Catenary riser supports

Abstract: A catenary riser support tube (20) that receives a riser (16) and is attached to a floating offshore structure (10). The support tube (20) is attached (A) to the floating offshore structure (10) at a slight angle from the vertical so as to be in line with the natural angle that the installed catenary riser (16) would assume on a calm day. The support tube (20) is attached to the floating offshore structure (10) at different points (A-D) along the length of the support tube (20) and thus flexes about its attachment points to the floating structure (10). This minimizes stresses on the catenary riser (16).

Description: The invention generally relates to the production of hydrocarbons from subsea formations, and more particularly to supports for risers used in such production. In producing hydrocarbons from subsea formations, it is common that a number of wells are drilled into the sea floor in positions that are not directly below or substantially within the outline of the structure used during production operations. The produced hydrocarbons are subsequently exported via subsea pipelines. This results in production and export risers that have a catenary curve in the risers between the structure and the sea floor. The movement of floating production platforms causes corresponding flex and stress in the risers. The current state of the art has accommodated the flex in the risers by incorporating ball joints at suitable locations in the joints between pipe segments in the riser. The ball joints present the problem of being more expensive and less reliable than pipe segments that are welded together. US-A-5 437 518 discloses a device for mounting a flexible pipeline on a structure. A riser guide tube, intended for the flexible pipeline, extends below the lower end of a flotation unit. The invention provides a floating offshore structure for producing hydrocarbons, in which a catenary riser extends from the sea floor to the floating offshore structure, the floating offshore structure including a support tube for the catenary riser, the support tube being attached at predetermined locations to the floating offshore structure so as to allow the support tube to flex about the attachment points at the predetermined locations, and such that the support tube exits the lower end of the floating offshore structure at an angle from the vertical, said support tube extending outboard the lower end of the floating offshore structure and being sized to receive and to allow support to be provided to the catenary riser through and along the length of said support tube. According to a preferred embodiment of the invention a support tube is attached to the floating offshore structure at an angle from the vertical so as to be in line with the natural angle that the installed catenary riser would assume on a calm day. The support tube is attached to the floating offshore structure at different points along the length of the support tube and thus flexes about its attachment points to the floating structure. The preferred support tube extends outwards from the floating structure such that the first attachment point is located at a distance from the lower end of the support tube. Additional flexibility in the support tube is attained by locating the second attachment point to the floating structure at a key distance further from the end of the support tube. The support tube may be provided with a bending stiffness that varies from the first attachment point to the lower end of the support tube. The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Fig. 1 is a side sectional schematic view that illustrates an embodiment of the invention on a floating offshore structure; and Fig. 2 schematically illustrates the shape of one version of the support tube. In Fig. 1, a floating offshore structure 10 is held in position by a plurality of mooring lines 12. The mooring lines 12 are attached at one end to the floating structure 10 and at the opposite end to anchors or pilings (not shown) in the sea floor 14. A catenary riser 16 extends from the sea floor 14 up to and through a centre well 18 provided in the floating structure 10 to production facilities (not shown). Support for the catenary riser 16 is provided in the form of a support tube 20. The support tube 20 has an internal diameter sized to receive the catenary riser 16 therethrough. The support tube 20 is attached at its upper end to the floating structure 10, indicated by A, at a slight angle from the vertical so as to be in line with the natural angle that the catenary riser 16 assumes when in its installed position in calm seas. The support tube 20 has a length that extends outboard the lower end of the floating structure 10 and is attached to the lower end of the floating structure 10, indicated by B, and at predetermined points indicated by letters C, D and E. The number and location of the attachment points between the support tube 20 and the floating structure 10 is determined by the flexibility desired in the support tube 20. For example, greater flexibility towards the lower end of the support tube 20 may be achieved by moving the attachment point C further away from the attachment point B at the lower end of the floating structure 10. Allowing the support tube 20 to flex about its attachment points to the floating structure 10 maintains the stresses on the catenary riser 16 within allowable limits. The support tube 20 can be provided with progressively greater flexibility from the attachment point B to its lower end such as by varying the wall thickness and/or diameter of the tube. As seen in Fig. 2, the support tube 20 may be constructed from a combination of curved and straight sections incorporated into its shape such that the upper end of the support tube 20 is substantially vertical and directs the catenary riser 16 to a desired above-water location on the floating structure in a substantially vertical orientation. In this example of an embodiment of the invention, a section 22 of the support tube 20 is straight, a section 24 is curved, and a section 26 is again straight. ,The floating structure 10 in this example is 55m (one hundred and eighty feet) tall. The section 22 of the support tube is 9m (thirty feet) long, the section 24 is 27m (ninety feet) long, and the section 26 is 18m (sixty feet) long. The section 24 has a radius of 132m (four hundred and thirty-two feet), which results in an exit angle of twelve degrees at the bottom of the floating structure 10 to accommodate the natural catenary curve of the riser 16. It should be understood that Fig. 2 is used merely as an illustration of one possible configuration of the invention, and that the entire length of the support tube 20 below the floating structure 10 is not shown in Fig. 2. Three different procedures may be used to install the catenary riser 16 in the support tube 20. In the pre-lay method, the riser pipe is placed on the sea floor 14 prior to the floating structure 10 being moored at the site. Once the floating structure 10 is secured in position, the end of the riser 16 is positioned at the bottom end of the support tube 20 and pulled through to a point where the end of the riser 16 is above the water. During the pull-in phase, the angle of the riser 16 and the support tube 20 would assume angles other than the natural catenary neutral (no bending stress) position. During the initial phase of the pull-in, the angle would be less than the desired neutral position. At the end of the pull-in phase, the exit angle could be equal to the desired neutral position or could be larger than the neutral angle. If the exit angle is larger than the desired neutral position once the end of the riser 16 reaches the surface, additional sections of riser pipe would be added. The upper end of the now-longer riser 16 is lowered, adding segments as needed, until the desired calm day neutral angle is achieved at the exit point from the support tube 20. Additional adjustments to the exit angle could be achieved by moving the floating structure 10 horizontally at the surface. It would also be possible with some floating structures to tilt the structure, and the attached support tube 20, using ballast, and thus more accurately accommodate the neutral riser catenary angle at the entrance point to the support tube 20 during installation. In the lay-to method, a riser installation vessel (not shown) would approach the floating structure 10 as the installation vessel lays riser pipe on the sea floor 14. The end of the riser 16 would be lowered with cables from the installation vessel until the riser 16 is at the entrance point to the support tube 20. A cable threaded through the support tube 20 would then be used to pull the riser 16 through the support tube 20 to a point above the water surface. The installation would then be completed as described above. In the lay-away method, the riser 16 would be pulled from the riser installation vessel by a cable threaded through the support tube 20. The appropriate length of riser pipe is suspended between the riser pipe installation vessel and the outside floating structure 10 to maintain the desired neutral entrance angle at the entrance point to the support tube 20. With this method, it would not be necessary to add segments of riser pipe at the end of the riser pipe at the floating structure 10. The above-described technique eliminates the need for expensive subsea flex joints and riser pipe connectors. With the pre-lay method, the riser pipe can be installed immediately after the floating structure is positioned and thus would be ready for use immediately. This could allow for oil and gas production to come on stream sooner. Also, the all-welded pipe is generally considered to be more reliable than pipelines with mechanical connections and flex joint elements. Because many varying and differing embodiments may be made within the scope of the claims and because many modifications may be made in the embodiment herein described it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.

Claims: A floating offshore structure for producing hydrocarbons, in which a catenary riser (16) extends from the sea floor (14) to the floating offshore structure (10), the floating offshore structure (10) including a support tube (20) for the catenary riser (16), the support tube (20) being attached at predetermined locations (A-D) to the floating offshore structure (10) so as to allow the support tube (20) to flex about the attachment points at the predetermined locations (A-D), and such that the support tube (20) exits the lower end of the floating offshore structure (10) at an angle from the vertical, said support tube (20) extending outboard the lower end of the floating offshore structure (10) and being sized to receive and to allow support to be provided to the catenary riser (16) through and along the length of said support tube (20). A floating offshore structure according to claim 1, wherein the portion of said support tube (20) that extends outboard the lower end of the floating offshore structure (10) is curved substantially to match the natural curve of the catenary riser (16) received therein. A floating offshore structure according to claim 1 or claim 2, wherein the portion of said support tube (20) that extends outboard the lower end of the floating offshore structure (10) is progressively increasingly flexible towards the lower end of said support tube (20). A floating offshore structure according to claim 1, claim 2 or claim 3, wherein said support tube (20) is formed from a combination of curved and straight sections (22,24,26) such that the upper end of said support tube (20) is substantially vertical and the lower end of said support tube (20) substantially matches the normal installed angle of the catenary riser (16) in calm seas.

Inventors: FINN LYLE DAVID, FINN, LYLE DAVID
Applicants: DEEP OIL TECHNOLOGY INC, DEEP OIL TECHNOLOGY, INCORPORATED
Application year: 1997
Main classification: E21B19
IPC codes: E21B17, E21B19