The invention is concerned with a method and apparatus for remote monitoring of a natural gas stream, and more particularly to remote monitoring of a high-pressure natural gas process stream.

It is often necessary to determine the properties of a natural gas stream. In particular, during the extraction, processing and distribution of natural gas, it is important to determine quality parameters (that is, parameters which affect the quality of the gas), such as gas composition, calorific value, H ₂ 0 content, H ₂ S content, C0 ₂ content, moisture content, mercury content, glycol content, hydrocarbon dew point, 0 ₂ content, methanol content, and so on.

Monitoring of a natural gas stream in order to determine its properties is, for example, carried out on production or processing platforms. A sample of the gas in the natural gas stream is removed and analyzed with measurement technology such as gas chromatography.

The natural gas stream is usually at a relatively high pressure (considerably above atmospheric pressure, and for example around 40 bar). However, measurement technology such as gas chromatography requires the gas sample to be depressurized to a pressure at or close to atmospheric pressure, and also to be vented to atmosphere after analysis. Further, it may be difficult to automate this type of measurement technology, and gas chromatographs in particular are relatively high-maintenance, and need considerable user intervention for maintenance and calibration. As a result, this type of monitoring is not easily applicable in situations where user intervention is difficult, such as on unmanned wellhead platforms.

There is increasing interest in subsea gas processing (that is, where the gas from the well is processed near the well at the seabed, rather than being brought to the surface for processing at topside infrastructure such as a production or processing platform). Existing technologies for monitoring of gas quality parameters are not normally suitable for subsea installations, for a variety of reasons. There are not only serious problems regarding user intervention (as the lack of accessibility introduces challenges with respect to maintenance and calibration/verification of monitoring devices), but also issues regarding the fact that an analysed gas stream cannot easily be vented to atmospheric pressure to allow depressurization of the sample. The invention has been made in view of the above circumstances, and it is an object of at least the preferred embodiments of the invention to provide a method and apparatus which can be used for gas quality monitoring at unmanned processing platforms or in subsea installations and also reduce operational cost of manned facilities.

According to a first aspect of the present invention, there is provided a method for remote monitoring of a high-pressure natural gas stream, comprising the steps of: taking a sample of gas from a first location in the natural gas stream; analyzing the gas with a high-pressure analyser which can analyse high-pressure gas to determine quality parameters; sending signals relating to the quality parameters to a control unit; and returning the sample of gas to the natural gas stream at a second location, wherein the pressure of the natural gas stream is higher at the first location than at the second location.

This method allows for remote monitoring of the quality parameters of the natural gas stream. Further, as the sample is taken from a higher-pressure location and returned to the stream at a lower-pressure location, the pressure difference drives the sample through the analyser, so that there is no need to provide separate pumping means for the sample for this purpose. It should be noted that the second location may be a considerable distance from the first location, and indeed may be in a different region of the stream.

As the natural gas stream (and thus the gas sample) is at a high pressure, it is necessary for the analyser to be a high-pressure analyser which can analyse high-pressure gas. As mentioned above, "high pressure" in the context of natural gas is considerably above atmospheric pressure, and for example around 40 bar. In a preferred form, the analyser is based on nuclear magnetic resonance, and is for example a nuclear magnetic resonance (NMR) unit.

In a preferred form, the NMR unit works with a method as described in WO 2015/090325. The hydrocarbon fingerprint can be different DEPT (Distortionless Enhancement by Polarization Transfer) pulse sequences at different angles.

Alternatively, the hydrocarbon composition can be determined by high resolution hydrogen (1 H) and/or carbon (13C) spectroscopy. The amount of nitrogen can be determined by nitrogen (14N) NMR. Other algorithms for other components have also been investigated.

Normally, NMR units work at atmospheric conditions, but they can work with high pressure gasses as long as high pressure sample tubes are applied. The first location and the second location can be anywhere in the natural gas stream, as long as the pressure difference is provided. However, in a preferred form, the first location is downstream of a compressor, and the second location is upstream of the same compressor. In an alternative form, the first location is upstream of the second location, and there is a pressure reduction from the first to the second location, such as a cooler. In a further alternative, the second location may be distinct from the natural gas stream, such as release to ambient; for subsea operations, ambient pressure will be sufficiently high.

Preferably, the first location in the natural gas stream is at a point where the natural gas stream contains a single phase gas stream. If the gas stream were in two phases (that is, if it contained gas and liquid), then a sample of the gas alone might not be representative of the entire flow. Taking the sample from a point where there is a single phase gas stream means that the sample will be properly representative of the overall flow.

As the sample taken from the natural gas stream may not be in an ideal state for analysis, it is preferred for the sample to be conditioned after it is taken from the natural gas stream and before it reaches the analyser.

This conditioning may take the form of changing (increasing or decreasing) the pressure of the sample, and in a preferred form, this is done by a hydraulic booster. In this case, the gas could be released to the first location, or the pressure at the second location may even be higher.

According to a second aspect of the invention, there is provided apparatus for remote monitoring of a high-pressure natural gas stream, comprising: a high- pressure analyser with an inlet connected to a first location in the natural gas stream and an outlet connected to a second location in the natural gas stream, the pressure of the natural gas stream being higher at the first location than at the second location; the analyser being adapted to analyse a high-pressure sample of gas from the natural gas stream to determine quality parameters and to output signals relating to these quality parameters to a control and logging unit remote from the apparatus.

As the pressure difference serves to drive the sample through the analyser, there is no need for the apparatus to include pumping means for this purpose, which allows the apparatus to be simpler and cheaper.

Preferably, the analyser is a nuclear magnetic resonance (NMR) unit. Preferably, the first location is downstream of a compressor in the natural gas stream, and the second location is upstream of the same compressor.

Preferably, the first location in the natural gas stream is at a point where the natural gas stream contains a single phase gas stream.

Preferably, the apparatus further comprises a hydraulic booster for changing

(increasing or decreasing) the pressure of the sample before it reaches the analyser. In a preferred form, the hydraulic boosting is achieved by a low vapour pressure liquid such as an ionic liquid piston compressor. The low vapour pressure means that the sample is less likely to be contaminated, and this contributes to more accurate monitoring.

The apparatus is suitable for use in inaccessible locations, and in a preferred form, the apparatus is located on an unmanned processing platform.

Further, as the gas sample is returned to the natural gas stream, the apparatus is suitable for use subsea, and in an alternative preferred form the natural gas stream is a subsea natural gas stream.

Of course, the apparatus is not limited to use in inaccessible locations, and in an alternative preferred form, the apparatus is located in a manned production or processing plant or platform, or onshore.

Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying Figures, in which:

Figure 1 is a schematic view of the last part of a gas processing plant with an apparatus for monitoring a high-pressure natural gas stream;

Figure 2 is a schematic view of a hydraulic booster which may be used to increase the pressure of a sample; and

Figure 3 is a schematic view of a further form of hydraulic amplifier.

The present invention is applied to a process stream of natural gas. Figure 1 shows a part of a process stream, in which a stream of natural gas 10 passes through a scrubber 12, which removes unwanted constituents from the natural gas stream, and into a compressor 14. The gas is compressed in the compressor 14, and leaves at a high pressure and high temperature, and preferably as a single phase. The compressed gas stream then passes through a cooler 6 to a pipeline 18.

As shown in Figure , a valve 20 is connected to the natural gas stream between the compressor 14 and the cooler 16. The valve 20 can be selectively actuated to allow a sample of the compressed gas to be drawn off from the natural gas stream 10. The method may be carried out intermittently, rather than continuously, in that the valve 20 is briefly opened at intervals to allow a small sample of the natural gas stream gas to be drawn off.

The gas sample drawn off from the natural gas stream is passed to an analyser 22 which can analyse the gas to determine quality parameters while it is still at a high pressure and high temperature. One suitable type of analyser would be an NMR unit, such as of the type described in WO 2015/090325, or an NMR spectroscopy unit. NMR units have the advantage of being able to operate on high- pressure samples, and need relatively little user intervention.

After analysis, the sample of the gas is returned to the natural gas stream

10 upstream of the compressor 14 via a second valve 24. Figure 1 shows the gas being returned to the scrubber 12, but it will be appreciated that the gas can be returned to the natural gas stream at any suitable point, as long as the pressure at the point in the natural gas stream where the gas is returned is less than the pressure at the point in the natural gas stream where the sample is taken. For example, the gas sample could be returned to the natural gas stream by being routed to another processing step which has a lower pressure than the first location. This allows the gas to be driven through the analyser 22 by the pressure difference, which removes the need for any additional pumping, and also allows the analysis to be carried out without dumping the gas sample.

The analyser 22 is connected to a control unit via line 26. Signals from the analyser 22 relating to the detected quality parameters can be sent to the control unit, a logging system or a control room, which can if necessary adjust the performance of the apparatus (for example, increasing or decreasing the power supply to the compressor), and/or document that the gas is within specifications and/or to calculate the value of the gas. Thus, the system shown in Figure 1 allows for online monitoring of the natural gas stream.

Since the NMR unit needs little user intervention, the apparatus of Figure 1 is suitable for use in inaccessible locations, such as unmanned wellhead platforms. Further, as the gas sample is returned to the natural gas stream, the apparatus can be used with subsea apparatus, in addition, as the flow of the gas sample through the analyser is achieved by the pressure difference in the natural gas stream, there is no need to provide pumping means, which simplifies installation and reduces costs. In a preferred form, the sampled gas is conditioned to increase the resolution of the high-pressure analyser. This conditioning consists of increasing the sample pressure by means of a hydraulic booster, as schematically shown in Figure 2. The hydraulic booster may be powered by hydraulic fluid, which is usually available in subsea installations for other purposes.

The hydraulic fluid 32 enters at the bottom of the booster 30 (in the orientation shown in Figure 2), and pushes on the wider end 36 of the mushroom- shaped plunger 34. The narrower end 38 of the mushroom-shaped plunger 36 is in contact with the gas sample, and the difference in size between the ends of the plunger 34 allows a large increase in the pressure of the gas sample to be achieved with relatively low pressure hydraulic fluid.

The hydraulic booster shown in Figure 2 is preferred to a normal piston, as the process gas to be analyzed never comes into in contact with solid walls that have previously been in contact with another fluid (such as water or hydraulic fluid). There is thus less chance of contamination of the sample, which could lead to inaccurate measurements of the quality parameters.

In those cases where there is no hydraulic power available, a hydraulic amplifier which is driven only by process pressure, as shown in Figure 3, can be used. In a further alternative, the piston of the amplifier can be driven by screw movement instead.

In a preferred form, the pressurization is controlled according to a pressurization-temperature curve. Depending on this curve, heating may be required. The aim of this is to reduce the risk of liquid contamination in the measurement section (for example, contamination by liquid ethylene glycol, triethylene glycol or water). By controlled pressurization and cooling outside the measuring section, constituents of the gas sample can be condensed to liquid in a region that is not measured by the analyser, so that the analyser only analyses the gas sample.

It is also possible use a hydraulic amplifier to reduce the pressure of the gas sample. This may be useful if an optical analyser (such as an analyser which uses near infra-red, Raman spectroscopy, or the like) is used, as these analysers work better at lower pressures. Since the natural gas stream is a high-pressure gas stream, the pressure of the sample will still be reasonably high, even if the pressure is reduced. Depending on the form of the analyser, calibration gases may be required. As the method is intended to be carried out without user intervention, and is adapted for use in inaccessible locations, these calibration gases may be provided as a local supply in gas bottles or remotely through an umbilical line,

According to at least the preferred embodiments of the invention, gas quality monitoring can be carried out in inaccessible locations such as unmanned wellhead platforms or subsea. There is thus no need for topside installation or infrastructure to be available, and so monitoring of qua!ity parameters can be carried out in locations where this was not previously practicable. However, it will be appreciated that the monitoring can also be carried out in accessibie locations, such as onshore, in laboratories, and the like.

Further, by providing means to change the pressure of the sample, the analyser can operate at a higher resolution.