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* August North Sea maintenance to reduce output by 18 mcm/d
* But healthy stocks, falling demand to prevent price spike
* Russian winter cut during Ukraine crisis seen as big risk
By Henning Gloystein and Nina Chestney
LONDON, July 22 (Reuters) - British natural gas prices fordelivery in late summer and autumn are set to rise in comingweeks as large-scale North Sea maintenance makes it moredifficult for utilities to stock up on reserves ahead of peakdemand in winter.
Prices could be pushed even higher by the Ukraine crisis,which many analysts fear will result in a disruption in thesupply of Russian gas destined for western Europe and flowingthrough Ukraine.
"We do expect prices to rally in the coming weeks based onmaintenance and profit-taking on recent price declines (closureof short positions)," Hans van Cleef, senior energy economist atABN Amro, told Reuters.
British natural gas prices for delivery a month ahead havealmost halved this year as healthy supplies have combined withlow demand due to mild weather, improving energy efficiency,increasing supplies of renewables and low population growth.
Yet analysts say North Sea maintenance is likely to reversethat trend, at least in the short term.
Maintenance is planned from Aug. 1-14 on the Fortiespipeline system and the Central Area Transmission System(CATS)Riser Platform, both in the North Sea.
The riser platform, operated by BP, feeds gas intothe CATS pipeline, which carries more than 48 million cubicmetres of gas a day (mcm/d) from the North Sea to Teesside innortheast England.
The Forties pipeline system, also operated by BP, receivesoil and gas liquids from more than 50 offshore fields anddelivers gas into the St Fergus terminal in Scotland.
Data from Thomson Reuters Commodities Research and Forecastsshow total British North Sea gas production is expected to dropby an overall 18 mcm/d during the first two weeks of August.
"The outage will constrain supply and should therefore leadto withdrawals from storage and push up prices, and it may cutback on exports to continental Europe," said Oliver Sanderson, asenior gas analyst at the Thomson Reuters' forecasting group.
"I wouldn't be surprised if day-ahead prices in Augustpushed towards 40 pence per therm during the peak maintenanceperiod, especially as there is a scheduled outage for Norwegiandeliveries to Britain for the second half of August," he added.
Current day-ahead prices and contracts for delivery inAugust were trading at 37.00 to 37.25 pence per therm onTuesday.
North Sea production changes often have a strong marketimpact, as seen in recent outages impacting the St Fergusterminal or at ConocoPhillips' J Block field.
RISES, BUT NO SPIKE
Despite the bullish price outlook for the coming weeks,analysts said that price increases could be limited, becausethere had already been plenty of stock-building over the pastfew weeks amid low summer demand, and because reserves werealready at healthy levels following a mild winter and spring.
Britain's gas storage sites are currently filled to anaverage of 89.2 percent, compared with just 40.65 percent atthis time last year.
European gas demand also has been falling steadily, from apeak of over 510 billion cubic metres (bcm) in 2010 to under 420bcm this year, and gas use is not expected to rise back above450 bcm, according to French bank Societe Generale.
"Barring a sudden acceleration in economic activity in H22014 - clearly not the consensus view at present - or a'Siberian' (cold) Q4 across the whole of Europe, we see limitedrisks to our forecast," the bank said in a note this week.
One such risk, which could lead to price spikes, could be apotential cut in Russia's supply of gas to Europe as a result ofthe Ukraine crisis.
Russia meets almost a third of European gas demand, and itpipes almost half of these supplies via Ukraine.
Previous disputes between Moscow and Kiev over gas pricingand volumes have led to three disruptions over the past decade,and analysts fear the tense situation this year may result in arepeated cut-off. (editing by Jane Baird)
