Annular pressure activated downhole tool

A method of activating a downhole tool comprising:

• introducing a tubular string supporting the downhole tool into a wellbore formed in a formation;
• adjusting pressure in an annulus defined about the downhole tool in the wellbore;
• activating a trigger at the downhole tool with the pressure in the annulus;
• disengaging the trigger from one of the mandrel and a power spring support of the downhole tool;
• releasing a spring upon activating the trigger;
• applying a force to a mandrel in the tubular string to activate the downhole tool;
• and
• wherein the downhole tool comprises a housing including an outer surface, an inner surface defining a passage, an annular projection projecting into the passage from the inner surface, and an opening exposing the passage to annular pressure;
• the mandrel extending into the passage, the mandrel having a first end and a second end;
• the power spring support detachably connected to the second end of the mandrel in the passage;
• the spring arranged about the mandrel and extending between the spring support and the annular projection;
• a trigger housing arranged in one of the mandrel and the spring support, the trigger housing being selectively aligned with the opening in the housing exposing the passage to annular pressure;
• and
• a trigger arranged in the trigger housing selectively engaged with the inner surface, the trigger being operable to disengage from the inner surface of the housing, releasing the spring, and shifting the mandrel along the passage when exposed to a selected annular pressure.