The process of in-service welding is frequently used in the repair, modification and extension of high-pressure pipelines.
A common technique used in this application is ‘hot tapping’, which facilitates the creation of connections within existing pressurised vessels, pipelines and networks without the need to empty any of these pressurised systems. In-service welding is also commonly used during pipe maintenance procedures, including installing sleeves to repair damaged sections within pipelines.
A thorough understanding of the factors that affect welding onto in-service pipelines helps avoid pipeline shutdowns and interruptions of service, thereby bolstering both economic and environmental benefits for operators and welders alike. Moreover, repairs can be undertaken efficiently, effectively and with full confidence.
According to Alistair Forbes, Weld Australia Technology Operations Manager, “The advantages of in-service welding are significant and wide-reaching, particularly in the gas & oil industries, where the process can be used to avoid costly shutdowns and service interruptions and maintain continuity of supply to customers.
“In comparison, in situations where in-service welding is impossible, sections of the pipeline undergoing repair or replacement must be sealed and degassed before welding can commence, and must then be repurged once welding is complete. This complicated process is not only costly and time-consuming, it is damaging to the environment, with considerable greenhouse gas emissions a frequent result.”
The ‘hot tapping’ procedure
Pipelines used to transmit natural gas require constant expansion and modification. In the past, making new connections meant sections of the pipeline had to be shut down and purged. This created a number of problems, ranging from the release of methane to the shutdown of service at the customer end, as well as the loss of sales and increased costs associated with the need to evacuate sections of the system.
Hot tapping allows the pipeline to remain in service. The procedure involves the attachment of a branch connection and valve on the exterior wall of the pipeline. Once this connection is attached, the pipeline wall can be cut out within the branch and wall sections can be removed through the valve. Hot tapping is not a novel procedure, but design improvements have significantly reduced the uncertainty that was associated with hot tapping. In most cases, the gas savings achieved by using the hot tap method are enough to make it more financially viable than using the old shutdown process for making new connections.
Decision makers need to answer four key questions when undertaking a cost-benefit analysis of hot tapping:
- Is the parent line in good condition?
- What would it cost to shut down a line, or a section of a line?
- How much will the hot tap procedure cost?
- What are the gas savings benefits of using the hot tap method?
In-service welding considerations
There are several factors that can make in-service pipeline welding difficult.
The first factor is that the gas or liquid flowing through the pipeline can cause a considerable loss of heat through the wall of the pipe. This, in turn, causes the weld to cool at an accelerated rate. Depending on the type of steel used to construct the pipeline, rapidly cooled welds can increase weld hardness, as well as the likelihood of cracking in the heat-affected zone (HAZ).
A second factor that can cause issues during in-service welding occurs when the strength of the pipe wall is reduced during the welding process. If the wall of the pipe becomes too weak, the structural integrity of the pipe can be compromised, causing the wall of the pipe to burst under the greatly increased internal pressure. This is also referred to as ‘burn-through’.
Both of these problems can be exacerbated by the steels most commonly used to construct pipelines in Australia. Australian pipeline fabricators often use high yield strength steels. X70 and X80 steels often have walls as thin as 5mm, which makes in-service welding difficult due to the increased risk of both accelerated weld pool cool down and burn-through during welding. These high strength steels are also especially susceptible to increased hardness at accelerated cooling rates.
Reducing the impact of in-service welding challenges
There are a number of ways these difficulties can be overcome to make in-service welding a viable option. By increasing the welding heat input, the effects of accelerated weld cooling rates are decreased. Increasing the heat input can, however, increase the chance of burn-through as weld penetration does increase with increased heat input.
To mitigate these factors, welders must ensure that the most suitable weld procedure is used, the HAZ hardness is not high enough to cause cracking, and heat input and penetration are correctly balanced to protect pipe wall integrity.
Weld Australia pipeline repair seminar
Facilitated by Weld Australia, the upcoming Pipeline Repair, Hot Tapping and In-Service Welding Course will include a review and critical analysis of available thermal analysis models, including the original Battelle model, the heat sink capacity method and the PRCI thermal model for hot tap welding.
Course attendees will learn why these models, while useful as planning tools, should not be regarded as ‘magic bullets’ against hydrogen cracking in hot tap welding. This course will give an unbiased analysis of the best strategies for avoiding burn through and the development of crack susceptible weld microstructures.
The two-day course will also cover the latest defect assessment methods for pipeline engineers and managers, from simple, quick assessments through to more detailed ‘fit for purpose’ analysis.
The course will be presented by William A Bruce, Group Leader -Welding Technology at DVN GL. With a 35-year career in pipeline welding research and its practical application, Bill is an American Welding Society representative on the American Petroleum Institute API 1104 Committee and is the Chairman of the Maintenance Welding Subcommittee. He has received numerous awards, including a Distinguished Researcher Award from the Pipeline Research Council International.
The Pipeline Repair, Hot Tapping and In-Service Welding Course will be held in Sydney from 23 to 24 October. For further information and to register, please visit: