Patent Publication Number: US-2015075802-A1

Title: Method and Device for Subsea Sampling

Description:
The invention relates to a method for keeping the content of a sample vessel at an essentially constant pressure. More specifically the invention relates to a method for maintaining the pressure conditions of a sample taken subsea while transporting the sample away from the location where it was taken. The invention also includes a device for practicing the method. 
     Cooling of fluids sampled subsea will cause contraction of the fluid and with the contraction a drop in pressure. The subsequent drop in pressure may lead to gases entrained within the fluid releasing. Once the gases are released they cannot be re-dissolved within the liquid. Therefore precise laboratory analysis cannot take place. 
     Devices are known that use pistons to allow evacuation of the sample vessel while keeping the fluid in phase state. 
     The object of the invention is to remedy or to reduce at least one of the disadvantages of the prior art, or at least to provide a useful alternative to the prior art. 
     The object is achieved by virtue of features disclosed in the following description and in the subsequent claims. 
     The overall objective of the invention is to improve the quality of laboratory analysis of fluid samples taken subsea, primarily in the oil- and gas industry. The sample content may be filled onto a sample vessel by a method and a device known per se, for example as described in Norwegian patent application no. 20110774, with the title “Method and device for filling a submerged sample bottle” and filed by the present applicant. By keeping the content of the sample vessel at an essentially constant pressure, where the essentially constant pressure may be essentially equal to or higher than the pressure at the location where the sample was taken, it is possible to maintain the sample in its original fluid state and thereby prevent volatile sample content from vaporizing. The invention thus makes it possible to analyse the subsea samples in their original fluid state in an on-shore or offshore laboratory facility. 
     In a first aspect the invention relates to a sampling device for subsea fluids, the sampling device comprising:
         a sample container including a first chamber and a second chamber, the first and second chambers being isolated by a movable separating body;   a first pressure sensing device for sensing the pressure in the first chamber of the sample container;   a second pressure sensing device for sensing the pressure in the second chamber of the sample container, wherein the sampling device further comprises a pressure compensating device, the pressure compensating device being adapted to compensate for pressure changes in the first chamber of the sample container by regulating the pressure in the second chamber of the container and thereby moving the separating body.       

     By keeping the sample at or above the original sampled pressure, the quality of sample may be maintained. The movable separating body of the sample container may in one embodiment be a piston, floating or connected to a piston rod. In other embodiments the movable separating body may be an elastic diaphragm or a totally enclosed bladder. 
     The pressure sensing devices, which may be conventional pressure gauges as known to a person skilled in the art, may sense the pressure directly in the chambers of the sample container, or indirectly in fluids lines or other devices connected to the two chambers. 
     In one embodiment the pressure compensating device may comprise an accumulator. The accumulator may be a compressed inert gas accumulator. The inert gas container may be external to the sample container, or the inert gas container may be integral with the sample container. 
     The accumulator may be directly connected to the second chamber of the sample container. Alternatively the accumulator may be indirectly connected to the second chamber of the sample container by means of an actuator cylinder. A piston rod of the actuator cylinder may be mechanically connected to a separating movable piston in the sample container. 
     In alternative embodiments the pressure compensating device may comprise a pump or a linear actuator, both in fluid communication with the second chamber of the sampling device. The pump may be reversible, and the linear actuator may be mechanic, hydraulic, or pneumatic. 
     In a second aspect the invention relates to a method for subsea sampling of fluids by means of a sampling device according to the above description, the method comprising the steps of:
         collecting a fluid sample in the first chamber of the sample container in a subsea environment;   moving the sample container from the subsea environment and to an offshore or on-shore location, wherein the method further comprises the step:   by means of a pressure compensating device to compensate for pressure changes in the first chamber of the sample container by regulating the pressure in the second chamber of the sample container and thereby moving the separating body.       

     The separating body and the pressure compensating device are used to overcome the large temperature fluctuations seen in the subsea environment, samples can be taken at wellhead pressures which can be in excess of 120° C. The sample container, once removed from the sample point, is exposed to sea bed temperatures close to 0° C. 
     Calculations in line with API  521  show the effect of temperature on fluids to have a significant effect on pressure. The pressure compensating device according to the invention will make up any pressure differential due to temperature changes as will be seen in a subsea environment. The pressure compensating device will also act as a safety device if the temperature increases. If the sampling device is in danger of becoming over pressurised by a temperature increase, the pressure compensating device will allow fluid expansion while keeping the pressure within design parameters. Upon the sampling device then cooling the pressure compensating device will then maintain pressure so keeping gases entrained. 
     Hereinafter, an example of a non-limiting, preferred embodiment is described and is depicted on the accompanying drawings, where: 
       FIG. 1  shows a schematic diagram of a first embodiment of a sampling device according to the present invention; 
       FIG. 2  shows a schematic diagram of a second embodiment of a sampling device according to the present invention; 
       FIG. 3  shows a schematic diagram of a third embodiment of a sampling device according to the present invention; 
       FIG. 4  shows a schematic diagram of a fourth embodiment of a sampling device according to the present invention; and 
       FIG. 5  shows a schematic diagram of a fifth embodiment of a sampling device according to the present invention; 
    
    
     In the following the reference numeral  1  indicates a sampling device according to the present invention. The figures are shown simplified and schematic. Identical reference numerals indicate identical or similar features in the figures. 
     In  FIG. 1  a first sampling device  1  according to the invention is shown. A sample container  11  is divided into a first chamber  111  and a second chamber  113  by means of a sealingly movable separating body  112  in the form a floating piston. A not shown sample is contained in the first chamber  111  of the sample container  11 . The not shown sample is collected subsea through sample fluid lines  14   a  closable by isolation valves  12 . The pressure of the sample in the first chamber  111  is measured by a first pressure gauge  13   a,  while the pressure in the second chamber  113  is measured by a second pressure gauge  13   b.  The second chamber  113  of the sample container  11  is fluidly connected to a pressurized inert gas accumulator  15 . One side of the accumulator  15  contains inert gas supplied via an inert gas line  151  while the other side of the accumulator is fluidly communicating with a hydraulic fluid control line  153 . As the collected sample is moved from the subsea environment to an offshore or on-shore laboratory facility, the surrounding temperature will change and thus change the sample pressure. The first pressure gauge  13   a,  which may be s connected to a not shown control unit, will sense the varying pressure. The accumulator  15 , which is fluidly communicating with the second chamber  113  of the sample container  11 , will then increase the pressure in the second chamber  113 , and thus move the sealingly movable floating piston  112  to compensate for the pressure changes in the first chamber  111 , thus maintaining the sample at an essentially constant pressure. 
     In  FIG. 2  a second sampling device  1  according to the invention is shown. A pressurized inert gas accumulator  15  is is connected to the sample container  11  via an actuator cylinder  16 . The accumulator  15  is connected to an inert gas line  151  and to hydraulic control fluid lines  154 ,  155 . The actuator cylinder  16  displaces the piston  112  in the sample container  11  by means of a piston rod  166  common to the actuator cylinder  16  and the sample container  11 . The actuator cylinder  16  comprises a first chamber  161  and a second chamber  163  separated by a piston  162  also connected to the piston rod  166 . The cylinder actuator  16  is regulated by pressurizing the first chamber  161  through an opening line  165  or the second chamber  163  through a closing line  164 . This embodiment of the invention is directly compatible with the previously mentioned Norwegian patent application no. 20110774. 
       FIG. 3  shows a third sampling device  1  according to the invention. An accumulator  2  is integral to the sample container  11  in that the second chamber  113  is provided with a pressurized gas cap  19  of inert gases, without any external connections, except from the pressure gauge  13   b.  Pressure conditions are maintained within the first chamber  111  of the sample container  11  solely by use of the gas cap  19  contained in the second chamber  113  of the sample container  11 . 
       FIG. 4  shows a fourth sampling device  1  according to the invention. A linear actuator  17 , connected to a not shown control unit, is used to simulate the action of an accumulator, thus to regulate the pressure in the sample container  11 . The linear actuator  17  may be mechanic, hydraulic, or pneumatic. 
     In  FIG. 5  a fifth sampling device  1  according to the invention is shown. A pump  18  is used to simulate the action of an accumulator, thus to regulate the pressure in the sample container  11 . 
     In general the above mentioned processes may be performed manually or automatically by means of control units communicating with the various components of the embodiments. 
     It should also be understood that components of the various embodiments can be combined to provide additional embodiments also within the scope of the present invention.