* Fredriksen remains top shareholder with 30% stake
* Asset impairment of $302 mln in third quarter
* Quarterly core operating profit beats firm's forecast (Adds impairment, market outlook)
OSLO, Nov 21 (Reuters) - Seadrill Chairman John Fredriksen, the oil rig firm's top shareholder, is stepping down with immediate effect and the company will write down asset values by more than $300 million.
The 75-year-old billionaire investor, who said his support for the company remained unchanged, would be replaced by Glen Ole Roedland, who has worked in shipping, oil, gas and other industries, Seadrill said.
The company, which is listed in Oslo and New York, emerged from U.S. Chapter 11 bankruptcy proceedings in July 2018, after converting unsecured bonds to equity and extending maturities on $5.7 billion bank loans.
Net earnings remain deeply in the red. On Thursday, it announced asset impairments of $302 million, stemming from the New York Stock Exchange's (NYSE) decision in September to delist Seadrill Partners, in which Seadrill is an owner.
The NYSE decision to suspend and delist Seadrill Partners was made after the unit's market value fell below the minimum required for a listing.
"While I have now decided to spend less time on board seats, my close involvement and strong support of Seadrill will remain unchanged," Fredriksen said in a statement, adding he would continue to push for consolidation rig industry players.
As of Feb. 28, Fredriksen's Cyprus-registered Hemen Holding Ltd was Seadrill's largest shareholder with a 30.2% stake.
In the third quarter, Seadrill had 22 rigs idle out of 55 rigs it owns or manages on behalf of its non-consolidated entities, such as Seadrill Partners, compared to 21 rigs idle in the previous quarter.
Seadrill said it saw some signs of improvement in the market for floater rigs, which are not attached to the sea floor, with tenders for longer term jobs increasing, especially in Africa and Brazil.
In the market for jack-up rigs, improving rates could lead to reactivating some high-specification units in 2020, it added.
The company reported $85 million in quarterly adjusted earnings before interest, tax, depreciation and amortization (EBITDA), beating its outlook of $70 million-$75 million from August.
In the fourth quarter, adjusted EBITDA was expected to decline to $40 million, the company said.
The company's third-quarter net loss, including impairments, more than doubled year on year to $522 million from a loss of $245 million a year ago.
(Reporting by Nerijus Adomaitis and Terje Solsvik; Editing by Sherry Jacob-Phillips and Edmund Blair)