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Three-Dimensional Oscillation Dynamics of the In Situ Piston Rod Transmission Between Buoy Line and the Double Hinge-Connected Translator in an Offshore Linear Wave Energy Converter
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2016 (English)In: Journal of Offshore Mechanics and Arctic Engineering-Transactions of The Asme, ISSN 0892-7219, E-ISSN 1528-896X, Vol. 138, no 3, 031901Article in journal (Refereed) PublishedText
Abstract [en]

Force and displacement measurements have been performed in situ on the piston rod mechanical lead-through transmission in the direct drive of the second experimental wave energy converter (WEC) 3 km offshore at the Lysekil research site (LRS) during a 130-day continuous full-scale experiment in 2009. The direct drive consists of a buoy line and a piston rod transmission with a double-hinged link (DHL) at the lower end connecting the point absorbing surface-floating buoy to the translator of an encapsulated permanent magnet linear generator on the seabed. The buoy line is guided by a funnel in the buoy line guiding system 3.2m above the generator capsule. The 3m long piston rod reciprocates through a mechanical lead-through in the capsule wall, sealing off seawater from entering the generator capsule. A setup of laser triangulation sensors measures the relative lateral displacement of the piston rod. This paper introduces a method and a system of equations for calculating piston rod relative tilt angle and piston rod azimuth direction of tilting from the relative lateral displacement measurements. Correlation with piston rod axial displacement and forces enables evaluation of the three-dimensional (3D) oscillation dynamics. Results are presented from 2 weeks after launch and from 3 months after launch in altogether four cases representing two different stages of wear in two different sea states. Piston rod tilting from accumulated wear in the buoy line guiding system is separated from tilting due to elastic displacement. Structural mechanical finite element method (FEM) simulations verify the magnitude of elastic displacement and indicate negligible stress and strain at the mounting point of the laser sensor setup. The proposed theory for piston rod 3D motion is validated by the experiment. As the experiment progressed, wear in the buoy line guiding system accelerated due to splitting of the buoy line jacketing compound, thereby increasing the piston rod tilt angles. Over 94 days into the experiment, 21.8mm of accumulated wear in the buoy line guiding system had altered the characteristics of the piston rod oscillations and increased the maximum piston rod relative tilt angle by 0.39 deg in the predominant azimuth direction of wave propagation. Further accumulated wear in the buoy line guiding system led to buoy line rupture 130 days after launch. The results presented in this paper have been used in assessments for improving the mechanical subsystems in subsequent experimental WECs based on the Uppsala concept.

Place, publisher, year, edition, pages
2016. Vol. 138, no 3, 031901
Keyword [en]
wave energy converter, offshore measurement, laser triangulation sensor, piston rod transmission, relative displacement, tilt angle, azimuth direction, oscillation dynamics
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:uu:diva-298070DOI: 10.1115/1.4031972ISI: 000375922900008OAI: oai:DiVA.org:uu-298070DiVA: diva2:944852
Note

Funding: The authors of this paper are affiliated with the Swedish Centre for Renewable Electric Energy Conversion at Uppsala University in Uppsala, Sweden. The paper is the product of research carried out within the Lysekil Wave Power Project. The research was supported by the Swedish Energy Agency, VINNOVA, Statkraft AS, Vattenfall AB, Fortum OY, Falkenberg Energy AB, Helukabel AB, Draka Cable AB, Seabased AB, Pro Enviro, the Gothenburg Energy Research Foundation, the Goran Gustavsson Research Foundation, Angpanneforeningen's Foundation for Research and Development, the Olle Engkvist Foundation, the J. Gust. Richert Foundation, the Swedish Association of Graduate Engineers' Environmental Fund, Vargon's Research Foundation, the Wallenius Foundation, and the Swedish Research Council Grant No. 621-2009-3417.

Available from: 2016-06-30 Created: 2016-06-29 Last updated: 2016-06-30Bibliographically approved

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