We introduce diffraction-theory-inspired analytic description of the metasurface comprising an array of graphene subwavelength hemispheres. Our theory describes light interaction with the random metasurface, in which the periodicity is broken by accidentally damaged meta-atoms in the nodes of a two-dimensional periodic lattice. Both numerical modeling and experiment show that such a nm-thin metasurface possesses giant broadband absorption in the THz spectral range that remains intact even when a substantial portion of meta-atoms, i.e. graphene hemispheres, is damaged. Moreover, defective fabrication of graphene free-standing metasurface may enhance the absorptive properties.