Flare gas recovery system saving 23 MMcm/year on Gullfaks

Field CO₂ payments cut by NKr 20 million

Schematic of UPT's gas recovery and flare ignition system. [53,229 bytes]
Flare gas recovery is becoming a growing market in the Norwegian sector of the North Sea. Traditionally the flare, visible from afar, symbolizes oil and gas production, but the new system is changing this potent image by doing away with flaring most of the time.

Umoe Process Technology says its own flare gas recovery system is playing a role off Norway. A number of advantages, both economic and environmental, accrue from the system, according to Åge Roar Wallentinsen, UPT's section head for technology and development.

The process enables the volume of carbon dioxide and nitrogen oxide emissions to the atmosphere to be substantially reduced, which in the Norwegian sector also means a significant decrease in carbon dioxide tax payments. Typical annual savings on Gullfaks A, the first platform on which the system was installed, are on the order of NKr 20 million.

The recovered gas also has value, whether it is used on the platform as fuel or for gas injection, or sold to a third party and exported. Gullfaks A now exports in a year 23 MMcm of gas that would have been flared previously.

The flare gas recovery system can either be retrofitted on existing production platforms or designed into the facilities for new ones. Statoil, which participated in the development of the system, has retrofitted it on the Gullfaks A and C platforms and on the Heidrun tension-leg platform, while Norsk Hydro has done the same on the Oseberg A platform. It has also been delivered to two production ships still under construction, Statoil's Åsgard A and Saga's Varg.

Also, the system is to be installed on the Åsgard B semi submersible platform, Hydro's Oseberg South and Oseberg Gas platforms, the Troll C semisubmersible platform, and Amoco's Valhall process platform.

Flaring process

By providing an outlet for excess gas, flaring is an important part of the safety system on an offshore platform. If uninterrupted flaring is to be discontinued, there must be a totally dependable system for resuming it when needed. The gas recovery system requires, then, not only a wholly reliable flare ignition system but also that platform personnel be convinced that this is the case. On Gullfaks A there was initial scepticism about the system, which gave way to confidence as it was shown to work.

The pellet is fired from an airgun powered by nitrogen under 200 bar pressure. The pellet contains two components - primer and zirconium. As the pellet hits the target plate at 200 meters/sec, the two components interact, causing a shower of sparks which stay alight for up to four seconds. There is no downfall of material as the zirconium burns up completely.

Two pellets are always fired. The first is sufficient to ignite a big release of gas through the flare system, such as is caused by a compressor trip. The second pellet, which is released typically 15 seconds after the first, is required for smaller releases when the gas takes longer to reach the flare tip.

The ignition system is installed in a self-contained cabinet complete with instrumentation and nitrogen bottles. The equipment cost for a fully automatic version of the ignition system is NKr 3 million.

In cases where the flare tower is angled away from the platform so that the tip is over the sea, the pellet is fired at the target plate through the air. However, in the case of vertical flare towers, where it is important to prevent any material from falling onto the platform, the pellet is fired through a conducting pipe.

Gas recovery

The gas recovery system works by directing the flare header into a knock-out drum from which it is routed back to the main process. While recovery is underway, the flare line which also leads out of the knock-out drum remains closed. The recovery system is designed to handle normal gas leakage rates. When leakage exceeds the design rate, the recovery line is isolated while a valve on the flare line opens. In case of failure, this valve will fail open. As a backup, a high-reliability rupture disc is installed in parallel with the valve.

UPT has now further developed the system for use in situations where the flare tower is located separately from the production platform, as is often the case in Middle Eastern and Asian offshore developments.

In this case, the pellet is propelled by compressed air through a conducting pipe installed alongside the gas line between the two platforms and terminating just below the flare tip. Immediately it exits from the conducting pipe, a special mechanism causes the pellet to burst.

The new system has been successfully tested for distances up to 250 meters, and the aim now is to extend this to one kilometer, Wallentinsen says.

The system has aroused interest in India and the United Arab Emirates, where moves are afoot to recover and use the large volumes of gas which are currently flared. Interest is also beginning to be shown in the UK sector, where the system is being evaluated for use on Chevron's Alba platform.