US3739411A - Low frequency wave absorbing device - Google Patents

Abstract

This disclosure is directed to a spar buoy system within which a compressible air bag or container has been installed to stabilize a platform. The compressible bag is positioned with its center at the center of gravity within a fixed area and pressurized such that the bag collapses or expands due to surrounding water pressure. The surrounding water applies a pressure onto the bag such that the pressure within the bag is equal to the pressure outside the bag.

Description

United States Patent 11 1 Ess June 19, 1973 [54] LOW FREQUENCY WAVE ABSORBING 3,680,160 8/1972 Heikki 9/8 R DEVICE 3,193,853 7/1965 Alexander 3,114,920 12/1963 Delaruelle et al 9/8 R [75] Inventor: John O. Ess, Alexandria, Va. 73 Assignee: The United States of America as r y EmmiflerDuane Reger represented b the Secretary f h Asszstant Exammer Gregory W. OConnor Navy, Washington, DC, Attorney-R. S. Sc1asc1a, Arthur L. Branmng, Philip Schneider et al. [22] Filed: Nov. 9, 1971 [21] App]. No.: 196,892 [57] ABSTRACT This disclosure is directed to a spar buoy system within [52] US. Cl 9/8 R, 9/8 P which a compressible air bag or container has been [51] Illlt. Cl B63!) 21/52, B631) 51/02 stalled to stabilize a platform The compressible g is [58] Field of Search 9/8 R, 8 P; positioned with its center at the center f gravity within l 14/05 R a fixed area and pressurized such that the bag collapses or expands due to surrounding water pressure. The sur- 1 References cued rounding water applies a pressure onto the bag such UNITED STATES PATENT that the pressure within the bag is equal to the pressure 3,471,877 10/1969 Bayles 9/8 R Outside the 3 Claims, 1 Drawing Figure 1 LOW FREQUENCY WAVE ABSORBING nEvtcE BACKGROUND OF THE INVENTION This invention relates to systems for stabilizing an ocean platform and more particularly to a system for stabilizing vertical movement of a buoy to which a platform is secured.

Heretofore, various methods and devices have been used to aid in stabilizing a buoy system or ocean platform. One method of providing a fairly stable ocean platform has been in the construction of large spar buoys. In those instances where damping is neglizible the way to obtain low amplitude buoy motion has been to design a buoy having a natural period of vibration much larger than the natural period of the ocean waves. In this way the ratio of buoy vertical motion to wave height is substantially reduced. For undamped motion the ratio of mass to spring constant determines the period of vibration. For undamped buoy motion the spring constant is the buoyancy per foot of spar while the mass is the total mass of the buoy including its entrapped water mass.

For an ideal buoy one would have a streamlined heavy mass which could support a payload with a very small spar. Outside of strength and buoy rotation the main problem with a small spar is that total system buoyancy must not change or the spar will either float too far out of the water or the whole system might sink. In general, this is no small problem as the payload supported either at the buoy or some distance below usually has a large volume for its in-water weight and thus small water temperature changes will change total system buoyancy.

If the buoy is tied by a floating line to a ship, then it can have a small diameter spar and the trim may be adjusted from the ship. One method would be to use an air hose to adjust a water ballast tank on the buoy. To keep the column of air from being compressed by a passing wave, which would change the total system buoyancy, the ballast tank should be filled or drained through a capillary tube sized to eliminate the problem of differential pressures.

Another method of providing trim, which would not have the disadvantage of pressure dependence, would be to use a closed water system with a pump in the buoy pumping water to and from the ship as required.

Either of these systems can be manually attended or be made to work automatically through some pressure switching arrangement having a long time constant. If the buoy is not attached to the ship the spar may be made somewhat larger so as to be less sensitive to secular changes, being trimmed just before release. With a little more difficulty a small spar may be retained with the triming done automatically from an enclosed air supply or water pump energized from a buoy contained power supply.

SUMMARY OF THE INVENTION When a wave passes a straight spar, the forcing function in the vertical direction consists primarily from a change of buoyency on the system. If there is an enlargement of the spar cross-section below the minimum waterline and the change to this enlarged area is very gradual, no additional forces on the buoy are induced in the vertical direction. To negate the forcing function of a passing wave on a spar a compressible air bag or container is inserted into the spar to counterbalance the change of system buoyancy.

One method of installing the bag in the spar is as follows: The bag is placed in a fixed size container, either flexible or firm and pressurized to a predetermined amount. It is then placed in the spar so that when the spar is placed in the water in its normally used position, the surrounding water which is at a greater pressure for that depth than what the bag is pressurized causes the bag to collapse so that the outside water pressure and inside air pressure are equal. The system now includes a collapsible and expandable container with a fixed volume, pressurized a fixed amount and located at some point in the spar. When a wave passes the spar, there is a change of buoyancy. When it is up, the buoy wants to go up and there is an additional upward force caused by the added buoyancy of more submerged spar. This upward wave, however, causes an additional external water pressure on the bag causing the bag to collapse further as the air pressure in the bag wants to stay at the same pressure as the external water pressure on the bag. If the bag is properly sized, pressurized and located in the buoy, the amount of collapse will substantially equal the amount of water displaced by the wave going up the spar. If the wave goes down, the bag expands and the gain due to expansion almost equals the loss of buoyancy caused by the water going down the spar. Thus, the forcing function on the spar has been practically removed in the vertical direction. It has been determined that the loss of buoyancy due to collapse of the bag should be slightly less than the gain caused by the water going up the spar because if the amount of collapse were more than the gain the buoy would start sinking causing more compression and further loss of buoyancy with the buoy ultimately sinking.

STATEMENT OF THE OBJECTS BRIEF DESCRIPTION OF THE DRAWING The drawing illustrates a spar buoy with a collapsible bag therein for stabilization of the buoy during wave action.

DESCRIPTION OF THE SYSTEM Now referring to the drawing there is shown by illustration a spar buoy 10 including therein a collapsible bag 11 which stabilizes the buoy. The spar buoy includes an upper pipe spar l2 and a lower pipe 13 between which is located an enlarged body section 17. The upper and lower ends 14 and 15 of the body are somewhat conical shaped including therein a filling of foam 16 which is not effected by water. The cylindrical hollow body section 17 is located between the conical main body section. If the collapsible bag is included within its own housing, then the bag housing will require openings or apertures therein through which the surrounding water will enter. The buoy may be provided with appropriate eyes or hoops 23 for handling purposes.

The size of the main body of the buoy, the amount of foam filling and the size of and amount of air in the collapsible bag will depend upon the pay load to be carried by the buoy. The amount of air pressure in the collapsible bag is a variable which may control the height of the exposed end of the spar buoy.

In carrying out the invention, the collapsible bag is placed in its own container or within the enlarge body portion of the buoy and pressurized to a predetermined pressure. The buoy is placed into the water in its desired location and will obtain its position depending on the pay load, the amount of foam filler and the amount of pressure in the collapsible bag. When the buoy reaches its level of buoyancy, the surrounding water at the level of the compressed bag will compress the collapsible bag until the pressure on the inside of the bag is at the same pressure as the pressure of the surrounding water. When a wave passes over the spar, the buoy has a tendency to go up due to its buoyancy because of the added portion of the buoy that is submerged. Simultaneously with more of the spar being submerged, the water depth becomes greater causing a greater water pressure surrounding the collapsible bag thereby causing the collapsible bag to collapse further bringing about an equalization of the internal and external pressure on the bag. The amount of collapse will almost equal the amount of water displaced by the wave going up the spar, therefore, the buoy will remain vertically stable. If the wave goes down, the external pressure is less about the collapsible bag thereby permitting the bag to expand to equalize the internal and external pressures. The amount of expansion is almost equal to the displacement of water going down the spar, therefore, the buoy will remain vertically stable.

It is realized that the subsurface pressure variations due to surface waves is not constant for a given water depth, that the ratio of change in pressure to wave height is dependent on the wave period. The ratio is smaller for shorter wavelengths, therefore, the collapsible bag system should be made to negate the forcing function ofa particular type of sea, being allowed to be less effective for small amplitude waves.

In order to extend the use of this invention for stabilization of a large platform, several spar buoys may be positioned in parallel to each other and secured together with the platform secured to each of the buoys.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

l. A system for stabilizing the vertical movement of a buoy on a body of water, which comprises,

an upper and lower elongated section,

a body section secured between said upper and lower sections a portion of said upper elongated section normally supported above the surface of said body of water,

said body section including upper and lower end sections secured to said upper and lower sections and a main body section secured between said upper and lower end body sections and positioned on the center of gravity of said buoy,

apertures in said main body section to permit surrounding water to flow into and out of said main body section,

a closed pressurized collapsible container for buoyantly supporting said buoy within said main body section with the center of said collapsible container located at the center of gravity of said buoy,

whereby surrounding water applies a pressure on said collapsible container to equalize the collapsible container pressure to that of the surrounding water pressure.

2. A system as claimed in claim 1, wherein;

said end sections of said body section are of conical shape and said conical shaped end sections are fitted with a buoyant material.

3. A system as claimed in claim 1; wherein,

said collapsible container is confined within a separate non-collapsible housing which is placed within a separate non-collapsible housing which is placed within said main body section and which exposes said collapsible container to the surrounding pressure.

Claims (3)

1. A system for stabilizing the vertical movement of a buoy on a body of water, which comprises, an upper and lower elongated section, a body section secured between said upper and lower sections a portion of said upper elongated section normally supported above the surface of said body of water, said body section including upper and lower end sections secured to said upper and lower sections and a main body section secured between said upper and lower end body sections and positioned on the center of gravity of said buoy, apertures in said main body section to permit surrounding water to flow into and out of said main body section, a closed pressurized collapsible container for buoyantly supporting said buoy within said main body section with the center of said collapsible container located at the center of gravity of said buoy, whereby surrounding water applies a pressure on said collapsible container to equalize the collapsible container pressure to that of the surrounding water pressure.

2. A system as claimed in claim 1, wherein; said end sections of said body section are of conical shape and said conical shaped end sections are fitted with a buoyant material.

3. A system as claimed in claim 1; wherein, said collapsible container is confined within a separate non-collapsible housing which is placed within a separate non-collapsible housing which is placed within said main body section and which exposes said collapsible container to the surrounding pressure.

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