Patent ID: 7073593

Claim:
A method of preparing and operating a subsea well, the method comprising the steps of: locating a plurality of drill-through subsea trees on a respective plurality of subsea wellheads, each of the plurality of drill-through subsea trees being connected by a manifold; sealingly connecting a high pressure drilling riser between one of the plurality of trees and a drilling platform at the sea surface; mounting a blow out preventer at the top of the riser; drilling the well through the drilling riser and the subsea tree; and establishing a production connection between the tree and the manifold through a production riser which is separated from the drilling riser; wherein the high pressure drilling riser comprises two concentric riser pipes, the inner of the pipes being a high pressure riser and the outer of the pipes being a low pressure riser; and wherein the low pressure riser is first connected between the tree and the drilling platform with a low pressure blow out preventer mounted at the top of the riser, the well is drilled to a first depth, the low pressure blow out preventer is removed, the high pressure riser is run into the low pressure riser, a high pressure blow out preventer is mounted at the top of the high pressure riser, and the well is drilled to a second, greater depth.