US3805725A

US3805725A - Method of and apparatus for dampening wave induced oscillations of floating vessels

Info

Publication number: US3805725A
Application number: US00865392A
Authority: US
Inventors: F Pease; J Sisk
Original assignee: Offshore Co
Current assignee: SONAT OFFSHORE DRILLING Inc
Priority date: 1969-10-10
Filing date: 1969-10-10
Publication date: 1974-04-23
Anticipated expiration: 1991-04-23

Abstract

A method of and apparatus for utilizing a portion of the waves to dampen wave induced oscillations of a floating vessel by trapping the portion of the wave above a dampening structure secured to the vessel to thereby create forces which are opposed to the buoyant forces of the waves on the vessel.

Description

United States Patent 1191 Inventors: Floyd T. Pease, Houston; James M.

Sisk, Liberty, both of Tex.

Assignee: The Offshore Company, Houston,

Tex.

Filed: Oct. 10, 1969 Appl. N0.: 865,392

U.S. Cl. 114/126, 114/5 D Int. Cl B63b 39/06, B63b 35/00 Field of Search 114/.5, .5 D, 121, 125,

[111 3,805,725 Pease et 'al. Apr. 23, 1974 [5 METHOD OF AND APPARATUS FOR 2,448,075 8/1948 Bortner 114/126 x D M N WAVE INDUCED 3,004,510 10/1961 Nawara...

3,224,401 12/1965 K0bus..... OSCILLATIONS 0F FLOATING VESSELS 3,366,087 1/1968 Hilliard 114/125 Primary ExaminerTrygve M. Blix "newer A method of and apparatus for utilizing a portion of the waves to dampen wave induced oscillations of a floating vessel by trapping the portion of the wave above a dampening structure secured to the vessel to thereby create forces which are opposed to the buoyant forces of the waves on the vessel.

3 Claims, 6 Drawing F igur es f PATENTEDAFRZS m 3.805725 i sum 10F 2 I FLOYD 7.7 PEAsE JAMES M. I $15K v INVENTORS.

BACKGROUND OF THE INVENTION Floating vessels such as drilling platforms are sometimes subject to substantial movements responsive to waves or swells because they strike the vessel at or near its natural frequency. Normally small waves or swells if they strike a very large vessel do not have any substantial effect on such vessels but if they strike such vessel at particular frequencies (near the natural frequency of the vessel) they induce very substantial movements of the vessel. With floating vessels such as drilling platforms, crane barges and pipe laying barges, it is important that such vessels should not be exposed to excessive movements responsive to the water movement during critical phases of operations of such vessel. For example, when the legs of a drilling platform are being lowered into the water to jack the platform up, excessive movement of the platform could cause the legs to strike bottom and cause severe damage to such legs. Any excessive motion of crane barges, pipe laying barges when they are lifting substantial loads from outboard is very undesirable.

Extended keels, flume stabilization systems and deep water fins have been used in attempts to dampen wave induced oscillations of floating vessels. Such devices have very limited effectiveness in dampening those oscillations for vessels of the types named above.

SUMMARY The present invention relates to an improved method of and apparatus for dampening the wave induced motion of a floating vessel.

An object of the present invention is to provide an improved method of dampening the oscillatory motions of a floating vessel which are at or near the natural frequency of the vessel.

Another object is to provide an improved method of dampening the wave induced motion of a vessel by utilizing portions of the waves to limit the forces causing such motion.

A further object is to provide an improved dampening structure for a floating vessel to utilize a portion of each wave tending to cause motion of such vessel to thereby limit the wave induced motion of the vessel.

Still another object is to provide an improved dampening structure for a floating vessel to temporarily trap a portion of each wave and produce a downward force against the upward buoyant force of the wave crests without adding to the downward force responsive to the passing of the wave troughs.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the pres ent invention are hereinafter set forth and explained in relation to the improved apparatus of the present invention shown in the drawings wherein:

FIG. 1 is a schematic perspective view of a drilling barge having the improved dampening device of the present invention installed thereon.

FIG. 2 is a partial perspective view of the drilling barge shown in FIG. 1 with the improved dampening device in its retracted position.

FIG. 3 is a schematic perspective of the drilling barge shown in FIGS. 1 and 2 and illustrates the wave patterns and the forces responsive to such waves on the barge and the improved dampening apparatus.

FIG. 4 is a partial schematic perspective view of a drilling barge having a modified formof dampening device of the present invention installed thereon.

FIG. 5 is an elevation view of the bow of a drilling barge having another form of the improved dampening device of the present invention.

FIG. 6 is a sectional view taken along line 6-6 in FIG. 5 to show the details of the relationship of the dampening device to the bow of the drilling barge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drilling barge 10 shown in FIGS. 1, 2 and 3 is generally rectangular in plan shape and has four legs 12 which normally would be lowered into engagement with the ocean floor to lift the barge above the water level for drilling. The stem of the barge 10 defines the recess 14 in which drilling operations are conducted.

The preferred dampening device of the present invention is positioned in the recess 14 and includes the

plates

16 and 18 which are pivotally mounted to the sides of the recess 14 and in their operation position extend generally horizontally to define a temporary water trap in recess 14 as hereinafter explained.

Plates

16 and 18 are provided with suitable framing 19 to support the forces developed thereon.

Suitable braces

20 and 22 are provided to retain the

plates

16 and 18 in their preselected operating position. The:

plates

16 and 18, being pivotally mounted, can be folded upward against the side walls of the recess 14 so that they do not interfere with drilling operations once the barge 10 has been converted to a drilling platform by being jacked above the water. Also, the

braces

20 and 22 are mounted to barge 10 to hold the

plates

16 and 18 in intermediate positions and thereby vary the dampening forces which it develops. Movement of the

plates

16 and 18 is provided by suitable jacking devices such as hydraulic or mechanical jacks. Such jacks may form a part of the end of

braces

20 and 22 within the barge 10 to move the

plates

16 and 18 to their preselected position.

The

plates

16 and 18 of the dampening device are secured to the vessel below the normal water line of the barge 10 but at a position above the lower level of the wave action. To perform the method of the present in vention, the barge 10 is oriented to face the stern recess 14 toward the waves or swells as shown in FIG. 3 so that as the crests of the waves engage the stern of the barge 10 to increase the upward buoyant forces represented by the

arrows

24 and 26, the crests also enter the recess 14. The

plates

16 and 18 with the cooperation of the walls of recess 14 trap the crests of the waves therein to provide the downward forces represented by the

arrows

28 and 30 on the

plates

16 and 18. With the stern facing the waves, the downward

dampening forces

28 and 30 are in substantial alignment with the

buoyant forces

24 and 26 so that the lift of the stem is minimized.

The

plates

16 and 18 are provided with suitable perforations so that the trapped water is dumped therefrom as the wave crest passes to avoid excess downward forces on the

plates

16 and 18 as the trough of the waves pass thereunder. The

plates

16 and 18 may be suitably perforated or take the form of a grid having sufficient structure to provide the desired downward forces while allowing the trapped wave crests to rapidly drain therefrom as the crest passes and the trough reaches the barge 10.

The modified form of the apparatus of the present invention shown in FIG. 4 provides a pair of

reaction plates

32 and 34 secured to the stern of a barge 36 similar to the barge on either side of the drilling recess 38. The side plates 40 are secured to the barge 36 in position extending out to the rear alongside the

reaction plates

32 and 34 to assist in the trapping of the crests of waves as they approach the stern of the barge 36. The

reaction plates

32 and 34 are preferably pivotally mounted to the barge 36 so that they may be folded upwardly against the stern when they are not in use. Also, it is preferred that some provision be made to be able to position the

plates

32 and 34 at positions intermediate their horizontal and vertical positions. In such intermediate positions they can still be used to trap the wave crests and control the oscillations of the barge 36 but they are effective only with respect to their projected area on a horizontal plane. Thus, if they are positioned at a 45 angle they will have a projected area which is 0.707 times their area and therefore will produce the downward forces which are only 0.707 times the forces which would be produced if they were in their horizontal positions. The addition of the side plates 40 may be omitted but the effectiveness of the

plates

32 and 34 will be reduced because less of the crest of the waves will be trapped above the

plates

32 and 34.

In some vessels, it may be desirable that the stern of the vessel not have to face into the waves or swells. For example with a driling barge, some operators prefer to have the stern of the barge face into the prevailing winds so that the wind does not blow past the drilling operations in the stern recess and then over the barge. In such situations, another modified form of the invention shown inFlGS. 5 and 6 may be used. In this form of the invention, the dampening device is positioned at the bow of the vessel or drilling barge 42. The bow of the barge 42 has an upwardly tapered surface 44 between the wells 46 which house the legs 48. The plate 50 is pivotally mounted below the water line on barge 42 to the surface 47 as best shown in FIG. 6. The outer end of plate 50 is held in position by the braces 52. The plate is pivotally connected to the barge 42 by the arms 54 and the pin 56 which is secured to the barge 42. When not in use, the plate 50 is moved into a position against the bow surface 44 as shown in dashed lines in FIG. 6. The arms 54 space the rear end of the plate 50 from the connection to the bow surface 44 to allow a quicker dumping of the water from the plate 50 when the wave crest passes. Further, the plate has a plurality of holes 58 extending therethrough to further assist in the dumping.

The improved dampening device illustrated in FIGS. 5 and 6 functions to trap the crest of the waves between the two wells 46 to create a downward force on the plate 50 when the wave crests create additional buoyant force on the bow of the barge 42 to thereby prevent the barge 42 from excessive oscillations responsive to the waves.

The improved dampening of the present invention provides a substantial reduction in motion for a small part of the cost and weight of other systems. On a vessel of 8,500 ton displacement, the improved dampening of the present invention greatly reduced vessel motion and the dampener weighed only tons. This is a relatively small amount of weight ascompared to a flume stabilization system. For example, in a floating vessel having a displacement of 1 1,400 tons, its flume stabilization system requires 2,400 tons of water which reduces the vessels allowable variable loads.

All of the improved dampening structures of the present invention, if oriented toward the waves or swells, trap water to increase downward forces responsive to the passing of the wave crests just s the buoyant forces are increased by the passing of the wave crests. By trapping this extra weight of water, the plates function as something other than a horizontal fin or extended keel. The plates when properly positioned provide improved dampening and prevent excessive movement of the floating vessel on which the plates are installed even when the waves engage the vessel at its natural frequency. The plates are designed to readily dump the trapped water as soon "as the crests pass to avoid the added downward force on the plates as the trough passes. Further, the plates can be mounted on the bow, the stern, the sides or in a recess of the floating vessel. By making the plate position adjustable, the effectiveness of the plate can be varied. With the variable effectiveness of the plate, the effective natural period of the vessel can be changed to prevent excess movement under all normal conditions.

What is claimed is:

l. A floating vessel, comprising a loat iid l nsi r e a n QMEEEWS PQYYEP! face between two leg wells, 21 plate extending outwardly from said barge in a direction toward the approaching waves,

said plate being pivotally mounted to extend forwardly from said inclined bow surface below the normal water line thereof.

2. A floating vessel, comprising a buoyant structure,

a plate secured to said buoyant structure below the normal waterline and being unobstructed to waves approaching said structure,

said plate being adjustably positioned to extend from a horizontal position to an upwardly extending position substantially parallel to the wall of said buoyant structure to which it is mounted, and

means defining two sides to said plate to at least partially trap the top portions of waves reaching said structure above said plate creating a force resisting the buoyant force on said structure to dampen the movement of said buoyant structure responsive to the waves.

3. A floating vessel, comprising a buoyant structure,

a plate secured to said buoyant structure below the normal waterline and being unobstructed to waves approaching said structure, and

means defining two sides to said plate to at least partially trap the top portions of waves reaching said structure above said plate creating a force resisting the buoyant force on said structure to dampen the movement of said buoyant structure responsive to the waves,

said vessel having an inclined flat bow service extending transversely of the longitudinal axis of said vessel,

said plate is pivotally mounted to extend forwardly from said bow surface.

Claims

1. A floating vessel, comprising a floating drilling barge having an inclined bow surface between two leg wells, direction a plate extending outwardly from said barge in a director toward the approaching waves, said plate being pivotally mounted to extend forwardly from said inclined bow surface below the normal water line thereof.

2. A floating vessel, comprising a buoyant structure, a plate secured to said buoyant structure below the normal waterline and being unobstructed to waves approaching said structure, said plate being adjustably positioned to extend from a horizontal position to an upwardly extending position substantially parallel to the wall of said buoyant structure to which it is mounted, and means defining two sides to said plate to at least partially trap the top portions of waves reaching said structure above said plate creating a force resisting the buoyant force on said structure to dampen the movement of said buoyant structure responsive to the waves.

3. A floating vessel, comprising a buoyant structure, a plate secured to said buoyant structure below the normal waterline and being unobstructed to waves approaching said structure, and means defining two sides to said plate to at least partially trap the top portions of waves reaching said structure above said plate creating a force resisting the buoyant force on said structure to dampen the movement of said buoyant structure responsive to the waves, said vessel having an inclined flat bow service extending transversely of the longitudinal axis of said vessel, said plate is pivotally mounted to extend forwardly from said bow surface.