ITTC Recommended Guidelines: Model tests for offshore wind turbines (7.5-02 -07-03.8)

The purpose of this document is to offer guidance to research organisations on performing model tests of offshore wind turbines (OWTs) according to the state of the art. These tests may include:

• measurements of foundation loads for bottom-mounted (fixed) OWTs;
• measurement of hydro-elastic response of OWT towers;
• measurements of global dynamic response of OWTs, including responses to specified design load cases, measurement of natural period of motion, and additional damping for floating offshore wind turbines (FOWTs);
• measurement of maximum offset of moored OWTs, allowing selecting appropriate length of the power umbilical;
• investigation of the interaction between the rotor aerodynamics and the dynamic response of the support structure;
• quantification of technical performance variables; validation of numerical models;
• investigation of operational and survivability characteristics limits, including responses to impulsive loadings, such as slamming or collision;
• investigation of transportation and installation methodology This guideline does not address the assessment of the aerodynamic performance of offshore wind turbines.

Many aspects of the experiments for floating offshore wind turbine structures are covered by the ITTC Recommended Procedure 7.5-02-07-03.1 Floating Offshore Platform Experiments.

However there are some key differences between model tests of OWTs and model tests of other offshore structures. The main distinctive features of the tests of OWTs may include:

• Requirement to simulate complex kinematics and material properties driving key interactions within the OWT system between aero-elastic response of blades, dynamics of rotor and nacelle assembly (RNA), hydroelastic response of towers and support structures, gyroscopic loading, and hydrodynamic response of floating platforms
• Special requirements for the model construction, in particular related to the rotor;
• Large size of full-scale structures: 7MW turbines are approaching 200m in height with rotor diameters up to 170m;
• Challenges related to dynamic similarity and scaling for aerodynamic and hydrodynamic flows for model testing in wind/wave tanks;
• Challenges of accurate simulation of realistic aerodynamic environment over large areas of test tanks;
• Requirement for analyses of design load cases from International Electrotechnical Committee (IEC) standards;
• Rapid evolution of design of FOWTs: diversity of innovative concepts, some presenting challenges for scaled model testing;
• Requirement for testing throughout the various experimental stages for novel floating concepts: the concept validation stage, the design validation stage, the system validation stage, and the prototype and demonstration stage.
• Potential requirement for tests of multiple scaled models corresponding to an array of OWTs.
• Particular challenges in simulation of unconventional, innovative mooring systems. The permanent mooring systems often include synthetic fibre ropes, which require special treatment in model testing.

These features place particular demands upon the experiment design, model construction; facility capability, and experiment procedure.