud’s properties change, the system will adjust the control strategy based on model calculations and updated parameters.
• Downhole fluctuations: If minor overflow or loss is detected, the system can automatically adjust the setpoint or response strategy.

Precise throttle control

• The control algorithm computes the required throttle‑valve opening adjustment command (to open wider or close tighter).
• This command is transmitted to the actuator—typically an electric or hydraulic drive—via a high-speed electro-hydraulic control system.
• The actuator adjusts the throttle valve opening with extremely high precision, with accuracy potentially reaching a few percent of the full opening.
• Changes in the throttle valve opening immediately adjust the wellhead backpressure, applying or relieving pressure on the annular fluid column, thereby directly affecting the equivalent circulating density (ECD) of the entire annular fluid column and the final bottomhole pressure (BHP).

Summary of the Closed-Loop Process

Real-time data acquisition → Model calculation of current bottomhole pressure/key-point pressures → Comparison of setpoints with actual values (calculation of deviation) → Generation of adjustment commands by the control algorithm → Precise actuation of the choke valve opening by the actuating mechanism → Modification of wellhead backpressure → Impact on bottomhole pressure → Acquisition of new data and feedback to the system… thereby establishing an uninterrupted, millisecond‑level high‑speed closed-loop control.

Key Features and Benefits

• Millisecond-level response: The system delivers exceptionally fast computational and execution speeds, enabling it to handle instantaneous changes in flow rate.
• High-precision control: Bottomhole pressure fluctuations can be maintained within an extremely narrow range (e.g., ±0.1 sg equivalent density or within ±50 psi).
• Dynamic pressure balance: Maintains dynamic (rather than static) stability of bottomhole pressure, continuously protecting the wellbore during drilling operations, including make‑up and other procedures.
• Automatic Diagnosis and Early Warning: The system can automatically detect even the slightest signs of overflow or leakage and trigger an alarm.
• Significantly enhanced safety: Effectively prevents complex incidents such as well kicks, blowouts, lost circulation, and stuck pipe.
• Enhance drilling efficiency: Safely and rapidly drill through previously difficult-to-reach “narrow-window” formations, thereby reducing non-productive time.

Structural Features of the Throttle Valve

• Plunger Design: A plunger is employed as the primary control element; by means of its vertical reciprocation, the cross-sectional area through which fluid flows is adjusted, enabling precise regulation of flow rate and pressure.
• Electric actuator: Equipped with an electric actuator and a servo motor, it drives the plunger’s motion, ensuring high-precision control.
• Sealing Structure: High-performance sealing materials are employed to ensure reliable sealing performance under high-pressure and high-temperature conditions.
• Valve body material: Typically made of high-strength low-alloy steel or stainless steel to withstand high pressure and corrosive media.
• Feedback System: Equipped with a built-in position sensor and pressure sensors installed upstream and downstream of the throttle valve, it continuously monitors plunger position and fluid pressure, generating feedback signals to enable closed-loop control.

Structural Advantages of the Throttle Valve

• High-precision control: The electric actuator, combined with a feedback system, enables precise regulation of flow and pressure, meeting the stringent requirements of drilling operations.
• Fast Response: Electric actuators offer rapid response, enabling swift adjustment of the plunger position to accommodate rapidly changing operating conditions during drilling.
• High degree of automation: It can be integrated with automated control systems to enable remote control and automated operation, reducing manual intervention and enhancing operational efficiency.
• High reliability: Utilizing high-performance materials and a sealed design to ensure long-term, dependable operation even in harsh environments.
• Easy maintenance: The structure is simple, making component maintenance and replacement convenient and reducing downtime.