Abstract:
A floating vessel is equipped with perforated plates which exhibit both an added-mass effect and a damping effect. The addition of porosity to an added mass plate phase-shifts the added mass force so that it becomes at least partially a damping force which does not depend on large velocities to develop a large damping force. Preferred porosity is in the range of about 5% to about 15% of total plate area. A semi-submersible drilling rig may have damper plates fitted between its surface-piercing columns and/or extending from the sides of its pontoons. A truss spar offshore platform may have damper plates installed within its truss structure intermediate its hull and ballast tank. Drill ships and similar vessels may be equipped with damper plates extending from the sides of their hulls to reduce both heave and roll. In certain embodiments, the damper plates are retractable so as not to interfere with docking and to reduce drag while the vessel is underway.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/182,629 filed Jul. 30, 2008, now U.S. Pat. No. 7,900,572 the disclosure of which is hereby incorporated by reference in its entirety. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to offshore platforms and vessels. More particularly, it relates to floating structures which employ porous, added-mass stabilizer plates for motion suppression. 
     2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98. 
     U.S. Pat. No. 3,986,471 describes an apparatus for damping vertical movement of a semi-submersible vessel having submerged pontoons and a small waterplane area which comprises a submerged damper plate equipped with valves for providing substantially greater resistance to upward movement of the plate than downward movement. The damper plate is supported deep beneath the semi-submersible vessel by flexible, tensioned supports such as chains or cables, at a depth beneath the water surface in the semi-submerged condition of the vessel where the amplitude of subsurface wave motion is less than the maximum heave amplitude which would be experienced by the semi-submersible vessel alone under identical sea conditions. The area of the damper plate is several times larger than the waterplane area of the vessel. An upward only-damping action is achieved due to the entrainment of large apparent masses of relatively still water by the damper plate. 
     U.S. Pat. No. 5,038,702 describes a semi-submersible platform supported on columns with pontoons extending between and outboard of the columns. Damper plates are provided by flat surfaces either on top of the outboard section of the pontoons or by plates positioned on the columns above the pontoons to provide heave and pitch stabilization and motion phase control in relation to the wave action such that when the platform is in the drilling mode, the heave phase of the platform is approximately 180° out of phase with wave action, and in the survival mode, heave action of the platform is substantially in phase with wave action. 
     U.S. Pat. No. 6,652,192 describes a heave-suppressed, floating offshore drilling and production platform that comprises vertical columns, lateral trusses connecting adjacent columns, a deep-submerged horizontal plate supported from the bottom of the columns by vertical truss legs, and a topside deck supported by the columns. The lateral trusses connect adjacent columns near their lower end to enhance the structural integrity of the platform. During the launch of the platform and towing in relatively shallow water, the truss legs are stowed in shafts within each column, and the plate is carried just below the lower ends of the columns. After the platform has been floated to the deep water drilling and production site, the truss legs are lowered from the column shafts to lower the plate to a deep draft for reducing the effect of wave forces and to provide heave and vertical motion resistance to the platform. Water in the column shafts is then removed for buoyantly lifting the platform so that the deck is at the desired elevation above the water surface. 
     U.S. Patent Publication No. 2002/0139286 describes a heave-damped floating structure that includes an elongate caisson hull and a plate set coupled to the hull. The plate set includes multiple heave plates located about an outer edge of the hull so as to form a discontinuous pattern generally symmetric about a vertical axis of the hull. 
     BRIEF SUMMARY OF THE INVENTION 
     The addition of porosity to an added mass plate phase-shifts the added mass force so that it becomes at least partially a damping force. This effect can develop fairly large damping forces without the need for the large relative velocities that drag damping forces typically require. A damper plate according to the invention can be configured to present a low profile to current forces thereby reducing station-keeping forces in high currents. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         FIG. 1  is a perspective view of a battered column, semi-submersible drilling rig equipped with added-mass stabilizer plates according to a first embodiment of the invention. 
         FIG. 2  is a perspective view of a stabilizer plate having slot-type damping apertures. 
         FIG. 3  is a perspective view of a stabilizer plate having generally square damping apertures. 
         FIG. 4  is a perspective view of a stabilizer plate having round hole-type damping apertures. 
         FIGS. 5A and 5B  are cross-sectional views of alternative embodiments having paired stabilizer plates. 
         FIG. 6  is a perspective view showing stabilizer plates according to the invention mounted between the columns of a battered-column semi-submersible drilling rig without pontoons. 
         FIG. 7  is a perspective view of a truss spar drilling rig equipped with a stabilizer plate having slot-type damping apertures. 
         FIG. 8  is a perspective view of the truss portion of the spar shown in  FIG. 7  equipped with a stabilizer plate having hole-type damping apertures. 
         FIG. 9  is a perspective view of the truss portion of the spar shown in  FIG. 7  equipped with a stabilizer plate having square damping apertures. 
         FIG. 10  is a front view (partially in cross section) of a drilling ship equipped with retractable roll stabilizers having damping apertures. 
         FIG. 11  is a top view of the drill ship shown in  FIG. 10 . 
         FIG. 12  is a front view of a drilling ship equipped with hinged roll stabilizers having damping apertures. 
         FIG. 13  is a top view of the drill ship shown in  FIG. 12 . 
         FIG. 14  is a plan view of a stabilizer plate having slot-type damping apertures. 
         FIG. 15  is a plan view of a stabilizer plate having generally square damping apertures. 
         FIG. 16  is a plan view of a stabilizer plate having round hole-type damping apertures. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Floating offshore oil platforms and drilling ships need to limit their motions as much as possible in order to conduct uninterrupted drilling and production operations. However, these vessels are subject to motion, particularly in the vertical direction (heave), due to the action of waves and swells passing the vessel&#39;s location. Accordingly, such vessels are often designed to have minimal waterplane area so that the vessel&#39;s buoyancy is affected as little as possible by wave action. 
     Increasing the added mass is a technique that has been used for some time to improve the motion characteristics of floating offshore platforms. The more massive an object is, the more resistant it is to motion in reaction to an applied force (e.g., a passing wave). Semi-submersible drilling rigs are often very large and heavy to take advantage of this effect. Whenever a floating object moves in a body of water, some of the water must move with the vessel. This “attached” water also has mass and thus “adds” to the apparent mass of the vessel. Certain structures may be designed to maximize this effect. For example, heave plates may be added to offshore platforms and other vessels to increase their effective mass and thereby increase their resistance to acceleration in the vertical direction. Heave plates are typically flat plates fixed in a horizontal position such that moving the plate in a vertical direction presents a large surface area to the surrounding water. This requires a relatively large mass of water to move with the heave plate thereby adding to the apparent mass (and motion stability) of the vessel. 
     Additionally, the heave plate provides increased drag in the vertical direction. Drag is a retarding force exerted on a body as it moves through a fluid medium such as water. It is generally comprised of both viscous and pressure effects. One characteristic of drag forces is that the force is proportional to the square of the velocity and thus large drag forces result from large relative velocities. 
     Damping is a resistive force to velocity. In a system in an oscillating condition (such as motion in waves), damping is any effect, either deliberately engendered or inherent to a system, that tends to reduce the amplitude of oscillations of the oscillatory system. Floating vessels exhibit a heave natural period (oscillation) when displaced vertically. To avoid potentially damaging resonance, it is desirable to design a floating vessel such that its heave period is outside the range of wave periods likely to be encountered. Dampers act to suppress oscillation and generally provide an opposing force that varies in proportion to the system&#39;s displacement from its neutral position or state and the velocity of the displacement. 
     Perforated heave plates exhibit another damping effect in addition to that associated with heave plates of the prior art. The addition of porosity to an added mass plate creates a phase shift in the added mass force so that the water pressure normally associated with added mass forces acts as a damping force. The porosity allows the water to lag behind the structure—i.e., it continues to flow through the plate after the plate stops and reverses direction in oscillatory motion. This is very significant in that the effect allows the development of large damping forces without the need for the large displacements and velocities that would be necessary to develop large damping by drag forces. 
     The invention may best be understood by reference to certain illustrative embodiments shown in the drawing figures. 
     A battered-column, semi-submersible drilling rig  10  according to a first embodiment of the invention is shown in  FIG. 1 . Deck  16  (upon which drilling equipment  18  is mounted) is supported on battered columns  12  projecting above the waterline. Buoyancy is provided by columns  12  and pontoons  14  which connect columns  12  and form the perimeter of central opening  24  through which drill string  22  may pass. The invention may also be practiced with conventional semi-submersible rigs—i.e., those having vertical columns. When drilling operations are being conducted, rig  10  is held in position by catenary anchor lines  20  which connect to anchors on the seafloor. The invention may also be practiced with dynamically positioned drilling rigs—floating platforms which maintain their position using vectored thrust rather than anchors. 
     Plate-type heave dampers  26  extend between columns  12  below the waterline and above pontoons  14 . Semi-submersible  10  shown in  FIG. 1  comprises a pair of dampers  26 . Other embodiments may have additional damper plates. Those skilled in the art will appreciate that it is desirable to locate the damper plates symmetrically about the center of the vessel. 
     In other embodiments of the invention (not shown), heave dampers  26  may be mounted to the vertical sides of pontoons  14 . Dampers  26  may be mounted on the interior surface (i.e., within central opening  26 ), exterior surface or both. Dampers  26  in this configuration may be cantilevered or braced as dictated by structural considerations. 
       FIG. 2  shows damper plate  26  in greater detail. Damper  26  comprises slotted plate  28  connected to support member  32 . Slots  30  provide openings through which water may flow from the upper surface of plate  28  to the lower surface of plate  28  and vice versa. Damper  26  may be constructed of any suitable material or combination of materials. One particularly preferred material is steel which provides relatively high strength at relatively low cost and may be worked using readily-available tools and equipment. 
     As shown in the exemplary embodiments of the drawing figures, support member  32  is a box beam. Other structures including, but not limited to, tubular members and flanged or un-flanged beams may similarly be used. Support members having a watertight internal cavity may also function as buoyancy members. It will be appreciated that damper plates according to the invention may be configured to present a relatively small frontal area to lateral movement of the vessel thereby minimizing the effects of currents and the station keeping forces necessary to hold the vessel in position. Low frontal area also is advantageous in reducing drag when the vessel is being moved from one location to another. 
       FIG. 3  shows one alternative damper plate  26 ′ in detail. Damper  26 ′ comprises perforated plate  34  connected to support member  32 . Square apertures  30  provide openings through which water may flow from the upper surface of plate  34  to the lower surface of plate  34  and vice versa. Damper  26 ′ may be constructed of any suitable material or combination of materials. One particularly preferred material is steel which provides relatively high strength at relatively low cost. 
       FIG. 4  shows yet another version of damper plate  26 ″ in detail. Damper plate  26 ″ comprises perforated plate  38  connected to support member  32 . Round apertures or holes  40  provide openings through which water may flow from the upper surface of plate  38  to the lower surface of plate  38  and vice versa. Damper plate  26 ″ may be constructed of any suitable material or combination of materials. One particularly preferred material is steel which provides relatively high strength at relatively low cost. 
       FIG. 5A  is a cross-sectional view of a fourth embodiment of a damper according to the invention. Paired-plate damper  42  comprises upper plate  44  and lower plate  46  both of which are connected to support member  32 . As shown in  FIG. 5A , holes  40  in upper plate  44  may be axially offset distance “O” from corresponding holes  40  in lower plate  46 . Alternatively, as illustrated in  FIG. 5B , holes  40  in upper plate  44 ′ of damper  42 ′ may be axially aligned with corresponding holes  40  in lower plate  46 ′. By selecting the extent (if any) of the offset “O,” the resistance to the flow of water from the upper surface of damper  42  to the lower surface of damper  42  (or vice versa) which may occur upon vertical movement of damper  42  may be modified, which may influence the damping effect. 
     A battered-column, semi-submersible drilling rig  48  according to another embodiment of the invention is shown in  FIG. 6 . Deck  16  (upon which drilling equipment  18  is mounted) is supported on battered columns  12  projecting above the waterline. Unlike the embodiment illustrated in  FIG. 1 , buoyancy is provided solely by columns  12  and there are no pontoons which connect columns  12 . Rather, columns  12 ′ are connected by truss structure  52 . Columns  12 ′ may have undersea section  50  of greater diameter to provide the buoyancy needed to support deck  16  without increasing the waterplane area of columns  12 ′. Perforated heave dampers  26  connect adjacent pairs of battered columns  12  and form the perimeter of central opening  24  through which drill string  22  may pass. The invention according to the embodiment of  FIG. 6  may also be practiced with semi-submersible rigs having vertical columns. When drilling operations are being conducted, rig  48  is held in position by catenary anchor lines  20  which connect to anchors on or embedded in the seafloor. Alternatively, rig  48  may be dynamically positioned. 
     A truss spar platform according to the present invention is shown in  FIG. 7 . Truss spar platform  54  comprises generally cylindrical hull  56 , truss structure  58  and ballast tank  60 , as shown. Deck  16 ′ is mounted to the top of hull  56 . Drilling equipment  18  may extend over the side of deck  16 ′ so that drill string  22  may be run to the seafloor. Alternatively, a moon pool may be provided in hull  56  for the drill string with corresponding openings in the damper and ballast tank. Ballast tank  60  (which may contain solid ballast) is sized and positioned so as to position the center of gravity of the vessel is below its center of buoyancy thereby ensuring its free-floating stability. The rig may be anchored in position by conventional catenary anchor lines (not shown). 
     At one or more points within truss structure  58  intermediate the bottom of hull  56  and the top of ballast tank  60  is heave plate  26 . In the embodiment shown in  FIG. 7 , heave plate  26  comprises a slotted plate.  FIG. 8  shows an alternative embodiment wherein heave plate  26 ″ comprises a perforated plate with holes.  FIG. 9  shows yet another embodiment of truss structure  58  wherein heave plate  26 ′ comprises a plate having substantially square apertures. 
     Another embodiment of the invention is shown in  FIG. 10 . In this embodiment, ship-shaped offshore vessel  62  comprising hull  64 , deck  65  and derrick  66  is equipped with retractable motion dampers  68  which may be extended from the sides of hull  64  below the waterline of the vessel. Motion dampers  68  may be retracted when the vessel is underway to reduce the drag acting on hull  64  or to permit the vessel to come alongside a dock or another vessel, such as a supply ship. Motion dampers  68 , when extended, act to reduce both roll and heave of the vessel. Depending on their position relative to the center of the vessel, dampers  68  may also act to reduce pitching motions of the vessel. 
       FIG. 11  is a top view of a portion of the drill ship  62  shown in  FIG. 10 . Motion dampers  68  may swing into retracted position  74  (shown in phantom) by pivoting about pivots  72 . As shown in  FIG. 10 , braces  70  may be attached between hull  64  and motion damper  68  to increase the structural rigidity of the extended dampers. 
     The motion dampers  68  shown in  FIG. 11  are of the slotted plate type. It will be understood that plates having other aperture shapes (such as those illustrated in  FIGS. 15 and 16 ) may also be used in the practice of the invention. 
     Another embodiment of the invention is shown in  FIG. 10 . In this embodiment, drill ship  62 ′ comprising hull  64 , deck  65  and derrick  66  is equipped with folding motion dampers  76  which may be extended from the sides of hull  64  below the waterline of the vessel. Motion dampers  76  may be retracted when the vessel is underway to reduce the drag acting on hull  64  or to permit the vessel to come alongside a dock or another vessel, such as a supply ship. Hinged motion dampers  76 , when extended, act to reduce both roll and heave of the vessel. Depending on their position relative to the center of the vessel, they may also act to reduce pitching motions of the vessel. 
       FIG. 13  is a top view of a portion of the drill ship  62 ′ shown in  FIG. 12 . Motion dampers  76  may be moved into retracted position  78  (shown in phantom) by swinging on hinges  80 . Braces (not shown) may be attached between hull  64  and motion dampers  76  to increase the structural rigidity of the extended dampers. 
     The motion dampers  76  shown in  FIG. 13  are of the slotted plate type. It will be understood that plates having other aperture shapes (such as those illustrated in  FIGS. 15 and 16 ) may also be used in the practice of the invention. 
     Damper plates according to the present invention preferably have between about 5% to about 15% porosity—i.e., the openings comprise about 5 to 15 percent of the total plate area (exclusive of support members). Particularly preferred is a damper plate having a porosity of about 10%.  FIG. 14  is a plan view (to scale) of a slotted plate  28  having slots  30  which comprise 10% of the plate area.  FIG. 15  is a plan view (also to scale) of a perforated plate  34  according to the invention which has substantially square apertures in a linear row-and-column configuration which comprise 10% of the plate area.  FIG. 16  is a plan view (to scale) of a damper plate  40  according to the invention having holes (round apertures)  40  in a linear row-and-column configuration which comprise 10% of the plate area. It will be understood that other aperture configurations are also possible and may be employed without departing from the scope of the invention. Particularly preferred are aperture configurations which are “screen-like”—i.e., those that have relatively smaller apertures spaced relatively close together as opposed to configurations having fewer and larger spaced-apart openings (even though the total porosity may be equal). 
     Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.