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BACKGROUND 
     Exploration, production, and monitoring of hydrocarbon and water deposits entails the measurement of subsurface characteristics and the evaluation of the obtained data to determine petrophysical properties of interest for the relevant formation or reservoir. A variety of techniques have been developed to measure subsurface characteristics. These techniques involve the subsurface deployment (usually through a borehole traversing the formations) of several different measurement and telemetry systems to provide data regarding the subsurface characteristics of interest, and data regarding the state of tools or instruments disposed downhole. 
     Among the various data collection and logging techniques routinely employed, systems for obtaining measurement data while drilling have proven to be cost effective. Logging While Drilling (LWD) and Measurement While Drilling (MWD) techniques are well known in the art. Logging While Tripping (LWT) systems have also been developed as an alternative to LWD and MWD techniques. In LWT, a small diameter “run-in” tool is sent downhole through the drill pipe, at the end of a bit run, just before the drill pipe is pulled. The run-in tool is used to measure downhole characteristics as the drill string is extracted or tripped out of the hole. In these types of systems, obtained data is either stored in downhole memory for later processing or transmitted to the surface using telemetry means, such as mud flow telemetry devices in the case of LWD/MWD systems. 
     Regardless of the conveyance means used for downhole tools, a shared requirement of the various measurement and telemetry systems is the need for electrical power. With the exception of wireline systems, it is difficult to convey electric power from the surface through the conveyance means to the components of the logging tools or the telemetry means. In these cases, electrical power can however be provided by downhole sources. Conventional systems obtain such power downhole either from a battery pack or a turbine-based alternator. Battery packs provide an energy storage medium. When using batteries, electrical power is made available until the battery is depleted. Turbine-based alternators provide an energy conversion device. In this case, electrical power is made available only when the energy source to be converted into electrical power is present. 
     Examples of alternators used in downhole logging tools are described in U.S. Pat. Nos. 5,517,464 and 5,793,625. An example of an alternator-like electrical torque-generator is described in U.S. Pat. No. 5,265,682. Turbine-based alternators employ rotors having impellers that are placed in the high-pressure flow of drilling fluid (“mud flow”) inside the drill string so that the impeller blades convert the hydraulic energy of the drilling fluid into rotation of the rotor. The rotor rotates at an angular velocity that provides enough energy to power the telemetry means and/or other components (e.g., sources/sensors) in the telemetry tool, and in some cases other tools in the downhole assembly. 
     Examples of battery packs used in downhole logging tools are described in U.S. Pat. Nos. 6,187,469 and 6,575,248. An example of a testing while drilling tool powered at least in part by a battery module disposed in the tool collar is described in U.S. Pat. No. 7,124,819 (assigned to the present assignee and entirely incorporated herein by reference). Battery packs are charged at the surface and provide electrical power to a single tool. Some batteries packs may be recharged downhole. 
       FIG. 1  shows a conventional land-based drilling rig  10  with connected drill pipe leading into a borehole  12  drilling through a subterranean formation F. At the tip of the drill string  14  is a drill bit  16  followed by a bottom hole assembly (BHA)  18  comprised of drilling, telemetry, and MWD/LWD tools. The borehole  12  is formed by rotary drilling in a manner that is well known. Drilling fluid or mud is pumped to the interior of drill string  14  to flow downwardly through the string. The drilling fluid exits drill string  14  via ports in the drill bit  16 , and then circulates upwardly through the annular space between the outside of the drill string and the wall of the borehole as indicated by the arrows. These conventional systems are powered both in a distributed fashion, wherein an individual tool in the string contains its own battery, and in a centralized fashion, wherein an individual tool draws power from a downhole turbine. 
       FIG. 2  shows a conventional design for supplying power in a downhole system such as the drilling assembly of  FIG. 1 . Each tool in the BHA has its own dedicated battery  20 . A single conductor, combined low electrical power and low speed communications bus  22  is used through all tools in the string. A low electrical power source is provided by a turbine, usually disposed in the telemetry tool, and energized by the mud flow. It should be appreciated that the bus  22  does not provide electrical coupling between the battery disposed in one tool and electrical components disposed in another tool. This configuration offers restricted spacing (a large spacing) between sources/sensors  24  in the string and presents handling and reliability issues. Thus a need remains for improved power distribution techniques for subsurface systems. 
     SUMMARY 
     The present disclosure provides a flexible architecture and modular method of supplying power to downhole tools and instruments. Aspects of the disclosure include a modular power sub to supply the power needs for downhole tools in a centralized manner in which a single power source can be used to simplify the BHA by removing the need for a power source in each tool or component in an assembly. The power subs disclosed herein provide more power in a flexible manner, such as to cover both flow and no flow measurements in while-drilling applications. In addition, it is also contemplated that the BHA may include one or more power subs, optionally with varying power outputs, which may provide the BHA with additional flexibility and modularity. For example, the BHA may be stacked with multiple power subs, running in series or parallel, that provide the BHA with additional or greater power. One potential benefit is to provide a more compact BHA, where the location of the power sub can be customized as desired for any drilling operation. An additional potential benefit of having power subs is that a frame of the power sub (e.g. a chassis and/or collar) may be used with a plurality of tools/BHAs, thereby reducing the inventory and design time associated with storing and/or designing frames for the various tools. Another potential benefit of centralizing and sharing the power sources amongst the tools of the BHA is to prevent failure of one particular tool caused for example by the depletion of its battery. In the proposed configurations, all tools of the BHA may be simultaneously powered and provide useful measurements as long as the centralized power source is available. While the failure of a single tool does not usually justify the end of the drilling operation, the depletion of a centralized power source may justify the end of the drilling operation. In this case however, the end of the drilling operation occurs at a time where an optimal use of the downhole power has been achieved. 
     Other aspects of the disclosure include a modular downhole tool having a measuring or testing section disposed in one tool collar and a modular power sub disposed in another tool collar. For example, during high temperature logging runs, which may expose the battery or any other component in the power sub to unacceptably high temperatures, the modular power sub may be removed from the tool and replaced with another modular power sub having a higher temperature rating, such as a turbine. Thus, the downhole tool may be equipped with any of a plurality of power subs having varying power outputs, thereby allowing the downhole tool to be configured with different electrical power configurations. An additional potential advantage provided by this configuration is that the measuring/testing section and the modular power sub of the downhole tool can be handled separately, thereby providing more compact collars (e.g. shorter than thirty five feet and/or lighter than four thousand five hundred pounds). Short collars are easier to transport to and from a drilling rig and easier to assemble to the BHA. 
     One embodiment introduced in the present disclosure provides an electrical power system comprising a support configured for interconnection within a subsurface drillstring, an electrical power unit coupled to the support, and a conductive link configured to distribute electrical power from the electrical power unit to at least one component coupled to the electrical power system within the subsurface drillstring. The support may comprise a drill collar and a central chassis internally coupled to the drill collar. The support may also comprise an annular space between the drill collar and the central chassis, wherein the annular space is configured to allow mud flow to or from the at least one component coupled to the electrical power system within the subsurface drillstring. The central chassis may comprise a first conductor, at least one power source, at least one electronics component, and a second conductor, wherein the at least one power source and the at least one electronics component are each coupled between the first and second conductors. The conductive link may be coupled, at least indirectly, to at least one of the first and second conductors. The conductive link may comprise at least one of the first and second conductors. One of the first and second conductors may comprise a pin conductor and the other of the first and second conductors may comprise a box conductor. The conductive link may comprise at least one multiple conductor member. For example, the conductive link may comprise a plurality of electrical conductors, including at least one first conductor configured for distributing high power and at least one second conductor configured for distributing communications. The plurality of electrical conductors may also include at least two first conductors configured for distributing high power and at least two second conductors configured for distributing communications. 
     The present disclosure also introduces a modular power system for a downhole tool, comprising a tool collar configured to operatively connect to a BHA, wherein the BHA comprises a plurality of BHA components; an electrical power source disposed in the tool collar and configured to provide electrical power to at least one of the plurality of BHA components; and a connector disposed between the electrical power source and one of the plurality of BHA components. The plurality of BHA components may include at least one drilling component, at least one telemetry component, at least one measurement-while-drilling (MWD) component, and/or at least one logging-while-drilling (LWD) component. The plurality of BHA components may not comprise any internal electrical power source and, thus, may only be powered by the electrical power source. The electrical power source may comprise an energy storage medium, including where none of the BHA components comprise an energy storage medium. The electrical power source may additionally or alternatively comprise an energy conversion device, including where none of the BHA components comprise an energy storage medium. The electrical power source may comprise an energy storage medium which is rechargeable downhole and/or at the surface. 
     The present disclosure also provides a modular drilling tool having a drill bit at and end thereof, comprising: a first collar including at least portions of a testing module; a second collar including at least portions of a telemetry module; and a third collar including an electrical power source operatively connected to the testing module; wherein removal of the third collar reduces a length of the tool. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of drilling apparatus according to the prior art. 
         FIG. 2  is a schematic view of a bottom hole assembly (BHA) according to the prior art. 
         FIG. 3  is a schematic view of a BHA according to one or more aspects of the present disclosure. 
         FIG. 4  is a schematic view of a power sub according to one or more aspects of the present disclosure. 
         FIG. 5  is a schematic view of an electrical connector according to the prior art. 
         FIG. 6  is a schematic view of an electrical connector according to one or more aspects of the present disclosure. 
         FIG. 7  is a schematic view of an electrical connector according to one or more aspects of the present disclosure. 
         FIG. 8  is a schematic view of an electrical connector according to one or more aspects of the present disclosure. 
         FIG. 9  is a schematic view of a power sub according to one or more aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a schematic view showing aspects of the present disclosure. This system configuration includes a centralized power scheme where a modular power sub  30  is disposed in the assembly. With a power section no longer required in each tool, their complexity and length decreases significantly. This in turn moves all of the measurements taken by these tools closer together (e.g., reduced spacing between sources/sensors  32 ) and to the drill bit. The close proximity to the bit creates a major advantage in well trajectory decision making as the well profile can be more precisely controlled with the measurements closer to the bit. In addition to the centralized power, a multiple conductor, high power and communications bus  34  is disposed in the string. Multiple conductors are preferable to maximize both the power transmission and the telemetry data rate. Also, while the tools shown in  FIG. 2  have a dedicated power source and hence may still be operative in case of a failure of the single conductor bus  22 , the bus  34  preferably provides redundant electrical paths between the modular power sub  30  and the tools in the BHA to minimize the impact of the failure of one of the paths in the bus  34 . Although aspects of the modular power sub  30  disclosed herein are shown powering the tools of a drill string BHA, one skilled in the art given the benefit of this disclosure will appreciate that the present disclosure is not limited solely to drilling applications. 
       FIG. 4  is a schematic view showing aspects of a modular power sub  30  of the present disclosure. A central chassis  36  resides inside a main drill collar  38  which withstands all of the drilling forces. The illustrated configuration allows mud flow within the annular space between the drill collar  38  and the chassis  36 . However, other configurations within the scope of the present disclosure may have the mud flow through a central channel (not shown) within the chassis  36 . Additionally, other aspects may be implemented with the chassis  36  offset from center of the collar  38 . 
     The chassis  36  may be comprised of four main components, such as an upper conductor  40 , a power unit/source  42 , electronics  44 , and a lower conductor  46 . The upper  40  and lower  46  conductors route power and communication to and from the power sub  30 . 
     The upper conductor  40  comprises a pin connector  48  and the lower conductor  46  comprises and a box connector  50 . It will be understood that aspects of the power sub  30  can be implemented with the pin and box connectors transposed (i.e., pin at lower position, box at upper position).  FIG. 5  shows a conventional connector pair  60 . This connector has a single-contact  62  configuration with seals  64 . Although conventional connectors may be used, aspects of the present disclosure are not limited to any particular connector design. Furthermore, the connector may be implemented in the collar wall, as known in the art. 
     Turning to  FIG. 6 , another aspect of a connector pair  70  of the present disclosure is shown. The connector pair  70  of  FIG. 6  is configured with a mating pair of dual contacts  72 ,  74 . Two or more contacts allow for efficient transmission of both power and communication. This connector pair maximizes contact length while minimizing the overall engagement length of the connector so as to easily fit within the envelope of a tapered drill collar thread connection. The short path between the connectors is also maximized and elastomeric seals  76  are used to isolate the connections. The radial arrangement of the smaller box socket comprising contacts  74  within the large pin could be expanded beyond two conductors as shown to three conductors and beyond. Any suitable materials may be used to implement the connector pairs of the present disclosure as known in the art. 
       FIG. 7  shows another connector pair  80  of the present disclosure. This aspect comprises multiple contacts  82  spaced axially on a single diameter separated by insulating seals  84 .  FIG. 8  shows yet another connector pair  90  of the present disclosure. This aspect also comprises multiple contacts  92  spaced axially and separated by insulating seals  94 , but at differing diameters as to increase the short path without increasing the axial distance between conductors. 
     Returning to  FIGS. 3 and 4 , the power source  42  section of the power sub  30  may be comprised of single or multiple power sources. These power sources can be divided into two types: energy conversion and energy storage. Energy conversion aspects include, but are not limited to, turbine power converting kinetic energy of the mud flow into electric power, vibrational energy generators, and piezo-electric power conversion. Energy storage aspects include, but are not limited to, batteries (rechargeable downhole or not), ultra-capacitors, fuel cells, and flywheel kinetic energy storage. 
     Batteries and rechargeable batteries offer a convenient power source for aspects of the present disclosure. Aspects may also be implemented using combinations of power sources. For example, an aspect may be configured for turbine power to provide power during mud flow while battery power is used to provide power with no flow present. The use of turbine or other energy conversion systems allows for aspects to be implemented wherein a battery or other energy storage device is charged downhole. Alternatively, such combinations could be used together to increase the power output of the power sub  30 . 
     In addition to providing power management of the power sources implemented in the power sub  30 , aspects of the electronics  44  section are configured to protect against spark and shock risk at the conductors during handling and connection with other tools. Indeed, while the tools shown in  FIG. 2  only share power source based on energy conversion (i.e., a turbine) that is inactive during the handling or connection with other tool, the power sub  30  preferably includes a power source based on energy storage (e.g., a battery) that is usually charged before it arrives at the drilling rig. Hence, it may be advantageous to implement one or more safety switches in the power sub  30 , as further detailed below. Other functions of the electronics  44  may include sensing, recording and transmitting power system status and diagnostics, both in real time via telemetry means and following an operation from recorded memory. Optional sensors may also be implemented in the electronics  44  section if desired (e.g., annular or stand pipe pressure sensors). 
     A tiered approach to safety is preferred, as well as the use of automated safety logic. Aspects of the power sub  30  may be implemented with a safety switch. One aspect is configured with the safety switch in the form of a hall effect sensor energized by a magnet and inserted into a read-out-port (ROP) disposed on the sub  30  (See item  45  in  FIG. 4 ) and operable from an exterior of the power sub  30  (e.g., an exterior of the collar  38 ). In this embodiment, when the magnet is inserted into the ROP  45 , the power source  42  is electrically disconnected or de-coupled from the conductors  40 ,  50  at either end of the tool. It will be understood by one skilled in the art that other means may be used to disconnect the power source  42 . For example, an alternative to using a hall effect switch is a mechanical switch that is closed when the power sub  30  is linked into the tools in the assembly (not shown). 
     In operations, the magnet is removed at the rig, for example once the BHA is assembled, thereby permitting the connection of the power source  42  to one or both connectors  40  and  50 . However further control of one or more of the power sources  42  may be achieved, if desired. Once in service downhole, the power source  42  can be switched on or coupled via downlink commands through telemetry means (or direct control via a wired drill pipe system). Such automated switch logic can also be configured for operation when the central chassis  36  is put into the BHA. Safety interlocks can be included for automatic coupling of the power source  42  when mud flow is confirmed. U.S. Pat. No. 6,649,906 (assigned to the present assignee and entirely incorporated herein by reference) describes safety interlock configurations that may be implemented with aspects of the present disclosure. The power source  42  can then remain coupled unless a safety interlock is violated (e.g., a voltage or current limit violation, or a predetermined timeout without mud flow) or a downlink telemetry decoupling command is given. 
     The electronics  44  of the power sub  30  may be configured to vary the output voltage delivered through the upper  40  and/or lower  46  conductors as needed. The electronics  44  can be implemented with appropriate circuitry to allow on-site programming of the output voltage, increasing the flexibility of the system. Additionally, the electronics  44  can also be used for the purpose of recharging a rechargeable power source  42 . 
     Aspects of the present disclosure may be implemented using multiple power subs  30  in a single assembly. The subs  30  could be used in parallel to increase the power capability of the system or used sequentially to increase the run life (e.g., in the case of an energy storage device). It should be appreciated that, in the configuration shown in  FIG. 2 , no electrical coupling between two power sources is provided. In contrast, when multiple power subs  30  are coupled to the bus  34  in the BHA, for example a battery and a turbine, it may be advantageous to implement power management systems between the multiples power subs. Thus, the electronics  44  in each sub  30  can be configured to prevent inadvertent charging or discharging of the power source  42  when multiple subs are used in combination. 
     Aspects of the present disclosure may also be configured with a retrievable power source  42  such that the source (e.g., a battery) could be replaced or initially inserted from the surface, with the BHA in position downhole. One such aspect can be implemented with the chassis  36  and collar  38  incorporating a latching mechanism (See item  49  in  FIG. 4 ) and a modified connector  48  at the upper end thereof. The latching mechanism  49  removably connects the retrievable chassis  36  to the drill collar  38 , while allowing for mud flow through the tubular. The modified upper connector  48  can be adapted for connection to a wireline, slickline, or other retrieval mechanisms as known in the art for retrieval of the unit to the surface. The retrievable chassis  36  may also be deployed into the downhole tool or subsurface assembly using a tractor, mud flow, gravity or other conveyance means. The retrievable chassis  36  is then secured in place using the latching mechanism  49 . Other retrievable configurations may be devised to implement aspects of the present disclosure. U.S. Pat. No. 6,577,244 and U.S. Patent Publication No. 20060260805 (both assigned to the present assignee and entirely incorporated herein by reference) describe retrievable tool configurations that may be implemented with aspects of the present disclosure. 
       FIG. 9  is a schematic view of a power sub  130  according to one or more aspects of the present disclosure. This system configuration includes a centralized power scheme where the modular power sub  130  is disposed in the assembly. One or more aspects of the power sub  130  are or may be substantially similar to corresponding aspects of the power sub  30  described above and shown in other figures. For example, the power sub  130  may include a multiple conductor, high power and communications bus which may be substantially similar in function to the bus  34  described above. The bus may provide redundant electrical paths between the modular power sub  130  and the tools in the BHA to minimize the impact of the failure of one of the paths in the bus. Although aspects of the modular power sub  130  disclosed herein are shown powering the tools of a drill string BHA, one skilled in the art given the benefit of this disclosure will appreciate that the present disclosure is not limited solely to drilling applications. 
     The modular power sub  130  shown in  FIG. 9  may also include a central chassis similar in function to the chassis  36  described above, coupled internally to a main drill collar  138  in a manner which allows mud flow within the annular space between the drill collar  138  and the chassis. However, other configurations within the scope of the present disclosure may have the mud flow through a central channel (not shown) within the chassis. Additionally, other aspects may be implemented with the chassis offset from center of the collar  138 . 
     The chassis may be comprised of four main components, such as an upper conductor module  140 , a plurality of rechargeable battery modules  142 , an energy conversion module  143 , an electronics module  144 , and a lower conductor module  146 . It is worth noting that the top-to-bottom sequence of these modules is not limited to the embodiment shown in  FIG. 9 . For example, the electronics module  144  may be positioned above the battery modules  142  and/or below the energy conversion module  143 , among other embodiments within the scope of the present disclosure. 
     The upper  140  and lower  146  conductor modules route power and communication to and from the power sub  130 . The upper conductor module  140  may comprise a pin connector and the lower conductor module  146  may comprise a box connector as described with regard to embodiments discussed above. It will be understood that aspects of the power sub  130  can be implemented with the pin and box connectors transposed (i.e., pin at lower position, box at upper position). Moreover, the particular connector pair employed by the upper  140  and lower  146  conductor modules may be any of those described elsewhere herein, conventional, or otherwise. 
     The power section of the power sub  130  comprises multiple power sources/modules. These power sources can be divided into two types: energy conversion and energy storage. Energy conversion aspects include, but are not limited to, turbine power converting kinetic energy of the mud flow into electric power, vibrational energy generators, and piezo-electric power conversion. In the embodiment illustrated in  FIG. 9 , for example, the energy conversion module  143  includes a turbine  143   a  configured to be driven by mud flow and cooperate with an alternator  143   b  to convert rotational kinetic energy into electric power. Energy storage aspects include, but are not limited to, batteries (rechargeable downhole or not), ultra-capacitors, fuel cells, and flywheel kinetic energy storage. In the embodiment illustrated in  FIG. 9 , for example, such energy storage is accomplished via the plurality of rechargeable battery modules  142  which are electrically coupled (at least indirectly) to the energy conversion module  143 , such that the rechargeable battery modules  142  may store the energy generated by the energy conversion module  143 . 
     The power sub  130  may also include a safety switch  145 . The safety switch may be or comprise a hall effect sensor energized by a magnet and inserted into a read-out-port (ROP) disposed on the power sub  130  and operable from an exterior of the power sub  130  (e.g., an exterior of the collar  138 ). For example, when the magnet is inserted into the ROP  145 , the battery modules  142  and/or energy conversion module  143  are electrically disconnected or de-coupled from the conductor modules  140 ,  146  at either end of the tool. It will be understood by one skilled in the art that other means may be used to disconnect the power sources  142  and/or  143 . For example, an alternative to using a hall effect switch is a mechanical switch that is closed when the power sub  130  is linked into the tools in the assembly (not shown). 
     The electronics  144  of the power sub  130  may be substantially similar, at least in function, to those describe above, such as being configured to vary the output voltage delivered through the upper  40  and/or lower  46  conductor modules as needed. The electronics  144  can be implemented with appropriate circuitry to allow on-site programming of the output voltage, increasing the flexibility of the system. Additionally, the electronics  144  can also be used in conjunction with recharging of the rechargeable battery modules  142 . For example, the electronics  144  may be configured to prevent inadvertent charging or discharging of the battery modules  142  and/or energy conversion module  143 , including when multiple subs are used in combination. 
     The sub  130  may also be configured with retrievable battery modules  142  such that they may be replaced or initially inserted from the surface, with the BHA in position downhole. One such aspect can be implemented with the chassis and collar  138  incorporating a latching mechanism (such as item  49  shown in  FIG. 4 ) and a modified connector at the upper end thereof. The latching mechanism may removably connect the retrievable chassis to the drill collar  138 , while allowing for mud flow through the tubular. The modified upper connector can be adapted for connection to a wireline, slickline, or other retrieval mechanisms as known in the art for retrieval of the unit to the surface. The retrievable chassis may also be deployed into the downhole tool or subsurface assembly using a tractor, mud flow, gravity or other conveyance means. The retrievable chassis may then be secured in place using the latching mechanism. Other retrievable configurations may be devised to implement aspects of the present disclosure. U.S. Pat. No. 6,577,244 and U.S. Patent Publication No. 20060260805 describe retrievable tool configurations that may be implemented with aspects of the present disclosure. 
     It will be appreciated by one skilled in the art that various tool configurations can be implemented using the modular power source techniques disclosed herein. For example, it will be appreciated that the disclosed power sub configurations can be implemented to include conventional sources and sensors to perform a variety of subsurface measurements as known in the art. Though not shown in full detail for clarity of illustration, the disclosed aspects can be implemented with conventional electronics, hardware, circuitry, housings and materials as known in the art. For example, embodiments can be implemented using composite materials to form the chassis and/or housing tubular as known in the art. U.S. Pat. Nos. 6,084,052 and 6,300,762 (both assigned to the present assignee and entirely incorporated herein by reference) describe composite-based tools and tubular configurations that may be implemented with aspects of the present disclosure. It will also be appreciated that aspects of the present disclosure may be used for any subsurface applications requiring a local power source including, but not limited to, LWD/MWD, LWT, run-in tools, production tubing, casing, and underwater applications. For the purposes of this disclosure it will be clearly understood that the word “comprising” means “including but not limited to” and that the word “comprises” has a corresponding meaning.

Summary:
An electrical power system comprising a support configured for interconnection within a subsurface drillstring, an electrical power unit coupled to the support, and a conductive link configured to distribute electrical power from the electrical power unit to at least one component coupled to the electrical power system within the subsurface drillstring.