PO047

Cross Comparison of two Analysis Tools for a Braceless Semi-Submersible Wind TurbineVersus Ocean Basin Test Results

Nikolaos Lampropoulos 1, Madjid Karimirad2 ,3
1Center for Renewable Energy Sources and Saving (CRES), Pikermi, Greece, 2Norwegian Marine Technology Research Institute (MARINTEK), Trondheim, Norway, 3Queen's University Belfast, Northern Ireland, Belfast, UK

Abstract

In the frame of the IRPWIND project (Integrated Research Programme on Wind Energy), a benchmark is going on for comparing the numerical tools versus results obtained from ocean basin
tests. A braceless semi-submersible wind turbine platform is considered herein. Real-time hybrid model (ReaTHMŪ) tests were done at 1:30 scale in MARINTEK’s Ocean Basin in 2015. The goal was to estimate the performance of a novel hybrid technique which avoids the use of a physical turbine for reproducing the aerodynamic loads. Instead, these being representative of the Northern North Sea wind conditions, were in real time calculated by a Blade Element Momentum (BEM) software and subsequently applied by a 5-degree-of-freedom actuators on the structure. The mooring system was typical of a three catenary mooring lines while the waves were produced by flapping mechanisms at basin's borders. The above mentioned system poduced experimental results that were taken as reference for calibrating numerical hydrodynamic and structural models. The former ones are based on panel methods derived from potential theory. In extreme weather conditions, the hull supporting the wind turbine may move significantly while the free surface of waves may break. The numerical simulations of such events are challenging and more research could advance better understanding the nonlinear dynamics associated to those events. A cross comparison between SIMA software based on a potential theory model with a CFD method as well as benchmarking against the experiment for two extreme wave regimes are attempted. The simulation focuses on the hull response to hydrodynamic and mooring forces.

Method

Two distinct methods are cross-compared in this work. On the one hand, SIMA (Advanced Analyses of Marine Operations and Floating Systems) is the state of the art numerical tools of MARINTEK for coupled/integrated simulations of floating offshore structures considering aero-hydro-servo-elastic formulations. It is about a nonlinear time-domain simulation tool which can capture all of the relevant hydrodynamic and aerodynamic loads, incorporate the control system actions and logic, and compute the structural response. On the other hand, the CFD method is a modular coupled algorithm comprised of the following open source tools: The two phase turbulent Navier-Stokes solver on moving meshes, OpenFoam, the library waves2Foam as a toolbox that generates and absorbs free surface water waves and the mooring lines MoorDyn code.

Results

CFD simulation are cross compared with SIMA output in terms of the global dynamic responses and mooring line tension exerted on the semi-submersible hull. Both simulations are benchmarked against experimental results at extreme sea state, i.e. with irregular waves present as reported by
tests no. 2420, 2410. It is verified that SIMA and test results agree well by tuning the viscous
excitation force especially near the free surface (splash zone). Good results produces also the CFD
method which reproduces exactly the hydrodynamic forces under any wave regime with no tuning
requirements. Nevertheless, it is verified that CFD induces a significant CPU overhead being
multiple of that required for panel methods.

Conclusions

This paper presents the simulation of a modular, open source high level tool for the response of
moored floating wind turbines (FWT) under two irregular wave regimes. The results are benchmarked against the predictions of a well calibrated panel method code (SIMA) and experimental data. It is proven that high fidelity tools (CFD) can predict hydrodynamic forces on
moving rigid objects under realistic conditions by incorporating models of the mooring systems by
design tools, such as FAST or SIMA. Vice versa, this end product can enhance design
software so that they can act as analysis tools for the combined aero-hydro-servo-elastic analysis of
FWTs.

Objectives

The present work will initiate the delegates into the concept of modular software. They will learn how to combine open source, high level (CFD) and design tools (SIMA or FAST). They will learn the pros and cons of CFD methods against potential ones and the applicability of eachone. They will realize that, even nowadays, the former are so computationally demanding so they can not substitute potential methods in design phase but they can act as standalone analysis and calibration tools, along with experimental results, of the latter.