Definition(s)
Concentric trees
Concentric trees are configured with their valves very much like those of the VXT design, but with the distinct difference being that the production bore is located concentrically within the tree and the annulus located off-centre (see Figure A.11).
The inherent feature of the design allows access only through the centrally located production bore for TH plug setting, and consequently other means are used for accessing the annulus, such as a flexible pipe run along the side of the completion/workover riser.
Source: API RP 17A, Design and Operation of Subsea Production Systems—General Requirements and Recommendations, Fourth Edition, Reaffirmed 2011. Global Standards