FIELD OF THE INVENTION

[0001] This invention relates to offshore drilling units, such as mobile gravity based structures, mobile offshore drilling units and submersibles, used in shallow water for drilling for hydrocarbons, as well as methods associated with drilling wells and maintaining stability of such offshore drilling units.

BACKGROUND OF THE INVENTION

[0002] In the search for hydrocarbons, many oil and gas reservoirs have been discovered over the last one hundred and fifty years. Many technologies have been developed to find new reservoirs and resources and most areas of the world have been scoured looking for new discoveries. Few expect that any large, undiscovered resources remain to be found near populated areas and in places that would be easily accessed. Instead, new large reserves are being found in more challenging and difficult to reach areas.

[0003] One promising area is the offshore Arctic. However, the Arctic is remote and cold where ice on the water creates considerable challenges for prospecting for and extracting hydrocarbons. Over the years, it has generally been regarded that six unprofitable wells must be drilled for every profitable well. If this is actually true, one must hope that the unprofitable wells will not be expensive to drill. However, in the Arctic, little, if anything, is inexpensive.

[0004] Drilling these wells can be done in the exploration phase with different drilling units. Drilling has been done with gravity based structures, drillships, and in the future possibly jack-up and submersible units. The bottom support units all need some downward force to hold them in place. The required force is dependent on environmental loads which can be very high in ice.

[0005] Once it is determined that a potentially profitable well could be drilled, explored or developed, an offshore drilling unit in the form of a very large, gravity based drilling or production system, or similar structure, may be brought in and set on the sea floor for the long process of drilling and producing the hydrocarbons. For just drilling wells, a submersible, jack- up or small gravity based structure could be brought in. The offshore drilling units are typically referred to as gravity based structures, submersibles or possibly a jack-up unit and all are bottom supported.

[0006] Conventionally, the gravity based structures or submersibles must be ballasted to create a downward force on the offshore drilling unit. In benign environments without ice, this ballasting can be done with water. However, in areas where ice forces must be resisted, sand, rocks or other solid materials have been used for ballast. This type of ballasting cannot be done on submersibles, and using solid ballast for gravity based structures is expensive, time consuming, creates environmental issues and limits movement to open water season only.

[0007] Therefore, a need exists for offshore structures that are cost efficient and can withstand ice conditions.

SUMMARY OF THE INVENTION

[0008] In one embodiment of the invention, an offshore drilling unit includes a main body and a ballast fillable with water to at least partially submerge the unit in a body of water with the unit resting on a sea floor. An anchoring assembly includes a pile secured to the sea floor. Further, a tensioning system operatively couples to the pile and to the main body at a location above the water and is configured to apply a downward force on the main body to resist ice loads imposed on the offshore drilling unit.

[0009] In another embodiment of the invention, a method of maintaining stability of an offshore drilling unit in potential ice conditions is provided. The method includes positioning the offshore drilling unit in a desired location in a body of water and filling ballast with water to at least partially submerge the unit until resting on a sea floor of the body of water. The method further includes securing an anchoring assembly having a pile to the sea floor. Imposing a downward force on a main body of the offshore drilling unit at a location above the water occurs utilizing a tensioning system operatively coupled to the main body and the pile.

[0010] In yet another embodiment of the invention, a method of extracting hydrocarbons from a well in ice prone waters is provided. The method includes positioning an offshore drilling unit having a platform in a desired location in a body of water and filling ballast with water to at least partially submerge the unit until resting on a sea floor. In addition, the method includes securing to the sea floor an anchoring assembly having a plurality of piles at least partially embedded in the sea floor. Imposing a downward force on the platform of the offshore drilling unit with a tension member of a tensioning system occurs using the tension member operatively coupled to the platform and the piles. The unit also enables extracting hydrocarbons from the well.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying figures by way of example and not by way of limitation, in which:

[0012] FIG. 1 is a schematic illustration of an offshore drilling unit having a tensioning system, where the offshore drilling unit is in an unanchored condition;

[0013] FIG. 2 is a schematic illustration of the offshore drilling unit in an anchored condition and resisting ice loads; and

[0014] FIG. 3 is a flow diagram illustrating a method of maintaining stability of an offshore drilling unit in potential ice conditions.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Reference will now be made in detail to embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not as a limitation of the invention. It will be apparent to those skilled in the art that various modifications and variation can be made without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the invention cover such modifications and variations that come within the scope of the appended claims and their equivalents.

[0016] FIG. 1 illustrates an offshore drilling unit 10. The offshore drilling unit 10 is a structure located or suitable for location over a relatively fixed position on a sea floor 12 of a body of water 14, which may be 20-25 meters deep. In one embodiment, the offshore drilling unit 10 is suitable for use in ice prone waters that often include bodies of ice. [0017] The offshore drilling unit 10 includes a main body 16 and is configured to be at least partially submerged in the body of water 14. As described below, the offshore drilling unit 10 may be a permanently installed offshore structure or a mobile offshore drilling unit. In one embodiment, the main body 16 is a platform suitable as a buoyant or floating type of platform submersible until supported on the sea floor 12 of the body of water 14. The platform of the offshore drilling unit 10 may be employed for various uses, including, but not limited to, offshore well drilling operations, hydrocarbon production, radar stations, heliports, and the like. Each of the aforementioned applications requires that the main body 16 of the offshore drilling unit 10 is relatively unaffected by tides, wave motion and by changes in the amount or distribution of the load. In ice prone waters, this is of particular concern due to potential contact with bodies of ice that may distribute a load on the offshore drilling unit 10.

[0018] An anchoring assembly 18 operatively couples to, and extends downwardly from, the main body 16 and secures the offshore drilling unit 10 to the sea floor 12. The offshore drilling unit 10 may include at least one, but typically a plurality of legs 22 extending downwardly from the main body 16. In some embodiments, the legs 22 connect the main body 16 with a base portion 24 of the offshore drilling unit 10. Prior to being ballasted, the offshore drilling unit 10 floats with the legs 22 and the base portion 24 submerged yet spaced from the sea floor 12 below.

[0019] FIG. 2 shows the offshore drilling unit 10 after filling ballasts, such as the legs 22 and/or the base portion 24, with water to lower the base portion 24 into resting contact with the sea bed 12 and maintain a fixed position of the offshore drilling unit 10. In addition to the base portion 24 thereby forming part of the anchoring assembly 18, numerous contemplated anchoring assemblies may be employed to interact with the sea floor 12 to facilitate anchoring of the offshore drilling unit 10. For some embodiments, the legs 22 may not couple to the base portion 24 but rather engage piled sockets pre-positioned on the sea floor 12.

[0020] The anchoring assembly 18 further includes a plurality of piles 20, with each of the plurality of piles 20 extending into the sea floor 12. The plurality of piles 20 are at least partially embedded within the sea floor 12 to fix the offshore structure's location. The plurality of piles 20 may be formed in any conventional manner, with an exemplary embodiment comprising suction piles. [0021] The offshore drilling unit 10 couples to the piles 20. For one embodiment, the anchoring assembly 18 includes the base portion 24 of the offshore drilling unit 10 in an operatively coupled relationship with the plurality of piles 20 that maintain the overall position of the offshore drilling unit 10. As noted above, the illustrated base portion 24 is configured to rest on the sea floor 12 to provide partial anchoring strength for the offshore drilling unit 10.

[0022] In contrast to a jack-up rig when raised having almost all weight above the water, the offshore drilling unit 10 when ballasted and resting on the sea floor may have more than 25 percent of its weight under the water and spaced (e.g., at least 10 meters) from the sea surface toward the sea floor 14. Such bottom weight tends to prevent tipping of the offshore drilling unit 10 as may occur with the jack-up rig. Sliding of the offshore drilling unit 10 along the sea floor 12 due to the ice loads may still present a problem.

[0023] In order to enhance the stability of the offshore drilling unit 10, and to balance a buoyancy force imposed on the main body 16, the anchoring assembly 18 also includes a tensioning system 26. As will be appreciated from the description below, the tensioning system 26 is employed to avoid the need for ballasting of the offshore drilling unit 10 with solid material denser than water. Avoidance of the need for solid ballasting advantageously allows quicker mobility of the offshore drilling unit 10 relative to gravity based structures with such solid ballast including sand, rocks or other expensive materials.

[0024] The tensioning system 26 includes a tension member 28 extending between, and operatively coupled to, the main body 16 above the water 14 and the piles 20. The tension member 28 is a line that may be formed of various suitable materials. The tensile strength of the material must be sufficient to sustain a tension force configured to impart a downward force on the main body 16. An exemplary embodiment of the tension member 28 comprises steel wire. Typically, multiple intertwined steel wires forming a cable are employed as the tension member 28. Steel is merely one example of a material that may be used as the tension member 28 and it is to be understood that numerous other materials may be suitable for operation of the tensioning system 26.

[0025] The tensioning system 26 also includes a tension member manipulation assembly

30. The tension member 28 is controlled by the tension member manipulation assembly 30. In one embodiment, the tension member manipulation assembly 30 includes a winch 30 used to adjust the tension of the tension member 28 during operation of the tensioning system 26. As shown, more than one tension member may be included in the tensioning system 26. For example, the tensioning system 26 may include a plurality of tension members and a plurality of tension member manipulation assemblies.

[0026] The tension member 28 operatively couples to the tension member manipulation assembly 30 proximate one end of the tension member 28 and operatively couples to at least one of the piles 20 proximate the other end of the tension member 28. Direct connection of the tension member 28 to the piles 20 without the piles 20 being otherwise secured to the offshore drilling unit 10 may facilitate relocation of the offshore drilling unit 10 since the tension member 28 may be released or severed from the offshore drilling unit 10 without requiring removal of the piles 20. After release of the tension member 28, the ballast may be refilled with air to float the offshore drilling unit 10 for movement if desired.

[0027] The precise attachment location of the tension member 28 may vary. In the illustrated embodiment, the tension member 28 extends through the base portion 24 of the anchoring assembly 18 and is fixed to the plurality of piles 20. In another embodiment, the tension member 28 is attached to the base portion 24. Alternatively, the tension member 28 is attached to a lower end of the plurality of legs 22.

[0028] In operation, irrespective of the precise attachment location of the tension member

28, the tensioning system 26 is configured to impose a downward force on the main body 16 of the offshore drilling unit 10 by manipulating the tension member 28 into tension. The desired downward force imposed on the main body 16 depends on the particular application, including factors such as weight and geometry of the main body 16, for example. The tension member manipulation assembly 30 provides relatively fast and efficient adjustment of the tension of the tension member 28 and therefore the downward force applied to the main body 16.

[0029] Application of the downward force with the tensioning system 26 avoids the need for solid ballasting, as discussed above. The downward force applied with the tensioning system 26 may provide an equivalent force as that of solid ballasting to resist ice loads, such as those imposed by one or more ice bodies 32. The tensioning system 26 does not require external materials, as is the case with solid ballasting, and can be implemented in conditions when ice is already present. Additionally, the tensioning system 26 may be employed with submersibles in shallow water utilizing water ballasting without relying on solid ballasting.

[0030] Referring to FIG. 3, a method 100 of maintaining stability of an offshore structure in potential ice conditions is also provided with reference to FIGS. 1 and 2. The offshore drilling unit 10 and, more particularly, the tensioning system 26, have been previously described and specific structural components need not be described in further detail. The method includes positioning the offshore structure in a desired location in a body of water and then submerging the structure by ballasting with water until resting on a sea floor, in an initial step 102. A piling step 104 includes securing an anchoring assembly to the sea floor of the body of water. During a loading step 106, downward force is imposed on a main body of the offshore structure with a tensioning system operatively coupled to the main body. Furthermore, the offshore drilling unit 10 is configured to extract hydrocarbons from a well once the offshore drilling unit 10 is properly positioned, secured to the sea floor and stabilized with the downward force provided by the tensioning system 26.

[0031] While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.