ABSTRACT

Forced rolling, swaying and roll-sway coupling tests of an U-tube type anti-rolling tank (ART) model are carried out to investigate the effect of sway motion on the roll reduction performance of an ART. The results of the forced swaying tests demonstrate that sway motion make water in the ART move violently, and the frequency of the violent motion is different from the natural frequency for roll motion. The results of the roll-sway coupling tests suggest that sway motion increases the roll damping created by the ART in lower-frequency range and decrease it in higher-frequency range. It can be said that sway motion causes negative roll damping or roll exciting moment to a ship in some phase difference between roll and sway motions.

INTRODUCTION

To reduce roll angle of a FPSO, bilge keels and an anti-rolling tank can be used because both roll-reduction tools are effective without forward speed of a ship (Park et al, 2004). Well-designed bilge keels can reduce roll angle at resonance condition by 50–60%. An U-tube type antirolling tank (ART) can reduce it by 40–50%, too. Therefore roll motion at resonance condition of a FPSO with bilge keels and an anti-rolling tank may be 20–25% of roll motion of it without them.

An anti-rolling tank is relatively cheap in the initial investment, and can be operated in low operational and maintenance costs. One of its defects is that it can reduce roll angle only near the natural frequency of the ART. This means that when the natural frequency of ART is different from the natural frequency of roll motion of a FPSO, no roll reduction can be expected. To solve the problem, anti-rolling tanks with a function of automatically adjusting the natural frequency of the ART to roll natural frequency of the ship have been developed.

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