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Title: Assessment of First- and Second-Order Wave-Excitation Load Models for Cylindrical Substructures: Preprint

Abstract

The hydrodynamic loads on an offshore wind turbine's support structure present unique engineering challenges for offshore wind. Two typical approaches used for modeling these hydrodynamic loads are potential flow (PF) and strip theory (ST), the latter via Morison's equation. This study examines the first- and second-order wave-excitation surge forces on a fixed cylinder in regular waves computed by the PF and ST approaches to (1) verify their numerical implementations in HydroDyn and (2) understand when the ST approach breaks down. The numerical implementation of PF and ST in HydroDyn, a hydrodynamic time-domain solver implemented as a module in the FAST wind turbine engineering tool, was verified by showing the consistency in the first- and second-order force output between the two methods across a range of wave frequencies. ST is known to be invalid at high frequencies, and this study investigates where the ST solution diverges from the PF solution. Regular waves across a range of frequencies were run in HydroDyn for a monopile substructure. As expected, the solutions for the first-order (linear) wave-excitation loads resulting from these regular waves are similar for PF and ST when the diameter of the cylinder is small compared to the length of the wavesmore » (generally when the diameter-to-wavelength ratio is less than 0.2). The same finding applies to the solutions for second-order wave-excitation loads, but for much smaller diameter-to-wavelength ratios (based on wavelengths of first-order waves).« less

Authors:
; ; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE), Wind and Water Technologies Office (EE-4W)
OSTI Identifier:
1346918
Report Number(s):
NREL/CP-5000-68131
DOE Contract Number:
AC36-08GO28308
Resource Type:
Conference
Resource Relation:
Conference: Presented at the International Society of Offshore and Polar Engineers Conference (ISOPE 2016), 26 June - 2 July 2016, Rhodes, Greece
Country of Publication:
United States
Language:
English
Subject:
17 WIND ENERGY; hydrodynamics; monopile; second-order forces; potential flow; strip theory

Citation Formats

Pereyra, Brandon, Wendt, Fabian, Robertson, Amy, and Jonkman, Jason. Assessment of First- and Second-Order Wave-Excitation Load Models for Cylindrical Substructures: Preprint. United States: N. p., 2017. Web.
Pereyra, Brandon, Wendt, Fabian, Robertson, Amy, & Jonkman, Jason. Assessment of First- and Second-Order Wave-Excitation Load Models for Cylindrical Substructures: Preprint. United States.
Pereyra, Brandon, Wendt, Fabian, Robertson, Amy, and Jonkman, Jason. Thu . "Assessment of First- and Second-Order Wave-Excitation Load Models for Cylindrical Substructures: Preprint". United States. doi:. https://www.osti.gov/servlets/purl/1346918.
@article{osti_1346918,
title = {Assessment of First- and Second-Order Wave-Excitation Load Models for Cylindrical Substructures: Preprint},
author = {Pereyra, Brandon and Wendt, Fabian and Robertson, Amy and Jonkman, Jason},
abstractNote = {The hydrodynamic loads on an offshore wind turbine's support structure present unique engineering challenges for offshore wind. Two typical approaches used for modeling these hydrodynamic loads are potential flow (PF) and strip theory (ST), the latter via Morison's equation. This study examines the first- and second-order wave-excitation surge forces on a fixed cylinder in regular waves computed by the PF and ST approaches to (1) verify their numerical implementations in HydroDyn and (2) understand when the ST approach breaks down. The numerical implementation of PF and ST in HydroDyn, a hydrodynamic time-domain solver implemented as a module in the FAST wind turbine engineering tool, was verified by showing the consistency in the first- and second-order force output between the two methods across a range of wave frequencies. ST is known to be invalid at high frequencies, and this study investigates where the ST solution diverges from the PF solution. Regular waves across a range of frequencies were run in HydroDyn for a monopile substructure. As expected, the solutions for the first-order (linear) wave-excitation loads resulting from these regular waves are similar for PF and ST when the diameter of the cylinder is small compared to the length of the waves (generally when the diameter-to-wavelength ratio is less than 0.2). The same finding applies to the solutions for second-order wave-excitation loads, but for much smaller diameter-to-wavelength ratios (based on wavelengths of first-order waves).},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Mar 09 00:00:00 EST 2017},
month = {Thu Mar 09 00:00:00 EST 2017}
}

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  • The hydrodynamic loads on an offshore wind turbine's support structure present unique engineering challenges for offshore wind. Two typical approaches used for modeling these hydrodynamic loads are potential flow (PF) and strip theory (ST), the latter via Morison's equation. This study examines the first- and second-order wave-excitation surge forces on a fixed cylinder in regular waves computed by the PF and ST approaches to (1) verify their numerical implementations in HydroDyn and (2) understand when the ST approach breaks down. The numerical implementation of PF and ST in HydroDyn, a hydrodynamic time-domain solver implemented as a module in the FASTmore » wind turbine engineering tool, was verified by showing the consistency in the first- and second-order force output between the two methods across a range of wave frequencies. ST is known to be invalid at high frequencies, and this study investigates where the ST solution diverges from the PF solution. Regular waves across a range of frequencies were run in HydroDyn for a monopile substructure. As expected, the solutions for the first-order (linear) wave-excitation loads resulting from these regular waves are similar for PF and ST when the diameter of the cylinder is small compared to the length of the waves (generally when the diameter-to-wavelength ratio is less than 0.2). The same finding applies to the solutions for second-order wave-excitation loads, but for much smaller diameter-to-wavelength ratios (based on wavelengths of first-order waves).« less
  • In this study, we assess the impact of different wave kinematics models on the dynamic response of a tension-leg-platform wind turbine. Aero-hydro-elastic simulations of the floating wind turbine are carried out employing linear, second-order, and fully nonlinear kinematics using the Morison equation for the hydrodynamic forcing. The wave kinematics are computed from either theoretical or measured signals of free-surface elevation. The numerical results from each model are compared to results from wave basin tests on a scaled prototype. The comparison shows that sub and superharmonic responses can be introduced by second-order and fully nonlinear wave kinematics. The response at themore » wave frequency range is better reproduced when kinematics are generated from the measured surface elevation. In the future, the numerical response may be further improved by replacing the global, constant damping coefficients in the model by a more detailed, customizable definition of the user-defined numerical damping.« less
  • To better access the abundant offshore wind resource, efforts across the world are being undertaken to develop and improve floating offshore wind turbine technologies. A critical aspect of creating reliable, mature floating wind turbine technology is the development, verification, and validation of efficient computer-aided-engineering (CAE) tools that can be relied upon in the design process. The National Renewable Energy Laboratory (NREL) has created a comprehensive, coupled analysis CAE tool for floating wind turbines, FAST, which has been verified and utilized in numerous floating wind turbine studies. Several efforts are currently underway that leverage the extensive 1/50th-scale DeepCwind wind/wave basin modelmore » test dataset, obtained at the Maritime Research Institute Netherlands (MARIN) in 2011, to validate the floating platform functionality of FAST to complement its already validated aerodynamic and structural simulation capabilities. In this paper, further work is undertaken to continue this validation. In particular, the ability of FAST to replicate global response behaviors associated with dynamic wind forces, second-order difference-frequency wave-diffraction forces and their interaction with one another are investigated.« less
  • This paper sets out to illustrate how a hybrid eigenfunction expansion and boundary integral equation based analysis of the fluid-structure interaction of the waves with the unique bottom founded steel caisson of arbitrary shape can be undertaken. Using direct pressure integration techniques the predicted first order velocity potentials are calculated for the second order mean forces and moments. The method allows a frequency domain analysis to be pursued which provides information about the hydrodynamic characteristics of the offshore caisson as it is installed in a wave environment.
  • Hydrodynamic loads on the platforms of floating offshore wind turbines are often predicted with computer-aided engineering tools that employ Morison's equation and/or potential-flow theory. This work compares results from one such tool, FAST, NREL's wind turbine computer-aided engineering tool, and the computational fluid dynamics package, OpenFOAM, for the OC4-DeepCwind semi-submersible analyzed in the International Energy Agency Wind Task 30 project. Load predictions from HydroDyn, the offshore hydrodynamics module of FAST, are compared with high-fidelity results from OpenFOAM. HydroDyn uses a combination of Morison's equations and potential flow to predict the hydrodynamic forces on the structure. The implications of the assumptionsmore » in HydroDyn are evaluated based on this code-to-code comparison.« less