Structural Control Strategies for Load Reduction of Floating Wind Turbines

Doktorgradsavhandling ved Fakultet for Teknologi og realfag, Universitetet i Agder, 2015Offshore wind energy has attracted great worldwide attention in recent years, while strong potentials have been found in deep sea areas in many places, such as the coastal lines of the United States, north Europe, and east Asia. According to extensive experiences in offshore industry, floating foundation for wind turbines is considered as an economical and applicable solution. So far, plenty of numerical investigations have been conducted by world-wide research institutions, and different kinds of prototype programs have also been launched, including OC3-Hywind, MIT/NREL TLP, ITI Barge, and Principle Power WindFloat, etc. One big challenge for floating windmills different from fixed bottom installations is the extra platform motion, which will heavily increase the load on turbine structure due to the high inertial and gravitational forces or even cause the failure of turbine control strategy. Special mechanical design or advanced control technique is required to improve wind turbine reliability, and effective load reduction methods are needed for the design of floating wind turbines. Among different approaches for load mitigation, structural control has offered a direct solution to dynamically compensate the vibrations of turbine structures and reduce their loads. This dissertation is mainly about the numerical investigations of different structural control ideas for load reduction of floating wind turbines. The state-of-the-art wind turbine simulator FAST-SC (customized for structural control analysis) is used in the simulation analysis, and different scenarios, including the below rated, rated, and parked situations, are considered respectively. Papers A and B are dealing with the parameter optimization problem of a spar-type floating wind turbine equipped with tuned mass dampers (TMDs). The passive structural control devices can either be installed inside the platform (Paper A) or along the nacelle (Paper B). Different performance indices and parameter optimization methods are adopted for TMD parameter determination, including frequency analysis, exhaustive search, and intelligent algorithms. Particularly, a mathematical model for wind turbine surge-heavepitch motion is established based on the D’Alembert’s principle of inertial forces. Paper C investigates the idea of installing tuned liquid column dampers (TLCDs) in floating wind turbines for load reduction, and the code FAST-SCTLCD is implemented based on FAST-SC for fully coupled high-fidelity wind turbine simulation with semi-active structural control channel. Optimal parameters are computed by using genetic algorithm based on the established model, while how to tune the head loss coefficient remains to be investigated. Paper D proposes a gain scheduling H2/H∞ active structural control deign for a hybrid mass damper (HMD) installed at the tower top of a floating wind turbine. The wind turbine dynamic model is improved in this work based on polynomial curve fitting approach, and different steady-state points are derived. The state feedback controller is designed by solving linear matrix inequalities (LMIs). However, full-state feedback controller is technically impossible to implement due to lack of sensors, while the observer-based control design could be a possible solution. Then, Paper E discusses this idea, and an observer-based guaranteed cost structural controller is developed

Modeling and Parameter Analysis of the OC3-Hywind Floating Wind Turbine with a Tuned Mass Damper in Nacelle

Floating wind turbine will suffer from more fatigue and ultimate loads compared with fixed-bottom installation due to its floating foundation, while structural control offers a possible solution for direct load reduction. This paper deals with the modelling and parameter tuning of a spar-type floating wind turbine with a tuned mass damper (TMD) installed in nacelle. First of all, a mathematical model for the platform surge-heave-pitch motion and TMD-nacelle interaction is established based on D’Alembert’s principle. Both intrinsic dynamics and external hydro and mooring effects are captured in the model, while tower flexibility is also featured. Then, different parameter tuning methods are adopted to determine the TMD parameters for effective load reduction. Finally, fully coupled nonlinear wind turbine simulations with different designs are conducted in different wind and wave conditions. The results demonstrate that the design of TMD with small spring and damping coefficients will achieve much load reduction in the above rated condition. However, it will deteriorate system performance when the turbine is working in the below rated or parked situations. In contrast, the design with large spring and damping constants will produce moderate load reduction in all working conditions

Analysis and real-time prediction of the full-scale thrust for floating wind turbine based on artificial intelligence

In this paper, numerous aero-hydro-servo-elastic coupled simulations are carried out in time-domain to observe the performance of the real-time thrust acting on the rotor of the OC3-Hywind offshore floating wind turbine. And the studying focuses on investigating the correlation between inputs (surge motion, pitch motion, wind conditions, etc.) and the targeted output (rotor thrust) in the time domain. Besides, artificial intelligence (AI) techniques are used to estimate a prediction model of real-time thrust based on the data from simulations. To predict the thrust, data for four comparative coupled environmental conditions are considered, by which the effect of turbulence and wave spectrum on the thrust force is also investigated. Moreover, a series of simulations of frequency-increasing regular wave conditions and speed-increasing wind conditions are carried out to observe their effect on the real-time rotor thrust. Additionally, the impact of the pitch and surge RAOs of the floating foundation and the wind velocity are quantitatively studied. It reveals that the high-frequency response of thrust is dominated by wave change, whereas low-frequency response is dominated by wind change. Besides, one simulation model of the thrust acting on the rotor is estimated regarding high-frequency and low-frequency response separately to account the dominating influence

Passive Inerter-Based Network Self-Induced Oscillations Damping for Spar-Buoy Floating Offshore Wind Turbines

This contribution analyses the influence of a passive inerter-based network on the stability of the 5MW NREL FOWT with a spar-buoy foundation when the system is subjected to the specific problem of self-induced oscillations. In this work, the idea of incorporating an inerter-based network with a classic tuned mass damper (TMD) in the nacelle is explored. The main objective is to show the effectiveness of the introduced network and demonstrate its benefit in the reduction of the oscillation amplitude when self-induced instabilities occur in comparison to the model equipped with TMD-only. It was demonstrated that the inerter-based network reduces the oscillation amplitude by over 90% and assures system stability when the loss of platform damping occurs

Enhancing the reliability of floating offshore wind turbine towers subjected to misaligned wind-wave loading using tuned mass damper inerters (TMDIs)

Floating offshore wind turbines (FOWTs) are the largest rotating structures on the earth. Dynamically sensitive structures such as these must be protected in these environments to ensure that they can continue to operate reliably and safely. In this paper structural dynamic models and probabilistic assessment tools are combined to demonstrate improvements in structural reliability when FOWT towers are equipped with a new type of damper — the tuned mass damper inerter (TMDI). A multi-body dynamic approach is used to model the wind turbine and the TMDI installed in the tower. The model is subjected to stochastically generated wind and wave loads of varying magnitudes to develop wind-induced probabilistic demand models for towers of FOWTs under model and load uncertainties. A focus is placed on the impact of the wind-wave misalignment on the lightly damped side-to-side mode. Numerical simulations are carried out to construct fragility curves which illustrate reductions in the vulnerability of FOWTs to wind and wave loading owing to the inclusion of the new damper. Results show that the TMDI delivers significant increases in structural reliability of FOWT towers

Complementary Airflow Control of Oscillating Water Columns for Floating Offshore Wind Turbine Stabilization

The implementation and integration of new methods and control techniques to floating offshore wind turbines (FOWTs) have the potential to significantly improve its structural response. This paper discusses the idea of integrating oscillating water columns (OWCs) into the barge platform of the FOWT to transform it into a multi-purpose platform for harnessing both wind and wave energies. Moreover, the OWCs will be operated in order to help stabilize the FOWT platform by means of an airflow control strategy used to reduce the platform pitch and tower top fore-aft displacement. This objective is achieved by a proposed complementary airflow control strategy to control the valves within the OWCs. The comparative study between a standard FOWT and the proposed OWC-based FOWT shows an improvement in the platform’s stability.This work was supported in part by the Basque Government, through project IT1207-19 and by the MCIU/MINECO through the projects RTI2018-094902-B-C21 and RTI2018-094902-B-C22 (MCIU/AEI/FEDER, UE)

STRUCTURAL CONTROL OF OFFSHORE WIND TURBINES USING PASSIVE AND SEMI-ACTIVE CONTROL

Offshore wind energy has the potential to generate substantial electricity production compared to onshore locations, due to the high-quality wind resource. Offshore wind turbines must endure severe offshore environmental conditions and be cost effective, in order to be sustainable. As a result, load mitigation becomes crucial in successfully enabling deployment of offshore wind turbines. A direct approach to reduce loads in offshore wind turbines is the application of structural control techniques. So far, the application of structural control techniques to offshore wind turbines has shown to be effective in reducing fatigue and extreme loads of turbine structures. However, the majority of previous research regarding the application of structural control to offshore wind turbine noted the needs for the high-fidelity analysis for structural control using a computer aided engineering (CAE) tool, such as FASTv8. In this dissertation, a structural control module coupled with FASTv8 is developed to meet the needs for high-fidelity analysis of structural control techniques for various OWTs. In addition, the developed control module is updated to analyze various structural control devices operating both passively and semi-actively. The dynamics of an omni-directional pendulum-type tuned mass damper and orthogonal tuned liquid column dampers (TLCDs) are mathematically modeled and incorporated into the structural control module. With the developed control module, several structural control devices are optimized through a variety of techniques (parametric study, exhaustive search and multi-objective optimization). Solving optimization problems not only provides the parameters for each control device that can be applicable to other multi-megawatts offshore wind turbines, but also provides insight into the effects of design variables on the control performance. Site-specific meteorological and oceanographic data that consists of a combination of wind and wave data are processed and compiled in order to establish key design load cases. With the optimal designs of structural control devices, non-linear fully-coupled time marching simulations are conducted by running a series of design load cases in order to investigate the impacts of passive and semi-active structural control on improving fatigue and extreme behaviors of fixed-bottom and floating offshore wind turbines. The simulation results demonstrate the effectiveness of various structural control techniques on reducing fatigue and extreme loadings

Performance Analysis on the Use of Oscillating Water Column in Barge-Based Floating Offshore Wind Turbines

Undesired motions in Floating Offshore Wind Turbines (FOWT) lead to reduction of system efficiency, the system’s lifespan, wind and wave energy mitigation and increment of stress on the system and maintenance costs. In this article, a new barge platform structure for a FOWT has been proposed with the objective of reducing these undesired platform motions. The newly proposed barge structure aims to reduce the tower displacements and platform’s oscillations, particularly in rotational movements. This is achieved by installing Oscillating Water Columns (OWC) within the barge to oppose the oscillatory motion of the waves. Response Amplitude Operator (RAO) is used to predict the motions of the system exposed to different wave frequencies. From the RAOs analysis, the system’s performance has been evaluated for representative regular wave periods. Simulations using numerical tools show the positive impact of the added OWCs on the system’s stability. The results prove that the proposed platform presents better performance by decreasing the oscillations for the given range of wave frequencies, compared to the traditional barge platform.This work was supported in part by the Basque Government, through project IT1207-19 and by the MCIU/MINECO through the projects RTI2018-094902-B-C21 and RTI2018-094902-B-C22 (MCIU/AEI/FEDER, UE)