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HDD Basics


2.1 Historical


The first pipeline river crossing utilizing the horizontal directional drilling technique occurred in the seventies It was realized that with then current technology In the oil industry. It was possible to guide a borehole to a predetermined target on the opposite side of a river or canal This was done using a downhole motor for drilling and magnetic single shot guidance, A single joint was drilled and a magnetic compass and plumb bob assembly was pumped down the drill string to land within a non-magnetic drill collar above the motor. The survey was taken and the instrument was withdrawn from the pipe. The single shot film was developed and the results calculated and plotted on the drilling plan. The next tool setting was then made and drilling progressed. Eventually the pilot bore reached the target side of the never where acceptance of punch-out was determined. If not acceptable, the assembly was withdrawn to a planned position, the bore sidetracked and drilled again to a secondary punch out. This process in some cases was repeated a number of times until an acceptable position was reached.

Generally, these jobs lasted some time primarily due to lack of instrument accuracy and the inordinate amount of time necessary to run and retrieve a single shot instrument. Once the Pilot hole exited in an acceptable position, hole- enlarging operations could commence, If the formation was soft a fly cutter or barrel reamer was used. If hard, oilfield hole openers were used. Finally, when the bore reached the planned diameter, the product line was pulled into position


Almost all of the early expertise migrated from the oil Industry. The rigs came from drilling contractors and directional drilling techniques came from oilfield directional contractors. The early rigs used cables and sheaves to generate push and pull and in fact some still do. A cable can generate extremely high pulling power in an economical rig package and some of the early rigs even today have the pulling records. These rigs tended to be large and heavy requiring a number of trailer loads to rig up on a crossing location, Geotechnical engineering firms provided local geological information.


The economics of the operation conversely was not oilfield It was provided by the pipeline construction Industry that generally incorporated fiver crossing into their bid to construct a complete spread, sometimes running hundreds of miles, Most ancillary equipment like power packages, mud pumps, mud cleaning equipment drill pipe down hole equipment instrumentation and general technique comes even today from the oil industry.


2.2 Development

Until the mid 1980s, the technique was almost solely confined to the pipeline market. Since most of the equipment and services came from the oil industry they tended to be expensive. The product line transmission companies were the only market able to finance the effort.


In the early 1980s more competitive companies were formed. These concentrated on rigs that were more easily handled from logistics standpoints and were generally hydraulically actuated. This provided a distinct economic advantage for these companies who were able to arrive on location and be rigged up and ready to spud much faster than the older cable rigs. The technique itself advanced with more knowledge available to be applied to the drilling problems. A 200 Ton rig in 1985 was doing jobs reserved for 400 ton rigs in 1980. The wire line steering tool was first used as guidance in the early eighties. The time expended to drill a pilot hole decreased by a factor of four or more. Although pullbacks and Sidetracks were still common and became the highest incidence of lost time


Ancillary equipment generally did not change much although the use of the equipment became more effective due to the overall Increase in knowledge. In the late eighties, the rig sizes continued to reduce down to the 20·ton area, These smaller rigs could easily do small short crossings and still install significant steel product lines. The "bread and butter" work of the larger rigs began to be taken over by the intermediate rigs. The economics of an intermediate rig was such that it became cost effective for the cable laying industry. Laying fibre cable infrastructure fuelled growth of many new companies


Coupled with this growth, Sharewell caused development of the TruTrack® coil system of locating boreholes. The information was a secondary direct measurement of borehole location to enable guidance engineers to correct their bore heading prior to punch out. Exiting the ground on target became the norm and significantly reduced target sizes over the following few years. As target sizes reduced tile amount of applications for directional drilling increased.


In the mid eighties small rigs began to be built for short cable runs alongside highways and within cities, These ngs used Similar techniques within the context of totally different markets and economics Depths were shallow allowing use of the surface locators or 'Walkover" systems. By 1990 these rigs had established themselves and were taking market from the intermediate sizes. From then to now, their numbers have continued to Increase into the thousands with many manufacturers. The directional drilling technique had come of age.

2.3 Future

The future of directional drilling can be measured In technology The technique has changed little since the first oil well 'blowout' was killed In the 1930's using controlled directional drilling Downhole tools will advance with the needs of the Industry instruments will become more accurate and usable. The science of drilling will enhance operations with more of an engineering approach to drilling plans. Most of the advances in development today will incrementally benefit the overall market. In the short term, large rigs will drill beyond the 7000' barrier. Intermediate rigs will install a 36" product line and small rigs will begin to drill and open holes in rock.

The technology exists today for these things to happen, although It is expensive. In the longer term, new guidance instrumentation will allow drilling safely In existing crowded pipeline corridors. More powerful downhole motors will allow larger diameter hole opening operations People are discussing 10000' bores. Drilling mud- handling problems will be solved Downhole sensors will be utilized to limit drilling problems. After the fact surveying will become necessary and mandatory. Finally, look around. The utilities you see above the ground today Will be underground within 2-3 decades


HDD Vocabulary

3.1 Definition

Controlled Directional Drilling is the science of steering a borehole along a predetermined course to a specified target employing the use of guidance and deflection tools. Pipeline and Utility Directional Drilling employs the use of directional drilling techniques and equipment in a specialty market segment.

3.2 Applications

The following list the market areas where directional drilling techniques have been utilized from utility and pipeline rigs Product Line River Crossings, Cable River Crossings, Canal and Road Crossings. Rail Road Crossings, Shore Approaches, Cemetery Crossings, Archeological Area Crossinqs, Wetland Crossings. Environmental Remedlation. Tar Sand Development. Building Hookups, Along Road Infrastructure, High Production Waterwells. Diaphragm Wall Construction

3.3 Borehole Geometry

The geometry of a borehole is largely determined by the limitations Imposed on the contractor. Many exotic profiles have been planned and drilled.

3.3.1 Conventional

Most standard crossings will be conventional defined as the same radius on the entry curve as the exit curve separated by a horizontal section. The curve may begin after a straight tangent section

3.3.2 Duel Radius Conventional

Same as conventional except that one of the curves will be a different radius

3.3.3 Single Radius

A radius curve begins at the entry point and ends at the exit point without and horizontal section dividing the curves. This is generally used where the total crossing length is too short to allow elevation limits to be satisfied and pipe or pulling radius is approaching the maximum.

3.3.4 Curve

A conventional profile except that the horizontal section dividing the vertical curves will be a horizontal curve. Used in avoiding obstacles or property lines.

3.3.5 "s" Curve

A conventional profile except that the horizontal section dividing the vertical curves will be two opposing horizontal curves. This profile is used when avoiding obstacles or property lines while the entry and exit points have been fixed.

3.3.6 Sag Bend

The part of a profile where the angle drops to a lower angle. This is generally used to reduce me exit angle to allow ease of pipe handling

3.3.7 Hog Bend

A part of a profile where the angle builds to a higher angle either from a larger radius to a smaller radius or from a tangent angle to an increasing angle This is used generally in shorter crossings where an elevation limit occurs on the exit side preventing a single radius causing a much smaller radius to be used In order to exit soon after the elevation limit is achieved.

3.3.8 Grade

A conventional plan, except that a slight angle higher or lower than horizontal separates the vertical curves. Used for some gravity feed product lines and sometimes to assist friction factors during pull back operations

3.3.9 Combined Curves

Any of the above except that vertical and horizontal curves occur at the same time. Used only where limits leave us no choice.

3.4 Downhole Assemblies - Pilot Hole

See rest of section

3.4.1 Drill Bits

There is a tremendous choice of drill bits in the market. There is a choice of Jet nozzle size to optimize hydraulic horsepower at the point of cutting. Generally, In order to make choices of the best type to use on a given project. We must understand the dynamics of how a roller cone bit actually cuts. The teeth of roller cone bits, either Milled Tooth. (MT) or Tungsten Carbide Inserts (TCI), come In a variety of shapes and sizes The long tooth MT cone is designed for softer formations while the dome shaped TCI IS for very hard formations The choice is made based upon the formation hardness and available push from the rig.

The teeth should penetrate the rock by approximately 1/3 of Its length through compression only. If they do not, the rate of penetration, (ROP) will be slow. If they penetrate the entire length of the tooth, ROP will also be slow. Since available compression force may be controlled by the driller, the highest ROP usually is achieved without the use of the rigs maximum available push given an optimum bit choice. Conversely, if the compressive strength of the rock is high and the rig does not have the power to make the cutter penetrate the rock, the rig is undersized for the Job. The bore may progress but the ROP will be low in the extreme. Bit choice and therefore Rates of Penetration can determine the difference In profit and loss on any bore.

3.4.1.2 PDC Bits

A Polycrystalline Diamond Cutter Bit (PDC) IS used on motors for high ROP?s In homogeneous rock formations. Many face shapes are available but since the bit is used for a directional drilling purpose, the flat face PDC should always be used. These bits are more expensive than roller cones and the benefits are difficult to measure. Where gravel or fractured rock is
present, a PDC should not be run.

Gravel and fractured rock can cause chipping or cracking of the PDC cutting surface. Due to the extremely hard but brittle nature of the PDC bits high impact forces can cause the bit to wear out within few joints.

3.4.1.3 Milled Tooth

A Roller Cone Milled Tooth Bit may be used on both jetting assemblies and motors Roller cones may be obtained with both sealed and open bearings As a general rule a sealed bearing bit should be run In combination. With a motor while It is unnecessary to have a sealed bearing bit on a Jetting assembly. Sealed bearing bits are significantly more expensive.

The MT Bits are more economical than TCI and are made for formations with compressive strengths near 20,000 PSI. In practice, since our application is horizontal causing high gauge wear, the life of MT Bits is significantly reduced above 8- 10,000 PSI rock. Additionally, formations with a crystalline structure are abrasive, increasing gauge wear and reducing life still further. In abrasive formations TCI and MT bits should be lugged. This is additional Tungsten Insert gauge protection located on the bit shirttails just behind the gauge cutting area on the bit body.

There are a variety of cutting structures on the cones from a minimum number of long teeth for very soft formations to a maximum number of short teeth for hard formations. The choice should be made with an effort to match the formation to be drilled as best as possible jet nozzles should be chosen to optimize the available motor power and hydraulic horsepower at the bit. Tables are available to calculate bit pressure drops and Hydraulic Horsepower.

3.4.1.4 TCI Bits

TCI Bits are run with motors and should have sealed bearings. They should also be lugged for additional gauge protection. Jet nozzles should be optimized. There are a number of different shapes of TCI cutters from an extremely aggressive chisel tooth shape to convex dome shapes. Remember, the cutter should penetrate the rock with compressive strength alone in order to obtain the highest ROP's. The TCI cutter does not need to penetrate as much as a MT cutter since the cutting action depends upon compressive force of the Insert. Care should be taken With TCI choice since ROP may vary drastically.

3.4.2 Jetting Assemblies

The majority of Pipeline and Utility Directional Drilling takes place with the use of a Jetting assembly, As with bit choice, a jet choice should be made based upon the type of formation to be drilled. More aggressive assemblies should be used in softer formations while less aggressive should be used in harder gettable formations. A poor choice will determine ROP and actual steer ability to a target.

3.4.2.1 Shovel Shaped Jet

The vast majority of the small rigs use this type of Jet. This is a derivative of an old Zublin bit of the 1940's and 50's commonly called a 'Spud Bit' It was used for directional control in soft formations and is highly directional. The Shovel Bit has a flat plate for a leading edge that is mounted at venous angles to the axis of the pipe. When oriented and pushed for- ward, the plate will slide through the formation in the direction of orientation. The mechanical advantage is generated within the first 12 Inches of the plate. Care must be taken to not push too far since the face of the Jet can generally change direction faster than the pipe or collars behind causing connections to break. The harder the formation, the more this effect is magnified. The Zublin Bit lost favour and was retired In the early 1950's due to Its tendency to cause losses downhole. In soft surface soils. There is no substitute for the control of these bits. They allow the directional driller to steer easily in water- saturated clay, peat and sand. As the formations increase in strength, the ROP generally drops due to the need to exercise a greater degree of directional control settings.

3.4.2.2 Bent Jetting Assembly

This assembly consists of a bent sub, extension sub and either a flat face Jet or an oriented roller cone MT bit. The mechanical advantage is generated by the fulcrum principle and the degree of lift at the bit from the axis of the pipe. Most bent subs have a one and one half degree bend Extension subs range in length between 6" and 18" depending upon exactly where the bend is in relation to the connection. As a general rule, the bit should be around 18 Inches from the bend generating about 1/2 Inch of lift at the bit from the axis of the pipe. This assembly will generate very short radius curves. Two nozzles of a roller cone bit should be blanked, leaving a single nozzle. This nozzle should be oriented exactly in the plane of the bent sub by using shims on the bit connection. Hydraulic horsepower IS utilized on the 'high side' on the bore assisting the bend in producing directional change. Care should be taken to ensure the nozzle size is correct for the amount of fluid intended. Special directional drilling, drill bits that have two roller cones and a single jet In the place of the third roller cone have been used in the oilfield in the past.

In softer formations, It Is unnecessary to use a roller cone bit at the face. A blank or flat face jet with and elevated nozzle may be substituted. The nozzle should have about a 10-degree elevation and also oriented to the 'high side' 0on the bent sub.

These assemblies enable the driller to steer his assembly when orientation is required and to rotate for faster penetration once the direction has been achieved, they are aggressive enough to drill in most jettable formations, yet are very controllable and much safer than the shovel type designs A direct result of better directional control is faster rates of penetration and therefore production.

3.4.3 Down Hole Mud Motors

A Down Hole Motor consists of a Power section. a Transmission assembly and a Bearing assembly. Above the bearing assembly is a bent member. The bent housing can be adjustable or fixed, generally between 1 and 2 degrees. This bent housing acts exactly the same as a bent sub on a jetting assembly providing, lift to the bit.

The power section converts the hydraulic fluid energy of the mud into mechanical horsepower using the Moineau principle. The stator is an elastomer bonded into a steel housing. The form is helically screwed along the length of the stator.

The rotor is produced, matching the lobe profiles to the helical pitch of the stator. When fluid is forced through the lobes, the rotor rotates within the stator.

The transmission assembly absorbs the thrust of the rotor and converts the elliptical and rotational motion of the rotor into concentric rotation of the output shaft. The bearing assembly surrounds the output shaft. A bit box is connected to the output shaft that couples the bit to the motor.


There are many downhole motors on the market. As a rule we need to optimize the motor parameters to the capabilities of a particular rig. All rigs have high-pressure mud pumps, although the flow rates and maximum pressures vary greatly. It is necessary to select a motor on the basis of available fluid and pressure, hole size requirements and maintenance costs.

3.4.4 Guidance Systems

Directional Drilling would not exist without some means of determining a bore location. In the early days, this was achieved by running a single shot camera to bottom and photographing a magnetic compass and plumb bob assembly. The resulting "picture", or survey, was read calculated and plotted on the drilling plan.

When the wireline steering tool was developed and obtained wide acceptance In the larger rig pipeline market, pilot hole drilling time was significantly reduced,

The early days of the small rigs saw the use of basic "pipe locators" which were limited in depth.These early detectors could only give a depth and a left right position determined by strength of Signal on the surface. Adding roll and pitch, deeper depths, less local interference to the Signal and dynamic transmission of results directly to the driller make the systems much more user friendly than on the late 1980's

Magnetic coil systems have been introduced to directly measure positions of bores which enhances the ability of the driller to know with confidence his bore location. Gyroscopic instruments have been used on occasion in areas of extreme magnetic interference to attempt to generate confidence. Their high cost normally precludes their use.

3.4.4.1 Surface Locators

There are a number of surface locators on the market..All work within their specifications and limitations. The Signal is an electromagnetic transmission on a preset generally low frequency, Low frequency radio waves will travel through the earth while high frequencies will not. The current depth limit is around 75 feet as long as little or no local EM interference exists. Locator beacons or sondes are placed within a steel sub and oriented to the Jet bits high Side or 12 o'clock position.The Sonde or Beacon sub has slots through which the EM Signal travels. The position of the slots when measured on surface with a receiver denotes tool orientation underground. Data obtained from these systems are direct readings of tool locations and orientations. Small variation in the surrounding EM field Will Yield Incorrect real positions that must then be averaged to determine real depth or left/right positions. After a job, It is normally not possible to generate a printout of positions with which to draw an 'as built' profile. In practice, it is very possible for a user to hide actual positions of the bore, since no data output exists. Finally due to the combination of highly directional shovel bits and walkover systems the actual profile of the bore normally dog legs up/down and left/right from position to position throughout the bore. This Single fact has produced many problems during pull back of the product line le stuck pipe, parted pipe loss of reamers and loss of drill pipe special care by an experienced drilling contractor should be exercised when ,"stalling steel product lines using a walkover guidance system.

3.4.4.2 Magnetic Guidance System

The MGS or Wireline Steering Tool has been used for some time it consists of a down hole probe containing tri-axial accelerometers and tri-axial magnetometers. The XL's measure gravity and resolve tool inclination in relation to vertical or horizontal. The Maq's measure the earth's magnetic field and dip angle and resolve the tools relationship to magnetic North.

The probe is connected via wireline from Its position near the bit to the surface, through the rig to a Probe Interface. The Interface is connected to a lap top computer and printer. The driller has a readout box directly in his sight line for him to be able 10 see Inclination, Azimuth and Tool Face (Roll pitch and direction)

The guidance engineer is able at all times While drilling to keep track of all necessary guidance data At the end of each drilled Joint, a survey is taken which is saved to disk. This forms the basis of the as built drawing when the bore is complete.

3.4.4.3 Coil Systems

An electromagnetic coil system was developed for use with wireline steering tools to assist positioning of bores in areas of high magnetic interference. The TruTrack coil has achieved great success and been responsible for much of the growth of the large and intermediate rig market

3.4.4.4 Gyroscopic Systems

GyroscopIc survey systems have been used to guide a few boreholes. The gyroscope is not affected by magnetism so In theory should be more accurate than magnetic systems. This is not the case due to the need to establish a line azimuth within tenths of degrees that today is not achievable on most rigs. Additionally gyroscopes have a tendency to change orientation slightly with the earth's rotation. The engineer must account for precession and can subjectively introduce error that Impacts punch out accuracy. Another problem in trying to guide the drilling assembly with a gyroscope is that the gyroscope can lose its orientation if rotated inside the drill string. They can be used with precision in a post survey mode. In this mode, the tool is pulled through the product line gathering data. The exact entry and exit locations are benchmarked to the data that yields an accurate representation of the bore path from entry to exit In this mode, there is real value In some situations.

3.4.4.5 EM Systems

Electromagnetic Steering Tools have been developed. These tools can be more accurately described as an MGS system without the wireline..The tools are battery powered and transmit normal steering tool data to the surface through the drill pipe. Today, the data rate is rather slow, giving us a data updates every 15 to 30 seconds. This seems fast but when a driller is attempting to orient the down hole tool he must wait after rotating some time to con-turn his orientation before beginning to drill once again. A wireless tool gives us an update in virtually real time so the driller can see the actual position of the tool face at all times.


These tools have real application as they become faster in the future. Electromagnetic Steering Tools have been developed. These tools can be more accurately described as an MGS system without the wireline. The tools are battery powered and transmit normal steering tool data to the surface through the drill pipe. Today the data rate is rather slow; giving us a data updates every 15 to 30 seconds. This seems fast but when a driller is attempting to orient the downhole tool, he must wait after rotating some time to con-firm his orientation before beginning to drill once again. A wireline tool gives us an update in virtually real time so the driller can see the actual position of the tool face at all times.

3.4.4.6 Ranging Systems

Magnetic steering systems are being developed which allow the driller/surveyor to determine the location of the steering tool in relation to a known magnetic current. These magnetic currents can be in the form of single surface laid wires or a wire inside an existing parallel pipeline. These tools If they prove reliable could provide the answer for steering in close proximity to existing pipelines or utilities. Thus far they have been somewhat inconsistent and are difficult to set up efficiently.

3.5 Down Hole Assemblies -

Hole Opening Within the market the widest variation of equipment may be found on the small rigs. Basically, when dealing with surface soils, most equipment focuses on compacting the ground around the borehole rather than removing the solids with drilling fluid. There are hundreds of designs which all seem to work some better than others On the larger rig markets and in hard ground, there are fewer workable designs. We will concentrate on these in this section

3.5.1 Fly Cutter Reamer

The purpose of a fly cutter reamer is to cut a concentric path with speed remove a significant amount of formation through flow hydraulics and slurry the balance of the formation not removed. The product line may then be pulled through this pre- reamed pathway.

Fly cutters have a central mandrel for drill pipe connections. Around this will be three or four spokes, sometimes holding an outside ring, the same diameter as the planned hole size. A number of let nozzles will be located on the spokes in various orientations. Jet nozzles are changeable to allow operators to optimize their hole cleaning hydraulics. Normally, 'Kennemetal Teeth' are placed In various concentrations on the spokes and the outside ring. These teeth provide the cutting action.

In the past, most fly cutters were made by the venous drilling contractors with little attention paid to the orientation (attack angle of the Kennemetal teeth), location of the tooth in relation to other teeth on the reamer or the angle of the jet nozzle, As a result most fly cutter reamers were made with way too many teeth resulting in a great deal of expense and unnecessary drag or torque during the hole enlarging process. Today a number of efficient and effective shop made fly cutters are available nom supply houses and service companies

The fly cutter is used In soft to medium hard ground With a maximum compressive strength of around 1.000 PSI or 7 MPa. As the strength approaches the higher numbers, around 650 PSI, the abrasiveness factor needs to be taken into account since fly cutters tend to wear rapidly. Length of crossing, abrasiveness and compressive strength are the factors used in choosing which reamer to use in specific ground conditions.

Normally, in soft ground. Fairly large fly cutters taking large formation bites can be pulled. In harder ground It may be necessary to make multiple passes, stepping up the diameter each time. If this method is opted for, It is generally necessary to centralize the fly cutter with barrel reamers and avoid 'tear dropping' the hole. Another way to accomplish this is to use a tapered fly cutter that lends to self centralize

3.5.2 Barrel Reamer

The purpose of a barrel reamer is to open a hole in softer ground compress the formation somewhat and leave a reasonably clean hole for the pipe. A barrel reamer pass can be slow due to the compression factor, Mud returns generally will flow to the rig side of the crossing since the reamer essentially forms a plug behind the Jet nozzles. The Barrel Reamer also consists of a central mandrel for pipe connections, Normally. pipe 'weld caps' are used In construction separated by a central section of line pipe the same diameter as the weld caps. Jet Nozzles are placed on the face of the reamer along with various numbers and designs of Kennemetal Teeth Most barrel reamers have a few nozzles and some teeth on the backside of the reamer to facilitate backing out of the hole If forward progress is blocked. Barrel reamers may also be run behind fly cutters to change direction of the mudflow and affect a cleaner hole with speed. Additionally, these reamers are often used as the leading tool while pulling the product line through the pre-reamed hole.

3.6.3 Bullet Nose Reamer

The purpose of the bullet nose is the same as the barrel reamer. Construction is almost identical. Instead of using end caps, the leading and trailing faces are swaged from the outside diameter to the mandrel at various angles. It is used in harder formations, where gravel is present and for product line pulls.

3.5.4 Hole Openers

Hole openers, like many other drilling devices In this industry, were Introduced from the oil and gas drilling industry. They consist of a mandrel with multiple cutters attached to the outer perimeter designed to penetrate and chip away at the rock formation. There are basically three designs for hole openers in the horizontal directional drilling Industry. (1) Conventional, (2) Split bit and (3) Lo-Torque. In addition to these hole openers there are lesser used types including PDC hole openers, drag hole openers yo-y0 hole openers and under reamers.

Of the four, all but the PDC have two basic cutting options, either milled tooth cutters or tungsten carbide insert (TCI) cutters.


3.5.4.2 Split Bit Hole Openers

These hole openers are manufactured by splitting a tri cone roller bit, either MT or TCI. and welding them on a hole opener mandrel. When done correctly the jet nozzle from the bit provides the flow in the same way as if it were still a drill bit. Although at first these hole openers seem like reasonable economic alternatives in the long run they tend to be expensive since the cutters are only changeable by replacing the complete arms complete with cutters. This normally is a welding shop job along with the acquisition of another tri-cone bit. Again it is extremely important for the welding shop to have experience with placing the cutters to avoid overloading one cutter or one cutter being out from the centre more than the others resulting in premature gauge wear.

3.5.4.3 Lo-Torque Hole Openers

Arguably one of the most innovative products to come to the trenchless industry since the steering tool, the Lo-Torque hole opener consists of a central mandrel with threaded spindles either cut directly on the body for the smaller openers or placed on a spindle block for the larger hole openers. The spindles normally accept up to four different sized cutters in two-inch increments providing the contractor with numerous hole-opening options. The cutters can be field changed and can have either TCI or milled tooth cutting structures

All single body Lo-Torque hole openers have the spindles cut from a large steel mandrel by a computer controlled machine insuring proper placement of the cutters both in relation to one another and assuring proper gauge is provided. The larger Lo-Torque hole openers utilizing spindle blocks are fabricated using special jigs to assure the same proper placement.

Lo-Torque hole opener cutters do not have outer arms that can wear out eliminating the fear of these cutters falling off In the hole as a result of an abrasive formation. There are 'push' and 'pull' bodies to assure the proper skew using the milled tooth cutters. The TCI cutters are tracked with special numbered cones to decrease the torque, decrease the amount of required weight on bit and increase the penetration rate. The TCI cutters can be assembled with an assortment of different inserts from domed to chisel to provide optimum penetration and durability

3.5.4.4 PDC Hole Openers

PDC hole openers have been used occasionally for specialty jobs. They work well in dense consolidated sedimentary rock although their very high cost normally precludes their use. They do not work in broken formations or where gravel is present. They work by scraping rather than chipping therefore are more effective at higher rprn’s than the cone type hole openers

3.5.4.5 Drag Hole Openers

These hole openers consist of a mandrel fitted with shearing blades. The cutting surface of the leading edge of the blade consists of tungsten carbide plates of various shapes. They work well in very soft rocks with compressive strengths up to about 2000 PSI. They do not work well in unconsolidated formations. Their reasonable cost and field replaceable blades make them a good choice for the smaller rigs.

3.5.4.6 Yo-Yo Hole Opener

A recent development, this tool takes Its name from its unique Yo-Yo shape. It consists of two concentric faces mounted flush against a mandrel They each turn Independent of each other as the mandrel is rotated. The outer shells are drilled to receive Kennemetal teeth that are field changeable. The Yo- Yo tool was developed to open large holes from a pilot hole without the need to make multiple stepped passes. Good idea but to date the cutting action produces high rates of wear on the teeth causing the tool to rarely complete an entire Job. Usually, conventional hole openers complete these Jobs. The tool also seems to be highly formation dependent. In very soft rock, the ROP IS very low while in hard rock tile teeth tend to break causing Immediate loss of progress. The working range of formation applicability seems to be narrow. With further development the tool has its uses.

3.5.4.1 Underreamers

An under-reamer is a hole-opening too: designed to run into a pilot hole of a particular size to a predetermined position. Once there a higher mudflow is used to open arms with cutters from the tool body. With rotation, the tool will begin opening the hole section to a larger diameter for a planned distance. When underrearnmq operations are complete, the mudflow IS reduced arms retracted into the body and the tool is withdrawn from the hole. This type of tool would only be used in specialty applications such as environmental re-mediation bores. It is expensive.

3.5.5 Cross Over Subs

Most hole openers and in fact down hole toots have threaded connections designed for high stress combined with ease of use. These connections generally are standard based on toot mandrel diameter. When connecting drill pipe to these tools, it is often necessary to install connection crossovers on each Side of the tool. In the small rig market crossovers are referred to as "adapters'' because they allow the contractor to adapt from one thread size to another.

3.5.6 Ring Centralizer

The purpose of any centralizer or stabilizer is to centralize the drill string and/or the down hole assemblies in relation to the pilot bore. Their construction is similar to the fly cutter but without teeth or jet nozzles. Centralization is normally unnecessary until the hole size exceeds 18 inches. After this point, the weight of the drill string hanging on each side of the opening assembly will cause fatigue fractures of the connections. It is also wise to ensure the opening assemblies remain as perpendicular as possible to the axis of the pilot bore. Otherwise, the reamer may attempt to cut up or down within the hole

3.5.7 Stabilizers

Stabilizers have the same purpose as centralizers. Their construction differs to the ring centralizer in that they have blades welded to the mandrel. These tools are much heavier than ring centralizers but have the advantage of operation with less additional torque.

3.5.8 Roller Centralizers

For rock applications where high torque due to either formation or hole size is experienced use of roller centralization is recommended. The tool consists of centralizer wings with rotating cylinders making contact with the bore walls. The cylinders are field changeable. The down side is the rollers have a limited bearing life and can be quite expensive. As a result they are not widely used.

3.5.9 Swivels

Once the bore has been enlarged to the planned diameter a pipe pulling assembly is rigged up to the product line. It usually consists of a reamer connected to a swivel connected to the product line. The purpose of the swivel is to allow the reamer to rotate while pulling a non-rotating product line. There are many swivels on the market from many manufacturers. Some, for smaller rigs, incorporate the swivel and the reamer to one tool.

4.1 Basic Principles

The basic principle of borehole guidance is to accurately ascertain the relative position of the bore from an entry point. In order to determine what directional decisions need to be made to direct the bore to a predetermined exit point.

Using a wireline steering tool, there are three basic measurements from which positions are calculated.

Pipe length - The distance measured along the course of the borehole from the entry point.

Inclination - The angle between the vertical and the axis of the borehole at a chosen distance from entry.

Azimuth - The angle between the horizontal component of the boreholes at a specified point measured in a clockwise direction from magnetic North.

All azimuths are expressed in the 0 - 360 Degree system

During orientation of downhole tools, a fourth critical measurement is made called "Tool Face" This is not used in calculations.

Tool face - Tool face is considered as a measurement of the position of the bias of a directional down hole assembly perpendicular to the axis of the borehole. From the above measurements a guidance engineer can use standard trigonometry to calculate from entry an elevation and left/right position of the bore at the position of the Instrument. From Tool Face he is able to orient a deflection tool in order to maintain or change the direction or elevation of the bore,

4.2 Instrumentation

Survey instruments within the borehole measure the inclination and azimuth readings, and tile distance away from entry is measured by direct pipe measurement at the rig. A wireline steering tool consists of a sensor section and a wlreline transmission section The sensors contained are three accelerometers and three magnetometers mounted orthogonally The gravity and magnetic data obtained from any attitude of the tool in space, resolve the tools inclination and azimuth.

The transmission section receives the sensor data, converts It from analogue to digital format and transmits. It along a single conductor wireline to the surface interface

The signal then moves from the interface to a lap or desktop computer. After processing the data are displayed on the computer screen and sent back to the Interface where it is provided through another wireline to a remote display located directly in front of the driller. The entire process occurs about once every second The data is used within the software to calculate and store survey calculations of current and previous bore positions.


Since the tool measures the earth's magnetic field In order to resolve magnetic North, It is Important to house the down hole probe in an area free of any extraneous magnetic interference. The bit, downhole motors, most subs and tile drill pipe are strong sources of magnetic fields. High carbon content of the high quality steel needed for the drilling process generates high residual fields. The probe is housed within a non-magnetic collar separating the drill pipe and the drilling assembly. Magnetic sensors therefore are spaced away from the interference fields of the assembly and the drill pipe

4.3 Coordinate Systems

Plans for drilling operations are represented on a flat sheet of paper, however the work is done practically In three dimensions on the curved surface of the earth It is not possible to represent a sphere exactly in two dimensions however since most jobs connect and entry and exit with a straight line, two dimensions are normally sufficient. Three dimensional coordinate systems such as Lat I Longs.

Lambert Projections and UTM that represent three dimensions are limited in this business to surface surveying techniques This will be discussed In a later chapter

Actual planned profiles normally begin at the entry point and terminate at the exit point. This is called a Local coordinate system, one that is determined locally for a specific project. A customer will accept data on a local system as long as the entry point is known in relation to local known points.

In many cases, a pipelines length from Origin is used to measure or position the entry point. This is called stations. A station has an origin point (0.0) at the beginning of the pipeline. Sometimes we are asked to represent our profile based on stations.

In every case, we are dealing with distance away from entry however represented, and elevations. Elevations also may be expressed in a local system with the engineer calling the entry point zero. Often Mean Sea Level (MSL) or another local datum is required.

It is the responsibility of the field engineer to determine from the customer how to express the profile data. Once the coordinate system is determined a drilling profile may be drawn.

4.4 Calculation Systems and Methods

The calculation of survey data for plotting on a directional plan involves a fundamental mathematical procedure. In this chapter various calculation methods are discussed and some operational recommendations for use in the field will be made. Read through the following definitions before beginning the chapter to reinforce important information.

Measured Distance - The actual total length of the drill pipe and that part of the down hole assembly only to the probes magnetic sensor, measured from the entry point.

Vertical Depth - The vertical distance from the surface reference Elevation datum to a particular underground horizon.

Inclination - The angle of tile borehole in degrees measured from the vertical or horizontal plane.

Entry Point - That point of entry chosen as the beginning of the vertical and horizontal profiles. Normally that point where the pipe enters the ground in front of the rig.

Exit Point - The target expressed in distance from entry elevation and a position left or right or directly on a centreline. This may be a planned exit or an actual exit point.

Horizontal Plan - A projection of the left or right position of tile bore against a planned centreline.

Profile - A projection of the vertical positron of the bore against a planned vertical profile.

Vertical Section - A mathematical calculation to express three dimensional positions in two dimensions.

Radius - An expression defining the exact rate of curvature of a line, expressed in feet or meters.

Dogleg Severity - Dogleg refers to the total three-dimensional change of angle between two given points. This is expressed by the calculation program in degrees per 100' and may be directly converted to radius between the two points. Data is obtained which is used to determine the position of point along a borehole. Instruments currently in use do not give these final values but produce raw data on which calculations must be performed to obtain the desired results. Many methods have been developed to increase the accurate calculation of the actual curved path of the bore between two survey stations. Since accuracy is dependent of the frequency of survey stations and surveying takes time, much effort has been expended in attempting to mathematically model the theoretical bore path between stations

They include the following:-
- Tangential Method
- Average Angle Method
- Radius of Curvature Method
- Driller Bias

None of the methods take into account the fact in our business that the driller can make or break the calculation method. He sees this angular position every foot drilled and makes corrections as needed. Normally he is given an inclination target he is to hit at the end of the joint to be drilled. Normally, he hits this target within the first third of the joint and uses the balance of the joint to hold his target angle. You can see that in a 30 foot joint, the first ten feet would then be a curve while the following 20 foot would be a straight line at the desired higher angle. Since the angles measured form the basis for the resulting calculated position the driller can bias the calculations by when he reached the desired angular target during the joint. The guidance engineer must carefully watch for this bias and attempt to account for it.

4.4.1 Tangential Method

For the distance above each survey station, this method assumes that the bore maintains the same inclination angle and hole azimuth.

Advantages
The method is easily calculated by hand. The error tends to show a slight increase in elevation over distance and therefore better reflects how most drillers drill a joint.

Disadvantages
No theoretical justification Automatically generates an Increase in elevation over distance

Recommendation
Always begin a job using this calculation method. Watch how the driller obtains his targets. If he aggressively chases the target early, stay with Tangential. If he waits to obtain his desired angle at the bottom of the joint, change to another method of calculation. Compare both methods to direct elevation readings from TruTrack Coil positions or walkover system positions. Use the best fit

4.4.2 Average Angle Method (also called the angle averaging method)

This calculation method assumes that the borehole is parallel to the simple average of both the inclination and hole azimuth angles between two survey stations. This method treats the bore as a straight line, but approximates the slope of the line by taking the average of the inclination angles at each end of the drilled section. This process is also carried out for the hole direction (azimuth) readings. These averages are used in a standard tangential calculation to determine elevations and left/right positions. This method becomes less accurate as the difference between either pair of angles increases or as the distance between survey stations becomes large within these limitations however, the results obtained with this method differ little from those obtained from more sophisticated methods.

Advantages
Fairly accurate, good repeatability with other more advanced methods Calculations are simple enough for field use with a non-programmable calculator

Disadvantages
No theoretical Justification.

Recommendations
On many crossings, this method works well we should always compare Tangential with Average Angle at various points during a crossing. Choice of methods should be based on driller bias and further comparison with surface location systems.

4.4.3 Radius of Curvature Method

In the Radius of Curvature Method, the data from two survey stations is used to define the assumed circular and trajectory of the bore hole between these points. The borehole is assumed curved in enter on both vertical and horizontal planes.

Advantages
Sound theoretical Justification

Disadvantages
Complex calculations that require a programmable calculator or computer Rarely different from average angle calculations. Not easily explained to customers

Recommendations
Use Average Angle Method where one or the other IS required otherwise the comments made about average angle apply

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