API Specification 6A, titled "Specification for Wellhead and Christmas Tree Equipment," is the primary industry standard published by the American Petroleum Institute (API) for the design, manufacturing, testing, and performance of wellhead and Christmas tree equipment for oil and gas drilling and production services.

• Scope: It defines requirements for materials, dimensions, testing, quality control, and marking for a wide range of equipment.
• Purpose: Its main goal is to ensure the safety, reliability, and interchangeability of equipment from different manufacturers.
• Application: It applies to equipment used for pressure containment and control in surface and subsea drilling and production operations.

A wellhead system is a complex assembly of spools, valves, and fittings installed at the surface of a well. Its primary components are built up in stages as the well is drilled.

1. Casing Head (or Starter Head): This is the lowest part of the wellhead. It attaches to the surface casing string and supports the next casing string. It provides a means of sealing the annulus between casing strings.
2. Casing Spool(s): Installed on top of the casing head, these spools support and seal subsequent, smaller-diameter casing strings. A well may have multiple casing spools.
3. Tubing Head (or Tubing Spool): This is the final spool installed on the wellhead. It supports the production tubing string, seals the annulus between the tubing and the final casing string, and provides access to that annulus.
4. Christmas Tree: Installed on top of the tubing head, this is an assembly of valves, chokes, and fittings used to control the flow of fluids from the well during production.

While often used together, they serve distinct functions:

• Wellhead:
  • Function: The primary function is to support the casing and tubing strings, provide a pressure seal between them, and allow access to the annuli.
  • Installation: It is installed *during* the drilling and completion phases of the well.
  • Components: Casing heads, casing spools, and the tubing head.
• Christmas Tree:
  • Function: Its primary function is to control the flow of well fluids *after* the well is completed and is ready for production.
  • Installation: It is installed on top of the wellhead *after* the tubing is run.
  • Components: Master valves, wing valves, swab valve, and choke.

In short, the wellhead is the foundation, and the Christmas tree is the control system for production.

PSLs define different levels of technical and quality requirements for equipment. They are chosen based on the service conditions and criticality of the well. There are four main PSLs:

• PSL 1: The baseline and lowest level. It covers basic quality and testing requirements suitable for general, low-pressure service.
• PSL 2: Has more stringent requirements than PSL 1. It includes additional quality control measures, such as traceable material properties and hydrostatic testing of each unit.
• PSL 3: Designed for high-pressure, critical service. Requirements are significantly higher than PSL 2, including:
  • More extensive Nondestructive Examination (NDE).
  • Mandatory hydrostatic testing of each unit with longer hold times.
  • More detailed material qualification and traceability.
• PSL 3G: Has all the requirements of PSL 3 plus an additional gas testing requirement. This is intended for equipment in high-pressure gas service where gas leaks are a major concern.

Material classes define the minimum material requirements based on the type of fluid being produced. They are based on resistance to Sulfide Stress Cracking (SSC), a form of hydrogen embrittlement caused by H₂S.

Material Classes based on NACE MR0175 / ISO 15156:

Temperature classes specify the minimum and maximum operating temperature range for which the equipment is designed and qualified.

API Temperature Ratings:

• K: -75°F to 250°F (-60°C to 121°C)
• L: -50°F to 250°F (-46°C to 121°C)
• P: -20°F to 250°F (-29°C to 121°C)
• S: 0°F to 250°F (-18°C to 121°C)
• T: 0°F to 180°F (-18°C to 82°C)
• U: 35°F to 250°F (2°C to 121°C)
• V: 35°F to 350°F (2°C to 177°C)

The standard operating range is often extended with high-temperature ratings, such as X (450°F), Y (650°F), etc.

PR levels define the validation testing requirements for a specific design of equipment. It proves the design's capability to perform under specified conditions. There are two main levels:

• PR1: The baseline performance level. A single prototype is subjected to a series of tests, including pressure and temperature cycling, to validate the design.
• PR2: A more stringent level requiring a higher number of pressure and temperature cycles, as well as testing of two prototype units. This provides a higher degree of confidence in the design's long-term performance and reliability.

PR levels are often specified along with PSL, for example, PSL 3, PR2.

API 6A specifies a set of standard nominal working pressures in pounds per square inch (psi):

• 2,000 psi (2K)
• 3,000 psi (3K)
• 5,000 psi (5K)
• 10,000 psi (10K)
• 15,000 psi (15K)
• 20,000 psi (20K)

1. Master Valves (Upper and Lower):
   • Lower Master Valve (LMV): Typically a manually operated gate valve, left fully open during normal production and only closed for long-term shut-ins or workovers.
   • Upper Master Valve (UMV): Can be manual or hydraulically actuated. It is the primary shut-in valve for routine and emergency shutdowns.
2. Wing Valves (Production and Kill):
   • Production Wing Valve: Located on the side of the tree, it controls the flow of hydrocarbons to the production flowline.
   • Kill Wing Valve: Located on the opposite side, it is used for injecting fluids (like corrosion inhibitors or methanol) into the well or for killing the well by pumping heavy fluids down the tubing.
3. Swab Valve (or Crown Valve): Located at the very top of the tree, it provides vertical access to the wellbore for wireline, coiled tubing, or workover operations. It is closed during normal production.

A flange is a method of connecting equipment (valves, spools, etc.) that allows for assembly and disassembly. API 6A specifies standard flange designs for pressure containment.

Key Features:

• Type: The most common type is the Type 6B flange for lower pressures (2K, 3K, 5K) and the Type 6BX flange for higher pressures (5K, 10K, 15K, 20K).
• Ring Groove: Flanges have a groove machined into their face to accommodate a metal ring gasket.
• Studs and Nuts: The connection is secured by a set of high-strength studs and nuts that clamp the two flanges together.
• Face-to-Face Makeup: Type 6BX flanges are designed for face-to-face contact between the flanges, which pre-loads the gasket and creates a very reliable seal.

Both are metal ring gaskets used to seal flange connections, but they have key differences:

• R-Type Gasket:
  • Profile: Oval or octagonal in cross-section. The octagonal is more common and provides a more efficient seal.
  • Application: Used in lower pressure API 6B flanges (up to 5,000 psi).
  • Sealing: Creates a seal by being compressed into the flange groove, causing it to deform and fill the imperfections. It is a pressure-energized gasket.
• BX-Type Gasket:
  • Profile: Square cross-section with tapered sealing faces.
  • Application: Used in higher pressure API 6BX flanges (5,000 psi and above).
  • Sealing: Creates a very high-integrity, face-to-face seal between the flanges. The initial compression creates a seal, which is then further energized by internal pressure. A BX gasket cannot be used in a 6B flange groove.

Hydrostatic testing (or hydro-testing) is a mandatory quality control test where a piece of equipment is filled with a liquid (usually water), bled of air, and pressurized to a specific level for a set duration to verify its integrity.

Purpose:

1. Strength Verification: It confirms that the equipment can withstand pressures well above its rated working pressure without yielding or failing. The standard test pressure is typically 1.5 to 2 times the rated working pressure.
2. Leak Detection: It proves that all seals, welds, and pressure-containing components are free from leaks under pressure.
3. Quality Assurance: It is a fundamental requirement under PSL 2, 3, and 3G to ensure every piece of equipment meets the standard.

A tubing hanger is a critical component that is landed inside the tubing head. It has several key functions:

• Support Production Tubing: Its primary function is to suspend the entire weight of the production tubing string.
• Provide Annulus Seal: It seals the annulus between the production tubing and the final casing string, isolating the producing formation from the surface.
• Downhole Control Line Ports: Modern tubing hangers often have ports or "penetrations" to allow for the passage of hydraulic or electrical lines used to control downhole equipment like a Surface Controlled Subsurface Safety Valve (SCSSV).

A gate valve is the most common type of valve used in wellhead and tree applications. It provides a positive, bidirectional shutoff.

Key Design Features:

• Slab Gate: A solid, single-piece gate that slides across the flow path to open or close the valve. It is a simple and robust design.
• Expanding Gate: A two-piece gate that expands when closed to create a tight mechanical seal on both the upstream and downstream seats. This is often preferred for applications requiring a very positive seal.
• Rising Stem vs. Non-Rising Stem: In a rising stem design (most common for wellheads), the stem rises as the valve is opened, providing a clear visual indication of the valve's position.
• Full Bore: Wellhead gate valves are typically "full bore" or "through-conduit," meaning the opening in the valve is the same diameter as the pipe, allowing for unrestricted flow and passage of tools.

A choke is a specific type of valve designed to precisely control the production rate from the well and manage the pressure drop from the wellhead to the flowline.

Functions:

• Flow Rate Control: By changing the size of the orifice through which the fluid flows, an operator can increase or decrease the production rate.
• Pressure Reduction: It takes the high wellhead pressure and reduces it to the lower pressure required by the downstream processing facility.
• Types:
  • Positive Choke: Has a fixed-size orifice or "bean." The flow rate is changed by physically replacing the bean with a different size.
  • Adjustable Choke: Uses a needle-and-seat or a cage-and-sleeve mechanism to allow the orifice size to be changed while the well is flowing, providing much finer control.

Traceability is the ability to track the history, application, and location of a component or a piece of equipment by means of recorded identification. In API 6A, it is a critical quality control requirement, especially for higher PSLs.

Key Aspects:

• Material Traceability: All pressure-containing and pressure-controlling parts must be traceable back to the original mill heat lot and the corresponding material test reports (MTRs). This ensures the material's chemical and mechanical properties are known and verified.
• Manufacturing Traceability: The manufacturing history, including heat treatment, NDE, and testing records, must be linked to the component's unique serial number.
• Documentation: This traceability is maintained through a comprehensive set of documents, often called a "data book" or "manufacturing record book" (MRB).

A back pressure valve is a one-way check valve that is installed in the tubing hanger to provide a temporary seal of the tubing bore.

Primary Uses:

• Well Control Barrier: It is installed after the tubing is landed to allow for the removal of the Blowout Preventer (BOP) and the installation of the Christmas Tree while maintaining well control.
• Prevents Backflow: It allows fluid to be pumped down the tubing but prevents any well fluids from flowing up.
• Retrieval: It is designed to be retrieved through the Christmas Tree under pressure using a special tool once the tree is installed and tested.

Hardness control is the single most important factor in preventing Sulfide Stress Cracking (SSC).

• SSC Mechanism: When steel is exposed to H₂S and water, hydrogen atoms can diffuse into the metal lattice. In high-strength (high-hardness) steels, these hydrogen atoms can cause embrittlement and lead to catastrophic, brittle failure at stress levels far below the material's normal yield strength.
• NACE MR0175/ISO 15156: This standard defines the acceptable hardness limits (typically measured in Rockwell C, HRC) for different materials to be considered resistant to SSC.
• Control Method: Hardness is controlled through careful material selection, heat treatment (quenching and tempering), and rigorous testing of the final product to ensure it does not exceed the specified limits.

A wear bushing is a sacrificial component that is installed in the wellhead during drilling operations.

Purpose:

• Protect Sealing Surfaces: It sits inside the casing head or casing spool and protects the internal sealing profiles from damage caused by the rotation of the drill string and the passage of drilling tools.
• Sacrificial: It is made of a softer material than the wellhead body and is designed to wear out instead of the expensive, permanent wellhead housing.
• Retrieval: After drilling is complete for a particular hole section, the wear bushing is retrieved before the next casing string and casing hanger are installed.

An actuator is a device mounted on a valve that uses an external power source to open and close it. On a Christmas Tree, these are critical for automated and emergency operations.

Common Types:

• Hydraulic Actuator: Uses pressurized hydraulic fluid to generate force. This is the most common type for Surface Controlled Subsurface Safety Valves (SCSSVs) and emergency shutdown (ESD) valves on the tree (often the UMV and Production Wing Valve).
• Pneumatic Actuator: Uses compressed air or gas. Less common on high-pressure trees but used in other plant applications.
• Fail-Safe Design: Actuators on safety-critical valves are almost always "fail-safe," meaning they use a large spring to automatically move the valve to its safe position (e.g., closed) upon loss of hydraulic pressure.

Both are used to suspend tubular strings in the wellhead, but they are used for different strings and have different features.

• Casing Hanger:
  • Function: Suspends a string of casing.
  • Location: Lands in the casing head or a casing spool.
  • Types: Can be slip-type (mandrel with slips that grip the casing) or mandrel-type (threaded directly to the casing).
  • Sealing: Activates a seal in the annulus between the casing string it supports and the previous one.
• Tubing Hanger:
  • Function: Suspends the production tubing string.
  • Location: Lands in the tubing head.
  • Features: More complex, often with ports for downhole control lines and a profile for a back pressure valve.

Side outlets are ports on the side of a wellhead spool (casing head, casing spool, or tubing head) that provide access to the annulus between casing or tubing strings.

Functions:

• Annulus Monitoring: They allow for the monitoring of pressure in the annulus, which is critical for well integrity.
• Fluid Injection/Bleed-off: They can be used to inject fluids into the annulus (e.g., for corrosion inhibition) or to safely bleed off any pressure buildup.
• Connection: They are typically threaded or flanged connections where a valve (often called an "annulus valve") is installed to control access.

Fire-resistant equipment is designed to maintain its pressure-containing integrity for a specified period of time during a fire. The standard for this is API 6FA, "Specification for Fire Test for Valves."

Design and Testing:

• Goal: The main objective is to prevent the loss of containment of flammable wellbore fluids during a fire, which could escalate the disaster.
• Mechanism: Since elastomeric or plastic seals will be destroyed, fire-tested designs rely on secondary metal-to-metal seals that become effective after the primary seals fail. Gaskets are often made of flexible graphite, which can withstand high temperatures.
• Testing: A prototype valve is subjected to an intense flame for 30 minutes, then cooled, and pressure tested to ensure it still provides an acceptable level of sealing.

A composite block tree is a modern design where multiple valves are integrated into a single, solid block of forged steel rather than being separate flanged components.

Advantages:

• Reduced Height and Weight: It is much more compact and lighter than a conventional tree made of individual flanged valves.
• Fewer Leak Paths: By eliminating multiple flange connections, the number of potential leak paths is significantly reduced, improving safety and reliability.
• Cost-Effective: While the initial forging is complex, the overall cost can be lower due to reduced machining, assembly time, and the elimination of many studs, nuts, and gaskets.

Clamp hub connectors are a type of mechanical connector that serves the same purpose as a flange but with a different design.

Features and Advantages:

• Design: Consists of two hubs, a metal seal ring, and a clamp that forces the hubs together over the seal ring.
• Compact and Light: Significantly smaller and lighter than an equivalent API flange, which is a major advantage on offshore platforms where space and weight are critical.
• Faster Makeup: Much quicker to assemble and disassemble than a multi-bolt flange.
• High Integrity Seal: The design creates a very strong, reliable, and reusable metal-to-metal seal.

NDE refers to a group of testing methods used to evaluate the properties and integrity of a material or component without causing damage. It is essential for finding defects that could lead to failure under pressure.

Common NDE Methods in API 6A:

• Visual Testing (VT): The examination of surfaces for visible defects.
• Magnetic Particle Testing (MT): Used to detect surface and near-surface cracks in ferromagnetic materials.
• Liquid Penetrant Testing (PT): Used to find surface-breaking defects in non-porous materials.
• Ultrasonic Testing (UT): Uses high-frequency sound waves to detect internal flaws like cracks, voids, or inclusions.
• Radiographic Testing (RT): Uses X-rays or gamma rays to create an image of the inside of a component, revealing internal defects.

The extent of required NDE increases significantly from PSL 1 to PSL 3.

Lockdown screws are threaded pins installed around the top flange of a casing head or spool.

Purpose:

• Energize Annulus Seal: In some designs, tightening the lockdown screws pushes down on the casing hanger or a compression ring, which energizes the elastomeric annulus seal.
• Secure the Hanger: They prevent any upward movement of the casing hanger that might be caused by thermal expansion of the casing or pressure in the annulus below. This ensures the seal remains engaged and the casing weight remains properly supported.

An SCSSV is a fail-safe safety valve installed deep inside the production tubing. While not part of the surface wellhead itself, it is directly controlled from the surface *through* the wellhead.

• Function: It is the primary downhole safety barrier, designed to automatically shut in the well in an emergency (e.g., damage to the Christmas Tree).
• Control: It is held open by hydraulic pressure supplied from a surface control panel. This hydraulic line runs from the surface, down the outside of the tubing, and connects to the SCSSV.
• Wellhead Interface: The hydraulic control line passes through a special port in the tubing hanger and tubing head to get from the outside to the control panel. Loss of this hydraulic pressure causes the valve to close and shut in the well.

A data book is a comprehensive package of documents that provides a complete record of an equipment's manufacturing history, as required by API 6A.

Typical Contents:

• Material Test Reports (MTRs) for all pressure-containing parts.
• Hardness test reports.
• Nondestructive Examination (NDE) reports (MT, PT, UT, RT).
• Heat treat charts and records.
• Hydrostatic and gas test charts and reports.
• Welding procedure specifications (WPS) and welder qualification records (PQR).
• Dimensional inspection reports.
• Certificate of Conformance and API Monogram certification.

The data book is the ultimate proof that the equipment was manufactured in full compliance with the specified API 6A requirements (PSL, Material Class, etc.).

These are two distinct pressure ratings defined in API 6A.

• Rated Working Pressure (RWP): This is the maximum pressure that the equipment is designed to be exposed to during service at the rated temperature. It is the pressure rating stamped on the equipment (e.g., 5,000 psi).
• Test Pressure: This is the higher pressure used during manufacturing to test the equipment's strength and integrity. It is a quality control check, not a service rating.
  • Hydrostatic Test Pressure: For most API equipment, this is 1.5 times the RWP. For a 10K valve, the hydro-test pressure is 15,000 psi.

It is critical never to subject equipment to its test pressure in the field; the maximum field pressure should never exceed the rated working pressure.

A frac tree is a specialized, high-pressure assembly used during hydraulic fracturing operations. It is designed for temporary, high-rate, abrasive service.

Key Differences:

• Size and Pressure: Frac trees typically have larger bores (e.g., 7-1/16") and higher pressure ratings (10K or 15K) to handle the high pump rates and pressures of fracturing.
• Configuration: They often have multiple side outlets ("frac heads" or "goat heads") to allow for multiple frac lines to be connected simultaneously.
• Valves: Valves are often manual gate valves as they are not typically used for automated production control. A key component is the "frac valve," which is the main isolation valve.
• Service Life: They are designed for short-term, erosive service, unlike a production tree which is designed for decades of service.

Grease fittings allow for the injection of heavy lubricant (valve grease) into the valve body and seat areas.

Functions:

• Lubrication: Reduces the torque required to operate the valve and prevents galling of the metal gate and seats.
• Sealing: The grease can help create a temporary seal if the primary metal-to-metal seal is slightly damaged or leaking. It can fill small scratches and imperfections.
• Cleaning: Injecting new grease purges old lubricant and contaminants from the seat area, helping to maintain the valve's performance.

The API Monogram is a licensed trademark of the American Petroleum Institute. When a manufacturer places the API Monogram on a product, it signifies that:

• The manufacturer has a license from API to use the mark.
• The product complies with all requirements of the relevant API specification (e.g., API 6A).
• The manufacturer operates a quality management system that meets the requirements of API Specification Q1.

It is a certification mark that provides end-users with confidence that the product meets a recognized industry standard for quality and performance.

This refers to the two primary ways of connecting a valve to the side outlet of a wellhead spool.

• Flanged Connection: The side outlet on the spool has an integral flange. The valve also has an integral flange. They are connected using a full set of studs, nuts, and a ring gasket.
• Studded Connection: The side outlet on the spool is a solid face with threaded holes. The valve is attached using studs that are screwed into these threaded holes. This is a more compact design than a flanged connection.

A test plug is a tool used to isolate and test specific seals within the wellhead assembly during installation.

Example Use:

• After installing a casing hanger and its annulus seal, a test plug can be landed in the casing head bore above the hanger.
• Pressure is then applied through a test port in the wellhead housing to the void between the test plug and the casing hanger seal.
• This allows for a direct pressure test of the casing hanger seal's integrity without having to pressurize the entire casing string.

Impact testing measures a material's toughness, which is its ability to absorb energy and resist fracture, especially at low temperatures.

Importance:

• Brittle Fracture Prevention: Many steels that are strong and ductile at room temperature can become brittle at low operating temperatures. A sudden impact or shock load could cause a catastrophic brittle fracture.
• Requirement: API 6A mandates impact testing for materials used in specific temperature classes (e.g., K, L, P) to ensure they retain sufficient toughness and are not susceptible to brittle failure in cold environments.

Galling is a form of severe adhesive wear that occurs when two metal surfaces slide against each other under heavy load. The surfaces can seize or weld together, and then tear apart, causing severe damage.

Dangers:

• It can cause valves to become inoperable (stuck open or closed).
• It can destroy sealing surfaces, leading to leaks.
• It can damage threaded connections, making disassembly impossible without cutting.

Prevention:

• Lubrication: Proper grease and anti-seize compounds are critical.
• Material Selection: Using dissimilar metals or metals with different hardness values can reduce the tendency to gall.
• Surface Coatings/Treatments: Applying low-friction coatings or surface hardening treatments can significantly improve resistance to galling.

An adapter is used to connect two pieces of equipment that have different sizes, pressure ratings, or connection types.

Common Uses:

• Size Change: Adapting from a 13-5/8" wellhead bore to an 11" BOP stack.
• Pressure Rating Change: Connecting a 10,000 psi rated Christmas Tree to a 5,000 psi rated tubing head (this would de-rate the entire system to 5,000 psi).
• Connection Type Change: Adapting from an API flanged connection to a clamp hub connection.

An adapter spool is a spacer spool with different connections on each end, while an adapter flange is a single flange with different properties on each side (e.g., different ring grooves).

A BOP, or Blowout Preventer, is a large, specialized valve or stack of valves used to seal, control, and monitor a well during drilling to prevent blowouts (the uncontrolled release of oil and gas).

Relationship:

• The BOP stack is installed on top of the wellhead (casing head or casing spool) after a casing string has been set and cemented.
• It forms the primary well control barrier during drilling operations.
• The wellhead provides the foundation and connection point for the BOP stack.
• Once the well is completed, the BOP is removed, and the Christmas Tree is installed in its place.

• Pressure Identification: Always assume all lines are pressurized until proven otherwise. Positively identify and verify the working pressure of all components.
• Line of Fire: Never stand directly in front of pressurized connections, valve outlets, or areas where a component could fail.
• Proper Isolation: Follow strict lock-out/tag-out (LOTO) and valve isolation procedures. Always confirm that the component being worked on is properly depressurized and isolated from the wellbore.
• Correct Tools: Use only tools that are rated for the job (e.g., calibrated torque wrenches for flange makeup). Avoid using impact wrenches on flanged connections unless specifically permitted by procedure.
• Personal Protective Equipment (PPE): Standard PPE (hard hat, safety glasses, steel-toed boots) is mandatory, with additional requirements like fire-retardant clothing (FRC) or H₂S monitors as needed.

This is another term for a composite block tree. It refers to a tree where the lower master valve, upper master valve, and swab valve are all integrated into a single forged block. This design minimizes height, weight, and potential leak paths compared to a conventional tree built from individual flanged components.

Cladding is a process where a layer of corrosion-resistant alloy (CRA), such as Inconel 625, is welded onto the surfaces of a component made from a lower-cost base material like low-alloy steel.

Purpose:

• Corrosion Resistance: It provides the high corrosion resistance of a CRA on the surfaces that are wetted by the process fluid, protecting the component from H₂S, CO₂, and chlorides.
• Cost Savings: It is more economical than manufacturing the entire component from solid CRA. The low-alloy steel base material provides the required strength at a lower cost.
• Application: It is commonly used for ring grooves, valve bores, and other critical sealing areas in sour or corrosive service.

The "bore" refers to the main vertical cylindrical opening through the center of the wellhead components and valves.

• Full Bore: It is critical that the bore of the wellhead and tree components is large enough to allow for the passage of the required casing strings, tools, and completion equipment.
• Nominal Size: The nominal size of a flange or valve (e.g., "7-1/16 inch") refers to the diameter of this bore.
• Consistency: The bores of all components in the vertical stack (casing head, spools, tubing head, tree valves) must be compatible and aligned to provide a continuous path into the well.

No, this would not be appropriate. The API/FCI leakage classifications define acceptable leakage rates, and they are not all equal.

• Class IV: Known as "metal-to-metal" shutoff. It allows for a very small, specified leakage rate. It is considered excellent for a metal-seated valve but is not bubble-tight.
• Class VI: Known as "soft-seat" classification. This is the tightest standard and is effectively a "bubble-tight" shutoff, meaning zero bubbles are observed during a low-pressure gas test.

For a critical gas well requiring zero leakage, a Class VI valve would be the appropriate specification.

• Full-Bore Valve (or Full-Port): The diameter of the opening (bore) through the valve is the same as the inside diameter of the pipe it is connected to. It offers no flow restriction when fully open. This is the standard for master and swab valves to allow tool passage.
• Reduced-Bore Valve (or Reduced-Port): The bore through the valve is smaller than the inside diameter of the connecting pipe. This creates some flow restriction and is generally not used for master or swab valves, but might be acceptable for a wing valve in some applications where maximum flow is not the primary concern.

A data acquisition system is used to monitor the well and Christmas Tree performance in real-time.

Components and Function:

• Sensors: Pressure and temperature transmitters (PTs and TTs) are installed on the tree and flowline to continuously measure process conditions.
• RTU/PLC: A Remote Terminal Unit or Programmable Logic Controller at the wellsite collects the data from the sensors.
• Communication: The RTU transmits the data via radio, satellite, or cellular network to a central control room.
• Benefits: This allows for remote monitoring of production, early detection of problems (like pressure anomalies), and automated shutdowns, improving both efficiency and safety.

Sustained Casing Pressure is pressure observed in a well annulus that rebuilds after being bled down. It indicates a leak path from a deeper, higher-pressure source.

Concerns:

• Well Integrity Failure: It is a direct sign that one or more barriers in the well (e.g., cement, casing, packer) have failed.
• Safety Hazard: The pressure could exceed the rating of the outer casing strings or the wellhead seals, potentially leading to a loss of containment at the surface.
• Regulatory Issue: Regulators have strict rules for monitoring and managing SCP. Wells with significant SCP often must be shut-in and repaired. Monitoring annulus pressure via the wellhead side outlets is a critical part of well integrity management.

A stack-up test is a pressure test performed in the workshop *before* shipping the equipment to the field. The full assembly (e.g., a Christmas Tree) is stacked up exactly as it will be in the field and is then subjected to a hydrostatic pressure test as a complete unit.

Purpose:

• Verify Fit and Function: It ensures that all components fit together correctly and that all ring gaskets and seals function as a complete system.
• Reduce Field Issues: It identifies any potential connection or sealing problems in the controlled environment of the workshop, which is far cheaper and safer than finding them during installation at the rig site.

An annulus seal assembly is the component within a casing head or spool that creates the pressure-tight seal between two concentric strings of casing or between the tubing and the final casing string.

Types:

• Elastomeric Seals ('P' seals, 'FS' seals): These are molded rubber or elastomer seals that are energized by compression (using lockdown screws) or by pressure from below.
• Metal-to-Metal Seals: For high-pressure, high-temperature (HPHT) or highly corrosive service, metal seals are used. These provide a more robust and reliable seal than elastomers.

The integrity of these seals is fundamental to the safety of the well.

A drift test is a procedure to verify that the internal bore of the wellhead and tree assembly is clear of any obstructions and meets the minimum required diameter.

Procedure:

• A precision-machined cylindrical bar, known as a "drift," is passed through the entire vertical bore of the assembled equipment.
• The diameter of the drift is slightly smaller than the nominal bore size of the equipment.
• If the drift can pass through smoothly from top to bottom, it confirms that there are no restrictions, misalignments, or foreign objects that would prevent the passage of downhole tools. This is a critical check before running completion equipment.