OMAE2012-83017: Wave Effects on Vortex-induced motion (VIM) of a large-volume semi-submersible platform

Rio de Janeiro | Brazil | July | 2012 31th International Conference on Ocean, Offshore and Arctic Engineering 1

WAVE EFFECTS ON VORTEX-INDUCED MOTION (VIM) OF A LARGE-VOLUME SEMI-SUBMERSIBLE PLATFORM

July | 2012

Rodolfo T. Gonçalves

Guilherme F. Rosetti

André L. C. Fujarra

Kazuo Nishimoto

Allan C. Oliveira

TPN – Numerical Offshore Tank

Department of Naval Architecture and Ocean

Engineering

Escola Politécnica – University of São Paulo

São Paulo, SP, Brazil


Outline

• Introduction

• Objectives

• Experimental Setup

• Results

– Only current

– Current + Waves

• Conclusions


VIM

VIV

Analytical

Experimental Numerical

Introduction

VIV on:

Flexible Risers

Steel Catenary Risers

Umbilical

Every slender body operating at offshore scenario

VIM on:

Spar platforms

Monocolumn platforms

Slender buoy

Large-volume Semi-submersible platforms

• The VIV is usually studied for rigid and flexible cylinders with large aspect ratio (L/D), for example in a riser dynamic scenario

• VIM is investigated for rigid bodies with low aspect ratio, e.g. spar, MPSO and slender buoys

• The current dimensions of the new semi-submersible platforms have increased, therefore promoting VIM

• The geometry of the semi-submersible implies more complex VIM than that single column platforms


Objectives

• Model test experiments were performed to verify the influence of concomitant presence of current and waves on VIM, such as:

– regular waves

– sea state conditions

45-degree incidence only current

45-degree incidence current + waves

waves


Experimental Setup

• Experiments were performed at the Institute of Technological Research (IPT) at São Paulo, Brazil

• Small-scale tests (1:100) of a Large-volume Semi-submersible platform: – Four rounded-square columns

– Rectangular closed-array pontoon

– Only the hydrodynamic important appendages were represented (riser support, hard pipe and mooring lines running above the columns)

• Equivalent mooring system: – Approximately parallel to the water surface

– Linear and symmetric stiffness

• Current velocity emulated by the towing carriage: – Six current velocities were carried out to represent

the main reduced velocity range in which the higher transverse VIM (only current) was observed;

– From 0.065m/s to 0.182m/s (model-scale)

– The Re range performed was 8,500 < Re < 56,000

• Measurements: • 6DOF motions using a commercial image system for

acquiring and processing (Qualisys)

• Forces at the 4 equivalent mooring lines

• 3 wave probes to measure the wave elevation;


Experimental Setup

• Three irregular waves (sea conditions) represented by a JONSWAP spectra were chosen to represent different environmental conditions at Campos Basin – Brazil, corresponding to distinct levels of unit motion;

• Three regular waves were chosen to represent different RAO values in the heave motion.

f [Hz]


Results: Characteristic Motions in the Transverse Direction

• On the other hand, the VIM was mitigated completely with the presence of regular waves;

• The motions are similar and very low for the three regular conditions.

• According to the results, the motions in the transverse direction decreased with the presence of sea conditions;

• Another issue is that the amplitudes are lower for sea conditions with higher significant amplitude.


Results: PSD of the Motions in the Transverse Direction

• PSD for the motions in the transverse direction only confirms that no VIM is evidenced for regular waves.

• The energy is considerable in the range of reduced velocities 5.0≤𝑉𝑟≤9.0.

• The energy is concentrated around the natural frequency of transverse direction, which corroborates the assumption that the VIM is a resonant behavior.

0

0.1

0.2

0.3

0

5

10

15

200

5

10

15

x 104

Frequency [Hz]Reduced Velocity [Vr]

PS

D [m

m2.s

]

fN

Cross-Flow

fN

Yaw

00.5

11.5

2

0

5

10

15

200

1

2

3

4

x 104


PS

D [m

m2.s

]

fN

Cross-Flow

fN

Yaw

fP Sea Condition

00.5

11.5

2

0

5

10

15

200

1

2

3

4

x 104


PS

D [m

m2.s

]

fN

Cross-Flow

fN

Yaw

fRW

Regular Wave

Only current Current + sea condition Current + regular wave

• VIM behavior for sea conditions tests occurs but with smaller amplitudes or energy density around the transverse natural frequency.


Results: PSD of the Motions in the In-Line Direction

• The energy for this degree-of-freedom is concentrated in the frequency of the regular waves performed, with no considerable energy in other frequencies.

• The results showed no considerable energy in the in-line direction.

Only current Current + sea condition Current + regular wave

• The energy in the presence of sea condition is higher and concentrated around the natural frequencies at the free surface plane, in-line;

• This is a resonant behavior in low frequencies caused by the irregular characteristics of the sea conditions.

0

0.1

0.2

0.3

0

5

10

15

200

5000

10000

15000


PS

D [m

m2.s

]

fN

Cross-Flow

fN

Yaw

00.5

11.5

2

0

5

10

15

200

2

4

6

x 104


PS

D [m

m2.s

]

fN

Cross-Flow

fN

Yaw

fRW

Regular Wave

0

1

2

0

5

10

15

200

2000

4000

6000

8000

10000


PS

D [m

m2.s

]

fN

Cross-Flow

fN

Yaw

fP Sea Condition


Iwagaki & Asana (1984) procedure for forced motions

• It is possible to consider the in-line motion due to the wave excitation as the imposed oscillatory motion and calculate the respective Keulegan-Carpenter number:

– Regular wave: 𝐾𝐶 =𝑈𝑀

𝑓𝑊𝐷

– Sea condition: 𝐾𝐶𝑟 =2𝜎𝑈

𝑓𝑃𝐷

• The effect of current and waves is calculated using the ratio 𝛼 =

𝜎𝑈

𝜎𝑈+𝑈

0 1 2 3 4 5 6 7 80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

KC

SS - Sea Conditions

SS - Regular Waves

Predominantly Drag(Viscous)

Predominantly Inertia

0 5 10 15 200

0.1

0.2

0.3

0.4

0.5

Reduced Velocity (Vr)

Ay /

L

45 degrees

no waves

regular wavefRW

= 0.91Hz H = 43.87mm

regular wavefRW

= 0.59Hz H = 78.91mm

regular wavefRW

= 0.38Hz H = 116.64mm

sea condition (JW)fP = 0.67Hz H

s = 65.23mm

sea condition (JW)fp = 0.56Hz H

s = 58.71mm

sea condition (JW)fp = 0.54Hz H

s = 51.80m


Iwagaki & Asana (1984) procedure for forced motions

• The results showed the regular wave condition and the sea condition incidence at a distinct behavior region, predominant inertia and predominant drag (viscous), respectively.

• It is possible to infer that the possibility of VIM existence depends on the imposed in-line motions due to the wave incidences to be at the predominantly drag (viscous) region, where the viscous forces or lift forces due to vortex shedding are considerable.

• The fact that VIM is not verified for regular wave tests is justified because imposed in-line motions due to waves are located at the predominant inertia region, i.e. the inertia forces are greater than the forces due to vortex shedding.

0 1 2 3 4 5 6 7 80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

KC

SS - Sea Conditions

SS - Regular Waves




Gonçalves et al. (2010) VIM of monocolumn platform

• The same procedure was applied to the results presented in Gonçalves et al. (2010) for a monocolumn platform subjected to current and regular wave incidence.

• Even for regular waves, the monocolumn platform experimented VIM, lower than with current incidence only, differently from the semi-submersible platform;

• However, the imposed in-line motion due to wave was located at the predominantly drag (viscous) force, as can be seen, in which VIM can be verified.

0 1 2 3 4 5 6 7 80

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

KC

MonocolumRegular WavesGonçalves et al. (2010b)



0

0.2

0.4

0.6

0.8

1

AY / D

0 5 10 15 200

0.1

0.2

Vrn = UT

n / D

AX / D

base case

T = 14.00 sH = 4.00 m

T = 16.00 sH = 5.22 m

T = 18.00 sH = 6.68 s


Conclusions

• The results for SS showed that, in regular wave tests, the VIM was completely mitigated. Motions in the transverse direction were not observed and the energy around the natural frequency of transverse motions could not be found;

• Differently, the SS results for the sea condition tests showed lower VIM when compared with the case without waves, but the PSD showed considerable energy levels around the natural frequency of transverse motion;

• This behavior is better understood by making plot 𝛼 vs 𝐾𝐶 using the in-line motions due to waves as the imposed oscillatory motion;

• The in-line response due to waves may be conjectured as the responsible to the possibility of VIM existence and not the wave nature (regular or irregular). However, the VIM amplitude also depends on the motion amplitudes of the other DOF, mainly heave, roll and pitch; as firstly discussed in Gonçalves et al. (2010).


Conclusions for the SS case studied

• The velocity ratio 𝛼 for the regular wave incidence is located at the predominantly inertia region, where the forces due to the vortex shedding are small and the VIM does not occur;

• However, the sea condition incidences are located at predominantly drag (viscous) region, where the forces due to vortex shedding are significant and the VIM may occur, but the VIM amplitudes also depend on the heave, roll and pitch motions;

• This assumption must be confirmed with more tests and studied in depth with fundamental experiments on simplified geometries such as bare cylinders, which has been done by the authors.


THANKS

[email protected]


References

• Gonçalves, R. T., Fujarra, A. L. C., Rosetti, G. F., & Nishimoto, K. (2010). “Mitigation of Vortex-Induced Motion (VIM) on a Monocolumn Platform: Forces and Movements”. Journal of Offshore Mechanics and Arctic Engineering, Vol. 132(4), p. 041102.

• Iwagaki, Y., & Asano, T. (1984). “Hydrodynamic Forces on a Circular Cylinder due to Combined Wave and Current Loading”. Proceedings of the International Conference on Coastal Engineering, No. 19.

Documents

OMAE2012-83017: Wave Effects on Vortex-induced motion (VIM) of a large-volume semi-submersible platform