Measuring and testing – Instrument proving or calibrating – Fluid pressure
Reissue Patent
1998-12-09
2003-04-01
Oen, William (Department: 2855)
Measuring and testing
Instrument proving or calibrating
Fluid pressure
C073S152510, C175S050000, C166S073000
Reissue Patent
active
RE038052
ABSTRACT:
BACKGROUND OF THE INVENTION
There are three main problems associated with the deployment of sensors in harsh environments. Firstly, there are the practical difficulties in getting the sensor to the correct position in the first instance and then, possibly, retrieving it later. Secondly, either the sensor must have a means of storing information, or it must have a means of communicating between the sensor itself and the information processing system. Thirdly, the sensor must be robust enough, not only to take the rigours of deployment, but also be able to function successfully for significant periods of time under harsh conditions. These three aspects are of particular importance in the oil extraction industry. The measurement of pressure at the bottom of an oil well is important for the efficient extraction of the oil reserves. The industry takes two different approaches. In the one case, a measurement system is lowered down the production tube of the well to take and record measurements for a short period before being returned to the surface. Although convenient for use with existing wells and only requiring short term stability of calibration, this approach gives limited information and interferes with the production of oil. In the other case, a measurement system is permanently installed during the construction of the oil well. This has the advantage of giving continuous measurements without interfering with oil production but places very severe requirements on the measurement stability of the sensor and its ability to withstand the high pressures and temperatures in the well for many years. Should such a sensor fail, or doubts be cast upon its accuracy, then the current practice would be to abandon it since it would be too expensive to replace. The permanent installation of a measurement system at the bottom of the oil well also requires significant technical problems to be solved in transferring information via metal or optical fiber cables, or radio links, up to the surface.
The present invention describes a pressure sensor which may be either temporarily or permanently deployed in a convenient fashion, using existing technology familiar to industries such as the oil industry.
SUMMARY OF THE INVENTION
An aim of the present invention is to provide apparatus for the measurement in a hostile environment preferably at a remote location.
BRIEF DESCRIPTION OF THE INVENTION
According to the present invention there is provided apparatus for measuring pressure, which apparatus comprises pressure communicating means for location at a point at which pressure is to be measured, a primary tube containing a fluid for linking the pressure communicating means to a control point where the pressure information is required, sealing means for sealing the primary tube so as to form a closed system when the pressure communicating means is closed, pressure control means for changing the pressure of the fluid within the primary tube, volume measuring means for measuring changes in the volume of the fluid within the primary tube as the pressure of the fluid is varied, pressure measuring means for measuring the pressure of the fluid at some point within the fluid, and remote from the control point, and transmitter means for transmitting pressure information from the pressure measuring means to the control point.
The pressure communicating means may be a device which may exist in a number of states according to the relative pressure difference between an external and an internal pressure, and whose volume depends upon this pressure difference. The external pressure is that which surrounds the pressure communicating means and is usually the pressure to be measured. The internal pressure is that of the fluid contained in the primary tube at the point of connection between the pressure communicating means and the primary tube. When the external pressure is higher than the internal pressure by more than a fixed limit, the pressure communicating means contains a fixed volume of the primary tube fluid which constitutes one state, the OFF state. Similarly; when the external pressure is lower than the internal pressure by more than a different fixed limit, the pressure communicating means contains a different fixed volume of the primary tube fluid, which constitutes a second state, the ON state. A third state, the ACTIVE state, occurs when the external pressure lies between these fixed pressure limits relative to the internal pressure. In which case, the volume of primary tube liquid contained by the communicating means depends upon the pressure difference in some consistent fashion and lies between the two volume limits. When the pressure communicating means takes this form, the whole apparatus is a sealed system with no direct contact between the fluid in the primary tube and the external environment.
Alternatively, the pressure communicating means may be a device which may exist in a number of states according to a control signal provided separately. In addition, the pressure communicating means may not contain a mechanical interface between the pressure in the primary tube and that in the region to be measured, and so there will be direct contact between the fluid in the primary tube and the external environment when the pressure valve is in the ACTIVE state.
The primary tube may be preferably narrow bore hydraulic tubing whose change in dimension with pressure is either small or well-known. The fluid contained by the primary tube may be a hydraulic oil, or any other fluid that is virtually incompressible or whose compressibility is well-known.
The sealing means may take any form that ensures that at all times the pressure within the primary tube may be adequately controlled both in accuracy and range. In particular, the pressure in the primary tube must be able to match the pressure to be found in the external environment to be measured, so that the pressure communicating means may operate as intended and the pressure be measurable.
The pressure control means may be any apparatus which permits the pressure to be changed from one value to another under some external control. Such apparatus could be a force acting on a piston inside a cylinder containing the-primary tube fluid.
The pressure measuring means may be any convenient apparatus which converts pressure into an indication such as a pressure dial gauge or a pressure transducer.
The volume measuring means may be any apparatus suitable for measuring the change in volume of the fluid within the primary tube and which has the sensitivity to detect the change in volume of the pressure communicating means as it changes state. An example of such apparatus would be the measurement of the position of a piston in a cylinder where the piston is used to determine the pressure in the primary tube fluid.
In one embodiment of the present invention, the means of changing the pressure, the volume measuring means, and the pressure measuring means, are all located together at the control point.
In a preferred embodiment of the apparatus, there is also provided means of measuring the temperature along the length of the primary tube. The temperature measuring means may be preferably an optical fiber distributed temperature sensor, either outside or inside the primary tube. An example of such a sensor is the York Sensors Limited DTS
80
which is commercially available.
In another preferred embodiment of the apparatus, there is also provided pressure sensing means of such a dimension and structure that it it is placeable inside the primary tube, locating means for locating the pressure sensing means at any desired point within the primary tube, and transmitter means for transmitting the pressure information from the pressure sensing means to the control point. More preferably there is also provided means for removing the pressure sensing means. The pressure sensing means may be a passive optical fiber sensor and the transmitter means may be optical fiber cables. More preferably, the pressure-sensing means may be a passive optical fiber sensor such as the polarimetric
Oen William
Sensor Dynamics Limited
Trop Pruner & Hu P.C.
LandOfFree
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