Specialist engineering company, STATS Group, has completed an 18 month design, manufacturing and installation project to enable the tie-in of a fifth pipeline on Statoil’s Troll A Platform.
Statoil’s Troll A platform, located off the west coast of Norway, utilises a system of seal tubes to tie-in risers which are run down the inside of a dry concrete leg. The seal tubes were concreted in place at the base of the riser leg on the gravity base structure in 1991. There are seven seal tubes – four designed for large bore pipe up to 36” - and three designed for smaller diameter pipe up to 20”.
When the platform was commissioned in 1995, three pipelines were pulled-in through the seal tube system - two 36” pipes for the export to Kollsnes and one 20” for an import line. In 1997 a fourth pipeline was tied-in. This time the line was a 16” which utilised the second smaller seal tube, however the seal cartridge was revised to utilise a compression seal rather than the inflatable seal used in the first three operations.
Statoil approached STATS to perform an engineering study to identify the procedures and apparatus required to perform another pipeline tie-in utilising one of the remaining unused seal tubes. The pipeline is a new 62.5 km, 36” pipeline (P12) which is to be installed between Troll A and Kollsnes in Norway.
STATS primary role was to design, manufacture and install the bespoke engineering interfaces on Troll A to allow the new pipeline to be pulled-in through the existing seal tube arrangement in the gravity base structure 300 metres below sea level, whilst still providing full isolation between the inside of the platform leg (at ambient) and the seawater pressure outside (30 Barg).
Pete Duguid, STATS Group chief executive officer, said: “The project was challenging as many of the original interfaces no longer complied with current standards and risk assessments and we developed a range of new tools that would provide safe isolations for the pull-in. The biggest challenges were to provide a minimum of two independent isolations during all of the phases of the pull-in, whilst also being able to monitor the pressures between these isolations at all times to provide a safe working environment within the concrete leg 300m below sea level.”
STATS conducted a site survey of the seal tube, (S1), to determine the external and internal condition of the seal tube and associated pipework and valves. The TSU (an early version of STATS’ current Tecno Plug™) hydraulic circuits were function tested to confirm expected fluid returns during setting and unsetting operations, additionally the seals were tested to confirm their condition. The TSU had been stored inside the seal tube for the past 16 years in one atmosphere of nitrogen. STATS confirmed the TSU was fully operational while NDT testing on the associated pipework of the seal tube confirmed wall thickness prior to conducting pressure testing on the system to reduce any risks.
STATS designed a 36” Temporary Anchor as the mechanism by which any axial forces generated by the 36” pipeline was transmitted into the seal-tube and in turn into the gravity base structure. This system stops the differential loading from the water pressure acting on the pipeline forcing the pull head into the platform which in turn could damage the seal tube. The hydraulic cylinders contained sufficient stroke to ensure maximum eccentricity conditions between pipe and Temporary Anchor was accommodated.
The 36” Seal Cartridge is a unique and bespoke interface which provided the seal between the P12 pipeline and the seal tube outer casing. The Seal Cartridge contains two 1.2m diameter compression seals, each weighing in excess of 80kg. The elastomer seals provided a pressure testable barrier system to prevent seawater under pressure from entering the internal end-cap. It also provided an additional sealing function to prevent grout ingress into the internal end cap during the grouting operations. The considerable radial forces generated by the Seal Cartridge elastomers have the added benefit of assisting the pipeline to centralise within the seal tube without causing damage to the pipeline. .
STATS also designed and manufactured the Grout Bung, a subsea installed isolation tool, which provided an external seal between the 36” P12 pipeline and the 50” Troll A seal tube outer casing. The Grout Bung utilises a two part specialist seal, integrated into a large hydraulic clamp assembly with integral locks, which can be remotely closed around the pipeline then manoeuvred into its set location via an arrangement of adjustable hydraulic circuits. The whole assembly was remotely controlled via hydraulic control panels mounted on the deployment frame. The Grout Bung installation was controlled by ROV and supervised by STATS technicians and once installed the void between the Seal Cartridge and Grout Bung could be grouted, providing corrosion resistance, support and stability to the new pipeline prior to installing the internal riser components.
During the project STATS were approached by Allseas, who were responsible for the pipe lay campaign, to design and manufacture the pipeline Pull Head. STATS utilised an existing model from Allseas and modified it to interface with the de-watering isolation tool supplied by STATS. A unique interface panel integrated in the Pull Head design, allowed the STATS technicians to communicate with the isolation tool in the event of a pipeline wet buckle. The isolation tool could then be used to dewater the repaired pipeline, saving weeks of downtime.
Mr Duguid added: “All stages were successfully completed to schedule by STATS, concluding with the removal of the pull head plug which was set inside the new pipeline to provide an isolation barrier during the final riser tie-in. Additionally, the pull head plug had the capability of being utilised as a dewatering plug in the event of a pipeline wet buckle during the pipe lay operation.
“The safety of people, assets, and the environment is the main focus on all STATS Group projects and our safety record on the P12 project is testimony to those objectives.”