Patent Publication Number: US-8540460-B2

Title: System for supplemental tensioning for enhanced platform design and related methods

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to tensioning of seabed-to-vessel marine risers. More particularly, this invention relates to tensioning the marine risers with a plurality of tensioning units in combination with a decoupled buoyancy can platform. 
     2. Description of the Related Art 
     A problem presented by offshore hydrocarbon drilling and producing operations conducted from a floating platform is the need to establish a sealed fluid pathway between each borehole or well at the ocean floor and the deck of the platform at the ocean surface. A riser typically provides this sealed fluid pathway. In drilling operations, the drill string extends through a drilling riser serving to protect the drill string and to provide a return pathway outside the drill string for drilling fluids. In producing operations, a plurality of production risers are used to provide a pathway for the transmission of hydrocarbons or other production fluid from multiple wells to the deck. 
     Each riser is typically projected up through an opening referred to as a moon pool in the vessel to working equipment and connections proximate an operational floor on the vessel. A riser pipe operating on the floating vessel in water depths greater than about 200 feet (34.72 meters) can buckle under the influence of its own weight and the weight of drilling fluid contained within the riser if it is not partially or completely supported. For floating platforms, the risers must be tensioned to maintain each riser within a range of safe operating tensions as the work deck moves relative to the upper portion of the riser. If a portion of the riser is permitted to go into compression, it could be damaged by buckling or by bending and colliding with adjacent risers. It is also necessary to ensure that the riser is not over-tensioned when the vessel hull moves to an extreme lateral or vertical position, such as, for example, when under extreme wave conditions or when ocean currents exert a significant side loading on the riser. 
     There are two primary types of tensioning systems: those that use long-stroke top-mounted tensioners (hydraulic, pneumatic, or hydra-pneumatic cylinders connected between the top of the riser and the vessel hull); and those that use buoyancy can tensioners (floatation devices connected to the upper portion of the each riser). The top-mounted tensioning systems function are either passive or active. The passive top-mounted tensioning systems include long-stroke tensioners that utilize cylinders with a stroke of typically between 15 and 30 feet in order to compensate for the movement expected due to deepwater operations. The active top-mounted tensioning systems further include a control system that actively adjusts hydraulic pressure of each long stroke tensioner cylinder to maintain a relatively constant tension on its associated riser. For both the passive and active top-mounted tensioning systems, abrupt lateral and vertical movements of the vessel hull are compensated for by the stroke of the tensioner. The buoyancy can tensioning systems, on the other hand, function by connecting buoyant cans, either individually, or collectively, to the top of each riser at a location below the water line to maintain a relatively constant tension, with abrupt lateral and vertical movements of the vessel hull being compensated for by allowing the buoyancy can and/or riser to slide up and down guide supports extending through the hull. 
     One of the problems related to offshore platforms that operate in deep and ultra-deep water (5000-10000+ foot water depth) is the amount or degree of lateral offset that is associated with the platform. The lateral offset, which results in a vertical differential between riser and vessel, is essentially controlled by the type of platform and the mooring system that is utilized. As water depths become deeper, however, regardless of the platform used, the lateral offsets increase. With floating production platforms such as SPAR&#39;s, which typically employ a top tensioned system of long-stroke tensioners, this lateral offset drives the total stroke requirements of the tensioning system. As a result, the stroke requirements can easily exceed 25-30 feet and, in active tensioning systems, require actively adjusting hydraulic pressure to increase pressure in the tensioning cylinders needed to maintain sufficient tension on the riser during a heave downward by the vessel and to reduce pressure in the tensioners to prevent the application of excessive tension to the risers during a heave upward by the vessel. 
     As such, it has been recognized by the inventors that the conditions associated with deep and ultradeep water inevitably result in a tensioning system made up of multiple cylinders, typically upwards of 25 feet in length, and capable of stroking the required 25-30 feet, along with significant space requirements within the vessel and/or an additional support frame or deck to support the non-stroking portion of the tensioner&#39;s cylinders, which can greatly add to the cost of the vessel to accommodate the 25-30 ft. stroke. By analogy, this can be equated to having to build a house with 25 foot high doorways and ceilings in every room of the house to accommodate the stroking portion of the tensioner&#39;s cylinders, rather than a normal single story having a six or eight foot doorway. Further, active tensioning systems can require a computerized control and feedback system and additional accumulators, gas pumps, pressure sensors, etc. These long-stroke tensioners can add significant extra weight to the hull supporting the production platform, and can significantly add to the costs of the riser management system. As exploration takes the industry into areas where the environmental and operational conditions, it is anticipated that there will be more and more instances where conditions exceed the current stroke capabilities of long-stroke top-mounted tensioning systems, which can lead to even higher costs. 
     Alternative designs to the long-stroke top-mounted tensioning systems have been employed to resolve the total stroke requirements. Such alternatives include the employment of a multi-buoyancy can system, described previously, which includes a set of individual buoyancy cans separately connected to a corresponding set of individual risers. Such systems, however, have some significant disadvantages. Such disadvantages include installation complexity, questionable storm resistance, stick-slip issues (e.g., due to contact with the side walls of the guide supports or columns), and buoyancy force limitations (e.g., resulting from a trade-off between the size of the individual buoyancy cans, the number of cans and risers supportable by the hull, and hull size. That is, for a given size hull, the larger the can the less number of risers supportable by the vessel. Similarly, for a given number of risers, the larger the cans, the larger the hull must be to support the risers, and the larger the costs of building, maintaining, and operating the vessel. 
     Another alternate platform design that solves some of these issues uses a “de-coupled” platform approach. Examples of such alternative platform design includes the riser support systems described, for example, in U.S. Pat. No. 7,537,416 and in U.S. Patent Publication No. 2009/009545, each incorporated by reference in its entirety. In essence, to employ such de-coupled approach, multiple production risers are immovably attached to one common, large air “mono-can” in such a fashion that the risers extend through the interstitial space between the air can cells, thus, de-coupling the production risers from the hull. In this design approach, the hull structure is laterally restrained and is independently moored and detached from the mono buoyancy can platform so as to allow the mono buoyancy can platform and risers to slide up and down guide supports extending through the hull. That is, this design approach employs the mono-can assembly as its substitute for long stroke tensioners to compensate for lateral offset. 
     Recognized by the inventors, however, is that while most of the total stroke requirements of a riser are directly related to vessel offset which generally effects each riser of a set of risers in a same manner and level, and thus, can be compensated for through utilization of a single buoyancy can platform, a small percentage of the stroke requirements are a result of factors which can affect each separate riser of the set of risers in a different manner or at least to a different level. These factors can include, for example, a change in riser initial length, riser initial weight, riser initial pre-tension, thermal growth, subsea wellhead and surface tree spacing distance, and pressure differentials between risers, which cannot be readily compensated for by a single de-coupled buoyancy can platform. Accordingly, it is recognized that although the single mono-buoyancy can- (multiple riser) decoupled platform system is an improvement upon the single-buoyancy can (single riser) platform approach, the mono-buoyancy can (decoupled) platform system still falls short of replacing long-stroke top-mounted tensioning systems, as it does not resolve this “small” but significant percentage of stroke requirements. It is further recognized that, as a result, such system will be expected to cause large tension variations between risers being held by the mono buoyancy can platform, as it assumes that environmental and operational variations have an equal effect on each riser in the set of risers, which can resultingly at least reduce the service life of one or more the risers, if not ultimately result in a catastrophic failure of one or more of the risers. 
     Accordingly, the inventors have further recognized the need for a riser tensioning system which can compensate for both lateral offset and additional factors such as thermal growth, subsea wellhead and surface tree spacing distance, and pressure differentials between risers, among others, without the need for long-stroke tensioners, or more significantly, the associated costs to the riser management system and the vessel associated with accommodating their significant size requirements. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, various embodiments of the present invention advantageously provide a riser tensioning system which can adequately compensate for both lateral offset and additional factors such as thermal growth, subsea wellhead and surface tree spacing distance, and pressure differentials between risers, among others, without the need for the vessel modifications needed to accommodate long-stroke tensioner units or the associated additional weight or associated additional costs. Further, various embodiments of the present invention can advantageously solve the problems associated with stroke variations on the “de-coupled” mono-can platform configuration, through use of a series of short-stroke tensioner units positioned atop the mono-buoyancy can platform, which provide a much lower cost tensioning system solution than that typically used on conventional SPAR or semi-submersible platforms. Advantageously, according to such configuration or configurations, the stroke variations among individual risers connected to the mono-buoyancy can platform can be handled by each individual tensioner unit while the tension requirements due to hull offset can be primarily handled by the “mono-can” platform. As a result, the variation in riser tension can be maintained nearly constant, or at least with a range of values, for variations in pressure, thermal growth and the various operating conditions separately affecting each individual riser. Further, as a result of application of a combination of short-stroke tensioner units with a mono-buoyancy can platform, even with a shorter stroke capability, various embodiments of the present invention can function adequately with a fixed gas volume and do not require active compensation. Also, as a result of application of a combination of short-stroke tensioner units with a mono-buoyancy can platform, various embodiments of the short-stroke tensioners may use a very small or even no gas volume to effectively work as a load/length adjusting device. 
     Specifically, according to an embodiment of the present invention, a riser management system can include a mono-buoyancy can platform operably coupled to a plurality of risers extending between subsea well equipment and a moored floating vessel and configured to be at least partially submerged, and a plurality of tensioner units each connected to a top portion of a separate one of the plurality of risers to provide tension to each of the plurality of risers. Advantageously, the mono-buoyancy can platform, operably coupled to the plurality of risers through the plurality of tensioner units and operably de-coupled from movement of the floating vessel, can provide tension to each of the plurality of risers sufficient to compensate for relative vertical movement between the risers and the vessel due to typically lateral vessel movement. This “vertical offset” generally affects each of the risers equally, within tolerances, while the tensioner units can simultaneously provide tension to compensate for one or more additional factors which can affect each riser differently—resulting in differential tension requirements between risers. 
     According to an exemplary configuration, the buoyancy can platform can include a plurality of buoyancy cans. Each of the plurality of buoyancy cans is operably coupled together to form the mono-buoyancy can platform configured to be at least partially submerged and positioned to collectively, rather than individually, provide tension to each of the risers sufficient to compensate for a vertical offset between the risers and the floating vessel. Similarly, each of the plurality of tensioner units include a plurality of cylinders having a top end portion or piston operably coupled to a riser connector for a respective one of the plurality of risers and a bottom end portion operably coupled to the mono-buoyancy can platform. Each of the cylinders for the respective tensioner unit can function collectively to provide tension to the riser to compensate for one or more additional factors other than/in addition to the vertical offset with the vessel. Further, according to the exemplary configuration, each of the risers, for example, via a riser connector, extend through the interstitial space between the plurality of buoyancy cans of the mono-buoyancy can platform. According to such configuration, the risers, although operably coupled to the respective plurality of tensioner units, due to the connection of the tensioner units to the buoyancy can platform, advantageously, the risers are operably de-coupled from movement of the floating vessel through coupling with the mono-buoyancy can platform. 
     Embodiment of the present invention also includes methods of maintaining a selected range of tension on a plurality of risers extending between subsea well equipment and a more floating vessel. The method can include coupling a plurality of risers to a corresponding plurality of tensioner units configured to adjust stroke length in response to movement of the respective riser in relation to a mono-buoyancy can platform decoupled from the vessel so as to allow vessel movement relative to a position of the buoyancy can platform; coupling the plurality of tensioner units to the mono-buoyancy can platform adapted to maintain tension on the plurality of risers within a certain range of tension values; and maintaining tension applied to each of the plurality of risers whereby tension is applied by a combination of both the plurality of tensioner units and the mono-buoyancy can platform, simultaneously, responsive to a change in the vertical offset in conjunction with a change in the one or more additional factors to thereby account for both the vertical offset and the additional factors 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the invention, as well as others which will become apparent, may be understood in more 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, which form a part of this specification. It is to be noted, however, that the drawings illustrate only various embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it may include other effective embodiments as well. 
         FIG. 1  is a perspective view of a riser management system for maintaining a selected range of tension on a plurality of risers extending between subsea well equipment and a floating vessel according to an embodiment of the present invention; 
         FIG. 2  is an enlarged perspective view of a plurality of short-stroke tensioner units of the riser management system of  FIG. 1  each having a plurality of cylinders, positioned atop an upper surface of a mono-buoyancy can platform and connected to a corresponding plurality of risers at a plurality of different strokes and supported by an upper support frame according to an embodiment of the present invention; 
         FIG. 3  is an enlarged perspective view of a single short-stroke tensioner unit of  FIG. 2  connected to a single riser and shown with each of its cylinders in a retracted position according to an embodiment of the present invention; 
         FIG. 4  is an enlarged perspective view of a single short-stroke tensioner unit of  FIG. 3  connected to a single riser and embedded within a support frame according to an embodiment of the present invention; 
         FIG. 5  is an enlarged perspective view of a single short-stroke tensioner unit of  FIG. 3  connected to a single riser and extending through a support frame according to an embodiment of the present invention; 
         FIG. 6  is an enlarged perspective view of a single short-stroke tensioner unit of  FIG. 3  connected to a single riser and extending through an upper support frame and landing upon a lower support frame connected to an upper surface of a buoyancy can according to an embodiment of the present invention; 
         FIG. 7  is an enlarged perspective view of a single short-stroke tensioner unit of  FIG. 3  connected to a single riser and landing upon to a surface of a support frame according to an embodiment of the present invention; 
         FIG. 8  is an enlarged perspective view of an example of a cylinder for a single short-stroke tensioner unit of  FIG. 3  shown in an extended position according to an embodiment of the present invention; 
         FIG. 9  is a schematic view of an example of a cylinder for a single short-stroke tensioner unit of  FIG. 8  according to an embodiment of the present invention; and 
         FIG. 10  is a schematic block flow diagram of a method of maintaining a selected range of tension on a plurality of risers extending between subsea well equipment and a floating vessel according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, which illustrate embodiments of the invention. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. Prime notation, if used, indicates similar elements in alternative embodiments. 
     As shown in  FIGS. 1-10 , various embodiments of the present invention employ and/or implement one or more of the above described principles in a new and unique manner in order to maintain a selected range of tension on a plurality of risers  21  extending between subsea well equipment  23  and a floating vessel  25  such as, for example, a conventional SPAR, TLP, or semi-submersible platform. 
       FIG. 1  illustrates an environmental view of subsea well equipment  23  positioned on a surface of a subsea floor  27  connected to the hull structure of floating vessel  25  that is laterally restrained and independently moored by a plurality of cables  29 , for example.  FIG. 1  also illustrates a riser management system  30  for maintaining a selected range of tension on the risers  21  extending between subsea well equipment  23  and the floating vessel  25 . The riser management system  30  includes a mono-buoyancy can platform  31  operably coupled to the risers  21  extending between the subsea well equipment  23  and the floating vessel  25 . The mono-buoyancy can platform  31  is configured to be at least partially submerged and positioned to provide sufficient tension to each of the risers  21 , simultaneously, to compensate, for example, for relative movement between the risers  21  and the floating vessel  25  caused by lateral offset of the floating vessel  25 . As further shown in  FIG. 2 , the mono-buoyancy can platform  31  includes a plurality of buoyancy cans  33  operably coupled to risers  21  such that each of the buoyancy cans  33  forming separate independent buoyant chambers are operably coupled together to form the mono-buoyancy can platform  31 . Note, it should be understood by one of ordinary skill in the art that a version of the mono-buoyancy can platform  31  having a single buoyant chamber is within the scope of the present invention. Such design, however, would not be preferred as it would be considered less survivable if damaged or punctured. 
     Referring again to  FIG. 1 , in a typical embodiment of a vessel, the vessel  25 , for example, supports an upper deck  41 . The vessel  25  also includes a middeck section  43  and a lower deck section  45  which can include compliant guides  46  positioned to provide lateral support between the vessel  25  and the buoyancy can platform  31  and to enable the vessel  25  and the buoyancy can platform  31  to rise and fall independently of each other in response to wave motions and/or changes in lateral offset between the vessel  25  and the subsea equipment  23 . 
     Referring back to  FIG. 2 , shown is an enlarged perspective view of a plurality of short-stroke tensioner units  51  of the riser management system  30 , each having typically three or four tensioning cylinders  53  (see also  FIG. 3 ), positioned indirectly atop an upper surface  55  of the mono-buoyancy can platform  31  with each unit  51  connected to one of the risers  21 , typically extending through a riser conductor  22 . In the illustrated example, an upper support frame  57  is connected to the upper surface  55  of the buoyancy can platform  31  through a plurality of support legs  59 . Also in the illustrated embodiment, the support legs  59  allow the buoyancy can platform  31  to be fully submerged with the tensioner units  51  being held above the waterline. In this illustrated example, as perhaps best shown in  FIG. 4 , a lower or bottom portion  61  of each tensioner cylinder  53  is embedded in a portion of the upper support frame  57 . Note, the positioning of the lower portion  61  of cylinders  53  is shown by way of example. 
     Other positioning methodologies are, however, within the scope of the present invention, to include, but not limited to, positioning each of the cylinders  53  so that the bottom portion  61  extends through a bottom surface  63  of the upper support frame  57  as shown, for example, in  FIG. 5 ; lands upon a lower support frame  65  as shown, for example, in  FIG. 6 ; or lands upon an upper surface  67  of upper support frame  57  as shown, for example, in  FIG. 7 . 
     As perhaps best shown in  FIG. 8 , each tensioning cylinder  53  has an upper end and a lower end. The upper end can include a rod end cap  71 . As perhaps best shown in  FIGS. 3 and 8 , the rod end cap  71 , and thus, the upper end of each cylinder  53  can be connected to a bridge  73  which is connected to a tensioner connection assembly  75  to provide the requisite tension to the tensioner connection assembly  75 , and thus, to the riser  21 . The tension connection assembly  75  can function to collectively provide tensioning forces from each of the tensioning cylinders  53  to a centrally located riser tensioning joint  77 , which is connected to a top portion of the riser  21 . In the exemplary configuration shown in  FIG. 3 , the tensioning connection assembly  75  includes a tensioner load frame  81  which is connected to a load ring  83  which is connected to the tensioning joint  77 . Note, a bottom portion of the tensioner load frame  81  can include a plurality of apertures  85  to allow easy removal of the tensioning cylinder  53  for replacement, and frame sheets  87  to increase the strength of the load frame  81 . 
     As perhaps best shown in  FIG. 9  for illustrative purposes only, the tensioning cylinder  53 , shown in the form of a high stiffness version containing mostly hydraulic fluid and little gas volume, includes a lower or bottom portion or barrel  61  which includes an outer cylinder barrel or main body  91  housing an inner cylinder barrel  93  each having a bore and an aperture on at least one end and having a pressurized fluid contained within. The main body  91  forms an accumulator having a preset volume of gas at a selected pressure set by a user to provide a range of tensioning for the operational environment. A piston  95  is slidably carried in the bore of the barrel  93 . The piston  95  of the each cylinder  53  is positioned to function independently of each other of the cylinders  53  for the respective tensioner unit  51 . Specifically, each piston  95  is individually positioned to increase pressure of the accumulator when the piston  95  strokes in the direction of the pressurized fluid (downward) during downward movement of the top end of the riser  21  to provide tensioning resistance, and to use pressure within the cylinder  53  to stroke upward to maintain tensioning on the riser  21  when the riser  21  moves upward due to various factors including, for example, thermal growth, a change in subsea wellhead and surface tree spacing distance, and a change in pressure differentials between risers  21 . As noted above, these functions are performed, simultaneously, with functions performed by the mono-buoyancy can platform  31  primarily providing tensioning due to relative vertical movement caused by lateral offset of the vessel  25 . 
     Note, although long-stroke tensioner units can be used in place of short-stroke tensioner units  51 , short-stroke tensioning units  51  having various stroke capabilities of approximately four feet, six feet, and eight feet, for example, depending upon vessel type and/or configuration and/or water depth, are preferred as they can have a total length of approximately six, eight, and ten feet, respectively, and thus, can allow use of much lower ceilings/spacing between horizontal vessel support structures and less weight to both the vessel  25  and the riser management system  30 , along with other advantages. Long stroke tensioner are generally much heavier and require more spacing between floor and ceiling feet. 
       FIG. 10  provides a high level flow diagram of a method of maintaining a selected range of tension on a plurality of risers  21  extending between subsea well equipment  23  and a floating vessel  25  according to an embodiment of the present invention. Specifically, according to an embodiment of such a method, the method can include the step of coupling the risers  21  to a corresponding plurality of tensioner units  51  configured to adjust stroke length in response to movement of the supported riser  21  in relation to a mono-buoyancy can platform  31  (block  201 ). According to a preferred configuration, each of the tensioning units  51  can include, e.g., three or four tensioning cylinders  53  including a top end portion or piston  95  adapted extend and retract responsive to changes in the one or more additional factors such as, for example, thermal growth of the supported riser  21 , a change in subsea wellhead and surface tree spacing distance, and pressure differentials between risers  21  unevenly compensated for by the buoyancy can platform  31 , described below, to maintain tension on the supported riser  21  within a certain range of tension values. Each of the tensioning cylinders  53  also includes a bottom end portion defining a barrel  61  configured to receive substantial portions of the piston  95  during retraction, and configured to be fixedly operably connected to the mono-buoyancy can platform  31 , as described below. According to one or more preferred configurations, the tensioning units  51  are short-stroke tensioning units  51  having various stroke capabilities of approximately four feet, six feet, and eight feet, depending upon vessel type and/or configuration and/or water depth. 
     Referring again to  FIG. 10 , the method of maintaining a selected range of tension on a plurality of risers  21  can include the step of coupling the plurality of tensioning units  51  to the mono-buoyancy can platform  31  which is decoupled from the vessel  25  so as to allow vessel vertical and lateral movement, which results in a vertical movement relative to a position of the buoyancy can platform  31  (block  203 ). This step can include the step of connecting the barrel  61  of each of the tensioning units  51  to a support frame  57  connected to a top portion of the mono-buoyancy can platform  31 , according to the various techniques. For example, according to one configuration, the barrel  61  is positioned embedded within a support frame  57  as shown, for example, in  FIG. 4 . According to another configuration, the barrel  61  extends below frame  57  as shown, for example, in  FIG. 5 . According to another configuration, the barrel  61  lands upon a lower support frame  65  as shown, for example, in  FIG. 6 . According to yet another configuration, the barrel  61  lands upon an upper surface is the  67  of support frame  57  as shown, for example, in  FIG. 7 . It should be understood, however, that other configurations are within the scope of the present invention. Further, it should be understood that support frame  57  can be separated from an upper surface  55  of the buoyancy can platform  31  or can land upon or be integral with the upper surface  55 . The step of coupling the plurality of tensioning units  51  to the mono-buoyancy can platform  31  can also include extending the risers  21  (e.g., housed within riser conductors  28 ) through interstitial space between the individual buoyancy cans  33  forming of the mono-buoyancy can platform  31 . 
     Referring again to  FIG. 10 , the method of maintaining a selected range of tension on a plurality of risers  21  can include the step of maintaining tension applied to each of the risers  21  whereby tension is applied by a combination of both the tensioner units  51  and the mono-buoyancy can platform  31  to thereby account for both a vertical offset with the vessel and the additional factors, described above (block  205 ). That is, according to an embodiment of the method, the step includes simultaneously applying tensioning responsive to tensioning requirements resulting from a change in the lateral offset of the vessel in conjunction with tensioning requirements resulting from a change in the one or more additional factors, with the mono-buoyancy can platform  31  primarily applying tensioning responsive to the change in vertical offset with the vessel  25 , and each of the tensioning units  51  separately primarily applying tensioning to its respective riser  21  responsive to the change in the one or more additional factors affecting the respective associated riser  21 . 
     Various embodiments of the present invention have several advantages. For example, various embodiments of the present invention allow an operator to ensure that proper tension to multiple risers  21  simultaneously is maintained due to both changes in the vertical offset with the vessel  25  and additional factors which can simultaneously affect each riser  21  differently, thus otherwise causing significant variations in tensioning requirements between risers  21  when connected to a single buoyancy platform  31 . Advantageously, embodiments of the present invention provide a set of multiple cylinders  53  to further support each of a plurality of risers  21  primarily supported by a single buoyancy can platform  31 . Advantageously, embodiments of the present invention can utilize short-stroke tensioner units  51  positioned atop the mono-buoyancy can platform  31 , which provide a much lower cost tensioning system solution than can be used on conventional SPAR and semi-submersible platforms. Advantageously, according to such configuration or configurations, the stroke variations among individual risers  21  connected to the mono-buoyancy can platform  31  are handled by each individual set of short-stroke tensioner units  51  while the tension requirements due to hull offset are primarily handled by the “mono-can”  31 . As a result, the variation in riser tension can be maintained nearly constant, or at least within a tight range of values, for variations in pressure, thermal growth and the various operating conditions separately affecting each individual riser  21 . Alternatively, it can be made very stiff (e.g., like a hydraulic jack) such that it primarily only adjusts for initial install variations such as, for example, overall riser length, weight, and pre-set tension. Other advantages have been described above and throughout. 
     In the drawings and specification, there have been disclosed a typical preferred embodiment of the invention, and although specific terms are employed, the terms are used in a descriptive sense only and not for purposes of limitation. The invention has been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification.