MITIGATION OF VORTEX-INDUCED MOTIONS OF A MONOCOLUMN PLATFORM

André L. C. Fujarra¹

Rodolfo T. Gonçalves¹ Fernando Faria² Marcos Cueva²

Kazuo Nishimoto¹ Elizabeth F. N. Siqueira³

¹ Department of Naval Architecture and Ocean Engineering

University of São Paulo, São Paulo, SP, Brazil ² Ocêanica Offshore, São Paulo, SP, Brazil

³ Research and Development Center (CENPES) Petrobras, Rio de Janeiro, RJ, Brazil

Honolulu, Hawaii

June 2009

Introduction

Experimental setup

Results

Heading Effects

Spoiler Plates

External Damping (Risers)

Simultaneous Presence of Waves

Change in the Draft

Conclusions

Summary

Model test experiments were performed to check the influence of many factors on VIM, such as: Different headings; Wave/current coexistence; Different drafts; Suppression elements, and the; Presence of risers.

The results of the experiments presented here are: Inline and cross-flow motion amplitudes; Ratios of actual oscillation and natural

periods; Motions in the XY plane, and; Coefficients of force will be published in

future work;

This is, therefore, a very extensive and important data set for comparisons and validations of theoretical and numerical models for VIM prediction.

Introduction

The experimental setup is characterized by a scale model (1:200) of the MonoBR floating (monocolumn platform) unit, secured by a set of equivalent horizontal moorings in the towing tank at IPT - Instituto de Pesquisas Tecnológicas do Estado de São Paulo.

The instrumentation was dedicated to monitoring: Displacement in the 6 degree od

freedom; Accelerations on the XY plane; Forces in each of the 4 springs; Towing velocity.

Experimental Setup

Diameter at the Bottom 138.60 m

Characteristic Diameter 108.00 m

Depth 55.00 m

Low Draft test 23.00 m

Full Draft test 42.00 m

Displacement in Low Draft 220,000 ton

Displacement in High Draft 372,000 ton

Mooring Line

UX

Y 12

3

4

(x01, y01, z01)

(x03, y03, z03)

(x04, y04, z04)

(x02, y02, z02)

(x1, y1, z1)

(x3, y3, z3)

(x2, y2, z2)

(x4, y4, z4)

Heading Effects

The first tests aimed at the verification of the influence of the hull appendages - particularly the fairleads;

Large amplitudes of cross-flow oscillation, 0º, come from the coexistence with inline motion;

The headings 45º and 135º show an evident decrease in the oscillation in this direction. This fact may be related to the decrease of emission correlation and, therefore, a weaker interaction between cross-flow and inline oscillations.

Heading Effects

For Vr > 11.0, 180º, there is an abrupt decrease in the amplitudes. This happens, possibly, because the separation point is fixed in both fairleads;

In general, whenever simultaneous inline and cross-flow oscillation was present, it was possible to identify Ty / Tny = 1.0 and Tx / Tny = 0.5;

From Vr > 11.0, it is possible to observe that inline oscillations raise, originating eight (8) shape trajectories.

Spoiler Plates A suppressor of innovative

geometry (as suggested in previous work) was tested aiming to attenuate VIM effects;

Reduction of the intensity of VIM in Vr > 10. Increase of the intensity of VIM in Vr < 10;

The ratio of periods of oscillation and the natural cross-flow period exhibit a unitary value differently to the usual ratio of Tx / Tny = 0.5;

The presence of suppressor elements causes the trajectories to be less coordinated.

External Damping (Risers)

A structural set of risers was built in the model to guarantee: Maintenance of coherence in

the emission pattern near the platform;

A known increase in damping, however not compromised with scaling the real values;

Dissipative forces cause a considerable decrease in the inline and the cross-flow motions;

The absence of the double period ratio has a clear effect over the trajectories. The eight (8) shape trajectory is no longer present.

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Simultaneous Presence of Waves

In order to best understand the phenomenon regular state sea was chosen;

The coexistence with waves seems to indicate a contribution to VIM mitigation, it also shows that it happens only under sever heave motions.

0 5 10 15 20 25 30 35 40 45 500

0.5

1

1.5

2

2.5

3

Time [s]

RA

O [

m/m

]

Numerical

T = 14.00s H = 4.00m

T = 16.00s H = 5.22m

T = 18.00s H = 6.68m

Change in the Draft

Within all investigated aspects, this is the most effective in attenuating VIM, (low aspect ratio);

The platform is tested in light weight draft;

In both degrees of freedom, inline and cross-flow, complete attenuation of VIM is noted;

No more coordinated trajectories in XY plane.

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1. The heading has showed to be decisive in the response of the platform;

2. Coexistence with waves has shown to be very important in attenuating VIM, mainly in the cases in which characteristic periods of the waves where near to the resonant periods of platform motions;

3. The spoiler plates did not perform as well as expected in suppressing VIM, as presented previously in Gonçalves, R.T. (2009);

4. Within all studied aspects, the decrease of draft was the most contributing aspect to the mitigating of VIM;

5. The tests with risers have shown that, besides modification of the emission pattern, their presence implicates the increase of damping and decrease the VIM response.

Conclusions

Thank You!

Rodolfo T. Gonçalves

(rodolfo_tg@tpn.usp.br)