1. Overview of the Article
This article will explain the hot tapping and line stopping operation from its typical uses and associated activities to illustrating the risks and benefits related to each operation.
Hot tapping and line stopping are executed in a variety of sectors, on various materials, as well as infrastructure (such as tanks or pipes) and environmental conditions both onshore as well as offshore. This article will focus on the use of hot tapping and line stopping onshore and on steel pipelines in the oil & gas sector.
The onshore hot tapping application is most visible in the following:
- Potable Water distribution
- Sewerage Treatment
- Oil and Gas
The same procedures apply to tanks with exception to minor differences within the procedures and requirements.
Typical prerequisites for hot tapping services are:
- Making a new connection for instruments such as pressure gauges, temperature gauges and intrusive type pig signalers into existing piggable pipelines. These are typically very small hot taps and are completed with the use of a custom twist bit type drill bit manufactured to attach to the tapping machine. When a twist bit is used, there will be no coupon recovered from the hot tap.
- Connection of new end users to existing pipeline networks such as a gas distribution system.
- Tapping into a pipeline or piping system to create a connection for chemical injection used to inhibit corrosion on the pipeline interior wall.
- Before a line stop can be inserted into the pipeline, a hot tap must be completed to gain access to the pipeline.
2. Introduction to Hot Tapping
It is typical for pipelines, piping systems and tanks to undergo modifications during their operational life. These modifications could be general maintenance, repairs (planned or emergency) or expansion of an existing system.
Due to the large product volumes and revenues typically involved in the oil and gas sector, any lost production time is critical. Therefore, shutting down a facility or pipeline requires careful planning and critical upfront analyses.
Before the development of hot tapping, the only option available for such modifications was an entire shutdown of the facility or pipeline.
Such shutdowns for pipelines are managed through operating specific valves and isolating a section of the facility from normal service. This is not a particular difficult task provided the facility was designed with operation flexibility. For tanks, shutdowns are particularly troublesome and not very economical, especially since they store large volumes of product. The asset owner is required to drain the entire contents of the tank and create a favorable entry environment before any modifications could start or entry into the tank is permissible. The drained product is sent to an alternative tank or a temporary storage facility.
The development of hot tapping brought relief to asset owners and operators, as it gave them an additional option for modification and repair work without shutting down entire or sections of their facilities.
2.1 Hot Tapping/Line Stopping Nomenclature
Hot tapping is a method of making a new connection to an existing pressurized pipeline without interrupting flow or emptying the section of pipeline or vessel. This benefit to the asset owner of the pipeline is, they can continue normal operation while maintenance or modifications are being executed.
Line Stopping or Plugging
A controlled method to isolate a section of pipeline through inserting a line stop plug into the pipeline while the system continues to operate normally allowing repairs and/or maintenance of critical components downstream.
The portion of the pipe that is removed from the pipeline by the hot tap cutter during the hot tapping operation.
Product Flow Rate
The measurement for the fluid, or gas, moving through a given cross-section of a pipe per unit of time.
Split Tee Fitting
A hot tap or line stop fitting with half of its body having a flanged branch connection and its other half completing the circumference of the sleeve. A Split Tee can have a branch outlet diameter equal to the main header or a reduced branch connection. The line stop fitting will typically have a flange with completion plug capabilities. The completion plug is held into the flange by use of segments or pins.
A branch connection fitting that is used for hot tapping branch connections typically when the branch size is smaller than half of the size of the pipeline being hot tapped.
2.2 Advantages of Hot Tapping over Conventional Shutdown Activities
Hot tapping has many advantages over a conventional shutdown to perform repair, maintenance, or network expansion activities. Examples of a few advantages are:
- There is no supply disruption to other pipeline users. This is the primary reason hot tapping has gained prominence over system shutdowns, especially in the gas distribution systems where new customers are continuously added into the existing gas distribution network. A new customer can be connected to the gas network without disrupting gas supply to other consumers.
- Uninterrupted revenue stream from product flow due to the continuous operation of the system.
- The coupon retrieved as a result from the hot tap can be evaluated to determine the rate of corrosion and the remaining wall thickness of the pipeline.
- Release of hazardous product into the environment is minimized ensuring a safer and healthier operation for the environment and population.
- No cost of draining, purging and possibly temporarily storing the pipeline’s content due to shutdown of a facility or a long section of pipeline.
2.3 Hot Tapping Capabilities
Through the years, there has been many improvements in hot tapping technology, which has been further extended to line stopping. Some of the capabilities of this technology are:
- Ability to hot tap into various pipe materials, including:
- Carbon steel
- Stainless Steel
- PVC and PE
- Increase of hot tapping on pipe sizes over 40”.
- Successful hot tapping on both onshore and offshore, on a platform or subsea, pipelines.
- Successful hot tapping executed on petroleum products in storage tanks (originally seen as a high-risk operation).
- Hot tapping and line stopping higher pressures using fully rated equipment and the use of engineering controls.
2.4 Tapping Orientation
Tapping may be performed in any of the following orientations. However, the orientation is subject to company approval and local regulations or requirements:
Vertical or Perpendicular to Main Pipeline
On horizontal surfaces, it is more convenient and safer to perform hot tapping in the vertical down position. The hot tapping machine is better supported in this position with support put in place beneath the fitting. The u-wires located on the pilot drill use gravity to actuate. In this position, the gravity will help the u-wires to remain activated against the coupon the duration of the hot tap.
Hot tapping may be performed horizontally into a horizontal or vertical pipe or a storage tank. Horizontal tapping is a common practice, especially for small diameter connections. Occasionally for the larger hot taps in this position, an alignment guide is used to compensate for drop in the cutter/boring bar. As the tapping machine is rolled from top of the pipe, the risk of losing the coupon is increased due to losing the force of gravity to actuate the u-wires.
Hot tapping may be performed at an angle from the vertical downward axis. Depending on the hot tap size as well as the capability of the hot tap machine, there are maximum angles that can be successfully completed. Before selecting a hot tap machine for angle hot tap, details must be provided to the vendor on the particular angle to determine whether the machine can successfully complete the angle hot tap. An angled hot tap guide is used on all angle hot taps to guide the pilot and cutter through the pipe. Without this guide, the pilot drill will walk up the pipe and eventually fail.
3. Components Description of a Hot Tapping Assembly
A hot tapping machine consists of various parts. It should be noted that the description of the components in this article may also consist of other smaller individual units see Figure 1 for a typical hot tap setup.
3.1 Measuring Rod
The measuring rod is inserted into the top of the hot tapping machine. It is used to gauge the travel distance of the cutting tool. Before the hot tap is started, the lower in and the total travel distance are marked on the measuring rod from the top of the hot tap machine. The lower in distance is the distance from the top of the hot tap valve to the top of the pipe. The travel distance is the distance the pilot bit extends from the cutter added to the distance the cutter will travel into the pipe to capture the coupon and clean up the hot tap hole. The measuring rod is scribed in 1” increments. With the visual aid of the measuring rod, the tapping machine operator can gauge when the cutter has completed the hot tap. Occasionally, the hot tap machine will have the measurement indicator scribed into the hot tap machine or integrated with the hot tap machine.
3.2 Hand Crank
The hand crank is used to mechanically raise and lower the boring bar. Before starting the hot tap, the hand crank is removed from the tapping machine due to the pinch point between the crank handle and the measuring rod.
3.3 Pilot Bit
The pilot bit is attached to the center of the cutter. Multiple U-wires are strategically affixed to the pilot bit to retain the cut coupon. The U-wires are cut and bent to form a “U” shape. They fold against the pilot bit into the milled relief area and actuate when the pilot bit drills through the pipe. When the top of the u-wire is slightly past the opposite pipe wall, the u-wire will fall into place to support the weight of the coupon. Note, the pilot bit penetrates the pipe wall before the cutter to allow purging the hot tap chamber to remove air.
The cutter is a special type of cutting tool like a hole saw. There are several different types of cutters such as, hot tap cutter, line stop cutter, etc. The cutter has teeth located on the cutting edge of the shell. The teeth are typically T-15 inserts although, tungsten carbide or diamond tips are used when the application suits such as concrete pipe. The cutter is securely mounted to the cutter holder with the pilot drill bit at the center. This assembly is attached to the boring bar of the hot tap machine assembly.
3.5 Hot Tapping Adaptor
The hot tapping adaptor houses the cutter assembly and is connected to the hot tap machine as well as the hot tap valve. The hot tapping adaptor is equipped with a flange of the appropriate size and rating to connect to the hot tap valve. A bleeder valve is installed on the hot tapping adaptor to purge air from the chamber or to bleed off pressurized fluid/gas when the hot tapping process is complete.
3.6 Hot Tap Machine
The hot tapping machine contains the mechanical ability to rotate the boring bar the pilot and cutter assembly is attached. It may be hydraulically, pneumatically, electrically, or manually operated. The tapping machine can be manually or automatically fed into the pipe to cut the coupon. The hot tapping machine will have a packing gland that seals with either a chevron style packing or o-rings. The seal is generally located at the boring bar close to the flange connecting to the hot tapping adaptor. Hot tapping machines vary in size and weight and most will require lifting support when attaching them to the hot tapping valve.
3.7 Hot Tap Fitting
There are various types of hot tap fittings which include weld-o-let fittings, split tees, reinforcing saddles and nozzles.
Only qualified and competent personnel should select fittings. Hot tap fittings must be sized to the end users operational and design requirements. The bore of the hot tap fitting must me large enough to accommodate the hot tapping cutter size.
The welded split tee is oversized on the run, “split” into two parts and welded on to the pipeline. The extruded split tee is extruded from a plate to form the oversized tee along with the outlet to weld the flange. The outlet of the split tee has a flanged connection welded to the end.
The weld-o-let (defined in section 2) is used when small branch taps are required. They are welded to the pipe and a flange is welded to the outlet.
The fittings are welded to the main line that will be hot tapped by welders certified and tested in the technique of welding on live pipelines.
3.8 Hot Tap Valve
The hot tap valve is attached to the fitting welded to the pipe. The hot tap valve may be flanged and must be full bore. The bore of the valve must be large enough to permit the passage of the cutter. Valve used for a hot tap are generally gate valves or ball valves. Sandwich valves are typically used for line stop activities, when pressure and temperature ratings permit, because of their slim body and the need to have studs and nuts for the line stop operation are not required. Some valve types cannot be used for hot tapping such as butterfly valves and globe valves.
4. Step by Step Description of Hot Tapping Sequence
It should be noted that only qualified and competent personnel should perform hot tapping activities. Below is a very brief description of the process to hot tap into a pipeline. The intention is to connect a new branch to the existing pipeline. Note, the same procedures may apply to piping and tanks except for changes in orientation and some other minor requirements. Please refer to API RP 2201 for additional requirements.
Prepare the hot tapping location on the pipeline. This includes excavating around the pipeline if it is buried and removing all external coatings on the pipeline hot tap point. The coating removal should be beyond the Heat Affected Zone that will result from welding activities. Test the pipe ultrasonically to verify ample wall thickness is available to weld the hot tapping fitting. Ensure there is adequate vertical clearance above the pipeline for the hot tapping machine.
- Weld the hot tap fitting (split tee or weld-o-let) to the pipeline
- Verify there are no leaks or defects in the weld. This can be accomplished through hydraulic or pneumatic leak testing or non-destructive inspection such as mag particle.
- Install the hot tapping valve to the welded fitting. Ensure the bore of the hot tapping valve is aligned properly with the bore of the welded fitting.
- Critical measurements and calculations are recorded to ensure only the top pipe wall is hot tapped.
- Close the hot tapping valve and install the hot tapping machine to the closed hot tapping valve. Ensure alignment between the hot tapping machine and the hot tapping valve.
- Open the hot tapping valve and advance the cutter/pilot drill bit assembly until it touches the pipeline.
- Verify the flange/threaded connections are leak tight. This can be achieved through hydraulic or pneumatic leak testing.
- Start the hot tap operation.
- After the pilot bit has broken through the pipeline, fluid in the pipeline will fill the void above the pipe. Purge the air in the void by opening the bleeder valve. Close the bleeder valve after all the air has been expelled.
- When the pilot bit has completely broken through the pipeline stop rotation of the machine and advance the pilot bit until the cutter contacts the pipe.
- When cutter touches the top of the pipeline, restart the hot tapping machine to commence cutting operation. The hot tapping operation will be complete when the coupon is cut free from the pipe and the pipe wall is cleaned up.
- When the hot tapping operation is complete, stop the hot tapping machine. Advance the cutter to confirm the cutting operation is complete; the cutter should move downward freely. Ensure pilot or cutter does not penetrate the opposite pipe wall.
- Retract the cutter with the coupon captured by the U-Wires until the cutter assembly is completely retracted into the hot tapping adaptor.
- Close the hot tap valve, bleed the pressure from the chamber, and remove the hot tapping machine.
- Connect the new branch pipeline to the hot tap valve or install a blind flange.
5. Introduction to Line Stopping
Line stopping or line plugging is a method of isolating an in-service pipeline or piping system to prevent product flow beyond the location where the plug is installed. The line stop plug serves as a temporary valve to isolate the downstream section of the system.
The hot tapping process mentioned above is the very first step of a line stop however, the line stop is complex and requires additional steps and equipment, including a line stop actuator, line stop valve, line stop head, line stop seals, and a fitting with a modified flange fitted with a completion plug that is set into the flange to recover the line stop valve. The completion plug is locked into place in the flange using either segment or jack bolts. The segments or jack bolts are fitted with o-rings or a packing seal so the product in the pipeline does not leak through the flange.
After completion of the hot tapping operation, the line stop plug is installed into the pipeline using the line stop actuator.
When two line stop plugs are installed into the pipeline in conjunction with a bypass line, product flow will continue through the bypass piping around the isolated section, thereby providing uninterrupted product flow.
Sealing elements are installed on the line stop heads to ensure there is an acceptable seal against the interior wall of the pipeline.
A sealing element is selected based on pipeline pressure, pipeline product, fluid temperature and the internal pipeline diameter.
5.1 Types of Line Stops
There are various types of line stops currently used in the oil and gas sector. Some of the most common types are listed below:
This is one of the first line-stopping technologies developed which uses a mechanical pivoting head actuated into position inside the pipeline. This line stop utilizes a fitting size greater than or equal to the size of pipeline. The line stop head is inserted through the hot tapped fitting using a line stop actuator. When the line stop heads wheel makes contact with the bottom of the interior pipe wall, the line stop head moves into place as the line stop actuators control bar is lowered.
The folding head, line stop is a mechanical type of line stop which uses a reduced branch fitting. The folding line stop head is inserted into the fitting in a folded position and mechanically expanded after contact with the bottom of the interior pipe wall.
These plugs can handle higher temperatures, pressures, and flow rates than conventional line stop plugs. They have a metal to metal seal and seal on the edges of the hole drilled through the pipe. The HTP Stop utilizes a special fitting welded on the pipeline since both the top and bottom portions of the pipeline are removed. The coupon will either be the whole section of pipe removed or there will be two separate coupons. The hot tap machine is mounted on the valve, and the cutting operation is performed. The plug is installed to isolate the line, and maintenance activities can be performed on the line. A completion plug is installed to recover the line stop valve when the project is complete. The HTP Stops are typically completed using gate valves that are bored to certain requirements, instead of using a sandwich type valve.
5.2 Line Stop Application
Line stops are used to:
- Re-direct an existing pipeline.
- A section of a pipeline is leaking, and the pipeline company requires the pipeline to remain in service. Two line stops can be utilized to create a temporary isolation and bypass on the pipeline while repair activities are completed. The bypass line can be integrated into the two line stop fittings/line stop housings, or two separate hot taps can be completed, creating the bypass for continuous flow until the repair activities are completed.
- Isolating a section of a pipeline for maintenance activities. Line stopping can be utilized to provide temporary isolation in the absence of an isolation valve in the pipeline system, while repair or maintenance activities are ongoing.
5.3 Step by Step Description of Line Stopping
The process starts with standard hot tapping procedures as described in section 5 and follows with the steps below of installing a line stop. Figures 10 through 13 show the installation of a line stop head.
- Complete the hot tapping as described in section 5 using a fitting with a special flange that permits the installation of a completion plug.
- With the line stop valve in the closed position, remove the hot tap machine and install the line stop actuator with the line stop head.
- Open the line stop valve and operate the line stop actuator to lower the line stop head into the pipeline.
- When the maintenance activities are complete, equalize pressure on the line stop head(s) and remove the line stop head(s) from the pipeline.
- To remove the line stop valve, install the completion plug into the flange by locking it into place in the flange below the valve.
- Remove the valve and install a blind flange.