1. Introduction
For the past few years, the development of offshore wind power in China has become increasingly attractive because of abundant, steady and strong wind resources and available spaces for offshore wind turbine installation in the southeast coast to meet the strong power needs in most electricity-poor developed cities. Traditional steel towers have been widely employed in offshore wind turbine support structures with various substructures, such as monopile, jacket, and gravity base foundations [
1,
2]. Wang [
3] presented a transformed linear Gaussian model for generating equivalent “nonlinear” irregular waves to assess the mechanical responses of an offshore jacket wind turbine support. Based on elastic and plastic analyses of monopile foundation, Campione [
4] proposed a simplified approach to calculate the soil–structure interaction (SSI) of offshore wind tower founded on a monopile member. Aidibi et al. [
5] investigated the stress concentration of the joints by using analytical calculations and numerical solutions. The results show that the stress concentration factors from the standards are more conservative than those from the finite element method. Unfortunately, the most widely used support structures for offshore wind turbines may not be economical enough for the construction of offshore wind turbines with huge capacity in deep seas [
6].
The concrete structure is an attractive alternative for lower production costs, better durability, and lower corrosion protection and maintenance costs, especially in salty atmospheres and harsh environments. Tu et al. [
7] investigated the nonlinear dynamic response of the concrete-based structure with infill aggregates for offshore wind turbines. The results show that the dynamic response of the gravity base foundation under combined wind and wave loads exhibits similar but less smooth curves to that under the wind loads only. Vølund [
8] compared the costs of utilizing concrete foundations against steel monopile foundations for offshore wind turbines and discovered that the concrete foundation can greatly reduce the investment of wind turbine support structures. Ma and Yang [
9] proposed and investigated a novel hybrid monopile foundation for an offshore wind turbine tower by replacing the conventional substructure with a steel–concrete hybrid structure. Lian et al. [
10] and Zhai et al. [
11] numerically and experimentally studied a suction bucket foundation with a prestressed concrete (PC) substructure for a 2.5 MW offshore wind turbine and verified its feasibility, respectively.
Prestressed concrete–steel hybrid (PCSH) wind turbine towers have been an attractive alternative for onshore wind turbine tower design. Chen et al. [
12] investigated the seismic responses of the PCSH wind turbine tower and indicated that time history analyses should be a necessary supplement for its seismic design. Huang et al. [
13] conducted a sensitivity analysis to study the relationship between the natural frequency and the dimensions of PCSH wind turbine towers and optimized a 160 m PCSH wind turbine tower. Li et al. [
14] proposed a two-scale model to explore the global and local mechanical behavior of PCSH wind turbine tower.
The cost of maintaining, installing, and manufacturing offshore wind turbine support structures occupies at least 30–50% of the overall capital cost [
15]. Several attempts have been made to find a cost-efficient support structure to make offshore wind energy compete with other traditional energy resources [
16,
17]. Optimization of all wind turbine support structures is critical for the realization of potential cost benefits and safety requirements. Kaveh and Sabeti [
18] optimized jacket support structures for offshore wind turbines utilizing the colliding bodies optimization algorithm and approximately halved the weight of the structure. Treating the frequency as the optimal objective, Natarajan et al. [
19] optimized the jacket offshore support structures for 10 MW wind turbines to alleviate the fatigue damage. Integrating kriging-based heuristic optimization, Mathern et al. [
20] carried out an approach to optimize the wind turbine foundation of a Swedish wind farm and concluded that the proposed method can provide good-quality designs with an initial sample size of only 20 designs.
In this paper, an offshore PCSH wind turbine support structure with a foundation composed of four steel piles is first proposed. The lower part of the traditional steel tube support tower and the substructure of a jacket-type offshore wind turbine platforms are replaced with PC tube. Then, taking a four-pile jacket-type 5.5 MW offshore wind turbine support structure with a hub elevation of 102.3 m as an example, the optimal design of the proposed alternative PCSH support structure with a four-pile foundation is investigated using a parallel modified particle swarm optimization (PMPSO) algorithm, where environmental influences, including the wind and wave loads, the seismic effect and the SSI are considered. The objective is to minimize the construction cost, and the optimization results for the PCSH support structure are compared with those of the original design. The results show that the optimal PCSH support structure can fulfill the design requirements but with a lower construction cost and comprehensive investment when compared with the original design. In addition, the optimal design results show that the steel tube of the optimized PCSH structure is suggested to occupy about 25.8% of the overall height of the PCSH support structure. Finally, the mechanical behavior of the optimal PCSH support structure with pile foundation is analyzed and compared with that of the traditional steel support structure.
2. Concept Description
In this paper, referring to the overall PC offshore wind turbine substructure proposed by Lian et al. [
10], a PC segment is adopted to replace the lower part of the traditional full-height tapped steel tube tower and the substructure of the offshore wind turbine support structure to decrease the construction cost of the offshore wind turbine. The proposed offshore PCSH support structure with a four-pile foundation consists of the PC substructure and PCSH tower, as illustrated in
Figure 1.
The lower part of the tower and the substructure are made of PC and the tower’ upper segment is a conventional tapered steel tube tower. Like the PC foundation for offshore wind turbines [
10], the offshore PCSH support can be fabricated onshore and towed to the designated construction site, and then submerged [
21]. Pile foundations are employed to reduce the weight of the structures when the wind turbine supports are built in soft clay [
22]. Compared with a traditional pure steel tube support structure, the PCSH structure reasonably results in a cheaper construction investment, lower center of gravity, better integrity, higher flexural stiffness, lower cost for corrosion protection, and less maintenance, especially in harsh environments. Therefore, the PCSH structure is a promising alternative for the offshore wind turbine support structures. The optimization of the proposed PCSH support structure with a pile foundation is critical and to be addressed in this study.
6. Conclusions
This paper presented a PCSH support structure for offshore wind turbines that consists of a PCSH tower, a PC substructure, and a pile foundation, and a PMPSO algorithm was adopted to optimize the design of the proposed PCSH support tower with the pile foundation used for the soft soil layer. The cost was considered as the optimization objective function under eleven optimization constraints. A 5.5 MW wind turbine supported by a steel tube and conventional jacket foundation was used as a reference model for comparison. The SSI and the effect of water pressure and earthquake were considered. The offshore PCSH wind turbine support structure with a pile foundation was modeled as a multi-degree-of-freedom system. A geometrically optimal result for the offshore PCSH support tower with the PMPSO algorithm was obtained and compared with that of the four-pile jacket-type offshore wind turbine support structure and the optimized results with PSO. Based on this study, the following conclusions can be made:
The cost of the optimized PCSH wind turbine support structure obviously decreases when compared with the original design, showing it is a cost-efficient alternative for traditional offshore wind support structures for lower cost requirements.
The height of the steel tube is recommended to occupy about 25.8% of the overall height of the PCSH support structure for offshore wind turbine.
Compared with the four-pile jacket-type offshore wind turbine support structure, the optimized offshore PCSH support structure can provide better mechanic behavior, including the first natural frequency, top deformation, and anti-overturning capacity.
The PMPSO algorithm provides better performance compared to the PSO algorithm. Fulfilling the design constraints, the PMPSO algorithm provides a more affordable primary optimal design for the PCSH support structures for offshore wind turbines with a pile foundation with considerably improved computational efficiency.
In this study, the PCSH support structure is assumed as a linear structure with small deformation. Further studies are desired for the optimization of the PCSH support structure with numerical and experimental analysis when the material’s nonlinearity, geometric nonlinearity, construction time and adaptability to site-specific conditions are considered.