Creep rupture in random polycrystalline aggregates is investigated numerically in terms of multi-grain cell studies using a Delaunay network modelling technique. The model involves a representation of the crystalline aggregate by means of special purpose elements attributed to each grain facet. These Delaunay elements account for elastic and creep deformations of the grains, free grain boundary sliding, as well as for the nucleation and diffusive growth of grain boundary cavities until coalescence leads to a facet microcrack. Damage accumulation is simulated numerically, until an excessive number of microcracks cause des-integration of the polycrystal. Primary attention is on the influence of randomness in the microstructure on creep rupture, either in terms of random variations of the size and shape of hexagonal grains, or in terms of random variations in the nucleation properties of grain boundaries. It is found that randomness always tends to decrease the life time. In particular, it is found that the life time depends sensitively on random variations of the geometry of the microstructure.