A 23-year-old woman is expected to appear in court today after reports of a bomb on a North Sea oil rig sparked a full-scale emergency operation involving the army, RAF and police.

According to one report, the scare started when a woman employee on the rig was overheard recalling a dream she had had about a bomb on the platform. Jake Molloy, general secretary of the Offshore Industry Liaison Committee, one of the biggest unions representing offshore workers, said: "It was complete madness. This girl had a dream about a bomb being on board and she was a bit shaken. The next thing anyone knew workers were being evacuated."

He said the rumour that a bomb was on the accommodation block - or "flotel" - had spread to senior managers within an hour. "It was complete madness on behalf of everyone. There was never any reason to evacuate the platform."

A Grampian Police spokesman last night said: "The female is expected to appear at Aberdeen sheriff court on Monday in connection with the matter."

Earlier police confirmed that they were treating the incident as a false alarm, adding that a 23-year-old woman was being brought ashore for questioning.

"The incident, which was reported to Grampian police force control room at 9.30am today is not considered terrorism related," said a police spokesman.

According to the company that runs the oilfield, Britannia Operator, the alarm was raised at about 9.15am when a member of crew on board the Safe Scandinavia, a flotel stationed next to a rig 190 miles north-east of Aberdeen, said there could be a suspicious device on board.

Within minutes emergency procedures swung into action and the block was evacuated, with about 500 people crossing the gantry linking it to the oil rig. A search of the platform began and army bomb squad officers were put on standby.

Five helicopters from RAF Kinloss and a Nimrod reconnaissance aircraft from RAF Waddington were scrambled to the scene. By mid-afternoon the company confirmed that nothing had been found, and workers began to return to the platform. The bomb squad was stood down and police said they were making arrangements to bring the woman ashore.

Kath McGill, managing director of Britannia Operator, said the company had acted sensibly. "We are very relieved this has turned out to be a false alarm but we obviously had to treat it seriously and act appropriately to ensure the safety and wellbeing of all our people."

However, the union described the decision to evacuate as "ludicrous".

Molloy said: "It appears that the whole thing was started when someone was a bit upset about a dream they had and that appears to have sparked one of the biggest security operations the North Sea has ever seen. The cost has been astronomical and there was never any need for it." He said he felt sorry for the woman who appeared to be at the centre of the alarm.

A spokesman for RAF Kinloss said the emergency services were following "precautionary measures". Squadron Leader Barry Neilson, from RAF Kinloss, said that although the incident had been "unusual" it had fallen within the "normal operating procedures."

A Scottish executive spokeswoman said during the operation: "A Scottish government civil contingencies official is based in the Grampian police control room, and the justice secretary is being regularly kept appraised of developments.

"There are well established procedures for dealing with incidents in the North Sea, and these are operating efficiently and effectively."

A Ministry of Defence spokesman confirmed their helicopters and planes had been stood down. "The helicopters that were there have been sent home. The two planes have been turned around and sent home as well."

Oil rigs have long been considered a potentially vulnerable and important target for terrorists and enemy forces. During the cold war Nato drew up contingency plans in the event of an attack in the North Sea by Soviet special forces.

Scottish Liberal Democrat leader Nicol Stephen, who represents Aberdeen South, paid tribute to the professionalism of those involved in the evacuation and said it was too early to say what had caused the incident.

"After more than three decades of North Sea oil this is the first serious security incident of this kind that I can recall," he said. "It is a tribute to everyone involved in the evacuation that it was carried out with such professionalism and efficiency.

"It is now important that all investigations are completed as quickly as possible and that work on the platform returns to normal."

Aberdeen coastguard said 161 of the 539 workers were airlifted from the rig before the alarm was called off. The workers who were flown to neighbouring platforms returned to the Safe Scandinavia last night.