Method and apparatus are described to reduce the tension required on a riser pipe used in offshore drilling between a floating vessel and a subsea wellhead. Heavy drilling fluid is circulated down a drill pipe and up the annulus between the drill pipe and the borehole wall to a point just above a subsea...http://www.google.ca/patents/US4291772?utm_source=gb-gplus-sharePatent US4291772 - Drilling fluid bypass for marine riser
