Martin Hofschroer, Bentec Drilling& Oilfield Services

HISTORICALLY THE DRILLINGindustry has tended to take a conserva-tive approach to implementing of newtechnologies. However, the challengesthrown up by more and more difficultformations, extended reach drilling,extreme environmental conditions onthe rig locations and the need to exploreincreasingly smaller reservoirs calls fora new approach.

All parties involved in the drillingprocess need to consider combiningtheir efforts through the application oftheir experience and the innovativestate of the art technologies that arenow available.

Offshore installations have been quickerto adopt a range of sophisticated tech-nologies, but there seems to have been aslower uptake in mobile land rig opera-tions.

Since everyone wishes to avoid downtime, anything that can improve wellquality and drilling performance, espe-cially in difficult well conditions, canonly benefit both the drilling contractorand the operator.

In the past, drilling appeared to be a

simple process. The drill bit, pusheddownwards by heavy weight drill pipes,is rotated either top drive or via rotarytable, high-pressure mud powers thedownhole motors, carries the cuttingsupwards and cools the bit.

Drilling in the future promises tobecome a more difficult and complexprocess. Mud needs to be available onhigher volume and higher pressure.

Deeper wells are calling for longer drill-strings putting higher stress to thematerials used.

Some formations can only be drilledwith very low pressure, called underbalanced drilling (UBD).

Downhole data needs to be transferredto the surface whereas the drillingprocess itself has to be as smooth aspossible to keep the bit life high to avoidunplanned round trips.

Drill pipe is opposed to very high frictionover almost the whole length sinceextended reach wells are more oftendrilled horizontally then vertically. Thepipe is just laying on the bottom of thebore hole.

The handling of the string on the surfaceasks for bigger set-back capacity on the

rig floor, gear-driven drawworks are thepreferred choice for powerful and flexi-ble operation.

The standard approach to drilling is tomake use of the physical limits of theequipment. Globally, engineers anddesigners are continually researchingmore efficient ways to drill.

Until these solutions are available to thefield, it is a “must” to optimize thedrilling process as we know it today.Experienced staff is definitely as impor-tant as the application of improved tech-nology. So every change can only bemade step by step.

Upgraded standard land rigs and newland rigs that are able to work accord-ing to future requirements are availabletoday.

C H A L L E N G E S U N M E T

Standard rigs are still equipped withchain driven drawworks. They are reli-able, known to all drillers and acceptedin the oilfield.

The weight on bit control (WOB) is donemanually, just through opening and clos-ing the mechanical brake, according tothe information supplied through thehook-load indicator.

36 D R I L L I N G C O N T R A C T O R September/October 2004

Automation aims for higher efficiency wells

Bentec Automatic drawworks control system

UPPER LIMIT/CROWN PREVENTION

PROGRAMMABLE

AUTOMATEDDRAWWORKS

CONTROL SYSTEM EDDY CURRENTBRAKE CONTROL SYSTEMOR EQUIVLAENT

FIELD SENSORSEDDY CURRENT BRAKE

DW-A

FT FT FTST EX

DW-B DW-CSAFEAREA

CONTROL CONTROL

DRAWWORKS

EDDY CURRENTBRAKE

SPEEDSENSOR

DRILLERS CABIN

WEIGHTSENSOR

WEIGHTSENSOR

LOWER SET POINT

PROGRAMMABLE

The torque and the RPM-value of thedrillstring is set manually as well, nottaking care enough of the responsesfrom down hole. Mud volume/pressure,the third important factor for the qualityof the bore process, is again just manu-ally controlled.

As we face the limits of this way of oper-ation for drilling rigs, some very impor-tant adaptations should be taken intoconsideration.

E N H A N C I N G A P P L I C A T I O N S

The “stop and go” operation of the brakehandle in order to keep the WOB withinthe required limits is not only exhaust-ing but also unacceptable for the qualityof the hole.

Stress on the material and vibrationwithin the drillstring will reduce theservice life of the whole equipment inuse.

The rate of penetration is poor sinceparameters will not be set to the opti-mum.

One way out of this situation would be aretrofit gearbox kit available to any kindof drawworks, attached to the extendedshaft of the eddy current brake.

Controlled via Variable Frequency Drive(VFD) driven AC motors, hoisting andlowering of the drillstring is achieved.

Closed loop control with respect to WOBand ROP can be set easily by the driller.

Hands-off operation with continuousfeed-off takes care of best possible holequality on optimized drilling speed. As aside effect, the system can be used forerecting the mast or as emergency hoist-ing system.

It offers a complete redundant draw-works drive connected to the emergencygenerator. If the downhole motor is usedfor directional drilling, the actual mudpressure can also used as input for con-trol of the string release rate.

J O Y S T I C K D R A W W O R K S

State-of-the-art SCR (Semi ConductedRectifier) or VFD systems with regener-ative braking capacity are used fordrawworks braking purposes. A joystickrather than a brake handle controls themovement of the travelling block assem-bly (TBA).

Applying this solution speeds up theoperation, round trips are much faster,and open hole periods can be reduced.

Beside the effect of using a joystick forthe drawworks operation, a monitoringsystem for the actual kinetic energywithin the system assures the highestpossible standard of safety for staff andequipment.

Braking distances are continuouslysupervised and interlocked against anyequipment, which can move into thetravelling path of the TBA. Upper andlower limits will not be exceeded.

Flexible defined working points can beused with a very high accuracy for stan-dard operation during round trips orduring the drilling period.

September/October 2004 D R I L L I N G C O N T R A C T O R 37

K E E P T H E B I T O N B O T T O M

Listening to the feedback informationout of the hole is the next step forincreasing the overall quality of the borehole. Top drives or rotary tables aremostly set to a constant RPM, puttingjust torque into the drillstring but do notadjust these values in order to get a con-stant revolution on the bottom holeassembly (BHA). Depending on the for-mation, the drill bit very often shows theso called stick-slip behavior, resulting intorsional drill-string vibration.

If the torque is exceeding a certain limit,the bit is released, revolving with a veryhigh RPM value until the torque isalmost zero. The same behavior willthen start again. Applying a mathematicalgorithm invented by Shell Research, itis now possible to read the feedbackdata of the top drive or rotary table.

This information is processed andadjusted through the drive control sys-tem (SCR or VFD), inducing a behaviorof the top drive or rotary table to damp-en these vibrations.

This information can be computed auto-matically, without any manual input.The former mass-spring model, whichwas used to receive the relevant data, isnow replaced by the real data, optimiz-ing the vibration “filter” function.

The effect of this system is again anincrease in bore hole quality and areduced number of round trips sinceservice life of bits and pipes are higher.

Overtorqued connections resulting frompipe vibration or even twist-offs can beavoided. The rate of penetration is sig-nificantly increased (60-100%) since thebit revolves on the bottom working withcontrolled constant speed.

M U D P R E S S U R E P U L S E S

Extended reach wells require high mudpressure. Beside the drill bit operation,the mud consistency and treatment hasa very high impact on the bore hole qual-ity.

One prerequisite for achieving thisrequired quality is constant mud flow,avoiding pressure peaks or volume fluc-tuation.

Whenever more than one mud pump isin operation, the common three-pistontype pump needs to be controlled in

order to shift the pressure peaks ofevery piston in a defined pattern.

Aiming for the smallest possible overallamplitude within the high pressure sys-tem, all pumps involved are operated ina way that the stroke sequence of allmud pumps is phased in correctly, i.e.three Triplex pumps work like one nine-cylinder pump.

Advantages of the application of such asystem are seen in reducing stress tothe pulsation dampeners and to the dis-charge valves and piping. Knowing thispattern, LWD or MWD measurementswill deliver much better results sincenoise, i.e. pressure peaks, can now befiltered out.

In extended reach wells, sometimes itmight even be impossible to receive trueMWD information without the synchro-nisation of the mud pumps.

W H A T E L S E ?

All electrically driven land rigs,equipped with any standard machinery,can be upgraded with one or all of thesesystems.

The sum of all single approaches willend up in a very efficient, simple drillingmachine, delivering the best possiblehole quality results.

Investment that really pays off very fastwill be beneficial to all. Operators canreduce the development phase due tofast-track drilling. The delivered holesare of high quality, even under difficultdrilling conditions. Contractors cantreat their equipment with much morecare that in turn will save maintenanceand spare parts costs.

Finally, applying state-of-the-art tech-nology increases the safety on the rig.LTI’s are prevented and job satisfactionis increased.

These drilling performance enhancingapplications can be seen as essential tomeet today’s challenges. Lookingbeyond the immediate focus of the directrelation to the hole quality, there areother innovative systems that will bringsome additional benefits with respect tooverall performance and safety.

Closed loop controlled mud farms withintegrated alarm and data logging canindicate a possible kick or other down-hole problems. Recorded values fromwells in the same area can be displayedas a guideline for the actual job.

Cylindrical mud tanks guarantee a highhomogeneity of the mud, zero spillrequirements and fast changes fromwater-base to oil-base mud can beachieved.

Remote access to drilling data via theInternet is easily available. Real-timeinformation fed to the teams at head-quarters can be valuable for supportingthe rig team onsite in finding the rightsolution to actual problems.

Much of this new technology is alreadyin use, but is not being fully exploited.Introducing rig automation must belinked to systematic training programsfor the drill crews.

This investment in technology and train-ing for the future of new rig technologyis both necessary and worthwhile.

It is of utmost importance to start imme-diately with the integration of such sys-tems in order to be prepared for the nextjobs to come. The standard rigs of todaymay well be assessed as no longer fit forpurpose sooner than we think. n

September/October 2004 D R I L L I N G C O N T R A C T O R 39

The feed off control system with an Emalgobrake is controlled via variable frequency drive(VFD) driven AC motors to hoist and lower thedrillstring. A retrofit gearbox kit is available forany kind of drawworks and is attached to theextended shaft of the eddy current brake.