Vortex-Induced Yaw Motion (VIY) was studied experimentally on a large-volume semi-submersible platform considering four different current incidence angles, 0, 15, 30 and 45 degrees. The results showed a synchronization range of reduced velocities from 6 up to 8, this one defined by means of the yaw natural motion period in still water. The maximum angular amplitude of yaw was approximately 5 degrees, verified for the 0 degree incidence. This periodic yaw motion of large amplitudes, together with the well known translational motions, named Vortex-Induced Motion (VIM), may contribute negatively to the fatigue life of mooring lines and risers; therefore, this new modality of yaw synchronization should be taken into account during the design of semi-submersible platforms. Furthermore, the effects of the lower draft condition and damping on this new type of synchronization were addressed, showing they are important for the model tests because they contribute to decreasing the yaw amplitudes.

1. VORTEX-INDUCED YAW MOTION (VIY) OF A LARGE-VOLUME
SEMI-SUBMERSIBLE PLATFORM
Rodolfo T. Gonçalves
Guilherme F. Rosetti TPN – Numerical Offshore Tank
Department of Naval Architecture and Ocean
André L. C. Fujarra Engineering
Escola Politécnica – University of São Paulo
Kazuo Nishimoto São Paulo, SP, Brazil
Allan C. Oliveira
Rodos| Greece | June | 2012
June | 2012 The Twenty-second (2012) International Offshore and Polar Engineering Conference 1

2. Outline
• Introduction
• Objectives
• Experimental Setup
• Results
– Current Incidence
Angles
– Damping Levels
– Draft Conditions
• Conclusions
• Ongoing Results
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3. Introduction
• The VIV is usually studied for rigid and
flexible cylinders with large aspect
Analytical
ratio (L/D), for example in a riser
dynamic scenario
• VIM is investigated for rigid
VIV VIM bodies with low aspect ratio, e.g.
spar, MPSO and slender buoys
Numerical Experimental
• The current dimensions of the
new semi-submersible platforms
have increased, therefore
VIV on: VIM on: promoting VIM
Flexible Risers Spar platforms
• The geometry of the semi-
Steel Catenary Risers Monocolumn platforms
Umbilical
submersible implies more
Slender buoy
Every slender body operating Large-volume Semi-submersible
complex VIM than that single
at offshore scenario platforms
column platforms
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4. Objectives
• Model test experiments
were performed to check
the influence on VIM,
such as:
– different current incidence
angles (or headings) 0-degree incidence
– different damping level
due to presence of risers
and mooring
– different draft conditions
• Verify the existence of a
vortex-induced yaw
motions (VIY)
45-degree incidence
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5. 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: • Measurements:
– From 0.044m/s to 0.292m/s (model-scale) • 6DOF motions using a commercial image system for
– These velocities were suitable to investigate the acquiring and processing (Qualisys)
entire range of synchronization for the VIM • Forces at the 4 equivalent mooring lines
– The Re range performed was 6,000 < Re < 90,000
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6. 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 Simulated external damping device
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.
Draft conditions at 0 and 45 degrees incidences
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7. Results:
Motions in the Transverse Direction
• The characteristic amplitude is
nondimensionalized by the column
face length, L. This choice permits
• Except for the headings of 0 degree,
to directly compare results from all other incidences showed a
different incidence conditions; synchronization range at 4 < Vr <10;
• The reduced velocity is defined as: • PSD showed high energy level around
– Vr = (U.T0) ⁄ D the natural frequency of the motions
– T0 is the transverse natural period in calm in the transverse direction;
water • These results confirms the resonance
– D=L(|sin ∅|+|cos ∅| ) of the motions in the transverse
direction.
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8. Results:
Yaw Motions
10
5
• The reduced velocity is defined as:
– Vr = (U.T6) ⁄ D
– T6 is the yaw natural period in calm water • PSD showed high energy level around the natural
frequency of yaw motion;
• According to those results, the 0-degree
incidence showed the largest amplitudes of yaw • These results appoint to a resonance behavior of
the yaw motions due to the vortex-shedding, then
motion, around 5 degrees for Vr around 7; a Vortex-Induced Yaw Motions (VIY);
• The yaw motions increase up to Vr=7 and drop • The galloping behavior has not been observed in
after this; the present work.
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9. 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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10. 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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11. 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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12. 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?
PRELIMINARY RESULTS
Regular waves
Sea conditions
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13. THANKS
rodolfo_tg@tpn.usp.br
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