Partial Safety Factors and Characteristic Values for Combined Extreme Wind and Wave Load Effects

2005 ◽  
Vol 127 (2) ◽  
pp. 242-252 ◽  
Author(s):  
Niels Jacob Tarp-Johansen
Keyword(s):  
Probabilistic Approach ◽  
Offshore Wind ◽  
Design Rule ◽  
Storm Event ◽  
Wave Loads ◽  
Wave Load ◽  
Safety Factors ◽  
Characteristic Values ◽  
Rule Method ◽  
Definition Of

Background: The present paper regards the concerted action of wind and wave loads on offshore wind turbines in the extreme storm event. The load combination problem involves the definition of the characteristic loads and safety factors. In wind engineering and offshore engineering well established practices for the definition of characteristic values and safety factors for wind and wave loads separately exist. The aim is to investigate the possibility of making a simple merger of these existing practices into a possibly conservative design rule. Method of Approach: The paper applies a simplified probabilistic approach giving an understanding of how the merging can possibly be established and finally gives first guidance on the choice of characteristic values and safety factors. Results and conclusions: Under the assumptions made herein, it is made probable that a simple combination rule can be established.

Analysis of the Fatigue Damage on the Offshore Wind Turbines Exposed to Wind and Wave Loads Within the Typhoon Area

2004 ◽  
Author(s):  
Atsushi Yamashita ◽  
Kinji Sekita
Keyword(s):  
Fatigue Damage ◽  
Wind Turbines ◽  
Offshore Wind ◽  
Wave Loads ◽  
Wave Load ◽  
Offshore Wind Turbines ◽  
Simple Addition ◽  
Recorded Data ◽  
Term Data

For the design of offshore wind turbines exposed to wind and wave loads, the method of combining the wind load and the wave load is significantly important to properly calculate the maximum stresses and deflections of the towers and the foundations1). Similarly, for the analysis of the fatigue damage critical to the structural life, the influences of combined wind and wave loads have not been clearly verified. In this paper fatigue damage at the time of typhoon passing is analyzed using actually recorded data, though intrinsically long-term data more than 10years should be used to properly evaluate the fatigue damage. This paper concludes that the fatigue damage of the tower caused by the wave load is not substantial and, thus, the fatigue damage by the combined wind and wave load is only 2–3% larger than the simple addition of the independent fatigue damages by the wind and the wave loads. The fatigue damage of the tower top, which is required to reduce the diameter in order to minimize the aerodynamic confliction with blades, is larger than that of the tower bottom. The fatigue damage at the foundation by the combined wind and wave load is 25% larger than the simple addition of the wind and wave damages, as the foundation is directly exposed to the wave load. For the foundation, the proper structural section can be designed in order to improve the structural performance against fatigue.

Validation and Development of Improved Methods for the Calculation of Wave Loads on XXL Monopiles

2018 ◽  
Author(s):  
Mareike Leimeister ◽  
Bastian Dose
Keyword(s):  
State Of The Art ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Wave Load ◽  
Wind Turbine System ◽  
Engineering Models ◽  
Dynamics Simulations ◽  
Diffraction Effects ◽  
Wave Induced

With the offshore wind industry aiming to deploy deeper water sites (> 30 m) while still utilizing monopiles, support structures with larger diameters are required. For the design and assessment of so-called XXL monopiles, wave-induced forces, which become more dominant with increasing diameter, have to be determined accurately. Thus, this study focuses on the identification of differences between state-of-the-art theories for wave load calculations with engineering models and the forces exerted on large monopiles from high-precision numerical reference methods. Within the framework of the research project TANDEM (Towards an Advanced Design of Large Monopiles) a 7 m diameter monopile is designed to support Fraunhofer’s IWT-7.5-164. This offshore wind turbine system is used as reference to determine wave-induced loads based on the MacCamy-Fuchs approach, implemented in models in Modelica. Different waves, defined in a simulation matrix, are investigated to elaborate the significance of diffraction effects, as well as the relevance of non-linear effects. The results are compared to CFD (Computational Fluid Dynamics) simulations. Deviations in the wave-induced forces are analyzed, taking into account the different capabilities of the applied tools, trends in the applicability of the engineering model are elaborated, and suggestions for improvement of the code based on state-of-the-art theories are given.

Soil–Structure–Wave Interaction of Gravity-Based Offshore Wind Turbines: An Analytical Model

2020 ◽  
Vol 143 (3) ◽  
Author(s):  
Dimitrios G. Pavlou
Keyword(s):  
Dynamic Response ◽  
Analytical Model ◽  
Wind Turbines ◽  
Initial Conditions ◽  
Offshore Wind ◽  
Design Parameters ◽  
Wave Loads ◽  
Wave Load ◽  
Offshore Wind Turbines ◽  
Dynamic Phenomena

Abstract The structural design of offshore wind turbines is based on the consideration of coupled dynamic phenomena. Wave loads cause the dynamic oscillation of the monopile, and the dynamic oscillation of the monopile affects the wave loads. The boundary conditions of the gravity-based foundation-monopile-turbine system are mostly affected by the flexural stiffness of the foundation plate, the elastic and creep behavior of the soil, and the inertia (translational and rotational) of the wind turbine mass. The design of the foundation should consider the dynamic response of the soil and the monopile, and the dynamic response of the soil and the monopile is affected by the design parameters of the foundation. The initial conditions of the system yield transient dynamic phenomena. A braking wave at t = 0 causes different dynamic response than the steady-state conditions due to a harmonic wave load. In the present work, an integrated analytical model simulating the above dynamic phenomena is proposed. With the aid of double integral transforms and generalized function properties, a solution of the corresponding differential equations for the monopile-soil-foundation system and the boundary and initial conditions is derived. A parametric study is carried out, and results of the effect of the design parameters and soil properties are presented and discussed.

Pore Pressure Under a Gravity Based Structure Under the Influence of Waves

2017 ◽  
Author(s):  
Erik Damgaard Christensen ◽  
Stefan Carstensen ◽  
Mikael Thyge Madsen ◽  
Peter Allerød Hesselbjerg ◽  
Christel Jeanty Nielsen
Keyword(s):  
Pore Pressure ◽  
Prediction Models ◽  
Direct Calculation ◽  
Wave Structure ◽  
Offshore Wind ◽  
Numerical Analyses ◽  
Wave Loads ◽  
Wave Load ◽  
Air Content ◽  
Seabed Interaction

The total wave load on a gravity based foundation for offshore wind turbines is influenced by the pore pressure from beneath the structure. The pore pressure is induced by the wave-structure-seabed interaction. Often the uplift force is included in a simplified way in the design of the gravity based foundation. This leads typically to very conservative designs in order to accommodate the uncertainties in the procedure. The experiments shall lead to better prediction models based on for instance CFD model’s with the direct calculation of pressure variations in the seabed and any erosion protection layer. Herewith, it will be possible to get a direct assessment of wave loads on the foundation, also under the seabed level. The study includes experiments as well as numerical analyses. A good agreement between the experimental results and the numerical analyses was found. In the numerical analyses, it was possible to investigate the effect of air content in the pores, which turned out to have an effect on the distribution of the pore pressure.

scholarly journals Uncertainty Assessment of CFD Investigation of the Nonlinear Difference-Frequency Wave Loads on a Semisubmersible FOWT Platform

2020 ◽  
Vol 13 (1) ◽  
pp. 64
Author(s):  
Lu Wang ◽  
Amy Robertson ◽  
Jason Jonkman ◽  
Yi-Hsiang Yu
Keyword(s):  
Low Frequency ◽  
Uncertainty Assessment ◽  
Difference Frequency ◽  
Offshore Wind ◽  
Wave Loads ◽  
Wave Excitation ◽  
Wave Load ◽  
Frequency Wave ◽  
Floating Offshore Wind Turbines ◽  
Frequency Excitation

Current mid-fidelity modeling approaches for floating offshore wind turbines (FOWTs) have been found to underpredict the nonlinear, low-frequency wave excitation and the response of semisubmersible FOWTs. To examine the cause of this underprediction, the OC6 project is using computational fluid dynamics (CFD) tools to investigate the wave loads on the OC5-DeepCwind semisubmersible, with a focus on the nonlinear difference-frequency excitation. This paper focuses on assessing the uncertainty of the CFD predictions from simulations of the semisubmersible in a fixed condition under bichromatic wave loading and on establishing confidence in the results for use in improving mid-fidelity models. The uncertainty for the nonlinear wave excitation is found to be acceptable but larger than that for the wave-frequency excitation, with the spatial discretization error being the dominant contributor. Further, unwanted free waves at the difference frequency have been identified in the CFD solution. A wave-splitting and wave load-correction procedure are presented to remove the contamination from the free waves in the results. A preliminary comparison to second-order potential-flow theory shows that the CFD model predicted significantly higher difference-frequency wave excitations, especially in surge, suggesting that the CFD results can be used to better calibrate the mid-fidelity tools.

Implied Reliability Levels in Different Load Models for Offshore Wind Turbines

2014 ◽  
Author(s):  
P. Agarwal ◽  
L. Manuel
Keyword(s):  
Wind Turbines ◽  
Large Diameter ◽  
Offshore Wind ◽  
International Electrotechnical Commission ◽  
Wave Loads ◽  
Support Structures ◽  
Wave Load ◽  
Load Effect ◽  
Offshore Wind Turbines ◽  
Inertia Forces

Assuring uniform reliability levels across various system configurations is the intent of design standards based on the Load and Resistance Factor Design (LRFD) methodology. One such design standard for offshore wind turbines developed by the International Electrotechnical Commission was based on the European experience and may not necessarily represent conditions suited for U.S. waters where several offshore wind energy projects are being planned. It is, hence, of interest to investigate how uniform is the reliability of offshore wind turbines under various levels of wind and wave loads. We assess the reliability of bottom-supported offshore wind turbines in ultimate limit states associated with the fore-aft tower bending moment at the mudline. We compare reliability index estimates for different characteristic load definitions and assumed coefficients of variation for wind and wave loads, as well as for various hydrodynamic to aerodynamic load influences. Effectively, such variations serve to describe different sites and turbine designs. Since large-diameter monopile support structures are dominated by inertia forces, while jacket or tripod support structures with smaller diameter members are dominated by drag forces, we extend an available combined wind-wave load effect model for offshore wind turbines, to include both drag and inertia forces. We show that reasonably uniform reliability levels may be achieved for various combinations of wind and wave loads. Results suggest that drag-dominated wave load cases result in smaller and less uniform reliability estimates than is the case for inertia-dominated wave load cases.

Transfer of Boussinesq Waves to a Navier-Stokes Solver: Application to Wave Loads on an Offshore Wind Turbine Foundation

2009 ◽  
Author(s):  
Erik Damgaard Christensen ◽  
Henrik Bredmose ◽  
Erik Asp Hansen
Keyword(s):  
Wind Turbine ◽  
Shallow Water ◽  
Offshore Wind ◽  
Navier Stokes ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Cfd Model ◽  
Wave Load ◽  
Boussinesq Model ◽  
Run Up

Wave load and wave run-up is a very important issue to offshore wind turbine foundations. These are often installed in relatively shallow water on for instance sand banks. Therefore the non-linear shoaling and subsequently the force and run-up are important to address. The paper presents a method to combine a Boussinesq model with a CFD model. This gives an accurate tool to estimate wave loads on the foundations at acceptable computational times.

scholarly journals Offshore Wind Turbine Nonlinear Wave Loads and Their Statistics

2019 ◽  
Vol 141 (3) ◽  
Author(s):  
Paul D. Sclavounos ◽  
Yu Zhang ◽  
Yu Ma ◽  
David F. Larson
Keyword(s):  
Nonlinear Wave ◽  
Machine Learning Algorithms ◽  
Offshore Wind ◽  
Support Vector ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Wave Load ◽  
Support Structure ◽  
Nonlinear Load ◽  
Floating Offshore Wind Turbines

The development of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom mounted and floating offshore wind turbines is presented. Explicit expressions are derived for the time-domain nonlinear exciting forces in a sea state with significant wave height comparable to the diameter of the support structure based on the fluid impulse theory (FIT). The method is validated against experimental measurements with good agreement. The higher order moments of the nonlinear load are evaluated from simulated force records and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed. The identification of the inertia and drag coefficients of a generalized nonlinear wave load model trained against experiments using support vector machine learning algorithms is discussed.

scholarly journals Offshore Wind Turbine Nonlinear Wave Loads and Their Statistics

2017 ◽  
Author(s):  
Paul D. Sclavounos ◽  
Yu Zhang ◽  
Yu Ma ◽  
David F. Larson
Keyword(s):  
Nonlinear Wave ◽  
Machine Learning Algorithms ◽  
Offshore Wind ◽  
Support Vector ◽  
Offshore Wind Turbine ◽  
Wave Loads ◽  
Wave Load ◽  
Support Structure ◽  
Nonlinear Load ◽  
Floating Offshore Wind Turbines

The development is presented of an analytical model for the prediction of the stochastic nonlinear wave loads on the support structure of bottom mounted and floating offshore wind turbines. Explicit expressions are derived for the time-domain nonlinear exciting forces in a seastate with significant wave height comparable to the diameter of the support structure based on the fluid impulse theory. The method is validated against experimental measurements with good agreement. The higher order moments of the nonlinear load are evaluated from simulated force records and the derivation of analytical expressions for the nonlinear load statistics for their efficient use in design is addressed. The identification of the inertia and drag coefficients of a generalized nonlinear wave load model trained against experiments using Support Vector Machine learning algorithms is discussed.

Impact of New Slamming Wave Design Method on the Structural Dynamics of a Classic, Modern and Future Offshore Wind Turbine

2017 ◽  
Author(s):  
Johan M. Peeringa ◽  
Koen W. Hermans
Keyword(s):  
Wind Turbine ◽  
Wave Train ◽  
Breaking Waves ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Breaking Wave ◽  
Wave Loads ◽  
Wave Load ◽  
Wave Impact ◽  
The Impact

In the WiFi-JIP project, the impact of steep (and breaking) waves on a monopile support structure was studied. Observations during model tests showed that large tower top accelerations occur due to a slamming wave. Using experiments and simulations results, a new formulation of the design load for a slamming wave was developed. Instead of the embedded stream function, as applied in industry, the wave train is generated with the nonlinear potential flow code Oceanwave3D. On the wave train a set of conditions is applied to find the individual waves, that are closest to the prescribed breaking wave and most likely cause a slamming impact. To study the effect of the new slamming load formulation on different sized offshore wind turbines, aero-hydroelastic simulations were performed on a classic 3MW wind turbine, a modern 4MW wind turbine and a future 10MW wind turbine. The simulations are performed with and without a slamming wave load. The slamming has a clear effect on the tower top acceleration. Accelerations due to the wave impact are highest for the 3MW model at the tower top and at 50m height. A serious tower top acceleration of almost 7m/s2 due to wave slamming is found for the 3MW turbine. This is an increase of 474% compared with the case of Morison wave loads only.

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