Abstract:
Disclosed are methods, systems and apparatus for circulating fluid within the annulus of a flexible pipe used in a riser in an offshore hydrocarbon production facility. Fluid, such corrosion inhibitors, can be introduced into the annulus by pumping fluid from a storage tank located on a platform or vessel into tubes within the annulus. 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 
     The disclosure relates to methods and systems for operating a subsea 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. The disclosure further relates to apparatus for circulating fluids in the annulus of a flexible pipe riser. 
     BACKGROUND 
     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. 
     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. 
     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. 
     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 
     In one aspect of the present disclosure, 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 a fluid into at least one tube having at least one opening within an annulus of a flexible pipe riser, the flexible pipe riser having one end terminating at a topsides riser end fitting in fluid communication with the at least one tube and another end terminating at a subsea riser end fitting. The fluid is discharged from the opening of the at least one tube into the annulus such that the fluid flows in the annulus and returns to the topsides riser end fitting. 
     In another aspect of the present disclosure, a flexible pipe apparatus for use in a riser system in an offshore hydrocarbon production facility is provided. The apparatus includes a tubular carcass layer defining a bore therein for transporting produced well fluids, a pressure sheath surrounding the carcass layer, an external sheath surrounding the pressure sheath and defining and annulus there between, at least two layers of armor wires within the annulus, and at least one tube within the annulus having at least one opening for introducing fluid to the annulus. 
     In yet another aspect of the present disclosure, a system for use in an offshore hydrocarbon production facility is provided. The system includes at least one flexible pipe riser terminating at one end at a topsides location and at another end at a subsea location, wherein the flexible pipe riser comprises an annulus having at least two layers of armor wires and at least one tube having at least one opening for introducing fluid to the annulus. A topsides end fitting is attached to a topsides end of the flexible pipe riser, having an inlet port in fluid communication with the at least one tube within the annulus and an outlet port in fluid communication with the annulus. A subsea end fitting is attached to a subsea end of the flexible pipe riser. The system further includes a pump at the topsides location in fluid communication with and between the outlet port and the inlet port for pumping fluid into the inlet port. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       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: 
         FIGS. 1A-1E  illustrate a system and apparatus for circulating fluid within the annulus of a flexible pipe riser in an offshore hydrocarbon production facility according to one embodiment. 
         FIGS. 2A-2D  illustrate possible embodiments of apparatus for circulating fluid within the annulus of a flexible pipe riser. 
         FIG. 3  illustrates an apparatus for circulating fluid within the annulus of a flexible pipe riser according to one embodiment. 
         FIGS. 4A-4D  illustrate possible embodiments of apparatus for circulating fluid within the annulus of a flexible pipe riser. 
         FIGS. 5A-5C  illustrate a flexible pipe riser according to one embodiment. 
         FIGS. 6A-6C  illustrate a flexible pipe riser in mating relation to a topsides end fitting according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     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, and the like 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. 
     The buffer fluid is circulated in a closed loop which includes at least one buffer fluid tube and the annulus of the flexible pipe riser. According to the present disclosure, the buffer fluid is introduced into the annulus of the flexible pipe riser through at least one opening in at least one tube in the annulus, also referred to herein as the “buffer fluid tube.” Referring to  FIG. 1A , a system is illustrated according to one embodiment in which one end of a flexible pipe riser  10  is connected to a topsides end fitting  12  at a production platform  1 . The other end of the flexible pipe riser  10  terminates on the seabed  3  at a touchdown point where subsea end fitting  14  rests on the seabed  3 . Subsea end fitting  14  is connected to an end fitting  15  of a flow line  60 . Buffer fluid is stored in fluid storage tank  30  on the production platform  1 . The fluid is taken from the tank  30 , through conduit  36  and pumped by pump  32  into at least one buffer fluid tube (to be described in more detail hereinafter) in the annulus of flexible pipe riser  10 . Once introduced into the annulus, the fluid flows in the interstices in the annulus. Fluid pressure drives the fluid within the annulus to rise through the annulus to return to the topsides end fitting  12  at the platform  1 . Upon returning to the topsides end fitting  12 , the fluid is directed to a port in the topsides end fitting in fluid communication with the tank  30  via conduit  31 . The fluid is returned to the tank where contaminants can be removed from the fluid, and the fluid can be recirculated in the loop including the at least one buffer fluid tube and the annulus by pump  32 . The fluid can be recirculated continuously or intermittently. 
       FIG. 1B  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 . The buffer fluid, introduced into the annulus from the buffer fluid tube, flowing in the interstices in the annulus is represented by  34 . 
     In one embodiment, at least one armor wire layer within the annulus includes at least one buffer fluid tube. In this case, the buffer fluid tube is embedded within the armor wire layer. The buffer fluid tube is generally similar in size and shape to an individual armor wire.  FIG. 1C  is an exploded view of a flexible pipe riser  10  showing each of the layers of the flexible pipe. Innermost is the bore  16  within and defined by the carcass  52 . The carcass  52  is surrounded by pressure sheath  54  which is in turn surrounded by the annulus  40 . The annulus  40  includes layers  50  which include pressure armor layer  56 , inner tensile armor wire layer  60 ′ and outer tensile armor wire layer  62 ′. In the embodiment illustrated, within inner tensile armor wire layer  60 ′ are tensile armor wires  61  and buffer fluid tubes  61 ′. Buffer fluid tubes  61 ′ have perforations  65  through which buffer fluid is introduced into the annulus. Similarly, within outer tensile armor wire layer  62 ′ are tensile armor wires  63  and buffer fluid tubes  63 ′ having perforations  65  through which buffer fluid  34  is introduced into the annulus. Surrounding the outer tensile armor wire layer is the external sheath  11 . The cross-section of the flexible pipe is shown in  FIG. 1D .  FIG. 1E  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. 
       FIGS. 2A-2D  and  FIG. 3  illustrate exemplary embodiments of the buffer fluid tube  61 ′ or  63 ′ having at least one opening therein for fluid to be introduced into the annulus.  FIGS. 2A-2C  illustrate buffer fluid tubes having one or more perforations  65  along the length thereof.  FIG. 2D  illustrates a buffer fluid tube having a solid wall, i.e., having no perforations. Such solid buffer fluid tubes have an opening  65  at one end thereof through which fluid can be introduced into the annulus.  FIG. 3  illustrates a buffer fluid tube  70  according to another embodiment having many small perforations  65  along the length thereof through which buffer fluid is introduced into the annulus such that it weeps from the tube the entire length of the riser. 
     In an alternative embodiment, the buffer fluid can be provided to the annulus by a tape having at least one buffer fluid tube therein. In one embodiment, the tape can include a plurality of buffer fluid tubes in a side-by-side arrangement. Referring to  FIG. 4A , tape  80  has a plurality of buffer fluid tubes  82  having openings  84  at the ends thereof. The tubes  82  are arranged side-by-side in a generally ribbon shaped tape.  FIG. 4C  is a cross-section of tape  80 . As shown, the number of tubes  82  within the tape may vary. The number of tubes  82  can vary between two and n tubes, where n is any convenient number. For example,  FIG. 4A  illustrates a tape with nine tubes  82 . Referring to  FIG. 4B , tape  80 ′ has a plurality of buffer fluid tubes  82  arranged side-by-side having openings  84  at the tube ends as well as perforations  86  along the length thereof.  FIG. 4D  is a cross-section of tape  80 ′. While in the cross-sectional views shown, all of the tubes are lined up in a straight row, they could have an alternative arrangement. For instance, the tubes could be staggered with respect to each other. 
       FIG. 5A  is an exploded view of a riser  10  incorporating tape  80  or  80 ′ (represented by  80 / 80 ′) within the annulus  40 . As shown, the tape  80 / 80 ′ is helically wound between armor wire layers  60  and  62 .  FIG. 5B  is a cross-section of riser  10 , and  FIG. 5C  is a detailed view of the wall of riser  10 . While the tape  80 / 80 ′ is shown between the armor wire layers  60  and  62 , the tape may also be provided between any two layers within the annulus, namely, between the pressure sheath  54  and the pressure armor layer  56 , between the pressure armor layer  56  and inner tensile armor layer  60 , between the armor wire layers  60  and  62 , between armor wire layer  62  and the external sheath  11 , and/or between any other additional layer which the annulus may contain and an adjacent layer. 
     Referring again to  FIG. 1A , the flexible pipe riser  10  is attached to topsides end fitting  12 .  FIG. 6A  is an exploded view of the flexible pipe riser  10  in mating relation with the topsides end fitting  12  according to one embodiment. In this view, the flexible pipe riser  10  as already described and shown in  FIG. 1C  is mated with topsides end fitting  12  such that the bore  116  of the end fitting is in fluid communication with the bore  16  of the flexible pipe riser  10 . The end fitting  12  includes an opening  99  and channel  98  in fluid communication with at least one buffer fluid tube in the annulus of the flexible pipe riser  10 , such that fluid can be pumped into the buffer fluid tubes  61 ′ and  63 ′ via the opening  99  and channel  98 . In the particular embodiment shown, segregating wall  106  separates the inlet portion  102  of the annulus (in fluid communication with opening  99 ) from outlet portion  104  of the annulus in fluid communication with an opening  101 . The segregating wall  106  helps direct the flow of fluid into the open ends  67  of the buffer fluid tubes. Opening  101  and channel  100  are in fluid communication with outlet portion  104  of the annulus. It will be appreciated by one of ordinary skill in the art that many other particular embodiments for introducing fluid into the buffer fluid tubes  61 ′ and  63 ′ could also be employed. 
       FIG. 6B  shows the apparatus of  FIG. 6A  with fluid  34  being introduced through inlet port  94  and channel  98 , entering the inlet portion  102  of the annulus and open ends  67  of perforated buffer fluid tubes  61 ′ and  63 ′ within the armor wire layers according to one embodiment. In one embodiment, the inlet port  94  is in fluid communication with the pump as previously described. The fluid  34 ′ flows through the annulus along the length of the flexible pipe riser  10  and returns to the topsides end fitting  12 , exiting through the outlet portion  104  of the annulus and the channel  100  and the outlet port  96 . The exiting fluid  34 ′ can then be returned to the storage tank.  FIG. 6C  illustrates a similar apparatus according to another embodiment in which the buffer fluid tubes  61  and  63  have solid walls. 
     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. 
     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. 
     From the above description, those skilled in the art will perceive improvements, changes and modifications, which are intended to be covered by the appended claims.