Abstract:
Disclosed is a process and system for circulating fluid within the annulus of a flexible pipe used in a riser in an offshore hydrocarbon production facility. Fluid, such as corrosion inhibitors, can be circulated in a closed loop which includes the annulus of the riser terminating at a platform or floating vessel, a fluid storage tank located on the platform or vessel and an umbilical tube terminating at the platform or vessel and at a subsea location. Use of the system to flow the fluid through the annulus can prevent or reduce corrosion of the steel members within the annulus and increase the fatigue life of the riser.

Description:
FIELD 
       [0001]    The disclosure relates to methods and systems for operating a riser in an offshore hydrocarbon production facility, the riser being formed of flexible pipe having a central bore and an annulus containing multiple functional layers. More particularly, the disclosure relates to methods and systems for circulating fluids in the annulus of a flexible pipe riser. 
       BACKGROUND 
       [0002]    Engineered flexible pipe is frequently used in riser applications in offshore hydrocarbon production facilities which convey hydrocarbon products from a subsea well to a topsides production platform or vessel. Such flexible pipe is formed of multiple layers, each layer designed for a specific function. In general, the innermost layer of the multiple layers is the carcass layer, made of corrosion resistant material, designed to resist collapse of the flexible pipe. Surrounding the carcass is a polymeric sealant layer or pressure sheath which is extruded around the carcass and sealed at flexible pipe end fittings to contain fluid within the bore. Surrounding the polymeric sealant layer is an annulus containing a number of metallic armor layers designed to impart strength against tensile loading (e.g. armor wires) and internal pressure loading (e.g. pressure armor). Surrounding these layers is another polymeric sealant layer or external sheath designed to avoid external sea water ingress into inner layers of the flexible pipe, which acts as an outer protective layer. The space between the two polymeric sealant layers is referred to as “the annulus.” Typically, the annulus contains one or two layers of circumferentially oriented steel members (referred to as pressure armor layers) designed to provide radial strength and burst resistance due to internal pressure. Surrounding the pressure armor layers are two or four layers of helically wound armor wires (referred to as armor wire layers) designed to provide tensile strength in the axial direction. 
         [0003]    Flexible pipe is terminated at each end by an end fitting which incorporates a flange for mating with other flanges. In use, flexible pipe risers are suspended from an offshore hydrocarbon production platform or host facility, thus placing high tensile loads on the armor wire layers. The loads along the riser are amplified due to the effects of environmental conditions and associated motions of the platform or host facility to which the riser is connected. 
         [0004]    Within the bore of the flexible pipe, in addition to hydrocarbon products, other components including hydrogen sulfide, carbon dioxide and water may be present. These other components can diffuse through the first polymeric sealant layer (pressure sheath) to the annulus. These components, hydrogen sulfide in particular, as well as water vapor, can accumulate within the annulus and eventually lead to corrosion of the steel wires therein via mechanisms including hydrogen induced cracking and sulfide stress cracking. Additionally, the annulus can be flooded with seawater due to damage of the outermost layer leading to corrosion of the armor wires. As noted, the armor wires in the flexible riser are particularly subject to dynamic cyclic loads, which can result in corrosion fatigue of the metallic armor wires in the annulus. Corrosion of the metallic wires in this region makes these wires particularly vulnerable to corrosion fatigue and potential acceleration of failure mechanism. 
         [0005]    It would be desirable to provide a way to prevent or reduce corrosion of the armor wires and other steel elements within the annulus of flexible pipe used in risers and in other dynamic applications. 
       SUMMARY 
       [0006]    According to one embodiment, a method is provided for circulating fluid within the annulus of a flexible pipe riser in an offshore hydrocarbon production facility. The method includes pumping the fluid into a closed loop at sufficient pressure to cause fluid to circulate through the loop. The loop includes the annulus of a flexible pipe riser terminating at a topsides riser end fitting at a production platform or an offshore vessel and at a subsea riser end fitting at a subsea location, and at least one umbilical tube within a subsea umbilical in fluid communication with the subsea riser end fitting, and terminating at an umbilical end fitting at the platform or vessel in fluid communication with the annulus. 
         [0007]    In another embodiment, a system is provided for use in an offshore hydrocarbon production facility. The system includes at least one subsea umbilical tube terminating at a production platform or offshore vessel and at a subsea location for conveying a fluid; at least one flexible pipe riser terminating at a production platform or offshore vessel and at a subsea location, wherein the flexible pipe riser includes an annulus in fluid communication with the at least one umbilical tube; end fittings at each terminal location of the flexible pipe riser, wherein each end fitting comprises a port in fluid communication with the annulus; a connector for placing the at least one umbilical tube in fluid communication with the port of the end fitting at the subsea location; and a pump for pumping fluid to circulate the fluid within a closed loop comprising the annulus and the at least one umbilical tube. 
         [0008]    In yet another embodiment, a method for retrofitting a riser system in an existing offshore hydrocarbon production facility is provided. The method includes disconnecting from a topsides venting system a port of an existing topsides end fitting of a flexible pipe riser including an annulus, wherein the flexible pipe riser has a topsides end fitting and a subsea end fitting having a venting port check valve in fluid communication with the annulus; and removing the venting port check valve from the subsea end fitting. The method further includes providing a recirculation kit on the production platform or offshore vessel, the recirculation kit including a fluid storage tank having a tank inlet and a tank outlet; a pump having a pump inlet in fluid communication with the tank outlet and a pump outlet; and piping for fluid connection between the tank outlet and the pump inlet. The port of the flexible pipe riser topsides end fitting is connected to the recirculation kit. A subsea end of an umbilical tube is connected to a port in the subsea end fitting of the flexible pipe riser. Finally, a topsides end of the umbilical tube is connected to the recirculation kit thereby establishing a closed loop including the annulus, the umbilical tube and the recirculation kit. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0009]    These and other objects, features and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where: 
           [0010]      FIGS. 1A-1I  illustrate systems for circulating fluid within the annulus of a flexible pipe riser in an offshore hydrocarbon production facility. 
           [0011]      FIG. 2  illustrates a method for retrofitting a riser system in an existing offshore hydrocarbon production facility. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    According to methods and systems of the present disclosure, the incidence of corrosion over time of armor wires and other steel elements (e.g. pressure armor layer(s)) within the annulus of flexible pipe, such as those used in flexible pipe risers in offshore hydrocarbon production facilities, can be reduced. 
         [0013]    The incidence of corrosion of the armor wires and related problems such as corrosion fatigue can be reduced by circulating a corrosion-inhibiting or a fluid containing surface passivating agents or other additives within the annulus so that the fluid flows in the interstices between the armor wires and other steel elements. A gas-flushing fluid to flush H 2 S, CO 2 , water vapor, etc. from the annulus can also be used. The fluid is referred to interchangeably herein as “buffer fluid,” “flushing fluid,” or simply “fluid.” The fluid can be circulated either continuously or intermittently. The fluid contacts and encompasses the armor wires and other steel elements, protecting them from corrosion. In another embodiment of the present disclosure, rather than or in addition to buffer fluid, image sensitive-materials can be circulated within the annulus of the flexible pipe riser, thus allowing the annulus to be imaged using known techniques. 
         [0014]    The buffer fluid is circulated in a closed loop which includes the annulus of the flexible pipe riser and at least one umbilical tube within a subsea umbilical. Referring to  FIG. 1A , a system is illustrated according to one embodiment in which a production platform  1  is connected to a flexible pipe riser  10  (at topsides end fitting  12 ). The flexible pipe riser  10  terminates on the seabed  3  at a touchdown point where end fitting  14  rests on the seabed  3 . End fitting  14  is connected to an end fitting  15  of a flow line  60 . Buffer fluid  34  is stored in fluid storage tank  30  on the production platform  1 . The fluid  34  is taken from the tank  30 , through conduit  36  and pumped by pump  32  into the annulus of flexible pipe riser  10 . Subsea umbilical  70  is connected to the flexible pipe riser end fitting  14  at port  72 . Fluid pressure drives the fluid  34  to rise through umbilical  70  to return to the tank  30  on the platform  1 . Arrows  34   a  indicate the direction of flow of the buffer fluid within the closed loop. 
         [0015]      FIG. 1E  is a longitudinal cross-section of the flexible pipe riser  10  illustrating a side view of the annulus  40  surrounding bore  16  having produced well fluids containing hydrocarbons  38  flowing there through. The armor wires and other steel elements within the annulus are represented by  50 .  FIG. 1F  is an exploded view of flexible pipe riser  10  showing each of the layers of the flexible pipe. Innermost is the bore  16  within the carcass  52 . The carcass  52  is surrounded by pressure sheath  54  which is in turn surrounded by the annulus  40 . The annulus  40  which includes layers  50 , including pressure armor layer  56 , inner tensile armor wire layer  60  and outer tensile armor wire layer  62 . Surrounding the outer tensile armor wire layer is the external sheath  11 . The cross-section of the flexible pipe is shown in  FIG. 1G .  FIG. 1H  is an expanded view of the wall of the flexible pipe, showing each of the layers previously described as well as the interstitial spaces  90  there between. Within these spaces, buffer fluid  34  flows. 
         [0016]      FIG. 1I  illustrates the subsea end fitting  14  of the flexible pipe riser  10  according to one embodiment. As shown in this embodiment, flexible pipe riser  10  is attached to end fitting  14  by bolts  84 . The end fitting  14 , including bore  86  therein, is designed to securely attach to the end of the flexible pipe and allow for attachment to an adjacent fitting. End fitting  14  also includes a port  72  in fluid communication with the annulus  40  of flexible pipe riser  10 . The umbilical  70  can be connected to port  72  which can be the location of a venting valve in a typical end fitting, thereby providing fluid communication between the umbilical  70  and the annulus  40 . While the figure shows  70  as a single umbilical tube, it should be understood that fluid  34  can flow through one or more individual umbilical tubes within a multicomponent subsea umbilical. 
         [0017]      FIG. 1B  illustrates an alternative embodiment similar to that of  FIG. 1A  in which the direction of buffer fluid flow in the closed loop, as indicated by  34   b , is reversed. In this embodiment, the fluid  34  is pumped from the storage tank  30 , through conduit  36  and pump  32  into at least one umbilical tube within a subsea umbilical  70 . As described above, umbilical  70  is connected via port  72  to flexible pipe riser end fitting  14 , such that fluid  34  passes from the umbilical  70  to the annulus  40  of the flexible pipe  10 . Fluid pressure drives the fluid  34  to rise through the annulus  40  to return to the tank  30  on the platform  1 . 
         [0018]      FIG. 1  illustrates an alternative embodiment similar to that of  FIG. 1A  in which fluid  34  flows through one or more individual umbilical tubes within a multicomponent subsea umbilical  70 .  FIG. 1D  shows the multicomponent subsea umbilical  70  in cross-section. Among the components within the umbilical  70  are individual umbilical tubes  71  through which fluid  34  flows. As shown in  FIG. 1C , fluid  34  is pumped into umbilical  70  via individual umbilical tubes  71 . Umbilical  70  terminates at a distribution unit  76  which can be any suitable manifold structure such as an umbilical terminal assembly (UTA). From the distribution unit  76 , a second umbilical  70 ′ can carry controls to various systems or equipment in the hydrocarbon production facility. One or more flying leads  74  can be used to transmit fluid  34  to flexible pipe riser end fittings  14  (other flexible pipe riser end fittings not shown). In this way, buffer fluid  34  can be circulated through multiple risers within a single hydrocarbon production facility.  FIG. 2  illustrates a method for retrofitting an existing riser system according to one embodiment. In an existing offshore hydrocarbon production facility  100 , a topsides structure  106  mounted on a platform receives produced well fluids from flexible pipe riser  10 , connected at  108 , and sends the well fluids for further processing indicated by Production  102 . Port  120  on topsides end fitting of riser  10  is typically connected to a venting system (not shown) for venting gases from the annulus of the flexible pipe riser  10 . At a subsea location, riser  10  terminates at subsea riser end fitting  110  where connection is established with flow line  60 . Flanges  114  and  116  connect subsea riser end fitting  110  to subsea flow line end fitting  118 . Subsea riser end fitting  110  typically has one or more venting port check valve(s)  113  which are in fluid communication with the annulus of the flexible pipe  10 . 
         [0019]    In order to retrofit the existing system, one of the venting port check valves  113  is removed from the subsea riser end fitting  110  and an umbilical  70  is connected to the port in its place. Port  120  on topsides end fitting of riser  10  is disconnected from the venting system (not shown). A recirculation kit  112  containing a fluid storage tank and pump are provided at the platform. The kit is connected to the port  120  (via line  122  as shown) and to the umbilical  70  thus establishing a closed loop including the annulus of the flexible pipe riser  10 , the umbilical  70  and the recirculation kit  112  through which fluid can be circulated. The kit can be connected so that the port of the flexible pipe riser topsides end fitting is connected to the pump outlet and the topsides end of the umbilical tube is connected to the tank inlet. Alternatively, the kit can be connected so that the port of the flexible pipe riser topsides end fitting is connected to the tank inlet and the topsides end of the umbilical tube is connected to the pump outlet. 
         [0020]    Where permitted, all publications, patents and patent applications cited in this application are herein incorporated by reference in their entirety, to the extent such disclosure is not inconsistent with the present invention. 
         [0021]    Unless otherwise specified, the recitation of a genus of elements, materials or other components, from which an individual component or mixture of components can be selected, is intended to include all possible sub-generic combinations of the listed components and mixtures thereof. Also, “comprise,” “include” and its variants, are intended to be non-limiting, such that recitation of items in a list is not to the exclusion of other like items that may also be useful in the materials, compositions, methods and systems of this invention. 
         [0022]    From the above description and appended drawings, those skilled in the art will perceive improvements, changes and modifications, which are intended to be covered by the appended claims.