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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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