Shock and vibration reduction in downhole tools
11732534 · 2023-08-22
Assignee
Inventors
Cpc classification
E21B17/10
FIXED CONSTRUCTIONS
E21B17/07
FIXED CONSTRUCTIONS
E21B17/1014
FIXED CONSTRUCTIONS
International classification
Abstract
Systems, methods, tools, and kits are used to reducing shocks and vibrations in tools, including downhole drilling tools. In at least some embodiments, shock and vibration reduction can be enhanced within tools that include an annular chassis within a collar. Existing locations where a gap exists are identified, and clamps, alignment features, joints, or other components can be modified or included to reduce the gap between the chassis and the collar. This includes on tool in which the chassis includes a flow tube or other tool with an axis that is offset from the axis of the chassis. Minimizing the gap can include providing three-point contact, which stabilizes the internal component within the collar. In a downhole environment, a downhole tool that includes the collar and chassis can perform a drilling or other downhole operation, and the internal vibration within the collar is reduced.
Claims
1. A downhole tool, comprising: a collar having an inner surface; a chassis having an outer surface disposed at least partially within the collar; a tube or rod at least partially within the chassis; and at least one clamp configured to fit at least partially around the tube or rod and secured to the chassis, the clamp including at least two bumper pads spaced from one another about the outer surface of the chassis, wherein the at least two bumper pads face the inner surface of the collar and provide points of support between the outer surface of the chassis and the inner surface of the collar to enhance contact and reduce a gap at discrete positions about the outer surface of the chassis; wherein the downhole tool is deployed in a wellbore or borehole.
2. The downhole tool of claim 1 wherein the at least two bumper pads and an outer surface of the clamp are configured to provide three or more points of support between the outer surface of the chassis and the inner surface of the collar.
3. The downhole tool of claim 1, wherein the clamp further includes at least one pocket, and the at least two bumper pads are secured to the clamp within the at least one pocket.
4. The downhole tool of claim 1, wherein the chassis includes a void opposite the clamp.
5. The downhole tool of claim 4, further comprising: a pulse neutron generator positioned within the void.
6. The downhole tool of claim 1, wherein the chassis has a longitudinal axis that is misaligned with a longitudinal axis of the collar.
7. The downhole tool of claim 1, which is part of a drill string.
8. The downhole tool according to claim 1, wherein the at least two bumper pads are disposed at different circumferential positions on the outer surface of the chassis.
9. The downhole tool of claim 1, wherein the at least two bumper pads comprise a polymer material.
10. The downhole took of claim 9, wherein the polymer material is selected from the group consisting of PTFE, PEEK, and UHMWPE.
11. The downhole tool of claim 1, wherein the at least two bumper pads comprise a non-polymer material.
12. The downhole took of claim 11, wherein the non-polymer material comprises a metal.
13. The downhole took of claim 12, wherein the metal is selected from the group consisting of: aluminium-bronze, titanium, and tungsten carbide.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) In order to describe the manner in which the above-recited and other features of the disclosure can be obtained, a more particular description will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. For better understanding, the like elements have been designated by like reference numbers throughout the various accompanying figures. While some of the drawings may be schematic or exaggerated representations of concepts, drawings of mechanical elements that are not identified as schematic should be considered as being to scale for some embodiments of the present disclosure, but are not limited to the illustrated scale. Understanding that the drawings depict some example embodiments, the embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
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DETAILED DESCRIPTION
(19) Embodiments of the present disclosure relate to systems and tools with increased reliability and decreased susceptibility to one or more of shock or vibration. In some embodiments, methods include modifying or manufacturing tools that have increased reliability and decreased shock and vibration. In still other embodiments, a kit is provided for retrofitting existing tools to reduce shock and vibration.
(20) Within a drilling BHA, the intensity of the drilling shocks and vibrations can be a function of the BHA design, while the drilling parameters may also provoke higher shock and vibration (S&V). Additionally, however, the internal architecture of particular tools can also have a strong influence on how damaging shocks and vibrations propagate to the sensitive elements where failure occurs. For instance, some sensitive elements include electronic boards, detector tubes, connectors, threads, harnesses, weldments, ports, locations of stress concentrations, and the like.
(21) An example drilling architecture can be referred to as an “annular chassis” design, where some or all sensitive functional components are arranged toward or on the outer diameter (OD) of a rod, cylinder, or tube. This rod, cylinder, or tube operates as the chassis as it carries these components. Such components may be directly on the outer surface, may be positioned through the outer wall of a tubular component (e.g., a port extending through the wall), or may be on the inside wall of a tubular chassis. A tubular chassis typically has a flow bore thru the center to enable the circulation of the drilling mud. Due to space constraints—particularly in smaller downhole tools—tools may have a “flow tube” to facilitate fluid flow. The flow tube of some downhole tools is arranged off-center relative to the chassis in order to make room for large functional components which otherwise may not fit around a central flow bore. For instance, radioactive receiver tubes on/off valves, and other components may be positioned around a flow tube. Some illustrative downhole tools include the ADNVISION, ARCVISION, DIGISCOPE, ECOSCOPE, GEOSPHERE, GEOVISION, IRISPHERE, MICROSCOPE, NEOSCOPE, OMNISPHERE, OPTIDRILL, PERISCOPE, POWERDRIVE, PROVISION, RHINO, SEISMICVISION, SONICSCOPE, SONICVISION, SPECTRASPHERE, STETHOSCOPE, and TELESCOPE tools available from Schlumberger in Houston, Tex., although embodiments of the present disclosure may be used in connection with tools of other companies.
(22) Within tools with an annular chassis design, there may be a gap between the chassis OD and the collar inner diameter (ID). When the tool is subject to shocks and vibrations, the gap can allow steel-to-steel internal impact to occur between the chassis and the collar. From these impacts, the shocks on the chassis may be higher than the shocks on the collar, and can have a higher frequency, which may end up exciting smaller structures with a higher natural frequency (e.g., boards or other elements). The term “shock transmissibility” can be used to express the amplitude ratio between the shocks on the chassis and the collar, or the ratio between the shocks on the chassis and the boards, and tools can be designed for increased or maximum transmissibility between these elements.
(23) In general, the cap can be seen as influencing the transmissibility of the shocks from the collar to the chassis, although quantifying this parameter for relation to the structural response of the tool is difficult. Assuming that if the gap was in fact zero—even if not achievable in practice—the internal impacts may not exist. Hence, as the gap increases, the impacts should theoretically be more significant. Conversely, minimizing the gap should reduce impact severity. The gaps used in the tools today varies depending on the tolerances, tool components, design, function, and other factors. In one tool, for instance, the collar may be sized to be to 124+0.1/−0 mm, and the chassis sized to 123.75+0.1/−0.5 mm. This means that there could be a gap anywhere from 150 to 850 microns, leaving the potential for internal motion and impacts. Minimizing the gap is not easy, however, as it means accurately matching the ID of the bore in the collar with the effective OD of the chassis within the tens of microns.
(24) While the gap between the chassis and the collar can influence the effects external shocks have on the internal shocks within the tool assembly, no detailed, systematic measurements and investigations are known to have been performed to relate the actual gap of a specific collar and chassis combination with the detailed shock response. This may have to do with the fact that measuring the actual gap along the entire chassis is quite difficult, including because the same gap may not exist everywhere as the parts are subject to machining tolerances. While measuring the chassis OD may be doable, measuring the ID of the collar continuously with micron accuracy over extended lengths (e.g., 5-10 m) is not an easy task. Hence it is likely that this gap—albeit important—has never been looked into in detail.
(25) Understanding the role of the gap, it can be seen that controlling the clearance between the collar and chassis to the lowest permissible value to minimize the transmissibility of shocks is desirable. However, this comes with increased manufacturing costs of requiring special tooling to maintain tight tolerances over lengths that can exceed 5, 7, 8, or even 10 m on the collar ID and chassis OD. Additionally, if the components are assembled by sliding the chassis within the collar, the close tolerances should still allow for ease of assembly.
(26) In some embodiments, rings around the chassis may be used for shock reduction. For instance, polymers (e.g., polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK), ultra-high-molecular-weight polyethylene (UHMWPE), etc.) may be fitted around the chassis. Instead of reducing the gap, such rings maintain the gap between the chassis and collar and instead rely on the ring providing a softer or dampened impact and thus a reduction in internal shock levels.
(27) In other embodiments, the relative motion between the collar and the chassis is minimized or potentially prevented by “jamming” or fixing them to one another radially. For instance, as shown in
(28) It is proposed that eliminating relative movement between chassis and collar is beneficial for improving shock performance. However, for existing tools this can involve significant modifications to the collar and chassis and the creation of new port holes, slots, and the like, which can increase cost due to re-work of the existing tool, but also because each port hole can create another site at risk of cracking, and therefore a risk of the tool being lost in hole (LIH).
(29) Some embodiments of the present disclosure relate to kits, methods, systems, and tools for re-working existing tools in a way that does not materially increase the sites at risk of cracking or failing. Embodiments used herein may be described in reference to a logging-while-drilling (LWD) tool, although application of these embodiments can be applied analogously to other drilling tools, whether new or existing. Examples of other tools include reamers, motors, rotary steerable tools, measurement-while-drilling tools, telemetry tools, jars, remedial tools, drill bits, other tools, or combinations of the foregoing. In some embodiments, the tool may include an annular chassis design and an off-center flow tube. Optionally, formation evaluation, sampling, drilling dynamics, or other sensing or measurement tools, or drilling tools (e.g., cutting tools, steering tools, etc.) may be integrated into the same tool. Hence, the tool may have a number of electronic boards, switches, valves, ports, and the like densely packed onto the chassis. Electronic boards in particular may have board failures that could appear to be related to circuit design, component selection, or firmware, yet instead be caused by mechanically induced shock or vibration. Therefore, implementing shock and vibration reducing techniques could in further improving the reliability of such tools.
(30) An example method of the present disclosure includes assessing the existing tool design and identifying opportunities to change existing, or to add new parts, to effectively control a chassis OD, collar ID, or both, and potentially to do so using least a “three-point support,” although four-point, five-point, or even continuous support may be used. In one embodiment, a tool includes clamps that hold a flow tube in place and improvements can be made to such claims. To illustrate this,
(31) In another step, a replacement or additional part is created. For instance,
(32) In the illustrated embodiment, two bumper pads 318 are shown on opposing lateral sides of the clamp 314-2. The bumper pads 318 may be removably secured to the outer surface of the clamp 314-2 for contact with the inner surface of a collar. In other embodiments, the bumper pads 318 may permanently secured to the clamp 314-2 using an adhesive, weld, or other mechanism. In at least some embodiments, the bumper pads 318-2 may be integrally formed with the body of the clamp 314-2, and thereby also permanently secured thereto.
(33) The bumper pads 318 may have a position that is fixed in any one or more of an axial, radial, or circumferential direction. The thickness may also be fixed. In other embodiments, the bumper pads 318 are adjustable in one or more directions. For instance, a method of embodiments of the present disclosure may include adjusting the radial position of the bumper pads. This may be done in any suitable manner, including by changing the bumper pad thickness (compression, expansion, replacement), or the position itself of the bumper pad (moving radially inward or outward). In some embodiments, adjusting the bumper pad radial position—and the amount of adjustment—can be critical to changing the shock and vibration experienced by the tool. In
(34) In the context of
(35) The radial position or bumper thickness can be measured using a ring gauge 520, as shown in
(36) Based on the measurements from the ring gauge 520, the bumper pads 518 may be adjusted. For instance, the bumper pads 518 may be ground or milled to size, supported with an appropriate number and size of shims, moved along a ramp, or otherwise expanded, compressed, or moved. In some embodiments, a gap of 50-100 microns between the surface of the bumper pad 518 and the collar ID (i.e., gap 409 of
(37) The methodology of some embodiments of the present disclosure may include modifications to one or more areas of a tool.
(38) In
(39) According to the same or other embodiments, an additional or alternative area that can be supported in an analogous way is a chassis joint. For instance, two portions 702-1, 702-2 of a chassis of
(40) Another or alternative aspect of the present disclosure may be to bring the internal component (e.g., chassis) and external component (e.g., collar) into contact via the force of a stiff spring or other biasing member. According to some embodiments of the present disclosure, an alignment key or other alignment feature may be used in connection with the downhole tool, and the alignment key may provide an opportunity for supporting the chassis by reducing the gap with the collar. The original function of an alignment key in some tools is to provide angular alignment between the chassis and the collar during assembly. This alignment may be desirable as certain features on the collar and on the chassis are connected via cables or wires (e.g., resistivity antennas, communication channels, power supplies, etc.).
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(42) Another embodiment that locks the chassis 902 against the collar 904 using the existing alignment key 934 is to introduce an intermediate set screw 942 or other threaded component, which engages in a thru-thread in the alignment key 934, as shown in
(43) In some embodiments, tightening the intermediate set screw 942, causes the body of the alignment key 934 to move upwards against the internal diameter (ID) of the collar 904 as the intermediate set screw 942 in turn pushes off the bottom of a pocket 944 in the chassis 902. This can then create a force that potentially locks or fastens the chassis 902 into the collar 904 and restricts and potentially eliminates any relative movement. A threaded hole in the center of the intermediate set screw 942 is optionally used to accept a port plug 944, which is a standard port plug in some embodiments, although a custom or proprietary port plug is used in other embodiments. One feature of the embodiment shown and described relative to
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(45) In the illustrated embodiment, the set screws 1042 can be tightened individually and at separate times, and their clamping forces (e.g., by pushing against the wall of the pocket 1044 in the chassis 1002 and causing the alignment key 1034 to push against the ID of the collar 104) will sum to provide an increased locking force between the chassis 1002 and collar 1004. The port plug 1046 can then be placed, threaded, or otherwise assembled in an original threaded hole.
(46) While
(47) Embodiments of the present disclosure may be used individually or in any of various combinations in order to stabilize one or more locations along a tool, such as a drill string.
(48) In the same or other embodiments, the downhole tool 1148 may include one or more flow tube clamps 1152 that also include bumper pads or other contact points to create three or more point support. Other or further stabilization features, including further flow tube clamps 1154 of other designs or function (see
(49) Tests have been conducted that show the effectiveness of embodiments of the present disclosure. A highly instrumented drilling tool was produced and used to quantify the difference between a standard commercial tool, and the same tool equipped with a shock reduction kit including embodiments of the present disclosure.
(50) It will be appreciated in view of the disclosure herein that there could be a number of variations to the tools, methods, and kids provided, and to the tooling and replacement parts (e.g., bumpers, clamps, etc.) that may be used. For instance, a precision ring or tube may be used in connection with or in lieu of a ring gauge with micrometer heads. In another embodiment, a feeler gauge may be used instead of the ring gauge with micrometer heads. Such embodiments could simplify adjustment and installation methods to make and use the tools increasingly user friendly and deployable in field locations.
(51) In some embodiments, bumper pads may be used as described herein. The bumper pads may have customized sizes; however, in other embodiments bumper pads may have a standard size and a system of shims or the like may be used to achieve a certain effective chassis outer diameter. Additionally, different materials may be used for the bumper pads. Polymers such as PTFE, PEEK, UHMWPE, or the like may be used, although non-polymers may also or alternatively be used. For instance, a metal such as aluminium-bronze, titanium, tungsten carbide, or the like may be used. Besides minimizing the gap, material selection could be used to add damping effects to the system. Also, so-called sandwich blocks that include combinations of materials (e.g., metal and rubber/plastic/polymer/dampening material) could be used. There could also be sizing changes to potentially improve bumper pads even closer to the collar ID.
(52) Additionally, different bumper blocks or bumper block kits could be made which are effective at different temperatures, pressures, sizes, for different tools, etc., depending on the job planning. Such bumpers may also be made from a material allowing for a certain amount of thermal expansion so that the heat downhole will help make the gap smaller. Bumper pads, including those with dampening effect, could be added to support point 1 in order to achieve an all-around “dampening suspension” of the chassis within the collar. Once the bumpers on a chassis are sized appropriately, the bumper size and location on the tool could further be recorded, which would make future servicing easier in case the bumpers get worn. Chasses and collars could also be kept together as pairs so that gap size is a known value.
(53) With example alignment keys, further variations could include a combination of a threaded and wedged locking/engagement mechanism, a threaded-only locking/engagement mechanism, or other engagement mechanisms. It is also contemplated that other locations of a downhole tool can include internal stabilization features, based on the degree of chassis modifications and space availability, so that spring suspensions, three-contact support, locking, dampening suspensions, or the like could be provided.
(54) Embodiments of the present disclosure can be used in a number of industries and applications. An example of such application is in a downhole environment when exploring for hydrocarbons or other natural resources, drilling a wellbore, or producing resources from the wellbore. By way of illustration only,
(55) In the drilling system 1460, a drillstring 1464 is suspended within the borehole 1462 and has a bottomhole assembly 1466, which includes a drill bit 1468 at its lower end. The surface system of the drilling system 1460 includes a platform and derrick assembly positioned over the borehole 1462 and including a top drive 1470, kelly 1472, hook 1474, and rotary swivel 1476. The drillstring 1464 is rotated by the top drive 1470 which engages the kelly 1472 at the upper end of the drillstring 1464. The drillstring 1464 is suspended from the hook 1474, attached to a traveling block (not shown), through the kelly 1472 and the rotary swivel 1476, which permits rotation of the drillstring 1464 relative to the hook 1474. A rotary table system could be used in other embodiments to rotate the drillstring 1464 in the wellbore and thereby rotate the drill bit 1468 against a face of the earth formation at the bottom of the wellbore.
(56) The surface system can further include drilling fluid or mud 1478 stored in a pit 1480 formed at the well site. A pump 1482 delivers the drilling fluid 1478 to the interior of the drillstring 1464 via a port in the swivel 1476, causing the drilling fluid 1478 to flow downwardly through the drillstring 1464 as indicated by the directional arrow 1484. The drilling fluid exits the drillstring 1464 via ports in the drill bit 1468, and then circulates upwardly through the annulus region between the outside of the drillstring 1464 and the wall of the wellbore 1462, as indicated by the directional arrows 1486. In this manner, the drilling fluid 1478 lubricates the drill bit 1468, cools the drill bit 1468, and carries formation cuttings up to the surface as it is returned to the pit 1480 for recirculation.
(57) A control unit 1488 may be used to control the top drive 1470 or other drive system. The top drive 1470 may rotate the drillstring 1464 at a rotation speed to produce desired drilling parameters. By way of example, the speed of rotation of the drillstring 1464 may be determined to optimize drilling speed (i.e., rate of penetration), to reduce drill bit wear, according to properties of the earth formation, or the like.
(58) The bottomhole assembly 1466 may include a logging-while-drilling (LWD) module 1490, a measuring-while-drilling (MWD) module 1492, a rotary-steerable system or motor 1494, and drill bit 1468.
(59) The MWD module 1492 may be housed in a special type of drill collar, and can contain one or more types devices for measuring characteristics of the drillstring 1464, the drill bit 1468, or other portions of the BHA 1466. The MWD module 1492 may further include an apparatus (not shown) for generating electrical power to the downhole system. This may include a mud turbine generator powered by the flow of the drilling fluid 1478, although other power (e.g., battery) systems may be employed. The MWD module 1492 may include one or more of the following types of measuring devices or other sensors: a weight-on-bit sensor; a torque sensor; a vibration and/or shock sensor (e.g., an accelerometer); a stick-slip sensor or inference device; a direction sensor; a rotational speed sensor (e.g., a gyroscope or tachometer); a speed sensor; or an inclination sensor.
(60) The LWD module 1490 may also be housed in a special type of drill collar, and can contain one or a plurality of types of logging tools. It will also be understood that more than one LWD module 1490 and/or MWD module 1492 can be employed. The LWD module 1490 may include capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. The LWD module 1490 may include a fluid sampling device. Typical LWD modules 1490 include, for example, natural gamma ray, spectral density, neutron density, inductive and galvanic resistivity, acoustic velocity, acoustic caliper, downhole pressure, and the like. Formations having recoverable hydrocarbons typically include certain physical properties, for example, resistivity, porosity (density), and acoustic velocity values in a certain range.
(61) The BHA 1466, including the LWD module 1490, MWD module 1492, and rotary steerable 1494 can be subjected to harsh downhole conditions while drilling or performing another downhole operation. These external conditions can produce in vibration along the BHA 1466 and the drillstring 1464. Additionally, the design of the BHA 1466 and drillstring 1464 itself can affect the vibrations that are felt. Vibrations may include both external vibrations (e.g., vibrations measured on the outside of the BHA 1466 or drillstring 1464), or internal vibrations (e.g., vibrations measure don the inside of the BHA 1466 or drillstring 1464).
(62) As discussed herein, when a flow tube, rod, or other tool is used within a tool (e.g., within the drillstring 1464, BHA 1466, LWD module 1490, MWD module 1492, or rotary steerable 1494), such a tool itself cause or impact the vibrational susceptibility of the drillstring 1464. A chassis within the drillstring 1464, BHA 1466, LWD module 1490, MWD module 1492, or rotary steerable 1494 and which contains or is coupled to another internal tool may help control the vibration. Embodiments of the present disclosure can include further use of methods, systems, tools, and kits to further control vibration by affecting the gap between the chassis and the collar of the BHA 1466, LWD module 1490, MWD module 1492, rotary steerable 1494, or other components, maintaining contact between the chassis and the internal surface of such components using three or more points of contact, and the like. In some embodiments, the vibration that is controlled includes internal vibration of a drill string or BHA where internal components vibrate relative to an outer collar, such as where one or more internal components are not coaxial or are not mass balanced.
(63) The embodiments of stabilization or vibration dampening features have been primarily described with reference to wellbore, downhole operations such as drilling; however, kits, methods, systems, and assemblies of the present disclosure may be used in other applications within a wellbore or other industries. In other embodiments, stabilization features according to the present disclosure may be used outside a wellbore or other downhole environment used for the exploration or production of natural resources. For instance, stabilization features of the present disclosure may be used in a borehole used for placement of utility lines. Accordingly, the terms “wellbore,” “borehole” and the like should not be interpreted to limit tools, systems, assemblies, or methods of the present disclosure to any particular industry, field, or environment. In further examples, a manufacturing, research, or other application that includes internal flow tubes or chasses within an outer tube or collar, or joints between tubes, and which suffer from shock and vibration may include embodiments of the present disclosure.
(64) Although one or more specific embodiments of the present disclosure are described herein, these described embodiments are examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, not all features of an actual embodiment may be described in the specification.
(65) The articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements in the preceding descriptions. It should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. For example, any element described in relation to an embodiment herein may be combinable with any element of any other embodiment described herein. Numbers, percentages, ratios, or other values stated herein are intended to include that value, and also other values that are “about” or “approximately” the stated value, as would be appreciated by one of ordinary skill in the art encompassed by embodiments of the present disclosure. A stated value should therefore be interpreted broadly enough to encompass values that are at least close enough to the stated value to perform a desired function or achieve a desired result. The stated values include at least the variation to be expected in a suitable manufacturing or production process in the related industry, and may include values that are within 5%, within 1%, within 0.1%, or within 0.01% of a stated value.
(66) A person having ordinary skill in the art should realize in view of the present disclosure that equivalent constructions do not depart from the spirit and scope of the present disclosure, and that various changes, substitutions, and alterations may be made to embodiments disclosed herein without departing from the spirit and scope of the present disclosure. Equivalent constructions, including functional “means-plus-function” clauses are intended to cover the structures described herein as performing the recited function, including both structural equivalents that operate in the same manner, and equivalent structures that provide the same function. It is the express intention of the applicant not to invoke means-plus-function or other functional claiming for any claim except for those in which the words ‘means for’ appear together with an associated function. Each addition, deletion, and modification to the embodiments that falls within the meaning and scope of the claims is to be embraced by the claims.
(67) The terms “approximately,” “about,” and “substantially” as used herein represent an amount close to the stated amount that still performs a desired function or achieves a desired result. For example, the terms “approximately,” “about,” and “substantially” may refer to an amount that is within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of a stated amount. Further, it should be understood that any directions or reference frames in the preceding description are merely relative directions or movements. For example, any references to “up” and “down” or “above” or “below” are merely descriptive of the relative position or movement of the related elements.
(68) The present disclosure may be embodied in other specific forms without departing from its spirit or characteristics. The described embodiments are to be considered as illustrative and not restrictive. The scope of the disclosure is, therefore, indicated by the appended claims rather than by the foregoing description. Changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.