Thesis Chapters by Nico Maljaars
Over the years, the installed capacity of offshore wind turbines has increased rapidly. However, ... more Over the years, the installed capacity of offshore wind turbines has increased rapidly. However, the LCOE (Levelized Costs Of Energy, average price per kWh) is still higher than the LCOE of traditional energy production methods like nuclear power or energy from coals or gas.
This research focuses on further decreasing the LCOE, by minimizing the weight of the support structure of the wind turbine. This is done in a so called integrated way: By optimizing the design of the tower and the foundation at the same time. Due to the interaction between wave loads and structure, many local optima are expected. Therefore an optimization strategy is proposed using a Particle Swarm Optimization (PSO) algorithm followed by a Gradient Based Algorithm on an objective which includes the mass and penalized constraints.
To reduce the optimization time, the evaluations of the objective function are done by using load estimations instead of extensive load calculations. To select reliable estimation methods, several methods are compared on a theoretical basis, after which the performance of a selection of these methods is examined on the problem. To improve the accuracy of the estimations, interaction of PSO and the estimators is proposed via estimator updating.
The proposed optimization approach is compared with the traditional design approach, by the optimization of several realistic case studies. This shows the potentials of using the estimator and optimization tool, developed during this project, in engineering practice.
Papers by Nico Maljaars
Aerodynamic Roll-Yaw Instabilities of Floating Offshore Wind Turbines
This paper presents an investigation of a newly discovered motion instability phenomenon for floa... more This paper presents an investigation of a newly discovered motion instability phenomenon for floating wind turbines. The instability is due to anti-symmetric coupling terms in roll and yaw caused by the turbine thrust force. For floaters with small separation between the uncoupled roll and yaw natural periods these coupling forces may result in rigid body roll and yaw oscillations. The paper explains the theory and the physics of the instability phenomenon, and analytical expressions for these stiffness coupling terms (K46 and K64) are derived. The instability phenomenon is demonstrated using several points of attack, by using time domain simulations, conservation of energy flow and eigenvalue stability analysis. The problem is stripped down to a simplified two degree of freedom roll-yaw model where analytical stability criteria are developed. The instability is also demonstrated in tailor made six degree of freedom time domain simulations, and in simulations using a fully coupled aero-hydro-servo elastic simulation tool including a BEM model. An important finding is that damping forces are needed to fully understand the observed instability. It is demonstrated, quite counter-intuitively, that damping reduces the stability margin. This is explained by considering the effect damping forces has on roll-yaw phasing, for the typical damping values relevant for a floating offshore wind turbine.
Reduced order modeling of non-linear monopile dynamics via an AE-LSTM scheme
Frontiers in Energy Research, Mar 6, 2023
Non-linear analysis is of increasing importance in wind energy engineering as a result of their e... more Non-linear analysis is of increasing importance in wind energy engineering as a result of their exposure in extreme conditions and the ever-increasing size and slenderness of wind turbines. Whilst modern computing capabilities facilitate execution of complex analyses, certain applications which require multiple or real-time analyses remain a challenge, motivating adoption of accelerated computing schemes, such as reduced order modelling (ROM) methods. Soil structure interaction (SSI) simulations fall in this class of problems, with the non-linear restoring force significantly affecting the dynamic behaviour of the turbine. In this work, we propose a ROM approach to the SSI problem using a recently developed ROM methodology. We exploit a data-driven non-linear ROM methodology coupling an autoencoder with long short-term memory (LSTM) neural networks. The ROM is trained to emulate a steel monopile foundation constrained by non-linear soil and subject to forces and moments at the top of the foundation, which represent the equivalent loading of an operating turbine under wind and wave forcing. The ROM well approximates the time domain and frequency domain response of the Full Order Model (FOM) over a range of different wind and wave loading regimes, whilst reducing the computational toll by a factor of 300. We further propose an error metric for capturing isolated failure instances of the ROM.
Proceedings of the 13th International Workshop on Structural Health Monitoring, Mar 15, 2022
In the field of structural health monitoring (SHM), inverse problems which require repeated analy... more In the field of structural health monitoring (SHM), inverse problems which require repeated analyses are common. With the increase in the use of nonlinear models, the development of nonlinear reduced order modelling techniques is of paramount interest. Of considerable research interest, is the use of flexible and scalable machine learning methods which can learn to approximate the behaviour of nonlinear dynamic systems using input and output data. One such nonlinear system of interest, in the context of wind turbine structures, is the soil structure interaction (SSI) problem. Soil demonstrates strongly nonlinear behaviour with regards to its restoring force and has been shown to considerably influence the dynamic response of wind turbine structures. In this work, we demonstrate the application of a recently developed nonlinear reduced order modelling method, which leverages Autoencoder and LSTM neural networks, to a nonlinear soil structure interaction problem of a wind turbine monopile subject to realistic loading at the seabed level. The accuracy and efficiency of the methodology is compared to full order simulations carried out using Abaqus. The ROM was shown to have good fidelity and a considerable reduction in computational time for the system considered.
Roll-Yaw lock: Aerodynamic motion instabilities of Floating Offshore Wind Turbines
Journal of offshore mechanics and Arctic engineering, Jan 28, 2022
This article presents an investigation of a newly discovered motion instability phenomenon for fl... more This article presents an investigation of a newly discovered motion instability phenomenon for floating wind turbines, first observed in numerical aero-hydro-servo-elastic simulations. The source of instability is identified as antisymmetric aerodynamic stiffness coupling terms in roll and yaw caused by the turbine thrust force. Analytical expressions for the roll–yaw and yaw–roll stiffness coupling terms are derived, using the actuator disk analogy of the rotor plane. It is demonstrated that a two degrees-of-freedom linear equation of motion without explicit external forcing qualitatively captures the instability. Based on this model, an analytical stability criteria is derived. An intuitive physical explanation of the phenomena is provided, considering the energy flow of the system. An important finding is that dissipative forces, in the form of aerodynamic and hydrodynamic damping, are needed to fully explain the observed instability. It is demonstrated, counterintuitively, that increased damping in yaw reduces the stability margin. This is explained by the fact that dissipative forces result in a phase difference between the harmonic roll and yaw motion in the coupled roll–yaw modes.
Reduced order modeling of non-linear monopile dynamics via an AE-LSTM scheme
Frontiers in Energy Research
Non-linear analysis is of increasing importance in wind energy engineering as a result of their e... more Non-linear analysis is of increasing importance in wind energy engineering as a result of their exposure in extreme conditions and the ever-increasing size and slenderness of wind turbines. Whilst modern computing capabilities facilitate execution of complex analyses, certain applications which require multiple or real-time analyses remain a challenge, motivating adoption of accelerated computing schemes, such as reduced order modelling (ROM) methods. Soil structure interaction (SSI) simulations fall in this class of problems, with the non-linear restoring force significantly affecting the dynamic behaviour of the turbine. In this work, we propose a ROM approach to the SSI problem using a recently developed ROM methodology. We exploit a data-driven non-linear ROM methodology coupling an autoencoder with long short-term memory (LSTM) neural networks. The ROM is trained to emulate a steel monopile foundation constrained by non-linear soil and subject to forces and moments at the top o...
Proceedings of the 13th International Workshop on Structural Health Monitoring
In the field of structural health monitoring (SHM), inverse problems which require repeated analy... more In the field of structural health monitoring (SHM), inverse problems which require repeated analyses are common. With the increase in the use of nonlinear models, the development of nonlinear reduced order modelling techniques is of paramount interest. Of considerable research interest, is the use of flexible and scalable machine learning methods which can learn to approximate the behaviour of nonlinear dynamic systems using input and output data. One such nonlinear system of interest, in the context of wind turbine structures, is the soil structure interaction (SSI) problem. Soil demonstrates strongly nonlinear behaviour with regards to its restoring force and has been shown to considerably influence the dynamic response of wind turbine structures. In this work, we demonstrate the application of a recently developed nonlinear reduced order modelling method, which leverages Autoencoder and LSTM neural networks, to a nonlinear soil structure interaction problem of a wind turbine mono...
フローティング風力タービンの連成動的解析のためのBHAWC空力弾性ソフトウェアとOrCAFLEX間の結合モジュールの開発【JST・京大機械翻訳】
Journal of Physics: Conference Series, 2019
Roll–Yaw Lock: Aerodynamic Motion Instabilities of Floating Offshore Wind Turbines
Journal of Offshore Mechanics and Arctic Engineering, 2022
This article presents an investigation of a newly discovered motion instability phenomenon for fl... more This article presents an investigation of a newly discovered motion instability phenomenon for floating wind turbines, first observed in numerical aero-hydro-servo-elastic simulations. The source of instability is identified as antisymmetric aerodynamic stiffness coupling terms in roll and yaw caused by the turbine thrust force. Analytical expressions for the roll–yaw and yaw–roll stiffness coupling terms are derived, using the actuator disk analogy of the rotor plane. It is demonstrated that a two degrees-of-freedom linear equation of motion without explicit external forcing qualitatively captures the instability. Based on this model, an analytical stability criteria is derived. An intuitive physical explanation of the phenomena is provided, considering the energy flow of the system. An important finding is that dissipative forces, in the form of aerodynamic and hydrodynamic damping, are needed to fully explain the observed instability. It is demonstrated, counterintuitively, that ...
Over the years, the installed capacity of offshore wind turbines is increasing rapidly. However, ... more Over the years, the installed capacity of offshore wind turbines is increasing rapidly. However, the Levelized Costs Of Energy (LCOE) is still higher than the LCOE of traditional energy production methods like nuclear power or energy from coals or gas. This research focuses on a further decrease of the LCOE, by minimizing the mass of a monopile support structure of a wind turbine. This is done in a so called integrated way: Optimizing the tower and the foundation together. The design variables used in this research are the wall thickness and the diameter of every +-3 meter section. These can even be cylindrical or conical. To simplify the problem, a parametrization of the designs is used, which reduces the design variables from around 180 to 28. This is checked with existing designs. Due to the interaction between mostly the first eigenfrequency and eigenmode, the diameter and the waves, it is expected that several local optima exist. Therefore, the proposed optimization strategy is...
Journal of Physics: Conference Series, 2019
Floating Offshore Wind Turbines (FOWTs) dynamic analysis can be quite challenging as it requires ... more Floating Offshore Wind Turbines (FOWTs) dynamic analysis can be quite challenging as it requires to model within the same calculation aerodynamics, hydrodynamics, controller, moorings and structure behaviour. Mooring and anchor design are strongly affecting FOWTs dynamic behaviour. Also, floater structural behaviour can be complex to model in a decoupled finite element analysis software. To do so, Innosea and Siemens Gamesa Renewable Energy developed a coupling between BHawC software and OrcaFlex software. BHawC is a nonlinear aeroelastic tool performing dynamic analysis of wind turbines. OrcaFlex is a generalist offshore marine engineering software that can perform the dynamic analysis of floaters and their mooring system. This paper describes the method and the mathematical background that is used to perform detailed time-domain simulations of a floating wind turbine coupling BHawC and OrcaFlex. The coupling is verified comparing time domain simulations on several load cases for r...
Aerodynamic Roll-Yaw Instabilities of Floating Offshore Wind Turbines
Volume 9: Ocean Renewable Energy, 2020
This paper presents an investigation of a newly discovered motion instability phenomenon for floa... more This paper presents an investigation of a newly discovered motion instability phenomenon for floating wind turbines. The instability is due to anti-symmetric coupling terms in roll and yaw caused by the turbine thrust force. For floaters with small separation between the uncoupled roll and yaw natural periods these coupling forces may result in rigid body roll and yaw oscillations. The paper explains the theory and the physics of the instability phenomenon, and analytical expressions for these stiffness coupling terms (K46 and K64) are derived. The instability phenomenon is demonstrated using several points of attack, by using time domain simulations, conservation of energy flow and eigenvalue stability analysis. The problem is stripped down to a simplified two degree of freedom roll-yaw model where analytical stability criteria are developed. The instability is also demonstrated in tailor made six degree of freedom time domain simulations, and in simulations using a fully coupled a...
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Thesis Chapters by Nico Maljaars
This research focuses on further decreasing the LCOE, by minimizing the weight of the support structure of the wind turbine. This is done in a so called integrated way: By optimizing the design of the tower and the foundation at the same time. Due to the interaction between wave loads and structure, many local optima are expected. Therefore an optimization strategy is proposed using a Particle Swarm Optimization (PSO) algorithm followed by a Gradient Based Algorithm on an objective which includes the mass and penalized constraints.
To reduce the optimization time, the evaluations of the objective function are done by using load estimations instead of extensive load calculations. To select reliable estimation methods, several methods are compared on a theoretical basis, after which the performance of a selection of these methods is examined on the problem. To improve the accuracy of the estimations, interaction of PSO and the estimators is proposed via estimator updating.
The proposed optimization approach is compared with the traditional design approach, by the optimization of several realistic case studies. This shows the potentials of using the estimator and optimization tool, developed during this project, in engineering practice.
Papers by Nico Maljaars
This research focuses on further decreasing the LCOE, by minimizing the weight of the support structure of the wind turbine. This is done in a so called integrated way: By optimizing the design of the tower and the foundation at the same time. Due to the interaction between wave loads and structure, many local optima are expected. Therefore an optimization strategy is proposed using a Particle Swarm Optimization (PSO) algorithm followed by a Gradient Based Algorithm on an objective which includes the mass and penalized constraints.
To reduce the optimization time, the evaluations of the objective function are done by using load estimations instead of extensive load calculations. To select reliable estimation methods, several methods are compared on a theoretical basis, after which the performance of a selection of these methods is examined on the problem. To improve the accuracy of the estimations, interaction of PSO and the estimators is proposed via estimator updating.
The proposed optimization approach is compared with the traditional design approach, by the optimization of several realistic case studies. This shows the potentials of using the estimator and optimization tool, developed during this project, in engineering practice.