Abstract:
The present invention relates to a device for transmitting fluid from a subsea producing module ( 8 ) to a subsea receiving module ( 9 ) comprising a choke valve ( 3 ) in a main flowpath ( 7 ) of the fluid. A bypass line ( 2 ) is arranged in parallel to the choke valve ( 3 ) and the bypass line ( 2 ) is connected to the main flowpath ( 7 ) by a bypass connection ( 1 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a hydrocarbon production system comprising a subsea well, a subsea production module, a subsea receiving module and a choke valve situated in a main flow path of production fluid from the well, between the production module and the receiving module. 
       BACKGROUND OF THE INVENTION AND PRIOR ART 
       [0002]    In a subsea installation, several wells or Xmas Trees are connected to a common manifold. The pressures in these wells are different, and to be able to transmit oil or gas to the common manifold, the pressure needs to be substantially equal. Thus, a choke valve is installed between each Xmas Tree and the manifold to adjust and control the pressure. 
         [0003]    It is a standard procedure to perform a clean-up process in a Xmas Tree as a basis for commissioning. Traditionally, the clean-up is done through the choke valve. The product of the clean-up can be fluids with different viscosity, and it can also comprise different types and dimensions of debris, e.g. sand, mud etc. This complex clean-up product comprising debris provides a substantial risk of choke malfunction and blockage when transported through the choke valve. There has been a preference, or in some cases even a requirement, from the oil operators that choke valves shall be designed to handle this clean-up process. On the other hand, no existing choke valve vendor can or will give a guarantee that the choke can tolerate this, as it is almost impossible to know what type debris will flow through the choke and what dimension the debris will have. Some choke valves might have a design which can protect the internal parts from debris compared to other choke valves. But still it will always be a risk for blockage of trim ports in the choke valve. 
         [0004]    The requirements for the features of the choke valve are different in the first phase of production of oil/gas than in the later phases of production in the Xmas Tree. Due to the high pressure in the first phase of production, good controllability of the choke valve is typically desired in that phase. In the later phases of the production, the pressure will decrease and the choke valve then needs to have features that provide maximum flow. Both good controllability and maximum flow are features required by the oil operators, but they are not sufficiently present in any choke valves on the market today. Therefore the solution has been to replace the choke valve with good controllability used in the first phase with a different choke valve that provides maximum flow for the later phases. This replacement procedure is a complex procedure that involves use of e.g. an ROV (Remote Operated Vehicle) and it also requires shutting down the installation during the replacement procedure. 
         [0005]    It is known from WO 2006/041820 an operating pressure and waste management apparatus for use during drilling operations. In this apparatus one of two choke valves is used to maintain a pressure in the system. Since the fluid flow out of the well is drilling mud, it will contain a substantial amount of drill cuttings. These will have to pass through the choke valve. As mentioned above there is today no choke valves on the market that are able to handle such an amount of debris without quickly becoming degraded, unless the pressure of the fluid and the flow rate is very low. The drill mud will generally have very low pressure and flow rates compared to production fluids. 
         [0006]    WO 2006/041820 describes a diverter line that may divert the drill mud flow outside of the two choke valves in the unlikely event that they both should fail. The arrangement of WO 2006/041820 would, however, not be considered used in a subsea production system, as the flow rates and the pressure is usually much higher in such a production system. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore a need for a solution that provides a remedy to the above mentioned problems regarding fluid containing debris flowing through a choke valve, providing a risk for choke malfunction and blockage, and the different features required in different phases of the production in a subsea installation. 
         [0008]    The objects of this invention are obtained as defined in the appended claim  1 . 
         [0009]    Advantageous embodiments are obtained by the features defined in the appended dependent claims. 
         [0010]    The objectives set forth above are achieved by providing, in a first aspect of the present invention, Hydrocarbon production system, comprising a subsea production module, operatively connected to a subsea well, a subsea receiving module and a main flow path, including a subsea production choke valve fluidly connecting the production module and the receiving module, wherein bypass line is arranged parallel to the choke valve and being connected to the main flow path by a bypass connection, to conduct production fluid around the choke valve. 
         [0011]    This prevents blockage and malfunction of the choke valve, and consequently the need for replacement of damaged chokes that could easily happen if the fluid was transmitted through the choke valve and not the bypass line. It also removes the problem of changing choke valves according to amount of fluid flow and production phase. 
         [0012]    In one embodiment the bypass connection comprises at least one valve capable of opening and closing the access from the main flow path to the bypass. This is a simple embodiment where the main flow path is always open, while the bypass can be closed or open. The fluid will flow the easiest way, which is through the bypass when this is open. 
         [0013]    In another embodiment the valve is a two-way valve that has a first position to open access to the choke valve and closing access to the bypass, and a second position to open access to the bypass and close access to the choke valve. This will close the access to the choke valve when the bypass is open, thereby eliminating any risk for debris to find its way to the choke valve. 
         [0014]    In yet another embodiment the valve is a three-way valve that has a first position to open access to the choke valve and closing access to the bypass, a second position to open access to the bypass and close access to the choke valve, and a third position to close access to both the choke valve and the bypass. This embodiment will enable access to both the choke valve and the bypass. In this embodiment it is possible to use the valve to shut the well down when maintenance is being performed on equipment downstream of the valve. 
         [0015]    In yet another embodiment the valve is a four-way valve that has a first position to open access to the choke valve and closing access to the bypass, a second position to open access to the bypass and close access to the choke valve, a third position to close access to both the choke valve and the bypass, and a fourth position to open access to both the choke valve and the bypass. This may be an advantage as the choke valve can be set to a fully open position and thereby provide an increased flow rate than what is possible through the bypass alone. 
         [0016]    In a preferred embodiment the bypass is connected to the main flow path by two connections, one at either side of the choke valve. It is thereby possible to remove the choke valve while production takes place through the bypass. 
         [0017]    In a preferred alternative embodiment of the device according to the first aspect of the present invention, the receiving module is a manifold. This enables the adding of more wells to the same manifold. 
         [0018]    In yet an alternative embodiment of the device according to the first aspect of the present invention, the subsea producing module is a Xmas Tree. 
         [0019]    In a further alternative embodiment of the device according to the first aspect of the present invention the bypass connection is arranged to transmit fluid in both directions of the bypass line to provide both production and injection of fluid. This leads to that the restrictions on choke valves regarding reverse fluid flow is no longer an issue. 
         [0020]    In still an alternative embodiment of the device according to the first aspect of the present invention the fluid comprises debris. 
         [0021]    In a second aspect of the present invention, a method is provided for performing clean-up of a subsea well and a subsea production module in a hydrocarbon production system, the subsea production module being adapted to produce a fluid transmitted through a main flow path including a choke valve, wherein during clean-up a bypass connection is opened to direct the produced fluid and/or treatment fluid from the well to a bypass around the choke valve in order to prevent debris in the flow from damaging or blocking the choke valve. 
         [0022]    The producing module produces a clean-up fluid comprising debris during the cleaning up, and the clean-up fluid is transmitted through the main flow path. A bypass connection directs the clean-up fluid through a bypass line arranged in parallel to the choke valve to prevent malfunction and blockage of the choke valve due to the fluid comprising debris. 
         [0023]    In a third aspect of the invention it relates to a method for controlling the flow in a hydrocarbon production system comprising a subsea production module, operatively connected to a subsea well, a subsea receiving module and a main flow path, including a subsea production choke valve fluidly connecting the production module and the receiving module, wherein during a situation with a pressure in the well higher than the pressure necessary to deliver the desired production flow rate, the choke valve is choked to limit the flow rate to a desired flow rate; and that during a situation when a flow rate is desired, which substantially corresponds to the maximum delivery rate of the well, a bypass line is opened to direct the flow outside the choke valve. This makes it possible to merely open the bypass when the pressure in the well has dropped to a certain level over time. It removes the necessity for replacing the choke valve with another valve with different properties, i.e. less restriction. It also makes it possible to open up for full pressure from the well in situations when it is not necessary to limit the flow from the well, e.g., if another well with lower pressure connected to the same manifold is shut down, and the present well can be allowed to produce at maximum without interfering with other wells. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0024]    The invention is described in more detail below with reference to the accompanying drawing, which illustrates the invention by way of an example. 
           [0025]      FIG. 1  is a schematic view of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    As seen in  FIG. 1 , a pipeline  7  has a connection to a Xmas Tree and a subsea well (commonly denoted by reference numeral  8 ) in one end, and the pipeline  7  is connected to a manifold  9  at the other end. A choke valve  3  for controlling the flow through the pipeline  7  is connected between the two end points  8 ,  9  of the line  7 . For illustration purposes only, typical building blocks are also shown in  FIG. 1 . Such building blocks can be a flow meter  4 , sand sensor  5  and injection line  6 . Pressure and temperature sensors PP, PT are also inserted in the pipeline  7 . In regular operation of the system seen in  FIG. 1 , fluid from the subsea well flows from the Xmas tree  8  through the choke valve  3  to the manifold  9 . During a clean-up process of the subsea well or Xmas Tree  8  the choke valve  3  can not safely handle the debris in the fluid. Thus a bypass line  2  is implemented as an additional line  2 , in parallel with the choke valve  3  to prevent the choke valve  3  from being damaged by debris. The bypass line  2  is connected to the pipeline  7  by a bypass connection  1 . The bypass connection  1  has a control function that directs the flow into the bypass line  2  or through the coke valve  3  according to the performed operation. During a clean-up process, the process product which often contains debris will flow through the bypass line  2  instead of the choke valve  3  (as in the current prior art), i.e. the debris will not be in contact with the choke valve  3  during the clean-up process, and the system will be able to perform well clean-up through the Xmas Tree in a much safer way than in known prior art techniques. By this, a potential failure of the choke valve  3  will be eliminated, and consequently a complex and costly process of changing and repairing a damaged choke valve  3  is avoided. A bypass line  2  will typically be made of the same material as the pipeline  7 , or integrated into the connector block which bridges the Xmas Tree  8  to the manifold  9 . The bypass line  2  is preferably mounted in the subsea installation at the same time as the other components of the installation. 
         [0027]    In addition to using the bypass line  2  for transmission of clean-up products, the bypass line  2  has a function especially for high rate production periods. In a subsea well the production periods can be high or low rate. In high rate production periods it will be preferable to provide flow of as much fluid as possible in the pipeline  7 . Without the bypass line  2 , the fluid will flow through the choke valve  3 , which will give a large pressure drop over the choke valve  3  and will not provide maximum flow. This will happen even if the choke valve is fully open. By letting the fluid flow through the bypass line  2  in high rate production periods, the pressure drop over the bypass line  2  will be minimal compared to the choke valve  3 , and maximum flow can be provided. Thus, the oil recovery can be increased during high rate production periods. 
         [0028]    The bypass line  2  also provides good controllability of the choke valve  3  during the first phases at the same time as it provides a maximum flow in later phases of the production in the well, without the need of replacing the choke valve  3  with another choke valve  3  with different features. The bypass line  2  provides a neglectable pressure drop over the choke valve  3 , and thus the Xmas Tree can operate with higher production rates than the choke valve  3  is designed for. 
         [0029]    Another additional function of the bypass line  2  is that it can be arranged to transmit fluid in both directions, to and from the Xmas Tree. In this way, the same Xmas Tree can be used for both production and injection. Reverse flow of the fluid (injection) through a choke valve  3  has many restrictions, and will require a higher injection pressure to overcome the pressure drop over the choke valve, but by using the bypass line  3  these restrictions are no longer an issue. 
         [0030]    Summarized, by providing a bypass line  2  in parallel with a choke valve  3  the present invention might add a good cost saver regarding increased oil recovery, installation cost and any re-installations/work-over operations could be reduced. The vulnerability and risk of malfunction of a choke valve  3  is reduced significantly during a clean-up period.