UPDATE 1-BP to cut Thunder Horse output by half -sources

* Thunder Horse platform in Gulf to be shut for weeks

* Total annual output to be cut by about 10,000 BOE/day (adds adds trader comment, industry comment, background, updates share price)

LONDON, April 9 (Reuters) - BP Plc (BP.L) will temporarily cut production at its 250,000 barrels of oil per day Thunder Horse platform in the Gulf of Mexico by half to replace undersea manifolds serving the platform, three sources familiar with the plans said on Friday.

The maintenance, which has been part of BP's plans for more than a year, is likely to last weeks and will cut total annual production from the major oil company by about 10,000 barrels of oil and gas equivalent per day in 2010, one source said.

BP has already factored the production losses into its outlook for output this year. The company said earlier it does not expect production to rise in 2010, although it sees output growth returning in 2011.

A BP spokesman declined comment.

Thunder Horse also produces up to 200 million cubic feet per day of natural gas.

The maintenance work will replace a temporary subsea gathering system that BP put in place at Thunderhorse earlier to help the platform begin oil production in 2008. BP will now replace the system with a permanent one, a source familiar with the project said.

The planned shut-in for Thunder Horse production has been widely informed to buyers of the Gulf of Mexico crude, oil traders and industry sources said.

"There may be a little extra spot buying to replace (Thunder Horse) crude, but for the most part everyone has probably planned for this, storing plenty of crude in advance," a trader said on condition of anonymity.

The crude, a light, low-sulfur oil with an API gravity rating of 34.5 and 0.61 percent sulfur content, can be replaced by ample supply and stockpiles of other crudes in the U.S. Gulf Coast region, oil traders said.

Differentials for Thunder Horse THH- grade crude in U.S. cash crude markets firmed slightly on Friday, with barrels being talked in the range of 30 to 10 cents below benchmark West Texas Intermediate CLc1, up from a range of 75 to 25 cents below WTI on Thursday, traders said. Most U.S. cash grades have strengthened this week.

Differentials for Southern Green Canyon SGC- crude, another offshore Gulf of Mexico crude that traders said may have firmed this week due to expectations of BP's maintenance program, strengthened to -$3.75 a barrel below WTI by Friday, up from -$4.60 a barrel on Monday.

BP shares were up 0.4 percent at 11:16 EDT (1616 GMT), lagging a 1.3 percent rise in the STOXX Europe 600 Oil and Gas index .SXEP. (Reporting by Tom Bergin in London and Joshua Schneyer in New York; Editing by Alden Bentley)