Abstract:
A tensioner for deploying a member such as a pipe or cable, such as a sub-sea cable deployed from a ship, comprises an endless track and drive means for driving the track. The track includes a plurality of traction elements mounted thereon for movement with the track. Each traction element includes one or more cushions for supporting the member, and a pair of opposed arms mounted on respective sides of the base of the traction element. One or more of the arms is pivotally mounted for movement between a clamping position and a release position. In an alternative arrangement, four moveable arms are linked in diametrically opposite pairs to move between a release position and a clamping position. Cushions on the arms clamp the member in conjunction with the cushions of the traction element, when the arms are in the clamping position. Guide means are provided to urge the arms into the clamping position to grip the member as it passes through the tensioner.

Description:
BENEFIT CLAIM 
     This application is a divisional of U.S. application Ser. No. 10/532,383, filed 21 Apr. 2005, now U.S. Pat. No. 7,178,708 which is a US National Stage of International Application No. PCT/GB03/004697 filed 31 Oct. 2003, which claims the benefit of GB 0225496.9 filed 1 Nov. 2002. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to apparatus for gripping an elongate member and applying a continuous longitudinal load to the member while allowing the member to move in a controlled manner through the apparatus. 
     More specifically, the present invention relates to tensioners for pipes and/or cables, in particular for laying pipes or cables offshore, and especially (but not exclusively) to such tensioners for use with so-called, Integrated Service Umbilicals (ISUs). 
     BACKGROUND OF THE INVENTION 
     Tensioners as such are known in the art and are used to grip pipes or cables as they are deployed from a ship to the sea bed during offshore installation. The tensioners maintain a constant tension in the pipe or cable (hereinafter “member”) as it is deployed over the side of the vessel. Conventional tensioners consist of a number of traction devices having a track, each track comprising a number of contacting elements arranged on a continuous belt or loop. The track is driven so that as the contacting elements move with the belt or loop they move along a path or section in which contacting surfaces the contacting elements bear against the member and so control the passage of the pipe or cable through the tensioner. Typically two, three or four tracks are provided arranged equidistantly around the member. Providing a higher number of tracks (for example four rather than two) achieves a more even distribution of the loading on the member gripped by the tensioner and, for a given coefficient of friction between the tensioner and the member reduces the length over which the member has to be gripped in order to achieve a desirably low tensile force. 
     ISUs are relatively stiff but delicate “cables” which consist of a number of elements such as hoses, power cables and signal cables in a single package, generally of circular cross section. ISUs cannot withstand high clamping loads which are conventionally applied when laying pipes for example. In order to avoid such high clamping loads, ISUs require very long pipe tensioning machines (even where four tracks are provided) to provide the required grip on the ISU while providing sufficiently high hold-back tension (typically of the order of 60 tonnes) which is necessary for laying cable from a ship into very deep water. 
     Conventional traction devices are arranged so that the clamping forces on the member are reacted by the traction devices themselves, opposing traction devices being urged by hydraulic or mechanical means onto the member to provide the gripping force. The traction devices are thus required to resist both the tensile force resulting from the laying of the member and the gripping forces normal to the longitudinal axis of the member. This necessarily means that each traction device is relatively large and heavy and this problem is multiplied where higher numbers of traction devices are needed. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention seeks to provide a tensioner in which the forces which traction devices must resist are minimised, so contributing to a reduction in the size of the tensioner and further to provide a gripping arrangement for a tensioner which reduces the required number of traction devices. 
     According to a first aspect of the invention there is provided a tensioner for deploying an elongate member comprising: 
     i) an endless track including a plurality of traction elements, each traction element comprising a base unit including at least a first gripping surface and first moveable arm and a second arm each including a further gripping surface, the arms being mounted at respective sides of the traction element and the first arm being operatively moveable about a pivot axis between a first position in which the further gripping surface of said first arm is spaced from the member and a second position in which said further gripping surface of said first arm can contact the member; 
     ii) drive means for driving the track so that the traction elements move along an endless path, said endless path including a path segment along which said first gripping surfaces operatively contact the member; 
     iii) first guide means which operatively co-operates with the first arm to urge the first arm from said first position to said second position such that the first arm maintains said second position as the traction elements move along said path segment; and 
     iv) second guide means which operatively co-operates with said second arm to form, in said path segment, a reaction surface for a reaction force substantially normal to said surface and to the longitudinal axis of the member. 
     In a much preferred embodiment of this aspect of the invention, the second arm is also operatively moveable about a pivot axis between a first position in which the further gripping surface of said second arm is spaced from the member and a second position in which said further gripping surface of said second arm can contact the member and said second guide means operatively co-operates with the second arm to urge the second arm from said first position to said second position such that the second arm maintains said second position as the traction elements move along said path segment. 
     Preferably in said path segment, the pivot axis of the or each arm is operatively substantially aligned with the longitudinal axis of the member. 
     Where only the first arm is moveable, preferably the first guide means presents a guide surface operatively co-operating with a contacting surface of the first arm, the guide surface extending generally longitudinally with respect to the member and being so shaped in a portion preceding said path segment to move said first arm from the first position to the second position and in a portion succeeding said path section to allow the first arm to move from said second position to said first position. 
     Where both the first and second arms are moveable, preferably the guide means present respective first and second guide surfaces operatively co-operating with a contacting surface of said respective arms, the guide surfaces extending generally longitudinally with respect to the member and being so shaped in a portion preceding said path segment to move said arms from the first position to the second position and in a portion succeeding said path section to allow the arms to move from said second position to said first position. 
     Preferably the first and second guide surfaces are defined on first and second guide rails. 
     In a preferred embodiment of this aspect of the invention the contacting surfaces of the or each arm comprises a surface of a roller mounted on the arm for rotation about an axis substantially perpendicular to the pivot axis of the first arm; in another preferred embodiment of this aspect of the invention, the base unit comprises two first gripping surfaces such that when the arms are in their second position the four gripping surfaces are equidistantly spaced. 
     According to a second aspect of the present invention there is provided a tensioner for deploying an elongate member comprising: 
     i) an endless track including a plurality of traction elements, each traction element comprising: 
     (a) a base unit; 
     (b) first and second arms mounted at respective opposite sides of the traction element and each including a gripping surface operatively disposed above the longitudinal axis of the member; and 
     (c) third and fourth arms also mounted at respective opposite sides of the traction element and each including a gripping portion operatively disposed below the longitudinal axis of the member, each said arm being operatively moveable about a pivot axis between a first position in which the gripping surfaces are spaced from the member and a second position in which the gripping surfaces can contact the member; 
     ii) drive means for driving the track so that the traction elements move along an endless path, said endless path including a path segment along which said gripping surfaces operatively contact the member; 
     iii) guide means which operatively co-operates with the first and second arms to urge the first and second arms from said first position to said second position such that the first and second arms maintain said second position as the traction elements move along said path segment; and 
     iv) a first linkage connecting the first arm and the fourth arm and a second linkage connecting the second arm and the third arm such that movement of the first and second arms between their first and second positions causes a corresponding movement of the third and fourth arms between their first and second positions. 
     Preferably, in said path segment, the pivot axes of the arms are operatively substantially aligned with the longitudinal axis of the member. 
     In a much preferred arrangement, respectively the first and third arms and the second and fourth arms pivot about a common axis. 
     In a further preferred arrangement, the common axes of the first and third and second and fourth arms, and the longitudinal axis of the member operatively lie in the same plane. 
     In a preferred embodiment of the second aspect of the invention, the first and third arms each include an extension portion extending below their pivot axis, and the first and second linkages comprises a link arm which is pivotally attached at a first end to a lower region of the extension portion of the respective first or second arm and which is pivotally attached at a second end to a lower portion of the third or fourth arm. 
     Preferably the guide means present respective first and second guide surfaces operatively co-operating with a contacting surface of said first and second arms, the guide surfaces extending generally longitudinally with respect to the member and being so shaped in a portion preceding said path segment to move said arms from the first position to the second position and in a portion succeeding said path section to allow the arms to move from said second position to said first position. The first and second surfaces are preferably defined on first and second guide rails. 
     In a preferred embodiment of this aspect of the invention the contacting surfaces of the first and second arms comprise a surface of a roller mounted on the arm for rotation about an axis substantially perpendicular to the pivot axis of the arm 
     In a much preferred arrangement of this aspect of the invention the gripping surfaces of the arms are, in use, equidistantly spaced about the circumference of the member in their second position. 
     In a preferred embodiment of the first and second aspects of the invention, the endless track comprises first and second track belts operatively driven in unison and wherein each traction element includes first and second attachment wings extending laterally from the base unit and fixedly mounted on a respective track belt. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       For a better understanding of the invention and to show how the same may be carried into effect, reference will be made, by way of example only, to the following drawings in which: 
         FIG. 1  is a perspective view of one example of a tensioner according to the invention; 
         FIG. 2  is a more detailed view of an end portion of the tensioner of  FIG. 1 ; 
         FIG. 3  is an end view of the tensioner of  FIGS. 1 and 2 ; 
         FIG. 4  is a plan view of a traction element of the tensioner of  FIG. 1 ; 
         FIG. 5  is a perspective view of the traction element of  FIG. 4 ; 
         FIG. 6  is a diagram schematically illustrating forces acting on a typical prior art tensioner; 
         FIG. 7  is a diagram schematically illustrating the absence of corresponding forces on the tensioner according to the invention; 
         FIG. 8  is an end view of a traction element of the second aspect of the invention in a gripping condition; 
         FIG. 9  is a view similar to  FIG. 8  with the traction element in a released condition; and 
         FIG. 10  is a view similar to  FIG. 9  of a partially disassembled traction element. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to  FIGS. 1 to 5 , the tensioner  1  is mounted in a frame  3  and comprises an endless track  5  consisting of two parallel track belts  5   a  and  5   b  which are driven by suitable drive means (not specifically illustrated). Mounted on the respective track belts  5   a ,  5   b  is a number of spaced traction elements  7 . Each traction element  7  includes a pair of lateral wings  9  by means of which it is attached to a respective track belt  5   a ,  5   b  for movement with the track belt. The track  5  is mounted and driven so that the traction elements  7  describe a path in the form of an endless loop. The path includes a path segment  11  in which the path is essentially straight and aligned with the longitudinal axis  13  of the member (pipe or cable or ISU etc)  15 . 
     Each traction element  7  is essentially similar and consists of a base unit  17  on which are mounted pads or cushions  19  which define gripping surfaces  21  which contact the member in use. The two lateral wings  9  depend from the respective sides of the base unit  17  and may be formed integrally therewith. At either side of the base unit  17  are mounted arms  23   a  and  23   b . On each arm is mounted a pad or cushion  25  which is generally similar to the pads or cushions  19  and which defines a further gripping surface  27   a ,  27   b . The arms  23  are mounted on the base unit so that they can rotate with respect to the base unit  17  about axes  29   a  and  29   b . The axes  29   a  and  29   b  are parallel to the longitudinal axis of the member  15  when the member is retained in the tensioner  1 . The rotation of the arms  23   a ,  23   b  about the axes  29   a ,  29   b  allows the arms  23   a ,  23   b  to move between a clamping position in which the member  15  is gripped by the further gripping surfaces  27   a ,  27   b  (in combination with the gripping surfaces  21  of the base unit  17 ) and a release position in which the further gripping surfaces  27   a ,  27   b  are spaced from the member.  FIGS. 4 and 5  specifically illustrate the clamping and release positions of arms  23   a  and  23   b . In these Figures, the arm  23   a  is shown in the clamping position and the arm  23   b  is shown in the release position. (NB: the positions of the arms as shown are for the purposes of illustration and explanation only. In normal use, the arms will move in tandem so that both are in the clamping position or both are in the release position). 
     In order to move the arms  23   a ,  23   b  from the release position to the clamping position, guide means  31  are provided which in the illustrated embodiment takes the form of a guide bar or rail  33 . In  FIGS. 1 ,  2  and  3  the guide rail for the arms  23   b  is omitted for reasons of clarity. The guide rails  33  are arranged generally parallel to the longitudinal axis of the member  15  as it passes through the tensioner and present a smooth guide surface  35  which contacts the arms  23   a ,  23   b . The guide surface includes a curved entrance portion which guides the arms  23   a ,  23   b  from the release position to the clamping position and a correspondingly curved exit portion which allows the arms  23   a ,  23   b  to move from the clamping position to the release position. Between the entrance and exit portions, the guide surface comprises a straight portion which is aligned with the longitudinal axis of the member  15  and which maintains the arms  23   a ,  23   b  in the clamping position. The straight portion corresponds to and is generally co-extensive with the path segment  11  of the track  5  which segment is essentially straight and aligned with the longitudinal axis  13  of the member  15 . The size of the base unit  17  and the arms  23   a ,  23   b , the spacing of the pads or cushions  19 ,  25  and the position and spacing of the guide rails  33  are all selected in accordance with the size of the member  15  to be deployed and the clamping force which is required. 
     In the illustrated embodiment, the arms  23   a ,  23   b  are provided with rollers  37  for co-operating with the guide surface  25 . The rollers are mounted on the ends of the arms  23   a ,  23   b  distal from the base unit  17  and rotate about an axis which is normal to the pivot axes  29  of the arms. 
     In use of the tensioner  1 , the track  5  is driven in the forward or reverse direction as appropriate to move the traction elements  7 . The member  15  is fed into the tensioner  1  so that it is accommodated on pads or cushions  19  of base units  17  entering and passing through the path segment  11 . As successive traction elements approach the path segment  11 , the guide rails  33  move the arms  23   a ,  23   b  from the release position to the clamping position so that in the path segment  11  the member is clamped by the pads or cushions  19  in combination with the arm pads or cushions  25 . The pads or cushions  19 ,  25  are so disposed that, with the arms  23   a ,  23   b  in the clamping position, the pads or cushions  19 ,  25  are equidistantly spaced about the circumference of the member  15 . At the end of the path segment  11 , the guide rails  33  release the arms  23   a ,  23   b . Thus, with the member clamped by the pads or cushions  19 ,  25  in the path segment  11 , movement of the member  15  is controlled by the tensioner  1  so that the member can be paid out or deployed, such as in sub-sea cable laying, at an appropriate tension. 
     Referring now to  FIGS. 8 to 10  which illustrate a traction element  107  for a tensioner according to a second aspect of the invention, the traction element is mounted on a track  5  (not shown) by means of a pair of lateral wings  109  which depend from respective sides of a base unit  117  and may be formed integrally therewith. First, second third and fourth arms  123   a ,  123   b ,  123   c  and  123   d  are mounted on the base unit for movement about pivot axes  129   a ,  129   b . The first and second arms  123   a ,  123   b  are essentially similar to one another and the third and fourth arms  123   b ,  123   d  are essentially similar to one another. The first and third arms  123   a ,  123   c  are mounted on one side of the base unit  117  and the second and fourth arms  123   b ,  123   d  are mounted on a second, opposite, side of the base unit  117 . The first and third arms  123   a ,  123   c  pivot about a first pivot axis  129   a  and the second and fourth arms  123   b ,  123   d  pivot about a second pivot axis  129   b.    
     Each of the four arms  123   a ,  123   b ,  123   c  and  123   d  is provided with a pad or cushion  125  which defines a gripping surface  127  for the member  15 . The pad or cushion  125  of the first and second arms  123   a ,  123   b  is disposed on an upper part  122   a ,  122   b  of the arm, above the longitudinal axis  13  of the member  15 . The third and fourth arms  123   c ,  123   d  extend in use principally below the longitudinal axis  13  so that the pads or cushions of these arms are disposed below the longitudinal axis  13 . 
     The first and second arms  123   a ,  123   b  comprise a roller  137  which co-operates with guide rails in an analogous manner to the first aspect of the invention to move the first and second arms  123   a ,  123   b  from a first (released) condition ( FIG. 9 ) to a second (gripping) condition ( FIG. 8 ). As can best be seen from  FIG. 10  (which shows the second and third arms only), the second arm  123   b  includes a lower portion  124   b  which extends below the pivot axis  129   b . First arm  123   a  includes a corresponding lower portion  124   a  extending below pivot axis  129   a . A link arm  139   b  extends between the lower portion  124   c  of second arm  123   b  and a lower part of third arm  123   c  and is pivotally attached to the third and second arms  123   c ,  123   b  at its respective ends  139 ′ and  139 ″. A corresponding link arm  139   a  links first arm  123   a  and fourth arm  123   d . The link arms provide that as the first and second arms  123   a ,  123   b  arm moved by the guide rail from the first (released) position to the second (gripping) position, the third and fourth arms  123   c ,  123   d  also move from a release position to a gripping position. In this way, members  15  of differing sizes can be accommodated by the tensioner, with the gripping surfaces  127  remaining centred about the axis  13  of the member  15 . That is, relative location of the member axis  13  with respect to the track  5  is independent of the member diameter. 
     A particular advantage of the construction according to the present invention is that the normal gripping forces are isolated from the traction device, allowing the tensioner to be driven by a single traction device. In the prior art designs, drive means are required for each track of the tensioner. Furthermore, in the construction according to the present invention the forces that individual traction devices are required to resist are minimised. In particular, the construction according to the present invention aligns the longitudinal force on the member  15  with the line of action of the reacting force of the traction device to minimise the resultant couples applied to the traction elements. This can be seen in particular by comparison of  FIGS. 6 and 7 . In the prior art device of  FIG. 6 , a force F 1  exerted by the member  15  is reacted by reaction force F 2  at the traction elements  7 ′ of tracks  5 ′ (tracks  5 ′ are shown in part only). Thus the traction elements  7 ′ are subject to a couple equal to (F 2 /2).d. This couple tends to tilt or skew the contact surfaces between the traction elements and the member, so reducing the contact area. As can be seen from  FIG. 7 , in the construction according to the invention, the force F P  exerted by the member  15  is reacted by reaction force F R  and there is no resulting couple.