Reducing rocking motion of marine floating structures

Abstract: A marine floating structure includes apparatus for reducing rocking motion thereof which includes a linear motor (6). The stator (5) of the linear motor is fixedly secured to the marine floating structure and the movable member acts as a solid mass which is moved to counteract rocking motion of the structure which is detected by a rocking motion sensor (12) whose output signal is phase-controlled by a controller (13) and supplied to the linear motor as a command signal.

Description: The present invention relates to marine floating structures, such as ships or barges and is concerned with apparatus for reducing or suppressing rocking motion, such as rolling and pitching, of such structures. Known apparatus of this kind include anti-rolling tanks and fin stabilizers. Figure 1 is a schematic view of an anti-rolling tank which is used to suppress rolling of a ship. A U-shaped water tank c, whose upper ends communicate through an air pipe b, is arranged in a main hull of a ship on a deck above the centre of gravity of the hull. It is designed such that in response to rolling of the hull a, the water d in the tank c is caused to effect passive resonance which lags the rolling in phase by 90°. Figure 2 is a schematic view of a fin stabilizer which comprises movable fins f which are driven by drives e and which are attached to the submerged bilge of the main hull of a ship substantially at the mid-point thereof in a fore-and-aft direction. The angle of rolling, the angular velocity and the angular acceleration of the main hull a are detected by a sensor g and the angle of elevation of each fin f is actively varied in response to the detected values so that the rolling of the main hull a is suppressed by dynamic lift produced on the fins f by the velocity of the ship through the water. An anti-rocking system using a solid mass has also been proposed, and such a system is shown schematically in Figure 3. This includes a base h with rails i mounted on the bottom, deck or the like of the main hull a of a ship and a weight k constituting a solid mass rides on wheels j on the rails i. A lead-screw 1 extends through and is in threaded engagement with the weight k and is supported by spaced bearings m. One end of the lead-screw 1 is connected through a coupling p to a reduction gear n and a motor o. When the main hull a rocks, the motor n is rotated in a clockwise or anticlockwise direction so that the weight k is displaced in a direction opposite to that of the external force acting on the hull a so as to reduce the rolling motion of the hull a. Anti-rolling tanks have the following problems: (1) Since the required total weight of the tank is generally 3 or 4% of the displacement of the hull in the case of smaller vessels and 1 or 2% in the case of larger vessels and since the anti-rolling mass is water, the system requires a large space over the upper deck, resulting in bad visibility, e.g. for steering. (2) In a smaller ship, the performance of the ship is adversely affected by the raised centre of gravity. (3) The anti-rolling effect may be comparatively satisfactory for a ship subjected to waves. But, when the waters are calm, the amount of free water increases so that any angle of listing or heeling of the ship is increased. (4) Once designed, the anti-rolling tank has a predetermined natural period so that when the actual rolling period of the ship is different to that of the anti-rolling tank, the anti-rolling effect is reduced. (5) Noise is produced by the flow of the water and the associated flow of the air through the connecting pipe so that the environmental conditions and personal comfort are adversely affected. Fin stabilizers have the following problems. (1) The anti-rolling effect cannot be ensured until the velocity of the ship becomes in excess of a minimum level at which the fins produce dynamic lift. In other words, the anti-rolling effect is not produced when the ship moves at a low velocity or stops. (2) Rigging of the fin stabilizer to the hull is very complicated. (3) As compared with anti-rolling tanks, fin stabilizers are extremely expensive (about five times as much as anti-rolling tanks). (4) There is a fear that the noise produced by fin stabilizers adversely affects the operation of sonar equipment. (5) Since fin stabilizers are attached to the ship's hull, the velocity of the ship is reduced. In the case of anti-rocking systems utilizing solid mass, a drive includes a rotating body so that in order to linearly drive the driven body linearly, an auxiliary system comprising gears and a screw (or a linkage) is required. As a result, the construction is complicated and system failure frequently occurs and consequently maintenance becomes problematic. In addition, the weight of the system is very high. GB-A-2213789, on which the precharacterising portion of Claim 1 is based, discloses a marine stabiliser including a moving mass which forms the whole structure of a linear motor. The reaction rail or stator of the motor is fixedly secured to the ship. The mass moves on a rail which extends across or along the ship. The linear motor is controlled in response to signals produced by accelerometers or the like. Against this background it is the object of the present invention to provide an apparatus for reducing rocking motion of a marine floating structure which is of simple lightweight construction, and to overcome the problems described above encountered in the known anti-rocking systems. According to the present invention a floating marine structure, such as a ship, includes apparatus for reducing rocking motion thereof, the apparatus including a solid mass, a linear motor to move the solid mass relative to the floating structure, the linear motor including a stator fixedly secured to the floating structure and a movable member which is constituted by the solid mass, a sensor for detecting rocking motion of the floating structure and a controller for delivering a phase-controlled driving command signal to the linear motor is characterised in that the movable member and the stator are in the form of concentric arcs. When rocking motion of the marine floating structure is detected by the rocking motion sensor, it produces a signal which is phase-controlled by the controller and is supplied as an output to the linear motor. In response to the received signal, the movable member is forced to move along an arcuate path in the direction reducing the rocking motion so that the movable member of the linear motor acts as a solid mass to reduce rocking motion of the marine floating structure. The movable body is forced to swing like a pendulum, thereby suppressing the rocking motion of a ship. Synchronizing the natural period of the movable body with that of the hull enables the movable body to act as passive means to reduce any rocking motion of the ship without operating the variable drive power supply at the synchronous or resonance point of the rocking. Thus in the marine floating structure of the present invention the apparatus for reducing rocking motion dispenses with gears, screws and other mechanical transmission components. Accordingly, the apparatus can respond immediately to rocking motion of the structure to reduce that rocking motion. Since a solid mass is used, as opposed to the water of anti-rolling tank systems, the installation space required is considerably reduced. In the case of ships, as opposed to other marine floating structures, the apparatus can be disposed in the main hull. As a result, steering operation of the ship is improved, the ability to maintain the ship in an upright position is enhanced and the space within the main hull is used effectively. In calm seas the solid mass constituted by the movable member can be moved appropriately to prevent the marine floating structure from listing. The apparatus for reducing rocking motion can inherently be of compact and inexpensive construction. Due to the use of a linear motor, friction is minimised and the solid mass can be displaced by a relatively low-power drive so that running costs are also reduced. If the number, position and orientation of linear motors is selected appropriately rocking motion in all directions of the marine floating structure can be reduced and thus the structure satisfactorily stabilized. Due to the elimination of gears and other mechanical linkages the noise produced during operation is substantially reduced. Further features and details of the present invention will be apparent from the following description of two constructions which are not in accordance with the invention and two further constructions which constitute embodiments of the invention which is given by way of example with reference to Figures 4 to 10 of the accompanying schematic drawings, in which:- Figure 4A is a side view of a ship with a first construction of apparatus for reducing its rocking motion which is not in accordance with the present invention; Figure 4B is a cross-sectional view thereof; Figure 5A is a side view of the apparatus for reducing rocking motion shown in Figures 4A and 4B; Figure 5B is a front view thereof; Figure 6 is a view, on an enlarged scale, of a linear motor; Figure 7 is a block diagram of a linear motor control system; Figure 8A is a side view of a second construction of apparatus for reducing rocking motion, which is not in accordance with the present invention; Figure 8B is a front view thereof; Figure 9 is a schematic view of a first embodiment of the present invention; and Figure 10 is a schematic view of a second embodiment of the present invention. In the construction shown in Figures 4A to 7, the marine floating structure is a ship generally indicated by reference numeral 1. A base 3 is mounted on the bottom floor of the engine room 2 and two parallel guide rails 7 are laid on the base 3. The linear motor 6 comprises a driven or movable means or body 4 and a stator means or body 5. The movable body 4 rides on wheels 8 on the rails 7 while the stator body 5 is securely arranged between the rails 7 and parallel therewith such that the top of the stator body 5 is spaced from the bottom of the movable body 4. The stator body 5 is electrically connected to an excitation power supply (not shown) and the movable body 4 is electrically connected to a variable drive power supply 15 (see Figure 7) so that when electric current is supplied to the movable body 4, a dielectric electromotive force is produced between the bodies 4 and 5 and consequently the movable body 4 is forced to move linearly as a solid mass along the stator body 5. As best shown in Figure 6, the movable body 4 of the linear motor 6 consists of a primary core with three-phase windings 9 and the stator body 5 consists of a secondary core with squirrel-cage windings 10. The movable body 4 is located on one side of the stator body 5. A rocking-motion sensor 12 and a controller 13 are located at any suitable position in the ship, typically substantially higher than the linear motor, in this case in the wheelhouse 11. The variable drive power supply 15 of the movable body 4 is energized by a control signal generated and delivered by the controller 13 in response to a signal from the rocking-motion sensor 12 so that displacement of the movable body 4 is controlled in relation to the rocking motion of the ship 1 and consequently the energy of rocking motion of the ship 1 is consumed or absorbed. As shown in Figure 7, the controller 13 processes the rocking-motion signal from the rocking-motion sensor 12 and delivers to the power supply 15 a phase and displacement signal which lags the rocking motion of the ship 1 in phase by 90°. The displacement signal applied to the movable body 4 is also fed back to the controller 13. In the first embodiment, the rocking-motion sensor 12 comprises an acceleration sensor and the acceleration is integrated twice in the controller 13 to generate the displacement signal, but it is possible to integrate the degree of acceleration only once to generate a velocity signal which in turn is converted into a reversed signal to be applied to the movable body 4 as a displacement signal. The flow of electric current through the primary core or movable body 4 of the linear motor 6 causes a dielectric electromotive force to be produced between the movable body 4 and the stator body 5 so that the movable body 4 is forced to move along the guide rails 7. Therefore, when rocking motion of the ship 1 is detected by the rocking-motion sensor 12, a phase-controlled signal based on the signal from the sensor 12 is transmitted from the controller 13 to the variable drive power supply of the movable body 4 so that the movable body 4 is forced to move in a direction to reduce the rocking motion in relation to the external force acting on the ship 1 and the movable body 4 functions as solid mass to immediately suppress the rocking motion of the ship 1. The movement of the movable body 4 can be controlled by phase-controlling the multi-phase alternating current and the velocity and acceleration can be controlled by changing the frequency. The ship's rocking motion can thus be reduced by a mechanism of simple construction without using gears, lead screws and the like. The second construction shown in Figures 8A and 8B, which is also not in accordance with the invention, is similar to the first construction shown in Figure 5 except that the linear motor 6 is in the form of cylinder. More specifically, the movable body 4 constituting the solid mass is a hollow cylinder through which a rod-like stator body 5 extends for movement of the movable body 4. Opposite ends of the stator body 5 are secured in position with respect to the base 3. This construction operates in a manner similar to the first construction. Figures 9 and 10 illustrate first and second embodiments of apparatus for reducing rocking motion in accordance with the invention which are modifications of the first and second constructions described above, respectively. In the first embodiment shown in Figure 9, the movable body 4 and the stator body 5 of the linear motor 6 are in the form of arcs and arranged concentrically. The movable body 4 is supported by support rollers 14 which are spaced apart along the length of the stator body 5. In the fourth embodiment shown in Figure 10, the cylindrical linear motor 6 is constructed again in the form of an arc. In both the first and second embodiments, the movable body 4 is forced to swing like a pendulum (in single harmonic oscillation), thereby suppressing the rocking motion of a ship. Synchronizing the natural period of the movable body 4 with that of the hull enables the movable body 4 to act as passive means to reduce any rocking motion of the ship without operating the variable drive power supply at the synchronous or resonance point of the rocking. In both embodiments of the invention, the apparatus is arranged across the width of the ship so as to reduce rolling of the ship; but when the apparatus is arranged along the length of the ship, pitching of the ship can be reduced. Thus if one apparatus is arranged in the widthwise direction of a ship while another is arranged in the lengthwise direction both rolling and pitching can be reduced. In both embodiments, one-sided or unilateral type linear motors are used, but it is to be understood that the present invention may utilise a two-sided or bilateral type linear motor. In this case, two movable bodies 4 can be arranged on opposite sides of the stator body 5. Furthermore, the movable body 4 may consist of a secondary core while the stator body 5 is a primary core. Whilst the movable body 4 has been described as being provided with wheels 8 or rollers 14, any mechanism may be employed such as linear guides, sliding bearings, a magnetic force system, air pressure system, hydraulic floating system and the like which can permit displacement of the movable body 4. In the described embodiments of the invention, the rocking-motion reducing apparatus is disposed on a ship; but it is to be understood that the apparatus may be mounted in any marine floating structures.

Claims: A marine floating structure, such as a ship (1), includes apparatus (6) for reducing rocking motion thereof, the apparatus including a solid mass (4), a linear motor to move the solid mass relative to the floating structure (1), the linear motor (6) including a stator (5) fixedly secured to the floating structure and a movable member which is constituted by the solid mass (4), a sensor (12) for detecting rocking motion of the floating structure and a controller (13) for delivering a phase-controlled driving command signal to the linear motor (6), characterised in that the movable member (4) and the stator (5) are in the form of concentric arcs. A structure as claimed in Claim 1 characterised in that the movable member (4) is in the form of a hollow cylinder, that the stator is in the form of rod (5) extending through the movable member and that the ends of the rod (5) are fixedly secured to the floating structure.

Inventors: MUTAGUCHI MASAO, SHIGEMATSU SHOZO, MUTAGUCHI, MASAO, SHIGEMATSU, SHOZO
Applicants: ISHIKAWAJIMA HARIMA HEAVY IND, ISHIKAWAJIMA-HARIMA JUKOGYO KABUSHIKI KAISHA
Application year: 1993
Main classification: B63B39
IPC codes: B63B39, F16F15