Patent Publication Number: US-10760241-B2

Title: Upgrading subsea foundations of mooring systems

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to retrofitting mooring systems, as used in the subsea oil and gas industry, to upgrade the capacity of their subsea foundations to resist tension in mooring lines. 
     A poster published in  Offshore Magazine , October 2013, entitled  Mooring systems for offshore floating installations: trends and technology  provides a comprehensive overview of the mooring technologies known in the art. That poster may be viewed online at http://www.offshore-mag.com/content/dam/offshore/print-articles/Volume%2073/10/MOORING-010814REV.pdf 
     Floating surface units or installations used in oil and gas production are moored to the seabed to remain in substantially the same location for many years of production. An example of such an installation is an FPSO (Floating Production, Storage and Offloading) vessel. 
     Some taut-leg floating production units are known but these require reinforced anchorages to be installed at the outset, which is generally achieved by injecting concrete or grout. The present invention is concerned with the more common catenary mooring lines that have generally catenary curvature when viewed in profile. 
     Typically, an FPSO will be held in place by a mooring system comprising catenary-shaped mooring lines arranged in groups to define semi-taut legs. When viewed from above, the mooring lines splay apart from neighbouring mooring lines in each group to define a passive spread pattern. 
     Usually there is one group of mooring lines at each quadrant of an FPSO, hence a total of four groups, radiating outwardly at 90° intervals when viewed from above. If each of the four groups contains three mooring lines, for example, this is known in the art as a 4×3 pattern. Some surface installations, such as turret-moored vessels, are moored with three groups of mooring lines, radiating outwardly at 120° intervals when viewed from above. 
       FIGS. 1 and 2  show one of the mooring lines  10  of a conventional mooring system for an FPSO  12  floating at the surface  14 , as an example of a moored surface installation. The mooring line  10  hangs with catenary curvature under tension between the FPSO  12  and the seabed  18 . Whilst curved, the mooring line  10  lies substantially in a vertical plane. 
     In this example, the mooring line  10  is anchored by a suction anchor or suction pile  16 . Most of the length of the pile  16  is embedded in the soil of the seabed  18  to ensure sufficient resistance to tension in the mooring line  10  when the FPSO  12  moves back. 
     It will be apparent from  FIGS. 1 and 2  that the mooring line  10  is attached to the pile  16  at a level below the mid-point of the length of the pile  16 , typically between half and two-thirds of the way down from the top of the pile  16 . A similar arrangement is also shown, for example, in WO 02/062653 to a predecessor of the Applicant. This arrangement optimises the balance of forces that act on the pile  16 : connecting the mooring line  10  at or nearer to the top would tend to cause rotation of the pile  16 , which would require a bigger pile to withstand a given tension in the mooring line  10 . 
     It follows that a short bottom part of the mooring line  10 , typically 10 m to 20 m long, is buried in the soil of the seabed  18  beside the pile  16 . That buried bottom part is sometimes called a ‘forerunner’. 
     A drag anchor, also embedded in the seabed  18 , may be used instead of a pile  16  as a mooring foundation for a surface installation such as an FPSO  12 . Again, a short bottom part of the mooring line  10  will similarly be buried in the soil of the seabed  18  if a drag anchor is used. 
     In more detail, the mooring line  10  comprises, in sequence from bottom to top: a bottom or ground chain  20  attached to the pile  16 ; a section of spiral strand wire (SSW)  22  attached to the ground chain  20 ; and a top chain  24  that joins the SSW section  22  to the FPSO  12 . 
     The SSW section  22  will usually be of coated steel but could be of a synthetic plastics material instead: references to ‘wire’ in this specification are not intended to limit the meaning only to metallic wires. 
     The SSW section  22  constitutes most of the length of the mooring line  10  because, for a given tensile strength, wire is lighter, more compact to store and less expensive than chain. Chains  20 ,  24  are used instead of wire at the bottom and top of the mooring line  10  to avoid damage to the wire at those vulnerable locations. As a non-limiting example, the ground chain  20  and the top chain  24  may each be about 200 m long whereas the SSW section  22  may be well over 1200 m long. 
     Various known connectors  26  join the successive components of the mooring line  10 . Different types of connector  26 , such as chain connectors and shackles, will typically be used at different locations along the mooring line  10 . 
     It will be apparent from the detail view of  FIG. 2  that the ground chain  20  is in two sections, namely a lower section  28  and an upper section  30 . Thus, in sequence from the bottom to the top of the mooring line  10 : the lower section  28  of the ground chain  20  is attached to a buried side wall of the pile  16 ; the upper section  30  of the ground chain  20  is connected to the lower section  28  by a first connector  26 ; the SSW section  22  is connected to the upper section  30  of the ground chain  20  by a second connector  26 ; and the top chain  24  is connected to the SSW section  22  by a third connector  26 . 
     The lower section  28  of the ground chain  20  is attached to the pile  16  before the pile  16  is overboarded from a surface installation vessel and lowered to penetrate the seabed  18 . Thus, the lower section  28  of the ground chain  20  and the first connector  26  are buried under the seabed  18  as part of the bottom ‘forerunner’ part of the mooring line  10 . 
     The upper section  30  of the ground chain  20  extends from the buried first connector  26  beneath the seabed  18  to the second connector  26  above the seabed  18 , where it joins the SSW section  22 . Thus, a short transitional portion of the upper section  26  of the ground chain  20  lies on or close to the seabed  18 . 
     A drawback of the partially-buried ground chain  20  arises from inevitable movement of the ground chain  20 , in use, relative to the surrounding soil of the seabed  18 . For example, all parts of the mooring line  10 , including the ground chain  20 , will move in response to motion of the FPSO  12  under wind and wave action. Similarly, all parts of the mooring line  10  will be moved by other seawater dynamics acting on the mooring system, such as ocean currents, especially in deeper water. 
     The resulting movements of the ground chain  20  also move the adjacent soil of the seabed  18 . Over time, this may create a trench without soil around and above the previously-buried part of the ground chain  20  beside the pile, hence potentially reducing the capacity of the pile  16 . Similarly, a drag anchor could slip and lose its intended position. The result is that, after several years, the actual capacity of the foundation may be significantly lower than was originally intended. 
     In other cases, the tension applied by a mooring line to a foundation may be greater than was originally intended. For example, the floating surface installation may be enlarged or a mooring line handling system may be upgraded. 
     For these reasons, it may be necessary to upgrade the capacity of a subsea foundation from time to time. Upgrading capacity may involve recovering original capacity that has been lost by a foundation over time. Alternatively, or additionally, upgrading capacity may involve improving the original capacity of a foundation. 
     Conventionally, upgrading the capacity of a subsea foundation involves installing a new, more efficient foundation in the seabed near the old foundation. Next, part of the mooring line is disconnected from the old foundation to reconnect it to the new foundation. The old foundation is then redundant. 
     Disconnecting a mooring line from an existing foundation, or even slackening a mooring line to reduce tension, disadvantageously increases the load on other parts of the mooring system. It is a lengthy and costly operation that introduces a risk of the surface installation losing its position, especially if sea conditions deteriorate during the operation. 
     WO 2008/129320, to a predecessor of the Applicant, discloses a frame that is used for connecting mooring line elements on the seabed. In a reverse operation, the frame can also be used to separate mooring line elements. The frame comprises pulling means to connect mooring line elements. However, the mooring line elements cannot have residual tension in them. Thus, the frame of WO 2008/129320 cannot be used on a live tensioned line; a line from the floating surface installation must first be slackened. 
     U.S. Pat. No. 5,061,131 discloses an alternative approach involving extra weights that sharply increase mooring resistance in the event of extreme drift. However, the possible size of such weights is limited on a permanent mooring and in any event would not provide a sufficient upgrade in tension-resisting capacity. Also, the weights apply a permanent shear stress to the mooring lines. 
     WO 94/16936 relates to a twin-anchor mooring arrangement for a floating vessel, in which a first drag anchor is attached at an end of a mooring line and a second drag anchor is slidable along the mooring line to a second anchoring position. 
     US 2009/123235 relates to a pile anchor system for an offshore structure. The system includes an original pile attached to a mooring line, and a supplementary pile installed in the vicinity of the original pile. The supplementary pile is attached to the original pile by means of a coupling member, but is not attached to the mooring line itself. 
     U.S. Pat. No. 7,976,246 describes a system for creating a deep water mooring spread by successively installing independent suction piles from a floating vessel. 
     BRIEF SUMMARY OF THE INVENTION 
     Against this background, the invention provides a method of upgrading the capacity of a mooring system, which system comprises a pre-existing subsea foundation at a first seabed location and a mooring line, which may be catenary-shaped, connected to the pre-existing subsea foundation and extending to a moored floating unit. The method comprises: installing a supplementary subsea foundation at a second seabed location, which may be spaced across the seabed from the first seabed location; and connecting the mooring line to the supplementary subsea foundation while maintaining the connection between the mooring line and the pre-existing subsea foundation. 
     A portion of the mooring line may be displaced at the second seabed location to make space for installing at least part of the supplementary subsea foundation. The displaced portion of the mooring line can then be moved into connection with at least part of the supplementary subsea foundation. 
     For example, the displaced portion of the mooring line may be lifted away from the seabed. In that case, at least part of the supplementary subsea foundation may be moved to a position beneath the displaced portion of the mooring line, for example by effecting translational movement of that part across the seabed or by lifting that part across the seabed. Before that, at least part of the supplementary subsea foundation may be lowered to a seabed location beside the mooring line. In another approach, the displaced portion of the mooring line may be diverted around the second seabed location and laid on the seabed. 
     Preferably, a frame of the supplementary subsea foundation is placed on the seabed; one or more foundation elements are embedded in the seabed and coupled to the frame; and the mooring line is connected to the frame. For example, a foundation element may be coupled to the frame by installing that foundation element into or through a guide structure provided in or attached to the frame. Another approach is to place a link between the frame and the foundation element, where the foundation element is embedded in the seabed and spaced apart from the frame. 
     Advantageously, the mooring line is placed between first and second laterally-spaced foundation elements. 
     A frame of the supplementary subsea foundation may be coupled with a foundation element of the supplementary subsea foundation pre-installed at the second seabed location. For example, the frame may be lowered onto the foundation element, such as onto the top of a pile. 
     If the supplementary subsea foundation is a pile, the mooring line may be connected to the pile via a connection mechanism integrated with the pile. 
     After installation of the supplementary subsea foundation is complete, the surface installation may continue to be moored using the existing subsea foundation and the supplementary subsea foundation in combination to resist tension in the mooring line without disconnecting the line from either foundation. 
     Preferably, a chain section of the mooring line is connected to the supplementary subsea foundation by clamping the chain section or by mechanically engaging links of the chain section. More generally, the mooring line is advantageously connected to the supplementary subsea foundation by embracing the mooring line. 
     The inventive concept extends to a related mooring system that comprises: a first subsea foundation at a first seabed location; a mooring line, which may be catenary-shaped, connected to the first subsea foundation and extending to a moored floating unit; and a supplementary subsea foundation at a second seabed location, connected to the mooring line by a remotely-operable mechanical connector. 
     The mechanical connector preferably embraces the mooring line and may connect the supplementary subsea foundation to a chain section of the mooring line by clamping the chain section or by engaging links of the chain section. 
     Advantageously, the mechanical connector defines an upwardly-opening receptacle to receive the mooring line. 
     The supplementary subsea foundation suitably comprises: a frame supporting the connector; and one or more foundation elements embedded in the seabed and coupled to the frame, for example via a guide structure provided in or attached to the frame or via a link extending between the frame and the or each foundation element, which foundation element is embedded in the seabed and spaced apart from the frame. 
     An interface may be provided on top of at least one foundation element and/or beneath the frame for coupling the frame to the foundation element. In another approach, the supplementary subsea foundation is a pile and the connector is integrated with the pile. 
     A supplementary subsea foundation for use in the mooring system of the invention comprises: a frame; a remotely-operable mechanical connector supported by the frame, which connector is arranged to connect with a mooring line; and at least one foundation element attachable to or integrated with the frame, arranged to be embedded in seabed soil. 
     The invention therefore provides a back-up or auxiliary mooring system that reinforces an existing mooring system without having to disconnect the mooring line. The invention embodies the principle of installing a subsea structure on the seabed near the existing mooring line, which is then lifted and connected to the structure. The subsea structure may be integrated with or attached directly to an anchoring foundation, such as a suction pile, or may be installed separately and anchored by driven piles to form a hybrid foundation anchor. 
     In summary, the invention provides a method and apparatus for upgrading the capacity of a catenary mooring line under tension, without disconnecting the line from its existing subsea foundation. This saves time and cost and reduces the risk of the anchored vessel drifting. The existing foundation remains connected to the mooring line during the upgrading operation and so remains available to restrain motion of the vessel. 
     The use of a supporting frame rather than a simple mechanical connection advantageously limits lateral motion of the part of the mooring line that is embedded in the seabed soil. The reduced lateral motion of the buried line portion reduces the trenching effect and so delays any additional loss of resistance to tension in the line. 
     The apparatus of the invention is designed to be installed and actuated in deep water from the surface, using known and readily-available means such as cranes and ROVs. 
     Embodiments of the invention provides a method to upgrade the resistance capacity of a mooring system. The method comprises: vertically lifting the mooring line near an initial foundation; inserting a supporting frame below a lifted section of the mooring line; releasing the vertical lift on the mooring line and mechanically connecting the frame to the mooring line; and coupling at least one additional foundation to the frame. 
     Embodiments of the invention also provide a device to upgrade the resistance capacity of a mooring system. The device comprises: a supporting frame inserted between the mooring line and the seabed; a remotely-actuated mechanical connector for connecting the mooring line to the supporting frame; and at least one additional foundation coupled to the supporting frame. 
     The additional foundation may comprise one or more piles, such as suction piles, gravity piles, pin piles or driven piles. Other forms of additional foundation are possible, preferably embedded foundations such as drag anchors. 
     At least one pair of additional foundations may be used, one foundation on each side of the mooring line. Thus, one foundation of each pair is preferably on a respective side of the mooring line. 
     Additional foundations such as piles may be inserted in slots, openings or guide tubes of, in or attached to the supporting frame. In another approach, additional foundations may be coupled to the frame by mooring line elements such as chains or wires, for example spiral strand wire. 
     Where an additional foundation is a suction pile, the top of the suction pile may serve as the supporting frame. The mooring line is suitably lifted aside during installation of the frame/pile. Mechanical connection between the mooring line and the frame/pile may be effected via a recess in the frame/pile. 
     The supporting frame may be arranged to lie on the seabed, for example taking the form of a generally flat mat or mudmat. Such a mudmat may have a peripheral skirt to engage with the seabed soil or may have sled formations designed to slide over the seabed, at least in an installation direction intersecting the general plane of the mooring line. 
     The additional foundation can be installed in or on the seabed before or after the frame. However it is preferred to install the additional foundation after the frame because it may be more challenging to position and couple the frame to a pre-installed foundation. 
     Mechanical connection between the frame and the mooring line may, for example, be effected by a chain stopper or by a clamp. 
     To illustrate the prior art background, reference has already been made to  FIGS. 1 and 2  of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a side view of a mooring line extending from a moored surface vessel to a suction pile; and 
         FIG. 2  is an enlarged detailed view of the ground chain end of the mooring line and the pile, corresponding to Detail II of  FIG. 1 . 
       In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the drawings in which: 
         FIG. 3  is a side view of a mooring line corresponding to  FIG. 1  but with the addition of a supplementary foundation interposed between the FPSO and the suction pile in accordance with the invention; 
         FIG. 4  is a schematic perspective view of a ground chain of a mooring line on the seabed before the addition of a supplementary foundation in accordance with the invention; 
         FIG. 5  corresponds to  FIG. 4  but shows the ground chain lifted above the seabed and a support frame placed on the seabed beside the ground chain; 
         FIG. 6  corresponds to  FIG. 5  but shows the support frame moved laterally across the seabed to lie under the still-lifted ground chain; 
         FIG. 7  is an enlarged schematic perspective view of the support frame of  FIG. 6  with the ground chain now lowered into engagement with a chain stopper mechanism on the support frame; 
         FIG. 8  corresponds to  FIG. 7  but shows the addition of piles that pin the frame to the seabed to complete the supplementary foundation of the invention; 
         FIG. 9  is a schematic perspective view of a second embodiment of the invention in which the supplementary foundation of the invention is completed by anchoring a support frame carrying a chain stopper mechanism to piles that are remote from the support frame; 
         FIG. 10  is a schematic perspective view of a third embodiment of the invention in which a support frame carrying a chain stopper mechanism is integrated with a pile to form a supplementary foundation, the ground chain being diverted laterally from its normal route to make space for installing the pile; 
         FIG. 11  corresponds to  FIG. 10  but shows the ground chain moved into engagement with the chain stopper mechanism on the pile; and 
         FIG. 12  is a schematic perspective view of a fourth embodiment of the invention in which a support frame carrying a chain stopper mechanism is being connected to a pre-installed pile. 
     
    
    
     DETAILED DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Referring firstly to  FIG. 3 , this largely corresponds to  FIG. 1  and therefore like numerals are used for like parts. Here, a supplementary foundation  32  that supports the mooring line  10  in accordance with the invention is shown interposed between the FPSO  12  and the suction pile  16 . The supplementary foundation  32  engages the ground chain  20  of the mooring line, where the ground chain  20  crosses or lies on the seabed  18 . 
     Thus, the pile  16  and the supplementary foundation  32  cooperate with each other to resist tension in the mooring line  10 . In other words, the supplementary foundation  32  adds to the capacity of the pile  16  to upgrade the overall capacity of the mooring system. Importantly, this upgrade is achieved without disconnecting the mooring line  10  from the pile  16 . Movement of the FPSO  12  is restrained continuously by the mooring line  10  throughout. 
     Advantageously, the lowermost portion of the mooring line  10  between the pile  16  and the supplementary foundation  32 , including the portion that is buried in the soil of the seabed  18 , is restrained against movement by the supplementary foundation  32 . The supplementary foundation  32  also isolates that portion of the mooring line  10  from movement of the remainder of the mooring line  10 . Thus, the capacity of the pile  16  will no longer degrade significantly due to trenching effects. The upper portion of the mooring line  10  between the supplementary foundation  32  and the FPSO  12  can, of course, continue to move but its amplitude of lateral motion is restricted. In any event, movement of the upper portion of the mooring line  10  takes place largely above the seabed  18 , where it cannot give rise to a trenching effect. 
       FIGS. 4 to 8  illustrate one way in which the supplementary foundation  32  of  FIG. 3  may be installed in accordance with the invention. 
       FIG. 4  shows a portion of the ground chain  20  of the mooring line  10  lying on the seabed  18 . Part of the catenary curvature of the mooring line  10  between the FPSO  12  and the seabed  18  is evident on the right side of  FIG. 4 , where the ground chain  20  rises above the seabed  18 . The pile  16  to which the ground chain  20  is attached is not shown but will lie to the left of the portion of the ground chain  20  shown in  FIG. 4 , at the end of a further portion of the ground chain  20  buried in the seabed  18 . 
       FIG. 5  shows a mid-point of the ground chain  20  lifted from and held above the seabed  18  by a wire  34  that hangs from a surface support vessel (not shown). The wire  34  suitably hangs from a winch on the vessel, such as an abandonment and recovery (A&amp;R) winch, but could instead hang from a crane on the vessel. 
     In  FIG. 5 , the surface support vessel has also lowered a frame  36  onto the seabed  18  in a holding location beside the raised portion of the ground chain  20 . The frame  36  carries a connection mechanism  38  that is arranged to grip or engage the ground chain  20 . The connection mechanism  38  may, for example, employ the principle of a clamp or a chain stopper as disclosed in CN 104802934. In this example, the frame  36  also carries one or more guide tubes  40  to receive pin piles, as will be described more fully with reference to  FIG. 8  of the drawings. 
     The frame  36  is suitably lowered from a crane on the surface support vessel but could instead hang from a winch on the vessel. An ROV  42  is shown in attendance to monitor the operation and to disconnect slings  44  that support the frame  36 . 
     In  FIG. 6 , the ground chain  20  is still held above the seabed  18  by the wire  34  but the frame  36  has now been moved across the seabed  18  to lie under the raised portion of the ground chain  20 . The direction of movement of the frame  36  is transverse to, preferably substantially orthogonal to, the general plane of the mooring line  10  so as to intersect that plane. 
     This translational, generally horizontal movement of the frame  36  could be achieved by using the crane on the surface support vessel to lift the frame  36  from the holding location on the seabed  18 . In that case, the ROV  42  shown in  FIG. 5  can, if needs be, disconnect and reconnect slings  44  to avoid the slings  44  clashing with the raised portion of the ground chain  20 . Another approach is to slide the frame  36  across the seabed  18  like a sled into its final position under the ground chain  20 . 
     The enlarged view of  FIG. 7  shows the ground chain  20  now lowered into, and engaged with, with the connection mechanism  38  on the frame  36 . The connection mechanism  38  defines an upwardly-opening receptacle for this purpose. The connection mechanism  38  embraces and receives the ground chain  20 . 
     Finally, as shown in  FIG. 8 , pin piles  46  are inserted into the guide tubes  40  on the frame  36  to secure the frame  36  to the seabed  18 . This completes the supplementary foundation  32  as also shown in  FIG. 3 . 
     The connection mechanism  38  is operable remotely to connect the ground chain  20  to the supplementary foundation  32 . The connection mechanism  38  clamps the ground chain  20  or engages links of the ground chain  20  in the manner of a chain stopper. 
     The FPSO  12  remains moored thereafter using the pile foundation  16  and the supplementary foundation  32  in combination to resist tension in the mooring line  10  without disconnecting the line  10  from either foundation  16 ,  32 . The line  10  remains connected to both foundations  16 ,  32  thereafter and may remain so connected at least until the whole mooring system is eventually decommissioned. This may therefore be for longer than a moored FPSO  12  or other surface installation remains on station, which could be for more than twenty years. 
     Turning next to  FIG. 9 , this shows a supplementary foundation  48  in a second embodiment of the invention, being a variant of the first embodiment shown in  FIGS. 3 to 8 . In this variant, a frame  36  supports a connection mechanism  38  as before but the guide tubes  40  are omitted. Instead, the frame  36  is anchored by remote foundation elements that are exemplified here as piles  50  but could take other forms such as drag anchors. The frame  36  is connected to the piles  50  by respective tensile links  52 , which could be of chain or wire. 
     Preferably, as shown in  FIG. 9 , the ground chain  20  is placed between laterally-spaced foundation elements such as the piles  50 . This balances forces on the system while leaving space for installing the piles  50  without having to divert the ground chain  20  laterally as shown in the next embodiments. 
       FIGS. 10 and 11  show a supplementary foundation  54  in a third embodiment of the invention, in which a connection mechanism  38  is integrated with a foundation member. The foundation member is exemplified here as a suction pile  56 . In effect, the top of the pile  56  is equivalent to the frame  36  of the preceding embodiments. 
       FIG. 10  shows the ground chain  20  pulled laterally across the seabed  18  to allow space for installation of the pile  56  along the normal route of the ground chain  20 . Once the pile  56  has been installed in the seabed  18 , the ground chain  20  is lifted into engagement with the connection mechanism  38  as shown in  FIG. 11 . 
     Finally,  FIG. 12  shows a supplementary foundation  58  in a fourth embodiment of the invention, in which a frame  36  that supports a connection mechanism  38  can be mounted to a pre-installed pile  60  via an interface structure  62 . Like  FIG. 10 , the ground chain  20  extending across the seabed  18  is shown here diverted to allow space for installation of the pile  60  along the normal route of the ground chain  20 . 
     As in the first embodiment, the frame  36  is suitably lowered from a crane on a surface support vessel but could instead hang from a winch on the vessel. An ROV like that shown in  FIG. 5  can monitor the operation and then disconnect slings  44  that support the frame  36 . 
     Once the frame  36  has been mounted onto the pile  60  via the interface structure  62 , the ground chain  20  is lifted into engagement with the connection mechanism  38  in a similar way to the arrangement shown in  FIG. 11 . 
     Many variations are possible without departing from the inventive concept. For example, the principle of the invention could be used when mooring a unit that floats at a subsea location rather than at the surface, such as a subsea buoy.