Abstract:
The present disclosure relates to a subsea catenary and a method of distributing ballast on a pipe. The subsea catenary includes a flexible pipe, and a selected amount of ballast attached to the flexible pipe, in which the ballast is distributed in a wave pattern on a sag bend of the subsea catenary. The method includes identifying a position of a sag bend in a flexible pipe of the subsea catenary, and applying ballast in a wave distribution along the subsea pipe at the identified position.

Description:
[0001]    BACKGROUND OF THE DISCLOSURE 
         [0002]    1. Field of the Disclosure 
         [0003]    The present invention relates to apparatus and methods of distributing ballast at a sag bend of a flexible catenary riser. 
         [0004]    2. Description of the Related Art 
         [0005]    U.S. Pat. No. 6,491,779, entitled “Method of Forming Composite Tubular Assembly,” filed Apr. 24, 2000, assigned to the assignee of the present application, and incorporated by reference in its entirety, discloses a flexible pipe made of lightweight composite materials for subsea use. Prior to the invention of the U.S. Pat. No. 6,491,779, it was generally known that conventional pipe (relatively heavy pipe, usually steel), when used subsea, formed a catenary from a vessel on the water&#39;s surface to the seabed. Such conventional pipe systems require expensive equipment at the surface or elsewhere to offset the heavy weight of the steel pipe. 
         [0006]    Subsequently, U.S. Pat. No. 7,073,978, entitled “Lightweight Catenary System,” filed Aug. 16, 2004, assigned to the assignee of the present application, and incorporated by reference in its entirety, disclosed a lightweight flexible pipe catenary system for deep sea installations. 
       SUMMARY OF THE CLAIMED SUBJECT MATTER 
       [0007]    In one aspect, the present disclosure relates to a subsea catenary. The subsea catenary includes a flexible pipe, and a selected amount of ballast attached to the flexible pipe, in which the ballast is distributed in a wave pattern on a sag bend of the subsea catenary. 
         [0008]    In another aspect, the present disclosure relates to a method to apply ballast weight to a subsea catenary. The method includes identifying a position of a sag bend in a flexible pipe of the subsea catenary, and applying ballast in a wave distribution along the subsea pipe at the identified position. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]    Features of the present disclosure will become more apparent from the following description in conjunction with the accompanying drawings. 
           [0010]      FIG. 1  shows a side elevation schematic of catenary risers. 
           [0011]      FIG. 2  shows a catenary riser in accordance with one or more embodiments of the present disclosure. 
           [0012]      FIG. 3  shows a catenary riser in accordance with one or more embodiments of the present disclosure. 
           [0013]      FIG. 4  shows a catenary riser in accordance with one or more embodiments of the present disclosure. 
           [0014]      FIG. 5  shows a catenary riser in accordance with one or more embodiments of the present disclosure. 
           [0015]      FIG. 6  shows a catenary riser in accordance with one or more embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    When a flexible catenary riser is connected to a floating production storage and offloading vessel (“vessel”), the flexible riser may suffer from high axial compression and violations of minimum bend radius due to a high curvature of a catenary at the touch-down zone, the point where the pipe touches down on the seabed. The violations may be made worse due to dynamic bending wave patterns that are formed in response to current flow and fluid dynamics at depth. 
         [0017]    To alleviate the compression loading and violation of minimum bend radius, ballast may be applied to the riser near the point of touch-down (at the point of sag bend) or to a portion of the riser close to the seabed. The ballast may provide weight and may prevent the dynamic bending wave patterns from forming or may minimize the effects of the dynamic bending wave patterns on the flexible catenary riser. 
         [0018]    Accordingly, pursuant to one or more embodiments of the present disclosure, distributions of ballast may be strategically positioned in the sag bend area of the flexible catenary riser to create bending waves during extreme response dynamics. As such, the dynamic bending wave patterns formed in the flexible catenary riser may be controlled and/or minimized due to the distributed ballast. 
         [0019]    Distributed ballast may significantly change the nature of the dynamic response of the flexible catenary riser to fluid dynamics at depth. The distributed ballast may cause dynamic bending waves to occur at specified locations and at pre-planned wavelengths. Accordingly, the dynamic bending wave patterns may be controlled and maintained within operable conditions for the flexible catenary riser. When the flexible pipe is subject to axial compression forces due to the wave loading and vessel motion, the wave pattern provided by the distributed ballast may minimize axial compression forces that the flexible pipe structure experiences. When the axial compression load is applied, the wave amplitude will increase. In other words, preferential buckling of the pipe in the direction of the wave will occur, rather than the compressive axial force causing compressive stresses in the pipe wall. 
         [0020]    The amplitude of the bending waves may be controlled by the relative magnitudes of ballast as distributed along the sag bend of the flexible catenary riser. Further, the bending waves may be controlled by the positional distribution of the ballast along the riser. Accordingly, the weight, the length of individual ballast segments, and the length of pipe covered by the distributed ballast may be controlled. The ballast may be distributed in half-wave segments such that a length of ballast (half-wave) is positioned on the riser for a distance followed by a length of riser (half-wave) with no ballast (bare pipe). A half-wave configuration may be repeated with the same or different weights and/or lengths of ballast in the wave pattern along the riser at the point of the sag bend. Preferably, the length of the wave pattern is uniform and each half-wave segment is of uniform length. The weight, therefore, may be distributed in half-wave segments of a pre-planned wavelength. 
         [0021]    Lightweight flexible pipe as used herein may be composite flexible pipe as described in U.S. Pat. No. 6,491,779. The flexible pipe may be entirely non-metallic or may be substantially non-metallic. Further, the flexible pipe may be of standard annulus construction and configuration. The flexible pipe may be of bonded or unbonded construction, as described in ISO 13628-2/API 17J or ISO 13628-10/API 17K, or it may be a composite riser as described in DNV-RP-F202 or it may be a thermoplastic composite riser as described in Airborne Composites white paper entitled “Thermoplastic Composite Riser.” The flexible pipe may include an internal pressure sheath to convey fluids, surrounded by layers of composite reinforcements and an outer sheath. The pipe may be vented at a topside (or vessel mounted) end fitting because permeated gas may build up in the annulus between the internal pressure sheath and the outer sheath. 
         [0022]    In accordance with one or more embodiments of the present disclosure, dynamic riser analysis (or modeling) may be used to determine an optimum distribution of ballast. The dynamic riser analysis may pre-determine the spacing, length of segments of ballast, weight of ballast, length of wave pattern, number of segments, and/or any other variables of ballast distribution prior to installation on a pipe. Accordingly, an optimum configuration may be pre-planned for any particular application of ballast in accordance with embodiments of the present disclosure. 
         [0023]    The dynamic riser analysis may account for extreme conditions at a particular location or proposed location. For example, a hundred-year environment may be determined, accounting for a hundred-year wave and a ten-year current. The dynamic riser analysis may also account for a near or far off-set. The off-set is the lateral distance from the mean position of the riser-top connection point at the vessel. Accordingly, the riser design and ballast distribution may be optimized for a particular maximum environmental condition, thereby anticipating loads that may be applied to the riser during installation and/or operation. 
         [0024]    Referring now to  FIG. 1 , a side elevation schematic of catenary risers is shown. In  FIG. 1 , a vessel  100  (a ship, platform, or any other riser support structure) is shown with two catenary risers. A first riser  101  may be a conventional pipe or umbilical made of steel and is non-buoyant, thereby forming a catenary when suspended from vessel  100 . A second riser  102  may be a lightweight flexible pipe, as described above, and may be buoyant when suspended as a catenary riser in seawater. 
         [0025]    As shown in  FIG. 1 , the catenary shape of the second riser  102  may not properly form due to the buoyancy of the flexible pipe. Weight may be added to the flexible pipe to provide tension within the pipe and thereby provide stabilization in dynamic situations as described in U.S. Pat. No. 7,073,978. Pursuant to one or more embodiments of the present disclosure, weight may be added in a pre-determined wave pattern along the flexible pipe near the seabed. At the point of the pipe where the catenary may be formed, the sag bend, weights may be provided to add tension to the pipe and provide stability, as discussed above. 
         [0026]    Referring now to  FIG. 2 , a lightweight flexible pipe having distributed ballast is shown in accordance with embodiments disclosed herein. A lightweight flexible pipe  202  may be suspended from a vessel  200  to the seabed. Ballast segments  220 ,  221 ,  222 ,  223 , and  224  may be distributed on the pipe  202  in a wave pattern, where each ballast segment corresponds to a half-wave length. Each of the ballast segments  220 ,  221 ,  222 ,  223 , and  224  may be of a different weight and/or length, as pre-determined by dynamic riser analysis and/or other means known in the art. 
         [0027]    As shown in  FIG. 2 , the ballast segments  220 ,  221 ,  222 ,  223 , and  224  are separated by segments of bare pipe at the sag bend, thereby forming the wave pattern of the ballast distribution. Further, as shown, the bare pipe sections may have curvature due to the buoyancy of the pipe, and the sections of pipe with ballast may allow for a controlled curvature at the sag bend. 
         [0028]    As shown in  FIG. 2 , the ballast segments  220 ,  221 ,  222 ,  223 , and  224  are distributed in five half-wave segments. However, those skilled in the art will appreciate that there may be more or fewer half-wave segments of ballast. Further, the segments of ballast may be uniformly distributed and may be of uniform length. Moreover, the segments of ballast may be of uniform weight or may be of varying and/or different weight. The length, weight, and/or number of the ballast segments may be determined by dynamic riser analysis, as discussed above. 
         [0029]      FIG. 2  shows the buoyant pipe rising between the half-wave segments of ballast. However, a pre-determined distribution of ballast may be configured to prevent the buoyant pipe from creating the wave-like shape shown in  FIG. 2 . For example, referring to  FIG. 3 , a pipe  302  may be suspended from a vessel  300  and may be equipped with sufficient ballast such that a smooth catenary may be formed. A distribution of ballast segments  320 ,  321 ,  322 ,  323 , and  324  may be provided to form the smooth catenary. As noted above, the weight of each segment of ballast may be varied such that a desired shape of the catenary may be formed. 
         [0030]    Now referring to  FIG. 4 , a ballast distribution in accordance with one or more embodiments of the present disclosure is shown. A pipe  402  may be suspended from a vessel  400  and may be weighted with ballast segments  420 . The weight of each segment of the ballast segments  420  may be varied to form a desired catenary shape. Further, as illustrated, the ballast distribution may be created from a series of smaller ballast segments. However, as shown in  FIGS. 2 and 3 , the ballast may be formed from larger ballast segments in which a single ballast segment forms the entire half-wave section. 
         [0031]    Referring still to  FIG. 4 , a dotted line  403  is shown as representing an unweighted pipe. The unweighted pipe  403  may be affected by the buoyancy of the pipe and/or by currents in the water thereby negatively affected a desired catenary. A Solid line  402 , the pipe  402 , represents the weighted pipe, with the addition of ballast segments  420 . As shown, the pipe  402  may be weighted down with the ballast segments  420  to achieve a desired catenary shape and/or to minimize the affect of currents in the water. 
         [0032]    Now referring to  FIG. 5 , a wave distribution of ballast, in accordance with one or more embodiments of the present disclosure, is shown. Pipe  502  may be suspended subsea from a vessel (not shown) to form a catenary near a seabed and may be a flexible composite pipe. Ballast weight may be distributed on the surface of the pipe  502  to form a desired catenary, as may be predetermined by dynamic riser analysis. As shown, four half-waves of ballast are configured to produce a desired catenary shape. Wavelengths  510 ,  511 ,  512 , and  513  of pipe  502  may each be a wave of a predetermined wavelength. Accordingly, wavelengths  510 ,  511 ,  512 , and  513  of pipe  502  may each be of a uniform length and may represent a wavelength (or wave) of weighted pipe. 
         [0033]    Each of the wavelengths  510 ,  511 ,  512 , and  513  of pipe  502  may include a half-wave (or half wavelength) of ballast  520 ,  521 ,  522 , and  523 , respectively. Further, each of the segments  510 ,  511 ,  512 , and  513  of pipe  502  may include a respective half-wave (or half wavelength) of bare pipe  530 ,  531 ,  532 , and  533 , respectively, that may correspond to half-waves of ballast  520 ,  521 ,  522 , and  523 . As noted above, although four waves of ballast-bare pipe (wavelengths) are shown, those skilled in the art will appreciate that more or fewer wavelengths may be provided to achieve a desired catenary shape, depending on the environmental and/or operational parameters present. 
         [0034]    As such, ballast may be distributed along a pipe in a wave pattern with half-waves of ballast and half-waves of bare pipe to form a catenary shape near a seabed. As shown in  FIG. 5 , half-waves of ballast  520 ,  521 ,  522 , and  523  may alternate with half-waves of bare pipe  530 ,  531 ,  532 , and  533 , respectively, thereby forming wavelengths of weighted pipe. Each half-wave of bare pipe may be of uniform length and of uniform weight, as each section of bare pipe may not have any additional weight added thereto. Each half-wave of ballast may be of uniform length, but, in contrast to the bare pipe sections, may be of varying weight. The varying weight may enable proper fluid dynamic response in environmental conditions, thereby forming a stable catenary shape near the touch-down zone. 
         [0035]    Now referring to  FIG. 6 , a catenary riser in accordance with one or more embodiments of the present disclosure is shown. A pipe  602  may be suspended from a vessel  600  and may have ballast and/or buoyancy attached thereto. As shown in  FIG. 6 , the pipe  602  may have one or more sections of ballast  640  attached thereto. Further, the pipe  602  may have one or more sections of buoyancy modules  650  attached thereto. Accordingly, a desired catenary shape may be formed by distributing combinations of both weighted ballast and buoyancy modules. As such, the dynamic wave forms of the pipe  602  may be configured to best suit a particular environment. 
         [0036]    As shown in  FIG. 6 , the sections of ballast  640  and sections of buoyancy modules  650  may be distributed in a wave pattern as discussed above. The sections  640  and  650  may each be half-wave sections with half-wave sections of bare pipe between adjacent sections of ballast and/or buoyancy modules. 
         [0037]    Advantageously, pursuant to one or more embodiments of the present disclosure, bending waves and fluid dynamics that may adversely affect a subsea catenary may be controlled, prevented, and/or minimized by application of half-wave segments of ballast distributed on the subsea catenary. Accordingly, a low cost means of providing a subsea riser with lightweight flexible pipe may be provided. 
         [0038]    Moreover, in accordance with one or more embodiments of the present disclosure, deep sea risers may be provided with lightweight flexible pipe. Ballast distributions, as disclosed herein, may allow for an alternative to conventional pipe. Accordingly, installation of buoyancy on a conventional pipe may be avoided and a simple and reliable catenary may be provided, thereby allowing for lower installed cost of the riser. 
         [0039]    Moreover, in accordance with one or more embodiments of the present disclosure, the riser may not necessarily be a lightweight flexible pipe. The pipe may be a steel armored unbonded flexible pipe as described in ISO 13628-2/API Specification 17J. For a heavier flexible pipe, installation of alternating ballast and buoyancy may be applied to the pipe to achieve either half waves or full waves in the ballast/buoyancy region. Alternatively, buoyancy modules may be applied to the lightweight flexible pipe to achieve partial waves that are less than half waves, to achieve full waves, or to increase the amplitude of the waves. The wave configuration may be optimized through optimization of the spacing of the ballast and/or buoyancy, which may form non-uniform spacing, creating lumping or grouping of ballast and/or buoyancy modules. Further, continuous mass sections may be distributed along the riser so as to minimize or eliminate pipe wall compression near the touch-down point. 
         [0040]    Moreover, in accordance with one or more embodiments of the present disclosure, uniformly distributed ballast may provide on-bottom stability to the catenary. Additionally, near and far off-sets may be accommodated with flexible pipe and distributed ballast, as disclosed herein. 
         [0041]    While the disclosure has been presented with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope of the invention should be limited only by the attached claims.