Modeling the TetraSpar Floating Offshore Wind Turbine Foundation as a Flexible Structure in OrcaFlex and OpenFAST

Abstract

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.

A Review of Numerical and Physical Methods to Analyze the Coupled Hydro-Aero-Structural Dynamics of Floating Wind Turbine Systems

“…Motions and loads predicted by the open-source OpenFAST and commercial OrcaFlex codes for an FWTS are compared together by [293] and it is shown that the predictions provided by these codes are generally in good agreement.…”

A review of numerical modelling and optimisation of the floating support structure for offshore wind turbines

“…The Simo/Riflex code was verified against some software of the Offshore Code Comparison project such as FAST, Adams and HAWC2 in Luxcey et al (2011); Ormberg et al (2011) while five numerical models that included some variations of a Simo/Riflex/Aerodyn were compared in Luan et al (2017). Motion and loads of the TetraSpar floating platform were compared between OpenFast and OrcaFlex, with a generally good agreement, including the flexibility of the structure (Thomsen et al 2021). The various studies discussed here have generally shown reasonable agreement bewteen numerical models and experimental studies.…”

Efficient multibody modeling of offshore wind turbines with flexible substructures

“…The Institute for Energy Technology in Norway describes in [11] an FEM-approach in their 3DFloat software to model flexible substructures, but 3DFloat is also relatively slow (several times slower than real-time), but it can be parallelized. In [12], flexible substructures of FOWT are simulated both with OpenFAST and OrcaFlex and the results are compared to rigid equivalents. A hydrodynamic potential flow assumption was implemented in a time-domain analysis using modal decomposition in [13].…”