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.

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.

scholarly journals A Comparative Study between Three-Legged and Tripod Sub-structures in Design of Offshore Wind Turbines in the Transition Water Depth

2018 ◽  
Vol 7 (3.36) ◽  
pp. 23
Author(s):  
Aliakbar Khosravi ◽  
Tuck Wai Yeong ◽  
Mohammed Parvez Anwar ◽  
Jayaprakash Jaganathana ◽  
Teck Leong Lau ◽  
...  
Keyword(s):  
Water Depth ◽  
Limit State ◽  
Cost Effective ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Ultimate Limit State ◽  
Offshore Wind Turbines ◽  
Welding Joints ◽  
Fatigue Limit State ◽  
Ultimate Limit

This research aimed at investigating tripod and three-legged offshore wind turbine substructures. A comparison between the two substructures based on their weight as well as the installation method of piles, i.e. pre-piling and post-piling, was carried out. The in-place (Ultimate Limit State), Dynamic, natural frequency check and fatigue (Fatigue Limit State) analyses were conducted considering aerodynamic and hydrodynamic loads imposed on substructures in 50m water depth. An optimisation process was carried out in order to reduce the mass of substructures. The results revealed that the three-legged substructure is more cost effective with 25% lesser structure mass. However, the construction of the three-legged structure usually takes more time due to increased number of members and subsequently welding joints. The results, furthermore, showed that the pre-piling method reduces the time and cost of offshore installation, and reduces the weight of piles by 50%.  

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. 

An Automated Approach for Optimizing Monopile Foundations for Offshore Wind Turbines for Serviceability and Ultimate Limit States Design

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
James P. Doherty ◽  
Barry M. Lehane
Keyword(s):  
Limit State ◽  
Offshore Wind ◽  
Ultimate Limit State ◽  
Trial And Error ◽  
Limit States ◽  
Foundation Design ◽  
Offshore Wind Turbines ◽  
Economical Design ◽  
Limit State Criteria ◽  
Ultimate Limit

Pile foundation design is conventionally conducted using a process of trial and error, where the dimensions of a pile are estimated and the performance is computed and compared with design criteria. The dimensions are varied and the process is repeated in order to converge to a safe and economical design. In this paper, this time-consuming and labor intensive process is replaced with an automated approach using the example case of an offshore monopile supporting a wind turbine. The optimum length and diameter of the monopile are determined with the aim of minimizing the pile weight while satisfying both serviceability and ultimate limit state criteria. The approach handles general soil and loading conditions and includes an ability to incorporate cyclic loading.

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.

DESIGN OF BOLTED CONNECTIONS SUBJECT TO TORSION AND SHEAR IN THE ULTIMATE LIMIT STRESS STATE

2019 ◽  
Vol 6 (1) ◽  
Author(s):  
Mohamed S. Abu-Yosef ◽  
Ezzeldin Y. Sayed-Ahmed ◽  
Emam A. Soliman
Keyword(s):  
Failure Modes ◽  
Focal Point ◽  
Design Method ◽  
Limit State ◽  
Bending Moment ◽  
Steel Structures ◽  
Ultimate Limit State ◽  
Shear Forces ◽  
Limit States ◽  
Ultimate Limit

Steel connections transferring axial and shear forces in addition to bending moment and/or torsional moment are widely used in steel structures. Thus, design of such eccentric connections has become the focal point of any researches. Nonetheless, behavior of eccentric connections subjected to shear forces and torsion in the ultimate limit state is still ambiguous. Most design codes of practice still conservatively use the common elastic analysis for design of the said connections even in the ultimate limit states. Yet, there are some exceptions such as the design method proposed by CAN/CSA-S16-14 which gives tabulated design aid for the ultimate limit state design of these connections based on an empirical equation that is derived for ¾ inch diameter A325 bearing type bolts and A36 steel plates. It was argued that results can also be used with a margin of error for other grade bolts of different sizes and steel of other grades. As such, in this paper, the performance of bolted connection subject to shear and torsion is experimentally investigated. The behavior, failure modes and factors affecting both are scrutinized. Twelve connections subject to shear and torsion with different bolts configurations and diameters are experimentally tested to failure. The accuracy of the currently available design equations proposed is compared to the outcomes of these tests.

Wind/Wave Misalignment Effects on Mooring Line Tensions for a Spar Buoy Wind Turbine

2018 ◽  
Author(s):  
Luigia Riefolo ◽  
Fernando del Jesus ◽  
Raúl Guanche García ◽  
Giuseppe Roberto Tomasicchio ◽  
Daniela Pantusa
Keyword(s):  
Wind Turbine ◽  
Wind Turbines ◽  
Wind Wave ◽  
Offshore Wind ◽  
Mooring Line ◽  
Ultimate Limit State ◽  
International Electrotechnical Commission ◽  
Wave Loads ◽  
Offshore Wind Turbines ◽  
Floating Offshore Wind Turbines

The design methodology for mooring systems for a spar buoy wind turbine considers the influence of extreme events and wind/wave misalignments occurring in its lifetime. Therefore, the variety of wind and wave directions affects over the seakeeping and as a result the evaluation of the maxima loads acting on the spar-buoy wind turbine. In the present paper, the importance of wind/wave misalignments on the dynamic response of spar-type floating wind turbine [1] is investigated. Based on standards, International Electrotechnical Commission IEC and Det Norske Veritas DNV the design of position moorings should be carried out under extreme wind/wave loads, taking into account their misalignments with respect to the structure. In particular, DNV standard, in ‘Position mooring’ recommendations, specifies in the load cases definition, if site specific data is not available, to consider non-collinear environment to have wave towards the unit’s bow (0°) and wind 30° relative to the waves. In IEC standards, the misalignment of the wind and wave directions shall be considered to design offshore wind turbines and calculate the loads acting on the support structure. Ultimate Limit State (ULS) analyses of the OC3-Hywind spar buoy wind turbine are conducted through FAST code, a certified nonlinear aero-hydro-servo-elastic simulation tool by the National Renewable Energy Laboratory’s (NREL’s). This software was developed for use in the International Energy Agency (IEA) Offshore Code Comparison Collaborative (OC3) project, and supports NREL’s offshore 5-MW baseline turbine. In order to assess the effects of misaligned wind and wave, different wind directions are chosen, maintaining the wave loads perpendicular to the structure. Stochastic, full-fields, turbulence simulator Turbsim is used to simulate the 1-h turbulent wind field. The scope of the work is to investigate the effects of wind/wave misalignments on the station-keeping system of spar buoy wind turbine. Results are presented in terms of global maxima determined through mean up-crossing with moving average, which, then, are modelled by a Weibull distribution. Finally, extreme values are estimated depending on global maxima and fitted on Gumbel distribution. The Most Probable Maximum value of mooring line tensions is found to be influenced by the wind/wave misalignments. The present paper is organized as follows. Section ‘Introduction’, based on a literature study, gives useful information on the previous studies conducted on the wind/wave misalignments effects of floating offshore wind turbines. Section ‘Methodology’ describes the applied methodology and presents the spar buoy wind turbine, the used numerical model and the selected environmental conditions. Results and the corresponding discussion are given in Section ‘Results and discussion’ for each load case corresponding to the codirectional and misaligned wind and wave loads. Results are presented and discussed in time and frequency domains. Finally, in Section ‘Conclusion’ some conclusions are drawn.

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