Abstract:
The invention is an insulation material comprising a novolac cured polysulfide polymer resin. The material is used as a thermal insulator for subsea gas and oil production equipment, such as pipes and wellhead equipment. In order to decrease the maximum exothermic temperature generated by the resin during curing, hollow glass beads are added to the resin. The amine content of the hardener in a standard resin was adjusted to further decrease the maximum exothermic temperature. A fumed silica thixotropic material was also added to increase the viscosity of the mixture. These modifications allow the material to be cast in thick sections without cracking, leaking, or excessive expansion during curing.

Description:
This application is based on U.S. Provisional Patent Application No. 60/197,319, which was filed on Apr. 14, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is related to an insulation material for use on subsea oil and gas production equipment. 
     When subsea oil and gas wells are located at depths of 5,000 feet or more, the pipelines and wellhead equipment are exposed to seawater which is just a few degrees above freezing. This same temperature can exist in shallow water at extreme latitudes, such as in the North Sea. During a temporary well shutdown, hot produced fluids within the production equipment become stagnant and are cooled by the surrounding seawater. If the stagnant fluids approach the seawater temperature, hydrates can form in the equipment and block the flow of the fluid. 
     Thermal insulation is sometimes used around subsea pipelines and wellhead equipment to slow the cooling process and delay hydrate formation until flow can be restored. To perform successfully in this environment, a thermal insulation material must have a low thermal conductivity, maintain acceptable insulating and mechanical properties under hydrostatic compression and long term exposure to seawater, have a low rate of water absorption under high pressure, be economical to install, repair and remove on complex or irregular shapes, cure without cracking or leaking from a mold, be flexible and impact resistant, and have good adhesion to the insulated surfaces. 
     One method of insulating undersea systems involves the use of pre-cast sections of rigid epoxy-syntactic foam. This material comprises a rigid epoxy resin mixed with a high volumetric proportion of hollow glass or ceramic spheres. Although this material exhibits excellent thermal conductivity, it is very brittle. The installation process, which is laborious and expensive, involves casting the material into sheets which are then cut and shaped piecemeal to match the surface of the subsea equipment. Due to the rigidity and brittleness of this material, it is easily damaged when subjected to sudden impacts or high stress levels. To compound this problem, rigid epoxy-syntactic foams are difficult to repair. Removal or replacement of this material is extremely difficult because the sections are bonded to the surface using adhesives or mechanical fasteners. 
     An alternative to pre-cast epoxy-syntactic foams is a cast-in-place, rigid epoxy-syntactic, such as Textron TyMar 10K™. Unfortunately, these materials are inherently brittle and exhibit a high exothermic temperature on curing, which causes excessive thermal expansion. This combination of thermal expansion and brittleness results in extensive cracking when the material is cast in large sections. This material also exhibits a high rate of water absorption. Furthermore, when cracking does occur during handling or service, a protective resin coated fiberglass wrap is required to keep the material in place. 
     Alternate materials include urethane syntactics. However, these materials exhibit a higher rate of water absorption, and are relatively expensive. Also, the typically short curing times of urethane syntactics make them difficult to cast in large or complex sections. 
     SUMMARY OF THE INVENTION 
     The present invention is a thermal insulation material which comprises a matrix made from a novolac cured polysulfide polymer resin. The preferred resin is a modified version of Thiokol® FNEC 2515, in which the amount of the tertiary amine in the resin hardener has been reduced to slow the curing reaction and thereby decrease the maximum exothermic temperature generated during curing. The thermal insulation material also comprises a plurality of preferably hollow glass beads contained within the matrix to decrease the exothermic heat generated during curing and also improve the thermal conductivity of the material. In addition, a fumed silica thixotropic material may be added to the thermal insulation material to increase its viscosity. 
     The thermal insulation material of the present invention exhibits many advantageous properties which make the material particularly beneficial for use on subsea oil and gas production equipment. The matrix material is highly flexible, which makes the insulation material resistant to cracking under thermal or mechanical stress. In addition, the reduced exothermic heat decreases the thermal expansion rate exhibited by the insulation material during curing. Also, because of its flexibility and minimal thermal expansion, the insulation material of the present invention can be cast-in-place in thick sections without cracking. Furthermore, the increased viscosity of the insulation material prevents the mixture from leaking through seams in the mold during application, further improving the cast-in-place performance of the material The insulation material also exhibits a low rate of water absorption, and excellent adhesion to both bare metals and epoxy coatings. Thus, the material as cast exhibits mechanical and thermal properties which are well within acceptable limits for subsea equipment applications. Furthermore, these properties remain within acceptable limits even after prolonged exposure to water at high temperatures and pressures. 
    
    
     These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of the thermal insulation material of the present invention; and 
     FIG. 2 is a cross sectional view of a subsea christmas tree having the thermal insulation of the present invention applied thereto. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is an insulation material suitable for subsea oil and gas production equipment. Referring to FIG. 1, the insulation material, which is indicated generally by reference number  10 , comprises a matrix  12  of a suitable polymer based resin material and a plurality of micro beads  14 . In addition, the preferred insulation material  10  also comprises a viscosity modifier to increase the viscosity of the mixture prior to curing. 
     The matrix  12  preferably comprises a polysulfide based resin material. A preferred matrix  12  comprises a novolac cured polysulfide polymer resin. The preferred matrix  12  is based on a flexible epoxy novolac coating modified with polysulfide, which is available from Polyspec Corporation of Houston, Tex. under the brand name Thiokol® FNEC 2515. This material is modified by reducing the amount of the tertiary amine in the resin hardener to slow the curing reaction and thereby decrease the maximum exothermic temperature generated during curing of the insulation material  10 . The amount by which the tertiary amine is reduced is determined empirically to maintain a suitably low exothermic temperature within an acceptable cure time. In a preferred embodiment of the invention, the tertiary amine is reduced by an amount sufficient to maintain the exothermic temperature under about 200° F. A suitable matrix  12  can be obtained by mixing the resin known as “99-NovoTherm® A” with a the resin hardener known as “99-NovoTherm® B”, both of which are available from Polyspec Corporation. Alternatively, the matrix material  12  could comprise any other suitable resin, such as a manganese oxide cured polysulfide elastomer. 
     The addition of the micro beads  14  also contributes to a reduction in the maximum exothermic temperature generated during curing of the insulation material  10 . The micro beads  14  are preferably hollow, preferably glass beads having a mean diameter of up to about 85 microns. In order to withstand the hydrostatic pressure of a deep sea environment, the beads preferably have an isostatic strength of at least approximately 4,000 psi. Suitable micro beads for use in this application are Scotchlite™ S38 Glass Bubbles available from 3M Corporation. While glass micro beads  14  are preferred, the micro beads could be made of any suitable material, such as ceramic or a polymer. 
     The insulation material  10  should include as large a quantity of the micro beads  14  as possible to facilitate effective thermal insulation while maintaining the degrees of water absorption and brittleness of the insulation material below maximum acceptable levels. Thus, prior to adding the viscosity modifier, the insulation material  10  should include about 50-95% by volume of the matrix  12  and about 5-50% by volume of the micro beads  14 . Preferably, the insulation material  10  should include about 75-90% by volume of the matrix  12  and about 10-25% by volume of the micro beads  14 . More preferably, the insulation material comprises about 82-87% by volume of the matrix  12  and about 13-18% by volume of the micro beads  14 . In one embodiment of the invention, an acceptable insulation material  10  was achieved by mixing 85% by volume of the matrix  12  with 15% by volume of the micro beads  14 , as measured prior to adding the viscosity modifier. It should be noted that micro beads  14  of two or more mean diameters may be combined in order to achieve a larger concentration of the micro beads in the insulation material  10 . 
     The viscosity modifier could be any suitable substance which is effective in increasing the viscosity of the insulation material  10 . The preferred viscosity modifier is CAB-O-SIL® TS-720 Treated Fumed Silica, available from Cabot Corporation of Boston, Mass. In the embodiment of the invention wherein the insulation material  10  is cast, the insulation material should comprise between about 5 and 30 grams of viscosity modifier per liter of matrix  12 . Preferably, the insulation material comprises between about 5 and 20 grams of viscosity modifier per liter of matrix  12 . More preferably, the insulation material comprises between about 8 and 10 grams of viscosity modifier per liter of matrix  12 . In one embodiment of the invention, an acceptable insulation material  10  which was suitable for casting comprised 9.2 grams of viscosity modifier per liter of matrix. 
     Obviously, a more viscous insulation material  10  may be achieved by adding more viscosity modifier. Thus, in applications where it is desired to trowel the insulation material onto the equipment to be insulated, for example, to make repairs to a previously applied insulation material, the insulation material may comprise more viscosity modifier than the limits mentioned above. For example, an insulation material  10  which can be troweled on the equipment to be insulated may comprise between about 30 and 50 grams of viscosity modifier per liter of matrix  12 . More viscosity modifier may be employed to achieve an even more viscous insulation material; however, the concentration of viscosity modifier should be below the amount which would make the mixture so dry as to no longer be coherent. 
     An exemplary mixture of the insulation material  10  is made by mixing the following constituent substances in any order: 
     20 parts by volume of the modified Polyspec Thiokol® FNEC 2515 resin; 
     3 parts by volume of the Scotchlite® S38 Glass Bubbles; and 
     9.2 grams of CAB-O-SIL™ TS-720 Treated Fumed Silica per litter of resin. 
     When cast to a thickness of approximately 2.5 inches, this mixture reached a maximum exothermic temperature of 189° F., and expanded approximately 5 to 6% relative to the original volume when poured. Both of these measurements are within acceptable limits for the intended application. Listed below are the relevant thermal and mechanical properties of this material, both before after hydrostatic testing. All of these properties are within acceptable limits for the intended application. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Property 
                 Value 
               
               
                   
                   
               
             
             
               
                   
                 Hardness 
                 46 Shore D 
               
               
                   
                 Tensile Strength 
                 940 to 1030 psi 
               
               
                   
                 Tensile Elongation 
                 35 to 45% 
               
               
                   
                 Compressive Strength 
                 30,000 psi 
               
               
                   
                 Thermal Conductivity (dry) 
                 0.12 BTU/(hr · ft · ° F.) 
               
               
                   
                 Density 
                 67 lbs/ft 3   
               
               
                   
                 Specific Heat 
                 0.52 BTU/(lb · ° F.) 
               
               
                   
                 Thermal Diffusivity (dry) 
                 0.003 ft 2 /hr 
               
               
                   
                 Glass Transition Temperature 
                 13° C. 
               
               
                   
                   
               
             
          
         
       
     
     The thermal insulation material of the above example exhibited the following properties after exposure to water at 230° and 300 psi for 15 days. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Property 
                 Value 
               
               
                   
                   
               
             
             
               
                   
                 Water absorption 
                 5% 
               
               
                   
                 Thermal Conductivity (wet) 
                 0.13 BTU/(hr · ft · ° F.) 
               
               
                   
                 Hardness 
                 33 Shore D 
               
               
                   
                   
               
             
          
         
       
     
     The insulation material of the present invention can be used to thermally insulate any object from a surrounding fluid. In the preferred embodiment, the material is used to insulate undersea pipes and wellhead equipment from seawater. Referring to FIG. 2, the insulation material  10  is shown applied to certain portions of a subsea Christmas tree  16 . The subsea Christmas tree with which the present invention may be used can be of any known type of subsea Christmas tree, including the so-called horizontal and conventional christmas trees. Using FIG. 2 as an example, such trees typically include an axial production bore  28  in communication with the well bore, a production outlet  30  connected to the production bore, one or more production valves  32  for controlling flow through the production outlet  30 , a choke  20  connected to the production outlet  30  via a flow loop  24 , an annulus outlet  34  connected to the tubing annulus surrounding the production tubing (not shown), one or more annulus valves  36  for controlling flow through the annulus outlet  34 , and a production flow loop  26  for connecting the production outlet with an undersea pipe (not shown). Ideally, the insulation material is applied to those portions of the christmas tree  16  which are most exposed to the surrounding seawater and through which the produced fluids will flow. For example, in FIG. 2 the insulation material  10  is shown applied to the production valve block  18  housing one or more of the production valves  32 , the choke  20 , the annulus valve block  22  housing one or more of the annulus valves  36 , and the flow loops  24  and  26 . Of course, the insulation material  10  may be applied to additional or fewer components of the subsea christmas tree  16  as desired or required under particular circumstances. The thickness of the insulation material  10  in the direction of heat transfer is preferably between about 0.25″ and 2″, although the thickness may vary depending on the environment and the geometry of the surface to be insulated. 
     The insulation material  10  can be installed using a variety of methods. In the preferred method, a form or mold is constructed around the object to be insulated. The material is then cast between the object and the mold and allowed to cure. Once the material has cured, the mold is removed. Alternatively, the insulation material can be pre-cast into sections which are shaped to complement the object to be insulated. Once the pre-cast sections have cured, they may be secured to the object using adhesives, mechanical fasteners, or any other suitable means. The insulation material can also be sprayed on the object using a spray nozzle or similar device. 
     In accordance with an alternative embodiment of the invention, both the micro beads and the viscosity modifier could be omitted from the insulation material  10 . Although the casting performance of this embodiment is inferior to the preferred embodiment, this alternate embodiment exhibits superior flexibility and water absorption characteristics. 
     In accordance with another embodiment of the invention, the insulation material  10  comprises a base layer and an outer coating overlaying the base layer. The base layer is preferably an inexpensive, flexible thermally insulating material, such as any of the polysulfide based resins mentioned above. Other suitable base layer materials include a metal oxide cured polysulfide resin and a peroxide cured polysulfide resin. The outer coating preferably exhibits low water absorption under high pressure so as to form an effective sealant for the base layer. Suitable outer coating materials include any of the preferred insulation materials  10  discussed above. 
     It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.