Abstract:
Dual-walled piping segments and pipelines are described that use an annular bulkhead to secure the jacket pipe radially outside of the carrier pipe near the axial ends of the piping segment. Pup joints are welded to each end of the carrier pipe, and the bulkheads are welded to the pup joints. The bulkheads have a number of features that provide improved load path control for axial forces induced by temperature differentials. There is a mechanical load-sharing interlock mechanism provided between the bulkhead and the interior pup joint and field joint closure joints designed to transmit stress loading to a plurality of ridges or threads, that may be enhanced by thermal contraction, and preclude axial movement between the jacket pipe and pup joint. A number of methods are described for creating the load-sharing interlock. Additionally, the bulkhead has a generally arcuate cross-section that defines an interior channel. The arcuate cross-section allows the bulkhead to be somewhat flexible to absorb axial and radial loading while reducing the available heat transfer rate. The bulkhead also contains several ports for pressure equalization and plugged ports for the pressure-thermal-chemical conditioning of the annular spaces.

Description:
[0001]     This application claims the priority of U.S. provisional patent application Ser. No. 60/557,259 filed Mar. 29, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates generally to dual-walled piping systems. In particular aspects, the invention relates to details and methods for constructing piping segments and pipelines formed of dual-walled piping segments.  
         [0004]     2. Description of the Related Art  
         [0005]     Dual-walled pipelines are often used at great depths in the ocean to carry fluids of extreme temperature. The annulus between the inner and outer pipes is typically filled with insulation to minimize fluctuations in temperature while the fluid is in transit through the pipeline. The pipelines are formed of a number of individual dual-walled piping segments that are interconnected in an end-to-end fashion.  
         [0006]     One longstanding problem with such pipelines is that temperature differentials between the product being piped and the environment cause great stresses upon the piping segments and, particularly, their connections. The fluids being piped are typically at extreme temperatures, and the function of the dual walled piping is to provide an annular insulated space protected from water ingress. For example, liquid natural gas is piped at a temperature of approximately (−)169° C. Some hydrocarbon products, on the other hand, are transported in pipelines at temperatures between 100°-200° C. Temperature differentials between the interior of the carrier pipe and the exterior of the jacket pipe create significant stresses at the interconnections between the carrier pipe and the jacket pipe. Additionally, stresses occur at, and near, the interconnections between the piping segments. In addition, because the inner carrier pipe and the outer jacket pipe of a pipe segment may be exposed to greatly different temperatures, they can each expand and contract in an axial direction placing high loads and great stress upon the mechanisms used to interconnect the inner and outer pipe segments. The brittleness of metals in extreme cold under the high stresses due to differential contraction can result in catastrophic failures in such pipelines.  
         [0007]     A number of prior art piping designs have attempted to solve the pipe in pipe problems, albeit with limited success. U.S. Pat. No. 6,145,547 issued to Villette, for example, describes a dual-walled piping arrangement having an inner tube and outer tube with an open pore-microporous material in between. An end ferrule underlies the outer jacket pipe and is welded to the inner tube. In response to temperature-differential induced forces, tremendous force, hence stress, is placed upon this inter-connection weld. The weld undergoing these high stresses with the commonly recognized stress intensification factors can weaken over time or fail in fatigue cycling, and once broken, would allow sea water to access the annulus and saturate the open pore-microporous material in between. The most troubling issue is controlling the stress levels in closure welds to prevent failures and increase reliability.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention provides an improved piping segment and pipeline features as well as methods for construction of these. An exemplary dual-walled piping segment is described that uses an annular bulkhead to secure the jacket pipe radially outside of the carrier pipe near the axial ends of a piping segment. Pup joints are welded to each end of the carrier pipe, and the bulkheads are welded to the pup joints.  
         [0009]     The pup joints and bulkheads have a number of features that provide improved load transfer control with resulting reduction of stresses induced by temperature differentials. First, there is a mechanical load-sharing interlock mechanism provided between the bulkhead and the interior pup joint. This mechanism is designed to transmit thermal force loading through a plurality of ridges or threads, that may be enhanced by thermal contraction, and thereby preclude axial movement between the jacket pipe and carrier pipe. A number of methods are described for creating the load-sharing interlock. Additionally, the bulkhead has a generally arcuate cross-section that defines interior channels. The arcuate cross-section allows the bulkhead to be somewhat flexible to absorb axial and radial loading. The cross section also minimizes heat transfer.  
         [0010]     Seal welds between the bulkhead and the pup joints provide the means of isolating the insulation in the annulus to ensure thermal efficiency. These isolating seal welds are made on a bulkhead section that is made more flexible by deep annular grooves, or reliefs. Due to the added flexibility afforded by the reliefs, the forces on the seal welds are diverted to the load-sharing interlock reducing the seal weld forces hence the resulting stresses thereby minimizing weld fatigue.  
         [0011]     In another aspect, the invention provides for an improved system and method of constructing a dual-walled pipeline consisting of two or more piping segments. The segments are interconnected in an end-to-end fashion using a field joint that engages radially outer sleeve lands on the bulkheads of the two adjoining piping segments. The load-sharing interlock provides means of compensating for pipe length variations as well as providing control in the weld root gap spacing during fabrication. The interlock further reduces heat transfer by interface resistance.  
         [0012]     Ports and plugs provide means of filling, emptying and pressure-thermal-chemical conditioning of the annular insulating spaces. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     For further understanding of the nature and objects of the present invention, reference should be made to the following drawings in which like parts are given like reference numerals and wherein:  
         [0014]      FIG. 1  is a side, partial cross-sectional view of an exemplary dual-walled piping segment in accordance with the present invention.  
         [0015]      FIG. 1   a  is an enlarged view of the cutaway portion shown in  FIG. 1 .  
         [0016]      FIG. 1   b  is a further enlarged cross-sectional view of a bulkhead used within the piping segment, surrounding components, and annulus port and plug.  
         [0017]      FIG. 2  is a side, cross-sectional view depicting an exemplary interconnection of two dual-walled piping segments in accordance with the present invention but with annular ports and plugs in alternate locations.  
         [0018]      FIG. 3   a  is a side view, partially in cross-section, of a push-on style of bulkhead now secured to an end of a dual walled piping segment.  
         [0019]      FIG. 3   aa  is an enlarged view of portions of  FIG. 3   a.    
         [0020]      FIG. 3   ab  is a cross-section taken along lines b-b in  FIG. 3   aa.    
         [0021]      FIG. 3   b  is a side view, partially in cross-section, of a threaded style of bulkhead now secured to an end of a dual-walled piping segment.  
         [0022]      FIG. 3   c  is a side view, partially in cross-section, of an inverted swaged style of bulkhead now secured to an end of a dual-walled piping segment.  
         [0023]      FIG. 4  is a side view, partially in cross-section, of a sliding sleeve style of field joint for interconnection of dual-walled piping segments.  
         [0024]      FIG. 5  is a side, cross-sectional detail of an alternative embodiment of the present invention that incorporates a plenum for the seal weld on the carrier pipe.  
         [0025]      FIGS. 6, 6A  are side, cross-sectional details of an alternative embodiment of the present invention which defines an interconnection arrangement with two pipe-in-pipe segments jointed with an insulated field joint closure.  
         [0026]      FIGS. 7A and 7B  are side cross-section details of an alternate embodiment that incorporates arcuate cross-sections defining interior channels. The field joint closure is also shown with the same features as well as alternate port and plug locations. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]      FIGS. 1, 1   a  and  1   b  illustrate an exemplary dual-walled piping segment  10  that of a type used within a piping system to transport extreme temperature fluids. The piping segment  10  includes a radially inner carrier pipe  12  and a radially outer jacket pipe  14 . The pipes  12  and  14  are preferably formed of steel alloys and other material suited for the thermal, pressure and corrosion issues as containment barriers. An annulus  16  is defined between the carrier pipe  12  and the jacket pipe  14  and is filled with insulating material  18 . The carrier pipe  12  defines an axial fluid flow bore  20  along its length. The piping segment  10  has two axial ends  22 ,  24 , which are preferably identical in construction. Details of the axial end  24  are shown in cross-section in  FIGS. 1 and 1   a . The end  24  includes a radially inner pup joint  26  that is of similar diameter as the carrier pipe  12  with a first axial end  28  that abuts the axial end  30  of the carrier pipe  12 . The second axial end  32  of the pup joint  26  is intended to abut a pup joint in an adjacent piping segment.  
         [0028]     An annular bulkhead  34  is used to interconnect the outer jacket pipe  14  to the pup joint  26 . The structure of the bulkhead  34  is best understood with reference to  FIG. 1   b . The bulkhead  34  is preferably fashioned of stainless steel, invar or nickel alloyed steel but may be formed of other materials having suitable strength and flexibility. The bulkhead  34  presents a generally arcuate or U-shaped cross-section of a simple form. The bulkhead  34  presents a radially inner portion  36  that presents a roughened, load-bearing inner surface  38 . The roughened inner surface  38  is formed to be complimentary to and interlock with roughened outer surface  40  on the pup joint  26 . The interlock may be enhanced by press fit or thermal shrinkage between the components. Additionally, an annular relief  41  is formed into the inner portion  36  adjacent the roughened surface  38 . The inner portion  36  is interconnected to a radially outer portion  42  by a central web, or hinge, portion  44 . A channel  46  is defined between the inner and outer portions  36 ,  42 . Additionally, the outer portion  42  presents a raised land  48 . It is noted that the pup joint  26  is welded to the carrier pipe  12  in an end-to-end fashion, as shown by weld  54 . A second weld  50  is provided between the bulkhead  34  and the jacket pipe  14 , while a third weld  52 , a seal weld, is provided to isolate the annular space  16  and secure the bulkhead  34  to the pup joint  26 .  
         [0029]     The design of the bulkhead  34  and its interconnection with the other components provides relief and absorption of stresses created by temperature differentials. First, a load-sharing mechanical interlock is provided by the engagement of the inner surface  38  of the bulkhead  34  and the outer surface  40  of the pup joint  26 . Axial movement between the outer jacket pipe  14  and the inner carrier pipe  14  and pup joint  26  is limited by the interlock, which may be further enhanced by the thermal contraction of the bulkhead  34 . Further, axial loads due to contraction or expansion are primarily borne by this interlock due to its high axial stiffness. The arcuate cross-section of the bulkhead  34  also provides flexibility in the connection so that the bulkhead  34  can absorb axial and radial stresses and loading. The web portion  44  of the bulkhead  34  provides a point of flexure between the inner and outer portions  36 ,  42  about the channel  46 . Additionally, the annular relief  41  provides an additional point of flexure within the inner bulkhead portion  36  which provides reduced axial forces hence stresses in the seal weld  52 .  
         [0030]      FIG. 2  illustrates an exemplary field joint interconnection of the piping segment  10  with an adjacent piping segment  10 ′. In this view, cross-hatching is used to depict insulated spaces rather than metal. The distal ends  32 ,  32 ′ of the pup joints  26 ,  26 ′ are brought into contact with one another and sealed with an annular weld  56 . A field joint closure sleeve  58  shown as a single sleeve surrounds the weld  56  and engages the raised sleeve lands  48  of the two bulkheads  34 . The closure sleeve  58  is then secured to each of the bulkheads  34  by an annular field joint closure weld  60 . An insulation space  62  is defined between pup joints  26 ,  26 ′, closure sleeve  58 , and the two bulkheads  34 . The insulation space  62  is preferably a vacuum or substantial vacuum in order to reduce heat transfer between the inner and outer pipes  12 ,  14 . Additionally, the space  62  is filled with insulation material  64 . Port  90  and plug  91  are shown in  FIG. 2 . These features are preferably provided in lesser stress regions and provide for evacuation of water vapor from the annulus for improved insulation properties.  
         [0031]      FIGS. 3   a ,  3   aa ,  3   ab ,  3   b , and  3   c  depict alternative methods for providing the load-sharing mechanical interlock between the bulkhead  34  and the pup joint  26 /carrier pipe  12 . In  FIG. 3   a , the roughened outer surface  40  is provided by a series of raised, ratchet-style ridges. The bulkhead  34  is secured to the pup joint  26  by first sliding it over the distal end  32  of the pup joint  26  and causing the roughened inner surface  38  of the bulkhead  34  to slide over the ridges of the outer surface  40 . The one-way ratchet tooth design of the outer surface  40  precludes movement of the bulkhead  34  in the opposite direction with respect to the carrier pipe  12 /pup joint  26 . Welds  50  and  52  are then formed. Additionally, as  FIGS. 3   aa  and  3   ab  depict, the inner portion  36  of the bulkhead  34  is colleted by longitudinal cuts to divide the inner portion  36  into segments, or fingers,  53  that may be radially spread apart from one another to assist in sliding the inner portion  36  onto the distal end  32  and over the outer surface  40 . Preheating, or in some cases cooling, the bulkhead  34  with controlled internal diameter dimensions further enhances the load carrying capacity for either cold or hot products. Longitudinal cuts made radially to or near the relief  41  ease the installation by reducing the load requirements for assembly. Tapering of the ridges of the roughened outer surface  40  and/or inner surface  38  assists by taking advantage of the accumulative deflection to allow the addition of larger ridges without overstressing the material.  
         [0032]     In  FIG. 3   b , the roughened outer surface  40   a  is formed of an outer screw thread that provides a series of individual ridges. The inner surface  38   a  on the bulkhead  34   a  is a complimentary thread.  
         [0033]     In  FIG. 3   c , the bulkhead  34   b  has a somewhat different design, with the inner and outer portions  36 ′,  42 ′ being joined by a non-arcuate web or hinge portion  44 ′ that acts as a point of flexure between the inner and outer portions  36 ′,  42 ′. The inner portion  36 ′ is swaged onto the outer surface  40   b  using a swaging tool (not shown) of a type known in the art. When swaged, the inner surface  38   b  of the inner portion  36 ′ becomes engaged with the outer surface  40   b.    
         [0034]     Referring now to  FIG. 4 , there is shown a portion of a further exemplary interconnection of piping segments  10 ,  10 ′. A field closure sleeve  58  surrounds the outer jacket pipe  14  of the two piping segments to be joined and rests upon the sleeve lands  48  of the bulkheads  34 . An annular sliding sleeve adapter  68  is welded by butt weld  70  to the end of the field closure sleeve  58 . The sliding sleeve adapter  68  has a radially inward-projecting portion  72  that contacts the outer surface of the jacket pipe  14 . Seal weld  74  is used to secure the adapter  68  to the jacket pipe  14 . Shims  76  are preferably positioned between the adapter  68  and the bulkhead  34  to help load the mechanical interlock of the bulkhead  34  with the pup joint  26 .  
         [0035]      FIG. 5  illustrates a further embodiment for a dual-walled piping segment that incorporates a plenum  80  in conjunction with the annular relief  41  of the bulkhead for reduction of stresses. In this instance, the annular relief  41 ′ is a chamfered shoulder upon the axial end  82  of the bulkhead  34 . The plenum  80  is an annular ring having flattened axial surfaces, in the manner of a washer. The plenum  80  surrounds the carrier pipe  12 /pup joint  26 , and a chamber  84  is formed between the bulkhead  34  and the plenum  80 . Bead weld  52  secures the plenum  80  to the carrier pipe  12 /pup joint  26 . An additional weld bead  86  secures the plenum  80  to the bulkhead  34 . In operation, this arrangement provides for stress loading upon the mechanical interlock (formed by inner surface  38  and outer surface  40 ). At the same time, the plenum  80  may deform in order to absorb deflection caused by clearances in the mechanical load-sharing interlock sections  38  and  40  while undergoing low fatigue stress.  
         [0036]      FIG. 6  illustrates a further embodiment field joint interconnection of the piping segment  10  with an adjacent piping segment  10 ′. In this view, cross-hatching is used to depict insulated spaces rather than metal. The distal ends  32 ,  32 ′ of the pup joints  26 ,  26 ′ are brought into contact with one another and sealed with an annular weld  56 . A field joint closure sleeve  58  surrounds the weld  56  and engages the raised sleeves  70  as a substitute for land  48  in  FIG. 2 . The closure sleeve  58  is then secured to each of the raised sleeves  70  which, in turn, are attached by welds  71  and  72  to outer jacket pipes  14  and  14 ′. Each outer jacket pipe  14  and  14 ′ is, in turn, attached to a bulkhead ring  80  by weld  83 . Bulkhead ring  80  is of such configuration to provide fully ultrasonic inspectable welds  81 ,  82  which connect to the carrier pipe pup joint  26 . Pup joint  26  is shown to be a thicker pipe section and has a transition  27  for the weld  54  to the carrier pipe  12 . A single relief  84  is shown in  FIG. 6  while  FIG. 6A  presents dual reliefs by a modification in the shape of the bulkhead ring  80 . This configuration, while not containing the adjustable features of the exemplary bulkhead system, presents economic advantage for those portions of a system where conditions and stress levels are found to be suitable. An insulation space  62  is defined between pup joints  26 ,  26 ′, closure sleeve  58 , and the two bulkheads  34  and is filled with insulation material  64 .  
         [0037]      FIGS. 7   a  and  7   b  illustrate further embodiments for piping systems wherein the field joint closure is composed of two portions  58  and  58 ′ with an adjoining butt weld  59  and seal welds as shown by  74 . The systems shown in  FIGS. 7   a  and  7   b  differ in that the relative diameter of sleeve lands  48  is of greater diameter the than roughened surface  40  in  FIG. 7A  and less than roughened surface  40  in  FIG. 7B . These differences allow the field joint closure halves  58  and  58 ′ to be either pre-installed over piping segments  10  and  10 ′ or installed separately. These feature differences are beneficial in field assembly procedures involving the handling of straight and curved bulkhead end segments for ease of construction and control of stresses. Providing the ability to pre-install the field closure halves  58  and  58 ′ over the piping segments allows for offshore lay barge installation. Providing the ability to add the field closure halves  58  or  58 ′ at assembly allows for tie-in spool fit-up operations in the field.  
         [0038]     As a further embodiment of the invention, within  FIGS. 7   a  and  7   b , are inner and outer sections  36  and  42  for the inner bulkhead and  37  and  45  for the outer bulkhead are shown to be fabricated of segments forming arcuate sections with joining welds  56  and  57 . These features allow for the most efficient thermal insulating paths due to the added lengths and reduced areas plus yield and an efficient means of machining ports  94  and  92  for pressure equalization across the interlocking surfaces  38 ,  40  and  39 ,  43 . When the arcuate features are as shown in  FIGS. 7   a  and  7   b , the heat transfer is greatly minimized while also minimizing stress levels for long fatigue life. The use of the roughened or mechanical interlock surfaces,  40  and  43  and their mating surfaces  38  and  39 , greatly increases the resistance to heat transfer due to thermal interface resistance recognized as a problem when heat transfer efficiency is the objective. The quantity of heat transferring decreases with reduced area; decreases with an increase in the length of the flow path and decreases with an increase of material and surface-to-surface (interface) thermal resistance. Advantage of thermal interface resistance is further taken advantage of by use of poorly conducting thread lubricants  19  as shown in  FIG. 7   c . Preferred lubricants are those which can tolerate the environmental temperature ranges and allow for the addition of insulating materials such as Teflon, a material with lubricity and thermal resistance.  
         [0039]     Also illustrated in  FIGS. 7A and 7B  are the use of pressure equalizing ports  94  and  95  and ports  90 ,  92  and plugs  91 ,  93  to allow pressure testing of the seal welds  52 ,  74 , the butt welds  50 ,  54 ,  56 ,  59  and the removal of water vapor and temperature/pressure conditioning of the annular spaces  16  and  21 . Also illustrated in  FIG. 7A  are the duplicate use of plenums  80  with attachment welds  85  and seal welds  52  and  74 .  FIG. 7B  illustrates the use of two annular reliefs  41  in lieu of plenums  80  to provide low stress and long fatigue life for seal welds  52 ,  74 .  
         [0040]     Those of skill in the art will recognize that numerous changes and modifications may be made to the exemplary systems and methods described herein without departing from the scope and spirit of the invention. In fact, the invention is intended to be limited only to the claims which follow and all permissible equivalents thereof.