TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe design of primary hydrocarbon production separation systems, given predicted wellstream flowrates and composition, is subject to nonlinear thermodynamic, hydrodynamic and mechanical constraints. Obtaining an optimal design for lifetime application represents a challenging problem because of the large changes in the production profiles that can be expected. In this paper we present a systematic and rigorous design method that generates the optimal lifetime design of such systems. The proposed method includes unit replacement and retrofit decisions. The method employ clustering theory and combinatorial and non-linear optimization to arrive at a solution. The method can be used during initial development planning and at any time during the lifetime as more knowledge of the field is gained. We exemplify the method on two fields with different economical models: one field with long lifetime based on an investment/salvage model and one field with a short lifetime based on a model for leased units.