Hydrodynamics: Setting the Scene*

Transcription

1 Hydrodynamics: Setting the Scene* *a selective view Prof. Rodney Eatock Taylor and Prof. Paul Taylor University of Oxford The Lloyd s Register Educational Trust (LRET) Marine & Offshore Research Workshop February, 2010 at Engineering Auditorium, NUS

2 Hydrodynamics: Setting the Scene* *a selective view Rodney Eatock Taylor and Paul Taylor University of Oxford LRET Marine and Offshore Research Workshop National University of Singapore, February 2010

3 Hydrodynamics and Modelling One of our aims must be to model the interaction of the most extreme waves with structures in the sea. And we must also consider hydrodynamic models motivated by design for operational conditions in waves Also Resistance, Manoeuvring, Sloshing, VIV/VIM, Hydroelasticity,.., which are not considered here Typhoon TLP from

4 Modelling using CFD From: Summary of CESOS Workshop 2007, by Marilena Greco

5 Issues raised Wave analysis Wave structure interaction Near-trapping by multiple structures in waves Wave-in-deck issues (waves + interaction): jackets and GBS Wave-induced sloshing in gaps Floatover A few comments on CFD

6 Wave analysis Time series plot of some large waves

7 NewWave Average of largest 50% linear profiles * compared with NewWave profile + * linearised using (average crest -average trough profile)/2 + average shape of extreme in linear random Gaussian process (Boccotti, Lindgren)

8 NewWave embedded in random record

9 Freak waves? Consider behaviour near an extreme crest: set-down will decrease crest elevation, while set-up will increase it. The famous New Year Wave measured at the Draupner platform in the North Sea. For convenience the time is set to zero under the extreme crest.

10 Setdown under extreme crest? T.A.A. Adcock & P. H. Taylor, 11th International Workshop on Wave Hindcasting and Forecasting, 2009.

11 Setup under Draupner crest The second order difference waves under the Draupner giant wave.comparison of filtered data and predicted second order difference waves based on a model of crossing seas. (Evidence of crossing seas from meteorology, measured forces, and absence of breaking in wave record from laser) T.A.A. Adcock & P. H. Taylor, 11th International Workshop on Wave Hindcasting and Forecasting, 2009.

12 Other evidence of set-up in crossing seas A. Toffoli et al. / European Journal of Mechanics B/Fluids 25 (2006)

13 Wave climate variability T.A.A. Adcock & P. H. Taylor, 11th International Workshop on Wave Hindcasting and Forecasting, 2009.

14 Wave-structure interaction 6 ka = ka = ka = 3 ray theory approx 6 ka = Diffracted wave field near single vertical cylinder

15 Diffraction by arrays, near-trapping and wave-in-deck 6 ka = , β = 0 o 6 ka = , β = 15 o ka = , β = 30 o ka = , β = 45 o

16 Near-trapping in square array Runup at cylinder 3, β = 45

17 Influence of local geometry Dimensionless wave amplitude near arrays of circular and non-circular cylinders ka= 1.831, β= 45 (simple models of a specific semisubmersible).

18 Application to a GBS

19 2 nd order behaviour simulated using Oxford program DIFFRACT near trapping frequency doubling : input waves at frequency f strong local surface response at 2f Body and free-surface grid for quadrant of the structure (using two planes of symmetry)

20 Incident wave End-on waves at near-trapping. β=0 o, f=0.126 Hz

21 Where does the high response occur? Linear amplitude response profile along the center-line of the structure at f=0.126hz. 2 nd order sum amplitude profile along the center-line of the structure at 2f =0.126Hz. Linear diffraction calculations provide useful guidance to 2 nd order behaviour

22 11m Predicted surface elevation between the rear legs with 11m incident focused wave group T p = 14.3 s η (1) η (2+) η (1+2) =TOTAL Practical implications for concrete GBS with closely spaced columns - water projection to high level - hitting the deck - air down to just above the caisson (slam loading on the top?) Walker et al., Ocean Engineering 2008

23 Waves in gap between closely spaced vessels SAFE OFFLOAD programme (EU) Noble Denton Experiments at DHI and Imperial College have confirmed importance of large free surface motions in the gap As has second order boundary element analysis at Oxford

24 Linear motions and elevations, beam sea 1 Floating Fixed

25 Relationship between first-order and second-order results for elevation(beam sea 1) point (-2,0) 7 point (-2,0) LINEAR nd order quadratic η (1) /A η (2) q ω (rad/s) point (-2,0) TOTAL 2 nd order ω (rad/s) point (-2,0) 2 nd order potential 0.76 η (2) η (2) p ω (rad/s) ω (rad/s) First-order Second-order

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27 Behaviour in transient waves Response in gap between 2 boxes with NewWavefocused group, beam seas, broad band spectrum

28 Experiments on GBS and Tanker (fixed) IC model scale unit: m water depth=0.82m

29 First-order wave elevation at centre line 5 4 T=0.780s(beam sea 2) Numerical (Oxford) Experimental (IC) T=0.889s(beam sea 2) Numerical (Oxford) Experimental (IC) 3 4 η (1) /A 2 η (1) /A y T=1.067s(beam sea 2) Numerical (Oxford) Experimental (IC) y T=1.455s(beam sea 2) Numerical (Oxford) Experimental (IC) η (1) /A η (1) /A y y

30 T p =1.2s incident y=0 Wave group excitation Beam sea, tanker exposed ζ t(s) First-order elevations at point (-0.03,0) in Beam Sea (T=1.01s) (T=1.03s) Numerical(Oxford) Experimental(IC) RAO for elevation at midpoint of gap η (1) /A (T=0.88s) (T=0.77s) Overall good agreement -freq of peak ~2% out - magnitude over-estimated f(hz)

31 Floatover operations Behaviour of flexibly connected bodies Chevron Olokola Floatover Installation Study Ocean Dynamics LLC

32 Relative heave motions of barges based on extension of diffraction analysis for flexibly connected vessels

33 Link to hydroelastic analysis of flexibly connected structures From: Wang, C. M. & Tay, Z. Y. HydroelasticAnalysis and Response of Pontoon-Type Very Large Floating Structures

34 Areas needing further understanding As discussed here Extreme wave statistics Directional nonlinear wave kinematics (with and without breaking) Wave structure interaction effects various, including wave-in-deck Other topics Coupled tank sloshing and vessel motions (parametric resonance, ) Impact pressures (compressibility effects, ) Hydroelasticity

35 Areas for further CFD development Unstructured adaptive grids for VOF modelling Overlapping grids Grid size (Re) 9/4 (for representation of disparate length scales in turbulent flow) Turbulence modelling for unsteady free surface flows LES SPH Hybrid approaches: simple modelling to identify critical parameters (wave frequency, direction, load case, geometry), followed by very detailed CFD e.g. strip theory + RANSE Coupled and multi-scale formulations Benchmark results for validation Integration of codes into optimisation tools (as has been done for nonlinear wave resistance)

36 Opportunities for marine CFD From:

37 Opportunities for marine CFD Courtesy:

38 Opportunities for marine CFD Courtesy:

39 Thank you Questions?