Patent Publication Number: US-9416649-B2

Title: Method and system for determination of pipe location in blowout preventers

Description:
BACKGROUND 
     Embodiments of the present invention relate generally to blowout preventers, and more particularly, to a method and system to monitor the position of a pipe in a blowout preventer. 
     Oil and gas field operations typically involve drilling and operating wells to locate and retrieve hydrocarbons. Rigs are positioned at well sites in relatively deep water. Tools, such as drilling tools, tubing and pipes are deployed at these wells to explore submerged reservoirs. It is important to prevent spillage and leakage of fluids from the well into the environment. 
     While well operators generally do their utmost to prevent spillage or leakage, the penetration of high-pressure reservoirs and formations during drilling can cause a sudden pressure increase (“kick”) in the wellbore itself. A significantly large pressure kick can result in a “blowout” of drill pipe, casing, drilling mud, and hydrocarbons from the wellbore, which can result in failure of the well. 
     Blowout preventers (“BOPs”) are commonly used in the drilling and completion of oil and gas wells to protect drilling and operational personnel, as well as the well site and its equipment, from the effects of a blowout. In a general sense, a blowout preventer is a remotely controlled valve or set of valves that can close off the wellbore in the event of an unanticipated increase in well pressure. Modern blowout preventers typically include several valves arranged in a “stack” surrounding the drill string. The valves within a given stack typically differ from one another in their manner of operation, and in their pressure rating, thus providing varying degrees of well control. Many BOPs include a valve of a “blind shear ram” type, which can serve to sever and crimp the drill pipe, serving as the ultimate emergency protection against a blowout if the other valves in the stack cannot control the well pressure. 
     In modern deep-drilling wells, particularly in offshore production, the control systems involved with conventional blowout preventers have become quite complex. As known in the art, the individual rams in blowout preventers can be controlled both hydraulically and also electrically. In addition, some modern blowout preventers can be actuated by remote operated vehicles (ROVs), should the internal electrical and hydraulic control systems become inoperable. Typically, some level of redundancy for the control systems in modern blowout preventers is provided. 
     During a blowout, when the valves of the BOP are activated, the shear rams are expected to sever the drill pipe to prevent the blowout from affecting drilling equipment upstream. The shear rams are placed such that the drill pipe is severed from more than one side when the valves of the BOP are actuated. Although BOPs are an effective method for preventing blowouts, the rams can sometimes fail to sever the drill pipe for several reasons including lateral movement of the pipe inside the BOP, and presence of a pipe-joint in the proximity of shear rams. 
     Given the importance of BOPs in present-day drilling operations, especially in deep offshore environments, it is important for the well operator to have confidence that a deployed BOP is functional and operable. Further, it is also desirable for the well operator to know the position of the pipe with respect to the BOP. In addition, the operator would also find it useful to determine the nature of movement of the pipe in the BOP. 
     As a result, the well operator will regularly functionally test the BOP, such tests including periodic functional tests of each valve to detect the presence of tool-joints in the BOP, periodic pressure tests of each valve to ensure that the valves seal at specified pressures, periodic actuation of valves by an ROV, and the like. Such tests may also be required by regulatory agencies. Of course, such periodic tests consume personnel and equipment resources, and can require shutdown of the drilling operation. 
     In addition to these periodic tests, the functionality and health of modern BOPs can be monitored during drilling, based on sensing signals produced by sensing systems placed in the BOP, and indirectly from downhole pressure measurements and the like. However, in conventional blowout preventer control systems, these various inputs and measurements generate a large amount of data over time. Given the large amount of data, the harsh downhole environment in which the blowout preventer is deployed, and the overwhelming cost in resources and downtime required to perform maintenance and replacement of blowout preventer components, off-site expert personnel such as subsea engineers are assigned the responsibility of determining BOP functional status. This analysis is generally time-consuming and often involves the subjective judgment of the analyst. Drilling personnel at the well site often are not able to readily determine the operational status or “health” of blowout preventers, much less do it in a timely and comprehensible manner. 
     In addition, sensing systems are sensitive to the presence of foreign material in the drill pipe and may produce erroneous results that lead to false positives. Examples of foreign material include, but are not limited to, debris caused due drilling and cutting, or water, or gas bubbles, and the like. Further, changes in environmental conditions may also lead to sensor drifts. The sensor drift may cause changes in output of the sensing systems thus causing errors in determination of position of the pipe in the BOP. 
     Since the corrective actions required to enable efficient operation of the BOP are dependent on determination of the pipe location with respect to the BOP, it is important for the sensing systems to produce accurate results. Hence, there is a need for a method and system that aids in determination of pipe location in a BOP while factoring movement of the pipe as well as the presence of pipe-joints in the BOP. 
     BRIEF DESCRIPTION 
     A system to detect a position of a pipe with respect to a blowout preventer (BOP) is provided. The system includes casing configured to be disposed around an outer surface of a section of the pipe. The length of the casing is greater than or equal to a length of the section of the pipe. Further, the system includes a plurality of sensing devices configured to generate a plurality of position signals. The plurality of sensing devices are arranged to form a plurality of arrays of sensing devices. Each of the plurality of arrays is disposed circumferentially around the casing and spaced from one another along the length of the casing. Furthermore, the system includes a processing unit that is configured to compute a distance between the pipe and each of the plurality of sensing devices based on the plurality of position signals. The processing unit is further configured to generate a first alert when the distance of the pipe determined from at least one sensing device is different from a reference distance between the pipe and the sensing devices. The processing unit to generate a second alert when the distance between the pipe and each sensing device of at least one array of sensing devices is different from the reference distance between the pipe and sensing devices. 
     A method for monitoring a position of a pipe with respect to a blow-out preventer (BOP) is provided. The method includes receiving a plurality of position signals from a plurality of sensing devices. The sensing devices are disposed on a casing to form a plurality of arrays of sensing devices along the length of the casing. The casing, on the other hand, is disposed on an outer surface of a section of the pipe. Further, the method includes computing a reference distance between the plurality of sensing devices and the section of the pipe. Furthermore, the method includes comparing a distance between each sensing device and the pipe with the reference distance. The method also includes generating at least one of a plurality of alerts when the reference distance is greater than at least one of a distance between at least one sensing device and the pipe or an average distance between sensing devices of at least one array and the pipe. 
    
    
     
       DRAWINGS 
       Other features and advantages of the present disclosure will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of certain aspects of the disclosure. 
         FIG. 1  illustrates a typical oil and gas exploration system that includes blowout preventers; 
         FIG. 2  illustrates a system for determination of a position of a pipe with respect to a BOP stack in an oil and gas exploration system, according to embodiments of the present invention; 
         FIG. 3  illustrates a system for determination of a position of a pipe in a blowout preventer, according to one embodiment of the present invention; 
         FIG. 4  illustrates a system for determination of a position of a pipe in a blowout preventer, according to another embodiment of the present invention; and 
         FIG. 5  illustrates a flowchart of a method for determination of position of pipe in a blowout preventer, according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will be made below in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals used throughout the drawings refer to the same or like parts. 
     Embodiments of the present invention provide for a system and method for determination of a position of a drill pipe in a blowout preventer (BOP). In oil and gas exploration system, drilling rigs are installed to drill through the sea surface and extract oil stored in the sea bed. The drilling process involves disposing multiple pipe sections to form pipe lengths that can stretch for multiple kilometers along with drill bits to drill through the sea bed. Pipes are installed in the drilling rigs to pump out the oil and gas discovered during drilling. Further pipes are also utilized to carry the waste material being cut by the drill bits and deposit it back in the sea bed. BOPs are installed around these pipes to prevent damage of equipment present on the sea floor caused by kicks and blowouts during drilling. The BOP, according to many embodiments, includes shear rams that can be electrically and/or hydraulically actuated. The rams are configured to sever the drill pipes when a blowout occurs. However, on certain occasions the shear rams may encounter pipe joints, which have a larger diameter than the remaining pipe, and may not be able to sever the pipe joints in the event of a kick. Further, BOPs installed with sensors to determine location of the pipe with respect to the shear rams may produce incorrect responses when characteristics of the fluid flowing the pipe changes. While the forthcoming paragraphs describe the method and system with respect to a shear ram, it may be obvious that the present embodiments may be applied to BOPs that include blind rams, pipe rams, annular rams, and the like. 
     Embodiments of the present invention, as described in the forthcoming paragraphs, provide for a method and system to detect the position of a pipe with respect to the BOP while eliminating the incorrect responses that may be caused due to presence of fluids. Further, embodiments of the system for determination of the position of pipe also detect the presence of pipe joints in the BOP. Accordingly, the present system includes a casing that is configured to be disposed circumferentially around an outer surface of a section of the pipe to be monitored. The length of the casing is selected to be longer than that of the section of interest of the pipe. The system further includes a plurality of sensing devices. The plurality of sensing devices are arranged to form a plurality of arrays of sensing devices. The arrays are arranged circumferentially on the casing and are placed along the length of the casing. The arrangement is made such that the plurality of sensing devices cover the length of the section of the pipe to be monitored and also cover the circumference of the section of the pipe at multiple locations. The sensing devices are configured to generate position signals that determine the position of the pipe with respect to each of the sensing devices. The position signals generated by the sensing devices are transmitted to a processing unit. The processing unit is configured to compare distances of the section of the pipe with respect to each of the plurality of sensing devices. Further, the processing unit is configured to generate a first alert when the distance between the section of interest of the pipe and at least one sensing device in any of the plurality of arrays is different from a reference distance. Furthermore, the processing unit is configured to generate a second alert when the distance between the section of interest of the pipe and each sensing device within at least one array is different from the reference distance. The reference distance is an expected distance between the section of interest of the pipe and sensing devices. The expected distance is a distance between the section of interest of the pipe and the sensing devices, when the pipe is parallel to the BOP stack and when the section of interest does not include a pipe joint. 
     A traditional offshore oil and gas installation  100 , as illustrated in  FIG. 1 , includes a platform  102  (or any other type of vessel at the water surface) connected via a riser/drill pipe  104  to a wellhead  106  on the seabed  108 . It is noted that the elements shown in  FIG. 1  are not drawn to scale and no dimensions should be inferred from relative sizes and distances illustrated in  FIG. 1 . 
     Inside the drill pipe  104 , as shown in the cross-section view, there is a drill string  110  at the end of which a drill bit (not shown) is rotated to extend the subsea well through layers below the seabed  108 . Mud is circulated from a mud tank (not shown) on the drilling platform  102  through the drill string  110  to the drill bit, and returned to the drilling platform  102  through an annular space  112  between the drill string  110  and a protective casing  114  of the drill pipe  104 . The mud maintains a hydrostatic pressure to counter-balancing the pressure of fluids coming out of the well and cools the drill bit while also carrying crushed or cut rock to the surface through the annular space  112 . At the surface, the mud returning from the well is filtered to remove the rock and debris and is recirculated. 
     During drilling, gas, oil or other well fluids at a high pressure may burst from the drilled formations into the drill pipe  104  and may occur at unpredictable moments. In order to protect the well and/or the equipment that may be damaged, a blowout preventer (BOP) stack  116  is located close to the seabed  108 . The BOP stack may also be located at different locations along the drill pipe  104  according to requirements of specific offshore rigs. The BOP stack may include a lower BOP stack  118  attached to the wellhead  106 , and a Lower Marine Riser Package (“LMRP”)  120 , which is attached to a distal end of the drill pipe  104 . During drilling, the lower BOP stack  118  and the LMRP  120  are connected. 
     A plurality of blowout preventers (BOPs)  122  located in the lower BOP stack  118  or in the LMRP  120  are in an open state during normal operation, but may be closed (i.e., switched to a close state) to interrupt a fluid flow through the drill pipe  104  when a “kick” occurs. Electrical cables and/or hydraulic lines  124  transport control signals from the drilling platform  102  to a controller  126 , which may be located on the BOP stack  116 . The controller  126  and the BOP stack  116  may also be at remote locations with respect to each other. Further, the controller  126  and the BOP stack  116  may be coupled by wired as well as wireless networks that aid transfer of data between them. The controller  126  controls the BOPs  122  to be in the open state or in the closed state, according to signals received from the platform  102  via the electrical cables and/or hydraulic lines  124 . The controller  126  also acquires and sends to the platform  102 , information related to the current state (open or closed) of the BOPs  122 . 
       FIG. 2  illustrates a system  200  for determination of a position of a pipe with respect to a BOP stack in an oil and gas exploration system, according to embodiments of the present invention. The oil and gas exploration system includes the system  200 , a drill pipe  214 , BOP stack  212 , a controller  216 , and hydraulic/electric lines  218  that couple the platform  102  to the controller  216  of the BOP stack  212 . The system  200 , according to certain embodiments, further includes a casing  202 , a plurality of sensing devices  204 , and a processing unit  206 . The casing  202  is configured to be disposed around a section of the drill pipe  214  that needs to be monitored. The section of the pipe  214  to be monitored, according to one embodiment, may be the section of the pipe  214  present in the BOP stack  212 . The casing  202  may be disposed around the section of interest of the pipe  214  when the pipe  214  is stationary. Further, the casing  202  may be disposed on the walls of the BOP stack  212  that face the pipe  214  when the pipe  214  is in motion. In other words, the casing  202  may be disposed in the BOP stack  212  such that the section of the pipe  214  present in the BOP stack  212  is covered by the casing  202 . In some other embodiments, the casing  202  may be disposed on a region of a stationary protective casing, such as the protective casing  114 , that is covered by the BOP stack  212 . According to certain embodiments, the casing  202  may have an adjustable length and the length of the casing  202  may be selected based on the length of the section of the pipe  214  to be monitored. The length of the casing  202  is selected such that it is greater than or equal to the length of the section of pipe to be monitored. Moreover, when the casing  202  is placed in the BOP stack  212 , the length of the casing  202  may be greater than or equal to the length of the BOP stack  212 . The casing  202 , according to certain embodiments, is a sheet made from a flexible material. Examples of flexible materials include, but are not limited to, elastomeric materials, rubber, fabrics, or any other suitable flexible materials. Adhesive materials may be disposed on two ends of the sheet such that when the two ends of the sheet are joined, they form a hollow cylindrical structure that is utilized as the casing  202 . According to certain other embodiments, the casing  202  may be made from a rigid material. The casing  202  may be a hollow cylinder made from rigid material that may be placed along the outer surface of the pipe  214  or the inner surface of the BOP stack  214 . 
     The sensing devices  204  are configured to generate a plurality of position signals. The sensing devices  204  may include transducers that are configured to generate signals that are incident on the pipe  214 . The section of the pipe  214  that is exposed to the incident signals from the sensing devices  204  causes the signals to deflect and/or reflect. The changes caused by the section of interest of the pipe  214  are referred to as the response of the section of interest to the signals. The position signals include a response of the section of the pipe to the incident signals. Examples of sensing devices  204  may include, but are not limited to, ultrasound sensing devices, a radio frequency identification transmitter and token pair, and the like. The sensing devices  204  can be unidirectional as well as bi-directional. Bi-directional sensing devices  204  are configured to generate the signals incident on the pipe  214  and further receive the response from the section of interest of the pipe  214 . Further, the sensing devices  204  are disposed on the casing  202  along the length of the casing  202  that is parallel to the direction of movement of the pipe  214  (from the platform  102  to the sea floor  108 ). The sensing devices  204  are grouped to form a plurality of arrays of sensing devices. One example of an array of sensing devices  204  is illustrated as reference numeral  220  in  FIG. 2 . Each array of sensing devices includes multiple sensing devices  204  that are placed proximate to one another to form a series of sensing devices  204 . The arrays of sensing devices are placed along the length of the casing  202 . According to one embodiment, when the casing  202 , along with the sensing devices  204 , is disposed on the outer surface of the section of the pipe  214  each sensing device  204  in an array of sensing device is configured to monitor the same portion along the length of the section of the pipe  214 . For example, the sensing devices  204  in the array  220  are configured to monitor a section  222  of the segment of the pipe  214  present in the BOP stack  212 . The section  222  is perpendicular to the length of the pipe  214 . The signals produced by the plurality of sensing devices  204  are incident on the section of the pipe  214  being monitored. The sensing devices  204  are further configured to receive the responses (position signals) of the section of interest of the pipe  214  to the transmitted signals. The position signals are transmitted to the processing unit  206 . 
     The processing unit  206 , in certain embodiments, may comprise one or more central processing units (CPU) such as a microprocessor, or may comprise any suitable number of application specific integrated circuits working in cooperation to accomplish the functions of a CPU. The processor  206  may include a memory. The memory can be an electronic, a magnetic, an optical, an electromagnetic, or an infrared system, apparatus, or device. Common forms of memory include hard disks, magnetic tape, Random Access Memory (RAM), a Programmable Read Only Memory (PROM), and EEPROM, or an optical storage device such as a re-writeable CDROM or DVD, for example. The processing unit  206  is capable of executing program instructions, related to the determination of position of the pipe in the BOP, and functioning in response to those instructions or other activities that may occur in the course of or after determining the position of the pipe. Such program instructions will comprise a listing of executable instructions for implementing logical functions. The listing can be embodied in any computer-readable medium for use by or in connection with a computer-based system that can retrieve, process, and execute the instructions. Alternatively, some or all of the processing may be performed remotely by additional processing units  206 . 
     The processing unit  206  is configured to compute a distance between each sensing device  204  and the section of the pipe  214  being monitored. The distance between the sensing device  204  and the section of interest of the pipe  214  is computed through the plurality of position signals. Further, the processing unit  206  is configured to compare the distance between each sensing device  204  and the section of the pipe  214  being monitored. Based on the comparison of the distances between the sensing devices  204  and the section of the pipe  214  being monitored, the processing unit  206  is configured to generate a plurality of alerts. The plurality of alerts include a first alert that is generated when the distance determined between at least one sensing device  204  and the pipe  214  is different from a reference or expected distance between the pipe  214  and the sensing devices  204 . The alerts also include a second alert that is generated when the distance between the pipe  214  and each sensing device  204  within at least one array of sensing devices is different from the reference distance between the pipe  214  and the sensing devices  204 . 
     The reference or expected distance between the sensing devices  204  and the section of interest of the pipe  214  that is utilized to generate the first and second alert, may be provided to the processing unit  206  through various channels. These channels include, but are not limited to, an input from an operator, a predetermined distance determined from a reference pipe, and dynamic determination by the processing unit  206 . Dynamic determination of the reference or expected distance by the processing unit  206  includes selecting an actual distance between the pipe  214  and one of the sensing devices  204  as the expected distance. To select one of the actual distances as the expected distance, the processing unit  206  may be configured to select a first set of sensor arrays from the plurality of arrays. The first set of sensor arrays includes those sensor arrays where the distance between the pipe  214  and each sensing device  204  within those arrays is equal. For example, during dynamic determination, the processing unit  206  may be configured to select the sensor array  220  to be one of the first set of arrays. The sensor array  220  is such that the distance between the pipe  214  and each sensing device  204  of the sensor array  220  is equal. Further, the processing unit  206  may also select sensor array  224  to be one of the first set of sensor arrays if the distance between each sensing device  204  of the array  224  and the pipe  214  is equal. Furthermore, the processing unit  206  compares the average distance observed by each array from the first set of arrays. For example, the average distance observed by the array  220  is compared with the average distance observed by the array  224  in the first set of sensor arrays. The processing unit  206  is further configured to select the average distance that is the largest among the average distances from the first set of sensor arrays as the reference or expected distance. For example, the average distance observed by the array  220  may be selected as the expected distance when the average distance of array  220  is greater than or equal to the average distance observed by the other array  224  in the first set of arrays. The processing unit  206 , thus, is configured to select the distance between the array  220  and the pipe  214  as the expected distance, when the array  220  is placed to detect a section of the pipe  214  that has the least diameter in comparison with the rest of the pipe  214 . For example, the array  220  may be disposed such that it is placed proximate to a section of the pipe that does not include a pipe joint. Whereas, the array  224  may be disposed such that it is proximate a pipe joint of the pipe  214 . In such a scenario, in dynamic determination of the expected distance, the processing unit  206  is configured to select the distance between the array  220  and the pipe  214  as the expected distance. 
     The first and the second alert, according to one embodiment, may represent at least one condition associated with the pipe  214 . The first alert, generated when one sensing device  204  of an array shows a measurement that is different from the other sensing devices  204  of that particular array, indicates that they pipe  214  may have displayed lateral movement. In other words, the first alert may be generated when the pipe  214  displays movement from the center of the protective casing  114  and/or the casing  202  towards one of the walls of the protective casing  114  and/or casing  202 . The processing unit  206 , while generating the first alert, compares the distance between each sensing device  204  and the pipe  214  to the expected distance. When the processing unit  206  determines, for a particular sensor array, that the distance between any one of the sensing devices  204  of that array and the pipe  214  is less than the distance between the remaining sensing devices  204  of that array and the pipe  214  or the expected distance, it generates the first alert. The second alert is an indication of the presence of a pipe joint in an operating range of the sensing devices  204  of the system  200 . The array of sensing devices  200  are positioned such that the distance between two sensing arrays is greater than the length of the pipe joint. To generate the second alert, the processing unit  206  compares an average distance between each array and the pipe  214  with the expected distance. If the processing unit  206  determines that the average distance between each array and the pipe  214  is equal to the expected distance, it is concluded that the sensing devices  204  are not in the vicinity of any pipe joint. Further, if the processing unit  206  determines that a difference between the average distance for each array and the expected distance is within a specified range, it is concluded that the sensing devices  204  are not in the vicinity of any pipe joint. Furthermore, if the processing unit  206  determines that a difference between the average distance for each array and the expected distance is greater than the specified range, it is concluded that at least one array is in the vicinity of a pipe joint. The processing unit  206  concludes that the array for which the average distance is the least among the average distance for all arrays is in the vicinity of a pipe joint. The processing unit  206 , thus, generates the second alert indicating that a particular array from the system  200  is in the vicinity of a pipe joint. The specified range for difference between the expected distance and the average distance is selected to be less than the difference between the diameter of a normal section of the pipe  214  and the diameter of the pipe joint. 
     The processing unit  206  is further communicably coupled with controller  216 . The controller  216 , based on the alerts generated by the processing unit  206 , may be configured to take corrective actions based on the position of the pipe with respect to the BOP stack  212 . Further, the processing unit  206  and/or controller  216  may communicate the alerts to the platform  102  through the hydraulic/electric lines  218 . Corrective actions may be initiated from the platform  102  when the position of the pipe  214  with respect to the BOP stack  212  is not as desired. For example, the platform  102  may cause the pipe  214  to move in a direction that is orthogonal to the platform  102  when the first alert is generated. Further, the platform  102  may also cause the pipe  214  to move further in a direction towards the sea floor when the second alert is generated. The controller  216  may also be configured to modify the actuation of the BOP rams when either the first or the second alert are generated, thereby avoiding the ram to attempt shearing the pipe  214  at the pipe joint location. 
     The system further includes a data repository  208  that is coupled to the processing unit  206 . The data repository  208  is configured to store prior pipe distances computed between the pipe and the sensing devices  204 . Further, the data repository  208  is also configured to store the expected distance between the pipe  214  and the sensing devices  204 . The processing unit  206  may also be configured to adjust the distance determined between each sensing device  204  and the pipe  214  with a compensation factor. The compensation factor may be dependent on characteristics of the fluid present between the space between the pipe  214  and the casing  202 , or presence of foreign material in the space between the pipe  214  and the casing  202 . The compensation factor helps in eliminating or reducing false alerts that may be generated by the processing unit  206  because of a change in the fluid characteristics in the pipe  214  as opposed to a comparison between distance of the pipe  214  with respect to the sensing devices  204  and the expected distance. The processing unit  206  compares the distance between each sensing device  214  and the pipe  202  with the expected distance between the sensing devices  214  and the pipe  202 . The difference between each sensing device  204  and the pipe  214  and the expected distance is considered as the offset or gain factor. The offset or gain factor is communicated to the calibration unit  210 . The calibration unit  210  adjusts subsequent measurements of each sensing device  204  with the appropriate compensation factor for each sensing device  204 . Subsequent measurements of the sensing devices  204  are compared with the expected distance to a need for compensation in measurement. 
     Exemplary configurations of the system for determination of a position of the pipe  214  in the BOP stack  212 , based on different type of sensing devices  204 , are explained in conjunction with  FIGS. 3 and 4 . 
       FIG. 3  illustrates an exemplary embodiment  300  of a system for determination of the position of a pipe  214  with respect to the BOP stack  212 . The system  300  includes a casing  302 , a plurality of sensing devices  304 , and a processing unit  306 . The casing  302 , as described in connection with  FIG. 2 , may be made from flexible materials or from rigid materials and is configured to be disposed around the outer surface of the section of the pipe  214  that is being monitored. In certain embodiments, the casing  302  is disposed around the inner surface of the BOP stack  212  such that a sections of the pipe  214  that are present in the BOP stack  212  when the pipe  214  is moving can be monitored. In the illustrated embodiment, the section of the pipe  214  that is being monitored is present in the BOP stack  212 . 
     Further, in the illustrated embodiment, the sensing devices  304  are disposed on the casing  302 . The sensing devices  304  are arranged on the casing  302  to form a plurality of arrays of sensing devices  308 ,  310 , and  312 . Each array of sensing devices  308 ,  310 , and  312  include one or more sensing devices  304  that are placed in a plane orthogonal to the length of the pipe  214 . The casing  302 , in one embodiment, is wrapped around the section of interest of the pipe  214 . The casing  302  is sealed at ends to define a cylindrical structure that is disposed around the pipe  214 . In another embodiment, the casing  302  provides for an opening to allow the pipe  214  to be surrounded by the walls of the casing  302 . When the casing  302  is wrapped around the pipe  214 , each array  308 ,  310 , and  312  encompasses a portion of the pipe in a circumferential fashion. Further, the arrays  308 ,  310 , and  312  are spaced apart from each other along the length of the casing  302  that is parallel to the direction of movement of the pipe  214  (from the platform  102  to the sea floor  108 ). During operation, when the casing  302  is disposed on the pipe  214 , the arrays  308 ,  310 , and  312  of the sensing devices  304  cover the length of the section of the pipe  214  being monitored as well as the circumference of the section of interest of the pipe  214 . The sensing devices  304  are configured to determine the distance between the sensing devices  304  and the pipe  214 . The sensing devices  304 , according to certain embodiments, may be unidirectional or bidirectional ultrasound sensing devices. 
     The sensing devices  304 , when provided with excitation signals, are configured to transmit signals that are incident on the pipe  214 . The signals get deflected and/or reflected from the surface of the pipe  214 . This signal response of the pipe  214 , also termed as position signal, to the signals transmitted by the sensing devices  304  is captured by the sensing devices  304 . The position signals are transmitted to the processing unit  306  that is configured to determine the distance between the pipe  214  and each sensing device  304 . 
     The processing unit  306  determines the distance between the pipe and each sensing device  304 , for example, by the time taken by the respective sensing device  304  to collect the reflections of the input signals from the pipe surface. The processing unit  306  is further configured to generate a plurality of alerts based on the analysis of distances between the pipe  214  and each sensing device  304 . In operation, the processing unit  306  compares the distance between each sensing device  304  and the pipe  214  with a reference or expected distance to generate the plurality of alerts. Specifically, the processing unit  306  generates a first alert when the distance between at least one sensing device  304  and the pipe is different from the reference distance. The second alert, on the other hand, is generated when the distance between the pipe and each sensing device  304  of at least one array  308 , or  310 , or  312  is different from the reference distance. 
     In one embodiment, the processing unit  306  receives the reference distance from the operator through a user interface. Further, the reference distance may also be determined from a reference pipe and provided to the processing unit  306 . Furthermore, the processing unit  306  may also dynamically determine the reference distance from the present distances determined between the sensing devices  304  and the pipe  214 . In dynamic determination, the processing unit  306  selects one of the actual distances between the sensing devices  304  and the pipe  214 . To select one of the actual distances as the expected distances, the processing unit  306  determines a first set of arrays from the plurality of arrays  308 ,  310 , and  312 . The first set of arrays includes an array where the distance between the pipe  214  and each sensing device  304  of that particular array is equal. For example, the first set of arrays may include sensor arrays  308  and  310  when the distance between each sensing device  304  of the array  308  and the pipe  214  is equal and the distance between sensing devices  304  of the array  310  and the pipe  214  is equal. Further, the processing unit  306  compares the average distance observed by each array from the first set of arrays. For example, the average distance observed by the array  308  is compared with the average distance observed by the other array  310  in the first set of arrays. The processing unit  306  is further configured to select the average distance that is greater than remaining average distances from the first set of arrays as the reference or expected distance. For example, the average distance observed by the array  308  may be selected as the expected distance when the average distance of array  308  is greater than or equal to the average distance observed by the other array  310  in the first set of arrays. The processing unit  306 , thus, is configured to select the distance between the array  308  and the pipe  214  as the expected distance, when the array  308  is positioned to detect a section of the pipe  214  that has the least diameter in comparison with the rest of the pipe  214 . For example, the array  308  may be disposed such that it is placed proximate to a section of the pipe that does not include a pipe joint. Whereas, the array  310  may be disposed such that it is proximate a pipe joint of the pipe  214 . In such a scenario, in dynamic determination of the expected distance, the processing unit  306  is configured to select the distance between the array  308  and the pipe  214  as the expected distance. 
       FIG. 4  illustrates another exemplary embodiment  400  of a system for determination of the position of a pipe in a BOP. The system includes a casing  402 , a plurality of sensing devices  404 , a processing unit  406 , and an identification token  408 . The sensing devices  404  are disposed on the casing  402  to define a plurality of arrays  410 ,  412 , and  414  of sensing devices  404 . The casing  402  is disposed on an outer surface of the section of the pipe  214  being monitored. The identification token  408  is placed at a predetermined location on the section of the pipe being monitored. The identification token  408  may be an active token as well as a passive token. 
     Each sensing device  404 , according to one embodiment, includes a transceiver that is configured to transmit interrogation signals to the section of the pipe  214  being monitored. In one embodiment, the interrogation signals may be radio frequency (RF) signals that are incident on the pipe  214  being monitored. The identification token  408  placed at the predetermined position on the pipe  214  being monitored, receives the transmitted interrogation signal and generates a response to the transmitted signal. The response, termed as position signals, is communicated to the processing unit  406 . The processing unit  406  is configured to determine the distance between the pipe and the sensing devices  404  based on the position signals. According to one embodiment, the processing unit  406  is configured to compute the distance between each sensing device  404  and the pipe  214  using the strength of the position signals received by the sensing devices  404 . The processing unit  406  may also include a plurality of signal processing components that are configured to eliminate noise from the position signals received from the sensing devices  404 . Further, the processing unit  406  may be configured to compute the distance between the sensing devices  404  and the pipe  214  by measuring a time taken to receive the position signal at each sensing device  404  from the token  408 . 
     In the case where identification tokens  408  are active identification tokens, the identification tokens  408  are configured to periodically transmit position signals to the sensing devices  404 . The processing unit  406  is configured to determine the distance between the sensing device  404  and the pipe  214  based on the strength of the position signals received by each sensing device  404 . 
     During operation, each sensing device  404  generates a signal directed towards the identification token  408  and receives a position signal from the identification token  408 . The processing unit  406  computes the distance between the pipe  214  and the sensing device  404  based on each position signal. Further, the processing unit  406  determines a reference distance for monitoring the pipe  214 . The reference distance is computed from the distance between each sensing device  404  and the pipe  214 . The processing unit  406  is further configured to generate alerts based on a comparison between the distance between the sensing device  404  and the pipe  214  and the reference distance. 
       FIG. 5  illustrates a flow diagram of a method for determination of a position of a pipe  214  in a BOP stack  212 . At  502 , the method includes receiving a plurality of position signals from a plurality of sensing devices. The plurality of position signals are generated as a response to an input signal generated by each of the plurality of sensing devices that is incident on the pipe being monitored. The sensing devices are disposed on a casing that is disposed on an outer surface of the pipe being monitored. The sensing devices are arranged on the casing to define a plurality of arrays of sensing devices. The arrays of sensing devices are arranges such that each array covers the pipe circumferentially and the arrays of sensing device cover the length of the casing. 
     Further, at  504 , a reference distance between the sensing devices and the pipe is computed. The reference distance between the sensing devices and the pipe is computed based on the determined distance between each sensing device and the pipe. The distance that is greatest among the determined distances may be selected as the reference distance. Further, at  506 , the method includes comparing the distance of each sensing device with respect to the pipe with the reference distance. At  508 , the method includes generating alerts when the reference distance is greater than the distance between at least one of the plurality of sensing devices and the pipe or when the reference distance is greater than the average of distances between sensing devices of at least one array of sensing devices and the pipe. 
     Various embodiments described above thus provide for a method and a system for determination of a position of a pipe in a blowout preventer. The system for determination generates alerts for a change in position caused by lateral and/or angular movement of the pipe within the BOP. Further, the system also generates an alert when a portion of the pipe that is larger in diameter than the remaining pipe is present in the BOP. The system includes dynamic determination of the reference distance, thus taking into account offsets caused in each sensing device due to the presence of foreign material that may interfere with the response signals from the pipe. Further, the system includes a self-calibration mechanism that allows for the system to be efficient and useful for determination of position of pipes even when the overall diameter of the pipe in the BOP changes. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its scope. While the dimensions and types of materials described herein are intended to define the parameters of the invention, they are by no means limiting and are exemplary embodiments. Many other embodiments will be apparent to those of ordinary skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” etc. are used merely as labels, and are not intended to impose numerical or positional requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure. 
     This written description uses examples to disclose several embodiments of the invention, including the best mode, and also to enable any person of ordinary skill in the art to practice the embodiments of invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. 
     Since certain changes may be made in the above-described system and method for determination of position of a pipe in a BOP, without departing from the spirit and scope of the invention herein involved, it is intended that all of the subject matter of the above description or shown in the accompanying drawings shall be interpreted merely as examples illustrating the inventive concept herein and shall not be construed as limiting the invention.