Rodos| Greece | June | 2012 The Twenty-second (2012) International Offshore and Polar Engineering Conference 1

VORTEX-INDUCED YAW MOTION (VIY) OF A LARGE-VOLUME SEMI-SUBMERSIBLE PLATFORM

June | 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

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Outline

• Introduction • Objectives • Experimental Setup • Results

– Current Incidence Angles

– Damping Levels – Draft Conditions

• Conclusions • Ongoing Results

Rodos| Greece | June | 2012 The Twenty-second (2012) International Offshore and Polar Engineering Conference 3

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

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Objectives

• Model test experiments were performed to check the influence on VIM, such as: – different current incidence

angles (or headings) – different damping level

due to presence of risers and mooring

– different draft conditions

• Verify the existence of a vortex-induced yaw motions (VIY)

0-degree incidence

45-degree incidence

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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: – From 0.044m/s to 0.292m/s (model-scale)

– These velocities were suitable to investigate the entire range of synchronization for the VIM

– The Re range performed was 6,000 < Re < 90,000

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

acquiring and processing (Qualisys)

• Forces at the 4 equivalent mooring lines

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Experimental Setup

• Different headings: – 0, 15, 30 and 45 degrees;

• A structural set of risers was built in the model to add damping on the system. Three different damping levels performed: – The first one represents only the

hydrodynamic damping due to the hull;

– The second and the third ones represent external damping levels higher than the first condition and include the damping device simulator. The difference between the second and the third external damping levels is the number of “simulated risers”;

• Two different draft conditions: – Full draft condition = 34 meters;

– Low draft condition = 16 meters.

Simulated external damping device

Draft conditions at 0 and 45 degrees incidences

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Results: Motions in the Transverse Direction

• The characteristic amplitude is nondimensionalized by the column face length, L. This choice permits to directly compare results from different incidence conditions;

• The reduced velocity is defined as: – Vr = (U.T0) ⁄ D

– T0 is the transverse natural period in calm water

– D=L(|sin ∅|+|cos ∅| )

• Except for the headings of 0 degree, all other incidences showed a synchronization range at 4 < Vr <10;

• PSD showed high energy level around the natural frequency of the motions in the transverse direction;

• These results confirms the resonance of the motions in the transverse direction.

Rodos| Greece | June | 2012 The Twenty-second (2012) International Offshore and Polar Engineering Conference 8

Results: Yaw Motions

• The reduced velocity is defined as:

– Vr = (U.T6) ⁄ D

– T6 is the yaw natural period in calm water

• According to those results, the 0-degree incidence showed the largest amplitudes of yaw motion, around 5 degrees for Vr around 7;

• The yaw motions increase up to Vr=7 and drop after this;

• PSD showed high energy level around the natural frequency of yaw motion;

• These results appoint to a resonance behavior of the yaw motions due to the vortex-shedding, then a Vortex-Induced Yaw Motions (VIY);

• The galloping behavior has not been observed in the present work.

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Results: Damping Levels

• The presence of risers and mooring lines increases the damping of the system, which reduces the VIM and VIY amplitudes, as can be seen in figures;

• The results showed lower amplitudes for higher external damping levels (or higher dissipative forces).

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Results: Draft Conditions

• The draft condition was the aspect that had greater effect on VIM amplitudes.;

• A complete attenuation of VIM and VIY is noted in the results;

• The decrease in amplitudes is correlated with the small immersed length of the platform columns, which does not provide a regular vortex shedding in the water surface plane capable to promote oscillating excitation forces.

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Conclusions

• The VIM of semi-submersibles is an important issue to be considered in the design of risers and mooring line systems due to changes in the fatigue life of these components;

• The yaw motions showed a resonance behavior such as motions in the transverse direction, with considerable amplitudes around 5 degrees for 0-degree incidence;

• The results showed an amplitude decrease with the increase in the external damping level, but still with considerable motion amplitudes and determined oscillation frequency;

• The low draft condition completely attenuated the VIM and VIY, i.e. no considerable motion amplitudes were verified or a dominant frequency.

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Ongoing Results

• How do the waves concomitant with current influence the VIM?

• What is the procedure to consider the VIM (current + waves) in the fatigue analysis?

Regular waves

Sea conditions

PRELIMINARY RESULTS

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THANKS

rodolfo_tg@tpn.usp.br