NO803303L - PROCEDURE AND DEVICE FOR DETERMINING DRILLING FLOW FROM A DRILL - Google Patents

Description

The present invention relates to a method and a device for determining the flow of drilling fluid from an underwater borehole, and more specifically, the invention relates to a method and a device for determining abnormal drilling conditions, e.g. the onset of a blowout or lack of circulation when heaving or vertical movement of a floating vessel, from which the drilling operation is carried out.

When drilling underwater boreholes, particularly oil and gas wells using rotary drilling methods, a hollow drill string extends from the surface to the bottom of the borehole. A drill bit is attached to the lower end of the drill string. Drilling fluid (mud) is circulated from the surface through the drill string and nozzles in the drill bit into an annular space that lies between the drill string and the inside of the borehole. The drilling mud then circulates upwards through the annular space to the surface where it is transferred to one or more tanks for treatment (e.g. removal of cuttings from the borehole, addition of chemicals etc.) before recycling to the borehole.

During the drilling operation, the drilling mud has several functions. most important is to hold back liquid under high pressure

in different soil formations. From time to time, the fluid under high pressure will enter the borehole and displace the drilling mud. This first penetration is referred to as a kick. If this takes place, it is correct that the pressure conditions are balanced as soon as possible, otherwise the liquid under high pressure will flow up through the borehole. This condition is known as a blowout. If weak soil formations are encountered during the drilling operation, however, the hydrostatic pressure from the drilling mud can break up the rock and the drilling mud can freely penetrate the formation from the borehole. This is referred to as lost circulation.

A blowout is most effectively prevented when the aforementioned kick or the first penetration of the formation's fluid is quickly detected and limited before this fluid displaces significant amounts of drilling mud from the borehole.

In the same way, lost circulation can be effectively limited when the beginning of the loss can be detected quickly and counteracted before a significant amount of the drilling mud has flowed from the borehole into the formation. Time is important when it comes to detecting these abnormal drilling conditions that can create dangerous situations.

Two basic methods are usually used in drilling operations to detect "kick" or lost circulation.

One method is based on a determination of the flow of drilling mud from the borehole. The second method is based on a determination of the volume of drilling mud that is carried from or to the borehole.

The first method involves determining the flow rate of the drilling mud returning from the borehole and comparing this rate with either (i) the rate of returning mud flow at earlier times or (ii) the rate of that drilling mud. which circulates down the borehole. The former method is in common use and is useful because the velocity of all circulating mud in the borehole often remains largely constant over long periods of time. The latter method has the advantage that it automatically compensates for normal changes in the sludge circulation rate. An increase in the mud return rate from the borehole over a corresponding increase in the circulation rate down the borehole is an indication of a kick. Similarly, an unexplained drop in the speed of the return flow will be an indication of lost circulation.

The other basic method is based on determining the volume of the drilling mud that is contained in the mud tanks on the surface, and which is in fluid connection with the borehole. These thoughts are in one or the other of two categories, namely active thoughts or trip thoughts.

Active tanks are those through which the drilling mud is circulated for removal of solids and other treatment before re-circulation. The volume in active tanks is affected by the differences between the volume of mud that is pumped down the borehole and the volume that returns from it. Although a number of normal processes can affect this volume (removal of solid particles, addition of water or other materials) an unexplained increase in volume will indicate a kick, while an unexplained drop will indicate lost circulation.

The trip tanks are usually much smaller than the active tanks and are therefore much more sensitive•to changes

in the sludge volume. It is connected to the borehole during periods when drilling mud is not circulated down the borehole through the drill string. Such non-circulating periods include (i) times when a kick is expected and mud circulation is stopped to determine if there is current in the wellbore, and (ii) times when the drill string is removed from or returned to the wellbore. This latter operation is called a trip and hence the name "trip tank". During the trips, the changes in the sludge volume in the trip tank are compared with the expected displacement on the ground

of inserting or removing a given length of drill string in or out of the borehole. In this way, unexplained increases or decreases in the trip tank's volume could be interpreted as kick or lost circulation.

Unfortunately, drilling offshore wells from a floating vessel will complicate monitoring the return flow of drilling mud or the volume of drilling mud at the surface. The drill rig is connected to the drill hole by a riser that serves as an extension of the drill hole between the seabed and the rig. The drilling mud returns to the vessel from the borehole through an annular space located between the outside of the drill string and the inside of the riser. In order to accommodate heave or vertical movement of the vessel, the riser usually has a telescopic section or sliding joint.

In the sea, the heaving movement of the vessel will cause the telescopic section to slide back and forth, thus lengthening or shortening the riser. In this way, the lower section (below the telescopic section) of the riser will remain stationary in relation to the seabed, while the upper part of the riser moves up and down together with the vessel. The forward-

and backward movement of the telescopic section increases and decreases the volume of the annular cavity and thus

the volume of the drilling mud that is in this space during backflow from the borehole. The resulting variations in the volume of the annular space in the telescoping sec- . tion affects the measurements of the mud flow from the borehole if one wishes to monitor this flow over the telescope section. In typical cases, one is interested in this because it is usually impractical to measure the current in the riser under the telescopic section due to difficulties caused by a rotating drill string.

The maximum and minimum flow rate of the drilling mud introduced by extension contraction of the riser can be many times greater or less than the actual or true flow rate from the borehole. E.g. variations can also occur in the measured return flow of drilling mud in the range from 9,000 liters per my. in the opposite direction (when the telescopic section expands) to approximately 22,500 liters per my. in the normal direction (when the telescoping section contracts) compared to a correct or true drilling mud return flow rate from the borehole of approximately 6750 liters per my. In addition, the variations in the volume of drilling mud in the telescopic section will introduce variations in the volume of drilling mud contained in the tanks which are in fluid connection with the riser. These variations complicate an accurate assessment of the increases or decreases in tank volume. For this reason, cyclic variations in the volume of the riser, produced by the movement of the vessel, will complicate an accurate detection of an abnormal drilling condition. A quick detection of a blowout or a lost circulation is very difficult without a means of correcting for the effects of the variations in the length of the telescoping member if one wishes to monitor the return flow of drilling mud or the volume above the telescoping member.

In the U.S. patent no. 3,602,322 describes a system for sensing a variation between the input and output currents from a borehole, above a certain tolerance limit. This system is one of several that apply to the determination of a blowout or lost circulation. Application of the system is, however, limited to a motionless system, e.g. on land. The described system cannot effectively exploit variations in the return flow of drilling mud due to the vessel's heaving motion.

In the following U.S. patents have addressed the problem of accurately detecting the true flow rate of returning drilling mud when monitoring the flow rate over the telescoping joint and while the offshore vessel is in heaving motion:

U.S. Patent No. 3,760,891

U.S. Patent No. 3,910,110.

U.S. patent No. 3,976,148.

The system described in U.S. Pat. patent No. 3,760,891 monitors the speed of the bores.lamb return flow at the vessel and produces an electrical signal proportional to the return speed. The signal is then processed and collected over preselected overlapping time intervals and compared to threshold values to determine presence

■ of a kick or lost circulation. The system requires preselection of time intervals over which collection takes place. The time interval is constant. However, this system will not enable monitoring of a telescoping member over time periods so that the effect on the final determination of the drilling mud flow from the borehole due to expansion and contraction of the telescoping member is eliminated.

U.S. patent no. 3,910,110 describes a system for detecting a kick or a lost circulation in an underwater borehole, where the return speed of the drilling mud flowing back to the vessel from the borehole is measured, and an electrical signal is produced proportional to the flow rate. The electrical signal is modified to compensate for rates of change in the mud volume inside the telescopic section. The modified electrical signal is then compared with a second electrical signal which is proportional to the flow rate of the drilling mud down the borehole. This patent describes a system for continuous modification of the electrical signal to compensate for a change in the volume of the flow path produced by the heaving movement of the vessel.

U.S. patent no. 3,976,148 also describes a method and a device for determining on board the vessel the flow rate of drilling mud coming from the borehole.

In this patent, however, a first, second and third electrical signal is produced corresponding respectively to (i) a flow rate of the mud flowing through a line on the downstream side of the telescope part, (ii) a rate of change for the volume of drilling mud contained in the riser above the point where the conduit between the sludge treatment system and the riser intersects the riser, and (iii) a rate of change of the volume of sludge in the telescopic section. The first, second and third signals are then combined to produce a fourth electrical signal which is proportional to the flow rate of the drilling mud flowing out of the borehole and into the riser. In the U.S. patent no. 3,976,148, however, continuous monitoring of the extension and contraction of the telescopic part is required in order to be able to accurately estimate the rate of change in terms of the volume of the drilling mud flowing through the riser.

Further background of general interest for systems for equalizing vessel heave and pressure control with drilling mud that circulates back is:

U.S. Patent No. 3,809,170

U.S. Patent No. 3,811,322

U.S. patent No. 3,815,673

U.S. patent No. 3,946,559

U.S. patent No. 4,085,509

U.S. patent No. 4,099,536

U.S. patent No. 4,099,582

U.S. Patent No. 4,121,806

U.S. Patent No. 4,138,886.

The present invention relates to a method and a device for determining an abnormal drilling condition by detecting penetration of formation fluids into the borehole or loss of drilling mud from it. In particular, the invention concerns a method and a device for measuring the flow of drilling mud coming from an underwater borehole. The invention particularly relates to offshore drilling from a floating vessel with a riser that forms a connection between the vessel and the borehole. A mud system is connected to the riser by means of a line. The riser comprises a telescopic joint or a sliding joint with an upper and lower cylinder which is displaceable in relation to each other to accommodate vertical movement or heave movement of the vessel. More specifically, the invention relates to determining the flow rate of the drilling mud coming from the telescopic part.

In short, the present invention comprises the measurement of

the volume of drilling mud coming from the telescopic part during the period when the telescopic part moves from a predetermined reference position and returns to this predetermined reference position. The reference position is a pre-selected mutual position between the upper cylinder and the lower cylinder. The volume of drilling mud in the telescopic part will thus always be constant in the reference position due to the preselected relative position of the two cylinders.

Another feature of the present method is to determine the specific times at which the telescopic part is in the reference position since the volume of drilling mud contained in the telescopic part is the same at these times. In this way, time periods or intervals are defined between the successive times which indicate that the predetermined reference position occurs. The flow of drilling mud from the riser will thus, when the average value is taken over one or more such time periods, be unaffected by the heaving of the vessel. The volume of drilling mud contained in any tank in close fluid communication with the riser will also be unaffected by the heaving movements of the vessel when measured at these particular times. In other words, the volume in the telescopic part at successive points when the predetermined reference position occurs will be unaffected by the vessel's heaving movement and this makes it unnecessary to measure exactly this volume for

having to correct for a change in this volume (as described in U.S. Patent Nos. 3,910,110 and 3,976,148) in order to determine either the flow of drilling mud leaving the borehole or the volume in surface mud tanks.

The present invention can be applied to different types of pipes between the riser and the sludge treatment system. At one plant, the line connecting the riser to the mud tanks is kept continuously full of drilling mud. In this way, errors that can be caused by variations in the mud volume in the line between the riser and the point where current or volume are measured are reduced to a minimum.

In another installation, shock absorber devices are used in fluid connection with the line to dampen the magnitude of the variations in the flow of drilling mud due to extensions and contractions of the telescopic part when the vessel pulls up. When shock absorber devices are used, the measurements of the volume of drilling mud flowing down through the telescopic part are preferably carried out on the downstream side of the shock absorber device itself. The flow variations on the downstream side of the shock absorber device are significantly smaller than if there were no shock absorber devices due to the devices' damping effect. When a shock absorber device is used, the method according to the invention thus includes measuring a volume change in the shock absorber device at the same time as the current is measured on the downstream side of the device. Summation of volume change in the shock absorber device and volume passing the meter on the downstream side of this device is the desired volume of sludge flowing from risers. In another embodiment, the shock absorber device is used as a pressure tank. A valve is fitted in the line on the downstream side of the shock absorber device. When mud is not circulated to the borehole, the valve closes and prevents flow through a meter on the downstream side of the shock absorber device. The volume of mud in the shock absorber device is therefore, when measured at the times indicating the reference position, unaffected by the heave movements of the vessel. In this way, unexplained increases or decreases in this volume can be detected much faster than was previously possible.

In a preferred embodiment of the invention for use on pipelines with or without shock absorber devices, the time period is measured between two consecutive reference positions. The measured volume of sludge passing the flow meter (summed together with the volume change in the sludge in the shock absorber device if one is used) during a certain time period is divided by the same time period to determine the average flow of sludge from the riser.

In another embodiment of the present invention, the volume of sludge contained in active sludge tanks is measured at the specific times when the telescopic part is in its reference position. The piping may, but need not, contain a shock absorber device on the upstream side of the active sludge tanks. If a shock absorber device is used, the volume of sludge in this is also determined with the telescopic parts in the reference position and it is summed together with the volume in the active tanks.

It is, however, a feature of the present invention that one is not limited to a single time period, but instead can produce consecutive time periods between consecutive signals. A signal is produced every time the relative position between the upper and lower cylinders is at the predetermined relative position. Each signal will end a previous time period and start the next one. Furthermore, the invention is not limited to equal periods of time. The duration of each time period is of no importance because the invention selects when controlling the telescopic section time periods so that the change in volumetric displacement in the telescopic section due to the lifting movement is eliminated. As the expression "consecutive" is used in this case, it differs from "successive" in that the invention is not limited to periods of time that immediately follow each other. The invention can be practiced over several time periods that lie between a predetermined number of signals.

When practicing the invention, consecutive time periods can be linked to their respective volumetric measurements. A single clock can record consecutive readings, or alternatively, a series of clocks can be designed to start and stop multiple gauges for consecutive measurement of the volume change in the shock absorber device and the volume of drilling fluid flowing on the downstream side of the shock absorber device.

The improved device for determining the current

of drilling fluid flowing from an underwater borehole primarily comprises a device for measuring the relative position of the upper and lower cylinders in relation to a predetermined relative position. The sensor device sends out a signal every time the cylinders are momentarily in the predetermined position. The device has a clock that measures the time between successive signals that define a time period or an interval. In addition, the device has means for measuring the volume of drilling fluid flowing on the downstream side of the telescope section during the aforementioned time period. In this way, the flow rate of the drilling fluid flowing from the borehole is determined by dividing the measured volume of drilling fluid flowing on the downstream side of the telescope section by the associated time period during which the said measurement was carried out.

In a modification of the apparatus, the measuring device has the ability to control, in series, a number of volumetric measurements over successive time periods to thereby enable the calculation of a series of flow rates in terms of flow from the borehole by dividing each volumetric measurement by the corresponding time period .

When determining a "kick" or lost circulation, the apparatus may include means for measuring the volume of drilling fluid flowing into the riser from the mud system during the aforementioned time period. In this way, the occurrence of a "kick" or lost circulation will be indicated as a difference between the measured volume of drilling fluid flowing on the downstream side of the telescopic section and the measured volume of drilling fluid flowing into the borehole from the mud system. When this value is determined, however, both volumetric flow measurements must be made during the same time period.

In another modification of the device where the shock absorber device is included in the lifting system, the improved device will include devices for measuring the volume change in the drilling fluid in the shock absorber device. The volume in the shock absorber device will be measured at the times when the telescopic section is in the predetermined reference position. Successive readings will be compared to estimate a volume change. The volume of drilling fluid flowing from the borehole is also measured on the downstream side of the shock absorber device. To determine the presence of an abnormal drilling condition, the following measurements are therefore made during the same time period: Difference between the measured volume of drilling fluid flowing into the riser from the mud system and the sum of the measured volume of drilling fluid flowing on the downstream side of the shock absorber device and the measured change in the volume of drilling fluid in the shock absorber device, this being an indication of either a "kick" or a lost circulation. If a trip was made, the improved device would only monitor the volume of drilling fluid in the shock absorber device to determine if a change was developing, which could indicate either a "kick" or lost circulation.

The invention is characterized by the features reproduced in the claims and will be explained below. in more detail with reference to the figures and tables of the drawings and where: Fig. 1, seen from the side, shows a floating offshore vessel drilling an underwater borehole using a conventional circulation system for drilling fluid,

fig. 2 represents a block diagram which schematically shows the flow of drilling fluid in the riser pipe and during volumetric changes in a telescopic section of this pipe due to the lifting movements of the vessel,

fig. 3 shows, seen from the side, the part of the circulation system for drilling fluid in fig. 1 as the present up-

invention applies to,

fig. 4 shows another embodiment for circulation system

for drilling fluid, where a rotating seal is used

the top of the riser and a shock absorber tank in connection with the line,

fig. 5A-5E show successive positions of the telescoping part during a lift cycle,

fig. 6 represents a block diagram of the present invention when used to determine the flow rate and volume of drilling fluid flowing from a subsea hole in a manner that is unaffected by the heave motion of the vessel,

fig. 7 shows a detail of a riser tensioner for

synthesized with a potentiometer that should sense the mutual style

ling between the upper and lower cylinders of the telescopic section,

fig. 8 shows an electrical connection core for a switch device, where a potentiometer is used for controlling the mutual position between the upper and lower cylinders in the telescopic part and for controlling the processing of the information according to the invention,

fig. 9 is another embodiment for a circulation system

to drilling fluid, modified for use of the present invention. It has the same structure as in fig. 4 except that it lacks the rotary seal,

fig. 10 is a graphical representation of the length of the telescopic section and a number of other parameters as a function of time based on the heave motion of the vessel,

fig. 11 is a block diagram of the present invention used to determine the presence of a "kick" or lost circulation in a subsea wellbore;

fig. 12 is. is a block diagram of an alternative embodiment of the present invention used to determine the presence of a "kick" or lost circulation in a subsea wellbore;

table 1 is a tabular arrangement of data which

is collected from the execution example and

table 2 is part of the same data that is reproduced

in table 1, but calculated over different time intervals.

With reference to fig. 1-12 and. tables 1 and 2.

and with particular reference to fig. 1, a vessel 20 has a tower 22 and platform 29 built up on the vessel 20 for underwater drilling of a borehole 24 (above referred to as well drilling) at a distance from land. The vessel is connected to the borehole by means of a riser 26 which extends from the platform 29 through a shaft 28 in the hull of the vessel 20 to the well or drilling on the seabed. The riser 26 is connected to the borehole 24 with per se known blowout prevention equipment (not shown). The riser is connected at its upper end to the platform 29 by means of a tensioning device 30 for the riser, and this tensioning device provides sufficient upward force which is necessary to support the riser.

The riser 26 also has a telescopic section or slip joint 32 near its upper end. The telescopic section comprises upper and lower cylinders 34, 36 which can slide telescopically into each other so that the upper cylinder 34 moves in the lower cylinder 36 during the vertical movement or heaving movement to which the vessel 20 is exposed due to waves, tides and currents . The tensioning device 30 secures the upper end of the lower cylinder 36. In this way, the vertical movement will be balanced by the tensioning devices 30. The tensioning devices are well known in the field in question here. E.g. may it be like the NL Shaffer's Riser Tensioner shown on page 4951 of Volume III of "The Composite Catalog of Oil Field Equipment and Services" 1978-1979, published by World Oil. The upper cylinder 34 slides along the inside of the lower cylinder 36 when the vessel heaves. The lower cylinder 36 remains stationary in relation to the seabed.

A drill string 38 hangs in a swivel 40 which in turn. hangs in a motion compensating device 41 in the tower 22.

An example of this compensation device for movement is NL

Shaffer's Drill S.tring Compensator shown on page 4945

in Volume III of the Composite Catalog of Oil Field Equipment and Services. The drill string 38 extends down through the riser 26 to the borehole as shown by dashed lines. A drill bit 42 which is used for drilling the hole is attached to , the lower end of the drill string 38. An annular space 44 which is formed between the inside of the riser 26 and the outside of the drill string 38 creates a return path for the drilling mud. A line 50 is connected to the upper part 25 of the riser 2'6 generally below the tower platform 29 and goes to a chip grater 46 and active sludge tanks 48.

A flow meter 45 is mounted in the conduit 50 between the riser 26 and the chip grater 46. The flow meter 45 measures flow rate or volume of sludge flowing toward the chip grater 46. Not shown measuring devices discussed in more detail below are included to measure the volume of sludge. in the active tanks 48. Main pipe 52 extends from the chip grater 46 to a flexible hose 53 which in turn is connected to the swivel 40 inside the tower. A flow meter 56 is mounted on the main pipe to measure the flow rate or volume of mud introduced into the drill string 38. A pump 54 sucks mud from the active tanks 48 and circulates up through the main pipe 52 to the flexible hose 53 and on to the swivel 50, as well as down through the drill string 38 towards the drill bit 42, and back to the vessel through the annular space 44. The returning mud comes out of the annular space through an opening 27 in the riser 26 and from there into the line 50 and flows to the chip grater 46, where solid particles are separated out, and the sludge is returned to the active tanks 48.

The mud comes out from the drill string 38 through the drill bit 42 and drags with it solid particles which are due to the drilling action, from the drill bit, at the same time as the drill bit cools. The drilled out solid particles are suspended in the mud and are carried back to the vessel in the return flow, up through the annular space 44. In order to maintain sufficient hydrostatic pressure on the underground formations that are being drilled,

the annular space 44 constantly full of sludge.

As explained earlier, an abnormal drilling condition, e.g. a "kick" or lost circulation is detected by observing unexplained changes in either the return flow of mud coming from the riser 26, or the volume of drilling mud in de-active mud tanks 48. In a stable state, e.g. on land, the flow rate measured by the flowmeter 45 and the sludge volume measured by instruments not shown in the tanks 48 are sufficient to indicate a "kick" or lost circulation. An example of a mud volume measuring instrument, which is well known in the art, is the "Mud Volume Totalizer", Series MVTX, manufactured by the Martin-Decker Company, Santa Ana, California, In water environments where heave motion of the vessel 50 displaces the upper cylinder 34 in the telescopic section 32 in relation to. the lower cylinder 36, significant volumetric changes would take place, and these would affect the measurements and limit the ability to detect potential problems regarding the control of the borehole.

By studying fig. 2, it will be easier to understand the phenomena to which the riser is in reality subjected. Fig. 2 indicates schematically that in a given time period the total volume of incompressible fluid (V^) added to a saturated fluid system will equal the total volume removed from the system (V^) plus any increase in the volume of fluid in the system (AV s). This relationship can be represented mathematically as:

Equation (1) can be applied to the entire sludge circulation system shown in fig. 1 or on any part thereof. E.g. when analyzing the effect of the heave motion of the vessel, V^n will be the volume of sludge entering at the bottom of the telescopic section 32 and V ^ will be the volume of sludge flowing past the flow meter 45. Therefore, AVs^s will be the volume change in the sludge in the flow path between these two points.

Fig. 3 shows, on an enlarged scale, part of the circulation system in fig. 1. As shown, the location of the opening 27 in the riser in relation to the chip grater 46 will be such that parts of the line 50 as well as the upper part 25 of the riser 26 will only be partially filled with mud. There exists a free surface 68 inside these components and this surface varies in height as a result of flow shocks caused by the vessel's heave motion. For this reason, the expression AV for this system will include not only the volume changes in the telescopic part 32, but also the changes in the mud volume in the upper part 25 of the riser 26 and the conduit 50. Because of the largely uniform cross-section of the telescopic part 32, it is well known in the art to relate the volume of sludge in the riser to its total length, which is easily measured. However, the complex relationship between the free surface 68 and the volume of sludge contained in the upper part 25 of the riser 26 and the line 50 will make it impractical to measure this volume.

From the state of the art, particularly the U.S. patent nos. 3910110 and 3976148 it is previously known to modify the flow path between the telescopic part 32 and the flow meter 45 to either eliminate the free surface.68 (U.S. patent no. 3,910110) or to create a geometry there. the line 50 remains full of sludge at all times, and where the free surface 68 exists only in the upper part 25 of the riser 26 above the opening 27 (U.S. patent 3976148).

Especially the U.S. patent 3976148 deals with determining the flow rate for the drilling fluid entering at the bottom of the telescopic part according to the following equation:

where Q represents the flow rate of drilling fluid entering at the bottom of the telescopic section, Q represents the flow rate of fluid passing through a flow meter in line 50 and ^S->/S represents the continuous volume change in the mud both in the telescopic

section 32 and the upper riser 25 as a function of time.

In the U.S. patent no. 3976148, both the length of the telescopic section and the mud level above the branch for the line 50 and the riser are continuously monitored to be able to directly provide a change in the flow rate.

The present invention which makes use of equation (1) to solve V. is based on the measurement of vout during certain time intervals which are chosen so that AV is zero. In this way, the measured volume vout will be an accurate indication of the volume entering the telescopic section Vn during the determined time intervals. The average flow of drilling fluid entering the telescopic section can thus be determined by dividing vout by the length of the time interval or period during which the volumetric measurement was made. In this way, the calculated average flow rate past the meter 45 will be largely equal to the flow rate where the drilling fluid flows in at the bottom of the telescopic section 32 and is unaffected by the heaving movement of the vessel.

As an alternative, the volume of sludge contained in the tanks in fluid connection with the riser can be measured at specific times when AV is zero and the volume measured is thus unaffected by the lifting movement.

Fig. 4 is a simplified drawing seen from the side of a preferred embodiment of the pipe arrangement, where the present invention can be used. The upper part of the riser 26 includes upper and lower cylinders 34 and 36 which form the telescopic section. The wire 50 protrudes on the side of the riser 26 at the opening 27 and extends to a chip grater 46. The annular space at the upper part 25 of the riser 26 and the drill string 38 is sealed by means of a rotating seal, e.g. a "Rotating blowout Preventer" manufactured by NL Shaffer of Houston, Texas and described in detail on page 4912 of Volume III of the "Composite Catalog of Oil Field Equipemtnt and Services". A shock absorber tank 43 is in fluid communication with the conduit 50 on the upstream side of the chip grater 46. As shown in fig. 4, the shock absorber tank 43 is connected directly to the line 50, but the tank 43 can also be connected directly to the riser 26 through an opening corresponding to the opening 27. A level sensor (not shown) but e.g. such as the Universal Trip Tank Monitoring System Series TTSX, manufactured by the Martin-Decker Company of Santa Ana, California), measures the height H of the free mud surface 68 in the buffer tank 43. A flow meter 45 is mounted in line 50, preferably on the downstream side of the buffer tank 43. The shape of the conduit 50 in the area of the flow meter 45 is chosen to maximize the accuracy of the flow meter by keeping the meter full of fluid at all times and by reducing the possibility of deposited solids blocking some of the flow area of the meter. The design shown applies to the use of a flow meter of the electromagnetic type, e.g. model 10D1435A/V Magnetic Flow Meter manufactured by Fischer & Porter Company of Warminster, Pennsylvania. As an alternative, a paddle-type flow meter, e.g. "Flo-Sho" manufactured by Warren Automatic Tool Company Houston, Rexas, is used in a horizontal portion 47 of conduit 50 provided that the conduit is designed to be kept partially full of fluid. Active sludge tanks 48 receive the sludge after it has passed through the chip grater 46. These tanks also contain a level sensor, e.g. Martin-Decker's TTSX system. A valve 66 is placed in the line 50 on the downstream side of the shock absorber tank 43.

The mutual heights of the gasket 58, the line 50, the shock absorber tank 43 and the chip grater 46 are chosen so that the mud surface 68 in the shock absorber tank 43 is the only free surface in the system. All other components on the upstream side of the flow meter 4 5 are kept filled with sludge at all times.

The shock absorber tank 43 dampens the force in which the shock flow meter 45, the chip grater 46 and the active tanks 48 are exposed. This is desirable because the undamped shocks and impacts due to the heave motion of the vessel and which come from the expansion and contraction of the telescopic section can be many times greater than the normal current previously discussed. By dampening these shocks, the components do not need to be designed to handle these extreme forces.

The present invention can be applied to the embodiment shown in fig. 4 to compensate for the vessel's heaving movements when either a system for measuring flow or a system for measuring volume is used. The invention will first be described in connection with a flow meter. system and then in connection with a volume measurement system.

Applying equation (1) to the system surrounded by the dashed line 67 in fig. 4 leads to the following equation:

where V is the return volume of the sludge that flows in at the bottom of the telescopic section (corresponding to V. in equation (1)), V is the volume that flows out of the system past the flow meter 45 and V is the volume that flows out of the system and into in the shock absorber tank 43. The sum (V + V,) corresponds to V , in - mt out equation (1). The expression AVts the change in the volume of the sludge in the telescopic section and corresponds to AVg in equation (1).

To highlight the importance of the present invention, reference should be made to fig. 5A-5E which reproduce the operation of the telescopic section. The dashed line 33 represents the typical lifting motion of the upper cylinder 34 which is attached to the vessel. For the sake of explanation, fig. 5A as reference position. The reference position is chosen so that it lies within the stroke length of the telescopic part, preferably near the midpoint of the cycle. At time tg, which is represented by fig. 5A, the telescopic section is in the reference position. The vessel heaves upwards as indicated by arrow 35, whereby the telescopic section is extended. Fig. 5B represents the relative orientation of the cylinders at a later time when the vessel is close to the limit of the upward movement. After this, the lifting movement will be downwards and at time t^ the telescopic section will again be in the reference position which is now represented by fig. 5C. The vessel will, however, continue further down as indicated by fig. 5D, where the movement reverses and the vessel again begins to move upwards. At time t~ (Fig. 5E), the telescopic•section will again be in the reference position for the second time after time t .

A suitable switch or similar contact device (not shown) is connected to the two cylinders so that whenever the cylinders are in the reference position (e.g. at times tg, and in Fig. 5A,' C and E) the switch or contact device will engage and send a signal. In this way, when the vessel pulls up, signals will be produced which indicate the times at which the telescope section is in the reference position. These signals will be generated by intervals that are from one to eight seconds.

At the point in time when the upper and lower cylinders are in the reference position (hereinafter sometimes called "determined points in time"), the expression ^ SyS in equation (3) will be zero. For this reason, equation (3) can be rewritten to

where the symbol (<*>) indicates the amount determined over the intervals between the specified times (hereinafter referred to as "determined time intervals" or "time periods"). In this way, one step in the method according to the invention will be the determination of the specific times at which the mud volume in the telescopic section is the same as in the reference position. By measuring the volume V<*>oa V<*>below it

3 m" t

certain time interval between the certain times, the effect of the volume in the telescopic section could be omitted. This clearly differs from the state of the art, which requires that the mud volume in the telescopic section be measured.

Equation (4) indicates that the desired quantity v<*>e-j-which is the volume of mud that returns' from the borehole, can be determined by measuring V^ and V<*> and adding these quantities together. These two volumes can be measured in many different ways that are well known to those skilled in the art, and two of the measurement methods will be described in more detail below.

The flow rate of the drilling mud that returns from the borehole is a very useful indicator of "kicks" or lost circulation. This quantity can also advantageously be calculated by measuring the specific time intervals represented by At<*.>The average flow rate of the returning sludge at the bottom of the telescopic section over a specific time interval, Q*^ can be calculated according to the following equation:

Even if Q<*> is an average value, the method according to the invention differs significantly from other previously known methods which depend on an average calculation over pre-selected, relatively long time periods in order to reduce the observed values of mud impact which are due the heave motion of the vessel. These previously known methods will have to result in a reduction of

. the sensitivity of the flow measuring instrument to the

. rings in the current from the borehole. With the present invention, the periods during which average calculations are performed will be short and the equalization for the lifting movement will be complete. Therefore, the instrument will also be extremely sensitive to changes in the return current, with very little delay in the reading. The specific time intervals At<*> referred to above need not be based on consecutive times. E.g. with reference to fig. 5A-5E may At<*>be the intervals t^to t^and t^to t^, or that

simple interval tQto t^. In general, At<*> can be the interval between successive specific times - or one or another multiple of these.

Opposite, the principles which are used when carrying out the method according to the invention are explained when it comes to compensating for the heaving movement of the vessel in instruments. systems, where the return flow of drilling mud is determined to detect any control problems with the borehole. The following paragraphs describe the practice of the present invention on systems where the volume of mud in the tanks on the vessel is determined in order to detect any control problems with the borehole.

As explained earlier, the mud volumes contained in the tanks which are in fluid connection with the borehole, e.g. the active sludge tanks 48 in fig. 4, be sensitive to "kick" and lost circulation. For the sake of discussion, the volume in the tanks 48 (also known as "pits") will be represented by V . For an onshore borehole, V will be the only one

P. P

volume in the sludge circulation system which varies particularly. In the structure shown in fig. 4, however, the mud volume in the telescopic section, V^g/volume in the buffer tank V^ and V will vary due to the heave motion of the vessel. Furthermore, the volume V^_ will be sensitive to "kick" and lost circulation; • tion very much in the same way as V . In reality, the buffer tank 43 can be considered as an extension of the system of active sludge tanks.

Exercising, of the present invention together with a volume measurement system, it is necessary to measure the volumes V^ and V^_ on the. certain times that have been previously defined. The volume V^_s does not need to be measured because it is the same at each of the specific times and is not affected by the conditions in the borehole. With reference to equations (1) and (3), this embodiment will be reduced to this:

where the double star symbol (<**>) indicates measurements carried out at the specified times and V<**> is the desired volume indication corrected for the effects of the vessel's heave motion.

A modification of the method described above is possible when there is no circulation in the borehole and a sensitive volume measurement is desired. This can be done in either a flow measurement system or a volume measurement system. In this case, the valve 66 in Fig. 4 should be closed to shut off the flow meter 45 and the active mud tanks 48 from the borehole. The shock absorber tank 43 will then act as a trip tank.

As the tanks 48 are shut off from the borehole and cannot be affected by "kick" or lost circulation, equation 6 is reduced which indicates that the desired volume measurement is the volume measurement of the shock absorber tank (or trip tank) at the specified times.

A preferred apparatus for carrying out the present invention is shown schematically in fig. 6. The reproduced device is able to measure both volume and flow with compensation for the effects of the lifting movement.

A lifting switch 60 gives a signal at each time or multiple of these when the cylinders of the telescopic section are in the reference position. A preferred device for performing the functions of the lift switch 60 will be described in more detail in the following.

As shown in fig. 6, the output from the switch will be a series of pulses corresponding to the specific times. Two of these pulses occur at the specific times t^ and t£ and are shown in the graphic representation 63 in fig. 6. These pulses are command signals and are transmitted via dashed lines 60a to an integrator 70, samplers 69 and 86 and a counter 78.

The integrator 70 integrates the signal Qm from the flow meter 45 over the period between successive command signals (ie from t^ to t^) and sends the integrated value at tv,. This output is the size previously defined. A further function of the integrator is to begin a subsequent integration at t 9 and continue this until the next command signal. The output of the integrator 70 is preferably held at the value obtained during the interval t^til until a subsequent integrated value is calculated. In this way, the output of the integrator 70 will be the value of V<*> for the last completed particular interval.

m

The implementation of the integrator 70 with either analog or digital technology is well known to those skilled in the art. The structure partly depends on the type of flow meter 45 that is used.

A preferred flow meter is the Model 10D1435A/U Magnetic Flow meter with two Model 50PZ1000A Flow Converters, manufactured by the Fischer & Porter Company of Warminster, Pennsylvania.

■i

These components can be built to form a bi-directional ■ flow meter capable of measuring flow in both directions. With this structure, you get two outputs, one corresponding to the flow rate in one direction and the other corresponding to the flow rate in the opposite direction. Each of these outputs comprises a pulse train whose frequencies are proportional to the flow rate of sludge through the magnetic flow meter in the direction concerned. For this reason, each pulse corresponds to a specific mud volume. With this type of flow meter, an integrator 70 is used which is known as an "up-down" counter which gives the total of the received pulses during the determined time interval. The counter will sum the pulses (volumes) that correspond to flow out of the system and subtract the pulses (volumes) that correspond to flow in the opposite direction. In this way, the integrator 79 will produce an output that indicates the net volume of sludge that has flowed out of the system past the flow meter, .

The sampler 69 samples an input value at the specific times indicated by the command signal and holds this value for the output until it is replaced by the next input value. Analog and digital samplers that perform this function are well known to those skilled in the art. As indicated in fig. 6, the input to the sampler 69 will be the sludge volume in the shock absorber tank V . As shown, this measurement is preferably obtained by measuring the level H of sludge in the shock absorber tank 4 3 with the help of a level sensor 62 and by comparing this measurement with the sludge

mets over f late area A^_ in the tank as indicated in an amplifier 64 . A suitable device for carrying out both of these functions

is the Universal Trip Tank Monitoring System, Series TTSX, manufactured by the Martin-Decker Gompany of Santa Ana, California,

which is previously mentioned.

The output from the sampler 69 therefore corresponds to V<**> which is the volume in the shock absorber tank 43 at the latest of the determined times. This size is used to calculate both volume and current.

For the determination of current, the output is fed from the sampler

to the input of a subtraction device 72 which subtracts

the last value of V<**> from the corresponding value measured at the previous specified time. The output will therefore be the value V<*> which represents the net increase of the volume of sludge in the shock absorber tank 43 over the last determined time interval. One will see with reference to fig. 4, that this will also be the net volume of sludge that flows out of the system and is indicated by dashed lines' 67 into the shock absorber tank 43. Naturally, a reduction of the volume in the shock absorber tank will be represented by negative values for

W*

As shown in fig. 6, the output from the integrator is 70'

(V<*>) and the subtractor 72 (V<*>) summed at a summation point 76. As indicated by equation (4), this sum is V<*>e^_ which is the volume entering at the bottom of the telescopic section during the last specified time interval.

A third device which receives a command signal from the switch 60 is the clock 78 which determines the time period At<*> corresponding to the most recently traveled determined time interval. These devices are well known to those skilled in the art.

A divider 80 is connected to the summing point 76 and receives the v<*>et signal from the point 76 and divides this by the associated determined time interval At<*>. As indicated by equation (5), this calculation results in the desired current velocity Q<*>et which is unaffected by the vessel's heave motion.

For volume determination, the output V<**> from the sampler 69 is added to the sampled volume V<**> from the active sludge tanks. The latter quantity is preferably determined by a pit-volume instrument 88, which e.g. a Mud Volume Totalizer, Series MVTX manufactured by the Martin-Decker Company of Santa Ana, California, as previously mentioned. The output V from the pit volume instrument 88 is fed to the second sampler 86. The operation of this device is controlled by the command signal from the lift switch 60 and corresponds in terms of construction and operation to the sampler 69 previously described. The output v<**> is then summed together with the output V<**> at summation point 82. The resulting value is V<**> in accordance with equation (6).

The apparatus of fig. 6 has two switches 74 and 84. Both work together with the valve 66 in fig. 4. They are shown in their normal position corresponding to the open position of valve 66. When valve 66 is closed, the switches transition to a zero input position (represented by the ground symbol in Fig. 6). This is electrically analogous to closing the valve and enables the sensor 62 to monitor volumes and flows only to the shock absorber tank 43. This mode of operation corresponds to the "tripping" operation previously described.

It is clear from the foregoing that the lift switch 60 performs a very critical function in the present invention. The switch arrangement may be a standard reversing switch which is mounted on the second cylinder and which is struck by an impact arm (not shown) mounted on the first cylinder each time the cylinders are in the reference position. Such a switch device is easily constructed by professionals. As an alternative, the switch can be mounted on the riser's tensioning device 30 (see fig. 1)

instead of on the telescopic section because the mutual movement of the tensioner relative to the vessel in phase corresponds to the movement of the first cylinder relative to the second cylinder.

Instead of a reversible switch, the switch device may include a potentiometer with a cord pull. Such potentiometers are well known and can be obtained e.g. of Humphrey Inc., of San Diego, California. As shown in fig. 7, the tensioning device 30 is attached to the lower cylinder 36 with a cable 31. The lower cylinder 36 hangs in the vessel 20 with the help of the tensioning device 30 and yet the cylinder 36 is stationary due to the balancing effect that the tensioning device 30 has. The tensioning device 30 has forward and reciprocating cylinders 30a, 30b which coincides with the reciprocating movement of the upper and lower cylinders 34, 36. The string tension potentiometer 33 measures a voltage variation originating from the extracted

cable 33a which is attached to the cylinder 30a, in relation to the potentiometer which is attached to the cylinder 30b.-

The electrical connections for the switch device 60 are shown in fig. 8. The string pull potentiometer 33 produces an output voltage which is analogous to the extension of the telescopic six ion. Although at first glance it may appear that a signal proportional to the length of the telescope section is being produced, it will be clear that the analog voltage output is only used to eliminate the effect of tidal fluctuations and to produce command signals at the desired specific times.

In fig. 8, the output voltage from a sliding contact 100 in the potentiometer 33 is divided into two branches. 102, 104. The first branch 102 is connected to an integration circuit 106. The circuit 106 serves as a low-pass filter. In other words, the circuit 106 will filter out all wave motion except for motions that have a very low frequency, e.g. periods of more than 5 min. This filtered signal represents the reference position of the telescope section. It filtered signal from the circuit. 106 is inverted and summed at the connection point 108 to the original signal passing along the branch 104. The summed voltage is then passed through an amplifier 110. In this way, the filtered signal from the circuit 106 when it is inverted and summed together with the output voltage from the potentiometer, reduce the voltage to oscillations about a modified zero voltage level (curve 111, Fig. 8) which cancels tidal action by adjusting the zero level. This also eliminates the need to physically move the switch on the tensioner or telescopic section. In this way, the circuit will automatically adjust for such long-term variations as tidal fluctuations. This is a principle advantage of the potentiometer compared to the mechanical reversible switch discussed above. As significant tidal variations can shift the natural zero line for the oscillations between the upper and lower cylinders 34, 36, contact between arm and switch will not be able to take place until the mechanical switch has been moved.

The output signal from the amplifier 110 is fed to a Schmitt trigger 112 whose output is a square wave signal (curve 113) with the same zero crossings and polarity as the input signal (curve 111). The square wave signal is then fed to a differentiating circuit 114 which converts the square wave signals from trigger 112 into a series of pulses (curve 115) which occur at each zero crossing of the wave signal. The polarity of each pulse depends on whether the zero crossing was positive-running or negative-running. The output pulse from the differentiation circuit 114 is fed to a non-inverting buffer 116 and an inverting buffer 118. The output from the non-inverting buffer is fed to a diode 120 which only conducts pulses with positive polarity. These pulses would e.g. respond to crossings of the reference position of the telescopic section as it extends (ie positive running crossings). The output from the inverting buffer 118 is fed to a similarly oriented diode 122 which only conducts positive pulses. Due to the voltage inversion, however, these pulses will correspond to negative crossings of reference positions (contraction of the telescopic section).

The lift switch in fig. 8 has two outputs, one corresponding to positively running crossings of the telescope section's reference position and one corresponding to negatively running crossings. One or the other can be used for the command signal in fig. 6. As an alternative, the two outputs can be summed and the result used as a command signal. In this case, a command signal will be produced at each crossing of the reference position regardless of the direction of movement.

When referring again to fig. 5A-5E, it is seen that a first signal or pulse is produced at the positive-going output when the cylinders are momentarily in the predetermined position, as shown in FIG. 5A and the telescopic section is extended. As the vessel is raised due to a wave crest in the sea, the first cylinder continues upwards in relation to the second cylinder as shown in fig. 5B. The vessel then sinks back into the wave valley, which causes the first cylinder to sink in relation to the second cylinder. At the time represented by fig. 5C, a signal is produced at the negative-going output and indicates that the telescopic section is again in the reference position and retracts. Even if a voltage from the potentiometer 33 varies proportionally with the length of the telescopic section, the output signals will thus only indicate the relative position of the cylinders in relation to the reference position.

On the basis of the preceding description, the utility will

of the present invention with the pipe arrangement shown in fig. 4, appear clearly because the only condition is that the drilling mud cannot be compressed. Given that

that the sludge does not have particular amounts of trapped air or gas, this is a reasonable assumption. There are, however, alternative pipe systems with which the invention can be used.

Fig, 9 is another arrangement where the invention is used.

It corresponds to what is shown in fig. 4 except that the rotation seal 5'8 has been removed. Rotational seals can cause frictional damage due to the movements that occur when the ship pulls up, and the damage is then found between the seal and the drill string 38. As a result of this, this component may require constant maintenance and it is not even desirable to apply this component. To prevent drilling mud from flooding, above the open upper end of the riser 26, the chip grater 46, the shock absorber tank 43 and the line 50 are lowered in fig. 9 in relation to the position they hold. fig. 4. The level of these components in relation to the outlet 27 of the riser 26'

is chosen so that the line 50 stays full. of sludge. Although the principles underlying the execution of these components are previously known, it must be pointed out that the limitations set by the drilling vessel's construction can prevent you from achieving what you want under all conditions when the vessel pulls up, when it applies to sludge flow and the ■ rheological properties of the sludge. However, it is believed that conduit 50 will be full of sludge under most conditions.

The effect of the modification shown in fig. 9 leads to a further free mud surface 59 in the upper part of the riser 26. It is clear that if the level of this mud surface can be measured the pipe could be considered as a further shock absorber tank according to the principles described above. However, under most conditions this will not be necessary. The area of the free sludge surface 59 in the riser (hereinafter referred to as A^) is usually small. compared to the area of the free surface in the shock absorber tank. As a consequence of this, errors in determining V* are not expected to be large even if the surface is free

right

59 are not included in the calculation. A recognition that the level variations on the free surface 59 in size and phase will correspond to the variations for the free surface 68 in the shock absorber tank 43 will, however, lead to a further improvement in terms of

■measurement of the shock absorber tank's level "H" (see fig. 9) which will then also be a measure of the sludge level in the riser. In the device in fig. 6, this can be achieved in a simple way by increasing the gain in the amplifier 64 from A^ to (At + A^).

Another pipe arrangement with which the present invention can be used is shown in fig. 3. With this arrangement, the line 50 and the upper part of the riser 26 are generally not full of sludge even under normal conditions. As explained above, this geometry is complex and makes it impractical to measure the volume of sludge in these components. When practicing the invention, however, the largest error component, namely the volume in the telescope section, is eliminated. For this application, the apparatus 6 can be used by removing the level sensor 62, the amplifier 64, the sampler 69 and the subtraction circuit 72 which are all connected to volume measurements in the shock absorber tank.

As explained above, it is well known to those skilled in the art to detect control problems in a borehole by comparing the flow rate of drilling mud returning from the borehole (Q<*>eti present invention) to either (i) the rate of return mud flow at earlier times or (ii) the rate on the mud that circulates into the borehole) The first-mentioned comparison can be made in different ways with the present invention, preferably by either recording the output Q<*>et on a printer or by setting alarms that come into operation when Q* , deviates by more than one on the fore-right ^

hand determined value from a value determined during previous trouble-free operations. The latter comparison can be made in many different ways which are well known to those skilled in the art, two of which are shown below.

Fig. 11 shows a modification of the apparatus in fig. 6 where the flow rate of mud circulating down the borehole Q . is subtracted from the flow rate 0*

c i ir c J- 6 X-on returning sludge to give a delta flow parameter AQ*e^_. Mathematically, this corresponds to the equations:

The signal Qcj_rcf is obtained from the flow meter 56, which is shown in fig. 1.

Another alternative comparison method for the flow rate is shown in fig. 12. The output from the flow meter 86 is fed to an integrator 75 which functions in the same way as the integrator 70. The integrator 75 integrates the signal Qc^rcover the period that lies between successive command signals (ie from t^ to t^) in and sends the integrated value as output at time t2• The output is the quantity V<*.>which is the volume of sludge pumped into

own the drill string 38 during the determined time interval. This integrated output was then subtracted from the sum of V<*>and V*^ at summation point 76 to form the value AV*et- Mathematically, this can be expressed as:

where A.V* is the difference between the volume of the drilling mud that has entered at the base of the telescopic section during the specified time interval and the volume of mud that has been pumped down the drill string 38 from the tanks 48 over the same interval. A positive value of AQ*e^_ indicates a "kick" while a negative value indicates lost drilling mud.

The signal Av*^et -can be processed in different ways. E.g. can be supplied to the divider 80 shown in fig. 12, to produce the delta flow parameter AQ*_et expressed as:

Equation (10) is. analog-with equation (5). However, equation (10) deals with the determination of a difference, namely the delta current. Alternatively, the AQ* , values can be fed to a totalizer

device 81 which sums the successive AV* to the total of previous values. The totalizer device is well known. In this way, net volumetric■ is determined

loss or gain of mud from the borehole, namely £AV* ^ . This quantity is equal to the change in the volume V<**>of sludge' in the sludge system on the surface, but is unaffected by increases or decreases of the material from the active sludge tanks 48. In the preferred embodiment, the totalizer device 81

be" activated whenever the parameter AQ<*>gt exceeds preset limits, to indicate a control problem regarding the well. The output £AV<*>gt would then indicate the total increase or decrease of the amount from the well due to of the behavior of the problem.This information is very useful when it comes to planning well control procedures.

Fig. 10 is a graphical representation of data that appears with the embodiment of the invention shown in fig. 6 and used together with a pipe arrangement as shown in fig. 4-, The curves in fig. 10 is based on an upper cylinder 34 with an outside diameter of 47 cm, a drill string 38 with an outside diameter of 12.7 cm and a shock absorber tank 4 3 with a cross section of approximately 65.77 dm 2 . Curve I shows the change in telescopic six the length of the ion as a function of time. The adjusted zero axis represents the reference position for the telescope section.

Curve II is a square curve showing the position of a mechanical switch or the output 113 from the Schmitt trigger 112 in fig. 8 as a function of time. E.g. starting at time tg, the telescope section is in the reference position as indicated by the location of curve I at the zero axis. This position corresponds to fig. 5A. When the upper cylinder moves upwards, the switch takes position 1 when the upper cylinder deviates from the reference position. The switch remains in position 1 until the telescoping section returns to the reference position at time t^' at which point the switch position changes to position 2. The upper cylinder then continues downward and reaches a minimum height, after which it reverses its motion 1 relative to the lower cylinder, as shown with the curve I until the time t- when the telescopic joint is again in the reference position. During all positive or upward movements of the upper cylinder in relation to the reference position, the switch will therefore be held in position 1. In the same way, all negative or downward movements of the upper cylinder in relation to the reference position will hold the switch in position 2. It is clear that position 1 and position 2 can be represented by different voltage levels in an electronic circuit.

Curve III is the same pulse train obtained by differentiating curve II. Curve III shows the generation of a positive pulse 200 each time the first cylinder moves upwards and crosses the reference position. A negative pulse 201 is generated each time the first cylinder moves downward and crosses the reference position. The use of these positive and negative pulses to generate command signals is explained above in connection with fig. 8.

A linear flow relationship was assumed when calculating the hydraulic behavior of this system as a function of time. For this reason, curve IV represents the behavior of the two parameters. As shown by the labeling of the left-hand ordinate, the graphical presentation indicates the variation in the level H of the sludge in the shock absorber tank around an average value which can arbitrarily be given the value zero. Because this level

(or liquid column) provides the driving force for the flow through the flow meter, the curve IV also represents the varia- . tion in this current Q . The right-hand ordinate indicates the size of these variations around an average flow rate of 2300 l/min, which is the assumed flow in at the base of the telescope section. When extension of the telescoping section takes place between tg and t^or t^and t^, the volume of fluid in the shock absorber tank and the flow rate through the flow meter will decrease as indicated by the values of H and Q between tQ and t^or t ? and t^. The tank level and flow rate

the density increases in the intervals when the telescopic section contracts, e.g. between t^and t2and t<- and tg.

Table 1 reproduces a more detailed arrangement of data from the execution example, where the calculated height at each of the times (tg, t^etc.) is indicated together with the calculated volume of fluid that has passed the flow meter on the downstream side of the shock absorber tank during the preceding specific time interval.

Knowing the sludge level H<**>in the shock absorber tank at each specific time and the volume passing the flow meter V<*>during each specific interval, the flow rate of sludge into the bottom of the telescopic section can be determined as follows:

If you now carry out the above calculations for each specific time interval, you get a constant flow rate of 2270 litres/min. which is the flow rate into at the bottom of the telescope section.

As pointed out above, the invention is not limited to the comparison of directly following intervals. It may even be advantageous to practice the invention by skipping every other time interval or a multiple of time intervals. Alternatively, one can also practice the invention by combining two or more intervals and redefining them as one interval. Or, one can define the time interval between zero crossings with the same sign by using only the positive or negative pulses in curve III. At time tg, in other words, the steepness of the curve I is positive because it increases. At time t^ the steepness of the curve I will be negative because it decreases. At time t^, however, the steepness of the line will again increase and therefore be positive. A reading taken between tQ and t 2 that can be

drawn as a positive zero crossing, will indicate the flow rate when the respective mud levels in the shock absorber tank together with the combined volume flow through the meter between tg and t^ are under consideration. If a calculation is also made in this case, the result will again be 2271 litres/min.

Table 2 summarizes the special values which are based on intervals between positive zero crossings (positive pulses). The data on which this table is based are the same as for

table 1. You will see that the volumes V<*>g and the times At<*> are both larger in table II than in table I. With a view to reducing errors, it is good practice to make the time intervals as large as possible, but also taking into account that sufficient reaction speed for the instrument must be maintained. It has been shown that values of At<*> of approximately 30 sec. is an appropriate time frame. If the average heave period for the vessel was about 8 sec, an interval consisting

'of eight consecutive reference crosses (or four

consecutive zero crossings with the same sign) be desirable. In the example in question here, this interval would be from tg to tg.

In that. above, the invention is described with reference to a number of exemplary embodiments. However, it is clear that many modifications and changes based on this description could be made by those skilled in the art. It is therefore intended to cover all such equivalent modifications and variations as fall within the scope of the invention.

Claims (17)

1. Method for determining the flow of drilling fluid during a drilling operation from a floating vessel, with riser pipe and telescopic section, characterized by measuring the volume of drilling fluid flowing from the telescopic section 1 during the time period when the telescopic section moves from a predetermined reference position and returns to it mentioned pre-determined reference position. 2. Method as stated in claim, characterized in that it includes determination of the flow rate for the drilling fluid flowing from the telescopic section into the riser, comprising measuring the duration of said time period and determining the flow rate of the drilling fluid flowing from the telescopic section into the riser by dividing said measured volume of drilling fluid by said duration of the time period. 3. Method for detecting an abnormal drilling condition during a drilling operation carried out from a floating vessel, comprising a riser, a telescoping section, a drill string extending through the riser and the telescoping section, and means for pumping drilling fluid down through said drill string and up through riser, and telescoping section, characterized by: (a) measuring the volume of drilling fluid flowing from the telescoping section during the time period when the telescoping section moves from a predetermined reference position and returns to said reference position, (b) measuring the volume of drilling fluid that is pumped into the drill string during said time period and (c) comparing the volume of drilling fluid measured during step (a) with the volume measured during step (b), as a difference will indicate an abnormal drilling condition. 4. Procedure for determining the flow rate for drilling fluid which. flows from an offshore borehole into a riser, where the borehole is drilled from a floating vessel and the riser connects the vessel to the borehole, and where a mud system in connection with the riser by means of a line circulates the said drilling fluid in the borehole and the riser, which riser has a telescopic section with an upper and lower cylinder which is to accommodate vertical or heaving movement of the vessel, characterized by cooling of the mutual position between the upper and lower cylinders in relation to a predetermined mutual position of the cylinders, where a first signal is produced when the cylinders vary from said predetermined position and start a clock, and a second signal is then generated when the cylinders return to said predetermined position and stop the clock thereby establishing a time period between the first and second signals, measuring the volume of drilling fluid flowing on the downstream side of the telescopic section during said time period, and determining the flow rate of the drilling fluid flowing from the wellbore into the riser by dividing the measured volume of drilling fluid flowing on the downstream side of the telescopic section by the mentioned time period. 5. Method as stated in claim 4, where the line is kept continuously full of drilling fluid, characterized by determining successive time periods by generating successive signals and measuring a volume of drilling fluid flowing on the downstream side of the telescope section within each time period, thereby enabling the determination of multiple flow rates. 6. Method for determining "kick" or lost circulation in an offshore well drilled from a floating vessel, where a riser connects the vessel to the well while a mud system in connection with the riser circulates a drilling fluid in the riser which includes a telescopic section with an upper and lower cylinder to accommodate vertical movement or heave movement the vessel performs, characterized by: sensing the mutual position of the upper and lower cylinders in relation to a predetermined mutual position of the cylinders, where a first signal is generated when the cylinders vary from the predetermined position and a second signal is then generated when the cylinders return to it said predetermined' position, measuring the volume of drilling fluid entering the borehole from said mud system between the first and second signals, measuring the volume of drilling fluid flowing on the downstream side of. telescoping section between the first and second signals, and determining a "kick" or lost circulation in the borehole when the measured volume of drilling fluid entering the borehole from the mud system is not substantially equal to the measured volume of drilling fluid flowing on the downstream side of the telescoping section . 7. Method as set forth in claim 6, characterized by generation of consecutive signals and measurement of a volume of drilling fluid entering the borehole from the aforementioned mud system and a volume of drilling fluid flowing on the downstream side of the telescope section between consecutive dignals .r to enable the determination of several differences between the measured volume of drilling fluid entering the borehole from the mud system and the measured volume of drilling fluid flowing on the downstream side of the telescoping section. 8. Method for determining the flow rate of drilling fluid flowing from an offshore borehole drilled from a floating vessel, where a riser connects the vessel to the borehole while a mud system with a shock absorber device in connection with the riser circulates a drilling fluid in the borehole and the riser, which riser has a telescopic section at the upper and lower cylinder to record vertical movement or lifting movement performed by the vessel, characterized by sensing the relative position between the upper and lower cylinder in relation to a predetermined relative position of the cylinders, where a first signal is produced when the cylinders deviate from said predetermined position for starting a clock, and a second signal is produced when the cylinders return to said predetermined position for stopping the clock and establishing a time period between the first and second signals, measuring the change in volume of drilling fluid in shock absorber devices n during said time period, measuring the volume of drilling fluid flowing on the downstream side of the shock absorber device during the same period, and determining the flow rate of the drilling fluid flowing from the wellbore to the riser by dividing the sum of the measured volume of drilling fluid flowing on the downstream side of the shock absorber device and the measured change in volume of drilling fluid in the shock absorber device with the mentioned time period. 9. Method as stated in claim 8, where the line is continuously kept full of drilling fluid, characterized by determining consecutive time periods by generating consecutive signals and measuring a volume of drilling fluid flowing on the downstream side of the shock absorber device within each time period to enable the determination of a number of flow rates. 10. Method for determining an abnormal drilling condition in an offshore borehole being drilled by a floating vessel with a riser pipe extending from the borehole to the vessel and a mud system with a shock absorber device and in connection with the riser pipe by means of a line extending from the mud system of the riser, which mud system circulates a drilling fluid in the borehole and the riser, while the line is kept continuously full of drilling fluid, which riser includes a telescoping section with an upper and a lower cylinder which is to record vertical movement or heaving movement the vessel performs, characterized by sensing a mutual position between the upper and the lower cylinder in relation to a predetermined mutual position of the cylinders, where a first signal is produced when the cylinders deviate from the said predetermined position and a second signal is then produced when the cylinders return to the said on predetermined position, measurement of the volume a v drilling fluid entering the borehole from the mud system between the first and the second signal, measuring the change in volume of drilling fluid in the shock absorber device between the first and the second signal, measuring the volume of drilling fluid flowing on the downstream side of the shock absorber device between the first and the second signal, and determination of an abnormal drilling condition until a borehole reaches the sum of. the measured volume of drilling fluid flowing on the downstream side of the shock absorber device and the measured change in volume of drilling fluid in the shock absorber device are not substantially equal to the measured volume of drilling fluid entering the borehole from the aforementioned mud system. 11. Method as specified in claim 10, characterized by generation of consecutive signals and measurement of a volume of drilling fluid entering the borehole from the mud system and a volume of drilling fluid flowing on the downstream side of the telescope section between consecutive signals, to enable determination of a plurality of differences between the measured volume of drilling fluid entering the borehole from the mud system and the measured volume of drilling fluid flowing on the downstream side of the telescoping section. 12. Device for determining the flow rate of a drilling fluid flowing in a riser from a borehole drilling ■ offshore from a floating vessel, where the riser extends from the borehole to the vessel, and comprising a mud system which is connected to the riser by means of a line where the mud system circulates drilling fluid through said borehole and riser pipe, which riser pipe has a telescopic section with an upper and lower cylinder to record vertical movements or lifting movements performed by the vessel, characterized by the fact that it includes means for sensing a mutual position between the upper and lower cylinder relative to a predetermined mutual position, capable of emitting signals when the cylinders are momentarily in said predetermined position, means for measuring time between . consecutive signals which establish a series of consecutive time periods, and a first device for. measuring the volume of drilling fluid flowing on the downstream side of the telescope section during any of said time periods, to determine the flow rate of the drilling fluid flowing from the borehole when the volume measured during a certain time period is divided by the time period. 13. Device as stated in claim 12, characterized in that the sensing devices comprise a switch which is controlled by the cylinders and sends the said signals. 14. Device as stated in claim 13, characterized in that the switch comprises a potentiometer which monitors the mutual position of the cylinders and produces a voltage which varies as a function of the mutual movement, a filter device which is connected to the potentiometer to eliminate all voltage variations over a predetermined frequency and allowing all filtered voltage variations below the predetermined frequency to fit, means for determining the difference between the filtered voltage variations and the produced voltage variation from the potentiometer to establish a modified zero axis about which the voltage varies, and wherein the modified zero axis represents the aforementioned predetermined position, and devices for converting the varying voltage into separate signals when the aforementioned voltage crosses the zero axis to thereby produce a signal when the cylinders are in the aforementioned predetermined position. 15. Device as stated in claim 12, characterized in that it comprises other means for measuring the volume of drilling fluid that enters the borehole from the mud system during each of the mentioned time periods, and devices for determining the difference between the volumes measured by the first and second devices during identical time periods, a difference indicating the presence of an abnormal drilling condition. 16. Improved apparatus for determining "kick" or lost circulation from a well in a system for offshore drilling from a floating vessel having a riser extending from the well to the vessel, and comprising a mud system' which is connected to the riser by means of a line, which mud system circulates drilling fluid through the borehole and the riser, where the riser includes a telescoping section with an upper and lower cylinder to absorb vertical movement or heave movement performed by the vessel and where the mud system includes shock absorber devices to dampen sudden changes in volumetric displacement of the drilling fluid in the telescopic section, characterized by means for sensing the relative position between the upper and lower cylinders relative to a predetermined relative position, capable of emitting the signal when the cylinders are temporarily in said predetermined position, means for measuring the time between successive signals for establishing a series of successive time periods, a first means for measuring the volume of drilling fluid flowing on the downstream side of the shock absorber devices during any one of the successive time periods , second means for measuring the volume of drilling fluid entering the wellbore from the mud system during any of said consecutive time periods, and third means for measuring a change in the volume of drilling fluid in the shock absorber device during each of said consecutive time periods consecutive time periods to allow determination of the presence of either a "kick" or lost circulation in the wellbore when the sum of the volume measured by the first means and the measured change in volume measured by the third means during identical time periods is not large set equal to the volume measured by the other means during the same time period. 17. Improved apparatus for determining a "kick" or loss of drilling fluid in a system for offshore drilling from a floating vessel having a riser extending from the wellbore to the vessel and a mud system connected to the riser by means of a line, wherein the mud system is capable of circulating drilling fluid through the borehole and the riser, and where the riser has a telescopic section with an upper and lower cylinder to accommodate vertical movement or heave movement exerted by the vessel, and where the mud system includes a tank device containing drilling fluid in connection with the said line during non-circulation of the drilling fluid, characterized by devices for sensing the mutual position between the upper and lower cylinder in relation to a predetermined reference position, where the sensing device sends out a signal when the cylinders are temporarily in the said reference position for determining a series of consecutive time periods between each other following generated signals, means for measuring the volume of drilling fluid in the tank device each time a signal is emitted from the sensing device, and devices for comparing the measured volume in the tank device so that a sudden change in the successive combined volumes measured over a series of for consecutive time periods will indicate a "kick" or loss of drilling fluid.