Abstract:
A pipe-laying vessel including a pipe-laying tower extending upwardly from the vessel, the vessel including a welding station for joining a new pipestring to an end of the pipeline held by the tower, a clamp assembly on the tower for engaging a pipestring with a lower end adjacent to the end of the pipeline held by the tower and with the pipestring extending upwardly from its lower end alongside the tower, wherein the clamp assembly includes a pipestring clamp that serves both the function of a transfer clamp for transferring the pipestring from a position alongside the tower but displaced from the pipe-laying path to a position approximately aligned with the pipe-laying path, and the function of a line-up clamp for lining up the pipestring with the end of the pipeline.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to pipe-laying vessels and to methods of laying a pipeline. Such vessels and methods are of particular use in the offshore industry relating to oil and gas production. The invention relates more particularly to a clamp assembly on a pipe-laying tower of a pipe-laying vessel. 
     One of the tasks that is carried out by certain vessels in the offshore industry is that of laying pipelines, either by S-laying or by J-laying. 
     In “S” laying, the pipeline leaves the vessel at little or no inclination to the horizontal, adopts a steeper inclination in the water and then returns to a generally horizontal disposition on the seabed. The tension in the pipeline is often accommodated by a series of track tensioners mounted along the pipe-laying path, on the vessel. The track tensioners support the weight of the pipeline and control passage of the pipeline. S laying is preferred in shallower water where the natural path of the pipeline is only ever inclined at a shallow angle as it passes to the seabed. Examples of S-laying arrangements are shown in WO 2006/085739 and WO 2007/000609. 
     In “J” laying, the pipeline leaves the vessel at a steep or vertical inclination and the inclination steadily reduces until the pipeline is in a generally horizontal disposition on the seabed. J laying often involves moving a new pipeline section from a substantially horizontal position (along the deck of the vessel) into a vertical position to align with a J lay tower mounted on the vessel. A lower end of the new pipeline section is welded to the upper end of the pipeline, which is held in place, suspended from the vessel, by a fixed clamp, located towards the bottom of the J lay tower. Once the new section of pipeline has been added to the existing pipeline, the fixed clamp is released and the pipeline lowered down the J lay tower. During such lowering the tension in the pipeline may be accommodated by a travelling block or track tensioners. The upper end of the newly lengthened pipeline (i.e. the upper end of the new section of pipeline) is then clamped by the fixed clamp, the travelling block, if used, is returned to its original position towards the top of the J lay tower and the process repeated. J-laying is used primarily for laying pipeline in deep water (that is typically deeper than 1,000 m). Examples of J-laying arrangements are shown in WO 2009/153352 and WO 2009/153354. 
     In the arrangements shown in WO 2009/153352 and WO 2009/153354, a pipestring is brought into a position alongside a J lay tower on a loading arm and is then transferred from that position to one aligned with the pipe-laying path by the clamps on the loading arm. Once the pipestring is in position on the pipe-laying path, it is engageable by three line-up clamps which position the pipestring in the correct position relative to the end of the pipeline to which the pipestring is to be joined. An internal line-up system (referenced 75 in the specifications) is mounted on the top of the tower structure. 
     It is an object of the invention to provide an improved pipe-laying vessel and an improved method of laying a pipeline. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the invention, there is provided a pipe-laying vessel including a pipe-laying tower extending upwardly from the vessel, the vessel comprising a welding station for joining a new pipestring to an end of the pipeline held by the tower, a clamp assembly on the tower for engaging a pipestring with a lower end adjacent to the end of the pipeline held by the tower and with the pipestring extending upwardly from its lower end alongside the tower, wherein the clamp assembly comprises a pipestring clamp that serves both the function of a transfer clamp for transferring the pipestring from a position alongside the tower but displaced from the pipe-laying path to a position approximately aligned with the pipe-laying path, and the function of a line-up clamp for lining up the pipestring with the end of the pipeline. 
     By providing a pipestring clamp that serves both for transferring a pipestring into a pipe-laying path and for lining-up the pipestring, the clamp arrangements can be simplified and some of the procedures speeded-up. 
     Preferably, the pipestring clamp is in the region of the top of the tower. Preferably, the pipestring clamp is for clamping around an upper end of the pipestring. 
     Preferably, the pipe-laying vessel also comprises a separate pipestring elevator for rotating a pipestring from a substantially horizontal orientation at the base of the tower to an orientation substantially parallel to the tower. Preferably, the clamp assembly is mounted not on the pipestring elevator. Preferably, the clamp assembly, when serving the function of the transfer clamp, transfers the pipestring from the pipestring elevator to the position approximately aligned with the pipe-laying path. 
     Preferably the pipestring clamp is a friction clamp. The pipestring clamp is preferably arranged to bear at least the major part of the weight of the pipestring. In that case it is especially significant that the same clamp is used for both transferring and lining-up the pipestring. 
     Whilst it is within the scope of the invention for there to be just a single pipestring clamp, there will usually be several such clamps. The “clamp assembly” according to the invention is exemplified in an embodiment of the invention described below by a clamp assembly referred to below as a “tower clamp assembly” and including a friction clamp as the pipestring clamp according to the invention. The vessel may further include one or more further line-up clamps for lining up the pipestring with the end of the pipeline. Whilst it is within the scope of the invention for those further line-up clamps also to serve the function of transferring the pipestring into the pipe-laying path, in an embodiment of the invention described below they do not. In that described embodiment there are two further line-up clamps and one further transfer clamp referred to in the embodiment as a “tower arm” for transferring a pipestring into the pipe-laying path. 
     The clamp assembly is preferably mounted on the tower. The clamp assembly is preferably mounted for pivotal movement about an axis substantially parallel to the longitudinal axis of the tower. The assembly may be mounted for movement about more than axis substantially parallel to the longitudinal axis of the tower and in a described embodiment of the invention is mounted for movement about two spaced parallel axes parallel to the longitudinal axis of the tower. This provides for a wide range of movements of the assembly in a horizontal plane. 
     The clamp assembly is preferably mounted for translational movement in a direction substantially parallel to the longitudinal axis of the tower. Preferably, the translational movement is limited to an upper portion of the tower. Preferably the translational movement is in addition to the mounting for pivotal movement and, for example, allows a pipestring to be lowered from a position above and spaced from an upper end of a pipeline to which the pipestring is to be joined to a position adjacent to the upper end of the pipeline. 
     Preferably the orientation of the pipestring clamp is adjustable. The orientation of the pipestring clamp is preferably adjustable through pivoting movement about an axis perpendicular to the longitudinal axis of the tower and, more preferably the orientation of the pipestring clamp is adjustable about two mutually perpendicular axes that are themselves perpendicular to the longitudinal axis of the tower. Such adjustment may be active or passive; in an embodiment of the invention described below, the adjustment is passive, with the orientation of the pipestring clamp being determined by the other line-up clamps. 
     The pipestring clamp is preferably also able to rotate the pipestring about its longitudinal axis. Preferably the pipestring can be rotated through more than 180 degrees; in an embodiment of the invention described below, the pipestring can be rotated through +/−190 degrees. Preferably this is an active adjustment. It may be controlled by an operator lining-up the pipestring held in the pipestring clamp. 
     The clamp assembly preferably further includes an internal line-up clamp arrangement. This enables the internal line-up clamp arrangement to be in fixed relationship with the pipestring clamp and therefore automatically aligned with the pipestring held in the pipestring clamp. 
     The internal line-up clamp arrangement preferably includes an internal line-up clamp garage and an internal line-up clamp. The internal line-up clamp arrangement preferably further includes an internal line-up clamp winch for raising and lowering the internal line-up clamp within the pipestring. The internal line-up clamp may take any conventional form. 
     Preferably the internal line-up clamp arrangement and the pipestring clamp are adjustably mounted for adjustment as a unit of their orientation relative to the tower. Preferably the internal line-up clamp arrangement and the pipestring clamp are fixed in relation to each other. Preferably the relative position and orientation of the internal line-up clamp arrangement and the pipestring clamp are non-adjustable. This helps to ensure that the internal line-up clamp arrangement is automatically lined up with a pipestring held in the clamp assembly. 
     The vessel may comprise a hang off clamp assembly provided at a lower region of the tower for clamping pipeline that has been deployed from the vessel and a travelling clamp with a travel path along a length of the tower for laying pipeline. Preferably the pipe-laying tower comprises a first lower section extending upwardly from a proximal end to a distal end above the main deck of the vessel, and a second upper section extending upwardly from the first lower section and having a length that is at least one third of the length of the lower section, wherein the travel path of the travelling clamp is limited to the lower section of the tower. The second upper section of the tower may be movably mounted on the first lower section of the tower, but it is possible for the first and second sections of the tower to be integral with one another. The preferred arrangement provides for a construction in which the upper section of the tower can be moved in relation to the lower section of the tower independently of the position of the travelling clamp. 
     The provision of a pipe-laying tower with upper and lower sections is described and claimed in UK patent application entitled “Pipe-laying Vessel and Method of Laying a Pipeline” with agent&#39;s reference “P015336 GB”, having the same filing date as the present application. In particular, the claims of the present application may be amended to include features relating to the tower and other associated features relating to the tower such as the elevator for conveying a pipestring to a position alongside the tower and various clamps for engaging different parts of the pipestring and pipeline. Some of those features are mentioned briefly below in a description with reference to the drawings. 
     The tower is preferably of an overall length sufficient to accommodate two pipe strings end to end. A working station is preferably provided partway up the tower. It is also preferred that a working station is provided in the region of the bottom of the tower. In an embodiment of the invention described below, there is a working station in the region of the bottom of the tower and another working station partway up the tower. A working station is a station at which the pipeline being formed can be welded and/or coated and/or inspected. 
     In a case where two working stations are provided, they are preferably spaced apart by the length of a pipestring. It is then possible for a junction of two pipestrings to be inspected or worked on at one working station while another junction is inspected or worked on at another working station. The working stations are preferably manned. 
     In a case where the tower comprises a first lower section and a second upper section, there is preferably a working station in the region of the top of the first lower section. 
     The feature of providing a clamp assembly that also comprises an internal line-up clamp arrangement is itself a special and novel feature of the invention. Accordingly, in a second aspect of the invention there is provided a pipe-laying vessel including a pipe-laying tower extending upwardly from the vessel, the vessel comprising: 
     a welding station for joining a new pipestring to an end of the pipeline held by the tower, 
     a plurality of clamps on the tower for holding a new pipestring with a lower end adjacent to the end of the pipeline held by the tower and with the pipestring extending upwardly from its lower end alongside the tower, 
     wherein the plurality of clamps includes a pipestring clamp for clamping around an upper end of the pipestring and the pipestring clamp is part of a clamp assembly that also comprises an internal line-up clamp arrangement, the internal line-up clamp arrangement and the pipestring clamp being adjustably mounted for adjustment, as a unit, of their orientation relative to the tower. 
     Preferably, the pipestring clamp is in the region of the top of the tower. The pipestring clamp is preferably arranged to bear at least the major part of the weight of the pipestring. 
     Preferably, the pipe-laying vessel also comprises a separate pipestring elevator for rotating a pipestring from a substantially horizontal orientation at the base of the tower to an orientation substantially parallel to the tower. Preferably, the clamp assembly is mounted not on the pipestring elevator. Preferably, the clamp assembly, when serving the function of the transfer clamp, transfers the pipestring from the pipestring elevator to the position approximately aligned with the pipe-laying path. 
     It will be appreciated that all of the features described above as optional or preferred in respect of the first aspect of the invention may be employed in the second aspect of the invention and indeed most of them are employed in an embodiment of the invention described below. 
     Similarly, the feature of providing a pipestring clamp that is arranged to bear at least the major part of the weight of the pipestring and that is also able to rotate the pipestring about its longitudinal axis is itself a special and novel feature of the invention. Accordingly, in a third aspect of the invention there is provided a pipe-laying vessel including a pipe-laying tower extending upwardly from the vessel, the vessel comprising: 
     a welding station for joining a new pipestring to an end of the pipeline held by the tower, 
     a plurality of clamps on the tower for holding a new pipestring with a lower end adjacent to the end of the pipeline held by the tower and with the pipestring extending upwardly from its lower end alongside the tower, 
     wherein the plurality of clamps includes a pipestring clamp in the region of the top of the tower, the pipestring clamp is arranged to bear at least the major part of the weight of the pipestring and the pipestring clamp is also able to rotate the pipestring about its longitudinal axis. 
     Preferably, the pipestring clamp is for clamping around an upper end of the pipestring. 
     Preferably, the pipe-laying vessel also comprises a separate pipestring elevator for rotating a pipestring from a substantially horizontal orientation at the base of the tower to an orientation substantially parallel to the tower. Preferably, the clamp assembly is mounted not on the pipestring elevator. Preferably, the clamp assembly, when serving the function of the transfer clamp, transfers the pipestring from the pipestring elevator to the position approximately aligned with the pipe-laying path. 
     It will be appreciated that all of the features described above as optional or preferred in respect of the first aspect of the invention may be employed in the third aspect of the invention and indeed most of them are employed in an embodiment of the invention described below. 
     The present invention still further provides a method of laying a pipeline from a pipe-laying vessel, in which the vessel is in any of the forms described above. 
     Thus, by way of a first example, the method may include the step of using a clamp assembly both to transfer a pipestring from a position alongside the tower but displaced from the pipe-laying path to a position approximately aligned with the pipe-laying path and to line up the pipestring with the end of the pipeline. By way of a second example the method may include the step of providing an internal line-up clamp arrangement and a pipestring clamp as a unit and allowing adjustment of the orientation of the unit relative to the tower. By way of a third example the method may include the step of providing a pipestring clamp on the pipe-laying tower for bearing at least the major part of the weight of the pipestring and also for rotating the pipestring. 
     As should already be understood, features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the features of the first, second and third aspects of the invention can all be combined and indeed are combined in a vessel embodying the invention and described below, and the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG. 1  shows a perspective view of a vessel according to an embodiment of the invention; 
         FIG. 2 a    shows a side view of the pipe-laying tower in an initial state, on the vessel of  FIG. 1 ; 
         FIG. 2 b    shows a front view of the pipe-laying tower, also in an initial state; 
         FIG. 2 c    shows a top view of part of the tower, also in an initial state; 
         FIG. 3  shows a perspective view of the string elevator, on the deck of the vessel of  FIG. 1 ; 
         FIG. 4 a    shows a perspective view of a tower clamp assembly; 
         FIG. 4 b    shows a front view of the tower clamp assembly; 
         FIG. 4 c    shows a side view of the tower clamp assembly; 
         FIG. 4 d    shows a bottom view of the friction clamp on the tower clamp assembly; 
         FIG. 5 a    shows a perspective view of a tower clamp mechanism; and 
         FIG. 5 b    shows a perspective view of part of the tower clamp mechanism. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a pipe-laying vessel  100 . The vessel  100  has a bow end  102  and a stern end  101 . The bottom of the vessel, known as the keel line, is labelled as  104 . On the deck  103  of the vessel, at the stern end  101  are various ramps defining S-laying apparatus  200 . Other S laying apparatus is provided towards the bow of the vessel as more fully described in WO2008/107186. At the stern end  101  is an S-lay opening  202  to allow the pipeline to enter the water near the stern of the vessel  100 . The vessel  100  also has a J-laying tower  300  in a middle portion of the vessel partway along the firing line for S-laying. 
       FIGS. 2 a  and 2 b    show more detailed views of the J-lay tower  300  and associated equipment. The tower  300  extends upwards vertically from the deck  103  of the vessel. 
     Below the tower is an opening  302  in the hull of the vessel to the water beneath. This opening allows passage of the pipeline P from the tower  300  to the water. 
     In the following description, the pipeline comprises a deployed pipeline (where the pipeline in question comprises pipestrings that have been welded together and substantially deployed off the vessel  100  below the keel  104  of the vessel  100 ). This deployed pipeline is labelled as P. The pipeline also comprises a pipeline that has not been deployed yet and is still held on the vessel  100 . This pipeline comprises a series of pipestrings PS, P 1 , P 2 , P 3  which are already or will be welded together and then deployed in due course. 
     In particular, PS is the pipestring already welded at its bottom end to the deployed pipeline P. Hence, pipestring PS is partly deployed below the keel  104  and partly still on the vessel  100 . P 1  is the pipestring welded to the top end of PS and held in a lower section  310  of the tower, P 2  is the pipestring that is being held in an upper section  311  of the tower and will be welded to the top of P 1  in due course and P 3  is the pipestring currently in a string elevator, which will be welded to the top of P 2  in due course. 
     Attached to the side of the opening  302  is a flute  320  (sometimes called a J-stinger) which controls orientation of the pipeline P as it enters the water. 
     Above the flute  320  and still below the deck  103 , is a hang off clamp assembly  330 . The hang off clamp assembly  330  is located close to the flute  320  and just above the sea water level. 
     Above the hang off clamp assembly  330  is a travelling clamp assembly  340 , mounted on two rails  342 . The rails  342  extend from above the hang off clamp assembly  330 , upwards through the opening  302 , passing through the deck  103  of the vessel  100  and upwards through the J-lay tower  300  to just below a welding station  500  located on the tower  300  at about mid-height. 
     The travelling clamp assembly  340  comprises a friction clamp  343  mounted on a trolley  341  on the rails  342 . The trolley  341  comprises four shoes  345  (one at each corner) which are made up of wheels and act as lateral guides to guide the travelling clamp assembly  340  up the rails  342 . At the top of each rail is an upper sheave block  344  with a pulley wheel  346 . The travelling clamp assembly  340  can be moved up and down the rails  342  by the use of cables  347  extending up, over these pulley wheels and attached to the travelling clamp assembly  340 . 
     The friction clamp  343  is lined up so that it is along the length of the pipe-laying path or axis  303 . The rails  342  extend parallel to this pipe-laying axis  303 . The friction clamp  343  has two front doors (not shown) that can open to allow a pipestring containing a bulky item to be introduced into the clamp. In addition, the friction clamp  343  is mounted to the trolley  341  by means of elastic mounts to allow an oscillation of +/−1 degree during bulky item introduction. 
     On the deck  103  of the vessel, to one side of the tower  300  is a coating station  600 . The coating station  600  is mounted on rollers  602  which run on track  601  in the deck  103  to allow the coating station  600  to be moved between a position adjacent the base of the tower  300  (a working position) and a position slightly removed from the base of the tower (a storage position). This allows the travelling clamp assembly  340  to pass by the coating station  600  and also allows bulky items to pass by. 
     The coating station  600  is separate from, and at a different height to, the welding station  500 . This allows a pipestring to be coated as it passes through the coating station  600  (i.e. as the pipeline P is being laid by the travelling clamp assembly  340 ) after it has been welded to the pipestring above. The coating and welding steps can take place independently and in parallel. The coating station  600  can also perform non-destructive tests (NDT) on the pipeline P and, if necessary, repair or replace any defective welds. 
     On the other side of the tower  300 , is a bulky item handling skid  700 . 
     Also on that other side of the tower  300  is a string elevator  400 . The string elevator is mounted on rails  405  which extend from the base of the tower  300 , along the deck  103  of the vessel so that the string elevator can be accommodated horizontally on the deck  103  of the vessel. The rails  405  also extend up substantially the whole length of that side of the tower  300 . The string elevator  400  holds a pipe-string P 3  made up of 3 joints (3J). When the string elevator  400  is raised up the tower  300  on the rail  405 , the pipe-string P 3  is lined up with a string axis  408 , which is parallel to the pipe-laying axis  303 . The string elevator will be described in more detail in relation to  FIG. 3 . 
     Various clamps and other laying equipment are located along the length of the tower  300  as will now be described. 
     Starting from the top of the tower, there is located a tower clamp assembly  800 , including an articulated friction line-up clamp  819 . The tower clamp assembly  800  is pivotable so as to move a pipe-string from the string axis  408  to the pipe-laying axis  303 . In  FIGS. 2 a  and 2 b   , the tower clamp assembly is shown clamping a pipestring P 2  at one side of the tower  300  in between the string axis  408  and the pipe-laying axis  303 . The tower clamp assembly  800  will be described in more detail in relation to  FIGS. 4 a    to  4   c.    
     A tower arm  371  is located further down the tower, above the height of the welding station  500 . The tower arm  371  is articulated (with a first and second arm) in the same way as the friction line-up clamp  819  of the tower clamp assembly  800 . Hence, as the friction line-up clamp  819  transfers a top end of the pipestring P 3  from the string axis  408  to the pipe-laying axis  303 , the tower arm  371  is controlled in the same way to guide a bottom portion of the pipestring P 3 . This way the pipestring P 3  stays vertical and can be lined up with the pipe-laying axis  303 . 
     A tower roller  370  is located about halfway between the tower clamp assembly  800  and the tower arm  371  and is located on the pipe-laying axis  303 . The tower roller  370  can be opened or closed around a pipestring P 2  on the pipe-laying axis  303 . 
     A line-up clamp mechanism  352  is provided above the tower arm  371  and is described in more detail in relation to  FIGS. 5 a    and  5   b.    
     The welding station  500  is provided with a roof  502  with a turntable  503  on the roof  502 . The turntable  503  is provided with a central pipe opening (not shown). Hence, the turntable allows a weld joint to be provided all the way round the circumference of the pipe strings. 
     This station welds the pipestring above it (for example P 2 ) to the pipestring below it (for example, P 1 ) which, by then, may be regarded as the upper end of the pipeline P. The welding station  500  also carries out non-destructive test (NDT) to check the pipestrings. 
     A line-up clamp mechanism  353  is provided on the roof  502  of the welding station  500 . This line-up clamp mechanism  353  is identical to the line-up clamp mechanism  352  higher up the tower  300 . 
     The welding station  500  is provided with a safety balcony  550  extending out from the welding station to under the string axis  408 . The safety balcony  550  prevents the pipestring (P 2 ) from falling down, while being transferred from the string axis  408  to the pipe-laying axis  303 . 
     Underneath the floor  501  of the welding station  500  is another clamp  372 . This clamp  372  acts to keep the pipeline P, including the previously welded pipestring P 1 , in the lower section  310  of the tower in position. 
     A similar clamp  373  is also provided in the lower section  310  of the tower. This clamp  373  is located towards the top of the lower part  312  of the lower section  310  of the tower. This clamp is mounted on a tiltable base so that it can be tilted into and out of the pipe-laying axis  303 . As the clamp  373  can be moved out of the pipe-laying axis  303 , this allows for the travelling clamp assembly  340  to travel up and down on the pipe-laying axis  303  and pass this clamp  373 . 
       FIG. 2 a    also shows a pivoted hydraulic ram  316  that is used to collapse the tower  300  into transit and bridge-passage configurations. 
       FIG. 2 c    shows a plan view of the tower clamp assembly  800  holding a pipestring on the string axis  408 . The figure also shows the position of the pipe-laying axis  303  of the tower  300 . 
       FIG. 3  shows the string elevator  400  at the base of the tower  300 . The string elevator comprises a main beam structure  409  with three clamps  401 ,  402 ,  403  distributed along the length of the beam  409 . These clamps  401 ,  402 ,  403  hold a pipestring P 3  on the string elevator. These clamps are fail-safe and are designed to remain closed in the absence of hydraulic power, for example. They are also designed not to open until a pipestring P 3  is being gripped by the tower clamp assembly  800  and tower arm  371 , as will be described later. These clamps are designed to take an axial load of 10% the weight of the heaviest pipestring to be used. 
     The string elevator is also provided with a shoe  404  at a second end furthest away from the tower  300 . The shoe  404  provides permanent support for the pipestring P 3  while on the string elevator  400  and is designed to take the weight of the heaviest pipestring to be used. The main beam structure  409  is mounted at its first end on a locomotion trolley  406 . This locomotion trolley runs up and down the tower  300  on rails  405   a . There are also further rails  405   b  that extend away from the base of the tower along the deck  103  of the vessel and provide a guide for a roller provided on the second end of the string elevator  400 . Importantly, the rails  405   b  are curved at the region at the base of the tower  300  so as to provide support for the second end of the string elevator  400  in this region and prevent it getting wedged in the corner at the base of the tower. 
       FIGS. 4 a , 4 b , 4 c  and 4 d    show detailed views of the tower clamp assembly  800  at the top of the tower  300 . The tower clamp assembly comprises a trolley  801  with rollers  804  on each of its four corners. The rollers  804  are engaged in two vertical parallel rails  802 , to allow the trolley  801  to run up and down the rails  802  by +/−1800 mm. A hydraulic cylinder  803  is connected to the top of the trolley  801  to control movement of the trolley  801  on the rails  802 . The hydraulic cylinder  803  also acts as a weight compensation system to accurately control the approach of the pipestring P 2  to the pipeline P to which it is to be joined in the welding station  500 . 
     A framework (first arm)  805  is mounted on the trolley  801  so that it can pivot about a vertical axis. The framework (first arm)  805  therefore acts as a first arm extending outwards horizontally from the trolley  801 . The framework (first arm)  805  can be rotated with respect to the trolley  801  by two hydraulic cylinders (not shown). The framework (first arm)  805  comprises a vertical end rod at a distal end, with a secondary arm  809  mounted on it. The secondary arm  809  comprises a sleeve portion which fits over the end rod to allow the secondary arm  809  to pivot about the vertical end rod. The secondary arm can be rotated with respect to the first arm framework (first arm)  805  by two rotary actuators  810 ,  811  mounted on the end rod at either end of the sleeve. 
     The pivotally mounted framework (first arm)  805  and secondary arm  809  allow the distal end of the secondary arm  809  to be pivoted from the string axis  408  to the pipe-laying axis  303 . This means that a pipestring P 2  held by the tower clamp assembly  800  (as will be described later) can be transferred to the pipe-laying axis  303  from the string axis  408 . It also allows the pipestring P 2  to be lined up on the pipe-laying axis  303  with the pipeline P, to which it is to be joined, beneath it. 
     On the distal end of the secondary arm  809 , is a mounting ring  813  with a vertically extending Internal Line-Up Clamp (ILUC) garage frame  820  within the mounting ring  813 . The mounting ring  813  is pivotally mounted on the secondary arm  809  for pivotal movement about a first horizontal axis and the garage frame  820  is pivotally mounted on the mounting ring  813  for pivotal movement about a second horizontal axis perpendicular to the first horizontal axis. These two pivotal mountings thus define a Cardan joint allowing the garage frame  820  to pivot about a vertical axis in any direction. Pivoting is restricted to about three degrees by a restraining ring  812  fixed to the secondary arm  809 . Furthermore retractable restraints are provided inside the ring  812 , which when not retracted, prevent any pivoting of the garage frame  820 . The garage frame  820  takes the form of an elongate cage. Within the cage is an ILUC guide  823  ( FIG. 4 b   ) in the form of an elongate tube extending along the longitudinal axis of the cage. The guide  823  has a diameter to correspond to an ILUC  827  contained in the guide  823 . The guide  823  is attached to the inside of the cage by a gimbal joint  826 . This gimbal joint  826  allows guide  823  to pivot slightly within the cage. At the bottom end of the guide  823  is a bevel protector  824 . This bevel protector  824  is installed around the top of the pipestring P 2  by 4 hydraulic cylinders  825 . 
     In addition, also at the bottom of the guide  823  are four radial safety pins  828  that can be moved in and out to prevent an ILUC  827  from falling through the guide  823 . As an additional/alternative safety mechanism, safety pins may be installed at a top end of the ILUC  827  to latch the ILUC on the top of the guide  823 . 
     Below, the mounting ring  813  is a further ring  814  for mounting a rotatable friction clamp  819  at the lower end of the garage frame  820 . A still further ring  817  is provided beneath the ring  814  and a rotatable sleeve  818  is provided in between the rings  814  and  817 . The sleeve  818  can rotate about the longitudinal axis of the garage frame  820 . Distributed around the circumference of the sleeve  818  are 6 radial cylinders  815  which can be moved radially in and out with respect to the longitudinal axis. At the inner end of each cylinder  815  is a friction pad  816 . The radial cylinders  815  can move in and out to allow these friction pads  816  to grip a pipestring P 2  and support its weight. As the friction pads  816  can be moved in and out a variety of pipestring diameters (from 8 inches to 36 inches diameter) can be accommodated. As can be seen in  FIG. 4 d   , the friction pads  816  are shaped to correspond to the rounded shape of the outer circumference of the pipestring P 2 . The sleeve  818  can be rotated +/−190 degrees about the longitudinal axis to allow the pipestring P 2  to be rotated to any orientation about that axis. This allows the gap between bevels and the out-of roundness between the pipestring P 2  and the pipeline P to which it will be joined, to be balanced out. 
     The ILUC  827  is connected at its top end to a cable  822  which is wound around an ILUC winch wheel  821 . This winch wheel is mounted at the top of the ILUC garage frame  820 . The winch wheel  821  can be rotated to deploy and retract the cable  822 , which causes the ILUC to be lowered and raised through the guide  823 . By fully deploying the cable  822 , the ILUC  827  can be lowered down through a pipestring P 2  held by the friction clamp  819  and down to the level of the floor  501  of the welding station  500 . 
     The friction clamp  819  and the ILUC garage frame  820 , are rotatable relative to one another about the axis of the garage frame but not otherwise adjustable, so that, when the clamp  819  is clamped to the top of the pipestring P 2 , the ILUC garage frame is automatically aligned with the pipestring P 2 . Furthermore that alignment is maintained when the garage frame and friction clamp are allowed to pivot about a vertical axis during alignment of a pipestring. 
     The tower clamp assembly  800  also comprises a pre-heating system (not shown) for induction heating the pipestring P 2 . 
     The clamps  401 ,  402 ,  403  on the string elevator  400  and the friction clamp  819  on the tower clamp assembly  800  are designed so that they cannot be open at the same time. This prevents a pipestring P 2  from being dropped. This is achieved by having a mechanical sensor roller (or rollers) on the tower clamp assembly  800  that detect when the pipestring P 2  is clamped in the friction clamp  819 . When a pipestring P 2  is held in the friction clamp  819 , the mechanical sensor roller abuts a wheel and operates a valve (or valves). This allows hydraulic fluid to flow in various flow paths and allow the string elevator clamps  401 ,  402 ,  403  to open. Importantly, in the absence of any hydraulic flow (i.e. in a natural state), the clamps  401 ,  402 ,  403  remain closed. 
       FIGS. 5 a  and 5 b    show a line-up clamp mechanism  352  (the line-up clamp mechanism  353  being of the same design). The line-up clamp mechanism  352  comprises a truss frame  360  upon which the clamp itself is mounted. The clamp itself comprises a main body  361  located in the truss structure  360  and two clamp fingers  362  protruding out from the main body  361  outside the truss structure  360 . 
     The outside edge of the main body  361  has an approximately semi-circular notch to accommodate one half of a pipestring. The two clamp fingers  362  are pivotally mounted on the main body  361  so as to be able to rotate inwards towards each other. Hydraulic cylinders  366  are provided to actuate the clamp finger  362 . Each finger  362  has a curved inward edge so as to form a circular hole with the main body  361  when in a closed position. A pipestring can be held in this circular hole. The main body  361  also comprises two rollers and each finger comprises one roller each at the curved edges. These four rollers  365  allow a pipestring to be held by the clamp in a certain horizontal position whilst still allowing the pipestring to be moved vertically up and down through the clamp. 
     Rollers  363  are mounted on the truss frame  360  to allow the truss frame  360  to move forwards and backwards in relation to the tower  300 . The main body  361  of the clamp is also provided with rollers  364  to allow the clamp to be moved sideways in relation to the truss frame  360 . 
     Laying of a pipeline using the tower  300  will now be described with particular reference to the operation of the tower clamp assembly  800 , including the ILUC arrangement. Details of the general operation away from the tower clamp assembly and the ILUC arrangement are generally not of relevance to the present invention and are not described fully here. A more detailed description of them can be found in UK patent application entitled “Pipe-laying Vessel and Method of Laying a Pipeline” with agent&#39;s reference “P015336 GB”, having the same filing date as the present application. As already indicated, the contents of that application are fully incorporated herein by reference. 
       FIG. 2 a   , which has already been described, shows the tower  300  and associated equipment in a first state. Here, pipestring P 1  is on the lower section  310  of the tower and is welded to pipestring PS and the deployed pipeline P. Pipestring P 2  is located on the upper section  311  of the tower. It is being held by the friction clamp  819  of the tower clamp assembly  800  and tower arm  371 . The ILUC  827  is partly deployed and is positioned mid-way down the length of the pipestring P 2 . As will be appreciated, because the friction clamp assembly and the ILUC arrangement are one unit, the insertion of the ILUC into the pipestring P 2  requires no lining up of those parts relative to one another. Pipestring P 2  is not held directly above pipestring P 1  on the pipe-laying axis  303 . Instead, it is held in a stand-by position in between the string axis  408  and the pipe-laying axis  303 . Pipestring P 3  is clamped to string elevator  400  on the deck  103  of the vessel. 
     In the state shown in  FIG. 2 a   , the friction clamp  343  holds the top of the pipestring PS. The hang off clamp  330  is open. The travelling clamp assembly  340  has just deployed pipeline after moving from the top of its travel path to the bottom of its travel path, as shown in  FIG. 2 a   . The line-up clamp mechanisms  352  and  353  are open and the friction clamp  819  is closed and holding the weight of the pipestring P 2 , while the tower arm  371  retains a lower part of the pipestring P 2  in position. 
     The hang off clamp  330  is then closed around pipestring PS to take the weight of the deployed pipeline P below it. Once the hang off clamp is taking this weight, the friction clamp  343  is released from PS. The framework (first arm)  805  and secondary  809  arms of the tower clamp assembly  800  (and the corresponding tower arm  371 ) are rotated to bring pipestring P 2  onto the pipe-laying axis  303 . 
     The ILUC  827  is lowered further down pipestring P 2  to the join of pipestrings P 2  and P 1 . Line-up clamp mechanisms  352 ,  353  and tower roller  370  are closed around pipestring P 2 . The tower arm  371  is then released from pipestring P 2  and rotated back to the stand-by position. 
     Line-up is performed using line-up clamp mechanisms  352 ,  353 . Friction clamp  819  and tower roller  370  follow the movement of the pipestring P 2  and are “slave” line-up devices. If necessary, friction clamp  819  is rotated to match the off-roundness of the upper end of pipestring P 1  with the lower end of pipestring P 2 . This line-up operation is performed by welding station  500  operators using a hand-held portable console. 
     Welding of pipestrings P 1  and P 2  is then performed in the welding station  500 , with the ILUC  827  in position inside the pipeline at the junction of the pipestrings P 1  and P 2 . Also the pipestring P 1  may be coated or otherwise treated in the coating station  600 . 
     Once welding of pipestrings PI and P 2  is complete, the ILUC is raised up through pipestring P 2  to the garage frame  820 . The tower roller  370  is energised so that the pipestring P 2  can be held in position. The friction clamp  819  is then released and the framework (first arm)  805  and secondary  809  arms of the tower clamp assembly  800  are rotated so that the friction clamp  819  is rotated to the stand-by position. The clamp  373  is folded into its stand-by position and the travelling clamp assembly  340  is raised past the clamp  373  towards the top of its travel path. 
     Friction clamp  819  and tower arm  371  are then moved from the stand-by position and closed around the next pipestring P 3  which has by then been raised on the string elevator  400  and the string elevator clamps  401 ,  402 ,  403  are released. The framework (first arm)  805  and secondary  809  arms of the tower clamp assembly  800  (and the tower arm  371 ) are then rotated to bring pipestring P 3  into the stand-by position above the safety balcony  550 . The ILUC  827  is lowered through pipestring P 3  to about mid-way along its length. When the travelling clamp assembly  340  has reached the top of its travel path just below the welding station  500 , it is clamped around the top region of the pipestring PI. The travelling clamp assembly  340  then takes the weight of the deployed pipeline P and pipestrings PS and PI. The hang-off clamp is then released and the travelling clamp assembly  340  is lowered to deploy pipeline. It is lowered to the bottom of its travel path (as in  FIG. 2 a   ). This pulls pipestring PI into the position occupied by pipestring PS in  FIG. 2 a   . It also pulls down pipestring P 2  to the position occupied by pipestring PI in  FIG. 2 a   . The line up clamp mechanisms  352  and  353  are released and other parts are also then returned to the first state of  FIG. 2 a    again, ready to deploy another pipestring of pipeline. 
     It can be seen that in this embodiment of the invention, the top section of the tower  311  is used for receiving the pipestring P 3  from the string elevator  400  and for line-up of the pipe-string on the pipe-laying path  303 . The bottom section of the tower  310  is used for welding the pipestrings P 3 , P 2 , P 1  and the pipeline P together and lowering the joined pipeline P from the top of the bottom section  310  using the travelling clamp assembly  340 . 
     Whilst the present invention has been described and illustrated with reference to a particular embodiment, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. 
     For example, the embodiment described above is designed for use in laying pipestring formed from 3 joints (3J). In other words, each pipestring is made up of 3 lengths of pipe, welded or pre-fabricated together in some way. This pre-fabrication usually takes place on the vessel. A typical 3J pipestring will be approximately 37.5 m long. However, other embodiments may be designed for laying 1J, 2J, 4J, or more. As will be understood, a 1J pipestring consists of a single length of pipe. 
     Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.