Patent Publication Number: US-2015060079-A1

Title: Riser assembly and method

Description:
The present invention relates to a riser assembly and method. In particular, but not exclusively, the present invention relates to a riser assembly suitable for use in the oil and gas industry, in which the configuration of the riser in the water is controlled. 
     Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater) to a sea level location. The pipe may have an internal diameter of typically up to around 0.6 metres. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe&#39;s functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer layers. 
     In many known flexible pipe designs the pipe body includes one or more tensile armour layers. The primary loading on such a layer is tension. In high pressure applications, in shallow water applications (for example less than around 500 metres depth), deep water (less than 3,300 feet (1,005.84 metres)) and ultra deep water (greater than 3,300 feet) environments, the tensile armour layer may experience high tension loads from a combination of the internal pressure end cap load and the self-supported weight of the flexible pipe. This can cause failure in the flexible pipe since such conditions are experienced over prolonged periods of time. 
     One technique which has been attempted in the past to in some way alleviate the above-mentioned problem is the addition of buoyancy aids at predetermined locations along the length of a riser. The buoyancy aids provide an upwards lift to counteract the weight of the riser, effectively taking a portion of the weight of the riser, at various points along its length. Employment of buoyancy aids involves a relatively lower installation cost compared to some other configurations, such as a mid-water arch structure, and also allows a relatively faster installation time. Examples of known riser configurations using buoyancy aids to support the riser&#39;s middle section are shown in  FIGS. 1   a  and  1   b , which show the ‘steep wave’ configuration and the ‘lazy wave’ configuration, respectively. In these configurations, there is provided a riser assembly  200  suitable for transporting production fluid such as oil and/or gas and/or water from a subsea location to a floating facility  202  such as a platform or buoy or ship. The riser may be provided as a flexible riser, i.e. including a flexible pipe, or as a composite riser, or a metallic riser, and includes discrete buoyancy modules  204  affixed thereto. The positioning of the buoyancy modules and flexible pipe can be arranged to give a steep wave configuration  206   1  or a lazy wave configuration  206   2 . 
     Wave riser configurations as shown in  FIGS. 1   a  and  1   b  may also be used in shallow water applications so as to allow for excursions of the vessel from the point where the riser contacts the sea bed. 
     During use, a riser may be subject to dynamic loading due to conditions such as motion of a vessel or platform on the sea surface. Surge and heave motion of such surface vessel can cause curvature changes in a riser configuration. Strong currents may also have a similar effect. It is generally advantageous to prevent shape changes or control such changes within predetermined limits. The attachment of buoyancy modules, for example in a wave configuration, is one technique for creating a predetermined nominal shape without constraining the pipe, although the effects of surface motion or current motion are still significant in the upper section of the riser and in the areas of the sag and hog bends of the wave configuration. A mid-water arch system has a comparatively higher degree of control and constraint on the pipe, as the pipe is typically clamped to a buoyancy module which has guides running across it for the pipe to lie in. However the size and weight of such mid-water arches is such that the costs of design, manufacture and installation can be very high indeed. 
     In addition, when two or more risers are located within a relatively short distance of each other, such as risers extending to the same turret of a vessel, wave motion of the surface vessel and/or motion from strong currents may cause the risers to collide. This can lead to damage of one or each flexible pipe. The damage may be minor in nature but nonetheless lead to a reduced lifetime of the riser, or the damage may be more severe, requiring emergency repair work. 
     WO2009/063163, incorporated herein by reference, discloses a flexible pipe including weight chains secured to a number of buoyancy modules on the pipe. The chains hang from the buoyancy modules, extending downwards to the sea bed and having an end portion lying on the sea bed. 
     Another known arrangement employs a mid-water arch structure (as briefly mentioned above), where a riser is laid over and attached to the mid-water arch, so that the weight of the riser in the water is partially taken by the mid-water arch, reducing tension loading and degrees of freedom of the pipe. These structures tend to be difficult to install because they are secured to and anchor or gravity base on the seabed in a specific location, and also very expensive to install. 
     It would be useful to provide an improvement or an alternative to the above-mentioned arrangements. 
     According to a first aspect of the present invention there is provided a riser assembly for transporting fluids from a sub-sea location, comprising:
         a riser;   at least one buoyancy compensating element attached to the riser for providing positive, negative or neutral buoyancy to the riser; and   a tethering element connecting a first point of the riser with a further point of the riser so as to form a hog bend or a sag bend in the riser.       

     According to a second aspect of the present invention there is provided a method of supporting a flexible pipe, the method comprising the steps of:
         providing a riser;   providing at least one buoyancy compensating element attached to the riser for providing positive, negative or neutral buoyancy to the riser; and   providing at least one tethering element for joining a first point of the riser to a further point of the riser so as to form a hog bend or a sag bend in the riser.       

     Certain embodiments provide the advantage that a riser assembly can be provided with a predetermined and controlled shape in the water. 
     Certain embodiments provide the advantage that regions of a riser having the greatest curvature can be controlled to prevent overbending, which may otherwise damage the pipe structure and affect the lifetime of the pipe structure. 
     Certain embodiments provide the advantage that a riser assembly and method of supporting a riser can be provided for use in water with relatively strong current and/or wave movement with reduced chance of damage to the flexible pipe structure, controlling the pipe configuration form but not constraining the pipe significantly. 
     Certain embodiments provide the advantage that a riser assembly can be provided at relatively low cost compared to some other known arrangements. 
    
    
     
       Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which: 
         FIG. 1   a  illustrates a known riser assembly; 
         FIG. 1   b  illustrates another known riser assembly; 
         FIG. 2  illustrates a flexible pipe body; 
         FIG. 3  illustrates another riser assembly; 
         FIG. 4   a  illustrates a riser assembly; 
         FIG. 4   b  illustrates an enlarged view of a portion of the riser assembly shown in  FIG. 4   a;    
         FIG. 5  illustrates another riser assembly; 
         FIG. 6  illustrates another riser assembly; 
         FIG. 7  illustrates another riser assembly; 
         FIGS. 8   a  and  8   b  illustrates another riser assembly; 
         FIG. 9  illustrates a perspective view of a yet further riser assembly; 
         FIG. 10  illustrates a plan view of an arrangement of riser assemblies; and 
         FIG. 11  illustrates another plan view of another arrangement of riser assemblies. 
     
    
    
     In the drawings like reference numerals refer to like parts. 
     Throughout this description, reference will be made to a flexible pipe. It will be understood that a flexible pipe is an assembly of a portion of a pipe body and one or more end fittings in each of which a respective end of the pipe body is terminated.  FIG. 2  illustrates how pipe body  100  is formed from a combination of layered materials that form a pressure-containing conduit. Although a number of particular layers are illustrated in  FIG. 2 , it is to be understood that the present invention is broadly applicable to coaxial pipe body structures including two or more layers manufactured from a variety of possible materials. For example, the pipe body may be formed from metallic layers, composite layers, or a combination of different materials. It is to be further noted that the layer thicknesses are shown for illustrative purposes only. It will also be understood that the present invention is also broadly applicable to umbilical lines comprising multiple fluid tubes and control cables in a broadly cylindrical arrangement. 
     As illustrated in  FIG. 2 , a pipe body includes an optional innermost carcass layer  101 . The carcass provides an interlocked construction that can be used as the innermost layer to prevent, totally or partially, collapse of an internal pressure sheath  102  due to pipe decompression, external pressure, and tensile armour pressure and mechanical crushing loads. The carcass layer is often a metallic layer, formed from stainless steel, for example. The carcass layer could also be formed from composite, polymer, or other material, or a combination of materials. It will be appreciated that certain embodiments are applicable to ‘smooth bore’ operations (i.e. without a carcass layer) as well as such ‘rough bore’ applications (with a carcass layer). 
     The internal pressure sheath  102  acts as a fluid retaining layer and comprises a polymer layer that ensures internal fluid integrity. It is to be understood that this layer may itself comprise a number of sub-layers. It will be appreciated that when the optional carcass layer is utilised the internal pressure sheath is often referred to by those skilled in the art as a barrier layer. In operation without such a carcass (so-called smooth bore operation) the internal pressure sheath may be referred to as a liner. 
     An optional pressure armour layer  103  is a structural layer that increases the resistance of the flexible pipe to internal and external pressure and mechanical crushing loads. The layer also structurally supports the internal pressure sheath, and typically may be formed from an interlocked construction of wires wound with a lay angle close to 90°. The pressure armour layer is often a metallic layer, formed from carbon steel, for example. The pressure armour layer could also be formed from composite, polymer, or other material, or a combination of materials. 
     The flexible pipe body also includes an optional first tensile armour layer  105  and optional second tensile armour layer  106 . Each tensile armour layer is used to sustain tensile loads and internal pressure. The tensile armour layer is often formed from a plurality of metallic wires (to impart strength to the layer) that are located over an inner layer and are helically wound along the length of the pipe at a lay angle typically between about 10° to 55°. The tensile armour layers are often counter-wound in pairs. The tensile armour layers are often metallic layers, formed from carbon steel, for example. The tensile armour layers could also be formed from composite, polymer, or other material, or a combination of materials. 
     The flexible pipe body shown also includes optional layers of tape  104  which help contain underlying layers and to some extent prevent abrasion between adjacent layers. 
     The flexible pipe body also typically includes optional layers of insulation  107  and an outer sheath  108 , which comprises a polymer layer used to protect the pipe against penetration of seawater and other external environments, corrosion, abrasion and mechanical damage. 
     Each flexible pipe comprises at least one portion, sometimes referred to as a segment or section of pipe body  100  together with an end fitting located at at least one end of the flexible pipe. An end fitting provides a mechanical device which forms the transition between the flexible pipe body and a connector. The different pipe layers as shown, for example, in  FIG. 2  are terminated in the end fitting in such a way as to transfer the load between the flexible pipe and the connector. 
       FIG. 3  illustrates a riser assembly  300  suitable for transporting production fluid such as oil and/or gas and/or water from a sub-sea location  321  to a floating facility  322 . For example, in  FIG. 3  the sub-sea location  321  includes a sub-sea flow line. The flexible flow line  325  comprises a flexible pipe, wholly or in part, resting on the sea floor  324  or buried below the sea floor and used in a static application. The floating facility may be provided by a platform and/or buoy or, as illustrated in  FIG. 3 , a ship. The riser assembly  300  is provided as a flexible riser, that is to say a flexible pipe  323  connecting the ship to the sea floor installation. The flexible pipe may be in segments of flexible pipe body with connecting end fittings.  FIG. 3  also illustrates how portions of flexible pipe can be utilised as a flow line  325  or jumper  326 . 
     It will be appreciated that there are different types of riser, as is well-known by those skilled in the art. Embodiments of the present invention may be used with any type of riser, such as a freely suspended (free, catenary riser), a riser restrained to some extent, totally restrained riser or enclosed in a tube (I or J tubes). 
     The present disclosure concerns embodiments of a riser assembly and methods for supporting a flexible pipe.  FIG. 4  illustrates a riser assembly according to one embodiment. The riser assembly  400  includes a riser  402 , extending from a location at the seabed  406  to a floating vessel  404 . The riser itself may be a flexible pipe, for example as described above, and therefore for brevity will not be described in further detail. 
     The riser assembly  400  also includes at least one buoyancy compensating element  408   1-n  attached to the riser for providing positive or negative buoyancy to the riser. It will be understood that a buoyancy compensating element may either provide positive buoyancy to the riser (e.g. a buoyancy aid or buoyancy module), or provide negative buoyancy to the riser (e.g. a ballast weight). In this case the buoyancy compensating element is a buoyancy module providing upward lift to the riser, for supporting a section of the riser. By use of the buoyancy modules  408 , the riser lies in the water in a ‘wave’ configuration. Although five buoyancy modules  408  are shown in  FIG. 4  as an example, it will be appreciated that any number of buoyancy modules, or buoyancy compensating elements, may be used depending on the specific requirements and circumstances of use. 
     The riser assembly  400  also includes a tethering element  410 , which in this case is a length of metal chain  410 . The chain ties a first buoyancy module to a further buoyancy module, so as to urge the riser into a bend configuration. 
     In this embodiment the tethering element  410  has a length shorter than the length of flexible pipe between the points of tying, so as to create the formation of a hog bend. Aptly the tethering element  410  has a length at least 5% shorter than the length of flexible pipe between the points of tying. The chain connects a first point  416  of the riser (i.e. a first specific location along the riser) with a further point  418  of the riser (i.e. a further specific location along the riser) so as to form the hog bend  412  in the riser (as shown in  FIG. 4   b ). In this case the chain connects the first and further points  416 ,  418  of the riser via buoyancy modules. Alternatively the chain may connect the first and further points of the riser via elements securing the buoyancy modules to the pipe. As can be seen in the figure, formation of a hog bend  412  also leads to the formation of a sag bend  414 . 
     In this embodiment a metal connector ring (not shown) is applied to each of the first buoyancy module and the further buoyancy module. The connector ring (or alternatively two or more rings) acts to attach to the buoyancy module, and create an attachment point for the chain to be connected to. The connector ring helps to achieve easy connection of tethering element to the riser, and thus easy installation of the riser assembly. Alternatively, a clamp or other connecting device may be used, or the chain may be affixed directly to the buoyancy module or flexible pipe (without a connecting device). 
     In use, a riser or flexible pipe may be supported in the water in a configuration such as that exemplified in  FIG. 4 , by providing a riser; providing at least one buoyancy compensating element attached to the riser for providing positive, negative or neutral buoyancy to the riser; and providing at least one tethering element for joining a first point of the riser to a further point of the riser so as to form a hog bend or a sag bend in the riser. 
     For example, the riser can be paid out from a vessel, with the at least one buoyancy compensating element being attached to the riser at a predetermined location as the riser is paid out. Aptly the buoyancy compensating element(s) may have a connector ring also attached at the time of being attached to the riser. Once the riser is located in the water, the tethering element  410  can be attached to the buoyancy modules so as to achieve the final riser shape. Alternatively, the necessary elements of the riser assembly may be provided in the factory at the stage of manufacture at the factory, or at the time of deployment of the riser, or a combination thereof. 
     It will be appreciated that the specific dimensions of the riser, the shape it assumes in the water, the buoyancy of the buoyancy modules, and the dimensions of the tethering element, etc., may be predetermined in accordance with the specific requirements for the particular use of the riser assembly. 
     The tethering element helps to control and support the shape of the riser in the water, to help prevent unwanted movement of the riser after installation. 
     It will be appreciated that the tethering element may be made from chain, wire or fibre rope, strip(s) of tensionable material, bar or such like, suitable to apply the tension required between the first and further riser locations in order to create the desired shape in the flexible pipe. 
     Alternatively or additionally to the configuration described above, the riser assembly may include a tethering element, e.g. a length of metal chain, to attach a first point on the riser to a further point on the riser, so as to urge the riser into a sag bend configuration. For example, a chain may extend from a location on the riser  402  on the hog bend (e.g. at the buoyancy module  408   n ) to a position along the riser closer to the vessel, i.e. across the sag bend region  414 . 
       FIG. 5  illustrates another riser assembly  500  according to a further embodiment. The riser assembly  500  has several elements in common with the riser assembly  400  described above, which will not be described again for brevity. However, in this case the riser assembly  500  includes a tethering element  510  that is formed of a number of sub-elements. Specifically, the tethering element  510  includes a plurality of lengths of chain  516   1-n . The plurality of lengths of chain  516   1-n  are each connected at a common point and respectively extend to various points along the riser, in this embodiment via buoyancy modules. The length of each sub-element chain  516   1-n  is predetermined so as to control the shape of the riser and urge the riser into a bend configuration. 
       FIG. 6  illustrates another riser assembly  600  according to a further embodiment. Again, the riser assembly  600  has various elements in common with the riser assembly  400  and  500  described above, which will not be described again for brevity. Here, a weight chain  618  is employed as a further sub-element of the tethering element  610 . The chain  618  is secured at one end to a position on the remainder of the tethering element, in this case to the common point where lengths of chain  616   1-n  are connected (so as to spread any load relatively evenly). The chain hangs down freely towards the sea bed  606 . A remaining end of the chain  618  rests upon the sea bed  606 . In this way a part of the weight chain rests on the seabed. 
     It will be appreciated that as conditions experienced by the flexible pipe change, such as when the density of content within the flexible pipe changes the result will be a tendency for the buoyancy modules and flexible pipe to move upwardly away from the sea bed or downwardly towards the sea bed. As such movement occurs more or less chain will rest upon the sea bed. For example, when the riser content density reduces the buoyancy will be balanced by the additional chain weight as it is lifted from the sea bed. When the riser content density increases the buoyancy will be balanced by reduced chain weight as the additional chain is laid on the sea bed. In this way the support provided to the flexible pipe is automatically and continually adjusted so as to maintain the flexible pipe in a desired configuration or at least in a configuration within predetermined threshold limits. 
     As an alternative to a weight chain extending down to the sea bed a top section of a weight chain may be replaced or provided by an alternative flexible filament such as a synthetic rope, wire, cable or the like. A weight chain is secured at a lower end region of the filament so that again a portion of the weight chain rests upon the sea bed. 
     The weight chain itself may be trimmed at the sea bed during installation to ensure that a section of the chain remains on the sea bed at the lightest riser configuration. The length of the chain on the sea bed will be determined for the largest potential change, for example in the riser contents. 
     During installation a length of chain is attached to each selected buoyancy module or to the flexible pipe itself as it leaves an installation deck of an installation vessel and before it reaches the surface water. The rest of the chain is then lowered into the water after it is attached to the flexible pipe or buoyancy module. The riser is then paid out continually until the next buoyancy module reaches the installation deck for weight chain attachment. 
     Use of the weight chain in combination with the buoyancy modules and tethering element acts as a self-adjusting device for automatically maintaining a flexible pipe configuration. Also, the weight chain acts to increase the support and control the shape of the riser in the water to help prevent unwanted movement of the riser after installation. 
     Alternatively, the upper portion of the tethering element, i.e. chains  616   1-n , may be tied to a fixed structure, which may for example be a gravity base (anchor weight) located on the seabed. A tether of rope or chain for example may be used to tie the upper portion directly to the fixed structure. Such an arrangement would help prevent unwanted movement of the riser assembly by restricting the horizontal and vertical movement of the riser. 
     As a further alternative to the configuration shown in  FIG. 4 ,  5  or  6 , another configuration is possible in which the upper portion of the tethering element (including the chain or chains that attach to the riser) are attached to a weighting element, such as a ballast weight hanging freely in the water (but not necessarily extending fully to the seabed or other structure), or to a buoyancy tank or element above a section of the riser, to which suitable lengths of chain are attached and which in turn attach at selected locations to the pipe, either directly or indirectly, to create the desired riser shape. Such a ballast weight or buoyancy tank or element may help ensure the tethering elements retain the required configuration. 
       FIG. 7  illustrates another riser assembly  700  according to a further embodiment. The riser assembly  700  is provided in a ‘double wave’ configuration. The riser assembly  700  includes a tethering element  710   3 . The tethering element  710   3  in this case includes a number of chains  716   1-3 , of predetermined length. The chain  716   3  is secured at one end to a position on the flexible pipe, in this case to the riser  702 . The chain hangs down freely towards the sea bed  706 . A remaining end of the chain  716   3  rests upon the sea bed  706 . 
     The chains  716   1  and  716   2  attach to the chain  716   3  at a common point. The chains each have a predetermined length, and the point of attachment is determined, so that the tethering element  710   3  is configured such that the central chain,  716   3 , extends to a lower overall height than the outer chains  716   1,2 . The tethering element is therefore arranged to create the formation of a sag bend. It will be appreciated that other arrangements may be possible, with other numbers of chains or other sub elements, to form a sag bend in a riser. 
     In this case the chains connect first and further points of the riser directly. In other embodiments the chains may connect to the riser via clamps or buoyancy modules or other intermediate feature, or directly onto at least one end fitting of a mid-line connection between two lengths of pipe, for example. 
     As an alternative to the configuration shown in  FIG. 7 , rather than include a chain that extends completely to the seabed, another configuration is possible in which the upper portion of the tethering element (including the chains that attach to the riser) are attached to a weighting element, such as a ballast weight hanging freely in the water ( FIG. 8   a  or  8   b ). Such a ballast weight would perform the task of ensuring the upper portion of the tethering element remains in the required configuration. This configuration may also be inverted to use a buoyancy tank/element attached at multiple locations to the pipe, so also creating a hog bend. 
     In the configuration as shown in  FIG. 7 , there are also two tethering elements  710   1,2  (similar to those described above with respect to  FIG. 6 ), and a further tethering element  710   4  similar to the tethering element  710   3 . Of course other arrangements are possible. 
       FIG. 8   a  illustrates another riser assembly  800  according to a further embodiment. Here the tethering element  810  includes various sub elements including chains  816   1-n  and a connector element  820 , which in this case is a cylinder beam (spreader beam). Each chain  816   1-n  is connected at one end to a respective buoyancy module  808   1-n  and at its other end to the cylinder beam  820 .  FIG. 8   b  shown a similar arrangement from a perspective view. 
     In this embodiment at least one connector ring (not shown) is applied to each of the buoyancy modules. The at least one connector ring acts to attach to the buoyancy module, and create an attachment point for the chain to be connected to. Alternatively, a clamp or other connecting device may be used, or the chains may be affixed directly to the buoyancy module or flexible pipe (without a connecting device). 
     The cylinder beam  820  in this case is negatively buoyant, i.e. acting as a ballast weight. The cylinder beam  820  therefore acts as a weighting element so as to ensure the chains  816   1-n  remain taught in the water. The cylinder beam may alternatively be neutrally or positively buoyant. The cylinder beam acts as a frame such that the remaining sub elements can be affixed thereto. 
     The cylinder beam may itself be constructed of suitable materials such as will provide the desired size shape and buoyancy. This may be determined as a fabricated steel or alloy tank filled with air (or another gas), water (or another liquid), or dense solid particles (for example sand or lead shot). Alternatively the cylinder beam may include a hollow polyurethane or polypropylene chamber; alternatively a steel or alloy sub-frame with a light, thin balloon or envelope attached at various locations to secure it to the sub-frame. It will be understood that other construction materials can also be envisaged. 
     The shape of the cylinder beam may be that of a cylinder, or barrel, or section of material of a suitable shape in order to provide sufficient stiffness through either material properties, design (second moment) or a combination of the two. The effect of different shapes of the cylinder beam would likely require a change to the lengths of the chains  816   1-n , minimising the risk of connecting chains to the incorrect positions on the pipe or the cylinder beam. 
     The chains  816   1-n  each have a predetermined length, which has been calculated so that the shape formed by the riser assembly is a hog bend in the water. As shown in  FIG. 8 , the central chain has a length longer than the chains either side. The specific dimensions of the chains and number of chains can be predetermined to suit the particular application of use, including the environmental conditions and the riser shape required. Together the chains  816   1-n  control the vertical movement of the riser pipe and to a lesser degree the lateral movement of the pipe, although the system as a whole is allowed to move together, within the limitations applied by the fixed ends of the pipe. 
       FIG. 9  illustrates another riser assembly  900  according to a further embodiment. As can be seen in the figure, the riser assembly  900  has chains  916   1-n  and a cylinder beam  920  in common with the riser assembly  800  described above, which will not be described again in detail for brevity. Here, the cylinder beam  920  is itself tethered to a fixed structure, in this case a heavy, concrete base  922  located on the seabed. The concrete base acts as an anchoring element. 
     As shown in  FIG. 9 , the cylinder beam  920  is tethered at two points via four ropes  924   1-4  to four points on the concrete base  922 , respectively. Of course other arrangements are possible. This embodiment requires that the tethering element is tied to a fixed location in the water. This helps to not only control the shape of the riser&#39;s hog bend but also control the accuracy of the position of the riser overall with respect to other fixed structures, e.g. the seabed, or other relatively fixed structures, e.g. a vessel or other risers in the water. 
     The cylinder beam  920  is positively buoyant, though it may alternatively be negatively buoyant or neutrally buoyant. The buoyancy of the cylinder beam may be predetermined in relation to the buoyancy of the riser and in relation to the contents of the riser in service (gas risers can be more positively buoyant than, for instance, a water injection riser). 
     Advantageously, in a configuration where a fixed gravity base is employed such as in  FIG. 9 , it may be particularly useful to use ropes or fibre tethers in place of chains for the connectors  916   1-n . This provides the advantage that the natural elasticity of those types of elongate elements ensures that there is no shock loading applied to the riser pipe from movements in the pipe reaching the extremes of the length of the tethering elements. Similar benefits can be gained in other configurations detailed herein where a weight chain may also be replaced with a tether and a gravity base. 
     In use, the arrangement of  FIG. 9  may be assembled, for example, as follows. The concrete base  922  may be positioned on the seabed and tied to the cylinder beam  920 , which has a positive buoyancy. Then (or before or at the same time), the buoyancy modules  908   1-n  may be mounted to the riser whilst the riser is held on a laying vessel. Then, the riser is installed into the water, if necessary (i.e. if the pipe is itself not sufficiently negatively buoyant either un-filled or filled with water), with temporary ballast material to counteract the buoyancy modules during installation. Finally, some or all of the buoyancy modules may be connected to the cylinder beam using chains  916  by divers or using a ROV (remotely operated underwater vehicle). 
       FIGS. 10 and 11  show a plan view of other variations to the embodiment of  FIG. 9 .  FIG. 10  illustrates risers  1002   1-3  with buoyancy modules  1008   1-n  extending from a vessel  1004  comprising a turret, to which the risers are connected. As is shown, each riser may be equipped with its own chain and cylinder beam  1020  configuration, but the cylinder beams  1020   1-3  are tied at different angles to two anchoring elements  1022   1,2 . The cross-tethering of the cylinder beams may help to strengthen the structure of the overall arrangement. As an alternative to cross-tethers, rigid cross connecting elements may be used directly between cylinder beams, or even directly between the risers themselves using additional or common connection points on buoyancy or clamps. The rigid connecting elements may be bars or tubes of metal or other suitable material but may also possess significant degrees of freedom of movement with respect to the risers at the respective connection points through the use or flexible or universal jointing systems, but be able to maintain a desired separation between adjacent risers due to their rigidity. 
     Also through the use of these cross tethers or cross connecting elements it may be possible to reduce the number or size of the cylinder beams or anchoring elements  1022   1,2 . 
     Significantly as a result of this embodiment the degree of lateral motion of the three risers shown is controlled so that there is little or no risk of clashing of the risers in extremes of weather and sea-states. 
       FIG. 11  is similar to  FIG. 10 , showing three risers with buoyancy modules  1108   1-n  and corresponding cylinder beams  1120   1-3  with cross-tethering of cylinder beams to anchoring elements  1122   1,2  via ropes  1124   1-n . In this case the risers are installed almost parallel as if hanging from the side of a platform or vessel. 
     The cylinder beam configurations may alternatively include a more complex framework of connecting elements for forming more complex structures to hold flexible pipes in a desired configuration. 
     Various modifications to the detailed arrangements as described above are possible. For example, whilst the tethering element has been described as a chain in some embodiments, the tethering element may be a rope or filament or cord, or cable, or the like, or a combination thereof. The tethering element may also include other parts in addition to the chain, rope, filament, cord, cable, etc. Aptly the tethering element is at least partly flexible. This may help the tethering element to react to various surrounding conditions, e.g. changes in the riser or movement of the surrounding water. 
     Aptly the tethering element  410  has a length (L te ) extending from the first point to the second point of the riser, and the riser has a length extending from the first point to the second point (L r ), and the length of the tethering element  410  (L te ) is shorter than the length of the riser between the first point and the second point (L r ). 
     Although the buoyancy compensating elements described above have been usually described as positively buoyant, they could also be negatively buoyant, e.g. ballast weight, for use in risers that require the addition of negative buoyancy. Alternatively a combination of both buoyancy modules and ballast modules may be used. 
     The number of buoyancy compensating elements could be any number, depending on the specific requirements of the particular use. The buoyancy compensating elements may be attached to or integrated with the flexible pipe or riser. 
     Although the above arrangements have been described with a riser extending between a floating facility (vessel) and the sea bed, the riser could alternatively extend between fixed or floating platforms or other structures at different heights above or below sea level. 
     With the above-described arrangement a riser assembly may be provided with a predetermined and controlled shape in the water. Also, regions of a riser having a relatively high curvature can have the shape controlled to prevent overbending of the riser. 
     Enhanced support may be provided to the riser assembly, which leads to improved riser performance. 
     The riser assembly may provide the same precision shaping and control to a pipe as a known mid-water arch structure, but without the associated high costs. 
     A riser assembly may be provided for use in water with relatively strong current and/or wave movement with reduced chance of damage to the flexible pipe structure. For example, where two or more risers are positioned in relatively close proximity, wave action and/or currents may otherwise cause the risers to clash together. This is often possible, particularly in shallower waters or even deep waters. This may be particularly so because, in these types of waters, a hog bend/sag bend combination is frequently used to give the riser the flexibility to adapt to movement of the surface vessel (because the sag bend effectively allows some slack or a margin of error in the position of the riser relative to the vessel). The corresponding hog bend may then induce a relatively large lateral drag force, particularly with a large number, or a large cross sectional area, of buoyancy compensating elements. The riser assemblies described herein help to prevent such movement, and clashing with adjacent structures and the related damage, because of the superior control over the shape of the riser and position in the water. 
     With the tethering elements that extend to a fixed location, e.g. the seabed (or platform or other fixed structure), a more precise control over the riser shape and location in the water can be achieved compared to known arrangements. 
     It will be clear to a person skilled in the art that features described in relation to any of the embodiments described above can be applicable interchangeably between the different embodiments. The embodiments described above are examples to illustrate various features of the invention. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
     The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.