Riser tensioner

Abstract: A riser tensioner (10) is provided for use in maintaining a tension on a marine riser (12) from a tension leg platform (14), the tension leg platform moving relative to the marine riser. The tensioner comprises an elastomeric assembly (20), adjustably deformable in pad shear, for maintaining the riser in tension during vertical movement of the platform relative to the riser. A gimbal assembly (18) pivotally connects the upper end of the elastomeric assembly (20) to the tension leg platform (14) to accommodate misalignment between the riser and the tension leg platform. The lower end of the elastomeric assembly (20) is secured to a base ring (34). A collar (36) is securely mounted on the riser, for resting within the base ring (34) to connect the lower end of the elastomeric assembly to the riser.

Description: This invention relates to offshore oil drilling and production, and specifically to a marine riser tensioner for use in a tension leg platform. In recent years, a great effort has been exerted in exploring for and producing oil from oil fields under water. The Gulf of Mexico and the North Sea are specific examples where a great effort has been exerted. Many techniques have been explored for efficient exploration and production of these undersea oil reserves. One recent development is the tension leg platform which can be used both for drilling and production. The tension leg platform (commonly referred to as TLP) is a floating structure, resembling a large semisubmersible drilling rig, connected to sea bed foundation templates by vertical mooring tethers. Buoyancy for the TLP is provided by watertight columns, pontoons and the like. The TLP is provided with an excess of buoyancy to keep the mooring tethers in tension for all weather and loading conditions. Three separate marine riser systems are commonly used for conducting fluids between the subsea template and the TLP during both drilling and production phases. These riser systems are the drilling, production and crude oil sales risers. The risers are secured at the sea floor on the subsea template and extend to the TLP. The risers must be maintained constantly in tension to avoid the risers collapsing from their own weight, despite movement of the TLP due to surface movement and weather extremes. In the past, active hydropneumatic systems have been used to maintain a tension on the risers in TLP systems. Such use is described in a paper entitled "TLP Riser Tensioners Stand Stresses by M.H.Frayne and F.L. Hettinger, in OFFSHORE, 1983, volume 43, no. 6, page 62. Tensioners disclosed in this reference incorporate hydraulic actuators which stroke up and down in response to TLP movements to apply a relatively constant tension to each riser. This system has several disadvantages. It is an active system which requires continuous supply of high pressure fluids for operation. Thus, if a malfunction occurs which eliminates the supply of this high pressure fluid, the system can fail. Further, a sophisticated and expensive control system must be provided which maintains the desired pressure in the system. I t is known to use a passive system for applying a tension in a marine riser, using an elastomeric spring for that purpose. (See World Oil, October, 1985, "New Technology stresses drilling a better hole" by Tom Muhlemann Jr, pages 61-64 at pages 61-62). This system uses an elastomeric spring that is connected to a tapered drum, and a line from the riser which is wrapped around the drum. Although the system is passive once adjusted, the drum nevertheless requires a motor to set the desired tension in the line. It is necessary to allow not only for vertical movement of the platform but also for tilting of the platform. US-A-4324194 shows a pipe string support and tensioning system which utilizes a hoist rig angularly mounted on a vessel. Hydraulic actuators are used in conjunction with resilient elements mounted in the rig base member to keep the hoist rig in parallel alignment with the central axis of the pipe string during pitching and rolling movement of the vessel. The resilient elements consist of two spherical sectors disposed around the pipe string. The upper resilient member functions essentially as a ball joint to allow the vessel and hoist rig to move freely about a slip bowl when the base member is locked into position by the hydraulic actuators. Attention is also directed to page 62 of the World Oil article mentioned above, which describes a hydraulic power swivel which has a set of tilting cylinders which allow the unit to be pivoted forward or backward. US-A-4359095 discloses a riser tensioner in accordance with the preamble of claim 1. The elastomeric assembly comprises a flexible column which is, in use, loaded in compression. In accordance with the present invention there is provided a riser tensioner for use in maintaining a tension on a marine riser from a tension leg platform, the tension leg platform being moveable relative to the marine riser and the marine riser having a center line, comprising: (a) an elastomeric assembly, adjustably deformable for maintaining the riser in tension during vertical movement of the platform relative to the riser, said elastomeric assembly having upper and lower ends; and (b) a base ring to which said lower end of said elastomeric assembly is secured; characterised in that the elastomeric assembly is deformable in pad shear and that the tensioner further comprises: (c) a gimbal assembly for pivotally connecting said upper end of said elastomeric assembly to the tension leg platform to accommodate misalignment between the riser and the tension leg platform; and (d) a collar, for securely mounting on the riser, to rest within said base ring to connect said lower end of said elastomeric assembly to the riser. In accordance with an embodiment of the present invention, the elastomeric assembly includes at least one first plate assembly operatively secured to the tension leg platform and a second plate assembly operatively secured to the riser. An elastomeric pad assembly is bonded between the first and second plate assemblies to be put in shear to tension the riser. The elastomer pad assembly can include a plurality of elastomeric pads separated by rigid plates. The plate assemblies of two elastomeric assemblies can be connected to increase the travel of the riser relative to the tension leg platform. In addition, elastomeric pad assemblies can be bonded on both sides of a plate assembly to increase tension forces. A more complete understanding of the invention can be had by referring to the following detailed description taken with the accompanying drawings, wherein: Figure 1 is a perspective view of an embodiment of the present invention for tensioning a marine riser on a tension leg platform; Figure 2 is a side view of the embodiment of Figure 1; and Figure 3 is a top view of the embodiment of Figure 1. Referring now to the drawings, there is illustrated a marine riser tensioner 10 forming an embodiment of the present invention. The marine riser tensioner 10 is intended to maintain a minimum tension on a marine riser 12 as the tension leg platform 14 moves under the influence of wave motion, weather and other factors. The marine riser tensioner 10 is capable of maintaining a desired tension on the marine riser 12, typically in the rare of 50-500 kips (2.2x10⁵-2.2x10⁶N or 50,000 - 500,000 lb-wt), despite vertical movement of the tension leg platform 14 relative to the marine riser 12 of perhaps as much as 6 feet (1.8 m) in either direction from the normal or equilibrium level, and for a tilting of the platform 14 relative to the marine riser 12, up to an angle of as much as 10°. The tensioner 10 is mounted on a deck 16 of the platform 14 with the majority of the tensioner extending below the deck through the hole in the deck through which the riser 12 passes. The tensioner 10 includes a gimbal assembly 18 which accommodates the pivoting of platform 14 relative to the riser 12 and an elastomeric assembly 20 which maintains the riser 12 in tension despite vertical movement of the platform 14 relative to the riser 12. The gimbal assembly 18 includes upstanding arms 22 which are rigidly secured to the deck 16 and pivotally support a first gimbal ring 24 for pivotal motion about the horizontal axis 26. A second gimbal ring 28 is pivotally secured to the first gimbal ring for pivotal motion about a horizontal axis 30 which is perpendicular the horizontal axis 26. Rollers 31 on ring 28 bear against the riser, but allow vertical motion of platform 14 relative to the riser. Thus, the gimbal assembly is capable of accommodating any pivotal misalignment between the marine riser and platform. The elastomeric assembly 20 is secured on the second gimbal ring 28. The elastomeric assembly 20 includes four identical elastomeric units 32 distributed at uniform radial positions about the riser. The units 32 are supported at their upper ends by the second gimbal ring and are secured to a base ring 34 at their lower end. A collar 36 is securely mounted on the riser 14. Collar 36 rests within the base ring 34 so that the riser is entirely supported on the platform 14 through the four elastomeric units 32. Each elastomeric unit 32 includes a threaded rod 38 extending through the second gimbal ring 28. The threaded portion of rod 38 receives a nut 40 which rests on the top of the second gimbal ring 28. By rotating nut 40, the vertical position of rod 38 can be varied to deform the elastomeric elements in unit 32 to provide the desired tension to the marine riser 12. The lower end of threaded rod 38 has a clevis 42 which receives a cross pin 44. Cross pin 44 supports an upper plate assembly 46 formed by bolting together plates 48 with threaded fasteners 53. Plates 48 each form a part of an elastomeric section 54. Each section 54 has rigid plates exterior 48, 56 and interior plates 58 with elastomeric pads 60 bonded between the plates to form a unitary structure which is designed for supporting a force acting through the elastomeric section directed along the planes of bonding between the elastomeric pads 60 and the various plates 48, 56 and 58 in pad shear. As can be seen in the figures, two upper elastomeric sections 54 are mounted side by side and attached by plates 48 to the threaded rod 38. A portion of the plate 56 of each of the upper elastomeric section 54 depends from the rest of the section to connect with a mating pair of lower elastomeric sections 54 directly beneath. The plates 56 of the mating elastomeric sections are bolted together by connector plates 62 and fasteners 64. The plates 48 of the lower pair of elastomeric sections 54 are also bolted together with threaded fasteners 52. The base ring 34 has a link 66 supporting a cross pin 68 which receives the plates 48 of the lower elastomeric sections 54. By use of four elastomeric units 32 distributed about the marine riser 12, vertical movement of the tension leg platform 14 relative to the riser is accommodated by deformation of the elastomeric pads 60 in pad shear. While any number of units 32 can be used, it is preferable to position the units so that the total force vector acting on the marine riser as a result of the units lies on the central axis of the riser. To set the desired tension on the marine riser, the elastomeric pads 60 are placed in pad shear by adjusting the nuts 40 to tension the marine riser at the equilibrium point of the motion of the tension leg platform 14 relative to the riser 12. The elastomeric assembly 20 will maintain sufficient tension on the marine riser as the platform 14 moves either direction around the equilibrium point relative to the riser by a suitable deformation of the elastomeric pads 60. Misalignment between the platform 14 and the riser 12 will be accommodated through the gimbal assembly 18. The material forming the elastomeric pads 60 can be selected for the desired operating characteristics. It is contemplated that the elastomeric pads 60 can be made of synthetic and/or natural rubber materials. For example, in service where wide fluctuations in temperature is expected, a blended natural rubber might be preferable. The elastomeric assembly can be made with the relationship between force and deflection either linear or nonlinear, as desired.

Claims: A riser tensioner for use in maintaining a tension on a marine riser (12) from a tension leg platform (14), the tension leg platform being moveable relative to the marine riser and the marine riser having a center line, comprising: (a) an elastomeric assembly (20), adjustably deformable for maintaining the riser (12) in tension during vertical movement of the platform relative to the riser, said elastomeric assembly having upper and lower ends; and (b) a base ring (34) to which said lower end of said elastomeric assembly (20) is secured; characterised in that the elastomeric assembly (20) is deformable in pad shear and that the tensioner further comprises: (c) a gimbal assembly (18) for pivotally connecting said upper end of said elastomeric assembly (20) to the tension leg platform (14) to accommodate misalignment between the riser and the tension leg platform; and (d) a collar (36), for securely mounting on the riser, to rest within said base ring (34) to connect said lower end of said elastomeric assembly (20) to the riser. A riser tensioner according to claim 1, wherein said elastomeric assembly (20) further comprises: (a) at least one elastomeric unit (32) having an upper elastomeric section (54) and a lower elastomeric section (54); (b) each of said elastomeric sections having a pair of rigid exterior plates (48, 56) and at least one elastomeric pad (60) bonded between said rigid plates and deformable in pad shear; (c) one (48) of said pair of rigid exterior plates of said upper elastomeric section being connected to the tension leg platform (14) via said gimbal assembly (18) and said other one (56) of said pair of rigid exterior plates of said upper elastomeric section being rigidly connected to one (56) of said pair of rigid exterior plates of said lower elastomeric section; and (d) said other (48) of said pair of rigid exterior plates of said lower elastomeric section being connected to the riser (12) via said base ring (34). A riser tensioner according to claim 2, wherein each of said elastomeric sections (54) further comprises a plurality of elastomeric pads (60) with a first of said pads being bonded to one of said rigid exterior plates (48, 56) and a second of said pads being bonded to the other of said rigid exterior plates (48, 56), and intermediate rigid plates (58) being bonded between adjacent elastomeric pads (60). A riser tensioner according to claim 3, further comprising means (38, 40) for adjusting the relative positions of said one of said pair of rigid exterior plates of said upper elastomeric section (54) being connected to the tension leg platform (14) and said other of said pair of rigid exterior plates of said lower elastomeric section (54) being connected to the riser, to provide a predetermined tension to the riser (12) by deforming said elastomeric pads (60) of said elastomeric sections in pad shear. A riser tensioner according to claim 4, wherein said elastomeric assembly comprises a plurality of said elastomeric units (32) distributed at uniform radial positions about the riser (12) to create a net tension force along the center line of the riser. A riser tensioner according to any one of claims 2 to 5, wherein each elastomeric unit (20) comprises: (a) a pair of said upper elastomeric sections (54), each of said one of said pair of rigid exterior plates (48) of each of said upper elastomeric sections which are connected to the tension leg platform (14) being secured together; (b) a pair of said lower elastomeric sections (54), each of said one of said pair of rigid exterior plates (48) of each of said lower elastomeric sections which are rigidly connected to the riser (12) being secured together; and (c) said remaining pair of rigid exterior plates (56) of said pair of upper elastomeric sections being rigidly connected to said remaining pair of rigid exterior plates (56) of said pair of lower elastomeric sections. A riser tensioner according to any one of claims 2 to 6, wherein said gimbal assembly (18) comprises: (a) a pair of upstanding arms (22) which are rigidly secured to the tension leg platform (14); (b) a first gimbal ring (24), pivotally supported by said pair of upstanding arms, for pivotal movement about a horizontal axis (26) passing through said pair of upstanding arms; (c) a second gimbal ring (28), pivotally secured to said first gimbal ring (24) for pivotal movement about a second horizontal axis (30) substantially perpendicular to said first horizontal axis (26), said second horizontal axis being in the plane formed by said second gimbal ring; and (d) means (38, 40, 42, 44) for connecting each of said elastomeric units (32) to said second gimbal ring (28). A riser tensioner according to claim 7, further comprises: (a) a threaded rod (38) extending through said second gimbal ring (28) and attached to the upper end of each of said elastomeric units (32); (b) a nut (40) threadable onto said threaded rod so that said nut rests on top of said second gimbal ring (28), whereby rotation of said nut varies the vertical position of said threaded rod to deform said elastomeric pads (60) in each unit to provide the desired tension.

Inventors: PEPPEL GEORGE WALTER, PEPPEL, GEORGE WALTER
Applicants: SHELL OIL CO, SHELL OIL COMPANY
Application year: 1987
Main classification: E21B19
IPC codes: E21B19