Abstract:
High reliability power cables for subsea application are provided. Example power cables provide enhanced resistance to partial discharge dielectric breakdown as well as resistance to explosive gas decompression, by eliminating micro-defects and voids at the interface between the insulation layer and the barrier layer during the cable manufacturing process. Lead metal, which is conventionally extruded as a primary barrier layer, is replaced in the example power cables by a gas-and-fluid-resistant thermoplastic that is co-extruded or tandem extruded with surface-modified insulation to promote bonding between the two layers. Elimination of lead metal in the example power cables also significantly reduces their overall weight. The improved resistance to partial discharge and resistance to rapid gas decompression translates to lower workover and lower cost of ownership.

Description:
RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of priority to U.S. Provisional Patent Application No. 62/020,888 to Xiang et al., filed Jul. 3, 2014 and incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    Electric submersible pump (ESP) power cables and motor lead extensions (MLE) incorporate high integrity insulation and armor. Conventional cables may rely on lead layers to resist high temperatures and pressures, and corrosives in the well fluid. While the lead sheaths provide excellent barrier properties, the regulation of lead metal and its use can results in leaded cables with reduced reliability and lower lifespan. 
         [0003]    In conventional cable-making, lead metal is extruded onto the insulation in its own separate extrusion process. The insulation, however, has already been post-cured and does not bond very significantly with the molten lead. Due to this lack of interfacial bonding and a difference in the coefficient of thermal expansion between these materials, gas-filled micro-voids and impurities carried over from previous post-curing processes are introduced between the conventional insulation layer and the conventional lead sheath. 
         [0004]    The dielectric constant within the unintended micro-voids and gaps is less than that of the surrounding dielectric, allowing the possibility of partial discharges—localized dielectric breakdowns of the insulation. 
         [0005]    Gas-filled micro-voids or packets between the insulation and the conventional lead barrier may also cause barrier layer damage, such as lead barrier rupture during conditions when the cable is rapidly being depressurized or undergoing cyclic pressurization. 
       SUMMARY 
       [0006]    High reliability power cables for subsea application are provided. In an implementation, a power cable for an electric submersible pump (ESP), comprises an electrical conductor in the power cable, an elastomeric insulation layer around the electrical conductor, a fluoroplastic barrier layer around the elastomeric insulation layer, and a bonding layer between the elastomeric insulation layer and the fluoroplastic barrier layer, the bonding layer formulated to prevent a dielectric breakdown of the power cable and a rapid gas decompression breakdown of the power cable. An example method includes extruding an elastomeric insulation layer onto one or more components of a power cable for an electric submersible pump (ESP), including at least a conductor, creating an adhesive surface layer on the elastomeric insulation layer, and extruding a fluoroplastic barrier layer onto the adhesive surface to molecularly cross-link with the elastomeric insulation layer. Another example method includes extruding an elastomeric insulation layer comprising a fluoroplastic-compatible modified EPDM rubber onto one or more components of a power cable for an electric submersible pump (ESP), including at least a conductor, then preserving the fluoroplastic-compatible modified EPDM rubber from an ambient environment, extruding a fluoro-plastic barrier layer onto the fluoroplastic-compatible modified EPDM rubber, and creating an interface bonding surface between the fluoroplastic-compatible modified EPDM rubber of the elastomeric insulation layer and the fluoroplastic barrier surface. 
         [0007]    This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Certain embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate the various implementations described herein and are not meant to limit the scope of various technologies described herein. 
           [0009]      FIG. 1  is a diagram of example layers of an example power cable with improved interfacial bonding. 
           [0010]      FIG. 2  is a diagram of an example triple extrusion process for making a power cable with improved interfacial bonding using a fluoroplastic compatible elastomeric insulation layer. 
           [0011]      FIG. 3  is a diagram of an example extrusion process for making a power cable with improved interfacial bonding using a conventional elastomeric insulation layer. 
           [0012]      FIG. 4  is a flow diagram of an example method of making a power cable with improved interfacial bonding using a conventional elastomeric insulation layer. 
           [0013]      FIG. 5  is a flow diagram of an example method of making a power cable with improved interfacial bonding using a fluoroplastic compatible elastomeric insulation layer. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0015]    This disclosure describes high reliability power cables for subsea applications. Since subsea applications provide one of the most challenging environments for cables, the high reliability power cables described herein may also be used for most other applications. 
         [0016]    In an implementation, the pitfalls of a conventional lead sheath extruded onto an insulation layer in an ESP power cable are overcome with a tough fluoroplastic barrier layer made to bond well with the insulation layer. “Barrier,” as used herein, means that the layer resists an aggressive environment that may expose the power cable to wear, to high temperatures and pressures, and also means that the layer is relatively impermeable to, and inert against, hot and pressurized corrosives in the well fluid, such as dissolved carbon dioxide CO 2 , methane CH 4 , and hydrogen sulfide H 2 S. The fluoroplastic barrier layer and its methods of construction provide several advantages. First, the environmental and regulatory issues that accompany using lead metal are avoided. Second, the bonding interface between underlying elastomeric insulation and the fluoroplastic barrier is superior to that of conventional lead metal extruded onto the insulation layer. This stronger interfacial bonding between the insulation layer and the overlying fluoroplastic barrier, as described herein, prevents formation of unwanted micro-voids and air gaps. These voids and gaps between layers can allow “partial discharge” (PD) dielectric breakdown at high voltage. Third, the superior bonding interface that avoids the micro-voids and gaps between the insulation layer and the overlying fluoroplastic barrier also eliminates the possibility of the voids trapping gases, which can expand during rapid decompression to damage the power cable. 
       Example Implementations 
       [0017]      FIG. 1  shows an example power cable  100  with an improved bonding interface  102  formed between a fluoroplastic primer-adhesive layer  104  and an insulation layer  106 . The improved bonding interface  102  is shown in comparison with conventional micro-voids  150  formed at the interface  152  between a conventional lead barrier layer  154  and a conventional insulation layer  156 . The example fluoroplastic primer-adhesive layer  104  (“adhesive layer”  104 ) described herein, in turn, bonds well with an example fluoroplastic barrier  108  overlying the adhesive layer  104 . 
         [0018]    The term “power cable”  100 , as used herein, also includes motor lead extensions (MLEs) and includes power cables  100  with one or more solid or braided conductors. For clarity, the explanation of layers focuses on layers around a single conductor, used as an example. 
         [0019]    The example power cable  100  may include the following layer components: a conductor  110 , such as copper with a corrosion-resistant coating  112 ; the insulation layer  106 , such as polypropylene, EPR, EPDM, polyimide, or PEEK; the bonding interface  102 , such as the fluoroplastic primer-adhesive layer  104 ; and the fluoroplastic barrier layer  108 . The insulation layer  106  may be cured EPDM rubber (ethylene propylene diene monomer), a synthetic elastomeric rubber. 
         [0020]    In some implementations, a jacket layer (not shown), such as EPR &amp; EPDM, nitrile, fluoroplastic, etc., is extruded over the fluoroplastic barrier layer  108 . In turn, an armor layer (not shown), such as galvanized steel, a Monel alloy, an Inconel alloy, and so forth, may be further extruded or otherwise formed over the jacket layer. In an implementation, the example fluoroplastic barrier layer  108  described herein also takes the place of the jacket layer. In an implementation, the example fluoroplastic barrier  108  described herein takes the place of both the jacket layer and/or the armor layer, so that no lead metal whatsoever is used in the example power cable  100 . 
         [0021]    As mentioned above, conventional methods for extruding lead metal onto elastomeric insulation have the vulnerability of a lack of interfacial bonding between the lead metal and the elastomeric insulation, made worse by a difference in their coefficients of thermal expansion. This scenario allows gas-filled voids and micro-defects to form between the conventional insulation layer and the conventional lead barrier sheath. 
         [0022]    Since the dielectric constant of these unintended voids is considerably less than that of the surrounding dielectric, the electric field across the voids is significantly higher than that across an equivalent distance of the dielectric. If the voltage stress across the void is increased above a corona inception voltage for the gas trapped within the void, partial discharge (PD) electrical activity begins to occur. Partial discharge (PD) is a localized dielectric breakdown of a small portion of a solid (or fluid) electrical insulation system under high voltage stress, but which does not bridge the entire space or distance between two adjacent conductors. 
         [0023]    In a similar manner, micro-voids containing gases introduced during extrusion may be one of the leading causes for the partial discharge (PD) phenomenon. The resistance of the power cable to rapid gas decompression may be compromised by the existence of such gas micro-voids or gas packets located at the interface between insulation and barrier. The accumulation of gases at these imperfect interfaces leads to barrier layer damage, such as lead barrier ruptures during conditions in which the system is rapidly depressurized or undergoing cyclic pressurization. The improved bonding interface  102  of the example power cable  100  can prevent these vulnerabilities. 
         [0024]      FIG. 2  shows an example triple extrusion process  200  that can be applied in two passes, for example. In the second pass, a primer layer  202  may be preheated by blowing hot air, dry nitrogen, etc., followed by extruding another thicker fluoroplastic barrier layer  210  as the effective impermeable layer on top of this primer layer  202 . The primer layer  202  can be a thin fluoroplastic layer that does not interfere with outgassing of cure byproducts of an EPDM insulation layer  204  underneath it during post-curing. 
         [0025]    The example triple extrusion process  200  includes applying the primer layer  202  after application of the insulation layer  204  to a conductor  110 , such as a tie layer coated conductor  110 . Extrusion of the primer layer  202  is followed by a curing process, for example in a steam tube  206 , and a post-curing process  208 . Then, the fluoroplastic barrier layer  210  is applied at a separate cross head  212 . The triple extrusion process  200  can be implemented by co-extrusion cross heads  214 , or by separate tandem extrusion steps (not shown). 
         [0026]    In an implementation, in order to reduce the effect of surface imperfection introduced during a single pass extrusion process, the interface between insulation  204  and barrier layers  210  is protected from (not exposed to) the ambient environment, before barrier layer  210  extrusion. The enhanced extrusion process  200  utilizes triple extrusion or tandem extrusion and minimizes interfacial contamination of the insulation-barrier interface. 
         [0027]    In an implementation, the example power cable  100  includes an electrical conductor  110 , an elastomeric insulation layer  204  around the electrical conductor  110 , a fluoroplastic barrier layer  210  around the elastomeric insulation layer  204 , and a bonding layer  202  between the elastomeric insulation layer  204  and the fluoroplastic barrier layer  210 . The bonding layer  202  is formulated to prevent a dielectric breakdown of the power cable  100  and rapid gas decompression breakdown of the power cable  100 . 
         [0028]    The elastomeric insulation layer  204  may be a cured ethylene propylene diene monomer (EPDM) rubber. For example, the elastomeric insulation layer  204  may be a fluoroplastic-compatible modified EPDM rubber. The bonding layer  202  may be a fluoroplastic primer-adhesive  202  between the fluoroplastic-compatible modified EPDM rubber  204  and the fluoroplastic barrier layer  210 . 
         [0029]    The bonding layer  202  may include an agent for stripping fluorine atoms from the face of the fluoroplastic barrier layer  210 . When the fluoroplastic barrier layer  210  is extruded to encapsulate the fluoroplastic-compatible modified EPDM rubber  204 , the agent strips fluorine atoms from the face of the fluoroplastic barrier layer  210  to allow cross-linking bonds to form with the fluoroplastic-compatible modified EPDM rubber  204 . The agent may be a metal oxide and a dehydrohalogenating chemical, such as an onium compound, an organo-onium, an amidine, DBU, or DBN. Chemically, DBN is 1,5-Diazabicyclo[4.3.0]non-5-ene, with the compound formula C 7 H 12 N 2 . DBN is an amidine base. Chemically, DBU is 1,8-diazabicyclo[5.4.0]undec-7-ene. 
         [0030]    The fluoroplastic barrier layer  210  may be one of the following: a polyvinylidene fluoride fluoroplastic, a polyvinylidene difluoride (PVDF) fluoroplastic, a polyvinyl fluoride (PVF), an ethylene tetrafluoroethylene (ETFE), or a chlorotrifluoroethylene (CTFE). Alternatively the barrier layer  210  may be selected from the polyaryletherketone family such as a polyetherketone (PEK), a polyether ether ketone (PEEK), or a polyetherketone-etherketoneketone (PEKEKK), etc. 
         [0031]    In an implementation, the bonding layer  202 , which is the primer or primer-adhesive  202  applied to the fluoroplastic-compatible modified EPDM rubber insulation layer  204 , can be a synthetic rubber fluoropolymer elastomer, a terpolymer of tetrafluoroethylene (TFE) and vinylidene fluoride (VF2) and hexafluoropropylene (HFP), such as VITON (DuPont Performance Elastomers LLC, Wilmington, Del.), or a fluorothermoplastic of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride, such as DYNEON THV 500 GZ (3M Company, St. Paul, Minn.). A reactive PVDF or fluoropolymer, such as THV (the above fluoroplastic consisting of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride), has excellent chemical and permeation resistance and low flammability and can be formulated into a hydrocarbon elastomer insulation compound such as EPDM/EPR which later reacts and bonds to the fluoropolymer barrier layer  210  applied next (EPR contains only the ethylene and propylene monomers, whereas EPDM also contains a diene monomer). 
         [0032]    The fluoroplastic barrier layer  210  may include a blocking agent to increase the impermeability of the fluoroplastic barrier layer  210 , such as clay, carbon black, graphene, graphite, talc, mica, silica, or metal particles/nanoparticles, and so forth. 
         [0033]    In an implementation, the fluoroplastic barrier layer  210  may also assume the role of a jacket layer in the power cable  100  or an outer armor layer of the power cable  100 . 
         [0034]      FIG. 3  shows an example tandem co-extrusion process  300  using an insulation layer  302  that may be a conventional EPDM rubber, for example, over a tie layer  304  on the conductor  110 . The example co-extrusion process  300  is followed by curing, e.g., via a steam tube  306 , and post-curing  308 , which is then followed in turn by a surface activation  310  of the insulation layer  302  via plasma activation or electron beam activation, for example. The activated EPDM insulation layer  302  may then undergo co-extrusion with an adhesive  312  and the impermeable fluoroplastic barrier layer  314 . The co-extrusion steps can be performed via respective co-extrusion cross heads  316  &amp;  318 , or in separate tandem extrusion processes. 
         [0035]    The plasma activation may be weakly ionized oxygen plasma, for example, to activate the surface of the insulation layer  302 . The adhesive fluoroplastic layer  312 , such as one of the PFA adhesives, can be coextruded with the impermeable fluoroplastic barrier layer  314 , with or without a blocking agent such as graphene nanoparticles, for example, as a filler to further increase the tortuosity of the permeation path for gases and liquids. 
       Example Methods 
       [0036]      FIG. 4  is a flow diagram of an example method  400  of making an ESP power cable. Operations of the example method  400  are shown as individual blocks. 
         [0037]    At block  402 , an elastomeric insulation layer is extruded onto one or more components of a power cable for an electric submersible pump (ESP), including at least a conductor. 
         [0038]    At block  404 , an adhesive surface layer is created on the elastomeric insulation layer. 
         [0039]    At block  406 , a fluoroplastic barrier layer is extruded onto the adhesive surface to molecularly cross-link with the elastomeric insulation layer. 
         [0040]    The elastomeric insulation layer may be a cured ethylene propylene diene monomer (EPDM) rubber, and creating the adhesive surface layer may include activating a surface layer of the EPDM rubber with a plasma activation or an electron-beam activation. 
         [0041]    Creating the adhesive surface layer on the elastomeric insulation layer may further include extruding a fluoroplastic primer or adhesive onto the elastomeric insulation layer prior to extruding the fluoroplastic barrier layer. 
         [0042]    The method  400  may use a triple extrusion process or tandem extrusion processes. 
         [0043]    The fluoroplastic barrier layer may be one of a polyvinylidene fluoride fluoroplastic, a polyvinylidene difluoride (PVDF) fluoroplastic, a polyvinyl fluoride (PVF), an ethylene tetrafluoroethylene (ETFE), or a chlorotrifluoroethylene (CTFE). Alternatively the barrier layer  210  may be selected from the polyaryletherketone family such as a polyetherketone (PEK), a polyether ether ketone (PEEK), or a polyetherketone-etherketoneketone (PEKEKK), etc. 
         [0044]      FIG. 5  is a flow diagram of an example method  500  of making an ESP power cable. Operations of the example method  500  are shown as individual blocks. 
         [0045]    At block  502 , an elastomeric insulation layer comprising a fluoroplastic-compatible modified EPDM rubber is extruded onto one or more components of a power cable for an electric submersible pump (ESP), including at least a conductor. 
         [0046]    At block  504 , the fluoroplastic-compatible modified EPDM rubber is preserved from an ambient environment; 
         [0047]    At block  506 , a fluoroplastic barrier layer is extruded onto the fluoroplastic-compatible modified EPDM rubber. 
         [0048]    At block  508 , an interface bonding surface is created between the fluoroplastic-compatible modified EPDM rubber of the elastomeric insulation layer and the fluoroplastic barrier surface. 
         [0049]    The method  500  may further include applying a fluoroplastic primer-adhesive as the interface bonding surface between the fluoroplastic-compatible modified EPDM rubber and the fluoroplastic barrier layer. The fluoroplastic primer-adhesive can be selected from the following: a synthetic rubber fluoropolymer elastomer, a terpolymer of tetrafluoroethylene (TFE) and vinylidene fluoride (VF2) and hexafluoropropylene (HFP), or a fluorothermoplastic of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride. 
         [0050]    The method  500  may further include adding an agent to the fluoroplastic primer-adhesive for stripping fluorine atoms from a face of the fluoroplastic barrier layer. When the fluoroplastic barrier layer is extruded to encapsulate the fluoroplastic-compatible modified EPDM rubber, the agent strips fluorine atoms from the face of the fluoroplastic barrier layer to allow molecular cross-linking between the fluoroplastic barrier layer and the fluoroplastic-compatible modified EPDM rubber. The agent may be a metal oxide and a dehydrohalogenating chemical, such as an onium compound, an organo-onium, an amidine, DBU, or DBN. 
       Conclusion 
       [0051]    Although a few embodiments of the disclosure have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.