Patent Publication Number: US-9422773-B2

Title: Relating to buoyancy-supported risers

Description:
This invention relates to subsea riser systems used to transport well fluids from the seabed to a surface installation such as an FPSO vessel or a platform. The invention relates particularly to buoyancy-supported riser (‘BSR’) systems. 
     A BSR system is an example of a hybrid riser system. Such systems are characterised by rigid riser pipes that extend upwardly from the seabed to a subsea support and by flexible jumper pipes that extend from the subsea support to the surface. The jumper pipes add compliancy that decouples the riser pipes from surface movement induced by waves and tides. The riser pipes experience less stress and fatigue as a result. 
     In a BSR system, the subsea support is a riser support buoy held in mid-water, tethered to a seabed anchorage under tension. The buoy is held at a depth below the influence of likely wave action but shallow enough to permit diver access and to minimise the possibility of collapse under hydrostatic pressure. A depth of 250 m is typical for this purpose but this may vary according to the sea conditions expected at a particular location, for example between 100 m and 300 m. 
     Riser pipes, typically of lined and coated steel, hang from the buoy. The riser pipes may extend substantially vertically along a riser tower or may splay away from one end of the buoy as steel catenary risers or ‘SCRs’. SCRs are a non-limiting example: other types of pipe are possible for the riser pipes. Jumper pipes hang as catenaries from an opposite end of the buoy to extend to an FPSO or other surface installation moored above, and offset horizontally from, the buoy. 
     Umbilicals and other pipes follow the general paths of the riser pipes and the jumper pipes to carry power, control data and other fluids. 
     In deep water, a surface installation such as an FPSO will usually have spread moorings. Spread moorings typically comprise four sets of mooring lines (each set being of say four to six mooring lines) with the sets radiating with angular spacing from the FPSO to anchors such as suction piles or torpedo piles embedded in the seabed. 
     In a spread-moored arrangement, a riser system is typically accommodated between neighbouring sets of mooring lines of the FPSO. Space may be limited such that in extreme conditions, there is a potential for interference or clashing between the mooring lines of the FPSO and the riser support buoy and/or the riser pipes. 
     It is necessary to ensure that BSR systems have enough stability to resist excessive movement of the riser support buoy in extreme conditions. The tension in the tethers created by buoyancy is a stabilising factor; so too are the horizontally-opposed forces applied to the buoy by the riser pipes and to a lesser extent by the jumper pipes. It may also be possible to apply additional stabilising balancing forces to a buoy, for example by means of guy lines extending to the seabed or to the FPSO or by interconnections between neighbouring buoys. However, such additional measures increase cost and there may be insufficient space to use them without introducing a risk of clashing. 
     Conventional moorings for subsea buoys fall into two categories, namely slack wire moorings and taut wire moorings. In slack wire moorings, the mooring lines are in a catenary shape such as the CALM (catenary anchor leg mooring) buoy shown in WO 96/11134. In taut wire moorings, tensioned wires may be substantially vertical as shown in GB 1532246 or opposed at substantial angles to the vertical as shown in GB 2273087. 
     U.S. Pat. Nos. 5,639,187, 6,780,072 and WO 2012/001406 disclose BSR systems having moorings comprising substantially vertical taut wire tethers. In each case, the riser support buoy is generally rectangular in plan view, defining 90° corners, and the tethers are attached to outer side walls of the buoy near those corners of the buoy. Generally the tethers are located at the sides of the buoy to be as far as possible from the riser pipes and the jumper pipes that hang from opposite ends of the buoy, in order to avoid clashing with those pipes. 
     For example, the buoy disclosed in WO 2012/001406 comprises a riser support member and a jumper support member defining the length of the buoy between them. The riser support member and the jumper support member extend in parallel between, and lie orthogonally with respect to, parallel side members. The buoy is moored by four pairs of tethers, each comprising a top chain connected to a central length of spiral strand wire. Two of those pairs of tethers are attached to each side member, with each pair being attached near a respective end of the side member. The tethers are all attached to the side members inboard of the length of the buoy, as measured by the length of the side members or between the lengthwise extremities of the riser support member and the jumper support member. 
     To meet operational requirements, it is important that a riser support buoy is maintained at an appropriate depth and at an appropriate location and orientation in the water. It is also important that the tethers each bear an appropriate share of the buoyant load, even though the tethers may extend differently and unpredictably in use. For these reasons, it is necessary to have a system for tension adjustment to balance loads in the tethers. WO 2012/001406, for example, discloses top connectors mounted on the side members that can serve as tensioning devices for respective tethers. The tensioning devices comprise chain stops functioning as ratchet mechanisms that engage with links of the top chains of the tethers. Each top connector is mounted on a respective hang-off porch that is cantilevered from an outer wall of the associated side member of the buoy. 
     It should be noted that the tethers in a BSR system will usually be slightly off vertical even in the absence of water currents, typically leaning toward the riser pipes which apply a greater horizontal pull to the buoy than the jumper pipes. Consequently, references in this specification to tethers being ‘substantially vertical’ are intended to cover instances where the tethers would assume a vertical orientation if the buoy was not subject to horizontal force components as from water currents or from the loads of jumper pipes and riser pipes. References to ‘substantially vertical’ are not intended to exclude instances where the tethers are off vertical merely as a consequence of such horizontal force components acting on the buoy, other than as may be imparted by opposing tethers that are themselves substantially off vertical as in GB 2273087. 
     Slack wire moorings and taut wire moorings at a substantial angle to the vertical are not appropriate for BSR applications. Excursion of the buoy has to be limited to limit pipeline fatigue, which rules out slack wire moorings. Also, as noted above, the riser support buoy and the pipes that it supports are located in a congested space between FPSO moorings, pipelines and umbilicals. Consequently, the footprint of the BSR mooring system has to be as small as possible, with the tethers adopting a minimal angle to the vertical so that the foundations take mainly vertical loads. However, this configuration is less efficient than taut angled moorings as disclosed in GB 2273087, as it offers less stability to dynamic solicitations caused by sea motion. 
     WO 03/093627 and WO 03/097990 disclose buoys that support flexible risers. The buoys are anchored by substantially vertical taut wire tethers. Stability and excursion issues are addressed by additional mooring lines arranged as catenaries. This catenary arrangement is expensive as it involves more mooring lines and it cannot fit into a congested subsea space. Similar problems afflict U.S. Pat. No. 5,480,264, which uses two or more taut mooring lines, one extending substantially vertically straight below the buoy and the other(s) being at a substantial angle to the vertical to reduce horizontal excursion. 
     CN 102418480 discloses a riser support device comprising a circular riser support buoy with angularly-spaced cantilever structures extending radially in plan view to support tethers that are outboard of the plan footprint of the buoy. Specifically, the buoy has a ‘starfish’ structure in which a circular central body is connected to three rectangular-section cantilever buoys at included angles of 120 degrees. 
     CN 102418480 is not concerned with stability, not least because a top-tensioned riser as used in CN 102418480 does not experience lateral loads applied by catenary risers. Instead, the purpose of the cantilever buoys in CN 102418480 is to achieve neutral buoyancy in different phases of the life of the riser system, during which the overall load on the buoy varies. For example, less buoyancy is needed during installation and more buoyancy is required when the risers are suspended from the buoy and full of oil. So, the length of the cantilever buoys can be varied to change their volume and hence to adjust their buoyancy. 
     As will be appreciated from the exemplary BSR system shown in  FIG. 1  of the accompanying drawings, the relative orientations of an FPSO and a riser support buoy means that roll of the FPSO tends to excite pitching motion of the buoy linked to the FPSO via jumper pipes. In this respect, pitch of the buoy means rotation around a transverse, widthwise axis parallel to the riser support member and the jumper support member, as opposed to roll of the buoy which would be rotation around an orthogonal axis parallel to the side members. The FPSO rolls about a longitudinal axis extending along its hull, which axis is orthogonal to a longitudinal axis of the buoy extending in the general flow direction of fluids through the jumper pipes. 
     To avoid mechanical resonance effects, the riser support buoy is designed to have a natural pitch period that is substantially different to (generally shorter than) the natural roll period of the FPSO. For example, as the natural roll period of an FPSO is typically between 11 and 13 seconds and most commonly between 11.5 and 12.5 seconds, the dimensions of the buoy may be calculated such that its natural pitch period is between 7 and 9 seconds and typically between 8 and 8.5 seconds. 
     If the number of suspended riser pipes increases and/or a BSR system is used in a greater depth of water so that the riser pipes must be longer, the riser support buoy must support a greater suspended mass. In that case, the dimensions of the buoy must be increased to provide the additional buoyancy necessary to support the additional mass. 
     For example, WO 2011/083268 discloses a riser support buoy that is generally U-shaped in plan view. Side members that are buoyant along their full length extend longitudinally far beyond an outboard edge of the riser support member at which loads are applied to the buoy by risers hanging from the buoy. This longitudinal offset of the side members shifts the centre of buoyancy toward the riser end of the buoy where the weight loads are greatest. The buoyant side members extend longitudinally almost as far beyond tether attachment points on the outside of the side members near the outboard edge of the riser support member. 
     Increasing the apparent mass of a riser support buoy lengthens its natural pitch period when tethers are connected to each end of the buoy. This necessitates using a greater number of tethers at each end of the buoy or using bigger tethers in order to keep the natural pitch period of the buoy below the natural roll period of the FPSO. However, increasing the size and/or the number of tethers may lead to greater problems in balancing the tensile loads in the tethers; designers may even encounter fabrication limits on tether size. 
     It is against this background that the present invention has been devised. 
     The invention resides in a subsea riser support buoy comprising: a positively buoyant riser support member and a positively buoyant jumper support member that extend generally parallel to each other and that define a lengthwise direction extending between them across the buoy; side members that extend in the lengthwise direction at ends of the riser support member and the jumper support member to join the riser support member and the jumper support member; and pontoons of negative or neutral buoyancy that extend lengthwise beyond the positive buoyancy of the riser support member and the jumper support member, the pontoons comprising attachment points for connecting tethers to the buoy. 
     The side members may also be positively buoyant, in which case the pontoons preferably extend lengthwise beyond the positive buoyancy of the side members. 
     The negative or neutral buoyancy in the pontoons is constant or they are not buoyant at all. The pontoons increase the spacing between tethers to increase the lever arm between the tethers with a minimal increase in the overall mass of the riser support buoy. The pontoons may, for example, extend the overall length of the buoy by 20% to 50% up to the attachment points, and preferably by 30% to 40%, relative to the length of the buoy across the riser support member and the jumper support member. 
     In summary, the invention solves the problem of limiting the natural pitch period of the riser support buoy while minimising the number and size of the tethers. The invention achieves this by adding extended pontoons suitably located at the corners of the buoy and by relocating top connectors to these pontoons, to which the tethers will be connected upon installation. The extended pontoons increase the rotational moment of the buoy without adding apparent mass to the buoy to the same extent. Consequently, the same number of tethers and similar sizes of tethers can be used as for a buoy of smaller overall dimension. 
     The pontoons suitably also extend in a widthwise direction beyond the side members. The pontoons may, for example, extend the overall width of the buoy by 5% to 20% up to the attachment points, and preferably by 10% to 15%, relative to the width of the buoy across the side members. 
     Within the inventive concept, the invention may be defined in alternative terms as a subsea riser support buoy comprising: a positively buoyant riser support member and a positively buoyant jumper support member that define a lengthwise direction extending between them across the buoy; and extended pontoons of negative or neutral buoyancy arranged to connect tethers to the buoy at respective attachment points that are spaced further apart lengthwise than lengthwise extremities of the riser support member and the jumper support member. 
     Correspondingly, the invention may be expressed as a method of altering the dynamic behaviour of a subsea riser support buoy that comprises a positively-buoyant riser support member and a positively-buoyant jumper support member defining a lengthwise direction extending between them across the buoy, the method comprising providing pontoons of negative or neutral buoyancy to space tether attachment points further apart lengthwise than the positive buoyancy of the riser support member and the jumper support member. 
     The inventive concept extends to a seabed-to-surface riser system comprising a subsea riser support buoy of the invention and tethers connected to the attachment points of the buoy and extending toward the seabed. 
     As the tethers are no longer connected at the sides of the riser support buoy and so are closer to the riser pipes and jumper pipes hanging from the ends of the buoy, the extended pontoons of the invention could increase the risk of clashing between the tethers and the riser pipes and jumper pipes. The length and the orientation of the extended pontoons relative to the members defining the underlying rectangular shape of the buoy must be calculated to avoid clashing. 
     Each pontoon is suitably angled in plan view relative to a side member from which the pontoon extends beyond the lengthwise extremity of an adjacent riser support member or jumper support member. The angle between the longitudinal axis of the pontoon and the longitudinal axis of the side member should preferably be from 0° to 45° and more preferably should be greater than 20° to avoid clashing with the riser pipes or the jumper pipes. Most preferably that angle will be between 25° and 35°. However, it is further preferred that the angle between the longitudinal axes of the pontoon and the side member is not greater than 45°, as otherwise the extended pontoon would have less or no effect on the natural pitch period of the riser support buoy. 
     The length of each pontoon along its longitudinal axis extending beyond the members to which it is attached must be sufficient to increase the rotational moment of the riser support buoy to a desired extent. However, the pontoons must not be too long as otherwise they may become too heavy and so disadvantageously increase the apparent mass of the buoy. Typically the length of each pontoon along its longitudinal axis is between 3 m and 8 m and preferably between 4 m and 7 m, in the context of a buoy that is 56 m wide and 40 m long by way of example. 
     The invention has various advantages. It allows an entire BSR system to have better overall dynamic behaviour and in particular offers a significant increase in the fatigue life or endurance of the tether system. It also provides a better response to the ‘one tether failure’ extreme design case of a BSR system. 
     The riser support buoy of the invention is more robust and so can better accommodate a payload increase than prior designs. The structural design of the buoy is also more efficient as it places the tethers further away from main ballast tanks of the buoy. This means that fewer or smaller ballast tanks are required for the same payload, which results in lower structural and piping weight. 
     The orientation and length of the extended pontoon can be adjusted in the design stage to avoid any potential clash between a tether and a riser pipe or jumper pipe. 
     It should be understood that horizontally-projecting pontoons are known to be used in floating structures in the offshore oil and gas industry, but that these known uses are not relevant to the present invention. Such pontoons are conventionally used for anchoring tensioned leg platforms or ‘TLPs’, whichever type of mooring is used. 
     One reason for pontoons in the prior art is the need for space between mooring legs to accommodate a wellhead located directly under a TLP. Examples are shown in WO 97/29942 and U.S. Pat. No. 5,421,676. In WO 01/62583, the pontoons of a TLP have the additional benefit of allowing sufficient space to add additional buoyancy modules below the platform. Another form of TLP is disclosed in JP 2010234965 for supporting an offshore wind turbine. 
     U.S. Pat. No. 6,447,208 teaches that the buoyancy of buoyant pontoons or wings can add stability to a TLP but this teaches away from the problem and solution that define the present invention. 
     U.S. Pat. No. 7,854,570 discloses a TLP whose legs are attached to piles without pontoons, teaching that a TLP without pontoons has a smaller subsea projected area than a conventional TLP with pontoons. This reduces the TLP&#39;s response to ocean currents and wave action and shortens its natural period, enabling the TLP to be deployed in greater water depths than a TLP with pontoons. U.S. Pat. No. 7,854,570 therefore teaches away from the present invention by suggesting that pontoons should be omitted and in any event is not relevant because a BSR is situated below the effects of wave action. 
     In conclusion, and as can be deduced from U.S. Pat. No. 7,854,570, the way that pontoons are used in TLPs is not relevant to the technical challenges faced by BSR systems. For example, the main vertical structure of the TLP adds an additional turning moment that decreases stability. The TLP design also has to accommodate sea motion at and near to the surface, including the splash zone. This is mitigated in TLPs by using the structure of the pontoons to provide additional buoyancy. 
    
    
     
       In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a riser installation to put the invention into context, the installation in this example comprising two BSR systems in conjunction with a single spread-moored FPSO; 
         FIG. 2  is a perspective view of a riser support buoy in accordance with the invention; 
         FIG. 3  is a schematic plan view of a riser support buoy in accordance with the invention; 
         FIG. 4  is a plan view of the riser support buoy shown in  FIG. 2 ; 
         FIG. 5  is an end view of the riser support buoy shown in  FIG. 2 , viewed from a jumper end of the buoy; 
         FIG. 6  is a side view of the riser support buoy shown in  FIG. 2 ; 
         FIG. 7  is a schematic side view showing the forces that act on a riser support buoy known in the prior art; 
         FIG. 8  is a schematic side view corresponding to  FIG. 7  but showing the forces that act on a riser support buoy in accordance with the invention; and 
         FIG. 9  is a schematic side view of a BSR system including a riser support buoy in accordance with the invention. 
     
    
    
       FIG. 1  of the drawings does not show the invention as such but instead explains its context. The remaining drawings show embodiments of the invention with the exception of  FIG. 7 , which shows a riser support buoy known in the prior art. Like numerals are used for like parts where appropriate. 
     Referring firstly then to  FIG. 1  to appreciate the background of the invention, a BSR system  10  comprises two riser supports  12  in this example, although the number of riser supports  12  is immaterial to the inventive concept. Each riser support  12  comprises a riser support buoy  14 , a seabed foundation  16  and a tether arrangement  18  extending between the foundation  16  and the buoy  14 . Each tether arrangement  18  comprises eight tethers in four pairs in this example, maintained under tension by the buoyancy of the buoy  14 . 
     Each buoy  14  supports a group of riser pipes  20  in the form of SCRs that each extend from respective PLETs  22  across the seabed, through a sag bend  24  and from there up to the buoy  14 . The riser pipes  20  converge upwardly toward the buoy  14  and each group of riser pipes  20  fans out across the seabed to the PLETs  22 . 
     Each riser pipe  20  communicates with a respective jumper pipe  26  that hangs as a catenary between the buoy  14  and an FPSO  28 . The FPSO  28  is moored with its hull extending parallel to an axis containing both buoys  14 , whereby the jumper pipes  26  connect amidships to one side of the FPSO  28 . 
     As noted previously, umbilicals and other pipes  30  generally follow the paths of the riser pipes  20  and jumper pipes  26 . These umbilicals  30  can be distinguished from the riser pipes  20  in  FIG. 1  as they do not terminate in PLETs  22 , and as they have a smaller bend radius at the sag bend  24 . 
     The FPSO  28  shown in  FIG. 1  is spread-moored with four sets  32  of six mooring lines  34 . Again, the number of mooring lines  34  is immaterial to the inventive concept. Two of the sets  32  of mooring lines  34 —one attached near each end of the FPSO  28 —are shown in  FIG. 1 . It will be clear that the riser installation  10  is accommodated so closely between these neighbouring sets  32  of mooring lines  34  that it is challenging to avoid interference between the mooring lines  34  and the riser supports  12 , the riser pipes  20  and the jumper pipes  26 . 
     Referring next to  FIGS. 2 to 6 , a riser support buoy  14  in accordance with the invention is generally rectangular in plan view. The buoy  14  comprises four buoyant members that are generally straight beams in plan view—namely a riser support member  36 , a jumper support member  38  and two side members  40 —which together surround a rectangular central opening  42 . 
     Each member  36 ,  38 ,  40  is hollow and is partitioned internally by bulkheads into compartments to define ballast tanks. The ballast tanks have adjustable buoyancy to aid installation of the buoy  14  and to keep the buoy  14  level in use, for example as successive riser pipes  20  are attached to the buoy  14 . 
     The riser support member  36  and the jumper support member  38  extend along parallel horizontal axes, spaced apart from each other and joined by the side members  40 . The side members  40  also extend along parallel horizontal axes, spaced apart from each other and extending orthogonally with respect to the riser support member  36  and the jumper support member  38 . The central opening  42  is defined by the spaces between the members  36 ,  38 ,  40 . 
     The members  36 ,  38 ,  40  have flat-bottomed cross-sections with bottom walls disposed in a common plane that is substantially horizontal when the buoy  14  is in use. 
     The riser support member  36  has a rectangular cross-section defined by generally flat walls, namely a bottom wall  44 , an inner wall  46 , an outer wall  48  and a top wall  50 . Each wall  44 ,  46 ,  48 ,  50  is disposed orthogonally with respect to the adjoining walls of the cross-section. Thus, the bottom wall  44  and the top wall  50  are substantially horizontal and the inner wall  46  and the outer wall  48  are substantially vertical when the buoy  14  is oriented for use. 
     The jumper support member  38  has an approximately quarter-circular cross-section defined by a flat bottom wall  52 , a flat inner wall  54  extending orthogonally from the bottom wall  52  and a top wall  56  that is convex-curved in cross-section. The top wall  56  curves smoothly between the top of the inner wall  54  and the outer edge of the bottom wall  52  to support the jumper pipes  26  and the umbilicals  30 . 
     The side members  40  each have a rectangular cross-section defined by generally flat walls, namely a bottom wall  58 , an inner wall  60 , an outer wall  62  and a top wall  64 . Each wall  58 ,  60 ,  62 ,  64  is disposed orthogonally with respect to the adjoining walls of the cross-section. Thus, the bottom wall  58  is substantially horizontal and the inner wall  46  and the outer wall  48  are substantially vertical when the buoy  14  is oriented for use. The top wall  64  is horizontal in cross-section but lies in an inclined plane as will be described. 
     The buoy  14  has a width defined as the horizontal distance between the outer walls  62  of the side members  40 , measured parallel to the riser support member  36  and the jumper support member  38 . The buoy  14  also has a length defined as the horizontal distance, measured parallel to the side members  40 , between the outer wall  48  of the riser support member  36  and the outer edge of the bottom wall  52  of the jumper support member  38  at its intersection with the curved top wall  56 . 
     In this non-limiting example, the width of the buoy  14  is 56 m and the length of the buoy is 40 m. It will therefore be apparent that the length of a buoy  14  may be less than its width. In this sense, the expression ‘length’ follows from the longitudinal direction in which fluids flow relative to the buoy  14  through the riser pipes  20  and the jumper pipes  26 . 
     The riser support member  36  is much larger in cross-section than the jumper support member  38  so as to provide greater buoyancy to support the heavier riser pipes  20 . To increase the cross-section of the riser support member  36  in this way without a corresponding increase in the length of the buoy  14 , the top of the riser support member  36  is higher than the top of the jumper support member  38 . As each side member  40  matches the height of the riser support member  36  at one end and the height of the jumper support member  38  at the opposite end, the top walls  64  of the side members  40  are inclined to reflect this difference in height. Consequently, the side members  40  are somewhat wedge-shaped in side view, tapering from the inner wall  46  of the riser support member  36  to the inner wall  54  of the jumper support member  38 . 
     As is well known in the art, the riser support member  36  carries an array of connectors  66  for connecting the riser pipes  20  to the jumper pipes  26 . Also, the riser support member  36  and the jumper support member  38  carry various guide structures  68  for supporting the jumper pipes  26  and the umbilicals  30 . Thus supported, the jumper pipes  26  and the umbilicals  30  cross the top wall  50  of the riser support member  36 , span the central opening  42  lengthwise and drape across the top wall  56  of the jumper support member  38 . From here, the jumper pipes  26  and the umbilicals  30  begin their catenary curve to the surface. 
     In accordance with the invention, pontoons  70  protrude from each corner of the buoy  14  in plan view so that tethers, represented here by top chains  72 , attach to the buoy  14  via the pontoons  70  at locations outboard of the riser support member  36  and the jumper support member  38 , and preferably also outboard of the side members  40 . In this embodiment, the pontoons  70  extend from the opposed ends of each side member  40 , beyond the lengthwise extremities of the riser support member  36  and the jumper support member  38  where the buoy  14  is viewed from one side. 
     The pontoons  70  do not contribute buoyancy. The buoyancy of the pontoons  70  is constant, whether neutral or negative. 
     The pontoons  70  also splay outwardly in plan view, each lying at an acute angle α to the longitudinal axis of the associated side member  40  as shown in  FIG. 3 , which angle is preferably between 20° and 45° and more preferably between 25° and 35°. The longitudinal axis of the side member  40  is parallel to the outer wall  62  of the side member  40  in this example, as shown schematically in  FIG. 3 . Consequently, in this embodiment, the pontoons  70  extend not only lengthwise beyond the riser support member  36  and the jumper support member  38  but also widthwise beyond the side members  40 . 
       FIG. 3  also shows the length L of each pontoon  70  protruding from the side members  40  up to the attachment points for the top chains  72 . In a typical buoy, by way of example, L may be between 3 m and 8 m and preferably between 4 m and 7 m. 
     In plan view, the pontoons  70  are narrower than the members  36 ,  38 ,  40  so as to minimise their effect on the apparent weight of the buoy  14 . For this reason, the pontoons  70  at the riser end of the side members  40  are also substantially lower in side view than the riser support member  36 , as will be appreciated in  FIGS. 2 and 6  especially. The pontoons  70  need have no added buoyancy, although this is optional. 
     As noted previously, relocating the tethers to the extended pontoons  70  reduces the space between the tethers and the riser pipes  20  and jumper pipes  26 . A complete series of in-place and installation analyses must be performed to determine the length L and the angle α of the pontoons  70  relative to the side members  40  for every intended system to which this solution will be applied in order to avoid any potential clashes. 
     Each pontoon  70  has parallel vertical side walls  74  and terminates in a chamfered, faceted vertical end wall comprising a central facet  76  that is orthogonal to the side walls  74 . The central facet  76  lies between outer facets  78  that, in plan view, lie at 45° to the central facet  76  in opposed directions and so lie orthogonally with respect to each other. 
     Cantilevered hang-off porches  80  extend outwardly like shelves from the outer facets  78 . The hang-off porches  80  support respective top connectors  82  that are engaged with the top chains  72  to set and maintain tension in the associated tethers. 
     The protruding length of each pontoon  70  along its longitudinal axis is typically between 3 m and 8 m and preferably between 4 m and 7 m. In this example, including the hang-off porches  80 , the pontoons  70  increase the overall length of the buoy  14  from 56 m to 64.2 m and the overall width of the buoy  14  from 40 m to 56 m. 
     It will be evident from the plan view of  FIG. 4  that the eight tethers all attach to the buoy  14  outside the lengthwise extremities of the riser support member  36  and the jumper support member  38 , far outside the centres of buoyancy of those members  36 ,  38 . Also, four of the tethers attach to the buoy  14  outside the widthwise extremities of the side members  36 , again far outside the centres of buoyancy of those members  40 . It will also be evident how each pontoon  70  extends beyond the underlying rectangular shape of the buoy  14  defined by the members  36 ,  38 ,  40 . 
     Moving on to  FIGS. 7 and 8 , these compare a prior art riser support buoy  84  shown schematically in  FIG. 7  and the buoy  14  of the invention shown schematically in  FIG. 8 . Forces acting on the respective buoys  14 ,  84  are apparent, as is the notably-increased gap between tethers  86  in the lengthwise direction in  FIG. 8  by virtue of the pontoons  70 , which gap acts especially to resist pitch of the buoy  14 . 
     Turning finally to  FIG. 9 , this shows schematically how the solution of the invention employing extended pontoons  70  also requires proper positioning of the riser support buoy  14  in the field, allowing proper mass and buoyancy balancing of the entire system and adjusting the tension in the tethers  86 . Correct positioning of the buoy  14  is mainly defined by setting proper azimuth angles for the jumper pipes  26  (β and δ) and for the riser pipes  20  (φ) and also by positioning the buoy  14  in a water depth WD that eliminates a risk of clashing between the tethers  86  and the riser pipes  20  and jumper pipes  26 . 
     In conclusion, if extended pontoons were not used, larger and heavier tethers or a greater number of tethers would have to be used to achieve similar pitch behaviour and fatigue endurance for the same main hull dimensions of the buoy and the same motions of the FPSO. Increasing the number and size of tethers in this way would significantly increase the installation complexity and cost of a project using a BSR system. 
     The extended pontoons concept of the invention confers much better dynamic behaviour on a BSR system and improves the responses of the system in extreme and tether-failure cases with reduced buoy motion and increased fatigue life for tethers, riser pipes and jumper pipes. So, for given main hull dimensions of the buoy and for a given tether system, the extended pontoons concept advantageously limits the pitch period of the buoy and minimises fluctuating loads on the tethers, increasing their endurance.