RELATIVE CONTRIBUTION FROM WIND AND WAVES TO LOADS ON OFFSHORE WIND TURBINES

RELATIVE CONTRIBUTION FROM WIND AND WAVES TO LOADS ON OFFSHORE WIND TURBINES Science Meets Industry Stavanger By Jørgen R. Krokstad WITH FOCUS ON SUPPORT STRUCTURES (with contributions from Loup Suja Thauvin and Lene Eliassen and others)

Content From innovation to scaling and optimization Highly integrated structures the support structures as a dynamic unit Metocean design basis in integrated design IEC-64100-3 and DNV-S-J101 in deriving wind/wave/fault design loads Analysis and statistical challenges Conclusions 2

Content From innovation to scaling and optimization Highly integrated structures the support structures as a dynamic unit Metocean design basis in integrated design IEC-64100-3 and DNV-S-J101 in deriving wind/wave/fault design loads Analysis and statistical challenges Conclusions 3

This was the world as we used to know it year 2010 a lot of innovation OWA competition Objective: Reduce foundation costs by up to 30% in 30-60m Shortlist Finalists Keystone Stage II focus Fabrication Gifford / BMT / Freyssinet Installation Airbus A320 SPT Offshore Universal Foundation Demonstration IHC Source: Carbon Trust Offshore Wind Accelerator 2010, IHC

This is more or less where we ended for bottom fixed SIF MT Hojgaard EEW SPC/Bladt EEW SPC/Bladt EEW SPC Horns Rev 1 Lynn London Array Baltic II Gode Wind II Future MP s 2.0 MW Water depth up to 14 m 2002 3.6 MW Water depth up to 18 m 2008 3.6 MW Water depth up to 25 m 2012 3.6 MW Water depth up to 27 m 2014 6 MW Water depth up to 35 m 2015 8+ MW Water depth up to 40 m 2018 L 34 m Ø 4 m 160 t L 45 m Ø 4.7 m 350 t L 68 m Ø 5.7 m 650 t L 73.5 m Ø 6.5 m 930 t L 80 m Ø 8.5 m 1050 t L >80 m Ø >9 m >1050 t Source: A2Sea News - Winter 2013 and EEW SPC

Design challenge of support structure with increasing rotor diameter Power spectral density Lower rotational speed of large turbines give lower 1P and 3P regions Wavespectrum 5MW 10 MW By Loup/Lene Statkraft/NTNU 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 Frequency [Hz] 6

Design trends on bottom fixed turbines Large turbines (6-10 MW, 150-200 meter diameter) Simple substructures mono-columns, jackets Possible integrated installation (foundation, tower, nacell and rotor in one piece) but has not shown to be economical so far INTEGRATED design optimize tower and foundation design 7

Design phases XL - monopiles Details in combination of wind/wave and fault loads driven by design phase: - Conceptual - FEED - Detailed USE METOCEAN AND LOAD DATA CONSISTENTLY THROUGHOUT THE DESIGN VERIFICATION PHASE Figure confidential 8

Content From innovation to scaling and optimization Highly integrated structures the support structures as a dynamic unit Metocean design basis in integrated design IEC-64100-3 and DNV-S-J101 in deriving wind/wave/fault design loads Analysis and statistical challenges Conclusions 9

The bottom fixed offshore wind turbine AN INTEGRATED structure The cut 1 0

Content From innovation to scaling and optimization Highly integrated structures the support structures as a dynamic unit Metocean design basis in integrated design IEC-64100-3 and DNV-S-J101 in deriving wind/wave/fault design loads Analysis and statistical challenges Conclusions 11

Design basis metocean requirements Wave growth by wind action Nonlinear wave-wave interaction Dissipation due to white-capping, bottom friction and depth-induced wave breaking. Refraction and shoaling due to depth variations If deemed relevant, wave-current interactions 12

Design basis metocean requirements Strong statistical coupling between hub height wind and sea state including water level (long time simulations 36 years 1 hour spectral simulations) Used by the industry Wind will drive wave but lacking backward coupling to turbulence (Siri Kalvig thesis 10 min versus one hour) Misalignment dependence (FLS and ULS) Joint probabilities of Water Level and Hs (ULS) Spectral shape and short term directional distribution questionable in relative shallow water short crestedness Breaking limits in intermediate water depths (energy dissipation) Time duration 1 hour (not 10 min or 3 hours, consistency between wind and wave) 13

Content From innovation to scaling and optimization Highly integrated structures the support structures as a dynamic unit Metocean design basis in integrated design IEC-64100-3 and DNV-S-J101 in deriving wind/wave/fault design loads Analysis and statistical challenges Conclusions 14

Some important considereations DNV-OS-J101 Combination of wind and wave loads a huge challenge for the offshore wind industry. Why? Consequence? 15

16

17

18

19

Content From innovation to scaling and optimization Highly integrated structures the support structures as a dynamic unit Metocean design basis in integrated design IEC-64100-3 and DNV-S-J101 in deriving wind/wave/fault design loads Analysis and statistical challenges Conclusions 20

FLS analysis - recommendation DLC 1.2 (combination of wave and operating wind loads) in addition to DLC 6.4 Use one hour time simulations and change seed for each yaw error change Include misalignment between waves and wind Separate wind driven and swell waves and combine final fatigue damage Include MacCamy correction by lumping force transfer functions to the FEM model (don t use constant Cm approximation without sea state dependence) Include short-crested waves may reduce fatigue from wave loading Predefine non-operational phase of total design life (3, 5 or 10%) Do a sufficient conservative damping estimate with turbine idling (1 2%) 21

ULS analysis - recommendation DLC 1.3 (combined wind and wave), 1.6a (combined wind and wave), and 6.1a (turbine idling but still aerodynamic loads): Use 1 hour time simulations and no embedding of non-linear stream function wave with a given H50 year reference Use Hs, Tp, contours as a function of nacelle mean velocity AND mean water level as a basis for short term extreme moment estimates. At least 20 seeds and a Gumbel fit must be added. The contour must be limited by the water depth wave breaking criteria. Use a non-linear wave load model validated against inertia dominated column structures. The model could be the extended FNV in combination with a slamming model or 2.order irregular wave kinematic model in combination with Morison (convective form). A 98% quantile is recommended as a basis but must be further calibrated against long term simulation or a wind/wave combination with a regular stream function wave Fault loads must be simulated with a sufficient number of seeds and maximum values should be treated statistically 22

Inconsistent wave theories due to shallow water affect strongly statistical method Hmax given as deterministic input destroy statistic 23

LADI tests stopped due to Statkraft strategy change Could have clarified the need for wave embedding against H50 year max wave once for all UF model D=6m D=8m D=10m XL monopile models 24

Conclusions It is possible to implement more optimal and consistent design methods closer to NORSOK without risking the certification approval More calibration of quantile level and verification against state of the art procedures are needed. Something for research? As long as you can prove cost saving opportunities with the proposed method without compromising safety DO IT! 25

THANK YOU www.statkraft.com