Bespoke Engineering For Complex 36" Pipeline Pull-In, Troll Alpha

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. Twin hydraulic circuits were utilised and as the Temporary Anchor is capable of restraining the maximum pipeline load of 350 tonnes on one hydraulic circuit, the system provided a built-in 100% back-up. The lock contact load created by the cylinder pressure is translated into a significant radial load. This load is transmitted into the pipe outer wall via a toothed profile lock contact surface. This set condition creates the internal locking mechanism. Inward movement of the pipeline when the locks are set further enhances the grip on the pipeline due the self-energising design of the temporary anchor.

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. Again, as with the Temporary Anchor, the Seal Cartridge utilised twin hydraulic circuits capable of setting the seals independently, thus the system provides a built-in 100% back-up.

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.

The Seal Cartridge also provides two 1” contingency grouting lines designed to be utilised in the event of an unplanned shutdown resulting in the existing platform grout system becoming blocked and unusable.

The Grout Bung is a bespoke subsea installed isolation tool, designed and manufactured by STATS to provide 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 is remotely controlled via hydraulic control panels mounted on the deployment frame. The Grout Bung installation is controlled by ROV and supervised by STATS technicians. Once installed, the void between the Seal Cartridge and Grout Bung can be grouted, providing corrosion resistance, support and stability to the new pipeline prior to installing the internal riser components.

The Grout Bung, like the Seal Cartridge, also provided additional contingency grouting lines designed to be utilised in the event of further unplanned shutdowns if the Seal Cartridge grouting system became unusable. The picture below shows the complete Grout Bung, including the spreader bar used to stabilise the assembly during deployment and recovery offshore.

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.

Over 10 additional bespoke interface tools were also designed and manufactured by STATS to assist in the pull-in of the new 36” pipeline. These included equipment for removing the external seal tube door and communicating with pre-installed equipment more than 16 years of age that were inaccessible by technicians during the project.

All stages were successfully completed to schedule by STATS and the 18 month project concluded in August 2011 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.