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Choke Manifold


The choke manifold is a system composed of valves, fittings, lines, and chokes that provide multiple flow paths to regulate the flow of mud, gas, and oil from the annulus during a kick. This section will cover the choke manifold’s parts and functions and IADC recommendations for its use in drilling operations.

When a kick occurs, well control is maintained by shutting in the well (secondary well control). It is crucial to regain primary reasonable control as soon as possible. A blowout may occur if the shut-in pressure becomes too high before this can be achieved. Chokes bleed off drilling fluid to keep the pressure below the Maximum Allowable Annular Surface Pressure (MAASP). At the same time, heavier mud is pumped into the well to replace lighter mud and reestablish primary reasonable control.

It’s essential to manage the amount of drilling fluid carefully bled off to avoid further imbalance in hydrostatic pressure caused by lighter fluids mixed with gas.
A high-pressure choke line leads to the choke manifold, which typically includes:

– Adjustable chokes
– An accurate, quick-response pressure gauge
– Control valves

Note: Chokes regulate fluid release at a controlled rate to limit casing pressure and allow for the replacement of mud in the well with heavier mud.

Choke Manifold Function


Typical Choke Manifold for 5,000 psi Working Pressure Service-Surface Installation

If the drilling fluid’s hydrostatic pressure is insufficient to control subsurface pressure, formation fluids will enter the well. Back pressure is applied to maintain control by routing the flow through adjustable chokes until the healthy flow condition stabilises. The chokes are connected to the blowout preventer stack via valves, fittings, and lines that provide alternate flow paths or can completely halt the flow. This assembly is known as the choke manifold.

Choke Manifold Components

A typical choke manifold, designed for 5,000 psi working pressure, includes the following:

1. Choke: A device with an orifice that restricts fluid flow. It controls the drilling mud flow rate out of the well when closed in with the blowout preventer during a kick.

2. Choke Line: A pipe attached to the blowout preventer stack, through which kick fluids and mud are pumped to the choke manifold when the blowout preventer is activated.

3. HCR (Hydraulic Control Remote) Valve: This is a pressure-operated gate valve used in the flow line. It is designed for high-pressure applications, with sizes ranging from 4-inch to 6-inch and working pressures from 3,000 to 5,000 psi.

4. Chokes Types :
Manual Adjustable Choke: A needle valve-type choke with a tungsten carbide seat for wear resistance, used for initial flow restriction.

Hydraulic Choke: A choke with a replaceable seat of different sizes used for maintaining constant flow rates over extended periods, such as during production tests.
Remote-Controlled Choke: Operated from a control panel, usually located on the rig floor, and includes pressure gauges, a pump stroke counter, and control switches.

5. Choke and Kill Line Outlet Valves: These must meet specific requirements to ensure operational flexibility and safety, including minimum inner diameters and configurations that prevent excessive erosion or fluid friction.

IADC Recommendations for Choke Manifold Planning & Installation

Pressure Rating: All components upstream of and including the chokes should have a working pressure equal to the blowout preventer stack’s rated pressure.
Components: Should comply with specifications for anticipated pressure, temperature, and fluid corrosivity.
Connections: For working pressures of 3M and above, use flanged, welded, or clamped connections.
Placement: The choke manifold should be easily accessible and ideally located outside the rig substructure.
Testing: The choke manifold should be tested to full working pressure during initial installation, after maintenance or repair, and regularly as part of the BOP stack inspection schedule.

2000 & 3000 PSI Manifold

5000 psi Manifold

10000 & 15000 psi Manifold

Following these guidelines, the choke manifold can effectively maintain well control and manage pressure during drilling operations.

Snubbing Units

Snubbing Units in Oil & Gas Operations

There is a common misconception that snubbing or hydraulic workover units are exclusively used for uncontrollable wells such as blowouts, which is inaccurate. The ability to run tubulars under pressure has allowed many wells to be controlled effectively, saving considerable costs for the industry. Moreover, these units enable routine workovers and well servicing tasks on live wells, reducing or eliminating the need for kill fluid. This approach protects the reservoir from damage and enhances the efficiency and cost-effectiveness of workover operations.

Hydraulic Workover Units (HWO)

A hydraulic workover (HWO) unit can fulfill the functions of workover and/or drilling rigs but offers distinct advantages in certain scenarios:
– It has a compact footprint, which is ideal for operations in limited space.
– It minimizes environmental impact compared to conventional rigs.
– It can be rapidly mobilized to offshore platforms without a drilling rig.
– It facilitates quicker mobilization to remote land wells than traditional drilling or workover rigs.
– Operating costs are lower than those associated with conventional rigs.
– It requires less extensive platform support due to its ability to be partially or fully supported by the well.

Disadvantages of HWO units include:

– Slower operation due to trip times, especially when making up joints.
– Additional equipment such as mud pumps and solids control is required, integral to workover or drilling rigs.

Snubbing Units

Traditionally used mainly for critical well control and high-risk operations, snubbing units have proven beneficial for enhancing well performance and reducing overall costs. They are particularly useful in operations where killing the well is undesirable or impractical, such as:
– Running and pulling completion strings.
– Performing through-tubing operations like gravel packing and cleaning out obstructions.
– Conducting well control operations during drilling and workovers.
– Addressing well abandonment and perforation needs.

Advantages of snubbing units over coiled tubing or wireline include:

– High load rating suitable for heavy-duty tasks like fishing and milling.
– It is capable of pipe rotation, ensuring effective hole-cleaning properties.
– Ability to handle live well operations without compromising pipe strength.
– Flexibility to operate under various wellhead pressures.

Disadvantages when used as a snubbing unit:

– Slower operation compared to coiled tubing or rigs due to trip times and single-joint handling.
– Higher operational costs relative to coiled tubing are partly due to longer mobilization and rig-up times.

Hydraulic Workover Operations (HWO)

HWO operations encompass a wide range of tasks including full workovers, clean-outs, re-perforating, and running or pulling completion equipment. These operations are executed similar to conventional rig operations with proper pressure control equipment (PCE) and adherence to standard well control procedures.

Snubbing Unit Operations

Snubbing involves inserting a plugged pipe into a live well using blowout preventers (BOPs) to create an external seal around the pipe. This method is preferred for tasks where maintaining well pressure is crucial, such as:
– Running or retrieving completion equipment.
– Milling and washing below production tubing.
– Performing through-tubing operations like gravel packing and acid/cement spotting.
– Addressing well control issues in deviated or high-pressure wells.

BOPs in Snubbing Units

Snubbing operations utilize stripping BOPs for primary well control, with backup BOPs employed on high-pressure wells. The design and configuration of snubbing units vary based on specific job requirements, typically categorized into short stroke, long stroke, and rig-assist (conventional) units.

Components of HWO/Snubbing Units

These units include essential components such as hydraulic jack assemblies, workbaskets, control panels, and safety systems. The hydraulic jack assembly provides the necessary push, pull, and torsional forces on the work string, which is essential for various downhole tasks. Key components include guide tubes, windows, traveling and stationary slips, rotary tables, and power tongs.

Long Stroke, Short Stroke, and Rig Assist Snubbing Units

Additional backup BOPs may be deployed on wells with very high pressure. For tapered workstrings, specific BOPs may be initially provided for each pipe size, resulting in potentially up to 10 BOPs being used.

Snubbing unit configurations are adaptable and customized based on job requirements, typically falling into three main designs:
1. Multiple Ram Configuration
2. Short Stroke unit, which is predominant due to its ability to manage higher snubbing forces, supported by tubing guidance (further descriptions of snubbing equipment primarily refer to Short Stroke units).
3. Long Stroke Unit
4. “Rig assist” or conventional units powered by drilling rigs.

High-pressure wells necessitate additional Well Control equipment, although this advantage is limited by the positioning of the short-stroke snubbing unit jack above all pressure containment devices. BOPs must be rated for specific tasks, such as 345 bar (5,000 psi), 689 bar (10,000 psi), 1034 bar (15,000 psi), etc.

HWO & Snubbing Units Rig-Up

HWO/Snubbing units are rigged directly onto the Xmas tree for through-tubing operations or onto the wellhead after removing the Xmas tree if pulling or running completion components. For washing out, they can be rigged onto a drill pipe.

 

Typical HWO/Short Stroke Snubbing Unit & Dimensions

Equipment is rigged as individual lifts or sub-assemblies and directly connected to the Xmas tree/Wellhead or the preceding component. Maximum lift usually involves loads up to approximately 6 tonnes with larger jacks, which typically do not pose issues. In cases requiring very tall rig-ups due to multiple BOPs, limitations may arise from the crane’s maximum reach. To address this, a portable rig-up crane may sometimes be necessary despite increasing rig-up time and costs.

All equipment is transported in baskets or on skids. As rig operations proceed, scaffolding is erected around the BOP and the window/jack for access. This scaffolding is standard for one-off jobs, but for multi-well campaigns, transport frames around the BOPs can prove cost-effective, serving as scaffolding post-rig-up.

Similarly, frames can be attached to the window and jack, functioning as platforms for individual platforms. Frame designs depend on rig-up heights and platform layout. Short-stroke units can even be rigged within a drilling rig’s derrick.

The snubbing system is categorized into these main components:
– Basic snubbing unit
– Well Control system
– Well barriers
– Auxiliary equipment

Basic HWO/Snubbing Unit Components

The basic HWO unit is a mechanical or hydraulic machine that generates push, pull, and torsional forces on the workstring for specific well tasks. The two common types are “standalone” and “rig-assist,” which share identical components.

HWO/Snubbing operations necessitate specialized support equipment, including a pipe handling system, transport containers, toolbox, pumping equipment, fluid storage/handling systems, and related support gear.

Equipment Description:

A HWO unit comprises:
– Hydraulic jack assembly featuring guide tube, window, traveling and stationary slips, rotary table, and power tongs.
– Workbasket, control panels, and safety systems.
– Hydraulic power pack, accessories, and hydraulic hoses.
– Circulating system with swivel, Kelly hose, tanks, and pumps.
– The BOP stack includes the control system and hoses.
– Guy wire/support system.

Hydraulic Jack Assembly in Snubbing Unit

The hydraulic jack assembly comprises cylinders configured to move the pipe in or out of the wellbore. Snubbing and pulling capacities depend on cylinder number, size, and hydraulic pressure. Some jacks use a regenerative hydraulic circuit to enhance pulling speed by circulating fluid from the snub side to the lift side.

Key components include:

– Guide tube: prevents work string buckling.
– Window: facilitates tool installation and stripper element changes.
– Traveling slips: grip the pipe during snubbing or lifting.
– Stationary slips: hold the pipe in place.
– Rotary table: for milling and fishing operations.
– Power tongs: hydraulic tools for making up connections.

Snubbing Unit Workbasket, Control Panels, and Safety Systems

The workbasket atop the jack serves as the crew’s platform for operations. It houses controls for jack and slips, BOPs, and counterbalance winches. Safety features include stairs for access and an emergency exit system like the “safety slide.”

Hydraulic Power Pack and Accessories

Ground-based power packs supply hydraulic pressure for jack, BOPs, rotary, winches, and tongs. Pressure regulation ensures safe operation.

Hydraulic Hoses in Snubbing Unit

These hoses transport hydraulic fluid to and from various unit components, tested to maximum working pressure after rig-up.

Circulating System Including Swivel, Kelly Hose, Tanks, and Pumps

The system includes necessary valves and must withstand high pressures for fluid circulation during operations.

Guy Wire Support System

Calculations are essential for supporting the unit’s height and determining cable dimensions, tension, and necessary weights to manage additional wellhead loads.

HWO and snubbing units offer specialized capabilities that enhance operational flexibility and efficiency in well intervention and servicing operations across diverse well environments.

Content Ref: Snubbing Units In Oil & Gas Field – Drilling Manual

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Types of BOPs

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