Fatigue of moorings and low voltage cables used for wave energy converters

Transcription

1 Fatigue of moorings and low voltage cables used for wave energy converters Jonas Ringsberg Professor in Marine Structures Department of Shipping and Marine Technology Chalmers University of Technology p. 1

2 Licentiate thesis presentation When: June 8 at Where: Chalmers Campus Lindholmen Discussion leader: Prof Lars Johanning, University of Exeter (U.K.) PhD student: Shun- Han Yang (Hedy) Supervisor: Professor Jonas W Ringsberg Co- supervisor: Adjunct Professor Erland Johnson The research project has been funded by: p. 2

3 Research project Propose a simulation methodology suitable for the analysis of WEC systems Fatigue assessment of the mooring system Different configurations and mooring materials Fatigue design and assessment of the (dynamic) cable between the WEC and the hub Dimensions, bending stiffness, length, mass Study of the power capture of the WEC Parametric studies: influence of environmental loads and factors Wave loads Ocean current Marine biofouling p. 3

4 Coupled analysis Motion and structural analysis of the WEC system Output: curvature, section axial force, and bending moments of cables Fatigue damage analysis Stress-based approach Rainflow cycle counting method Output: fatigue damage and fatigue life prediction p. 4

5 Parametric study: case study WEC system WEC Point absorber Geometry, mass, COG, and COB SHIPPING AND MARINE TECHNOLOGY Hub Stationary point (fixed in six degrees of freedom) Cable Circular tube with a small inner diameter Hydrodynamic and structural properties Environmental conditions (hydrodynamic model) Waves Sea water Ocean current Seabed Biofouling p. 5

6 Variation in environmental loads Environmental loads Studied cases Current [-] Yes / No Current direction [deg] 0, 90, 180 Wave height, H s [m] with interval of 1 meters Wave period, T z [s] with interval of 1 seconds H s [m] T z [s] Sum Sum (Unit: ) p. 6

7 Variation in cable data Reference values Length, Lc [m] 70 Outer diameter, dout [m] 0.04 Inner diameter, din [m] 0.02 Axial stiffness, EA [N] Bending stiffness, EI [Nm 2 ] 400 Torsional stiffness, GK [Nm 2 /rad] Mass, Mc [kg/m] 2.75 Normal flow added mass coefficient, CMn [-] 1 Normal flow drag coefficient, CDn [-] 1.2 Tangential flow drag coefficient, CDt [-] 0 Rayleigh damping coefficient, β [-] Parametric study Properties of the cable Mc [kg/m] EI [Nm 2 ] Lc [m] Case 1 Case 2 Case 3 Case 4 Case 5 Case 6 Case 7 Case 8 Case 9 Case 10 Case 11 Case 12 Case 13 Case 14 Case 15 Case 16 Case 17 Case 18 p. 7

8 Current and wave directions Power cable p. 8

9 p. 9

10 Instantaneous dynamic behaviour (1) p. 10

11 Instantaneous dynamic behaviour (2) p. 11

12 Influence from ocean current p. 12

13 Fatigue damage evaluation p. 13

14 Current and wave directions p. 14

15 Future work Continue to study Energy harvesting model of WEC Mooring systems and materials Local cable model (intrinsic failure mechanisms) Life cycle cost analysis of WEC system Ocean basin tests: WEC system Full scale tests: Runde (Norway) We look forward to fruitful cooperation with both national and international partners. For detailed project information, please contact: PhD student Shun-Han Yang Professor Jonas W Ringsberg (Jonas.Ringsberg@chalmers.se) Adjunct Professor Erland Johnson (Erland.Johnson@sp.se) p. 15