Offshore wind turbines are designed and analyzed using comprehensive simulation tools (or codes) that account for the coupled dynamics of the wind inflow, aerodynamics, elasticity, and controls of the turbine, along with the incident waves, sea current, hydrodynamics, mooring dynamics, and foundation dynamics of the support structure. This paper describes the latest findings of the code-to-code verification activities of the Offshore Code Comparison Collaboration Continuation project, which operates under the International Energy Agency Wind Task 30. In the latest phase of the project, participants used an assortment of simulation codes to model the coupled dynamic response of a 5-MW wind turbine installed on a floating semisubmersible in 200 m of water. Code predictions were compared from load case simulations selected to test different model features. The comparisons have resulted in a greater understanding of offshore floating wind turbine dynamics and modeling techniques, and better knowledge of the validity of various approximations. The lessons learned from this exercise have improved the participants’ codes, thus improving the standard of offshore wind turbine modeling.

In the design, certification, and optimization of offshore wind turbines, extensive loads simulation is inevitable to develop reliable and cost-effective turbines. Description of the marine environment is based on a variety of techniques taking the stochastic nature of both, the wind and the water waves into account. The wind turbine is a highly dynamic system including effects of heavy rotating machinery and other significant nonlinearities leading to static, cyclic, transient, and stochastic loads. Due to the nature of the external loading, the system properties and the turbine design lifetime, offshore wind turbines are prone to fatigue-driven failure. For loads assessment, aero-hydro-servo-elastic tools are used including the coupled effects of the environment and the turbine to simulate the overall lifetime of the turbine in the harsh marine environment. These tools are constantly further developed and adapted to the needs of a fast-growing industry and newly arisi ng turbine concepts. Engineering approaches to allow for certification and reliable design are summarized in extensive guidelines and standards supporting engineers in daily design work

This work presents the results of a benchmark study on aero-servo-hydro-elastic codes for offshore wind turbine dynamic simulation. The codes verified herein account for the coupled dynamic systems including the wind inflow, aerodynamics, elasticity and controls of the turbine, along with the incident waves, sea current, hydrodynamics and foundation dynamics of the support structure. A large set of time series simulation results such as turbine operational characteristics, external conditions, and load and displacement outputs was compared and interpreted. Load cases were defined and run with increasing complexity to trace back differences in simulation results to the underlying error sources. This led to a deeper understanding of the underlying physical systems. In four subsequent phases-dealing with a 5-MW turbine on a monopile with a fixed foundation, a monopile with a flexible foundation, a tripod and a floating spar buoy-the latest support structure developments in the offshore wind energy industry are covered, and an adaptation of the codes to those developments was initiated. The comparisons, in general, agreed quite well. Differences existed among the predictions were traced back to differences in the model fidelity, aerodynamic implementation, hydrodynamic load discretization and numerical difficulties within the codes. The comparisons resulted in a more thorough understanding of the modeling techniques and better knowledge of when various approximations are not valid. More importantly, the lessons learned from this exercise have been used to further develop and improve the codes of the participants and increase the confidence in the codes' accuracy and the correctness of the results, hence improving the standard of offshore wind turbine modeling and simulation. One purpose of this paper is to summarize the lessons learned and present results that code developers can compare to. The set of benchmark load cases defined and simulated during the course of this project-the raw data for this paper-is available to the offshore wind turbine simulation community and is already being used for testing newly developed software tools. Despite that no measurements are included, the large number of participants and the-in general-very fine level of agreement indicate high trustworthy results within the physical assumptions of the codes and the simulation cases chosen. Other cases, such as large prebend flexible blades, large wind shear, large yaw error or transient maneuvers, may not show the same level of agreement. These cases were deliberately left out because the focus is on the specific offshore application. Further on, this benchmark study includes participating codes and organizations by name (contrary to several previous benchmark studies) that gives the reader a chance to find results from one particular code of interest

At Fraunhofer IWES a Modelica Library including all major components needed for load calculations of current offshore wind turbines is developed. The library additionally includes models for external conditions, like wind, soil and waves, and their respective influence on the structures. The library constitutes a large effort in the creation of a highly coupled multiphysics model with Modelica for an industrial project. The results obtained with this library are compared to the results from the IEA Wind Task 23 project OC3 1 (Offshore code comparison collaboration). The OC3 project is an international effort to define a set of loadcases and a reference wind turbine that are used to verify simulation systems on a code-to-code basis. In this paper the status and the implemented theories of the individual models at IWES are explained and verification results are presented and discussed.

The main objective of the Offshore Code Comparison Collaboration Continuation, with Correlation (OC5) project, is validation of aero-hydro-servo-elastic simulation tools for offshore wind turbines (OWTs) through comparison of simulated results to the response data of physical systems. Phase III of the OC5 project analyzes the Senvion 5M wind turbine supported by the OWEC Quattropod from the alpha ventus offshore wind farm. This paper shows results of the verification of the OWT models (code-to-code comparison). A subsequent publication will focus on their validation (comparison of simulated results to measured physical system response data). Based on the available data, the participants of Phase III set up numerical models of the OWT in their simulation tools. It was necessary to verify and to tune these models. The verification and tuning were performed against an OWT model available at the University of Stuttgart – Stuttgart Wind Energy (SWE) and documentation provided by Senvion and OWEC Tower. A very good match was achieved between the results from the reference SWE model and models set up by OC5 Phase III participants.