With the deployment of the TetraSpar demonstrator, a significant cost-reduction is realized in the field of offshore floating wind turbines. The TetraSpar floating wind turbine foundation brings a milestone that emphasizes on a modular and fully industrialized foundation that consists of main components already widely available in the current wind energy supply chain. In an effort to provide an open approach to the development of the concept, this paper aims at giving a description of the design in order to enable an educated discussion of different design philosophies and their influence on material usage and production times. The description of the different subcomponents of the system should allow any entity to build a model for comparison and/or benchmarking any of their own findings against this concept. It is the authors’ expectation that this open approach to technological discussion is paramount to obtaining continued cost-reduction in the area of floating offshore wind—for this concept and others.

New floating wind turbine designs are needed to reduce production costs and to increase mass production feasibility. The TetraSpar floating wind turbine achieves these goals by being constructed using components highly suitable for standardization and industrialization. The design makes use of a suspended submerged counter weight to obtain a low center of gravity of the floating system, while also allowing a low draft during transport and installation. This novel concept requires a multibody modeling approach to perform a dynamic load and response analysis, as the stiffness between the floating platform and the counter weight is provided by chains. Additional design criteria are required for the counter weight system dependent on a combination of chain capacity and maintaining positive tension in all of the lines. To satisfy these design criteria a global hydrodynamic load and response analysis of the floater and counter weight is performed. In this concept, the counter weight depth contributes significantly to the dynamic properties of the system and therefore a parametric study is conducted. The global response parameters of the rigid-body motion natural frequencies, nacelle accelerations, counter weight chain tensions, and maximum platform-pitch angles are compared. Design recommendations are made for the configuration of counter weight depth and suspension system layout.

Numerical models have been used extensively in the design process of the TetraSpar floating offshore wind turbine (FOWT) foundation to optimize and investigate the influence from a number of structural and environmental conditions. In traditional offshore design, either the Morison approach or a linear boundary element method (BEM) is applied to investigate the hydrodynamic loads on a structure. The present study investigated and compared these two methods and evaluated their applicability on the TetraSpar FOWT concept. Furthermore, a hybrid model containing load contributions from both approaches was evaluated. This study focuses on motion response. In the evaluation, hydrodynamic data from BEM codes are applied, while the commercial software package OrcaFlex is utilized for time series simulations of the coupled structure. The investigation highlights the difference between the modelling approaches and the importance of particularly drag and inertia contributions. By optimizing the input coefficients, reasonable agreement between the models can be achieved.

Floating offshore wind turbine technology has seen an increasing and continuous development in recent years. When designing the floating platforms, both experimental and numerical tools are applied, with the latter often using time-domain solvers based on hydro-load estimation from a Morison approach or a boundary element method. Commercial software packages such as OrcaFlex, or open-source software such as OpenFAST, are often used where the floater is modeled as a rigid six degree-of-freedom body with loads applied at the center of gravity. However, for final structural design, it is necessary to have information on the distribution of loads over the entire body and to know local internal loads in each component. This paper uses the TetraSpar floating offshore wind turbine design as a case study to examine new modeling approaches in OrcaFlex and OpenFAST that provide this information. The study proves the possibility of applying the approach and the extraction of internal loads, while also presenting an initial code-to-code verification between OrcaFlex and OpenFAST. As can be expected, comparing the flexible model to a rigid-body model proves how motion and loads are affected by the flexibility of the structure. OrcaFlex and OpenFAST generally agree, but there are some differences in results due to different modeling approaches. Since no experimental data are available in the study, this paper only forms a baseline for future studies but still proves and describes the possibilities of the approach and codes.

The potential of offshore wind is enormous. It could meet Europes electric energy demand seven times over, and the United States energy demand four times over. However, much of the offshore potential is at water depths that can only be served by floating systems. In order to truly enable floating offshore wind, the cost of energy needs to reach the level of fixed-bottom offshore wind. At the present time, a number of suppliers are offering floating offshore wind foundations, but at cost levels that are prohibitive for large-scale application. The root cause of the high cost levels is that existing designs have emerged from the offshore oil and gas sector; they are manufactured using conventional, non-industrialized methods, weights are measured in thousands of tons and manufacturing times are measured in months. In contrast, the TetraSpar concept is based on the application of proven design and manufacturing technologies from the highly competitive wind industry. As a result, the weight is only a fraction of the weight of other floating wind turbine foundations, manufacturing takes place in factories using industrialized methods, and assembly and installation is measured in days or weeks, not months. The foundation and wind turbine can be installed in any port of a reasonable size using a standard, land-based crane, and the complete assembly can be towed to site and hooked up to the moorings and the electrical cable using standard tugs. This paper presents how these desirable economic traits of the TetraSpar design are achievable, and how the near future feasibility of offshore floating wind turbines may develop as a consequence of this radical change in cost levels.