In this study, a three-dimensional (3D) model is adopted to investigate the problem of fluid-seabed-structure interactions (FSSI) around offshore detached breakwaters. The Volume-Averaged Reynolds-Averaged Navier-Stokes (VARANS) equations are used to simulate the 3D fluid motion in the vicinity of breakwaters, including reflected, diffracted waves and non-linear turbulence. The partially dynamic Biot's equations (i.e., the u−p approximations) and poro-elastoplastic constitutive model are adopted to describe fluid-seabed interactions in a porous seabed. A one-way coupling algorithm is established to integrate the fluid motion and seabed response. By using the present model, this study addresses the long term cyclic loading induced seabed response and liquefaction potential of a loosely packed foundation. In addition, the effects of the perpendicular longshore currents on the hydrodynamic properties and liquefaction potential are also taken into account. The numerical results reveal that the loose sandy seabed around the offshore detached breakwaters experienced a gradual decrease of stiffness until liquefaction occurs under successive loading cycles. Longshore currents tend to exacerbate the wave field and increase the risk of liquefaction of seabed foundation, especially in the shallow soil layer near the seabed surface.