Abstract:
A riser tensioner has a cylindrical barrel and a piston rod that telescopingly engage each other. An external accumulator containing a hydraulic fluid and a gas is mounted outside of the barrels for communicating the hydraulic liquid to an end of the barrel. Depending on the stroke of the piston rod, the liquid level in the accumulator will fluctuate and the gas pressure vary. A composite rod within the accumulator is embedded with an array of PVDF sensors. The PVDF sensors provide an output signal to indicate the liquid level in the accumulator when the liquid comes in contact with the sensor, allowing for reliable monitoring of liquid levels.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates in general to a riser tensioner system used during the offshore drilling and production phases and, in particular, to a standalone liquid level sensing device for monitoring liquid level in a composite gas accumulator of the tensioner system. The proposed sensing device and methodology can be extended to any other pressure vessel for liquid level sensing. 
         [0003]    2. Description of the Related Art 
         [0004]    Risers are used in offshore oil and gas drilling and production for conveying drilling mud as well as production fluids through a subsea wellhead to a floating production platform, also known as tension leg platforms (TLP) or SPAR. Tensioners are employed at the platform to apply tension to the risers. During the drilling or production phases, a riser system is typically kept in tension in order to avoid structural instability of the drilling or production stackup. A typical tensioner comprises a telescoping piston and cylinder arrangement supplied with gas pressure from accumulators. Waves and currents cause the piston and cylinder to extend and retract. 
         [0005]    In one type of tensioner design, the piston component comprises a barrel that slidingly engages the cylinder or other barrel. Each barrel has a closed end and an open end, the open end being in fluid communication with each other. The interiors of the barrels serve as the chamber for receiving gas pressure. A plurality of pistons may be utilized. 
         [0006]    A fluid within the chamber provides lubrication to dynamic seals. Thus, liquid level inside the accumulator is critical to ensure effective lubrication of dynamic seals. By using the liquid level information inside the accumulator, gas volume can be measured. The gas volume can be used to provide damping stiffness for the riser tensioner system. A novel technique for monitoring the liquid level inside an accumulator is desired. 
       SUMMARY OF THE INVENTION 
       [0007]    One embodiment of a system, method, and apparatus for a standalone liquid level sensor for a riser tensioner has a cylindrical barrel and a piston rod that telescopingly engage each other. An external accumulator or vessel is mounted outside of the barrels for communicating a gas quantity to one side of the barrel and a hydraulic liquid to an opposite side of the barrel. Depending on the stroke of the piston rod, the liquid level in the accumulator will fluctuate. The piston rod moves in and out of the cylindrical barrel in response to waves and currents. The external accumulator can be utilized as a Drilling Riser Tensioner (DRT) or as a Production Riser Tensioner (PRT). 
         [0008]    A composite rod is mounted co-axially within the external accumulator. Sleeves at each end of the composite rod are used to secure the rod in place. Each sleeve has holes that allow gas or liquid to flow in or out of the accumulator, maintaining the status-quo. The composite rod is embedded with an array of polyvinylidene fluoride (PVDF) sensors. The PVDF sensors are disposed on the composite rod at desired intervals which provide an output signal to indicate the liquid level in the accumulator when the liquid comes in contact with the sensor, allowing for reliable monitoring of liquid levels. The PVDF sensor array produces a different output voltage depending on whether gas or liquid is in contact with the sensor. 
         [0009]    The standalone liquid level sensing apparatus may advantageously be utilized with existing riser tensioner systems to accurately monitor liquid levels in an existing or third-party gas accumulator, can provide tensioner feedback to maintain riser system in constant tension, utilize low cost piezoelectric sensors, and possible recalculation of gas pressure and volume. No additional design changes to the accumulator are required. The dynamic gas volume measurement also facilitates detection of the gas leakage inside the accumulator, if any. 
         [0010]    The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    So that the manner in which the features and advantages of the present invention, which will become apparent, are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof that are illustrated in the appended drawings which form a part of this specification. It is to be noted, however, that the drawings illustrate only some embodiments of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
           [0012]      FIG. 1  is a perspective view of one type of tensioning system, constructed in accordance with the invention; 
           [0013]      FIG. 2  is a perspective view of one type of tensioning system, constructed in accordance with the invention; 
           [0014]      FIG. 3  is a perspective view of a piston assembly of  FIG. 1 ; 
           [0015]      FIG. 4  is a sectional view of a piston assembly of  FIG. 2 , constructed in accordance with the invention; 
           [0016]      FIG. 5  is a sectional view of a composite bar in the accumulator tank of the piston of  FIG. 4 , constructed in accordance with the invention; 
           [0017]      FIG. 6  is a plan view of a sensor on the composite bar of  FIG. 5 , constructed in accordance with the invention; 
           [0018]      FIG. 7  is a sectional view of a sleeve for mounting top end of the composite bar, constructed in accordance with the invention; 
           [0019]      FIG. 8  is a sectional view of a sleeve for mounting at bottom end of the composite bar, constructed in accordance with the invention; 
           [0020]      FIG. 9  is a perspective view of the sleeves of  FIGS. 7 and 8 ; 
           [0021]      FIG. 10  is a high level flow diagram of one embodiment of a method in accordance with the invention. 
           [0022]      FIG. 11  is an additional embodiment for a piston assembly, constructed in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring to  FIG. 1 , one type of riser tensioning system (“RTS”)  10  is depicted. The RTS  10  may be utilized to maintain tension on a drilling riser or a production riser. A piston assembly  12  is shown that connects at an upper end to a frame  11  of RTS  10  and at a lower end to a tensioner ring  13 . In this embodiment, the piston assembly  12  is oriented at an angle relative to the riser. Referring to  FIG. 2 , another type of RTS  14  is depicted. A piston assembly  16  is shown that connects at an upper end to a frame  17  of the DRT and at a lower end to a tensioner ring (not shown). In this embodiment, the piston assembly  16  is oriented parallel to the riser. The piston assemblies  12 ,  16  for both RTS  10  and RTS  14  orientations are similar in their components and are further explained below. 
         [0024]    Referring to  FIG. 3 , an embodiment of the piston assembly  12  of  FIG. 3 , is illustrated. It is understood that the piston assembly  16  is similar to the piston assembly shown in  FIG. 1 , with only difference in angle of piston barrel inclination with the vertical. Thus, this section in  FIG. 3  serves to also describe such a configuration. The piston assembly  12  has a cylindrical barrel  18  that slidingly receives a piston rod  20 . The barrel  18  may have an upper connection point  22  for connecting to the frame  17  ( FIG. 3 ). The piston rod  20  has a connection point  24  for connection to a component such as a tensioner ring (not shown) ( FIG. 2 ), which can heave as a result of waves and currents. Consequently, the piston rod  20  can also move in and out of the barrel  18  in response to such waves and currents. 
         [0025]    Continuing to refer to  FIG. 3 , an accumulator or composite gas accumulator  26  is shown connected to the cylindrical barrel  18 . The accumulator  26  is in fluid communication with a lower end of the cylindrical barrel  18  via a lower tube  28  and in fluid communication (N2 [nitrogen] gas or other suitable gas) with an upper end of the cylindrical barrel via an upper tube  30 . Upper and lower tubes  30 ,  28  form a closed loop system with the barrel  18  and the accumulator  26 . The connection of upper and lower tubes  30 ,  28  to the barrel  18  and the accumulator  26  will be explained further below. Upper tube  30  does not open within the barrel  18 . The accumulator  26  may be cylindrical in shape and may contain a compressible gas and an incompressible fluid. The accumulator  26  and related components are further explained below. The accumulator  26  may be made of a carbon epoxy composite matrix, which can be manufactured using filament-winding technology. Further, a thermal and pressure relief device  31  may be located on the upper tube  30  and a low pressure relief device  32  may be located at the upper end of the cylindrical barrel  18 . A hurricane isolation valve (not shown) may also be located on the barrel  18  at about mid-length on the barrel  18 . 
         [0026]    Referring to  FIG. 4 , a sectional view of the piston assembly  16  is illustrated. During operation, the piston rod  20  will move in or out of the barrel  18  in response to waves and currents, thus maintaining a constant tension on a riser (not shown). A chamber  25  within the accumulator  26  in this embodiment contains both a gas  27  and a liquid  29 . The gas  27  may be nitrogen N2 gas and the liquid  29  may be ethylene glycol, also known as erifon fluid. Other suitable gases and liquids may be utilized. In this embodiment, the N2 gas tends towards the upper end of the accumulator  26  and the ethylene glycol tends towards the lower end of the accumulator. 
         [0027]    In this embodiment, when the piston rod  20  moves axially outward from the cylindrical barrel  18 , a portion of ethylene glycol trapped below a piston  40  is forced out of the barrel  18  and into the accumulator  26  via the lower tube  28 . The N2 gas is restricted from flowing into the barrel  18 , resulting in the N2 gas occupying a smaller volume in the accumulator  26  which in turn results in the N2 gas being at higher pressure within the composite gas accumulator  26 . Alternatively, the N2 gas can be allowed to flow out of the accumulator and into the barrel  18  via the upper tube  30 . When the piston rod  20  moves axially inward from the cylindrical barrel  18 , N2 gas inside the accumulator expands, reducing the gas pressure in the accumulator. As a result, the N2 gas in the accumulator  26  displaces ethylene glycol out of the accumulator and into the barrel  18  via the lower tube  28 . The N2 gas remaining in the accumulator  26  occupies a larger volume and is thus at a lower pressure. 
         [0028]    Referring to  FIG. 4 , a composite bar  42  is mounted from an upper end  44  and a lower end  46  within the accumulator  26 . The composite bar  42  functions to indicate liquid levels, such as ethylene glycol, in the accumulator  26 . Although the composite bar  42  is shown centrally located within the accumulator  26 , it may also be located eccentrically. Composite bar  42  is located on a longitudinal axis of accumulator  26 . The bar  42  may also be fabricated from materials other than a composite. 
         [0029]    Referring to  FIGS. 5 and 6 , the composite bar  42  is further illustrated. The composite bar  42  may be cylindrical in shape and has an axial passage  43  extending from the lower end  46  to the upper end  44 . The hollow composite bar  42  is open at the upper end  44  ( FIG. 4 ) and closed at lower end  46  ( FIG. 4 ) to prevent fluid from entering the interior of the composite bar. In this embodiment, the composite bar  42  includes a sensor array  50 . The sensor array  50  may be a plurality of sensors  52  that may be in the form of films or strips and are made from PVDF embedded or bonded to an outer laminate  54  of the composite bar  42 . The sensors  52  are further coupled to an electrode  56  ( FIG. 6 ) that may be located within the hollow area or passage  43  of the composite bar  42 . The electrodes  56  may extend radially inward into the hollow area  43  of the bar  42 . The number of PVDF sensors  52 , n, may vary and each may be monitored as an individual channel, as explained in a later section. Further, the sensors  52  may be fabricated in various types of shapes, including but nor limited to circular, square, rectangular, and triangular. 
         [0030]    PVDF is a piezoelectric material, which produces electricity in response to mechanical strain. Mathematically, the electrical-mechanical coupling of the PVDF can be described using strain-charge and stress-charge piezoelectric coupling equations. PVDF is a semi crystalline polymer consisting of long chain of molecules with CH 2 CF 2  as a repeating unit. Although various types of piezoelectric materials may be utilized, PVDF has numerous advantages over other piezoelectric materials and is available in the form of thin films, which can be embedded or bonded to a composite. Some of these advantages include:
       1. Highest tensile strength of all processable fluorocarbons;   2. Good radiation resistance;   3. Melt processable, allowing PVDF to be metalized and embedded with composite materials;   4. High abrasion and chemical resistance;   5. Can be used at temperatures of up to 150° C. (300° F.);   6. High flexibility, ruggedness, and light weight, and   7. Low acoustic impedance.       
 
         [0038]    In this embodiment, the PVDF sensors  52  can have a thickness of up to 250 microns. Referring to  FIG. 6 , each electrode  56  may include an inner coating  58  on the inner surface of the sensor  52  and an outer coating  60  on the outer surface of the sensor. Inner and outer coatings  58 ,  60  are made from conductive silver ink to facilitate conduction. Each electrode  56  may be connected to a wire and routed upwards as a conductor to form an output wire assembly or bunch  62  that internally gathers each conductor signal from the lower end  46  ( FIG. 4 ) to the upper end  44  ( FIG. 4 ) of the composite bar  42 . When one of the film sensors  52  is strained, the voltage response of the film sensor can be transmitted through the wire assembly  62  as an analog voltage output that can be analyzed further. Use of the voltage output is discussed further below. 
         [0039]    Referring to  FIGS. 7 ,  8 , and  9 , an upper mounting sleeve  70  and a lower mounting sleeve  72  are shown that secure the upper end  44  and lower end  46  of the composite bar  42 , respectively. The sleeves  70 ,  72  may be metallic and may be installed onto each end of the accumulator  26  via interference fit. Alternatively, the sleeves  70 ,  72  may be threaded onto accumulator  26 . Mounting sleeves  70 ,  72  may be used to mount the composite bar  42  in an existing accumulator of an existing tensioner system such as RTS  14  ( FIG. 2 ) or RTS  10  ( FIG. 1 ). Each mounting sleeve  70 ,  72  can have holes or passages  74 ,  76  as shown in  FIG. 9 . In upper mounting sleeve  70 , the holes  74  allow N2 gas to exit accumulator  26  for the purposes of allowing a pressure gauge to display a pressure reading. In the lower mounting sleeve  72 , the holes  76  allow the ethylene glycol fluid to enter the accumulator  26 . As explained earlier, the lower end  46  of the composite bar  42  is closed to prevent fluid from entering the composite bar. 
         [0040]    The embedded PVDF sensor array  50  at outer surface  54  of composite bar  42  thus can function as a liquid level sensing device that facilitates analog voltage output response, which is different for the ethylene glycol (liquid) and the N2 (gas). Voltage signals due to piezoelectric effect of from each individual sensor of sensor array  52  can be taken to a platform via wire conduit  80  for monitoring and analysis by a data acquisition system (DAQ)  82 , digital signal analyzer (DSA)  84 , and a processor  86 , to determine a level, L, of ethylene glycol inside the accumulator  26 . Due to RTS  10 , being very near to the platform (not shown), voltage signals are efficiently carried to the platform with minimal noise. In addition to monitoring the signal response at the platform, the frequency response from each individual sensor  52  will also be monitored because the frequency changes if the liquid level drops and that particular sensor is exposed to gas. 
         [0041]    During operation, the piston rod  20  ( FIG. 4 ) may move axially outward or inward from the cylindrical barrel  18  ( FIG. 4 ) in response to heaves or currents. When the piston rod  20  moves axially outward, a portion of ethylene glycol trapped below a piston  40  is forced out of the barrel  18  and into the accumulator  26  ( FIG. 4 ) via the lower tube  28  ( FIG. 4 ). The ethylene glycol is allowed to flow into the accumulator  26  through the holes  76  formed in the lower mounting sleeve  72  ( FIG. 8 ). Thus, the level of ethylene glycol in the accumulator  26  rises and compresses the N2 within the accumulator, which at that point occupies a smaller volume within the accumulator. As illustrated by flow chart in  FIG. 10 , the DAQ  82  collects voltage output signals from the sensor array  50  where they signals are separated into input channels by the DSA  84 . The channels correspond to the number, n, of sensors  52  in the sensor array  50 . Voltage output signals from each sensor  52  may thus be monitored through the use of software, such as LabView®. Because contact with a liquid such as ethylene glycol causes a strain in the sensor  52  differs from a strain placed on the sensor by a gas such as N2, a voltage output signal that characterizes it, is produced that allows a sensor response to be distinguished between that of ethylene glycol or N2 gas. The DSA and processor can then provide an appropriate active sensor location that corresponds to maximum ethylene glycol level, L, as measured from the bottom of the accumulator  26 . The appropriate sensor location can then be processed and the level or column height of ethylene glycol can then be displayed. Further, because N2 gas is a compressible gas, the processor can be utilized to calculate N2 gas pressure and volume in the accumulator  26  by applying polytropic process equation: 
         [0000]    
       
         
           
             
               
                 P 
                  
                 
                     
                 
                  
                 1 
               
               
                 P 
                  
                 
                     
                 
                  
                 2 
               
             
             = 
             
               
                 ( 
                 
                   
                     V 
                      
                     
                         
                     
                      
                     2 
                   
                   
                     V 
                      
                     
                         
                     
                      
                     1 
                   
                 
                 ) 
               
               n 
             
           
         
       
       
         
           
             where: 
             P1 is the initial gas pressure; 
             V1 is the initial gas volume; 
             P2 is the dynamic gas pressure; 
             V2 is the dynamic gas volume. And 
             n=1.1 for nitrogen, n is a gas constant 
           
         
       
     
         [0048]    As previously explained, the response of each individual sensor  52  will be directly proportional to the strain experienced by the sensor. Thus, the strain experienced due to gas pressure will be different from the strain developed due to fluid interaction with the individual sensor  52 . Thus, the first sensor can be located from the bottom side of the bar  42 , which shows the response from the max liquid level in composite gas accumulator  26  ( FIG. 4 ). An output response trend for the sensor may be determined for both gas and liquid interaction under specified conditions. 
         [0049]    It is understood that one of ordinary skill in the art that the above methodology for obtaining, analyzing, and processing voltage output signals from the sensor array  50  is also valid for when the piston rod  20  moves axially inward of the barrel  18 . In such a scenario, ethylene glycol in the accumulator  26  would flow into the barrel  18  via the lower tube  28 , causing the level of ethylene glycol in the accumulator to drop. As explained in the previous section, the DAQ  82 , DSA  84 , and processor would collect, analyze, and process the resulting voltage outputs of the sensor array  50  to determine the level of ethylene glycol in the accumulator  26 . In an additional embodiment, the piston assembly  12  of  FIG. 1  is shown in  FIG. 11 . The piston assembly  12  is identical to piston assembly  16  ( FIG. 4 ), having a barrel  100  and a piston rod  102  that axially moves inward and outward from the barrel  100  in response to heaves and currents. The piston assembly  12  also has a piston  104  that is connected to an upper end of the piston rod  102 . The piston  104  traps liquid, such as ethylene glycol, below it. The piston assembly  12  is in fluid communication with a lower tube  110  for ethylene glycol liquid flow. An upper tube  108  may allow N2 gas to communicate with a pressure reading device such as a gauge but does not allow communication with the barrel  100 . A composite bar  112  is mounted within the accumulator  106  as described in a previous section for the piston assembly  12 . Further, a PVDF sensor array  114  made from a plurality of sensors  116  is embedded in the composite bar  106  as described in  FIG. 5  and functions in the same manner to obtain, analyze, and process voltage output signals from the sensors  116 . However, the piston assembly  12  is tilted at an angle θ from the horizontal, which can vary between 0 and 90 degrees. Another difference is in bonding or embedding the sensors in  FIG. 11 , sensors strips may be bonded or embedded exactly parallel to the horizontal given the inclined axis. 
         [0050]    The level of the liquid ethylene glycol can thus be determined by the sensors  116  and subsequent processing by taking into account the tilt of the piston assembly  12 . 
         [0051]    While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.