LONDON, England (CNN) -- An apparent false alarm early Sunday triggered a massive air-sea rescue operation to evacuate over 500 workers from an oil rig in the North Sea, police and the British coast guard said.

A 23-year-old woman who works on the rig was taken on shore in connection with the incident, according to the local police force coordinating the operation.

Grampian police are expected to formally detain and question the British woman once she is on land, a spokesman for the force said.

Jake Molley, from the British Offshore Workers Union, said it appeared the alert was the result of "someone with an overactive imagination."

"I expect we're going to have one very embarrassed individual," Molley told CNN by phone.

In a statement, Britannia Operator Limited, the oil company operating the rig, said the alert was raised following an allegation by a worker that there was a suspicious device on the installation.

The British Press Association said the incident was sparked by threats made by a woman on board the vessel.

A total of 14 helicopters, including five Royal Air Force helicopters, were scrambled to the Safe Scandinavia rig -- described as "a floating accommodation unit" -- after emergency services were contacted early Sunday, British coast guard spokesman Fred Kaygill told CNN.

A bomb disposal unit was also deployed to the scene. Kaygill called it a "security incident" and said "it is in no way, shape or form related to terrorism."

Britannia said 161 of the rig's workers were evacuated to two nearby platforms before the operation came to a halt. Those workers are being returned to the rig.

A bomb disposal team had been called to the Safe Scandinavia rig, which is owned by Norwegian-based firm Pro-Safe and provides accommodation for workers from the oil company Britannia Operator Limited, according to a British defense ministry spokesman.

He later confirmed that the military assistance operations had ceased.

In the initial rescue effort, a total of 539 staff were being evacuated to nearby installations from the rig, around 280 kilometers (175 miles) northeast of the Scottish coastal town of Aberdeen.

Pro-Safe, the Norwegian-based owners of the rig, provides off-shore support to oil and gas companies. The Safe Scandinavia rig is described on the company's Web site as "a floating accommodation unit" with 583 beds.

A Pro-Safe official in Aberdeen told CNN by phone that the rig was contracted out to the oil company Britannia Operator Limited and that it was their employees who were being evacuated from the vessel. E-mail to a friend