Work is planned to re-route the Frigg Transportation System pipeline which carries gas from UK and Norway fields into the UK St. Fergus gas terminal near Aberdeen, Scotland.

In order for Total to decommission the Frigg field installation, the 24-in. gas pipeline from the Alwyn field has to be diverted to directly connect to the 32-in. Frigg pipeline. It is currently routed over the Frigg TP1 platform to gain access to the Frigg line 1 pipeline system.

The solution adopted for this diversion is a pigging skid which allows the pipeline system to be diverted in a way which:

• Provides the pipeline bore change in a removable pipe section;
• Minimizes pipeline flooding for installation operations;
• Allows local dewatering and recommissioning of the pipe system at installation;
• Provides full pipe bore functionality when in operation;
• Provides the ability to handle pigging operations of both pipeline sizes;
• Provides a smooth transition from 24-in. to 32-in. pipelines for dual diameter pigs;
• Provides a bypass path for the product during pigging operations;
• Provides an expansion facility for three additional inlets to the system; and
• Provides restraint to the pipework to allow flange pipe connections in the structure.

The bypass joins a 24-in. pipe from Alwyn to the 32-in. Frigg system. Currently both systems are piggable separately to receiver/launchers on the TP1 platform. If a direct bypass was installed, then the full system could only be pigged using dual diameter pigs. This requirement is complicated further by the Bruce field Wye connection requiring all pigs passing this location to be Wye piece piggable. By installing the bypass with a removable transition piece each section of the pipeline can, if required, be pigged alone with production maintained via a cross over arrangement. With the removable transition piece in place the system has the same functionality as if the bypass was directly-installed.

During the installation operations the pigging skid has several advantages over a direct welded-in bypass. A high pressure isolation tool (HPIT) is planned to be pigged into place from the TP1 platform approximately 1.8 miles (3 km) from the bypass location. It will be set to provide isolation from the pressure and content of the pipelines. Once the bypass is installed, each leg of the pipeline can be brought back into service by a dewatering and recommissioning train placed in the pipe ahead of the isolation HPITs which are released and run into their appropriate receivers for recovery.

This has eliminated the need for development of 24-in. dual diameter and Wye piggable HPITs installed in the 24-in. line and subsequently removed by pigging all the way to St Fergus - a distance of 237 miles (380 km). During this removal run the 24-in. HPITs would have been in a 32-in. pipeline. They would also have had to pass through the Bruce Wye and all areas of risk for the pig sticking, which would have caused potentially severe delays to start up of the many fields connected into the Frigg trunk line.
In providing the ability to carry out pigging operations at the skid, intelligent pigging operations can be carried out with existing technology and without the need to develop specialist dual diameter Wye piggable units for future integrity assurance.

The skid design has incorporated three blanked off future tie-in locations, two of 12-in. diameter and one of 16-in. diameter. These tie-in locations are "hot tap" tees - tees with an integral steel closure membrane beneath the branch flange. This provides the infrastructure to directly bolt on a valve and hot tap tool to form the tie-in. The protection structure has also been designed to provide space for the future tie-in pipework and pipework entry porches into the structure.

Being a gas line, Total specifications require all non-restrained joints of 24 in. or greater in diameter to be hyperbarically welded. This has driven the requirement for single part spools for the tie-in spools from the skid to their tie-in location to the pipelines. It was made possible by sitting the skid in or near a crossing point of the existing pipelines where the distance between the pipelines is approximately 180 ft (55 m).
The skid has been developed from its original concept to be installed from a Dive Support Vessel. The unit will be installed in four main parts, the base frame, 24-in. piping unit, 32-in. piping unit and the roof structure, with modular lifts no greater than 90 tonnes. It was due to be installed by the DSV Toisa Polaris in late May with the heaviest module being 80 tonnes.

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