The Lloyd’s Register Educational Trust (LRET)

Marine & Offshore Research Workshop

16-18 February, 2010 at Engineering Auditorium, NUS

Hydrodynamics: Setting the Scene*

*a selective view

Prof. Rodney Eatock Taylor and Prof. Paul Taylor

University of Oxford

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, 16-18 February 2010

Typhoon TLP

from http://blog.kir.com/archives/002473.asp

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

Hydrodynamics and Modelling

From: Summary of CESOS Workshop 2007, by Marilena Greco

Modelling using CFD

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

Time series plot of some large waves

Wave analysis

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)

NewWave

NewWave embedded in random record

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.

Setdown under extreme crest?

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

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

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)

Setup under Draupner crest

Other evidence of set-up in crossing seas

A. Toffoli et al. / European Journal of Mechanics B/Fluids 25 (2006) 649–661

Wave climate variability

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

Wave-structure interaction

ka = 0.4

-5 0 5-6

-4

-2

0

2

4

6

0.5

1

1.5

ka = 1

-5 0 5-6

-4

-2

0

2

4

6

0.5

1

1.5

ka = 3

-5 0 5-6

-4

-2

0

2

4

6

0.5

1

1.5

ka = 5

-5 0 5-6

-4

-2

0

2

4

6

0.5

1

1.5

ray theory approx

Diffracted wave field near single vertical cylinder

1 2

34

ka = 2.2729, β = 0o

-5 0 5-6

-4

-2

0

2

4

6

1 2

34

ka = 2.2729, β = 15o

-5 0 5-6

-4

-2

0

2

4

6

1 2

34

ka = 2.2729, β = 30o

-5 0 5-6

-4

-2

0

2

4

6

1 2

34

ka = 2.2729, β = 45o

-5 0 5-6

-4

-2

0

2

4

6

2 4 6

Diffraction by arrays, near-trapping and

wave-in-deck

Near-trapping in square array

Runup at cylinder 3, β = 45

Dimensionless wave amplitude near arrays of circular and non-circular cylinders

ka = 1.831, β = 45 (simple models of a specific semisubmersible).

Influence of local geometry

Application to a GBS

2nd 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)

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

Incident wave

Linear amplitude response profile along

the center-line of the structure at f=0.126Hz.

2nd order sum amplitude profile along the

center-line of the structure at 2f =0.126Hz.

Where does the high response occur?

Linear diffraction calculations provide useful guidance to 2nd order behaviour

─── η(1) ─── η (2+)

─── η (1+2)=TOTAL

Predicted surface elevation

between the rear legs with

11m incident focused wave group

Tp = 14.3 s

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

11m

Noble Denton

Waves in gap between closely spaced vessels

SAFE OFFLOAD programme (EU)

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

Linear motions and elevations, beam sea 1

Floating Fixed

Relationship between first-order and second-order results for elevation (beam sea 1)

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

1.351.291.111.20

1.04

0.95

0.88

0.81

0.755

point (-2,0)

|η(1

) |/A

ω (rad/s)

0.3 0.4 0.5 0.6 0.7 0.80

1

2

3

4

5

6

70.75

point (-2,0)

|η(2)

q|

ω (rad/s)

0.3 0.4 0.5 0.6 0.7 0.80

2

4

6

8

10

12

0.76

0.670.71

0.640.600.560.520.475

0.44

0.405

0.38

point (-2,0)

|η(2)

p|

ω (rad/s)

0.3 0.4 0.5 0.6 0.7 0.80

3

6

9

12

15

0.76

0.68

0.640.600.560.520.4750.44

0.405

0.38

point (-2,0)

|η(2) |

ω (rad/s)

First-order 0.755 0.81 0.88 0.95 1.04 1.11 1.20 1.29 1.35

Second-order 0.38 0.405 0.440 0.475 0.52 0.56 0.60 0.64 0.67

LINEAR 2nd order quadratic

2nd order potentialTOTAL 2nd order

Response in gap between 2 boxes with NewWave focused

group, beam seas, broad band spectrum

Behaviour in transient waves

Experiments on GBS and Tanker (fixed)

– IC model scale

unit: m water depth=0.82m

First-order wave elevation at centre line

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.20

1

2

3

4

5T=0.780s(beam sea 2)

Numerical (Oxford)

Experimental (IC)

|η(1) |/A

y

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.20

1

2

3

4

5

6

7T=0.889s(beam sea 2)

Numerical (Oxford)

Experimental (IC)

|η(1) |/A

y

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.20

1

2

3

4

5

6

7

8T=1.067s(beam sea 2)

Numerical (Oxford)

Experimental (IC)

|η(1) |/A

y

-1.2 -0.8 -0.4 0.0 0.4 0.8 1.20.0

0.5

1.0

1.5

2.0

2.5

3.0T=1.455s(beam sea 2)

Numerical (Oxford)

Experimental (IC)

|η(1) |/A

y

0 10 20 30 40 50 60 70-30

-20

-10

0

10

20

30 incident

y=0

ζ

t(s)

Tp=1.2s

0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

0.9703

(T=1.03s)

1.2987

(T=0.77s)

1.13636

(T=0.88s)

0.9901

(T=1.01s) Numerical(Oxford)

Experimental(IC)

|η(1

) |/A

f(Hz)

First-order elevations at point (-0.03,0) in Beam Sea 2

Wave group excitation

Beam sea, tanker exposed

RAO for elevation at mid-

point of gap

Overall good agreement

- freq of peak ~2% out

- magnitude over-estimated

Floatover operations

Behaviour of flexibly connected bodies

Chevron Olokola Floatover Installation Study © Ocean Dynamics LLC

Relative heave motions of barges

based on extension of diffraction analysis for flexibly connected vessels

From: Wang, C. M. & Tay, Z. Y. Hydroelastic Analysis and Response of

Pontoon-Type Very Large Floating Structures

Link to hydroelastic analysis of flexibly connected structures

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

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)

Opportunities for marine CFD

From: http://wiki.manchester.ac.uk/spheric

Opportunities for marine CFD

Courtesy:

Opportunities for marine CFD

Courtesy:

Thank you

Questions?