JACK-UP DRILLING UNIT WITH TENSION LEGS

Abstract

The invention relates generally to a method for supporting non-buoyant offshore drilling units that are anchored to the sea floor and are subject to heavy lateral loads. Specifically, tension tendons are attached to piled foundations to prevent jack-ups from being overturned by the loads. Tension can also be applied to the tendons to provide additional hold down forces to the offshore units.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application which claims benefit under 35 USC §119(e) to U.S. Provisional Application Ser. No. 61/754,856 filed Jan. 21, 2013, entitled “JACK UP DRILLING UNIT WITH TENSION LEGS,” which is incorporated herein in its entirety.

FEDERALLY SPONSORED RESEARCH STATEMENT

Not Applicable

REFERENCE TO MICROFICHE APPENDIX

Not applicable.

GENERAL FIELD

The disclosure relates generally to offshore drilling units that are anchored to the sea floor, and particularly to jack-up rigs.

BACKGROUND

The oil and gas industry has been expanding its exploration and production operations from land to sea since the 1890s. The first submerged oil well was drilled in fresh waters in Ohio in 1891. In 1897, the first derrick was placed atop a wharf about 250 feet from the California shoreline. However, true offshore drilling and production did not take off until the first well was drilled completely out of site of land in 1947. Since then, advancing technology has allowed for the drilling of wells and recovery of oil and gas at greater water depths. However, difficulties still remain, especially in waters known for large waves, moving sea ice or icebergs, and landfast ice.

FIG. 1 displays commonly used offshore drilling units. Some units, such as the semi-submersible and drilling ship are moored to the sea floor to minimize lateral and vertical displacement. Such mooring systems can include catenary mooring, where the lines are loose, and buoyancy-corrected weight of the suspended part of the mooring line must balance the station-keeping moment in the catenary mooring system. Another option is a taut mooring system, where the lines are light, tightened to near straightness, and axial elasticity accommodates wave motion. Semi-taut systems are also available.

Others, such as the platform rig, are permanently anchored to the sea floor. Of particular interest is the jack-up rig because it is a mobile offshore drilling unit that anchors to the sea floor and is raised above the water for use.

The jack-up is designed to be towed (though some may be self-propelled) to the drilling site and be ‘jacked up’ above the water so that the wave action of the sea only impacts the legs, and not the main body of the decking. Because the legs generally have a smaller cross section, the wave action can pass by without significantly moving the JU. However, jack-up rigs are limited to water depth of less than 400 feet due to the limits on leg length.

Generally, jack-up platforms are triangular in shape with three legs that bear the weight of the unit equally. However, the platform shape and number of legs can change to accommodate a larger unit. Furthermore, during a ‘preloading’ phase, ballast water and the platform are used to weigh down the legs and drive them into the sea floor such that they cannot penetrate further during operations, thus shifting the deck while in use.

Jack-up units typically include the drilling, production, and/or workover equipment, a leg jacking system (typically a rack and pinion system), crew quarters, loading and unloading facilities, storage areas for bulk and liquid materials, helicopter landing deck and other related facilities and equipment.

The advantage of the jack-up rig is that as a floating platform it can be moved easily from site to site. However, many jack-ups are now operating for extended periods at one location, especially those acting as production units. Furthermore, many jack-up rigs are self-elevating. A rack and pinion system located on the platform is used to lower the legs to the sea floor when they continued to be ‘jacked’ until the hull is raised above the water. By having the ‘jacking’ system on board, the rig can quickly be moved in case of an emergency.

However, jack-up rigs do have some disadvantages. Jack-up rigs are subject to a number of internal and external loads, which when combined, can lead to bending, compression, buckling, fatigue or overturning. As such, jack-up rigs have limited resistance to external loads, which is a function of the jack-up design and available downward force, limited by the weight and loads on the jack-up. The horizontal environmental loads, due to wave, wind, current, and ice, as applied to the deck and legs, especially when the unit is ‘jacked up,’ can generate enough force to overturn the structure. Ice loads in particular can be very high exceeding wave and winds loads by a very high factor and cause a jack-up to slide.

For instance, jack-up rigs are finding increasing use in offshore Arctic drilling, especially in more shallow waters. However, the Arctic is a remote and harsh location where ice on the water creates considerable challenges to prospecting for and producing hydrocarbons. The ice floes found in the Arctic can collide with the rig legs and cause structural damage to the legs or push the rig off its mooring. The horizontal loads, combined with the internal compression loads caused by needing a larger platform, more equipment, and ice weight on the jack-up itself, can also facilitate buckling of the rig. This is problematic because even small shifts can result in a hydrocarbon leak. Any risk of such a leak is completely unacceptable in the oil and gas industry, to the regulators and to the public, particularly in pristine Arctic environments.

Many inventions have been made to increase the stability of jack-up rigs, especially in the Arctic. For instance, US20120125688 proposes an ice-worthy jack-up (IWJU) wherein the hull includes an ice bending shape that starts near the deck and extends downwardly to the bottom of the hull. The hull is lowered into the water into an “ice defensive configuration” in the absence of ice. Thus, when ice moves towards the IWJU or when there is landfast ice, the legs of the IWJU are protected by the ice bending hull. While this protects the IWJU from structural damage, it does not address the issue of overturning.

Even in milder conditions, jack-up rigs face threats of being overturned by currents, waves, and wind. U.S. Pat. No. 4,265,568 discloses a jack-up rig with a single leg attached to a polygonal-shaped gravity base that is capable of penetrating the sea floor. The gravity base provides some protection against overturning forces. Other designs well know in the oil and gas field include attaching the legs to spud cans or mats anchored onto the sea floor.

What is needed in the art is a better method of supporting offshore drilling units anchored to the sea floor, such as the jack-up, against heavy wind, wave or ice loads.

Furthermore, what is needed in the art is a method of supporting jack-ups against horizontal loads wherein the support system can be released quickly.

SUMMARY

The present disclosure relates to a method of protecting offshore drilling units that are anchored to the sea floor, such as a jack-up (JU), from being overturned by horizontal loads. Specifically, tendons are attached to the sea floor anchoring foundation and to the rig to prevent the unit from overturning from heavy horizontal environmental loads.

U.S. Pat. No. 4,604,001 describes a tension-leg platform that uses jack-up legs to secure the platform while tendons are attached and tightened. However, in use this platform is submerged to a depth below the level of the majority of the surface forces, such as, for example, 150 to 200 feet. Thus, the design and specifications are quite different from a jack-up rig. Further, the legs are only used during placement of the tendons, and then jacked-up out of the way again.

The devices and methods of the present disclosure is similar in some respects, however, to the tension-leg platforms (TLP) (see e.g., FIG. 1B-C). The TLP is a compliant structure that responds dynamically to wave excitation. In a free floating condition, the platform would behave like a semi-submersible and move in the six degrees of freedom that are associated with all floating bodies; surge, sway, heave, roll, pitch and yaw. The tension legs resist heave, pitch and roll, but surge, sway and yaw still occur and are dependent on wave excitation and the response characteristics of the platform.

In the present disclosure, the tendons are used to offset horizontal loads that may overturn the anchored offshore unit, thus providing additional support. This is done by tensioning the tendon, which places the units leg in compression. Thus, the combination of jack-up legs together with tension tendons provides resistance to horizontal movement and overturning.

The tension tendons (aka tension legs) can be used with any type of jack-up leg, including but not limited to open-truss legs and columnar legs. Furthermore, the tension tendons are independent and in addition to other stabilization systems, including the legs and anchoring foundations such as mats and spud cans.

While it is possible to integrate the tendons into the jack-up legs, this may not be a preferred configuration because the tendons may be too heavy for the leg structure, especially where tubular joints are used for the tension legs. Also, external tendons may be advantageous because they will be easier to replace. Thus, attachment points are required to reversibly couple or attach the tendons to both the offshore unit and to some foundation structure on the sea floor. Such attachment points can be any place convenient, but should be attached to something configured to handle the load, and include the leg structures and/or the hull.

Top connectors for TLPs, include those described in US2009290939 and which permit some rotational movement of the connector, although it is expected that little rotational movement need be accommodated herein since the rig also has solid legs. In the connector of US2009290939, the tension leg buoy has a bowl-shaped extension that transfers the tension of the tendon to the flex connector. A dome-shaped structure at least partially surrounds the bowl-shaped extension, but is spaced apart from it to permit at least some rotational movement of the connector. The dome-shaped structure may be attached by clamps or other mounting means to a flange member affixed to the tendon porch. This arrangement provides a load path when the tendon top connector is reverse loaded that extends from the tendon length adjustment joint, through slips, through the bowl-shaped extension, through the dome-shaped structure, through the segmented clamps, and finally into a flange attached to the tendon porch. In this way, detachment of the tendon top connector is prevented if a reverse load is applied such as may occur during extreme ocean conditions.

U.S. Pat. No. 4,871,282 describes another top connector. In this patent, the upper connector for each tendon includes a housing with a conical shoulder located therein. A terminal segment on the upper end of each tendon extends through the housing. Dogs are carried on the shoulder of the housing, each having threads on the interior for mating threads formed on the terminal segment. A cam ring moves the dogs from an outer retracted position to an inner engaged position. The cam ring also will rotate the dogs relative to the terminal to mesh the threads of the dogs with the threads of the terminal segment.

U.S. Pat. No. 5,020,942 describes a TLP top connector that has a housing with a bore containing a conical shoulder. Several segments locate on the conical shoulder and slide between an upper retracted position to an engaged position. In the engaged position, threads on the interior of the segments engage threads formed on the exterior of the tendon. A cam plate slides the segments down when the cam plate is rotated. A guide ring mounts outward of the segments. The guide ring has fingers that engage slots in the backs of each of the segments, and the fingers and vertical slots allow the segments to move axially, but prevent them from rotating relative to the guide ring. A clutch ring applies a frictional force to the guide ring to resist rotation until the segments engage the tendon threads.

The bottom end of the tendon is attached to a foundation—any structure that is anchored to the sea floor—and is preferably an existing foundation, although foundation anchor points can be added if needed. Tendons can be connected to a foundation through any suitable anchor connector, and many are available in the art. Anchor connectors are described, for example, in U.S. Pat. No. 4,498,814, U.S. Pat. No. 4,611,953, EP0319419, U.S. Pat. No. 4,943,188, U.S. Pat. No. 4,374,630, U.S. Pat. No. 4,907,914, U.S. Pat. No. 5,004,272, and U.S. Pat. No. 6,568,875. OTC: 4947-MS describes the connectors used in Conoco's Hutton field TLP.

ConocoPhillips has also developed tendon anchoring devices. U.S. Pat. No. 4,844,659 describes an apparatus for attaching a floating tension leg platform to an anchoring base template on the subsea floor includes an external mooring porch for each tendon, the porches being mounted on the outside surfaces of the platform's columns. The number of porches may exceed the number of columns by a factor of at least two or three.

U.S. Pat. No. 5,324,141, also by ConocoPhillips, describes another such bottom connector, wherein the apparatus includes a tendon with an enlarged connector on one end, the connector being formed by a frustoconical bearing surface extending away from the end to which it is connected. A connector shroud partially surrounds the frustoconical surface and is connected to it by an elastomeric bearing. An inwardly sloping surface engages a supplementarily sloped surface formed on the attachment receptacle load ring to increase hoop strength. A side-entry bottom receptacle is also disclosed.

The tendon attachment point(s) can be modified for use with the various offshore units. In one embodiment, the tendons can be attached to a piled foundation or sockets (depending on the type of offshore unit). In another embodiment, a single deep-set, large diameter pile can serve as the attachment point for multiple tendons.

U.S. Pat. No. 6,036,404 describes a foundation system for tension leg platforms without use of foundation templates, wherein each tendon is directly connected to a socket inside the pile, said piles being positioned for driving purpose by means of a pile-driving template which is employed as a spacing device. The pile-driving template is positioned with the aid of pins that slot into guides built into the well template. After the groups of piles needed to anchor a corner of the platform have been driven in, the pile-driving template is withdrawn and repositioned so as to enable the piles for the other group of legs to be driven; this process continues until all of the pile-driving is finished.

Mooring systems are described in US2011206466, which includes tendons supported at tendon porches directly at the four column outboard lower corners, without additional radially-extending tendon support structures.

A hydraulic jack assembly or other tensioning device can be used to tension the tendons. Usually, the tendons are configured in such a way as to form a straight line alongside (or inside) the legs, but can also be at an angle, e.g., about 45° from horizontal, although this is less likely.

The tendons can be tensioned in place using a hydraulic jack assembly, winch, or by opposing forces (buoyance v. hold down force or stretching as the jack-up is raised). In other embodiments, the tendons can be tensioned as described in US2006210362, U.S. Pat. No. 5,551,802, or U.S. Pat. No. 7,044,685. For example, the tension can first be applied to a pull-down line (such as a steel chain) by a winch, and later transfer the tension to the tendon by engaging the tendon to a fixed sleeve and release the pull-down line.

Self tensioning legs are also available, and may be particularly preferred as easy to install. See e.g., US2010232886. This patent describes a self-tensioning tendon (of the pipe type) having a hydraulically controlled Length Adjustment Joint (LAJ) containing an integral cylinder with external threads and a piston rod. A hydraulic source actuates the integral cylinder to pull the TLP down to the target draft position, significantly reducing the time needed to ballast the hull of the TLP with millions of gallons of water. A Top Tendon Connector (TTC) ratchets down along the cylinder to lock the TLP at the final draft position.

US20110052327 describes a floating hull and a method of connection of such a hull to tension legs, which avoids large bending forces on the connector and which can be rapidly anchored to the sea bed via pre-installed tension legs without the need for installation support vessels. The attachment means comprise a guide member for lowering a tensioning member section by a predetermined length, which tensioning member section at a free end is provided with a complementary connector for attaching to the connector on the upper end of the tensioning member, the tensioning member section comprising at an upper end a stopper for engaging with the base and for fixing the upper end in a vertical direction, the floating construction comprising a pulling device attached to the tensioning member section, for lowering the uncoupled tensioning member section along the guide member.

Tension legs can be any known in the art, and in the past have typically been tubular pipes, joined end to end. As an example, the world's first commercial tension leg platform by Conoco at Hutton field utilizes a plurality of tubular joints thirty feet in length having a ten-inch outer diameter and a three inch longitudinal bore. However, modern methods also use tension leg cables, which can be easier to transport and deploy due to their flexibility. Pressurized gas filled tendons are also known (U.S. Pat. No. 4,521,135), as are pressurized liquid filled tendons (U.S. Pat. No. 4,664,554).

Corrosion resistant cables are taught in U.S. Pat. No. 4,285,615 by Conoco, and such may be particularly preferred. These multi-strand cables have voids between adjacent strands, a fluid tight covering, and a fluid, such as nitrogen or argon, filling all spaces between the two.

The first installed TLP, on Conoco's Hutton field, had 10.25 inch (0.26 meter) OD, thick-walled tubular tendons, but in general, industry is tending toward larger diameters with D/t ratios of 20 to 30. Thus, the tendons may preferably be large diameter and thin walled, typically 48 inch (1.22 meter) outside diameter (OD) with a diameter to thickness (D/t) ratio greater than 30. See OST: 5937-MS. Compared with heavy-wall, small diameter tendons, the advantages of large D/t tendons are that they: (1) have greater buoyancy, which reduces the weight; (2) permit a leak-before-break inspection philosophy; (3) can be fabricated by more efficient pipe rolling methods, thereby reducing overall tendon cost, and (4) may have greater steel area in each tendon, thereby reducing the number of tendons and tendon support hardware.

One cable that may be preferred is a 122 mm diameter cable with a breaking strength elf 8000 KN. The core is composed of layers of round wires 7 mm in diameter, made of drawn high carbon steel. An amorphous polypropylene filling medium is pressure applied during spinning. On this center two layers of Z profile wires are stranded 7 mm thick. They are made of cold drawn and rolled high carbon steel. The peripheral layer is composed of pure zinc Z wires and form a continuous concentric anode. An outer sheath of polyurethane is then pressure-extruded onto this full locked type assembly. See e.g., OTC: 4052-MS. This cable is allegedly easy to handle because of its plastic jacket and its internal wire arrangement; and well protected against water penetration because of the inside filling material, the presence of three layers of Z wires, and the presence of the external sheath.

Where the tendon needs to be compiled from multiple lengths, connectors will be needed. Such connectors are well known in the art, and include threaded joints for tubular legs, releasable joints (U.S. Pat. No. 4,869,615), and the like.

Length-adjustment unit(s) may also be provided in a tendon to either increase or reduce the tension for different purposes. For example, when ice or high seas nudges the hull upward to an extent that the tendon or the hull may be damaged, extending the length of the tendon can alleviate the stress.

For jack-up rigs in particular, the tendons are expected to increase the flexibility of ice worthy JUs for Arctic operations, and increase the ability of rigs to operate in areas with high wind, wave and ice loads.

The phrase “horizontal environmental loads” or “horizontal loads” are used interchangeably and mean any environmental or climatic force (wind, waves, ice, current, etc.), generally perpendicular to gravity, which acts on a structure causing it to drift, slide or topple. As used herein, this also applies to horizontal loads that can cause structural damage to supports, i.e. ice hitting legs on a jack-up.

The phrases “offshore unit”, “drilling unit”, and “unit” are used interchangeably and mean any unit used in water for oil and gas drilling, production, or completion. This includes both open and closed waters.

The phrase “legs” as used herein mean any solid structure (as opposed to tendons, which are flexible) used to support a sea floor anchored offshore oil and gas unit. These can include, but is not limited to, open-truss legs, columnar legs, ballasts, and other gravity-based support structures.

By “tendon” what is meant is a flexible tube, bar, wire or cables. Cable structure consists of multiple wires bonded, twisted or braided together to form a single assembly, but preferred cables are 7×19 stainless steel wire rope, 3×19 galvanized cable, spiral strand cable, and the like, and may be wrapped or otherwise coated for increased corrosion resistance. For example, mooring cables are often jacketed in polypropylene. In this application heavy tube would be the most likely tendon as it could carry heavier loads that a cable and stretch less and would be easier to tension.

By “tensioned” tendon what is meant is that force is applied to tendon, tightening it in place, and not applied by an external, variable force as would be the case with e.g., a loose mooring line being variably tightened and loosened by wave action. The tensioning force will tend to pull the hull down, whereas the jack-up legs will oppose that force.

By “flexible” in relationship to tendons what is meant is that the tendons are too flexible to support the weight of the rig, as opposed to the legs, which are constructed to support the weight of the rig. Thus, a tubular tension leg, even though not flexible over a short length, will be too flexible over the entire length to support a jack-up unit that is no longer buoyant.

Preferred materials are corrosion and salt resistant, and include stainless steel, carbon fiber reinforced steel, and properly protected steels. Deep sea mooring cables in synthetic materials are also known, but in certain situations may be less preferred as subject to biological attack.

The use of the word “a” or “an” when used in conjunction with the term “comprising” in the claims or the specification means one or more than one, unless the context dictates otherwise.

The term “about” means the stated value plus or minus the margin of error of measurement or plus or minus 10% if no method of measurement is indicated.

The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or if the alternatives are mutually exclusive.

The terms “comprise”, “have”, “include” and “contain” (and their variants) are open-ended linking verbs and allow the addition of other elements when used in a claim.

The phrase “consisting of” is closed, and excludes all additional elements.

The phrase “consisting essentially of” excludes additional material elements, but allows the inclusions of non-material elements that do not substantially change the nature of the invention.

The following abbreviations are used herein:

CPM  Conical piled monopods
GBS  Gravity based structures
IWJU Ice-worthy Jack-up
JU   Jack-up
TLP  Tension leg platform

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A. Common offshore drilling units and preferred use depth.

FIG. 1B. A perspective view of a prior art tension leg platform (TLP), showing vertical columns that are connected together by a ring-shaped pontoon, which is located inboard of the columns.

FIG. 1C. is a plan view in cross section taken along lines 4-4 of FIG. 1B of the hull (columns and pontoons) of the tension leg platform.

FIG. 2A illustrates a jack-up rig on location before the legs are lowered.

FIG. 2B illustrates a jack-up rig attached to spud cans resting on the pile. Tendons have been lowered and attached to the foundation.

FIG. 2C illustrates a jack-up rig on location and hull or platform raised above the water level.

FIG. 3 illustrates an ice-worthy jack-up (IWJU) in an “ice defensive configuration” with tendons attached to the pilings.

FIG. 4 illustrates the hydraulic jack assembly.

FIG. 5A-C. One tensioning method.

DESCRIPTION OF EMBODIMENTS

The present disclosure provides a novel method and devices for preventing an offshore unit from overturning due to horizontal environmental and climatic loads using tendons capable of being tensioned. This support method can easily be modified for use with jack-ups (JUs), ice-worthy jack-ups (IWJUs, e.g., a JU having an ice hull), conical piled monopods (CPMs), and gravity based structures (GBS).

One of the novel aspects of this method includes the use of tendons, capable of being tensioned, attached to the foundation and the offshore unit. Essentially, the tendons are attached adjacent to the legs and are attached to the sea floor foundation. The tendons are then ‘tensioned’ by the opposing forces (i.e. buoyant or jacked-up platform and immobilized foundation) present. Thus, the novel units have both legs and tendons and thus greatly improved stability.

The invention may include one or more of the following embodiments:

A method for reinforcing an offshore oil and gas unit that is anchored to the sea floor comprising:

a) providing an offshore oil and gas unit (OOGU) that is that is anchored to the sea floor with at least one leg;

b) extending a tendon from the OOGU to a foundation on the sea floor; and

c) tensioning said tendon sufficiently to reinforce the OOGU.

The offshore drilling unit can be a jack-up, an ice-worthy jack-up, a conical piled monopod, or any gravity based structure, or any unit also having solid legs.

Another embodiment is a jack-up rig for offshore drilling for hydrocarbons comprising:

a) a flotation hull having a relatively flat deck at the upper surface thereof;

b) at least three legs that are positioned within a perimeter of the flotation hull wherein the legs are arranged to be i) lifted up off the seafloor so that the Jack-up may be towed through shallow water and ii) extend to the sea floor and extend further to lift the hull partially or fully out of the water;

c) a jack-up device associated with each leg to raise and lower said leg; and

d) a plurality of tendons having a top end attached to the jack-up unit and a bottom end configured to be attached to a sea floor foundation.

The tension legs or tendons can have a top end is attached to the hull or to one of said legs.

The foundation can be any suitable foundation, and preferably uses existing foundation structures, such as a piled foundation. One or more than one tendon is attached to said foundation. The tendons can be tubular tendons, or cable tendons, but preferably include a self tensioning tendon.

In other embodiments, an improved jack-up rig is provided, jack-up rigs including a buoyant hull, legs traversing said hull, and a jackup device for each leg such that the buoyant hull can be raised out of the water for use, the improvement comprising tension legs having a top end attached to said jack-up rig and a bottom end attachable to a foundation on the sea floor.

In yet another improved jack-up rig, the improvement includes tension legs having a top end attached to said jack-up rig and a bottom end attached to a foundation on the sea floor, said tension legs further having a self-tensioning unit for adjusting the length and thus tension of each tension leg.

Some embodiments of the disclosure are exemplified with respect to FIGS. 2-5.

Although the systems, processes, and alternative designs included herein have been described in detail, these figures are exemplary only. It should be understood that various changes, substitutions, and alterations could be made without departing from the spirit and scope of the invention. Those skilled in the art will be able to identify other ways to practice the invention that are not explicitly described herein. The following examples are intended to be illustrative only, and not unduly limit the scope of the appended claims.

Prior Art Tension Leg Platform

A prior art TLP (from US2011206466) is shown in FIGS. 1B and 1C. FIGS. 1B-C show a tension leg platform 10 for use in offshore applications. The platform 10 has a hull 11 including vertical support columns 12 and a central pontoon structure 14 disposed inboard of the columns at a lower end thereof. TLP 10 includes a deck structure 13 supported by the upper ends of the columns 12.

The interior of both the columns 12 and the pontoon structure 14 is preferably subdivided by structural bulkheads (not illustrated) to strengthen the structure, to provide enclosed spaces for locating and storing various equipment (e.g., anchors, chains, propulsion mechanisms, etc.), and to provide a plurality of separate tanks for purposes of ballasting the vessel and storing various fluids, equipment, and other materials which may be required or desired during drilling or production by the well.

TLP 10 is anchored by a plurality of vertical or near vertical mooring tendons 17 that are connected to tendon porches 18 on the lower end of the outboard face of the columns 12. Each column 12 is designed to mate with at least one, but usually two or more tendons 17. The tendon porches are positioned near the keel elevation and contain connection sleeves (not illustrated) to receive the upper tips of the tendons 17 and clamp thereto. The connection sleeves may be ring-shaped, requiring vertical entry of the tendons 17, or they may be slotted to allow side entry of the tendons 17.

Various types of risers 19 can be supported by the hull 11, including near-vertical top tensioned risers (TTR), flexible risers, or steel catenary risers (SCR). The flexible risers or steel catenary risers (SCRs) can be supported on the inboard or the outboard side of the central pontoon structure 14, and extended to the deck 13 by either a single span spool piece or by piping supported on the hull. The top tensioned risers (TTRs) can be supported on the deck 13, and can also be supported laterally at the pontoon elevation by riser keel joints (not illustrated).

Although any suitable shape may be used, the central pontoon structure 14 is octagonal-shaped, having four orthogonally-oriented side segments 14a intervaled with four diagonally-oriented corner segments 14b that are connected to the pontoon structure 14 to form a unitized structure centered about the platform central vertical axis C. In the embodiment shown in FIGS. 1B-C, the central pontoon structure 14 includes a central moonpool opening 14c, which is illustrated as an octagonal opening but may have any other suitable shape. Side and corner segments 14a, 14b are each preferably characterized by generally rectangular transverse cross section surrounding a central horizontal axis or horizontal centerline HC.

Each of the vertical columns 12 has a lower end 12a and an upper end 12b. The columns 12 preferably have a quadrilateral transverse (horizontal) cross-section, which may be a generally rectangular or trapezoidal-shaped configuration. FIGS. 1B-C show columns 12 as rectangular, having a transverse cross-sectional shape with a major axis A1 oriented radially outward from a center point C of the hull 11.

Specifically, columns 12 define a rectangular transverse cross section formed of two parallel spaced wider lateral side walls 12c connected to narrower inner and outer side walls, 12d, 12e, respectively. Thus, each vertical support column 12 defines a major axis A1 extending between the inboard and outboard side walls, 12d, 12e, and a minor axis A2 extending between the two lateral side walls 12c. Each vertical support column 12 defines a vertical longitudinal axis or vertical centerline VC at the intersection of major axis A1 and minor axis A2. The major axis A1 of each of the vertical support columns 12 is preferably oriented radially outward from the center C of the platform. A lower portion of inboard side wall 12d of each vertical support column 12 abuts and is joined to a respective diagonal corner segment 14b of the pontoon structure 14.

Vertical support columns 12 are disposed substantially outboard of the central pontoon structure 14. The vertical axis VC of each column 12 is disposed a distance D1 outwardly from the outer periphery of corner segment 14b of the pontoon structure 14 and a distance D2 outwardly from the central horizontal axis or horizontal centerline HC extending through the pontoon corner segment 14b. Thus, with this hull configuration the central pontoon structure 14 is positioned inboard of the vertical support columns 12, such that a line S defined between the vertical centerlines VC of two adjacent columns 12 lies outside the horizontal centerline HC of the pontoon side segments and, more preferably, outside the outer periphery of the pontoon structure 14. This design feature differs from previous tension leg platform designs, which typically have individual pontoons centered between the columns, with the vertical centerlines of the support columns intersecting the horizontal centerlines of the adjacent pontoons.

One or more of the TLP features (especially tendon legs, connectors, tensioning systems and the like) may be incorporated into the jack-up rig and other structures claimed herein.

Installing Jack-Up with Tension Tendons

FIG. 2A shows a jack-up rig 210 at a pre-determined location. The generally planar deck 221, where the drilling equipment 230 is located, of the floating hull 220 rest above the water line 212. The feet (e.g., the bottom of the legs) 226 have been connected to spud cans 228 before being towed to the pre-determined location. Once positioned above the pilings 236 located on the sea floor 215, the rig legs 225 will be lowered through openings 227 in the hull. The spud cans 228 will be connected to the piled socket 235.

In FIG. 2B, the legs 225 have been lowered and the spud cans 228 have been connected directly to the piled sockets 235 anchored to the sea floor 215. The piled sockets 235, after having been precisely positioned on the sea floor 215, are secured to the seafloor by pilings 236 that keep the sockets 235 from lifting up or settling lower into the sea floor. Once the legs 225 are connected to the foundation, the tendons 237 can be lowered and attached to the piled sockets 235 or another foundation means. Once the tendons are in place, the hull or platform of the jack-up rig can be raised.

In an alternative configurations (not shown), the tendons can be attached to a single pile, resulting in tendons not running parallel to the legs. In yet alternative configurations not shown, the tendons can be attached to a multiple piles outside the perimeter of the rig, resulting in tendons angled away from the vertical.

In FIG. 2C, the deck 221 has been raised above the water line 212. The tendons 237 are stretched between the top of the legs 225 and the sockets 235. Generally, the tendons will experience some tension because, much like rubber bands, they will want to return to their original length but cannot due to being anchored by the foundation. A hydraulic system (not shown) can be used to increase the tension of the tendons 237. Increase in tension is especially necessary for harsh environmental conditions found in places such as the Arctic. Moving ice and land fast ice can displace an ice worthy jack-up even when it is in an ice defensive position (see FIG. 3).

The jack up rigs can have one or more legs, and more commonly three or four legs, however, for simplicity, only two legs are shown here. Furthermore, while open-truss legs are shown in the figures, columnar legs can also be used because the tendons can be outside the leg structure. Generally, only one tendon is used per leg, however, in strenuous environments, such as the Beaufort Sea, more tendons can be used.

Spud cans are typically used on independent-legged jack-up rigs and are designed to spread the load so that the unit does not sink too deeply into the sea floor. Other types of supports for the rig legs include mat-supported which distribute the weight of the unit across the ocean's bottom, much like a snow shoe. Any type of support system can be used with the present devices and methods.

When support systems are not used, the jack-ups are usually ‘preloaded’ into the sea floor to simulate the maximum expected leg loads. This occurs by using the weight of the platform, hull and ballast to drive the legs into the sea floor. In theory, the legs will not penetrate any further into the sea floor during operations. For these types of set ups, a single pile can be installed and used as the foundation attachment point.

Tensioning Tendons

In the current devices and methods, the tendons can be tensioned using a hydraulic jack assembly to provide additional hold down forces in the foundation. The operation and utilization of this tension control system begins after the tendons and legs have been attached to the foundation or support system, and generally after the j

FIG. 4 illustrates an exemplary hydraulic jack assembly. This system includes one or more hydraulic jacks 467. Each jack has a cylinder 463 coupled through a load block 462 to a tension tendon 446, and a floating piston 466 cooperating with the load block plug (not shown) coupled to the deck. A collar means 468 is disposed about the tendon and is adapted for movement in cooperation with the anchoring plate (not shown) on the offshore unit to grip the tendon. A flange 464 helps to hold the load block to the offshore unit. The floating piston 466 is contained inside the cylinder 463 and is moveable. The bore of the load block is aligned with the core of the cylinder such that the load block plug can slide through the bore of the load block and abut the floating piston.

An accumulator 460 supplies hydraulic fluid 61, under pressure, to the jacks to compensatingly adjust the relative position of the floating piston 66 and cylinder 67 to selectively maintain the tension loading of the tendon as the rig hull is ‘jacked up.’

Alternative Tensioning of Tendons

In the alternative, tension can be applied to the tendons on site. Referring to FIG. 5A, a winch 444 located on the deck 448 can be used to wind a pull-down line 446, which may be a steel chain. At the lower end of the pull-down line there is provided an engaging means 532A, which will be coupled with the tip 532B of a tendon 512, as shown in FIG. 5B. The hull 516 preferably also has corresponding number of sleeves 522 to securely lock the tendons in position.

As shown in FIG. 5C, the pull-down line 546 is first guided inside the sleeve 522 to further lower into the water, where the engaging means 532A will couple with a corresponding tendon 512 at its tip 532B. After the engaging means of the pull-down line 546 is coupled with the tendon 512, the winch 544 then applies tension to the pull-down line by reeling up the pull-down line. When the tendon 512 is in place within the sleeve 522, as shown in FIG. 5D, the sleeve can then clamp the tendon 512 or equivalently lock the tendon 512 in position. At this point of time, relieving the tension by the winch 544 to slack the pull-down line 546 will transfer the tension to the tendon 512. Additional tension can be applied if the tendons stretch, which may be of significance if the cable tendons are used that were not pre-stretched before deployment.

Additionally, a length adjustment unit can be provided for each tendon. The length of the tendon can be adjusted by the length adjustment unit. Such adjustment may be useful if relieving a certain amount of tension is beneficial, for example if the ice load pushes hull upward to the extent of damaging the tendon, or if slack is needed for other reasons.

The following references are incorporated by reference in their entirety.

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Claims

1. A method for reinforcing an offshore oil and gas unit that is anchored to the sea floor comprising:

a) providing an offshore oil and gas unit (OOGU) that is that is anchored to the sea floor with at least one leg;

b) extending a tendon from the OOGU to a foundation on the sea floor; and

c) tensioning said tendon sufficiently to reinforce the OOGU.

2. The method of claim 1, wherein said offshore drilling unit is a jack-up, an ice-worthy jack-up, a conical piled monopod, or a gravity based structure.

3. A jack-up rig for offshore drilling for hydrocarbons comprising:

a) a flotation hull having a relatively flat deck at the upper surface thereof;

b) at least three legs that are positioned within a perimeter of the flotation hull wherein the legs are arranged to be i) lifted up off the seafloor so that the jack-up may be towed through shallow water and ii) extend to the sea floor and extend further to lift the hull partially or fully out of the water;

c) a jack-up device associated with each leg to raise and lower said leg; and

d) a plurality of tendons having a top end attached to the jack-up unit and a bottom end configured to be attached to a sea floor foundation.

4. A jack-up rig as in claim 3), wherein said top end is attached to said hull.

5. A jack-up rig as in claim 3), wherein said top end is attached to one of said legs.

6. A jack-up rig as in claim 3), wherein said foundation is a piled foundation.

7. A jack-up rig as in claim 4), wherein more than one tendon is attached to said piled foundation.

8. A jack-up rig as in claim 3), wherein said tendons are tubular tendons.

9. A jack-up rig as in claim 3), wherein said tendons are cable tendons.

10. A jack-up rig as in claim 3), wherein said tendons comprise a self tensioning tendon.

11. An improved jack-up rig, jack-up rigs including a buoyant hull, legs traversing said hull, and a jack for each leg such that the buoyant hull can be raised out of the water for use, the improvement comprising tension legs having a top end attached to said jack-up rig and a bottom end attachable to a foundation on the sea floor.

12. An improved-up rig that includes a buoyant hull, legs traversing each hull and a jack-up device for each leg such that the buoyant hull can be raised out of the water for use, the improvement comprising tension legs having a top end attached to said jack up rig and a bottom end attached to a foundation on the sea floor, said tension legs further having a self-tensioning unit for adjusting the length and thus tension of each tension leg.