System and method for improving a jack up platform with asymmetric cleats

A mobile drilling unit having a floatable platform for use in a body of water with a plurality of vertical support legs. With each vertical support leg configured to move with a cleat at the lower end of each support leg. Each cleat having a lower surface to transmit gravitational force from the unit to the sea floor. The cleats are asymmetric with respect to the legs which allows expansion of the center of pressure on the cleats to be beyond the vertical support legs.

BACKGROUND

Generally, an offshore jacket is comprised of at least three substantially vertical legs that are interconnected by framing or cross-bracing members to form a triangular or rectangular base, wherein a leg is disposed at each corner of the base. In its upright position, the jacket rest on the sea floor with the bottom of the legs resting on the sea floor or slightly penetrating into the soil. The jacket is secured to the sea floor with piles which are either driven through the legs or driven through sleeves attached to the legs.FIG. 1shows a traditional offshore jacket. The flared jacket25provides wider base (b) for greater stability when attached to the sea floor.

In many areas of the world, the soil of the sea floor is unconsolidated and very soft resulting in very low allowable bearing pressures. These soft sea floors occur frequently near the mouths of large rivers that empty into the oceans. Sea beds in the world which exhibit high hydrocarbon content but are characterized by soft soils from river deltas include areas in the Gulf of Mexico, west Africa and southeast Asia.

The low bearing pressures of these unconsolidated sea floors create jacket support problems during installation of offshore platforms. Specifically, without adequate support, the legs of a jacket will sink into the sea floor, causing the jacket to either fall onto its side or settle lower than design specifications. In any case, jacket settling due to a soft sea floor can negatively affect the alignment of the jacket as it is positioned at the drilling site. In this same vein, difficulties often arise during pile driving operations, which are generally completed within one to two weeks of placing a jacket in position on the sea floor.

One solution to the difficulties associated with unconsolidated sea floors is to provide a structure that spreads the downward forces applied to the jacket over a larger area of the sea floor. The most common structure for accomplishing this task is called a mudmat. A mudmat has a very large surface area that rests against the sea floor (as opposed to the comparatively small surface area of a jacket leg), distributing the load of the jacket over a larger sea floor, thus allowing the jacket to properly stand on the soft sea floor and to provide stability during pile-driving operations. The bearing plate rests against the sea floor and provides the large surface area for force distribution.

There are several different types of units. Of course one of the first developed was the fixed platform in which the legs or supports of the rig are permanently installed, penetrating the floor of the body of water in which the well is to be drilled as discussed previously inFIG. 1. Such a structure is limited by water depth and does not provide the mobility and flexibility of the mobile or portable type unit.

One form of unit is the self-elevating platform, sometimes called “bootstrap” or “jack-up”, units which are moved to a use site. These units with a plurality of legs, usually three, are lowered from a floating platform through the water for engaging sea floor The footings (cleats or feet), engage with the sea floor, then the platform is jacked up a sufficient distance above the water surface to get the platform above the wave action. U.S. Pat. Nos. 3,996,754 to Lowery and 4,265,568 to Herrmann et al. are representative of this type. Although such units are highly mobile and stable when in place, they are less stable when floating and when in transit from site to site and are limited to a range of water depth while the unit is afloat. In areas of extreme weather conditions, the three or more legs of such rigs may not have the required stability as the base (b) is limited by the size of the platform. This example is shown inFIG. 2.FIG. 2shows platform10supported by three legs20, connected to cleats or feet50on the sea floor1. The cleats or feet50are symmetric with respect to the legs such that the center of pressure51exerted by the sea floor1is congruent with the center of the legs20. While those cleats or feet50are shown as octagonal, many other symmetric shapes are commonly used, circles, squares, rectangles, ovals, etc. However, each shape is symmetric with respect to the leg to ensure the center of pressure is under the leg, and external bending moments on the legs20are minimized.

The present subject matter provides the mobility, low cost and stability in a self-elevating type unit by extending the base beyond the traditional limits of self-elevating platforms, enabling compact transportation and a distributing leg reactions over a larger base (b′) on the sea floor.

These and many other advantages of the present subject matter will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of preferred embodiments.

DETAILED DESCRIPTION

The self-elevating unit of the present subject matter combines desirable features of (fixed) jackets and jack-up units. The self-elevating unit is made up of three major components as shown inFIG. 3: a plurality of legs20, a respective plurality of cleats or feet50and a floatable platform10. The plurality of legs20are each attached to their respective cleat/foot50and extend upwardly through a respective leg well21provided therefore in the platform10. The platform10supports a drilling derrick, living quarters and other equipment necessary for drilling.

The cleats or feet50which preferably provide additional buoyancy for supporting the unit while it is floating and is in transit from site to site. Should it be desired to increase the stability of the unit during transit, say for heavy seas, the cleats or feet may be partially filled with water and lowered to a partially submerged position, lowering the center of gravity of the unit and increasing its stability.

The cleats or feet50of the disclosed subject matter differ from that of the prior art in that they are asymmetric with respect to the legs20(e.g. are not centered on the leg). Prior art cleats or feet are centered on the leg20as such to align the center of pressure51under each respective leg.FIG. 2shows the force provided by the cleat or foot50being directed vertically up the leg20. The center of pressure as used in this disclosure being predominately a function of cleat or foot bottom surface in contact with the sea floor. The cleats or feet as shown inFIG. 3, are configured to have a center of pressure outboard of the leg well21and the leg20, such that a base b′ is greater than the base b of the prior art. The bearing force is also directed though the leg20to the platform but additionally introduces a moment component as shown inFIG. 3. The larger base b′ provides stability.

In the initial or transit position, enough ballast is removed from the cleats or feet so that the platform is floating, and the combined buoyancy is supporting the remainder of the unit. In this position, the unit may be moved. The unit may be attached to an ocean going tow vessel for transit to a preselected site. Alternatively, the rig may be positioned on a traditional barge, however this is not preferred. Should heavy seas be encountered during transit, ballast may be introduced into the cleats or feet, at least partially submerging the platform. In this position, the center of gravity is lowered, increasing the stability of the unit . When these adverse conditions have subsided, the ballast may be removed and the platform returned to the above-described floating or transit position.FIGS. 4a-cshows another advantage of the present disclosure. The initial or transit portion of the cleats or feet20may be rotated to reduce overall size or align with the direction of transport to reduce drag. The rotation may be a function of the elevating jacks, in which the legs20are rotated, or may be a function of a rotation system that rotates the cleats or feet50relative to the leg20.FIGS. 4a-cshow a configuration in which all of the legs are rotated under the platform, are rotated with respect to the direction of travel and shown in a square platform configuration oriented with respect to the direction of travel.FIG. 4bin which the cleats or feet50are within the bounds of the platform10is the minimum size configuration, but also is aligned with the direction of travel.

Upon reaching the selected site, the cleats or feet50may be oriented to their operational positions when fully submerged until it and the support legs are fully supported on the floor of the body of water. In the initial stages of this movement, the platform moves upwardly to assume the partially floating position. The relative movement of the legs20is permitted by the elevating jacks being engaged to drive the legs20. At this point, with the feet50in contact with the sea floor, the platform which is now floating on the surface of the body of water may be elevated, by means of elevating mechanisms, to a selected height above the surface of the body of water. Then the unit is capable of drilling. If desired, the elevating mechanisms may be removed after drilling has been completed and the entire unit converted to a permanent or semi-permanent platform. However, if it is desired to move the unit to a different location, it is only necessary to move the derrick to its non-interfering initial position, lower the platform until it is floating in the water, and raise the cleats or feet and legs. Then the unit may be moved to another site.

The self-elevating unit of the present invention offers the advantages of traditional and jack-up rigs without some of the disadvantages inherent in each of these designs. Further objects and advantages of the invention will become apparent from the description which follows in conjunction with the accompanying drawings.

FIG. 5also shows the cleat or foot50providing a force→Fand moment→Mto the leg20, resultant from center of pressure51of the cleat or foot50. The portion of the cleat or foot50engaging the sea floor is shown as symmetric inFIG. 5, such that during bottoming of the cleat or foot, a lateral force is not imparted to the leg20.

Another aspect of the disclosed subject matter is the ability and advantages of rotating the cleats of feet50on an axis oblique52to the center axis22of the legs20. As shown inFIGS. 6aand 6b, the cleat or foot50may be rotated about the center axis22of the leg20, such that the depth of the cleat or foot to the platform remains substantially constant. However, as shown inFIGS. 6cand 6d, if the axis of rotation52is oblique to the vertical center axis22of the leg20, a vertical change in the distance between the platform10and the cleat50may also be accomplished. As shown D1>D2, where the axis of rotation to inclined away from the platform, such a change may advantageously lower the center of gravity during transport and provide greater stability without increasing drag. Moreover, the lower surface of the cleat or foot50may be optimized for contact with the sea floor, while the surface presented to the sea in the oblique rotation may be optimized to reduce drag and increase stability, tracking during transport.

As can be seen from the foregoing description and accompanying drawings the self-elevating unit of the present invention offers a low center of gravity for ocean tow with a high degree of ocean tow stability at much less cost than self-elevating units designed for comparable water depths. By having asymmetric cleats or feet, the disclosed subject matter provides the support and greater in-place stability afforded by bottom resting units .