Duration and size of outage uncertain, says Gassco

Shell works to restore capacity as soon as possible

Adds outage extended to Tuesday, Shell comment, price in paragraphs 2, 4, 6-7

OSLO, Dec 16 (Reuters) -Norway's Nyhamna gas processing plant suffered a partial outage on Monday owing to a disturbance in local power supply caused by stormy weather, operator Shell SHEL.L said.

Norwegian gas system operator Gassco announced an unplanned outage of 30.9 million cubic metres (mcm) at Nyhamna for both Monday and Tuesday, leaving the plant to operate at an estimated reduced capacity of 48.9 mcm.

Gassco said in a regulatory message that the exact size and duration of the outage was uncertain.

"We're working to restore this back to normal as quickly as we can," a Shell spokesperson said.

The Nyhamna plant on Norway's west coast receives gas from the offshore Ormen Lange and Aasta Hansteen fields, among others, and exports the gas via pipelines to Britain and continental Europe.

Norway still planned to export 329 mcm of gas on Monday, Gassco said.

The day-ahead gas prices for Britain TRGBNBPD1 and the Netherlands TRNLTTFD1 were trading about 5% down at 97 pence per therm and 39.30 euros per megawatt hour (MWh) respectively by 1332 GMT.