Abstract:
A cable for monitoring a tubular structure. The cable comprising a fiber optic bundle arranged for simultaneously sensing a plurality of parameters along a length of the tubular structure that the cable is interfaced with.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       TECHNICAL FIELD 
       [0002]    The present invention generally relates to subsea asset monitoring and in particular, but not by way of limitation to a composite monitoring fiber optic cable for monitoring subsea assets, such as subsea tubular structures: umbilicals, risers (including rigid, semi-rigid and flexible risers), flowlines, and offload lines. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the oil and gas industry there are certain installations in which various connections and conduits are necessary. In the subsea domain, for example, subsea hydrocarbon production systems using sea surface facilities of any sort require petroleum fluids to flow from the seabed to the surface through various connections and conduits. Such connections and conduits, also referred to herein as tubular structures or pipe structures, may include any combination of rigid, semi-flexible or flexible risers, flowlines, flexible pipes and umbilicals. 
         [0004]    Due to the various forces acting on these tubular structures, for example as the sea surface rises and falls with waves and tides and the facilities are moved vertically, laterally and rotationally, such structures are subject to structural failure due to fatigue. Additional damage may occur in the form of corrosion, erosion, or blockage which can be caused by the interior deposition of one or more of the flow components (such as wax, hydrates, asphaltenes, scales, etc.). 
         [0005]    In the surface operating domain, whether onshore or offshore for example, pipe structures may experience structural failure due to periodic installation and disassembly, or jarring from high pressure fluid flow. 
         [0006]    Accordingly, there is a need to monitor the condition of such pipe structures for effective operation so that remedial measures may be taken before such structural failure occurs. 
         [0007]    WO2009087371 discloses a monitoring system for use in floating production installations such as those used in offshore oil and gas production. The disclosure teaches a continuous optical fiber distributed sensor installed as part of a flexible (or partially flexible) pipeline, wherein the sensor provides a distributed measurement of temperature and/or strain. 
       SUMMARY OF THE INVENTION 
       [0008]    According to a first aspect of the invention there is provided a cable for monitoring tubular structures including subsea pipelines, umbilicals, risers and flowlines. The cable may comprise a fiber optic bundle arranged for simultaneously (or near simultaneously) sensing temperature, vibration and strain along a length of the monitored structure in which the cable is interfaced. Such cable may be used to provide distributed measurements (for example, using OTDR techniques) and discrete, localized measurements (for example, using FBG techniques). 
         [0009]    According to a further aspect of the invention the cable may comprise: a housing encapsulating a central fiber optic bundle and a first and a second tube; the central fiber optic bundle extends axially along a central-axis of the cable in between the first and the second tube extending axially and substantially adjacent to a respective side of the cable. The first and second tubes preferably have a fiber optic sensor located in each. 
         [0010]    According to yet a further aspect of the invention the cable may comprise a central tube having a loosely-fitted fiber optic; a fiber optic bundle stranded around and fixedly-fitted to an outside surface of the central tube; and a housing that surrounds both the central tube and the fiber optic bundle so as to form a substantially circular cross-section. 
         [0011]    According to another aspect of the invention there is provided a cable for monitoring a tubular structure, the cable may comprise: a first and a second tube, each of the tubes having a loosely-fitted fiber optic located therein and a fiber optic bundle fixedly connected around an outside surface of each tube; and a housing for surrounding both the first and the second tubes, the housing having a surface shaped to fit substantially flush with the monitored asset. 
         [0012]    Still other aspects, features, and advantages of the present invention are readily apparent from the entire description thereof, including the figures, which illustrates a number of exemplary embodiments and implementations. The present invention is also capable of other and different embodiments, and its several details can be modified in various respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
           [0014]      FIG. 1  shows a substantially flat cable arrangement according to an embodiment of the invention; 
           [0015]      FIG. 2  shows a circular cable arrangement according to an alternative embodiment of the invention; and 
           [0016]      FIG. 3  shows a substantially flat cable arrangement according to yet another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Subsea Umbilical, Risers and Flowlines (SURF) are used for communication, control, power and fluid conveyance in the production of hydrocarbons from beneath the seabed. Embodiments of the invention are concerned with fiber optic based monitoring cables for operational and integrity management of SURF assets, but may be expanded to include monitoring of a variety of onshore and offshore pipe structures as will be understood by those skilled in the art. For purposes of conciseness, however, embodiments of the present invention will primarily be described as related to monitoring SURF assets. 
         [0018]    One example of a SURF asset includes the umbilical. Umbilicals are typically complex pipe structures which may include electrical, optical and hydraulic pipes or cables which allow for power, data communication, control and fluid injection between the surface and the various subsea installations. 
         [0019]    Another example of a SURF asset includes the riser and flowline. Risers and flowlines typically also use complex pipe structures primarily for carrying hydrocarbons in a subsea environment. Flexible risers for example often operate in harsh conditions where environmental loading can cause fatigue and thus it is desirable to having a strain sensing system in place. Many different types of risers and flowlines may be used including flexible, semi-rigid or rigid risers and flowlines. For example rigid flowlines are often times used, which for example take the form of an inner metallic pipe surrounded by an insulating polymer layer, which in turn might also be coated with cement. 
         [0020]    Monitoring SURF assets, which are typically long slender structures, often requires measurements at regular intervals along the complete length or partial length of the structure. The monitoring systems are typically tied-back to the surface production platform, enabling real-time access to the monitoring data, or remotely located for periodic retrieval of the monitoring data. 
         [0021]    The cable, or cables, conceived by the present invention is preferably equipped with fiber optic sensors for monitoring strain, temperature and acoustic vibration along the length, or a portion of the length, of the SURF structure. The cables effectively bundle comprehensive monitoring capability within a single package and simplify installation of these systems in SURF assets. The cable shape is readily manufactured for ease of integration with the SURF structure. The monitoring cable may be either embedded within the SURF asset or externally attached. 
         [0022]    Fiber optic sensing technologies to be used include discrete, point measurements along an array of Fiber Bragg Gratings (FBG) and distributed measurements based on numerous optical time domain reflectometry (OTDR) techniques (e.g. Rayleigh, Brillouin, Raman and coherent Rayleigh). 
         [0023]    Fiber Bragg Gratings (FBG), a well established technology, reflects particular wavelengths of light, while transmitting others, which is achieved by adding a periodic variation to the refractive index of the fiber core. Fiber Bragg gratings can be used for localized/semi-distributed strain measurement, acting as direct strain gauges. A large number of gratings can be multiplexed on a single optical fiber. Several systems for interrogating FBGs have been proposed, and will be well know to the person skilled in the art. Based on current technology, it is believed to be possible to interrogate up to a few hundred FBG sensors on a single optical fiber. Typical measurement time is around 1 s. 
         [0024]    Optical time domain reflectometry (OTDR) is a well-known technique for measuring the distribution of a number of parameters of an optical fiber, such as attenuation, core diameter, numerical aperture, and even chromatic dispersion. When a narrow-band source is used in an optical time domain reflectometer to interrogate an optical fiber for example, Rayleigh backscattered light is produced in response to an interrogating pulse launched into the fiber. In essence, the interrogating pulse can be thought of as occupying a certain length of the fiber and, assuming that the pulse is coherent, all the electric dipoles arising from the non-homogeneity of the glass have a fixed (though random) phase relationship to one another. The resulting backscatter signal for a particular section of the fiber is then treated as the coherent sum of all the electric fields of these dipoles. This sum is, of course, dependent on the phase as well as the amplitudes of each dipole. For a fixed optical source frequency and a fixed state of the fiber (i.e., a fixed temperature, strain, etc.), the backscatter return (relative to the pulse energy) from a particular location is fixed, but randomly related to the backscatter return from any other section of fiber. 
         [0025]    Application of these sensing technologies on SURF structures preferably includes robust interface packaging of the fiber optic while simultaneously ensuring integrity of the sensor measurement. Traditionally, FBG-based SURF monitoring systems have embedded the fiber optic sensor package within clamps, collars, strips and rods. Distributed fiber optic measurements are less established with SURF applications and more commonly deployed in downhole (subsurface) temperature measurement where stainless steel tubing provides the main structural protection. Temperature monitoring of flowlines and ancillary thermal management systems is an emerging application where the fiber optic is also deployed in a stainless steel tube. 
         [0026]    The basic parameters to be preferably measured include: strain, temperature and acoustic vibration. Traditionally, the data acquired from these separate sensors is processed and interpreted for conditions that provide valuable information for operational and integrity management of SURF assets. Examples include: tensile and compressive loading, bending and buckling, feedback for flow assurance operations (thermal monitoring, liquid slugs, sand production, leaks), alarms for classified thresholds, correlation of in-service performance with design predictions and ongoing integrity assessments with in-service measurement data. 
         [0027]    Strain sensing requires that the fiber optic is closely coupled to the structural deformation of the SURF asset. Temperature and acoustic measurements can also be performed with strain-coupled fiber optics, provided a strain-insensitive measurement technique is used. For example, Raman-based OTDR provides a strain-insensitive temperature measurement. Temperature and acoustic measurement can also be acquired from strain-free fiber optic in loose filled tubes. 
         [0028]    Fiber optic based OTDR and FBG sensors are well suited for monitoring the complete, or partial, length of SURF assets. Thus, according to a preferred embodiment of the invention, the various distributed optical fiber sensors are bundled into a single package for interfacing with the monitored SURF asset. Specifically, the package may be a cable having a bundle of various fiber optic sensor arrangements, which easily attach to and interfaces with the SURF asset for monitoring. Because the various sensor arrangements are all bundled into a single cable arrangement, the package is able to simultaneously perform strain, temperature and acoustic monitoring of the SURF asset. It should be appreciated for example, that the different sensory sub-assemblies within the package may utilize FBG and OTDR technologies. 
         [0029]      FIG. 1  shows a cross-sectional view of a monitoring cable  10  according to an embodiment of the present invention. Shown therein, a strain sensing fiber optic assembly  11  packaged in a central tube and substantially aligned along a central-axis member  19  of the cable  10 . The strain sensing fiber optic assembly  11  comprises a bundle of fibers  18  closely wrapped together around the central-axis member  19 . In the embodiment of  FIG. 1 , eight (8) fibers  18  are shown arranged around the central-axis member  19 . However, any number of fibers  18  may be arranged around the central-axis member so as to provide a useful strain measurement. The strain sensing fiber optic assembly  11  of the present embodiment is shown to further comprise a layer  15  made of a gel, such as silicone, that surrounds the bundle of fibers  18  as a protective layer between the fibers  18  and the central-axis member  19 . An additional layer  9  may be positioned between layer  15  and the outer diameter of the central tube of the strain sensing fiber optic assembly  11 . 
         [0030]    The monitoring cable further comprises respective a distributed temperature fiber optic sensor  16  and an acoustic vibration fiber optic sensor  17  located inside tubes  12  and  13  on either side of the strain sensing fiber optic assembly  11 , and near the sides of the monitoring cable  10 . The tubes  12  and  13  may be made of a metallic material such as stainless steel or Inconel and may be designed to add strength to the cable  10  for withstanding a certain amount of tension placed on the SURF asset. The strain sensing fiber optic assembly  11  and tubes  12  and  13  are preferably encapsulated within a housing made of a thermoplastic material  14 , such as polyethylene, to protect the cable  10  from environmental damage. Many different thermoplastic materials could be used provided that the cable  10  can withstand the harsh environmental conditions. The encapsulation is typically made using well-known techniques such as pressure extrusion so that the thermoplastic material can fill substantially all available space  14  between the tubes  12 ,  13  and the strain sensing fiber optic assembly  11 . 
         [0031]    The fiber optics used (i.e.  16 ,  17  and  18 ) may be any suitable fiber optics for sensing temperature, vibration and strain, respectively. In the embodiment of  FIG. 1 , fibers  18  can be of the single or multimode type and are preferably designed to sense strain along the length of the cable. Fiber  16  can be of the single or multimode type and is preferably designed to sense temperature along the length of the cable  10 . Likewise fiber  17  can be of the single or multimode type and is preferably designed to sense vibration along the cable  10 . 
         [0032]    The monitoring cable  10  preferably has a generally rectangular cross section with flat or slightly curved sides  5  and  6  that sit substantially flush when held firmly against a component surface of the SURF asset, e.g. pipe, tube, sheath or flat-sided armor wires. The flush interface preferably increases measurement accuracy by allowing for effective strain, temperature and vibration transfer to the monitoring cable  10 , and thus the fibers  16 ,  17  and  18  located within the cable  10 . 
         [0033]    The cable  10  can either be firmly sandwiched within a multilayer SURF construction, or attached such as for example by strapping it or bonding it to the external surface of SURF asset. The flat or slightly curved sides  5  and  6  of the cable facilitate interfacing the cable with SURF constructions. Two or more runs of the cable  10  could be used along the SURF asset to increase the level of contingency or redundancy of the encapsulated fiber optic sensors. Moreover, by using three or more runs as for example separated by 120 or 90 deg can allow the bending or buckling events to be resolved in two planes. 
         [0034]      FIG. 2  shows a cross-sectional view of a monitoring cable  20  according to another embodiment of the present invention. In this embodiment, the cable  20  is shown to have a substantially circular shape. The cable  20  is configured to be preferably helically stranded around or within a SURF structure, or installed along the central axis of a SURF asset, such as an umbilical. The external surface of the cable should be circumferentially supported against adjacent components within the SURF structure for effective strain coupling. These supporting components may, for example, include shaped void fillers in the case of umbilical. 
         [0035]    Monitoring cable  20  may comprise a central tube  26  made preferably from a metallic material for providing strength. A loose-fit fiber optic  28  is located therein for performing temperature and/or acoustic monitoring. Alternatively, instead of a single optical fiber  28 , two optical fibers (not shown) could be used: one for performing temperature and one for performing acoustic monitoring. As previously mentioned in brief, the tube  26  may also act as a strength member for handling operations before it is embedded within a SURF structure. An additional layer  23  made of metallic material may be disposed around the tube  26  to impart additional tensile strength. 
         [0036]    Fiber optics  24  may be helically stranded around, and anchored to, the outside of the tube  26  for strain measurement. The embodiment of  FIG. 2  can include strain-coupled optical fibers  24  as well as strain-free optical fiber(s)  28 . The optical fibers  24  are preferably encapsulated by a layer  29  made of silicone or the like for protection. Optionally a tight-buffer jacketing (not shown) made preferably of thermoplastic material may surround each fiber  24 . The cable  20  is preferably jacketed with a thermoplastic housing material  30 , which may further have optional metallic or non-metallic stranded armoring. 
         [0037]      FIG. 3  shows a cross-sectional view of a cable  60  according to yet another embodiment of the present invention, which combines certain features from the cables  10  and  20  shown in  FIGS. 1 and 2 , respectively. In general, the cable  60  comprises at least one of the cable  20  of  FIG. 2  embedded within a flat housing similar to that of  FIG. 1 . Such arrangement potentially increases measurement accuracy by obtaining the flush-fit shape with, or external to, the SURF asset. 
         [0038]    In one implementation of the embodiment shown in  FIG. 3 , a fiber optic bundle  36  can provide temperature and strain monitoring while another fiber optic bundle  38  can provide vibration and strain monitoring. Alternatively, in another implementation of the embodiment shown in  FIG. 3 , the fiber optic bundles  36  and  38  within the cable  60  may provide the exact same monitoring for contingency or redundancy purposes. 
         [0039]    Tubes  48  and  56 , within each bundle  38  and  36 , are preferably made of a metallic material for providing tensile strength. The tubes  48  and  56  of each bundle arrangement  38  and  36  may accommodate loose-filled fiber optics  46  and  58  for strain-free temperature and/or acoustic sensing. An outer layer of helically stranded optical fibers  40  and  54  of each arrangement  38  and  36 , are preferably strain coupled to the tubes  48  and  56  for strain measurement. 
         [0040]    The cable  60  is preferably encapsulated with a thermoplastic material (e.g. polyethylene) that is compatible with the environment, and protects the bundles  36  and  38  from damage. The encapsulation may be pressure extruded so that thermoplastic fills all available space within the cross-section. The cable  60  may incorporate all types of fiber-optic based FBG and OTDR sensing systems applicable to SURF assets. The cable  60  can either be firmly sandwiched within a multilayer SURF structure or attached (e.g. strapped or bonded) to the external surface (not shown). The flat or slightly curved sides of the cable facilitate interfacing the cable with SURF structure. Two or more runs of the cable along the SURF asset may further increase the level of contingency or redundancy of the encapsulated fiber optic sensors. Three or more runs (e.g. separated by 120 or 90 deg) allow the bending or buckling events to be resolved is two planes. 
         [0041]    While the present invention has been described in connection with a number of exemplary embodiments, and implementations, the present invention is not so limited, but rather covers various modifications, and equivalent arrangements, which fall within the purview of the appended claims.