about 2 500 feet from the water surface if the riser disconnect 24 is placed about 500 feet below the water surface the displacement depth would thus be about 2 000 feet below the riser disconnect 24 the particular depth to which gas should displace liquid in the riser 18 will depend on the weigh in water of the particular riser and the length of the riser below the disconnect 24 in one embodiment of a method according to the invention to prepare the wellbore 16 and the riser 18 for moving the platform 10 from the location first the drill string 14 is lifted out of the wellbore 16 such that it is above the bop 20 the bop 20 is then closed to seal the wellbore 16 below the bop 20 when the bop 20 is closed the control line 20 a may be retrieved therefrom and withdrawn to the platform 10 then the drilling fluid in the riser 18 is displaced with sea water in one implementation the sea water is pumped into the interior of the drill string 14 so as to displace all the drilling fluid in the riser 18 up the annular space between the drill string 14 and the riser 18 by displacing all the drilling fluid which may have a specific gravity of as much as 2 2 or more the weight of the riser 18 in the water 11 is reduced after the drilling fluid in the riser 18 is displaced the drill string 14 is withdrawn from the riser 18 and a riser.