Abstract:
A method, device, and system for determining whether liquid enters a subsea system includes a flow line having an inside bore capable of carrying producing fluids from a subsea wellhead to a floating production storage and offloading vessel (FPSO). An annulus is defined between an innermost sheath that surrounds the inside bore and an outermost sheath at an exterior of the flow line. The device further includes a housing element disposed within the annulus and in hydraulic communication with the annulus; a sensor element disposed at a predetermined location within the housing element; and data acquisition equipment capable of: determining environmental characteristics of the annulus based on measurements made by the sensor element, and determining at least one of the presence, location, and rate of collection of liquids in the subsea system based on the environmental characteristics of the annulus.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application, pursuant to 35 U.S.C. §119(e), claims priority to U.S. Provisional Application Ser. No. 61/099,573, filed Sep. 24, 2009. That application is incorporated by reference in its entirety. 
    
    
     FIELD OF DISCLOSURE 
     Embodiments disclosed herein generally relate to a device, method, and system for determining the presence of liquids in the annulus of a flexible riser or flow line in a subsea system. 
     BACKGROUND 
     Subsea production for many oil companies is projected to increase significantly in the next 5-10 years. In addition, offshore fields are being exploited in deeper and deeper waters. However, producing from floating production, storage, and offloading vessel (FPSO) presents many challenges, which increase as the water depth increases. 
     Referring to  FIG. 1 , produced fluids are often carried from a wellhead  10  on the seabed  20  to an FPSO  30  through flow lines such as flexible risers  100 . Additionally, a riser clamp  40  and tethers  50  may be employed to retain the end of the flexible riser  100  to the seabed  20 , while buoyancy modules  60  may be connected to various positions along the flexible riser  100  and a bend stiffener  80  may be included at the connection to FPSO  30  so as to allow for surface movement of the FPSO  30  and movements due to wave action without putting undue stress upon the flexible riser  100 . As labeled in  FIG. 1 , the bends created in the flexible riser  100  by the buoyancy modules  60  are commonly referred to as “hog bends” at high points and “sag bends” at low points. 
     Flexible risers  100  bring many advantages allowing produced fluids to flow from the fixed seabed wellhead  10  to the FPSO  30 , which will move with tidal and wave action. Additionally, flexible risers  100  may be manufactured in long continuous lengths that allow for a simpler and more efficient installation. The use of flexible risers  100  is well documented and known to one of ordinary skill in the art. 
     Referring to  FIG. 2 , a typical flexible structure  100  consists of many layers, each of which plays a different role from providing structural strength to providing isolation between the inside bore  102 , which carries producing fluids, from the outside sea water. The steel reinforcing layers (armours  108  and pressure vault  106 ) are contained within a very confined environment called the annulus, which is located between the innermost sheath that surrounds the inside bore  102 , such as inner polymer sheath  104  and the outermost sheath that contacts the sea water, such as external polymer sheath  110 . The inner polymer sheath  104  is the barrier to the conveyed production fluids and the external polymer sheath protects  110  against the seawater environment. 
     If water is present in the annulus, then the longer term integrity of the flexible riser  100  may be compromised due to corrosion. It should be noted that although the inner sheath  104  and outer sheath  110  are impermeable, under high temperature and pressure conditions small amounts of gases can permeate through the inner sheath  104 . Corrosive gases are often present in production fluids (e.g. H 2 S, CO 2 , and water vapor), plus hydrocarbons such as CH 4 , and can diffuse through the inner sheath and accumulate in the annular space. This results in a corrosive environment in contact with the carbon steel members, which can significantly reduce the life of the flexible riser  100 . 
     In addition, it is possible that the outer sheath  110  may be damaged during installation, which can allow a slow ingress of seawater over time. If the outer sheath  110  becomes seriously breached, the annulus may become flooded. Also, a slow diffusion of water through the outer sheath  110  is also possible. In all such cases, water enters the annulus causing corrosion of the steel wire structures, which can result in premature failure of the flexible riser  100 . 
     The failure of a flexible riser  100  can be very costly due to lost production and associated installation services, which may vary widely depending on the availability of such services. Also, the failure may result in catastrophic damage to the environment. However, if failures are detected early and monitored, repair or replacement can be scheduled in order to significantly reduce the risk of environmental damage and minimize the down-time of production. 
     Determination of the presence of liquid in the annulus is presently achieved by periodically monitoring the vented gas flow rate from the annulus and vacuum testing. If water collection occurs, then the gas displaced thereby is vented at surface. However, this approach is not very accurate and small amounts of water intrusion are difficult to detect. In addition, flexible risers  100  are often installed with a buoyancy modules  22  in the middle and, if water collects in the low lying section around a sag bend, gas may become trapped and not vent at the surface. As a result, pressure within the annulus may increase to the point where the outer sheath  110  is ruptured, thereby causing catastrophic failure. 
     Other approaches to measure the pressure in an annulus have been developed. See, for example, EP 1492936B1 (hereinafter “Technip”). Technip describes a method and device that measures the pressure in the annulus at the bottom of the flexible riser along with the hydrostatic pressure of the seawater column and compares the difference between the two measurements. This approach requires that specific measurements both inside the annulus and outside the flexible riser are made. Also, the method requires a special measurement component at the end of the flexible riser and requires electrical wiring to run to surface with connections between this wiring and the measurement sub. Also, this method and device would not likely be able to identify water collection in low lying areas around a sag bend. 
     Another existing method involves periodically pulling a vacuum on the vent lines at the surface that connect to the annulus. The degree to which a vacuum can be held is used to give indication of any leaks in the inner or outer sheaths. In practice, this method is generally recognized as difficult to control and not very reliable. 
     SUMMARY OF DISCLOSURE 
     In one aspect, one or more embodiments of the present invention relate to a device for determining whether liquid enters a subsea system, the device comprising: a flow line comprising an inside bore capable of carrying producing fluids from a subsea wellhead to a floating production storage and offloading vessel (FPSO), wherein an annulus is defined between an innermost sheath that surrounds the inside bore and an outermost sheath at an exterior of the flow line; a housing element disposed within the annulus and in hydraulic communication with the annulus; a sensor element disposed at a pre-determined location within the housing element; and data acquisition equipment capable of: determining environmental characteristics of the annulus based on measurements made by the sensor element, and determining at least one of the presence, location, and rate of collection of liquids in the subsea system based on the environmental characteristics of the annulus. 
     In one aspect, one or more embodiments of the present invention relate to a method for determining whether liquid enters a subsea system comprising a flow line having an inside bore capable of carrying producing fluids from a subsea wellhead to a floating production storage and offloading vessel (FPSO), wherein an annulus is defined between an innermost sheath that surrounds the inside bore and an outermost sheath at the exterior of the flow line, the method comprising: disposing a housing element within the annulus and in hydraulic communication with the annulus; disposing a sensor element at a pre-determined location within the housing element; determining environmental characteristics of the annulus based on measurements made by the sensor element; and determining at least one of the presence, location, and rate of collection of liquids in the subsea system based on the environmental characteristics of the annulus. 
     In one aspect, one or more embodiments of the present invention relate to a system for determining whether liquid enters a subsea system, the system comprising: a flow line coupled from a subsea wellhead to a floating production storage and offloading vessel (FPSO), wherein the flow line comprising an inside bore capable of carrying producing fluids from a subsea wellhead to FPSO, wherein an annulus defined between an innermost sheath that surrounds the inside bore and an outermost sheath at the exterior of the flow line; a plurality of open-ended tubes disposed within the annulus and in hydraulic communication with the annulus; wherein the plurality of open-ended tubes have different lengths; a plurality of fiber optic pressure sensors disposed at a pre-determined locations within the plurality of open-ended tubes, wherein at least one fiber optic pressure sensor is disposed in each of the plurality of open-ended tubes; a plurality of connecting fibers disposed within the plurality of open-ended tubes and spliced to the fiber optic pressure sensors; and data acquisition equipment coupled to the plurality of connecting fibers, wherein the data acquisition equipment is capable of: determining environmental characteristics of the annulus based on measurements made by the sensor element, and determining at least one of the presence, location, and rate of collection of liquids in the subsea system based on the environmental characteristics of the annulus. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a typical subsea production system using a flexible riser. 
         FIG. 2  shows a cut-away view of a conventional flexible riser. 
         FIG. 3  shows a deployed flexible riser in accordance with one or more embodiments of the present invention. 
         FIG. 4  shows a system having a three sensor configuration in accordance with one or more embodiments of the present invention. 
         FIG. 5  shows water collection in the annulus of a flexible riser in accordance with one or more embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Specific embodiments of the present disclosure will now be described in detail with reference to the accompanying Figures. Like elements in the various figures may be denoted by like reference numerals for consistency. 
     Further, in the following detailed description of embodiments of the present invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the embodiments disclosed herein may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description. 
     Referring to  FIG. 3 , in one aspect, embodiments disclosed herein generally relate to a system for determining the presence of liquids in the annulus of a flow line such as flexible riser  200  in a subsea system  210 . One or more embodiments use specific measurements by sensor elements, such as optical interrogation of fibers, to determine environmental characteristics, such as pressure, at one or more locations within the flexible riser  200 . The system involves a subsea wellhead  10  on the seabed  20  coupled to an FPSO  30  through flow lines such as flexible riser  200 . Additionally, a riser clamp  40  and tethers  50  may be employed to retain the end of the flexible riser  200  to the seabed  20 , while buoyancy modules  60  may be connected to various positions along the flexible riser  200  and a bend stiffener  80  may be included at the connection to FPSO  30  so as to allow for surface movement of the FPSO  30  and movements due to wave action without putting undue stress upon the flexible riser  200 . 
       FIG. 4  shows a schematic representation of a system  210  according to one or more embodiments of the present invention. The system  210  employs a flexible riser  200  having sensor elements located at various locations along the length of flexible riser  200 , for example, at positions  230 ,  240 , and  250 . Multiple housing elements, such as open-ended tubes  220 , are placed in the annulus of the flexible riser  200 . As shown in the enlarged inset of  FIG. 4 , each of the open-ended tubes  220 , which are wound in the annulus of the flexible riser  200 , is provided with a sensor element, such as a fiber optic pressure sensor  270 , placed close to the open-end. The multiple open-ended tubes  220  are of different lengths so as to run within the flexible riser  200  to the pre-determined locations  230 ,  240 , and  250 . Thus, if water enters into the annulus of the flexible riser  200 , the water will collect at the low points  240  and  250 , where the ends of open-ended tubes  220  are located. 
     In one or more embodiments, the fiber optic pressure sensors  270  may be spliced to a connecting fiber  280  that runs from data acquisition equipment, such as surface instrumentation  290  on the FPSO  30 . The connecting fiber  280  may be interrogated from the surface using the surface instrumentation  290  to give any or all of the following measurements: the pressure at the end of the fiber, distributed temperature along the fiber from surface, and the optical attenuation along the fiber, which can give an indication of hydrogen presence in the annulus and water ingression. 
     The use of fiber optic sensors to measure hydrogen in an annulus has been described in U.S. Patent Application Ser. No. 60/989,688, filed on Nov. 21, 2007, assigned to the same assignee as the present application, and the entirety of which is incorporated herein by reference. Also, the use of optical fiber pressure sensors for measuring water ingression is described in U.S. Patent Application Ser. No. 60/990,147, filed on Nov. 26, 2007, assigned to the same assignee as the present application, and the entirety of which is also hereby incorporated by reference. 
     In one or more embodiments, at least one fiber optic pressure sensor  270 , which is spliced to a connecting fiber  280  and embedded in the annulus of the flexible riser  200 , may be a side-hole polarimetric fiber sensor that creates birefringence and provides measurement resolution of less than about 0.01 PSI (pounds per square inch). In one or more embodiments, other types of fiber optic sensors  270  may be used, such as those using a fiber Bragg grating as described in Yamate, T, Ramos, R. T., Schroeder, R. J., Udd, E, “Thermally Insensitive Pressure Measurements up to 300 C Using Fiber Bragg Gratings Written onto Side Hole Single Mode Fiber,” 14th International Conference on Optical Fiber Sensors OFS—Venice, Italy, 11-13 Oct. 2000, the entire contents of which is incorporated herein by reference. 
     In one or more embodiments, the phenomena known as “pressure induced birefringence” may be used to measure the pressure that a “side-hole” fiber experiences. A side-hole fiber is a section of an optical fiber having two holes offset from the center of the fiber and running along the length of the sensing element. The principle is described in detail in Clowes, J. R. and Zervas, M. N, “Pressure Sensitivity of Side-Hole Optical Fibre Sensors,” IEEE Photonics Technology Letters, Vol. 10, No. 6, June 1998 and in Clowes, J. R, McInnes, J., Zervas, M. N. and Payne, D. N, “Effects of High Temperature and Pressure on Silica Optical Fibre Sensors,” 12th International Conference on Optical Fibre Sensors, 1997 OSA Technical Digest Series, Vol. 16, Optical Society of America, both of which are incorporated by reference in their entirety. 
     The connecting fiber  280  with the fiber optic pressure sensor  270  spliced on its end may be contained within the open-ended tube  220  that is installed in the annulus of the flexible riser  200  during the manufacturing thereof. The process of installing such a tube in the annulus of a flexible riser is known in the art. It should be noted that the housing elements may be open-ended tubes  220  as shown, or alternatively, the housing elements may have slots or openings so as to allow hydraulic communication with the annular space. Therefore, if water collects in the annulus to a level above the open-end of the tube  220 , (or the slots or openings in alternate housing elements), then the level of water within the housing element will be the same as in the annulus. The fiber optic pressure sensors  270  may measure the hydrostatic head of water above the measurement point. This measurement provides an indication of water in the annulus. In addition, monitoring changes in this pressure with time may give an indication of the rate of water collection. 
     Because embodiments of the present invention use of optical fiber pressure sensors, the overall device or instrument may have a very small diameter, preferably less than about 1 millimeter (mm) The small diameter allows the fiber optic to be easily fit within any tube located in the annular space. As discussed above, the fiber optic pressure sensor  270  comprises a section of fiber spliced to a connecting optical fiber  280 , which passes through the flexible riser  200 . The connecting fiber  280  may be interrogated by surface instrumentation  290  to determine distributed temperature measurements. These measurements may be used to identify so-called “cold spots” in which seawater has diffused into the annulus. Thus, a location indication, which may facilitate repair of the flexible riser, may be obtained. In addition, interrogating the same optical fiber may also provide an indication of the presence of hydrogen in the annulus. 
     Optical interrogation of fibers is a technology that has been available for many years and currently has several commercial applications. For example, distributed temperature sensing (DTS) makes use of the Raman backscattered Stokes and anti-Stokes wavelengths in the optical fiber. DTS can provide a distributed temperature measurement along the fiber. This technique has been used in many applications, such as fire detection, power line monitoring, downhole oilfield applications, and measuring temperature variations along the side of a flexible riser on a subsea platform offshore. 
     One or more embodiments may include the use of at least one open-ended tube  220  with a connecting fiber  280  and a fiber optic pressure sensor  270  deployed therein. As discussed above, the open-ended tube  220  can be wrapped within the armor layers of the flexible riser  200  during the manufacture thereof using well known techniques such as those described in: Felix-Henry, A. “Prevention and Monitoring of Fatigue-corrosion of Flexible Risers&#39; Steel Reinforcements” OMAE2007-29186, Proceeding of the 26th International Conference on Offshore Mechanics and Arctic Engineering, June 10-15, 2007, San Diego, Calif., USA, the entirety of which is incorporated by reference. 
     Referring to  FIG. 5 , the operation of the system will be discussed with reference to a schematic of a section of the flexible riser  200  in accordance with one or more embodiments of the present invention. As water level  300  rises in the annular space  260  and into the open-ended tube  220 , the fiber optic pressure sensor  270 , which is spliced to the connecting fiber  280  in the open-ended tube  220 , measures the increase in pressure due to the hydrostatic head H of water above the sensor  270 . Thereby, using surface instrumentation  290  that interrogates the fiber optic pressure sensor  270 , the level, or the rate of rise, of the water  300  in the annulus 
     As discussed above, possible methods of obtaining the connecting fiber  280  with the fiber optic pressure sensor  270  spliced on the end include, but are not limited to, deploying the fiber within the open-ended tube  220  during manufacturing of the flexible riser  200 . In such a case, the connecting fiber  280  may be placed inside the open-ended tube  220  using well established techniques in the industry. There are many ways to achieve this and there exist several companies that provide small tubing with fiber already deployed inside, for example, the commercially available SENSATUBE. The open-ended tube  220  may then be wrapped with wire supporting members during manufacture of the flexible riser  200  in accordance with known techniques. 
     It is also possible to place multiple connecting fibers  280 , each with a fiber optic pressure sensor  270  spliced on the end, within the one or more open-ended tubes  220  before incorporation within the flexible riser  200 . These connecting fibers  280  may be of different lengths so that pressure is measured at multiple points along the flexible riser  200 , preferably including any sag bends in the flexible riser  200 . 
     It may be necessary to provide hydraulic connectivity between the inside of the housing element and the annulus in the proximity of each sensor element, depending on how the open-ended tube  220  is incorporated into the annulus. This may be achieved by placing small holes or slots in the open-ended tube  220 . In one or more embodiments, one such pressure measurement would be made close to surface giving the surface annular pressure and other sensors would be placed at low lying points, such as at the bottom of the flexible riser  200  near the seabed  20  and at the bottom of any sag bends. 
     As stated above, in accordance with one or more embodiments, any number of housing elements may be deployed in the flexible structure each running to different lengths within the flexible riser, each open-ended to provide hydraulic communication with the annular space and each with connecting fiber and a fiber pressure sensor close to the open-end. This may provide pressure measurements at multiple locations within the flexible. While a three sensor system was shown, those of ordinary skill in the art will appreciate there may be more or less sensors, depending on the deployment configuration of the flexible riser  200 . 
     In accordance with one or embodiments, another methodology of deployment of the connecting fiber  280  with fiber optic pressure sensor  270  spliced on the end is to incorporate the open-ended tube (or tubes)  220  within the flexible annulus structure  260  during manufacture, but without including the connecting fiber  280  and fiber optic pressure sensor  270 . The connecting fiber  280  with the fiber optic pressure sensor  270  spliced on the end may then be pumped into the open-ended tube (or tubes)  220 . The deployment methodology is well known in the art. Additionally, the fluid used to pump the connecting fiber  280  with the fiber optic pressure sensor  270  spliced on the end into the open-ended tube  220  may be a corrosion inhibitor. 
     Once the connecting fibers  280  with fiber optic pressure sensors  270  are installed, the surface end of the connecting fiber  280  may be connected to various different interrogation equipment, such as surface instrumentation  290  on the FPSO  30 , which may provide real-time, or periodic, pressure measurements at each fiber optic pressure sensor  270 . Changes in the pressure measurements with time may be used to determine whether water is collecting at various points in the annulus of the flexible riser  200 . Additionally, the pressure measurements may also be used to determine the change in the water level  300  with time. For example, a sudden increase in water level will give an indication of a flooding of the annulus, whereas a slow increase in level may be an indication of a small leak or condensation. 
     In addition, if a fiber optic pressure sensor  270  located in a sag bend is showing water collection, but another fiber optic pressure sensor  270  located at the bottom of the flexible riser  200  shows no presence of water, then it can be deduced that a leak is occurring above the sag bend. By comparing the changes in pressure at different points within the flexible riser  200 , it may be possible to better diagnose the position and cause of the leak in the inner sheath  104  or outer sheath  110 . 
     One or more of the connecting fibers  280 , which connect the fiber optic pressure sensor  270  to data acquisition equipment, such as surface instrumentation  290 , may also be interrogated for DTS, according to known techniques. This may provide a continuous real-time log of temperature along the annulus of the flexible riser  200 . If there is a breach of the outer sheath  110 , then it would be expected that a “cold spot” is detected by the fiber optic pressure sensor  270  where the cold seawater enters the annulus of the flexible riser  200 . This cold spot may give an indication of the location of the leak and facilitate the repair thereof. 
     One or more embodiments of the present invention may provide one or more of the following advantages. First, the tube and fiber optic configurations may be made in a very small size. The sensing element is a section of optical fiber which may have a diameter of about 200 microns and of length of a few tens of centimeters. This allows deployment of the sensors into very small size tubes which fit into the annulus of a flexible riser. 
     Second, the use of these types of sensors may give a direct measurement of pressure with a very high resolution. One or more embodiments may measure changes in pressure of less than 0.01 PSI (pounds per square inch). This may facilitate the identification of small amounts of water collection in the annulus. 
     Third, the sensor may be interrogated at surface, which removes the need for electronics or electrical connections within the flexible, thus simplifying the system design, deployment, and operation. 
     Finally, because fiber optic pressure sensors may be spliced to a connecting fiber running to surface, the connecting fiber between surface and the sensor may be interrogated to give distributed temperature and distributed attenuation. DTS and distributed attenuation measurements may provide complimentary measurements to help identify leak location and the presence of hydrogen or H 2 S in the annulus. It should be noted that, typically, corrosion generates hydrogen. Thus, the detection of hydrogen may also be used to identify zones where corrosion is taking place inside the annulus. 
     While the present disclosure has been described 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 disclosure as described herein. Accordingly, the scope of the invention should be limited only by the attached claims.