| Drilling fluid plays an important role in the process of drilling. Whether can we detect the density of drilling fluid accurately to control the material ratio of each composition of drilling fluid is related to the safety and efficiency of drilling.At present, there are many equipment and devices for detecting liquid density on the market, such as float type densimeter, the static pressure type densimeter, capacitive fluid densimeter and radioactive isotopic densitometer and vibration tube type liquid density meter, etc. However, all kinds of equipment and devices in the above have limitations in detecting density of liquid. In addition, the environment factors of drilling site are relatively harsh. Therefore, all kinds of the density testing equipment and devices above are not good enough to meet the requirements of testing the drilling fluid density accurately, safely and efficiently.In order to test the density of drilling fluid accurately, safely and efficiently, the vibration condition and temperature condition of drilling site should be considered when the liquid density densimeter is being choiced. Meanwhile, in combination with velocity condition, stress condition and physical properties and chemical properties of drilling fluid, etc., the vibrating tube drilling fluid density detection system is proposed and established. On the basis of this detection system, the vibrating tube type liquid densitometer is redesigned, and a cantilever type drilling fluid densitometer is developed. Furthermore, various properties of this densitometer are researched by the finite element analysis. The main works accomplished are as follows:(1) The research of liquid density detection equipment and devices in domestic and foreign is summarized and analyzed, and the vibration tube liquid densitometer is selected to detect density of drilling fluid. Combining with the vibration condition and temperature condition of drilling site and the pressure condition and velocity condition of drilling fluid, the vibrating tube drilling fluid density detection system is proposed and established.(2) Considering the function of each composition in the vibration tube drilling fluid density detection system, it is concluded that the various properties of this detection system can be represented by various performance characteristic of vibration tube drilling fluid densitometer. With reference to the traditional design of Coriolis mass flowmeter, and combined with physical and chemical properties of drilling fluid, this paper proposes a new type of vibration tube type liquid densitometer, cantilever type drilling fluid densitometer. The sensitive elements of cantilever densitometer are cantilever beam and measurement tube. In addition, the design and calculation of the cantilever type drilling fluid densitometer are completed.(3) The first order vibration mode and the corresponding natural frequency of the sensitive element S2 are obtained by modal analysis, the result shows that the lower limit of the excitation frequency is 1591.0Hz, the upper limit is 1691.0Hz, and the step size is 0.05 Hz. Modal analysis of the densitometer’s shell is done, and the analysis results show that the shell will not resonates with the sensitive element S2. Then the rationality of design for structure and size of the cantilever type drilling fluid densitometer is verified.(4) The harmonic response analysis of the sensitive element S2 is done, the analysis results show that under the action of vibration force, the average equivalent stress and average equivalent strain of the sensitive element S2 are mainly concentrated at the location of the excessive of the a and b parts of the cantilever beam. That means when the cantilever type drilling fluid densitometer is working, the position near a and b parts of the cantilever beam prones to be damaged.(5) The harmonic response analysis of the sensitive element S2 is done when external vibration of the drilling site is available, the results show that displacement along z axis direction of the sensitive element S2 caused by the external vibration is small. It can be approximated that the vibration performance of the cantilever beam type drilling fluid densitometer is not affected by vibration of the drilling site. Further more, the mass flow measurement error of the cantilever beam type drilling fluid density meter caused by external vibration is 0.0011%, the value is far less than the density meter’s accuracy 2%. In addition, the extenal vibration does not affect the natural frequency of the sensitive element S2, that means the external vibration has small effect on the mass flow and density measurement of the cantilever beam type drilling fluid density meter, as a result, the vibration performance of the cantilever type drilling fluid densitometer is excellent, the densitometer is able to adapt to the vibration condition of drilling site.(6) The model analysis of the sensitive element S2 is done under the action of the temperature condition of the drilling site. The results show that the density measurement error caused by the limit temperature of drilling site is 0.0075g/cm3, the value can satisfy the density measurement accuracy 0.01g/cm3, that means the temperature of drilling site has small effect on the density measurement, and the temperature performance of the cantilever beam drilling fluid densitometer is good.(7) The structure analysis of the sensitive element S2 is done under the action of the temperature field of the drilling site, the analysis results show that the displacements along the z axis of the nodes on the left side cantilever beam J1 is equal to the displacements along the z axis of the nodes on the right cantilever beam J2, that means the vibration displacement of J1 and J2 caused by the temperature of drilling site are the same. That means the the temperature has small effect on the mass flow measurement of the cantilever beam type drilling fluid densitometer, so the temperature performance of the cantilever beam type drilling fluid densitometer can adapt to the temperature condition in the drilling site.(8) The modal analysis of the sensitive element S2 is done under the action of drilling fluid, the results show that the density measurement error caused by the limit of pressure design of drilling fluid is 0.0022g/cm3, the value can satisy the densitometer’ density measurement accuracy of 0.01g/cm3. Thus, we can know that the density measuremet error caused by the pressure of the drilling fluid is small and the pressure performance of the cantilever beam drilling fluid densitometer is good.(9) The structure analysis of the sensitive element S2 is done under the action of the drilling fluid’s pressure, the analysis results show that when the pressure of drilling fluid reaches the limit of pressure design, displacement along all the directions are tidy, and the mass measurement flow error is 0.0003%, the value is far less than the mass flow measurement accuracy 2%. Then, it can be known that the mass flow measurement of the cantilever beam drilling fluid density meter is very small, and the pressure performance of the densitometer can adapt to the working pressure. In addition, the maximum average equivalent stress is far less than the maximum allowable stress of the material of the sensitive element S2.(10) Considering the density gauge of system error, the density and the mass flow measurement error caused by the external vibration, temperature and drilling fluid pressure, it is concluded that the sum of density measurement error is 0.0097g/cm3, and the sum of mass flow measurement error is 0.0014%. Both of them can satisfy the precision of density meter, so the vibration performance, temperature performance and pressure performance are excellent, all of them can adapt to the working conditions of the drilling site. |
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