July 2024, Vol. 251, No. 7
Features
Meeting Challenges of Safely Repairing Online Pipelines
By Darran Pledger, STATS Group
(P&GJ) — Oil and gas operators the world over face the same set of challenges, best summed up as requiring that they identify and implement the safest and most cost-effective way of maximizing hydrocarbon output.
With safety the top priority, they seek to improve efficiency and to control costs by minimizing downtime while executing maintenance and integrity work on ageing assets and infrastructure. Increasingly, those objectives must be met while reducing emissions through the elimination of venting or flaring.
Isolating and evacuating a pipeline can be costly and meeting downstream throughput requirements may restrict an operator to how and when they can perform necessary pipeline maintenance and improvement activities.
Methods of performing repair or maintenance work on a pipeline in-service include blanking or blinding a pipeline section from the pressurized system, venting and purging, or displacing the hazardous substance. Depending on the infrastructure in place, it may not be possible to disconnect and blind or blank off the pipeline, isolation valves may not be double block and bleed or do not sufficiently hold pressure.
Downstream requirements may limit the duration in which a pipeline can be out of service, among various other reasons isolating and preforming maintenance on a pipeline can be challenging.
Double Block and Bleed Regulations
The term “double block and bleed” is most used to define a level of isolation provided by valves in a pipeline or process system.
The term is used to describe a safe method of isolation from a pressurized or hazardous medium to facilitate breaching of the pressure system for maintenance and modifications. This terminology is also utilized to identify the relative integrity of temporary isolation systems.
However, although “double block and bleed” is a universally used term to specify a level of isolation, the definition of the term is by no means universal.
For example:
USA, OHSA, 1910.146(b)
“‘Isolation’ means the process by which a permit space is removed from service and completely protected against the release of energy and material into the space by such means as: blanking or blinding; misaligning or removing sections of lines, pipes, or ducts; a double block and bleed system; lockout or tagout of all sources of energy; or blocking or disconnecting all mechanical linkages.”
Alberta, Occupational Health and Safety Code (A.Reg. 191/2021): Part 15, Section 215.5(1)
“To isolate piping or a pipeline containing a substance under pressure, an employer must ensure the use of:
(a) a system of blanking or blinding, or
(b) a double block and bleed isolation system providing
(i) 2 blocking seals on either side of the isolation point, and
(ii) An operable bleed-off between the 2 seals.”
The U.K. Health and Safety Executive guidance document, The Safe Isolation of Plant and Equipment1, defines double block and bleed as “an isolation method consisting of an arrangement of two block valves with a bleed valve located in between.”
Testing and monitoring the effectiveness of the isolation is specified in the guidance document which states (ref. article 165), not must but should:
“Prove the integrity of all isolation points of an isolation scheme before proceeding with intrusive work (unless your risk assessment has indicated that the use of non-proved isolation is acceptable):
- each part of the isolation should be proved separately, e.g. prove each valve in a double block and bleed scheme
- each part should be proved to the highest pressure which can be expected within the system during the work activity. Care is required when there is a low differential pressure across valves where sealing mechanism is activated by pressure
- where possible, each part of the isolation should be proved in the direction of the expected pressure differential.”
Many global operators have developed their own guidance with regards to double block and bleed which is typically derived from the HSE guidance document HSG 253.
Hot Tapping and Line Stopping
Pipeline hot-tapping and line-stopping are two critical techniques used for maintenance, repair, and modification of pipelines without interrupting the flow of the product, reducing downtime and increasing efficiency.
Pipeline hot-tapping involves cutting a hole into a pressurized pipeline to create a new branch connection without disrupting the flow of the product. The hot-tapping process requires specialized equipment and highly skilled technicians to perform the procedure safely and efficiently.
Line stopping, however, involves stopping the flow of product through a pipeline temporarily. This technique is used to perform maintenance on a short section of the pipeline without depressurizing the entire line when incumbent valves are not available or not providing the required level of isolation. During line stopping, a temporary plug is inserted into the pipeline to prevent the flow of the product.
This allows operators to work on the pipeline in a controlled environment, with the isolated section of pipeline vented and purged reducing the risk of leaks or other safety concerns. If pipeline flow must be maintained, a standalone bypass or line stopping tools with integrated bypass can be employed.
Both pipeline hot tapping and line stopping are critical techniques used by operators to ensure the safety and reliability of pipelines. One of the primary benefits of pipeline hot-tapping and line stopping is the ability to minimize downtime and maintain product flow.
Shutting down a pipeline for maintenance or repair can be a costly and time-consuming process, and it can result in significant product losses, adverse effects to the environment, integrity threats and increased risk.
By using these techniques, operators can perform necessary work on the pipeline while the product is still flowing, reducing the requirement to depressurize, vent and purge large sections of pipeline.
Line stopping however, requires in-service welding of fittings on the pipeline, often requires land permits and excavation work which can be labor-intensive, and leaves fittings on the pipeline that pose as a potential future leak-path.
In-Line Isolation Tools
In-line isolation pigs are a piggable tool that can provide a fully proven, double block and monitored isolation that eliminates the need for in-service welding and additional fittings on the pipeline.
In-line isolation tools can also vastly reduce associated costs such as excavation, in-service welding and non-destructive examination, may eliminate land permit requirements, and reduces (or in some cases eliminates) outage time required when compared to traditional means of pipeline isolation.
Consider replacing a receiver trap valve; by using an in-line isolation pig, the tool can be loaded into the receiving trap, pigged to the isolation location just meters upstream of the valve using pipeline product, water, or nitrogen, then set in place to provide the necessary isolation.
From the time of opening the receiver valve once the outage starts to when the tool is set in place providing the isolation can take only a few hours, depending on the hold times for seal testing. Product behind the tool can be recompressed or drained to sump and the valve can be replaced in a short outage window.
On the other hand, the alternative may be utilizing an upstream mainline valve likely many kilometers away from the receiving trap to provide the isolation. Product between the upstream mainline valve would need to be purged, vented, flared, or recompressed which could take a significant amount of time and effort associated with gas purging, recompression, or product handling.
Another scenario is the use of dual in-line isolation tools to provide a mainline isolation for riser pipework repair, pipeline spool replacement or re-routing.
The isolated section of pipework can be reduced to meters compared to many kilometers between mainline valves. Furthermore, the line can remain operational as the tools are pigged to and from the target isolation location.
In-line isolation pigs also make it possible to hydro or leak test newly installed pipework immediately after isolation by adding an additional isolation module.
Energy Transition
As the oil and gas industry transitions to more sustainable energy, and investment in carbon capture and storage (CCS) and hydrogen accelerates, there is a growing focus on how existing pipeline infrastructure can be repurposed for CO2 and hydrogen transportation.
With this, the requirement for isolation and intervention technology to isolate these pipelines safely and efficiently for repair, maintenance and modifications is increasingly important. STATS double block and bleed isolation tools can provide high integrity isolation in traditional hydrocarbons as well as hydrogen and high-pressure liquid CO2 pipelines.
STATS is the only company in the world with an extensive track record of providing hot tapped isolation of high-pressure CO2 pipelines, operating at up to 138 bar.
Isolating liquid CO2 pipelines has many challenges and project success required the careful control of CO2 phase-change during venting, and the appropriate selection of steel and elastomer materials to accommodate CO2 service.
Reducing Venting
As legislation around methane emissions becomes stricter relating to CO2 emissions, emission-less isolations will become more important if not essential as part of the measures required to meet climate targets.
Pipeline operators face challenges associated with eliminating greenhouse gas emissions during intervention activities such as hot-tapping and line isolation for mainline replacement and facility upgrades.
STATS’s inline pig (Remote Tecno Plug®) and hot-tap installed isolation (BISEP) tools are contributing to reducing carbon emissions for their customers.
When using leak-tight, double block and bleed pipeline isolation technologies, localized repair and maintenance worksites can be safely isolated without the need to depressurize large sections of the pipeline, thereby avoiding the need to discharge significant quantities of greenhouse gases into the atmosphere.
In the case of large-diameter gas pipelines, this can prevent the potential discharge of thousands or tens of tons of thousands of methane emissions into the atmosphere.
In addition, emission-less temporary pipeline isolation can be undertaken when conducted alongside cross compression/recompression units, where instead of venting or flaring the gas, a mobile recompression unit transfers the gas from the isolated section into the flowing pipeline or to an adjacent pipeline, eliminating flaring and venting completely.
Innovation
STATS is committed to incorporating sustainability into its core activities to assess and measure its social and environmental impact. As the company strives to support the industry move to a lower carbon future, it will continue to innovate and develop new technologies to enable energy transition and support net-zero targets.
The United Nations Environment Programme global methane emissions report states that existing technologies can reduce methane emissions by 30% by 2030. The greatest potential for cost saving is in the oil and gas sector, where captured methane adds to revenue instead of being discharged to atmosphere(1).
The technologies the U.N. refer to don’t require innovation funding or development and have an existing track record. These technologies are available now and can have a real impact, technologies like the Remote Tecno Plug and the BISEP. Both tools are qualified and available for use onshore, offshore and subsea.
With innovative solutions and a dedication to safety, efficiency, and cost control, STATS strives to support operators in meeting their ESG goals and reduce emissions while maintaining operational integrity.
REFERENCES
- United Nations Environment Programme and Climate and Clean Air Coalition (2021). Global Methane Assessment: Benefits and Costs of Mitigating Methane Emissions.
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