Abstract:
A method for forming a connection between two tubular sections having a polymeric outer surface jacket, using electrofusion to fusion bond a casing of similar, non-crosslinked polymer to the outer surface of the tubular sections.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for forming a connection between two tubular sections having a polymeric outer surface jacket. The method utilizes electrofusion to bond a casing to the polymeric outer surface jacket of the tubular sections. The bond may be of a casing of a single layer of non-crosslinked polymer, or may be of an inner layer of a two (or more) layer polymer casing comprising an outer, cross-linked polymer layer. 
       BACKGROUND OF THE INVENTION 
       [0002]    Pre-insulated pipeline is fabricated by putting together lengths of manufactured pipe. The pipe lengths are manufactured in various diameters and lengths, and are typically steel pipe, optionally epoxy coated, covered with a layer of foam insulation, then covered with a polymeric outer surface jacket. The metal pipe is exposed at the two ends of the pipe length, along a defined length of pipe. One exposed length of the metal pipe is girth welded to the end of the pipeline, forming a cut-back region of bare steel pipe, having at its centre a girth weld, and surrounded on both sides by the foam insulation and polymeric outer surface jacket. Ideally, a pipeline should have continuous insulation and outer polymer jacket layers, so the cut-back region is filled using a variety of different methods and means. Typically, a casing of polymer is attached to the outer polymer jacket layer on both ends of the cut-back region, and injection filled with foam insulation. 
         [0003]    Attachment of the polymer casing to the outer polymer jacket is typically done in one of three methods: electrofusion, induction fusion, or adhesively bonding of the inner layer of the polymer casing to the outer layer of the polymer jacket. 
         [0004]    Certain methods of electrofusing a casing to a field joint are well known in the art. For example, U.S. Pat. No. 4,629,216 published Dec. 16, 1986 and incorporated herein by reference, describes non-shrink plastic casings and employs electric heating elements to form a bond between the plastic casing and the plastic jacket of preinsulated pipes. U.S. Pat. No. 4,866,252, published Sep. 12, 1989 and incorporated herein by reference, discloses a connection between preinsulated pipes having a casing and sleeve articles, one disposed over each end of the casing where it overlaps the jacket of the preinsulated pipe. The articles have a bonding material that will form a fusion bond to the jacket, an outer heat shrink layer and a built in electrical heating element in contact with the heat shrink layer for heating and shrinking the outer layer. Since the heat flux from the built in heating element is relatively small, the heat shrink layers are made thin to permit heat penetration and shrinking of the heat shrink layer, and according to the patent the thickness of the article before heat shrinking may be up to 6 mm. The article is less useful where thicker casing members are desired for use with large diameter preinsulated pipes. Use of electric heating elements for binding casings is also described in the Mounting Instructions for the Electric Welder for BelmaFlex Welding Joints (BelmaFlex, Farsø, Denmark). The BelmaFlex casing system comprises a welding band which is a heating element, fitted to the outer surface jacket of the pipe on each end of the pipe joint. A casing is then slid over the pipe joint, and the ends of the casing are heat shrunk to the outer surface jackets on each end of the pipe joint. Once the casing has cooled, buckles are placed around the heat shrunk sections, and current is applied to the welding band, through electrical connections connected to the welding band and extending beyond the casing, between the casing and the outer surface jacket. The application of electric current causes the welding band to heat, which melts and fuses the casing to the outer surface jacket. The casing is then drilled, foam insulation is injected into the drill holes to fill the gap between the casing and the exposed pipe joint, and the drill holes are capped. 
         [0005]    Some of the present inventors have also disclosed various methods for electrofusing of a casing member to a cut-back region of a preinsulated pipeline, and casing members for use in those methods, for example, in PCT publication WO 2011/143733, incorporated herein by reference. Generally, the casing members have a first, cross-linked, heat shrinkable outer layer, and a second, inner, non-crosslinked (or less cross-linked) layer. The casing members have an electrically heatable member spaced inwardly from the first layer, for fusion bonding the second layer to the tubular sections. 
         [0006]    In use, when the first layer of the casing member is heat shrunk, in one preferred form it applies a hoop stress to the second layer. When the second layer is heated with the electrically heatable members, in a preferred form, a fusion bond is created between the material of the second layer and of the outer surface of the tubular section. A fusion bond is created when two compatible plastics materials melt and fuse together under fusion or welding conditions. A fusion bond results in a continuously homogeneously weld portion. The above-mentioned hoop stress facilitates formation of the fusion bond. 
         [0007]    The WO 2011/143733 publication further describes the use of an electrically heatable member as a particularly effective and convenient way of heating the second layer and the tubular section surface substrate to create the fusion bond. The electrically heatable member, may for example, be in the form of electrical resistance heating elements or electrically inductively heatable elements which may be disposed inwardly from the second layer, or may be disposed integrally within the second layer. 
         [0008]    Generally, a proper electrofusion of two surfaces has excellent mechanical properties and is difficult to unattach. However, some prior art electrofusion methods have been criticized as sub-optimal for water penetration of the seal. 
         [0009]    A known alternative method to electrofusion is to bond the outer polymer surface layer of the pipe to the casing using an adhesive. Typically, the adhesive is in the form of an inner layer of the casing. In some methods, the adhesive is a liquid adhesive which is applied to the outer polymer surface layer or to the inside of the casing (or to both). In most instances the adhesive must be heated to bond to at least one of the two surfaces. While adhesive-based methods are very well known and used with much success, some adhesive-based methods are criticized for less than optimal mechanical properties, and are known to detach with time and/or application of external environmental factors. 
       SUMMARY OF THE INVENTION 
       [0010]    According to one aspect of the present invention is provided a method of attaching a casing to an outer surface jacket of a pipeline at a pipe joint, comprising: wrapping or fitting a heat shrinkable casing around the pipe joint so that a side portion of the casing overlaps the end portion of the outer surface jacket on either side of the pipe joint, to form an interface region on each side of the pipe joint, said interface region defined as being between the end portion of the outer surface jacket and the side portion of the casing; said interface region, end portion, and/or side portion having an electric heating element around its circumference; and wrapping an adhesive tape around said side portions to provide pressure between the side portions and the end portions at the interface region; applying an electric current to said electric heating element of a sufficient energy to heat said electric heating element to a temperature sufficient to fuse said side portions to said end portions. 
         [0011]    In certain embodiments, the electric heating element is wrapped around the end portion of the outer surface jacket prior to wrapping or fitting the casing around the pipe joint. 
         [0012]    According to certain embodiments, the casing is heat shrinkable and at least the side portions of the casing are heat shrunk before the wrapping step. 
         [0013]    In certain embodiments, the adhesive tape having an adhesive providing stickiness at room temperature and which stays adhesive to a temperature of at least 240° C. 
         [0014]    According to a further aspect of the invention is provided a method of attaching a casing to an outer surface jacket of a pipeline at a pipe joint, comprising: wrapping or fitting a casing around the pipe joint so that a side portion of the casing overlaps an end portion of the outer surface jacket on either side of the pipe joint, to form an interface region on each side of the pipe joint, said interface region defined as being between the end portion of the outer surface jacket and the side portion of the casing; said interface region, end portion, and/or side portion having both an electric heating element and an adhesive strip around its circumference; and applying an electric current to said electric heating element of a sufficient energy to heat said electric heating element to a temperature sufficient to fuse said side portions to said end portions. 
         [0015]    In certain embodiments, heat is applied to the side portion of the casing to melt the adhesive strip, resulting in a bonding of the outer surface jacket to the casing. 
         [0016]    According to a further aspect of the invention is provided a method of attaching a casing to an outer surface jacket of a pipeline at a pipe joint, comprising:wrapping or fitting a casing around the pipe joint so that a side portion of the casing overlaps an end portion of the outer surface jacket on either side of the pipe joint, to form an interface region on each side of the pipe joint, said interface region defined as being between the end portion of the outer surface jacket and the side portion of the casing; said interface region, end portion, and/or side portion having an electric heating element around its circumference; applying a first electric current to said electric heating element, said first electric current of insufficient energy to fuse said side portions to said end portions, but of sufficient energy to warm and render more uniform a temperature of the heating element; and applying a second electric current to said electric heating element of a sufficient energy to heat said electric heating element to a temperature sufficient to fuse said side portions to said end portions. 
         [0017]    According to a further aspect of the present invention is provided a heating element for use in electrofusing a casing to an outer surface jacket of a pipeline at a pipe joint, said heating element comprising: a wire mesh body and two ends, said body of sufficient length and flexibility to be capable of snugly wrapping around a circumference of said outer surface jacket in a manner that the two ends are proximate to one another but not touching; each of said two ends having a labyrinth configuration and a lead extending therefrom for connection to a power source. 
         [0018]    According to a further aspect of the present invention is provided a spacer for use in conjunction with a heating element for use in electrofusing a casing to an outer surface jacket of a pipeline at a pipe joint, said heating element comprising a wire mesh body and two ends, said body of sufficient length and flexibility to be capable of snugly wrapping around a circumference of said outer surface jacket in a manner that the two ends are proximate to one another but not touching; each of said two ends having a lead extending therefrom for connection to a power source; said spacer capable of holding the two ends at a desired distance; said spacer comprising: a gap region of a width of about the desired distance and a height equal or greater than the height of the ends; and two tongues, each extending from opposing sides of said gap region and having a height that is less than said gap region; each tongue having at least one button extending therefrom to a height equal or greater than the height of the ends, said button of a shape, size and configuration that it is capable of extend into a hole in the wire mesh when said wire mesh is placed on top of said tongue. 
         [0019]    According to a further aspect of the present invention is provided a spacer for use in accurately positioning a thermocouple proximal to a heating element for use in electrofusing a casing to an outer surface jacket of a pipeline at a pipe joint, said heating element comprising a wire mesh body and two ends, said body of sufficient length and flexibility to be capable of snugly wrapping around a circumference of said outer surface jacket in a manner that the two ends are proximate to one another but not touching; each of said two ends having a lead extending therefrom for connection to a power source; said spacer capable of accurately positioning a thermocouple proximal to a heating element, said spacer comprising: a backing; a plurality of buttons extending from said backing, each of which being of a shape, size and configuration that it is capable of extending into a hole in the wire mesh when said wire mesh is placed on top of said backing; a channel within the backing, of a diameter similar to the diameter of a thermocouple probe; said channel having an aperture at one end for receiving a thermocouple probe and a stopper at an opposing end for stopping movement of said thermocouple probe beyond said channel. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0020]    The invention will be more fully described by way of example only with reference to the accompanying drawings. 
           [0021]      FIG. 1  shows a schematic view of a cross section of a cut-back region of a pre-insulated pipeline, as known in the prior art. 
           [0022]      FIG. 2  shows a somewhat schematic view of a cross section of a cut-back region of a preinsulated pipeline, having a casing attached thereto. 
           [0023]      FIG. 3  shows a somewhat schematic view of a cross section of a cutback region, with a casing applied thereto according to the present invention. 
           [0024]      FIG. 4  shows a photograph of one end of a cut-back region according to one aspect of the present invention. 
           [0025]      FIG. 5  shows a close-up view of a portion of a mesh heating element and lead according to one aspect of the present invention. 
           [0026]      FIG. 6  shows a perspective view of a spacer according to one aspect of the present invention. 
           [0027]      FIG. 7  shows a close-up of the lead region of the mesh heating element according to one aspect of the invention. 
           [0028]      FIG. 8  shows a polyethylene insert according to one aspect of the invention. 
           [0029]      FIG. 9  shows a photograph of a casing clamped onto an outer surface layer of a pre-insulated pipe utilizing adhesive tape instead of a traditional clamp. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]      FIG. 1  shows a somewhat schematic view of a cross section of a cut-back region of a preinsulated pipeline, as known in the prior art. The region comprises steel pipe  10 , which has been girth welded to the end of the pipeline  11  at girth weld  12 . Both the pipe  10  and the pipeline  11  are encased in an insulation layer  14 , which in turn is encased in outer surface jacket  16 , which is typically a polyolefin layer which provides structural strength and water resistance. 
         [0031]    The pipeline region  18  is connected to the pipe region  20 , leaving a cut-back region  22  which is bare steel, which does not comprise insulation or polyolefin jacket layers. 
         [0032]      FIG. 2  shows a prior art casing  36  placed on the cut-back region  22 . As shown, the casing  36  comprises two layers: an outer casing layer  34 , which is cross-linked and heat shrinkable, and an inner casing layer  32 , which is not cross-linked, or alternatively is less cross-linked than the outer casing layer  34 . Shown are heating elements  24 ,  26 , each having electrical connector leads  28 ,  30 , respectively. The heating elements  24 ,  26  are typically bands or meshes of copper or another resistive metal. An electrical current (not shown) is applied to the heating elements  24 ,  26 , which is converted to heat by the resistance properties of the heating elements  24 ,  26 . The heat is of a sufficient energy to melt a portion of the outer surface jacket  16  and a portion of the inner casing layer  32 , to fuse the two layers together. As would be understood by a person of skill in the art, the polymer of the inner casing layer  32  and of the outer surface jacket  16  are selected to be compatible and fusable together with the presence of sufficient heat. It is noted that in typical methodology, the heating elements  24 ,  26  are incorporated within the joint and not removed after fusion. 
         [0033]    It has been found that it is desirable to maintain a certain amount of hoop stress in the casing  36 , most particularly at the ends of the casing  36  which are to be fused. The hoop stress significantly enhances the fusing process. In prior art methods, as described in WO 2011/143733, this hoop stress is created by utilizing a heat-shrinkable outer casing layer, and heat shrinking said outer casing layer before electrofusion takes place. Thus, the method has two main steps—a heating step where the ends of casing  36  are heat shrunk to the outer surface layer  16 , and an electrofusion step, where electricity is passed through heating elements  24 ,  26 , fusing the casing to the outer surface layer  16 . 
         [0034]      FIG. 3  shows a somewhat schematic cross-section of one embodiment of a method of applying a casing to a cut-back region of a pipeline according to the present invention. Like in prior art method of  FIG. 2 , the casing  36  is fitted to the cut-back region  22 ; heating elements  24 ,  26  are fed electricity through leads  28 ,  30  to electrofuse the inner surface of the casing  36  to the outer surface of the outer surface layer  16 . However, in this embodiment, the method also comprises a further step. An adhesive strip  38 ,  40  is applied to one of the outer surface of the outer surface layer  16  or the inner surface of the casing  36 , before application of the casing  36  to the cut-back region  22 . Thus the casing  36  is bonded to the outer surface layer  16  twice on each side of the cut-back region—a first electrofusion bond as previously described, and an adhesive bond distal to the cut-back region  22  relative to the electrofusion bond. The electrofusion bond provides structural strength and integrity to the bond, whereas the adhesive bond provides a better prevention of water penetration. The use of the two bonds, side by side, also provides a certain degree of redundancy and a greater overall strength to the casing-outer surface jacket bond. 
         [0035]    Heating elements  24 ,  26  may be, for example in the form of resistively heatable wire, rods, wire mesh or netting in a tape form, or electric wire or flat metallic strip coated with a polymer compatible with the fusion bonding of the casing second layers to the pipe jackets, for example polyethylene. In preferable embodiments, heating elements  24 ,  26  are a wire mesh. 
         [0036]      FIG. 4  shows a photograph of one end of a cut-back region in an alternative configuration of the embodiment shown at  FIG. 3 . Here, in contrast to the example shown in  FIG. 3 , the heating element  24  is distal to the cut-back region  22  in comparison to the adhesive strip  38 . Note that adhesive strip  38  has backing  42 , which is removed immediately before or (preferably) immediately after the placing of the casing around the cut-back region, in order to prevent sand, dust, and other environmental impurities from attaching to and reducing the efficacy of the adhesive strip  38 . 
         [0037]      FIG. 5  shows a close-up view of a portion of a mesh heating element  24  and lead  28  of certain embodiments of the present invention. The heating element  24  wraps around the outer surface jacket  16  of the pipe. The heating element  24  is of a mesh configuration, having metal portions  44  and gaps  46 . As would be appreciated to a person of skill in the art, lead  28  comprises two separate electrical leads  28 A,  28 B, and an electrical current is applied by connecting one of these to each of a positive and a negative terminal of an electrical source. It is necessary to have a sufficient gap  54  between leads  28 A and  28 B, to prevent short circuiting of the heating element. In previous electrofusion methods, the gap between the leads was estimated by the operator. This led to significant quality control issues—if the lead gap  54  between the leads  28 A  28 B was too large, there was potential for a gap in the electrofusion, and a resultant gap in the fusion interface between the outer surface layer  16  and the casing  36 . This would allow ingress of water and/or air and a resultant deterioration of the insulating layer and corrosion of the metal pipe. If the lead gap  54  between the leads  28 A  28 B was too small, there was potential for a short circuiting of the heating element  24  resulting in poor or completely failed electrofusion, or a ‘burn’ or bubbling at the site of the leads, resulting in poor regional fusion. According to one aspect of the invention is therefore provided spacer  48 . Spacer  48  comprises spacer region  56  which provides optimum lead gap  54  between leads  28 A and  28 B. Spacer also comprises tongues  52 A and  52 B which attach to a plurality of gaps  46  on either side of spacer region  56  utilizing buttons  50  which poke through gaps  46  and keep the spacer  48  in place. Spacer  48  is made of one piece of polymer, preferably polyethylene, of a type that is fusion compatible with both the outer surface layer  17  and the casing  36 , and is designed to be disposable—like the heating element  24 , after fusing, it is incorporated within the fused layers. 
         [0038]      FIG. 6  shows a perspective view of spacer  48  comprising spacer gap  56 , buttons  50 , and tongues  52 A,  52 B. As can be seen, spacer gap  56 , like buttons  50 , extends above the thickness of the heating element  24  to designed to hold leads  28 A  28 B, and thus heating element  24 , in place. 
         [0039]      FIG. 7  shows a further embodiment of the invention, which may be combined in preferred embodiments with the spacer of  FIG. 6 , modified in configuration in a manner which would be evident to a person of skill in the art.  FIG. 7  shows a close-up of the lead region comprising leads  28 A,  28 B of the heating element  24 . Here, the lead region is configured in a labyrinth shape, to maximize the heated surface area at the lead gap  54 . It has been found that labyrinth shaped lead region maximizes the heated surface area at the critical connections gap, thus maximizing the weldable surface area and improving the strength and reliability of the weld between the leads. 
         [0040]    It has been known that the temperature attained when electrofusing is very important to the integrity of the process. If too low a temperature is utilized, the polymer will not melt sufficiently and thus will fail to provide a strong, sealed bond between the outer surface coating and the inner surface of the casing. Paradoxically, if too high a temperature is utilized, blistering, melting and dripping of the polymer, and other problems can also contribute to a failure to provide a strong, sealed bond between the outer surface coating and the inner surface of the casing. For this reason, prior art methods utilizing electrofusion have utilized either a disposable or a re-usable thermocouple to measure and control the temperature at the interface between the outer surface coating and the inner surface of the casing. A thin, rod-like thermocouple is inserted between these two layers, and readings are taken while the electrofusion takes place. In some embodiments, the thermocouple automatically controlled the current running through the heating elements, thus controlling the temperature at the interface. 
         [0041]    It has been surprisingly found that the positioning of the thermocouple is critical. For example, the thermocouple readings at metal portions  44  will be different than thermocouple readings at gaps  46 —sufficiently different that the strength of the electrofusion bond is affected. Using prior art systems and methods, it is virtually impossible to determine whether the thermocouple is reading at a metal portion  44  or at a gap  46 , short of using an integrated, disposable thermocouple (which is undesirable and expensive) and, as a result, there was significant variation in overall heating temperature, and bond strength, from one electrofused casing to another. 
         [0042]    Accordingly, provided as a further embodiment of the present invention, and as shown in  FIG. 8 , is a polyethylene insert for securing the re-usable thermocouple within the heating mesh element to better and more accurately control the temperature during a thermofusion welding process between the outer surface coating and the inner surface of the casing. The use of such an insert provides temperature reading accuracy similar to the accuracy achieved in systems with integrated thermocouples while allowing re-use of the thermocouple element, thus reducing the cost of the system. 
         [0043]    Polyethylene insert  58  comprises backing  60  and buttons  62 , which are of a size, shape and configuration to poke through gaps  46  in the heating element  24  mesh and hold the polyethylene insert  58  in place. Insert  58  also comprises aperture  64  and channel  66 , both of a diameter designed to receive a standard and/or desired thermocouple probe (not shown). The thermocouple probe is inserted into aperture  64  and travels along channel  66 , to provide accurate placement of the probe for a temperature reading at metal portion  44 . Stopper  68  further positions the probe by prevention of movement beyond channel  66 . 
         [0044]    Polyethylene insert  58  is made of a material fusion-compatible with both outer surface coating and the inner surface of the casing, and is disposable in that it is melted and integrated within the fusion bond between the outer surface coating and the inner surface of the casing. 
         [0045]    In prior art methods of electrofusing a casing onto a cut-back region of a pipe, it was found to be desirable to add pressure to the casing before initiating the electrofusion. Adding this pressure in the form of hoop stress was found to improve the strength and integrity of the electrofusion bond. This addition of hoop stress was through the use of a heat-shrinkable casing, which was heat shrunk to the outer surface coating. However, this heat shrink step sometimes would not provide sufficient hoop stress, resulting in incomplete or sub-optimal electrofusion. A second method habitually used to add such pressure to the casing was the use of clamps which surround the casing and apply pressure, pushing the casing and the outer surface coating together. Use of clamps works well, but requires heavy, cumbersome clamps. In addition, different clamp sizes are typically required for different sizes of pipe. A better option was desired. 
         [0046]    According to an aspect of the present invention is provided a method for applying sufficient pressure to the casing to provide an improved electrofusion of the casing to the outer surface coating. 
         [0047]    Surprisingly, it has been found that sufficient pressure application can be provided with an adhesive tape. Accordingly, provided is a method for providing sufficient pressure between the inner surface of the casing and the outer surface coating by applying wrap around, self-adhering tape, after heat shrinking the ends of the (heat shrinkable) casing. The tape generally conforms to the shape of the pipe, an advantage over previously known rigid mould systems, which often result in pressure gaps, and provides uniform distribution of pressure around the entire circumference of the pipe. In a preferred embodiment, the self adhering tape is a fiber mesh reinforced, paper backed tape, with an adhesive that provides stickiness at room temperature, and which stays adhesive to a temperature of at least approximately 240° C. In this manner, the tape does not disintegrate, nor does it dissociate from the casing, at electrofusion temperatures. In ideal embodiments, the tape has the properties of low expansion and low elongation at high temperature (240° C.) and the adhesive is non-flowing at the polyethylene welding temperature of 240° C. 
         [0048]    The use of tape is also ideal in situations where the casing is cross-linked and not pre-stretched, or where the casing is not cross-linked. 
         [0049]    Tape  70  applied to the ends of a casing  36  applied onto a cut-back region of a pipe is shown in the photograph shown as  FIG. 9 . 
         [0050]    It has also been found that running a pre-warm cycle, to normalize the temperature around the heating element  24 , is desirable, immediately before the welding cycle is initiated. A pre-warming cycle normalizes the temperature around the circumference of the pipe, eliminating or decreasing undesirable temperature variations around the pipe. 
         [0051]    Typically, the inner and outer layers of the casing member as well as the jacket material  46  comprise polyolefin, more typically polyethylene. 
         [0052]    Thus, according to one embodiment of the invention is a method for attaching a casing  36  to a cut-back region  22  of a pipeline, wherein a casing  36  is disposed around the pipe joint, overlapping the ends of the outer surface jacket  16  on pipes  10  on either side of the cut-back region  22 . Annular electrically heatable heating elements  24 ,  26  are disposed on the outer side of the outer surface jacket  16  in the overlap region. 
         [0053]    Such heating elements  24 ,  26  are in themselves known for other purposes, and need not be described in detail herein. 
         [0054]    The side portions of the casing  36  are pressure clamped to the outer surface jacket  16  by tightly wrapping tape  70  around them. The heating elements  24 ,  26  are then energized using power sources (not shown) connected to leads  28 A,  28 B to cause fusion bonding between the casing  36  and the outer surface jacket  16 . 
         [0055]    Following fusion bonding of the sides of the casing  36  to the outer surface jacket  16 , the interior of the casing receives a precursor of a foam through a fill hole (not shown), and the foam is allowed to fully form and cure. 
         [0056]    In an alternative embodiment, instead of utilizing the tape  70 , the casing  36  has at least one heat shrinkable layer and the sides of the casing  36  are heat shrunk to the outer surface jacket  16  to create hoop stress at least equivalent in strength to the pressure provided by the tape  70 . 
         [0057]    In a further alternative embodiment, a strap (not shown) can be placed around the tape  70 . The strap is narrower than the metal heating element  24 ,  26  and is generally elongation-resistant at the temperatures it is subjected to in the welding process. During the welding process by which the casing  36  is fused to the outer surface jacket  16 , diameter of the casing  36 , outer surface jacket  16 , and/or tape  70  increases substantially. Since the strap is generally elongation-resistant, the diameter of the strap does not increase substantially during the welding process. This causes the polyolefin material of the outer surface jacket  16  and/or the casing  36  to be displaced from underneath the strap. Surprisingly, it has been found that such displacement—the actual movement of melted material—produces a higher quality weld than a wider strap system that extends beyond the edges of the heating elements. The amount of polyolefin displacement can be seen through ridges formed on both sides of the strap after completion of the welding process. Height and width of the ridges can be correlated to properties of the achieved weld.