Conventional pipeline maintenance and modification work requires removal of fluids from inside the pipeline section where work will be performed. In order to complete this task, the pipeline must either be depressurized (blown down) or temporarily isolated.

Depressurization can be costly because the pipeline cannot stay in service during the time of decommissioning, intervention and re-commissioning, and depend significantly on factors like pipeline size, length and pressure. As a result, valuable production is lost and downstream customers may be affected. Also, the significant environmental issues associated with the removal of pipeline inventory further escalate the overall costs and complexities of the maintenance/repair process. Depressurization might not even be an option for repair/maintenance work on long pipeline sections.

An alternative to depressurization of the entire pipeline is isolation of the pipeline section requiring maintenance or repair. This method allows the pipeline to stay in service, production downtime and loss of pipeline product are kept to a minimum, with associated environmental and economic benefits.

A wide range of methodologies, both intrusive (such as hot tapping and plugging) and non-intrusive (inline isolation plugs), can be used to isolate in-service pipeline sections. Both approaches are well accepted in the industry, with the choice of one over the other being largely governed by factors such as location and accessibility of the pipeline, operating pressures, pipeline inventory, and costs.

Some maintenance/ repair operations using isolation methods require a facility shutdown. These cases often require double barriers to safeguard personnel and facility equipment during pipeline maintenance. Although the philosophy, definition, and requirements of double barriers have been described in various international codes and standards, certain misconceptions persist surrounding double barrier terminology and its application to pipeline pressure isolation tools. The objective of this paper is to clarify the concepts of double barrier isolation, and to examine how both intrusive and non-intrusive methods can be used to provide double barrier isolation that meets the accepted requirements. The paper also addresses methods that can be used when standard isolation is not practical.

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