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.

The catenary mooring system is a well recognized station keeping method. However, there could be economical and environmental benefits of reducing the footprint. In the last decades, more focus has been given to synthetic mooring lines and different mooring layouts to optimize the levelized cost of energy (LCOE) for offshore renevable energy converters such as wave energy converters. Therefore, this work presents a parametric study of two important parameters, namely the mooring line angle and line pretension, for a taut mooring configuration focusing on the dynamic response when applied to the TetraSpar floating foundation compared to a catenary mooring system. The work is based on experimental results conducted in the wave basin at Aalborg University (AAU) and compared to analytical stiffness calculations. In addition, a numerical model was tuned based on the main dynamics to achieve the tension response. The results showed satisfying dynamic behavior where the angle and pretension mainly influenced the surge and yaw natural periods. The motion response showed similar behavior between the chosen parameters, and larger pitch amplitudes were found compared to the catenary system.

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.

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.

As part of the process of deploying new floating offshore wind turbines, scale model testing is carried out to de-risk and verify the design of novel foundation concepts. This paper describes the testing of a 1:43 Froude-scaled model of the TetraSpar Demo floating wind turbine prototype that shall be installed at the Metcentre test facility, Norway. The TetraSpar floating foundation concept consists of a floater tetrahedral structure comprising of braces connected together through pinned connections, and a triangular keel structure suspended below the floater by six suspension lines. A description of the experimental setup and program at the Alfond W2 Ocean Engineering Lab at University of Maine is given. The objective of the test campaign was to validate the initial design, and contribute to the development of the final demonstrator design and numerical models. The nonlinear hydrodynamic characteristics of the design are illustrated experimentally and the keel suspension system is shown to satisfy design criteria.