Patent Publication Number: US-6910438-B2

Title: Oscillation suppression and control system for a floating platform

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to resonant oscillation suppression systems for offshore floating platforms. 
   Tension Leg Platforms (TLPs) are floating platforms that are held in place in the ocean by means of vertical structural mooring elements called tendons, which are typically fabricated from high strength, high quality steel tubulars, and include articulated connections on the top and bottom (tendon connectors) that reduce bending moments and stresses in the tendon system. Many factors must be taken into account during the design of the tendon system to keep the TLP safely in place including: (a) limitation of stresses developed in the tendons during extreme storm events and while the TLP is operating in damaged conditions; (b) avoidance of any slackening of tendons and subsequent snap loading or disconnect of tendons as wave troughs and crests pass the TLP hull; (c) allowance for fatigue damage which occurs as a result of the stress cycles in the tendons system throughout its service life; (d) limit natural resonance (heave, pitch, roll) motions of the TLP to ensure adequate functional support for personnel, equipment, and risers; and (e) vibrations in the platform system arising from vortex-induced vibrations. 
   As water depth increases beyond about 4,000 ft, the TLP system cost begins to be driven by the cost of the tendon system due to the length and wall thickness of tendons and by fatigue considerations. To provide adequate platform motion control and to limit the amount of fatigue damage caused by each stress cycle, it has been thought necessary to limit the natural resonance periods of the TLP system (heave, pitch and roll) to the 3-4 second range by increasing the cross-sectional area of the tendon (i.e., by stiffening the “spring” since the “mass” of the platform is set mainly by operational considerations). The increasing requirement for more steel cross-sectional area in addition to length in deeper water causes the tendon system to become heavier, thus increasing the tendon cost and reducing the payload carrying capacity of the platform system, i.e. more and more platform buoyancy is ‘consumed’ merely supporting its own mooring system. This combination of increasing tendon length and tendon wall thickness causes the tendon system to dominate total installed cost of the entire TLP system in deepwater installations, i. e. beyond 6000 ft water depth. 
   It is therefore an object of the present invention to provide a floating platform system including a passive oscillation suppression system that inhibits resonant responses in the platform system leading to better motions for personnel, equipment and riser support, and to lighter and lower cost tendon systems. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, an oscillation suppression system is provided to inhibit resonant oscillations of a floating platform. The oscillation suppression system includes energy absorbtion chambers that may be integrated into or be separately attached to the hull of the floating platform. The chambers are comprised of air (or other gas) in the upper portion, which may be closed or partially vented to the atmosphere, and water in the lower portion, which is open at the bottom. The enclosed air in the upper portion of the chamber acts as an air spring reacting between the floating platform and the water mass. Suppression of resonant oscillations of the floating platform is accomplished through air pressure variations in phase opposition to external forces on the floating platform. The dimensions of the chambers are chosen to produce natural periods of water mass oscillation near the resonant periods of the floating platform. Pressure changes result from changes in the air chamber volume caused by the vertical motion of the water mass relative to the floating platform. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features, advantages and objects of the present invention are attained can be understood in detail, a more particular description of the invention briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       FIG. 1  is a side view of a mono-column floating platform depicting energy absorption chambers of the oscillation suppression system of the present invention attached to the hull of the floating platform; 
       FIG. 2  is a section view of the floating platform of the present invention taken along line  2 — 2  in  FIG. 1 ; 
       FIG. 3  is a section view of an energy absorption chamber of the present invention; 
       FIG. 4  is a section view of an energy absorption chamber of the present invention depicting valve venting means thereon; 
       FIGS. 5A-5F  are section views of alternate embodiments of energy absorption chambers of,the present invention; 
       FIGS. 6A  is a side view of a mono-column floating platform depicting stepped diameter energy absorption chambers of the present invention secured to the hull of the floating platform; 
       FIG. 6B  is a section view of the floating platform of the present invention taken along line  6 B— 6 B in  FIG. 6A ; 
       FIG. 7  is a partially broken away side view of a mono-column floating platform depicting an annular energy absorption chamber of the oscillation suppression system of the present invention incorporated in the hull of the floating platform 
       FIG. 8  is a section view of the floating platform of the present invention taken along line  8 — 8  in  FIG. 7 ; 
       FIG. 9  is a section view of an alternate embodiment of the oscillation suppression system of the present invention depicting multiple energy absorption chambers incorporated in the hull of the floating platform; 
       FIGS. 10  is a partially broken away side view of a multi-column floating platform depicting the oscillation suppresion system of the present invention incorporated within the four support columns of the floating platform; 
       FIGS. 11  is a section view of the floating platform of the present invention taken along line  11 — 11  in  FIG. 10 ; 
       FIGS. 12-17  are side and section views depicting alternate embodiments of the oscillation suppression system of the present invention; 
       FIG. 18  is a schematic diagram representing a platform and the oscillation suppression system of the present invention; and 
       FIG. 19  is a schematic diagram representing the oscillation suppression system of the present invention including controlled venting means. 
   

   DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
   Referring first to  FIG. 1 , a mono-column hull floating platform generally identified by the reference numeral  10  is shown. The floating platform  10  includes a column or hull member  14  projecting above the water surface  16  supporting a platform deck  15  thereon. Pontoons  18  extend radially outward from the base of the hull  14 . The floating platform  10  is anchored to the seabottom by tendons  20 . 
   In a typical tendon design, steel tendons are utilized to secure the floating platform  10  to the seabottom. As exploration and production of oil reserves expand into deeper waters, the design of the tendon system becomes more critical and begins to dominate the platform costs. The tendon system must be designed to operate between tolerable minimum and maximum tensions, to restrict natural resonance motions, and to limit the fatigue damage caused by each stress cycle. The latter two are typically accomplished by increasing the cross-sectional area of the steel tendon, which increases the tendon axial stiffness. But this increases the weight of the tendon and reduces the payload carrying capacity of the platform  10 . 
   Including an oscillation suppression system in the platform design may lessen the cost premiums associated with motion limiting and fatigue-driven tendon design. The oscillation suppression system inhibits vertical and rotational resonance in the tendon system by applying an out-of-phase force on the TLP system, compensating external forces. 
   In accordance with the present invention, counteracting expected or unexpected vibrations in a platform system is accomplished by providing compensating forces through a tuned vibration absorber oscillation suppression system. The tuned vibration absorbing system is similar in function to such systems used to prevent vibrations in machinery or swaying of tall building structures, but in this application is composed of water masses and air springs. Referring to  FIG. 18 , the tuned oscillation suppression system of the present invention is conceptually similar to a two-degree-of-freedom oscillator pair, in which energy associated with a large mass-spring system, mass M, spring stiffness K, is naturally transmitted to a smaller mass-spring system, mass m, spring stiffness k. There is a supplementary spring k g  which represents the hydrostatic restoring of the water level in the energy absorption chambers of the present invention, and which makes the solution somewhat different than the classic case. Referring to  FIG. 19 , in the present invention, the platform  10  is the large mass M P , the tendons  20  are the large spring K P , water in one or more energy absorption chambers acts as the smaller mass, m w , and air in the upper portion of the energy absorption chambers acts as the smaller spring stiffness, k a . Air flow {dot over (m)} a  through a valve or throttle plate provides a damping effect to the air spring k a . and is used to adjust the tuned oscillation suppression system damping. 
   In summary, the air-water chambers of the oscillation suppression system of the invention operate as parasitic mass-spring systems transferring energy from the floating platform to the water. 
   Specification of the oscillation suppression system is controlled by the requirement that the natural frequency of the vertical oscillation of the water mass in the chambers be near the natural frequency of the floating platform system. The oscillation suppression system&#39;s natural oscillation frequency depends on the ratio of the combined air-spring and water-column stiffness to the water-column mass. To maintain a fixed ratio between the oscillation suppression system&#39;s natural period and the floating system&#39;s natural period, changes in the stiffness and water mass of the oscillation suppression system must occur in the same proportion. 
   For the passive oscillation suppression system described herein, pressure changes result from changes in the air chamber volume caused by the vertical motions of the water mass relative to the floating platform. The net force from the pressure changes that acts on the floating platform is proportional to the aggregate waterline area of the oscillation suppression system. Individual oscillation suppression chambers should have small transverse dimensions compared to in-water column length to inhibit secondary, horizontal water mass displacements. 
   Increasing the in-water column length of the oscillation suppression system increases the water mass, reduces the relative influence of surface gravity waves within the chamber, and reduces the relative effects of the hydrostatic spring noted as k g  above. 
   While it is theoretically possible in the absence of any damping in the tuned-oscillator to entirely negate resonant motions of the floating platform for a very narrow range of frequencies, in practice, exciting forces and responses are likely to occur over a relatively broad range of frequencies. With an oscillation suppression system, the resonant frequencies of each of the floating platform&#39;s vertical mode resonant responses are split into two distinct frequencies, shifting the resonance to higher and lower frequencies. External forcing at these new resonant frequencies, with low oscillation suppression system damping, will result in larger than desired resonant responses of the floating platform. With increased damping of the oscillation suppression system, the response near the original resonant frequency will increase, but the response at the new resonant peaks will diminish. An optimal damping can be found that minimizes the maximum response of the floating platform over all frequencies. 
   Referring again to  FIG. 1 , the platform  10  of the invention is provided with one or more energy absorption chambers secured on the hull  14  of the platform  10 . In the configuration shown in  FIG. 1 , the energy absorption chambers comprise three cylinders  30  equally spaced about the hull  14 . The cylinders  30  include an open bottom end  32  and a closed or partially vented upper end  34 . The cylinders  30  are partially filled with a water mass  36 . The upper portion of the cylinders  30  is filled with air or other gas, which forms an air spring  38 . The water mass  36  oscillates vertically against the air spring  38  within the cylinders  30  and thereby inhibits resonant oscillations of the platform  10 . 
     FIGS. 3 and 4  show a means of damping of the oscillation suppression system of the invention without frictional or hydrodynamic drag forces acting on the water mass in the cylinders  30 . By controlled venting of air through an orifice  33  or a control valve  35 , it is possible to damp the oscillation suppression system of the platform  10  and to remove large energy pulses from the system before the occurrence of large platform resonant oscillations and their associated high tendon stresses. 
   Various energy absorption chamber configurations may be utilized for increasing or decreasing the turbulence of the flow within the energy absorption chambers to vary the energy absorption characteristics of the oscillation suppression and control system of the platform  10 .  FIGS. 5A-5F  illustrate several embodiments of energy absorption chambers. In  FIG. 5A  the energy absorption chamber is a cylinder  40  having an open bottom and a closed top. The energy absorption cylinders  40  may include a screen or baffle plates  42  in the water mass portion ( FIG. 5B ) or in the air mass portion ( FIG. 5C ) of the cylinders  40 . Screens or baffle plates may also be incorporated in both the air and water mass portions of the cylinders  40 . In  FIG. 5D  the cylinder  40  includes a sharp lower end  44  and in  FIG. 5E  the lower end  46  of the cylinder  40  provides a smooth flared entry into the bottom of the cylinder  40 . In  FIG. 5F , the cylinder  40  includes pipe  48  concentrically mounted within the cylinder  40  to control sloshing and to provided additional damping surfaces. The energy absorption characteristics of the oscillation suppression and control system of the invention may also be adjusted by shortening or lengthening the water mass portion and/or the air mass portion of the energy absorbing cylinders  40 . However, excessive hydrodynamic or frictional damping of the water mass may render the oscillation suppression system ineffective and should be avoided. 
   Referring now to  FIGS. 6A and 6B , the oscillation suppression system of the invention comprises energy absorbing chambers  50  mounted about the hull  14  of the platform  10 . The chambers  50  are stepped diameter cylinders including a lower portion  52  having a diameter less than the diameter of an upper portion  54 . Trapped air in the upper portion  54  forms an air spring  56 . The stepped diameter configuration of the energy absorbing chambers  50  permits the platform designer the flexibility to limit the height of the energy absorbing chambers  50  while still controlling the volume of the air spring  56 . While the diameter of the water portion  52  is preferably constant for a particular design, flexibility is provided by altering the size and shape of the air spring  56  and thereby changing the volume of the upper portion  54  of the energy absorbing chambers  50  for fine tuning the oscillation suppression system of the invention. Fine tuning of the oscillation suppression system may also be accomplished by increasing the diameter of the lower portion  52  rather than the upper portion  54  of the energy absorbing chambers  50 . 
   Referring now to  FIGS. 7 and 8 , an alternate embodiment of the oscillation suppression of the invention is depicted wherein a platform  60  includes an annular configuration of the oscillation suppression system incorporated into the structure of the platform hull. The oscillation suppression system comprises a vertical annular chamber  62  open at the bottom  63  and closed or partially vented at the top  65  thereof. The outer surface  64  of the annular chamber  62  may define the outer diameter of the platform hull. Integrating the annular chamber  62  into the hull structure of the platform  60  may result in fabrication cost savings and may make it possible to economically obtain a large capacity oscillation suppression system. The capacity of the oscillation suppression system may be altered by changing the external diameter of the platform hull, or the diameter of the inner wall  66  of the annular chamber  62 . 
   The energy absorption characteristics of the annular chamber  62  may be altered further by partitioning the annular chamber  62  into multiple chambers  68  as shown in FIG.  9 . The chambers  68  are formed by installing partitions  70  in the annular chamber  62  between the inner and outer surfaces  64  and  66  forming the annular chamber  62 . Not all segments of the partitioned annular chamber  62  need be utilized for energy absorption chambers. 
   In  FIGS. 10 and 11  an embodiment of the oscillation suppression system for a multi-column platform is shown. In this embodiment, the oscillation suppression system of the invention includes one or more energy absorbing chambers  82  mounted within the four columns  84  of a platform  80 . The energy absorbing chambers  82  are preferably located within the columns  84 . The upper ends of the absorbing chambers  82  are closed by plates  86  which secure the chambers  82  within the platform support columns  84 . Flange plates  88  circumscribing the open lower ends of the chambers  82  close off the bottom ends of the platform support columns  84 . The energy absorbing chambers  82  may also be attached to the outer surface of the platform columns  84  in a manner similar to that of the embodiment of the invention shown in FIG.  1  and described hereinabove. 
   Referring now to  FIGS. 12-17 , various alternate embodiments of the oscillation suppression system of the invention are shown which may be desired because of environmental and/or platform design criteria. The alternate oscillation suppression system configurations include spherical air spring chambers  90  (FIGS.  12  and  13 ), arcuate energy absorbing chambers  92  (FIGS.  14  and  15 ), and energy absorbing chambers  94  designed integral to a platform hull (FIGS.  16  and  17 ), or mounted in a moonpool of a platform. 
   Although the energy absorbing chambers shown in the figures and referred to in the discussion above are primarily referred to as single chambers, there may be vertical partitioning of any of the energy absorbing chambers to limit the horizontal extent of the free surface within a chamber. Vertical partitioning will prevent gravity waves from occurring, which may disrupt the dynamics of the oscillating mass. The vertical partitions may extend only near the water line, or extend up to the full length of the energy absorbing chambers. 
   In all cases, a gas or gases may be substituted for the use of air in the description of the invention above. Such gases, for example carbon dioxide or nitrogen, include elastic properties which fulfill the function of the air in the description of the invention, and may add other desirable qualities, such as better corrosion control or better control of pressure/volume behavior. 
   While various embodiments of the invention have been shown and described, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.