Comparison of laboratory wave generation techniques on response of a large monopile in irregular sea

As the offshore wind industry moves toward larger monopile turbines, model testing and validation of hydrodynamic load models become more important for new designs of turbines. Wave generation is an important aspect of hydrodynamic model testing. When generating irregular waves with a piston-type wavemaker, first order wavemaker theory is commonly used. This leads to generating spurious free waves in the tank. Using second order wavemaker theory reduces the generation of these spurious waves. In this study, the two wave generation techniques have been used in the measurement of the dynamic responses of a monopile with (full-scale) natural period of 5 sec. The effect of superharmonic spurious waves on the response statistics was minor. A marginal improvement in experimental repeatability of the second mode response in large wave events was observed by using second order wavemaker theory.publishedVersio

Experimental study on the effect of second order wavemaker theory on the response of a flexible large diameter monopile in irregular sea

Motivated by the need for larger offshore wind turbines, large diameter monopile foundations are being developed. To ensure safe design, there is a need for model testing and validation of hydrodynamic load models. Scaled model tests with a piston-type wavemaker commonly apply first order wavemaker theory for irregular waves. This approach results in the generation of second (and higher) order spurious (also known as parasitic) free waves in the tank. In this study, the effect of superharmonic spurious waves on the response of a monopile with eigenfrequency close to three times the wave peak frequency is examined experimentally. The bending moment response statistics are not found to be significantly affected by the wavemaker correction. Different wave breaking patterns are observed for individual events, but our results do not indicate any clear relation between breaking waves and the wave generation technique.publishedVersio

Preliminary Sizing and Optimization of Semisubmersible Substructures for Future Generation Offshore Wind Turbines

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Application of a PI-controller to a 25 MW Floating Wind Turbine

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On Common Research Needs for the Next Generation of Floating Support Structures

The world is facing several industrial and societal challenges, such as providing enough renewable energy and enough safe and healthy food as formulated in the United Nations sustainable development goals. Using floating stationary structures, the ocean can contribute to solving several of the challenges. New applications need new types of structures, with which we have limited experience. These support structures will be diverse, but also have essential research needs in common. Design of novel floating structures need reliable descriptions of the marine environment. This is particularly challenging for semi-sheltered coastal regions, with complex topography and bathymetry. Novel structures are likely to be compliant, modular and/or multi-body, requiring increased understanding and rational models for wave-structure interaction. Structures with sustainable, safe, and cost-efficient use of materials, including untraditional ones, must be developed. Smart, affordable, and reliable mooring systems and anchors for novel applications are necessary for station keeping. Digital solutions connecting the various stages of design and operation, as well as various design disciplines, researchers, and innovators, will be necessary. Sustainability will be an integral part of any new design. To unlock the potential of novel floating structures, we need to understand the requirements of the applications, as well as the associated technology gaps and knowledge and research needs. This paper highlights research needs for innovation within floating offshore wind, floating solar power plants, novel aquaculture structures, and coastal infrastructure.acceptedVersio

Analysis of difference-frequency wave loads and quadratic transfer functions on a restrained semi-submersible floating wind turbine

One of the concerns regarding numerical simulation of floating wind turbines (FWTs) in waves is underprediction of resonant responses in the low-frequency range. In the present work, the difference-frequency wave loads on a restrained semi-submersible FWT subject to bichromatic waves are investigated by higher-fidelity tools (Computational Fluid Dynamics, CFD) and simplified engineering tools based on potential flow theory with Morison type drag. The effects of mean pitch angle (trim) and the wave force distribution on the multimember semisubmersible are assessed. Compared to the CFD results, wave loads estimated by engineering models are in good agreement at the wave frequencies, while slightly larger differences occur at the surge and pitch natural frequencies. The most significant underprediction of the surge force at the surge natural frequency occurs in the heave plate of the floater. Compared to the upright floater, the increased wave loads on the trimmed floater at the surge natural frequency are more significant than those at the pitch natural frequency. Furthermore, quadratic transfer functions (QTFs) are estimated based on the CFD model with a set of bichromatic wave cases. A new approach is found to use the CFD results to modify the QTFs in lower-fidelity engineering tools. This approach is validated against experimental measurements in irregular waves. Good agreement is achieved between measured and numerically estimated difference-frequency wave loads by engineering tools with modified QTFs

Validation and application of nonlinear hydrodynamics from CFD in an engineering model of a semi-submersible floating wind turbine

Nonlinear hydrodynamics play a significant role in accurate prediction of the dynamic responses of floating wind turbines (FWTs), especially near the resonance frequencies. This study investigates the use of computational fluid dynamics (CFD) simulations to improve an engineering model (based on potential flow theory with Morison-type drag) by modifying the second-order difference-frequency quadratic transfer functions (QTFs) and frequency-dependent added mass and damping for a semi-submersible FWT. The results from the original and modified engineering models are compared to experimental data from decay tests and irregular wave tests. In general, the CFD results based on forced oscillation tests suggest increasing the frequency-depending added mass and damping at low frequencies compared to first order potential flow theory. The modified engineering model predicts natural periods close to the experimental results in decay tests (within 5%), and the underprediction of the damping is reduced compared to the original engineering model. The motions, mooring line tensions and tower-base loads in the low-frequency response to an irregular wave are underestimated using the original engineering model. The additional linear damping increases this underestimation, while the modified QTFs based on CFD simulations of a fixed floater in bichromatic waves result in larger difference-frequency wave loads. The combined modifications give improved agreement with experimental data in terms of damage equivalent loads for the mooring lines and tower base

Experimental and numerically obtained low-frequency radiation characteristics of the OC5-DeepCwind semisubmersible

Added mass and damping play a significant role in accurate prediction of floating wind turbine (FWT) motions, especially near the resonance frequencies. This paper investigates the still-water hydrodynamic characteristics of a semi-submersible FWT around the natural periods of surge, heave and pitch motion. A higher-fidelity tool (Computational Fluid Dynamics, CFD) based on OpenFOAM is employed in the numerical computations. The tool is validated against experimental measurements (decay tests and forced surge motions) and then applied to investigate the hydrodynamic characteristics of the whole floater and each column at different amplitudes of forced motions. The heave and pitch decay match well with the experimental measurements, whereas the CFD simulations underestimate the damping in the surge decay. However, better agreement is obtained between measured and numerically estimated surge force in the forced oscillations in surge. Furthermore, the added mass derived from the CFD simulation is around 12% larger than that estimated by the potential flow theory, except the estimated heave added mass under the largest heave motion (up to 35% larger). This additional added mass in the CFD simulations is due to the viscous effects. The damping shows a small dependence on the oscillation period and a larger dependence on the oscillation amplitude within the tested period range. At these frequencies, radiation damping is completely negligible compared to the viscous damping due to vortex shedding, and the accuracy of Morison's drag forces in capturing the viscous damping is sensitive to the drag coefficient

Design, structural modeling, control, and performance of 20 MW spar floating wind turbines

As floating wind turbines (FWTs) increase in size and power, the relative contribution of wave and wind loads to their global responses differs from what has been observed for 5–10 MW units. In addition, the larger deflections at the platform, increased natural period range for some degrees of freedom, and larger RNA weight at higher heights invite a review on structural modeling methods, design constraints, dynamic analysis, and control systems. This paper explores these topics through the design and structural analysis of three spar-type 20 MW FWTs, with different constraints on the static pitch angle at rated wind speed. Time-domain simulations are performed with a non-linear aero-hydro-servo-elastic software, and sectional fatigue damage and extreme motions and axial stresses for the three designs are compared. The platform is modeled as a flexible body, with hydrodynamic loads evaluated with potential theory and distributed over the hull. A control system with a motion compensation strategy is adopted, ensuring the same controller bandwidth for the three FWTs and showing significant performance improvements compared to detuning the controller gains. In addition to impacting steel and ballast mass, the static pitch angle at rated thrust affects the platform dynamics and fatigue damage/extreme loads significantly. The platforms with larger restoring in pitch present less fatigue damage at the platform, but more at the tower. Extreme stresses are largely affected by gravitational loads, such that the designs with larger pitch at rated thrust have the highest extreme stresses at the platform and most of the tower sections. Load cases associated with the rated wind speed often govern the extreme loads, unlike previous studies with 5 MW and 10 MW FWTs

Low-frequency dynamic wake meandering: comparison of FAST.Farm and DIWA software tools

Most of the global dynamic response models used today for the design of wind turbines are based on the aero-hydro-servo-elastic analysis of one single turbine. However, research on bottom- fixed offshore wind turbines has shown that interactions among turbines in a farm influence both the power production and the structural loading. Furthermore, floating wind turbines (FWTs) are sensitive to low-frequency variations, and therefore to wake meandering perturbations. In the current work, we use the Dynamic Wake Meandering (DWM) model as implemented in DIWA and FAST.Farm, to study the low-frequency content of the meandering at a target turbine placed 8 diameters downstream. At frequencies in the range of the natural frequencies of rigid body motions of semisubmersible floaters, the two models yield different results. These differences are seen for every wind speed and wind turbine model, even though they decrease as the wind speed increases. The observed differences may affect low-frequency motions and consequently mooring system design