Optimization of geo-mechanical-structural drilling with diamond crowns Optimizacija geomehansko strukturnega vrtanja z diamantnimi kronami Jurij Šporin1, Željko Vukelic2 1IRGO, Slovenčeva ulica 93, SI-1000 Ljubljana, Slovenia; E-mail: jurij.sporin@irgo.si 2University of Ljubljana, Faculty of Natural Sciences and Engineering, Department of Geotechnology and Mining, Aškerčeva cesta 12, SI-1000 Ljubljana, Slovenia; E-mail: zeljko.vukelic@ntf.uni-lj.si Received: June 21, 2007 Accepted: July 10, 2007 Abstract: For successful projecting and performance in all segments of mining, geo-technological and construction projects relating or depending on rock conditions where work is going on, quality geo-mechanical-structural drilling is of extreme importance. In article special attention will be focused on performing research drilling using diamond crowns because this is the way how most quality samples of rock which are later on fully examined in laboratory, can be obtained. Significance of optimal drilling for geo-mechanic-structural wells will be presented. Izvleček: Za uspešno projektiranje in izvajanje del pri vseh segmentih rudarskih, geotehnoloških in gradbenih projektov, ki se nanašajo oz. so odvisni od pogojev hribine v kateri se izvajajo, je izjemnega pomena izvajanje kvalitetnega geomehansko-strukturnega vrtanja. V članku se bomo predvsem osredotočili na izvajanje raziskovalnega vrtanja z uporabo diamantnih kron, saj na ta način pridobimo najkvalitetnejše vzorce hribin, ki jih nato lahko detajlno preiščemo v laboratoriju. Prikazali bomo pomen optimalnega načina vrtanja pri izvedbi geomehansko-strukturnih vrtin. Key words: research drilling, crown loading, crown, core Ključne besede: raziskovalno vrtanje, obremenitev na krono, krona,jedro Introduction For successful projecting and performance in all segments of mining, geo-technological and construction projects relating or depending on rock conditions where they are carried out, quality geo-mechanical-structural drilling is of extreme importance. In article special attention will be focused on performing research drilling using diamond crowns since in this way most quality samples of rock which will be fully examined in laboratory, can be Core sampling Core sampling is a process of drilling using drilling devices with advancing tool construction shaped in a way which enables them to take rock and soil samples. Sample - core is accumulated in a special tube, called core tube, which enables to bring the sample to surface. The aim of core sampling is gaining quality, intact core suitable for further research in laboratory. Drilling method and used equipment have major influence on core quality. Less influence to the core quality is later produced by manipulating with core in course of investigation. Core sampling with core crowns and core tubes At core sampling an optimum among the following parameters is striven for: • rotation speed of drilling accessories and tool; • force magnitude on crown; • ways of well flushing out (quantity, pressure, quality and flushing type). Regarding soil and rock properties we can state the following: 1. Because of great diversity in soil and rock quality the probability of proper advancing speed determination is low irrespective of small number of drilling parameters. Rock materials are complex and heterogeneous. Although microstructure and composition (minerals, grain size, bonds between grains etc.) are obtained. Significance of optimal drilling for geo-mechanical-structural wells will be presented. In Slovenia in general a so called core tubes made according to Swedish (Craelius) metric standard are used. Core tube diameters move from 36 mm to 146 mm. The following core barrels are in use: • single tube core barrel; • double tube core barrel; - rigid double tube core barrel - double tube core barrel with bearing • three wall core tube; • core sampling according to "wire line". the same, macrostructure (cracks, frequency etc.) varies because of different factors such as surface loadings which affect advancing speed. 2. Crown activity on rock material influenced by various drilling parameters is mutually dependent and complex. Independent variables while drilling are the following: - characteristics of drilling tools - crown (number, shape and size, used matrix and geometry of the crown) rock type (hardness, solidness, abrasion, mineralogical composition, cracks etc.); - drilling method (moments, axis force, rotation speed etc.). Dependent variables that affect drilling advancement are: - wear out of teeth; - rock fracture; - detritus size and shape; - input specific energy during drilling. ".Characteristic wear-out of teeth is defined by the drilling mode: - Drilling advancement (m/h) unsuitable; - Loading on single tooth is too low so teeth have not adequate contact with rock. Teeth penetration obstructed dew to increased surface arising from polishing teeth and getting blunt. Friction between teeth and rock is low. - Drilling by too high loading on crowns resulting in teeth breaking, high friction, unsuitable well washing out, unsuitable crown cooling and well bottom cleaning. Penetration speed does not grow up resulting in so called burning drilling crown. Optimal drilling advancement (m/h): - Rock fracturing and borehole particles formation uniform and constant. - Teeth and crown matrix wear out evenly, adv ancement, number of revolutions and loading on crown are also even. Figure 1. Presentation ofdrilling crown tooth wear out in dependence of loading Slika 1. Prikaz obrabe zob vrtalne krone v odvisnosti od obtežbe Regarding above stated facts we can conclude: - Loading of the crown is at optimal drilling dependent on strength and hardness of the rock and on teeth and crown conditions. - Equilibrium between wear out of teeth and crown begins at maximal advancement at optimal loading which is a little over minimal needed specific energy by individual drilling mode and rock type. - Wear out of teeth is the best indicator on drilling mode. - Teeth of smaller dimensions require higher loadings for effective drilling, achieve better advancements and produce finer particles detritus. - Tests and investigations showed that major part of input energy is not used for rock fracturing but for secondary crushing and grinding of bore particles. Therefore quality washing out of well bottom and bore quantity are very important. - Work done for crown loading is much smaller than the work input to overcome the torque. For better understanding of entire drilling mode operation it is necessary to show the principle of tooth crown cutting. Drilling mode is described in following phases: • Ploughing In very soft formations teeth can penetrate into rock and cause local overcoming of shear strength as shown in Figure 2. During crown rotation around axis and loading action on it teeth scratch bottom surface of the well like plough while ploughing field. Axial loadings are higher than shear ones so major part of work is done by axial loading. Provided that teeth are set in matrix in way that one furrow overlaps the other one the next tooth pushes some particles of the former tooth and the depth of furrow is equal as at former - in this case crown advances into rock. It is very important to wash out relatively big particles of the borehole. • Stress relaxation Stress relaxation occurs when pressure strength of the rock is high and static loading on single tooth is too low for immediate penetration into rock. Fracturing is triggered by tension discharge in the tooth furrow when the furrow traversed and caused a series of characteristic cracks and their widening as shown in Figure 2. In general a tooth during cutting never penetrates into rock if the latter had not been damaged before. • Grinding, abrasion In final phase grinding and abrasion are present and used at drilling into very hard rocks and at very high rotary speed. In principle that mechanism is very similar to mechanism of tension relaxation, the only difference are much more shallow cracks present here. ploughing action LOAD AREA > STRENGTH OF ROCK LOAD STRESS RELAXATION ( PROPAGATION OF CRACKS) MOVE I 3 3 - < STRENGTH OF HOCK AREA POLISHED DIAMOND (NO PROPAGATION OF CRACKS) diamond exposure Figure 2. Operation of crown tooth depending on loading Slika 2. Delovanje zob vrtalne krone v odvisnosti od obtežbe For rock demolition crown tooth has to operate in accordance to the following principle: 1. Under influence of axial loading single crown tooth impresses into rock 2. Crown's turning torque generates shear force which fracturing the rock Axial loading on single tooth is given by the equation: Where is: F0 - axial loading on single tooth [N] F - axial loading on whole crown created by drilling device [N] m - number of crown teeth [/] Advancement into rock is possible only under condition: F (1) F„> S-a p Where is: F0 - axial loading on single tooth [N] S - touching surface of tooth [m2] <7p - one axis pressure strength of rock [N/m2] Fo Figure 3. Crown tooth with technical elements Slika 3. Zob krone s tehničnimi elementi Figure 4. Scheme of the crown with teeth Slika 4. Shema krone z zobmi S = b-c (3) Where is: b - tooth width [m] c - tooth thickness [m] Tooth width is therefore given by expression: b = R-r (4) RMZ-M&G 2007, 54 Where is: R - external crown radius [m] r - internal crown radius [m] In case of a new and unused tooth its surface is given by expression: S = a-b = h0-tgP-R-r) (5) Where is: j8 - sharp edge angle of tooth [°] h0 - penetration depth of tooth into rock [m] Axial loading on single tooth is equal to expression: Fo= S-Op=ho-tgP-(R-ryap^ho= R --(6) In this case at m number of teeth in crown and n number of crown revolutions, advancement in a unit of time is: G , t Fo -m-n-t F-n-t (7) L0 = h0 - m-n-t =--.-r-=--.-r--(7) ^ 0 tg$-{R-r\ap tgp-(R-r)-Op Where is: b - tooth width [m] c - tooth thickness [m] j8 - sharp edge angle of tooth [°] h0 - penetration depth of tooth into rock [m] R - external crown radius [m] r - internal crown radius [m] t - time of crown operation until teeth wear out [s] n - number of tool revolutions [rev/s] It seems that during drilling when tooth traverses very cracked regions cutting depth is very small compared to crack size. This shows a more complex relaxation system of rock and removing borehole than foreseen from the above stated drilling modes. It is very likely that in praxis the turning and tension relaxation mechanism is used in every softer rock formation. Every time when tooth travels across a furrow it causes changes according to the same principle of drilling mode under surface. Operational effect of every drilling mode depends on hardness and rock tension and relations among tooth hardness, rock grain size and rock non-homogeneity. Drilling modes mentioned above do not consider washing out of detritus from the crown operation area. Nevertheless experiences show how great influence washing out, tool cooling, flushing medium leading mode, quantity of washed medium etc. have on gained core quality, advancing speed and wear out of drilling tools and accessories. Influencing factors on core samp-ling quality From the drilling theory we know that the following factors have major influence on advancement speed: - rotational speed; - single tooth penetration depth into rock; - physical mechanical properties of rock; - drilling tool loading; - teeth shape and conditions in tool; - rinsing medium quality, flow and type. Cutting speed Drilling tool rotational speed has great influence on cutting speed. For quicker and more convenient calculation of cutting speed we can assume that it is equal to: v = n-D-n (8) Where it is: v - cutting speed [m/min] D - drilling tool diameter [m] n - number of tool revolutions [rev/min] Figure 5 shows a graph for quick determination of cutting speed regarding rotational speed and tool diameter. Hiy:ili:*ri Snm'-i! rm' Figure 5. Graph for cutting speed determination Slika 5. Grafza dolocitev hitrosti rezanja Lower or higher speed has influence on advancement speed but it depends on rock formation properties. Final rotational speed is determined during drilling and mainly depends on rock properties and drilling techniques used. Crown loading Proper loading of tool - crown, is as important as proper rotational speed. During drilling crown loading is most easily determined by multiplying teeth number and the force that single tooth can withstand. This force is dependent on single tooth material quality. As shown above axial loading on single tooth is equal to the expression: Fo= S-op=K ■tg$-{R-r)-op^ho= F --(9) In this case at the number of crown teeth m and crown number of revolutions n time advancement is equal to: r F0 •m-n-t F-n-t G0 = h0 -m-n-t =-7-r-=-7-r--(10) ^ 0 tg$-{R-r)-op tgP-(R-r)-Op In a definite time t crown teeth wear out for an amounty. Therefore tooth height effected by axial force F0 equals to: h = h„-y (11) Where it is: h0 - height of a new and unused tooth [m] y - tooth wear out [m] Tooth wear out extent in time t can be expressed by equation: v '2-tgPi^r) (12) 2 Tooth alloy material wear out can be expressed by: v = a-A (13) Where it is: ft) - coefficient of volume tooth friction wear out on every Nm [m3/Nm] Friction work for thin wall crown in time t can be expressed by equation: A = F0-f-Ji-(R + r)-n-t (14) Where it is: f - coefficient of friction between rock and tooth The last expression can be put into equation for tooth material wear out so we get the following: v = (a-F0- f -n -(R + r)-n-t 2 r v = y-gfi-jR-r) (15) Equalizing both equations we get: '2{R-r) = CQ-F0- f -K - (R + r)-n-t Arranging the equations can express tooth wear out extent by: 2(t) -F0- f -n - {R + r)-n-t tgß-(R-r) (16) (17) If we know values offt) and/we can determine tooth sharpness loss (bluntness) in time t and tooth penetration depth into rock material under influence of axial force F0 in the same time t. Actually it is h = h0 — y as shown in the Figure 6. Figure 6. Crown tooth with technical elements with geometrical laws taken in account Slika 6. Zob krone s tehničnimi elementi ob upoštevanju geometrijskih zakonitosti Considering tooth wear out it follows: h = F0 -F0- f-n-(R + r)-n-t (19) Solving equation after time t and using relation F ■ m for axial force F0 we get crown working time until teeth bluntness. F02 -tgp-(R-r) __ t, max o2- (R~rf ■tg 2p-a>-2-Fo- f-n-{R + r)-n (20) 2 dp -tgp-(Q-f-n-{R + r\R-r)-n-m From the above expression it follows that we can increase crown working time tmax by increasing axial force F proportionally with crown teeth bluntness. With regard to above stated expression we can conclude: - crown working time until teeth bluntness tmax quickly falls by increasing one axial pressure rock strengthen; - increasing number of teeth in crown m shortens crown working time to bluntness tmax. Data about single tooth loading capability can be obtained from crown manufacturer. In practice it applies that loading on single tooth should be 2/3 of maximal allowed loading. Advancement speed The most important parameter in drilling practice is exactly advancement speed provided that the other costs (e.g. tool and equipment) of achieving fastest progress are acceptable or as low as possible. During drilling we must find optimal rotational speed, tool loadings and rinsing for every rock formation separately. Crown advancement time until full teeth wear out can be expressed, provided we add meaning of tmax for time t, from above expression. We get following: G0-t " n ' t max 2 ¡2-(0 F ■ f -n-{R + r)-n-m-t. max Op-(R-r)-tgP 3^ (R-r)-tgP _FnF_ Vp-{R-r)-tgP-!