US3097622A - Stabilization of floating bodies - Google Patents

Description

' July 16, 1963 J. BELL STABILIZATION OF FLOATING BODIES 3 Sheets-Sheet 1 Filed March 28, 1962 J. BELL STABILIZATION OF FLOATING BODIES Jul 16, 1963 3 Sheets-Sheet 2 Filed March 28, 1962 fillllllpllllmll -il lliwidrix, X m m w m e .K H Y WWV N %N 0e 6 A Pm m N 00 0 S 5 A RH V.

July 16, 1963 J. BELL v3,097,622

STABILIZATION OF FLOATING BODIES Filed March 28, 1962 3 Sheets-Sheet 3 This invention relates to the stabilization of floating bodies.

More specifically, the invention relates to cancelling or reducing the rolling and/01' pitching motion of ships, and stabilizing of floating platforms, such as those used by floating cranes and floating docks, against movement in any direction from the horizontal plane.

Stabilization of ships using moving masses either liquid or solid has been proposed but there are various practical objections to their use. In the case of a liquid mass, considerable power has to be expended to move it rapidly enough from one side of the vessel to the other, and in the case of a solid mass, a unit of the order of 2 percent of the ships weight may be regarded as dangerous.

A particular kind of liquid mass stabilizer employs tanks fitted on both sides of the ship, the bottoms of which are open to the sea. The amount of water contained in each tank is controlled differentially by pumps or power-operated pistons in such a manner that the desired stabilizing torque is obtained at any instant by draw. ing water from or discharging it directly into the sea, thereby altering the buoyancy of the tank.

In a specific example, a ship may be provided with live tanks having a working height of, say 20 feet operating from a mean sea level of 10 feet and a capacity of W tons of water above the mean sea level. Thus, for each operation to full stroke of the stabilizer having two such tanks, the water is lifted or depressed by a mean amount of 5 feet and the work done per cycle is 4X5 W tons feet. The stabilizing torque or couple for such an installation is Wb tons feet where b is the athwartship separation of the two tanks. It is estimated that for a ship having a rolling period of 27 seconds and a stabilizing operation of 4 seconds duration, eflective twice in each rolling period, 600 HP. is required to operate the stabilizer.

It is an object of the invention to reduce the work done in operating a ship stabilizer by reducing the effective height of the tank or container from, say, 20 feet as given in the example to, say, 2 feet or any convenient height related to the dimensions of the vessel without substantially altering the volumetric capacity of the tank appropriate for a given ship.

A further object is to provide such a stabilizer to operate either without power means or with a minimum of power, the power being mainly derived from the rolling energy which it is desired to kill or damp out.

The invention consists of a stabilizer for a floating body comprising means mounted on the body for displacing part of the medium in which the body floats by gas, and means for controlling the displacement for the purpose of resisting the movement to be stabilized.

The control means act so as to open and close a valve or valves for a period of time necessary to enable the tank to fill, or partly fill, with water when it is in or near the downwards limit of its vertical movement due to rolling or pitchin" and to open and close the valve or valves for a period of time necessary to enable the tank to empty when it is in or near to the upward limit, thus adjusting the water level in the tank in such a manner as to counteract or damp the vertical motion of the body.

More than one tank may be provided, for example, spaced in opposite substantially horizontal directions from the horizontal axis of motion or two sets may be provided each spaced about a horizontal axis, the two horizontal axes being at right angles.

The valve may be used for transferring gas such as air from the tank at one side of the vessel to the other and back again with simultaneous displacement of water at the sides of the vessel corresponding to the gas thus transferred.

A phase control may be inserted in the control means to advance or retard the opening and closing of the valve or valves in order to maintain maximum efliciency or optimum change in the hydrostatic head of the water tank.

In one embodiment of the invention the tanks are placed on each side of the floating body, for instance, a ship in the case of a roll stabilizer and in the bow of the ship in the case of a pitch stabilizer.

Considering an application of a roll stabilizer, the bottoms of the tanks are open to the sea and an air pipe in which a stop valve is placed interconnects the tops of the tanks. Thus, when the ship is in an upright position in still water and the stop valve is open, the hydrostatic pressure of the sea would cause water to enter both tanks. Air is pumped into the interconnecting pipe and the water level in the tanks will fall, if the vessel is on an even keel, until all the water is driven out.

As the vessel rolls air will escape from the highest tank and under stabilizing conditions the tanks may be regarded as each being half full. The air supply is small in amount and is required only to maintain this state against leak-age.

When the ship rolls to port or to starboard due to the forces imposed on it by the sea, one side is more deeply immersed while the other side is correspondingly raised or less deeply immersed but since the change in the hydrostatic pressure is small very little change will occur in the amount of water in each tank while the stop valve is shut. The difference in water level in the tanks, when appropriately controlled by the stop valve, may be utilised to produce a torque about the rolling axis of the ship which acts in opposition to the rolling torque, thereby reducing the rolling motion to small proportions. A suitable control means may comprise a velocity sensitive or rate gyroscope which senses the velocity of the roll and causes actuating means to open and close the stop valve at desired intervals.

The invention will now be described in greater detail with reference to the accompanying drawings, in which:

FIGS. 1 to 5 show diagrammatically a cross section of a ship through a liquid mass stabilizer with the ship in various roll positions indicating the functions of the liquid mass stabilizer at various stages of the roll;

FIG. 6 shows, by means of graphs, the functions of said stabilizer over one complete roll period;

FIG. 7 shows a control means and actuating means for a stop valve according to the invention;

FIG. 8 shows an anti-rolling tank adapted to counter the pitching motion of a ship.

FIGURE 1 shows a ship 1 on an even keel with tanks 2 and 3 located on either side of the lower portion of the ship below the water line 4. The tanks are connected by a pipe 6 having a valve 5 connected to it and they are at least half filled with air supplied by means of pump 7.

FIGURE 2 shows the ship rolling in an anti-clockwise direction. By suitable control means a stop valve 8 in the pipe 6 is closed when rolling motion starts and hence the amount of water in each tank is substantially unchanged. The control is opened again whenever motion stops at the limits of roll and closes when it starts again.

FIGURE 3 shows the conditions at the time of maximum roll angle. The stop valve it has opened with the decrease in rolling velocity and due to the difference in hydrostatic pressure in the two tanks 2 and 3 the 3 water level in tank 2 has risen while the water level in tank 3 has fallen by a corresponding amount.

At the commencement of the rolling motion in a clockwise direction the stop valve 8 closes and FIGURE 4 shows the conditions as the ship rolls through the vertical or even keel position, lifting tank 2, nearly filled with water, which by gravity acts in the sense required to damp the rolling motion.

If the magnitude of the damping effect is insufficient to arrest the rolling motion at the position of zero roll the ship will heel over in a clockwise direction to the end of the roll, stop valve 8 will then open, enabling tank 2 to empty, while tank 3 fills and damping will be continued during the ensuing anti-clockwise roll.

FIGURE 5 illustrates these conditions and FIGURE 6 illustrates, by means of graphs, the functions of the sta bilizer for one complete roll period, that is to say, one complete roll from port maximum to starboard maximum and the return roll to port maximum.

FIGURE 6a is a roll angle curve, the portion above the zero line representing the list to port and the portion below the zero line, the list to starboard.

FIGURE 6b is the corresponding roll velocity curve which also may represent the precession of a velocity sensitive or rate gyroscope controlling the stop valve 8.

FIGURES 6d and 6e are curves showing the height of water in the port and starboard tanks respectively during a complete roll period.

FIGURE 6a illustrates the period during which stop valve 8 is open and it will be seen that it is, in fact, open for a period long enough to allow one tank to empty and the other tank to fill. During the remainder of a half period of roll, stop valve 8 is closed. Furthermore, it will the seen that stop valve 8 opens slightly before zero roll velocity (which corresponds to maximum roll angle) and closes slightly after zero roll velocity as indicated by the full line curve. The efiiciency of the stabilizer may be increased by advancing the phase of the velocity control signal to take into account the inherent lapse of time required to allow one tank to fill and the other to empty. Thus, it can be arranged that the change-over in the water levels in the two tanks is completed at or before the time of zero roll velocity (maximum roll angle) as shown by the broken lines (x) in FIGURES 6c, d and e.

Furthermore, the sensitivity of the control may be adjusted, and so vary the length of time that stop valve 8 remains open, to obtain the optimum change of water level in the tanks.

A preferred control and actuating means for a ship stabilizer which is adjustable for phase and sensitivity according to the invention is shown in FIGURE 7. In the figure, 9 is a velocity or rate gyroscope of known kind mounted to respond to the velocity of roll of the ship. As is well known the deflection of the centralising springs 22 is proportional to velocity.

The rate lgyroscope 9 serves to operate either directly or through a servo linkage 11 and arm 1% which is free to turn about pivot 12. Yoke 13 attached to the upper end 14 of arm is adapted to carry two electrical contacts 15 and 15 mounted in insulated carriers 16 and 16' which are slidably mounted on rail 17. Thumbwheel 18 is attached to the centre of screw 19, the two halves of which are threaded in opposite senses and engage correspondingly threaded holes in the insulated carriers 16 and 16'. Thus, the spacing between contacts 15 and 15' may be continuously adjusted towards or away from each other by turning thumbwheel 18 in either direction. Sector shaped plate 29, made of a suitable insulating material, is also mounted on pivot 12. beneath arm 14 but with a degree of friction such that it will not be influenced by the initial movement of arm 14 from the central. position. Plate 20 carries sector shaped electrical contacts 21 and 21 so disposed that contact 15 will make contact with 21 when arm 14 swings to the left and contact v15' will make contact with 21' when arm 14 swings to the right. Also mounted on plate 20 are adjustable stops 23 and 23' symmetrically disposed about the central position of arm 14. As shown in the figure, contacts 15 and 15' are electrically connected to one pole of a DC. supply 24, the other pole of which is connected to one end of solenoid coil 25. Sector shaped contacts 21 and 21' are similarly connected to the other end of solenoid coil 25. Thus it will be seen that solenoid coil 25 will be energized whenever arm 14 departs from the central position in either direction. When solenoid coil 25 is energized the solenoid armature 26 is raised and thus the stop valve 27 in air pipe 28 is closed over linkages 29 and 39. Spring 31 returns the stop valve 27 to the open position when solenoid coil 25 is de-energized.

The function of the control and actuating means above described can be explained as follows, assuming a rolling motion of the vessel.

When the rate gyroscope 9 is in the central position indicating that the roll velocity is zero, that is to say, the ship is at the extremity of a roll either to port or to starboard, arm 10 will also be in the central position so that solenoid coil 25 is dc-energized and, consequently, stop valve 27 is open. As soon as the ship commences to return to the vertical the gyroscope will precess and arm 10 will therefore be deflected. Suppose, for the purpose of explanation, that arm 11B is deflected to the left. The upper portion 14 will be deflected to the right and electrical contact 15 will make contact with 21' thereby energizing solenoid coil 25 and closing valve 27. As the rolling motion continues the upper part 14 of arm 14 will engage stop .23" and with further movement of arm part 14, plate 2.0 will be forced to turn to the right. As the end of the roll is approached and consequently the roll velocity is decreased, the gyroscope defiexicn also decreases causing arm part 14 to move to the left and break contact between 15 and 21'. It will be apparent, however, that because plate 21 has been moved to the right, contacts 15 and 21' will separate before arm 10 has regained the central position and thus the opening of valve 27 will have been advanced in the phase. As the roll velocity builds up again in the oposite direction after passing the maximum roll angle it will 'be seen that contact 15 makes contact with contact 21, solenoid coil 25 is energized and valve 27 is closed in advance. Similar conditions will obtain when gyroscope 9 processes to the right in response to the reverse roll. The contact gaps, the distances between stops 23 and 23' and the amount of travel of plate 20 can be adjusted for optimum sensitivity and advance.

It will be clearly seen that adjustment stops 23 and 23" will control the magnitude of the phase advance and adjustment of the spacing of 15 and 15 by means of thumbwheel 18 will determine the length of time that valve 27 remains open for adjustment of the water levels in the tanks. This adjustment constitutes a sensitivity control. It will be understood that it might be desirable to amplify the relatively weak forces derived from gyroscope 9 to actuate arm 15. For this purpose any form of mechanical power amplifier may be used, for instance, a hydraulic relay.

[For stabilizing a ship with regard to pitching motion an anti-roll tank according to the invention is placed as far forward as possible in the ship. FIGURE 8 shows such a tank 39 which, conveniently, may conform to the shape of the hull located in the bow part of the ship 46*. Openings or vents 41 allowing access to the sea are provided in the bottom of tank 39 and stop valve 42. is located in the top or" said tank, the stop valve 42 being operated by a control system (not shown) as indicated in FIGURE 7 but in which the sensing means is sensitive to motion about the pitching axis of the ship.

The top of anti-rolling tank 39 is normally below the exterior water line 43. In this embodiment an air chamber is is located immediately above tank 39 and is kept under pressure from air pump 47 so that the normal water level in tank 39 is as indicated by the full line 48.

In practice, the volume of chamber 46 is greater than the volume of tank 39 in order to limit the air pressure diiferential due to the pitching of the ship and, consequently, increasing the quantity of water entering and leaving tank 39. In action the stop valve is opened and closed by the control means approximately at the end of each upward and downward movement of the bow, but by keeping the air in chamber 46 under sufii cient pressure as shown, the difference between water levels 44 and 45 due to pitching is still maintained.

In this embodiment a phase control may be included in the control means to advance or retard the opening and closing of the stop valve in order to obtain maximum efliciency, that is to say, maximum change in the hydrostatic head of the water in the tank.

It will be understood that no restriction is made to the particular forms of embodiment and details of construction illustrated in the drawings which are quoted merely by way of example and that numerous modifications are quite possible within the meaning of the above description without departing from the scope of the invention.

I claim:

1. A floating body having a stabilizer tank the top of which is below the water line and open so that water fills the tank up to a level determined by pressure of gas in the tank, pump means for pressurizing the gas in the tank so that the tank is normally substantially filled with gas when the tank is in its central position of vertical movement, valve means controlling the flow of gas into and out of the tank and means for opening the valve means for a period when the tank is near the downward limit of its vertical movement to permit water to enter the tank and for opening the valve means for a period when the tank is near its upward limit of vertical movement to permit water to leave the tank.

2. A ship having a pitch stabilizer tank located in the bow of the ship which is normally below the water line and the bottom of which is open so that the water fills the tank up to a level determined by pressure of gas in the tank, a gas chamber above the tank, means for pressurizing the gas chamber, valve means venting to the gas chamber for controlling the flow of gas into and out of the tank and means for opening the valve means for a period when the tank is near the downward limit of its vertical movement to permit water to enter the tank and for opening the valve means for a period when the tank is near its upward limit of vertical movement to permit water to leave the tank.

References Cited in the file of this patent UNITED STATES PATENTS 1,700,406 Hammond Jan. 29, 1929 2,892,435 Ljungstrom June 30, 1959 2,923,874 Bell Feb. 2, 19'60 FOREIGN PATENTS 313,461 Germany July 14, 1919 OTHER REFERENCES Ser. No. 132,695, Hort (A.P.C.), published May 11, 1943, now abandoned.

Claims (1)

1. A FLOATING BODY HAVING A STABILIZER TANK THE TOP OF WHICH IS BELOW THE WATER LINE AND OPEN SO THAT WATER FILLS THE TANK UP TO A LEVEL DETERMINED BY PRESSURE OF GAS IN THE TANK, PUMP MEANS FOR PRESSURIZING THE GAS IN THE BANK SO THAT THE TANK IS NORMALLY SUBSTANTIALLY FILLED WITH GAS WHEN THE TANK IS IN ITS CENTRAL POSITION OF VERTICAL MOVEMENT, VALVE MEANS CONTROLLING THE FLOW OF GAS INTO AND OUT OF THE TANK AND MEANS FOR OPENING THE VALVE MEANS FOR A PERIOD WHEN THE TANK IS NEAR THE DOWNWARD LIMIT OF ITS VERTICAL MOVEMENT TO PERMIT WATER TO ENTER THE TANK AND FOR OPENING THE VALVE MEANS FOR A PERIOD WHEN THE TANK IS NEAR ITS UPWARD LIMIT OF VERTICAL MOVEMENT TO PERMIT WATER TO LEAVE THE TANK.

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