Patent Publication Number: US-2012040573-A1

Title: Deep water and ultra deep water mooring system

Description:
This invention relates to a mooring and flowline system for use in floating facilities for handling petroleum and petroleum products. The system is particularly, but not exclusively applicable to the mooring and loading of floating production vessels operating in waters in excess of 1000 m deep. 
     Production from offshore wells can be exported ashore by subsea pipeline, but this requires a very large capital expenditure. Recently there has been a move to exploiting more marginal fields by the use of a floating production, storage and offtake (FPSO) vessel. Commonly, the floating production facility has been provided by conversion of an existing tanker. Such arrangements have worked well, but there is a continuing need for a substantial reduction in installation costs in order to improve the economics of marginal field development and production. 
     Tanker conversions used hitherto have required extensive conversion. In some cases a turret mooring is used which includes a rotary oil flowline joint. This requires major structural work on the tanker in addition to the complex mooring turret. In other cases, a flexible riser to the tanker has been used, but has required large quick disconnect (QDC) valves with a physical size and weight requiring installation outboard of the tanker bow on a specially installed and relatively large structure. 
     A Tri-Catenary Mooring System has more recently been proposed in U.S. Pat. No. 5,944,448 and U.S. Pat. No. 6,435,124 (assigned to Mooring Systems Limited of Aberdeen, UK) and such a system has proved very effective in operation in the Oudna field, offshore Tunisia, in approximately 260 m water depth. However, it is acknowledged that it would not be technically possible to use the same Tri-Catenary Mooring Systems in water of a depth greater than 500 m. 
     The exploitation of more marginal fields often requires operation in water at depths in excess of 500 m and sometimes in excess of 1000 m. Operating at these increased depths presents many challenges when using a FPSO vessel, including but not exclusively, the design and shape of the mooring lines, and risers used to transport the oil from the wellhead to the vessel. 
     An object of embodiments in accordance with the present invention is to provide an improved mooring and flowline system which is quickly and fully disconnectable from the FPSO vessel, which is simple and economical to install and which is suitable for use in Deep Water (i.e. at depths greater than 500 m) and Ultra Deep Water (i.e. at depths greater than 1000 m). It is also an object of embodiments in accordance with the invention to provide a system which requires minimum use of heavy lifting gear located on the FPSO vessel or other support vessel. 
     Accordingly, the present invention provides a mooring and flowline system comprising:
         means for mooring a floating production and/or storage vessel to the seabed; and   one or more flowlines for connection to one or more producing wells or to a facility;   the mooring means comprising at least two anchors in the sea bed and a respective anchor riser extending from each of the anchors, each of the anchor risers having one end secured to its anchor and the other end secured to a common mooring node member, and a mooring pendant extending from the common mooring node member, the mooring pendant having an upper end for connection, in use, to the vessel;   each flowline comprising a lower flowline extending from the direction of the well or facility partway toward the vessel into the vicinity of the mooring means, and a flowline riser extending from the lower flowline to the vessel;   wherein each flowline riser has part of its length secured to the mooring pendant by one or more guide frames to control sideways and rotational movement so that, in use, each flowline riser is capable of helixing around the mooring pendant in response to turning of the vessel;   characterised in that buoyancy means are provided on, in-between, for or by at least one of the one or more guide frames, wherein the buoyancy afforded by the buoyancy means is sufficient to support the common mooring node member and mooring pendant below the surface of the sea and above the seabed when the mooring pendant is disconnected from the floating production and/or storage vessel.       

     Typically, the system comprises two or more guide frames and more preferably comprises more than two guide frames such as in the region of 6 to 12 guide frames. 
     Preferably, the buoyancy means are distributed along the length of the mooring pendant, between the common mooring node member and the upper end of the mooring pendant for connection to the vessel and more preferably are distributed substantially equi-distant along the length of the mooring pendant. 
     Preferably, the buoyancy means are provided on, in-between, for or by each of the one or more guide frames such that the one or more guide frames support the mooring pendant when disconnected from the floating production and/or storage vessel and more preferably, the buoyancy means are provided by each of the one or more guide frames. 
     Preferably, the mooring and flowline system is arranged such that when the mooring pendant is connected to the floating production and/or storage vessel, in use, at least one of the guide frames is located above the surface of the sea and at least one of the guide frames is located below the surface of the sea. 
     Preferably, the one or more guide frames are sealed pressure vessels. Optionally, the one or more guide frames consist of sealed chambers that are filled with air. Alternatively, the sealed chambers are filled with any suitable buoyant material, for example foam. 
     Optionally, the buoyancy of the one or more guide frames can be adjusted to control the position in use, of the common mooring node member and mooring pendant below the surface of the sea and above the seabed, when the mooring pendant is disconnected from the floating production and/or storage vessel. 
     Preferably, further buoyancy means is provided for, on or by the mooring means to reduce the likelihood of the anchor risers clashing or interfering with the flowline risers when the system is connected to and disconnected from the vessel. 
     Preferably, the system is capable of operating at depths up to 1000 m, and more preferably at depths greater than 1000 m such as between 1000 and 1500 m of water. 
     Optionally, disconnection of the system can be made without the need to lower the system into the water using a winch. 
     Preferably, when the mooring pendant is disconnected from the floating production and/or storage vessel, the one or more guide frames are located below the surface of the sea and above the sea bed and more preferably, the buoyancy provided by the buoyancy means of the one or more guide frames supports the mooring pendant and one or more flowline risers attached thereto when disconnected at a depth in the region of 50 m to 100 m and more preferably in the region of 70 m. Preferably, once disconnected from the vessel, the system adopts a stable submerged configuration. 
     Optionally, the buoyancy on the flowlines is in the form of buoyancy tanks. 
     Preferably, yet further buoyancy means is provided for, on or by the anchor risers such that the profile of the anchor risers is increased in that the anchor risers are raised and held above the flowlines. Increasing the profile of the anchor risers helps to reduce the likelihood of the anchor risers clashing or interfering with the flowline risers. 
     Optionally, a riser jumper provides fluid communication between each riser tower and the floating production and/or storage vessel. 
     Optionally each riser jumper is a continuous conduit with a single axial rotational coupling, located between the uppermost guide frame and a riser jumper hang-off point on the vessel. 
     Optionally, the buoyancy tanks support a connection between the riser tower and the riser jumper at a depth of 150 m. 
     Optionally the buoyancy tanks pull on the connection via 20 m of chain. 
     Preferably the system is fully disconnectable in 10 hrs, more preferably in 8 hrs, and in less than 8 hrs being particularly preferred. 
     Preferably, there are at least three anchors in an angular array, three anchors at mutual 120° spacings being particularly preferred. The anchors may be arranged to point north, south-west and south-east. 
     Preferably, there is no mooring swivel between the anchors and the upper end of the mooring pendant; the common mooring node member may be a plain ring, shackle or plate. 
     In a preferred embodiment of the invention, a conventional tanker is used as the production vessel with minimal conversion, the chafe chain brought inboard of the tanker to a standard chain stopper or to a remotely controlled release stopper, the riser jumpers are brought inboard of the tanker over a chute to have their inboard ends lying near the tanker deck, and quick disconnect valves are located near the tanker deck between said riser jumpers and a tank manifold of the tanker. 
     Optionally, the chafe chain may comprise two parallel chains, one passing over each bow of the vessel. 
     Typically the anchor risers are chains between 120 and 147 mm in diameter of Grade R3 or optionally of higher strength material. 
     Typically the chafe chain is 147 mm in diameter of Grade R3 or optionally of higher strength material. 
     Preferably, the vessel is a tanker. 
    
    
     
       An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which: 
         FIG. 1  is a schematic side view of an embodiment of a mooring and flowline system in accordance with the present invention, in use with a production tanker; 
         FIG. 2  is a plan view corresponding to  FIG. 1 ; 
         FIG. 3  is a schematic side view of the chafe chain and guide frames in particular of  FIG. 1 ; 
         FIG. 4  is a schematic side view of the embodiment of  FIG. 1 , the mooring and flowline system having been disconnected from the production tanker; 
         FIG. 5  is a schematic perspective view of a guide frame of  FIG. 3 ; 
         FIG. 6  is a schematic perspective view of the connection between the chafe chain and a guide frame of  FIG. 3 . 
     
    
    
     Referring particularly to  FIG. 1 , a floating production tanker or FPSO  10  is anchored to the sea bed  12  by means of a three-leg anchor system consisting of anchor risers  14   a ,  14   b ,  14   c  and mooring buoys  16   a ,  16   b ,  16   c ,  16   d ,  16   e  and  16   f . The anchor risers  14   a ,  14   b  and  14   c  are connected to a common mooring node  18 , from which a mooring pendant  20  is secured to the tanker  10 . The anchor system is in the vicinity of a wellhead (not shown in the drawings) from which oil is transferred via a number of flowlines, as will now be described. 
     The flowlines consist of riser towers  22   a ,  22   b  and  22   c  and riser jumpers  24   a ,  24   b  and  24   c . The riser towers  22   a ,  22   b  and  22   c  are provided with buoyancy tanks  26   a ,  26   b  and  26   c . The riser towers  22   a ,  22   b  and  22   c  may be tensioned steel catenary risers. 
     The upper part of the riser jumpers  24   a ,  24   b  and  24   c  are secured along the mooring pendant  20  by the guide frames  32  that are each rigidly connected to plates  56  integral to the chafe chain  36  as shown in  FIG. 6 . 
     The riser jumpers  24   a ,  24   b  and  24   c  are taken aboard the tanker  10  via a turret  34 . The riser jumpers  24   a ,  24   b  and  24   c  are flexible with sufficient dynamic properties to accept movement of the ship&#39;s bow and movement of the anchor risers  14   a ,  14   b  and  14   c.    
     The riser jumpers  24   a ,  24   b  and  24   c  are continuous conduits with a single axial rotational coupling (not shown), located between the uppermost guide frame  32  and a riser jumper hang off point (not shown) on the turret  34  of the tanker  10 . 
     The mooring buoys  16   a ,  16   b ,  16   c ,  16   d ,  16   e  and  16   f  increase the profile of the anchor risers  14   a ,  14   b  and  14   c  such that anchor risers  14   a ,  14   b  and  14   c  are held above and clear of the riser jumpers  24   a ,  24   b  and  24   c , reducing the likelihood of the anchor risers  14   a ,  14   b  and  14   c  clashing or interfering with the riser jumpers  24   a ,  24   b  and  24   c.    
     The riser jumpers  24   a ,  24   b  and  24   c  can be unbonded or bonded pipes of a wide variety of sizes and specifications. 
     When the mooring pendant  20  is connected to the vessel  10 , some of the guide frames  32  are located above and some of the guide frames  32  are below the surface of the sea  66 . 
     The mooring pendant  20  is particularly for use with a mooring and flowline system as described herein that can be operated in Ultra Deep Water. The mooring pendant  20  can also be used with a lazy-wave arrangement of flowlines such as that described in U.S. Pat. No. 5,944,448 and U.S. Pat. No. 6,435,124 (assigned to Mooring Systems Limited of Aberdeen, UK), and such systems could be operated in Deep Water (but would not likely be suited for Ultra Deep Water). 
     As seen in  FIG. 2 , the anchor system is set such that a 120° angle may be presented towards the sea bed  12  with one leg of the anchor system pointing north.  FIG. 2  also shows umbilicals  28   a ,  28   b  and  28   c  provided with flotation collars  30   a ,  30   b  and  30   c . The umbilicals  28   a ,  28   b  and  28   c  provide communication between the vessel  10  and wellhead (not shown). 
     The anchors (not shown) may be any suitable form of conventional mooring anchor. At least some of the length of each anchor riser  14   a ,  14   b  and  14   c  is chain. In this embodiment the on-bottom section is chain, the lower part of the anchor risers  14   a ,  14   b ,  14   c  is chain and the upper 350 m of the anchor risers  14   a ,  14   b ,  14   c  is preferably of synthetic rope. 
     The use of rope for part of the anchor riser  14   a ,  14   b ,  14   c  has several advantages. Principally, it reduces the weight which has to be lifted when the tanker  10  is connecting to the system, as is discussed more fully below. It also simplifies stowage on the vessel  10  used to deploy the system. When rope is used it is desirable to have the final 30 metres or so of anchor riser  14   a ,  14   b ,  14   c  nearest the node  18 , of chain, to reduce the risk of kinking the rope. 
     Referring to  FIG. 3 , the node  18  is a plate that connects together the three anchor risers  14   a ,  14   b  and  14   c  and the pendant  20  with adequate mechanical strength. A suitably sized master ring may be used, or a triangular plate arrangement, together with conventional shackles. No swivel is incorporated in the node  18 . The lower most end of the chafe chain  36  is secured to the node  18  with a suitable shackle or the like. Guide frames  32  are secured to the chafe chain  36  as shown in  FIG. 6  and described below. 
       FIG. 3  shows the disposition of the three riser jumpers  24   a ,  24   b  and  24   c  passing through nine guide frames  32 , although ten guide frames  32  are shown in  FIG. 1 , secured to the chafe chain  36 . This shows how the riser jumpers  24   a ,  24   b  and  24   c  approach the node  18  from different directions and are brought into a parallel array at the vessel&#39;s bow at position  40 . The guide frames  32  are evenly distributed along the length of the chafe chain  36 . In this particular embodiment, as shown in  FIG. 3 , there are nine guide frames  32  secured to the chafe chain  36  at 4.5 m intervals along a 51 m length of chafe chain  36 . 
     In a typical installation, the tanker  10  is a segregated ballast tanker of 600,000 to 750,000 bbl capacity and the mooring is designed to hold the node  18  at a depth of 30 metres, and thus up to about 15 metres beneath the tanker hull. In these circumstances, the chafe chain  36  will be of the order of 40 to 60 m in length. 
     The preferred form for the mooring pendant  20  comprises a single large-size chafe chain  36  extending from the node  18  and connected to the tanker  10 . A suitable chain has a diameter in the region of 120 mm to 147 mm, which will fit many standard chain stoppers (not shown). This arrangement simplifies the design of the node  18  and minimises modification to the tanker  10 . 
     A suitable tanker bow installation or turret is used that requires minimum modification of standard shipping practices, as described in for example U.S. Pat. No. 6,435,124. 
     Installation of the system requires each anchor (not shown) to be laid in turn, and the anchor risers  14   a ,  14   b ,  14   c  to be run back to a common point. As it is laid, each anchor riser  14   a ,  14   b ,  14   c  is buoyed off on a pendant wire (not shown). Tensioning of the anchors can be achieved using the bollard pull of suitable vessels or with the inclusion in the system of a standard chain tensioner. One of the mooring legs (not shown) includes a standard seabed chain stopper for correct pre-tensioning of the system. When all three anchor risers  14   a ,  14   b ,  14   c  have been laid, the anchor handler (not shown) recovers all three anchor risers  14   a ,  14   b ,  14   c  over the stern roller (not shown). 
       FIG. 4  depicts the mooring and flowline system disconnected from the vessel. The buoyancy provided by the guide frames  32  provided on the mooring pendant  20 ; mooring buoys  16   a ,  16   b ,  16   c ,  16   d ,  16   e  and  16   f ; and buoyancy tanks  26   a ,  26   b  and  26   c , maintain the necessary profile of the riser jumpers  24   a ,  24   b  and  24   c  and anchor risers  14   a ,  14   b  and  14   c . In a preferred embodiment, once disconnected from the vessel  10 , the system adopts a stable submerged configuration, the mooring pendant  20  preferably being arranged to sink to a depth of 70 m. 
       FIG. 4  illustrates a pick-up rope  42  attached to the upper end of the mooring pendant  20  and a buoy  46 . 
     In use, when tanker  10  approaches location, the pick-up rope  42  is recovered from buoy  46 , either by the tanker  10  itself or by a support vessel (not shown) and fired across to the tanker using a compressed air rocket gun (not shown). The tanker  10  pulls on the pick-up rope and starts to retrieve the mooring pendant  20 , securing the chafe chain  36  in the chain stoppers (not shown). The winches (also not shown) of a typical tanker  10  can be supplemented by a block and tackle or a temporary winch to achieve sufficient pull for this application, which may require a pull of up to 400 tonnes. The guide frames  32  provide buoyancy and therefore an upward force which reduces the pull or recovery force required to lift the mooring pendant  20 , and the rest of the mooring and flowline system, from a depth of 70 m up to the sea surface  66 . Therefore, the guide frames  32  provide sufficient buoyancy to the mooring and flowline system such that the force provided by the buoyancy of the guide frames  32  (when the upper most guide frame  32  is in the region of 70 m water depth) is substantially equal and opposite to the weight of the rest of the mooring and flowline system and thus provides stability to the mooring and flowline system in the water after it has been disconnected from the tanker  10 . 
       FIG. 5  shows a guide frame  32  in more detail. The guide frame  32  is a sealed pressure vessel. The central slot  48  receives the chafe chain (not shown). Ports  50   a ,  50   b  and  50   c  receive riser jumpers  24   a ,  24   b  and  24   c . Ports  52   a  and  52   b  receive two of the umbilicals  28   a ,  28   b  and  28   c . Port  54  receives a gas export pipe (not shown). 
       FIG. 6  further depicts the plate  56  used for connecting the guide frame  32  to the chafe chain  36 . The embodiment of the invention shown in  FIG. 6  comprises chambers  60  filled with air, although they could be partially or fully evacuated to provide a partial or full vacuum. The chambers  60  are sealed to provide buoyancy. The chafe chain  36  is secured to the guide frame  32  by bolts  62  that pass through plate  56  and guide frame  32 . 
     A breakaway-cum-emergency release unit (not shown) may be provided, suitably a Gall-Thomson coupling is included and consists of bolts which shear at a predetermined axial load, a chute and a short section of flexible pipe ensuring that the load on the coupling is always axial. The breakaway-cum-emergency release unit (not shown) also seals the pipe both sides of the unit as the unit breaks apart. A manually operated ball valve would be used to close the connection in normal disconnection routines. The hydraulically or gas pressure activated release system (not shown) within the breakaway-cum-emergency release unit can be operated remotely, for example, from the bridge of the vessel, when it is decided to make an emergency disconnection of the tanker. The emergency shutdown valve is included to provide shutdown of flow without disconnection and would normally be part of the tanker&#39;s equipment even if not located close to the release system. 
     It will be appreciated from the foregoing description that the arrangement in operation will cause the riser jumpers  24   a ,  24   b  and  24   c  to helix around the mooring pendant  20 . There will therefore be a restriction on the number of turns the vessel  10  can make, since turning full circle will effectively twist the riser jumpers  24   a ,  24   b  and  24   c  and the mooring pendant  20  round each other. Contrary to previous practice, the present inventors believe and can demonstrate that the limit of this helixing is not a practicable restriction in a floating production situation. It is believed that the weather patterns actually found in practice may make a vessel  10  do complete turns; however there are more than sufficient lighter weather periods when the tanker  10  can turn back, either using its own engines and rudder (not shown), or being pulled round by a tug (also not shown). 
     During normal operation the vessel  10  aligns with its stern  64  meeting the waves (not shown). The vessel  10  is able to weathervane up to three times before it must turn back to unwind the riser jumpers  24   a ,  24   b  and  24   c ; umbilicals  28   a ,  28   b  and  28   c ; and gas export pipe (not shown) from around the chafe chain  36 . 
     The invention provides a mooring and flowline system which can handle one or more flowlines making use of a minimally converted tanker and relatively small, standard components. In particular the present invention may use individual hose swivels each typically 600 mm long, weighing 120 kg and costing about GBP 20,000, made as standard components by manufacturers such as Woodfield. In contrast, prior art systems require the use of a multi-path (toroidal) flowline swivel which can be up to 10 m high, weigh several tens of tons, and cost GBP 3-4 million. 
     Modifications and improvements may be made to the embodiments hereinbefore described without departing from the scope of the invention. For instance, buoyancy can be added to the chafe chain  36  in-between guide frames  32  either instead of the inherent buoyancy provided by the sealed pressure vessel guide frames  32  disclosed herein or in addition thereto, to support the mooring pendant  20  when disconnected from the vessel  10  and submerged.