The right combination

Darran Pledger, STATS Group, UK, explores how combining equipment can lead to safe and efficient isolations for subsea pipeline repair and maintenance.

The global oil and gas industry is reliant on the long term dependability of pipelines. Pipelines are vital to the safe and efficient transportation of hydrocarbons and failures of these pipelines can have huge energy supply, environmental, safety and reputational implications. As the global demand for energy increases, so do the consequences of any potential pipeline failure.

In many instances, pipeline operators encounter difficulties when attempting to isolate and depressurise a section of their pipeline to facilitate repair or maintenance activities, especially when suitably located valves are not available or are not functioning correctly. Often it is the isolation valves themselves that need to be replaced, repaired, or retrofitted.

There are many reasons for removing a section of subsea pipeline. Sometimes a degraded or corroded section needs to be removed to increase the operating life of a pipeline. Other reasons for compromised pipeline integrity could be dents or gouging from external mechanical damage or highly stressed deformations. Although, more often the situation is that even well managed pipelines with good integrity need to be isolated and have sections removed, to allow infrastructural reconfiguration, as offshore facilities are being decommissioned and removed.

When a pipeline section must be removed, it may initially be assumed that to conduct the intervention with as low risk as possible, the entire pipeline needs to be fully depressurised, thus removing any pressure threat, and that the pipeline needs to be flushed and flooded to displace all hydrocarbons, so removing any environmental impact.

However, in many cases achieving the lowest possible risk is not reasonably practicable; for instance, when the pipeline has many assets producing into it, or when venting off the line pressure, flooding the pipeline and then dewatering and drying the line, would lead to a production outage of many months and large discharges to the atmosphere or environment.

If it turns out that it is not reasonably practicable to depressurise and hydrocarbon-free the pipeline, then it may be necessary to break containment with the pipeline pressurised and full of its production inventory. In such a scenario a safe, pressure capable

isolation barrier complying with industry isolation standards and recommended practices will be required – typically this is a proved double block and bleed barrier.

Subsea tie-in and pipeline reconfiguration To enable new field developments the most cost-effective solution may require a tie-in or connection into existing subsea pipeline infrastructure. When no pre-installed tie-in point exists, it will be necessary to install a new tie-in connection into the pipeline. Ideally the new tie-in connection can be done by disconnecting a pipeline flanged spool and replacing it with a pipeline tie-in, such as a tee or wye spool. However, in some cases a suitable removable spool is not available at the required location, and a tie-in connection must be retrofitted, either by fitting a tie-in clamp and performing a hot tap or by removing a section of the pipeline and installing a new tee.

The strategic use of STATS’ piggable Remote Tecno Plug® isolation tool and SureTap® hot tapping equipment facilitated a recent subsea pipeline reconfiguration in the North Sea. The pipeline infrastructure redevelopment was required as two offshore platforms were being negatively affected by seabed subsidence, and redevelopment activities would enable economic production of the fields’ tail-end production profile. While the platforms were being redeveloped, the production pipelines needed to be disconnected from the platforms to enable efficient reconnection of the 26 in. and 22 in. production pipelines to the platforms’ Subsea Isolation Valve Skids. The subsea pipeline reconfiguration was done without depressurising the entire 100 km long 26 in. gas export pipeline, operating at 85 bar. The use of a Remote Tecno Plug isolation tool provided a fully proved double block and monitored isolation which enhanced safety, reduced the project execution time, and led to significant cost savings.

In order to provide fuel gas to the platform during an extended shutdown a new 8 in. pipeline was tied-in to a 24 in. pipeline using a class 900 mechanical hot tap fitting. STATS supplied the mechanical hot tap clamp and SureTap hot tap machine to cut an 8 in. hole into the 24 in. pipeline and recover the pipe coupon. The 8 in. pipeline was then successfully connected to the tie-in valve safely completing the tie-in workscope.

New field development In 2021, STATS were contracted to provide a 12 in. tie-in point into an existing 18 in. pipeline to allow a new gas field to be brought online in the Bass Strait, offshore Australia. To enable this, STATS provided a subsea mechanical clamp and SureTap hot tap machine. The clamp was designed and manufactured based on the pipeline parameters and to the client’s exact specification. The 18 in. clamp was designed to class 900 with the 12 in. branch designed to class 1500.

STATS mechanical clamp design incorporates dual seals and locks – the locks are mounted outside the pressure boundary, which positions the locks away from the pipeline contents and provides optimal loading to the pipeline. Clamp seal compression flanges were initially actuated to set the seals, which are energised against the pipe wall, without setting the clamp locks. This allowed a pressure test of the dual seals, via the annulus cavity, which confirmed both seals were leak-tight and the dual seal pressure boundary was sound. Once the seal test was successfully completed the structural grip locks were set. Independently activated lock segments ensure that each lock fully engages onto the pipe wall. This feature is particularly beneficial on pipelines that are oval, ensuring full structural gripping and reinforcement of the pipeline. A final seal test ensured the clamp integrity prior to hot tapping into the pipeline.

Gareth Campbell, Regional Manager for Asia Pacific at STATS Group, said: “The subsea tie-in was safely completed while the pipeline remained at 72 bar and provided the new tie-in point for the 6 km flowline. The project was successfully completed without incident and is testament to the teamwork and good communication between all parties.”

Subsea pipeline repair During the offshore installation phase of a new field development in the North Sea, spool pieces where installed and the new piping infrastructure was leak-tested between the subsea isolation valve and platform riser. While conducting the leak-test, damage was discovered on the sealing face of the platform riser base flange, allocated for the tie-in of the pipeline. At this stage the operator put plans in place to carry out an in-situ repair of the damaged flange. However, in case the repair was unsuccessful a contingency solution was developed. This allowed the damaged flange to be cut and removed and replaced with a 14 in. class 2500 mechanical flanged connector, installed directly onto the bare pipe end. This would ensure the schedule for commissioning the new tie-in would not be jeopardised if the subsea flange repair failed.

Figure 1. SureTap ST910-90 hot tap machine and 18 in. mechanical hot tap clamp class 900 with a 12 in. branch designed to class 1500.

Figure 2. 14 in. class 2500 mechanical connector.

Figure 3. Remote Tecno Plug inline isolation tools.

STATS provided two mechanical connectors as, for technical assurance, one of the connectors was subject to bend fatigue testing, and as the fatigue test was a test to destruction, a primary connector could not be used. A factory acceptance test was carried out first at STATS headquarters in Aberdeenshire, Scotland, before the connector was shipped to a third party for the bend test, with all testing witnessed by an independent verification body and the client.

In operation the connector is offered onto the pipe end and a seal compression flange is actuated to set the seals. The seals can then be pressure tested to prove the effectiveness of both seals prior to setting the locks. Once the seals are verified, the lock bowl is actuated, by torquing a separate array of bolts to set the locks onto the pipe. A final localised code-compliant strength test and seal leak test verifies that the connector is structurally secured to the pipeline and the seals are leak tight. An environmental seal is included in the compression flange to allow the lock cavity to be flushed with an inert fluid.

Subsea pipeline isolation and repair When an 8 in. gas condensate export pipeline had been dragged out of position by a vessel anchor and needed repaired in the Gulf of Thailand, STATS were contacted to provide an isolation and intervention solution. The pipeline was situated at a water depth of 60 m and was operating at pressure ranging between 7 and 21 bar.

STATS utilised its range of subsea products to facilitate the repair, which included hot tap installed BISEP’s, slab valves, hot tap clamps, completion plugs, end connectors and abandonment plugs. This comprehensive suite of tools was engineered, manufactured, tested and deployed in-field, allowing successful pipeline repair to be completed.

Following a detailed engineering and risk assessment, a repair methodology and solution was developed, whereby an 8 in. bypass was installed on the seabed as a permanent repair to re-route the pipeline medium away from the damaged pipeline section.

The isolation and repair solution allowed the damaged section of pipeline to be isolated and safely removed from the system, with no impact to production, the environment or risk to diver safety.

STATS’ patented BISEP was selected to provide leak-tight isolation of the pressurised pipeline. The BISEP is deployed through a full bore hot tap penetration at the isolation location through a mechanical hot tap clamp. The DNV type approved BISEPs provided a fully proved, fail-safe, double block and bleed isolation barrier from the pressurised pipeline. This level of isolation complies with industry guidance on isolation and intervention for diver access to subsea systems, ensuring diver and worksite safety. The pipeline repair solution incorporated installing a permanent bypass pipeline, which allowed the operations to be completed without the need for shutting-in pipeline production or depressurising, thus, ensuring production was maintained uninterrupted.

In order to provide a barrier to the marine environment for long term wet storage of the damaged pipe section, STATS supplied three 8 in. class 150 subsea abandonment plugs. The mechanically activated abandonment plugs incorporate a set of taper lock grips and a single seal.

Once the damaged pipeline section was cut away, the bare ends of the original pipeline were left exposed. To provide a pressure competent termination, STATS supplied subsea mechanical connectors, complete with a blind flange that were fitted to the three bare pipe ends.

Pipeline decommissioning In recent decades, the North Sea and Gulf of Mexico have seen an increase in decommissioning activity as hydrocarbon reservoirs are depleted. As oil and gas wells and installations developed from the 1970s onward reach the end of their useful lives, decommissioning hot spots are now emerging around the globe. Over the coming years, hundreds of offshore assets are scheduled to be decommissioned worldwide.

As fields reach the end of their economic life, specific parts of the pipeline system naturally become redundant, and with no potential future use, are required to be decommissioned. Isolation plugs and intervention equipment can be used to safely and effectively decommission, abandon or re-route hydrocarbon pipelines and infrastructure.

Conclusion STATS pipeline isolation plugs (Tecno Plug and BISEP) are Type Approved and fully certified by DNV to verify that the design criteria satisfy the requirements for pipeline isolation plugs to provide dual seal and isolation in accordance with Offshore Standards; DNV-ST-F101 (Submarine Pipeline Systems) and recommended Practices; DNV-RP-F113 (Subsea Pipeline Repair). These devices provide the highest level of safety for divers and protection to the environment during subsea pipeline repair and maintenance.

“STATS pipeline isolation, intervention and repair technology provides clients with high integrity, cost effective solutions, which increase levels of safety and reduce system downtime during subsea pipeline repair and maintenance” said Ron James, Sales Director for STATS Group. “Our extensive global track record and commitment to first-class delivery, provides operators a turnkey service for pipeline repair and maintenance on piggable and unpiggable pipelines.”

Often, utilising a single product line cannot on its own achieve the desired end state, but when inline isolation plugs and hot tap installed plugs are used in combination, safe, pressure competent access points into piping systems and pipelines can be made, allowing the operator to safely isolate or disconnect infrastructure from hydrocarbon production.