This paper presents a systematic analysis of optimisation formulations for the LNG process. It focuses on the construction and testing of eight objective functions with an aim to identify the most appropriate formulation. Four objective functions relate to the operational aspect of the LNG process, while four concentrate on the design aspect. It was found that the most effective operation optimisation objective function is the minimisation of the major operating cost, being compressor power (W-s). For the design objective functions, the minimisation of Net Present Value (NPV) is favoured where no restriction exists on the area available for LNG plant construction while minimising the objective function (W-s-UA) is favoured in case where a limit on the plant area is imposed. Finally, a methodology is constructed for using both design and operation objective functions over the life of the LNG plant, considering gas field feed profile. (c) 2012 Elsevier Ltd. All rights reserved.