Engineers at STATS Group had a hat-trick of challenging issues to overcome when commissioned to isolate a 42” subsea pipeline.
The dead leg pipeline was housed within an oil storage tank and the Middle East NOC owner was concerned the integrity of the pipe was suspect after identifying irregular flow characteristics.
The Operator was concerned that unless the pipeline was successfully isolated, that over time the problem would worsen, with the potential for water contamination of the oil export.
Aberdeenshire-based STATS Group had to overcome three main challenges. Firstly, as the pipeline was constructed of thin wall pipe, consideration needed to be given to ensure the weight of the isolation plug would not pose a risk of rupturing the pipeline during the deployment process.
Secondly, the isolation plug is required to provide leak-tight isolation for at least 25 years, so suitability of seal material for long-term deployment was essential.
Finally, the storage tank had to remain fully operational throughout the workscope, so STATS had to ensure that the subsea deployment, setting and testing of the isolation plug, could be conducted by divers within a strict 14-day window.
The 42” dead leg is connected to a 96” Pump Plenum, housed within the oil storage tank which supplies suction to three topside pumps and the main 36” NB feed transfer line. The solution required the provision of a secure isolation which could be deployed subsea via divers into the 42” dead leg through an open flange entry point.
The isolation plug would need to negotiate two mitre joints and travel up a five-degree incline before reaching its final isolation position 38 metres away, directly ahead of the Plenum weld. A triple seal mechanical isolation plug was chosen and provided the option to remove the plug at a later date, if required.
STATS proposed the use of a modified Tecno Plug®. These non-intrusive inline isolation tools have an extensive track record of providing pressurised isolation of onshore and subsea pipelines in sizes from 3” to 48” and in a variety of applications.
Due to the thin wall pipe of the dead leg, a standard Tecno Plug would be too heavy to deploy safely on this occasion, therefore, the plug was re-engineered to reduce the overall weight.
As standard, STATS Tecno Plugs feature two elastomeric seals, the dual seal configuration providing an annulus void which is pressure tested to verify both seals are leak-tight. However, as a contingency measure a third seal was incorporated into the design, this additional seal provided a facility to fill the seal annulus with grout if additional sealing was required.
Once the Tecno Plug was deployed and set at location, each seal was leak tested at 110% of the maximum potential isolation pressure in turn, and once the seal integrity was proved, the annulus was then vented to ambient to create a zero-energy zone, providing effective double block and bleed isolation.
The large section elastomer seals are highly compatible with poor pipe surfaces, particularly corroded pipework, ensuring a leak-tight seal even in ageing assets. Due to the sealing capabilities of the Tecno Plug the additional contingency measure of grout-filling the seal annulus was not required.
As an additional fail-safe feature, a mechanical locking mechanism was incorporated into the plug design. Once the Tecno Plug was hydraulically set and following the successful seal integrity tests, the plug was locked in the set position with the aid of a ratchet mechanism, providing a further mechanical lock in place of the hydraulics to permanently set the plug once abandoned in the pipeline.
To allow the Tecno Plug to reach the desired isolation location, STATS engineered a hydraulically-actuated deployment frame to allow stem bars to be installed in sections, pushing the Tecno Plug to location in a controlled manner. Flexible control lines attached to the rear of the Tecno Plug provide communication to set the plug, test and monitor the seals.
Once the Tecno Plug was at location and successfully set and tested, the stem bar deployment system was attached to a specially adapted blind flange and bolted to the open flange, successfully completing the isolation and abandonment workscope within the tight deadline.
Prior to the offshore deployment, STATS conducted a client witnessed Factory Acceptance Test at their operational base in Musaffah, Abu Dhabi, which also included two-days of diver familiarisation training on a full-scale replica pipe fixture.
This case study was first published in the February 2017 edition of World Pipelines
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