A new active gyrostabiliser system for ride control of marine vehicles
Introduction
Motion control systems are fitted to a variety of marine structures in order to provide a stable platform for mission deployment and/or for human comfort (Burger and Corbet, 1966). As indicated in the examples provided in Table 1, systems that have been developed to control undesirable ship motions can be classified as either external or internal systems. Furthermore, they may be actively forced, or more simply, passively react in order to reduce vessel motions.
External systems generate motion-controlling forces and moments outside the hull of the ship and generally rely on hydrodynamic interactions. Internal systems generate moments and forces entirely within the hull. Most commonly, internal systems use moving weights to generate stabilising moments. A common example of this approach is water-tank stabilisation systems in which a transfer of water is used to provide righting moments, acting to counter, say, roll motion (Gillmer and Johnson, 1985; Lewis, 1986). Whereas, appropriate, actively controlling the actuation of a system usually improves its performance compared to the passive equivalent (Bennett, 1970).
Both internal and external systems, when appropriately applied, can provide adequate solutions to the challenge of reducing undesirable ship motions in a seaway. However, as indicated in Table 1, these two distinctly different approaches also have associated disadvantages. Consequently, lighter external systems are used on weight-critical vessels (e.g. passenger ships), whereas weight based internal systems are used on deadweight carriers (e.g. offshore supply vessels) wherein the stabilisation weight also often provides ballast weight. Alternatively, the use of gyroscopes to stabilise marine vehicles is one method to generate stabilising moments entirely within the hull of the vessel without simply relying on providing sufficient movable weight. Historic and current gyroscopic systems are however limited by the magnitude of stabilising moments they can generate.
Recent industrial interest in providing an internal stabilisation system, that does not simply rely on the use of additional weight, has led to the development of an active gyro stabilisation system that uses a new mode of operation to improve the stabilisation performance compared to historic or current systems (Townsend, 2005). In this paper, a theoretical and physical experimental study of this active gyrostabiliser is carried out at model-scale. Furthermore, the viability of the proposed system for use at full-scale is theoretically demonstrated.
Section snippets
Background to gyro stabilisation
A gyrostabiliser uses the inertial property of a rotating flywheel to apply moments to a vehicle (or other object). These moments alter the amplitude of oscillatory motions that a vehicle suffers when subject to external excitation (e.g. wave excitation of a ship). While the particular method presented in this paper is novel, it is notable that the principle of gyroscopic stabilisation has been successfully used in a number of different applications historically and in recent times.
In a
Principles of operation of internal stabilisation systems
In order to demonstrate the advantages provided by a gyroscopic approach to stabilisation as compared to the more common weight-based systems, the principle of operation of each system is explained and compared in the following sections. This is followed by an explanation of the physical principles associated with current active control for gyro stabilisation before the new mode of operation is presented.
The experimental gyrostabiliser system
Fig. 3 provides a sketch of the gyro stabilisation system used for practical experimentation. Consistent with the principles outlined in Section 3.5, the new gyro stabiliser uses a pair of gyroscopes nutating at a constant rate about their free axes. To remove the effects of gyroscopic moments acting about the undesired axis of ship rotation, it is necessary to spin the flywheels in opposite directions while also nutating the gyroscopes in opposing directions. This latter point is well
Experimental investigations
The aim of the experimental investigations was to demonstrate the effectiveness of the new device, described in Section 4, as a ride control system for marine vehicles. These experiments were carried out on a ship-model in a towing tank. The ship-fixed axis of rotation about which the undesirable motions are targeted using the gyro stabiliser is arbitrary. For the experiments reported here, the targeted motion was pitch. This readily permitted testing with the model ship at forward speed in a
Presentation of results
The experimental apparatus readily permitted the operation of the gyro stabiliser at two spin and two nutation rates. The two spin rates were 4000 and 6200 rpm and the two rates of nutation were 61 and 85 rpm. The heights of the regular head waves used were approximately 29% of the model ship draught.
Fig. 6 provides the relative amplitude of the induced bow motion when the gyroscope was operated, at the different specified spin and nutation rates, on the vessel advancing in otherwise calm water.
Discussion
The maximum amplitude of the bow motion induced by the operation of the gyroscope on the model vessel advancing in otherwise calm water were observed to be of the order of magnitude of the wave heights selected for head sea experiments. This provided confirmation that the gyroscopic parameters had been sufficiently well specified for the intended head sea experiments. Furthermore, in Fig. 6, for the vessel advancing in otherwise calm water, it is notable that the variation in experimentally
Conclusions
The initial motivation for this research was to examine alternative ride control systems that are capable of providing levels of motion reduction, on ships, comparable to existing external systems, while also overcoming the associated hydrodynamic disadvantages. Furthermore, the new system was required to exploit alternative approaches to generating moments within the hull in order to circumvent the weight penalties incurred by simply providing sufficient movable mass to generate stabilising
Acknowledgements
This research was funded by FBM Babcock Ltd. and the Engineering and Physical Science Research Council, EPSRC. We are grateful to Mr. N. Warren for helpful discussions on this subject.
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