In horizontal subsea tree systems, the tree is installed on the wellhead and then the tubing hanger is installed inside the tree. The tubing hanger forms the connection between the production/injection tubing and the tree.
Figure A.14 shows a typical configuration with a production guidebase as part of the stack-up. This is to allow tree retrieval without disturbing the flowline and umbilical. Clearly, with the reduced likelihood of having to retrieve the tree, there is less need for a base and, in certain circumstances, the production guidebase may be integrated with the XT spool. This saves a running operation, but at the expense of reducing system flexibility, i.e.: restricts installation of the flowline and umbilical until after the XT is installed; disturbs the flowline and umbilicals if the XT ever has to be recovered.
Source: API RP 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations, Fourth Edition, Reaffirmed 2011. Global Standards
Subsea tree with production and annulus bore valves located external to the tree, where the tubing hanger or dummy tubing hanger is installed after the tree.
Source: API RP 17G, Recommended Practice for Completion/Workover Risers, Second Edition, July 2006 (Reaffirmed April 2011). Global Standards
A system of valves installed on a subsea wellhead that has a master valve in the horizontal outlet from the vertical bore rather than in the vertical bore.
Source: API RP 96, Deepwater Well Design and Construction, First Edition, March 2013. Global Standards
Tree that does not have a production master valve in the vertical bore but in the horizontal outlets to the side.
Source: API SPEC 17D, Design and Operation of Subsea Production Systems—Subsea Wellhead and Tree Equipment, Upstream Segment, Second Edition May 2011 (Errata September 2011). Global Standards