Not Applicable
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).
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
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 groves 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.
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:
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.
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:
Referring to
Together, clamp 14, flex bearing 16 and base 18 comprise tendon top connector 48.
In
Tendon tensioning tool 30 comprises hydraulic cylinders 32 containing hydraulic rams 34 which act against plate 36 so as to apply additional tension to tendon 28—i.e., more tension than that due to the buoyancy of the TLP applied via the load path comprising tendon porch 26, lower load ring 24, load cell 22, upper load ring 20 and flex bearing 16. The net result of the additional tension applied by tendon tensioning tool 30 is a reduction in the load borne by the top connector slips 15.
The upper portion of tendon tensioning tool 30 temporarily functions as a tendon top connector during portions of the process.
Also shown in
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 1: Supplemental buoyancy module 40 is attached to tendon 28 undergoing flex bearing 16 replacement; [this state is illustrated in
Step 2: Air is added to buoyancy module 40 to take/apply force greater than the in-water weight of tendon 28;
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 tool 30;
Step 5: The tendon top connector 48 is unlatched;
Step 6: A temporary spacing device 44 is inserted between tendon 28 and the interior of tendon porch 26 (to keep tendon 28 from rubbing on porch 26 while the top clamp is removed; [this state is illustrated in
Step 7: The tendon top tension is reduced by slacking off on tendon tensioning tool 30 until weight in water of tendon 28 carried by buoyancy module 40; [this state is illustrated in
Step 8: Tensioning tool 30 is removed; the tendon top connector is removed and flex element 16 is replaced; [this state is illustrated in
Step 9: The tendon top connector (or, alternatively, a replacement top connector) is placed onto tendon porch 26 (with the replacement flex bearing 16);
Step 10: Tendon tensioning tool 30 is installed; tension is applied to tendon 28 by reacting against the top of the tendon top connector;
Step 11: Temporary spacing device 44 is removed;
Step 12: Air is released from supplemental buoyancy module 40;
Step 13: The tendon tension is adjusted with tensioning tool 30 to match adjacent tendon(s) and the tendon top clamp is engaged; [starting position shown in
Step 14: Supplemental buoyancy module 40 is removed;
Step 15: The tendon tension is checked and tendon tension reapplied with tensioning tool 30 and adjustment of the top clamp, if necessary; and,
Step 16: Tendon tensioning device 30 is removed. [returning the system to the state illustrated in
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.
This application claims the benefit of U.S. Provisional Application No. 62/108,353, filed on Jan. 27, 2015, the contents of which is hereby incorporated by reference in its entirety.
Number | Date | Country | |
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62108353 | Jan 2015 | US |