US3407766A

US3407766A - Stabilized floating structure

Info

Publication number: US3407766A
Application number: US581291A
Authority: US
Inventors: Gunnar B Bergman; Ross A Mcclintock
Original assignee: PIKE CORP OF AMERICA
Current assignee: PIKE CORP OF AMERICA
Priority date: 1966-09-22
Filing date: 1966-09-22
Publication date: 1968-10-29
Anticipated expiration: 1985-10-29
Also published as: GB1188849A; SE349994B; ES345292A1; JPS5020357B1; NO128811B; GR35523B; NL6713012A; DE1531648A1

Description

Oct. 29, 1968 c. B. BERGMAN ET AL 3,407,766

STABILIZED FLOATING STRUCTURE I Filed Spt. 22, 1966 4 Sheets-Sheet 1 17 JeossA. M an/mag,

INVENTORS.

Oct. 29, 1968 G. B. BERGMAN ET AL 3,407,766

STABILIZED FLOATING STRUCTURE Filed Sept. 22, 1966 4 Sheets-Sheet 2 Ha ripnl'al Gav/mi? floss A. M 62. {ma-w,

Oct. 29, 1968 G N ET AL 3,407,766

I STABILIZED FLOATING STRUCTURE Filed Sept. 22, 1966 4 Sheets-Sheet 5 A fixaa.

- J ll/@1 4.

Ivrgymks Gum/.41? ,8- amt-M4446 Ross A. M a/ymcw,

.5 7 751 flrmeueysf Oct. 29, 1968 BERGMAN ET AL STABILIZED FLOATING STRUCTURE 4 Sheets-Sheet 4 Filed Sam. 22, 1966 115 Gum/A? R RE/PQMAM INVENTORS.

United States Patent STABILIZED FLOATING STRUCTURE Gunnar B. Bergman, Pasadena, and Ross A. McCliutock,

Huntington Harbour, Califl, assignors to Pike Corporation of America, Los Angeles, Calif., a corporation of California Filed Sept. 22, 1966, Ser. No. 581,291 21 Claims. (Cl. 114--.5)

ABSTRACT OF THE DISCLOSURE A sub sea drilling vessel stabilized against rolling and pitching movements induced by wave action by a stabilizer body positioned a substantial distance beneath the drilling vessel below the severe wave and surface action of the water. The stabilizer body is so constructedand arranged as to be freely movable in a vertical direction and to resist movement in the horizontal position transverse to the vertical direction. The stabilizing body is shown in various embodiments as an open ended cylinder or as parallel plates which act to entrap a mass of water to resist forces tending to move the body horizontally. The stabilizing body is connected to the drilling vessel such that a force moment is transmitted to the drilling vessel in a direction oppositely to that imparted by wave action to offset the rolling or pitching movement of the vesel.

This invention relates to the stabilization of floating vessels and structures.

Exploration and exploitation of oil and other mineral resources on and below the ocean floor depend upon the stability of floating vessels and structures under a variety of wave and tide conditions to conduct operations. Other operations at sea require a stable platform or a platform of predictable and controllable movement characteristics. Although not limited thereto the present invention is particularly applicable to off-shore drilling operations for oil and will accordingly be described in connection with such operations. It will be seen, however, that the present invention is also applicable to the stabilization of other floating structures including such as, floating observation platforms, dry docks, temporary loading platforms, and buoys.

In the present state of the art drilling beneath a body of water is accomplished from floating vessels, semi-submersible platforms and fixed platforms. Fixed platforms are supported from the ocean floor by legs which position the drilling platform above the wave action of the water surface. Such platforms are, of course, severely limited in the depth of water inwhich they can be utilized. Semisubmersibles and drilling vessels are capable of working in waters of any depth. Semi-submersibles, as well known, rely upon buoyancy chambers positioned beneath the wave action to support the drilling platform above the wave action. Such semi-submersibles are of many designs, extremely expensive and subject to rough weather problems, tidal actions, and other force factors since they are substantially fixed in location when set up for operation.

Drilling vessels in which the drilling operation is carried on from a floating vessel are well known and are shown, for example, in US. Patent No. 3,177,954. These vessels offer many advantages over semi-submersibles, one advantage being that of mobility since they can move to and from any location quickly. Additionally, the cost of such vessels while presently considerably less than semisubmersible platforms can be further reduced if widths can be reduced while maintaining or increasing stability. A primary disadvantage of such drilling vessels is the relative instability of the deck of the vessel, which deck having 3,407,766 Patented Oct. 29, 1968 ice the drilling derrick and equipment positioned thereon, constitutes the drilling platform. That is, since the drilling vessel is freely floating it is subject to wave action. This is true even though the vessel is firmly anchored in position since anchor cables must necessarily include catenaries suflicient to allow wave action on the vessel including heave, pitch, sway, roll, yaw and surge. When the roll or pitch of the vessel becomes excessive drilling operations become difficult. The adverse effects of roll of the vessel are magnified at the crown block of the drilling derrick. Drilling equipment, pipe string or other drilling tools suspended in the derrick will swing in response to vessel roll and since such equipment is exceedingly heavy, property damage and personal danger can result.

As employed in this specification roll refers to rotational movement about the longitudinal axis of the buoyant body (i.e., vessel); pitch refers to the rotational movement about the transverse axis of the buoyant body; sway refers to horizontal movement of the buoyant body; and heave refers to vertical movement of the buoyant body.

It can be seen that the optimum roll may vary dependent upon the use of the floating structure. That is, the definition of optimum rolling conditions depends upon the characteristics of the structure and the operation to be achieved therefrom. For many operations conducted from a floating structure such as visual tracking platforms, radar platforms, missile tracking stations and the like, no rolling motion would be desirable. In certain subsea operations such as drilling operation, stresses in drill string and riser pipe. can be minimized if the ship leans slightly into the wave or has a negative roll factor as discussed more fully hereinafter. A still different definition of optimum roll for a vessel or structure is obtained if the working conditions of the crew in handling objects are to be optimized. Under such criteria it is desirable that at all times the resulting forces of gravity and inertial forces acting upon crew members and objects they are handling are perpendicular to the deck upon which they are working. In accordance with the present invention such optimization can be achieved for specific purposes as described more fully hereinafter.

In connection with the application of the present invention to floating drilling vessels, vertical movement or heave of the ship in response to wave or tidal action causes little or no difiiculty since drilling apparatus is the ship. Additionally, since most vessels have a mean that is small compared to frequently occurring wave length the rolling motion of the ship combined with the sway is the movement of most concern and pitch in response to Wave action is normally tolerable.

It is a primary object of the present invention to provide means for stabilizing a floating structure to reduce to predetermined limits the amount by which the decks or normally horizontal components of the structure are inclined from the horizontal plane in response to wave action.

It is another object of the present invention to provide apparatus and means in combination with a floating structure to minimize the pitch and roll of such a structure in response to wave action.

Another object of the present invention is to provide apparatus in combination with a floating vessel which allows the vessel to 'sway and heave in response to wave action but controls the roll of the vessel to predetermined tolerable limits.

It is another object of the present invention to increase the stability of a floating structure or vessel which serves as-a platform from which undersea operations are performed. a

The present invention comprises in general terms a floating structure from which is suspended to a depth below the most severe wave and surface action of the water an underwater body so constructed and arranged that it effectively traps a large quantity of water to resist forces tending to move the body horizontally but does not trap water when moved in the vertical direction. Such a mass trap is affixed to the floating structure in such manner as to impart a force moment to the structure oppositely to that imparted by wave action. The underwater body can be fixed or retractable.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which a presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

In the drawings:

FIGURE 1 is a view in elevation of an illustrative embodiment of the present invention;

FIGURE 2 is a cross-sectional view taken along line 22 of FIGURE 1;

FIGURE 3 is an end view of the vessel of FIGURE 1 reduced in scale and partially schematic;

FIGURE 4 is a view in elevation of an alternative embodiment of the present invention utilizing a single underwater stabilizer in connection with a buoy and rigidly connected thereto;

FIGURE 5 is a view similar to FIGURE 4 showing an alternative embodiment in which the underwater stabilizer is elastically connected to the buoy;

FIGURE 6 is a schematic wave amplitude diagram;

FIGURE 7 is a wave and vessel position diagram illustrating the operation of the present invention;

FIGURE 8 is an illustrative view of a drilling vessel utilizing two underwater bodies in accordance with the present invention elastically connected to the vessel;

FIGURE 9 is a sectional view taken along line 9-9 of FIGURE 8;

FIGURE 10 is an end view of the embodiment of FIG- URE 8;

FIGURE 11 shows illustrative graphical solutions of angle of roll at various predetermined spring constants of the elastic coupling to the stabilizer body;

FIGURE 12 is a sectional view of a third alternative embodiment of the underwater body;

FIGURE 13 is a sectional view of a fourth alternative embodiment of the underwater body;

FIGURE 14 is a view in elevation of an embodiment with the underwater body positioned non-vertically; and,

FIGURE 15 is a schematic diagram showing certain physical relationships and factors.

The present invention comprises a passive means for stabilization of a floating body. The roll stabilization of floating bodies has been a problem for which many solutions have been proposed. Such stabilizers are of the active or passive type. Among the passive type are bilge keels and resonant water tank systems, which are advantageous for some uses but are not sufficiently effective for stabilization of the types of structures described herein especially in a confused sea. The active systems require shipboard power for their operation and to be effective become very expensive and complex. Active stabilizers are primarily used on moving vessels such as passenger ships. The device of the present invention is of the passive type and is primarily adapted for vessels or structures at anchor or moving slowly.

Referring now to FIGURES 1, 2 and 3 there is shown schematically a buoyant body such as a vessel including apparatus of the present invention. The vessel designated as 10 is shown as a typical offshore drilling vessel having a center well through which drilling operations are conducted with a drilling derrick mounted thereabove. Drilling operations are carried out in a Well known manner from the operating deck 12 and in the figure the drilling vessel is shown with a riser pipe 14 connected from the vessel to the ocean floor. The method and apparatus for drilling from the vessel are well known to the art and form no part of the present invention. It should be noted, however, that such apparatus typically includes a base plate 15 anchored to the ocean floor 16 with latching means 17 for connecting and disconnecting the riser pipe to the well head. The riser pipe, sometimes termed conductor tube, is provided with a deflection joint 18 and slip joint (not shown) to allow horizontal and vertical movement respectively of the vessel relative to the well head or base plate. Mud circulation means, packer means, blowout preventers and vessel anchoring means of the well-known type are also provided.

In accordance with the present invention, suspended beneath the vessel are forward and aft stabilizing bodies 20 each of which is mounted to the hull of the vessel by suitable force transmitting members, as described more fully hereinafter. In the embodiment shown, each of the stabilizing bodies comprises an open-ended cylinder. The open-ended cylindrical body is vertically oriented and symmetrical with respect to the centerline of the vessel. The cylinders are each comparable in construction and are affixed in similar manner to the vessel. Accordingly, each is not described separately in detail hereinafter. The cylindrical body 20 is elongate and is affixed to the hull by means of

struts

21 and 22. The struts are structural members of suitable strength such as steel I-beams and are alfixed to the cylinder in any suitable structural manner as, for example, by welding to the cylinder wall at opposite points of the transverse diameter thereof or by forming a spider 23 in the interior of the cylinder and affixing the struts to the cylinder wa'll proximate to opposite ends of one of the spider members 23 as shown in FIGURE 2. The spider is such as to leave the interior of the cylinder substantially open to the unimpeded passage of water therethrough. The

struts

21 and 22 of each cylinder 20 then extend to the hull of the ship where they are aflixed at opposite sides of the hull. As shown in FIGURE 3 the strut 21 is afiixed to the starboard side of the hull and the strut 22 is afiixed to the port side. The

struts

21 and 22 are afiixed to the hull in such a manner as to impart a force moment about the longitudinal centerline or roll axis of the ship, hereinafter referred to as the longitudinal centerline, as a roll stabilizing force. For that reason it is preferable that the struts be affixed to a frame member or other point of structural strength of the hull and the struts themselves are rigid members of sufiicient strength to transmit such forces. In the embodiment shown in FIGURES 1, 2 and 3 the struts are pivotally affixed to the hull as shown at 25 and 26 for pivotal movement longitudinally with respect to the hull. For each of maintenance and repair it is preferable that the struts be aflixed to the hull above the waterline and preferably at the deck line or as near thereto as structurally feasible. This pivotal connection, when stabilization of the rolling motion of the platform or vessel is the primary stabilization to be accomplished, makes possible the retraction of the underwater bodies during towing or in shallow water. Suitable retraction means, such as cables (not shown) are employed to swing the underwater bodies upward to a nonoperative position. In addition. in the embodiment of FIGURES 1, 2 and 3 stabilization against pitch or com bined pitch and roll can also be achieved by utilizing such cables of sufficient strength to retain the underwater bodies,

in a fixed horizontal position. That is, as will become more apparent in connection with the embodiment of FIGURE 4, the stabilizing bodies can be utilized to control pitch as well as roll or the combination thereof by fixing the stabilizing bodies longitudinally relative to the vessel. Cables, or other suitable structural means can be employed for this purpose. If cables are employed the longitudinal position of the stabilizing bodies can be -fixed by extending and tensioning cables in opposite longitudinal directions from each stabilizing body to a structurally suitable affixing point on the hull of the vessel.

In the illustrative embodiment as shown in FIGURES 1, 2 and 3, the height and diameter of the cylinder provide a substantial area in the vertical plane but the thickness of the cylinder wall is such that little or no substantial area is presented in the horizontal plane, as shown in FIGURE 2. From the foregoing it can be seen that the volume within the cylinder is freely accessible to, and filled with, water to define a captive mass in the horizontal direction transverse to the centerline of the ship as discussed more fully hereinafter. There is in addition, the effective mass of the surrounding water adding up to a considerable total inertial mass. The cylindrical body is thus freely movable in the vertical direction while being substantially immovable in the horizontal direction.

As discussed hereinbefore, the cylindrical underwater mass traps in the embodiment of FIGURES 1, 2 and 3 are each rigidly afiixed to the hull of the vessel by means of

struts

21 and 22 at points 25 and 26 longitudinally spaced on the hull. Ea'ch cylindrical mass trap is open ended and is vertically oriented and symmetrical with respect to the centerline of the vessel. The cylinder is at a depth below the severe wave and surface water effects and is so oriented that its axis lies vertically in the vertical plane of the centerline of the hull. In the illustrative embodiment shown, wherein two such underwater bodies are used for illustration they would also preferably be symmetrically oriented with respect to the length as well as the beam of the vessel. In a typical'embodiment wherein a drilling vessel of 10,000 tons is employed, a cylinder 25 feet in diameter, 25 feet in height and affixed at a depth 200 feet below the waterline would typically be utilized. When such a cylinder is suspended beneath the vessel it can be seen that it will offer little or no resistance to vertical movement. (The weight of the cylinder itself is treated as being negligible.) However, since it is open ended and surrounded with water it contains within its walls a cylindrical volume of water 25 feet in diameter. The weight of such a mass of water, assuming the weight of the cylinder to be relatively negligible would be approximately 350 tons. If a force is exerted horizontally on the cylinder this mass is effectively trapped and the force required to accelerate it horizontally would be a function of the mass of the captive water plus the elfective mass of the surrounding water. There is in addition, some drag force due to the horizontal projection of the cylinder alone. Since the mass of water is entrapped during any horizontal movement the resistance of the cylinder to horizontal forces is due primarily to its inertial mass and drag forces can be neglected.

. In order to more clearly describe the operation of the present invention, there is shown in FIGURE 4 a simplified buoyant structure illustrative of a buoy or similar platform 30 having a deck 32 which it is desired to stabilize for any of many reasons, typically because of equipment to be positioned or operated thereon. In the embodiment shown in the buoyant structure is a closed hollow cylinder or cylinder of such material as to be buoyant. Positioned beneath the platform is an underwater stabilizing body 20 in accordance with the present invention and comparable to the stabilizing body of the embodiment of FIGURES l, 2 and 3. The underwater body is again an open-ended cylinder having a sub stantial vertical cross-section, i.e., height and diameter, and an inconsequential area of horizontal cross-section, i.e., wall thickness. In the embodiment of FIGURE 4 the underwater stabilizer body 20 is affixed to the buoyant body hereinafter termed buoy 30 by a single rigid member 33 in the form of an elongate column, such as an I- beam which is affixed to the vertical centerline of the buoy and to the underwater body 20 by suitable structural means such as a spider 34. The connection between the buoy and stabilizer body 20 is thus such that any force exerted by the underwater body is transmitted to the buoy. Since the embodiment of FIGURE 4 is symmetrical about any scross-section taken vertically along the diameter of the buoy and underwater body the connection is comparable to that of afiixing the underwater body by means of rigid struts to any opposed points on a given diameter through the buoy as shown by the dotted lines 21-22 in FIGURE 4, and the longitudinal axis of the buoy would be that horizontal axis perpendicular to the indicated diameter. Thus, in the buoy embodiment of FIG- URE 4 the dotted lines are comparable to

struts

21 and 22 of FIGURES 1, 2 and 3. In FIGURE 6 there is shown the form of a wave and FIGURE 7 schematically orients relative to the wave the buoyant body, such as a vessel, having an underwater body rigidly affixed as in the embodiment of FIGURES 1, 2 and 3 or a buoy as in FIGURE 4.

As discussed hereinbefore, among the six different ways in which a vessel responds to wave action, i.e., heaving, surging, swaying, pitching, yawing and rolling, rolling is the most objectionable movement particularly with respect to vessels which are performing subsea drilling and similar operations. Various of the motions which, intrinsic to buoyant platforms, are impractical to eliminate. Particularly, heaving and swaying are inherent in any buoyant floating structure since the: structure, such as a vessel, is strongly coupled to the surrounding water surface for these modes of response.

In analyzing the rolling motion of a vessel the roll factor R can be defined as the ratio of maximum roll angle to maximum Wave slope when the vessel is in steady state motion in a small amplitude wave traveling in a direction perpendicular to the longitudinal axis of the vessel. To account for the fact that the rolling motion of the vessel, or other platform, is not necesasrily in phase with the wave motion, a phase angle cc is introduced. The vessels roll characteristics are thus given by the two quantities R and or both of which are functions of the wave period, Optimum roll characteristics, as dis cussed hereinbefore, are dependent upon the structure being stabilized and the type of operations being carried out thereon.

The action characteristics of a buoyant structure including a vessel without stabilization are such that a given buoyant structure has a characteristic roll resonance which is related to the metacentric height and the effective moment of inertia about the longtiudinal axis. The period of resonance may be in the range of 5-10 seconds. When a ship is in a wave of a period much longer than the resonance period, R is close to unity and u is close to zero. The decks are in order words at all times very nearly parallel to the tangent plane to the wave at the location of the ship. But as the wave period becomes shorter (shorter than the resonance period), R gradually decreases and sooner or later becomes less than unity. Then, however, on approaches which means that the vessel leans into the wave. It may be concluded that, in general, the natural response of the vessel is never close to optimal under any of the roll criteria hereinbefore discussed.

In connection with FIGURES 6 and 7 the buoyant body is described as a vessel, it being understood that the discussion is applicable to a drilling vessel, such as shown in FIGURES l, 2 and 3, or any platform shown as a buoy in FIGURE 4. The discussion of wave motion and induced roll will be applicable to the motion and forces occurring about the longitudinal axis of the vessel of FIGURES 1, 2 and 3 but will be applicable also to the motion and forces occurring about any horizontal axis of the buoy of FIGURE 4. As is well known, the vessel is caused to heave and sway by wave action due to its coupling to the water surface. That is, the vertical position of the vessel is caused to change by the wave action as the water surface rises and falls relative to a position fixed with respect to the floor beneath the water. Fixed position is hereinafter referred to as that position extending vertically upward from a fixed point on the floor beneath the body of water. Accordingly, in addition to heave (vertical motion) the vessel is caused to move horizontally (i.e., sway) with respect to the fixed position by wave action. The swaying movement, as is well known, is caused by the movement of water particles in a wave. Thus, referring to FIGURES 6 and 7 the fixed position is indicated by the vertical line 35 and is understood to be horizontally fixed with respect to the floor beneath the water. The fixed position 35 is transposed to indicate different positions relative to the wave, it being understood, however, that the fixed position line 35 is constant in horizontal position and it is the wave that is moving. The method of showing wave movement is well known and is employed here for ease of illustration. Thus, in FIGURES 6 and 7 the wave is illustrated as moving from left to right and the line 35 or fixed position line is fixed although shown transposed to four locations relative to the wave. In a wave moving from left to right the movement of water particles in the wave is clockwise and is shown in FIGURE 6 at the crest and trough of the wave as well as at the midpoint on the descending and ascending waves. The water movement is illustrated at the crest as 56, the midpoint of the descending wave as 37, the trough as 38 and the midpoint of the ascending wave as 39. At the crest and trough of the wave 36 and 38 the buoyant body will be at the upper and lower limits of vertical movement (heave) but there will be no movement from the horizontal position (sway). At point 37, i.e., the midpoint of the wave of descending slope, the buoyant body will be at the midpoint of its vertical excursion but will be forced to the left of the fixed position by the distance x in the figures by the clockwise direction of water movement and similarly the right of the fixed position by the distance x at the midpoint 39 of the ascending wave. As discussed above the underwater stabilizing body 20 strongly resists motion in the transverse, or horizontal, direction due to its effective horizontal mass but offers practically no resistance in the vertical direction. The roll factor R can be computed as follows. The Wave form is of the following mathematical form as shown in FIGURES 6 and 7:

where T is the period of the wave, L is the wave length,

and A is the wave amplitude.

The slope is:

dY 27r x t COS The sway X is given by a good approximation as:

:v t X-A cos 21r( (3) The slope of the deck 21 is then:

cos 2 1T1) L T 4 negative R means, however, that the ship leans into the.

wave although with an arbitrarily small angle.

The action of the underwater body in stabilizing the vessel can be further seen by reference to FIGURES 7a, 7b, 7c and 7d wherein the vessel is shown at four different wave positions. In FIGURE 7a the vessel is in the descending slope of the wave. The rolling force upon the vessel is in the clockwise direction about the center of gravity of the vessel. The normal position of the vessel without stabilization would be with the deck 12 sloping downward and to the right in the figure, i.e., in the direction of slope of the wave. The location of the underwater body 20 is horizontally fixed but vertically movable. The height of the underwater body is therefore approximately at its mean depth. It can be seen that if the underwater body was freely movable both horizontally and vertically it would be swung to the left in the FIGURE 7a by the rolling forces or force moment upon the vessel. Since the underwater body is horizontally immovable it creates a force moment through the connecting member 33 of FIGURE 4 or the

struts

21 and 22 of opposite direction to the rolling moment applied by the wave action to the vessel. Thus, the strut to the left in FIGURE 7 termed port strut 22 is in tension and the starboard strut 21 is in compression whereby a counterclockwise force moment is applied to the vessel about its center of gravity to thereby prevent the clockwise roll. In FIGURE 71) at the trough of the wave the underwater stabilizing body is below its mean depth but the deck of the vessel is horizontal and no force moment is applied through the struts. In FIGURE 7c the rolling force due to the wave action imparts a counterclockwise force moment about the center of gravity of the ship as the vessel tries to assume a slope at which the decks are parallel to the slope of the wave. The underwater body 20 again rises to the mean depth but remains in the same horizontal position. It thus resists the forces trying to move it to the right in the figure and thereby imparts a clockwise force moment to the ship to thereby resist the rolling force. Thus, in this position the starboard strut 21 is in tension while the port strut 22 is in compression. In FIGURE 7d at the peak of the wave the underwater body is at its highest point above its mean depth but is again in the same horizontal position with no moment forces applied.

An alternative and presently preferred embodiment of the present invention is shown in FIGURES 8, 9 and 10 in which the underwater stabilizing body 40 is afiixed to the hull of the vessel by means having a predetermined degree of elastic compliance. In the embodiment shown, four

cables

42, 43, 44 and 45 are utilized to suspend each of the two similar underwater stabilizing bodies 40 beneath the vessel 10. The two underwater bodies 40 shown in FIGURES 8, 9 and 10 are identical and are similarly attached to the vessel. Accordingly, only one stabilizing body and the manner of attachment to the vessel will be described in detail. The underwater bodies in this embodiment comprise two or more spaced

parallel plates

45 and 46 which are generally square or somewhat rectangular in configuration. The

plates

45 and 46 extend parallel to the longitudinal centerline of the ship and are symmetrical with the ship centerline in the embodiment shown. Suitable structural members 47 which may include an intermediate plate 48 interconnect the

plates

45 and 46 to impart structural rigidity to the parallel opposed plates. Each of the two stabilizing bodies 40 are affixed to the vessel by the four cables which are attached to the four upper corners of the stabilizing body and extend upwardly to connect the starboard plate 45 to the starboard side of the vessel and the port plate 46 to the port side of the vessel. The cables are affixed to the hull of the vessel in such manner as to transmit a moment arm tothe vessel about the longitudinal centerline of the vessel. The cables are connected to the vessel through means for applying an elastic restoring force thereto whileallowing the length .of the cables to vary within prescribed limits. Thus, in the embodiment shown in FIGURES 8, 9 and 10, the starboard cables 42-43 are extended from the stabilizing body 40 over pulley 49 aflixed to the starboard side of the vessel at spaced points forward. and aft of the vertical axis of the stabilizer body. The pulleys are for the purpose of protecting the hull and allowing movement of the cables with respect theretoand other well known means can also be employed. In the embodiment of FIGURES 8, 9 and 10 a spring-loaded winch is shown as the'elastic restoring force for each cable-The starboard winches are designated as 50 and the port winches as 51. Such winches are of. the type well known to the art and "other similar means for applying an elastic restoring force can also be utilized such as fluid-operated motors, springs and air winches. Such e'lastic restoring force can also include elasticity in the cables themselves such as nylon cables or cables having an inherent spring force. By the use of elastic restoring forces such'as spring loaded winches the elastic restoring force or spring constant can be varied for different operating conditions as described more fully hereinafter.

The parallel plates of the stabilizing body 40 entrap therebetween a volume of water which forms an inertial mass to be moved if the plates are moved in the horizontal plane transverse to the centerline of the ship as discussed hereinbefore. The stabilizing body 40 of the embodiment of FIGURES 8, 9 and 10 'by havingits inertial mass transverse to the longitudinal centerline of the vessel is intended to stabilize the vessel only with respect to the roll component of any wave or surface action exerted upon the vessel. The elastic restoring forces and cables are oriented only to exert a force moment about the longitudinal centerline of the vessel. It should be noted that in most circumstances where drilling vessels are operating a current in one direction will be present and relatively constant. By allowing free movement of water through the mass-trap, or stabilizingbody 40, in both the vertical and longitudinal direction the current forces exerted upon the stabilizing bodies are minimized by proper orientation of the vessel. If the stabilizing body is moved vertically or longitudinally substantially no resistance to movement occurs and no force istrans-mitted to the vessel from the stabilizing bodies. In the embodiment of FIGURES 8, 9 and 10 the underwater body must be of sufiicient weight to maintain the cables in tensionat all times. As compared to theembodiment utilizing rigid struts, the embodiment having cables with means for providing an elastic restoring force provides greater efllciency in maintaining the deck level without overcorrection to cause the vessel to lean into the wave.

By reason of the cable suspension of FIGURES 8, 9 and '10 the depth at which the stabilizing bodies as positioned beneath the vessel can be varied. Also, the stabilizing bodies can be raised toja non-operative position for shallow water operating or towing. The forward stabi- 10 stabilizing body to movement in the horizontal transverse direction. This resistance imparts a force moment through rigid connecting members, as in the embodiment of FIGURES 1, 2, 3 and 4, to the vessel around its longitudinal centerline and opposite to the direction in which the deck is attempting to deviate from the horizontal.

When the structural connections between the stabilizing lizer body is shown in phantom in such a retracted position. From such position cranes or derricks operated from the working deck of the vessel can raise the stabilizer bodies to the deck for extended towing or inoperative periods.

The compliance of the embodiment shown in FIG-

URES

8, 9 and 10 in effect adds one more degreeof freedom to the system. It is this fundamental difference which allows complete removal of the rolling motion. The same fundamental dilference also allows the depth at which the water trap must be located to be greatly diminished. Thus, as a vessel is pulled onto a wave (FIG- URE 7a) the deck would, without the stabilizing apparatus of the present invention, tend to deviate from the horizontal as it tilts with the slope of the wave. However, the deviation from the horizontal does not occur with the present invention because of the resistance of the body and the vessel are rigid the force moment imparted to the vessel sometimes overcompensates for the tilting motion caused by the wave; as a result the vesselleans in to the wave and the deck deviates from the horizontal in a direction opposite the slope of the wave. When, however, an elastic restoring force is provided for the connecting cables or other members, as in the embodiment of FIGURES 8, 9 and 10, the cables vary in length by a predeterminable amount so that a desired degree of rolling motion is achieved. Thus, overcompensation by the stabilizing body is prevented by allowing spring means 30 and 30a to adjust or vary the relative length of cables 34 and 34a (i.e., cable 34 becomes longer and cable 34a becomes shorter or vice versa or only one cable changes in length) at a point in time at which the vessel would otherwise lean into the wave beyond the horizontal plane.

In order to more clearly describe'the operation of the present invention in connection with elastic connections between the underwater stabilizer body and the vessel or other buoyant body, a simple buoy or similar structure 10 is again shown in FIGURE 5. The underwater stabilizer body is again shown as an open-ended cylinder 20 but is connected to the buoyant body by four equally spaced

connectors

62, 63, 64 and 65 which include elastic means such as, springs 66 shown schematically which function as the spring loaded winches 50 and 51 in the embodiment of FIGURES 8, 9 and 10. From the foregoing discussion it can be seen that the connecting

members

62, 63, 64 and 65' comparable to those in the preceeding embodiments can vary in length and the force moment about a given centerline through the buoyant body in which the connectors are placed in tension and compression can be varied or predetermined by the springs. In the construction as shown in FIGURE 5, the underwater stabilizer body and connectors are symmetrical about the vertical centerline through the structure so that a force to roll and pitch or any combination thereof wouldbe opposed by the stabilizing action. With the Wave action in a single direction, i.e., parallel to the plane of FIGURE 5 the op eration of the embodiment of FIGURE 5 is similar to that of the embodiment of FIGURES 8, 9 and 10 in which, because of spaced parallel plates forming the underwater stabilizer body, roll of the vessel is counteracted.

Thus, the analysis made in connection with rigid connections, as in FIGURES 1 through 4, can be continued for elastic connections as in FIGURES 5, 8, 9 and 10 as follows with reference to FIGURE 15:

and a turning moment M is acting on the vessel of magnitude M,,,=KDA cos 21.6

It is again assumed that the sway is given by (3) which is a sufiiciently good approximation. The moment M must now cancel the opposite moment associated with the slope 1 1 of the wave. This moment M is given to a good approximation by v where W is the weight of the vessel and M the me tacentric height. Putting M and M equal leads to Q v 21r I K D WM Gr (11) WM DL 12 For a given wave length L, there is consequently a spring constant K for which there will be no rolling motion. It is an important fact that this is true for all wave periods, including a wave period equal to the resonance period of the vessel without the stabilizer of the present invention.

The case R= is a special case. The general case R0 may be analyzed as follows. The equation of motion for the ship in roll is of the type where I is the effective moment in inertia of the ship in roll e.g., (13) can be rewritten Il3+(k +k ),6=(k Bk C) sin wt (14) As k and K in (6) are related (in a manner to be shown later), the effect of changing K can conveniently be shown by finding a solution to (14) for a selection of k /k values. These solutions have the following characteristics:

(a) For every k /k value, the characteristic resonance behavior is obtained. The resonance frequency is x/ 1 k2 I For k =0 this frequency is which is the roll resonance frequency for the vessel without stabilizer. (b) For very small w, the solution to (14) is the amplitude has a negative sign and becomes C for a ratio of infinity,

(e) For all values of k /k the phase angle at goes from zero for small to through at resonance and approaches as w grows larger.

For each k /k value, a solution to (14) as a function of no can be defined. Four examples are shown in FIG- URES Ila-d.

These solutions can be discussed as follows. As shown in FIGURES lla wherein w w as the k /k ratio increases from zero, both the amplitude of response and the phase angle decrease (the latter because the resonance frequency becomes farther removed from w). The amplitude goes through zero for 2/ k =B/ C and then becomes negative;

(i.e., the vessel then leans into the wave).

As shown in FIGURE llc wherein w w for vessels with reasonable roll damping, it appears that as the k k ratio increases from zero, the amplitude of response decreases. The phase angle on also decreases from a value greater than 90. These changes in R and a take place in such a way that the vessel leans into the wave at first, but a decreasing angle of roll. It then begins to lean with the wave. Ultimately, however, it will again begin to lean into the wave. For the special case of (and keeping B/ C constant), the vessel always leans into the wave except for k /k =B/C when the angle of lean is zero.

In summary, it can be said that in the instance in which at is less than w as shown in FIGURE 11a, the introduction of the stabilizer provides all the roll control needed. In the instance in which or is greater than m the degree of control remains substantial. The stabilizer permits complete elimination of the rolling motion, but a close-tooptimal combination of roll and sway is harder to achieve primarily due to the increased difficulty of controlling the phase angle.

The relation between the quantities k and k and parameters used earlier are as follows:

M =m+ AwC where m is the body structure the density of the water A: the area of a baffle w: the distance between the baffles C a geometric form factor typically about 2.

The inert mass M resisting the vertical motion is given by To describe the motion, a very large mass M and total roll removal can be considered. From (8) and (12) is obtained Z DA cos wt The corresponding force F is given by.

To qualify. as a large mass, the excursion of the mass must be considerably smaller than A and the equation of motion is to a good approximation DL 22) and in WMG m2 MtrDLA cos wt (23) and 21rg WMG 2 MaDL 1 (2 27rg WMG DL (25 or But- T :a g g L 21r T2 21W2 2T0? Therefore WMG :2 Mtr (.0 D 21rg WM G The motion of the baffles will be further reduced due to drag and due to the penetration of the wave motion to the depth where the battles are located. This wave motion tends to cancel the bafile motion. The amplitude A of the wave at depth D is If the constant K is chosen so that a certain desired R value is obtained for a given wave period, then R will decrease as the wave period is increased (keeping K constant), Sooner or later R will become negative and the ship will begin to lean into the wave. This leaning into the wave is enhanced by the approach of the resonance condition in whichthe mass Mg acts as a pendulum with a restoring force consisting of the gravitational force on the baggle system augmented by the spring force in the connecting structure and the equivalent spring force of the ship stiffness, the latter two in series. For even longer wave periods, this resonance is passed and the ship again leans with the wave.

'.As discussed hereinbefore in various instances it can be seen that the complete elimination of roll of a buoyant body would not necessarily be the optimal roll characteristics. For example, in a drilling vessel the minimization or,elimination of forces acting on men and equipment parallel to the working deck due to roll of the drilling vessel would be the most desirable operating conditions. Thus, for example, if a heavy piece of drilling equipment is suspended from the crown block to the working deck its position relative to the working deck and a man thereon should optimally be stationary. As discussed hereinbefore, in accordance with the present invention this is achieved if the combined sway and roll of the vessel are such that the resultant forces of gravity and inertia acting upon crew members and objects they are handling are perpendicular to the deck upon which they are working. Such optimum roll can be found as follows and can be achieved by means of the present lHV6I1tl0l1.ASSl1lTl6 a wave form:

14 where X and Y are horizontal and vertical surface water particle coordinates. We then have Sway acceleration n 2 X= A cos 21 The slope of the wave dY 2 a; t a r 45?) 31 Horizontal displacement D of point of height h relative to reference point on ship at mean waterline for roll factor R and assuming phase angle a to be 'zero (ZY 2n It i D,. Rh- ARh cos 21r (32) Acceleration 1 5 associated with this displacement 21r hr x i i COS 27F Slope S of total inertial and gravitational force relative Ex. 1: For h=0 (means water level) R=l Ex. 2: For h=L/21rR=1/2 As R changes with h, the optimum conditions strictly speaking apply only to the handling of objects of vertical dimensions much smaller than the wave length L. This is not necessarily the case. For example, sections of drill pipe may be hanging in a cable suspension from the top of a derrick. In this case, the average R for upper and lower end of pipe may be optimal.

In the embodiment of the present invention as shown in FIGURES 1, 2, 3, 8, 9 and 10, two underwater bodies functioning as mass traps are shown since such an arr rangement is the more practical one for use with a drilling or other operational vessels. Further, in all embodiments shown herein the mass trap is shown vertically beneath the vessel or buoyant body. In some instances wherein depth of water or unusual Water conditions are a consideration the mass trap can be oriented away from the vertical as shown in FIGURE 14. In FIGURE 14 the stabilizing body is again shown as spaced apart parallel plates parallel to the longitudinal axis of the vessel but offset from the vertical plane of such centerline. The op eration of the apparatus is comparable to "that previously 15 described in connection with the rigid connection of the stabilizing body to the vessel. In connection with this embodiment it can be seen that the stabilizing body provides an inertial mass which resists movement in the direction transverse to a plane passing through the center of gravity of the vesseland the stabilizer body.

Referring now to FIGURES l2 and 13, there are shown various alternative arrangements of underwater bodies which will function in accordance with the present invention. Thus, in FIGURE 13 a body having a square cross-sectional configuration with open upper and lower ends is shown. In FIGURE 12 crossed members are shown. In both of the embodiments of FIGURES '12 and 13 the underwater body will again function to trapa large mass of water in the horizontal direction while allowing freedom of movement of the underwater bodies in the vertical direction.

What is claimed is:

1. A floating structure stabilized against rolling and pitching movements induced by wave action comprising:

a buoyant body supported by a body of water;

stabilizing body means; means for connecting said stabilizing body means to said buoyant body a substantial distance beneath said buoyant body and below the severe wave and surface action of the water supporting said buoyant body and for transmitting a force moment from said stabilizing body means to said buoyant body;

said stabilizing body means comprising means for entrappi'ng a mass of water upon movement of said bouyant body in a direction transverse to a line between said buoyant body and said stabilizing body means and for permitting water to flow freely therethrough upon movement of said buoyant body in a direction substantially parallel to said line.

2. A floating structure stabilized against rolling movements induced by wave action comprising;

a buoyant body having a longitudinal centerline supported by a body of water; stabilizing body means; means for connecting said stabilizing body means to said buoyant body a substantial distance beneath said buoyant body and below the severe wave and surface action of the water supporting said buoyant body and for transmitting a force moment from said stabilizing body means to said buoyant body;

said stabilizing body means comprising means for entrapping a mass of water upon movement of said buoyant body in a direction transverse to said longitudinal centerline and for permitting water to flow freely therethrough upon movement of said buoyant body in a direction substantially parallel to a line between said buoyant body and said stabilizing body means;

whereby a force moment is transmitted from said said stabilizing body means to said buoyant body; said longitudinal centerline thereof oppositely to that imparted by wave action.

3. A floating drilling vessel stabilized against rolling movements induced by wave action comprising:

a bouyant drilling vessel supported by a body of water;

stabilizing body means;

means for connecting said stabilizing body means to said vessel a substantial distance beneath said vessel and below the severe wave and surface action of the water supporting said vessel and for transmitting a force moment from said stabilizing body means to said vessel;

said stabilizing body means being of relatively light weight and comprising means for permitting water to freely flow therethrough in the vertical direction and in the direction parallel to the longitudinal centerline of the vessel, whereby to allow freedom of movementof said stabilizing body means in said vertical and parallel directions in response to such movement of said vessel, and for entrapping a large mass of water upon movement of said stabilizing body means in a direction transverse to the longitudinal centerline of said vessel, whereby to provide resistance to movement about said longitudinal centerline of said stabilizing body means and to transmit a force moment to said vessel by said connecting means. 4. A floating drilling vessel stabilized against rolling 1O movements induced by wave action comprising:

a buoyant drilling vessel supported by a body of Water and having a longitudinal centerline; stabilizing body means; means for connecting said stabilizing body means to said buoyant vessel a substantial distance beneath said buoyant vessel and below the severe wave and surface action of the water supporting said buoyant vessel and for transmitting a force moment from said stabilizing body means to said buoyant vessel;

said stabilizer body means comprising means of light weight and providing a substantial area only in vertical planes parallel to the longitudinal centerline of said vessel, whereby to allow the free flow of water through said stabilizing body means vertically or longitudinally and to trap a large mass of water to resist movement of said stabilizing body means in the direction transverse to said longitudinal centerline of said vessel.

5. The apparatusas defined in claim 1, in which the one horizontal direction is transverse to a longitudinal centerline of the buoyant body.

6. The apparatus as defined in claim 2 in which said buoyant body is a sub-sea drilling vessel.

7. The apparatus as defined in claim 1, in which the connecting means between said buoyant body and said underwater body comprises connecting members having a moment arm about both pitch and roll axes of said buoyant body.

8. The apparatus as defined in claim 2 in which the connecting means between said buoyant body and said underwater stabilizer body comprises connecting members having a moment arm about the longitudinal centerline of said buoyant body.

9. The apparatus as defined in claim 1 in which the stabilizer body comprises a vertically oriented open ended cylinder.

10. A floating drilling vessel stabilized against rolling motion induced by wave action comprising:

a floating drilling vessel;

first and second stabilizer bodies longitudinally spaced and vertically positioned beneath the longitudinal centerline of said vessel; each of said stabilizer bodies comprising parallel, vertically oriented baflles, said balfles being parallel to and spaced at opposite sides of said vessel centerline;

said stabilizer bodies being spaced beneath said vessel at a depth below severe surface and wave action of the water;

said stabilizer bodies each being connected at opposite sides of the hull of the vessel equidistant from the longitudinal centerline thereof by structural means; and,

said structural means including elastic means for providing an elastic compliance in said connecting means relative to the moment arm about said longitudinal centerline.

11. The apparatus as defined in claim in which said structural connecting members are cables and said elastic means provide variable predetermined elastic compliance 70 in each of said cables.

12. A floating structure stabilized against rolling and pitching movements induced by Wave action comprising:

a buoyant body;

a stabilizer body positioned beneath said buoyant body at a predetermined depth below wave action and connected thereto, said stabilizer body being so constructed and arranged as to be vertically movable but to entrap a mass of water and thereby provide inertial resistance to acceleration in at least one horizontal direction, said underwater body being connected to said buoyant body to impart a force moment to the buoyant body oppositely to that imparted by wave actionrand connecting means between the stabilizer body and said buoyant body, said connecting means including elastic means for applying predetermined forces at opposite sides of the moment arm about the pitch and roll axes of the buoyant body.

13. The apparatus as defined in claim 12, in which the connecting means between said vessel and said stabilizer body comprises rigid members having a moment arm about the longitudinal centerline of said vessel.

14. The apparatus as defined in claim 13, in which the connecting means between the stabilizer body and vessel are rigid in the transverse direction and pivotal at the vessel in the longitudinal direction.

15. The apparatus as defined in claim 13, in which the underwater body comprises a pair of spaced parallel plates parallel to and on opposite sides of the plane of the longitudinal centerline of the vessel.

16. A floating structure stabilized against rolling move ment induced by wave action comprising:

a buoyant body having a longitudinal centerline;

a stabilizer body positioned beneath said buoyant body at a predetermined depth below wave action;

said stabilizer body being so constructed and arranged as to be vertically movable but to entrap a mass of water to provide inertial resistance against movement in the horizontal direction transverse to said longitudinal centerline;

said underwater body being connected to said buoyant body by means such that a force moment is transmitted from said stabilizer body to said buoyant body about said longitudinal centerline thereof oppositely to that imparted by wave action, said connecting means between the stabilizer body and buoyant body including elastic means for applying predetermined forces at opposite sides of the moment arm about the longitudinal centerline of the buoyant body.

17. A floating structure stabilized against rolling and pitching movements induced by wave action comprising:

a buoyant body;

a stabilizer body positioned beneath said buoyant body and connected thereto at a predetermined depth below wave action, said stabilizer body being so constructed and arranged as to be vertically movable but to entrap a mass of water and thereby provide inertial resistance to acceleration in a horizontal direction transverse to a longitudinal centerline of the buoyant body;

said stabilizer body being connected to said buoyant body to impart a force moment to the buoyant body oppositely to that imparted to wave action; and

connecting means between the stabilizer and said buoyant body said connecting means including struts affixed at opposite sides of the longitudinal centerline of the vessel and including means for varying the elastic compliance of the coupling force between the stabilizer body and opposite sides of the vessel with regards to force moments about the horizontal lon-.

gitudinal centerline through the vessel.

18. The apparatus as defined in claim 17 in which the stabilizer body comprises a pair of spaced parallel plates, said plates being parallel to the longitudinal centerline of the vessel.

19. The apparatus as defined in claim 18 in which said elastic means comprised a variable force means connected to cables, said cables comprising said connecting means between said buoyant body and said stabilizer body.

20. The apparatus as defined in claim 12 in which the elastic compliance in said connecting means can be varied to predetermined compliance.

21. The apparatus as defined in claim 16 in which the elastic compliance in said connecting means can be varied to predetermined compliance.

References Cited UNITED STATES PATENTS 83,420 10/1868 Stoner et al. 1l4124 131,719 9/1872 Stoner 114-124 3,279,404 10/ 1966 Richardson 1 14-.5

FOREIGN PATENTS 305,134 1/1933 Italy.

MILTON BUCHLER, Primary Examiner.

T. M. BLIX, Assistant Examiner.