Requirements for a Full Drill through Subsea Wellhead and Tree System

Abstract This paper describes the requirements realized in the development of a full drill through subsea wellhead and tree specification. Drill through wellhead and tree systems provide an opportunity for capital and operating expense savings. Given a target development, definition of requirements determines potential savings, performance, and necessary contingencies. Current industry practice considers both slim bore and drill through wellheads and trees. However, operational interfaces may not be fully apparent. The case considered is an 18¾-in. wellhead with a 13⅝-in. nominal hanger bore. The system interfaces a standard 21-in. outside diameter riser and an 18¾-in. horizontal subsea tree. The full drill through system is capable of drilling out and running a 10¾-in. casing and installation of the lower and upper completion with the subsea tree installed. Primary design considerations include the casing program, sequence of operations, protection of seal bores, and primary and contingency operations. Seal surface protection is important for the reduced wellhead bore design. The reduced bore increases the exposure of casing hanger and running tool seal areas. Normally, primary operations take place through the tree; however, contingencies must consider operations without the tree installed. Thorough understanding of the requirements achieves an optimized drill through system.