Video system and method for determining and monitoring the depth of a bottomhole assembly within a wellbore

Video systems and methods for determining the length of objects to be inserted in a wellbore, and for summing the lengths to obtain an accurate determination of the depth at which a bottomhole assembly is located at any given time. The video systems and methods of the present invention are also used in conjunction with hookload and traveling block location information to determine bottomhole assembly depth while drilling, or tripping-in or tripping out of a well. Also disclosed is a method of accurately determining the transition a drillstring undergoes and its associated movement when passing from in-slips to out-of-slips.

TECHNICAL FIELD 
The present invention relates to systems and methods for determining and 
monitoring the depth at which a drilling rig is operating, and more 
particularly relates to systems and methods for determining and monitoring 
the depth at which a bottomhole assembly is located within a wellbore. The 
present invention further relates to systems and methods for accurately 
determining the length of an object before it is placed in a well. 
BACKGROUND OF THE INVENTION 
In common rotary drilling methods and systems used in drilling oil field 
boreholes, power rotating means delivers torque to a drill pipe, a 
plurality of which forms a drill string, via a kelly and a rotary table. 
The drill string in turn rotates a bit located at its lowermost end that 
drills a borehole through the sub-surface formation. The drill string is 
supported for up and down movement by a drilling mast located at the 
earth's surface. A drill line or cable supported by the drilling mast and 
coupled to the drill string is used in conjunction with a rotating drum to 
facilitate the up and down movement. The drill line is anchored at one end 
called the dead line anchor, which is typically located adjacent to one 
leg of the drilling mast. The drill line extends from the anchor upwardly 
to a crown block formed of a plurality of rotatable sheaves at the top of 
the mast. The drill line is reaved around the sheaves in the crown block 
and extends downwardly between the crown block sheaves and rotating 
sheaves in a traveling block. The drill line then extends from the crown 
block downward to a rotating drum or drawworks. that moves the crown block 
up and down by reeling the drill line in or out. 
As will be appreciated by those skilled in the art, determining and 
monitoring the depth at which a component of the bottomhole assembly (BHA) 
is located at any given time in a wellbore is important for many reasons. 
For example, the drilling rig operator needs to know the depth at which 
the bottom hole assembly is located during trips in and out of the well so 
that he can be cautious when passing through sensitive zones such as 
bridges, ledges, or key seats. In addition, by correlating information 
gathered from offset wells, a driller needs accurate depth measurement 
information while drilling subsurface formations to anticipate trouble 
zones, e.g., high gas-pressured gas zones, in order to take appropriate 
precautionary measures. Also, accurate depth information is extremely 
valuable when performing directional or horizontal drilling operations. 
In recent years, many developments have been made in the area of gathering 
borehole data while the drilling operation is being conducted. These 
services, which are commonly referred to as measurement-while-drilling 
(MWD), logging-while-drilling (LWD), and formation evaluation while 
drilling (FEWD), typically incorporate various sensing devices into the 
bottomhole assembly to gather information related to, for example, 
formation lithography, downhole environment, and tool operating 
parameters. The raw or processed data gathered by such devices are 
typically either transmitted to the surface in "real time" by using, for 
example, a mud pulse telemetry system, or stored in a memory device 
located in the downhole tool for later retrieval when the BHA is brought 
back to the earth's surface, or simultaneously transmitted in real time 
and stored downhole. For much of this information to be of significant 
value, particularly lithography data, it must be correlated to the 
particular depth at which the information was obtained. Accordingly, it is 
extremely important for MWD or LWD service providers to have an accurate 
depth measurement system and apparatus. 
Present day depth systems and methods typically include a combination of 
keeping a tally indicating the length of each drill pipe inserted into the 
borehole, and measuring the incremental length of the last drill pipe 
being lowered into the borehole during the drilling or tripping operation 
by monitoring the movement of the traveling block. Traveling block 
movement is commonly determined by monitoring the motion of the drilling 
line as it is fed from the drawworks, e.g., with a sensor coupled to the 
rotating drum or one of the sheaves in the crown block. This general type 
of system, however, contains many sources of errors and inaccuracies. For 
example, the length of a particular pipe section is simply inaccurately 
measured or noted erroneously, or added to the drill string in an order 
different from that noted in the tally. In addition, with respect to 
monitoring the motion of the drilling line through drum rotation to record 
the length of the last pipe, since the drill line cable stretches over 
time and because the cable is wound in layers around the rotating drum, 
the rotation of the drum itself does not accurately correlate to the 
length of the last drill pipe being lowered. 
Further inaccuracies with prior methods typically occur during the 
procedure when pipe is added or subtracted to the drill string either 
while conducting the drilling operation or while tripping in or out of the 
well. For example, when the rig's traveling block has reached its maximum 
downward movement during a drilling operation and a new section of pipe 
must be added, the traveling block and connected drill string are first 
raised a short distance by reeling in the drill line cable, followed by 
placing slips in the rotary table. After the slips are inserted, the 
traveling block is lowered a short distance such that the slips support 
the drill string, which allows the kelly to be unscrewed. In the process, 
cable is reeled out while the BHA remains stationary. The disparity in 
movement is due to the release of tension in the cable since the cable is 
no longer supporting the weight of the drill string. On the other end of 
the procedure when the kelly is swung over to the pipe and the new pipe is 
attached onto the kelly, and the kelly and new pipe are swung back and 
attached to the drill string, the traveling block first moves upward to a 
point where the slips can be removed. When the slips are removed, again 
misallocations regarding drum rotation and traveling block movement with 
respect to the drill string movement are made with resulting depth 
determination inaccuracies. These small errors at each transition can 
translate into an accumulated error of several feet during the course of 
drilling a well. 
An additional problem with tracking BHA position based on traveling block 
altitude is that such systems, for a variety of reasons, often loose track 
of the block position. Systems that determine block position based on 
encoders connected to the drawworks loose block position accuracy, for 
example, because of cable stretch over time and changes in the way the 
cable wraps on the drumwork's rotating drum. Systems that place encoders 
on the fast sheave in the crown block typically loose block position 
accuracy, for example, because of cable slippage and cable stretch. Both 
of these general types of systems typically lack a reliable way of 
resetting block position that does not affect or interfere with the 
drilling operation. 
In order to overcome some of the inaccuracies inherent in most prior art 
depth techniques, several different methods and apparatus have been 
proposed. For example, in U.S. Pat. No. 4,114,435 to Patton et al, it is 
proposed to measure different traveling block reference points that relate 
to when the cable on the drawwork drum reaches different layers of 
unwinding, and then to determine the location of the traveling block via 
an equation, the reference points, the rotation of the drum, etc. The 
Patton et al system, however, still provides inaccuracies because it fails 
to account for the dynamic nature of the cable layering process. Moreover, 
an account for the cable stretching over time is not provided for. 
U.S. Pat. No. 4,787,244 to Mikolajczyk proposes to automatically determine 
the drill bit depth by tracking the movement of the cable. Movements of 
the cable are only tracked when the weight carried by the traveling block 
exceeds a certain minimum threshold as determined by a tensiometer on a 
cable. However, this prior technique fails to account properly for 
movements of the cable during the slips-in and slips-out procedure when 
the transition is made through the threshold. Similar types of errors are 
believed to be inherent in the system proposed in U.S. Pat. No. 4,616,321 
to Chan. 
U.S. Pat. No. 4,610,005 to Utasi proposes a video system that monitors the 
position and movement of the traveling block to determine borehole depth. 
In Utasi's system, a video camera is positioned to track the vertical 
movement of the traveling block. However, Utasi's systems seems to be 
fairly impractical and inaccurate because of the remote distance that the 
camera must be positioned to view the entire rig. In addition, the 
distance between the camera and the rig renders the system susceptible to 
interference from the rig structure, lighting changes, equipment movement, 
etc. 
In light of the above, a principal object of the present invention is to 
provide a system for and method of accurately determining and monitoring 
the depth at which a bottomhole assembly is located within a wellbore. 
A further object of the present invention to provide a system for and 
method of accurately measuring and recording the length of an object 
before it is inserted into a well. 
Another object of the present invention is to provide a means for verifying 
and resetting a depth determination to substantially reduce accumulated 
errors. 
A further object of the present invention is to provide a system for and 
method of accurately measuring and recording the depth at which a 
bottomhole assembly is located while substantially not affecting or 
interfering with the normal operation of a drilling rig and its crew. 
SUMMARY OF THE INVENTION 
The present invention provides systems for and methods of accurately 
determining the length of an object before it is inserted in a wellbore, 
and accurately determining the depth at which a bottomhole assembly is 
located within a wellbore. In a preferred embodiment of the present 
invention, a video camera is positioned near the rig floor and focused 
above the mouse hole. The camera is associated with a video display having 
moveable cursors superimposed thereon by a measuring device. After the 
measuring device associated with the camera and video display has been 
calibrated to build a table of pixel distance between cursor position 
versus length within a computer, the length of an object placed within the 
mouse hole, e.g., a section of drill pipe, is determined by moving the 
cursors on the video display adjacent to the image of the portion of the 
object protruding from the mouse hole, and equating the pixel distance 
between the cursors to a length based upon the pixel distance/length 
table. This length is added to the previously-determined length that pipes 
extend below the rig floor into the mouse hole to obtain the object's 
overall length. The computer is programmed to sum up the total length of 
pipes added to the drillstring via the mousehole during either drilling or 
tripping operations. 
In another preferred embodiment of the present invention, two cameras are 
positioned on a rig to measure the length of joints suspended within the 
rig's mast as they are added to or subtracted from a drillstring. Both 
cameras and the images displayed thereby on a video display and the 
measuring device associated therewith are calibrated to generate a pixel 
distance versus length table which is used in determining the length of 
added or subtracted joints, which are summed by the computer in 
determining depth. 
In other preferred embodiments of the present invention, traveling block 
movement and position information and hookload information are used to 
determine depth with the video systems being used in association therewith 
to verify the accuracy thereof and provide the basis for making resets and 
offsets when necessary.

DETAILED DESCRIPTION OF THE INVENTION 
The depth determining and monitoring systems and methods of the present 
invention may be used and practiced in association with a wide variety of 
drilling rigs that are commonly used in the industry, for example, 
on-shore, off-shore, floating platforms, rotary table drives, top drives, 
mud motor drives, etc. In addition, the systems and methods of the present 
invention may be used to determine and sum the lengths of any objects as 
they are placed within a wellbore, e.g., drill pipe, drill collars, MWD 
subs, tubing, casing, etc. With reference to the Figures in which the same 
numeral is used to indicate common apparatus and application components, 
FIG. 1 schematically illustrates a typical drilling rig generally 
indicated as 10 that is representative of most rigs commonly used in the 
art. In FIG. 1, rig 10 includes a vertical derrick or mast 12 having a 
crown block 14 at its upper end and a horizontal rig floor 16 at its lower 
end. Drill line 18 is fixed to deadline anchor 20, which is commonly 
provided with hook load sensor 21, and extends upwardly to crown block 14 
having a plurality of sheaves (not shown). From block 14, drill line 18 
extends downwardly to traveling block 22 that similarly includes a 
plurality of sheaves (not shown). Drill line 18 extends back and forth 
between the sheaves of crown block 14 and the sheaves of traveling block 
22, then extends downwardly from crown block 14 to drawworks 24 having 
rotating drum 26 upon which drill line 18 is wrapped in layers. The 
rotation of drum 26 causes drill line 18 to be taken in or out, which 
raises or lowers traveling block 22 as required. Drawworks 24 may be 
provided with sensor 27 which monitors the rotation of drum 26. Sensor 27 
may be, for example, a quadrature incremental encoder that produces pulses 
as drum 26 rotates as is well known in the art. Alternatively, sensor 27 
may be located in crown block 14 to monitor the rotation of one or more of 
the sheaves therein. 
Hook 28 and elevators 30 are attached to traveling block 22. Hook 28 is 
used to attach kelly 32 to traveling block 22 during drilling operations, 
and elevators 30 are used to attach drill string 34 to traveling block 22 
during tripping operations. Drill string 34 is made up of a plurality of 
individual pipe members, a grouping of which are typically stored within 
mast 12 as joints 35 (singles, doubles, or triples) in a pipe rack. Drill 
string 34 extends down into wellbore 36 and terminates at its lower end 
with bottom hole assembly (BHA) 37 that typically includes a drill bit, 
several heavy drilling collars, and instrumentation devices commonly 
referred to as measurement-while-drilling (MWD) or logging-while-drilling 
(LWD) tools. Mouse hole 38, which typically has spring 39 at the bottom 
thereof, extends through and below rig floor 16 and serves the purpose of 
storing next pipe 40 to be attached to drill string 34. 
During a drilling operation, power rotating means (not shown) rotates a 
rotary table (not shown) having rotary bushing 42 releasably attached 
thereto located on rig floor 16. Kelly 32, which passes through rotary 
bushing 42 and is free to move vertically therein, is rotated by the 
rotary table and rotates drill string 34 and BHA 37 attached thereto. 
During the drilling operation, after kelly 32 has reached its lowest point 
commonly referred to as the "kelly down" position, new pipe 40 in mouse 
hole 38 is added to drill string 34 by reeling in drill line 18 onto 
rotating drum 26 until traveling block 22 raises kelly 32 and the top 
portion of drill string 34 above rig floor 16. Slips 44, which may be 
manual or hydraulic, are placed around the top portion of drill string 34 
and into the rotary table such that a slight lowering of traveling block 
22 causes slips 44 to be firmly wedged between drill string 34 and the 
rotary table. At this time, drill string 34 is "in-slips" since its weight 
is supported thereby as opposed to when the weight is supported by 
traveling block 22, or "out-of-slips". 
Once drill string 34 is in-slips, kelly 32 is disconnected from string 34 
and moved over to and secured to new pipe 40 in mouse hole 38. New pipe 40 
is then hoisted out of mouse hole 38 by raising travelling block 22, and 
attached to drill string 34. Traveling block 22 is then slightly raised 
which allows slips 44 to be removed from the rotary table. Traveling block 
22 is then lowered and drilling resumed. 
"Tripping-out" is the process where some or all of drill string 34 is 
removed from Wellbore 36. In a trip-out, kelly 32 is disconnected from 
drill string 34, set aside, and detached from hook 28. Elevators 30 are 
then lowered and used to grasp the uppermost pipe of drill string 34 
extending above rig floor 16. Drawworks 24 reel in drill line 18 which 
hoists drill string 34 until the section of drill string 34 (usually a 
"triple") to be removed is suspended above rig floor 16. String 34 is then 
placed in-slips, and the section removed and stored in the pipe rack. 
"Tripping-in" is the process where some or all of drill string 34 is 
replaced in wellbore 36 and is basically the opposite of tripping out. 
In some drilling rigs, rotating the drill string is accomplished by a 
device commonly referred to as a "top drive" (not shown). This device is 
fixed to hook 28 and replaces kelly 32, rotary bushing 42, and the rotary 
table. Pipe added to drill string 34 is connected to the bottom of the top 
drive. As with rotary table drives, additional pipe may either come from 
mouse hole 38 in singles, or from the pipe racks as singles, doubles, or 
triples. 
The depth of a component of the BHA, whether the bit or an MWD device for 
example, during the drilling operation at any instant in time is the sum 
of the distance between the lower edge of the lowermost pipe of 
drillstring 34 to the BHA component, the total length of drill string 34, 
and the length of the portion of kelly 32 extending below rig floor 16, 
which is typically the reference point or "zero" for all depth 
determinations. The depth of a component of the BHA while tripping in or 
out of a well is the total of the distance between the lower edge of the 
lowermost pipe of drillstring 34 to the BHA component, plus the total 
length of drill string 34, minus the portion of the uppermost pipe 
extending above rig floor 16. The depth determination and monitoring 
systems and methods of the present invention accurately determine the 
depth of a BHA component during drilling or while tripping in or out of 
wellbore 36. 
In one preferred embodiment of the present invention and still referring to 
FIG. 1, lower camera 50 is positioned within the lower portion of mast 12 
of rig 10 on or near rig floor 16 such that its field of view is directed 
to the rotary table and the top of drill string 34 extending above the 
rotary table when present. The field of view of lower camera 50 is also 
preferably directed to the upper portion of next pipe 40 stored within 
mouse hole 38 that extends above drilling floor 16. On some drilling rigs, 
it might not be possible or practical to position lower camera 50 such 
that its field of view is directed to both the rotary table and the mouse 
hole. In such instances, two lower camera units are preferably used. In 
another preferred embodiment of the present invention to be described in 
greater detail later herein, upper camera 52 is located in the upper 
portion of mast 12 and positioned such that its field of view is directed 
to where the top edge of a joint 35 would be located during a tripping 
operation. 
FIG. 2 schematically illustrates the main components of the video depth 
determination systems of the present invention. Camera 54 represents any 
one of the cameras used in the present invention and located on the rig, 
whether it be lower rotary table/mouse hole camera 50, upper camera 52, or 
a separate rotary table camera and a mouse hole camera. Camera 54 may be 
of any standard video format, black and white or color, such as model No. 
WVBL202 available from Panasonic. Camera 54 acquires an image of the 
object to be measured and supplies a corresponding video signal to 
measuring device 56 such as that available from Boeckeler Instruments, 
Inc., model number VIA 100. Measuring device 56 displays the image 
received from camera 54 on video screen or display 58 such as model No. 
VM4509 available from Sanyo, and superimposes moveable cursors 60 and 62 
thereon. The location of cursors 60 and 62 on video display 58 is 
independently controlled by an input signal to measuring device 56 from 
control console 63. The distance between cursors 60 and 62 appearing on 
video display 58 is measured in pixels by measuring device 56 and supplied 
to computer 64, which may be any computing device capable of accepting 
data from measuring device 56 and making the required computations. 
Computer 64 may be any computer, microcomputer, microprocessor, 
microcontroller, etc. such as an N286 available from ACUDATA, Inc. of 
Houston, Tex. U.S.A. In an alternate embodiment, measuring device 56 
superimposes only one moveable cursor on disply 58, which in operation is 
functionally equivalent to the two cursor embodiment shown in FIG. 2. 
In making depth determinations with a particularly preferred embodiment of 
the systems and methods of the present invention to be described 
hereinafter in greater detail, the position and movement of traveling 
block 22 and hookweight or hookload are preferably obtained and imputed 
into computer 64. Hookweight measurements are made by hookload sensor 21 
located, for example, in conjunction with deadline anchor 20 as shown in 
FIG. 1 although as those skilled in the art will appreciate, hookload may 
be measured at any one of many locations such as at hook 28, in crown 
block 14, on drill line 18 etc. The position and movement of traveling 
block 22 may be obtained from traveling block sensor 27 such as a 
drawworks sensor that monitors the rotation of drum 26 as drill line 18 is 
reeled in and out of drawworks 24. Traveling block sensor 27 may be, for 
example, an encoder directly or indirectly attached to the rotating shaft 
of rotating drum 26 as is presently known in the art. Alternatively, block 
position and movement may be determined by a sensor located in crown block 
14 that monitors the rotation of one or more of the sheaves therein, or 
monitors the movement of drill line 18 as it passes through the crown 
block 14 or near drawworks 26. As noted previously herein, determining 
depth based upon traveling block location by monitoring movement of drill 
line 18 and monitoring hookload alone as presently done in the art is 
replete with sources of error that individually or cumulatively result in 
inaccurate depth measurements. However, the video depth systems and 
methods of the present invention are equipped with means for detecting and 
substantially eliminating these errors as will be hereinafter explained in 
greater detail. 
When determining depth while drilling or tripping, it is important to 
accurately measure the length of a pipe being added to or subtracted from 
the drillstring and keeping an accurate record of the length of each of 
these pipe sections. FIGS. 3 and 4 illustrate the procedure of a preferred 
embodiment of the present invention that uses lower camera 50 for 
measuring the length of next pipe section 40 located within mousehole 38 
to be added to drillstring 34, or that was removed from drillstring 34. 
Briefly, the procedure includes a calibration step and an actual measuring 
step. The calibration procedure produces the length that pipes extend 
below rig floor 16 and into mouse hole 38 when a pipe is placed therein 
that loads spring 39 (if present), and a table of coefficients used to 
measure the section of the pipe extending above the rig floor. The total 
length of pipe 40 mouse hole 38 is then obtained by adding the two lengths 
together. 
First referring to FIG. 3, face-to-face length "A" of any pipe placed 
within mouse hole 38 is equal to length "B" of the pipe extending below 
rig floor 16 excluding the length of male thread 41', plus length "C" that 
the pipe extends above rig floor 16. Since the length of male threads or 
"pin" 41' of all pipes to be measured is fairly constant and held to a 
tolerance set by the API, this length is ignored. In the first step of the 
calibration procedure, total face-to-face length A of reference or 
calibration pipe 41 is accurately measured before it is placed in 
mousehole 38 with a steel tape, for example, and entered into computer 64. 
When pipe 41 is placed within mousehole 38, the weight thereof loads and 
compresses spring 39, and since spring 39 is very rigid, all subsequent 
pipes placed within mousehole 38 will compress spring 39 approximately the 
same amount. Then, with reference to FIG. 4, after reference pipe 41 is 
placed in mousehole 38, calibration rod 70 having a plurality of 
calibration marks 72 thereon is placed adjacent to the upper portion of 
pipe 41 extending above rig floor 16, both pipe 41 and rod 70 preferably 
being approximately the same distance away from lower camera 50. In a 
preferred embodiment, marks 72 on rod 70 are spaced an equal distance from 
one another, e.g., 0.5 feet (15.25 cm), the number and spacing of marks 72 
depending on the degree of accuracy desired to overcome the apparent 
displacement of non-equidistant objects associated with camera 50. 
The image recorded by camera 50 is displayed on display 58 which has 
superimposed thereon reference cursor 62 and measurement cursor 60 by 
measuring device 56 as shown in FIG. 4. From control console 63, reference 
cursor 62 is moved and placed where calibration rod 70 contacts rig floor 
16 and remains in this position during both the calibration and length 
measurement procedures. Measurement cursor 60 is then first placed over or 
adjacent to the next mark 72 on rod 70 up from rig floor 16. When so 
placed, an entry is made on control console 63 which sends a signal to 
computer 64 through measuring device 56 to determine the number of pixels 
between cursors 60 and 62 and to equate that number to the known distance 
the between the bottom and first marks 72 on rod 70. Measurement cursor 60 
is then moved up through each successive mark 72 on rod 70 and signals are 
sent to computer 64 at each point. Once all marks 72 on rod 70 have been 
recorded in this fashion (or as many marks as accuracy requires and time 
and circumstances permit), computer 64 has compiled a table of pixel 
distance between cursors 62 and 60 versus length, or actual height above 
rig floor 16 in this case. In an alternate form of the present invention, 
display 58 has only one moveable cursor superimposed thereon by measuring 
device 56, which functionally is the same as the two cursor embodiment 
just described by the one cursor serving as both a reference cursor in one 
mode and as a measurement cursor in the other mode. 
In the final calibration step, measurement cursor 60 is placed adjacent to 
the top edge of pipe 41 as shown on display 58 in FIG. 4. Based on imput 
from measuring device 56, computer 64 then equates the pixel distance 
between reference cursor 62 and measurement cursor 60, which corresponds 
to length C (FIG. 3) of pipe 41 extending above rig floor 16, to a length 
(feet or meters and fractions thereof) by using the earlier-generated 
pixel distance versus length table. A linear interpolation, a curve 
fitting algorithm, or any similar algorithm known to those skilled in the 
art can be used to solve for points that fall between the calibration 
points. In this manner, length C (FIG. 3) is obtained, which is subtracted 
from earlier-determined total length A of pipe 41 to determine length B of 
the pipe extending below rig floor 16. Length B is stored in computer 64 
for future use. 
The video system of the present invention illustrated in FIG. 4 is then 
fully calibrated and ready to accurately measure and automatically tally 
the length of each new pipe 40 of unknown length before it is added to 
drillstring 34, or after it is removed therefrom, via mouse hole 38. In 
the measurement procedure, after a pipe is placed in mouse hole 38, 
measurement cursor 60 is lined up with the very top edge of the pipe with 
reference cursor 62 remaining where it was placed during the calibration 
procedure, and an entry is made in control console 64. From the pixel 
distance versus length table generated during the calibration procedure 
and stored in computer 64, computer 64 equates the distance between 
cursors 62 and 60 into a length, and then adds the previously-determined 
and stored length B of mousehole extension thereto, which gives the total 
face-to-face length of the pipe about to be added to drillstring 34. Each 
time the length of a new pipe is measured in this fashion and the pipe is 
added to drillstring 34, an entry is made on control console 64 which 
through measuring device 56, updates a summation program in computer 64 to 
add the new length to a running total length. Alternatively, each time a 
pipe is removed from drillstring 34 and the length thereof determined as 
just described, an entry is made on control console 64 which through 
measuring device 56, updates a subtraction program in computer 64 to 
subtract the length of the removed pipe from the running total length. In 
a preferred embodiment, the total length of all pipes making up 
drillstring 34 is displayed on display 58, and also recorded on tape for 
playback if desired. 
In an alternate version of the embodiment of the present invention just 
described, measurement device 56 is replaced with a video digitizer 
equipped with digitization software such as a TARGA M8 available from 
Dawson and Associates of Houston, Tex. U.S.A. In this alternate 
embodiment, the video digitizer is resident on the computer 64 bus, for 
example, or as a component separate from computer 64 as with measuring 
device 56. The image recorded by the camera is digitized, written to the 
computer's videomemory, and displayed on the display or screen associated 
with the computer along with cursors also generated by the digitization 
software. The software moves the cursors upon operator command and 
determines the pixel distance between the cursors as was done in the 
measurement device embodiment. 
The calibration and measurement procedures from an operator viewpoint are 
basically the same. 
Another implementation of this alternate embodiment automates the 
measurement procedure with computer 64 making the measurement with little 
or no operator input. Specifically, a map, for example, representing the 
approximate shapes of calibration rod 70 and the objects to be measured 
are stored in computer 64. During the calibration procedure, calibration 
rod 70 is placed the same as previously described. Computer 64 recognizes 
its basic shape as well as each calibration marking 72 thereon and stores 
the pixel location of each mark 72 in its memory. Computer 64 then uses 
the pixel distance between each mark 72 along with its previous knowledge 
of the distance between each marking to generate a pixel distance versus 
length table as previously described. 
Reference pipe 41 is placed in mousehole 38 and its total length entered 
into computer 64. The computer then examines and recognizes the section of 
pipe 41 extending above rig floor 16 and uses the image in determining the 
length thereof with the pixel distance versus length table, and solves for 
length B of FIG. 4. Thereafter, computer 64 through image recognition 
determines the length of any pipe in mousehole 38 as previously described. 
In instances where a confusing background might undermine or interfere 
with the ability of computer 64 to recognize shapes and outlines of 
object, a constant shade backdrop or backlight is preferably used. 
In another preferred embodiment of the present invention, the length of 
single, double, or triple joints 35 stored in the pipe racks of rig 12 may 
be accurately measured and tallied as they are added to or subtracted from 
drilling 34 either while tripping with a rotary table drive rig, or when 
adding pipe during drilling or tripping with a top drive rig. Referring 
briefly to FIG. 1, upper camera 52 is positioned in the upper portion of 
mast 12 and is used in conjunction with lower camera 50 to make the 
required length measurements. As with the previous embodiment used to 
measure the length of a pipe in mousehole 38, the pipe rack embodiment of 
the present invention includes first a calibration step followed by the 
actual measurement steps, either in digitized or non-digitized format. 
FIG. 5 illustrates the calibration procedure used for the two camera 
embodiment of the present invention, which will generate two separate 
tables of pixel distance versus length in computer 64, one for each 
camera. In calibrating the system, upper camera 52 is focused on the upper 
portion of reference pipe 80, which may be a single, double, or triple, 
that has been previously measured by any accurate technique such as by 
hand with a steel tape, or with the previously-described mousehole 
embodiment as reference pipe 80 is assembled and attached to the portion 
of drillstring 34 extending above rig floor 16. Calibration rod 82, which 
is essentially identical to previously-described rod 70 and includes a 
plurality of markings 83 thereon, is positioned adjacent to the upper 
portion of reference pipe 80 such that both the top portion of pipe 80 and 
rod 82 are within the field of view of upper camera 52 and preferably 
being approximately the same distance away from camera 52. The view 
recorded by upper camera 52 is displayed on display 90 having reference 
cursor 84 and measurement cursor 85 superimposed thereon although as noted 
earlier herein, one cursor may be used that is functionally equivalent to 
cursors 84 and 85. Similarly, lower camera 50 is positioned such that its 
field of view is directed to the lower portion of reference pipe 80 and 
second calibration rod 88 having a plurality of marking 89 thereon held 
adjacent to pipe 80. The view from lower camera 50 is displayed on display 
58 having reference cursor 92 and measurement cursor 93 superimposed 
thereon. Lower camera 50 and display 58 may be the same as those used in 
measuring the length of pipe located in mousehole 38 as just described, or 
may be a separate third camera if it is desired to leave the mousehole 
camera undisturbed in order, for example, to preserve the mousehole 
calibration. In an alternate embodiment, both the view from upper camera 
52 and the view from lower camera 50 may be displayed on a single display 
in a split screen format, or alternately on the same display on command. 
In calibrating first upper camera 52, reference cursor 84 is placed on or 
adjacent to the lowest marking 83 on upper calibration rod 82 as shown in 
FIG. 5. Since reference cursor 84 will remain in this position for all 
subsequent measurements, care should be taken that the position of 
reference cursor 84 will be below the top end of each joint that is 
planned to be measured when it is added to drillstring 34. Next, 
measurement cursor 85 is placed on the next marking 83 up from the 
lowermost marking. An entry is then made into computer 64 via control 
console 63 and measuring device 56 that records the pixel distance between 
cursors 84 and 85, and also the length that this pixel distance is equal 
to, e.g., 0.5 feet (15.25 cm). After this entry has been made, measurement 
cursor 85 is moved up along each successive marking 83 on upper 
calibration rod 82 with an entry being made into computer 64 for each 
marking such that a pixel distance versus length table is generated and 
stored inside computer 64. In the final calibration step, measurement 
cursor 85 is placed adjacent to the top edge of reference pipe 80 as shown 
in FIG. 5. Based on the pixel distance versus length table stored in 
computer 64, the pixel distance between reference cursor 84 and 
measurement cursor 85 is converted into length with this length "F" as 
indicated on display 90 being stored in computer 64 for future use as will 
be hereinafter explained. 
The calibration of lower camera 50 is done in essentially the same manner 
as with upper camera 52 by using lower calibration rod 88, reference 
cursor 92, and measurement cursor 93 except that the final calibration 
step records the pixel distance between reference cursor 92 and 
measurement cursor 93 when the later is placed adjacent to the lower edge 
of reference pipe 80 as shown on display 58 in FIG. 5. This pixel distance 
is converted into a length based on the pixel distance versus length table 
generated for lower camera 50 and stored in computer 64. This length "G" 
as indicated on display 58 is stored in computer 64 for future use as will 
be hereinafter explained. 
FIG. 6 illustrates how length "D" between lowest marking 89 on lower 
calibration rod 88 (which corresponds to reference cursor 92), and lowest 
marking 83 on upper calibration rod 82 (which corresponds to reference 
cursor 84), is determined, length D being needed to compute the length of 
a new joint being added to or subtracted from drillstring 34 during the 
measurement procedure. As noted previously, total length "E" of reference 
pipe 80 was previously measured by using any accurate technique and 
entered into computer 64. Length F between lowermost marking 83 on rod 82 
(where reference cursor 84 is fixed) and the top edge of reference pipe 80 
was measured and recorded during the calibration procedure and is 
therefore also known. Similarly, length G between lowermost marking 89 on 
rod 88 (where reference cursor 92 is fixed) and the lower edge of 
reference pipe 80 was also measured and recorded during the calibration 
procedure and is therefore also known. Length D therefore is equal to 
length E plus length G minus length F. Once length D is determined in this 
fashion and stored in computer 64, the length of any new pipe joint added 
to or subtracted from drillstring 34 can be determined from the equation: 
unknown pipe length=length D (known)-length G (to be determined)+length F 
(to be determined), lengths G and F being determined in the following 
manner. 
After the calibration procedure of upper camera 52 and lower camera 50 is 
complete, the video system of the present invention is ready to determine 
and record the length of any joint being added to or subtracted from 
drillstring 34, this procedure being illustrated in FIG. 7. In FIG. 7, 
lower camera 50 records the lower portion of unknown pipe 100 and displays 
this view on display 58. Measurement cursor 93 is placed adjacent to the 
lowermost edge of unknown pipe 100 (uppermost edge of drillstring 34) with 
reference cursor 92 remaining where it was placed and fixed during the 
earlier-described calibration procedure. An entry is made in control 
console 63 which instructs computer 64 via measuring device 56 to compute 
length G based on the pixel distance versus length table generated and 
stored in computer 64 during the calibration procedure. Similarly, with 
the view of the upper section of unknown joint 100 displayed by upper 
camera 52 on display 90 (or on display 58 in a split-screen format), 
measurement cursor 85 is placed adjacent to the uppermost edge of unknown 
pipe 100 with reference cursor 84 remaining where it was placed and fixed 
during the calibration procedure. An entry is made into control console 63 
which instructs computer 64 via measuring device 56 to compute length F 
based on the pixel distance versus length table generated and stored in 
computer 64 during the calibration procedure. Once lengths F and G are 
determined in this fashion, computer 64 computes the length of unknown 
pipe 100 according to the equation: unknown length=D-G+F. This known 
length is then stored in computer 64 and used in adding or subtracting the 
lengths of all pipes that have been added to or subtracted from 
drillstring 34 either while the drilling operation is being conducted, or 
while tripping in or out of the well. In an alternate embodiment, the 
calibration and measurement procedures may be performed in a digitized 
format as was described earlier in conjunction with the mousehole 
embodiment. 
In particularly preferred embodiments, the video systems and methods of the 
present invention find particular use in determining and verifying the 
depth at which a component of a BHA, e.g., the drill bit or a particular 
sensor of an LWD sub, is located at any given moment in a wellbore in 
order to, for example, reset BHA position, reset traveling block position, 
determine maximum bit penetration, and measure incremental bit 
penetration. Referring to FIG. 8, there is shown in simplified form 
drillstring 34 extending below rig floor 16 and into borehole 36. 
Drillstring 34 includes BHA 37 at its lower end and portion 102 of the 
last measured pipe added to drillstring 34 extending above rig floor 16. 
Camera 50 is positioned such that its field of view is directed to the 
portion 102 of drillstring 34 extending above rig floor 16. Camera 50 may 
be the same camera as that used to measure the length of a pipe placed 
within the mousehole as described earlier herein in conjunction with FIGS. 
3 and 4, or another camera if it is not possible to focus in on both the 
mousehole and immediately above the rotary table. In the alternative, 
camera 50 may be the same as lower camera 50 used in determining the 
length of a joint added to drillstring 34 as described earlier herein in 
conjunction with FIGS. 5-7. In whatever case, a pixel distance versus 
length table (hereinafter referred to as the "rotary table calibration 
table") is generated and stored in computer 64 by following the 
calibration procedure as described earlier herein, and the table is used 
in determining the depth of BHA 37 at any given time in the following 
manner. 
In FIG. 8, depth "H" at any given time in borehole 36 is simply the 
summation of length "I" (the overall length of drillstring 34) minus 
length "J" (the length of section 102 extending above rig floor 16). 
Length I is determined by following the calibration, measurement, and 
summation procedures described earlier herein by using the one-camera 
technique of measuring the length of a pipe when it is in the mousehole 
before being connected to drillstring 34, or the two-camera technique of 
measuring the length of a joint while it is suspended in mast 12. Length J 
of portion 102 of drillstring 34 extending above rig floor 16 is 
determined by following the same basic calibration and measurement 
technique used for measuring length C of pipe 40 extending out of 
mousehole 38, which was described earlier herein in conjunction with FIGS. 
3 and 4 and therefore believed unnecessary to be repeated. Once length J 
is determined by computer 64 through the use of the rotary table 
calibration table, computer 64 determines the depth H at which BHA 37 is 
positioned by subtracting length J from the summation of all lengths, or 
length I. 
The ability to accurately determine the depth of BHA 37 at any given moment 
in time as shown in FIG. 8 is particularly useful in verifying and 
resetting block position and depth as recorded by a block position 
sensor/hookload sensor type of depth system used in association with the 
video systems of the present invention. For example, if drillstring 34 is 
placed "in-slips" as shown in FIG. 8 whether during a drilling or tripping 
operation, the video system of the present invention can determine depth H 
as just described and compare that depth with that indicated by the 
traveling block movement sensor 27 and hookload sensor 21. In a 
particularly preferred embodiment of the present invention as shown in 
FIG. 2, signals from traveling block movement sensor 27 and hookload 
sensor 21 are sent to computer 64, which continuously compares the depth 
indicated by sensors 27 and 21 with that determined by the procedure 
indicated in FIG. 8. If a discrepancy exists, computer 64 automatically or 
on command resets the position of the traveling block as indicated by 
sensors 27 and 21, which as noted earlier herein, is a major shortcoming 
of prior systems in that resetting block position is a slow and disruptive 
process. 
FIG. 9 illustrates how one embodiment of the present video system can be 
used in resetting block position in an alternate manner. In FIG. 9, lower 
camera 50 (FIG. 8) displays an image on display 58 of the top portion of 
drillstring 34 being grasped by elevators 30 with reference cursor 92 and 
measurement cursor 93 superimposed on this image by measuring device 56. 
Reference cursor 92 is fixed in the same position it was placed when the 
rotary table calibration table was generated, and measurement cursor 93 is 
placed adjacent to the lowermost edge of elevators 30, which is the 
preferred reference point. Based on the rotary table calibration table, 
computer 64 converts the pixel distance between cursors 92 and 93 to an 
actual length. This distance between the lowest point of elevators 30 and 
rig floor 16 is the same as (or a known distance from) the traveling block 
altitude. If this newly measured distance varies from that indicated by 
traveling block position sensor 27, computer 64 either on command or 
automatically resets block position. FIG. 10 illustrates a method of an 
embodiment of the present invention that is used in measuring incremental 
bit movement and in determining maximum bit penetration. In FIG. 10, 
incremental bit movement is measured by placing measurement cursor 93 
adjacent to a mark 110 on kelly 32 with reference cursor 92 remaining in 
its calibration position (i.e., on rig floor 16). The length corresponding 
to the pixel distance between cursors 92 and 93 is measured by computer 64 
with the rotary table calibration table. After a period of drilling time, 
kelly 32 will have move downward, for example, to where mark 110 on kelly 
32 is shown in phantom. At that time, measurement cursor 93 (shown in 
phantom) is moved adjacent to mark 110 on kelly 32 and the distance 
between cursors 92 and 93 is again measured. The difference between the 
two measurements is the amount of bit penetration, which is recorded as 
such by computer 64. In a preferred embodiment, the bit penetration is 
added to the previously-measured and determined sum of the lengths of all 
the pipe in the borehole to give BHA depth at any given moment in time. In 
a particularly preferred embodiment, computer 64 is provided with a clock 
or timing circuitry means which records bit penetration versus time to 
yield rate of penetration, which is a valuable parameter to the operator 
of the drilling rig. 
The systems and methods of the present invention can be further used in 
conjunction with block position sensor 27 and hookload sensor 21 in 
accurately determining depth while drilling, tripping-in, or tripping-out, 
all as shown operationally in FIG. 2. Hookload is monitored to determine 
when traveling block movement, as monitored by traveling block position 
sensor 27, can be equated to drillstring or BHA movement. In general, a 
high hookload indicates that the drillstring is supported by the traveling 
block, i.e., the string is out-of-slips, and therefore movement of the 
traveling block can confidently be equated to drillstring movement. A low 
hookload indicates that the drillstring is supported by the slips, i.e., 
the string is in-slips, and therefore movement of the traveling block 
should not be equated to drillstring movement. 
Hookload sensors are typically hydraulic or load cell driven and are 
commonly placed on or near the deadline anchor. Unfortunately, using 
hookload sensors in determining the slips-in versus slips-out transition 
is somewhat inaccurate with most of the inaccuracies arising because of 
delays imposed on the hookload sensor signal. For example, delays are 
imposed by the mechanics of the hydraulic system or the electronics of the 
load cell, and the mechanics of the cable as it stretches and contracts. 
There are also delays induced by electrical and electronic components of 
the data acquisition circuitry, both intentional and parasitic. These 
delays and their magnitude, which typically vary from sensor to sensor and 
rig to rig, adversely affect hookload measurements because they mask the 
slips transition point, i.e., the exact point at which the drillstring and 
BHA start moving when taken out-of-slips. This problem is made more acute 
at slips transition points because both traveling block position and 
hookload are typically changing rapidly. 
Hookload measurements can also vary appreciably because the drillstring is 
typically suspended at the end of more than a thousand feet of cable. This 
cable stretches and acts like a spring whenever the drawworks plays out or 
takes in cable, which causes momentary overshoots and undershoots on the 
hookload signal that are more a function of driller or rig action than 
string weight. 
Friction of the string against the formation, especially in a deviated 
well, can also cause overshoots and undershoots in hookload during 
movement, depending on whether the string is going in out of the hole. 
These false drops and rises during movement can also occur in a well that 
has both high mud weight and a bit with small jets. False hookload 
measurements can also occur in a string that is stationary because some of 
the string weight can be supported by the formation wall in the case of a 
deviated well, and/or by the formation itself when the bit is on-bottom. 
In addition, the accuracy of hookload sensors, especially hydraulically 
driven ones, are susceptible to temperature changes such as those caused 
by changes in sunlight patterns or precipitation. 
Traveling block movement sensors are usually up/down counters and are 
commonly placed on the drawworks or the fast sheave in the crown block. 
There are also cable movement sensors, also mounted in the crown block or 
near the drawworks, that can be used to determine block movement and 
position. When placed on the drawworks, they actually function as a 
drawwork position sensor. In all cases, a calibration must be performed to 
relate drawworks movement or cable movement to traveling block movement or 
position. This is typically done by positioning the traveling block at its 
lowest point and setting the drawworks position in a computer. A series of 
periodic measurements of block height and corresponding drawworks position 
are then made at certain prescribed intervals as the drawworks turns, 
thereby taking in cable and raising the traveling block. When the block is 
at its highest position, the calibration procedure is complete and these 
drawworks-position/block-height coefficients make up a table that is used 
to relate drawworks position to block height. When the traveling block 
sensor is placed on the fast sheave in the crown block, a conversion is 
preformed based on sheave diameter so that sheave rotation can be equated 
to drawworks movement. Since sheave diameter is constant, only initial 
block position needs to be entered and then traveling block position can 
be tracked. 
The drawworks cable is subject to both elastic and plastic stretch when 
under tension. Elastic stretch is temporary, i.e. the cable returns to its 
previous length when tension is removed. Since the cable wraps over the 
drawworks drum in several layers, the relationship between drawworks 
position and block altitude is neither constant nor linear. These layers 
are applied at different times under different hookloads (different 
tension) and therefore the cable does not always change layers at the same 
exact place with respect to block position. The effects of plastic 
stretch, which typically occur with a new cable or when a cable is 
subjected to a higher then normal stress, are not removed with a reduction 
in tension. Therefore, drawworks calibrations should be preformed whenever 
the cable is replaced, after new cable has been "worked in", and whenever 
the cable has been subjected to excessive loads such as after the freeing 
of a stuck drill string. 
A rotation sensor mounted on the dead sheave can be used to detect cable 
stretch because the dead sheave only moves an amount proportional to cable 
stretch. However, a cable stretch sensor adds a third sensor to the cost 
of the system, does not address the cable slippage problem, is subject to 
fouling, and is difficult to install and maintain because it is located at 
the top of the derrick. 
Cable movement sensors can also be used to determine block movement. These 
sensors typically detect cable movement by sensing the strands on the 
cable through hall effect sensors or some other means. They basically 
preform the same as, and have the same drawbacks as, sheave sensors. That 
is, they are expensive, difficult to install and maintain, and are subject 
to fouling. 
In using the present video system and methods in conjunction with a 
conventional hookload/block position depth system, two hookload thresholds 
are preferably used to provide hysteresis in the slips transition 
determination algorithm programmed within computer 64. An in-slips 
threshold is selected low enough such that whenever hookload is below the 
threshold, it may be confidently assumed that the string is definitely 
in-slips. An out-of-slip threshold is selected high enough such that 
whenever hookload is above the threshold, it may be confidently assumed 
that the string is definitely out-of-slips. Hookload must pass through 
both thresholds before a slips transition can be said to have occurred. 
FIG. 11 illustrates the operation of a hookload monitoring method used in 
conjunction with the video systems of the present invention that employs 
hysteresis. The top graph of FIG. 11 illustrates hookload 112 versus time 
while the lower graph illustrates traveling block position or traveling 
block altitude (TBA) 115. In FIG. 11 in conjunction with FIG. 2, computer 
64 scans the hookload signal 112 imputed to computer 64 from hookload 
sensor 21 at a rate sufficient to ensure the necessary accuracy and 
resolution. Computer 64 saves each hookload measurement as well as the 
corresponding block position measurement 115 from block position sensor 27 
in a buffer. This buffer preferably contains a sufficient number of 
samples so that when a slips transition occurs, computer 64 is able to 
scan back through the buffer and find the block position corresponding to 
the hookload value at the previous threshold. 
During an in-slips transition, hookload 112 can be seen to fall through the 
out-of-slips threshold at point 113 and then through the in-slips 
threshold at point 114 at which time drillstring 34 is firmly in-slips. 
Block position 115 also falls during this time since the traveling block 
is being lowered as drillstring 34 is being placed in-slips. Computer 64 
scans the hookload samples stored within the buffer back from point 114 
until it finds point 113, which is above the out-of-slips transition 
threshold. At point 113, computer 64 takes the corresponding block 
position at point 116 as the point where the drillstring stopped moving. 
The dynamics of an out-of-slips transition is different from those of an 
in-slips transition because the bit does not start moving until the 
hookload is above the out-of-slips threshold. Computer 64 monitors 
hookload 112 as it rises above the in-slips threshold at point 117 and 
then to out-of-slips threshold at point 118. At this time, computer 64 
selects block position 115 at point 119, which corresponds to point 118 of 
hookload 112 as the point the bit started moving. Computer 64 then uses 
in-slips block position 116 subtracted from out-of-slips block position 
119 and equates this the length of the pipe just added to or subtracted 
from drillstring 34. 
Incremental depth measurements while the drillstring is out-of-slips are 
usually quite accurate in a properly functioning hookload/cable movement 
type depth system. Errors usually occur during slips transitions, and 
these errors are usually small. Unfortunately, these errors accumulate and 
over a period of time can become significant, well over several feet in a 
trip-in or trip-out operation. The second source of inaccuracy typically 
occurs in measuring the length of pipe added to or subtracted from the 
drillstring by the hookload/drawworks measurement routine. Since the 
primary purpose of these pipe measurements is a check of the driller's 
manual tally, this check should be more accurate and reliable than the 
measurement it is being used to verify. 
A particularly preferred embodiment of the present invention significantly 
improves the depth measurement algorithms with the addition of a rig 
calibration algorithm. It also has a video measurement capability to 
provide frequent depth resets that eliminate the accumulation of depth 
errors. This video based measurement is independent of conventional 
sensors, and does not interrupt or affect the normal drilling operation of 
the rig or its crew. 
The Rig Calibration Algorithm of the present invention uses the same 
hookload/traveling block position pairs saved by the just described 
measurement algorithm and adds two offsets called the In-Slips Look Back 
(ISLB) and the Out-of-Slips Look Back (OSLB). These offsets are used by 
the hookload/TBA measurement algorithm to calibrate the hookload/block 
position measurement. 
In calibrating a rig, a reference pipe being added to the drillstring is 
first accurately measured with a tape or with the video systems and 
methods described earlier herein. This pipe becomes a reference for use by 
the rig calibration algorithm. The reference pipe is subsequently added to 
the drillstring and the length thereof measured by the hookload/TBA 
measurement algorithm. If the length of the reference pipe as measured by 
the measurement algorithm is not the same as that actually measured 
manually or by video, the operator can select a ISLB offset and/or an OSLB 
offset so that length of the pipe is indeed accurately determined by the 
measurement algorithm. This same offset is then applied to all subsequent 
measurements automatically. 
FIG. 12 and accompanying Table 1 illustrate an example of the calibration 
process where only an out-of-slips look back is used, the in-slips look 
back being essentially identical in principle and therefore believed not 
necessary to be also described in detail. Samples taken by the software 
are numbered on the graph of hookload 112 versus time and shown in Table 1 
along with corresponding traveling block altitudes (TBAs). In the example, 
a reference pipe is measured as 30.00 feet (9.14 m) and in-slips TBA of 
45.92 feet (14.00 m) is recorded. ISLB and OSLB are both initially set at 
zero. The out-of-slips threshold (OST) is set at 90 Klbs (40.8 KKG). 
At sample #6, hookload is 91 Klbs (41.3 KKG) which is above the OST 
threshold of 90 Klbs (40.8 KKG). The string is now out-of-slips and the 
measurement algorithm looks back in time at each sample until it finds one 
at or below the OST of 90 Klbs. Sample 5 is 89 lbs (40.4 KKG), which is 
below the OST. At this point, the out-of-slips TBA is 74.98 feet (22.85 
m). 
The in-slips TBA was measured during the previous transition and was 45.00 
feet (13.72 m) and therefore the length of the pipe is out-of-slips TBA 
(75.98 ft. (23.16 m)) minus in-slips TBA (45.92 feet (14.00 m)) or 30.06 
ft (9.16 m), which is 0.06 ft longer than the reference pipe actually is. 
The operator therefore adjusts the OSLB to correspond to sample #3 which 
has an out-of-slips TBA of 75.92 (23.14 m). After making this OSLB 
adjustment, the length of the pipe as computed by the measurement 
algorithm is the out-of-slips TBA as OSLB adjusted (75.92 ft/23.14 m), 
minus the in-slips TBA (45.92 ft/14.00 m)=30.00 feet (9.14 m). Now every 
time the measurement software steps back through the trace buffer it will 
look back an extra three times (the out-of-slips look back OSLB) to obtain 
the out-of-slips TBA. 
In a particularly preferred embodiment of the present invention, the video 
measurement allows bit position to be checked and reset at frequent 
intervals, thereby preventing the accumulation of depth errors. Bit 
position can be reset when drillstring 34 is in-slips and a portion of the 
string, called the stem, extends above the rig floor. Bit position can 
also be reset while drilling and the kelly is fully extended into the 
hole. 
The video system also measures pipe independent of the hookload/block 
position sensors. This video measurement can be used to resolve 
discrepancies between the driller's and the hookload/block position sensor 
measurement. Also, since all pipe is preferably video-taped while going 
into the hole, the tape can be reviewed at a later time to construct a 
complete depth-versus-time log or to resolve specific depth anomalies. The 
video system can also be used to reset the traveling block position should 
it get out of calibration. Block position must be known at all times 
because it is the basis of both incremental bit position and 
hookload/block position measurements. 
FIGS. 13A-13D illustrate a method of a preferred embodiment of the present 
invention that uses the previously-described video rig floor calibration 
and measurement procedures in conjunction with block movement sensor 27 
and hookload sensor 21 (shown in FIGS. 1 and 2) to determine depth while 
tripping in. In FIG. 13A, the trip-in procedure begins by measuring the 
height "K" of portion 120 of drillstring 34 extending above rig floor 16 
as shown on display 58 having cursors 92 and 93 superimposed thereon. All 
of the video measurements are made while the string is in-slips to ensure 
the string is motionless. In FIG. 13B, new joint 122, either a single, 
double, or triple, is added to drillstring 34, and string 34 is taken out 
of slips. The out-of-slips block altitude "L" as indicated in FIG. 13B is 
measured by block movement sensor 27. Height L is shown referenced to the 
lower edge of box 124 of joint 122 because this is the point the elevators 
contact the string. Actual block height as recorded by block movement 
sensor 27 may not be this exact point but will always be a constant 
distance from this point. Because of that constant distance relationship, 
block height distance differences cancel out. 
Referring to FIG. 13C, drillstring 34 is placed in-slips once new joint 122 
has been lowered into the wellbore and the in-slips block height M is 
measured by block position sensor 27. In FIG. 13D, camera 50 displays the 
top portion of joint 122 on display 58. Measurement cursor 93 is placed at 
the top edge of joint 122 with reference cursor 92 in its reference 
position. Computer 64 calculates height "N", the length of the portion of 
joint 122 extending above rig floor 16, using the rig floor calibration 
table. The length of added pipe 122 is the out-of-slips block height L 
minus the start height K plus the end height N minus the in-slips block 
height M. 
FIG. 14 illustrates a composite of the above measurements showing rig floor 
16 before pipe 122 is added, and rig floor 16 (shown in phantom) after the 
pipe is added. The length "O" of joint 122 is equal to the out-of-slips 
block height L minus the video-measured start height K plus the 
video-measured end height N minus the in-slips block height M. 
FIGS. 15A-15D illustrates the method of a preferred embodiment of the 
present invention that uses the rig floor video measurement system in 
conjunction with block position sensor 27 and hookload sensor 21 to 
measure joints while tripping out. In FIG. 15A, the trip-out measurement 
procedure begins by measuring the height "P" of portion 130 of drillstring 
34 extending above rig floor 16 as recorded by camera 50 and displayed on 
display 58. All of the video measurements are preferably made while the 
string 34 is in-slips to ensure the string is motionless. 
Referring to FIG. 15B, the out-of-slips block altitude "Q" is measured by 
the block position sensor 27 and hookload sensor 21 when string 34 is 
placed out of slips and just before string 34 is raised from the borehole. 
This height Q is shown referenced to the bottom edge of box 132 because 
this is the point the elevators contact string 34. Actual block height as 
recorded by block position sensor 27 may not be this exact point but will 
always be a constant distance from this point. Because of that constant 
distance relationship, these block height differences cancel. 
Referring to FIG. 15C, drillstring 34 is shown raised such that joint 134 
to be removed from drillstring 34 extends above rig floor 16. String 34 is 
placed in-slips and the in-slips block height "R" is measured by block 
position sensor 27. In FIG. 15D, camera 50 records the top portion of 
drillstring 34 extending above rig floor 16. Measurement cursor 93 is 
placed at the top edge of drillstring 34 (which corresponds to the lower 
edge of removed joint 134) and computer 64 calculates height "S" of the 
portion of drillstring 34 extending above rig floor 16 using the rig floor 
calibration table. The length of removed joint 134 is the in-slips block 
height R minus the video-measured end height S plus the video-measured 
start height P minus the out-of-slips block height Q. 
FIG. 16 shows a composite of the above measurements showing rig floor 16 
before joint 134 is removed from drillstring 34, and rig floor 16 (shown 
in phantom) after joint 134 is removed. Again, length "T" of joint 134 is 
equal to the video-measured start height P minus the out-of-slips block 
height Q plus the in-slips block height R minus the video measured end 
height S. 
The previously-described video depth determination systems and methods are 
particularly suited for accurately determining depth in conjunction with 
providing services such as measurements-while-drilling (MWD), 
logging-while-drilling (LWD), and formation evaluation while drilling 
(FEWD). In providing such services, downhole parameter sensing tools 
typically either telemeter information to the surface in "real time," 
and/or record downhole information in a memory device in an information 
versus time log for later retrieval and evaluation at surface. In the case 
of realtime telemetered data, BHA depth as measured and recorded versus 
time with the systems and methods of the present invention is synchronized 
with downhole information as it is received at the surface. In the case of 
recorded data, the downhole recorded information versus time log is 
retrieved from the LWD tool when it is brought back to surface and 
sychronized with the depth versus time log recorded by the systems and 
methods of the present invention to generate a downhole information versus 
depth log. 
Systems and methods for accurately determining depth are thus provided. The 
systems described and illustrated herein have been somewhat simplified so 
that a person skilled in the art may readily understand the present 
invention and incorporate it into any application by making a number of 
modifications and additions thereto, none of which entailing a departure 
from the spirit and scope of the present invention. Accordingly, the 
following claims are intended to embrace such modifications.