The invention relates to measuring the amount of liquid entrained in a gas flow.

It is often necessary to measure the liquid content of a gas flow. In particular, in the gas outlet of separators and scrubber units, it is important to measure the amount of liquid hydrocarbons and/or water in the gas, as the presence of too much liquid in the gas can cause serious damage to equipment intended to operate solely on gas (such as compressors and contactor units). It is particularly important to measure the liquid content of gas emerging from a device for separating liquid from the gas, to monitor the performance of the device.

Various methods have been proposed for carrying out this measurement. For example, some known methods use radiation attenuation, where a radiation detector detects radiation which has been emitted by a radiation source and has passed through the gas. When the amount of liquid entrained in the gas increases, the amount of radiation detected decreases, and this decrease can be correlated with the amount of liquid in the gas. Similarly, the amount of liquid in the gas flow can be calculated from measurements of the density of the flow, the dielectric properties of the flow, and pressure drops.

However, these methods can be inaccurate and complex. The online measurement of amounts between 0.1 and 100 parts per million of liquid volume fraction, which would allow remote monitoring of the amount of liquid, is currently very difficult, if not impossible. It is an aim of at least the preferred embodiment of the invention to provide a more reliable and accurate method of measuring the amount of liquid entrained in a gas flow.

The invention arose from a consideration of the characteristics of flexible risers, which are widely used in the oil and gas industry. A problem which has been encountered in the past with flexible risers is the generation of high levels of noise and vibration when gas flows though the riser above a critical speed. This problem is referred to as "singing". It has been determined that the singing arises as a result of the corrugated structure of the carcass (the innermost layer of the riser); when gas passes through the carcass, vortex shedding occurs at each of the internal corrugations, and when the flow conditions are such that the frequency of vortex shedding coincides with the resonant frequency of the pipe, singing occurs. Singing is normally considered to be a serious problem with risers, as the resonances involved can lead to fatigue cracking of the riser and associated equipment to which the riser is connected. It has been found experimentally that singing can be prevented by adding small amounts of liquid to the gas flow (see the document entitled "Flow Induced Pulsations due to Flexible Risers" by S.P.C.

Belfroid et al, available at http://e-book.lib.sjtu.edu.cn/otc-2009/pdfs/otc19904.pdf). The invention seeks to utilize the effect of liquid on the gas flow to measure the amount of liquid entrained in the gas flow, and is not limited to measurements involving risers.

It is well known that both the vortex shedding effect (which produces the sound) and the resonator effect (which amplifies the sound) are damped by the presence of a small amount of liquid in the gas flow, in some cases causing some pitches to disappear.

According to a first aspect of the present invention, there is provided a method of detecting the amount of liquid entrained in a gas flowing in a process pipe, comprising the steps of: generating sound inside the pipe; providing a medium at least partially composed of the process stream for the sound to travel and/or resonate in; detecting sound in the pipe; determining the frequency and/or amplitude of at least selected pitches of the detected sound; and determining the amount of liquid entrained in the gas flowing in the process pipe based on the determined frequency and/or amplitude of the detected sound.

The sound may be generated by the gas flowing in the process pipe, as with the "singing" risers discussed above. However, the method can also be applied to smooth pipes, in which case the sound may be generated by sound generation means such as speakers provided within the pipe.

According to a second aspect of the present invention, there is provided apparatus for detecting the amount of liquid entrained in a gas flowing in a process pipe, comprising: a detection device for attaching to the process pipe such that the detection device can detect sound in the pipe, and an analysis device for connection to the detection device such that the analysis device can determine the frequency and/or amplitude of the sound detected by the detection device, and determine the amount of liquid entrained in the gas flowing in the process pipe based on the determined frequency and/or amplitude of the detected sound. The detection device may be a microphone or an accelerometer. The precise form of the detection device used will of course vary depending on the particular situation in which it is to be used.

The apparatus may further comprise one or more speakers to serve as a source of sound. Alternatively, one or more mechanical sound sources such as corrugations on the inner surface of the process pipe may be provided.

In order to ensure consistent measurement, the apparatus may further comprise a flow conditioning device located upstream of the detection device, to ensure that at least a part of the liquid is entrained in the gas as droplets.

It would be possible to mount the detection device in an opening made in the wall of the pipe. However, in order to reduce process piping modification, it is preferred for the detection device to be installed between flanges joining sections of the process pipe. Similarly, if speakers are used as the source of the sound, these may also be installed between flanges.

Preferably, the analyzing device analyzes several different pitches, which are sensitive to different liquid thresholds, to provide a non-analogue signal measurement.

A presently preferred embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of an apparatus for performing a preferred method of the invention using a sound source;

Figure 2 is a schematic view of a part of an apparatus for performing the preferred method of the invention with a corrugated pipe as the sound source; and Figure 3 is a view of an experimental set-up used to test the method.

Figure 1 shows a section of pipe 10 carrying a process stream of gas with entrained liquid (as indicated by arrow 20). The gas stream can be the output of a scrubber or separator, and can be conditioned before reaching the section of pipe 10 to ensure that any liquid is entrained in the gas a droplets. Further, after leaving the section of pipe 10, the gas stream may enter liquid sensitive equipment, and so it is important to monitor the amount of liquid in the gas stream.

The pipe is divided into sections 12, 14, 16; the whole of section 14 is shown, but only the downstream part of section 12 and the upstream part of section 16 are shown. Between sections 12 and 14 is a short pipe section 30, which includes a speaker 32. The speaker can generate sounds, which will propagate through the process stream.

Between sections 14 and 16 is a second short pipe section 40, which includes a detection device 42 in the form of a microphone (although an

accelerometer could also be used). The microphone can detect sound in the pipe, and is connected to an analysis device 44, which can analyse the detected sounds to determine their frequency and/or amplitude.

The speaker and the microphone are located in short sections of pipe, which can be bolted between the flanges on the ends of the sections 12, 14 and 16. This avoids the need to modify the sections of process pipe themselves.

Of course, it is also possible to attach the speaker and the microphone directly to the sections of process pipe. The speaker and microphone could simply be attached to the outer wall of the pipe, but this may cause difficulties as sound waves would then need to pass through the outer wall of the process pipe twice (from the speaker into the process stream and from the process stream to the microphone) in order to be detected. Alternatively, openings could be formed in the wall of the process pipe sections to accommodate the speaker and the microphone, but these would then need to be sealed.

As mentioned above, the resonator effect (which amplifies the sound) is damped by the presence of a small amount of liquid in the gas flow, which can lead in some cases to some pitches disappearing. Furthermore, the transmission of sound waves through the process stream is affected by the amount of liquid in the gas. If the speaker emits sound of a constant volume (constant amplitude), then a change in the amplitude (as detected by the microphone and the analysis device) can be correlated to a change in the amount of liquid in the gas, and so the amount of liquid in the gas can be measured.

The frequency of the sound emitted by the speaker can be freely chosen. The resonant frequency of the pipe can be used, but this may potentially cause problems with resonance of the pipe if the sound is emitted continuously. Short bursts of sound could be emitted, but this would mean that constant monitoring of the amount of liquid in the gas stream would not be possible. It may be preferable to use a sound with a frequency other than the resonant frequency of the pipe, and emit this continuously. Figure 2 shows, in a schematic form, a corrugated pipe 50 which carries a gas flow (as indicated by arrow 52). In practice, the corrugated pipe may be the carcass of a flexible riser. A microphone 54 is connected to the corrugated pipe 50, and can detect sound in the corrugated pipe. The microphone 54 is also connected to an analysis device 56, which can determine the pitch of the sound in the pipe.

As gas flows through it, the corrugated pipe 50 acts as an acoustic resonator, with specific resonance frequencies (or eigenfrequencies) for the principal resonance modes. The pitch of the sound detected by the microphone 54 as determined by the analysis device 56 will correspond to these eigenfrequencies.

However, the presence of traces of liquid in the gas flow will change the amplitude of the principal mode, and possibly the frequency for the principal mode. The change in the frequency and amplitude varies with the amount of liquid in the gas. This change in frequency and amplitude will be detected by the microphone 54 and the analysis device 56 as a change in the pitch/amplitude of the sound in the corrugated pipe 50, and the correspondence between the amount of liquid in the gas and the change in pitch and amplitude (for a given resonator geometry) can be correlated.

It is an attractive benefit that such liquid-in-gas measurement devices are robust, easy to install, and require minimal client-specific calibration.

Figure 3 shows an experimental set-up used to prove the concept behind the invention.

As can be seen, a length of pipe 60 is arranged horizontally, and gas flows through the pipe. A gas flow meter is arranged upstream of the pipe, so that the gas flow rate can be measured. A microphone 62 is arranged at the downstream end of the corrugated pipe (the gas outlet), to detect sounds in the corrugated pipe, and is connected to a computer which serves as the analysis device to determine the pitch of the detected sound.

In the experiment, dry gas was allowed to flow through the corrugated pipe, and the sound made by the dry gas flowing through the corrugated pipe was detected. Liquid was then injected into the corrugated pipe through syringe 64, and the sound detected by the microphone changed, with the pitch amplitude varying depending on the amount of liquid injected.

The method of the invention is well suited for subsea implementation in view of its simplicity and the very low energy consumption; all that is required is the microphone (and speaker, if one is to be used) and the associated electronics of the analysis device.