TILTED GAMMAMETER

This invention relates to a gamma-ray flow measuring instrument for measuring the density of a fluid in a volume in a container, especially in a fluid flow in a pipe or similar. More specific the invention relates to a construction where a source and detector are partially inserted into the pipe wall and the pipe opening area has a conical shape so as to reduce the vulnerability of the pipe wall and at the same time let the source come as close as possible to the detector.

In oil and gas production it is important to monitor the quality and composition of the production flow, and among these the density of the fluid flow, where the fluid may comprise both gas and liquids like oil and water. For density measurements gamma sources are often used. These are based on positioning a gamma source and detector on opposite sides of the flow and measuring the differences in detector signal depending on the density of the flow. The present systems often require the use of high activity gamma sources of the CsI 37 type in the range of 1100-190OkBq (30-50mCurie) and energies in the range of 660keV, and thus requires special handling of radioactive materials in addition to constituting large units to be mounted on the pipe.

The object of the present invention is to provide a more compact solution which provides sufficiently good measurements and also requires gamma sources with lower activity. This is obtained with an instrument as specified in the independent claims.

In the solution according to the invention the gamma source is inserted into a recess in the pipe wall corresponding to the outer shape of the source housing. Depending of the photon energy generated by the source the source housing may be constituted by an outer lead filled steel housing or titanium housing with a tungsten or steel core or source holder including the gamma source preferably positioned coaxially therein. The core or holder also includes a channel from the embedded gamma point source toward the pipe centre. Coaxial with the core and channel a conical protrusion is provided surrounding an extension of the channel and forming a point into a corresponding cavity in the centre of the recess in the pipe wall.

This leaves a relatively short propagation path through the pipe material before entering the flow, which results on low beam attenuation in the pipe material and also avoids weakening the pipe more than necessary. In addition a sufficiently concentrated beam through the flow and to the detector is obtained.

The invention is especially suited for measurements in wet gas flows the line between the source and detector has an angle relative to the flow axis. This is advantageous because a longer propagation path provides improved measurement sensitivity as the wet gas otherwise has too low beam attenuation at the relevant photon energies. In these cases an Am241 source may be placed in a steel core and e.g. having titanium housing. Because of the angle of the beam the attenuation may be relatively high, and cavity in the inner wall of the pipe may be made, or the holes is made through the pipe wall so as to reduce the propagation path length through the wall. Depending on the changes of clogging or deposits into the cavity the removed part of the pipe wall may be may be replaced by a portion of an essentially transparent material for the gamma radiation, e.g. a PEEK material low attenuation of the gamma radiation and also being capable of withstanding the conditions in the flow. Thus a continuous inner pipe wall may be obtained.

According to an alternative to one embodiment of the invention for measuring density of multiphase flows the recess is made into the pipe wall leaving a part of the pipe wall between the top of the core and the flow. In this case the source emits relatively high energy radiation, e.g. a Csl37 source, which results in a requirement for the source housing to provide a good shielding, e.g. being made from lead or a lead filled steel casing with a tungsten core.

In the other side of the pipe a detector is inserted in a suitable recess, preferably having a cavity in front of the detector with a smaller diameter to reduce the material thickness of the pipe directly between the detector and the flow.

The invention will be described below with reference to the accompanying drawings, illustrating the invention by way of examples.

Figure 1 illustrates an embodiment of the invention adapted for the use with a Cs 137 source in a multiphase flow.

Figure 2 illustrates an embodiment of the invention adapted for the use with a

Am241 source in a wet gas flow.

Figure 3 illustrates an alternative embodiment of the source housing and holder. Figure 4 illustrates a possible variation in the essentially conical shape of the source holder.

Figure 1 illustrates a gamma instrument mounted in a pipe 1 as seen along the axis of the pipe. The pipe 1 is made from steel and in addition the illustrated pipe comprises a PEEK (polyetheretherketone) material 3 on its inner surface. The latter may be required if the gamma instrument is combined with other measuring instruments, e.g. with electrodes in contact with the flow, but is not important to the present invention. PEEK is often used in relation to multiphase hydrocarbon flows because of its mechanical durability, and chemical neutrality in combination with the components in the flow in varying temperatures.

As is seen in figure 1 a gamma source housing 2 is positioned in a recess in the pipe wall and being fastened therein by any available means. The housing and recess is also sealed O-rings 8 to avoid leaks through the recess. According to the preferred embodiment for measurements in multiphase fluids comprising a Cs 137 source the housing 5 is preferably made from lead, but in some cases, e.g. when using less active gamma sources, steel may be used. In the illustrated example a lead filled 5 steel housing 6 is shown. The source 4 is positioned in a core or source holder 7 positioned coaxially in the housing, and this core has a channel from the source toward the flow inside the pipe. Depending on the source material the core may be of different types but in the illustrated example the core is made from Tungsten or other absorbing materials, and with less active sources a combination of steel housing and enlarged Tungsten core may be used.

In order to reduce the gamma-ray attenuation in the pipe material it is advantageous that the thickness through the material is as small as possible, but without weakening the pipe wall. This is obtained by providing an essentially conically shaped tip 10 on the core which protrudes into the pipe wall wherein the channel in the core extends through the cone and out from the tip. This conical shape provides a sharp beam cross section as it provides a long channel through the absorbing material, but does not weaken the

wall in the same degree as would be the case if the core had the same cross section along its length. The essentially conical shape of the tip 10 may in practice deviate from the smooth cone, e.g. due to machining considerations, for example in a stepwise reduction of the radius toward the end of the tip, as is illustrated in figure 4.

In the drawing approximately 5mm of pipe material remain in front of the core channel. This reduces the requirements for source strength and thus may reduce the problems related to handling of radioactive sources in density measuring instruments. As stated above the preferred source 4 in this embodiment is a Cs 137 source, preferably in the range of 3.7-37 x 10 ⁷ Becquerel (1-10 mCurie), especially 18.5 x 10 ⁷ Becquerel (5mCurie).

Figure 3 illustrates an alternative embodiment of the source housing and core, where the housing 2 is provided as a steel shell 20 enclosing a Tungsten inner part 21, and in which the core/source holder 22 is also made from Tungsten, and contains the source 24. The core 22 is mounted into the inner part 21 by a threaded coupling 23.

As with the source housing the detector 9, which may be on any suitable type, is positioned in a recess in the pipe wall on the other side of the flow. With the Cs 137 source the energy being detected is in the range of range ~660eV, as lower energies from the source will be absorbed by the steel in the pipe wall.

In the illustrated embodiment a cavity is also provided in front of the detector 9 to reduce the wall thickness in front of the detector, thus to reduce the attenuation in front of the detector. In this case the pipe wall in front of the detector and the source is approximately 5mm, but this may be chosen according to the specific use of the instrument.

In figure 1 the detector is positioned on the opposite side of the flow relative to the source. The detector may also be provided with a frustoconical shape in order to reduce the weakening of the pipe wall. Other configurations may be contemplated, such as several detectors positioned off the beam axis for detecting radiation scattered from the flow, as discussed in Norwegian patent application No. 1999.2988 for the purpose of obtaining salinity independent density measurement in the flow.

According to another embodiment of the invention the radiation may be transmitted directly from the one or more sources through different parts of the flow. This way it will be possible to provide measurements of the density on the flow centre as well as close to the pipe wall, so as to make it possible to detect a layered structure where different fluids are found at different distances from the flow centre.

A radiation beam aimed at measuring close to pipe wall will both leave and enter the pipe wall at an angle. In high pressure environments the detector and source housing has to be positioned at a distance from the inner pipe wall so as not to weaken the wall. When the beam has to leave the wall at an angle this also means that it in these cases has to propagate through more pipe wall material before entering the flow. In order to avoid this, a cavity may be provided into the inner pipe wall so as to reduce the propagation length through the pipe wall, and this cavity may be filled with a material being relatively transparent to the radiation, e.g. PEEK material. A related solution is illustrated in figure 2, where the beam has an angle relative to the flow direction and the pipe wall.

As illustrated in figure 2 the gamma beam is, according to the invention aimed at an angle relative to the flow. This is especially suitable when measuring the density of wet gas flows, where the fluid has a low attenuation rate and the sensitivity may be increased by increasing the propagation length through the fluid flow. The angle will depend on the attenuation in the flow and sensitivity of the detector relative to the beam energy, but will usually be below 75 degrees so as to obtain a significant increase in the beam path length.

In the embodiment illustrated in figure 2 a source 14 with lower energy may be used, such as Am241, thus requiring a reduced wall thickness to avoid too much attenuation in the steel pipe walls 11. As is illustrated this is solved by constructing the recess by making a hole through the pipe wall and positioning the source housing in the outer parts of this opening, and sealing it for avoiding leaks though the opening in any suitable way being available for a person skilled in the art, e.g. by using O-rings as illustrated in the drawings. Also, because of the angle against the pipe wall and the required thin metal in front of the source housing a recess is left in the inner pipe wall.

In order to avoid turbulence and materials deposited in the recess a window 13 for the gamma radiation, e.g. made from PEEK or similar materials, may be position in the recess. In figure 2 the source housing 12 is made from an outer casing 15 of a low density materials such as titanium alloys with a steel core 17 containing the source 14. As in the first embodiment the core protrudes with a conical shape 16 toward the flow, but in this case the housing 12 covers the tip of the cone and provides a wall for the gamma rays in the outer end of the channel closest to the flow, so as to protect the source from interactions with leaks etc from the flow passing the peek material 13. In the preferred embodiment of this solution the housing 12 may have a thickness of 5mm on front of the cone 16. Titanium alloys are especially advantageous in this application as it combines mechanical strength with low absorption, thus acting as a window for the radiation in front of the cone opening. This could also be solved with a thin steel plate in the end of the channel, at the cost of mechanical strength or increased absorption.

According to the embodiment illustrated in figure 2 adapted for performing measurements in wet gas flows an Am241 source in the range of 180-1100 x 10 ⁷ Becquerel (5O-3OOmCurie) is used and a detector 19 suitable for detecting in the energy range of ~59,5keV is preferred.

Because of the low energies a solution similar to the solution related to the source is used in relation to the detector, with a radiation window made from PEEK or similar materials is used between the detector and the fluid flow.

The embodiments disclosed in figures 1 and 2 are suitable for different applications. The compact gamma density measuring instrument illustrated in figure 1 represents the preferred embodiment in situations where the flow is a multiphase flow comprising e.g. oil, water and gas. In this case the source and detector is positioned directly opposite each other with a beam having a 90° angle relative to the fluid flow direction.

The gamma density measuring instrument in figure 2 represents a preferred embodiment in the case where the flow is constituted by wet gas, i.e. mainly gas with some contributions of water and/or oil.

In both situations the invention is aimed at providing a compact solution providing good dynamical range of measurements performed without using strong gamma sources.

Although this invention is mainly adapted to measurements in pipes containing fluid flows passing the measuring instrument it may also be used for containers containing fluids, e.g. for measuring the density of a pressurized fluid inside a tank.