The duct keel is a plated box/tunnelled keel allowing passage right forward. It provides additional buoyancy, together with a through passageway for cables and pipelines running in the fore and aft direction.
In other words we can explain:
An internal passage of water tight construction (two longitudinal girders spaced not more than 2.0 m apart) running same distance along the length of the ship, often from the forepeak to the forward machinery space bulkhead.
To carry the pipe work, and an entrance is at forward machinery space via a watertight manhole.