Monopile head stiffness for servicibility limit state calculations in assessing the natural frequency of offshore wind turbines

2017 ◽  
Vol 12 (3) ◽  
pp. 267-283 ◽  
Author(s):  
Mohammed Hemza Aissa ◽  
Djillali Amar Bouzid ◽  
Subhamoy Bhattacharya
Keyword(s):  
Natural Frequency ◽  
Wind Turbines ◽  
Limit State ◽  
Offshore Wind ◽  
Offshore Wind Turbines

Simulation Requirements and Relevant Load Conditions in the Design of Floating Offshore Wind Turbines

2018 ◽  
Author(s):  
Ricardo Faerron Guzmán ◽  
Kolja Müller ◽  
Luca Vita ◽  
Po Wen Cheng
Keyword(s):  
Wind Turbines ◽  
Limit State ◽  
Power Production ◽  
Offshore Wind ◽  
Ultimate Limit State ◽  
Offshore Wind Turbines ◽  
Floating Offshore Wind Turbines ◽  
Simulation Time ◽  
Number Of Seeds ◽  
Load Conditions

Aligned with work performed in deliverable D7.7 of the H2020 project LIFES50+, this study supports the definition of the numerical setup in the design of floating offshore wind turbines. The results of extensive simulation studies are presented, which focus particularly on determining the requirements for the load simulations in the design process. The analysis focusses on the cases of: (1) fatigue during power production and (2) ultimate loads during power production and severe sea state. For the fatigue load case, sensitivity study is performed in order to determine relevant load conditions and the expected impact of a variation in the environmental loading. Additionally, focus is put on the requirements regarding the run-in time, number of seeds and the simulation length for both fatigue and ultimate limit state (FLS, ULS) analysis. Another topic addressed is the benefit of using an increased number of seeds rather than extending the simulation time of single seeds, when a given total simulation time is required as described in the guidelines. The run-in time may be shortened when using predetermined steady states as initial conditions. Requirements for the steady state simulations are also determined and presented.

Hydrodynamic Modeling of Large-Diameter Bottom-Fixed Offshore Wind Turbines

2015 ◽  
Author(s):  
Erin E. Bachynski ◽  
Harald Ormberg
Keyword(s):  
Wind Turbines ◽  
Environmental Conditions ◽  
Limit State ◽  
Second Order ◽  
Offshore Wind ◽  
Stokes Wave ◽  
Intermediate Water ◽  
Hydrodynamic Loads ◽  
Offshore Wind Turbines ◽  
Wave Kinematics

For shallow and intermediate water depths, large monopile foundations are considered to be promising with respect to the levelized cost of energy (LCOE) of offshore wind turbines. In order to reduce the LCOE by structural optimization and de-risk the resulting designs, the hydrodynamic loads must be computed efficiently and accurately. Three efficient methods for computing hydrodynamic loads are considered here: Morison’s equation with 1) undisturbed linear wave kinematics or 2) undisturbed second order Stokes wave kinematics, or 3) the MacCamy-Fuchs model, which is able to account for diffraction in short waves. Two reference turbines are considered in a simplified range of environmental conditions. For fatigue limit state calculations, accounting for diffraction effects was found to generally increase the estimated lifetime of the structure, particularly the tower. The importance of diffraction depends on the environmental conditions and the structure. For the case study of the NREL 5 MW design, the effect could be up to 10 % for the tower base and 2 % for the monopile under the mudline. The inclusion of second order wave kinematics did not have a large effect on the fatigue calculations, but had a significant impact on the structural loads in ultimate limit state conditions. For the NREL 5 MW design, a 30 % increase in the maximum bending moment under the mudline could be attributed to the second order wave kinematics; a 7 % increase was seen for the DTU 10 MW design.

Dynamic Responses of Jacket-Type Offshore Wind Turbines Using Decoupled and Coupled Models

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Muk Chen Ong ◽  
Erin E. Bachynski ◽  
Ole David Økland
Keyword(s):  
Wind Turbines ◽  
Limit State ◽  
Coupled Model ◽  
Dynamic Responses ◽  
Offshore Wind ◽  
Ultimate Limit State ◽  
Coupled Models ◽  
Offshore Wind Turbines ◽  
Thrust And Torque ◽  
Rotor Model

This paper presents numerical studies of the dynamic responses of two jacket-type offshore wind turbines (OWTs) using both decoupled and coupled models. The investigated structures are the OC4 (Offshore Code Comparison Collaboration Continuation) jacket foundation and a full-lattice support structure presented by Long et al., 2012, “Lattice Towers for Bottom-Fixed Offshore Wind Turbines in the Ultimate Limit State: Variation of Some Geo metric Parameters,” ASME J. Offshore Mech. Arct. Eng., 134(2), p. 021202. Both structures support the NREL 5-MW wind turbine. Different operational wind and wave loadings at an offshore site with relatively high soil stiffness are investigated. In the decoupled (hydroelastic) model, the thrust and torque from an isolated rotor model were used as wind loads on the decoupled model together with a linear aerodynamic damper. The coupled model is a hydro-servo-aero-elastic representation of the system. The objective of this study is to evaluate the applicability of the computationally efficient linear decoupled model by comparing with the results obtained from the nonlinear coupled model. Good agreement was obtained in the eigen-frequency analysis, decay tests, and wave-only simulations. It was also found that, by applying the thrust force from an isolated rotor model in combination with linear damping, reasonable agreement could be obtained between the decoupled and coupled models in combined wind and wave simulations.

scholarly journals Optimal Design of Jacket Supporting Structures for Offshore Wind Turbines Using CBO and ECBO Algorithms

2017 ◽  
Author(s):  
Ali Kaveh ◽  
Sepehr Sabeti
Keyword(s):  
Optimal Design ◽  
Wind Turbines ◽  
Heuristic Algorithms ◽  
Limit State ◽  
Offshore Wind ◽  
Ultimate Limit State ◽  
Offshore Wind Turbines ◽  
Supporting Structure ◽  
Colliding Bodies Optimization ◽  
Resultant Forces

Structural optimization of offshore wind turbines is a tedious task due to the complexity of the problem. However, in this article, this problem is tackled using two meta-heuristic algorithms - Colliding Bodies Optimization (CBO) and its enhanced version (ECBO) - for a jacket supporting structure. The OC4 reference jacket is chosen as a case study to validate the methods utilized in this research. The jacket supporting structure is modeled in MATLAB and its optimal design is performed while both Ultimate Limit State (ULS) and frequency constraints are considered. In the present study, it is presumed that both wind and wave phenomena act in the same horizontal direction. As a result, all resultant forces and moments will act in-plane and the substructure can therefore be modeled in 2D space. Considerable weight reduction is obtained during the optimization process while fulfilling all constraints. 

System Reliability for Offshore Wind Turbines: Fatigue Failure

2013 ◽  
Author(s):  
S. Márquez-Domínguez ◽  
J. D. Sørensen
Keyword(s):  
Wind Turbines ◽  
Limit State ◽  
Wind Farms ◽  
Offshore Wind ◽  
Design Standards ◽  
State Equations ◽  
Offshore Wind Farms ◽  
Offshore Wind Turbines ◽  
Cost Of Energy ◽  
Wake Effects

Deeper waters and harsher environments are the main factors that make the electricity generated by offshore wind turbines (OWTs) expensive due to high costs of the substructure, operation & maintenance and installation. The key goal of development is to decrease the cost of energy (CoE). In consequence, a rational treatment of uncertainties is done in order to assess the reliability of critical details in OWTs. Limit state equations are formulated for fatigue critical details which are not influenced by wake effects generated in offshore wind farms. Furthermore, typical bi-linear S-N curves are considered for reliability verification according to international design standards of OWTs. System effects become important for each substructure with many potential fatigue hot spots. Therefore, in this paper a framework for system effects is presented. This information can be e.g. no detection of cracks in inspections or measurements from condition monitoring systems. Finally, an example is established to illustrate the practical application of this framework for jacket type wind turbine substructure considering system effects.

Lattice Towers for Bottom-Fixed Offshore Wind Turbines in the Ultimate Limit State: Variation of Some Geometric Parameters

2011 ◽  
Vol 134 (2) ◽  
Author(s):  
Haiyan Long ◽  
Geir Moe ◽  
Tim Fischer
Keyword(s):  
Wind Turbines ◽  
Limit State ◽  
Steel Structures ◽  
Offshore Wind ◽  
Ultimate Limit State ◽  
Offshore Wind Turbines ◽  
Lattice Towers ◽  
Visual Impact ◽  
Physical Impact ◽  
Ultimate Limit

Optimal solutions for offshore wind turbines (OWTs) are expected to vary from those of their onshore counterparts because of the harsh offshore climate, and differences in loadings, transportation, access, etc. This definitely includes the support structures required for service in the sea. Lattice towers might be a competitive solution for OWTs due to less physical impact from waves and less concern for visual impact. This paper addresses the design methodology of lattice towers for OWTs in the ultimate limit state and presents a FEM code that has been developed to implement this methodology. The structural topologies are specified in terms of tower cross-section geometry, the inclination of bracings, and the number of segments along the tower height. For each topology a series of towers is designed in which the bottom distance between the legs has been varied; the resulting tower mass is evaluated as a major parameter for the cost assessment. The study was conducted using the NREL 5-MW baseline wind turbine for an offshore site at a water depth of 35 m. The optimal design is searched for according to tower mass and fabrication complexity. The most economical tower geometry appears to have a constant inclination of bracing owing to its simplicity of fabrication and strong antitorsion capacity. Three-legged and four-legged alternatives have different advantages, the former having simpler geometry and the latter offering better torsion resistance. As a design driver for offshore steel structures, the fatigue life of the towers designed in the ultimate limit state should be assessed and the structures are consequently modified, if necessary. However, fatigue assessment is out of the scope of this paper and will be done in a later work.

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