Offshore platform with outset columns

A hull, suitable for use as a tension leg platform (TLP) or as a semi-submersible vessel, comprises columns having a generally polygonal transverse cross section at least one axis of which is generally radially aligned with the central vertical axis of the hull. Buoyant, subsurface pontoons interconnect adjacent columns. The pontoons are generally rectangular in cross section and the outboard, generally vertical surface of each pontoon is connected to a side surface of an adjoining column at a location which is substantially inboard of the outermost face of the column. In certain embodiments of the invention, tendon porches (configured to receive the upper tendon connectors of a TLP) are mounted to the outboard surface of one or more pontoons.

ROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to offshore platforms. More particularly, it relates to tension leg platforms (TLPs).

A tension leg platform (TLP) is a vertically moored floating structure typically used for the offshore production of oil and/or gas, and is particularly suited for water depths greater than about 1000 ft.

The platform is permanently moored by tethers or tendons grouped at each of the structure's corners. A group of tethers is called a tension leg. The tethers have relatively high axial stiffness (low elasticity) such that virtually all vertical motion of the platform is eliminated. This allows the platform to have the production wellheads on deck (connected directly to the subsea wells by rigid risers), instead of on the seafloor. This feature enables less expensive well completions and allows better control over the production from the oil or gas reservoir.

A semi-submersible is a particular type of floating vessel that is supported primarily on large pontoon-like structures that are submerged below the sea surface. The operating decks are elevated perhaps 100 or more feet above the pontoons on large steel columns. This design has the advantage of submerging most of the area of components in contact with the sea thereby minimizing loading from wind, waves and currents. Semi-submersibles can operate in a wide range of water depths, including deep water. The unit may stay on location using dynamic positioning (DP) and/or be anchored by means of catenary mooring lines terminating in piles or anchors in the seafloor. Semi-submersibles can be used for drilling, workover operations, and production platforms, depending on the equipment with which they are equipped. When fitted with a drilling package, they are typically called semi-submersible drilling rigs.

The DeepDraftSemi® vessel offered by SBM Atlantia, Inc. (Houston, Tex.) is a semi-submersible fitted with oil and gas production facilities that is suitable for use in ultra deep water conditions. The unit is designed to optimize vessel motions to accommodate steel catenary risers (SCRs).

A variety of TLP and semi-submersible designs are known in the art. The following patents describe various examples.

U.S. Pat. No. 7,462,000 discloses a tension leg platform that 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.

U.S. Pat. No. 4,585,373 describes a tension leg platform with exterior buoyant columns located outside the normal tension leg platform structure. The exterior columns are designed to decrease the pitch period of the tension leg platform away from the point of concentration of the largest wave spectrum energy encountered at a particular marine location. This modification of the pitch period of the tension leg platform is said to reduce the cyclic fatigue stresses in the tension legs of the platform thereby increasing the useful life of the platform structure.

U.S. Pat. No. 6,024,040 describes an off-shore oil production platform that includes an upper barge above the level of the sea. The barge is connected to a completely submerged hollow lower base by partially submerged vertical connecting legs forming a buoyancy tank. The legs along their submerged height include at least two successive portions. A first portion with solid walls delimits a closed space and forms a buoyancy tank. A second portion with openwork sidewall has an interior space that is open to the surrounding marine environment.

U.S. Pat. No. 6,652,192 describes a heave-suppressed, floating offshore drilling and production platform with vertical columns, lateral trusses connecting adjacent columns, a deep-submerged horizontal plate supported from the bottom of the columns by vertical truss legs, and a topside deck supported by the columns. The lateral trusses connect adjacent columns near their lower end to enhance the structural integrity of the platform. During the launch of the platform and towing in relatively shallow water, the truss legs are stowed in shafts within each column, and the plate is carried just below the lower ends of the columns. After the platform has been floated to the deep water drilling and production site, the truss legs are lowered from the column shafts to lower the plate to a deep draft for reducing the effect of wave forces and to provide heave and vertical motion resistance to the platform. Water in the column shafts is then removed, lifting the platform so that the deck is at the desired elevation above the water surface.

U.S. Pat. No. 3,982,401 describes a semi-submersible marine structure for operation in offshore waters that comprises a work deck which is supported by a buoyant substructure. The substructure includes a separable anchor unit which can be lowered to the floor of the offshore site and thereafter weighted in order to regulate the position of the floating structure. Tensioning lines extending between the anchor and the structure draw the latter downward below its normal floating disposition. Outboard anchor lines are used to locate the structure laterally with respect to its position over a drill site.

U.S. Pat. No. 6,347,912 describes an installation for producing oil from an off-shore deposit that includes a semi-submersible platform, at least one riser connecting the platform to the sea bed, and devices for tensioning the riser. The tensioning devices for each riser include at least one submerged float connected to a point on the main run of the riser for hauling it towards the surface, and a mechanism for hauling the riser. The mechanism is installed on the platform and applied to the top end of the riser.

U.S. Pat. No. 5,558,467 describes a deep water offshore apparatus for use in oil drilling and production in which an upper buoyant hull of prismatic shape has a passage that extends longitudinally through the hull. Risers run through the passage and down to the sea floor. A frame structure connected to the hull bottom and extending downwardly comprises a plurality of vertically arranged bays defined by vertically spaced horizontal water entrapment plates providing open windows around the periphery of the frame structure. The windows provide transparency to ocean currents and to wave motion in a horizontal direction to reduce drag. The frame structure serves to modify the natural period and stability of the apparatus to minimize heave, pitch, and roll motions of the apparatus. A keel assembly at the bottom of the frame structure has ballast chambers for enabling the apparatus to float horizontally and for stabilization of the apparatus against tilting in the vertical position.

U.S. Pat. No. 4,850,744 describes a semi-submersible, deep-drafted platform which includes a fully submersible lower hull, and a plurality of stabilizing columns which extend from the lower hull to an upper hull. At least one column has means adapted to reduce the water plane area within a portion of the dynamic wave zone of the column and to increase the natural heave period of the platform.

U.S. Pat. No. 4,723,875 describes a deep-water support assembly for a jack-up type marine structure that comprises a support base, pile guides in the base through which piles are driven to anchor the support base to a marine floor, a receptacle containing a grouting material and adapted to mate with the jack-up structure for providing an anchoring foundation, and a support structure for supporting the receptacle at a fixed height below the marine surface. In one version, a tension leg support assembly is provided in place of the tower assembly. The tension leg assembly also comprises a support base structure, means for anchoring the support base structure to the marine floor, and receptacle means containing a grouting material and adapted to mate with the jack-up structure for providing an anchoring foundation. However, the receptacle means is provided with ballasting and de-ballasting chambers which permit the receptacle means to be employed as a tension leg platform which can be supported from the base structure by tension cables acting in opposition to the buoyancy forces created by de-ballasting the platform once the cables have been secured to the ballasted receptacle means during assembly.

U.S. Pat. No. 3,837,309 describes a floating offshore device that includes a water tight hull, which is adapted to be ballasted to a submerged stage and, when submerged, retained in position by buoying means that can sway relative to the hull. Structural columns fastened to the vessel extend above the water and support a floatable platform above the water when the device is in operable working position. The platform rests on the vessel when the device is being moved.

U.S. Pat. No. 4,169,424 describes a tension leg buoyancy structure for use in seas exposed to wave action that includes a buoyancy section, an anchor section which rests on the sea bed, and a plurality of parallel tethers connecting the buoyancy section with the anchor section to permit the buoyancy section to move relative to the anchor section. Design parameters are selected such that the natural period of the buoyancy section for linear oscillation in the direction of wave travel, the natural period of the buoyancy section for linear oscillation in a horizontal direction perpendicular to the direction of wave travel, and the natural period of the buoyancy section for rotational oscillation about a vertical axis of the buoyancy section structure are greater than 50 seconds.

U.S. Pat. No. 4,906,139 describes an offshore well test platform system that comprises a submerged buoy restrained below the surface of the water by a plurality of laterally extending, tensioned cables, a platform structure connected to a submerged buoy with an upper portion that extends above the surface of the water, and a flexible riser that connects the well to a well test platform deck above the surface of the water.

U.S. Pat. No. 5,012,756 describes a floating structure with completely or partially submersible pontoons that provide the buoyancy for an offshore drilling platform, with a deck that is located on columns attached to the pontoons. A separate, submerged ballast unit is attached to the pontoons to help stabilize the floating structure and improve its motion in waves. The ballast unit is approximately the same size in the horizontal plane as the extent of the pontoons and is attached to the floating structure at each corner by at least three vertical struts that extend through and below the pontoons. The struts are attached so that they can be connected or removed from a locking device on the top side of the pontoons. At the upper end of the struts, an attachment head is provided which can be connected and removed from a lifting device such as a wire driven by a winch mounted on the platform.

U.S. Pat. No. 4,829,928 describes an ocean platform that has a negatively buoyant pontoon suspended from the balance of the platform to increase the heave resonant period. Tendons suspend the pontoon to a depth where dynamic wave forces do not materially act directly on it in seas of normally occurring periods of up to about 15 seconds but do in seas of periods above about 15 seconds. Columns and an upper pontoon provide buoyancy for the platform.

U.S. Pat. No. 4,864,958 describes an anchored platform of the Ship Waterplane Area Protected (SWAP) type. This platform is of similar design to a SWAP-type free floating platform with the additional elements of a downward extension of a vertical hollow column, tensioned anchor chains, catenary mooring lines and anchors, a foundation including a pontoon, ballast, anchoring arrangements and a well template.

U.S. Pat. No. 5,707,178 describes a tension base for a tension leg platform. A buoyant base is submerged below the water surface and is retained with base tendons to a foundation on the sea floor. The buoyant base is attachable to the mooring tendons of a tension leg vessel positioned above the buoyant base. The buoyant base can be selectively ballasted to control the tension in the base tendons. Additional buoyant bases and connecting tendons can extend the depth of the total structure. Mooring lines can be connected between the buoyant base and the sea floor to limit lateral movement of the buoyant base. The buoyant base creates a submerged foundation which is said to reduce the required length of a conventional tension leg platform. The tension leg platform can be detached from the buoyant base and moved to another location.

U.S. Pat. No. 4,626,137 describes a submerged multi-purpose facility which employs anchored tethers and a balanced buoyant/ballast to keep the facility in location. Drift is controlled by tethering the facility to the sea bottom using one or more cables or other slightly flexible tie-down means.

U.S. Pat. No. 6,478,511 describes a floating system held in position on the sea bed by one or several vertical or nearly vertical tensioned lines made of a material that is not very sensitive to fatigue stresses and the tensioned line or lines are sized in a manner independent of the fatigue phenomena associated with the dynamic behavior of the floating system under the effect of external loadings.

U.S. Pat. No. 4,585,373 describes a pitch period reduction apparatus for tension leg platforms. A tension leg platform is provided with exterior buoyant columns located outside the normal tension leg platform structure. The exterior columns decrease the pitch period of the tension leg platform away from the point of concentration of the largest wave spectrum energy encountered at a particular marine location. Modification of the pitch period of the tension leg platform in this manner is said to reduce the cyclic fatigue stresses in the tension legs of the platform, and thereby increase the useful life of the platform structure.

U.S. Pat. No. 6,431,167 illustrates a variety of offshore platforms of the prior art and additionally describes a tendon-based floating structure having a buoyant hull with sufficient fixed ballast to place the center of gravity of the floating structure below the center of buoyancy of the hull. A support structure coupled to an upper end of the hull supports and elevates a superstructure above the water surface. A soft tendon is attached between the hull and the seafloor. A vertical stiffness of the soft tendon results in the floating structure having a heave natural period of at least twenty seconds.

U.S. Pat. No. 6,718,901 describes an “extendable draft platform” that has a buoyant equipment deck on a buoyant pontoon with elongated legs on the pontoon, each comprising a buoyant float, that extend movably through respective openings in the deck. Chains extending from winches on the deck are reeved through fairleads on the pontoon and connected back to the deck. The chains are tightened to secure the deck to the pontoon for conjoint movement to an offshore location. The chains are loosened and the pontoon and leg floats ballasted so that the pontoon and leg floats sink below the floating deck. The chains are then re-tightened until pawls on the leg floats engage the deck. The buoyancy of at least one of the pontoon and leg floats is increased so that the deck is thereby raised above the surface of the water. The chains are connected to mooring lines around an offshore well site, and the raised deck and submerged pontoon are maintained in a selected position over the site with the winches.

U.S. Patent Publication No. 2005/0084336 A1 describes a deck-to-column connection for an extendable draft platform, a type of deep-draft semi-submersible platform. The extendable draft platform has a deck and buoyancy columns installed in leg wells in the deck for vertical movement from a raised position to a submerged position. A connection arrangement secures the columns to the deck when the columns are in the submerged position. In the connection arrangement, a plurality of first guide elements near the top of each column is engageable by a plurality of complementary second guide elements secured to the deck around each leg well when the column is lowered to its submerged position. A locking mechanism is operable between the columns and the deck when the first guide elements are engaged with the second guide elements. The first and second guide elements may be configured so that the connection between the deck and the columns may be enhanced by over-ballasting the columns and/or by welding the columns to the deck.

U.S. Pat. No. 7,854,570 discloses a pontoonless tension leg platform (TLP) that has a plurality of buoyant columns connected by an above-water deck support structure. The design eliminates the need for subsea pontoons extending between the surface-piercing columns. In certain embodiments, the buoyancy of the columns is increased by the addition of subsea sections of increased diameter (and/or cross-sectional area) to provide the buoyancy furnished by the pontoons of the TLPs of the prior art. A pontoonless TLP has a smaller subsea projected area in both the horizontal and vertical planes than a conventional multi-column TLP of equivalent load-bearing capacity having pontoons between the columns. This reduction in surface area produces a corresponding reduction in the platform's response to ocean currents and wave action and consequently allows the use of smaller and/or less costly mooring systems. Moreover, the smaller vertical projected area results in a shorter natural period which enables a pontoonless TLP according to the invention to be used in water depths where conventional TLPs cannot be used due to their longer natural periods. The absence of pontoons in a multi-column TLP also has the added benefit of providing an unobstructed path for risers to connect with the deck of the platform.

U.S. Pat. No. 6,447,208 describes an extended-base tension leg substructure for supporting an offshore platform where the substructure includes a plurality of support columns disposed about a central axis of the substructure and interconnected by at least one pontoon. Each column comprises an above water and submerged portion. The substructure also includes a plurality of wings or arms radiating from the columns and/or the pontoons, each wing securing at least one tendon extending from a wing to an anchor on the seabed. It is said that the wings minimize translational movement and rotational flex in the substructure reducing fatigue in the tendons and their connections.

U.S. Pat. No. 7,140,317 describes a central pontoon semi-submersible floating platform for use in offshore applications which has a hull configuration that includes vertical support columns, a central pontoon structure disposed inboard of the columns at a lower end thereof, and a deck structure supported at an upper end of the columns. The vertical columns and pontoon structure are constructed substantially of flat plate. The vertical columns are adjoined to the outer periphery of the central pontoon and have a transverse cross sectional shape with a major axis oriented radially outward from a center point of the hull, and a central vertical axis disposed a distance outward from the pontoon outer periphery. Risers can be supported on the inboard or outboard side of the pontoon and extended to the deck, and the structure can be anchored by mooring lines extending along the outboard face of the columns extending radially outward and downward from their lower ends.

DETAILED DESCRIPTION OF THE INVENTION

The invention may best be understood by reference to certain illustrative embodiments.FIG. 1depicts a TLP100according to a first embodiment of the invention installed at an offshore location. As is conventional for tension leg platforms, the buoyant hull of the vessel (comprised of columns110and pontoons112) is anchored to the seafloor by tendons132which are tensioned to hold the vessel such that the waterline in its installed condition is above the waterline when in its free-floating state. This arrangement eliminates most vertical movement of the structure.

TLP100comprises a deck138which may be configured to suit the particular needs of the owner or operator. A typical deck layout for an oil and gas production operation is shown inFIG. 1and includes process equipment144, helicopter landing facility148, crew quarters150and loading cranes146. Catenary risers134and/or vertical risers (not shown) may be supported by the TLP from riser supports136on pontoons112, columns110or deck138.

Columns110have a generally rectangular transverse cross section and are oriented such that the major axis of the transverse cross section is generally aligned with the central axis of the vessel. As shown in the embodiment illustrated inFIG. 1, columns110may comprise a plurality of different sections. For example, the lowermost section has straight sides and corners with tendon porches130attached to outboard face126and adjacent side face128. Upper sections of column110have curved corner sections118which connect adjoining side panels and inboard or outboard panels which may be generally flat. Curved sections118may have a single radius of curvature, a compound radius of curvature or a generalized curve shape. A transition section of column110may join the upper section having curved corners118to the lower section having square corners120with blending corner piece122. In certain metocean conditions, columns with curved corner sections may exhibit more favorable hydrodynamic properties in response to waves and currents than columns having only straight corners.

Deck structure138extends above upper surface116of columns110. In certain embodiments, deck138(or cellar deck140) may be supported on column upper surface116whereas in other embodiments, separate deck support means may be provided, as described more fully, below.

Pontoons112interconnect adjacent columns forming a pontoon ring which defines central opening114. In the embodiment ofFIG. 1, inboard face124of column110is generally flush with the inboard faces of the adjacent pontoons112. Outboard surface152of pontoons112intersects side surface128of column110at a location intermediate inboard face124and outboard face126. One or more curved sections153on outboard face152and/or inboard face125may be incorporated such that there is a generally orthogonal intersection of pontoon112with side face128of column110. In the illustrated embodiment, the longitudinal axis of pontoon112intersects the base section of column110at approximately its midpoint.

Pontoons112may comprise a plurality of internal compartments (not shown) which may comprise buoyancy tanks, ballast tanks and/or storage tanks as is conventional in the art.

Deck138may be a separate, detachable unit thereby facilitating both fabrication and installation. In certain applications, it has been found advantageous to set the deck on the columns of the TLP using heavy-lift barge cranes subsequent to installation of the hull portion of the structure at the operations site.

Deck support members142structurally interconnect upper deck138and cellar deck140. In certain embodiments, the upper deck, cellar deck and deck support members may comprise a truss-type structure. The geometry of the deck need not be the same as that of the hull or of the deck support structure.

The distance between the nominal waterline of the platform in its installed condition and the underside of cellar deck140is known as the air gap. This distance is typically selected to exceed the wave height of the platform's design storm so that the platform does not experience a possibly catastrophic uplift force which might occur if waves were allowed to strike the deck.

A second embodiment of the invention is illustrated inFIG. 2. This embodiment is a semi-submersible production vessel200moored in position using a plurality of catenary anchor lines260which connect to anchoring means in the seafloor (not shown). Anchor lines260are routed through fairleads258proximate the lower ends of columns210and up the outboard face of columns210to winches254mounted on winch balconies256. Winches254may be used to tension anchor lines260. In certain situations, winches254may be used to selectively adjust the payout and tension of anchor lines260so as to effect lateral movement of semi-submersible200.

The hull of semi-submersible200is of the same configuration as that used in TLP100, illustrated inFIG. 1.

FIG. 3shows a third embodiment of the invention which is a semi-submersible drilling rig300. The hull and anchoring means of drilling rig300may be substantially the same as those of semi-submersible200. However, deck338is equipped with derrick362which may be used to support a drill string contained within vertical riser364that connects to a wellhead on the seafloor.

Also shown (in phantom) inFIG. 3are optional azimuthal thrusters366which may be components of a dynamic positioning (DP) system. Dynamic Positioning is a station-keeping system for floating units that uses thrusters to compensate for wind, wave and current forces in a dynamically controlled mode to keep the unit on a predetermined location and heading at sea. A dynamic positioning system may be used in lieu of catenary anchor lines360.

The hull structure, alone, used in TLP100and semi-submersibles200and300(FIGS. 1,2and3, respectively), is shown inFIGS. 4A and 4B. Visible inFIGS. 4A and 4Bare deck support posts468which may be used to support a deck structure438(shown in phantom) on hull400.

Deck support posts468may connect to both column upper surface416and inboard column surface424such that column face424is a major load-bearing member. In this way, the internal structure of a column may be reduced from that which would be required were deck438supported only by upper surface416.

A second embodiment of the invention is illustrated inFIGS. 5A and 5B. In this embodiment, the pontoon ring extends inboard from the inboard face524of columns510and includes inner pontoon portions570located immediately inboard from the lower portion of inboard face524of each column510. In this embodiment, the longitudinal axis of pontoons512intersects side face528of columns510at a point that is intermediate the midline of column510and the juncture of inboard face524and side face528.

An embodiment of the invention having tapered columns is illustrated inFIGS. 6A and 6B. Columns610have portion672wherein the cross sectional area of the column progressively increases with distance from upper surface616. Inboard face624of columns610may be substantially the same as inboard face124of TLP100(as illustrated inFIG. 1) and may be the principal load-bearing member for deck support posts668. Outside surface674may be inclined from the vertical and a curved surface676may join outboard surface674and side surface628. The use of tapered columns610may allow TLP hull600to be constructed using less material than that required for the hull of TLP100.

An embodiment of the invention having the pontoon structure of TLP hull500(seeFIGS. 5A and 5B) and the tapered column structure of TLP hull600(seeFIGS. 6A and 6B) is shown inFIGS. 7A and 7B. The pontoon ring extends inboard from the inboard face724of columns710and includes inner pontoon portions770located immediately inboard from the lower portion of inboard face724of each column710. In this embodiment, the longitudinal axis of pontoons712intersects side face728of columns710at a point that is intermediate the midline of column710and the juncture of inboard face724and side face728. Columns710have portion772wherein the cross sectional area of the column progressively increases with distance from upper surface716. Inboard face724of columns710may be substantially the same as inboard face124of TLP100(as illustrated inFIG. 1) and may be the principal load-bearing member for deck support posts768. Outside surface774may be inclined from the vertical and a curved surface776may join outboard surface774and side surface728. The use of tapered columns710may allow TLP hull700to be constructed using less material than that required for TLP hull500.

FIGS. 8A and 8Bshow an embodiment of the invention wherein the columns810are outboard of the pontoon ring. The pontoon ring extends inboard from the inboard face824of columns810and includes inner pontoon portions880located immediately inboard from the lower portion of inboard face824of each column810. The inboard surface878of the pontoon ring may include one or more curved sections884between adjacent straight sections. In this embodiment, the outboard face852of pontoons812(which may include curved section882) intersects columns810at the juncture of side face828and inboard face824.

A sixth embodiment of the invention is shown inFIGS. 9A and 9B. In this embodiment, columns910have a curved corner section986which extends to substantially the bottom of the column910. In this embodiment, tendon porches930and930′ may be closer together than in embodiments with column bottom sections having square corners (e.g., the hulls shown inFIGS. 1-4). Curved corner sections986may improve the hydrodynamic properties of the hull900in response to ocean waves and currents. The outboard surface952of pontoons912may include curved portions988configured such that the intersection of pontoon outboard surface952with column side surface928is substantially orthogonal.

A seventh embodiment of the invention, shown inFIGS. 10A and 10B, includes a three-abreast set1090of tendon porches1030on outboard face1026of each column1010.

FIGS. 11A and 11Bdepict an eighth embodiment of the invention. TLP hull1100has the same general configuration as the hulls shown inFIGS. 1-4. However, hull1100has pontoon-mounted tendon porches1192attached to outboard face1152of pontoons1112. These pontoon-mounted porches1192may be in addition to the more conventional column-mounted tendon porches1130which are attached to adjacent side face1128of columns1110. Outboard face1126of columns1110may be devoid of tendon porches, if so desired.

FIGS. 12A and 12Bdepict a ninth embodiment of the invention. Unlike the hulls shown inFIGS. 1-4that have generally rectangular columns, TLP hull1200has 5-sided columns1210with two, generally orthogonal side surfaces1228and three, adjacent outboard surfaces1226a,1226band1226cconnected between the side surfaces. Columns1210may have curved corner sections1218which connect side panels1228and outboard panels1226aor1226cwhich may be generally flat. Additionally, curved sections1218may be used to connect outboard panels1226ato1226band1226bto1226c. Curved sections1218may have a single radius of curvature, a compound radius of curvature or a generalized curve shape.

A plurality of buoyant pontoons1212are connected between adjacent columns1210, the pontoons having a generally rectangular cross section and an outboard, generally vertical surface1252connected to a side surface1228of an adjacent column at a location that is inboard of the outboard surfaces1226of column1210.

Tendon porches1230may be mounted to one or more of outboard surfaces1226a,1226band/or1226c. In yet other embodiments, tendon porches may be mounted to column side surfaces1228outboard of the juncture of column surface1228and pontoon surface1252. Such tendon porches may be in addition to or in lieu of the tendon porches on outboard surfaces1226aand1226cor1226b.

FIGS. 13A and 13Bdepict a tenth embodiment of the invention. TLP hull1300has rectangular columns1310with two, opposing, side surfaces1328aand1328bwith outboard surface1326and opposing inboard column surface1324connected between the side surfaces. Columns1310may have curved corner sections1318which connect side panels1328to outboard panel1326and/or inboard panel1324which may be generally flat. As shown in the illustrated embodiment, curved corner sections1318may be used in the upper portion of columns1310while the lower portion has square corners. Curved sections1318may have a single radius of curvature, a compound radius of curvature or a generalized curve shape.

A plurality of buoyant pontoons1312are connected between adjacent columns1310, the pontoons having a generally rectangular cross section and an outboard, generally vertical surface1352connected to a side surface1328of an adjacent column at a location that is inboard of the outboard surface1326of column1310. The inboard surfaces of the pontoons may be flush with the inboard surfaces1324of columns1310. In this embodiment, the pontoons1312are comprised of substantially straight sections.

Tendon porches1330may be mounted to one or more of outboard column face1326and side surfaces1328aand/or1328boutboard of the juncture of column surface1328and pontoon vertical surface1352. It will be appreciated that a TLP according to the present invention permits tendon porches to be located on adjacent faces of a column when a plurality of tendon porches are connected to a single column.

Referring now toFIG. 14, it may be seen that a tension leg platform according to one embodiment of the invention may comprise a plurality of buoyant columns10having a polygonal transverse cross section with at least two tendon porches30on each column, adjacent tendon porches30aand30bbeing configured such that a first line L1normal to the surface28of the column on which a first tendon porch30ais mounted and passing through the center of the tendon seat31of the first tendon porch lies at an angle α that is greater than zero degrees and less than or equal to 90 degrees to a second line L2normal to the surface26of the column10on which an adjacent second tendon porch30bis mounted and passing through the center of the tendon seat of the second tendon porch30band, a plurality of buoyant pontoons12connected between adjacent columns10.

Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims.