Sliding Tension Leg Tower

This invention relates to offshore structures for drilling and producing operations. In particular the invention is concerned with a compliant structure suitable for use in water depths in excess of 1,000 feet.

The use of offshore structures for drilling and producing operations has become relatively commonplace in recent years. However, as more petroleum fields are being developed in deeper waters, the search continues for structures capable of withstanding the hostile wind and wave forces encountered without being prohibitive in cost.

Three structures proposed in the prior art for operation in water depths greater than 1,000 feet are the guyed tower, the tension leg platform and the buoyant articulated tower. The guyed tower is a trussed structure that is supported on the ocean floor with a spud can or with pilings. Guy lines run from the deck to fairleads below the water surface to clump weights on the ocean floor. Since the tower will sway a few degrees during the passage of large waves, the well conductors must flex at the tower base. Preferably the fairleads are positioned at about the same elevation as the center of pressure of the applied design wave and wind loads. The environmental forces are therefore, more or less, colinear with the mooring system and the moment transmitted to the tower base is minimized. Beyond the clump weights, the guy lines are attached to suitable fixed anchors. Thus, the clump wieghts may be lifted from the bottom by heavy storm waves permitting further displacement of the tower.

An articulated buoyant tower differs from the foregoing fixed structure in several important respects. An articulated joint, such as a universal or ball joint, attaches the tower to a pile base thereby permitting the tower to tilt in response to environmental forces. A set of buoyant chambers provide the necessary righting moment and the upward force is effectively negated by a ballast chamber located near the bottom of the tower. The primary objection to such articulated systems arises as a result of the tower's lack of redundancy and the difficulty of inspection and/or replacement of the articulated joint.

A tension leg platform is a buoyant floating structure held in place by vertical tension cables anchored to the sea floor. The flotation chambers are designed to minimize the platform's response to weather and wave conditions.

The present invention combines the better features of the above systems in a new and ingenious manner to produce a superior structure for offshore drilling and producing operations.

The present invention relates to a compliant offshore drilling and producing structure. In accordance with the invention a plurality of axial load piles installed in the sea floor extend upwardly therefrom to a point beyond the upper surface of the water. A rigid platform is provided having a plurality of open ended sleeves affixed thereto and extending downwardly therefrom in a substantially vertical orientation over each of the axial piles. Buoyant means affixed to the sleeves below the water line are used to provide a buoyant upward force in excess of the weight of the platform, equipment and sleeves. Means are also provided for counterbalancing the buoyant forces in excess of the platform weight from the plurality of axial load piles. Preferably these latter means comprise pistons attached to the ends of the axial piles which extend downwardly into hydraulic cylinders secured to the platform. Means are provided for injecting hydraulic fluid into each of the cylinders and preferably groups of the cylinders are connected to a single hydraulic circuit.

Bearings are provided between the axial piles and the sleeves to facilitate vertical movement of the sleeves and platform relative to the fixed axial piles. The buoyant chambers should be compartmented to prevent a compressive load from being applied to the axial piles in the event of a rupture in the chambers. If required, skirt piles may also be installed near the base of the structure to provide additional lateral support.

The drawing is a schematic diagram of apparatus suitable for use in the present invention.

Referring to the drawing there is shown a structure in accordance with the present invention, generally referred to by reference numeral 10. A plurality of axial load piles 12, preferably at least 3 in number, are driven into the sea floor 14 to a suitable depth to provide an adequate resistance against the environmental forces, primarily wind and wave, which may occur. As illustrated, the piles extend upwardly from the sea floor beyond the water's surface 16.

A platform 18 which provides the necessary working space for the drilling and producing operations and which may also provide housing and office space for the crew is situated above the water line beyond the height of the maximum anticipated storm sea.

A plurality of sleeves 20 are rigidly attached in any conventional manner to the platform 18 and extend vertically downward over each of the axial piles. Preferably, the sleeves will extend below the water line at least 75% of the distance to the sea floor. The sleeves are also preferably cross braced with stiffening trusses 22 substantially along their underwater lengths.

Bearings 24 are provided between the sleeves 20 and the piles 12 to facilitate relative axial movement therebetween. The bearings may be of any suitable and conventional design to lower the frictional forces which would otherwise develop and provide lateral support to the axial piles. Under the conditions of use, the bearings should preferably be designed as a permanent system which will not require replacement during the life of the structure. Where this is not possible, sufficient access should be provided to the components to the bearing system so that it is possible to replace critical elements with minimum dismantling of adjacent components.

Preferably 101-105% of the weight of the entire structure, including the platform and its associated equipment, and excluding the shear piles, will be supported by buoyancy chambers 26 conventionally affixed to the sleeves beneath the water line. Buoyancy chambers 26 provide a righting moment to the tower whenever it sways from a true vertical orientation due to environmental forces. These chambers should be compartmented so that unexpected sealing failures will not unduly burden the foundation pilings.

Normally two sets of buoyant chambers will be used for the structure's tow and installation at the drilling site. The chambers provided for supporting the lower portion of the sleeves during transportation may be flooded to submerge the structure, removed, or shifted towards the upper end of the unit.

The upper end of each axial pile extends through its associated sleeve as shown in the drawing and is connected by cross arms 28 to pistons 30. Each piston is housed in a hydraulic cylinder 32 affixed to the platform in a load bearing relationship. Preferably at least one cylinder attached to each axial pile is serviced with hydraulic fluid via lines from a single fluid reservoir housed in the platform. As shown in the drawing, line 34 provides a flow path for hydraulic fluid from reservoir 36 to the outer cylinders and line 38 provides a flow path for hydraulic fluid from reservoir 40 to the inner cylinders.

The excess buoyant force over the weight of the platform and sleeves is counterbalanced by tension in the axial piling through the hydraulic cylinders, fluid and pistons. This system gives the overall structure the desired degree of compliancy of rotation about the sea floor, but resists platform heave or vertical motion.

To provide additional lateral support, skirt piles 42 may be installed in the sea floor near the base of the platform. Vertically slidable sleeves 44 transmit lateral loads from the skirt piles through a truss 46 rigidly affixed to sleeves 20. Bearings 48 may be inserted between the skirt piles 42 and sleeves 44 to facilitate relative axial movement.

While use of the hydraulic means as set forth above is preferred for coupling the structure sleeves and platform to the axial load piles, it is within the spirit and skill of this invention to use conventional mechanical systems to accomplish the same end.