Patent Application
20070201955
Not Applicable
Not Applicable
This invention relates to floating offshore storage, drilling or production platforms and more particularly, platforms of the Spar type. More specifically, the invention relates to an improved interface connection assembly between the Spar hull and a caisson extending downwardly therefrom.
Floating platforms are typically employed in water depths of about 500 ft. (approximately 152 m) and greater, and are held in position over a seabed well site by mooring lines anchored to the sea floor, or by motorized thrusters located on the sides of the platform, or both. Although floating platforms are more complex to operate because of their movement in response to wind and water conditions, they are capable of operating in substantially greater water depths than are fixed platforms, and are also more mobile, and hence, easier to move to other well sites. There are several different types of known floating platforms, including so-called “drill ships,” tension-leg platforms (“TLPs”), “semi-submersibles,” and “Spar” platforms.
Spar-type platforms comprise an elongate, substantially cylindrical (or multi-cylindrical) buoyant hull that supports one or more decks above the surface of the water when floating in an upright operating position, in which an upper portion of the platform hull extends above the waterline and a lower portion is submerged below it. Because part of the platform hull is above the water, it is subject to forces of wind, waves, and water currents which cause a constant movement of the platform. Generally, a Spar-type floating platform is designed to be installed for a 25 to 30 year service life.
Despite their relative success, Spar-type platforms include some aspects that need improvement to accommodate various applications and/or a larger spectrum of environmental conditions. For example, in typical Spar designs and configurations, at least one tubular caisson extends downwardly from the bottom of the Spar hull. The caisson(s) may extend some distance below the Spar, or to the sea floor. Often the inside of the caisson will include a plurality of tubulars and/or control bundles. Because the caisson extends below the floating spar hull, there is typically relative, flexing motion at the interface connection area between the Spar hull and the caisson. Currently existing hull-to-caisson interface connections do not accommodate very high tensile loads, the bending/flexing relative motion between the spar hull and the caisson for the life of the installation, and the very demanding fatigue life. Since the wind, waves, and water currents are always present in various intensities, the hull/caisson interface connection must be designed to accommodate the very demanding fatigue life due to the high loads and constant relative motion.
It would therefore be desirable for an improved Spar hull-to-caisson interface connection that accommodates high tension loads between the spar hull and the caisson while still facilitating angular, flexing motion (rotation) while also simultaneously constraining lateral movement and vertical axial movement. The result would be highly-improved fatigue characteristics of the interface between the Spar hull and the caisson.
In accordance with the present invention, a Spar hull-to-caisson interface connection assembly is provided that accommodates high tension loads between the hull and caisson of a Spar-type platform, while facilitating angular, flexing motion (rotation) and simultaneously constraining lateral movement and vertical axial movement. Broadly, the interface connection assembly comprises an annular bearing shoulder provided at the top end of caisson, and a laminated bearing assembly mounted in the bottom end of the hull and defining a passage through which the upper portion of the caisson passes, so that the caisson shoulder seats against the upper end of the bearing assembly. The bearing assembly comprises a laminated structure of alternating steel and elastomer flex elements that are bonded to each other so that they flex together as a unit, rather than sliding relative to each other. The laminated structure of the bearing assembly supports the vertical tensile loads applied by the weight of the caisson on the hull, while also allowing a smooth and efficient accommodation of the angular and rotational loads applied between the caisson and the hull, so as to reduce fatigue at the hull/caisson interface.
A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings.
Referring first to
The platform 10 further comprises at least one cylindrical caisson 20 that extends vertically down from the hull 12. The caisson 20 is coupled to the hull 12 by means of an interface bearing assembly 22 (
Drilling, production, and export from the platform 10 all require vertical conduits through the water column between the seabed 28 and the platform 10. These conduits are typically provided by a plurality of risers 30 that extend from the seabed 28, upwardly through the caisson 20 and the hull 12 to the deck 16. In a typical Spar-type platform, the risers 30 may be secured to the deck 16 by tensioning devices (not shown), or they may be supported on the deck and descend therefrom in a modified catenary shape to the seabed 28. Other means, well-known in the art, may be employed to support individual risers, especially those of a smaller diameter.
Turning now to
The configuration of one of the steel flex rings 38 is illustrated in
Although the flex elements 38, 40 are shown, in this first exemplary embodiment, as being continuous annular elements, they may be configured as a plurality of discrete cylindrical flex element stacks disposed in an annular arrangement, as discussed below in connection with the embodiment shown in FIGS. 8 an 9.
From
In a preferred embodiment of the invention, there are preferably about ten to about thirty each of the steel rings 38 and the elastomeric rings 40, vulcanized and bonded together (and to the upper bearing ring 34 and the lower support ring 36) by any suitable means known in the art to form a laminated structure in which the flex elements 38, 40 flex together, instead of sliding, in response to angular motions of the caisson 20 relative to the hull. By thus flexing, instead of sliding, the flex elements 38, 40 avoid the so-called “slip-stick” effect, in which relative motion between the caisson 20 and the hull 12 would only occur only when the static friction forces between adjacent flex elements are overcome by the angular flexing and bending loads to which the caisson 18 and the hull 12 are subject. This “slip-stick” effect would thus cause a “jerking” action, inducing erratic bending moments in the caisson, with a resultant reduction in the fatigue life of the hull/caisson interface. By eliminating this “slip-stick” effect, the bearing assembly 22, with its laminated flex elements 38, 40, starts to flex with any flex-inducing load applied to the caisson 20 and/or the hull 12, instead of requiring a load that exceeds the friction forces between unbonded flex elements. Thus, the laminated (bonded) flex element arrangement in the bearing assembly 22 substantially eliminates erratic bending moments.
The upper end of the caisson 20 may advantageously include a stress joint portion 42 where the caisson 20 joins the shoulder 32. The stress joint portion 42 is formed with a tapered wall thickness that gradually increases as it approaches the shoulder 32 along a radiused juncture 44. This feature provides an improved distribution of stress within a bending tubular member, such as the caisson 20.
The function of the hull/caisson interface connection assembly of the present invention is illustrated in
A hull/caisson interface connection assembly, in accordance with a second preferred embodiment of the invention, is illustrated in
A bearing assembly, comprising a plurality of substantially cylindrical flex element stacks or blocks 78, is mounted between the lower surface 64 of the caisson shoulder 62 and the support surface 72 of the receptacle 70, so as to surround coaxially the upper portion of the caisson 60 that is above the receptacle 70. Specifically, each of the flex element stacks or blocks 78 has a steel top bearing element 77 that is seated in one of the upper sockets 66, and a steel bottom support element 79 that is seated in a corresponding one of the lower sockets 74, so that the stacks or blocks 78 are arranged around the periphery of the caisson 60 and extend radially outward therefrom, much like spokes on a wheel, as best shown in
As shown in
Each of the flex element stacks or blocks 78 flexes as a unit, as does the above-described arrangement of annular flex element rings 38, 40 in the first embodiment. Furthermore, the above-described advantages of the first embodiment are also achieved in this second embodiment. Although eight flex element stacks or blocks 78 are shown in the illustrated embodiment, the number of the stacks or blocks 78, their specific physical dimensions, and the number of individual flex elements 80, 82 forming each stack or block 78, will vary according to the specific needs and demands of the Spar structure in which they are employed. One advantage of this second embodiment is that the use of multiple flex element stacks or blocks removes constraints on the size of the individual flex elements.
Although an exemplary embodiment of the invention has been described above by way of example only, it will be understood by those skilled in the field that modifications may be made to the disclosed embodiment without departing from the scope of the invention, which is defined by the claims that follow.
