UK GAS-Oversupply weighs on within-day prices, day-ahead gains

* British gas market oversupplied by 24 mcm

* Wind power set to peak to 6.6 GW

* Centrica (LSE: CNA.L - news) to decommission Rough 47/8A platform

Sept 9 (Reuters) - British gas prices for immediate delivery fell on Friday morning as increased domestic supply coincided with high output from wind power turbines which reduced gas demand.

Gas for immediate delivery was down 0.9 pence or 3.7 percent at 23.45 pence per therm by 0957 GMT after falling more than 5 percent.

Britain's gas system was oversupplied by 24 million cubic metres (mcm) per day, with demand forecast at 187 mcm and supply at 211 mcm, National Grid (LSE: NG.L - news) data showed.

Total (LSE: 524773.L - news) 's Bacton SEAL gas terminal was ramping up output after an unexpected outage on Thursday cut its output by 9 mcm/day.

Wind power output in Britain was expected to peak at about 6.6 gigawatts (GW) on Friday compared to installed capacity of almost 9 GW, meaning less demand for gas to generate power.

Wind turbines were generating more than 13 percent and combined cycle gas turbines almost 47 percent of power, National Grid data showed.

Meanwhile, British gas prices for delivery on Monday gained 1.05 pence or 4.37 percent to 25.05 pence/therm as Norwegian gas deliveries were likely to fall.

Flows through the Langeled pipeline, Britain's main subsea gas import route, were expected to drop after the end of maintenance at the Sleipner platform, allowing some gas to be redirected from Britain to Belgium, analysts at Thomson Reuters (Dusseldorf: TOC.DU - news) said.

They expected Langeled flows to drop to its pre-outage level of 5 mcm/day from 21 mcm/day on Friday morning.

Centrica, the operator of Britain's largest underground gas storage Rough, said on Friday it would permanently withdraw from service its 47/8A installation at the site after 40 years of operation.

The operator said its decision would have minimal impact on Rough capabilities as only 4 of its wells were used out of some 30 in the depleted offshore gas field turned to storage.

Centrica confirmed it expected to have 20 wells available for gas withdrawal at Rough from Nov. 1 as previously planned.

Further along the curve, contract for gas delivery in October eased by 0.08 pence to 30.37 pence/therm, while gas for winter was steady at 38.10 pence/therm.

In the Netherlands, the day-ahead gas price at the TTF hub fell 0.16 euro or 1.4 percent to 11.02 euro/MWh.

In the European carbon market, the benchmark Dec (Shanghai: 600875.SS - news) . 16 contract was up 0.02 euro or 0.49 percent, to 4.10 euro a tonne. (Reporting by Nerijus Adomaitis in Oslo; editing by Jason Neely)