WO1991006833A1

WO1991006833A1 - Method and measuring device for determining and monitoring of interface areas in at least two different media

Info

Publication number: WO1991006833A1
Application number: PCT/NO1990/000168
Authority: WO
Inventors: Kjell M. Haugstad
Original assignee: Haugstad Kjell M
Priority date: 1989-11-02
Filing date: 1990-11-02
Publication date: 1991-05-16
Also published as: NO894368D0; NO894368L; AU6618490A

Abstract

In a method for determining the interface between different media, particularly water, oil and gas in storage tanks for crude oil a number of thermal sensor units are employed provided in a vertical tube immersed in the medium or the media. Each sensor unit comprises two sensor elements (T1, T2) which measures a first and a second absolute temperature (t1, t2) respectively, which are detected and generated as a function of the vertical position of the sensor unit and employed for indicating a thermal resistance (Δt) which is the difference between (t1 and t2). Based on the measured temperatures (t1 and t2) and the thermal resistance values of the respective media threshold values for the interfaces between the different media are determined and the position and the level of interfaces are found as intersection points of respectively the function of the second absolute temperature (t2) or the thermal resistance (Δt) and the determined threshold values. A measuring device for performing the above-mentioned method is also disclosed.

Description

METHOD AND MEASURING DEVICE FOR DETERMINING AND MONITORING OF INTERFACE AREAS IN AT LEAST TWO DIFFERENT MEDIA

The present invention relates to a method for determining and monitoring interfaces and interface areas between at least two different media, particularly water, oil and gas in storage tanks for crude oil according to the introduction of claim 1. The invention also relates to a measuring device for determining and monitoring interfaces and interface areas between at least two different media, particularly water, oil and gas in storage tanks for crude oil according to the introduction of claim 5.

The main object of the method and the measuring device according to the invention is the monitoring of the interface areas and interface zones of water, oil and gas in large subsea storage tanks. Equipment mounted inside such storage tanks or storage cells will not be accessible for service throughout their entire useful life. The method and measuring device according to the invention are thus principally applied on concrete production platforms at sea where the ashore transport of oil is performed by tankers and not by pipelining.

The crude oil is stored in tanks which initially are filled with seawater. As the oil is produced and enters the tank, water is forced out of the cells. When the oil is shipped, water flows in and replaces the oil. Production and shipping are taking place on several storage tanks simultanously and the level of the liquid in the tanks varies differently in the various tanks or storage cells. The method and measuring device according to the invention are intended to prevent that the oil produced is vented to the open gea and that seawater are pumped aboard the tankers.

Prior art systems based on thermal sensors measuring temperature differences based on vertical thermocouple

SUBSTITUTE SHEET technology do not fully solve the problem of detecting oil/water interfaces. The reasons for this are that: a. Turbulent oil as found in near empty or near full conditions are not detectable. These are the conditions under which the level detection is most important. b. Oil/water signal discrimination is generally in the order of 1.5:1 to 2:1, which is too low to cater for unfavourable conditions. c. The vertical spacing of the thermocouple produces unwanted effects when passing the interface zone, due to a large vertical temperature gradient.

NO-PS 133 517 discloses a prior art system wherein each of the sensors is provided with a number of connected thermocouples or thermal converters, the connection being such that each sensor consists of two thermal converter groups, one designed as the measuring section and supplied with heat from a heating element and another designated as the reference section positioned vertically spaced from the measuring section. The measurement of the interface level is related to the difference in the specific heat between the liquids, which will influence the measuring signal in that heat transfer from the measuring section will vary according to the specific heat of the surrounding liquid and generate a corresponding variation in the difference signal between the measuring section and the reference section respectively.

NO-PS 162 093 is based on a corresponding prior art in so far as concerns the arrangement of the measuring means or the sensors, but is based on measuring the heat transfer between a thermal reference element being in thermal^■ contact with a known heating medium which flows through a vertically extending tube and sensors which are in thermal contact with the measuring medium, i.e. the surrounding liquid. Variations in the heat transfer between the reference mediu and the surrounding medium appears as temperature differences detected by sensors provided on the heat conductors which connect the reference medium thermally with the surrounding medium. A

SUBSTITUTE SHEET variation in the difference signal from the respective sensor elements in the sensor units hence becomes an indication of the heat transfer and thus of a change in the thermal properties of the surrounding liquid.

The measuring device according to the present invention measures both thermal resistance and liquid temperature by comparing temperature differences by detecting the absolute temperature in the horizontal plane. Thus, two methods for determining the interface level will be able to cooperate; viz. the thermal resistance method and the absolute temperature method. These will separately propose candidate values for the interface level and intelligent data processing and detailed process knowledge will effectively cover the total requirements for this kind of detection, as will be presently described:

a. The absolute temperature signal will not be affected by turbulence of the liquid. b. Oil/water signal discrimination of 4:1 is readily obtained from the combination of the two principles. c. There are no vertical spacing of the sensors to cause signal disturbances in zones of large temperature gradient.

The method of the present invention is based on the following physical principles:

a. Thermal resistance of liquids and gas, resulting in a thermal resistance as measured by the measuring device according to the invention of typically 1:3:6 in relative units for water/oil/gas respectively. b. The absolute temperature of liquids and gas, based on the fact that the crude oil which enters the storage cell holds about 37°C while the seawater temperature in the ocean at the seabed typically is 3-8°C.

The method according to the present invention is characterized in the features disclosed by independent claim 1, whereas

SHEET further features and advantages of the method will appear from the appended dependent claims 2-4. A measuring device according to the invention is characterized by the features disclosed by independent claim 5.

The invention shall now be described in greater detail in the following with reference to the accompanying drawing wherein

Fig. 1 shows a simplified diagram of the electrical layout of the measuring device according to the invention,

fig. 2 shows the arrangement of a sensor rod in a storage tank,

fig. 3 shows a horizontal section through a sensor of the measuring device according to the invention,

fig. 4 shows the arrangement of sensor elements in the sensor rod of the measuring device according to the invention,

fig. 5 shows an ideal thermal resistance signal display,

fig. 6 shows a practical thermal resistance signal display,

fig. 7 shows an of absolute temperature signal display and

fig. 8 shows a display of combined thermal resistance and absolute temperature detection.

The measuring principle of the method according to the ^*••**^• invention shall first be briefly elucidated. The thermal resistances of water/oil/gas may be detected by measuring the heat distribution around a constant power heat source. The temperature close to the heat source will increase in order to allow for the dissipation of the heat. A sensor embedded in a medium of larger thermal resistance will be exposed to a peak surface temperature reaching a higher value as the temperature increase are distributed further away from the source. A location for detecting the temperature increase in the vicinity

TUTE SHEET of the heat source along a defined conducting part will referred to the reference temperature of the surrounding medium, generate a signal which is a function of thermal resistance of the medium. Hence the absolute temperature of the surrounding medium may be measured by placing another temperature measuring element in a location on the same horizontal plane at a thermally infinite distance from the heat source.

The construction of the measuring device will now be described with reference to figures 1-4. The measuring device comprises a number of sensors, the electrical of which arrangement which is shown schematically in the accompanying figure 1. Each sensor comprises one (or two for redundancy) heating resistor R and two temperature detectors with sensor elements Tl, T2 provided inside a metal tube. With the tube in an upright position the sensors are located close to the inner wall with one temperature detector, i.e. a thermocouple or thermal converter Tl adjacent to the heating resistor R and the other temperature detector or thermal converter T2 on the opposite side of the tube relative to the heat source R.

As shown in figure 2, in a large tank any number of sensors may be arrayed vertically and molded inside a metal tube. Such a tube which constitutes an essential part of the measuring device according to the invention may be called a sensor rod. The 'sensor rod arrangement is shown schematically in figure 2 and provides a system of the discrete sensors spaced vertically apart and providing a vertical measuring resolution. In large tanks the sensor rod will preferably be constructed from prefabricated modules. The modules may be assembled on site with all electrical connections. In order to do this in a practical manner, the electrical conductors are passed through a dedecated plastic tube wherein he conductors from one module •joins the conductors from the modules below. A horizontal section through a sensor module illustrating the design of a sensor unit, showing the arrangement of the sensor element Tl closed to the heating resistor R and the other sensor element

TE SHEET T2 on the opposite side of the tube, with the heating conductors passing through the larger plastic pipe and the sensor signal conductors passing through the smaller plastic pipe. Figure 4 shows a perspective view of a sensor unit arranged in the sensor rod module, i.e. the metal tube and how the conductors in module join the conductors from the modules below. The sensor elements are molded in a soft polyurethane disc placed horizontally in the sensor rod around the plastic conductor tubes as is easily seen in figure 4. By locating the conductors of the heating circuits in a separate plastic tube the noice susceptibility of the sensor signal circuit is reduced.

Now shall the determination of an interface or interface areas in the medium surrounding the sensor rod shall be described.

The thermal resistance measurement is a good method for determining the interfaces of an stored liquid with several immiscible components. The sensor signals may be arranged as shown in figure 5. Of course the oil floats on the top of the water and the interface shown may be found by simple comparison of the signal amplitudes as evident from figure 5. In practice a deposit of various elements will be formed on the sensor rod surface, especially in the lower third of the storage cell. The effect of this is to increase the signal amplitudes since the thermal resistance at this level increases, but also to increase the oil-water signal discrimination. The effect of a rod deposit is illustrated in figure 6. This adverse effect of a rod deposit may be partly be overcome by individual calibration of threshold values.

The effect of turbulence in the liquids is to reduce the thermal resistance and hence decrease the sensor signal level. This phenomena occurs in the near empty or near full condition of the storage cell. The main problem arises when oil are pumped out of a near empty cell. The oil is turbulent due to a small oil volume and the main oil inlet tube being close to the

SUBSTITUTE SHEET sensor rod. The thermal resistance of the oil as measured by the sensor decreases and reaches the water value. This problem may be solved by looking at the absolute temperature signal from the sensor.

The absolute temperature is not affected by turbulence. If the system detects different candidate values for the level, the one determined by the absolute temperature should be dominant under turbulent conditions.

The temperature sensing element T2 measures the absolute temperature t2, while the element Tl measures the increased temperature t^ caused by the integrated heat element giving as a measure the thermal resistance ^Δt=tι-t2.

The water on which crude oil is stored and floats, is seawater with a fairly constant temperature, e.g. about 6-7°C, although it will vary a few degrees around this value with the seasons. Crude oil is however controlled to be at 36-37°C when it is introduced in the storage cell in the course of its production.

Hence the situation arises or having fairly warm oil stored on the top of cold water. The time constant of heat transfer from oil to weater in the volume of an average storage cell of for instance a Condeep pipe may be shown to be in the order of one year. The loss to the ocean water through the cell is difficult to calculate, as it depends on a number of parameters including whether the cell in question is surrounded by the ocean or other cells and with the relative filling volume of adjacent cells. However, this constant is certainly large and very much larger than that of the production, storage and a shipping cycle.

From figure 7 which shows a display of the detected absolute temperature it appears that a temperature profile through the storage cell clearly indicates the thermal transition through

SUBSTITUTE SHEET the interface zone, and that the sensor is not affected by turbulence.

The thermal resistance method and the absolute temperature method may be combined in order to give a sufficient oil/water discrimination basis for all conditions as shown in figure 8. The display represented in figure 8 gives a very clear indication visually as well as numerically. Furthermore it should be noted that there is an inherent element of redundancy in the sensor principle employed in the method according to the invention.

As described above, the sensors of the measuring device according to the invention measures the thermal resistance by means of the two separate sensor elements, these as mentioned being in the form of thermocouples or thermal converters. The thermal converters measure separately the absolute temperature of the reference point and of the hot spot. A value for the thermal resistance is found by computing the temperature increase of the hot spot by subtracting the reference value from the hot spot value.

The temperature increase will be a few degrees only. Measured in an absolute (Kelvin) scale one sees that in fact a measurement is performed of two temperatures which are about equal. If this are to be realized in practice each sensor element or thermal converter must have an extremely high absolute precision.

Such demands on the sensor elements also require a careful of selection and possibly tolerance trimming by means of laser. Such sensor elements are found in qualities with a precision from +5K to versions trimmed by laser with a precision of +0,25^κ. Other parameters as resolution, long term stability, linearity etc. are unchanged. The greatest difference lies in their price.

SUBSTITUTE SHEET With regard both to economy and quality, the cheapest ungraded version may be employed while sufficient presicion and correction for a possible drift are taken care of by using two types of calibration, viz. pre-calibration and operating calibration.

The pre-calibration may be a part of the quality assurance program in the production of sensor modules. After the finished sensors are molded in modules, correction tables are generated for all sensor elements. This may be done at ambient (room) temperature, e.g. 20°C. If a sensor indicates 23,6°C, its correction factor is -3.6°C. Another sensor element indicates 19.8°C and is assigned a correction factor of +0.2°C. In this way cheap sensor elements may be calibrated to +0.1°C. Individual testing of each signal element is however required for quality reasons.

Each module then has its specific correction table, one for the reference sensor element and one for the hot spot element. When the modules are assembled into sensor rods, the correction tables are assembled in a similar manner.

The pre-calibration in this way results in two tables for each sensor rod and theses tables are for instance loaded in a data recording system to be used with the method and the measuring device according to the invention. These tables are a correction table for the reference element and a correction table for the hot spot element. The correction values are subtracted from the measured values and a cheap, ungraded sensor element behaves as sensor elements with a precision of +0.1°C as a result of the pre-calibration..

The correction table for the reference element are now stored as a constant and forms the base for a precise measurement of absolute temperatures. In this case a precision of +1°C or +2°C is sufficient.

SUBSTITUTE SHEET The correction table for the hot spot elements are stored as a variable which may be updated according to need.

Operating calibration requires updating the correction table for the hot spot element during operation. The precision requirement is in this case greater than that of the absolute temperature measurement, being at least +0,5°C.

The operating calibration may be performed in connection with reporting the status of the sensor elements delivered from the measuring device to the operator. The calibration takes place by the power of the heating element being switched off. Then one waits until equilibrium is reached and the sensor element values are read. The correction table for the hot spot elements are now updated relative to the reference values which have been corrected by means of the constant correction table for the reference elements.

This ensures correction of a possible drift and secures a precise value of the thermal resistance.

SUBSTITUTE SHEET

Claims

PATENT CLAIMS

1. A method for determining and monitoring interfaces and interface areas between at least two different media, particularly water, oil and gas in storage tanks for crude oil, wherein use is made of a number of mutually, vertically spaced thermal sensor units are produced in a vertical tube toghether with electrical conductors connected with respective heating resistors (R) in each sensor unit and signal conductors for transmitting sensor signals from each sensor unit to a measurement data processing device, wherein each sensor unit comprises respectively at least one heating resistor (R) and two thermal sensor elements (Tl, T2) at the inner wall of the vertical tube such that the sensor element (Tl) is in thermal contact with the heating resistor (R) and the sensor element (T2) in thermal contact with the surrounding medium, c h a r c t e r i z e d i n that the heating resistor (R) is supplied with a constant electrical power, that a first absolute temperature (t^) is measured at the sensor element (Tl) and another absolute temperature ( 2) at the sensor element (T2) , that the measured absolute temperatures (t^, t2) are detected and generated as a function of the vertical arrangement of the vertical position of the sensor units, that a thermal resistance is indicated by means of a proportional value (Δt) , this value being the difference between the measured absolute temperatures (t^ and t2) that the thermal resistance (Δt) is detected and generated as a function of the vertical position of the sensor unit and that based on the measured absolute temperatures (ti, t2) and the indicated thermal resistance values for water, oil and gas respectively for each of the said functions a threshold*value is determined for the interface water/oil and oil/gas, the points of intersection of respectively the functions of the second absolute temperature (t2) and thermal resistance (Δt) with the threshold valuing in question determining the interface position.

STITUTE SHEET

2. A method according to claim 1, c h a r a c t e r i z e d i n that the functions for respectively the absolute temperature and the thermal resistance after scaling the function of the thermal-resistance are integrated to a combined function and that the separate, predetermined threshold values for each function are integrated to a combined threshold value for the combined function, the intersection point of the combined function and the combined threshold value determining the position of the interface.

3. A method according to claim 1, c h a r a c t e r i z e d i n that the sensor elements before use are calibrated against a known temperature.

4. A method according to claim 3, c h a r a c t e r i z e d i n that the sensor element (Tl) is calibrating during use, as the power supplied to the heating resistor (R) are switched off and the absolute temperature measured at the sensor element (Tl) is read when thermal equilibrium is achieved, whereupon the calibration of the sensor element (Tl) takes place against the reference value given by the simultaneously read, calibrated value for the absolute temperature of the sensor element (T2) in the same sensor unit.

5. A measuring device for determining and monitoring interfaces and interface areas between at least two different media, particularly water, oil and gas in storage tanks for crude oil, wherein a number of mutually, vertically spaced thermal sensor units are provided in a vertical tube together with electrical conductors connected with the respective heating resistors (R) in each sensor unit and signal conductors for transmitting sensor signals from each sensor unit to a measurement data processing device, wherein each sensor unit comprises at least one heating resistor (R) and two thermal sensor elements (Tl, T2) at the inner wall of the vertical tube such that the sensor element (Tl) is in thermal contact with

SUBSTITUTE SHEET the heating resistor (R) and the sensor element (T2) in thermal contact with the surrounding medium, c h a r a c t e r i z e d i n that the heating resistor (R) is a constant heating source, that the thermal sensor elements (Tl, T2) in each sensor unit are provided in essentially the same horizontal plane in the vertical tube and such that the sensor element (Tl, T2) are not in thermal contact, and that each sensor element (Tl, T 2) are adapted for measuring the absolute temperature.

SUBSTITUTE SHEET