Title:
Motion compensated blowout and loss circulation detection
United States Patent 3910110
Abstract:
A system for detecting the commencemenet of a blowout or lost circulation in a subaqueous well being drilled from a floating vessel in which drilling fluid is being utilized. The rates of flow of the drilling fluid into and out of the well are monitored, compared, and a signal is generated proportional to the difference therebetween; the electrical signal is modified to compensate substantially instantaneously for the change in volume of the flow path of the drilling fluid caused by the heaving motion of the vessel. Alternatively, the rate of flow of the drilling fluid out of the well is monitored and an electrical signal is generated proportional thereto; the electrical signal is modified to compensate substantially instantaneously for the change in volume of the flow path of the drilling fluid caused by the heaving motion of the vessel.


Inventors:
Jefferies, Robert Kennedy (Reston, VA)
Wickline Deceased., Glenn D. (late of Houston, TX)
, Wickline, executrix; by Barbara Alice (Houston, TX)
Mirdadian, Kian Mohammed (Houston, TX)
Application Number:
05/508883
Publication Date:
10/07/1975
Filing Date:
09/26/1974
Assignee:
The Offshore Company (Houston, TX)
Primary Class:
Other Classes:
73/152.29, 175/48
International Classes:
E21B21/00; E21B21/08; (IPC1-7): E21B47/10
Field of Search:
73/155 175
View Patent Images:
US Patent References:
Primary Examiner:
Myracle, Jerry W.
Attorney, Agent or Firm:
Edwards, Joe Martin Julian Clark E.
Parent Case Data:


BACKGROUND AND OBJECTS OF THE INVENTION

This application is a continuing application of our pending application Ser. No. 403,380, filed Oct. 4, 1973, entitled "Motion Compensated Blowout and Loss Circulation Detection", now abandoned.
Claims:
What is claimed is

1. A method of detecting the commencement of blowouts or lost circulation in a subaqueous wellhole connected through risers or the like with a floating vessel and in which drilling fluid is being pumped from a mud system aboard the vessel into the wellhole and circulated back to the mud system, including the steps of:

2. A method of detecting the commencement of blowouts or lost circulation in a subaqueous wellhole connected through risers or the like with a floating vessel and in which drilling fluid is being pumped from a mud system aboard the vessel into the wellhole and circulated back to the mud system, including the steps of:

3. A method of detecting the commencement of blowouts or lost circulation in a subaqueous wellhole connected through risers or the like with a floating vessel and in which drilling fluid is being pumped from a mud system aboard the vessel into the wellhole and circulated back to the mud system, including the steps of:

4. A method of detecting the commencement of blowouts or lost circulation in a subaqueous wellhole connected through risers or the like with a floating vessel and in which drilling fluid is being pumped from a mud system aboard the vessel into the wellhole and circulated back to the mud system, including the steps of:

5. In a vessel floating in water and connected to a subaqueous wellhole through risers and the like, a method of monitoring the flow rate of drilling fluid being circulated from a mud system aboard the vessel down into the wellhole and back to the mud system, wherein the heaving motion of the vessel in the water is compensated for, including the steps of:

6. An apparatus for detecting the commencement of blowouts or loss circulation in a subaqueous wellhole connected through risers or the like with a floating vessel and in which drilling fluid is being pumped from the mud system aboard the vessel into the wellhole and circulated back to the mud system, including:

7. An apparatus for detecting the commencement of blowouts or loss circulation in a subaqueous wellhole connected through risers or the like with a floating vessel and in which drilling fluid is being pumped from the mud system aboard the vessel into the wellhole and circulated back to the mud system, including:

8. An apparatus for detecting the commencement of blowouts or lost circulation in a subaqueous wellhole connected through risers or the like with a floating vessel and in which drilling fluid is being pumped from a mud system abroad the vessel into the wellhole and circulated back to the mud system, including:

9. An apparatus for detecting the commencement of blowouts and lost circulation in a subaqueous wellhole connected through risers or the like with the floating vessel and in which drilling fluid is being pumped from a mud system aboard the vessel into the wellhole and circulated back to the mud system, according to claim 8, including:

10. An apparatus for detecting the commencement of blowouts or lost circulation in a subaqueous wellhole connected through risers or the like with a floating vessel and in which drilling fluid is being pumped from a mud system aboard the vessel into the wellhole and circulated back to the mud system, including:

11. An apparatus for detecting the commencement of blowouts and lost circulation in a subaqueous wellhole connected through risers or the like with the floating vessel and in which drilling fluid is being pumped from a mud system aboard the vessel into the wellhole and circulated back to the mud system, according to claim 10, including:

12. In a vessel floating in water and connected to a subaqueous wellhole through risers and the like, an apparatus for monitoring the flow rates of drilling fluid circulating from a mud system aboard the vessel down into the wellhole and back to the mud system, wherein the change in the volume of the flow path of the drilling fluid caused by the motion of the vessel the water is compensated for substantially instantaneously, including:

Description:
This invention relates to a motion compensated blowout and loss circulation detector and method to be utilized in connection with floating vessels from which drilling operations are being performed.

In drilling a well, particularly an oil or gas well, there exists the danger of drilling into an earth formation that contains high pressure fluids. When this occurs, the high pressure fluid from the formation flows into the well and displaces the drilling fluid (mud) up the well. If this occurrence is not controlled rather quickly, the drilling fluid may be substantially displaced and the high pressure fluid may flow freely up the well. This is termed a blowout. On the other hand, the well may be drilled into an earth formation which is very porous. In such a situation, there may be a tendency for the drilling fluid to flow freely from the well into the surrounding earth formation. This is termed lost circulation.

Blowout prevention is most effective when the commencement of an influx of high pressure fluid into the well can be quickly detected and controlled before an appreciable amount of the drilling fluid is displaced from the well. Loss of drilling fluid is kept to a minimum when the commencement of the loss can be quickly detected and the flow of the fluid controlled before an appreciable amount has passed from the well into the earth formation. It is known in the art to detect such an influx or loss of fluid by comparing the flow rate of the drilling fluid into the well and the flow rate of the fluid returning out of the well. A substantial increase in the rate of the return fluid flow when there was no corresponding increase in the rate of the fluid flow into the well, is indicative of a blowout. A substantial decrease in the rate of the return fluid flow when there was no corresponding decrease in the rate of the fluid flow into the well, is indicative of lost circulation. A readily available commercial apparatus for measuring the inflow and outflow rates of the drilling fluid, comparing such rates, and recording their differences, is the WATCO Differential Flowmeter manufactured by Warren Automatic Tool Company, 3915 Tharp Street, Houston, Texas. It is also known in the art to detect such an influx or loss of flow by monitoring only the flow rate of the drilling fluid returning out of the well. So long as the flow rate of the returning drilling fluid is substantially constant, it indicates there is no blowout or lost circulation. A substantial increase in the rate of the fluid flow out of the well is indicative of a blowout. A substantial decrease in the rate of the fluid flow out of the well is indicative of lost circulation. A readily available commercial device measuring the outflow rate of the drilling fluid is the Flowmeter Model No. 2806-SABQ-AS manufactured by Foxsboro.

However, in drilling offshore subaqueous wells from floating vessels, such as ships, platforms, or semisubmersibles, the measurement of the return fluid flow from the well is complicated by the heaving motion of the vessel. This heaving motion produces cyclic variations in the volume of the flow path of the returning fluid so that it becomes difficult to determine whether a substantial decrease or increase in the return flow is due to a blowout, lost circulation, or the movement of the vessel. For example, the subaqueous wellhole is usually connected to the floating vessel by a marine conductor or marine riser. To accommodate the motion of the vessel, the marine conductor or marine riser is usually provided with a telescoping joint. A hollow drill string extends downwardly from the vessel, through to marine conductor or marine riser, and into the well. A drill bit is connected to the lower end of the drill string. To accommodate the motion of the vessel, the drill string also is usually provided with a telescoping joint (often called a "bumber sub"). Drilling fluid is generally pumped from the vessel through the hollow drill string downwardly to the drill bit. The drilling fluid flows out into the well through ports in or adjacent the drill bit and circulates back up to the vessel through the annulus between the drill string and the casing or marine riser.

The motion of the vessel strokes the telescoping joints in the marine riser and the drill string causing them to expand and contract, thereby increasing and decreasing the volume of the flow path of the drilling fluid. This results in pulsations in the rate at which the returning drilling fluid is received by the mud system onboard the vessel. The instantaneous maximum and minimum flow rate of the returning drilling fluid induced by the heaving ship may be several times greater or less than the steady state or real flow rate. Thus, the real flow rate is masked by the contracting and expanding action of the telescoping joints, making it difficult or impossible to detect quickly any changes in the real return flow rate.

Gadbois, in his U.S. Letters Pat. No. 3,760,891, discloses a method and apparatus for rapidly detecting blowouts and lost circulation in a drilling well, which method and apparatus has particular application in a well being drilled at sea from a heaving vessel. The Gadbois system monitors the return rate of flow of the drilling fluid and generates an electrical signal proportional thereto. The electrical signal is monitored, accumulated, compared with selected samples of the accumulated signal, and compared with selected threshold values, to determine the existence of the blowout or lost circulation. The Gadbois system is very advantageous but does not provide a signal which is continuously and substantially instantaneously proportional to the flow rate of the drilling fluid and the volume of the drilling fluid flow path. Gorsuch, in his U.S. Letters Pat. No. 3,602,322, discloses a system for determining an inbalance between the rates of flow of the drilling fluid into and out of a well. Gorsuch, however, does not disclose a system which can effectively deal with the oscillations in the rates of flow of the drilling fluid in a well being drilled at sea from a heaving vessel.

It is, therefore, an object of this invention to produce an improved method and apparatus for monitoring the flow rate of the drilling fluid leaving from and returning to the mud system aboard a floating vessel, for generating a signal which is proportional to the difference therebetween, and for continuously modifying the signal to compensate substantially instantaneously for the change in volume of the drilling fluid flow path caused by the heaving motion of the vessel.

Another object of this invention is to provide a method and apparatus for detecting rapidly the commencement of blowouts and lost circulation in subaqueous wells being drilled from floating vessels wherein the flow rate of the drilling fluid to and from the mud system aboard the vessel is monitored and a signal is generated which is proportional to the difference therebetween and the volume of the drilling fluid flow path.

It is also an object of this invention to provide a method and apparatus for monitoring the flow rate of the drilling fluid returning from a well to the mud system aboard a floating vessel, for generating a signal proportional thereto, and for modifying such signal to compensate substantially instantaneously for the changing volume of the drilling fuid flow path caused by the heaving motion of the vessel.

Another object of this invention is to provide a method and apparatus for detecting rapidly the commencement of blowouts and lost circulation in wells being drilled from floating vessels wherein the flow rate of the returning drilling fluid is monitored and a signal is generated which is proportional to such return flow rate and the volume of the drilling fluid flow path.

These and other objects and advantages of the present invention will be set forth and explained and will become apparent in the following drawings, specifications, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings in which like reference numerals indicate like parts:

FIG. 1 is an elevation view of a typical subaqueous well being drilled from a floating semisubmersible vessel, the drilling fluid flowing from the mud system aboard the vessel through the drill string into the well and returning to the mud system aboard the vessel through the annulus between the drill string and the marine riser.

FIG. 2 is a combined block diagram and schematic diagram of the components comprising a first preferred embodiment of this invention.

FIG. 3 is a chart of change in elevation of water compared with time depicting the operating boundaries of the first preferred embodiment of this invention.

FIG. 4 is a schematic diagram of the components comprising an alternate embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates schematically certain portions of a typical offshore well drilling apparatus. A semisubmersible vessel 4 for floating on a body of water 5 is engaged in drilling a subaqueous well 6 in the seabed 7. The vessel 4 mounts on its deck a derrick 8 which includes a draw works (not shown) and other usual apparatus for conducting drilling operations. Extending between the vessel and the wellhole in the seabed is a marine riser indicated generally at 9 which includes at its lower end the usual blowout preventor apparatus 10 and, at its upper end, a telescoping joint 11. Telescoping joint 11 includes an upper cylindrical portion 12 which is mounted from and is movable with the vessel 4 and a lower cylindrical portion 13 which remains stationary with respect to the seabed. Upper portion 12 telescopes into lower portion 13. The upper and lower portions are slidably sealed with respect to each other. Drill casing 14 is installed from the blowout preventer 10 downwardly a selected distance into the wellhole.

A drill string indicated generally at 15 is supported from a swivel 16 within the derrick. The swivel 16 is suspended from a traveling block 17 which in turn is connected through cables to the crown block 18 at the top of the derrick. The drill string extends downward through the marine riser 9, blowout preventers 10, well casing 14, and into the wellhole. A bit secured to the lower end of the drill string drills the wellhole in the earth. The drill string also includes a telescoping joint 20. The telescoping joint 20 includes cylindrical portion 21 which surrounds and is sealed against an upper cylindrical portion 22 and a lower cylindrical portion 23 of the drill string. The upper and lower portions of the drill string are slidable sealed against and telescope into the outer portion 21.

In the customary fashion, drilling fluid for flushing out dirt and rock chips during drilling of the well is pumped from the mud tank 24 on the vessel 4 by pump 25 through standpipe 26 to the swivel 16. The drilling fluid is circulated down the bore of the drill string 15 and out ports 27 in the drill bit 19. The drill fluid returns to the vessel through the annulus 28 between the drill string 15 and the casing 14 and between the drill string 15 and the marine riser 9. At the vessel, the drilling fluid returns to the mud tank 24 through conduit 29. The mud tank 29, pump 25, flexible hose 26, and conduit 29 are referred to herein as the mud system aboard the vessel.

It is desirable to monitor the flow rate of the drilling fluid returning to the mud system aboard the vessel and generate a signal porportional thereto. This preferably is accomplished by inserting a monitoring apparatus 30 into the conduit 29 returning the drilling fluid to the mud tank. The monitoring apparatus 30 may be any of numerous commercially available apparatus, such as a Foxsboro Flowmeter Model No. 2806-SABQ-AS. It is also desirable to monitor the flow rate of the drilling fluid leaving the mud system aboard the vessel and generate a signal porportional thereto. This is preferably accomplished by inserting a monitoring apparatus 31 into the standpipe 26. The monitoring apparatus 31 also may be any of numerous commercially available devices such as a Foxsboro Flowmeter of a type similar to that described above. In the first preferred embodiment of this invention, it is preferable that both the rate of flow of the drilling fluid away from and back to the mud system be monitored and the difference between such flow rates be determined. This may be accomplished through the use of numerous commercially available apparatus, such as the WATCO Differential Flowmeter manufactured by Warren Automatic Tool Company, 3915 Tharp Street, Houston, Texas. A variation of the WATCO Differential Flowmeter is illustrated in U.S. Letters Pat. No. 2,966,059. The WATCO Differential Flowmeter utilizes Foxsboro Flowmeters, standard discrete electronics, and a Varian Recorder Model No. G-11A-B1.

It is desirable to employ a means for determining the heaving motion of the semisubmersible vessel 4 relative to the wellhead as such heaving motion has the effect of increasing or decreasing the volume of the flow path of the drilling fluid. This can be accomplished in numerous ways well known to those skilled in the art. It can be accomplished through sophisticated electronics utilizing accelerometers and other commercially available position sensors. However, the preferred manner of determining that motion of the vessel which increases or decreases the volume of the flow path of the drilling fluid is to attach cables 32 (only one of which is shown) between the outer portion 13 of the marine riser 9 and riser tensioner assemblies 33 (only one of which is shown) attached to the vessel. Each riser tensioner assembly 33 preferably comprises a cylinder 34 attached to the vessel 4 and a piston and piston rod mounted for longitudinal movement therein. A sheave 36 is attached to the end of the piston rod 35 extending from the cylinder. The cable 32 is reaved around the sheave 36 and fastened to the semisubmersible vessel 4. Hydraulic fluid is supplied into the cylinder 34 and against a selected side of the piston as is well known in the art. As the semisubmersible vessel 4 moves relative to the wellhead, the cylinder 34 and piston rod 35 expand and contract relative to each other. It is desirable that the linear movement of the piston rod 35 relative to the cylinder 34 be transduced into data proportional thereto. This may be accomplished through the use of any of numerous commercially available apparatus. For example, a position transducer for transducing linear motion into an electrical signal proportional thereto can be attached to the cylinder 34 in contact with the piston rod 35. Preferably, the linear movement of piston rod 35 is transduced, by the use of an extension arm 37, cable 38, and sheave 39, into rotary motion. The rotary motion is then transduced into an analog electrical signal proportional thereto by any of numerous commercially available devices, such as, a potentiometer.

One of the advantages of a measuring system connected to the marine riser 9 is that it takes into account the manner in which the vessel 4 heaves in the water. The vessel 4 moves substantially vertically responsive to the tide because the vertical change of the water responsive to the tide coming in or out is very slow. But when the vessel 4 heaves responsive to wave action, its movement is not entirely vertical. Even though the vessel 4 is anchored, the vessel 4 will move laterally as well as vertically responsive to wave action. This movement of the vessel responsive to wave action is referred to as heave. The marine riser 9 is relatively flexible and the upper end of the marine riser will move with the vessel 4. Thus, the monitoring system which is connected to the upper end of the marine riser 9 produces data indicative of an increase or decrease in the flow path of the drilling fluid whether such increase or decrease is caused solely by vertical movement of the vessel or the combination of the vertical and lateral movement of the vessel.

FIG. 2 illustrates a first preferred apparatus according to this invention for determining the rate of flow of the drilling fluid out of the well without substantial error being produced in such determination by the change in the volume of the flow path of the drilling fluid caused by the heaving motion of the vessel. A means 45 is employed to measure the flow rates of the inflowing and outflowing drilling fluid, comparing such flow rates and determining the difference therebetween, and recording such difference. There are numerous commercially available apparatus for performing the function of such means 45. For example, the WATCO Differential Flowmeter manufactured by Warren Automatic Tool Company may be utilized. The WATCO Differential Flowmeter 45 includes two Foxsboro Flowmeters 31 and 30 for monitoring the flow rate of the inflowing and outflowing drilling fluid, respectively, and for generating electrical signals proportional thereto. The two electrical signals generated by the Foxsboro Flowmeters 31 and 30 are coupled to the means 46 within the WATCO Differential Flowmeter 45 for comparing such signals, determining their difference, and generating an electrical signal porportional to the difference therebetween. The electrical signal proportional to the difference therebetween is coupled to the input of a Varian Recorder 47. The electrical signal coupled from the comparing means 46 to the Varian Recorder 47 vacilates from zero voltage to a plus or minus voltage depending upon the difference between the outflow and the inflow of the drilling fluid. This electrical signal is recorded on a strip-type chart by the Varian Recorder 47. If the outflow and inflow are at an equal rate, the recording will be zero, in the center of the chart. If the outflow is greater than the inflow, the difference is recorded to the right of zero as a "gain." Conversely, if the inflow is greater than the outflow, the loss of drilling fluid in the hole is recorded to the left of zero as a "loss." The output of the comparing means 46 is also coupled to the input of an alarm means 48 which functions to sound an alarm whenever the electrical signal generated by the comparing means 46 exceeds preselected threshold limits.

A means 59 is employed for generating a signal which accurately, continuously, and substantially instantaneously compensates for the change in the volume of the drilling fluid flow path caused by the heaving motion of the vessel. Preferably, a potentiometer circuit 49 is physically manipulated by a shaft or other means coupled to sheave 39. The means for operating a potentiometer circuit are well known to those skilled in the art. The electrical signal generated by the potentiometer circuit 49 has a voltage proportional to the motion of the vessel relative to the wellhead. The electrical signal generated by the potentiometer circuit 49 is resistively coupled to the input of a buffer amplifier, preferably a type 741 such as is manufactured by numerous manufacturers. The output of amplifier 50 is also an electrical signal proportional to the heave of the vessel relative to the wellhead. The vessel moves relative to the wellhead responsive to two general types of movement of the water: the movement of the water as a result of the tide and the movement of the of the water as a result of normal wave action. The portion of the electrical signal generated by amplifier 50 proportional to the movement of the vessel responsive to the tide may be thought of as a DC portion of the signal because of its very long time period. The portion of the electrical signal generated by the amplifier 50 proportional to the movement of the vessel responsive to the wave action may be thought of as the AC portion of the signal because of its relatively short time period.

The electric circuitry according to this invention to compensate for the heaving movement of the vessel relative to the wellhead need not provide total compensation for such movement. Generally, compensation for movement of the vessel responsive to the tide is not needed because the period of the tide is so long. On the other hand, when the period of the waves becomes very short in time and the amplitude of the heave of the vessel very great, drilling operations are not conducted. For example, referring to FIG. 3, the electrical circuitry preferably is designed to ignore all wave action having a longer period than 900 seconds and the circuitry need not be accurate for wave action to the left of line 51, such as a 40 foot wave moving from crest to crest in twelve seconds or less, because drilling operations are not conducted in such conditions.

The output of amplifier 50 is coupled to a capacitance 52 chosen to eliminate the substantially DC portion of the signal having a period longer than 900 seconds. The output of capacitance 52 is resistively coupled to the input of amplifier 53. Amplifier 53 is preferably a type 741 manufactured by numerous manufacturers. It is wired in a differentiator mode to differentiate the electrical signal supplied to its input and to generate from its output an electrical signal proportional to the rate of heave of the vessel within the operating limits of the circuitry. It is desirable to determine the rate of heave of the vessel, rather than the mere amplitude of the heave of the vessel, in compensating for the heave of the vessel because the movement of the vessel responsive to the wave action is not at a constant velocity. The rate of heave of the vessel is zero at the crest and trough of a wave and increases to a maximum as the vessel moves between the two. The error introduced into the electrical signal output by comparing means 46 arises from the change in volume of the flow path of the drilling fluid. And the change in volume of the flow path of the drilling fluid varies in response to the rate of the heave of the vessel.

The electrical signal generated by amplifier 53 is modified or scaled to take into account the cross-sectional area of the drill pipe and the cross-sectional area of the annulus between the drill pipe and the marine riser. This preferably is accomplished by resistively coupling the electrical signal output by amplifier 53 to the input of amplifier 54, a type 741 amplifier operated in the differential mode. By varying the ohmage of the resistances supplied to the input of amplifier 54, the electrical signal output from amplifier 53 can be modified or scaled. Preferably the output of amplifier 53 is coupled over line 55 through a bank of resistances to one of the inputs of amplifier 54, the resistance of the bank being variable in accordance with the cross-sectional area of the drill pipe. The electrical signal output by amplifier 53 is combined with a variable resistive feedback signal and coupled over line 56 to the other input of amplifier 54. The resistance of the feedback signal may be varied in accordance with the cross-sectional area of the marine riser.

The electrical signal generated by amplifier 54 is proportional to the rate of heave of the vessel and the cross-sectional area of the flow path of the drilling fluid. This electrical signal is coupled to the input of amplifier 57 which functions to scale the electrical signal so that it will match the scale factor of the recorder 47 being utilized. The output of amplifier 57 is applied across resistances which are coupled into the low voltage line between the comparing means 46 and the recorder 47 and alarm 48. The voltage dropped across resistance 58 compensates the electrical signal generated by comparing means 46 for the change in volume of the flow path of the drilling fluid as the vessel heaves relative to the wellhead.

Thus, the first preferred embodiment of this invention provides an improved method and apparatus for monitoring the flowing rates of drilling fluid into and out of a well being drilled at sea from a floating vessel, generating a signal proportional to the difference between such flow rates and continuously modifying such signal to compensate substantially instantaneously for the change in the volume of the flow path of the drilling fluid caused by the heaving motion of the vessel. Such method and apparatus has particular use in rapidly determining the existence of blowouts and loss circulation.

An alternate embodiment of the invention employs the means 59 for compensating for the change in the volume of the drilling fluid flow path caused by the heaving motion of the vessel in combination with an apparatus which monitors only the flow rate of the drilling fluid out of the well and generates a signal proportional thereto. As shown in FIG. 4, preferably a flowmeter 30, such as the Foxsboro Flowmeter previously described, is employed on the conduit 29 carrying the drilling fluid out of the well and back to the mud system. The flowmeter 30 generates an electrical signal proportional to such return flow rate. The electrical signal generated by flowmeter 30 is coupled to the input of a summing amplifier 60, such as is readily available on the commercial market. Also coupled to the input of summing amplifier 60 is the output of the means 59 for compensating for the change in the volume of the drilling fluid flow path caused by the heaving motion of the vessel. The summing amplifier 60 sums the two electrical signals supplied to its input and generates an electrical signal which is proportional to the flow rate of the returning drilling fluid and the volume of the drilling fluid flow path. The electrical signal generated by the summing amplifier 60 may be coupled to a chart recorder, such as the previously described Varian Recorder, or to an alarm apparatus.

Optionally, flowmeter 31 may be employed on the conduit 26 carrying the drilling fluid into the well, and the electrical signal generated by the flowmeter 31 proportional to the flow rate of the drilling fluid into the well may also be coupled to the chart recorder. The modified electrical signal proportional to the flow rate of the returning drilling fluid and the volume of the drilling fluid flow path may be compared with the electrical signal proportional to the flow rate of the drilling fluid into the well. This comparison may be accomplished by the operator of the system observing the chart recorders or by an electrical apparatus such as is well known to those skilled in the art.

Thus, this alternate embodiment of the invention provides an improved method and apparatus for monitoring the flow rate of drilling fluid out of a well being drilled at sea by a floating vessel, gnerating a signal proportional thereto, and continuously modifying such signal to compensate substantially instantaneously for the change in the volume of the flow path of the drilling fluid caused by the heaving motion of the vessel. Such method and apparatus has particular use in rapidly determining the existence of blowouts and loss circulation.

This invention provides an improved blowout and loss circulation detection method and apparatus in which there is substantially instantaneous compensation for the change in the volume of the flow path of the drilling fluid as a result of the heaving movement of the vessel relative to the wellhead. The invention has been shown and defined with reference to two particular embodiments. However, many variations and modifications of the invention will now be apparent to those having skill in the art. For instance, the discrete electronics for compensating the electrical signal generated by the comparing means 46 may be arranged and constructed in a myriad of different forms. A different means 45 for monitoring the inflow and outflow of the drilling fluid and generating an electrical signal proportional to the difference there-between may be utilized. For example, the output of the comparing means 46 may be coupled to an analog-to-digital converter, such as a C.I.I. Model 7311, which converts the analog electrical signal into digital data. Accordingly, the output of amplifier 57 may be coupled to an analog-to-digital converter, such as a C.I.I. Model 7311, to transduce the analog electrical signal into digital data. The two streams of digital data may then be correlated to produce a compensated digital signal responsive to the difference between the inflow and outflow of the drilling fluid modified by the movement of the vessel relative to the wellhead. Additionally, rather than electrical signals being employed throughout, the flow rate or rates could be measured hydraulically and such hydraulic signals modified to compensate for the change in the flow path of the drilling fluid caused by the heaving motion of the vessel. It is believed, therefore, that the foregoing disclosure and description of the invention are only illustrative and explanatory, and various changes in the circuitry and the components may be made within the scope of the appended claims without departing from the spirit of the invention.