A method of drilling a subterranean well bore comprising, monitoring the well bore for influx of formation fluid into the well bore, and, on detection of an influx, a) stopping any pump pumping fluid into the well bore, b) operating a first blow out preventer so that it closes within a first period of time, c) operating a second blow out preventer so that it closes within a second period of time, the second blow out preventer being located below the first blow out preventer, the second period of time being longer than the first period of time, d) circulating the influx out of the well bore via a flow line extending from below the second blow out preventer.