A great deal of work has been developed on the spar and monocolumn vortex-induced motion (VIM) issue. However, there are very few published works concerning VIM of semi-submersible platforms, partly due to the fact that VIM studies for this type of platform recently became interesting particularly due to the increasing semi-submersible dimensions (columns diameter and height. In this context, a meticulous experimental study on VIM for this type of platform concept is presented here. Model test experiments were performed to check the influence of many factors on VIM, such as different headings and hull appendages. The results comply with in-line, cross-flow and yaw motion amplitudes, as well as with combined motions in the XY plane.
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Rotterdam| The Netherlands | June | 2011 30th International Conference on Ocean, Offshore and Arctic Engineering 1 EXPERIMENTAL STUDY ON VORTEX-INDUCED MOTIONS (VIM) OF A LARGE-VOLUME SEMI-SUBMERSIBLE PLATFORM June | 2011 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
Transcript
Rotterdam| The Netherlands | June | 2011 30th International Conference on Ocean, Offshore and Arctic Engineering 1
EXPERIMENTAL STUDY ON VORTEX-INDUCED MOTIONS (VIM) OF A LARGE-VOLUME SEMI-SUBMERSIBLE PLATFORM
June | 2011
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
Rotterdam| The Netherlands | June | 2011 30th International Conference on Ocean, Offshore and Arctic Engineering 2
Outline
• Introduction • Objective • Experimental Setup • HHT for Signal Analysis • Results
– Transverse Characteristic Amplitude – Yaw Characteristic Angle – Time History
• Conclusions • Ongoing Results
Rotterdam| The Netherlands | June | 2011 30th International Conference on Ocean, Offshore and Arctic Engineering 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)
– hull appendages • Hard pipes in columns
(black)
• Fairleads and mooring chains in columns (red)
• Riser supports in pontoons (green)
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Experimental Setup
• Experiments 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 in the y-direction (cross-flow)
• Different headings: – 0, 15, 30, 45, 180, 195, 210 and 225 degrees
• Measurements: • 6DOF motions using a commercial system for
acquiring and processing
• Forces at the 4 equivalent mooring lines
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Hilbert-Huang Method for the Signal Analysis
Time History
EMD
IMFs
Hilbert Transform
Hilbert Spectrum H (ω,t)
Marginal Spectrum
Instantaneous Energy Level Hilbert-Huang
Spectrum
Characteristic motion
amplitude
Characteristic motion
frequency
ω
E
t
E
t
ω H
See Gonçalves et al. (OMAE2010) for details
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Results: Transverse Characteristic Amplitudes
• 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 ∅| )
• According to those results, the 30, 45, 210 and 225 degrees showed the largest VIM amplitudes in the transverse direction
0,00
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No
nd
imen
sio
nal
Am
pli
tud
e (A
y/L)
Reduced Velocity (Vr)
0º 15º 30º 45º
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0,05
0,10
0,15
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No
nd
imen
sio
nal
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pli
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e (A
y/L)
Reduced Velocity (Vr)
180º 195º 210º 225º
• Except for the headings of 0 and 180 degrees, all other incidences showed a synchronization at 4 < Vr <10
• It is not possible to define one oscillation frequency for Vr > 14
• The appendages influence on VIM can be verified by comparing the headings:
– 0 and 180 degrees
– and also 15 and 195 degrees
• Differences may be attributed to the presence and position of the hard pipes in the columns
Rotterdam| The Netherlands | June | 2011 30th International Conference on Ocean, Offshore and Arctic Engineering 8
Results: Yaw Characteristic Angles
• Considering the TRANSVERSE-T0, a synchronization range of the yaw is identified for Vr > 10
• Possible existence of “Vortex-induced Yaw Motion (VIY)”
• Again, it is possible to observe the appendages influence by comparing the 0 and 180 degrees, and also 15 and 195 degrees heading
• In previous work, Waals et al. (2007) proposed that the yaw oscillation was a consequence of a galloping phenomenon
• The same behavior has not been observed in the present work
0,00
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Yaw
Am
pli
tud
e [d
egr
ee]
Reduced Velocity (Vr)
0º 15º 30º 45º
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Yaw
Am
pli
tud
e [d
egr
ee]
Reduced Velocity (Vr)
180º 195º 210º 225º
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• By using the natural period of yaw (model test value), T6, to calculate the reduced velocity, a typical VIM behavior, for this degree of freedom, is observed – Vr=U T6 / D
• The largest yaw angles occur in Vr = 8, a very similar result to that usually obtained for VIM in the transverse direction
• The amplitudes decrease for a high value of Vr, characterizing a auto-controlled phenomenon, like VIV
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Am
pli
tud
e [d
egr
ee]
Reduced Velocity (Vr=U T6 / D)
0º 15º 30º 45º 180º 195º 210º 225º
Yaw Characteristic Angle
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• Time history of motions in the in-line (x/L), transverse direction (y/L) and yaw motion for the heading of 45 degrees
• Vr=3.78 corresponds to a region at the beginning of the transverse synchronization • Vr=6.76 corresponds to the peak of oscillation inside the region of the transverse synchronization. The
yaw motion presents frequency similar to the transverse oscillation • Vr=12.06 corresponds to the peak of yaw motion, i.e. in the region of the yaw synchronization. The
frequency of the yaw motion is clearly defined
Comparing Time Histories Vr=12.06 Vr=6.76 Vr=3.78
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Conclusions
• The VIM phenomenon was experimentally observed for a Large-volume Semi-submersible Platform
• The largest VIM in the transverse direction was observed at 30, 45, 210 and 225 degrees of heading
• In general, the VIM in the transverse direction occurs in a range of 4.0<Vr<14.00 with peaks around 7.0<Vr<8.0. The largest amplitudes obtained were Ay/L=0.4 (where L is the characteristic dimension of the rounded-square column)
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Conclusions
• Considering the headings, an important asymmetry was observed by comparing the 0 and 180 degrees incidences. Among other appendages, the hard pipes may be the reason for the differences observed
• Considerable yaw motion oscillations were verified in these tests and a synchronization region could be well identified, herein named as “Vortex-Induced Yaw Motion (VIY)”
• The largest yaw motions were verified for the 0 and 180 degrees of incidence, corresponding to angles around 4.5 degrees.
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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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