This article details the technical and safety requirements for executing a hot tap on hydrocarbon pipelines, piping, or tanks.
Hot tapping is a method of making a new connection to an existing pressurized pipeline or vessel without interrupting the flow or reducing the pressure in a section of pipe or vessel. Hot tapping allows a pipeline to continue to operate while doing modifications or maintenance.
Another definition according to API RP 2201: Hot tapping is a technique of attaching a mechanical or welded branch fitting to piping or equipment in service and creating an opening in that piping or equipment by drilling or cutting a portion of the piping or equipment within the attached fitting.
Hot tapping has an extensive application range in the oil and gas sector. It is often difficult to shut down pipelines due to disruptions it causes in product supply to customers and other facilities. Hot tapping facilitates the tie in of new connections to an existing pipeline in operation without interruption to flow. And in combination with line plugging, it can be used to divert flow around a pipeline or piping section that is under repair or maintenance.
2 CRITICAL ASPECTS TO CONTEMPLATE BEFORE HOT TAPPING – A PLANT OR PIPELINE OWNER’S PERSPECTIVE
As a prerequisite to any hot tap operation, perform a detailed job analysis encompassing both safety and technical considerations.
A hot tapping operation is a highly safety-sensitive activity and should not be considered a routine activity. Consider the following aspects during the hot tap assessment:
- Safety. Hot tapping typically involves hot work, i.e., welding on pipelines, piping, or tanks in operation. And so, a proper hot work risk assessment must be done. Safety is the most critical element in every hot tapping operation. If the safety of the personnel and the environment is not guaranteed, then do not perform hot tapping. Any safety aspect will not be complete without considering the risk that may arise from pollution or the release of any toxic product in case of hot tap failure.
- Condition of the piping, pipeline, or tank identified where hot tapping would take place. Critically analyze the state of the equipment or piping to determine if it is safe to perform the hot tap. Do not proceed to hot tap unless there is a high degree of confidence in the condition of the plant, piping, pipeline, or any other applicable structure.
- System Configuration. The configuration of the system must be such that hot tapping can proceed without any hindrance. This is usually a more critical consideration for piping configurations and tanks than for pipelines. Where access restrictions cause an unsafe work environment, hot tapping should not continue. Surrounding environmental conditions such as soil conditions, water tables, vegetation overgrowth, unsafe trench excavation, and other factors are also part of this consideration.
- Operating conditions.As will be discussed later, there are various operational limitations to hot tapping, such as process temperature, operating pressure, flow velocity, corrosive environments, and, in some cases, the location of a hot tap,g., on a bend. The operating data of the system must be critically analyzed to ensure that hot tapping is successfully executed within these parameters.
- In-house capability consideration. Availability of suitable hot tapping equipment: The available equipment to perform the hot tap must be able to execute the task successfully. Equipment selection is crucial because selecting the wrong hot tap machine could result in a failed hot tapping operation. The hot tap machine must be able to work under the operating conditions such as temperature, pressure, flow rate, total cutting distance and an array of other aspects that will influence the outcome of this operation. Do not proceed with the planned hot tap activity if a hot tap machine that can effectively operate under all identified conditions is not available.
- In-house capability consideration. Availability of suitably trained and qualified technicians to safely execute a hot tap. As with the comment on suitable equipment to perform a hot tap, so too is the requirement for a suitably qualified and experienced technician to execute the hot tap operation. Many detailed nuances will make the difference between failure or success, none of which are apparent to the untrained eye.
- An outsourced capability consideration. Availability of suitably vetted hot tap and linestop service provider to execute the required work. Experience level, safety record, established processes and procedures, equipment range, and a host of important factors to be considered.
- The economic aspect of the hot tap operation must be thoroughly evaluated. In the unlikely event where the cost of hot tapping is significantly higher than the cost of total shutdown and performing routine maintenance, the hot tapping should be avoided. See our article on this site that expands on this economic evaluation of a shutdown vs. hot tapping.
- Welding Suitability. Can safe welding of the hot tap fitting be performed on the pipeline? Can welding be performed on the pipeline at the operating conditions and material of the tank or pipeline/piping? Determine suitability before performing a hot tap. There may be instances where a mechanical bolt-on fitting may be suitable for use. Still, these are all to be carefully considered as alternative options when looking at the weldability.
- Regulatory requirements. Carefully evaluate all regulations (be that industry codes, local bylaws, etc.) before performing any hot tapping operation and do not proceed if the regulations are not met.
3 GENERAL SAFETY RESTRICTIONS
Although, over time, there have been significant improvements in hot tapping technology, there are still several restrictions focussed on increasing the safety of the process.
It must be noted that these restrictions vary among companies and are also dependent on the capability of hot tap machines and personnel.
Typically, additional consideration should be given before hot tapping is to proceed under the following conditions:
- Pipelines, piping or storage tanks that contain a mixture of flammable gas and air
- Heat exchangers or pressure vessel
- Jacketed storage tanks or piping
- Storage tanks or piping that require Post Weld Heat Treatment (PWHT). If PWHT is needed, further analysis and consultation with professionals shall be performed
- Storage tanks and piping containing unsaturated hydrocarbon that undergo exothermic decomposition due to high temperature as a result of welding activities. Ethylene and propyleneare typical examples.
- The decks, roofs, floating roofs or pontoons of storage tanks
- Cladded pipes or tanks, unless approved after critical examination by qualified personnel. However, hot tapping may be performed on internally flow coated pipes or equipment
- Pipelines or tanks when the fluid content is below 0 Celsius (32 Fahrenheit) or above 200 Celsius (392 Fahrenheit) unless permitted by the client after critical evaluation of the tapping operation
- Equipment under vacuum, i.e., pressure less than atmospheric pressure unless a thorough assessment by engineering
- Equipment and piping containing hazardous materials. As defined in SHELL DEP 184.108.40.206-Gen, Section 3.2, Table 1, hazardous materials include Acetylene, Acetonitrile, Butadiene, Caustic soda / Sodium Hydroxide, Chlorine, Hydrogen sulfide, Hydrofluoric acid, Oxygen, Propene (Propylene), Propene oxide, Sulphuric acid.
4 DOCUMENTATION REQUIRED – EXECUTING HOT TAPPING
Before executing a hot tap, detailed planning and design shall be performed and shall require preparing different documents that are reviewed and approved by the customer and other stakeholders. Some of the documentation to prepare may include:
- Design documentation. Applicable code selection, design factor, wall thickness calculations, design and operating parameters, corrosion allowance, and additional detail to make an accurate selection of the best bespoke solution for hot tapping or line stopping.
- The customer may prepare drawings to include general arrangement drawings of the piping network, sections, and details,location of hot tap / linestop. From the hot tap service provider, you can expect design drawings with supporting calculations of each fitting needed for each particular job.
- Material Take-off. The document will aid in the procurement of all materials required to execute the hot tap successfully. It will show the necessary quantities of valves, bolts and nuts, flanges, pipes for branch connection, hot tap fitting, blind flanges, and other appurtenances.
- Safety plan and documentation, which shall include a detailed Job Safety The safety plan should, at minimum, cover escape routes, firefighting, safety equipment, gas detectors, and first aid facilities, identified risks and mitigation measures.
- Transportation procedures. The procedures shall cover how the hot tap machines, components, and personnel are transported to and from the site. Transportation considerations can become crucial if the hot tap location is remote or hard to reach.
- Site preparation procedures. For hot tap locations in congested areas, site clearing is required. In the absence of an access road to the hot tap location, temporary roads shall be created (if feasible). For buried pipelines, the excavation shall be carried out around the pipeline.
- Hot tapping procedures. The procedure shall indicate the general information about the process conditions, the activities to be performed, the resources required, reference documentation, machine operation procedure right up to the point where the coupon is handed to the customer.
- Welding procedures. The welding procedure is a critical document reflecting the type of welding, base material qualified, type of electrodes to use for welding, progression of weld pool, welding currentand travel speed.
- Non-Destructive Testing (NDT) procedures. This document usually will highlight how the welded joints’ integrity shall be validated. It may cover activities, including leak testing, UT,or MPI.
- Leak test These procedures may be part of the NDT procedure or a stand-alone procedure. This procedure will show the test pressure, medium to use for the test, duration of the test,and various other considerations to safely execute a successful leak test.
- Hot tap checklist. Complete a summary of all the activities before the commencement of hot tapping. It is usually in a tabular form with activities completed checked and the personnel responsible indicated.
- Hot tapping close-out report. After successful completion of a hot tap, the close-out report is a summary of all activities performed, duration, delays, a review of lessons learned, and company-specific requirements to allow for continuous improvement of the process.
5 HOT TAPPING REQUIREMENTS
This section highlights some of the essential requirements before performing a hot tapping operation. Although each hot tap is unique and should be evaluated on merit, these points do give a general overview.
5.1 Hot Tapping on Pipelines or Piping
Tapping into pipelines and piping is the most common type of hot tapping operation. Some requirements are:
- The pipe material and the fittings used for the hot tapping operation shall be compatible with the material of the run pipe.
- The pipes and fittings shall have a rating consistent with that of the existing run pipe as outlined in the relevant code.
- Welding procedures shall be prepared and reviewed by an experienced engineer to confirm that the welding can be successfully performed on the pipeline or piping.
- Even though the preference is performing a hot tap or linestop in the vertical position, hot tapping can be performed in all o’clock positions of the pipeline. Considerations discussed in this document will be relevant to any of these cases. Before performing an angled hot tap, a detailed technical analysis shall be completed.
- Hot tapping and welding can also be performed on flare/vent lines, but if there is the possibility of oxygen ingress, special measures need to be taken. If hot tapping is performed on flare lines, it must be confirmed that the line is free of oxygen ingress – if this cannot be confirmed, the line must be continuously purged with inert gas or steam during a welding and hot tap
5.2 Requirements for Tapping into Tanks or Vessels.
The main danger in welding on the exterior of a tank or vessel is the ignition of a flammable atmosphere in the vapor space above the liquid level.
There are additional requirements that must be fulfilled to successfully perform hot tapping into a tank or vessel containing hydrocarbon products. Some of these requirements are clearly stated in section 9.1 of API RP 2201. Below is a summary of these requirements:
- Welding on the exterior of the tanks shall not be performed unless there is a procedure on the ground to prevent flammable gas/vapor from reaching the vicinity of the welding location. Reliable gas detecting devices must be located at a strategic location. Work shall be stopped immediately when gas/vapor is detected.
- There must be mitigation for every identified possible hazard. Hazards may result from:
- Tank venting, which may circulate vapor around the location where welding is performed. These vapor results in the formation of a flammable atmosphere around the welding location.
- Overflowing of products within the tank which may result in spillage of products into the welding area.
- Inadvertently allowing the level of liquid in the tank to fall below the welding point. The welding point is not the direct point of welding alone but a defined region around the weld point. When the liquid in the tank falls below the welding point, the heat sink provided by the liquid is lost thereby exposing the vapor space in the tank to the welding point, which is a possible source of ignition.
- Welding shall not be performed above the liquid level on a cladded tank or double-walled tank unless approved by qualified personnel.
- The minimum liquid level above the point of welding shall be 3ft as recommended by API RP 2201 and ExxonMobil GP 03-01-04. SHELL DEP 220.127.116.11-Gen also recommends 39 inches.
- As much as possible, there should be no flow into or out of the tank.
- Welding shall be performed by qualified personnel, preferably with previous experience in welding on live pipelines.
- During welding, burn-through the tank or vessel wall must be prevented. Adequate precautions, including heat/temperature control, must be put in place and strict adherence to a suitably qualified WPS is of critical importance.
- Hot tapping or welding activities on floating roofs, pontoons, or decks are prohibited. These areas of a tank are subjected to flammability hazards.
5.3 Other Requirements for Considerations
5.3.1 Welding Requirements
A welding procedure shall be developed for any hot tap welding projects. All welders shall be qualified per applicable codes and specifications. Consideration may be given to welders with previous experience. As stated in API RP 2201 section 6.8, welding shall not be permitted closer than 18 inches to a flange or threaded connection or approximately 3 inches to a welded seam (including a longitudinal seam of welded piping) unless determined by an engineering review to be acceptable. All welds shall be visually inspected and tested before the connection of the hot tap machine. Dye Penetrant Testing, Ultrasonic, or Magnetic Particle Inspection (MPI) may also be utilized; however, pressure tests may be used to validate weld integrity. This topic will be discussed in much more detail in another article to appear on this website.
5.3.2 Weld Testing
Welded joints must be tested to validate their integrity. If the temperature of the piping, pipeline or tank permits it, a leak test should be used to test welded joints per applicable codes. The test pressure should be a minimum of 10% more than the operating pressure; however, if an internal collapse is envisaged, the test should not exceed 10% of the operating pressure. The test pressure may be reduced as per applicable codes. It is common practice to use Nitrogen to conduct a leak test. Normally a leak test is done when the hot tap machine is bolted onto the hot tap fitting prior to commencing, ensuring a proper assembly of the hot tap machine onto the fitting as well as a snoop test around the weld area.
5.3.3 Process Fluid Velocity Requirements
It is always preferable to perform any welding on a pipeline while there is flow, but it must be noted that this could be potentially hazardous. A well-developed WPS will allow welding to proceed on pipeline systems with zero flow, but this has to be a known factor when making the initial assessment for in-service welding and using experienced welders becomes mandatory. In a flowing pipeline the heat is carried from the welding location, reducing the risk of possible ignition. Unless determined otherwise by qualified personnel, the flow velocities below are recommended for pipelines or piping.
The minimum recommended flow velocity for gas pipelines or piping as per SHELL DEP 18.104.22.168-Gen is 1.3 ft./s, which is also the recommended flow velocity as per ExxonMobil GP 03-01-04.
For Liquid Lines, the minimum recommended flow velocity as per SHELL DEP 22.214.171.124-Gen is 1.41ft/s.
Note, when the flow rate is too high, there is a tendency of rapid cooling of the welding area, which will affect the welding process and weld quality. More heat input is required to ensure full weld fusion and higher flow rates tend to result in rapid quenching, which in turn contributes to cracking.
The maximum recommended flow velocity for gas piping or pipelines as per SHELL DEP 126.96.36.199-Gen is 30ft/s while 32.8ft/s is recommended flow as per ExxonMobil GP 03-01-04.
For Liquid Lines, the maximum recommended flow velocity as per SHELL DEP 188.8.131.52-Gen is 5.75ft/s, while 16.4ft/s is the recommended flow velocity as per ExxonMobil GP 03-01-04.
5.3.4 Pressure Requirements
All hot tapping machines have pressure ratings at which they can operate effectively. The pressure rating of the hot tap machine must be confirmed before the start of hot tapping. The operating pressure of the pipeline must be below the maximum pressure rating of the hot tap machine.
Also, the maximum pressure resulting in the pipeline during the operation should not be higher than the maximum allowable pressure. The maximum allowable pressure can be calculated from ASME B31.4 for liquid pipelines, ASME B31.8 for gas pipelines, and ASME B31.3 for process piping. Consideration shall be given to increase in temperature resulting from welding activities in calculating the maximum allowable pressure.
For safety reasons, as recommended in SHELL DEP 184.108.40.206-Gen, hot tapping shall not be performed on the pipeline when the fluid pressure exceeds 1015 psi. However, there are hot tapping machines rated for 1480 psi and more.
5.3.5 Hot Tapping Loads
External loads are a significant concern when performing hot tapping. All external load effects on the pipeline must be carefully analyzed and mitigated. When performing hot tapping, the weight of the machine and personnel performing activities may be evenly or unevenly distributed on the pipeline, which can cause pipeline failure. All lines to be hot tapped must be adequately supported to counter the effect of external loads. Also, when performing this analysis, the corroded or as-is state of the pipeline should be used to perform the analysis.
5.3.6 Minimum Wall Thickness of Pipeline (Run Pipe)
Before performing a hot tap on a pipeline, piping, or tank, the wall thickness shall be confirmed. This may be confirmed utilizing Ultrasonic Thickness measurement.
The measured wall thickness shall be used to run an analysis to confirm that the pipe can withstand all loads resulting from operating conditions and external loading resulting from hot tapping.
Also, the thickness of the pipeline goes a long way in influencing the success of the hot tapping operation. For pipelines with thickness less than ¼”, additional investigation into the welding parameters is recommended to address the concern of possible burn-through. However, this is mitigated by a properly formulated WPS, PQR, and suitably experienced and qualified welders.
Hot tap welding or in-service welding should only start after wall thickness verification.
5.3.7 Minimum Thickness of Branch Connection
The new pipeline to be connected using hot tapping shall be analysed for all loads after its wall thickness has been calculated. Generally, the wall thickness may be calculated as per ASME B31.4 for liquid pipelines, ASME B31.8 for gas pipelines, or ASME B31.3 for process piping.
5.3.8 Temperature Considerations
Allowable temperature shall be considered for hot tapping machines, fittings, and pipelines.
High temperature applications are made possible with the use of specially designed and manufactured cooling spools.
The maximum temperature resulting from the hot tapping operation shall be critically examined by qualified personnel. Also, the minimum surrounding temperature should be evaluated prior to commencement of welding activities. As recommended by API RP 2201 Section 6.7, welding should not be performed when the ambient temperature is colder than -50oF (-45oC) unless special precautions are taken. These precautions include providing temporary shelter for welding, providing space heaters.
Welding procedures shall be developed, indicating if preheating is required, the electrodes to be used for welding and a carefully considered welding procedures is a key component to a successful completion.
The maximum inner wall temperature resulting from welding shall be calculated. The maximum temperature shall also take into account the reaction of the fluid transported.
5.3.9 Hot Tap Location
Selecting the location of the hot tap is crucial. Hot tap locations shall be readily accessible and free of obstructions.
The other considerations may include the proximity to a weld seam or pipe fitting as there are numerous guidelines governing the hot tapping through a welded pipe seam or the spacing between 2 welds on a pipe.
5.3.10 Hot Tap Valve or Block Valve
Hot tap service valves shall be appropriately rated to suit the branch connection required. The rating should consider pipeline pressure, temperature, line content and in cases whether or not a full port valve is to be used. For linestop applications, linestop valves should be used; however, gate valves are also a viable option. All valves, bolts, nuts and gasket shall be new because they may form part of the system. All valves shall be tested as per applicable codes and standards, including a test for seat leakage.
5.3.11 Hot Tapping Machine
Hot tapping machines will be of appropriate rating in terms of temperature and pressure. It is required that the hot tapping machine must be centred on the valve and aligned with the fitting to ensure uninterrupted cutting of the coupon.
One key factor that must be considered when selecting a machine is the Total Travel Distance of the machine. This is specific to each machine.
Before the start of hot tapping, it shall be confirmed that the selected machine can travel the required distance from the hot tap valve until the entire coupon is cut.
Total Travel Distance may be calculated using the formula below.
Typical Hot Tapping Checklist
As stated in previous sections, hot tapping is a high safety-sensitive operation requiring multi-disciplinary activities. Before commencing the operation, some considerations shall be fulfilled. These considerations are clearly stated in API RP 2201 and other companies’ specific regulations.
The Checklist below is an extract from API RP 2201; therefore, all referenced sections in the Checklist are to API RP 2201. The attached Checklist should be modified to suit each hot tap job to be performed.
API RP 2201: Safe Hot Tapping Practices in the Petroleum & Petrochemical Industries
ASME B31.4: Pipeline Transportation Systems for Liquids and Slurries
ASME B31.8: Gas Transmission and Distribution Piping Systems
ExxonMobil GP 03-01-04: Hot Tapping
SHELL DEP 220.127.116.11-Gen: HOT-TAPPING ON PIPELINES, PIPING AND EQUIPMENT