Abstract:
A mounting system for supporting elongate, metallic structural members used in offshore applications and subject to dynamic forces comprising a support assembly having a socket that at least partially encircles the structural member, an attachment for securing the support assembly to a positioned structure, such as an offshore platform, and a sleeve received in the socket, the sleeve being disposed in surrounding relationship to the structural member and comprising an electrically nonconductive, fiber-reinforced polymer composite.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to mounting or support systems and, more particularly, to mounting or support systems for supporting elongate structural members found in offshore applications and subject to dynamic axial and lateral loads. 
     2. Description of the Prior Art 
     As the search for oil and gas is extended to progressively deeper offshore waters, greater demands are placed upon the structural components used in those activities. For example, marine riser systems may extend thousands of feet from a wellhead assembly to an offshore platform or vessel. These riser systems, which are sections of tubular components joined together, e.g., by threads, can be subjected to very high dynamic forces that act both axially and laterally on the riser strings. While typically the riser strings are constructed of steel, in an attempt to reduce hang-off loads and/or diminish static and cyclic bending stresses, as well as transmitted moments that generally increase with riser size and/or wave action, titanium alloy components are being incorporated into the riser strings. While steel and other ferrous-based riser strings and offshore components are effectively protected from the effects of corrosive seawater environments by sustained application of cathodic protection, if titanium alloy components are incorporated into the riser strings, it may be necessary to totally isolate impressed cathodic currents/potentials from these components to avoid long-term hydrogen adsorption and damage. Additionally, it is also prudent to preclude the detrimental effects of galvanic coupling between mating or adjoining dissimilar riser system alloys exposed to seawater and/or produced fluid brine through electrical isolation. 
     For dynamic riser systems, flex joints comprised of interspersed steel and rubber-layer laminate flex elements provide electrical isolation at the riser topside or subsea termination point. Alternative means of electrical isolation in dynamic riser systems using traditional, common electrical insulation materials, such as ceramics and polymers, has been largely unsuccessful because of the undesirable properties offered by these materials. Although ceramics possess elevated compressive strength, they are highly susceptible to physical damage and cracking due to their intrinsic brittleness and low toughness. Additionally, their high stiffness (high modulus) and low shear strength require extremely tight dimensional tolerances and close fit, which are often not achievable in the interface cross-section sizes required in riser systems. Although thermoset and thermoplastic polymers exhibit reasonable ductility, toughness, and durability, and are readily applied as coatings or sheet forms, their compressive bearing, creep, and fatigue strength properties are generally too low for these dynamic systems and rapidly diminish with increasing service temperature. 
     Over and above the problems of electrically isolating the titanium components from the steel components, dynamic loading on the titanium components can become a severe problem. Typically, elongate titanium alloy structural components, such as risers, are supported by steel support pods or sockets. Because of the cyclic lateral forces exerted on the elongate titanium components, there is a tendency for the titanium components to undergo fretting and fatigue failure generally at the junction of the titanium structural component and the steel support pod or socket. Thus, aside from the problems associated with electrical isolation between the steel components and the titanium components, there is also the problem of diminished fatigue life in the titanium component as a result of fretting at the juncture of the titanium component and the steel support. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a mounting system for structural members used in offshore applications and subjected to dynamic loading. 
     Another object of the present invention is to provide a mounting system for elongate structural members used in offshore applications that permits electrical isolation of the structural member from associated structures such as offshore platforms. 
     Another object of the present invention is to provide a mounting system for supporting elongate structural members used in offshore applications that minimizes fretting of the elongate structural member due to dynamic lateral loading. 
     The above and other objects of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. 
     In accordance with the present invention, there is provided a mounting system for elongate, metallic structural members used in offshore applications, e.g., titanium tubing, that are subject to dynamic forces. The mounting system includes a support assembly, the support assembly including a socket that at least partially encircles the structural member. There is an attachment for securing the support assembly to a positioned structure such as an offshore platform. A sleeve is received in the socket, the sleeve being disposed in surrounding relationship to the structural member. The sleeve comprises an electrically nonconductive, fiber-reinforced polymer composite having a through-thickness compressive strength above 25 ksi at temperatures at least as high as 120° C. and an S-N fatigue life under cyclic compressive through-thickness loading in excess of 1 million cycles at maximum compressive stress levels of ≦20 ksi. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view, partly in section, showing the mounting system of the present invention used to hang off a catenary riser system from an offshore platform; 
     FIG. 2 is a view taken along the lines  2 — 2  of FIG. 1; 
     FIG. 3 is a view taken along the lines  3 — 3  of FIG. 1; 
     FIG. 4 is a view similar to FIG. 3 showing another embodiment of the mounting system of the present invention wherein an elongate structural member is supported by both horizontal and vertical bearings; 
     FIG. 5 is a perspective view of an offshore platform anchored by a tension leg system; 
     FIG. 6 is an enlargement of the area of FIG. 5 circumscribed by the circle; and 
     FIG. 7 is an elevational, cross-sectional view of another embodiment of the present invention showing the tieback connector supported at a subsea wellhead. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference first to FIG. 1, the mounting system of the present invention is shown as being used to support a metallic catenary riser string extending from an offshore platform to a subsea wellhead (not shown). The platform, shown generally as P, has a hull portion, shown generally as H, that is below the seawater surface. Attached to the hull H is a framework, shown generally as F, having a bracket  10  that extends laterally from hull H and to which are secured upper and lower, generally T-shaped members  12  and  14 , respectively. Members  12  and  14  partially encircle and are secured to a socket, shown generally as  16 . Socket  16  has a first, upper frustoconical wall  18  and a second, lower, generally frustoconical wall  20 , frustoconical wall  20  terminating in a radially inwardly extending lip  22 . As best seen in FIG. 2, members  12 ,  14  and socket  16  do not form full annular structures, but rather collectively define a laterally opening slot  24  framed by a pair of guide plates  26  and  28 . 
     Received in socket  16  is a portion of a riser assembly, shown generally as R. Riser assembly R comprises a section of steel piping  32  that is connected to the top deck (not shown) of platform P. Steel piping  32  is connected via a flange connection  34  to a titanium stress joint  36 , which in turn is connected to a steel catenary riser  38  that extends to a subsea wellhead (not shown). As best seen with reference to FIG. 3, a removable clamshell-type bushing  28  surrounds stress joint  36 . Bushing  28  also has an inwardly facing frustoconical surface  30 . Because of slot  24 , it will be seen that the stress joint  36  carrying bushing  28  can be easily inserted into the socket  16  and, once positioned, will be prevented from any downward axial movement, bushing  28  resting on lip  22 . As best seen in FIG. 3, titanium stress joint  36  has a tapered upset portion  38 , the tapered upset portion  38  having an outer frustoconical surface  40  that is complementary to frustoconical surface  30  formed in bushing  28 . Disposed between surface  30  of bushing  28  and surface  40  of stress joint  36  is a sleeve  42  of a polymer composite, described more fully hereinafter. As can be seen, sleeve  42  is essentially sandwiched between stress joint  36  and bushing  28 . 
     It will be understood that the riser assembly R, which may extend for thousands of feet from the hull H of platform P, will be subject to cyclic forces due to current and wave action. Since the stress joint  36  is made of titanium or titanium alloy having a relatively low modulus of elasticity, and the socket  16 , including any bushings received therein are made of steel having a relatively high modulus of elasticity, the bending forces acting on the stress joint  36  can cause the stress joint  36  to undergo fretting, generally at the juncture of the titanium stress joint and the steel socket  16 , resulting in reduced fatigue life. Additionally, contact of the titanium stress joint with the steel socket could set up a galvanic couple and permit cathodic potentials to be infused on the titanium components. Therefore, it is necessary to electrically isolate the titanium stress joint from the steel socket. According to the present invention, this is accomplished by the use of the sleeve  42 , which serves. both as an electrical insulator and an antifretting bearing member. 
     Sleeve  42  is made of a polymeric composite that is electrically nonconductive and provides a high load-bearing, fatigue-resistant interface between the stress joint and the socket. The polymeric composites used to form the sleeves of the present invention will (a) have a through-thickness (short transverse) compressive strength above 25 ksi to temperatures at least as high as 120° C. and (b) provide an S-N fatigue life under cyclic compressive through-thickness loading in excess of 1 million cycles at maximum compressive stress levels of equal to or less than 20 ksi. Additionally, the material ideally should retain minimum compressive strength and fatigue properties after long-term seawater exposure to temperatures at least as high as 120° C., exhibit insignificant creep under compressive (through-thickness) bearing stresses of equal to or less than 25 ksi to temperatures at least as high as 120° C., and remain durable, monolithic, and electrically nonconductive in seawater up to at least 120° C. over an extended service life. 
     As used herein, the term “composite” or “composite material” refers to a combination of two or more materials (reinforcements and composite matrix binders) differing in form or composition on a macro scale. The constituents retain their identities-i.e., they do not dissolve or merge completely into one another, although they act in concert. Subject to meeting requirements (a) and (b) above, the composite can be comprised of a reinforcing filler supported in a polymeric matrix selected from the group consisting of thermoplastic resins, thermosetting resins, and mixtures thereof. Non-limiting examples of such thermosetting resins include expoxy resins, bismaleimide resins, polyimide resins, phenolic resins, polyurethanes, etc., and mixtures thereof. Non-limiting examples of thermoplastic resins that can be used in the composites to form the sleeves used in the present invention include polyether etherketones, polyphenylene sulfides, polyetherimides, polyamideimides, polyurethanes, etc., and mixtures thereof. It will also be appreciated that in certain cases it may be possible to use mixtures of thermoplastic and thermosetting resins, just as it is possible to use more than one type of filler or reinforcement in the composites. Preferred resins useful in forming the sleeves of the present invention are those characterized by a high degree of cross-linking (in the case of thermoset resins) and/or crystallinity and high-glass transition temperatures (T G ) and heat deflection temperatures (HDT). Typically, T G  and HDT values should exceed about 110° C. to provide sufficient compressive strength and modulus, and creep resistance under the high static and cyclic bearing loads to which the structural members are subjected. Additionally, to resist environmental attack, the composites should be highly resistant to hydrolysis and exhibit minimal water absorption (&lt;0.5 wt. %) to ensure good long-term property retention in seawater up to at least 120° C. 
     Non-limiting examples of reinforcements include glass fibers, aramid fibers, polybenzimidazole fibers, boron fibers, silicon carbide fibers, aluminum oxide fibers, etc. Preferred fibers are glass or aramid fibers, glass fibers, especially S-2 structural glass fibers, being preferred due to their high strength properties. Furthermore, a high density of continuous filaments is generally preferred over woven, fabric-mat, or short-chopped (random) fibers. An especially preferred composite is a laminate consisting of &gt;50 wt. % (typically 60-75 wt. %) continuous glass fibers in a polyether etherketone matrix. Generally speaking, the fiber-reinforced composites used in making the sleeve of the present invention are laminated with unidirectional (or fabric) layers at discrete angles to one another, such as in plywood, thereby distributing the in-plane load in several directions. These laminated structures can be obtained by hand or machine lay-up and filament winding. In lay-up, material that is usually in prepreg form is cut and laid up, layer by layer, to produce a laminate of the desired thickness, number of plies, and ply orientations. In filament winding, a fiber bundle or ribbon is impregnated with resin and wound upon a mandrel (in this case, the stress joint) to produce the sleeve. It will be understood that filament winding may use wet (or melted) resin or prepreg. In either case, the fiber placement process is followed by some type of cure or molding process. 
     Sleeve  32  can be formed in several ways. For example, the sleeve can be prefabricated on a mandrel to final shape and high dimensional tolerances and then positioned around the structural member using an environmentally resistant adhesive, e.g., an epoxy-based adhesive. Once the sleeve has been positioned on the structural member, the assembly can then be mated to the socket. Additionally, it is also possible to use the structural member as the mandrel to form the sleeve on the structural member. 
     In a specific example, a sleeve made of a nine-ply laminate (approximately 1.4 mm thick) comprised of 75 wt. % (61 vol. %) S-2 glass fiber-reinforced continuous glass fibers in a polyether etherketone matrix was formed. It was found that this composite had a T g  of about 143° C., provided exceptional long-term resistance to seawater exposure up to at least 120° C., and had a dielectric constant at 1 MHz, 24° C. of 4.60. 
     With reference now to FIG. 4, there is shown another embodiment of the present invention wherein a riser string R 1  is supported both laterally, or vertically, and horizontally using a sleeve and a bearing made of a composite material according to the present invention. With reference to FIG. 4, a socket, shown generally as  50 , comprises an outer or bowl  52  in which is received a bushing  54 . Received in bushing  54  is a collar  54  having a bore  58  through which a titanium stress joint  60  extends. Titanium stress joint  60  differs from stress joint  36  in that it is cylindrical rather than having an upset, frustoconical portion, such as  38  on stress joint  36 . Stress joint  60  also is provided with a radially outwardly, annularly extending flange  62 . Received in a recess  64  of collar  56  is a sleeve  66  made of a composite as described above. Collar  64  is also provided on its upper surface with a counterbore  68  in which is received a bearing  70 , bearing  70  being made of a composite material as described above. It can be seen that stress joint  60  is electrically isolated from socket  50  by means of bearing  70  and sleeve  66 . Additionally, the fretting of stress joint  60  is minimized by sleeve  66  much in the same manner that sleeve  42  acts to minimize fretting of stress joint  36 . It will be appreciated that socket  50  can be attached or secured to a positioned structure, such as an offshore platform, in a manner similar to that described above with respect to the embodiment shown in FIGS. 1-3. 
     With reference now to FIG. 5, there is shown a perspective view of an offshore platform, shown generally as  70 , platform  70  having an upper deck  72  on which is supported a derrick  74 . A subsurface deck  76  is tied to tension legs  78 , which in turn are secured to anchors or moorings  80  on the seabed. The attachment of tension leg  78  to platform  70  is shown in detail in FIG. 6, FIG. 6 showing an enlargement of the circled area A of FIG.  5 . With reference then to FIG. 6, a socket  82  is attached to platform  76  by a suitable mounting bracket  84 . Received in socket  82  is a bushing  86  that receives the upper terminus of tension leg  78 , tension leg  78  having a flared upper end  88  that has a frustoconical outer surface complementary to a frustoconical inner surface formed in bushing  86 . Sandwiched between the frustoconical surface in portion  88  of tension leg  78  and the frustoconical surface of bushing  86  is a frustoconical sleeve  90  made of a composite as describe above. Although sleeve  90  can be used to electrically isolate tension leg  78  from socket  82 , such is not normally a consideration since the tension legs  78  are frequently made of a metallic material similar to that of socket  82 ; however, sleeve  90  serves as a structural member to prevent fretting of tension leg  78  brought on by lateral loading from current or wave action. It will be understood that tension leg  78  can be secured to anchors  80  in the same manner in which it is secured to platform  70 . 
     With reference now to FIG. 7, there is shown yet another embodiment of the present invention wherein a tubular member, e.g., a tieback  90 , is attached to a wellhead  92 . A socket  94  has a radially outwardly extending flange  96  that is secured to wellhead  92  via nut and bolt assemblies  98  that are threadedly received in threaded bores  100  in wellhead  92 . Socket  94  has an inwardly facing frustoconical surface  102 . Received in socket  94  is the lower end  104  of tieback  90 , end  104  having a frustoconical outer surface  106  that is complementary to the frustoconical surface  102 . Disposed between socket  94  and the surface  106  of tieback  90  is a sleeve  108 , sleeve  108  being sandwiched between tieback  90  and socket  94 . As can be seen, flow path  110  through tieback  90  is in register with a flow path  112  in wellhead  92 , fluid-tight sealing between tieback  90  and wellhead  92  being accomplished by means of an annular fluid-tight seal  114 . 
     As used herein, “titanium” includes titanium itself, as well as alloys thereof. Additionally, while the invention has been described primarily with reference to titanium elongate structural members being supported by steel support assemblies, it will be understood that it is not so limited. 
     The foregoing description and examples illustrate selected embodiments of the present invention. In light thereof, variations and modifications will be suggested to one skilled in the art, all of which are in the spirit and purview of this invention.