Modeling Breaking Waves for Fixed-Bottom Support Structures for Offshore Wind Turbines

2018 ◽  
Author(s):  
Hannah M. Johlas ◽  
Spencer Hallowell ◽  
Shengbai Xie ◽  
Pedro Lomonaco ◽  
Matthew A. Lackner ◽  
...  
Keyword(s):  
Wind Turbines ◽  
Water Depth ◽  
Adaptive Mesh Refinement ◽  
Adaptive Mesh ◽  
Breaking Waves ◽  
Offshore Wind ◽  
Breaking Wave ◽  
Intermediate Water ◽  
Offshore Wind Turbines ◽  
Ocean Conditions

Fixed-bottom offshore wind turbines (OWTs) are typically located in shallow to intermediate water depth, where waves are likely to break. Support structure designs for such turbines must account for loads due to breaking waves, but predictions from breaking wave models often disagree with each other and with observed behavior. This variability indicates the need for a better understanding of each model’s strengths and limitations, especially for different ocean conditions. This work evaluates and improves the accuracy of common breaking wave criteria through comparison to Computational Fluid Dynamics (CFD) simulations of breaking waves. The simulated ocean conditions are representative of potential U.S. East Coast offshore wind energy development sites, but the discussion of model accuracy and limitations can be applied to any location with similar ocean conditions. The waves are simulated using CONVERGE, a commercial CFD software that uses a Volume of Fluid (VOF) approach and includes adaptive mesh refinement at the evolving air-water interface. First, the CFD model is validated against experimental data for shoaling and breaking wave surface elevations. Second, 2D simulations of breaking waves are compared to widely-used breaking wave limits (McCowan, Miche, and Goda) for different combinations of wave height, wavelength, water depth, and seafloor slope. Based on these comparisons, the accuracy and limitations of each breaking limit model are discussed. General usage guidelines are then recommended.

Experimental Investigations of Breaking Wave Impact Forces on a Monopile Substructure for Offshore Wind Turbines Under Regular Breaking Waves

2017 ◽  
Author(s):  
Vipin Chakkurunni Palliyalil ◽  
Panneer Selvam Rajamanickam ◽  
Mayilvahanan Alagan Chella ◽  
Vijaya Kumar Govindasamy
Keyword(s):  
Circular Cylinder ◽  
Wind Turbines ◽  
Breaking Waves ◽  
Offshore Wind ◽  
Offshore Wind Turbine ◽  
Breaking Wave ◽  
Wave Impact ◽  
Offshore Wind Turbines ◽  
Impact Forces ◽  
The Impact

The main objective of the paper is to investigate wave impact forces from breaking waves on a monopile substructure for offshore wind turbine in shallow waters. This study examines the load assessment parameters relevant for breaking wave forces on a vertical circular cylinder subjected to breaking waves. Experiments are conducted in a shallow water flume and the wave generation is based on piston type wave maker. The experiments are performed with a vertical circular cylinder with diameter, D = 0.20m which represents a monopile substructure for offshore wind turbines with regular waves of frequencies around 0.8Hz. The experimental setup consists of a 1/10 slope followed by a horizontal bed portion with a water depth of 0.8m. Plunging breaking waves are generated and free surface elevations are measured at different locations along the wave tank from wave paddle to the cylinder in order to find the breaking characteristics. Wave impact pressures are measured on the cylinder at eight different vertical positions along the height of the cylinder under breaking waves for different environmental conditions. The wave impact pressures and wave surface elevations in the vicinity of the cylinder during the impact for three different wave conditions are presented and discussed.

Detailed Study on Breaking Wave Interactions With a Jacket Structure Based on Experimental Investigations

2017 ◽  
Vol 140 (2) ◽  
Author(s):  
Jithin Jose ◽  
Olga Podrażka ◽  
Ove Tobias Gudmestad ◽  
Witold Cieślikiewicz
Keyword(s):  
Wind Turbines ◽  
Wave Breaking ◽  
Offshore Structures ◽  
Offshore Wind ◽  
Wave Forces ◽  
Breaking Wave ◽  
Offshore Wind Turbines ◽  
Wave Parameters ◽  
Wave Slamming ◽  
Breaking Wave Forces

Wave breaking is one of the major concerns for offshore structures installed in shallow waters. Impulsive breaking wave forces sometimes govern the design of such structures, particularly in areas with a sloping sea bottom. Most of the existing offshore wind turbines were installed in shallow water regions. Among fixed-type support structures for offshore wind turbines, jacket structures have become popular in recent times as the water depth for fixed offshore wind structures increases. However, there are many uncertainties in estimating breaking wave forces on a jacket structure, as only a limited number of past studies have estimated these forces. Present study is based on the WaveSlam experiment carried out in 2013, in which a jacket structure of 1:8 scale was tested for several breaking wave conditions. The total and local wave slamming forces are obtained from the experimental measured forces, using two different filtering methods. The total wave slamming forces are filtered from the measured forces using the empirical mode decomposition (EMD) method, and local slamming forces are obtained by the frequency response function (FRF) method. From these results, the peak slamming forces and slamming coefficients on the jacket members are estimated. The breaking wave forces are found to be dependent on various breaking wave parameters such as breaking wave height, wave period, wave front asymmetry, and wave-breaking positions. These wave parameters are estimated from the wave gauge measurements taken during the experiment. The dependency of the wave slamming forces on these estimated wave parameters is also investigated.

scholarly journals Effects of Spilling and Plunging Type Breaking Waves Acting on Large Monopile Offshore Wind Turbines

2020 ◽  
Vol 7 ◽  
Author(s):  
Ye Tang ◽  
Wei Shi ◽  
Dezhi Ning ◽  
Jikun You ◽  
Constantine Michailides
Keyword(s):  
Wind Turbines ◽  
Breaking Waves ◽  
Offshore Wind ◽  
Offshore Wind Turbines

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.

Using Nonlinear Wave Kinematics to Estimate the Loads on Offshore Wind Turbines in 3-Hour Sea States

2018 ◽  
Author(s):  
Tim Bunnik ◽  
Erik-Jan de Ridder
Keyword(s):  
Wind Turbine ◽  
Nonlinear Wave ◽  
Wind Turbines ◽  
Wave Model ◽  
Breaking Waves ◽  
Model Tests ◽  
Offshore Wind ◽  
Wave Loads ◽  
Offshore Wind Turbines ◽  
Wave Basin

The effects of operational wave loads and wind loads on offshore mono pile wind turbines are well understood. For most sites, however, the water depth is such that breaking or near-breaking waves will occur causing impulsive excitation of the mono pile and consequently considerable stresses, displacements and accelerations in the monopile, tower and turbine. As has been shown in earlier, recent publications, Computational Fluid Dynamics (CFD) can be used to accurately analyze wave impacts on offshore wind turbines. However, it is not yet well suited to study the statistical variability of wave impact loads in long-duration sea states, and thus estimate the ULS and ALS loads for which a wind turbine has to be designed. An alternative, simplified approach, is the use of a Morison model in which the kinematics (water particle velocities and accelerations) from a nonlinear wave model are used. For long-crested waves the nonlinear wave model can be run in a 2D mode and is therefore relatively cheap. In this paper model tests for steep and breaking waves on an offshore wind turbine are compared with results from the Morison model. First, a deterministic comparison is made between the wave loads from the model tests and the simulation model (simulating the same 3-hour wave realization as in the basin), which turns out to be difficult because of differences between wave reflections in the wave basin (a physical beach) and the numerical wave model (absorbing boundary condition). Second, a statistical comparison is made by comparing with different wave realizations measured in the wave basin.

Installation of Monopiles for Offshore Wind Turbines—By Using End-Caps and a Subsea Holding Structure

2011 ◽  
Author(s):  
Arunjyoti Sarkar ◽  
Ove T. Gudmestad
Keyword(s):  
Wind Turbines ◽  
Offshore Wind ◽  
Intermediate Water ◽  
Offshore Wind Turbines ◽  
Correct Location ◽  
End Caps ◽  
Jack Up ◽  
Basic Philosophy ◽  
Water Depths ◽  
Design Challenges

Monopiles are commonly used as foundations for offshore wind turbines at sites with shallow to intermediate water depths (say, up to 40m water depth). The installation of a monopiles is normally carried out by using a bottom supported platform (e.g., a jack-up vessel) which holds the pile at the correct location vertically while driving it into the seabed. In this paper, a methodology for installing a monopile is described which can be applied either by a bottom supported platform or by a floating vessel. The basic philosophy behind this methodology is to support the monopile initially by buoyancy and then by a subsea holding structure. Thus the requirement for a large crane working offshore is eliminated and the marine operation is no longer dependent on the motions of the supporting vessel. Brief geotechnical calculations are presented to support the feasibility of this methodology. Some of the possible design challenges of the installation aids are listed in the conclusion.

scholarly journals Load Mitigation on Floating Offshore Wind Turbines With Advanced Controls and Tuned Mass Dampers

2018 ◽  
Author(s):  
John Cross-Whiter ◽  
Benjamin B. Ackers ◽  
Dhiraj Arora ◽  
Alan Wright ◽  
Paul Fleming ◽  
...  
Keyword(s):  
Fatigue Damage ◽  
Wind Turbines ◽  
Water Depth ◽  
Operating Conditions ◽  
Offshore Wind ◽  
Tuned Mass Dampers ◽  
Offshore Wind Turbines ◽  
Floating Offshore Wind Turbines ◽  
Advanced Controls ◽  
Load State

General Electric, the National Renewable Energy Laboratory (NREL), the University of Massachusetts Amherst (UMass), and Glosten have recently completed a US Department of Energy (DOE)-funded research program to study technologies for mitigating loads on floating offshore wind turbines through the use of advanced turbine controls and tuned mass dampers (TMDs). The analysis was based upon the Glosten PelaStar tension leg platform (TLP) with GE Haliade 150 turbine, a system developed in a previous front end engineering design (FEED) study funded by the Energy Technology Institute (ETI) in the UK. The platform was designed for the WaveHub wave energy research site, with a mean water depth of 59-m. Loads were analyzed by running time-domain simulations in four 50-year return period (50-YRP) ultimate load state (ULS) conditions and 77 fatigue load state (FLS) environmental conditions. In 50-YRP conditions advanced controls are not active. The influence of TMDs on ULS loads have been reported previously (Park et al. [2]). In FLS conditions advanced controls and TMDs afford dramatic reductions in fatigue damage, offering the potential of significant savings in tower structural requirements. Simulations in turbine idling conditions were run in OrcaFlex, and simulations in operating conditions were run in FAST. Simulations were run with a baseline turbine controller, representative of the current state of the art, and an advanced controller developed by NREL to use collective and individual blade pitch control to maintain rotor speed and reduce tower loads. UMass developed a number of TMD types, with varying system configurations, including passive nonlinear dampers and semi-actively controlled dampers with an inverse velocity groundhook control algorithm. Loads and accelerations in FLS conditions were evaluated on the basis of damage equivalent loads (DELs), and fatigue damage was computed by Miner’s summations of stress cycles at the tower base. To study sensitivity to water depth, loads were analyzed at both the 59-m WaveHub depth and a more commercially realistic depth of 100 m. TMDs reduce fatigue damage at the tower-column interface flange by up to 52% in 59-m water depth and up to 28% in 100 m water depth. Advanced controls reduce fatigue damage at the tower-column flange by up to 22% in 59-m water depth and up to 40% in 100 m water depth. The most effective load-mitigation strategy is combining advanced controls with TMDs. This strategy affords a 71% reduction in fatigue damage in both 59-m and 100-m water depths.

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