Mechanical damper system for a floating structure

Abstract: A mechanical damper system (20) is disclosed for a floating structure (10). The floating structure (10) has a framework (21) for supporting a tensioner system (23) and a mechanical brake system (22) operatively coupled to the tensioner system. A long member (25) has a bottom end (32) anchored to the seabed (16) and a top end (34). The tensioner system suspends the long member's top end (34) from the framework. The tensioner system, in use, applies a predetermined tension on the top end of the long member, which is frictionally varied by the brake system in dependence on the heave of the floating structure, thereby exerting corresponding damping forces thereon.

Description: This invention refers to a damper system according to the features of the introductory part of claim 1 (see especially prior art of US-A-4617998), and in particular to a platform used for conducting drilling and production operations over a seabed site. With this type of structure a long member has a lower end secured to a stationary underwater object in the seabed and a movable upper end extending upward to a deck on the platform. There is small osillatory up and down heave between the movable platform and the independently movable upper end of the long member, which typically is relatively rigid. Tensioner means support the movable upper end for continuously pulling thereon with an initial upward-acting force that induces in the movable upper end an initial tension, as the platform heaves within an expected maximum heave range in response to anticipated environmental forces. Applicant's own US-Patent 48 50 744 shows a damper for damping the heave of a floating semi submersible platform for drilling and production operations. This platform, which is known as ELDORADO, can operate in very deep waters without being tied to the seabed by tension legs. It uses a cluster or risers, each having a lower end fixedly connected to a submerged well on the seabed site, and an upper end connected to an above water wellhead tree. The riser's movable upper end and its wellhead are suspended from a wellhead deck by a relative-heave tensioner which allows up and down small vertical relative motion or heave between the platform and the riser's upper end. US-Patent 43 79 657 discloses a pneumatic-hydraulic tensioner typically used for inducing in the riser an upward, substantially constant tension, as the riser slowly heaves relative to the platform in response to environmental forces acting on the platform and the riser and regardless of the waves falling within an expected maximum wave range. According to US-Patent 43 79 657 and US-Patent 47 33 991 great precautions have been taken to design the relative motion tensioners so as to maintain a nearly constant unchanged tension within the respective risers (see also GB-Patent 20 62 068, page 2, lines 17 - 25). As described in US-Patent 43 95 160, column 2, lines 24 - 55 it was generally believed to those skilled in the art that if the tensioners were not to induce a constant tension in their risers, then the risers might buckle or otherwise become damaged. According to US-Patent 47 33 991 (column 1, lines 18 - 19 and column 2, lines 25 - 26) it was also generally believed in the art that such tensioners can be used for drilling and production operating only when the platform has "very small vertical oscillations". However, seas may produce waves that might, although very rarely, fall outside of the expected maximum wave range, and which can induce heaves having a frequency approaching the platform's resonant period T n . Applicant's ELDORADO platform (US-Patent 48 50 744) has a low heave response. It has a natural (resonant) heave period T n that is greater than the longest expected period of any wave with substantial energy in the surrounding waters. However, because the determination of this longest expected frequency is based on historical statistical data, it may be possible that the selected tensioner for the riser may exceed its effective range in seas which produce wave energies that fall outside of the expected maximum wave range. Such an event may present the risk of buckling, crimping or otherwise damaging the risers leading to consequential oil and gas spills into the water. US-Patent 41 67 147 (Figure 14) proposes a passive damping system which requires a tensioned flexible cable the lower end of which is anchored to a stationary weight on the sea floor. After passing over a sheave, the cable's upper end is fixedly secured to the platform's upper deck. This Patent does not offer a solution for damping a riser's upper end, which moves relative to the platform. Similarly, US-Patent 46 17 998 uses brakes on a drilling vessel to grasp and then stop the vertical motion of the upper end of a riser by securing it to a deck on the platform, "whenever the riser is disconnected from the wellhead" (see Abstract). But, after the riser's lower end is disconnected from the wellhead on the sea bottom, the riser's lower end cannot be placed under tension by the tensioner, as is required in damping an offshore structure with a long stiff member. It is an object of this invention to improve mechanical damper systems according to the introductory part of claim 1 in such a manner that the floating offshore structure is also protected against wave energies that fall outside of the expected maximum wave range, and to avoid the risk of buckling, crimping or otherwise damaging the risers leading to consequential oil and gas spills into the water. It is a further and more specific object of the invention to provide a braking system which prevents excessive heave in the platform by slowing it down in high waves. According to the subject invention these objects are solved by the features of the characterising part of claim 1. Further embodiments of this invention are the subject of the subclaims. The damper system according to this invention is very practical and yet very economical because it can make use of the onboard-relative motion tensioners that already are associated with the risers. These already existing onboard relative-motion tensioners are incorporated into novel damper-tensioners to generate anti-heave damping forces, which preferably are substantially constant and are generated only in anticipation of severe rough seas and only when the platform heaves up during each oscillatory heave cycle. When the upward buoyant forces on the platform exceed the design capacity of the brakes, the brakes will start to slip and at the same time will slow down the continued upward vertical displacement of the platform due to the constant frictional braking forcs exerted by the brakes against the opposite polished surfaces, of the fins. When the brakes start to slide relative to the fins, they dissipate energy due to the frictional forces (known as Coulomb friction or damping). Because the brakes apply frictional forces against the fins when the platform excessively heaves up, and then they are deactivated as soon as the platform starts to heave down, the platform's down motion will be limited, which will avoid excessive energy dissipation. Constant frictional damping forces most efficiently suppress resonant heave motions of the platform. The nearly constant frictional damping forces will be much larger than damping forces that are dependent on the heave velocity of the platform (Newtonian damping). Specific embodiments of the invention will be described by the way of example only in connection with the accompanying drawings, wherein: Fig. 1 is a schematic side elevation view illustrating applicants' prior semi-submersible floating platform together with the mechanical brake system of the present invention; Fig. 2 is a view taken along line 2-2 on Fig. 3; and Fig. 3 is a plan view of the framework surrounding the arrays of the linear friction brakes. Many different types of floating semi-submersible structures are known and presently employed for hydrocarbon drilling and/or production, and principles of the present invention are applicable to many of these, and also to floating structures of other types. All such structures are subject to resonant heave in a seaway. The invention is illustrated for use with a production platform 10 (FIG. 1) described in applicant's U.S. patent No. 4,850,744. Platform 10 is a column-stabilized, semi-submersible floating structure which is especially useful for conducting hydrocarbon production operations in relatively deep waters over a seabed site 16 which contains submerged oil and/or gas producing wells 17. Platform 10 has a fully-submersible lower hull 11, and an above-water, 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 and its maximum deck load. A wellhead tree (not shown) is coupled to an individual well 17 through a production riser 18. A tensioner (not shown) suspends riser 18 from the upper hull 12 above waterline 19. In use, platform 10 is moored to seabed 16 by a spread catenary mooring system (not shown), which is primarily adapted to resist large horizontal excursions of the platform. Platform 10 is designed to have a very low-heave response to the most severe wave and wind actions that are expected. In accordance with the present invention, the damper system 20 comprises a framework 21 for supporting a tensioner system 23 and a mechanical brake system 22. Framework 21 (FIGS. 2-3) consists of vertical and horizontal I-beams 21a and 21b, respectively, all securely attached to the structure of platform 10. Mechanical brake system 22 includes friction brakes 44 and a hollow brake cylinder 24 having an outer surface 24′ and top and bottom inner braces 24a-24b. Tensioner system 23 is a pneumatic-hydraulic tensioner system of type commonly used to suspend drilling or production risers, and is described in U.S. patents 4,733,991, 4,379,657 and 4,215,950. Tensioner system 23 comprises a pneumatic-hydraulic reservoir (not shown) for supplying through a pipe 26 pressurized hydraulic fluid to a hydraulic cylinder 27 having a power piston 28 and a movable piston rod 29. Pipe 26 connects the bottom of the hydraulic reservoir with the bottom of hydraulic cylinder 27. Hydraulic cylinder 27 is coupled to a transverse beam 21b of framework 21 by a pivot 30. Piston rod 29 extends downwardly and is connected by a pivot 31 to a top brace 24a inside hollow cylinder 24. A very long member 25 has a bottom end 32 tied to a very strong anchor 33 in seabed 16. The upper end 34 of long member 25 is attached by a pivot 35 to a bottom brace 24b inside cylinder 24. Long member 25 preferably is a 95/8″ diameter steel pipe extending down to seabed 16 in several hundred to a few thousand meters of water. Tensioner system 23 suspends cylinder 24 and therefore top end 34 from framework 21 so as to allow relative up and down heave between top end 34 of long member 25 and floating structure 10. A top array 36 (FIGS. 2-3) and a bottom array 37 of centralizing, spring-loaded bearing wheels 38 ride on the outer surface 24′ of brake cylinder 24, which has a circular shape in section. In this manner, wheels 38 restrict the tendency of brake cylinder 24 to rotate and/or to displace laterally, while allowing platform 10 to have limited heave relative to cylinder 24. Fins 40 are angularly spaced apart and are secured to outer surface 24′ by bolts 43. Fins 40 are made of long, flat metal bars each having a rectangular section defining polished opposite surfaces 41, 42. Framework 21 supports arrays of linear, hydraulically activated, friction caliper brakes 44, which carry friction pads 45 adapted to bear against the opposite, polished surfaces 41, 42 of fins 40. Mechanical friction brakes 44 are operated by hydraulic power means (not shown) under the control of an electronic module 47, which is responsive to motion sensors in a line 48 and to load sensors (not shown) on brake pads 45 for the purpose of monitoring a parameter of the heave of floating structure 10, such as the heave's direction, velocity, or acceleration, etc., thereby controlling the operation of the mechanical brake system 22. In use, brake cylinder 24 is always maintained suspended above water line 19. The relative motion between platform 10 and long member 25 is caused by wave and tidal actions. Piston 28 reciprocates in cylinder 27 within a fixed stroke range calculated to compensate for the maximum expected up and down heave of platform 10 relative to brake cylinder 24. For any position of piston 28, piston-rod 29 will apply through cylinder 24 a continuous, predetermined, upward-acting force, which induces a corresponding positive tension on top 34 of long member 25, regardless of the heave and heave velocity of piston-rod 29. The largest expected relative heave of platform 10 must be within this stroke range in order to ensure the structural integrity of long member 25. Tensioner system 23 maintains long member 25 under a large amount of tension, while permitting relative motion between platform 10 and cylinder 24. It is the object of the frictional forces developed by friction brakes 44 to prevent excessive heave in platform 10 by slowing it down, but preferably only in high waves, i.e., waves which create a sufficient buoyant force to overcome the static frictional force which is designed into the brakes. Consequently, the particular draft of platform 10 might be deeper than the nominal draft, and a moderate size wave could cause friction brakes 44 to slip. However, if the platform had already been driven to a higher position (less than nominal draft), a much larger wave would be required to cause brakes 44 to slip. In one embodiment, friction brakes 44 are deactivated when platform 10 heaves-down, but this energy will be stored as potential energy due to the deeper draft. Brakes 44 are preset to lock cylinder 24 with a static frictional design force. This design force is greater than the tension that will be applied to cylinder 24 by the anticipated smaller waves. However, this design force is less than the tension that will be applied to brake cylinder 24 by the anticipated larger waves. Accordingly, friction brakes 44 and fins 40 are designed to be able to first stop the upward displacement of platform 10 in response to these smaller waves. But, when the upward buoyant forces on platform 10 exceed the design capacity of brakes 44, the brakes will start to slip and at the same time they will slow down the continued upward vertical displacement of platform 10 due to the constant frictional braking forces exerted by brakes 44 against the opposite polished surfaces 41, 42 of fins 40. When brakes 44 will start to slide relative to fins 40, they dissipate energy due to the frictional forces (known as Coulomb friction or damping). Because brakes 44 apply frictional forces against fins 40 as soon as platform 10 starts to heave up, and then they are deactivated as soon as platform 10 starts to heave down, the platform's down motion will be limited, which will avoid excessive energy dissipation. When platform 10 is stopped by the brakes, it acts as if it had a taut mooring. Since the braking forces are derived from mechanical brakes 44, the heave energy pumped into platform 10 by the sea waves is converted only into heat or is stored as potential energy due to draft changes. This heat can be conventionally absorbed by platform 10, by heat exchangers, by circulating sea water through fins 40, etc. Mechanical brakes 44 develop frictional forces that are independent of the velocity of the platform's displacement. Accordingly, brakes 44 will generate downward-acting damping forces which are substantially constant and also independent of heave velocity of platform 10. Constant frictional damping forces most efficiently suppress resonant heave motions of platform 10. The nearly constant frictional damping forces will be much larger than damping forces that are dependent on the heave velocity of platform 10 (Newtonian damping). In another embodiment, brakes 44 are activated when platform 10 heaves up and down. Therefore, mechanical brake system 22 increases the tension on top 34 of long member 25 when floating structure 10 heaves up, thereby exerting a downward-acting damping force on the floating structure, and decreases the tension on top of long member 25 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 long member 25 to prevent buckling.

Claims: A mechanical damper system for a floating platform (10), a framework (21) forming part of said structure, a long member (25) having a bottom end (32) and a top end portion (34) near and opposite to said framework, said framework carrying an extensible tensioner system (23) adapted to support said long member under an upward tension, and a mechanical brake system (22) between said framework (21) and said top end portion (34), characterized in that said brake system (22) comprises a brake cylinder (24) having at least one longitudinal fin (40) and a linear mechanical friction brake (44) coupled to said long member (25) and adapted to selectively exert frictional forces against said fin to increase the applied tension in said long member, thereby damping the excessive heave of said structure (10) relative to said upper end portion (34) of said long member (25) in response to severe dynamic sea conditions and while said lower end (32) remains anchored to a very strong anchor (33). A damper system according to claim 1, characterized in that a plurality of said fins (40) are circumferentially-spaced on the cylindrical outer surface (24′) of said brake cylinder (24) and a corresponding plurality of linear mechanical friction brakes (44) operate to selectively exert frictional forces against said fins, thereby to increase said tension in said long member, and said increased tension exerts a corresponding damping force on said structure in a direction opposite to the relative heave between said structure (10) and said top end portion (34) of said long member (25). A damper system according to claims 1 and 2, characterized in that said linear mechanical friction brakes (44) are mounted on said framework, said brake cylinder (24) is on said top end portion (34) of said long member (25) near and opposite to said framework, and said brake cylinder together with said top end portion has vertical reciprocating motion relative to said framework depending on said heave of said structure. A damper system according to claims 2 and 3, characterized in that said outer surface (24′) of said cylinder (24) has a circular shape in section, said fins (40) are angularly-spaced, vertically-elongated, and extend radially and outwardly from said outer surface (24′), each fin is flat and has opposite brake surfaces (41, 42), and said brakes (44) are hydraulically-activated friction brakes, each having friction pads (45) adapted to selectively frictionally bear against the brake surfaces of a corresponding fin. A damper system according to claims 3 and 4, characterized in that top and bottom spring-loaded bearing wheels (38) are adapted to ride on said outer surface (24′) of said cylinder (24) to restrict the tendency of said cylinder to rotate and/or to displace laterally, while said structure (10) heaves relative to said cylinder. A damper system according to claims 4 and 5, characterized in that said brake pads (45) of said brakes (44) and said brake surfaces (41, 42) of said fins (40) engage only when said structure heaves up during a portion of each heave cycle, and they are disengaged during the remainder portion of each heave cycle when said structure heaves down. A damper system according to claim 1, characterized in that said brake system (22) increases said applied tension to said long member only when said structure (10) heaves up, thereby exerting a downward-acting damping force on said structure, and said brake system is deactivated when said structure heaves down. A damper system according to claims 1 - 5, characterized in that said linear friction brakes (44) exert constant frictional forces against said fins (40) to increase said applied tension to said long member (25) by a substantially constant value such that when said structure (10) heaves down, the reduced tension in said long member still remains greater than the minimum tension required to maintain the structural stability of said long member. A damper system according to claims 1 - 8, characterized in that motion control and sensing means (47) are positioned in proximity to said structure (10) for sensing the value of a selected parameter of said heave of said structure and controlling said braking system (22) in accordance with said sensed value of said parameter.

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: E21B15
IPC codes: E21B19, B63B21, E21B17