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Subsea dual bore trees have a production and annulus bore passing vertically through the tree body with production and annulus master valves and swab valves oriented vertically in the main block of the tree.

They are designed to allow vertical access to the main production bore and to the annulus bore during installation and workover operations.  When a dual bore subsea Christmas tree is connected to a subsea wellhead it must interface with the tubing hanger previously installed in the wellhead.

Example of a Compact Dual Bore Tree is shown below (Courtesy of OneSubsea)