NOVELTY - The turbine (100) has a first set of guide vanes (116) intersecting a U-shaped flow passage (104) and placed proximal to a first port (112). The first set of guide vanes include a first set of stators (116a) that is positioned between the first port and a second set of stators (116b), where the stators turn fluid flowing from the first port toward rotor blades (120) and are clocked relative to each other. A second set of guide vanes (118) intersects the flow passage, and is placed proximal to a second port (114), where the guide vanes are offset from a central axis (103) by a distance (D2).    USE - Oscillating water column turbine for use in marine application.    ADVANTAGE - The rotor blades can be vary in turning angle from a root to a tip to provide third-dimension of shape, while variance in turning angle depends on design and height of a rotor such that circumferential chord length can increase proceeding from the tip toward the root to eliminate separation of the rotor blades and enable the rotor blades to produce non-axial downstream flow as proceeding toward the guide vanes, thus reducing pressure loss when the non-axial downstream flow reaches the guide vanes, and hence providing more efficient passing of the non-axial downstream flow and increasing efficiency and reliability of the turbine.    DETAILED DESCRIPTION - The rotor blades are crescent shaped rotor blades. INDEPENDENT CLAIMS are also included for the following:    (1) an apparatus for extracting energy from bi-directional fluid flow    (2) a method for extracting energy from an oscillating water column.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an oscillating water column turbine.    Distance between rotor blades and central axis (D1)    Distance between guide vanes and central axis (D2)    Oscillating water column turbine (100)    Central axis (103)    U-shaped flow passage (104)    Ports (112, 114)    Guide vanes (116, 118)    Stators (116a, 116b)    Rotor blades (120)