An improved offshore drilling method and apparatus are disclosed which are useful in preventing formation fracture caused by excessive hydrostatic pressure in a drilling riser. Gas is injected into the riser to provide the lift necessary to return the drilling fluid to the surface and to reduce the density...http://www.google.es/patents/US4099583?utm_source=gb-gplus-sharePatente US4099583 - Gas lift system for marine drilling riser
