Drawdown apparatus and installation method for a floating platform

Description

BACKGROUND OF THE DISCLOSURE

The present invention relates to floating platform systems for testing and producing hydrocarbon formations found in offshore waters. More particularly, the invention relates to a platform drawdown method and system for maintaining stability of a floating structure while lowering the platform down to its installation or lock-off draft.

The exploration for oil and gas deposits in offshore waters, and recovery of the oil and gas therefrom is very expensive. Various methods and offshore production systems have been utilized to locate and recover offshore oil and gas deposits. Exploration and production systems such as converted Mobile Offshore Drilling Units (“MODU”), Tension Leg Platforms (TLP) and other floating structures typically used in offshore waters are very expensive to manufacture and difficult to install.

Installation of an offshore platform, such as a TLP, may require that the platform hull be wet towed to the installation site. The hull of a single column TLP comprises the central column and a plurality of pontoons extending radially outwardly from the central column. The hull of a single column TLP is quite stable floating on the pontoons as it is wet towed to the installation site because of the large water plane area provided by the central column and pontoons. During a typical TLP installation, the hull is ballasted down for connection to a plurality of pre-installed tendons which anchor the hull to the seabed. As the hull is ballasted down, the hydrostatic stability of the platform decreases significantly as the pontoons submerge below the water surface and the platform water plane area at the water surface is substantially reduced. To minimize stability problems during installation, the hull is typically installed without the deck and a stabilizing upward force is applied at the top of the hull as it is ballasted down to connect with the pre-installed tendons. The upward stabilizing force or hook load is typically supplied by an installation support vessel. After the hull is lowered to the lock-off draft, connected to the tendons and deballasted, the platform is sufficiently stable so that the deck may be installed. The installation support vessel or other lifting equipment may be used to lift the deck and safely set it on the platform hull. However, only a limited number of installation support vessels are available worldwide capable of providing the hook load required by a typical TLP and the rental rates for installation support vessels is very high.

For some offshore platform installations, it may be very advantageous commercially to utilize an installation alternative that eliminates the need for expensive installation support vessels during hull and deck installation. One such installation method, would for example, include installing the deck on the hull onshore or in a less exposed, shallower water location with less expensive lifting means. The hull-deck assembly would then be wet-towed to the installation site and lowered to its installation draft and connected it to the pre-installed tendons without using an installation support vessel.

It is therefore an object of the present invention to provide a method for installing a floating platform without using an installation support vessel to provide stability to the hull or hull-deck assembly of the platform during transition of the platform from the wet tow draft to its installation or tendon lock-off draft.

It is a further object of the present invention to provide a method for installing a floating platform including the steps of assembling the platform, including the hull, deck, drawdown system and most production equipment at or near the fabrication site; wet towing the assembled platform to the installation site; and actuating the drawdown system for lowering the assembled platform to its installation draft without an installation vessel.

It is a further object of the present invention to provide a floating platform utilizing a drawdown system to rapidly lower the platform to its installation draft.

SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, a floating platform includes a hull and a deck mounted on the uppermost end of the hull. Pontoons extend radially outwardly from the lower end of the hull. The platform is anchored to the seabed by a plurality of tendons connected to the hull at the upper ends thereof and secured to the seabed at the lower ends thereof. The platform includes a removable drawdown system for lowering the platform to lock-off draft without utilizing an installation vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages and objects of the present invention are attained can be understood in detail, a more particular description of the invention briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.

It is noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

FIG. 1 is a side view of a floating platform illustrating the platform at the wet tow draft positioned above pre-installed tendons and rigged-up in accordance with the invention;

FIG. 2 is a side view of the floating platform illustrating a step in the installation method of the invention wherein the drawdown system is attached to the tendons and the platform is transitioning between the wet tow draft and the installation draft;

FIG. 3 is a side view of the floating platform illustrating a step in the installation method of the invention wherein the tendons have passed through the tendon connectors and the platform is ready for lock-off;

FIG. 4 is a side view of the floating platform at lock-off draft and the drawdown system has been dismantled and recovered;

FIG. 5 is a side view of the drawdown system of the invention;

FIG. 6 is an end view of the drawdown system of the invention;

FIG. 7 illustrates the tensioning adapter member of the drawdown system of the invention;

FIG. 8 is a side view of a floating platform illustrating the platform at the wet tow draft positioned above the pre-installed tendons and rigged-up in accordance with an alternate embodiment of the drawdown system of the invention;

FIG. 9 is an enlarged partial side view of the drawdown system of the invention illustrated in FIG. 8;

FIG. 10 is an enlarged partial side view of the floating platform illustrating a step in the installation method of the invention wherein the drawdown system illustrated in FIG. 8 is attached to the tendons and the platform is ready to be lowered to the lock-off draft;

FIG. 11 is a side view of the floating platform illustrated in FIG. 8 at lock-off draft;

FIG. 12 is a side view of the floating platform illustrating a step in the installation method of an alternate embodiment of the invention wherein the drawdown system is mounted on temporary stability columns secured to the platform and the platform is transitioning to the installation draft; and

FIG. 13 is a side view of the floating platform illustrating a step in the installation method of the invention illustrated in FIG. 12 wherein the tendons have passed through the tendon connectors and the platform is ready for lock-off.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring first to FIG. 1, the floating platform of the invention is generally identified by the reference numeral 10. The platform 10 includes a hull 12 which provides positive buoyancy and vertical support for the platform deck 14. In a preferred embodiment of the invention illustrated in FIG. 1, the hull 12 comprises a central column 13 and pontoons 18 extending radially outwardly from the lower end of the central column 13. One or more decks 14 are supported on the central column 13. Drilling and/or production equipment necessary for the recovery and processing of oil, gas and water are secured on the decks 14.

The central column 13 extends upward from the base or keel of the platform 10. The base node of the platform 10 is at the intersection of the central column 13 and the pontoons 18 extending radially outwardly therefrom. The platform 10 is anchored to the seabed by tendons 20 secured at one end thereof to the pontoons 18, as shown in FIG. 3, and at the opposite ends thereof to foundation piles (not shown in the drawings) embedded in the seabed.

In accordance with the present invention, the platform 10 includes a drawdown system mounted on each pontoon 18. The drawdown system includes strand jacks, generally identified by the reference numeral 28, temporarily mounted on the distal ends of the pontoons 18. The strand jacks 28 include tower masts 30 mounted on the pontoons 18 and extending upwardly therefrom. The tower masts 30 comprise one or more tower mast sections 32 vertically stacked one on top of the other to the design height. The height of the tower masts 30 is sufficient to maintain the tops thereof above the water surface at the installation draft of the platform 10 and provide some hydrostatic stability for the platform 10 at all intermediate drafts as it is lowered to the lock-off draft. The lowermost mast section 32 is mounted on a mast base 34 secured to the top surface of the pontoons 18. Oppositely facing towers 30 are erected on each of the pontoons 18 and are interconnected by cross-ties 36 to form a rigid drawdown support structure. Additional support is provided by brace members 38 extending at an angle from the tower masts 30 to the pontoons 18 and secured thereto. A strand jack base 40 is mounted across the tops of the oppositely facing tower masts 30. Strand jack controls 42 for each strand jack 28 are mounted on the base 40.

In the preferred embodiment shown in FIG. 1, the drawdown system of the invention utilizes six strand jacks 28, one for each tendon 20. Strand jacks are commercially available lifting jacks used for heavy lifting operations. Each strand jack 28 includes a bundle of strand cables 44 that passes through a strand guide or umbrella 46 and is connected to a lifting block 48, shown in greater detail in FIG. 7. The strand jacks 28 are individually controlled by the controllers 42 which are linked for equalizing the load among them.

A tensioning adapter 50 is connected to the lower end of the lifting block 48. The tensioning adapter 50 is sized to pass through the tendon connection porch 21. An internal connector 52 (floating ball type, J-lock, internal shear keys or similar type connector) is fixed to the lower end of the tensioning adapter 50. The connector 52 is adapted for mating engagement with a length adjustment joint 54 which is welded or otherwise secured to the upper ends of the tendons 20. The length adjustment joint 54 is externally threaded or grooved and adapted to extend through the tendon porch 21. A tendon lock off connector assembly mounted on the length adjustment joint 54 permits adjustment of the tension of the tendons 20.

Referring now to FIGS. 1-5, the installation sequence will be described. The platform 10, including the hull 12, the deck 14 mounted on top of the central column 13 and the drawdown system mounted on the pontoons 18, is wet-towed to the installation site and maneuvered over the tendons 20 which have been pre-installed and connected at the lower ends thereof to foundation piles embedded in the seabed. In the configuration shown in FIG. 1, the hull 12 and deck 14 mounted thereon are stable floating on the pontoons 18 because of the large water plane area provided by the pontoons 18. The assembled hull 12 and deck 14, however, tends to be unstable at the transition draft where the pontoons 18 are fully submerged below the water line 16. While the tower masts 30 and the strand cables 44 provide some buoyancy and stability for the platform 10 by providing additional water plane areas, the assembled hull 12 and deck 14 is not fully stable and out of danger of capsizing until the tendons 20 are connected to the pontoons 18. In the final installed configuration of the platform 10, the tendons 20 provide stability for the platform 10.

Once the platform 10 is positioned over the tendons 20, the tensioning adapters 50 are lowered through the tendon porches 21 and connected to the length adjustment joints 54 of the tendons 20. After all tensioning adapters 50 have been connected to the tendons 20, the platform 10 may be lowered to the installation draft. In operation, each of the strand cables 44 of the drawdown system is fed through top and bottom anchor heads or slips (not shown in the drawings) that open and close to release or grip the strand cables 44. Initially, the bottom anchors of the strand jacks 28 are opened and release the strand cables 44. An upward lifting force is simultaneously applied to the top anchors of the strand jacks 28 drawing the strand cables 44 with them. At the top of the stroke, the bottom anchors of the strand jacks 28 are closed and grip the strand cables 44. The top anchors are then opened and lowered to their starting position. The cycle is repeated until the platform 10 is lowered to the installation draft and the length adjustment joints 54 extend through the tendon porches 21. The tendons 20 are then locked off and the tension in the strand cables 44 is transferred to the tendons 20 and tendon porches 21. The platform 10 may be de-ballasted to increase the tension in the tendons 20 to the installation tension, if necessary, to complete the lock off operation. The tensioning adapters 50 are uncoupled from the length adjustment joints 54 and the drawdown system is dismantled and removed.

In another embodiment of the invention illustrated in FIGS. 8-11, the drawdown system of the invention is similar to that described in FIG. 1 with the exception that the drawdown system for lowering the platform 10 to its lock-off draft includes ratchet connectors and hydraulic cylinder tensioning assemblies, one for each tendon 20. The tensioning assemblies, as best shown in FIGS. 9 and 10, include vertically oriented hydraulic cylinders 60 secured at the lower ends thereof to the tendon connection porches 21. The opposite ends of the hydraulic cylinders 60 are connected to dual ratchet connectors 62. Two or more hydraulic cylinders 60 support each of the dual ratchet connectors 62.

A lateral guide structure comprising two or more upright posts 64 and an angular brace member 66 support guide sleeves 68 above the dual ratchet connectors 62. The guide sleeves 68 are spaced from and vertically aligned with the dual ratchet connectors 62 so that upon assembly the tensioning adapters 50 extend through the guide sleeves 68 and the dual ratchet connectors 62 as illustrated in FIGS. 9 and 10. The tensioning assemblies are individually controlled by the controllers (not shown in the drawings) which are linked for equalizing the load among them.

The installation sequence utilizing the hydraulic cylinder drawdown system is similar to that described above and depicted in FIGS. 1-4. Once the platform 10 is positioned over the tendons 20, the tensioning adapters 50 are lowered through the tendon porches 21 and connected to the length adjustment joints 54 of the tendons 20. After all tensioning adapters 50 have been connected to the tendons 20, the platform 10 may be lowered to the installation draft. The hydraulic stroking cylinders 60 are activated to simultaneously apply an upward force against the dual ratchet connectors 62. The upward lifting force causes the dual ratchet connectors 62 to grip the tensioning adapters 50 and thereby apply an opposite downward force for lowering the platform 10. At the top of the cylinder stroke, the direction of the stroking cylinders 60 is reversed and the dual ratchet connectors 62 disengage from the tension adapters 50. Simultaneously therewith, slip bowl connector assemblies supported by the tendon porches 21 grip and hold the tensioning adapters 50 while the cylinders 60 return to their starting position. The cycle is repeated until the platform 10 is lowered to the installation draft. The tendons 20 are then locked off and the platform 10 may be de-ballasted to increase the tension in the tendons 20 to the installation tension, if necessary, to complete the lock off operation.

In another embodiment of the invention illustrated in FIGS. 12 and 13, the drawdown system of the invention is similar to that described in FIG. 1 with the exception that the drawdown system for lowering the platform 10 to its lock-off draft is mounted on temporary stability columns 70 supported on the pontoons 18 of the platform 10 at the distal ends thereof. The stability columns 70 are floodable and provided with the necessary plumbing, including a fill port and vent, for connection with a ballast system used during installation of the platform 10. Pin and bracket connectors 72 secure the stability columns 70 to the pontoons 18. A strand jack platform 74 secured on top of each of the stability columns 70 provides a mounting platform for the drawdown system of the invention. The width of the platforms 74 is greater than the width of the stability columns 70 so that they extend sufficiently beyond the sidewalls of the stability column 70 to enable alignment of the strand cables 44 with the tendon porches 21. The strand jack controls 42 for each strand jack 28 are secured on the platforms 74.

The alternate embodiment of the invention illustrated in FIGS. 12 and 13 further includes one or more heave compensation devices 76. The heave compensation devices 76 may comprise a plurality of hydraulic cylinders or the like mounted between the strand jack platforms 74 and each of the strand jack bases 40. The heave compensation devices 76 aid to eliminate overloading of the drawdown system of the invention in sea states exceeding the installation criteria for the platform 10. Additionally, the heave compensation devices 76 facilitate transitioning of the platform 10 from a floating structure to a tension leg platform in a controlled manner.

The installation sequence utilizing the temporary stability columns 70 is substantially the same to that utilizing the tower masts 30 described above. In the event additional height is required to maintain the strand jacks 28 above the water line, mast sections 32 may be mounted on the temporary stability columns 70, as required, to form strand jack towers 30 of sufficient height extending above the temporary stability columns 70. Upon completion of the alternate embodiment installation procedure illustrated in FIGS. 12 and 13 and lock-off of the platform 10 at the installation draft, the drawdown system of the invention and the temporary stability columns 70 are dismantled and removed.

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.