Damper system for a floating structure

Abstract: The damper system (20) for a floating structure (10) is moored to the seabed by at least one mooring line (22) and comprises a framework (26) forming part of the floating structure. A tensioner system (51) is mounted on the framework. The tensioner system includes an upper gripper (44) adapted to grip the mooring line. A loadbeam (30) supports the upper gripper. Extensible double-acting ram means (31) on the framework support the loadbeam. The loadbeam, the upper gripper, and the gripped portion of the mooring line together have vertical reciprocating motion relative to the framework depending on the reciprocating motion of the ram means in response to dynamic sea conditions. A mechanical brake system (52) is coupled between the loadbeam and the framework.

Description: This invention is concerned, in general, with a mechanical damper system for a floating offshore structure such as a platform used for conducting drilling and production operations over a seabed site. Applicant's US-A-4 850 744 describes such platform (known as ELDORADO). It can operate in very deep waters without being tied to the seabed by tension legs. It is subject to pitch, roll, heave, surge, sway, and yaw. It uses very long catenary mooring lines which resist yaw motions and large horizontal excursions of the platform. However, very strong waves and other environmental forces may have the capacity to induce heaves having a frequency approaching the platform's resonant period T n . What was needed before was additional protection against the more severe waves although they may never occur. The needed machine should be compact enough so that it could be positioned at a location not used as deck space or other work space which are limited and therefore very valuable. This is accomplished by starting with a known linear winch and increasing its capabilities to serve as a mooring line tensioner and as a platform motion dampener. For example, a linear winch pulling a mooring line is described in Johnson's US-A- 4 446 807. A tensioner compensating apparatus is described in Haines' U.S. reissue patent 28 218 starting on column 3, line 47. And the desirata for a damper machine are described by Bergman in US-A 4 167 147. US-A-4 446 807 describes a pull machine on a vessel between guide means and fairleads in encompassing relation to a mooring line. This machine includes an upper gripper mounted on a crosshead, a lower gripper and a pair of rams on which crosshead is supported. Upper gripper and lower gripper include gripper jaws activated by hydraulic cylinder-pistons. In use, the various cylinders are selectively activated to pull line in an upward direction until the desired tension is achieved in the line. This machine selectively exerts a high force to draw in or to pay out on a mooring line, holds the mooring line against outward movement, or moves jaws into clearing relation with respect to the mooring line. In the known linear puller machines, the upper end of the flexible cable encompassed by the gripper jaws is not suspended for vertical displacements relative to its framework and hence relative to the floating structure in response to wave action and environmental forces. It was generally believed that motion compensating tensioners can be used for drilling and production operating only when the platform has "very small vertical oscillations". (Myers US-patent 4 733 991, column 1, lines 18 - 19, column 2, lines 25 - 26). It is a primary object of this invention to enable a floating structure such as the ELDORADO to remain functional in severe sea waves that might, although very rarely, fall outside of the expected maximum wave range. This object is solved by the subject invention, as described in the following, and by the characterising features of the main claim. Further embodiments of this invention are the subject of subclaims. The system for damping the heave of an offshore structure or platform floating over a seabed site includes a framework forming part of the structure for supporting a linear line winch. A flexible long loadline has a lower end tied to a stationary underwater object, and an upper end encompassed by the winch which includes a loadbeam supported by a pair of hydraulic rams for pulling on the loadline. A hydraulic liquid network activates the winch. Under normal sea and environmental conditions, the loadline is restrained by a lower gripper whose two jaws firmly grip the loadline, while the rams are fully retracted. The framework supports the damper system which comprises a tensioner system and, preferably, a mechanical force system operatively coupled between the tensioner system and the framework. In the tensioner mode, upper gripper and its jaws grip the upper end of the loadline, thereby suspending it from the framework. The lower gripper is then deactivated to release the upper end. The output flow from the accumulator is directed into the lower ends of the rams, and the pressure of the hydraulic fluid confined in the rams is at all times maintained by the accumulator at a substantially constant selected value. For any position along their strokes, the piston-rods in the rams apply a substantially constant, upward-acting force against the loadbeam, resulting in a substantially constant tension in the loadline. As the platform cyclically heaves up and down during each oscillatory cycle, hydraulic fluid is alternately pushed in and out of the rams while their piston-rods, the loadbeam, and the upper gripper vertically oscillate in response to wave action on the floating structure, without appreciably modifying the applied selected tension in the loadline. When there is a need to increase the tension in the loadline, the damper means is activated and preferably only, when the platform heaves up, and is deactivated as the platform starts to heave down, thereby appreciably dampen the reciprocation of the loadbeam and consequentially the linear and angular motions of the platform. Preferably, the force means is a mechanical brake system having first and second frictionally engageable flat brake surfaces. The present system avoids the constraints according to the prior art (for example US-A-4 733 991); it remains functional in sea waves that might fall outside of the expected maximum wave range and can induce heaves having a frequency approaching the platform's resonant period Tn. It was generally believed by those skilled in the art that if in a tensioner the force exerted on the loadline were increased substantially then it might snap (deJong, US-A-4 395 160, column 2, lines 24 - 55). In applicant's system, the tensional force exerted on the loadline is purposely increased substantially to achieve the desired damping function. Applicant's linear line puller winch can selectively accomplish all three functions of Johnson's machine. But, it can, in addition, selectively serve in a relative-heave line tensioner, and in a damper system to increase the tension in the mooring lines whenever they may be needed and for whatever purpose. In so doing, the present invention increases the effectiveness of the catenary mooring system, such as is used on the ELDORADO platform. The system of this invention is very practical, compact and yet very economical because it can be positioned away from scarce deck space or other working space. It is positioned externally and on the side of the platform, thereby saving very valuable deck space. Under normal sea and environmental conditions, the mooring line is held in place under tension by the linear winch in a conventional manner. Under above average sea and wind conditions, the upper end of the line becomes suspended from the framework so as to allow relative heave therebetween. The tension in the line is maintained substantially constant despite the oscillating heave of the platform relative to the line's anchor in the seabed. This constant tension can be on the order of 200 tons or more for an ELDORADO type platform, thereby reducing the spring stiffness of its catenary mooring system, which correspondingly increases the natural periods of oscillation of the pitch, roll, heave, surge, sway, and yaw motions of the moored platform. Under severe sea and environmental conditions, the upward force on the upper end of the line is increased substantially which produces a downward-acting desired damping force on the platform. The damping force is preferably substantially constant and calculated to achieve a substantial decrease in the amplitude of a particular linear or angular motion, especially when the platform is about to approach the resonance state for that particular motion. Specific embodiments of the invention will be described, by way of example only, in connection with the accompanying drawings, wherein: FIG. 1 is a schematic side elevation view illustrating a semi-submersible floating production platform equipped with the damping system of the present invention; FIG. 2 is a schematic side elevation view of a preferred embodiment of the damping system; FIG. 3 is a sectional view taken on line 3-3 of FIG. 2; and FIG. 4 is a diagrammatic representative of the hydraulic network used to activate the damping system shown in FIG. 2. In a non-limited way, the present invention is illustrated for use with a production platform 10 (FIG. 1) described in US-A-4,850,744 and US-A-4,913,952. Platform 10 has a fully-submersible lower hull 11 and an upper hull 12 having a top deck 13. Lower hull 11 together with large cross-section, hollow, buoyant, stabilizing, vertical columns 14 support the entire weight of upper hull 12. Platform 10 is used for oil-and-gas drilling and/or production operations in a very deep body of water over a well site 16 which contains submerged oil and/or gas producing wells 17. By virtue of the platform's relatively low-motion responses to wave and wind action, surface-type, production wellhead trees (not shown) can be suspended from upper hull 12 above waterline 18. Each pipe 19, called "production riser", has a bottom end connected to a particular well 17 and an upper end connected to a wellhead tree. A tensioner (not shown) suspends each riser 19 from upper hull 12 so as to allow relative up and down vertical motion or heave therebetween. Such tensioners are described in US-A-4,733,991, US-A-4,379,657 and US-A-4,215,950. A catenary spread-type mooring system 21 includes one or more very-long catenary mooring lines or cables 22, each extending from a powered spool 23 on deck 13, through a lower pulley 15 on a column 14, and thence downwardly and outwardly, in catenary form, toward seabed 16. Cable 22 is followed by a long and heavy chain 24 which is tied to a strong anchor schematically represented as 25. Catenary mooring system 21 resists yaw motions and large horizontal excursions of platform 10. At a convenient location below spool 23 is securely attached to the structure of platform 10 a support framework 26 (FIG. 2) made up of vertical and horizontal I-beams 27 and 27′, respectively. Framework 26 supports a damper system 20 (FIG.1) which comprises a tensioner system 51 (FIG.4) and, preferably, a mechanical brake system 52 operatively coupled between tensioner system 51 and framework 26. Tensioner system 51 includes a linear winch 28 (FIG.2) operated by a hydraulic network 35. Linear winches are described, for example, in US-A-4,615,509 and US-A-4,427,180. A linear winch 28 can also be used for deploying and retrieving mooring line 22. Linear winch 28, per se, does not form part of this invention. Linear winch 28 includes a pair of grippers 43,44, adapted for gripping cable 22, and a loadbeam 30 which is supported by a pair of double-acting rams 31 having cylinders 32 and piston-rods 33. Cable 22 passes through the center of loadbeam 30 (FIG. 3). The bottoms of cylinders 32 are supported by a base plate 34 secured to a transverse beam 27'. Gripper 43 is mounted on base plate 34 between rams 31 and gripper 44 rests on top of and moves with loadbeam 30 and piston-rods 33. Internally, each cable gripper includes two wedge-shaped jaws 45,46, (FIG.4) which are activated by hydraulic cylinders 45',46', respectively. In each cylinder 32, piston-rod 33 reciprocates in response to fluid pressure from hydraulic network 35, which includes valves 36,37 and a tandem valve 38. A pump unit 41 supplies hydraulic fluid through a regulator 40 to an accumulator bank 39. Brake system 52 includes vertical fins 54 which are preferably secured by bolts to the opposite ends of load beam 30 and are therefore replaceable. Each fin 54 is a long, flat metal bar defining opposite polished brake surfaces 59,60. The vertical structural member 27 of framework 26 opposite to a fin 54 supports a vertical array of linear caliper brake 56, each having pads 61,62 adapted to frictionally bear against the opposite, polished surfaces 59,60 of fin 54. Linear brakes 56 are operated by hydraulic power means (not shown) under the control of known instrumentation modules which include motion sensors and load sensors on brake pads 61,62. Under normal sea and environmental conditions, cable 22 is restrained by gripper 43 whose two jaws 45,46 firmly grip cable 22, while rams 31 are fully retracted. In anticipation of a storm, gripper 44 is activated through valve 36 and its jaws 45,46 grip cable 22; gripper 43 is then deactivated so that its jaws 45,46 release cable 22; and tandem valve 38 is shifted from its normal position to its activated position, whereupon valve 37 becomes isolated, and the output flow from hydraulic-fluid accumulator bank 39 is directed into the lower ends of rams 31. As platform 10 cyclically heaves up and down during each oscillatory cycle, hydraulic fluid is alternately pushed in and out of cylinders 32. As a result, gripper 44, loadbeam 30, piston-rods 33, and the gripped portion of cable 22, acquire heave motion relative to platform 10. For any position along their strokes, piston-rods 33 will apply a continuous, substantially-constant, predetermined, upward-acting force against loadbeam 30, which has the effect of increasing the tension in cable 22. This tension can be on the order of 200 tons or more for a platform 10 of the type above described. It will be apparent, therefore, that under dynamic sea conditions tensioner system 51 (1) grips and suspends the upper end of cable 22 from framework 26 so as to allow relative heave therebetween, and (2) applies a substantially constant tension to the upper end of cable 22, thereby reducing the spring stiffness of catenary mooring system 21, which correspondingly increases the natural periods of oscillation of the pitch, roll, heave, surge, sway, and yaw motions of moored floating platform 10. The increased tension in cable 22 also produces anti-motion forces on platform 10, which are calculated to achieve a substantial decrease in the amplitude of a particular linear or angular motion, especially when the platform is about to approach the resonance state for that particular motion. Consequently, the damping forces will assist in maintaining within acceptable limits, the resonant motion responses of platform 10 to wave energies exceeding the maximum periods of expected waves over the well site 16. Additionally, as platform 10 cyclically heaves up and down during each oscillatory cycle, brake system 52 is activated. Pads 61,62 of caliper brakes 56 apply frictional forces against the opposite surfaces 59,60 of fins 54 only when platform 10 heaves upward. These frictional forces (Coulomb friction) dissipate energy as soon as platform 10 starts to heave up, and then brakes 56 are deactivated as soon as platform 10 starts to heave down. The applied frictional forces have the effect of increasing the tension in cable 22, thereby generating corresponding downward-acting, anti-motion forces on platform 10 which are substantially constant and independent of the velocity of the platform's displacements. These anti-motion forces most efficiently suppress resonant heave motions of platform 10. Since the frictional forces are produced by mechanical brakes 56, the motion energy pumped into platform 10 by the sea waves is converted into heat energy, or it is stored as potential energy due to draft changes. This heat energy can be absorbed by platform 10, by heat exchangers, or by circulating sea water over fins 54. Brakes 56 can be activated also when platform 10 heaves up and down. Therefore, mechanical brakes system 52 increases the tension on top of cable 22 when floating structure 10 heaves up, thereby exerting a downward-acting damping force on the floating structure, and decreases the tension on top of cable 22 when the floating structure heaves down, thereby exerting an upward-acting damping force on floating structure 10. The decrease in tension is such that there will always remain sufficient positive tension in cable 22 to prevent buckling. It will be apparent that the opposite ends of loadbeam 30 can carry brakes 56, and each longitudinal structural member 27 opposite to an end of loadbeam 30 can support the linear fin 54.

Claims: A damper system (20) for a floating structure (10) moored to the seabed (16) by a mooring system (21) including at least one mooring line (22), said damper system (20) comprises a framework (26) forming part of said floating structure, and a tensioner system (51) including an upper gripper (44) adapted to grip said mooring line; a loadbeam (30) for supporting said upper gripper; extensible double-acting ram means (31) on said framework for supporting said loadbeam, characterised in that said loadbeam (30), said upper gripper (44), and the gripped portion of said mooring line (22) together have vertical reciprocating motion relative to said framework (26) depending on the reciprocating motion of said ram means (31) in response to dynamic sea conditions. A damper system according to claim 1, characterised in that said framework (26) supports a linear winch (28) having said loadbeam (30) supported by a pair of hydraulic rams (31), each having a cylinder (32) and a reciprocating piston-rod (33), a flexible long loadline (22) having a lower end (24) tied to a stationary underwater object (25) and an upper end forcibly gripped by said winch, and for any position along its stroke, each piston-rod (33) receives a continuous supply of hydraulic fluid at a selected, substantially-constant pressure value and applies a continuous, substantially constant, predetermined, upward-acting force against said loadbeam (30), thereby causing said gripped upper end of said loadline to become suspended from said framework (26) as said loadbeam (30), piston-rods (33), and said gripped upper end of said loadline (22) together cyclically heave up and down relative to said framework (26) in response to wave and environmental action on said structure (10), and as hydraulic fluid is alternately pushed in and out of said cylinders (32). A damper system according to claim 1, charaterised in that said tensioner system (51) suspends said gripped portion of said mooring line (22) which has a bottom end anchored to the seabed (16), whereby, in use, said tensioner system applies a substantially constant tension to said gripped portion of said mooring line, and said tension being effective in assisting to maintain the resonant motion responses of said structure to wave energies exceeding the maximum periods of expected waves, and in reducing the spring stiffness of said mooring system (21) which results in an increase in the natural periods of oscillations of said structure. A damper system for damping the heave of an offshore structure (10) according to claims 2 and 3, wherein said winch (28) includes said upper gripper (44) on top of said loadbeam (30), characterised in that said upper gripper (44) grips and holds said upper end of said loadline (22), which permits said floating structure (10) to shift vertically relative to said stationary object without appreciably modifying said upward acting force against said loadbeam (30). A damper for damping the heave of an offshore structure (10) according to claims 1 - 4, characterised in that said damper system (20) comprises damping means (52) coupled between said loadbeam (30) and said framework (26) for slowing down said up and down heave of said loadbeam, said piston-rods (33), and said gripped upper end of said loadline (22). A damper system for damping the heave of an offshore structure (10) according to claim 5, characterised in that said damping means (52) is a mechanical brake system comprising first (59, 60) and second (61, 62) frictionally engageable brake surfaces for mechanically slowing down said up and down heave of said loadbeam (30), said piston-rods (33), and said gripped upper end of said loadline (22). A damper system according to claims 1 and 2, characterised in that said damper system (20) comprises a mechanical brake system (52) coupled between said tensioner system (51) and said framework (26), and said brake system, in use, frictionally varies the applied tension to said mooring line (22) in dependence on the relative heave between said mooring line and said structure, thereby exerting corresponding damping forces on said structure in a direction opposite to the relative heave. A damper system (20) for damping the heave of an offshore structure (10) according to claim 6, characterised in that said first (59, 60) and second (61, 62) surfaces engage for frictional interaction only when said beam (30) heaves up during an upper end of each heave cycle, and they are disengaged during the remainder portion of each heave cycle when said beam heaves down. A damper system (20) according to claims 1 - 8, characterised in that said loadbeam (30) carries on the opposite ends thereof, vertically-extending fins (54), each fin has a rectangular section defining opposite flat brake surfaces (59, 60) and said framework carries opposite linear friction brakes (56) having friction pads (61, 62) positioned to become frictionally and slidingly engageable with said brake surfaces (59, 60). A damper system (20) according to claim 9, characterised in that said framework (26) has opposite to said fins (54) upright vertically-extending structural members (27) each supporting said linear friction brakes (56). A damper system for damping the heave of an offshore structure (10) according to claims 2 and 4, characterised in that said piston-rods (33) receive said continuous supply of hydraulic fluid from a hydraulic network (35) including an accumulator (39), and a pump unit (41) supplying hydraulic fluid to said accumulator, and said hydraulic fluid is alternatively pushed in and out of said cylinders (32) during each up and down heave cycle of said beam (30), and said upward acting force against said loadbeam (30) produces downward-acting damping forces on said structure (10) to thereby reduce the relative heave between said loadline (22) and said structure. A damper system for damping the heave of an offshore structure (10) according to claims 1 through 11, wherein said structure (10) is a semi-submersible drilling and/or production platform adapted to operate in very deep waters, said platform is moored to said seabed by said mooring system (21) including a plurality of spread out mooring lines (22), said long loadline is one of said mooring lines, said underwater object (25) is a strong anchor in said seabed (16), and said linear winch (28) is positioned externally and on the side of said platform (10), characterised in that said upward acting force against said loadbeam (30) reduces the spring stiffness of said mooring system (21), which increases the natural periods of oscillation of the pitch, roll, heave, surge, sway, and yaw motions of said moored floating platform (10).

Inventors: PETTY TERRY DON, REHMANN WILLIAM H JR, PETTY, TERRY DON, REHMANN, WILLIAM H., JR.
Applicants: ODECO INC, ODECO INC.
Application year: 1990
Main classification: B63B21
IPC codes: B63B21, E21B17