With the increase in need for offshore wind exploitation a research is set up with aims to develop a preliminary design tool. The introduction begins with background information on what role floating wind will play in the offshore wind industry and why this is a relevant topic. A general overview of floaters is presented with a comparison of the advantages of each type of floater. As well a chronological review of approaches to preliminary design. After which a numerical load and response model is developed. First theory is discussed regarding load and response modelling as well as the fatigue damage calculations. After the theory is covered, a test case is set up using the IEA 15MW reference turbine. The turbine is subjected to aerodynamic and hydrodynamic loading. This loading is calculated using Morison’s equations and a simplified thrust coefficient. In the test case the IEA15MW reference turbine is put atop a large floating substructure. For the environmental condition a site off the coast of Norway is used with an adjusted depth to allow for the size substructure. The response calculation is then, where the equations of motion are set up in matrix form, and terms in all of the matrices are then derived for the mass, added mass, stiffness and damping matrix. The stiffness matrix considers hydrodynamic stiffness and mooring stiffness. The terms in the damping matrix are derived from aerodynamic and hydrodynamic damping. Hydrodynamic damping only considers viscous damping terms. The model is then tested for a set of regular and irregular wave and wind conditions to analyse the system’s behaviour. For the fatigue calculation the substructure is divided into welded areas that will be investigated for lifetime global fatigue. This response model is validated by comparing natural frequencies of a smaller well documented 5MW floating spar. Optimization theory is covered, where multiple gradient and non-gradient based approaches are discussed. The simulated annealing algorithm is developed and tested on a set of test functions: Ackley & Booth. The algorithm shows quick convergence for areas that have small changes. The optimization problem for this research is formulated. The design space is visualised and compared to the most similar test function. After which, the constraints used by the optimiser are presented. The first constraint set consists of logical design constraints determined by the spar geometry. After which, the constraint set also includes limitations on the extreme response in time domain simulation. In the results chapter, it is found that this slows down the process so much that it is not feasible to include fatigue damage in the constraint function. Furthermore, it is found that global fatigue damage wouldn’t be a governing constraint. The results chapter discusses the importance of mooring stiffness in this optimization. Present the results from constraining the optimiser with the time domain response and compare the fatigue lifetime of the optimal designs. It is found that global fatigue is not a governing design consideration. It is found that both geometrical and response constraints are relevant for this type of optimization. However using global fatigue as a constraint is both irrelevant and computationally infeasible for the optimization of a floating substructure. Finally, a discussion is presented where the work is critiqued, and recommendations are made for further work.