VORTEX-INDUCED MOTION OF A MONOCOLUMN PLATFORM: NEW ANALYSIS AND COMPARATIVE

STUDY

Rodolfo T. Gonçalves¹ André L. C. Fujarra¹

Guilherme F. Rosetti¹ Kazuo Nishimoto¹

Marcos Cueva² 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

Analysis Procedure

Results

IPT-Brazil (2005)

NMRI-Japan (2007)

NMRI-Japan (2008)

Conclusions

Summary

This paper presents a new analysis and a comparison of results obtained from Vortex-Induced Motion (VIM) model tests of the MonoGoM platform;

The tests were performed in three different basins;

The objective of this work is to discuss the most relevant issues regarding the concept, execution and procedures to analyze comparatively the results obtained from model tests;

The approach employed in the tests was designed to build a reliable data set for comparison with theoretical and numerical models for VIM prediction, especially that of Monocolumn platforms.

Introduction

The experimental setup is characterized by a scale model of the MonoGoM floating (monocolumn platform) unit, supported by a set of equivalent horizontal moorings, in three different basins: Towing Tank of the Instituto de Pesquisas

Tecnológicas do Estado de São Paulo (IPT), Brazil, in September 2005;

Model Ship Experimental Towing Tank (Middle Towing Tank) at the National Maritime Research Institute (NMRI), Japan, during March 2007, and;

400 meter Experimental Tank of NMRI, Japan, during June 2008

The instrumentation was dedicated to monitoring: Movements in the 6 degree of freedom; Accelerations on the XY plane; Forces in the springs (only in NMRI); Towing velocity.

Experimental Setup

Breadth at the bottom 118.85 m

Main Breadth 100.00 m

Depth 58.00 m

Draft test 39.50 m

Displacement 262,000 ton

The main characteristics of the tests are:

Basically the difference are: Dimensions of the basin; Model scale; Number of springs; Reynolds number.

Experimental Setup

Tank

Dimensions

L x W x D

[m]

Model

Scale

Number

of

Springs

Reynolds Number VrnTotal of

runs

IPT - Brazil

(2005)

280.0 x 6.6 x

4.01:200 4 or 2 2x10

4 – 1x10

5 2.5 – 13.5 63

NMRI - Japan

(2007)

150.0 x 7.5 x

3.51:90 3 1.5x10

5 – 3.5x10

5 6.0 – 11.0 41

Low 1.5x105 – 3.5x10

5 7.0 – 11.5 57

High 5.5x105 1x10

6 3.5 – 9.5 14

NMRI - Japan

(2008)

400.0 x 18.0 x

8.01:90 3

Mooring Line

SEincidence

UX

Y

3

4

Mooring Line

UX

Y 12

3

4

NWincidence

Mooring Line

0 degreeincidence

180 degreeincidence

U UX

Y3

1

2MPSO

Spring

Spring

Wire

CCD camera

Spring

1 2

3 4

IPT (Brazil)

NMRI (Japan)

Analysis Procedure

The standard procedure of results analysis intends to be robust and was structured based on the current literature regarding fluid structure interaction, in particular, the VIV and VIM phenomenon.

Restoration Forces

The restoration forces are measured in the

springs

Non-Dimensional Motion Amplitudes A/D is defined by the authors as the average

of the 20% highest peaks of the motion signal

Hydrodynamic Forces The linear rigid body motion equations for a

platform with two uncoupled DOF. are represented by Sarpkaya, T. (2004) as shown below:

Then the lift and drag coefficients can be write:

Analysis Procedure

Added Mass

Fujarra, A.L.C. and Pesce, C.P. (2002), proposes a classical analysis on the frequency domain for estimating the added mass coefficient.

According to that analysis, the following relation can be stated:

Reduced Velocity

The reduced velocity is defined for each drift level, taking into account the eventual alteration on the added mass and restoration, thus:

And:

Results – IPT (2005)

The results for the different headings are similar until Vrn = 10 for all the non-dimensionals;

The non-dimensional motions increase in a greater speed from Vrn = 10 , mainly on the NW heading;

In the Vrn = 10 region, the in-line motion in the double frequency of the motion coexists with the transversal motion;

In the same region, there is a high level of dynamic amplification;

Mooring Line

SEincidence

UX

Y

3

4

Mooring Line

UX

Y 12

3

4

NWincidence

Results – IPT (2005)

A new class of suppressor of innovative geometry was tested aiming to attenuate VIM effects;

The motions due to VIM are mitigated in the region of Vrn < 10. In region of Vrn > 10 nothing could be concluded;

Regarding added mass a great decrease can be observed in this region, due to the change in the pressure field which resulted from the change caused by the spoiler plates on the flow.

Results – IPT (2005)

The polar motion graphics on the XY plane have clearly shown the presence of the coexistence of the inline and cross-flow motions in both cases. On the NW heading, the formation of the typical eight shape trajectory is clear.

0.5

1

1.5

2

30

210

60

240

90

270

120

300

150

330

180 0

0.5

1

1.5

2

30

210

60

240

90

270

120

300

150

330

180 0

0.5

1

1.5

2

30

210

60

240

90

270

120

300

150

330

180 0

Concerning the use of spoiler plates, the result of the motion on the XY plane, shows that there is no presence of any usual observed shape.

SE NW spoiler

plates

Results – NMRI (2007)

The non-dimensional amplitude has shown maximum values for of about 0.9; these values are closer to those verified in the IPT tests;

A great dispersion can be observed. This can be explained by the reduced acquisition time. Due to the length of the tank, L = 150m and the model scale (1:90), a short running time was possible, hence few cycles were verified on the motion registry.

Results – NMRI (2008)

The execution of the same tests in a new basin aimed at obtaining a temporal series with more cycles in fully developed condition as well as higher Reynolds number, obtained by the increase of the carriage speed;

The non-dimensional amplitude shows a similar pattern as those obtained in the previous tests;

The results regarding the drag coefficient were practically constant Cd = 0.7. The difference occurs where the dynamic amplification phenomenon for values Vrn > 10 was verified.

The present paper has presented an extensive database on the effects of VIM of monocolumn platforms which can be used advantageously for the initial design of such platforms;

The results coming from different basins can be considered similar, exempting their own peculiarities;

When dimensioning VIM experiments, it is necessary to choose the model scale in such a way as to allow for the greatest acquisition time to improve statistics;

Tests for Vrn > 12 are necessary to allow not only the identification of the lock-in region, but also better behaved motions with clear observations of double frequency and eventual motion drop;

The paper that will be presented in the next aims to complement the previous work in the literature and mainly to provide continuity to this present work, in the way VIM tests are made for a new configuration of a monocolumn platform.

Conclusions

Thank You!

Rodolfo T. Gonçalves

(rodolfo_tg@tpn.usp.br)