There were no injuries in the ditching and all 19 people on board are being taken by helicopter to Kirkwall in Orkney, the coastguard said.
It is understood the helicopter was flying from Aberdeen to the West Phoenix drilling rig, about 86 miles (140km) north-west of Shetland.
A spokesman for the coastguard said the weather in the area was good.
"It has been quite calm today and that has been favourable in terms of getting the rescue crews to the scene quickly."
More top news
A former British soldier who served in Afghanistan is one of a 'growing number' of westerners joining Kurds in the fight against Isis.
Growing numbers of people on low incomes are turning to food banks to survive, new research has revealed.
It will be cold and frosty across the north of the UK tonight, with some icy patches in the west.