Abstract:
A deep water support platform, suitable for use as a hydrocarbon exploration or production facility in very deep waters of 10,000 ft or more is presented. The platform is attached to the floor of the ocean with a buoyant pile that includes buoyant members attached about the periphery of the pile. The buoyant pile and buoyant members include tubular members that can be filled with water, oil, air or other materials to produce a structure that has improved buoyancy and stability over prior platforms. Embodiments include configurations of buoyant members that have constant and equal diameter and spacing, and other configurations where the diameter and/or spacing of the buoyant members changes along the pile. In addition, the buoyant members are arranged about the pile to reduce vortex induced vibrations on the platform by interfering with current flow about the support structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority as a division of allowed U.S. patent application Ser. No. 10/308,299, published Jun. 3, 2004, (Pub. No. US 2004/0105724), the entire disclosure of which is incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to permanently affixed support structures for conducting operations in deep-water and, in particular, structures used to support deepwater, offshore platforms used in connection with oil and gas exploration and extraction.  
       BACKGROUND OF THE INVENTION  
       [0003]     Offshore platforms are used to provide stable and safe locations above the ocean surface for drilling and other operations associated with the exploration and extraction of oil and gas resources. While offshore platforms have been used by the oil and gas industry for many years in relatively shallow waters, such as the Gulf of Mexico or the North Sea, the increasing demand for energy has created the need to exploit oil and gas resources from deepwater locations. Many of the traditional offshore platform designs used for shallow water applications are not practically adaptable for use in deeper waters. In addition, the platform designs that are in use, or which have been proposed for use in deep water locations have various disadvantages and limitations.  
         [0004]     The design of offshore platforms presents many structural engineering challenges. Such platforms are subjected to severe environmental forces associated with the movement of the surrounding water and air. The platform responds to these forces by moving, to some degree, in several ways, including horizontal movement along the surface in direct response to an applied force, rolling (side-to-side rocking along an axis in the direction of the prevailing current), pitching (side-to-side rocking along an axis perpendicular to the direction of the prevailing current), yawing (rotation about the vertical), heaving (up and down motion), surging (an offset in the direction of the current about the anchorage), and swaying (an offset sideways about the anchorage). The structure must be able to withstand periodic forces that are capable of inducing vibration, possibly causing at oscillating frequencies of the structure. These movements, while unavoidable, must be constrained within acceptable limits by the structural design of the platform. This, in turn, imposes limitations on the various components used in the design. The limits on what constitutes acceptable movement of the platform is normally determined by the nature of the operations that are intended to occur on or near the structure, such as the operation of drilling equipment and the docking of ships or landing of helicopters on a platform, the protection of risers from the seabed to the platform, and the support of risers that pass into the seabed. The structure and any occupants must also be able to safely ride the high winds and seas of storms.  
         [0005]     Deepwater platforms in use, or which have been proposed, include (1) tension leg platforms (TLPs) that are fixed at a location with generally vertical tendons anchored to the seabed that are in tension and are connected to a floating platform, (2) catenary moored systems such as semi-submersible floating structures and spar-like floating structures that are stabilized with cables anchored to the seabed and forming a catenary between the floating platform, and (3) buoyant leg structure (BLS), sometimes referred to as a buoyant “pile” structure. Buoyant leg structures are described in the following U.S. patents, incorporated herein by reference: U.S. Pat. Nos. 5,118,221, 5,443,330, and 5,683,206 to Copple, and U.S. Pat. No. 6,012,873 to Copple et al. (the “Copple patents”). For reasons described in the Copple patents, the buoyant upper portions of a BLS provide added stability against environmental forces.  
         [0006]     There are several features that are common to buoyant leg structures. A BLS includes one or more hollow members that form a column that extends downwards from the surface of the water towards the seabed. The hollow members can be formed, for example, from stacked compartments or from an elongated hollow member, such as a pipe or tube. The column is anchored to the seabed, either directly or by a tether. The hollow members have a lower portion that is partially filled with seawater or can be used for storing oil, and an upper portion that is emptied to provide predetermined buoyancy. For BLSs formed from elongated hollow members, a watertight bulkhead provides partitioning between the lower portion and upper portion. The center of buoyancy is above the center of gravity, so that when the top of the BLS is displaced by currents or winds, a righting moment tends to straighten the BLS. Another characteristic of a BLS is that the lower portion of the BLS is in tension and the upper portion of the BLS is in compression.  
         [0007]     While the BLS designs described in the Copple patents disclose structures wherein the buoyant leg is directly anchored to the seabed, subsequent BLS designs contemplated by the inventors are anchored by a tether enabling use at water depths much greater than alternative deepwater platforms-perhaps to depths of 10,000 feet or more. At these greater depths, the natural oscillating period of a BLS increases in heave, and may correspond to periods having substantial wave energy. When this occurs, energy from the waves can couple into the BLS, producing large up and down platform motions. Prior buoyant leg structures have a limited ability to design around this problem.  
         [0008]     Therefore, it is one aspect of the present invention is to provide a BLS having added stability in deep water.  
         [0009]     It is another aspect of the present invention is to provide an offshore deep-water platform suitable for use at great depths that has increased buoyancy.  
         [0010]     It is yet another aspect of the present invention is to provide an offshore deep-water platform suitable for use at great depths that has increased buoyancy for supporting heavier platforms.  
         [0011]     It is one aspect of the present invention to provide an offshore deep-water platform that is less susceptible to vortex shedding and to vortex induced vibrations.  
         [0012]     Another aspect of the present invention is to provide an offshore deep-water platform that is simple in design, and which is relatively easy and inexpensive to construct, moor and operate.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention solves the above-identified problems of prior BLS systems by providing a BLS having increased buoyancy and mass. In accordance with one aspect of the present invention, the BLS has a buoyant leg anchored to the seabed and provides added buoyancy through a plurality of buoyant members attached to the upper end of the buoyant leg. In one embodiment, the buoyant members are cylindrical, and they are aligned with and connected to the upper portion of the buoyant leg.  
         [0014]     In accordance with another aspect of the present invention, a tethered BLS having additional ballast in the buoyant unit is provided having a natural period in heave that does not correspond with the energy spectra of the water.  
         [0015]     In accordance with yet another aspect of the present invention, a deep-water support system for supporting a structure adjacent to the surface of a body of water at a pre-selected site is provided by an apparatus having at least one buoyant pile and at least two buoyant tubular members having elongate shapes. The pile has a lower end anchored to the bottom of a body of water and an upper portion for mounting the structure. The pile is also at least partially filled with a buoyant material. The tubular members are connected to the upper portion and are also at least partially filled with buoyant material to increase the buoyancy of the pile. In another embodiment of the present invention, the pile is anchored to the bottom of a body of water by a tether.  
         [0016]     In accordance with another aspect of the present invention, a deep-water support system for supporting a structure adjacent to the surface of a body of water at a pre-selected site is provided to reduce vortex-induced vibrations in the structure.  
         [0017]     In accordance with yet another aspect of the present invention, vortex-induced vibrations are reduced by providing spacing between buoyant members that varies along the length of the members. In accordance with another aspect of the present invention, vortex-induced vibrations are reduced by providing buoyant members diameters that vary along the length of the members. In accordance with yet another aspect of the present invention, vortex-induced vibrations are reduced by providing about the structure buoyant members of different.  
         [0018]     A further understanding of the invention can be had from the detailed discussion of the specific embodiments below. A BLS platform according to the present invention may include buoyant or non-buoyant members that differ from these embodiments, or may be assembled in way that differ from these embodiments. It is therefore intended that the invention not be limited by the discussion of specific embodiments.  
         [0019]     Additional objects, advantages, aspects and features of the present invention will become apparent from the description of embodiments set forth below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The foregoing aspects and the attendant advantages of the present invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0021]      FIG. 1  is a side view of a first embodiment of a buoyant leg structure of the present invention;  
         [0022]      FIG. 2  is a sectional view through the buoyant unit of the first embodiment, indicated as section  2 - 2  in  FIG. 1 ;  
         [0023]      FIG. 3  is side view of the first embodiment where the platform is laterally displaced;  
         [0024]      FIG. 4  is a side view of a second embodiment of a buoyant leg structure of the present invention having a multiple member restraining unit;  
         [0025]      FIG. 5  is a sectional view through the restraining unit of the second embodiment, indicated as section  5 - 5  in  FIG. 4 ;  
         [0026]      FIG. 6  is a side view of a third embodiment of a buoyant leg structure of the present invention wherein the restraining unit is tethered to the seabed;  
         [0027]      FIG. 7  is side view of the third embodiment where the platform is laterally displaced;  
         [0028]      FIG. 8  is a side view of a fourth embodiment of a buoyant leg structure of the present invention having buoyant members of varying spacing;  
         [0029]      FIG. 9  is a side view of a fifth embodiment of a buoyant leg structure of the present invention having buoyant members of varying diameter; and  
         [0030]      FIG. 10  is a sectional view through an alternative buoyant unit embodiment as section  2 - 2  of  FIGS. 1, 6 ,  8 , or  9 , and having buoyant members of different diameters. 
     
    
       [0031]     Reference symbols are used in the Figures to indicate certain components, aspects or features shown therein, with reference symbols common to more than one Figure indicating like components, aspects or features shown therein:  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     To facilitate its description, the invention is described below in terms of specific embodiments and with reference to the Figures.  FIG. 1  is a side view of a first embodiment of a BLS  100  of the present invention. In general, BLS  100  includes an upper buoyant unit  110  and a lower restraining unit  120 . BLS  100  is shown with buoyant unit  110  supporting a platform  10  with a frame  20  above a surface S of a body of water and with restraining unit  120  moored to seabed B, in a depth of water depth L, with an anchorage  30 . While BLS  100  is shown above surface S, it is understood that waves may occasionally rise above the BLS, possibly to the level of platform  10 . Platform  10 , frame  20 , and anchorage  30  are conventional or conventionally designed items that are shown to place the invention in context of one use of a BLS, and are not intended to limit the scope of the present invention.  
         [0033]     Buoyant unit  110  extends from an upper end  119  to a lower end  112  at transition unit  115 , and restraining unit  120  extends from an upper end  121  at the transition unit to a lower end  123 . Also extending at least a portion of the length of BLS  100  is an elongated tubular member  111  that is similar to those described in the Copple patents. In general, elongated tubular member  111  can be anchored to the seabed, as in the first embodiment, or can be tethered to the seabed, as discussed subsequently. For embodiments where member  111  is anchored to the seabed, it is proper to refer to this member as pile  111 . Since the elongated tubular member of the BLS is not necessarily anchored, it is thus understood that the term “pile” is not meant to limiting the scope of the claims, but is used only to denote elongated tubular member of an anchored BLS. Pile  111  of the first embodiment extends the length of buoyant unit  110  and restraining unit  120 , and includes a transition unit  115 . Several elongate buoyant members  113  are arranged about and attached to pile  111  to form part of the buoyant unit  110 , while restraining unit  120  consists primarily of the pile. Restraining unit lower end  123  is moored to seabed B by anchorage  30 .  
         [0034]     Pile  111  is generally an elongate structure of tubular, watertight construction. At least one bulkhead  117  is provided at an intermediate location along the length of pile  111 , such as near transition unit  115 , to divide the pile into an upper, buoyant portion and a lower, non-buoyant portion, and to prevent or control movement along the pile of materials such as water, air, oil or other buoyant or ballast materials. In a preferred embodiment, the cross-sectional area of pile  111  decreases from the buoyant unit upper end  119  to the restraining unit lower end  123 , with the change in area being either step-wise, or tapered.  
         [0035]     The upper portion of BLS  100 , including an end of upper buoyant unit  110  and a portion of buoyant members  113 , penetrates the surface S of the body of water, as shown in  FIG. 1 . Platform  10  is attached to the ends of upper buoyant unit  110  and buoyant members  13  that protrude above surface S. In addition to providing a stable platform for mechanical and/or human operations, buoyant unit  110  and restraining unit  120  can provide protection for and lateral bracing of drilling and production risers (not shown) which extend from the seabed to the surface of the platform. Preferably, the risers are routed within the BLS for their entire length, or for at least a portion of their length within the pile of the buoyant unit, and pass through the transition unit to the outside of the BLS.  
         [0036]     While tubular pile  111  is shown as having a circular cross-section. Tubular pile  111  and all tubular or circular members herein are understood to include a variety of other cross-sectional shape members. It is preferable that the cross-sectional shapes be symmetric. Exemplary shapes include round, square, and many-sided regular and irregular shapes.  
         [0037]      FIG. 2  is a sectional view through buoyant unit  110  of the first embodiment, indicated as section  2 - 2  in  FIG. 1 . In a preferred embodiment, tubular buoyant members  113  each have the same diameter, D-1, and are evenly distributed about pile  111  having diameter D-2, where the diameter D-1 is less than the diameter D-2. Pile  111  and buoyant members  113  are joined, supported, and spaced by a plurality of web plates  201  that are aligned with the length of the pile and diaphragm plates  203  that are aligned perpendicular to the length of the pile. Plates  201  and  203  are intermittently spaced between pile  111  and buoyant members  113  to provide spacing of the pile and buoyancy members and to provide rigidity to buoyant unit  110 . The size, shape and placement of plates  201  and  203  are selected to connect pile  111  and buoyant members  113  in a structurally satisfactory manner that will prevent structural failure and limit movement between the pile and buoyant members.  
         [0038]     Buoyant members  113  are spaced a distance Z from each other, giving a center-to-center spacing of W, and the buoyant members are spaced from pile  111  by a distance Y. The number, spacing and rigidity of plates  201  and  203  depends on the diameters of pile  111  and buoyancy members  113 , taking into account the worst case environmental conditions which may be encountered where the BLS  100  is moored. In a preferred embodiment, plates  201  and  203  provide the required rigidity at an acceptable cost, while minimizing the forces on the BLS  100  from the wind, waves, and currents, and also reduces or eliminates the formation of localized vortices that may result in vortex induced vibration. In an alternative embodiment spacing between buoyant members  113  and pile  111  are provided by at least one truss.  
         [0039]     The interior of buoyant members  113  is hollow and is at least partially filled with a buoyant material such as air, and may also include a ballast material, such as water or crude oil. In the embodiment shown in  FIG. 2  the diameter of pile  111  is not the same as the diameter of buoyant members  113 . In general, the diameter of pile  111  and members  113  can be the same or they can be different. The symmetric distribution of buoyant members  113  reduces yawing forces on BLS  100  that can result from non-symmetric wave, current or wind forces. Pile  111  has a skin  211  and buoyant members  113  each have a skin  213 . Pile  111  and buoyant members  113  include ring stiffeners  205  and longitudinal stiffeners  207  to provide additional support under internal and external forces.  
         [0040]     According to one aspect of the present invention, buoyant members are used to increase the overall buoyancy of BLS  100 . In general, the center of buoyancy of buoyant unit  110 , including pile  111  and buoyant members  113 , is located above the center of gravity of the BLS  100 , providing a righting force to maintain platform  10  above surface S and to prevent unwanted tilting of the surface of the platform, i.e., departure of the platform surface from a horizontal orientation. Bulkhead  117  divides pile  111  into an upper buoyant portion and a lower non-buoyant portion. The overall buoyancy of BLS  100  depends on the density and distribution of buoyant material and ballast within the pile, the cross-sectional shape of the pile, and the location of bulkhead  117 . The selection of the buoyancy, including the center of buoyancy, and weight, through the addition of ballast, provide a means for modifying the stability of the BLS  100  under the action of wind and water forces. The amount of ballast, which can be water, oil or any other material that is heavier than sea water, is added to limit the pitch and roll of the BLS, and can alternatively be added to control tension in the tendons during storms or can be changed in response to the weight of the platform.  
         [0041]     As is well known, vortices are sometimes formed in a cross-flow across one or more bodies, such as a current flow in the plane of section  2 - 2 . These vortices include pressure variations that can locally interact with the bodies to induce vibrations (vortex induced vibrations). Several embodiments of the present invention address the reduction of these vibrations through structures that minimize either the shedding of vortices or the interaction of these vortices with portions of the BLS.  
         [0042]     One configuration that reduces vortex induced vibrations has alternating buoyant member diameters. A specific alternative embodiment is illustrated in  FIG. 10 , which shows a cross-sectional view  2 - 2  of a buoyant unit  110 ′ having three buoyant members  113   a ′ each with a diameter D-1 and three buoyant members  113   b ′, each with a diameter D-3, and where the diameter D-3 is larger than diameter D-1. As is illustrated in  FIG. 10 , buoyant members  113 ′ are evenly distributed about pile  111 ′. It is preferable that BLS  100  be symmetric about the center of the cross section to reduce the tendency of the structure to rotate by providing buoyant members  113  that are symmetrically placed about pile  111 .  
         [0043]     Another configuration that reduces vortex induced vibrations varies the spacing of the buoyant members along the length of the buoyant unit.  FIG. 8  is a side view of a fourth embodiment of a buoyant leg structure of the present invention having buoyant unit  110 ″ with buoyant members  113 ″ of varying spacing. The cross-sectional view  2 - 2 , as illustrated in  FIG. 2  or alternatively in  FIG. 10 , has symmetric spacing between buoyant members  113 . However, the spacing between buoyant members  113 ′ of the fourth embodiment is shown as decreasing with distance from surface S. Thus the fourth embodiment has values of W and Z that vary along the length of buoyant unit  110 ″. In general, the spacing may vary by having a spacing that varies along the length to reduce the tendency of the BLS  100  to vibrate. This may include portions where the spacing remains constant, or where the spacing changes with increasing depth.  
         [0044]     Yet another configuration that reduces vortex induced vibrations includes buoyant members  113  having diameters that vary along the length of buoyant unit  110 .  FIG. 9  is a side view of a fifth embodiment of a buoyant leg structure of the present invention having a buoyant unit  110 ′″ with buoyant members  113 ′″ of varying diameter. Specifically, buoyant members  113 ′″ are segmented into four sections:  113   a ′″,  113   b ′″,  113   c ′″, and  113   d ′″. The cross-section of members  113 ′″ is illustrated in cross-sectional view  2 - 2 , as illustrated in  FIG. 2  or alternatively in  FIG. 10 , with each section of members  113 ′″ having different values of spacing (W and Z)  
         [0045]     In the absence of lateral forces, BLS  100  assumes a vertical orientation as shown in  FIG. 1 . Buoyant unit  110  provides an upward force that is balanced by the weight of the BLS  100  and the holding force exerted by anchorage  30 . A schematic showing the effect of lateral forces on BLS  100  is shown in  FIG. 3  as a side view, where platform  10  is laterally displaced by a distance A from a line  300  representing the unperturbed position of BLS  100 . Since BLS  100  is a moored, buoyant structure, it has limited horizontal and vertical movement about the mooring. Buoyant member  110  maintains a substantially vertical orientation, while restraining unit  120  accommodates the lateral movement of BLS  100  by bending. Since restraining unit  120  is in tension and is relatively flexible in comparison with the remaining structure, it bends in response to the lateral forces, as depicted in  FIG. 3 , with the bending occurring mostly at upper end  121  and lower end  123 , so the restraining unit  120  remains relatively straight in the central portion.  
         [0046]     As the result of the lateral forces (i.e., wind, current, waves) acting on BLS  100 , combined forces represented by an external force  307  act on BLS  100 , displacing the structure distance A. Also shown in  FIG. 3  is a center of buoyancy  301  and a center of gravity  305  corresponding to the positions where a buoyancy force  303  and a gravitational force  317  may be viewed as acting on BLS  100 , respectively.  
         [0047]     In general, a vertical reaction force  313  is exerted by anchorage  30  to counteract the buoyancy and gravitational forces  303  and  317 , and in reaction to external force  307 , a horizontal reaction force  311  is exerted on BLS  100  at the anchorage. As a result of the displacement A, buoyancy force  303  and gravitational force  317  are displaced horizontally with respect to reaction force  313 . Although buoyancy force  303  and gravitational force  317  can also be displaced vertically, this is a secondary effect since the lateral displacement is generally small in comparison to the height of BLS  100 . The lateral displacement of the vertical forces  303 ,  317 , and  313  generates a righting moment where the BLS  100  is fixed to anchorage  30  that tends to right the BLS. It is an important feature of the present invention that center of buoyancy  303  is located above center of gravity  305 . This relationship between the vertical forces provides stability in the vertical direction by maintaining platform  10  vertical and above surface S and by resisting pitching of the platform, and generates a righting moment. In a preferred embodiment, buoyant unit  110  contains ballast  315  to lower the center of gravity and further increase the resistance of platform  10  to pitching motions.  
         [0048]     Restraining unit  120  accommodates the external forces on BLS  100  through tension and lateral forces that stretch and bend the unit. BLS  100  is adapted for use in very deep waters with restraining unit  120  having a length-to-diameter of several hundred to several thousand to one, allowing the restraining unit to flex a significant amount. It is important that the flexure occurs without high bending stresses that may fatigue the restraining unit material and limit its lifetime.  
         [0049]     A preferred embodiment of the present invention useful for deepwater operation may be constructed within the following parameters. The depth of water L can range from approximately 600 ft to approximately 10,000 ft or more, and is preferably more than 1,000 ft. Buoyant unit  110  preferably extends from above surface S to a depth D of hundreds of feet, preferably at least approximately 400 ft. Buoyant members  113  have equal diameters d that may be in the range of from 10 to 35 ft, or larger. In one embodiment there are six buoyant members of approximately  20  ft in diameter, symmetrically distributed about a center pile  111 . Pile  111  of buoyant unit  110  also has a diameter d, as shown in  FIG. 2 , that is larger than that of buoyant members  113 , though other embodiments may include piles of different diameter than the buoyant members. Alternatively, pile  111  is bigger than the surrounding members  113 . For example, pile  111  may be up to 50 feet or greater in diameter, and members  113  are 10 to 35 ft, or larger, in diameter. Those skilled in the art will appreciated that the various structural components, such as buoyant members  113 , pile  111 , restraining unit  120 , webs, etc., are preferably constructed of steel suitable for marine use.  
         [0050]      FIG. 4  is a side view of a second embodiment BLS  400  having a multiple member restraining unit  420 , and  FIG. 5  is a sectional view  5 - 5  through the restraining unit of the BLS. BLS  400  has a pile  411  extending the length of the BLS, and has an upper, buoyant unit  410  and lower restraining unit  420 . The portion of pile  411  within buoyant unit  410  has a construction similar to that of buoyancy unit  110 , as indicated by the common buoyant unit sectional view  2 - 2 . The portion of pile  411  forming restraining unit  420  has more than one member; specifically it includes three legs  501  that are moored at anchor  30 . The use of a restraining unit with multiple legs provides several benefits that are realized for two or more legs. For example, such a construction enhances the overall strength of the pile, provides redundancy in the event that one of the legs of the restraining unit fails and may reduce the likelihood or amplitude of forces from vortex shedding.  
         [0051]     In the embodiment of  FIGS. 4 and 5 , structure is included for spacing legs  501  so that the legs do not move axially and impact one another. Legs  501  are interconnected by horizontal diaphragms  503  and longitudinal webs  505  that are distributed along the length of restraining unit  420 . Alternatively, the legs  501  can be held together by circumferential bands and an elastic material, such as rubber, to provide spacing between the legs (not shown). This alternative allows for a small amount of relative longitudinal movement between legs  510 .  
         [0052]     Pile  411  changes from the cross-sectional shape of  FIG. 2-2  to that of  FIG. 5-5  over some length of BLS  400  indicated as a transition portion  415 . In one embodiment, the multiple member restraining unit  420  has a bulkhead  117  at the top of portion  415 , and portion  415  flooded with ballast.  
         [0053]     A BLS has many modes of oscillation that depend on the stiffness, mass, buoyancy and length of the structure. In order to maintain a stable platform, it is important that the periods of these modes do not correspond with periods of water or air motion that might excite a natural mode of the BLS. When the modes of oscillation of the BLS have periods that overlap with the energy spectra of the surrounding water, there is a possibility that oscillations of the BLS can be amplified, producing a very unstable situation and, possibly, catastrophic failure of the platform support structure.  
         [0054]     One particular mode of concern for deep-water BLS is the up-and-down motion of heave. The wave energy spectrum for deep water is particularly strong in the range of 6 to 18 seconds. For water depths below 7000 ft, the heave natural period for the BLS shown in  FIGS. 1-5  is approximately 5 seconds. As the water depth increases, the heave natural period of a BLS increases and can approach the 6 to 18 second range of the deep-water energy spectra. One way to decrease the natural period of the BLS is to change the axial stiffness of the buoyant unit by tethering the restraining leg to the anchor and by the selection of the buoyancy and weight of the BLS. Specifically, by selecting a tether having an elastic modulus less than that of the restraining leg, the axial stiffness can be decreased to acceptable level with a natural heave period greater than 18 seconds. Either a single or multiple strand cable of either steel or polyester can obtain the appropriate elastic modulus.  
         [0055]      FIG. 6  is a side view of a third embodiment of a BLS  600 , wherein the restraining unit is tethered to the seabed, and  FIG. 7  is side view of the third embodiment, where the platform is laterally displaced a distance C. BLS  600  has buoyant unit  110  and elongated tubular member  111  similar to those of the first embodiment BLS  100 . A restraining unit  620  includes the lower end of member  111  and a tether  640  that extends from a lower end  643  attached to anchor  30  to an upper end  641  that is attached to the lower end  123  of member  111 . It is preferable that the lower end of restraining unit  620  is long and flexible enough so that most or all bending occurs in restraining unit  620 . This has the advantage of reducing stress concentrations in tether  640  and restraining buoyant unit  110  from pitching and rolling.  
         [0056]     The use of tether to moor BLS  600  may allow the structure to yaw more easily that a BLS having a pile connected to the seabed, as in BLS  100 . If necessary, the increased tendency to yaw can be overcome by the addition of supplemental moorings.  
         [0057]     The invention has now been explained with regard to specific embodiments. Variations on these embodiments and other embodiments may be apparent to those of skill in the art. It is therefore intended that the invention not be limited by the discussion of specific embodiments. It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims.