Steel Jackets and Monotower Foundations for Offshore Wind Turbines

2012 ◽  
Author(s):  
Kasper Wåsjø ◽  
Morten Bjerkås ◽  
Tore Søreide
Keyword(s):  
Cost Benefit ◽  
Offshore Wind ◽  
Support Structures ◽  
Base Shear ◽  
Offshore Wind Turbines ◽  
Steel Jacket ◽  
Important Benefit ◽  
Comparable Magnitude ◽  
Water Depths ◽  
Offshore Wind Industry

Steel jacket support structures for offshore wind have an increasing popularity for water depths larger than 20 m. Traditionally, support structures such as monopiles and gravity based foundation have dominated at shallow water. Offshore wind industry deals gradually with larger water depths, and light weight foundations as steel jackets are believed to give a cost benefit. The present study shows that the quasi static overturning moment for monotower structures and steel jackets are of comparable magnitude. It is also shown that the most important benefit of steel jackets as support structures is to reduce the base shear. At last it is shown that foundation size is primarily driven by wave and current loads at water depth around 50–60 m.

scholarly journals Tripod-Supported Offshore Wind Turbines: Modal and Coupled Analysis and a Parametric Study Using X-SEA and FAST

2019 ◽  
Vol 7 (6) ◽  
pp. 181 ◽  
Author(s):  
Pasin Plodpradit ◽  
Van Nguyen Dinh ◽  
Ki-Du Kim
Keyword(s):  
Wind Turbines ◽  
Natural Frequencies ◽  
Offshore Wind ◽  
Computational Time ◽  
Coupled Analysis ◽  
Physical Interaction ◽  
Support Structures ◽  
Coupled Models ◽  
Support Structure ◽  
Offshore Wind Turbines

This paper presents theoretical aspects and an extensive numerical study of the coupled analysis of tripod support structures for offshore wind turbines (OWTs) by using X-SEA and FAST v8 programs. In a number of site conditions such as extreme and longer period waves, fast installation, and lighter foundations, tripod structures are more advantageous than monopile and jacket structures. In the implemented dynamic coupled analysis, the sub-structural module in FAST was replaced by the X-SEA offshore substructure analysis component. The time-histories of the reaction forces and the turbine loads were then calculated. The results obtained from X-SEA and from FAST were in good agreement. The pile-soil-structure interaction (PSSI) was included for reliable evaluation of OWT structural systems. The superelement concept was introduced to reduce the computational time. Modal, coupled and uncoupled analyses of the NREL 5MW OWT-tripod support structure including PSSI were carried out and the discussions on the natural frequencies, mode shapes and resulted displacements are presented. Compared to the uncoupled models, the physical interaction between the tower and the support structure in the coupled models resulted in smaller responses. Compared to the fixed support structures, i.e., when PSSI is not included, the piled-support structure has lower natural frequencies and larger responses attributed to its actual flexibility. The models using pile superelements are computationally efficient and give results that are identical to the common finite element models.

On the fatigue behavior of support structures for offshore wind turbines

2014 ◽  
Vol 18 (2) ◽  
pp. 117-134 ◽  
Author(s):  
N. Alati ◽  
V. Nava ◽  
G. Failla ◽  
F. Arena ◽  
A. Santini
Keyword(s):  
Wind Turbines ◽  
Fatigue Behavior ◽  
Offshore Wind ◽  
Support Structures ◽  
Offshore Wind Turbines

scholarly journals Larger MW-Class Floater Designs Without Upscaling?: A Direct Optimization Approach

2019 ◽  
Author(s):  
Mareike Leimeister ◽  
Athanasios Kolios ◽  
Maurizio Collu ◽  
Philipp Thomas
Keyword(s):  
Optimization Procedure ◽  
Offshore Wind ◽  
Intermediate Step ◽  
Optimization Approach ◽  
Support Structures ◽  
Direct Optimization ◽  
Offshore Wind Turbines ◽  
Existing Structures ◽  
Automated Optimization ◽  
Floating Platforms

Abstract The trend towards larger offshore wind turbines (WTs) implies the need for bigger support structures. These are commonly derived from existing structures through upscaling and subsequent optimization. To reduce the number of design steps, this work proposes a direct optimization approach, by which means a support structure for a larger WT is obtained through an automated optimization procedure based on a smaller existing system. Due to the suitability of floating platforms for large MW-class WTs, this study is based on the OC3 spar-buoy designed for the NREL 5 MW WT. Using a Python-Modelica framework, developed at Fraunhofer IWES, the spar-buoy geometry is adjusted through iterative optimization steps to finally support a 7.5 MW WT. The optimization procedure focuses on the global system performance in a design-relevant load case. This study shows that larger support structures, appropriate to meet the objective of the hydrodynamic system behavior, can be obtained through automated optimization of existing designs without the intermediate step of upscaling.

scholarly journals A fracture mechanics framework for optimising design and inspection of offshore Wind Turbine support structures against fatigue failure

2020 ◽  
Author(s):  
Peyman Amirafshari ◽  
Feargal Brenan ◽  
Athanasios Kolios
Keyword(s):  
Fracture Mechanics ◽  
Wind Turbine ◽  
Fatigue Failure ◽  
Cost Benefit Analysis ◽  
Cost Benefit ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Support Structures ◽  
Benefit Analysis ◽  
Support Structure

Abstract. Offshore Wind Turbine (OWT) support structures need to be designed against fatigue failure under cyclic aerodynamic and wave loading. The fatigue failure can be accelerated in a corrosive sea environment. Traditionally, a stress-life approach called the S-N curve method has been used for design of structures against fatigue failure. There are a number of limitations in S-N approach related to welded structures which can be addressed by the fracture mechanics approach. In this paper the limitations of the S-N approach related to OWT support structure are addressed, a fatigue design framework based on fracture mechanics is developed. The application of the framework to a monopile OWT support structure is demonstrated and optimisation of in-service inspection of the structure is studied. It was found that both the design of the weld joint and Non-destructive testing techniques can be optimised to reduce In-service frequency. Furthermore, probabilistic fracture mechanics as a form of risk-based design is outlined and its application to the monopile support structure is studied. The probabilistic model showed to possess a better capability to account for NDT reliability over a range of possible crack sizes as well as providing a risk associated with the chosen inspection time which can be used in inspection cost benefit analysis. There are a number of areas for future research. including better estimate of fatigue stress with a time-history analysis, the application of framework to other types of support structures such as Jackets and Tripods, and integration of risk-based optimisation with a cost benefit analysis.

How to Detect Local Out-of-Plane Vibrations in Jacket Support Structures for Offshore Wind Turbines

2014 ◽  
Author(s):  
Sebastian Schafhirt ◽  
John M. Hembre ◽  
Michael Muskulus
Keyword(s):  
Fatigue Damage ◽  
Wind Turbines ◽  
Sequential Analysis ◽  
Offshore Wind ◽  
Support Structures ◽  
Local Vibration ◽  
Offshore Wind Turbines ◽  
Simulation Based ◽  
Out Of Plane ◽  
The Impact

There has been an ongoing debate whether local out-of-plane vibrations of braces exist in jacket support structures for wind turbines. The issue has been raised with the sequential analysis of offshore wind turbines, where increased fatigue damage for bracings was observed. Local vibration modes, excited by rotor harmonics, were detected as a reason for it. A methodology to remove global motion of the jacket from the displacements of the central joint in a brace is presented and the amplitude of local out-of-plane displacements is analyzed, using an integrated wind turbine simulation based on a flexible multibody solver. Finally, the impact on fatigue damage is calculated. Results indicate that the extent of local vibrations is much less than previously thought or predicted in other studies.

scholarly journals Numerical Simulations for Installation of Offshore Wind Turbine Monopiles Using Floating Vessels

2013 ◽  
Author(s):  
Lin Li ◽  
Zhen Gao ◽  
Torgeir Moan
Keyword(s):  
Time Domain ◽  
Coupled System ◽  
Dynamic Responses ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Support Structures ◽  
Offshore Wind Turbines ◽  
Motion Responses ◽  
Sensitivity Studies ◽  
Jack Up

Monopiles are the most commonly used support structures for offshore wind turbines with up to 40m water depth due to the simplicity of the structure. The installation of turbine support structures can be carried out by a jack-up vessel which provides a stable working platform. However, the operational weather window using jack-up vessels is very limited due to the low sea states required for jacking up and down. Compared to jack-up installation vessels, floating vessels have more flexibility due to fast transportations between foundations. However, the vessel motions will affect the motion responses of the lifting objects, which might bring installation difficulties. Therefore, it is necessary to examine the dynamic responses of the coupled system to ensure safe offshore operations. In this paper, the installation operation of a monopile using a floating installation vessel is studied by a numerical model. Time domain simulations were carried out to study the installation process of a monopile, including lowering phase, landing phase and steady states after landing. Sensitivity studies were performed focusing on the effects by the gripper device stiffness and landing device stiffness. Comparisons of critical responses by using floating vessel and a jack-up vessel were also studied in the paper.

scholarly journals Active Load Mitigation to Counter the Fatigue Damage Contributions From Unavailability in Offshore Wind Turbines

2018 ◽  
Author(s):  
Stian H. Sørum ◽  
Emil Smilden ◽  
Jørgen Amdahl ◽  
Asgeir J. Sørensen
Keyword(s):  
Fatigue Damage ◽  
Expected Value ◽  
Offshore Wind ◽  
Design Standards ◽  
Offshore Wind Turbines ◽  
Active Load ◽  
Design Value ◽  
Unused Capacity ◽  
Conservative Values ◽  
Offshore Wind Industry

The offshore wind industry continues to grow, but there is still a need for more economical designs. As unavailability conditions can be critical for the fatigue damage in support structures, design standards use conservative values for availability. This leads to most turbines having an unused fatigue capacity at the end of the lifetime. This paper investigates the potential for reducing this unused capacity in order to reduce the capital expenses. The proposed strategy is to design the turbines for a higher availability, closer to the expected value. For individual turbines that experience lower availability than the design value, active load mitigation is imposed to reduce the fatigue damage. The potential of this methods is explored, together with its limitations. It is found that the effect of faults occurring early in the turbines lifetime can be reduced. This is not the case for faults occurring towards the end of the lifetime.

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