DUBAI, Dec 8 (Reuters) - Emirates EMIRA.UL , the world's biggest long-haul airline, expects to introduce "premium economy" tickets within the next 12 to 18 months, according to its president, Tim Clark.

The Dubai-based airline, faced with pressures on margins caused by a strong dollar and tougher competition, has recently added new revenue streams.

It introduced an advanced seat selection fee for economy passengers in October and has said it is considering other additional fees.

"We are at the stage of finding what form [premium economy] will take," Clark told reporters in London on Wednesday in comments confirmed to Reuters by the airline.

"I would think within the next year to 18 months, we will have it in the airline, hopefully up and running."

Clark first said on May 10 that the airline was considering adding premium economy, a class between economy and business that could prove attractive to corporate passengers affected by the decline in the oil price and tighter travel budgets.

Emirates reported on Nov. 9 a 75 percent fall in first-half profit to 786 million dirhams ($214 million).

The airline filled an average of 75.3 per cent of seats in the six months to Sept. 30, a decline of 3 percentage points.