Use of hot taps for gas pipelines can be expanded

Nov. 30, 1998
Burke Delanty TransCanada PipeLines Ltd. Calgary This 30-in. branch (left) was installed on a 48-in. (880-psi MOP) pipeline under full-flow conditions near Caronport, Sask. Note stiffener ring and centering device as well as the nondestructive examination (MPI) work. (Photograph courtesy TransCanada Hot Taps, Calgary) [38,590 bytes] Welders install a 30-in., round-body gate valve near Caronport, Sask. (Photograph courtesy TransCanada Hot Taps, Calgary) [32,689 bytes]
John A. McElligott, Joe Delanty
TransCanada PipeLines Services Ltd.
Calgary
Burke Delanty
TransCanada PipeLines Ltd.
Calgary
TransCanada PipeLines Ltd., Calgary, has relied on hot tapping to complete more than 700 large diameter (12-30 in.), horizontal, high-pressure hot taps without incident since 1960.

Its subsidiary, TransCanada Hot Taps (TCHT), has further refined the field procedures and completed forty-one 30 and 24-in. assemblies in 1998.

All taps were performed with no restrictions on flow or pressures (880, 1,000, and 1,440 psi maximum operating pressures), negligible emissions, and no deterioration in system integrity.

For TransCanada, the direct advantages of a hot tap over a cold connection resulted in the avoidance of gross revenue losses of $1 million (Canadian) or more per hot tap, no environmental emissions, seamless service, and no effects whatsoever on its shippers.

A hot tap is a method of joining a new tie-over pipe to an existing and loaded facility such as a pipeline or tank. The objective of the procedure is to cut an interconnection hole in the facility through a valved hot-tapping assembly without the need to evacuate the contents.

A successful hot tap will minimize operational effects and result in no spills or emissions. In this discussion, the generic term "hot tap" will include all of the technical operations necessary to prepare for and complete the interconnection of a new pipeline with an operating pipeline. The actual cutting of a hole in the pipeline will be referred to simply as "tapping" or "boring."

The design, installation, and quality-control operations of a hot tap are described later.

Operator dilemma

Connecting a new pipeline lateral or loop to an existing high-pressure pipeline system has always been fraught with high costs and the potential for major system effects. Pipeline owners and operators historically have had to choose between a traditional cold connection with its high associated costs and a less-expensive but less familiar hot tap.

Although the cost savings of a hot tap have always been considerable, operators have not always felt comfortable with them because of the potential risk of complications during the branch weld or hot tap or during subsequent system operation.

Despite their extraordinary costs and throughput effects, the perceived certainties of cold connections were often sufficient to justify their regular use. The advent of two important factors, however, has radically improved the viability of hot tapping as an interconnection technology.

In the first, a variety of design developments has significantly enhanced the long-term integrity of a hot-tapped assembly.

In the second, the 1997 Kyoto Protocol on Climate Change has resulted in new commitments by world governments to reduce greenhouse gas emissions. For the North American gas industry, these initiatives could result in voluntary compliance objectives, incentive-based programs, or legislated reforms, any of which will significantly affect current practices. The role of hot tapping in this is explained later.

TransCanada has concluded that the technological evolution of hot tapping makes it a superior choice in almost all interconnect scenarios.

TransCanada's primary need for hot taps arises during pipeline expansions, although smaller hot taps will also be frequently required for the installation of new laterals, ultrasonic meters, or sales taps.

The rationale for using hot taps instead of cold connections is based upon detailed comparative analyses of the benefits and disadvantages of each operation with respect to risks, direct and indirect costs, and shipper, operations, and environmental effects.

TransCanada believes it has developed practices to the point that system integrity is not diminished and economic and environmental benefits are clearly superior to any alternatives.

Risk mitigation

Ultimately, the risk inherent in any individual hot tap will depend upon the quality of effort invested in design, preparation, and installation. The risk-management objective is to install an assembly that meets the short and long-term integrity standards of the overall pipeline system.

As a less popular technology, it must be at least as good as the industry-standard cold tee installation.

When discussing risk, it is important to consider that a completed hot tap is like any other component on a high-pressure pipeline system. Conclusive proof of its integrity can only be had when the assembly is retired from service.

Therefore, the decision to employ a hot tap or a cold tee must be based upon an analytical comparison of the risks and benefits.

A risk assessment must address the potential for installation difficulties and long-term operating problems, while the benefits must consider all environmental, economic, and operating impacts. Although successful removal of the carrier pipe coupon after a hot tap effectively puts the assembly into service and is certainly indicative of good design and workmanship, it is from that point forward that the integrity of the assembly will truly be tested.

The "hot" and otherwise specialized nature of a hot tap has justifiably earned it a reputation as an operation with a "higher-than-average" risk. When comparing the risks of cold tees and hot taps on gas pipelines, however, it is important to remember that the hot cuts around gas/air mixtures that are required for a cold tee installation come with their own safety and installation problems.

In fact, TransCanada believes that safety risks are easier to manage during installation of a hot tap than on a cold tee. TransCanada has successfully managed hot-connection risks by continual research and development and good control practices.

Direct, indirect costs

Overall, TransCanada estimates that the total cost of a hot tap is approximately 60-80% less than the total cost of a cold tee. The actual variance will depend on circumstances unique to each location or system.

Hot tap cost advantages occur in three basic categories:

  • Direct cost savings. The direct costs of a hot tap are often significantly less than are those of a cold tee.

Although the material costs, welding requirements, and quality-control expenses would be similar, a hot tap will not incur any expenses for evacuation (blow-down) of the pipeline or for the purging and loading process that is necessary once a cold connection is completed.

The on-site heavy equipment and general labor requirements would also normally be reduced for a hot tap. This is in part because a manager has more effective control over the schedule of work events.

During a cold tee, the requirement to minimize system downtime usually entails a "hurry-up" schedule that results in several different union personnel on stand-by (and charging) on-site for the time when they will be needed. The scheduling flexibility of a hot tap usually obtains excellent on-site production efficiencies.

  • Indirect costs-maintenance of transportation revenues.

Implicit in the reasons underlying any system expansion is that the existing plant is probably operating at capacity. Any service reductions or stoppages will automatically decrease revenues.

In a natural-gas transmission business, interruptible or other discretionary transportation is usually the most valuable class of service and will also be the first one to be affected during a system shutdown. Therefore, any system interruption will affect revenues disproportionately.

With larger interruptions, revenues can be further affected, even when the line is back in service, because the previously discretionary capacity may be required to rebuild firm-service obligations. The lost transportation revenues during a cold connection will normally exceed the direct costs of a hot tap by an order of magnitude or greater.

  • Indirect costs-commodity saved. Making a cold connection on a gas pipeline typically requires reducing the operating pressure to somewhere between 2,750 kPa (400 psi) and 1,700 kPa (250 psi) before the gas is vented.

For a typical 25-km (15.5-mile) section of 1,219-mm OD (48-in.) pipe, the volume of gas at 1,700 kPa represents about 500,000 cu m (18 million cu ft) of line pack, which is owned by the operator. At $1.63 (Canadian)/gigajoule, this represents more than $30,000 (Canadian) without including transportation costs.

Affecting shippers, operations

A hot tap will usually allow an operator to avoid, or at least reduce, the service impacts of a cold interconnect. During welding on new pipe, there should be no service impacts to a shipper; for older pipe, the interruption could be either a restricted flow or a full shut-in to manage cooling rates.

In either case, the interruption should be limited to 5 hr or less. Depending on operating pressures and which equipment is employed, the tapping work should not require any flow or pressure restrictions.

In a multiple loop system, a 5-hr shut-in should normally be transparent to shippers. In a single-line system, a small interruption can be scheduled to minimize or completely avoid any shipping effects.

Alternately, a cold tee will require 3-4 days or more of system shutdown that may entail considerable negotiations with shippers.

A system interruption for an expansion interconnection could justifiably be viewed by a current shipper as an unacceptable imposition on its business for the sake of future shippers. The effects may not qualify as "force majeure" interruptions and could be contrary to shipping contract provisions.

The consequences of a 4-day shutdown for a cold connection on shipping contracts and shippers' downstream obligations could also seriously undermine an operator's customer relations. The expense of preparing customers for a shutdown by rearranging contracts and subsidizing or otherwise mitigating shippers' consequences could also be extraordinary.

In most cases where pipelines less than 15 years old are being hot tapped, system impacts are invisible to a shipper.

Large downtime requirements can affect several different stakeholders and demand a lot of time-consuming and potentially disruptive transportation planning and revision. When a system restriction is required at a hot-tap site for pipe-chemistry reasons (that is, the worst-case scenario), it would typically create less than 5% of the outage effect of a cold tee.

Finally, the flexibility inherent in a hot-tapping operation enables an operator to adapt a schedule quickly for variables such as weather or operating system changes.

Environmental effects

The environmental advantages of a hot tap have made the technology indispensable to a federally regulated gas-transmission company like TransCanada. Commitments made by both the U.S. and Canadian governments at the Kyoto Protocol on Climate Change ("Kyoto") to reduce greenhouse-gas emissions could result in a variety of incentives and penalties related to greenhouse-gas emissions for all industries in North America.

On a $1(Canadian)/ton of carbon equivalents investment basis, a hot tap promises to be one of the most cost-effective investments that a natural-gas transmission company can make with respect to emissions reductions.

Although natural gas is known as a clean-burning fuel, the U.S. Environmental Protection Agency reports that methane (CH4) has a capacity for trapping heat in the atmosphere that is 21 times more effective than that of CO2.

In the example cited earlier, the methane contained in a 25-km section of 1,219-mm OD (48-in.) pipeline at 1,700 kPa (250 psi) represents the carbon equivalent of approximately 8.0 kilotons of CO2.

Based on this information, an owner/operator has two very important reasons for not releasing methane to atmosphere during an interconnect.

    1. A significant commodity loss is avoided, and the gas remains available for conversion to CO 2, (that is, CH 4 + 2O 2 = 2H 2O + CO 2).

    2. Responsibility for the methane or CO2 greenhouse-gas emissions remains with the shipper or end consumer.

A hot tap clearly allows a gas-transmission pipeline operator to avoid the liability of a significant greenhouse-gas release.

Furthermore, as Kyoto commitments are enacted in legislation and social or industrial incentives, an owner/operator should be able to obtain satisfaction (at the very least) or even some form of economic credit for an active and measurable effort in controlling emissions.

It is significant to note that emissions credits are now a genuine commodity, and major industries in North America are already voluntarily engaged in their purchase and sale.

When compared with the costs of achieving emissions reductions through other technological developments such as dry low-NOx controls for compressor units, a hot tap can be economical for a gas-transmission company to achieve voluntary and potentially mandatory greenhouse-gas emissions targets.

In liquids pipelines, greenhouse-gas emissions are not a concern. A hot tap has the ability, however, to isolate and otherwise minimize the potential sources of spills in addition to all of the other downtime-avoidance advantages it offers.

Facility design

On a main line-looping project, TransCanada typically attempts to install a new block valve parallel to and adjacent existing block valves (Fig. 1 [64,601 bytes]). The new pipeline is connected to the existing system with tie-overs (or "crossovers") that are upstream and downstream of the block valves.

For a typical 16 in. or larger hot tap, a vertical hot-tapping alignment is usually not feasible because of insufficient ground cover. In addition, TransCanada has always attempted to keep as much of its facilities from view as possible for security.

Another advantage of a horizontal alignment is that the weight load that would be introduced to the carrier pipe would require significantly more design and installation work to create the necessary mechanical support.

Fig. 2 [69,949 bytes] shows TransCanada's standard horizontal hot-tapping assembly.

The main disadvantage of a horizontal hot-tapping assembly is that it requires significant refinements to the design and installation procedures systematically to nullify the gravity-induced complications that would not affect a vertical hot tap.

Carrier pipe

The carrier pipe is the one component in the assembly that cannot be designed for and, therefore, the assembly design must be adapted to it. Consequently, the reconnaissance exercise is an essential part of the design and quality-control process.

Although a designer can preselect the exact location for a hot tap, field reconnaissance will usually result in considerable savings to the owner/operator. A 5-m movement of a branch site to the adjacent pipe joint can result in completely different flow and design parameters for the hot weld.

The supervisor of the investigation must be familiar with all available as-built data for the site as well as all of the geometric and metallurgical design requirements of a hot tap. This will enable selection of the optimum site and minimize costs and field work.

The branch should be located so that the end of the vertical split tee will be well clear of the nearest girth weld. Wherever possible, it should also avoid crossing the longitudinal or spiral factory seam weld.

If there are no alternatives, however, the area intersected by the branch and factory weld must be carefully examined for general corrosion and stress corrosion cracking (SCC).

The circular site for the branch weld must also be examined and free of general corrosion, SCC, and laminations. The portion of the factory long-seam or spiral weld that will lie under the split tee must also be examined and determined to be free of irreparable weld flaws because it will be ground flush before the split tee is fitted. The unanticipated start-up of a nearby compressor unit or cooling unit during a hot weld could materially impair the weld quality. A prudent weld design must be based on the best available information. But unlike joining new pipe, a branch weld is often performed on pipe of uncertain heritage. Therefore, as-built pipe data that may be deficient or not reliably or accurately indicate a pipe's chemical composition must be rejected and replaced with reliable information.

TransCanada has used laboratory methods to analyze carrier-pipe chemistry. This approach requires coating and pipe mill scale to be removed from a small area inside the location of the future branch and the use of an industrial burr grinder to remove steel particles from the pipe.

A simple paper cone and magnet system is then used to fill a 35-mm film container approximately half full of steel filings.

Although the field-sample collection method now produces good data, Trans Canada had to spend considerable time and effort to develop a system that produced consistently reliable data.

Occasionally, the thin skin of high-carbon mill scale present on the outside surface of a pipe was mixing enough with samples to skew the analysis. The errors would result in specifications for restricted flow conditions during hot welding that were later proven to be unnecessary.

A major disadvantage of the laboratory-analysis system is the time delay between sample collection and results. To avoid the high costs of returning to a site to examine alternate locations, it is often prudent to collect samples from all possible sites in case the preferred location is unsuitable.

The additional field work and lab analyses can increase costs considerably by reducing crew efficiencies and introducing nuisance problems while the excavation is open. A spectrometer can eliminate these delays. Even with newer pipe and good as-built data, it is an excellent tool for verifying metallurgical information.

TransCanada now prefers real-time spectroscopic techniques to confirm local metallurgy. The technology has now advanced to the point where accurate pipe chemistry analysis can be achieved in real time.

The equipment is highly portable and has the accuracy to make significant reductions in field costs and turnaround times for the operator. Redundant investigations can be eliminated, and the time delay between investigation and installation can be reduced to zero in some cases.

Branch

The branch (also known as the "stub," "nipple," or "nozzle") is a component that could be easily and wrongly dismissed as "just another piece of pipe." The parent pipe for the branch must be compatible with the design of the system with which it will connect in terms of maximum allowable operating pressure, temperature rating, and basic metallurgy.

Nevertheless, a few other issues must be addressed to create an acceptable component.

A branch must be systematically fabricated to ensure its enduring integrity. The factory weld cap must eventually be removed so that the outlet half of the split tee can be fitted over it. The weld cap must also be removed in advance of the hydrostatic test of the parent pipe so that the test results remain valid.

A branch-welding specification must address the weld bevels at the valve and carrier pipe ends of the branch. The valve side of the branch will normally be beveled for a standard field butt weld. The weld bevel at the carrier pipe end of the branch is a little more complicated on account of the scarfed edge of the branch.

The bevel angle will vary as it follows the scarf from 30° at the 6 and 12 o'clock positions to 45° at the 3 and 9 o'clock positions. The angles will also vary with the relative difference in diameters of the carrier pipe and branch.

The length of the branch must be short enough to minimize the overall travel distance of the hot tap cutter and long enough to enable accurate fitting and welding.

Split reinforcing tee

The reinforcing tee is designed with a vertical split and is welded (rather than bolted) together with a lapping steel bar on top and bottom. Although the split tee is also connected to the branch with a fillet weld, there are no encirclement welds to the carrier pipe.

Since a standard vertical split tee cannot be pretested, it does not qualify as a pressure-retaining device.

TransCanada also employs a patented reinforcement system on the hot-tap assembly that prevents yielding of the pipe into the annulus area between the carrier pipe and the tee once the coupon has been removed.

This practice has significantly enhanced long-term integrity by maintaining the design geometries of the materials and reducing the internal stresses on the assembly. TransCanada now employs these assemblies as permanent fittings.

Gate valve

TransCanada's standard tie-over design uses full-bore ball valves for main line block valves and gate valves in the tie-overs. A "round-body" design is used for the gate valve to enhance its sturdiness.

In general, a pretested weld-by-flange gate valve is preferred for tapping because the cavity on the flange side will comfortably accommodate most big tapping cutters when the gate is closed.

Gate valves have an additional advantage over ball valves because a gate valve will be a little farther from the pipe wall than a ball valve during a tap and the gate chamber tends to catch errant shavings.

The end result is that gate valves are not as susceptible as ball valves to seal damage from migrating tapping cuttings. Ball valves are usually adequate for vertical applications.

Hot weld, flow design

The ideal hot weld for an operator is one that does not require system-flow restrictions. Notwithstanding the operator's needs, however, the primary objective of a welding design engineer is to produce a specification that will result in an acceptable weld.

The engineer's secondary objective is to accommodate the operator by minimizing any impacts on flow without impairing weld quality. The chemistry and cooling rate of the carrier pipe are the two critical factors for development of a valid specification. High cooling rates, in combination with poor controls, can lead to hydrogen cracking in the weld and the potential for subsequent in-service failure.

Modern pipeline steels typically have a carbon equivalence (CE) of less than 0.38 and should be compatible with parameter requirements for welding under many standard operating conditions. Pre-1960 pipelines usually have high CEs and require shut-in or restricted flow conditions for welding. Regardless of the vintage, it is always prudent to verify the CE of every operating pipeline prior to a hot weld.

The second critical factor for an acceptable weld is the cooling rate at the weld location. If a 50-mm (2-in.) diameter circle on a pipeline with a CE of 0.38 or greater cools to 100° C. from a preheated level of 250° C. in less than 40 sec, flow restrictions will almost certainly be required during the branch weld.

For every hot tap, a qualified engineer must address the carrier pipe's CE and cooling rate in development of the weld specification. Additionally, the operator must maintain system operating conditions compatible with design assumptions during the weld.

Quality control

A good quality control program for materials and procedures is vital to ensuring all components meet specifications and integrity objectives. Welders must be prequalified and technicians trained and skilled in equipment maintenance, repair, set-up, and operation.

Field staff must be familiar with documented procedures, fundamental safety practices, and quality control objectives. None of the seven hot-tap welds in the assembly are normally proven by hydrostatic test. Because six of the seven are fillet welds (Fig. 3 [71,873 bytes]) which are difficult to examine reliably by X-ray or ultrasonics, rigid adherence to welding specifications is essential. Welders with higher-than-average skill levels are also preferred, especially for work on thin-wall carrier pipes.

The critical nature of the branch weld requires its examination by magnetic-particle inspection (MPI) methods after each weld pass. The five other fillet welds are only examined by MPI when they are complete.

The branch-to-valve weld is the only one suitable for examination by X-ray or ultrasonic techniques.

With adequate preparatory work, the tapping operation can be one of the simplest parts of the hot-tapping process. A successful tap presumes that equipment is in good repair and precisely adjusted, that all welds meet specifications, and that the assembly is horizontal and square to the carrier pipe.

On site, TransCanada employs a variety of procedures to ensure a successful tap. Stiffeners are used to prevent the pipe coupon from springing flat during the tap and potentially causing the cutter to bind in the hole.

Additionally, alignment devices are used to bring the heavy cutter and pilot bit back onto the horizontal dead center of the carrier pipe. Failure to compensate for the gravity-induced deflection of the cutter could result in broken cutter teeth or cutter binding and eventual tap failure.

Future refinements

The North American high-pressure transmission pipeline industry has not universally adopted hot tapping technology for several reasons.

Primarily, a limited but noteworthy number of failure incidents has resulted in a legitimate mistrust of their long-term integrity. Hot taps also do not technically comply with code requirements in Class 1 locations. And, a large-diameter hot tap can create a pigging trap and damage electronic pigging equipment.

In response to the integrity issue, hot taps simply cannot provide an interconnection solution for all scenarios. Older installations on TransCanada's system, however, have consistently proven the integrity and value of the technology when constructed appropriately.

Recent developments in supplementary reinforcing technology have been proven to enhance further the long-term integrity of standard assembly designs. An incident-free record with older designs fortifies the conclusion that quality control and design are critical in the long-term operating risk of TransCanada's hot-tap assemblies.

Currently, national design codes only permit the completion of a hot tap in heavy-wall pipe locations. Even though the technology is compatible with the objectives of national design codes in terms of resistance to fracture propagation, no increase in societal risk, and maintenance of system integrity, designs would have to be adapted to meet the strict requirements of the codes in Class 1 pipe locations.

However, the procedure's compeling technical, environmental, and economic advantages warrant review when interconnecting in any location.

Finally, hot-tapped branches can be impediments to certain types of pigs when the branch-to-carrier-pipe-diameter ratio is large. TCHT is currently looking at technology to install pigging bars at hot-tapped branches.

The Authors

John McElligott is the vice-president and general manager of TransCanada Hot Taps, a wholly owned subsidiary of Trans Canada Pipelines Services Ltd., Calgary. He joined TransCanada in 1990 and has worked in the right-of-way and pipeline engineering departments.
McElligott holds a Bachelors Degree in Technology from Ryerson Polytechnical University and a BS from the University of Toronto.
Joseph Delanty is a mechanical millwright, recently retired from TransCanada and consultant to TransCanada Hot Tapping. In his 40 years of service with TransCanada, he was involved in more than 400 hot-tap completions.
Burke Delanty is a project leader in the technical services department of TransCanada PipeLines Ltd. (TCPL), Calgary. He joined TCPL in 1986 after receiving a BS from the University of Toronto. He is a registered professional engineer in Ontario.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.