This study proposes a multi-objective mathematical optimisation model which includes minimisation of the investment costs of new offshore wind farm units, investment costs of new transmission lines, fuel costs, emissions of the fossil fuel generators as well as the maximisation of incentives for new generating units. The mathematical model can be used to determine the optimal expansion planning for generation and transmission facilities with consideration of the impact of location and feed-in tariffs for offshore wind power on the overall system capacity and efficiency. The proposed model is solved using CPLEX 12.6.3 solver in advanced interactive multidimensional modelling system and tested on the modified Garver 6-bus test system and the IEEE 24-bus reliability test system. Simulation results indicate that improving the incentive support scheme and minimisation of the total cost will improve the effective utilisation of offshore power.