Environmental Conditions Inducing Extreme Mooring Loads on a

1. Environmental Conditions Inducing Extreme Mooring Loads on a Dynamically Responding Moored Structure 03/05/2014 – SuperGen Summer class Violette Harnois Research Associate – PhD fellow V.Harnois@exeter.ac.uk Supervisors: Lars Johanning, Philipp R. Thies University of Exeter, Cornwall Campus

2. SuperGen Summer class Introduction • How to design a mooring system? • → follow oil and gas standard such as DNV-OS-E301 • What does DNV-OS-E301 recommend?

3. SuperGen Summer class DNV-OS-E301 • Envelope line 100 years • Is that still valid for a dynamically responding moored structure? → investigations at the SWMTF

4. SuperGen Summer class The South West Mooring Test Facility (SWMTF) Instrumented buoy

5. SuperGen Summer class Location of the SWMTF Depth: 27m -32.4m Depth SWMTF and FabTest

6. SuperGen Summer class Methodology • Assessment • of wave • conditions • Scatter • diagram

7. SuperGen Summer class Methodology • Assessment • of wave • conditions 2. Percentage occurrence

8. SuperGen Summer class Methodology • Assessment • of wave • conditions 3. Contour lines

9. SuperGen Summer class Methodology • B. Selection of extreme mooring loads • Peak Tension Threshold K • Comparison with standard score of max load • 2. Minimum Tension Threshold τ • Comparison with a minimum mooring load

10. SuperGen Summer class Methodology

11. SuperGen Summer class Methodology

12. SuperGen Summer class Results • 1st of October 2010 -31th of December 2010 • →13245 data files (10 minutes) • Mooring line 3 • K=7.5 • τ = 10kN (three times the average during the whole deployment of the mean mooring load values of each 10 minute dataset) • →70 files containing extreme mooring loads (0.53%)

13. SuperGen Summer class Results: waves

14. SuperGen Summer class Sensitivity analysis • K=0 • τ = 0kN • → 100 %files

15. SuperGen Summer class Sensitivity analysis • K=5 • τ = 0kN • → 18%files

16. SuperGen Summer class Sensitivity analysis • K=10 • τ = 0kN • → 0.04%files

17. SuperGen Summer class Sensitivity analysis • K=7.5 • τ = 0kN • → 0.63%files

18. SuperGen Summer class Sensitivity analysis • K=7.5 • τ = 20kN • → 0.51%files

19. SuperGen Summer class Results: wave directionality

20. SuperGen Summer class Results: Current

21. SuperGen Summer class Discussion-Conclusion • Oil and gas standards should be adapted for highly dynamic responding moored structures → Consider several additional Hs/Tp combinations INSIDE the scatter diagram • Investigate low tide conditions in detail

22. SuperGen Summer class Discussion-Conclusion • Consequences of this more detailed design: • reduced FOS, reduced costs • Reduction of occurrences of extreme mooring loads: • Increase pre-tension (but consider power production, reasonable tension at high tide) • Increase number of lines (cost/power production) • Orienting the mooring to avoid worst wave climate

23. SuperGen Summer class Discussion-Conclusion • Limitations of this study: one site, one mooring type, short period of time • Further work: • Investigate another MEC device • Investigate wave spectrum

24. SuperGen Summer class Next step: why do extreme mooring loads occur? • Validation of a numerical model (Orcaflex) with tank tests • Video tank test +

25. SuperGen Summer class Thank you for your attention Violette Harnois V.Harnois@exeter.ac.uk