# drill string design

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drill string components and design principals
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(1)   DRLG STRG DESIGN  11:54 AM 7/13/2007 M A-Mohse  DRILL STRING DESIGN   1.4.1 INTRODUCTION   When designing a drill string the aim is :- ã   Keep the maximum stress at any point in the drill string less than yield strength derated by a design facto ã   Select components and configure assemblies to retard fatigue as much as economically practical. ã   Provide equipment that is resistant to Hydrogen Sulphide (H2S) if H2S is expected. This Topic covers simple drill string design steps for vertical and directional wells, including so considerations for extended reach drilling. Loads applied by tension, torsion, combined tension and torsio  burst pressure, collapse pressure, slip crushing and stability forces are considered. It does not cover techniqu for vibration analysis, torque and drag modelling, hydraulics design, directional control or jar placeme Proprietary software programs for performing the calculations described in this Topic is now available i most OUs. As always, however you should be aware of what data they are using and what they are doing wi it. 1.4.1.1 DESIGN ASSUMPTIONS For simplicity, the assumptions outlined below are built into the calculations in this section. ã   Tension is approximated using the Buoyed Weight method when doing manual calculations. Althoug this method ignores the effects of circulating pressure on tension, it continues to be very popular f tensile design. This is no doubt due to the following: o   It's the way we've always done it (Tradition!), o   It's simpler than the more exact Pressure-Area method, which is used by Torque/Dr software such as Well plan for Windows, and o   The error that it causes is easily compensated when selecting the Margin of Overpull (MOP ã   Buckling is assumed to occur only below the point where buoyed string weight equals weight on bi This point will be called the Neutral Point . This assumption ignores some pressure forces in order simplify design calculations. In fact, unless the drill pipe is stuck, the neutral point will never be abo this point except temporarily when pump rate is increased with the bit on bottom. ã   Increasing hole angle at the BHA reduces the fraction of BHA weight available for bit weight. ã   Tension calculations assume the string is hanging vertically. For high angle and extended reach drillin this assumption will be modified as it would otherwise result in too conservative a design. ã   Material yield strength is the specified minimum for the component being considered. ã   Drill pipe tube wall thickness is the minimum for the stated drill pipe class. ã   Connection torsional strength and makeup torque are calculated using the A.P. Farr formula from A RP 7G.  (2)   DRLG STRG DESIGN  11:54 AM 7/13/2007 M A-Mohse  1.4.1.2 DESIGN FACTORS Design factors are numbers that are used to derate the load capacities of components and assemblies. Desig factors provide an extra margin of capacity to take care of inexactness in our assumptions about materi  properties, loads and hole conditions. For drill strings, the following design factors will be used: For Tension (DF T ): This factor is divided into a component's maximum tensile load capacity to determine t maximum allowable load that we're comfortable applying to that component. Design Factor for tension (DFshould be 1·15, based on SIEP recommendations. Margin of Overpull (MOP): This is excess tensile capacity above the normal hanging or working load (P w ) t account for factors such as hole drag. MOPs may vary from 50,000 to 150,000 lbs, depending on ho conditions. For excess BHA weight (DF BHA ): This factor is multiplied by desired weight on bit (WOB) to determi minimum buoyed BHA weight. This excess weight in the BHA provides an extra margin to keep the neutr  point below the top of the BHA. The recommended value for DF BHA is 1·15. For Torsion:  Applied torsion is limited to tool joint makeup torque. Standard makeup torque is either 60% 50% of tool joint torsional yield strength, according to the Class of the pipe, and tool joints are almost alwa weaker in torsion than the tubes to which they're attached. Therefore a design factor in torsion isn't necessa for drill string design. For Collapse Pressure (DF C ): Collapse pressure capacities are first derated to account for the effect of a simultaneous tension, then the derated capacity is further reduced by dividing by the collapse design factor. Dshould be 1·125. For Burst Pressure (DF B ) Burst pressure loading is rarely a concern in drill string design because surfa  pressures rarely approach the burst capacities of most drill pipe. The design factor for burst is divided into component's burst pressure capacity to give the maximum permissible burst pressure that may be applied to th component. DF B  should be 1·176. Burst capacity is increased by simultaneous tension, but this benefit normally ignored in drill string design. 1.4.2 DESIGN PROCEDURE Design is a multi-step process which usually begins at the bottom of the hole and works upward. The followi is a discussion of a simple tension and torsion design. These steps and considerations will be present in mo design situations. Special considerations for extended reach drilling are also included. The following steps i the process are discussed: ã   Selection of drill collar diameter ã   Selection of BHA connections and features ã   Stabiliser and jar placement ã   Determine length of drill collar section ã   Determine length of the heavy weight drill pipe section ã   Other checks to make ã   Drill pipe tension design nomenclature ã   Calculate allowable load (P A )  (3)   DRLG STRG DESIGN  11:54 AM 7/13/2007 M A-Mohse  ã   Set margin of overpull (MOP) ã   Calculate working load (P W ) ã   Calculate the maximum length of the first drill pipe section ã   Calculate the maximum length of the second drill pipe section ã   Calculate the maximum length of the third drill pipe section ã   Burst pressure ã   Collapse pressure ã   Combined loading ã   Stability forces and drill pipe buckling ã   Slip crushing ã   Buoyancy factor for non-steel components ã   Special considerations for extended reach wells 1.4.2.1 SELECT DRILL COLLAR DIAMETER Unless geometric sticking (see Topic 1.2.1 of Section 5, Part 1 - Stuck pipe prevention and fishing operation is a problem, the largest diameter drill collars consistent with other needs are generally best. Their increase stiffness means more directional stability. Also, they will have fewer connections for a desired weight on bi They allow shorter BHAs which can lessen the likelihood of differential sticking. Larger OD collars in a give hole also mean less lateral freedom of movement in the BHA. This decreases buckling stress and the rate connection fatigue. In practice however, drill collar size is often determined by existing rig stocks. Other facto which come into play are: ã   Fishability considerations. ã   Capabilities of the rig handling equipment. ã   Directional control requirements. ã   Hydraulics. ã   Desired exterior features (spiral grooves, elevator groove, or other features). 1.4.2.2 SELECT BHA CONNECTIONS AND FEATURES The following points apply not only to drill collars and HWDP from the rig inventory, but also to the ma specialised tools that find their way into the hole. Stabilisers, motors, MWD and LWD tools, hole opener under-reamers, jars and other tools are all subject to fatigue. Bending strength ratio: The predominant consideration, especially in selecting larger BHA connections is Bending Strength Rati (BSR). BSR is a ratio of the relative stiffness of the box to the pin for a given connection type. If we select connection with either a pin or box that is out of balance with the other member, we tend to increase the stre level and accelerate fatigue in the weaker member (see Figure 2.1.28)  (4)   DRLG STRG DESIGN  11:54 AM 7/13/2007 M A-Mohse    Figure 2.1.28 : The effect of BSR of 2.5 on fatigue life. The traditional target BSR  is 2·5, and acceptable BS ranges centre on this point. However, BSR ranges a rough guidelines established by experience and shoul not be used as strict operating limits. Staying withi recommended BSR  guidelines does not elimina connection fatigue failures, nor does exceeding th recommended ranges always lead to fatigue failures. In theory, high BSRs should cause accelerated pin failur and low BSRs should cause accelerated box failures.  balanced BSR  should provide maximum connection lif However, field experience shows that larger OD colla (8 and up) suffer predominantly from box fatigue crac even when they operate at or near the ideal BSR of 2·5. This indicates that higher BSRs may be more appropria for these sizes. On the other hand, 43/4 collars with BS as low as 1·8 are widely used but rarely experience bo fatigue cracks. Therefore, the suggested BSR ranges i Table 2.1.5 are probably better. In every case howeve experience under given conditions should be a maj factor in BSR selection.   One frequently overlooked connection is the one between the top drill collar and the bottom joint of HWDP. If a straight (non- bottleneck) crossover sub is used, and the collar OD is larger than the HWDP tool joint OD, the resulting BSR of that one connection will be exceedingly high. Pin failures in the  bottom oint of HWDP are not uncommon, and this is the probable reason. The problem is helped by using a bottleneck sub to smooth the change in cross section. Table 2.1.5 : Recommended BSR ranges

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