Abstract:
Blowout preventer (BOP) stack and lower marine riser package for sealing an undersea well. The blowout preventer stack includes a frame extending along an axis and configured to be attached to a head of the well; an accumulator attached to the frame and configured to provide high pressure; two or more BOPs attached to the frame, the two or more BOPs being disposed within the frame and attached one on top of the other along the axis; each BOP having a body and a pair of bonnets attached to the body, where the bonnets are configured to be detachably attached to the body; each BOP having a corresponding cavity extending along the axis through which a fluid from the well is circulating; each BOP being configured to seal the well when the high pressure from the accumulator is released to the BOPs; plural brackets attached to the body of at least one BOP; and a safety bar attached to the plural brackets and configured to partially encircle the body of the at least one BOP.

Description:
BACKGROUND 
     1. Technical Field 
     Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to techniques for providing a safety mechanism for a blowout preventer. 
     2. Discussion of the Background 
     A subsea blowout preventer (“BOP”) stack is used to seal a wellbore during drilling operations, both for safety and environmental reasons. As shown in  FIG. 1 , a drilling system  10  used for extracting oil and/or gas from under the sea floor may include a lower blowout preventer stack (“lower BOP stack”)  14 , which may be rigidly attached to a wellhead  16  upon the sea floor  12 , while a Lower Marine Riser Package (“LMRP”)  18  is retrievably disposed upon a distal end of a marine riser  20 , extending from a drill ship  22  or any other type of surface drilling platform or vessel. As such, the LMRP  18  may include a stinger  24  at its distal end configured to engage a receptacle  26  located on a proximal end of the lower BOP stack  14 . 
     In typical configurations, the lower BOP stack  14  may be rigidly affixed atop a subsea wellhead and may include (among other devices) a ram-type blowout preventer (BOP)  28  useful in controlling the well as it is drilled and completed. Similarly, the LMRP  18  may be disposed upon a distal end of a long flexible riser  20  that provides a conduit through which drilling tools and fluids may be deployed to and retrieved from the subsea wellbore. Ordinarily, the LMRP  18  may include (among other things) a ram-type blowout preventer at its distal end such that, when desired, ram-type blowout preventers of the LMRP  18  and the lower BOP stack  14  may be closed and the LMRP  18  may be detached from the lower BOP stack  14  and retrieved to the surface, leaving the lower BOP stack  14  atop the wellhead. 
     During the operation of the lower BOP stack  14  and/or the LMRP  18 , various incidents or simply maintenance events may occur that require access to the BOP  28 . For this purpose, the well might be shut down and the part necessary to be worked on may be lifted to the vessel  22 . If the equipment necessary for repairing the BOP  28  is not available on the vessel  22 , the BOP  28  is shipped to dry locations where the equipment is available. As the BOP  28  is connected to other parts of the lower BOP stack  14  and/or the LMRP  18 , which parts may be heavy and linked to the BOP in a complicated manner, it is the practice to remove the whole lower BOP stack  14  and/or LMRP  18  and bring it to the surface for repairs or inspection. 
     Thus, the weight of the entire lower BOP stack may be in the range of millions of kg and a height of the stack may be up to 10 m. Especially when the repair and/or inspection of the lower BOP stack  14  and/or LMRP  18  is performed on the vessel  22 , the sea waves may sometimes tilt these devices so violently that maintenance personnel working around the lower BOP stack  14  and/or LMRP  18  may get injured or even lose their lives. 
     Accordingly, it would be desirable to provide systems that avoid the afore-described problems and drawbacks. 
     SUMMARY 
     According to one exemplary embodiment, there is a blowout preventer (BOP) stack and lower marine riser package for sealing an undersea well. The blowout preventer stack includes a frame extending along an axis and configured to be attached to a head of the well; an accumulator attached to the frame and configured to provide high pressure; two or more BOPs attached to the frame, the two or more BOPs being disposed within the frame and attached one on top of the other along the axis; each BOP having a body and a pair of bonnets attached to the body, wherein the bonnets are configured to be detachably attached to the body; each BOP having a corresponding cavity extending along the axis through which a fluid from the well is circulating; each BOP being configured to seal the well when the high pressure from the accumulator is released to the BOPs; plural brackets attached to the body of at least one BOP; and a safety bar attached to the plural brackets and configured to partially encircle the body of the at least one BOP. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a schematic diagram of a conventional offshore rig; 
         FIG. 2  is a schematic diagram of a lower BOP stack according to an exemplary embodiment; 
         FIG. 3  is a schematic diagram of plural BOPs according to an exemplary embodiment; 
         FIG. 3A  is an enlarged view of the three BOPs of  FIG. 3 . 
         FIG. 4  is a longitudinal cross section through a body of a BOP according to an exemplary embodiment; 
         FIG. 5  is a detailed view of a corner of a body of a BOP and a safety bar according to an exemplary embodiment; 
         FIG. 6  is a transversal cross section through a body of a BOP according to an exemplary embodiment; 
         FIG. 7  is a schematic diagram of a safety bar with a t-slot according to an exemplary embodiment; 
         FIG. 8  is a schematic diagram of a safety bar having a spherical profile portion according to an exemplary embodiment; and 
         FIG. 9  is a schematic diagram of a safety bar having a rectangular profile portion according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of BOP stacks. However, the embodiments to be discussed next are not limited to these systems, but may be applied to other systems that require the presence of a person at a high height or in an unstable environment, as for example, the LMRP. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     As discussed above with regard to  FIG. 1 , the lower BOP stack  14  and/or LMRP  18  may include plural BOPs  28  and other components for performing the undersea drilling. An example of a lower BOP stack  14  is shown in  FIG. 2  and the lower BOP stack  14  includes, among other things, a frame  40 . The same structure may be used in a LMRP and for this reason only the lower BOP stack  14  is discussed next. The BOPs  28  are not visible in  FIG. 2 . However, one or more accumulators  42  and plural ports  44  are visible. At the bottom of frame  40  there is a head  46  which is configured to be attached to wellhead  16 . 
     A more detailed view of the lower BOP stack  14  is shown in  FIG. 3 , in which plural BOPs  28 - 1  to  28 - 3  are shown assembled on top of each other along an axis Z. Each BOP has a cavity (not shown) extending along the axis Z. A fluid from the well is configured to pass these cavities while being pumped to the vessel. The plural BOPs  28 - 1  to  28 - 3  are provided inside frame  40  and they may extend 10 m in height and about 3 m wide. When maintenance to one or more of the BOPs  28 - 1  to  28 - 3  is performed, specialized personnel build a scaffold around the lower BOP stack  14  and perform part of the maintenance operations from the scaffold and part of the operations by directly stepping on the BOPs. This approach may be dangerous as the maintenance personnel may slip and/or trip on the BOP, thus potentially falling down from the BOPs. Given the relative high heights of the BOPs, such a fall may be lethal. This danger is further amplified when maintenance is performed on the rig or vessel, which may experience unstable conditions due to motion caused by ocean waves. 
     According to an exemplary embodiment, at least one BOP  28  of the plural BOPs may be provided with a safety bar  50  that is firmly (permanently) attached to a body of the BOP. Having the safety bar  50  provided around the BOP, the maintenance personnel may attach a safety clip (not shown) to the safety bar  50 , thus, securing a harness that is worn by the maintenance personnel to the safety bar  50 . For example, the safety bar  50  may be welded, screwed or otherwise attached to the body of the BOP  28 - 2  as would be recognized by those skilled in the art. According to an exemplary embodiment, the safety bar  50  is provided to completely encircle the body of the BOP  28 - 2 , as shown in  FIG. 4 . BOP  28 - 2  is selected as an example but any of the BOPs may be used.  FIG. 4  shows the body  52  of the BOP  28 - 2  having extensions  54  that are configured to accommodate internal movable parts of the BOP, e.g., ram blocks, ram block pistons, etc. The safety bar  50  is attached to the body  52  of the BOP  28 - 2  via brackets  56 . However, according to an exemplary embodiment, the safety bar  50  is provided only partially around body  52  of the BOP  28 - 2 . 
     Referring to  FIG. 3A , although safety bar  50  is not mounted to the upper BOP  28 - 3 , the upper surface  51  of body  52  of BOP  28 - 3  is the same as that of middle BOP  28 - 2 . A cylindrical flange  53  of a next upward element bolts to upper surface  51 . The cylindrical flange  53  that bolts to upper surface  51  of BOP  28 - 2  connects BOP  28 - 2  to upper BOP  28 - 3 . The upper portion of body  52  is rectangular, thus cylindrical flange  53  results in four triangular-shaped upward facing shoulders  55 , one at each corner of body  52 . Threaded holes  57  are formed in each upward facing shoulder  55 . 
     A detailed view of a bracket  56 , according to an exemplary embodiment, is shown in  FIG. 5 , which corresponds to detail A in  FIGS. 3 and 3A .  FIG. 5  shows bracket  56  being fixed by three screws  58  to one of the upward facing shoulders  55  of body  52  of the BOP  28 - 2 . Screws  58  engage the threaded holes  57  ( FIG. 3A ). The three screws  58  are exemplary and more or less screws may be used. Each bracket  56  extends outward from body  52  a short distance along a radial line of the axis of body  52 . The safety bar  50  is fixed to bracket  56  via one or more screws  60 . One skilled in the art would recognize that instead of screwing bracket  56  and safety bar  50  to each other and to the body  52  of the BOP, these elements may be welded in place.  FIGS. 3 and 4  show four different safety bars  50  attached with four brackets  56  to the body  52  of the BOP  28 - 2 . In one application, less or more safety bars may be used. 
     According to an exemplary embodiment shown in  FIG. 6 , the safety bars  50  are placed on an upper part of the BOP  28 - 2 , such that the safety bars  50  are above the extensions parts  54 . One reason for having this placement of the safety bars  50  is to give the maintenance personnel the freedom to work on the extensions parts  54 , which are bonnets that need to be removed in order to reach inside the body  52  to the ram blocks. As the bonnets may weight in the range of hundreds of kg, for example, from 100 to 500 kg, the manipulation of these bonnets, which may be attached by hinges  58  to the body  52 , may trigger the fall of the maintenance personnel. Having their safety clip attached to the safety bar  50 , in the event of an accidental slip off the BOP, the maintenance personnel is still attached to the safety bar  50 , thus preventing a dangerous fall. 
     In addition, the distribution of the safety bar  50  at the top part of the BOP and around the body  52  of the BOP offers the maintenance personnel the freedom to move around the BOP for various maintenance operations. According to another exemplary embodiment, the safety bars  50  may be used to restrain the movement of the maintenance personnel at the periphery of the lower BOP stack  14  while allowing them to work inside the frame  40 . 
     Carbon steel 4130 may be used to produce the safety bars  50 . Carbon steel 75 ksi (ksi=1000 psi) tensile strength may be used for 1-, 1.25- and 2.0-inch diameter safety bars, 95 ksi may be used for 1.50 in and 60 ksi may be used for 1.75 in. These dimensions of the safety bar ensures that the bars are configured to withstand a minimum of 5000 lbs per rig personnel. 
     According to an exemplary embodiment, the safety bar  50  may have a flat band profile having a t-slot  60  machined at a side  62  as shown in  FIG. 7 .  FIG. 7  shows the safety bar  50  disposed with a side  64  next to the body  52  of the BOP  28 - 2 .  FIG. 7  is a cross sectional view of the BOP  28 - 2  through a middle portion of the body  52  such that brackets  56  are not visible. A tie-off ring (not shown) that is attached to a harness strap (not shown) of the maintenance personnel may be provided inside the t-slot  60  for securing the tie-off ring to the safety bar  50 . Providing the t-slot  60  into the safety bars  50 , all the way around the body  52 , ensures that the maintenance personnel is free to move around the BOP  28 - 2 . 
     According to another exemplary embodiment as shown in  FIG. 8 , the safety bar  50  may have a plate shape portion  51  with a round profile  70  at an end portion. A C-clamp  72  may be provided to encircle the round profile  70  such that the C-clamp  72  cannot detach from the safety bar  50 . A tie-off hole  74  may be provided in the C-clamp  72  such that the maintenance personnel may attach their harness to the tie-off hole  74 . Optionally, internal bearings  76  may be provided on an internal face of the C-clamp  72  for facilitating a relative motion of the C-clamp  72  relative to the safety bar  50 . The C-claim  72  may include a mechanism  78  for opening it when there is a need to remove the clamp from the safety bar  50 . 
     According to another exemplary embodiment shown in  FIG. 9 , the C-clamp  72  may be replaced with a square clamp  80  that slides along a square profile  82  of the safety bar  50 . A tie-off hole  74  is provided in a side of the square claim  80  and the internal bearings  76  and mechanism  78  are similar to the embodiment shown in  FIG. 8 . 
     The disclosed exemplary embodiments provide a system for increasing the safety of operating personnel when working on a BOP. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. 
     This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other example are intended to be within the scope of the claims.