When she arrives at the oil rig by helicopter, Hanna is not wearing a transit or immersion suit. Wearing this is obligatory when travelling to off-shore installations in the North Sea.
In order to fly out to the rig, Hanna would have had to do a three-day off-shore survival course. There seems to be no indication that she does this in the film.
If an employee requires medical treatment on an off-shore installation he would immediately be evacuated off.