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welding in-service (hot tap) pipelines
The ability to weld onto in-service (Hot Tapping) lines enables both the installation of repair sleeves and hot-tap branch connections without interruption of flow. While there are economic benefits to this practice, certain issues must be addressed to ensure that public, environmental, and worker safety is maintained both during and after welding.
Specifically, high cooling rates occur when welding onto an in-service pipeline because the flowing contents quickly remove heat from the pipe wall. These cooling rates promote the formation of hard heat-affected zone microstructures, making these welds susceptible to hydrogen cracking during or soon after welding, and to sulfide stress cracking in subsequent sour service.
The practice of welding onto in-service pipelines (hot tapping) is not uncommon. Nevertheless, there is a need to standardize how in-service weld repair procedures are qualified and selected. The use of properly qualified procedures provides a cost-effective solution while still minimizing the risk of hydrogen cracking by eliminating the high costs associated with using unnecessarily complex procedures.
A group-sponsored project completed by EWI on this topic included the input of gas transmission pipeline operators, diversified oil companies, and service providers. The results of the project (in-service welding guidelines) are available to the general industrial community.
In-Service Welding Guidelines
The guidelines contain a compendium of welding procedures which are documented by both Welding Procedure Specifications (WPSs) and Procedure Qualification Records (PQRs). These procedures were qualified to the requirements of API 1104, API 1107, ASME Section IX, BS4515, BS6990, and CSA Z662. The procedures were developed using both laboratory mock-ups and full-scale pipes, and were validated by making field welds onto live gas pipelines. They apply to both hot tap branch connections and sleeve repairs (both the fillet and groove welds). Procedures are also provided which satisfy the 22 HRC requirement of NACE MR0175 for sour service.
The welding procedure for a particular situation is determined based upon four factors:
(1) the hydrogen level of the welding process
(2) the pipe wall thickness
(3) the expected cooling rate of the weld
(4) the chemical composition
Methods for determining the cooling rate and chemical composition of an in-service line are provided. Guidelines are provided for selecting the least complicated of the qualified procedures (i.e., the least expensive to execute) required for a particular situation. These procedures rely upon the following factors, singularly or in combination, to produce satisfactory welds: heat input control (between 15 and 40 kJ/in.), preheat temperature control at 200°F (for thick wall pipe only), a temper bead deposition sequence, or use of austenitic weld metal as a hydrogen “sponge.” Procedures have been developed for the following range of conditions:
- Any steel having a carbon equivalent (per IIW) of 0.50 or less
Any diameter pipe operating at any flow rate
- Any combination of pipe wall, branch wall, and sleeve wall thickness
- Both GMAW and SMAW using both low hydrogen and cellulosic electrodes
- Welding both up-hill and down-hill
- Any gas or liquid pipeline contents
Cost Incentive
Benefits attributable to welding onto in-service pipelines arise from both uninterrupted product flow and from using the most economic welding procedure available.
Uninterrupted Flow: Maintaining product flow eliminates the income lost through non-delivery. Furthermore, this practice eliminates the cost and potential environmental impact of draining liquid pipeline contents and venting gas to the atmosphere. Some companies have arbitrary limits that prohibit in-service welding for certain applications (e.g. for materials with a carbon equivalent above a certain level). If the use of these guidelines allows in-service welding where it would have otherwise been prohibited, the cost of the guidelines could be more than offset by a single application.
Best Welding Practice: The information provided by this offering will enable recipients to minimize costs associated with in-service welding. Employing these qualified procedures can significantly reduce the time required for welding and the potential for failures attributable to poor welding practice. In the past, three widely publicized incidents associated with poor in-service welding have resulted in loss of life, destruction of facilities, and a significant product spill in a residential area, resulting in a repair program with costs exceeding $6 million.
How to Obtain In-Service Welding Guidelines
These in-service welding (hot tapping) guidelines are available to EWI members for $12,500 (US) and to non-EWI members for $16,000 (US). The revenue generated by this offering will be used to perform work aimed at extending the scope of applicability of these procedures. The results of this work will be provided to purchasers of these guidelines at no additional charge.
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Laboratory Testing
