Method and apparatus for replacing a tendon flex bearing on a tension leg platform

A method and apparatus for replacing a tendon flex bearing on a tension leg platform includes installing a tendon tensioning tool on the Length Adjustment Joint (LAJ) of a tendon and providing a supplemental buoyancy module on the tendon. The tendon tensioning tool is used to disengage the slips from the top tendon connector. The buoyancy of the supplemental buoyancy module may then be adjusted to support the tendon whereupon the tendon tensioning tool may be disengaged permitting removal and replacement of the tendon flex bearing. Following installation of the new (or refurbished) flex bearing, the steps of the procedure may be reversed to return the tendon to its normal operating state. In certain embodiments, top clamp actuation means is provided in the tendon tensioning tool for removing slips from the top clamp.

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to floating platforms for the offshore production of oil and gas. More particularly, it relates to tension leg platforms (TLPs).

The tendon flex bearing is a structural element that allows for relative rotation between the top of the tendon and the tendon porch, i.e. the TLP hull. The tendon flex bearing is in the direct load path of the primary station keeping forces. A typical TLP full life-cycle operational plan does not call for the replacement of these elements.

Station keeping of a typical TLP is provided by a minimum of six tendons. A TLP is designed for extreme and survival conditions, as well as for conditions wherein one tendon missing (i.e., the TLP is re-ballasted to maintain proper tendon tension) and for a broken tendon condition (i.e., no immediate ballast compensation). Though the TLP is designed for such conditions, remedial actions are generally not developed as part of the design process.

Piles driven into the seafloor are attached to the lower ends of the tendons. The upper end of the tendon connects to the TLP hull. The upper end of the tendon itself comprises tendon body elements and, above such, a length adjustment joint (LAJ). The diameter of the LAJ is generally smaller than the diameter of the tendon body. A section of the LAJ is equipped with concentric grooves which allow slips having corresponding projections to clamp onto the tendon (LAJ). These slips, called “the clamp,” are supported by the flex bearing. The flex bearing allows for differential rotation between the tendon body and TLP. The flex bearing may be supported by a load ring which is supported by load cells used to measure the tendon tension. The load cells may be supported by a second load ring that is rigidly attached to the tendon porch which is connected to TLP hull. Alternatively, some TLPs are configured such that the tendon flex bearing is supported directly on the tendon porch or on a ring on the tendon porch. Most of these devices are circular in cross section and, when installed, have a common longitudinal axis. As such, the LAJ (and tendon) are at the center and protrude vertically through the centers of the clamp, the flex bearing, the centers of both load rings and the geometric center of the load cells.FIG. 1illustrates a typical layout of a tendon and its associated flex bearing.

BRIEF SUMMARY OF THE INVENTION

The methodology and hardware disclosed herein enables the in situ replacement of a tendon flex bearing on a tension leg platform.

The challenge with replacing a flex bearing is to remove this element from the primary load path under in-situ conditions in a cost-efficient manner while assuring platform integrity and safety. Replacing the flex bearing requires the following steps:A. releasing the clamp from the LAJ;B. removing the clamp;C. removing the flex bearing;D. installing a new (repaired or refurbished) flex bearing;E. reinstalling the clamp; and,F. securing the clamp to the LAJ.

Typically, the pretension in each of the tendons is approximately one-third to one-half the maximum allowable load. One cannot simply release the clamp (as stated in step A, above). The tension in the tendon (at the clamp) must first be significantly reduced. However, the tension must remain sufficient to prevent any buckling of the tendon. Furthermore, the clamp cannot be unloaded by applying a load to the top of the LAJ, because the configuration does not allow for the existing flex bearing to be removed and a new one to be installed (due to interference issues).

In a method according to the invention, a supplemental buoyancy module is attached to the tendon and its buoyancy is adjusted to apply a force greater than the in-water weight of the tendon.

A tendon tensioning tool is used to disengage the slips from the top tendon connector. The buoyancy of the supplemental buoyancy module supports the tendon whereupon the tendon tensioning tool may be disengaged permitting removal and replacement of the tendon flex bearing.

DETAILED DESCRIPTION OF THE INVENTION

Herein, a method is disclosed for replacing a tendon flex bearing on an installed TLP.

The invention may best be understood by reference to the drawing figures wherein one particular method according to the invention is shown sequentially. In the drawing figures, the following reference numbers are used to designate the listed elements:10length adjustment joint (LAJ)12LAJ grooves14clamp15slips16flex bearing18flex bearing base20upper load ring22load cell24lower load ring26tendon porch28tendon body30tendon tensioning tool32hydraulic cylinders34hydraulic rams36plate40supplemental buoyancy module42messenger line44spacer46messenger line adapter48tendon top connector

Referring toFIG. 1, an installed tendon28is shown secured in its normal operating state to tendon porch26of a TLP (not shown). Upper portion10of tendon28comprises the length adjustment joint (LAJ) which may have a reduced diameter relative to the main body portion of tendon28. LAJ grooves12are provided in a portion of the outer surface of LAJ10to engage slips15in clamp14. Clamp14bears against flex bearing16supported on flex bearing base18. Flex bearing base18bears against upper load ring20which bears against load cell22supported on lower load ring24so as to provide a measure of the tension in tendon28. It will be appreciated that this is merely an exemplary configuration and other configurations used in the industry may also employ the method of the invention. For example, in some configurations, the load cell is incorporated into the body of the tendon; in yet other configurations, the flex bearing is supported directly on the tendon porch.

InFIG. 2, the tendon assembly shown inFIG. 1is illustrated with tendon tensioning tool30attached. Messenger line42(connected to the LAJ via messenger line adapter46) may assist in the installation of tendon tension tool30on LAJ10of tendon28. As will be appreciated by those skilled in the art, the apparatus illustrated in the drawing figures is below the waterline of the TLP on which it is installed. Accordingly, messenger line42may act to guide tendon tensioning tool30during installation and removal.

Tendon tensioning tool30comprises hydraulic cylinders32containing hydraulic rams34which act against plate36so as to apply additional tension to tendon28—i.e., more tension than that due to the buoyancy of the TLP applied via the load path comprising tendon porch26, lower load ring24, load cell22, upper load ring20and flex bearing16. The net result of the additional tension applied by tendon tensioning tool30is a reduction in the load borne by the top connector slips15.

The upper portion of tendon tensioning tool30temporarily functions as a tendon top connector during portions of the process.

Also shown inFIG. 2is removable supplemental buoyancy module40attached to tendon28at a point below tendon porch26. In certain embodiments, supplemental buoyancy module40has means for adding or releasing a gas (which may be air) to displace or admit water into the interior chamber(s) of supplemental buoyancy module40thereby adjusting its buoyancy.

In certain embodiments, top clamp actuation means for the top clamp is provided in the tendon tensioning tool. Top clamp actuation tools of the prior art normally fit in the same interface area where the tendon tensioning tool must react on the top of the top clamp body. This makes use of both an actuation tool and a tendon tensioning tool at same time impossible. Providing top clamp actuation means in the tendon tensioning tool overcomes this problem.

The exemplary, illustrated embodiment of the invention comprises the following steps:

Step 2: Air is added to buoyancy module40to take/apply force greater than the in-water weight of tendon28;

Step 3: The buoyancy of the TLP is adjusted to reduce the tendon tension in the tendons at the particular corner of the TLP at which flex element replacement is required;

Step 4: Tension is applied to the tendon (undergoing flex element removal) between the top of the exposed tendon top and the top of the tendon top connector, using tendon tensioning tool30;

Step 5: The tendon top connector48is unlatched;

Step 6: A temporary spacing device44is inserted between tendon28and the interior of tendon porch26(to keep tendon28from rubbing on porch26while the top clamp is removed; [this state is illustrated inFIG. 3]

Step 7: The tendon top tension is reduced by slacking off on tendon tensioning tool30until weight in water of tendon28carried by buoyancy module40; [this state is illustrated inFIG. 5]

Step 8: Tensioning tool30is removed; the tendon top connector is removed and flex element16is replaced; [this state is illustrated inFIG. 4]

Step 9: The tendon top connector (or, alternatively, a replacement top connector) is placed onto tendon porch26(with the replacement flex bearing16);

Step 10: Tendon tensioning tool30is installed; tension is applied to tendon28by reacting against the top of the tendon top connector;

Step 12: Air is released from supplemental buoyancy module40;

Step 13: The tendon tension is adjusted with tensioning tool30to match adjacent tendon(s) and the tendon top clamp is engaged; [starting position shown inFIG. 5]

Step 15: The tendon tension is checked and tendon tension reapplied with tensioning tool30and adjustment of the top clamp, if necessary; and,

Step 16: Tendon tensioning device30is removed. [returning the system to the state illustrated inFIG. 1]

The foregoing presents particular embodiments of a system embodying the principles of the invention. Those skilled in the art will be able to devise alternatives and variations which, even if not explicitly disclosed herein, embody those principles and are thus within the scope of the invention. Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.