* Activists from 18 nations charged over offshore platform protest
* Could be sentenced to 15 years in prison if convicted
* Greenpeace says activists accused of "imaginary offence"
By Steve Gutterman
MOSCOW, Oct 3 (Reuters) - Russia has pressed piracy charges against all 30 people arrested after the environmental group Greenpeace staged a protest at an offshore oil platform in the Arctic, investigators said on Thursday.
The accused could be sentenced to 15 years in prison if convicted over the protest last month, in which a Greenpeace ship approached a platform belonging to state-controlled energy firm Gazprom and two activists tried to scale the rig.
Greenpeace has dismissed the piracy charges as absurd and unfounded. It says the protest at the Prirazlomnaya platform was a peaceful effort to draw attention to what it says are grave dangers posed by drilling in the fragile Arctic environment.
"Our activists have been charged with a crime that did not happen, they are accused of an imaginary offence," Greenpeace International Executive Director Kumi Naidoo said. He said the group's campaign against Arctic drilling "will not be silenced".
The 30, who were aboard the Greenpeace ship Arctic Sunrise during the protest, are being held in custody in the northern Russian city of Murmansk.
They include activists and crew members from 18 nations on five continents as well as a British videographer and a Russian photographer who were documenting the protest.
Greenpeace released a letter in which Faiza Oulahsen, a 26-year-old Dutch activist who was charged on Wednesday, said it was "ice cold" in her cell and the lights were never turned off.
Russia's Investigative Committee, which answers to President Vladimir Putin, said all the accused had denied guilt and refused to give substantive testimony relating to the piracy charges. It said the investigation was continuing.
The Arctic holds 13 percent of the world's undiscovered oil and 30 percent of its undiscovered gas, the U.S. Geological Survey estimates. However, its economic viability, as well as its environmental safety credentials, remain a matter of debate.
Prirazlomnaya is Russia's first offshore Arctic oil rig, giving it an important role in the energy-reliant nation's effort to extract resources in the region.
Global majors including ExxonMobil, Eni and Statoil have agreed deals with Russia's state-owned Rosneft to enter Russia's Arctic offshore waters, projects seen as crucial to maintaining the 10 million barrels a day of oil flow from the world's No. 1 producing nation.
Russia's economy has slowed in recent years after growing robustly during Putin's initial stint as president in 2000-2008.
Putin, whose current term ends in 2018, told a meeting of his ruling United Russia party on Thursday that the Arctic was an "extremely important region" in terms of both national security and economic development.
Putin said last month that the Greenpeace activists were clearly not pirates but that they had violated international law. Prime Minister Dmitry Medvedev said on Wednesday that concern for the environment could not justify illegal activity.
Gazprom unit Gazprom Neft Shelf said in an email to Reuters that the company is on track to begin oil production at Prirazlomnoye, the project which is served by the Prirazlomnaya platform, by the end of the year.
Gazprom has cited technical reasons for past delays to the start of production at Prirazlomnoye, which has estimated reserves of 526 million barrels of oil, and the firm expects to reach peak production of 120,000 barrels per day in 7-8 years.
Greenpeace believes efforts to drill in the Arctic for fossil fuels that contribute to climate change and ice melt are "a vicious circle we need to break", said Ben Ayliffe, head of the group's Arctic oil campaign.
He said that there is no known way of effectively cleaning up an oil spill in ice and that research showed that a major disaster at Prirazlomnaya could pollute over 3,000 kilometres (nearly 1,900 miles) of coastline.
"Gazprom is simply not prepared to deal with an oil spill at Prirazlomnaya," he said.
Gazprom said in the email that it was capable of tackling any possible spills at the project.