The Horizontal Subsea Tree: A Unique Configuration Evolution
Authors: Skeels H.B. , Hopkins B.C. , Cunningham C.E. 
Published: October 2008

ABSTRACT
The horizontal (or spool) subsea Christmas tree is the next generation of "Large bore" subsea wellhead completion arrangement developed for high maintenance, and recently, electrical submersible pump (ESP) completions. The large bore configuration allows the operator to install and retrieve downhole equipment through the tree without having to disturb the tree or its external connections to flowlines. service lines, or control umbilicals. The configuration also provides well control and vertical access to downhole equipment through a Conventional marine chilling riser and Subsea BOP; eliminating the need for costly and specialized completion risers.
As subsea technology matures and operators strive for more cost effective field developments, the potential equipment reduction and cost savings associated wit h the horizontal tree are resulting in ever-increasing inquiries into the subject. This paper compares both conventional and horizontal subsea tree concepts, illustrates system advantages/imitations, and presents some of the pressing operating practice issues so that informed choices, both technically and economically, can be made with regard to using a horizontal tree.

BACKGROUND
The horizontal subsea tree concept evolved from ear]y though bore tree concepts as a means to retrieve production tubing and downhole equipment "through the bore" of the subsea tree in an effort to simplify equipment and workover operations. especially for troublesome well where frequent workovers are necessary. Through bore trees were limited to single string completions and equipment whose diameter could pass through 7 inch master and swab gate valves. Other penetrations such as hydraulic stabs und electrical connectors has to exit via side outlets in a tubing head, below the tree. With the advent of subsea ESP completions and critical service applications. a need has arisen for subsea equipment to be configured in a way to meet additional demands, such as:providing a means to incorporate a high power electrical feed-through connection which allows both halves of u wet-mute power connector and providing full wellbore access for pumps and packers for subsea ESP completions.providing larger multi-bow tubing hangers with additional penetrations to access and control downhole equipment
To meet these requirements, two other subsea configurations have been considered for their suitability:A "BOP rum" style or drill stem test style Tree.A special design tubing head (or tubing spool) with well and flow control valves mounted horizontally outside of the production bore, allowing a clear path to the well; he horizontal tree.
Of the three, the horizontal tree makes the most conventional sized equipment, has the most compact profile (optimizing loud transfer into the wellhead), and offers a reduced number of valves for well control while maintaining the required number of pressure barriers.
Operators and suppliers are not giving serious thought to the horizontal tree as the nest generation of subsea completion equipment for natural drive wells. Site specific water depth and environmental requirements tend to generate one-of-a-kind installation/workover completion risers for use in Conventional subsea tree intervention.