Battered column tension leg platform

A tension leg platform includes a deck supported on the upper ends of three or more columns interconnected at the lower ends thereof by horizontally disposed pontoons. The columns are battered inwardly and upwardly from the pontoons to the deck. Tendons connected at the columns anchor the platform to the seabed. The footprints of the base of the battered columns and the tendons are larger than the footprint of the deck supported on the upper ends of the columns.

BACKGROUND OF THE DISCLOSURE

The present invention relates to offshore floating platforms, more particularly to a tension leg platform (TLP) for installation in water depths from less than 1,000 to 10,000 ft.

TLPs are floating platforms that are held in place in the ocean by means of vertical structural mooring elements (tendons), which are typically fabricated from high strength, high quality steel tubulars, and include articulated connections on the top and bottom (tendon connectors) that reduce bending moments and stresses in the tendon system. Many factors must be taken into account in designing a TLP to safely transport the TLP to the installation site and keep it safely in place including: (a) limitation of stresses developed in the tendons during extreme storm events and while the TLP is operating in damaged conditions; (b) avoidance of any slackening of tendons and subsequent snap loading or disconnect of tendons as wave troughs and crests pass the TLP hull; (c) allowance for fatigue damage which occurs as a result of the stress cycles in the tendons system throughout its service life; (d) limit natural resonance (heave, pitch, roll) motions of the TLP to ensure adequate functional support for personnel, equipment, and risers; (e) maximizing the hydrostatic stability of the TLP during transport and installation; and (e) accommodating additional requirements allowing for fabrication, transportation, and installation.

These factors have been addressed in the prior art with varying degrees of success. Conventional multi-column TLP's generally have four vertical columns interconnected by pontoons supporting a deck on the upper ends of the vertical columns. Tendons connected at the lower ends of the columns anchor the TLP to the seabed. In such conventional TLP designs, the footprints of the deck, the vertical columns and the tendons are substantially the same and therefore hydrostatic stability of the TLP can be a problem. Some TLP designs address this problem by incorporating pontoons and/or structures that extend outboard of the column(s) to provide a larger tendon footprint limit natural resonance (heave, pitch, roll) motions of the TLP. In U.S. Pat. No. 6,447,208, a TLP having an extended base substructure is disclosed. Vertical columns supporting a deck on the upper ends thereof form the corners of the substructure. A plurality of wings or arms extends radially out from the outer perimeter of the substructure. The arms increase the radial extension of the base substructure between about 10% and about 100%. The arms include tendon connectors affixed at the distal ends thereof for connection with tendons anchoring the TLP to the seabed. The tendons footprint is substantially larger than the footprint of the substructure.

The present invention, in its various embodiments, addresses the above-described factors to accommodate different payload requirements, various water depths and to improve TLP response. Improvement of TLP performance may be obtained by battering the deck support columns, thereby reducing tendon tension reactions, increasing the free floating stability of the TLP, and reducing overall system costs.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, a tension leg platform includes a deck supported on the upper ends of at least three columns interconnected at the lower ends thereof by horizontally disposed pontoons. The columns are battered inwardly from the pontoons to the deck. Tendons connected at porches extending outwardly from the lower ends of the columns anchor the platform to the seabed. The footprint of the tendons is substantially the same or slightly larger than the footprint of the battered columns, whereas the footprint of the deck is smaller than the footprint of the columns. The battered columns also contribute to platform stability during free floating operations by providing a large water plane dimension at shallow draft.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first toFIG. 1, a preferred embodiment of a TLP system in accordance with the present invention is generally identified by the reference numeral10. The TLP10includes four columns12having upper ends projecting above the water surface14for engaging and supporting a platform deck16thereon. Horizontally disposed pontoons18interconnect adjacent columns12proximate the lower ends thereof. The TLP10is anchored to the seabed by tendons20. The upper ends of one, two or more tendons20are connected at each column12and the lower ends thereof are anchored to the seabed. Tendon porches22mounted proximate to and outboard of the lower ends of the columns12secure the tendons20to the columns12.

The columns12and pontoons18form an open structure hull for supporting the deck16and the equipment mounted thereon above the water surface14. The deck16is supported above the water surface14on the upper ends26of the columns12. The open structure of the columns12and pontoons18provides improved wave transparency and further defines a moonpool24providing access to the seabed from the deck16. The columns12form the corners of the hull and are battered or inclined inwardly toward the central longitudinal axis of the hull. Preferably, the columns12are battered inwardly at an angle less than 20 degrees from vertical

Referring still toFIG. 1, the columns12include a substantially vertical section28forming the lower ends of the columns12and an inclined or battered section30terminating at the upper ends26of the columns12. The lower ends28of the columns12provide a vertical perimeter structural surface for connection of the pontoons18thereto. The tendon porches22are fixed to and extend outward from the lower ends28of the columns12. Connectors23may be fixed to and extend outward from the pontoons18for supporting risers25, flow lines or the like from the pontoons18. In addition, the TLP10may be provided with one or more catenary mooring lines or one or more lateral mooring lines to compensate for the weight of any risers or midwater pipelines connected to the TLP10.

The payload capacity of a TLP system is controlled by the displacement of the structure, as well as the ability of the system to resist overturning moments due to wind, waves, and current. The overturning resistance is lost when a tendon goes slack. For a given displacement and pretension, the overturning resistance is increased by having a larger horizontal plan baseline, i.e., a larger distance between tendons. In a conventional four column TLP, the deck is supported by vertical columns interconnected by pontoons or similar structural members. Consequently, the perimeter dimensions or footprints of the deck and the vertical support columns of a conventional TLP are about equal. The tendon plan dimension is limited to much this same perimeter dimension. The overturning capacity of the TLP is therefore limited by the overall dimensions of the deck and columns. This limitation is overcome by the TLP10of the present invention by battering the columns12so that the columns12footprint, defined by the perimeter dimension of the lower ends28of the columns12, is larger than the deck16footprint defined by the perimeter dimension of the upper ends26of the columns12. Also the battered columns12provide an efficient load transfer path for balancing deck weight, hull buoyancy, and tendon tension loads. All loads are direct acting through the columns12, without large cantilevers or large structural moments. As best shown inFIG. 2, the radial distance R1of the tendons20footprint from the central longitudinal axis of the TLP is substantially equal to or slightly greater than the radial distance R2of the columns12footprint. Since the moment force generated by the tendons20increases as the radial distance R1of the tendons20increases, minimizing the difference in radial distance between the columns12footprint and the tendons20footprint is desirable. Preferably, the radial distance R1of the tendons20footprint is less than 10% greater than the radial distance R2of the columns12footprint, thereby minimizing the tendons20moment force.

Various modes of transportation may be utilized to transport the TLP or components thereof to the installation site. When the hull and deck are assembled at the fabrication yard, the hull-and-deck assembly may be free floated to the installation site. For free floating conditions of the hull-and-deck assembly (such as deck integration, loading and unloading from a transport vessel, and towing to the installation site), hydrostatic stability is most lacking at shallow draft when the vertical center of gravity of the hull-and-deck assembly is high. The battered columns12of the TLP10provide a larger water plane dimension at shallower drafts of the free floating hull-and-deck assembly than a conventional TLP with vertical columns. As best illustrated inFIG. 3, the water plane dimension of the hull-and-deck assembly at the water surface14for a first draft position is represented by the line D1. At a shallower second draft position, the larger water plane dimension of the hull-and-deck assembly is represented by the line D2. Unlike the water plane dimension of a conventional TLP, which is the same at all drafts, the water plane dimension of the TLP10increases at shallower drafts of the free floating hull-and-deck assembly. The battered columns12therefore provide additional water plane dimension for maximizing TLP stability at shallower drafts where it is most needed, and thereby maximizing the payload capacity of the deck16during free floating phases of the TLP.

The balancing of hydrodynamic loads in waves is another aspect of the design of TLPs, semisubmersibles, and other column/pontoon structures. These platforms are typically optimized with regard to the ratio of volumes of surface piercing structure (vertical columns) and submerged structure (pontoons) in order to minimize the vertical forcing of waves. Under the crest of a wave, the upward force on the surface piercing structure is maximum upward, while the upward force on a submerged structure is maximum downward. Under a wave trough these are reversed. This balance is affected by the draft of the structure and the period of waves. Normally a structure is designed to have the vertical forces balanced and canceling in the most energetic wave periods. For a TLP, these are not the only forces acting, nor the only constraints on geometry, and the final design is a compromise of many factors of which this is one. However, for battered columns, the column begins to have pontoon characteristics with increasing batter. This may be used in the balancing of the structural proportions of the hull in order to provide best performance in waves for a particular site.

As noted above, inclination of the columns12imparts pontoon-like properties to the columns12which may be best understood by visualizing a horizontal cross section through the columns12at the water surface14and a shadow (shown in phantom inFIG. 3) formed by the sun located directly above. The portion P1of the columns12that is not under the shadow of the surface water plane has water acting both above and below, whereas the portion P2of the columns12that is under the shadow of the surface water plane has water acting only from below. The balance between the surface piercing buoyancy of the columns12and the non-surface piercing buoyancy of the pontoons18may therefore be modified without changing the actual dimensions of the columns12and pontoons18by increasing or decreasing the draft of the TLP10.

Referring now toFIG. 4, another embodiment of the battered column TLP of the present invention is generally identified by the reference numeral100. The TLP100is substantially the same as the TLP10described hereinabove with the exception that two of the columns12are battered toward each other above the pontoons18. It is understood however that the columns12may be inclined inwardly in any radial direction between 0° (shown in solid line) and 90° (shown in phantom). Thus, the TLP design of the present invention may accommodate various sizes and shapes of the deck16and payload capacity without changing the actual dimensions of the columns12and the pontoons18.

Referring now toFIG. 5, another embodiment of the battered column TLP of the present invention is generally identified by the reference numeral200. The TLP200is substantially the same as the TLP10described hereinabove with the exception that the lower ends of the columns12do not include a vertical dimension. The columns12illustrated inFIG. 4are inclined inwardly from the lower ends228to the upper ends26thereof.

Referring now toFIG. 6, another embodiment of the battered column TLP of the present invention is generally identified by the reference numeral300. The TLP300is substantially the same as the TLP10described hereinabove with the exception that the columns12include a battered section330extending inwardly from an intermediate point332between the upper ends26and the lower ends28of the columns12.

Referring now toFIG. 7, another embodiment of the battered column TLP of the present invention is generally identified by the reference numeral400. The TLP400is substantially the same as the TLP10described hereinabove with the exception that the columns12include a substantially vertical section426forming the upper ends of the columns12and an inclined or battered section430extending between the upper ends226and the lower ends28of the columns12.

Referring now toFIG. 8, another embodiment of the battered column TLP of the present invention is generally identified by the reference numeral500. The TLP500is substantially the same as the TLP10described hereinabove with the exception that the hull of the TLP500comprises three battered columns12interconnected by the pontoons18at the lower ends28and supporting the deck16at the upper ends26thereof.

It will be observed that the columns12and pontoons18are depicted as cylindrical members in the various embodiments of the present invention. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms and not intended to be limiting.

While a preferred embodiment of the invention has been shown and described, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims which follow.