Method and apparatus for deepwater drilling

Apparatus for drilling an offshore well in water depths exceeding 3000 feet, and similar wells where a pressure gradient from a drilling fluid is likely to be abnormally high, said drilling being accomplished through a previously installed subsea wellhead from an above-surface platform without the use of a conventional riser, said apparatus comprising:

• (a) a blowout preventer stack attached to said subsea wellhead;
• (b) an upper stack package affixed to the upper portion of said blowout preventer stack;
• (c) a drill string extending through said wellhead, said blowout preventer stack and said upper stack package, said drill strings conveying drilling mud from said platform to a drill bit;
• (d) a rotating head assembly detachably secured in said upper stack package to isolate the seawater above said rotating head from said drilling mud therebelow, said rotating head slidably receiving said drilling string;
• (e) a running collar fixedly attached to said drill string at a particular location above said drill bit said collar being initially attached to said rotating head assembly by shear pins for running into said upper stack package;
• (f) a mud return line extending from said upper stack package to said platform to convey said drilling mud and resulting cuttings from said drill bit to said platform;
• and
• (g) pump means positioned in said mud return line near said upper stack package to pump said mud returns to said above-surface platform, said pump means being powered by a hydraulic fluid;
• whereby the pumping of the mud returns to the above-surface platform serves to reduce said abnormally high pressure gradient by an amount equal to a differential gradient between that which is caused by a surface-to-seabed column of drilling mud and that which is caused by a column of seawater of equivalent length.