Patent Publication Number: US-8523491-B2

Title: Mobile, year-round arctic drilling system

Description:
REFERENCE TO PRIORITY APPLICATION 
     This application is the National Stage entry under 35 U.S.C. 371 of PCT/US2007/003903 that published as WO 2007/126477 and was filed on Feb. 13, 2007 and claims the benefit of U.S. Provisional Application 60/787,602, filed 30 Mar. 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art. 
     The present invention relates to a mobile, year-round arctic drilling system, also referred to herein by the acronym “MYADS.” It is a drilling system for drilling offshore wells and/or performing other offshore activities at multiple, successive locations in a “sub-Arctic” environment. The system combines the ability to move to different locations and the strength to resist ice loading when on location and when ice-covering is present in the sub-Arctic environment. 
     The “sub-arctic” offshore environment is characterized by yearly, seasonal incursions of ice. This environment is less severe than that of the “high” arctic environment that may have ice present year-round. However, even the sub-arctic environment presents problems for the use of standard offshore drilling systems. The standard offshore drilling systems are primarily designed to resist loading from waves, winds and currents, and, where necessary, earthquakes, but not from ice. In a sub-arctic environment, the overall or global loading due to ice impingement on an offshore drilling system could be an order of magnitude higher than that associated with wave, wind and current loading. Thus, the structure of a typical offshore drilling structure would not able to withstand the significantly higher forces in a sub-arctic environment. 
     Ice impingement can also create large pressure forces in small, local areas of any drilling equipment structure. For a typical offshore drilling system, these high local forces would damage unprotected frame brace elements since these elements are typical offshore structures designed solely to resist wind, waves and current. 
     The advantage of mobility is that it allows the drilling equipment to operate at widely different locations without the need to build a permanent structure to support the drilling equipment at each location. 
     Some current drilling structures have been designed for sub-arctic conditions. However, most of these structures are configured as permanent (non-mobile), production/drilling/quarters (PDQ) platforms. Various kinds of icecrush resistant drilling structures are also known. Brick-type systems, such as the Concrete Island Drilling System (CIDS) described in U.S. Pat. No. 4,011,826, are one type of an ice crush resistant structure. Another example is the structure disclosed in U.S. Pat. No. 5,292,207. Each of these systems is a large, permanent, walled structure configured to receive drilling rigs. 
     Other existing systems require some major structural components to be permanently on location (i.e., only the drilling facilities themselves are mobile). One example is the Deck Installation System for Offshore Structures disclosed in U.S. Pat. No. 6,374,764. Another example is the monopod jack-up configuration disclosed in U.S. Pat. No. 4,451,174. In these systems, a different sub-structure anchored to the seabed is required for each new drill location. 
     Another example of a monopod jack-up system is the offshore platform erection system and method of U.S. Pat. No. 4,648,751, which utilizes a single leg attached to a permanently installed substructure. The single-leg structure is jacked up by a retractable jacking system. Once at operating height, the deck is secured to the single leg, and the drilling derrick is moved into position to drill. The monopod jack-up is intended to drill exploration wells in an arctic environment. However, this configuration is only designed for exploration drilling with no provision for re-deployment over an active well site. Further, the single-column design may not be structurally sound for seismically-active locations. 
     Existing mobile drilling systems for non-arctic conditions, such as the conventional jack-up system, cannot operate in areas where the structure may come into contact with ice floes. There are two types of such conventional jack-ups: (1) those supported on open lattice structural legs and (2) those supported on closed cylindrical legs. Neither of these existing designs is capable of resisting local and global loading due to sub-arctic ice. 
     The open-lattice leg design is not suitable to resist the local ice forces as individual members of the lattice structure would be bent or crushed by the local ice forces. The closed-cylindrical leg design improves on this drawback. However, current designs are not suitable to resist the high local ice loads as the legs are primarily designed to resist much smaller wave loading. Some current closed-cylindrical leg designs have moments of inertia as low as 1.1 meters to the fourth power (m 4 ). 
     Neither of the above designs is capable of resisting the global ice loads typical of sub-arctic regions. These global ice loads can easily be an order of magnitude higher than the wave and wind loads to which conventional jack-ups are designed to resist. 
     Accordingly, a need exists to configure a structure that can support offshore drilling operations while able to withstand both global and local ice loading that will occur during the yearly, seasonal incursions of ice. In addition, the structure should have the capability to relocate to a new drilling site during the relatively ice-free time of the year, and return, if necessary. Preferably, the relocation time may be relatively short and require no significant offshore logistics support (i.e., nothing more than a few towing vessels). 
     Other related material may be found in at least U.S. Pat. No. 4,249,619; U.S. Pat. No. 5,228,806; U.S. Pat. No. 5,288,174; and U.S. Pat. No. 5,290,128. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the present invention, a mobile drilling system is provided. The mobile drilling system comprises a hull; at least two legs adapted to be lowered through the hull to contact a seabed and elevate the hull out of the water; at least one foundation associated with at least one of the at least two legs; and a drilling rig located on the hull. Each of the at least two legs has a closed structure comprising an outer plate and an inner plate forming an annulus, wherein a bonding agent is disposed in the annulus. 
     According to other aspects of the invention, each leg may be of cylindrical shape with an outer plate diameter of about 10 meters of greater, or about 15 meters or greater or about 20 meters or greater. The thickness of the outer plate may be about 25 millimeters (mm) to about 50 mm. Further, the leg may be of cylindrical shape with an inner plate diameter of about 14 meters. The thickness of the inner plate may be about 25 mm to about 50 mm, but preferably less than the outer plate thickness. The bonding agent may comprise at least one of grout or elastomeric agent. The foundation may have a diameter of about 25 meters to about 35 meters. One or more of the foundation structures may be capable of securing wellheads when the system is removed from its location. Additionally, the moment of inertia of the mobile drilling system may be between about 100 m 4  and about 130 m 4 . Further, the mobile drilling system may be utilized in a sub-arctic environment. 
     According to another embodiment of the present invention, a method of offshore drilling is provided. The method of offshore drilling comprising providing a mobile drilling system, wherein the mobile drilling system comprises a hull; at least two legs adapted to be lowered through the hull to contact a seabed and elevate the hull out of a body of water; at least one foundation associated with at least one of the at least two legs; and a drilling rig located on the hull, wherein each of the at least two legs having a closed structure comprising an outer plate and an inner plate forming an annulus, wherein a bonding agent is disposed in the annulus. The method further comprises drilling through at least one of the at least two legs. 
     According to yet another embodiment of the present invention, a method of producing hydrocarbons is provided. The method of producing hydrocarbons comprising providing a mobile drilling system comprising a hull; at least two legs adapted to be lowered through the hull to contact a seabed and elevate the hull out of a body of water; at least one foundation associated with at least one of the at least two legs; and a drilling rig located on the hull, wherein each of the at least two legs having a closed structure comprising an outer plate and an inner plate forming an annulus, wherein a bonding agent is disposed in the annulus. The method further includes drilling through a leg of the drilling system. The drilling may include drilling through an ice-resistant caisson. 
     According to still another embodiment of the present invention, a method of installing an offshore drilling system is provided. The method of installing an offshore drilling system comprising transporting a mobile drilling system to a location in a body of water. The mobile drilling system comprises a hull; at least two legs; at least one foundation associated with at least one of the at least two legs; and a drilling rig located on the hull, wherein each of the at least two legs having a closed structure comprising an outer plate and an inner plate forming an annulus, wherein a bonding agent is disposed in the annulus. The method further includes lowering the at least two legs to a seabed; elevating the hull above a surface of the body of water; penetrating the at least one foundation into the seabed; and positioning the drilling rig over a drilling location. 
     According to a fifth embodiment of the present invention, a method of removing an offshore drilling system is provided. The method of removal comprising providing a mobile drilling system in a first location in a body of water, wherein the mobile drilling system is installed at the first location. The mobile drilling system comprises a hull; at least two legs; at least one foundation associated with at least one of the at least two legs; and a drilling rig located on the hull, wherein each of the at least two legs having a closed structure comprising an outer plate and an inner plate forming an annulus, wherein a bonding agent is disposed in the annulus. The method further includes securing at least one of the at least one foundation to protect a wellhead located in the at least one of the at least one foundation; lowering the hull into the body of water; raising the at least two legs; and transporting the mobile drilling system to a second location. 
     According to a sixth embodiment of the present invention a method of re-installing an offshore drilling system is provided. The method of re-installing an offshore drilling system comprising providing a mobile drilling system on a body of water. The mobile drilling system comprises a hull; at least two legs; at least one foundation associated with at least one of the at least two legs; and a drilling rig located on the hull, wherein each of the at least two legs having a closed structure comprising an outer plate and an inner plate forming an annulus, wherein a bonding agent is disposed in the annulus. The method further includes transporting the mobile drilling system to a drilling location, wherein the drilling location includes a first foundation; lowering the at least two legs to a seabed, wherein one of the at least two legs is lowered into the first foundation; elevating the hull above a surface of the body of water; penetrating the foundation of the remaining legs of the at least two legs into a seabed; and positioning the drilling rig over a drilling location. Additionally, the foundation may provide well protection to subsea wellheads and one of the legs may be lowered into the first foundation utilizing a guide system. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The foregoing and other advantages of the present invention may become apparent upon reviewing the following detailed description and drawings of non-limiting examples of embodiments in which: 
         FIG. 1  is an exemplary illustration of a side view of a MYADS in accordance with the present invention; 
         FIG. 2  is an exemplary illustration of an isometric view of an installed MYADS in accordance with the present invention; 
         FIGS. 3A-3D  are exemplary illustrations of a sequence of an initial installation process of the MYADS of  FIGS. 1 and 2  in accordance with the present invention; 
         FIGS. 4A-4D  are exemplary illustrations of a sequence of a removal process of the MYADS of  FIGS. 1 and 2  in accordance with the present invention; 
         FIGS. 5A-5D  are exemplary illustrations of a sequence of a re-installation process of the MYADS of  FIGS. 1 and 2  in accordance with the present invention; 
         FIG. 6A  is an exemplary illustration of drilling with a foundation well protection structure utilizing the MYADS of  FIGS. 1 and 2 ; 
         FIG. 6B  is an exemplary illustration of drilling over a wellhead structure utilizing the MYADS of  FIGS. 1 and 2 ; 
         FIGS. 7A-7B  are exemplary illustrations of a cross-section of a leg of the MYADS of  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description section, the specific embodiments of the present invention are described in connection with preferred embodiments. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present invention, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the invention is not limited to the specific embodiments described below, but rather, it includes all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims. 
     It may be economically advantageous to develop offshore oil and gas reservoirs by locating well centers at de-centralized locations. Having several drilling centers may allow better reservoir recovery, for example. Also, if one section of a reservoir is discovered to have lower than expected recovery, a smaller, de-centralized well center can be decommissioned more easily. A decentralized well center may be particularly advantageous in sub-arctic regions where it may be desirable to move equipment due to ice impingement or other environmental conditions. 
     The primary disadvantage to prior designs of drilling systems is the increased cost associated with building a permanent drilling structure at every location identified for drilling. 
     Rather than construct several permanent structures for each drilling location, a single mobile drilling structure can drill all locations using the same structure at a significantly reduced manufacturing cost. Therefore, the present invention addresses the problem of configuring a mobile structure that can support facilities for drilling offshore wells and/or performing other offshore activities at multiple, successive locations in a sub-arctic environment. 
     The present structure, referred to as the “mobile, year-round artic drilling system” (MYADS), combines the mobility to move to different drilling locations and the strength to resist ice loading when on location. Some embodiments of the MYADS may comprise a floating hull having supporting legs which are lowered through the hull to touch down on the seabed and may elevate the hull out of the water for performing offshore activities. 
     Turning now to the drawings,  FIG. 1  is an exemplary illustration of a side view of a MYADS in accordance with the present invention. The MYADS  1 , having a hull  10 , at least two legs  11  adapted to be lowered through the hull  10  to contact a seabed  100  and elevate the hull out of the water  110 , a foundation system  12 , which may be a suction caisson foundation, and a drilling rig  13  supported on skid beams  14  for positioning the drilling rig  13  over at least one subsea wellhead silo system  15 . In some embodiments of the present invention, the MYADS may have three legs or four legs, or five legs, or more, the legs  11  being adapted to be lowered through the hull  10  to contact a seabed  100  and elevate the hull  10  out of the water  110 . The hull  10  provides buoyancy to the structure when the legs  11  are elevated. Short distances may be traveled by towing the hull  10 , while long distances may be traveled on a transport vessel (not shown). As also shown in  FIG. 1 , in some embodiments of the present invention the MYADS  1  may comprise an ice-protective cone  5  and scour skirt  16  on each of the legs  11 , as well as protective jackhouse  17  for supporting the elevating and clamping systems. The MYADS  1  may also comprise living quarters, a helideck  18 , and any other facilities know to those of skill in the art that may be found on an offshore drilling platform. 
     Referring now to the legs  11  of the MYADS  1 , a person of skill in the art understands that the shape of the legs may be significant, but that numerous cross-sectional shapes are applicable to the present invention. Preferably, the legs  11  are cylindrically shaped, in which cases the legs  11  have a circular cross-sectional shape. The legs  11  may have any cross-sectional shape, provided such cross-sectional shape permits the legs  11  to withstand the anticipated ice loads. For example, in alternative embodiments, the legs  11  may be of oval, elliptical, hexagonal, pentagonal, square, triangular cross-sectional shape, or a combination of shapes. In each case, the MYADS&#39; legs  11  will be of the closed type (as opposed to the lattice type). In some embodiments, the closed legs  11  have a moment of inertia of about 20 m 4  or greater, or about 50 m 4  or greater, or about 100 m 4  or greater, or about 110 m 4  or greater, or about 120 m 4  or greater, or about 130 m 4  or greater. As used herein, “moment of inertia” is the moment of inertia also known as “second moment of area,” or “area moment of inertia” and is known to those skilled in the art. Generally, it is a measure of a shape&#39;s resistance to bending and deflection and is dependant on the shape of the member being measured. 
     Some embodiments provide a mobile drilling system comprising: a hull  10 ; at least two legs  11  adapted to be lowered through the hull  10  to contact a seabed  100  and elevate the hull  10  out of the water  110 ; a foundation  5  associated with each leg  11 ; and a drilling rig  13  supported on a skid beam  14 , wherein each leg  11  is a closed cylindrical or closed non-cylindrical type having a moment of inertia of about 20 m 4  or greater. In some embodiments, each leg  11  is a closed cylindrical type. In some embodiments of the present invention, each leg  11  has a moment of inertia of about 100 m 4  or greater. 
     In yet some other embodiments, a method of producing hydrocarbons comprising: drilling a well in a hydrocarbon reservoir using an embodiment of the MYADS of the present invention and recovering the hydrocarbons from the well is described. 
       FIG. 2  is an exemplary illustration of an isometric view of an installed MYADS in accordance with the present invention. In one or more embodiments, to resist ice forces, the legs  11  of the MYADS are configured as large diameter cylinders. The cylindrical shape minimizes ice loading forces from any particular direction. The large diameter of the legs  11  provides the strength and stiffness required to resist global ice forces. Global ice forces are forces that may cause a structure to fall over or collapse. The legs  11  may be built entirely of steel. To accommodate design requirements for resisting local ice loading or ice forces, in one or more embodiments a composite (“sandwich”) construction may be used. Local ice forces are forces that may puncture or damage a structure at a particular location. The composite construction preferably comprises two steel layers separated with a filler material such as a bonding agent. The bonding agent is preferably grout, but other known materials, such as elastomeric agents may be used.  FIG. 2  illustrates an embodiment of the invention in which drilling rig  13 A is positioned over leg  11 D such that the MYADS  1  drilling can be carried out by drilling through leg  11 D (also referred to herein as “drilling through a leg.”) 
     A jack-up structure, like the MYADS, resists sub-arctic ice forces using “portalling action,” in which the primary resistance to ice loading is mobilized through bending of the legs. Portalling action is the reaction of a portal frame to a load or force and is particularly relevant to the resistance of a bending force. A portal frame is a structure having multiple columns and at least one rafter or equivalent structural member. In the present invention, the portal frame includes the legs of the MYADS and the lintel or platform connected to the legs. A higher moment of inertia is beneficial in resisting ice forces and an increased leg  11  diameter yields a larger moment of inertia. Thus, an increased diameter is preferable to increase the bending load resistance, which resists the ice forces. 
     To further enhance the portalling action, each leg  11  is preferably supported on a foundation system having a foundation member  12  and skirt member  16 , collectively, a foundation system  12 ,  16 . The foundation system  12 ,  16  provides strength and stiffness to allow the MYADS  1  to resist the loads associated with sub-arctic ice. 
     To resist local ice forces, the legs  11  of the MYADS  1  are configured as strengthened plates. The strengthening is preferably achieved by combining the outer plate with an inner plate separated with an internal bonding agent. The bonding agent may include an elastomer and the preferred bonding agent is grout. This “sandwich” configuration provides resistance to local ice forces. Alternative strengthening is possible. One such approach may be to apply stiffening members to the inner walls of the legs  11 . Some “alternative strengthening” may actually be used concurrently with the strengthening techniques described herein. 
     In some other embodiments, the MYADS  1  is configured such that drilling is performed through one of the legs of the structure (see  FIG. 2 ). In some embodiments, the MYADS may be configured to drill through an ice-resistant caisson either through a moonpool arrangement or in a cantilever arrangement more typical of conventional jack-ups. The moonpool arrangement locates the drilling rig over an opening in the hull. This arrangement only allows the jack-up to drill over a subsea wellhead system. In the cantilever arrangement, the drill rig is located on a cantilever beam structural system that locates the drill rig outboard of the stern of the jack-up structure. This arrangement allows the jack-up to drill over an existing surface-piercing structure that supports well heads above the surface of the water (e.g. a “dry tree”). 
     Some methods of operation of the present invention include: initial installation, removal of the installation, and re-installation, some exemplary illustrations of which may be seen in  FIGS. 3A-D ,  4 A-D, and  5 A-D, respectively. For purposes of illustration, simplified views of the MYADS  1  are shown. It will be understood, however, that, where not explicitly shown, the remainder of the MYADS structure is implicitly present. 
       FIGS. 3A-3D  are exemplary illustrations of a sequence of an initial installation process of the MYADS of  FIGS. 1 and 2  in accordance with the present invention. Accordingly,  FIGS. 3A-3D  may be best understood by concurrently viewing  FIGS. 1 and 2 . In  FIG. 3A , the MYADS  1  is towed to the location with foundations (not shown) attached to the legs  11  and the drilling structure  13  is in the “transport” position. The ice-protective cone  5  and scour skirts  16  may be located within the hull  10  during transport and are thus not shown. Once on site at the location, the MYADS  1  is moored to stay on location. As shown in  FIG. 3B , the MYADS legs  11  are then lowered to the seafloor. The marine motions of the MYADS  1  are reduced due to the extension of the legs  11  below the hull  10 , as is well known to those of skill in the art. As shown in  FIG. 3C , the foundations  12  are penetrated into the seafloor  100 . This penetration is accomplished by applying the weight of the MYADS  1  as the hull  10  is lifted or elevated out of the water  110 , as shown in  FIG. 3D , by application of additional weight by adding water to “pre-load” tanks in the hull, and/or by applying suction underneath the foundations  12  and/or by using a jetting system that disturbs the soil sufficiently to ease penetration or other method and apparatus for applying additional weight to the structure to force the foundations  12  to penetrate the sea floor  100 . Once on location, the MYADS  1  drilling structure  13  is skidded over the drilling leg  11 D, and the well or wells may be drilled. 
       FIGS. 4A-4D  are exemplary illustrations of a sequence of a removal process of the MYADS of  FIGS. 1 and 2  in accordance with the present invention, which may be accomplished after the initial installation process of  FIGS. 3A-3D . Accordingly,  FIGS. 4A-4D  may be best understood by concurrently viewing  FIGS. 1 ,  2 , and  3 A- 3 D. In  FIG. 4A , a foundation  12  is first removed from the seafloor  100 . This removal is accomplished by applying the upward, buoyant forces as the hull  10  is lowered into the water  110 , by applying pressure underneath the foundations  12  and/or by using a jetting system that disturbs the soil sufficiently to ease removal. Referring to  FIG. 4B , the foundation system  12 A that contains one or more wells may be left in place as protection for the wellheads in the sub-arctic environment. As shown in  FIGS. 4C-4D , the MYADS legs  11 ,  11 D are then raised from the seafloor  100 , leaving one or more portions  12 A of the foundation system  12 ,  16  to protect one or more wells contained therein. The MYADS  1  is then towed to another drilling location if all foundations  12  remain attached. If a foundation  12 A remains on location to protect wellheads, then the MYADS  1  may be towed to a location for installation of a replacement foundation  12 A or to a location at which a foundation  12 A is already in place. 
       FIGS. 5A-5D  are exemplary illustrations of a sequence of a re-installation process of the MYADS of  FIGS. 1 and 2  in accordance with the present invention, which may be accomplished after the removal process of  FIGS. 4A-4D . Accordingly,  FIGS. 5A-5D  may be best understood by concurrently viewing  FIGS. 1 ,  2 , and  4 A- 4 D. The re-installation operation may be utilized to locate the MYADS on a site where the MYADS has already drilled. Referring to  FIG. 5A , the MYADS is towed to a location with one foundation not attached. A guide system  50  locates the drilling leg  11 D over the in-place foundation. Once in place, the MYADS legs  11  are lowered to the seafloor  100  and the foundations  12 B that have not penetrated the seafloor  100  are then penetrated into the seafloor  100  using one or more of the techniques described above, which is shown in  FIGS. 5A-5D . Again, the marine motions of the MYADS  1  are reduced due to the extension of the legs  11  below the hull  10 . The remaining foundations  12  are penetrated into the seafloor  100  as described above. 
     Thus, in one or more embodiments, the MYADS  1  provides a foundation system that: (1) provides access to drilling wells, (2) provides protection to the wells after the MYADS  1  structure leaves, and (3) allows the MYADS  1  to reconnect for future operations at a given site. 
     The foundation system of the MYADS is enhanced over designs for conventional jack-ups. The foundation system may be structurally enhanced with a variety of structural members, such as central caissons and perimeter skirts. In some preferred embodiments, the foundation diameter is between about 25 meters to about 35 m. In one or more embodiments the central caisson is the same diameter as the legs, which may be from about 10 meters to about 20 meters. One preferred embodiment comprises legs having a diameter of about 15 meters. 
     In sub-arctic conditions, it is preferable that production wells have either: (1) a subsea protection structure in the case of subsea wellheads or (2) a surface-piercing structure in the case of dry trees. A “dry tree” is a wellhead that is not located under water. In this case, all of the control valves and manifolds of the surface-piercing structure are preferably located above the water  110  to provide easy access. A subsea wellhead system may be deployed on the seafloor, generally within a protective structure, such as the provided foundation  12  of the present invention. In one preferred embodiment of the present invention, the valves and manifold controls are handled remotely. The MYADS  1  of the present invention may be adapted for use with either of these two methods.  FIG. 6A  illustrates an alternative embodiment of a MYADS  1  used in connection with a subsea wellhead  60  enclosed in a subsea silo  61  formed by the MYADS foundation system  12 ,  16 , i.e., part of the foundation for the drilling leg  11 B. In this embodiment drilling is performed through the leg  11 B.  FIG. 6B  shows another alternative embodiment in which MYADS  1  is used in connection with dry wellheads  60  and a surface-piercing structure  62  to protect the dry wellheads  60 . Drilling rig  13  is positioned over the structure  62  on a cantilever beam or similar member and drilling is performed through the surface-piercing structure  62 . 
     In some embodiments of the present invention, the foundation  12  system may incorporate at least one subsea wellhead silo system as illustrated in  FIGS. 1 and 6A . As described above in connection with  FIG. 1 , this structural system can be a suction caisson, potentially augmented with an ice-protective cone  5  and scour-protecting skirt  16 . Subsea wellheads are located inside the silo and above ground level. Referring to  FIG. 1 , the drilling leg of the MYADS may connect mechanically to the subsea silo by preferably a clamping system  6  or other system known to those skilled in the art. 
     In the MYADS  1 , the legs are preferably about 15 meters in diameter, but in any of the embodiments disclosed herein the legs may have a diameter of about 10 meters or greater, or about 15 meters or greater, or about 20 meters or greater. The length of the legs  11  is determined by the requirements of the water depth and “air gap” (clearance between the water surface and the bottom of the hull in the elevated condition). The thicknesses of the outer and inner plates preferably range from about 25 millimeters (mm) to about 50 mm or higher. (The maximum thickness is generally limited by the availability of steel). Preferably, the diameter of the legs  11 , the thickness of the inner and outer plates and other structural considerations should be chosen with the overall moment of inertia in mind. As previously stated, the moment of inertia is preferably higher than that of conventional systems and preferably in the range of about 50 meters to the fourth (m 4 ) to about 130 m 4 . 
     The large diameter of the legs provides the MYADS lateral stiffness and strength to resist global ice loads, can be a detriment to the local strength of the leg. Locally, high ice loads can occur as ice impinges on the leg. As the diameter of the leg is increased, the ability to resist these local ice loads is also diminished because the local profile of the leg becomes more “flat” and less “rounded” as the leg diameter increases. Thus, depending on the leg size or diameter and the expected local ice loads, it may be desirable to strengthen the leg walls. 
     Leg wall strengthening in the MYADS may be accomplished by stiffening the leg wall such as is done, for example, in ship construction, and, with some modification, for hull strengthening on ice-breaking ships. In some embodiments of the present invention, leg stiffening is accomplished by adding a second wall with an intermediate material between the first wall and the second wall (i.e., a “sandwich” design). This embodiment provides localized strength by increasing the local stiffness of the wall at all locations on the leg; this option may also minimize construction costs in many cases, although that potential is site-dependent. 
       FIGS. 7A-7B  show an exemplary cross-section of the legs  11  of the MYADS  1  of  FIGS. 1 and 2 . Accordingly,  FIGS. 7A-7B  may be best understood by concurrently viewing  FIGS. 1 and 2 . Referring to  FIGS. 7A and 7B , a cross-section of a “sandwich” leg wall design is shown wherein a MYADS leg is made of an outer plate and an inner plate with bonding agent filled between the outer plate and the inner plate.  FIG. 7B  shows an enlarged view of one embodiment of the sandwich leg wall design that may be used in any of the embodiments of the present invention. In the embodiment shown in  FIGS. 7A and 7B , the outer plate  80  has a thickness  83  of about 50 mm, the inner plate  81  has a thickness  84  of about 35 mm, and the bonding agent  82  has a thickness  85  of about 195 mm. The bonding agent  82  may be Class  300  concrete, and inner wall  81  and outer wall  80  may be made from extra high strength steel having a yield strength of about 690 megapascals (Mpa). As mentioned above, low cost concrete, grout or elastomer material may be used as the bonding agent between the walls of the sandwich design. Calculations have shown that a leg based on the exemplary structure shown in  FIGS. 7A and 7B  have a moment of inertia of about 113 m 4 . As is known in the art, moment of inertia is a measure of bending stiffness. 
     It should be noted that although the MYADS system is disclosed with reference to a sub-arctic environment. However, the present invention may also be applied to an arctic environment or other environment having seismic activity and or floating ice or other debris that may impinge on the legs of a drilling structure. Other elements such as the shape of the legs, type of drilling operation, size of the legs, type of equipment on the platform, etc. may also be varied significantly and still be taught by the present disclosure. 
     While the present invention may be susceptible to various modifications and alternative forms, the exemplary embodiments discussed above have been shown only by way of example. However, it should again be understood that the invention is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present invention includes all alternatives, modifications, and equivalents falling within the true spirit and scope of the invention as defined by the following appended claims.