Pipeline isolation tool, assembly and method

An isolation tool for use in isolating a section of a pipe, the isolation tool comprising:

• a seal unit configured for location in a section of the pipe via a single opening in a wall of the pipe, the seal unit comprising:
• a mandrel, and
• a seal arrangement configured to engage the wall of the pipe to isolate the section of the pipe, wherein the seal arrangement is self-energized by an isolated pressure differential across the seal unit, wherein the seal unit is reconfigurable from a first, axially extended, configuration to a second, axially retracted, configuration, and movement of the seal unit to the second, axially retracted, configuration urges the seal arrangement towards the wall of the pipe to isolate the section of the pipe;
• a primary activation arrangement configured to move the seal unit from the first configuration to the second configuration, wherein the primary activation arrangement comprises an activating member disposed on and axially moveable relative to the mandrel, and the activating member comprises or defines an upstream pressure head configured to be exposed to upstream fluid pressure in the pipe;
• and
• a secondary activation arrangement configured to apply a force on the activating member of the primary activation arrangement to maintain the seal unit in the second, axially retracted, configuration, wherein the secondary activation arrangement comprises or takes the form of a fluid-powered activation arrangement, the secondary activation arrangement comprising:
• an activating member disposed on and axially moveable relative to the mandrel, the activating member of the secondary activation arrangement comprising or taking the form of a piston, and
• a chamber configured to receive fluid which causes the activating member of the secondary activation arrangement to be urged towards the activating member of the primary activation arrangement.