Towards Multiscale Green Sea Loads Simulations in Irregular Waves - PowerPoint PPT Presentation

Towards Multiscale Green Sea Loads Simulations in Irregular Waves with the Naval Hydro Pack Inno Gatin, Vuko Vukˇ cevi´ c, Hrvoje Jasak Faculty of Mechanical Engineering and Naval Architecture, Zagreb, Croatia 25/July/2017 FSB 1/20 Motivation Procedure and methods Conclusion 25/July/2017

Different procedures and methods for different scales are presented. Motivation, Procedure and methods, Preliminary results. FSB 2/20 Motivation Procedure and methods Conclusion 25/July/2017

The objective is a complete numerical framework for green sea load calculation. • A multiscale framework comprising CFD and a large scale method, • The large scale method takes into account the statistical nature of wave loads , • CFD uses the results from the large scale method to compute highly nonlinear wave loads due to green sea. FSB 3/20 Motivation Procedure and methods Conclusion 25/July/2017

Three–scale procedure is proposed in this work. 1. Linear seakeeping → exploration of multiple sea states and heading angles → selection of the most adverse condition, 2. Coarse CFD → conducting a three hour seakeeping simulation for the selected sea state → detecting green sea events , 3. Fine CFD → conducting a detailed green sea simulation on the critical part of the deck structure → loads on deck structures. FSB 4/20 Motivation Procedure and methods Conclusion 25/July/2017

Procedure and methods FSB 5/20 Motivation Procedure and methods Conclusion 25/July/2017

Step 1: Hydrodynamic coeffs obtained 1 with linearised free surface solver. • Single–phase simulations with linearised free surface model, • Efficient wave diffraction and radiation simulations. 1.534e+09 1.532e+09 1.53e+09 F Z , N 1.528e+09 1.526e+09 1.524e+09 0 50 100 150 Time, s FSB 6/20 Motivation Procedure and methods Conclusion 25/July/2017

Linearised free surface solver agrees 1 well with potential flow methods. 4 3.6 1.4e+08 3.2 Naval Hydro 1.2e+08 Naval Hydro HydroSTAR HydroSTAR 2.8 1e+08 2.4 φ Fz , rad F Z , N/m 8e+07 2 1.6 6e+07 1.2 4e+07 0.8 2e+07 0.4 0 0 0.2 0.3 0.4 0.5 0.6 0.2 0.3 0.4 0.5 0.6 ω , rad/s ω , rad/s 6 6e+09 5 Naval Hydro 5e+09 HydroSTAR 4 Naval Hydro HydroSTAR 4e+09 φ My , rad M Y , Nm/m 3 3e+09 2 2e+09 1 1e+09 0 0 0.2 0.3 0.4 0.5 0.6 0.2 0.3 0.4 0.5 0.6 ω , rad/s ω , rad/s FSB 7/20 Motivation Procedure and methods Conclusion 25/July/2017

Step 2: Use linear seakeeping methods 2 to assess green water probability. • Using the calculated hydrodynamic coefficients, calculate the green water probability for a large number of sea states , • Select the most adverse sea state, which will serve as a starting point for the next step. FSB 8/20 Motivation Procedure and methods Conclusion 25/July/2017

Step 3: Calibrate the input spectrum 3 using Higher Order Spectrum method. Higher Order Spectrum (HOS) method: • Pseudo-spectral method for solving nonlinear boundary conditions for free surface waves , • Takes into account nonlinear wave–wave interaction and modulation , • Appropriate for efficient nonlinear irregular sea state propagation, • Applicable for coupling with CFD, • Low CPU expense. FSB 9/20 Motivation Procedure and methods Conclusion 25/July/2017

Directional wave spectrum is efficiently 3 propagated using HOS. 3 hours of real time simulated in 5 minutes of CPU time. FSB 10/20 Motivation Procedure and methods Conclusion 25/July/2017

Low CPU expense of HOS enables 3 fast calibration of input spectrum. • The selected wave energy spectrum is calibrated using HOS in order to produce the target spectrum, • Up to 100 three–hour realisations using HOS needed for the calibration → a bit difficult with CFD. . . 200 Target: JONSWAP 150 HOS result after calibration HOS result before calibration 2 s S ζζ , m 100 50 0 0.3 0.4 0.5 0.6 0.7 ω , rad/s FSB 11/20 Motivation Procedure and methods Conclusion 25/July/2017

3 • Comparing HOS and CFD wave spectrum reveals that minimal wave damping occurs in CFD. 200 HOS CFD 150 2 s S ζζ , m 100 50 0 0.2 0.3 0.4 0.5 0.6 0.7 ω , rad/s FSB 12/20 Motivation Procedure and methods Conclusion 25/July/2017

Step 4: Perform a three hour CFD 4 seakeeping simulation. • SWENSE method is used to couple potential flow and CFD, • Fast and robust simulations with coarse temporal (200 time–steps/period) and spatial (600 000 cells) resolution, • Small number of nonlinear iterations per time–step is enabled using enhanced 6–DOF–fluid flow coupling . 3e+06 2e+06 1e+06 F X , N 0 -1e+06 3e+06 2e+06 -2e+06 1e+06 0 -3e+06 -1e+06 1000 2000 3000 4000 5000 6000 7000 Time, s -2e+06 3900-3e+06 3650 3700 3750 3800 3850 FSB 13/20 Motivation Procedure and methods Conclusion 25/July/2017

Enhanced coupling reduce 4 CPU time by a factor of 4. Strongy coupled 6 23 -0.28 Enhanced Ref. solution 5.8 22.5 -0.29 γ RT1 , rad R T0 , N R T1 , N 5.6 22 -0.3 5.4 21.5 -0.31 5.2 21 5 20.5 -0.32 0 4 8 12 0 4 8 12 0 4 8 12 N N N -0.16 -1.6 0.93 -0.165 -1.7 0.9 γ z1 , rad -0.17 z 0 , m z 1 , m -1.8 0.87 -0.175 0.84 -1.9 -0.18 -0.185 -2 0 4 8 12 0 4 8 12 0 4 8 12 N N N -0.065 1 -0.07 -0.54 0.995 γ φ 1 , rad φ 0 , rad φ 1 , rad -0.075 -0.55 0.99 -0.08 -0.56 0.985 -0.085 -0.57 0 4 8 12 0 4 8 12 0 4 8 12 N N N FSB 14/20 Motivation Procedure and methods Conclusion 25/July/2017

Step 5: Detect green water events and 5 perform detailed CFD simulation. • Customised post–processing tools detect the situations where water on deck occurred in the three hour simulation , • Select the green water incident which is considered the most dangerous, • Conduct a detailed CFD simulation with fine spatial and temporal resolution, including complex geometries. FSB 15/20 Motivation Procedure and methods Conclusion 25/July/2017

Detailed V&V of green sea loads has been performed. Experiments performed for a static FPSO model at SNU: Lee, H.H., Lim, H.J. and Rhee, S.H.: Experimental investigation of green water on deck for a CFD validation database (2012). • Green water pressure is compared at ten locations on deck , • Nine incident waves are considered, • isoAdvector geometric VOF method is used for interface tracking, • Grid, temporal and periodic uncertainty is assessed . FSB 16/20 Motivation Procedure and methods Conclusion 25/July/2017

isoAdvector preserves a sharp interface for green water simulations. Author: Dr. Johan Roenby, DHI . FSB 17/20 Motivation Procedure and methods Conclusion 25/July/2017

Pressure peaks and pressure impulses are compared to experiments. A detailed verification study has been performed for nine waves: • Numerical uncertainties = periodic + discretisation uncertainties, • Experimental uncertainties = periodic + measuring uncertainties, • 20 wave periods simulated to achieve periodic convergence, • Four grid levels used: from ≈ 200 000 to ≈ 4 000 000 cells. 700 8000 600 500 6000 400 p, Pa p, Pa 4000 300 200 2000 100 0 0 6 8 10 12 14 16 14 16 18 20 22 24 26 Time, s Time, s Location further from the Location near the breakwater. breakwater. FSB 18/20 Motivation Procedure and methods Conclusion 25/July/2017

Results agree well with experiments. H = 13.5 cm, λ = 2 . 25 m H = 15.0 cm, λ = 3 . 0 m 600 CFD CFD 800 EFD EFD 500 700 600 400 p max , Pa p max , Pa 500 300 400 300 200 200 100 100 0 0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 Pressure gauge label Pressure gauge label Pressure peaks, Pressure peaks, 300 300 CFD CFD EFD EFD 250 250 200 200 P, Pa s P, Pa s 150 150 100 100 50 50 0 0 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 Pressure gauge label Pressure gauge label Pressure impulses Pressure impulses (time integrals). (time integrals), FSB 19/20 Motivation Procedure and methods Conclusion 25/July/2017

All the components of the procedure have been thoroughly validated. A comprehensive procedure for green water load assessment includes: • Stochastic nature of ocean waves, • Complicated geometries. • Validated numerical methods : � Higher Order Spectrum method, � Linearised free surface solver, � SWENSE method with enhanced 6–DOF–fluid flow coupling algorithm, � isoAdvector and Ghost Fluid Method for highly resolved green water simulations. Future work: • Conduct the complete procedure for an example vessel. FSB 20/20 Motivation Procedure and methods Conclusion 25/July/2017

All the components of the procedure have been thoroughly validated. A comprehensive procedure for green water load assessment includes: • Stochastic nature of ocean waves, • Complicated geometries. • Validated numerical methods : � Higher Order Spectrum method, � Linearised free surface solver, � SWENSE method with enhanced 6–DOF–fluid flow coupling algorithm, � isoAdvector and Ghost Fluid Method for highly resolved green water simulations. Future work: • Conduct the complete procedure for an example vessel. Questions? FSB 20/20 Motivation Procedure and methods Conclusion 25/July/2017