Refining estimation of design loads for wave slam events experienced by offshore wind turbines (original) (raw)
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Advanced Numerical Modelling of Wave Loading on Monopile-Supported Offshore Wind Turbines
2017
In the vast majority of cases, winds are considered to be stationary and as such are treated in the same way for design purposes. However, not all wind is the same, and non-stationary winds such as thunderstorm downbursts and tornadoes need to be considered using a different framework. This paper outlines two possible analytical models for such winds and introduces a possible framework which may be of use to the urban designer.
Influence of wave load variations on offshore wind turbine structures
2017
Nowadays, much attention is payed to the development of renewable energy resources. Wind energy plays a major role in this issue. That is why there is a growing interest for improving the design process of wind turbines at many aspects. This study compares two types of offshore wind turbines structures, the monopile and the jacket structure, in their dependency on wave load characteristics’ variations. The examined wave characteristics are significant wave height and wave peak period. The jacket structure showed lower influence of increase of wave height to stresses in the cross section at the bottom of the structure compared to the monopile structure. The monopile structure showed slight dependency of stresses on increasing wave frequency, while the jacket structure showed nearly no dependency, due to its more complex geometry and higher stiffness.
This study presents comparisons between two methods to estimate wave loads on wind turbine foundations. The analyses have been restricted to analyses of loads on mono-piles. The traditional method is based on a combination of undisturbed wave kinematics and a Morison force model. A refined model is based on CFD techniques where the wave interaction with the structure is modelled directly. The comparisons are made for regular waves, irregular waves, and breaking waves. The effect of scour holes/scour protection has also been analysed.
Numerical study of irregular breaking wave forces on a monopile for offshore wind turbines
Energy Procedia
Offshore wind turbine substructures consisting of cylindrical members are exposed to highly non-linear and breaking waves in shallow waters [1]. Those structures experience extreme impulsive loads of short duration from breaking waves that can cause permanent structural damage[2]. The main purpose of the present paper is to investigate the wave impact forces on a slender cylinder from plunging breaking waves in shallow waters both experimentally and numerically. The present study consists of two major parts: laboratory measurements and numerical simulations. The laboratory experiments are performed with regular waves. Plunging breaking waves are generated and free surface elevations are measured around the cylinder. Next, numerical simulations are carried out in the three-dimensional numerical wave tank REEF3D. The model is based on the incompressible Reynolds-averaged Navier-Stokes equations together with the k − ω for turbulence and the level set method for free surface. The numerical results are compared with the laboratory measurements in order to validate the numerical model. A good agreement between the computed results and the experimental data is seen for the breaking wave properties. Further, the breaking wave forces and the free surface deformations during the interaction of plunging breaking waves with a vertical cylinder are investigated and they are reasonably well represented in the numerical simulations.
Consequences of Steep Waves and Large Wave Forces to Offshore Wind Turbine Design
Offshore wind farms on still deeper waters include growing wave loads compared to the wind loads. This requires accurate determination of wave loads and modelling of the dynamic interaction between wind and wave loads. For cases with extreme design the wave loads are not estimated accurately if modelled traditionally, e.g. by Wheeler stretching the pressure and velocity profile. Accurate load determination requires either model tests or a more refined wave kinematics calculation model with due account of the fact that wave crests may be 2-4 times the wave trough. Wheeler stretching does not give a satisfactory accuracy in the range 0.25 < H s /h < 0.5, where H s = significant wave height and h = water depth. Up to now only a single wave model (Stokes 5th order or Stream function) has been available. By means of a new accurate Boussinesq model, time series of wave loads may be generated so the required time series input to offshore wind turbine foundations may be defined. Conse...
Measurements of breaking wave forces on vertical circular columns have been re-analysed, where substantial magnification over forces due to non-breaking nonlinear waves may occur, by a factor of up to 2.8. The analysis shows that this factor increases as the depth parameter kd decreases, being close to unity for kd>1.5 (k is wave number and d is depth). Non-breaking wave forces are predicted reasonably by nonlinear stream function wave theory using Morison's equation with empirical drag and inertia coefficients. A study was then made of the magnitude of wave overturning moment in relation to hub moment due to wind on standard 2MW and 5MW turbines with a 6 m diameter column. This showed that that wave moment in extreme conditions is greater than wind 'hub' moment for depths greater than about 7 m and 13 m for the 2MW and 5MW turbines respectively. Monopiles are normally used in depths below about 30 m and extreme moments due to waves occur when waves are depth-limited and defined by the Miche criterion, well below the limiting value of kd for the onset of depth-induced breaking. The maximum moment for these depths is expected to be predicted reasonably.
Ocean Engineering, 2017
We present experimental data from MARIN on a bottom-fixed offshore wind turbine mounted on a monopile in intermediate water depth subjected to severe irregular wave conditions. Two models are analysed: the first model is fully flexible and its 1 st and 2 nd eigenfrequencies and 1 st mode shape are representative of those of a full-scale turbine. This model is used to study the structural response with special focus on ringing and response to breaking wave events. The second model is stiff and is used to analyse the hydrodynamic excitation loads, in particular the so-called secondary load cycle. The largest responses are registered when the second mode of the structure is triggered by a breaking wave on top of a ringing response. In such events, the quasi-static response accounts for between 40 and 50% of the total load, the 1 st mode response between 30 and 40%, and the 2 nd mode response up to 20%. A statistical analysis on the occurrences and characteristics of the secondary load cycle shows that this phenomenon is not directly linked to ringing.
Critical Assessment of Non-linear Wave Loads in the Design of Offshore Wind Turbines
2014
Good wind conditions make Dogger Bank a well suited site for offshore wind industry. The harsh environment combined with shallow water, create significant non-linear wave loads which must be accounted for in the ultimate limit state design calculations (ULS). The application of different non-linear wave models have been given a wide critical assessment throughout the thesis. A monopile substructure is considered at depths of 25 and 45 meter. The DNV standard OS-J101 give the industrial practice for design of offshore wind turbines, and is followed by large contractors in the industry. The standard defines load cases involving the 50 year sea state parameters and the 50 year wave height. The irregular 2nd order wave model is presented, including stretching and extrapolation which are methods to account for the surface elevation. Linear and 2nd order kinematics, stretching of linear kinematics to the 1st and 2nd order surface, and 1st and 2nd order extrapolation models, have all been ...
Energies
Estimation of wave run-up has been of increasing concern for offshore wind structures and a critical aspect for designers. The highly nonlinear phenomenon makes the study difficult. That is the reason for the very few design rules and experimental data available to estimate it. Actual wave run-up is greater than commonly predicted. The goal of this research is to benchmark the theoretical formulations with the results of the physical model tests performed by Deltares in the field of crest elevation, run-up, forces and pressures. The laboratory reproduced in a wave tank (75 m length; 8.7 m width; 1 m depth; and a 1:60 scale, with Froude similarity) an offshore power converter platform located at intermediate water depths (25–43.80 m) in the Southern North Sea, designed by the Norwegian company Aibel. The purpose of this research is to offer a preliminary design guide for wave run–up using theoretical expressions both for cylinders and gravity based structures (GBS), leaning on the ci...