When some wind farms are close to the end of its lifetime, an aging fleet could occur in the wind farm. Then, there are several options to be considered by its wind turbine operators: dismantling the turbine, embarking on a lifetime extension or repowering [
2]. Repowering is the process of replacing older wind turbines with newer ones and of using the same place with the output capacity of power in the wind farm increases. By repowering old wind turbines with new upgrades, the increased size and efficiency of the new turbines will increase the amount of energy from a given wind farm. The repowering can happen in two different approaches involving partial repowering and full repowering. Partial repowering is as small as upgrading the main old components, particularly the rotor and gearbox with new ones whereas retaining other elements such as the foundation and tower. For full repowering, it means that the entire wind turbine systems including the towers and foundations are updated by new units to obtain higher energy efficiency. The repowering of wind farms can keep the wind power plant running and save the total investment costs, as some of the decommissioning and installation expenses can be shared and the wind resource is well known to lower the risk of the project. With less costs and a higher energy output, the repowering process is excessively beneficial. Wind turbines, being the principal technology for the generation of electrical power as wind energy converters, have been extensively investigated with respect to their capacity, effectiveness and safety. A large collection of research results has been accumulated with reference to the structural response of wind energy converters. For instance, Tziavos et al. [
3,
4] performed the structural performance of grouted connections under large moments by using a nonlinear finite element analysis. Li et al. [
5] analyzed the reasons for wind turbine tower collapse under extreme wind loads, and they proposed a robust design for wind turbine towers against typhoons. Kilic et al. [
6] measured and predicted the behavior of wind turbine towers by using wireless sensor networks and accelerometers. Binh et al. [
7] proposed evaluation formulas for the design wind load on the supporting structure in complex terrains, and these formulas have been validated by comparing analytical solutions with the respective finite element model simulations. Kim et al. [
8] carried out seismic analysis of offshore wind turbine towers by considering the soil–pile interaction. The critical displacement was obtained to assess the structural safety under seismic loads by using pushover analysis. Tondini et al. [
9] reported the structural response of high strength steel circular columns subjected to fire loading by comparing numerical and experimental results. Van der Woude et al. [
10] performed parametric studies on base isolation systems to improve the structural response of wind turbine structures during strong earthquake events; it was concluded that the use of base isolation systems reduces possible excessive dynamic displacements of the structures in seismic zones. Tran et al. [
11] described the influence of the door opening on the strength of wind turbine towers by means of detailed finite element models. Do et al. [
12] studied the structural response of towers by taking into account fatigue due to wind loads, aiming to minimize the cost of structural steel, and to optimize the design parameters of the tower base and to achieve a longer fatigue life of the towers. Schneider et al. [
13] presented the structural response of ring-stiffened cylindrical shells of 50 m height under wind loads using finite element analysis. Valamanesh et al. [
14] compared the predicted results with those from a baseline wind turbine tower in operation and at rest, where a reasonable agreement seems to have been achieved. Guo et al. [
15] performed a series of bending tests on tower tubes with stiffeners, to investigate the effect of section slenderness on the behavior of the steel tower tubes, and the respective experimental results are in accordance with the AS4100 design code. Ghazijahani et al. [
16] considered the effect of an opening on the structural response of a cylindrical shell under axial compression. Sabouri-Ghomi [
17] studied the relevant design parameters and in particular, the quantities and dimensions of the stiffening rings with the aim of analyzing their effect on the structural stability of reinforced concrete cooling towers. By using numerical analysis, a method to determine the parameters of the stiffening rings, which could increase the buckling capacity of the cooling towers, was proposed. Perelmuter et al. [
18] formulated an optimization problem for the design of steel wind turbine towers by considering the wall thickness, the diameters of the cross-section and the height as design variables. Sim et al. [
19] reported a parametric study in which a numerical simulation was compared with experimental results on the flexural buckling strength of a wind turbine tower. Hu et al. [
20] studied the effect of varying the number of stiffening rings with respect to wall thickness variation, on the structural response of steel wind turbine towers. Within this framework the most efficient method for selecting the number of stiffening rings and for reducing the wall thickness in order to strengthen the towers and minimize costs was proposed for each height case. Negm et al. [
21] chose the cross-sectional area, radius of gyration and height of each segment as design variables, and formulated the design problem as a nonlinear mathematical programming problem. Shi et al. [
22] investigated the overall buckling of tubular columns composed of high strength steel by applying experimental testing and numerical simulation, and the numerical and experimental results were compared with reference to the analytical solutions obtained by applying current code provisions. Zhu et al. [
23] studied the optimized mesh size and performed a parametric study of steel oval hollow section columns by using one hundred numerical models. Karpat [
24] developed a virtual tool to perform the cost optimization of wind turbine steel towers with rind stiffeners by using the MATLAB procedure, it was found that the variations of the wall thickness and diameter have an important effect on the mass and cost of wind turbine towers. Chen [
25] studied the stress and strain distribution of the reinforced concrete beam–slab foundation under various loading states, they thought that a proposed circumferential pressing technique could reduce the tensile stress of concrete on the top surface of the foundation pier. Ding [
26] monitored the floating performance of an offshore wind turbine tower with a composite bucket foundation during transportation, they found that the wind turbine could meet the specified acceleration value limits during towing.
To repower a turbine, the wind turbine systems including the tower should be replaced. The higher tower and longer blades can generate more output energy with fewer turbines as higher space has faster and more stable winds. As the tower height is closely related to the energy yield, the appropriate supporting structure for a wind turbine should be designed by taking into account cost effectiveness. To facilitate transportation, wind turbine towers are manufactured in sections that are connected in situ during the erection. Typical tubular steel wind turbine towers are composed of cylindrical or conical shells interconnected by bolted flanges. Obviously, the geometric variation of the stiffening rings greatly affects both the strength and the stability of the towers. To improve the economy in the design of such towers, the wall thickness, the mid-section width-to-thickness ratio and the spacing of the stiffening rings of the wind turbine tower at different height levels should be considered as the critical design variables for repowering to update the old wind farm. Wind turbine tower can be repowered based on the conclusion of the efficiency repowering range of design variables so that the upgradation of wind turbine system in a wind farm can be performed more efficiently. Therefore, the results of efficiency repowering range of design variables can be used to propose a new optimum design of the wind turbine system when repowering a wind farm.