In-Situ Measurements of Relative Displacement between Piston Rod and Seal Housing with Estimations of Wear on a Sealing System in a Submerged Transmission of an Offshore Underwater Wave Energy Converter
(English)In: Journal of Renewable and Sustainable Energy, ISSN 1941-7012Article in journal (Other academic) Submitted
A type of submerged piston rod mechanical lead-through transmission has been designed and implemented inside the two first full scale wave energy converter (WEC) prototypes at the Lysekil wave energy research site on the Swedish west cost. The transmission constitutes a direct drive which transfers the mechanical force, absorbed from the waves by a heaving surface-floating buoy, through an encapsulation and into a permanent magnet linear generator bolted to a foundation on the seabed, while sealing off the corrosive seawater. A set-up of 7 laser triangulation sensors has been installed in the second wave energy converter prototype to measure the relative displacement of the piston rod and the seal housing in-situ. This paper focus on investigating the relative tilt angles of both objects, the differential tilt angle between the piston rod and the seal housing and how they move in the time and space as they tilt with the ocean waves. Estimations on wear of the sealing components in the dynamic sealing system are made. The measurements are performed for two sea states with 2.5 month separation. The paper describes the experimental set-up and the sensor measuring systems. Equations for calculating the relative differential tilt angle between piston rod and seal housing are presented. The calculated differential tilt angle is correlated with measurements of the piston stroke length and translator position performed with the draw-wire sensor. Tilt angles, azimuth angles and differential tilt angles are presented for one wave period in each measurement period. The wear is estimated by calculating the relative change in traversed clearance gap by the piston rod across the interface inside the seal housing. Results show the expected differential tilt angle early in the experiment. A substantial increase in differential tilt angle developed over 3 months time as a consequence of large wear between buoy line and guiding funnel causing a large tilt angle on the piston rod and unintentionally large normal forces between piston rod and the dynamic sealing system inside the seal housing. The results indicate the need of a more flexible mechanical lead-through design, which is presented at the end of the paper. The results are discussed and conclusions are drawn with specific emphasis on improving the sealing system with stronger guide elements and the patented mechanical lead-through design.
wave energy converter; linear generator; direct drive; piston rod; seal housing; mechanical lead-through; submerged transmission; relative displacement measurement; laser triangulation sensor; tilting; tilt angle; azimuth angle; estimation of wear; sealing component.
Research subject Engineering Science with specialization in Science of Electricity
IdentifiersURN: urn:nbn:se:uu:diva-179737OAI: oai:DiVA.org:uu-179737DiVA: diva2:545960
ProjectsThe Lysekil Wave Power Project