A study on moving mass roll control for ships

Abstract

Due to a low restoring force as well as lack of damping, the roll motion of a
vessel is usually the motion with the highest amplitude of all degrees of
freedom. Therefore, in history many have tried to stabilize rolling ship
starting with Watts in 1883. He described the damping of the roll motion by
having a group of men running to the "high" side of the vessel. Since then,
a great deal have been written about the subject of roll damping for
stationary vessels. The principle however has been the same ever since, a
shift of mass to counter the roll motion. Water tanks, carts on a curved track and sliding masses, all tend work with this principle. In 1904 Schlick introduced another device to damp roll motions was introduced, the gyroscope. By controlling the precession angle, the roll motion could be damped. Following from an increase in computational power more advanced control strategies could be applied, leading to a renewed interest in the subject as well as different methods to model the vessel for controller design.

In the current paper, the performance of a gyroscope is compared with the performance of a moving mass, modeled as a double pendulum, for dynamic performance of a showcase vessel, the Huisdrill P10000. Results of this analytic study are promising, a reduction of more than 80% for both systems was
achieved at the natural frequency of the Huisdrill. The decay time after a roll amplitude of 5.7 degree was reduced from about 250 seconds to 60 seconds for the gyroscope controller model and 70 seconds for the double-pendulum controller model. A static heel control system is modeled based on the double-
pendulum equations. This proves to be able to compensate for the heeling moment depending on the weight at the crane tip as well as the weight and distance available to compensate. Combing the static an dynamic approach is possible with the note that there should be sufficient space for the mass to deviate after compensating for static heel.

In irregular waves the 4-DoF model shows good performance for the showcase of the Huisdrill P10000. The RMS value for the roll-roll motion was decreased by 65% For other degrees of freedom, similar reductions were obtained. From this it can be included that for the given seastate, significant reductions can be achieved using the ballast train. In the time domain, it was shown that great reductions in most probable maximum of reduction of roll amplitude were achieved for time periods close to the natural frequency of the vessel. For no time period, reductions in performance were measured. It was found that
the most probable maximum roll angle linearly increased with the wave height, as well as the most probable deviation did.

Interpreting the results of this study, it can be stated that both the gyroscopic control system as well as the double-pendulum system are able to introduce great reduction in dynamic roll motions. The main benefit of using the double-pendulum system is its ability to compensate for the heeling moment induced
by the swiveling crane as well as compensating for the dynamic motions that a vessel will undergo offshore.

It is recommended to do a simulation of a non-linear time-domain model or to perform model tests for verification of the results found in this report. The gap between static and dynamic control should be filled by a master control which is to be designed and run in the time domain.