A GAS DETECTION SYSTEM

Abstract: A system for detecting escape of gas from a borehole (3) comprises an array of ultrasonic emitters (7) and absorption detectors (8) on opposite sides of the borehole (3), an ultrasonic Doppler-shift detector (4), and a gas collector (30) surrounding the drill pipe (1) to intercept at least a proportion of gas escaping from the borehole (3). The gas collector concentrates the gas through a chimney (32) in which are provided a gas analyser (33) and a gas flow meter (34).

Description: A Gas Detection System The invention relates to the detection of gas escaping from an underwater bore hole. In some areas of the offshore drilling world, thin deposits, or pockets of gas a few hundred to a few thousand feet below the seabed cause added hazard to offshore drilling. Some offshore drilling rigs initially begin an offshore bore hole by drilling into the seabed, with return fluids and solids going into the sea. This is called "open hole drilling". Gas which is released from a penetrated subsea gas pocket will then rise uncontrolled from the well opening to the ocean surface, frequently reaching the surface without warning, around and underneath the rig, producing a dangerous explosion and fire hazard. A number of offshore shallow-gas incidents have occurred due to this and have resulted in loss of life, and extensive property damage. After intermediate casing is set and cemented, a blowout preventer assembly (BOP) can be installed, providing the means for shutting in and controlling a kick. However, intermediate casing can only be set after 1000 to 5000 feet of open-hole drilling. Until the casing is installed and cemented, and the BOP assembly is installed, the present method of detecting a subsea gas release is a 24-hour manned "gas watch" by rig personnel, stationed around the rig periphery, carefully watching the water's surface. This method has the major disadvantage that the gas can be visually spotted only after it reaches the water surface - almost too late for rig personnel to take emergency action. In accordance with the present invention, a monitoring system for detecting gas released from an underwater bore hole comprises a detector adjacent to or below the sea bed which detects the presence of gas in the vicinity of the detector and an indicator at the surface to indicate the detection of gas by the detector. By detecting released gas before it reaches the surface, the invention provides extra warning time to rig personnel to turn off equipment, take their safety evacuation stations, and take other appropriate action. In one example of the invention, the detector comprises an emitter and a receiver so that a change in the signal received by the receiver from the emitter indicates the presence of gas in the water surrounding the detector. The detector could be an absorption detector in which gas located between the emitter and the receiver absorbs the signals emitted by the emitter so that a reduced signal is received by the receiver. Alternatively, the emission of gas from the well could be detected by a Doppler shift detector in which the signals are waves emitted by the emitter. The waves are reflected from rising bubbles of gas the movement of which causes a frequency shift in the reflected waves which is detected by the receiver. In another example of the invention, the system may also comprise a gas trap in which any gas emitted from the well is collected and the detector could be a suitable gas detector which detects the collection of gas within the gas trap. Preferably, the system comprises the Doppler shift detector, the absorption detector and the gas trap in combination and typically, the Doppler shift detector detects the presence of gas, the absorption detector monitors the volume and rate of gas escaping and the gas trap also comprises gas analysers which analyse the composition of the gas escaping. Preferably, the signals emitted by the emitter are ultrasonic waves. However, it may also be possible to use other types of signals such as electromagnetic radiation for the absorption detector. Preferably, there are a number of emitters and a corresponding number of receivers to enable substantially all of the bore hole to be monitored for escaping gas. Typically, the system is mounted on a support which is connected to the surface structure and which may be lowered and raised using the drill string as a guide. Preferably, the detector may also comprise a processor to process the signals and the processed signals are fed to the indicator which may be a display. Typically, the indicator may also comprise an alarm device which may be audible and/or visible to warn an operator when gas is detected by the system and which may also warn when the amount of gas being released reaches a predetermined release rate. Typically, the ultrasonic waves have a frequency of substantially 200kHz and are preferably modulated. Typically, the modulation frequency may be about 1kHz. An example of a gas detection system in accordance with the invention will now be described with reference to the accompanying drawings, in which:- Fig. 1 is a schematic diagram showing the system in use; Fig. 2 is a schematic diagram showing the gas monitoring part of the system of Fig. 1 in more detail; and, Fig. 3 is a detailed circuit diagram of the gas monitoring part of the system shown in Fig. 2 Fig. 1 shows a drill string 1 which extends from a surface structure (not shown) to the seabed 2. At the seabed 2 the drill string 1 extends into a well bore 3 which is being drilled by the drill string 1 and the drill string 1 rotates around its longitudinal axis in the direction shown by the arrow A to impart a rotational cutting motion to the drill bit (not shown) at the bottom of the well bore 3. Also shown is a Doppler detector 4 which comprises an ultrasonic transmitter 5 and an ultrasonic receiver 6. The transmitter 5 operates at a high frequency, which could be in the region of 200kHz and has a continuous wave operation. The transmitter 5 is directed downwards towards the open well bore 3 from a position just outside the cylindrical boundary that defines a possible drill string or drill bit position, so that the transmitter floods the upper opening of the well bore with a continuous 200kHz signal. The ultrasonic receiver 6 is arranged in a similar manner so that it is also directed downwards into the well bore 3 and observes the area immediately above the well bore 3. The return signal from the receiver 6 is amplified via an amplifier (not shown) and electronically mixed in a non-linear mixing circuit which produces a frequency difference output for a frequency difference exists between transmitted and received signals. The apparatus for carrying out the amplification and the mixing is well-known conventional electronics for detecting Doppler shift frequencies and could be mounted adjacent the Doppler detector 4 on the seabed or could be remotely located on the surface structure. If objects are in vertical motion within the range of the detector 4, the received signal contains a Doppler shift component, that is, the signal is higher or lower in frequency than the frequency emitted by the transmitter 5. If bubbles are rising from the well bore 3, then the signal is slightly higher in frequency. The general formula for calculating the Doppler shift component f D is:- f D = f e v b /c where:- f e is the frequency of the signal emitted by the emitter; v-_ is velocity of the bubbles; and, c is the velocity of the signal emitted by the emitter. In this example, sound travels at roughly 1,400ms-- 1 - in sea water and the frequency is 200kHz. Hence, as bubbles rise at approximately 0.30ms -1 , which is the nominal rise rate for bubbles in sea water at 10°C, a 43Hz increase in the nominal 200kHz signal emitted by the transmitter 5 would be expected. If the signal received by the receiver 6 is processed to look for increases in frequency and a substantial sustained Doppler shift component between 15Hz and 50Hz is detected it can be assumed that gas bubbles are being liberated from the sub-sea well bore 3. A warning signal can then be produced on the surface structure that causes the driller to stop drilling and take preparatory action to reduce potential fire hazards from gas releases in the sea. Also shown in Fig. 1 is a linear array of transmitters 7 and a linear array of receivers 8 on diametrically opposite sides of the well bore 3. The transmitters 7 and the receivers 8 are mounted on a portable sub-sea frame 9 so that they lie in an approximately horizontal plane a few feet above the seabed 2. If gas flows from an undersea well bore 3 which is open to the sea during initial drilling operations, the gas bubbles will alter the acoustic (sound carrying) properties of the sea water surrounding the well opening. Gas bubbles, being compressible, will absorb acoustic energy. Due to the lower density of gas with respect to sea water, gas bubbles will cause acoustic waves to reflect off the bubble's surface and change the wave's direction, bouncing some wave energy between bubbles until the acoustic energy is largely dissipated. The degree of acoustic energy reduction is a function of the gas to sea water ratio in the acoustic transmission path and so as the gas bubble density increases, the acoustic signal path degrades and a reduced signal is received by the receivers 8. When calibrating the absorption detector an acoustic signal with a frequency of approximately 200kHz is introduced into sea water by the transmitters 7 and directed across the well opening to the acoustic receivers 8, where the received signal amplitude, unchanged due to the absence of gas in the signal path, is recorded and displayed as a reference. As gas bubbles are released from the well bore, they pass through the acoustic signal path, and cause a reduction in received signal amplitude due to energy absorption, reflection, and signal dispersal. As the gas/water ratio in the signal path approaches roughly 50% signal attenuation approaches virtually 100%, reducing the received signal lever to near zero. Variations in the gas/water ratio produce corresponding variations in the received signal level (indicating roughly the gas percentage in a circular cross-section above the well bore), and since gas bubbles migrate upward at constant rate, it is possible to roughly calibrate an instrument to indicate the flow rate of released gas, knowing the rough equivalent > cross-section of gas, and assuming an average upward migration rate of gas through sea water. The well bore 3 also contains the drill string 1, which can and does move around randomly as it rotates and the well is drilled. The drill string is typically a hollow, heavy-walled steel pipe about 8" in diameter. It causes acoustic interference by reflecting and absorbing acoustic energy directed towards it. This in combination with the random lateral movements of the drill string 1 as it rotates causes irregular, varying baseline readings as the drillpipe wanders around in the acoustic path. This instability effect caused by the moving drillpipe is greatly reduced or eliminated in the invention by using several parallel acoustic energy emitters 7, as shown, thus creating a "line source" of acoustic energy across the well bore, and by the use of several individual acoustic receivers 8 fixed in a line opposite the several emitters. This is explained in more detail below. Figs. 2 and 3 show the arrays 7,8 and the associated electronic circuitry in more detail. A waveform generator 10 generates a 200kHz signal which is modulated by 75% by a 1kHz signal, which is amplified by the power amplifier 11 and fed to the transmitter array 7. The transmitter array 7 emits the 200kHz modulated signal which is then detected by the receiver array 8. The output signals from the receiver array 8 are transmitted via individual receiver channels 12 to detectors 13 which comprise a tuned amplifier detector 14 which extracts the 1kHz modulation signal from the output signal from the corresponding receiver and amplifies it before outputting it to an audio detector 15 which comprises a full wave rectifier which converts the 1kHz audio signal to a DC level signal. The DC level is dependent on the amplitude of the signal output from tuned amplifier detector 14 to the audio detector 15. The outputs from the detectors 13 are then output to a processor 16 which comprises a summing operational amplifier 17 which sums all the output signals from the audio detectors 15, to produce a single output signal. All the electronics described so far are located adjacent the arrays 7,8 near the seabed 2 and the bore hole 3 and could, for example, be mounted within the portable subsea frame 9. The output from the summing operational amplifier 17 is then output via a cable to the surface structure where it is passed through another operational amplifier 18 which controls the zero setting of a display 19 and a chart recorder 20 via a zero reference potential adjuster 21. The full scale deflection of the display 19 and chart recorder 20 are adjusted via a variable resistor 22. As the drill string 1 moves about, it interferes with the same number of emitters and receivers and therefore by using a linear array causes a relatively fixed percentage level of signal interference which produces a relatively constant output from the summing operational amplifier 17. As the drill string acoustic signal interference causes a fixed overall reduction in the total receive signal across the well bore 3, it can be easily calibrated. Typically, five channels 12 would be effective in reducing the drillpipe effect and ten channels 12 would be even more effective. In addition, on the sea bottom tidal currents can sweep away gas bubbles exiting from the well bore 3 before they are detected. Hence, it is desirable to install a water current shield vertically outside the acoustic signal path area which causes gas bubbles to rise more vertically for the first few feet after leaving the well bore 3. This permits more reliable detection by the acoustic system before the bubbles disperse. As the bubbles tend to absorb the acoustic energy emitted by the emitter array 7, the display 19 and chart recorder 20 will display a maximum reading when there is no gas being emitted from the well bore 3 and the reading will decrease inversely with the amount of gas exiting the well bore 3. An alarm horn 23 is connected to the display device 19 and is triggered when the reading drops below a preset level, that is, when the amount of gas exiting from the bore hole 3 rises to a pre-determined level. The alarm 23 warns the operators on the surface structure of a dangerous release of gas from the well bore 3. Also shown in Fig. 1 is a gas trap 30 which is mounted above the Doppler detector 4 and the portable subsea frame 9 so that gas exiting the well bore 3 is collected in the gas trap 30 after it has passed through the Doppler detector 4 and the subsea frame 9. The gas trap 30 has a central aperture 31 through which the drill string 1 extends. The gas from the well bore 3 collects in the gas trap 30 and due to the design of the gas trap 30 migrates towards an analysis chimney 32 where sensors 33 can confirm the movement of gas bubbles from well bore 3. The gas trap 30 enables gas samples to be collected and their properties analysed on the sea bottom. For example commercial combustible gas detectors are available which use infra-red means to determine whether or not a gas is combustible. If a sufficient sample of gas is collected, the sample will displace surrounding sea water from the infra-red detector which will allow its use underwater to detect the possibility of combustible gas being present. A flow meter 34 can also be incorporated into the chimney 32 to enable the rate of gas flow to be determined. The combustible gas monitor 33 and the flow meter 34 could be linked to the surface by means of a cable (not shown) which would give a read-out on the surface of the amount of combustible gas present and the amount of gas flow. The Doppler detector 4 enables gas exiting the well bore 3 to be detected and after detection of gas by the Doppler detector 4, the linear arrays 7,8 and the associated circuitry is activated in order to monitor the exiting gas to determine the amount of gas which is exiting from the well bore 3. The gas trap 30 is then used to analyse the gas in order to determine whether or not the escaping gas is combustible. Although, the system has been described here with all three methods of detection, it is possible that only one method could be used to detect the gas or that a combination of any two methods could be used. In use, after the initial exiting gas from the well bore 3 has been detected and the alarm tripped and acknowledged, the read-out from the display 19 and the chart recorder 20 shows approximate gas flow in cubic feet per day at the seabed. This helps the driller make critical judgements to reduce the hazard caused by the escaping gas. Changes in gas liberation rate from the well bore alters the sea waters acoustic properties which produces increases and decreases in gas flow rate indication on the display 19 and the chart recorder 20. The benefit of the chart recorder is that it provides a valuable permanent record of the gas release event time, event duration and approximate flow magnitude. Hence, a valuation of the chart record by the driller can help separate short time, harmless puffs of well bore gas from serious kicks. This system has the benefits that it enables prompt detection of a gas release at the seabed which provides an increase safety margin over any topside gas watch, especially in reduced visibility conditions. This allows the driller extra minutes to alert all the rig personnel, shut off potential ignition sources, and activate other hazard reduction plans, while the instruments, chart recording and display allow the driller to get an idea of the volume of released gas and its strength, for example, is it increasing or decreasing as he takes remedial actions. The invention also has the advantage that the continuous well bore monitoring by instrumentation reduces multiple shift personnel, that is, the gas watch, requirements, and lowers the rig risk exposure which would possibly reduce the insurance costs. In addition, the driller can better judge the size and trend of a gas release event which reduces the instance of false alarms to the rig. The permanent chart record also allows future administrative review of data recorded against action taken by rig personnel and having been provided with a faster warning rig personnel have time to take more economical as well as safer remedial action to a gas release. Modifications and improvements may be incorporated without detracting from the scope of the invention. MURGITROYD AND COMPANY MITCHELL HOUSE 333 BATH STREET GLASGOW G2 4ER

Claims: CLAIMS 1 A monitoring system for detecting gas released from an underwater borehole, comprising a detector adjacent to or below the sea bed which detects the presence of gas in the vicinity of the detector, and an indicator at the surface to indicate the detection of gas by the detector. 2 A monitoring system according to Claim 1, in which the detector comprises an emitter and a receiver arranged such that the signal received by the receiver from the transmitter is changed by gas released from the borehole. 3 A monitoring system according to Claim 2, in which the detector is an absorption detector. 4 A monitoring system according to Claim 2, in which the detector is a Doppler shift detector. 5 A monitoring system according to any of Claims 2 to 4, in which the emitter produces ultrasonic waves. 6 A monitoring system according to Claim 3, in which a plurality of emitters are arranged in a linear array at one side of the borehole, and a corresponding plurality of detectors are arranged in a linear array at an opposite side of the borehole. 7 A monitoring system according to Claim 1, including a gas trap adapted to be positioned adjacent and above the borehole to collect gas emitted from the borehole. 8 A monitoring system according to Claim 7, in which said detector is a gas detector within the gas trap. 9 A monitoring system according to Claim 7, including one or more emitters and absorption detectors beneath the gas trap, a further emitter and Doppler detector adjacent the gas trap, and analyser means in the gas trap for analysing the composition of gases escaping from the borehole. 10 A monitoring system according to Claim 9, in which said one or more emitters comprise ultrasonic transducer means transmitting a continuous wave at a relatively high frequency modulated at a relatively low frequency. 11 A monitoring system according to any preceding claim, the system being mounted on a support which is connected to a surface structure and which may be raised and lowered using as a guide a drill string extending from the surface structure into the borehole.

Inventors: SMITH GEORGE EDWARD, YOUNG GARETH JOHN, SMITH, GEORGE, EDWARD, YOUNG, GARETH, JOHN
Applicants: ELECTRO FLOW CONTROLS LTD, PREMIER RES & DEV LTD, ELECTRO-FLOW CONTROLS LIMITED, PREMIER RESEARCH AND DEVELOPMENT LIMITED
Application year: 1990
Main classification: E21B47
IPC codes: E21B47, E21B49, G01N29