Abstract:
A method and apparatus for analyzing sag in drilling fluids or solid bearing fluids wherein a cylindrical high-pressure cell assembly ( 80 ) capable of withstanding high pressure and high temperature with a coaxial cylindrical rotor ( 33 ) driven to rotate inside cell assembly ( 80 ), a sample port ( 12 ) for testing sample subtraction used for further analysis, and a sample inlet port ( 74 ) for adding testing sample. This said cell assembly ( 80 ) is supported on a pivotal cell support ( 90 ) so that it can be tilted and fixed at any angle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND 
     1. Field of Invention 
     The present invention relates to apparatuses and methods for monitoring, measuring, or analyzing the sag of a weighted material in a drilling fluid. 
     2. Description of Prior Art 
     This tester applies to “sagging” of oil well fluids. Sagging is the settling of weighted additives in the wellbore during drilling conditions, as well as times of static activity. This directly applies to dense materials, such as Barite, used in the drilling fluid. These materials serve little purpose other than a weighting agent. The density measurement is the focus of this tester. 
     Settling or sag is not an issue with vertical or near vertical drilling, but problematic with higher drilling angles. As these modern angles increase, sag becomes more of an obstacle. Sagging can reduce the rate of penetration, fluid flow, and cutting removal. Sag can occur when the flow of the fluid ceases, as when the drill string stops. Sag can also concentrate in one area during this “static” time. 
     Addressing this concern has been an issue since the 1920s. Many adjustments to fluids have been made in an effort to alleviate this problem. Until now, minimal substantial test data were available for such research. Drilling fluid changes were made based upon guesswork in the field in the past. Data supporting downhole mud density is critical for modern production and performance. 
     A few types of arrangements have been applied to measure the sag of drilling fluids. In U.S. Pat. No. 6,330,826, an apparatus consists of a conically or frustro-conically shaped inner body; an outer body having an opening with contours closedly matching those of inner body such that in conjunction inner and outer body are separated by an arrow gap defining a conically or fustro-conically volume with a vertex; a motor drive for rotating the inner body with respect to the outer body; and a sampling access to determine the density of said drilling fluid with a localized part of said volume. One of the drawbacks of this setup is that it can not simulate downhole mud conditions which are under high pressure and high temperature. One other drawback of this apparatus is that the inner and outer body shapes are considerably different from the real drilling conditions in which both bore hole and drilling pipe are cylindrical. In U.S. Pat. No. 6,584,833, a device was disclosed for measuring dynamic and static sag of drilling fluids under high temperature and high pressure conditions. One of the drawbacks of this invention is its complexity. It consists of a very complicated testing cell and delicate electronics and control systems. As of 2007, its cost to build is about 8 times of the current invention. Another drawback of this invention is that it is difficult to achieve more than 5,000 psi due to the nature of its design. Because it needs to sense tiny shifts of center of gravity, it can not use a heavy high-pressure vessel. The current invention can easily test samples up to or more than 30,000 psi. Another drawback of this invention is that it is very difficult to operate and difficult to clean due to its many components and complex design. 
     It is an object of this invention to provide a sag tester wherein dynamic and static sag caused by settling of weighting materials in drilling fluids or other solids bearing fluids can be accurately determined under conditions closely simulating down-hole environments. 
     It is another object of this invention to provide a sag tester wherein dynamic and static sag of weighting materials in drilling fluids or other solids bearing fluids can be accurately determined under any inclined angle which simulates any high or low angle drilling operation. 
     It is another object of this invention to provide a sag tester that requires substantially less maintenance work yet meets industry standards of accuracy, reliability, durability, dependability, and ease of cleaning. 
     SUMMARY 
     A sagging tester in accord with the present invention conveniently comprises of a cylindrical high-pressure vessel capable of withstanding high pressure and high temperature with a coaxial cylindrical rotor assembly located inside, which could be driven to rotate through an outside magnetic coupling. One testing sample addition port and at least one testing sample extraction port are attached to the high-pressure vessel for pressure maintenance and testing sample subtraction. Density changes, composition and other properties of subtracted samples can be obtained with a density meter, pycnometer, fluid analyzer, etc. 
     The apparatus and method of the present invention provide an alternative way to measure or analyze dynamic and static sag caused by settling of weighting materials in drilling fluids or other solids bearing fluids under high-pressure high-temperature conditions. It can also measure the viscosity of a testing fluid in addition to measuring weighted material sagging. 
    
    
     
       DRAWING FIGURES 
       Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with accompanying drawing in which: 
         FIG. 1  is a cross-section view of the cell assembly  80  in preferred embodiment of the invention; 
         FIG. 2  is the flow diagram of this invention; 
         FIG. 3  shows an alternative configuration of cell assembly  80 A; 
         FIG. 4  is the flow diagram with an alternative cell assembly  80 A; 
         FIG. 5  shows a flow diagram with another alternative cell assembly  80 B with just one port. 
       
         
           
                 
               
                 
                 
                 
               
             
                 
                     
                 
                 
                   Reference Numerals in Drawings 
                 
                 
                     
                 
               
               
                 
                     
                 
               
            
             
                 
                     
                    12 
                   sample port 
                 
                 
                     
                    12A 
                   sample port 
                 
                 
                     
                    12B 
                   sample port 
                 
                 
                     
                    22 
                   sampling tube 
                 
                 
                     
                    26 
                   o-ring 
                 
                 
                     
                    26A 
                   o-ring 
                 
                 
                     
                    27A 
                   piston 
                 
                 
                     
                    28A 
                   o-ring 
                 
                 
                     
                    30A 
                   screw thread 
                 
                 
                     
                    32 
                   support bearing 
                 
                 
                     
                    33 
                   rotor 
                 
                 
                     
                    33A 
                   inside rotor 
                 
                 
                     
                    34 
                   coupling magnet 
                 
                 
                     
                    35 
                   cell wall 
                 
                 
                     
                    35A 
                   cell wall 
                 
                 
                     
                    36 
                   magnet holder 
                 
                 
                     
                    37A 
                   dynamic seal 
                 
                 
                     
                    38 
                   driving-magnet 
                 
                 
                     
                    39 
                   thermal couple 
                 
                 
                     
                    39A 
                   thermal couple 
                 
                 
                     
                    40 
                   magnet mount 
                 
                 
                     
                    41 
                   straight bore 
                 
                 
                     
                    41A 
                   thermal couple port 
                 
                 
                     
                    42 
                   bearing 
                 
                 
                     
                    43 
                   conical surface 
                 
                 
                     
                    44 
                   bearing 
                 
                 
                     
                    45A 
                   shaft 
                 
                 
                     
                    46 
                   lock nut 
                 
                 
                     
                    47 
                   cell bottom 
                 
                 
                     
                    47A 
                   cell bottom 
                 
                 
                     
                    48 
                   bushing 
                 
                 
                     
                    49A 
                   motor 
                 
                 
                     
                    50 
                   screw 
                 
                 
                     
                    52 
                   heater 
                 
                 
                     
                    52A 
                   heater 
                 
                 
                     
                    54 
                   pivot 
                 
                 
                     
                    62 
                   cell body 
                 
                 
                     
                    62A 
                   cell body 
                 
                 
                     
                    63 
                   screw thread 
                 
                 
                     
                    63A 
                   screw thread 
                 
                 
                     
                    74 
                   sample inlet port 
                 
                 
                     
                    74A 
                   pressurization port 
                 
                 
                     
                    76 
                   cell cap 
                 
                 
                     
                    76A 
                   cell cap 
                 
                 
                     
                    78 
                   thread 
                 
                 
                     
                    80 
                   cell assembly 
                 
                 
                     
                    80A 
                   cell assembly 
                 
                 
                     
                    80B 
                   cell assembly 
                 
                 
                     
                    90 
                   cell support 
                 
                 
                     
                    90A 
                   cell support 
                 
                 
                     
                    90B 
                   cell support 
                 
                 
                     
                    92 
                   piston 
                 
                 
                     
                    92B 
                   piston 
                 
                 
                     
                    94 
                   pressurization oil chamber 
                 
                 
                     
                    94B 
                   pressurization oil chamber 
                 
                 
                     
                    96 
                   liquid pump 
                 
                 
                     
                    96A 
                   liquid pump 
                 
                 
                     
                    96B 
                   liquid pump 
                 
                 
                     
                    98 
                   sample reservoir 
                 
                 
                     
                   100 
                   relief valve 
                 
                 
                     
                   100A 
                   relief valve 
                 
                 
                     
                   100B 
                   relief valve 
                 
                 
                     
                   102 
                   sample chamber 
                 
                 
                     
                   102B 
                   sample chamber 
                 
                 
                     
                   104 
                   sampling valve 
                 
                 
                     
                   104A 
                   sampling valve 
                 
                 
                     
                   104B 
                   3-way valve 
                 
                 
                     
                   106 
                   high pressure sample vessel 
                 
                 
                     
                   106A 
                   high pressure sample vessel 
                 
                 
                     
                   106B 
                   high pressure sample vessel 
                 
                 
                     
                     
                 
               
            
           
         
       
     
    
    
     DESCRIPTION 
     FIGS.  1  and  2 —Preferred Embodiment 
       FIG. 1 , is a cross-section view of a cell assembly  80  with a cell body  62  and a cell cap  76 . Cell body  62  is detachable from cell cap  76  via a screw thread  63 . An o-ring  26  assures against the escape of fluid through screw thread  63 . Inside of cell body  62  and below screw thread  63  is a cylindrical cell wall  35 , which extends downward to a cell bottom  47 . A tapered hole with a conical surface  43  and a straight bore  41  is located in the center of cell bottom  47 . A pivot  54 , which is secured to cell bottom  47  by a lock nut  46  through a thread  78 , is seated into said tapered hole on conical surface  43 . Lock nut  46  is tightened to provide initial seal on conical surface  43  between cell bottom  47  and pivot  54 . A thermal couple  39  is inserted into the center of pivot  54 . Radially outward of the outer surface of pivot  54  is a bushing  48 . Bushing  48  is made of Rulon, Teflon or equivalent plastics. A magnet holder  36  and a coupling magnet  34  are positioned radially outward of bushing  48 . A screw  50  secures magnet holder  36  and coupling magnet  34  to the bottom of a rotor  33 . A support bearing  32  provides vertical support of the assembly of rotor  33 , magnet holder  36  and coupling magnet  34 , which can rotate freely on the same central axis of pivot  54 . A sample inlet port  74  is provided to maintain the inside pressure of cell assembly  80  at a constant value. Sample can be added or bleed off from sample inlet port  74 . A sample port  12  is also provided just to subtract sample from cell assembly  80  for measurement and analysis. A sampling tube  22  connects to the end of sample port  12  tightly and their connection is sealed from the sample at the top of cell assembly  80 . The other end of sampling tube  22  extends to the bottom of cell assembly  80 . This ensures that the sample subtracted from cell assembly  80  is from its bottom. A magnet mount  40  is rotationally supported on the outside of cell body  62  by a bearing  42  and a bearing  44 . Magnet mount  40  can be rotated by any conventionally means such as gearbox or motor. A driving magnet  38  is mounted on magnet mount  40  at approximately the same level where coupling magnet  34  is mounted inside of the cell body  62 . In  FIG. 2 , Cell assembly  80  is supported on a cell support  90  and can be tilted and fixed at any inclined positions from 0 to 90 degree corresponding to the horizontal plane. A sample reservoir  98  is connected to sample inlet port  74 . A pressurization oil chamber  94  and a sample chamber  102  are inside of sample reservoir  98  and are separated by a piston  92 . Piston  92  effectively prevents the mixing between pressurization fluid and testing sample. A relief valve  100  and a liquid pump  96  are both connected to sample reservoir  98  at pressurization oil chamber  94  side. A sampling valve  104  connects to sample port  12 . High pressure sample vessel  106  is optional and is connected to sampling valve  104 . 
     OPERATION 
     FIGS.  1  and  2 —Preferred Embodiment 
     In  FIG. 1 , Pivot  54  is secured to cell body  62  by lock nut  46  and can be cleaned together with cell body  62 . During installation, screw  50  holds magnet holder  36 , coupling magnet  34  and rotor  33  together. Bushing  48  is pushed into the bottom of magnet holder  36 . This said subassembly is dropped into cell body  62  and rotationally supported by pivot  54 . A motor or gearbox drives magnet mount  40  to rotate carrying driving magnet  38 . Due to the magnetic coupling between driving magnet  38  and coupling magnet  34 , rotor  33  rotates at the same revolving speed as magnet mount  40  does. Test sample is poured into cell body  62  so that sample surface submerges the top of rotor  33 . Screw down cell cap  76  with o-ring  26  in place. Add more test sample fluid from sample port  12  until sample starts to come out from sample inlet port  74  in order to expel all air inside of cell assembly  80 . A heater  52  heats up cell body  62  while thermal couple  39  provides temperature feedback for temperature control. In  FIG. 2 , cell assembly  80  is then mounted on cell support  90  and is tilted to desired angle. Relief valve  100  and sample valve  104  are turned off. Next, liquid pump  96  starts to pump pressurization oil to pressurization oil chamber  94  inside of sample reservoir  98 . Piston  92  is moved by pressurization oil and pushes more testing sample inside of sample chamber  102  into cell assembly  80  through sample inlet port  74 . After desired pressure is reached, pump  96  is turned off. Magnet mount  40  is driven to rotate at desired speed and heater  52  heats up cell assembly  80  to desired temperature. If the pressure inside of cell assembly  80  is above desired pressure, relief valve  100  will be turned on briefly to bleed off small amount of pressurization oil until the pressure inside of cell assembly  80  is dropped back to desired value. 
     After a desired duration of time, sampling valve  104  is opened. High pressure sample vessel  106  is to receive sample under high temperature and high pressure conditions and to cool the sample off before discharging to atmospheric environment. High pressure sample vessel  106  is optional since sample can be directly discharged to atmospheric environment if evaporation of sample is not an issue. Because of sampling tube  22  of  FIG. 1 , the sample withdrawn from cell assembly  80  is near the bottom of cell assembly  80 . This discharged sample is further analyzed for its composition and its density is measured. At last, tested sample sagging information is derived from those data. 
     While subtracting sample from cell assembly  80 , liquid pump  96  pumps more pressurization fluid into sample reservoir  98  which in turn adds more sample to cell assembly  80  to maintain the inside pressure of cell assembly  80 . 
     In  FIG. 1 , viscosity of tested sample at an elevated temperature and pressure condition is also obtained by measuring the power consumption of the driving device that keeps driving magnet  38  rotating at a constant speed. Because cell wall  35  is static and rotor  33  is rotating, there is a drag due to the viscosity of the tested sample applied on the outside surface of rotor  33 . At a constant rotating speed, a thicker tested sample causes more drag on the outside surface of rotor  33 . Thus, more energy is consumed in the driving device to overcome this drag. 
     DESCRIPTION 
     FIGS.  3  and  4 —An Alternative Cell Assembly Embodiment with Different Sample Withdrawn Configuration 
       FIG. 3  shows a cross-section view of a cell assembly  80 A with a different sample withdrawn configuration. Cell assembly  80 A consists of a cell body  62 A and a cell cap  76 A. Cell body  62 A is detachable from cell cap  76 A via a screw thread  63 A. An o-ring  26 A assures against the escape of fluid through screw thread  63 A. Inside of cell body  62 A and below screw thread  63 A is a cylindrical cell wall  35 A that extends downward to a cell bottom  47 A. A shaft  45 A driven by a motor  49 A inserts into the bottom of cell assembly  80 A. An inside rotor  33 A connects to the top of shaft  45 A through a screw thread  30 A. Inside rotor  33 A is cylindrical shape and located approximately in the center of cell body  62 A. A dynamic seal  37 A provides seal between cell bottom  47 A and shaft  45 A. A thermal couple  39 A is inserted into the bottom of cell assembly  80 A through a thermal couple port  41 A. A pressurization port  74 A is provided to maintain the inside pressure of cell assembly  80 A at a constant value. Sample can be added or bled off from pressurization port  74 A. A sample port  12 A is also provided just to subtract sample from cell assembly  80 A for measurement and analysis. Sample port  12 A is located considerably at the lower portion of cell body  62 A and connects to cell bottom  47 A in radial direction. This ensures that the sample subtracted from cell assembly  80 A is from its bottom. A piston  27 A is located inside of cell body  62 A. Below piston  27 A is filled with testing sample and above piston  27 A is filled with pressurization fluid. An o-ring  28 A provides the seal between testing sample and pressurization fluid. In  FIG. 2 , Cell assembly  80 A is supported on a cell support  90 A and can be tilted at any inclined positions from 0 to 90 degree corresponding to the horizontal plane. A relief valve  100 A and a liquid pump  96 A are both connected to pressurization port  74 A. A sampling valve  104 A connects to sample port  12 A. High pressure sample vessel  106 A is optional and is connected to sampling valve  104 A. 
     OPERATION 
     FIGS.  3  and  4 —An Alternative Cell Embodiment with Different Sample Withdrawn Configuration 
     In  FIG. 3 , Shaft  45 A sticks into the bottom of cell body  62 A. Then screw inside rotor  33 A to the end of shaft  45 A. Pour predetermined amount of testing sample into cell body  62 A. Insert piston  27 A. Add some pressurization fluid on top of piston  27 A. Screw down cell cap  76 A with o-ring  26 A in place. More pressurization fluid can be added from pressurization port  74 A. A heater  52 A heats up cell body  62 A while thermal couple  39 A provides temperature feedback for temperature control. In  FIG. 4 , cell assembly  80 A is then mounted on cell support  90 A and is tilted to desired angle. Relief valve  100 A and sample valve  104 A are turned off. Next, liquid pump  96 A starts to pump pressurization oil to cell assembly  80 A. After desired pressure is reached, pump  96 A is turned off. Motor  49 A drives inside rotor  33 A to rotate at desired speed and heat  52 A heats up cell assembly  80 A to desired temperature. If the pressure inside of cell assembly  80 A is above desired pressure, relief valve  100 A will be turned on briefly to bleed off small amount of pressurization oil until the pressure inside of cell assembly  80 A is dropped back to desired value. 
     In  FIG. 4 , After a desired duration of time, sampling valve  104 A is opened. High pressure sample vessel  106 A is to receive sample under high temperature and high pressure conditions and to cool the sample off before discharging to atmospheric environment. High pressure sample vessel  106 A is optional since sample can be directly discharged to atmospheric environment if evaporation of sample is not an issue. This discharged sample is further analyzed for its composition and its density is measured. At last, tested sample sagging information is derived from those data. While subtracting sample from cell assembly  80 A, liquid pump  96 A pumps more pressurization fluid into cell assembly  80 A to maintain the inside pressure of cell assembly  80 A. 
     In  FIG. 3 , viscosity of tested sample at an elevated temperature and pressure condition is also obtained by measuring the power consumption of motor  49 A. Because cell wall  35 A is static and inside rotor  33 A is rotating, there is a drag due to the viscosity of the tested sample applied on the outside surface of inside rotor  33 A. At a constant rotating speed, a thicker tested sample causes more drag on the outside surface of rotor  33 A. Thus, more energy is consumed in motor  49 A to overcome this drag. 
     DESCRIPTION 
     FIG.  5 —An Alternative Configuration with Only One Port on Pressure Cell Assembly 
     In  FIG. 5 , a cell assembly  80 B is supported on a cell support  90 B and can be tilted and fixed at any inclined positions from 0 to 90 degree corresponding to the horizontal plane. Cell assembly  80 B has similar inside configuration compared to cell assembly  80  in  FIG. 1 , except it does not have a designated sample inlet port as sample inlet port  74  in  FIG. 1 . A sample reservoir  98 B is connected to a 3-way valve  104 B. A pressurization oil chamber  94 B and a sample chamber  102 B are inside of sample reservoir  98 B and are separated by a piston  92 B. Piston  92 B effectively prevents the mixing between pressurization fluid and testing sample. A relief valve  100 B and a liquid pump  96 B are both connected to sample reservoir  98 B at pressurization oil chamber  94 B side. 3-way valve  104 B connects to a sample port  12 B on cell assembly  80 B. High pressure sample vessel  106 B is optional and is connected to 3-way valve  104 B. 
     OPERATION 
     FIG.  5 —An Alternative Configuration with Only One Port on Pressure Cell Assembly 
     In  FIG. 5 , cell assembly  80 B is then mounted on cell support  90 B and is tilted to desired angle. Relief valve  100 B is turned off. 3-way valve  104 B is set to connect sample port  12 B to sample reservoir  98 B. Next, liquid pump  96 B starts to pump pressurization oil to pressurization oil chamber  94 B inside of sample reservoir  98 B. Piston  92 B is moved by pressurization oil and pushes more testing sample inside of sample chamber  102 B into cell assembly  80 B through sample port  12 B. After desired pressure is reached, pump  96 B is turned off. If the pressure inside of cell assembly  80 B is above desired pressure, relief valve  100 B will be turned on briefly to bleed off small amount of pressurization oil until the pressure inside of cell assembly  80 B is dropped back to desired value. 
     After a desired duration of time, 3-way valve  104 B is switched to connect sample port  12 B to high pressure sample vessel  106 B. High pressure sample vessel  106 B is to receive sample under high temperature and high pressure conditions and to cool the sample off before discharging to atmospheric environment. High pressure sample vessel  106 B is optional since sample can be directly discharged to atmospheric environment if evaporation of sample is not an issue. This discharged sample is further analyzed for its composition and its density is measured. At last, tested sample sagging information is derived from those data. 
     Ramifications 
     Rotor  33  and inside rotor  33 A do not have to be cylindrically shaped. They could be a blade, frame or any geometry shape. Furthermore, rotor  33  and inside rotor  33 A could be eliminated if shearing of fluid is not required. 
     In  FIG. 1 , multiple sample ports which are similar to sample port  12  can be provided along with sampling tubes which are similar to sampling tube  22  with their ends at different locations with cell assembly  80 . With this arrangement, samples at various height of cell assembly  80  can be subtracted at relatively the same time. Thus, the distribution of density inside of cell assembly  80  can be obtained. 
     Cell wall  35  in  FIG. 1  and cell wall  35 A in  FIG. 3  could be conical shape instead of cylindrical. 
     Testing sample subtracted from the bottom of cell assembly  80  could also be analyzed for other properties besides density. 
     In  FIG. 1 , driving magnet  38  could be driven to rotate in an oscillatory fashion as well instead of just constant direction, while power consumption of driving device is monitored. Similarly, in  FIG. 3 , insider rotor  33 A could be driven to rotate in an oscillatory fashion instead of just constant direction, while power consumption of motor  49 A is monitored. Thus visco-elasticity of tested sample could be obtained as well. 
     In  FIG. 1 , the end of sample tube  22  does not have to be located at the bottom of cell assembly  80 . The end of sample tube  22  could be located at any height to study the density change over time at that particular location. Similarly the breakthrough point of port  12 A into cell assembly  80 A in  FIG. 3  does not have to be located at the bottom of cell assembly  80 A. 
     In  FIG. 2 , sample reservoir  98  does not have to be a piston style. It could be a bladder type accumulator or anything equivalent. 
     In  FIG. 2 , once sample is withdrawn and shut in high pressure sample vessel  106 , total weight of high pressure sample vessel  106  could be measured to calculate the density of testing sample without have to discharge it out. 
     In  FIG. 1 , driving magnet  38  does not have to be located radially outside of cell assembly  80 . It could locate beneath of cell assembly  80  as long as it can generate a magnetic coupling with coupling magnet  34 . 
     In  FIG. 2 , besides density, other properties of extracted sample in high pressure sample vessel  106  from cell assembly  80  could be measured with other kind of equipment to determine the sagging of drilling fluids as well. 
     In  FIG. 3 , piston  27 A can be removed if mixing between pressurization fluid and testing sample would not be a problem or liquid pump  96 A in  FIG. 4  can pump testing sample directly. 
     Conclusion, and Scope 
     Accordingly, the reader will see that this invention can be used to construct a pivotal high pressure vessel from which sample can be subtracted under high pressure and high temperature conditions for density change monitoring. This said structure could also provide shear to testing sample at a desired rate. It satisfies an eminent drilling industry need. 
     Objects and Advantages 
     From the description above, a number of advantages of my sagging tester become evident:
         (a) Drilling fluids under high temperature and high pressure can be subtracted from high pressure testing vessel for density and other analysis without reducing the pressure inside of testing vessel.   (b) Due to limited number of components, current invention is easy to operate and maintain.   (c) The pressure rating of current invention will only be limited to the pressure rating of its pressure vessel, tubing and valves, which can be up to 60,000 psi. Previously no sag information of drilling fluids has been obtained under more than 5,000 psi pressure conditions.   (d) Current invention can test drilling fluids dynamically and statically under high pressure, high temperature and various inclined positions.       

     Further objects and advantages of my invention will become apparent from a consideration of the drawings and ensuing description.