The present invention relates to a method and apparatus for supporting risers in offshore applications. More particularly, the present invention relates to a method and system for supporting risers connecting subsea facilities such as wells or manifolds at the ocean floor to surface facilities such as Christmastrees or other production facilities provided on spar-type platforms or the like.
In traditional bottom founded platforms having fixed or rigid tower structures known as platform jackets, risers are provided lateral support with drive pipes or conductors installed through the length of the platform's structural framework. The conductors are large diameter tubular goods connected to the framework with conductor guides installed at frequent intervals along the length of the platform jacket. Although ultimately supporting production risers, the conductors also serve to guide the drill string, subsea valve placement, and other drilling and completion equipment prior to installing the production risers.
However, traditional bottom founded platforms have been taken to the logical depth limitations in the development of offshore oil and gas reserves. Economic and technical considerations suggest that alternatives to this traditional technology be used in the development of deepwater prospects.
Spar platforms provide an alternative which can support offshore developments in very deep water more economically than traditional fixed platforms. Spars platforms may be adapted to a number of configurations, including drilling platforms and drilling and production platforms. Further, spar platforms lend themselves well to a promising design premiss using minimal structures, e.g., spars with minimal completion and workover facilities or even minispars dedicated exclusively to production operations.
Unlike traditional fixed platforms, spar platforms and other deepwater concepts are designed to "give" in a controlled manner in response to dynamic environmental loads rather than to rigidly resist those forces. However, an attraction of the spar design is its characteristic resistance to heave and pitch motions. Nevertheless, spars are subject to sufficient motion at the ocean surface that this can be a significant design parameter for connected components such as risers which may be subject to significant relative motion with facilities on the spar platform. The risers are subject to buckling failure should a sufficient net compressive load develop in the riser. This would collapse the pathway within the riser necessary for drilling or production operations. Similarly, excess tension from uncompensated support can also damage the riser.
Thus, direct support from the spar platform to the riser requires a riser tensioning system with motion compensation capability to accommodate this relative motion. Whether active or passive, this requires major structural components with long life and moving parts in the harsh offshore environment.
Alternatively, the riser may be independently supported, e.g., by sufficient, submerged buoyancy modules to ensure a net tension (or at least to avoid an excess compressive load) to protect riser integrity throughout the range of riser motion. However, a riser to platform interface is still required that can accommodate relative motion between the riser and the spar. Yet this interface must not interject excessive bending moments and/or wear on the riser. This may be a particular problem with spars which have substantial lengths of vertically depending structure through which the risers must pass to connect topside facilities with equipment at the ocean floor.
This length of vertical structure raises other difficulties in just passing production risers or even the drill string or other equipment to the wells or subsea manifold.
An advantage of a deepwater riser system in accordance with the present invention is that it securely supports the riser and accommodates relative motion between the riser and platform while avoiding the need for complex motion compensating machinery and protects the riser from loads and wear at an interface.
A further advantage of the riser system of the present invention is that it provides lateral support to a substantial length of the production riser within the spar structure.
Yet another advantage of a riser system in accordance with the present invention is its availability for guiding a drill string, production riser, or other equipment run or installed through the vertical span of the spar platform.