Roll Minimization by Blige Keels and Moving Weight Systems

ABSTRACT The objective in this paper is to discuss the technical and economic considerations arising in the process of minimizing the roll motion of marine vehicles through the use of bilge keels and moving weight systems .. Given a statement of the mission and owner';s requirements for any vessel, the procedure for selecting the most appropriate anti rolling device is delineated on the basis of a trade-off analysis among candidate systems for such an application. Design methods are developed for moving weight systems and bilge keels on the basis of linear and nonlinear mathematical models of the interacting ship-stabilizer motions. The paper concludes with the description of a computer program developed for stabilizer design, some example applications and a brief review of the economic advantages accrued by the incorporation of roll stabilizers. INTRODUCTION During the last decade, the maritime industry has witnessed an impressive trend toward the deployment of "roll stabilizers" onboard different types of marine vehicles. Contrary to what their name might imply, the function of such systems is to minimize or suppress a vessel';s roll motion, rather than enhance its transverse stability. Typical anti rolling devices range from the time-honored bilge keels to sophisticated fin arrangements and activated tanks, each providing a differing roll reduction effectiveness depending on the particular application. Unlike other ship motions, roll is readily susceptible to containment, mainly because the inherent damping properties of most hulls are low and also because the external couples acting on the vessel are not too excessive. Under such conditions, it becomes relatively easy to conceive of a system that produces, at the expense of a small amount of energy, the appropriate torque counteracting that induced by the environment. The required energy may either be extracted from the rolling vessel itself, as in the case of passive anti rolling systems or may be supplied externally by separate sources, as in the case of activated devices. The question of whether a given vessel should be equipped with one or more anti rolling devices cannot be answered objectively unless a study is made of her dynamic behavior in the expected environment, given the mission objectives and hull characteristics. The first step in such a study is to attempt to define critical measures of roll (or its time derivatives) which can be used as yardsticks of what constitutes acceptable roll behavior, Ref. (1). The second step is to find how the vessel behaves in a variety of operational conditions and to judge whether her responses are acceptable. If they are, the question of stabilization is by-passed. If they are not, the third step is to examine whether certain changes in geometry and mass distribution might reduce roll.