Slenderwell Wellhead Benefits and Opportunities of Selected 13" Option
Authors: Theiss David H. 
Published: October 2008

Abstract
The benefits and opportunities of the 13" slenderwell wellhead are examined. A smaller diameter, lighter weight drilling riser and/or BOP stack, reduces the riser tension requirements, deck load requirements, deck space requirements and drilling fluid requirements allowing smaller and older generation drilling rigs to be used in deeper waters. Another benefit is the opportunity to install a drill through horizontal completion tree or drill through tubing spool prior to the installation of the BOP stack. This eliminates the need to retrieve and reinstall the BOP stack in order to install the tree after drilling operations are completed. Opportunities to batch set surface and intermediate casing strings offer even more savings on mult-well projects. The BOP can be moved from well to well to drill for and to run the intermediate casing strings. The drill through horizontal completion tree can then be installed and the BOP run and moved from well to well to batch set the production casing strings and completions. The 13" slenderwell wellhead also provides additional life of well costs savings by allowing well intervention and workovers to be accomplished with a slender riser system and a smaller, less expensive intervention rig. The synergy of this slender wellhead system with other technologies such as expandable casing and slenderwell completions will be examined. The technical challenges encountered and overcome in the design of the wellhead system will also be discussed.

Introduction
Presently the vast majority of subsea wells are drilled and completed with an 18-3/4" subsea wellhead system and an 18- 3/4" subsea BOP stack run on a 21" nominal OD, 19" nominal ID drilling riser system. Substantial savings in rig costs, rig time, drilling costs and completion costs can be obtained by using a slenderwell subsea wellhead and a smaller ID and lighter weight riser system. The smaller ID and lighter weight riser system reduces the amount of riser buoyancy required and yields reduced riser tension and deck load requirements which allows use of a smaller or older generation drilling rig in deeper waters. The drilling fluid volume in the riser is substantially reduced which in turn reduces the rig topside mud handling, storing, and processing equipment requirements. The slenderwell requires fewer and smaller casing strings and the removal of less drill cuttings, thus reducing the well drilling time.
The slenderwell subsea wellhead technology can be used to reduce costs in deepwater drilling and completions worldwide and it is particularly well suited for deepwater prospects with reservoirs which can be reached with one or two casing strings drilled for through the BOP stack.

Comparison of Conventional and Slender Risers
A conventional riser for an 18-3/4" drilling system normally has a central pipe that is 21" OD and a wall thickness of between 0.50" and 0.875" to meet the required tension, bending, internal pressure, and external pressure loads expected. The riser is equipped with external auxiliary lines which include a high pressure choke line and a high pressure kill line for well control and circulation.