Semi-submersible Wind Turbine Research Articles

Development and verification of a time-domain approach for determining forces and moments in structural components of floaters with an application to floating wind turbines

Structural design of the floater is an important aspect in developing cost efficient and reliable floating wind turbines. It is difficult to well account for the effect of strong non-linear dynamic characteristics and transient loading events, e.g. wind turbine faults, of floating wind turbines in a frequency-domain finite element analysis. The time-domain approach which implements the Morison's formula cannot accurately account for the hydrodynamic loads on the hull of floating wind turbines. While, the conventional hybrid frequency-time domain approach (based on the potential flow theory) fails to capture structural responses of the hulls since a rigid-body global model rather than a finite element model of the hull is employed. The present paper deals with the development and verification of a time-domain approach that can be easily implemented in various state-of-the-art computer codes for wind turbine analysis, e.g. Simo/Riflex/Aerodyn, OrcaFlex and FAST + CHARM3D, to extend their capabilities to analyze global forces and moments in structural components of a generic floater subject environmental loads from e.g. wind and waves. The global forces and moments in the structural components might be used as inputs of design formulas for structural strength design checks and/or used as boundary conditions in a sub-model finite element analysis to determine structural responses such as stresses. The proposed approach focuses on modeling of the inertia and external loads on the hull and mapping of the loads in the finite element model of the hull. In the proposed approach, floating wind turbines are considered as a system of several structural components, e.g. blades, rotational shaft, nacelle, tower, mooring lines, columns, pontoons and braces, rather than one rigid-body, while a finite element model for the hull is developed to represent the global stiffness of the structural components. The external and inertial loads on the hull are modeled as distributed loads rather than the integrated forces and moments. The conventional hybrid frequency-time domain approach, which is available in the state-of-the-art computer codes, is implemented to model the hydrodynamic loads on each structural component with essential modifications with respect to the corresponding hydrodynamic coefficients, e.g. added mass and potential damping coefficients and wave excitation forces. Approaches for modeling the hydrostatic pressure forces, gravity loads, drag forces and inertial loads on each structural component are also illustrated. Second order and higher order terms of the hydrostatic and hydrodynamic loads and the hydroelasticity effects are not accounted for in the present paper but can be further included. So far, the proposed approach has been implemented in the computer code Simo/Riflex/Aerodyn to analyze global forces and moments in the hull of a semi-submersible wind turbine. Good agreement between the reference values and the simulation results has been observed and indicates that the developed time-domain numerical models are reliable. The simulation results show that the low-frequency aerodynamic loads and fluctuations of hydrostatic pressure forces on and gravity of the floating wind turbine are important contributions to the structural responses, in particular, in the low-frequency range.

Experimental study of the functionality of a semisubmersible wind turbine combined with flap-type Wave Energy Converters

In the present paper the functionality of the Semisubmersible wind energy and Flap-type wave energy Converter (SFC) is examined experimentally. In order to study the functionality of the SFC, the focus is on operational environmental conditions. SFC is a combined concept that utilizes offshore wind energy and ocean wave energy for power production. Details are presented as far as the physical modelling of the wind turbine with the use of a redesigned small-scale rotor and of the Power Take-Off mechanism of the Wave Energy Converters (WECs) with the use of a configuration that is based on a mechanical rotary damper. Tests with quasi-static excitation, motion decay, regular and irregular waves without and with wind that is uniform are conducted on an 1:50 scale physical model. The experimental data are compared with numerical predictions obtained by a fully coupled numerical model using Simo/Riflex tool. A good agreement is observed between experimental and numerical predictions. The combined operation of WECs doesn't affect the tension of mooring lines nor the acceleration of nacelle and the bending moment in tower's base. The produced power of the WECs of the SFC and consequently the functionality of the SFC is estimated.

Mooring System Design and Classification of an Innovative Offshore Wind Turbine in Different Water Depths

In the middle of 2013, the offshore installed wind capacity was 6.5 GW in European countries while by 2020 the total installed capacity is expected to reach 43 GW mainly by application of offshore wind turbines; the growth of offshore wind industry necessitates introducing concepts with potential of being classed without being much affected by different water depths. Initially, recent patents of offshore wind turbines are presented in this article. Afterwards, stochastic dynamic motion responses and generated power of a semisubmersible floating wind turbine for different water depths of 100 m and 200 m are examined for selected environmental conditions. The mooring line design is correspondingly modified to provide proper station keeping as well as appropriate interactive influence on the dynamic responses of the semisubmersible wind turbine without affecting the generated power. Fully coupled aero-hydro-servo-elastic numerical models have been developed and integrated time-domain approach has been applied in order to investigate the dynamics of the semisubmersible floating wind turbine. The motion responses, mooring lines effective tension as well as wind turbine performance are compared for different water depths and environmental conditions. Hence, the present semisubmersible floating wind turbine can be classified which means an optimized design can easily be modified for new offshore site with different water depth without the need of major modifications. Keywords: Offshore floating wind turbine, Classification, wind and wave induced dynamics.

V-shaped semisubmersible offshore wind turbine: An alternative concept for offshore wind technology

The design aspects of a 5-MW V-shaped semisubmersible floating wind turbine considering the floater main dimensions and configurations are presented in this paper. Initially, the effect of different geometry parameters that correspond to different design cases have been investigated on the hydrostatic stability of the semisubmersible support platform through the comparison of righting arm and righting moments. Afterwards, the dynamic behavior and performance of the V-shaped semisubmersible wind turbine are presented for one of the examined design cases. Aero-hydro-servo-elastic numerical modeling has been applied for achieving coupled integrated time-domain analysis in order to investigate the dynamics of the V-shaped semisubmersible offshore wind turbine. The water depth is selected to be 100 m in order to study the feasibility of such concept in moderate water depth. The wave-induced as well as wave–wind-induced motions, tension of mooring lines and functionality of wind turbine are presented and discussed for selected environmental conditions. In general, the results show that the presented in the present paper V-shaped semisubmersible offshore wind turbine is a promising concept which can enhance the offshore wind industry.

Time domain analysis procedures for fatigue assessment of a semi-submersible wind turbine

Long term time domain analysis of the nominal stress for fatigue assessment of the tower and platform members of a three-column semi-submersible was performed by fully coupled time domain analyses in Simo-Riflex-AeroDyn. By combining the nominal stress ranges with stress concentration factors, hot spot stresses for fatigue damage calculation can be obtained. The aim of the study was to investigate the necessary simulation duration, number of random realisations and bin sizes for the discretisation of the joint wind and wave distribution. A total of 2316 3-h time domain simulations, were performed.In mild sea states with wind speeds between 7 and 9 m/s, the tower and pontoon experienced high fatigue damage due to resonance in the first bending frequency of the tower from the tower wake blade passing frequency (3P).Important fatigue effects seemed to be captured by 1 h simulations, and the sensitivity to number of random realisations was low when running simulations of more than 1 h. Fatigue damage for the tower base converged faster with simulation duration and number of random realisations than it did for the platform members.Bin sizes of 2 m/s for wind, 1 s for wave periods and 1 m for wave heights seemed to give acceptable estimates of total fatigue damage. It is, however, important that wind speeds that give coinciding 3P and tower resonance are included and that wave periods that give the largest pitch motion are included in the analysis.

Frequency Versus Time Domain Fatigue Analysis of a Semisubmersible Wind Turbine Tower

The current paper deals with a study of a semisubmersible wind turbine (WT), where short-term tower base bending moments and tower fatigue damage were estimated by a frequency domain (FD) method. Both a rigid structure assumption and a generalized degree-of-freedom (DOF) model for including the first flexible mode of the turbine tower were investigated. First, response to wind and wave loads was considered separately, then superposition was used to find the response to combined wind and wave loading. The bending moments and fatigue damage obtained by these methods were compared to results from a fully coupled, nonlinear time domain (TD) analysis. In this study a three column, catenary moored semisubmersible with the NREL 5 MW turbine mounted on one of the columns was modeled. The model was inspired by the WindFloat concept. The TD simulation tool used was Simo-Riflex-AeroDyn from Marintek and CeSOS. The FD method gave a good representation of the tower base bending moment histories for wave-only analyses, for the moderate sea states considered in these analyses. With the assumption that the structure is completely rigid, bending moments were underestimated, but including excitation of the elastic tower and blades, improved the results. The wind-induced low-frequency bending moments were not captured very well, which presumably comes from a combination of nonlinear effects being lost in the linearization of the thrust force and that the aerodynamic damping model was derived for a fixed turbine. Nevertheless, standard deviations of the bending moments were still reasonable. The FD model captured the combined wind and wave analyses quite well when a generalized coordinates model for wind excitation of the first bending mode of the turbine was included. The FD fatigue damage predictions were underestimated by 0–60%, corresponding to discrepancies in standard deviations of stress in the order of 0–20%.

A study on variable amplitude load fatigue and an expert system for fatigue assessment of highway steel bridges. (Dissertation): Melhem, H.G.Diss. Abstr. Int. Sept. 1990 51, (3), 373 pp

This study investigates the effect of variability and uncertainty of wind and wave conditions on the short-term fatigue damage of a 10-MW floating wind turbine drivetrain. Global dynamic responses of a semi-submersible wind turbine are calculated by aero-hydro-servo-elastic simulations in various environmental conditions. Then, rotor and generator loads, as well as nacelle motions from the global analysis are provided to a drivetrain model to investigate its dynamics. One-hour fatigue damage of the drivetrain bearings is calculated based on the bearing loads and speeds, and the effect of uncertainties related to wind and waves is assessed. The results indicate that the variations of mean wind speed, turbulence intensity and wind shear have great effects on the studied drivetrain. The effect of uncertainties of irregular waves on the drivetrain fatigue damage is small. Five wind and wave random samples are sufficient for dynamic analysis of the drivetrain to achieve accurate results at a reasonable computational cost. Among the drivetrain components, the main bearings are generally most sensitive to the investigated environmental variables. Finally, this study discusses the variation of environmental variables in terms of their relative importance for drivetrain analysis. The results provide a basis for establishing improved design standards and engineering practice for design and analysis of floating wind turbine drivetrains.