* Says ultra-deepwater rig market still challenging
* To maintain dividend policy well into 2016
* Shares down 4.5 pct (Adds detail)
OSLO, Aug 27 (Reuters) - Seadrill, the world's biggest offshore driller by market capitalisation, reported second-quarter earnings below forecasts on Wednesday and offered a cautious outlook for the rig market, sending its shares lower.
Seadrill, the crown jewel in shipping tycoon John Fredriksen's business empire, has been hit like other rig firms by oil companies reining in spending to counter rising costs.
Seadrill's earnings before interest, tax, depreciation and amortisation (EBITDA) for the quarter came in at $641 million, below forecasts of $663 million in a Reuters poll of analysts and down from the $665 million it posted a year ago.
"The near-term market for ultra-deepwater drilling units continues to be challenging, partly driven by a reduction in exploration drilling that has led to a slower growth rate in overall upstream spending," the company said in a statement.
Shares in Seadrill were among the worst performers on the Oslo bourse. They were down 5.1 percent by 0753 GMT, lagging behind a flat Oslo benchmark index.
The company intends to prioritise returning cash to shareholders, it said, adding that it can maintain a quarterly dividend of 1 dollar per share well into 2016, even if the rig market fails to make a significant recovery.
Seadrill will also refrain from ordering new rigs until it becomes easier to read the market, it said. Seadrill has 18 new rigs yet to be delivered. (Reporting by Gwladys Fouche; Editing by David Goodman)