Cause of Asgard Vibrations Found

Following an extensive test program, Statoil has determined the cause of the vibrations in the export pipelines on the Asgard B gas platform in the Norwegian Sea.

"The vibrations occur as a result of acoustic waves produced when the gas flows over uneven surfaces inside the flexible riser from Asgard B down to the seabed," reports operations vice president Ola Krumsvik.

He explains that the acoustic waves are transmitted to the platforms fixed pipe system and that vibrations occur due to resonance at certain wave frequencies. By making some alterations to the gas-carrying pipelines on board, the problem can be reduced without affecting export capacity.

A system for monitoring the vibrations has been installed. On board, reinforcements have been carried out so that exports of gas now total more than 20 million scm per day. Further modifications will be made in June, when a third riser will also be installed. Statoil expects that Asgard B will be in full production following the work in June. The platform will however undergo modification in September, in connection with a planned turnaround at the Karsto gas reception terminal, north of Stavanger. Welding joints on the pipelines that connect the installations on the Asgard field to the Smorbukk reservoir will also be rectified. So far, 46 of the 72 welds have been repaired, reports Idar Grytdal, operations manager for the subsea installations.

The repair work is very much dependent on weather conditions. Pipeline ends with poor welds are disconnected with the aid of remotely controlled subsea equipment before being raised to the surface for welding. Technically speaking, the repairs have gone according to plan, but bad weather has led to a slight delay. However, with efforts being made to catch up, the work should finish on schedule in the summer.

Contracted gas volumes will be delivered with help from other fields in periods where Asgard has reduced export capacity.