System for drilling a subsea well

A system for use in the drilling of a subsea well includes a drilling riser which extends from the drill ship to a lower marine package installed on a blowout preventer on the upper end of the subsea wellhead. The riser includes a riser pipe having a line ML carried by and extending along its side to return drilling fluid from the annulus about the drill string extending through the package to the drill ship, and another line NL also carried thereby and extending along its side to inject gas from a source at the drill ship into the drill fluid for commingling therewith as it is returned thereby lightening the head of drill mud in the well bore. The riser pipes making up the marine riser are connected end to end by bolted flanges to which the ends of the lines are connected the flanges having an outer side edge which is generally oval shaped to reduce its weight.