Abstract:
A collapse tolerant flexible pipe and method of manufacturing same according to which an inner tubular layer is provided within an outer tubular layer in a coaxial relationship thereto. The inner layer maintains the maximum allowable strain on the outer layer below a value that will cause damage to the outer layer when the pipe collapses.

Description:
BACKGROUND  
         [0001]    Flexible pipes currently used in offshore oil and gas fields for the transport of fluids underwater between the subsea wellhead and the surface facilities are designed to retain a circular cross-section when subject to external hydrostatic pressure. This is usually achieved by the inclusion of metallic layers which extend around and support a polymer fluid barrier layer and which resists collapsing under the external hydrostatic pressure. However, for deep water applications, the strength and the weight of the metallic layers required to resist collapse becomes a limiting factor in flexible pipe design.  
           [0002]    Also, in these designs the innermost barrier layer is designed to contain the fluid or gas. Thus, when the pipe collapses or is squashed, the barrier wall will experience excessive localized over-bending, which can cause structural damage to the barrier layer and result in failure of the pipe.  
           [0003]    Therefore, what is needed is a flexible pipe that can tolerate relatively high hydrostatic pressure yet eliminate the disadvantages of the metallic layers discussed above while avoiding potential structural damage to the barrier layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is an isometric view of a pipe according to an embodiment of the invention.  
         [0005]    [0005]FIGS. 2A, 2B,  3  and  4  are enlarged transverse sectional views of the pipe of FIG. 1, depicting various collapsed modes.  
         [0006]    [0006]FIG. 5 is an enlarged longitudinal sectional view of the pipe of FIG. 1.  
         [0007]    [0007]FIG. 6 is an isometric view of a pipe according to an alternate embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0008]    Referring to FIG. 1 of the drawings, the reference numeral  10  refers, in general, to a pipe according to an embodiment of the invention. The pipe  10  is designed to receive a fluid at one end for the purpose of transporting the fluid. The pipe  10  includes a barrier layer  12  and an inner layer  14  disposed within the barrier layer in a coaxial relation thereto, with the inner layer normally conforming to the corresponding inner surface of the barrier layer in an abutting relationship, for the entire length of the latter layer.  
         [0009]    The barrier layer  12  can be fabricated from a material that has reasonable ductility and elasticity such as a plastic or elastic polymer. The material forming the inner layer  14  can also be a plastic or elastic polymer, and preferably is selected so that it has sufficient ductility to survive after being subjected to large strain levels a number of times, and sufficient elasticity to tend to recover from a collapsed state when the pipe is repressurized.  
         [0010]    The wall thickness of the inner layer  14  relative to the wall thickness of the layer  12  is selected so that damage to the barrier layer  12  is prevented when both the barrier layer and the inner layer are collapsed in response to a hydrostatic load placed on the pipe. For example, and assuming the layers  12  and  14  are fabricated from a polymer material as discussed above, their relatively thicknesses are selected so that, when the pipe  10  collapses under a hydrostatic load, a maximum strain on the layer  12  will occur that is no greater than approximately 7% which is below the value that will cause damage to the barrier layer for most polymer material. Thus, the thickness of the inner layer  14  relative to the thickness of the layer  12  is selected to limit the bending of the outer layer to within safe levels of strain. In this context, it is understood that the thickness of the inner layer  14  relative to the barrier layer  12  can vary from a value in which the former is less or greater than the latter based on the relative dimensions of the layers  12  and  14  and the material of the layers. Thus, the relative thicknesses of the layers  12  and  14  shown in the drawing are for the purposes of a non-limitative example only.  
         [0011]    [0011]FIGS. 2A and 2B depict the pipe  10  after application of an external pressure to the barrier surface of the barrier layer  12  sufficient to collapse the pipe. In the case of FIG. 2A, one area of the pipe  10  has collapsed, whereas in FIG. 2B, diametrically opposite portions have collapsed. In both cases, the outer radius R of the inner layer  14  forms a cushion that limits the bending of the barrier layer  12  at an area where the maximum strain on the barrier layer normally occurs. The thickness of the inner layer  14  is selected so that the maximum possible bending of the barrier layer  12  is limited to an amount less than the bending that would cause strain on the barrier layer sufficient to damage it.  
         [0012]    If the external pressure acting on the pipe  10  remains sufficiently high after the initial collapse shown in FIGS. 2A and 2B, then the pipe may be further forced into a post-buckled mode shown in FIG. 3. In this situation, one portion of the barrier layer  12  and the inner layer  14  (in the example shown, the upper halves of the layers) attain maximum deformation, and the collapse is such that the flow path through the inner layer  14  is completely closed. As in the situation of FIGS. 2A and 2B, the collapsed inner layer  14  forms a cushion with round radii R which limit the maximum possible bending of the barrier layer  12  and thus protect it from damage.  
         [0013]    The collapse of the pipe  10  can also result in small gaps G at two ends of the cross section of the pipe, as shown in FIG. 4. As in the situation of FIGS. 2A and 2B, the collapsed barrier layer  12  and inner layer  14  form a cushion with round radii R where the maximum strain on the barrier layer occurs. However, due to the gaps G, the radii R will be greater than the radii R in the example of FIG. 3. As a result, relative lower strain is expected on the barrier layer  12 . By taking this phenomenon into consideration, the relative thickness of the inner layer  14  (and therefore the ratio of the inner layer thickness over the thickness of the barrier layer  12 ) can be reduced from a value used when the gaps G are not present.  
         [0014]    In each of these situations, the inner layer  14  can suffer localized structural damage, such as crazing or localized yielding, especially after several collapses, but this damage will not affect the function of the pipe and can be tolerated. When the inner layer  14  is, in fact, damaged, it functions as a sacrificial layer.  
         [0015]    The accumulation of permeated fluid and/or gas in the interface between the barrier layer  12  and inner layer  14  can cause separation between the barrier layer  12  and inner layer  14  prior to collapse of the pipe  10 . This separation could result in an undesirable collapse mode other than those shown in FIGS. 2 and 3 since the inner layer  14  may not be able to protect the barrier layer from over-bending and subsequent structural damage. A technique to eliminate this accumulation and thus to insure that the pipe  10  collapses properly to the collapse modes (shapes) shown in FIGS. 2 and 3 is depicted in FIG. 5.  
         [0016]    Specifically, a series of small radially-extending and axially and angularly-spaced holes  14   a  are formed through the inner layer  14  in any known manner, such as by drilling. During operation, the holes  14   a  will promote the flow of the trapped fluid/gas from the interface F, and into the interior of the inner layer  14  as shown by the solid arrows. This is caused by two effects—a “vacuum” effect due to low pressure at the inner side of the holes  14   a  which is generated by the flowing fluid/gas inside the inner layer  14  in the direction shown by the dashed arrow, and a “squeezing” effect as the internal flow pressure (with possible external pressure on the outer surface of the inner layer  12 ) pushes the inner layer  14  and the barrier layer  12  against each other. This flow through the holes  14   a  avoids separation of the barrier layer  12  and inner layer  14  so that they will thus remain in contact in their designed, abutting, coaxial configuration, thus avoiding the undesirable separation and enabling the pipe  10  to return from its collapsed condition to its normal condition shown in FIG. 1.  
         [0017]    The pipe  10  thus can tolerate relatively high hydrostatic pressures while eliminating the disadvantages of the metallic layers discussed above and avoiding potential structural damage to the barrier layer. In addition, the pipe  10  can be wound on a storage reel in a collapsed, substantially flat form, an advantage from a storage and transportation standpoint.  
         [0018]    A pipe  20  according to an alternate embodiment is shown in FIG. 6 and is designed to receive a fluid at one end for the purposes of transporting the fluid. The pipe  20  includes a barrier layer  22  and an inner layer  24  which are identical to the barrier layer  12  and the inner layer  14 , respectively, of the previous embodiment. Thus, the inner layer  24  is disposed in the barrier layer  22  in a coaxial relation thereto, with the inner layer normally conforming to the corresponding inner surface of the barrier layer in an abutting relationship, for the entire length of the barrier layer.  
         [0019]    A protective layer  26  extends over the barrier layer  22 , a reinforcement layer  28  extends over the protective layer  26  and an additional protective layer  30  extends over the layer  28 . Although only one layer  26 ,  28 , and  30  are shown, it is understand that additional layers  26 ,  28 , and  30  can be provided. The protective layers  26  and  30  can be made from plastic or elastic polymer, or plastic or elastic polymer tapes with or without reinforcement fibers. The reinforcement layer(s) can be made from metallic or composite strips with or without interlocking.  
         [0020]    The pipe  20  thus enjoys all of the advantages of the pipe  10  and, in addition, enjoys additional protection and reinforcement from the layers  26 ,  28 , and  30 .  
         [0021]    It is understood that variations may be made in the foregoing without departing from the scope of the invention. For example, the pipe can be provided with one or more protective layers and/or one or more reinforcement layers extending over the outer layer.  
         [0022]    The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.