Abstract:
A subsea lubricator system is disclosed which includes a lubricator tube adapted to be positioned subsea above a subsea well, a pressure control head adapted to be positioned above the lubricator tube, at least one pressure sensor adapted for sensing at least one of a pressure in the subsea well or an ambient seawater pressure proximate the pressure control head, and at least one pump that is adapted to be positioned subsea to inject a lubricant into the pressure control head at a pressure that is greater than the sensed pressure. A method of operating a subsea lubricator system positioned above a subsea well, the lubricator system including a pressure control head, is also disclosed which includes monitoring at least one of a pressure within the well and an ambient seawater pressure proximate the lubricator system, and injecting a lubricant into the pressure control head at a pressure that is greater than the monitored pressure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a method for controlling grease injection to a subsea intervention system and an apparatus comprising a housing containing a control unit and a pump for grease. 
         [0003]    2. Description of the Related Art 
         [0004]    When performing intervention in a hydrocarbon well, it is necessary to isolate the well from the environment. Intervention is often carried out using wireline techniques (braided wire, composite cable or slickline). To contain the pressure in the well during operations and avoid hydrocarbons escaping to the environment, intervention operations involve the use of a stuffing box which is part of a pressure control head (PCH). The PCH provides a dynamic seal between the cable and the wellbore enclosures to maintain pressure control and prevent wellbore fluids from leaking into the environment. However, because of its braided (wire rope like) exterior, the cable has a bumpy, crevice-filled surface which is difficult for the PCH to seal around as the cable passes through the PCH as it travels into and out of the well. 
         [0005]      FIG. 1  is a schematic drawing showing a prior art subsea lubricator system  100  attached to a subsea well  105 . The subsea well  1055  extends into a subterranean formation and has a Christmas tree  106  attached to the wellhead and a flowline/umbilical  107  extending to a process facility. The subsea lubricator stack  100  includes a pressure control unit (BOP)  111 , a lubricator (pipe)  112  and the pressure control head (PCH)  113 . 
         [0006]    The lubricator system  100  further comprises a control system (IWOCS)  115  with a separate workover umbilical  117  extending to the surface. The control system  115  controls the system  100 . In prior art operations, grease is pumped down the line  117  and further through line  123  to the PCH  113  to maintain a seal between the braided wire or cable  109  and the seawater environment. 
         [0007]    Current practice is to inject grease into the PCH body  113  at a higher pressure than that of the well. In addition, grease has to be replenished at some rate to replace grease lost to the surface of the braided cable  109  as it passes through the ends of PCH  113  (going into or out of the well). The grease injection rate is controlled by periodic visual monitoring of the sealing ends of the PCH  113  for leakage and monitoring the grease injection pressure. 
         [0008]    This operation gets complicated when performing this practice subsea on a subsea well. This involves the use of a subsea riserless light well intervention (RLWI) stack. For RLWI, the PCH  113  is now remote and difficult to monitor; making it difficult to determine when and how much grease needs to be injected. Furthermore, as the stack is run in deeper water, the length of the grease supply line feeding the PCH  113  grows longer, making it increasingly difficult to pump viscous grease down to the PCH  113  at a reasonable surface pressure or pump rate. The long grease lines and viscous grease becomes more problematic as deeper colder environments are encountered. To do that requires pumping grease at some empirical rate monitored visually. In subsea situations, the pumping pressure is exacerbated by the length of the grease line going down to the subsea PCH  113  and the rate is often a pure guess, often resulting in sending too much grease down to conservatively compensate for the unknown conditions. 
         [0009]    Current practice for subsea grease injection requires the surface deployment of grease lines as shown in U.S. Pat. No. 4,821,799, which is hereby incorporated by reference in its entirety. That patent discloses the use of an accumulator to enable a better control of injection pressures. 
         [0010]    There is also a more subtle problem associated with grease injection to a subsea PCH  113 , namely, water ingress. Normally, the PCH  113  is lowered to the lubricator tube  112  together with the tool. However, in some operations, the PCH  113  is run independently after the wireline tools, cable, etc. are landed in the RLWI stack&#39;s lubricator tube  112 . As the PCH assembly is lowered down to the sea floor, the braided cable  109  passes through the PCH  113 . If grease is not supplied at a sufficient pressure and rate to offset the increase in ambient seawater pressure, and the loss of grease to the cable  109  passing by, seawater could weep past the seal ends of the PCH  113  into the main cavity of the PCH  113  and/or the tube  112 . If this occurs, there is an increased risk that the water will help to form a hydrate plug inside the PCH  113  (later exposed to wellbore pressure and fluids) and prevent the cable  109  from freely moving through the PCH  113 . 
         [0011]    The present invention is directed to methods and devices solving, or at least reducing the effects of, some or all of the aforementioned problems. 
       SUMMARY OF THE INVENTION 
       [0012]    The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
         [0013]    The present subject matter is generally directed to a method and a device for controlling grease injection to a subsea intervention system, where there is provided an at site pressure compensated system for providing the grease at a pressure higher than the outside pressure, this being either the well pressure, the pressure of the water around the subsea system, outside pressure, or both of these pressures. 
         [0014]    According to one aspect, the present subject matter may be employed in an intervention workover control system (IWOCS) that is all electric or electro-hydraulic that may comprise a processor with the capability to handle information, for example, to record outside ambient seawater pressure, pressure inside the PCH and below the PCH (inside the well). As mentioned above, the purpose of the grease and PCH is to create a dynamic seal that generates a slightly higher (grease) pressure inside the PCH than the pressure of the environment above the PCH or the pressure in the well below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
           [0016]      FIG. 1  is a schematic depiction of an illustrative prior art subsea lubricator system; 
           [0017]      FIG. 2  shows a sketch of an intervention system on a subsea well; 
           [0018]      FIG. 3  is a diagram showing the grease injection module in IWOCS mode; and 
           [0019]      FIG. 4  is a diagram showing the grease injection module in autonomous mode. 
       
    
    
       [0020]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0021]    Illustrative embodiments of the present subject matter are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
         [0022]    The present subject matter will now be described with reference to the attached figures. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
         [0023]      FIG. 2  is a schematic drawing showing a subsea lubricator system  10  described herein attached to a subsea well  5 . The subsea well  5  extends into a subterranean formation and has a Christmas tree  6  attached to the wellhead and a flowline/umbilical  7  extending to a process facility. The subsea lubricator stack  10  includes a pressure control unit (BOP)  11 , a lubricator (pipe)  12  and the pressure control head (PCH)  13 . 
         [0024]    The lubricator system  10  further comprises a control system (IWOCS)  15  that controls the system  10 . Electrical power is supplied to the control system by electrical power line  17 . In one illustrative embodiment, a grease injection module  21  is attached to the PCH  13 . An electric cable  23  connects the grease injection module  21  with the control system  15 . With the grease injection module  21  attached to the PCH  13 , it can be raised and lowered together with the PCH  13  and operate in an autonomous mode. In another embodiment, the grease injection module  21  is not attached to the PCH  13 , but rather is made as a part of the control system  15 . In that case, an additional fluid line  25  (shown as a dashed line) is employed to supply grease or lubricant to the PCH  13 . In some embodiments, a single line from the control system  15  to the grease injection module  21  may contain both electrical and fluid lines. 
         [0025]    The subject matter disclosed herein proposes the elimination of a grease line (like the grease line  117  shown in  FIG. 1 ) to or from the surface to supply lubricant to the PCH  13 . In one embodiment, as shown in  FIG. 3 , lubricant may be supplied to the PCH  13  by use of a depth compensated accumulator  31  filled with a lubricant or grease. The grease injection module  21  comprises an accumulator  31  for grease operatively connected via line  33  to a pump  35 . The outlet grease line  37  from the pump  35  is connected to the PCH  13 . The pump  35  is controlled by an electric motor  36 . A first power supply cable  32  connects the control system  15  with the electric motor  36  for the pump  35 . The grease line  37  has a one way valve  43 , a shut-off valve  44  and a pressure and temperature sensor  45 . 
         [0026]    In the embodiment shown in  FIG. 3 , there is also provided a second pump  38  with associated motor  39 , having a separate power supply cable  34 . A second grease line  41  connects the pump  38  with the PCH  13 . As above, the second grease line  41  includes a one way valve  46 , a shut-off valve  47  and a pressure and temperature transmitter  48 . The second pump  38  may be added to provide for redundancy in the system, in case of failure of the first pump  35 . Providing dual pumps  35 ,  38  also makes it possible to generate higher grease pumping rates in case of emergency, with both pumps operating together. They may also be used for the rare times when the cable  9  is travelling very quickly through the PCH  13  and may require more grease than one electric motor/pump can supply. 
         [0027]    As an alternative, grease may be wiped from the cable  9  as it passes out of the PCH  13  and returned to a container in the grease injector module  21 . For example, as shown in  FIG. 3 , a return grease line  52  that is in fluid communication with a canister  54  may be provided. In this way, very little, if any, grease will be released to the environment. 
         [0028]    In addition, an ROV attachment  22  may be added to provide a means to periodically replenish the grease in the accumulator  31  for long duration jobs. 
         [0029]    In operation, the control system  15  closely monitors the pressure of the environment outside of the PCH  13 , the pressure inside the PCH  13  and/or the pressure in the well  5 . Periodically, the control system  15  actuates one or both (depending upon the situation) of the grease pumps  35 ,  38  to pump grease into the PCH  13 . The grease pressure is closely monitored and the pump(s)  35  and/or  38  are regulated to generate a very small pressure differential between the PCH  13  and the well  10 , e.g., a differential of approximately 15 psi. Stated another way, the grease is injected at a pressure that is a set or established value above at least one of the monitored pressures. 
         [0030]    The close in-situ monitoring of the various pressures by the control system  15  minimizes the amount of grease or lubricant needed because the differential pressure can be kept to a minimum value, e.g., a 15 psi differential pressure. A lower differential pressure or set value may also be employed. This is a significant benefit as compared to prior art systems where operators merely guessed as to the volume of grease needed, and the associated difficulties trying to pump the grease down a grease line. Keeping the differential pressure or set value to a minimum also lessens the amount of grease that works itself past the seal elements (not shown) in the PCH  13  into the well and/or the environment. By employing two pumps  35  and  38 , the grease may be injected into the PCH  13  in two locations (again opening one or two lines to compensate for situations of high cable speed, rapid loss of grease, etc.). There also may be a third grease injection line  51  in a location below the PCH  13  for better control of the differential pressure between the PCH  13  and the well, if necessary. 
         [0031]    In the embodiment shown in  FIG. 4 , the grease injection module  21  is equipped with its own separate control unit  60  configured as an autonomous version of the control system  15 . The autonomous control unit  60  comprises a processor and data storage (not shown) and is preferably powered by a battery  62 . Thus, the electric control can be separated from the main control system  15 , while retaining the monitoring and injection control features for grease injection into the PCH  13 . This embodiment simplifies the packaging of the PCH  13  assembly by eliminating the need for the subsea electrical connection  23  ( FIG. 2 ) after the PCH  13  is lowered separately and latched to the rest of the intervention (RLWI) stack. However, this autonomous feature adds two new capabilities. First, as the PCH  13  assembly is lowered to the sea floor, it independently monitors the increase in ambient seawater pressure and can adjust by injecting grease into the PCH  13  at just a slightly higher than ambient pressure differential, e.g., a 15 psi differential, to keep seawater from entering the cavity in the PCH  13 , thereby avoiding the hydrate plugging issues. The control unit  60  is battery powered to maintain its autonomy. Second, in the event that the surface vessel needs to depart and/or the cable is cut somewhere outside of the PCH  13  and the control system  15  is disconnected, the grease injection pressure containment feature of the PCH  13  is maintained even though the rest of the control system  15  is shut down, for as long as battery power is present. 
         [0032]    Another issue is the grease itself. Current practice is to use some form of viscous petroleum based grease that has a certain amount of stickiness to adhere to the surface of the seals (not shown) in the PCH  13  and the rough exterior of the cable  9 , creating a pseudo smooth surface on the braided cable. However, this creates its own “leakage to the environment” as the grease laden cable  9  emerges out the top of the PCH  13  during wireline retrieval. In addition, the ambient seawater environment may be as low as 4° C. (39° F.), which may lead to an increase in the grease&#39;s viscosity or lead to a hardening condition. To alleviate this condition, it is contemplated to replace petroleum grease with a bio-degradable, non-hydrocarbon lubricant, such as a fish oil based lubricant, e.g., cod liver oil, so as to significantly lower the viscosity of the lubricant and eliminate hydrocarbon discharge to the environment. 
         [0033]    The benefit of the present invention is that its architecture is substantially depth insensitive, eliminating the pressure flow rate problems associated with pumping viscous grease longer distances (at higher surface pump pressures) and eliminates waste by using environmentally friendly lubricants that are injected at much lower differential pressures because the injection process is monitored. It also eliminates a line going into the water which is beneficial for better line management; critical for deepwater (&gt;500 m˜1500 ft.) operations. 
         [0034]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.