Abstract:
In a system for mooring offshore drilling units, a first mooring assembles installed at a first drilling venue, after which the mobile offshore drilling unit is moored by connection to the mooring lines. A second mooring assembly is installed at a second drilling venue while drilling operations are carried out at the first drilling venue. In this manner the mobile offshore drilling unit can be relocated between successive drilling venues with minimum down time. Less than complete mooring assemblies can be used to temporarily secure the mobile offshore drilling unit.

Description:
This application is a division of pending application No. 08/948,227 filed Oct. 9, 1997, U.S. Pat. No. 6,009,825. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to mooring systems, and more particularly to recoverable systems for mooring mobile offshore drilling units in deep water. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     As is well known, exploration for and recovery of oil and gas has long since extended into offshore venues. Early offshore drilling operations were concentrated in relatively shallow waters. However, the number of shallow water drilling sites is finite, while the world&#39;s appetite for oil and gas is seemingly unlimited. It has therefore become necessary to conduct offshore drilling operations in waters as deep as 10,000 feet or more. 
     Offshore drilling operations are frequently conducted from floating platforms known as mobile offshore drilling units (MODUs). While the mooring of offshore drilling units in shallow water is relatively straightforward, the successful mooring of MODUs in deeper water can be problematic. 
     The traditional method of mooring MODUs in deeper water involves the use of drag embedment anchors and mooring lines which are stored on the MODU, and which are deployed from the MODU using anchor handling vessels. Some of the latest generation MODUs can carry adequate lengths of wire and chain on board, and are equipped with combination wire/chain mooring winches to moor at maximum depths of 5,000 feet of water. Large anchor handling vessels are capable of deploying and recovering such mooring legs and anchors. In even deeper water, however, the amount of wire and chain that would have to be carried on the MODU becomes too large, and even large anchor handling vessels would have difficulty deploying and recovering such mooring systems in the traditional manner. 
     Older generation MODUs typically cannot carry enough mooring line to moor in water deeper than about 2,000 to 3,000 feet. This water depth limit can be extended by inserting sections of wire in each mooring leg, or by pre-installing mooring legs prior to arrival of the MODU at location. Both types of extended water depth mooring legs (insert or preset) typically use modern high holding power drag embedment anchors. Large anchor handling vessels are used to install the wire inserts during mooring leg deployment or to pre-install the preset mooring legs. 
     One drawback to deep water MODU moorings using drag embedment anchors is that such anchors typically cannot handle uplift (vertical load), which requires both that the mooring leg is very long, and that the anchor is set very far from the MODU. In water depths over 6,000 feet the horizontal distance to the anchors can become a problem, since it could be as large as 12,000 feet or 2 nautical miles, and each mooring leg could be as long as 15,000 feet or 2.5 nautical miles. This requires an anchor spread diameter of about 4 nautical miles. 
     If an anchor system can be used which can handle substantial uplift or vertical load, the anchor radius and mooring line length can be reduced significantly. Driven anchor piles are capable of handling uplift, but cannot be installed in deep water nor are they recoverable. For this reason, driven anchor piles have never been used for MODU moorings. 
     Mooring systems employing anchors other than drag embedment anchors and driven piles have been proposed heretofore. For example, U.S. Pat. No. 4,318,641, granted to Hogervorst on Mar. 8, 1982, discloses mooring systems employing suction embedment anchors, which are capable of taking significant uplift or vertical load. However, prior to the present invention, there has not been provided a successful system for installing and recovering suction anchors in very deep water thereby facilitating ultra deep water drilling operations. 
     The present invention comprises a system for mooring mobile offshore drilling units which overcomes the foregoing and other difficulties long since associated with the prior art. In accordance with the broader aspects of the invention, mobile offshore drilling units are moored using anchors which are recoverable and reusable upon the completion of drilling operations. In accordance with a preferred embodiment of the invention, recoverable suction anchors are employed, and in accordance with another embodiment of the invention vertically loaded anchors are employed. Recoverable drag embedment anchors can also be used in the practice of the invention, if desired. 
     Regardless of the anchor type employed, the present invention comprises a method of mooring MODUs wherein a first set of recoverable preset mooring legs with suitable anchors are pre-installed at spaced apart locations surrounding an offshore drilling venue. After the recoverable preset mooring legs are installed, a MODU is positioned at the drilling venue, and connected to the preset mooring legs by short lengths of the mooring chain or-wire extending from mooring winches on the MODU. 
     While drilling operations are being conducted from the MODU, a second set of mooring legs is preset at spaced apart locations around a second drilling venue. When drilling operations have been completed at the first drilling venue, the mobile offshore drilling unit is moved from the first drilling venue to the second drilling venue, and is secured in place at the second drilling venue by connection to the second set of preset mooring legs surrounding the second drilling venue. The mobile offshore drilling unit is then used to conduct drilling operations at the second drilling venue in the usual manner. 
     While drilling operations are being conducted at the second drilling venue, the first set of mooring legs is recovered from the first drilling venue and is moved to a third drilling venue. The mooring legs from the first drilling venue are then preset at spaced apart locations surrounding the third drilling venue with the installation thereof being completed prior to the movement of the mobile offshore drilling unit from the second drilling venue to the third drilling venue. This process continues until drilling operations have been completed at all of the drilling venues within a particular area, whereupon all of the mooring legs and the MODU are removed to a different area. 
     The present invention may also be practiced using less than two complete sets of mooring legs. In such instances, a reduced number of mooring legs, for example, one half of a complete set, is installed at a second drilling venue. The MODU is then moored to the second drilling venue and secured in place using the preset mooring legs then in place. Next, the remainder of the mooring legs comprising a complete set are installed, whereby the MODU is fully secured. This procedure is repeated until drilling operations are completed at a particular location. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention may be had by reference to the following Detailed Description when taken in connection with the accompanying Drawings wherein: 
     FIG. 1 is a diagrammatic illustration of the method of mooring mobile offshore drilling units comprising the present invention; 
     FIG. 2 is an illustration of a preset taut mooring leg with a recoverable anchor in its pre-installed configuration useful in the practice of the invention. 
     FIG. 3 is a view similar to FIG. 2 showing the mooring leg attached to a mobile offshore drilling platform; 
     FIG. 4 is a side view of a suction anchor useful in the practice of the invention; 
     FIG. 5 is an illustration similar to FIG. 4 wherein the suction anchor is rotated 90 degrees; 
     FIG. 6 is a top view of the suction anchor of FIG. 4; 
     FIG. 7 is an enlargement of the pad eye of the suction anchor of FIG. 4; 
     FIG. 8 is a sectional view of the pad eye of FIG.  7 : 
     FIG. 9 is a top view of a submerged buoy useful in the practice of the invention; 
     FIG. 10 is a longitudinal sectional view of the buoy of FIG. 9; 
     FIG. 11 is a bottom view of the buoy of FIG. 9; 
     FIG. 12 is a top view of an installation vessel useful in the practice of the invention; 
     FIG. 13 is a side view of the vessel of FIG. 12; 
     FIG. 14 is an illustration of an early stage in the installation of a suction anchor in accordance with the present invention; 
     FIG. 15 is an illustration of a somewhat later stage in the installation of the suction anchor; 
     FIG. 16 is an illustration of a still later stage in the installation of the suction anchor; 
     FIG. 17 is an illustration of a still later stage in is the installation of the suction anchor; 
     FIG. 18 is an illustration of a still later stage in the installation of the suction anchor; 
     FIG. 19 is an illustration of a still later stage in the installation of the suction anchor; 
     FIG. 20 is an illustration of a still later stage in the installation of the suction anchor; 
     FIG. 21 is an illustration of a still later stage in the installation of the suction anchor; 
     FIG. 22 is a top view of a pumpskid useful in the practice of the invention; 
     FIG. 23 is a side view of the pumpskid of FIG. 22; 
     FIG. 24 is an end view of the pumpskid of FIG. 22; 
     FIG. 25 is an illustration of the final stages in the installation of the suction anchor; 
     FIG. 26 is an illustration of an early stage in the removal of the suction anchor in accordance with the present invention; 
     FIG. 27 is an illustration of a later stage in the removal of the suction anchor; 
     FIG. 28 is an illustration of a still later stage in the removal of the suction anchor; 
     FIG. 29 is an illustration of a recoverable system for mooring offshore drilling units comprising a second embodiment of the invention; 
     FIG. 30 is an illustration of a recoverable system for mooring mobile offshore drilling units comprising a third embodiment of the invention; 
     FIG. 31 is an illustration of a recoverable system for mooring offshore drilling units comprising a fourth embodiment of the invention; 
     FIG. 32 is an illustration of a recoverable system for mooring mobile offshore drilling units comprising a fifth embodiment of the invention; 
     FIG. 33 is an illustration of a recoverable system for mooring mobile offshore drilling units comprising a sixth embodiment of the invention; 
     FIG. 34 is an illustration of a first type of vertically loaded anchor useful in the practice of the invention; 
     FIG. 35 is an illustration of a second type of vertically loaded anchor useful in the practice of the invention; and 
     FIG. 36 is an illustration of the vertically loaded anchor of FIG. 35 showing the anchor in the installed configuration. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the Drawings, and particularly to FIG. 1 thereof, there is shown an area  30  comprising a portion of the ocean or other water body suitable for underwater drilling operations. Area  30  includes at least three drilling venues,  32 ,  34 , and  36 . Venue  32  comprises a previously drilled location at which drilling operations have been completed. Venue  34  comprises a current drilling location wherein drilling operations are currently underway. Venue  36  comprises a yet-to-be drilled location at which drilling operations have not yet begun. 
     In accordance with the present invention, a mobile offshore drilling unit  38  (MODU) is employed to conduct drilling operations at venues  32 ,  34 , and  36 . During the time that drilling operations were in progress at venue  32 , the mobile offshore drilling platform  38  was secured in place at venue  32  by a first mooring assembly  40  comprising a plurality of mooring legs  42  each including a mooring line  44  connected to the mobile offshore drilling unit  38 . Although the mooring assembly  40  is illustrated as comprising eight mooring legs  42 , it will be understood that the invention is not limited to any particular number of mooring legs, with the actual number employed being dependent upon the requirements of particular applications of the invention, and that in some cases  9 ,  10 , or more mooring legs will be required in order to properly moor a particular MODU. 
     After drilling operations are completed at drilling venue  32 , the mobile offshore drilling unit  38  is disengaged from the first mooring assembly  40  and is towed or otherwise repositioned at drilling venue  34 . Prior to the repositioning of the mobile offshore drilling unit  38  from venue  32  to venue  34 , a second mooring assembly  46  also comprising mooring legs  42  including mooring lines  44  extending therefrom is preset at venue  34 . The use of multiple mooring assemblies and the installation thereof at drilling venues prior to the positioning of the mobile offshore drilling unit at the drilling venue comprises an important feature of the invention in that it allows the mobile offshore drilling unit to be secured in place and ready for operation very rapidly. 
     After the mobile offshore drilling unit  38  has been disengaged therefrom, the first mooring assembly  40  is recovered from drilling venue  32  and is transported to drilling venue  36 . The mooring legs  42  and the mooring lines  44  associated therewith comprising the first taut-leg mooring assembly  40  are preset at drilling venue  36  while drilling operations are progressing at drilling venue  34 . Later, after drilling operations are completed at drilling venue  34 , the mobile offshore drilling unit  38  is disconnected from the mooring assembly  46  and is towed or otherwise repositioned at drilling venue  36 . The mobile offshore drilling unit  38  is then secured in place at drilling venue  36  utilizing the first mooring assembly  40 . The foregoing steps are repeated until drilling at all of the venues comprising a particular one has been completed, whereupon the mooring assemblies and the MODU are moved to a different area. 
     FIG. 1 further illustrates two alternative procedures for mooring MODUs in accordance with the present invention. The first alternative procedure involves the use of a single mooring assembly, for example, the mooring assembly  40 . The mobile offshore drilling unit  38  is initially secured at the first drilling venue  32  utilizing the mooring assembly  40 , including all of the mooring legs  42  thereof. 
     After drilling operations are completed at the first drilling venue  32 , half of the mooring legs  42  comprising the mooring assembly  40  are disengaged and are repositioned at the second drilling venue  34 . The mobile offshore drilling unit  38  is then repositioned to the second drilling venue  34  and is secured in place utilizing the drilling legs  42  comprising part of the mooring assembly  40  which have been preset at the second drilling venue  34 . Thereafter, the remainder of the drilling legs  42  comprising the mooring assembly  40  are installed at the second drilling venue  36 , whereupon the mobile offshore drilling unit  38  is fully secured. This procedure is repeated until all of the drilling venues in a particular area have been drilled, after which the mooring legs  42  comprising the mooring assembly  40  are recovered and the mooring assembly  40  and the mobile offshore drilling unit  38  are removed to a new drilling area. 
     The second alternative procedure involves the use of one complete mooring assembly  40  and a second mooring assembly  46  comprising, for example, one half of the number of mooring legs utilized in the complete mooring assembly  40 . The mobile offshore drilling unit  38  is initially secured in place at the first drilling venue  32  utilizing the complete mooring assembly  40 . While drilling operations are in progress at the first drilling venue  32 , the partial mooring assembly  46  is preset at the second drilling venue  34 . 
     Upon completion of drilling operations at the first drilling venue, the mobile offshore drilling unit  38  is relocated to the second drilling venue  34  and is secured in place utilizing the mooring legs  42  comprising the partial mooring assembly  46 . Thereafter, half of the mooring legs  42  from the complete mooring assembly  40  are installed at the second drilling venue  34 , whereupon the mooring assembly  46  becomes a complete mooring assembly and the mooring assembly  40  becomes a partial mooring assembly. During the time that drilling operations are in progress at the second drilling venue  36 , the partial drilling assembly  40  is preset at the third drilling venue  36 . 
     After drilling operations are completed at the second drilling venue  34 , the mobile offshore drilling unit  38  is repositioned to the third drilling venue  36  and is secured in place by the drilling legs  42  comprising the partial drilling assembly  40 . Thereafter, half of the mooring legs comprising the taut-leg mooring assembly  46  are installed at the third drilling venue  36 . This procedure is repeated until all of the drilling venues at a particular drilling location have been drilled, whereupon all of the drilling legs comprising the drilling assemblies  40  and  46  are recovered and are moved to a new drilling area along with the mobile offshore drilling unit  38 . 
     Referring now to FIG. 2, the mooring legs  42  and the is mooring lines  44  associated therewith comprising the mooring assemblies  40  and  46  are illustrated in greater detail. The mooring legs  42  preferably comprise taut-leg mooring legs which include suction anchors  43  constructed either from steel or from concrete. For example, a steel suction anchor comprising a cylindrical tube 12 feet in diameter, 60 feet in length, and having a wall thickness of 1 and ½ inches may be utilized in the practice of the invention. Special drag embedment anchors designed for high vertical loading, also known as Vertically Loaded Anchors (VLAs) may also be utilized in the practice of the invention, if desired. 
     In the case of a drilling venue having a water depth of 7,500 feet, the mooring line  44  of each mooring leg  42  comprises a 5,500-foot long, 3.88-inch diameter first segment of riser line  50  attached to and extending from the anchor  43 . A 4 and ½-inch diameter connecting link  51  of the type manufactured by Kenter, Baldt, Bruce, or Ramnäs is attached to the distal end of the riser line  50 . A 5,500-foot second segment of riser line  52  is attached at its proximal end to the connecting link  51 . A 3.38-inch diameter, 15-feet long, buoy pigtail chain  53  is attached to the distal end of riser line segment  52 . A plurality of submerged buoys  54  having a 145-kip total net buoyancy are attached to the distal end of the buoy pendent wire  53 . 
     Below each buoy  54  there is a 15-foot×3.38-inch chain section  53 , and there is a 100-foot×3.88-inch wire rope pendant  58  between the buoys  54 . A 3.88-inch diameter, 50-foot length of wire  60  extends from the upper buoy  54  to a connector  62 . A 15-foot 2.5-inch diameter wire  63  extends from the connector  62  to a submerged buoy  64  having a buoyancy of between about 3 and about 5 kips. A buoyant line  65  formed from synthetic material, for example Samson® ultrablue 3.5-inch diameter line extends to a floating marker buoy  66 . 
     In FIG. 3 there is illustrated the connection of the mooring line  44  to the mobile offshore drilling unit  38 . A 3.88-inch diameter wire  68  of about 800 feet length deployed by the MODU from its mooring winch, and secured to a 150-foot long×3.38-inch diam. K 4  chain  69 , which is inserted by the hook-up vessel between the preset mooring line and the MODU&#39;s mooring wire. The line  63 , the submerged buoy  64 , the line  65 , and the floating marker buoy  66  are removed when the MODU  38  is connected to the taut-leg mooring leg  42 . 
     It will be understood that the foregoing indications of length, diameter, and type of the component parts of the mooring line  44  are representative only, and that the actual dimensions of the component parts of the mooring line  44  will depend at least on three factors: first, the depth of the water in which the mooring line is used; second, the particular material selected for use in the fabrication of each component part of the mooring line  44 ; third, the size of the MODU. It will be further understood that the use of one or more submerged buoys  54  as a component part of each mooring line  44  comprises an important feature of the invention in that it allows the mobile offshore drilling unit  38  to be secured rapidly in place by a taut-leg mooring system. In addition, the use of the buoys  54  significantly improves the performance of the taut-leg mooring system and reduces the vertical loads imposed on the MODU by the mooring legs. 
     Turning now to FIGS. 4 through 8, therein is shown a steel suction anchor  70  useful in the practice of the invention. The suction anchor  70  is a right circular cylinder 12 feet in diameter and 60 feet in length, having a wall thickness of 1.5 inches. Skids  71 , which may comprise lengths of angle iron or lengths of pipe cut in half longitudinally are welded to the cylinder comprising the anchor  70  to prevent it from rolling on the deck of the installation vessel. 
     The suction anchor  70  is open on the lower end  72  and closed at the upper end  74  by a plate  76 . A pad eye  78 , for receiving mooring line  44 , is attached on an exterior side of suction anchor  70  approximately 40 feet from the top. The top closure plate  76  on the upper end  74  of suction anchor  70  includes ports  82  which allow water to flow through the closure plate  76  as the anchor  70  heaves up and down during lowering to and retrieval from the sea floor. The ports  82  are opened and closed by worm gear actuators  83  which are in turn operated by a manipulator extending from a remote operation vehicle (ROV)  300  which is located relative to the skid  80  by docking posts  84 . 
     Alignment of the anchor  70  is determined using a camera on the ROV  300  which observes a bullseye level  85 . The ROV  300  also adjusts the horizontal alignment of the suction anchor  70  by checking the suction anchor&#39;s heading with a gyrocompass on board the ROV. If the horizontal alignment is out of tolerance, the ROV  300  rotates the suction anchor  70  by activating thrusters on the ROV. The placement of the ROV  300  on the outer edge of the closure plate  80  ensures that the ROV&#39;s thrusters can apply adequate torque to rotate the suction anchor  70  about its axis. 
     Pad eyes  86  are used to connect the anchor to a recovery bridle. An alternate pad eye  87  may be used with a single recovery pendant or with double recovery sling. A suction port  88  having a clamp down hub is engaged by the ROV  300  to effect pumping of water into or out of the anchor  70 . 
     The submerged buoys  58  utilized in the mooring lines  44  are further illustrated in FIGS. 9,  10 , and  11 . Each buoy  54  comprises a frame  90  including a hollow central shaft  92  and a bottom plate  94  secured thereto. A pad eye  96  is secured at the bottom of the shaft  92  for use in securing the buoy  54  in the mooring line  44 . A top plate  98  is mounted at the upper end of the shaft  92  and is secured in place by a bolt  100 . The upper end of the shaft  92  is provided with a pad eye  102  for use in securing the buoy  58  in. the mooring line  44 . 
     The buoy  58  further comprises a primary buoyancy member  104  and a plurality of auxiliary members  106  which are added to or removed from the buoy  54  depending on the amount of buoyancy required by the water depth and the particular applications of the invention. All of the buoyancy members  104  and  106  comprise syntactic foam. An outer protective layer of glass reinforced polyester may be provided around the buoyancy members  104  and  106 , if desired. It will be understood that the top plate  98  is secured in place on the buoy  54  by extending the bolt  100  through selected holes  108  formed in the shaft  92  depending upon the number of auxiliary buoyancy members  106  that are utilized in a particular application of the invention. 
     Referring now to FIG. 12, therein is illustrated a top view of the deck  110  of an installation vessel  112 . The deck  110  of the vessel  112  supports suction anchors  70  during transportation to a drilling venue. FIG. 13 is a side view of the transportation vessel  100  and the anchors  70 . 
     Referring now to FIG. 14 therein is illustrated the initial deployment stage of the suction anchor  70 . The installation vessel  110  is positioned at the drilling venue with its bow into the prevailing seas. Recovery pendent line  142  and a recovery buoy  146  are rigged to the top of the suction anchor  70  by connection to the pad eyes  87 . 
     It is also possible to install and recover the suction anchor  70  without using the recovery buoy  146 . In such instances there is provided a doubled sling secured to the top of the suction anchor  70  which is laid down across the suction anchor and onto the sea floor. For recovery of the suction anchor, the ROV  300  connects a special hook to the doubled sling. This option comprises an important feature of the invention since recovery buoys suitable for use in very deep water are expensive to purchase and maintain. 
     The recovery buoy  146 , if used, is secured to a nylon stretcher  139  which is in turn secured to a multi-strand lowering wire  140  spooled off an auxiliary vessel  200 . The nylon stretcher  139  allows the stern of the auxiliary vessel  200  to heave in the seas without overloading the lowering wire  140 . An upper drum work wire  150  is rigged over an A-frame gantry  158  and connected to the suction anchor  70 . A lower drum work wire  152  is also connected to the suction anchor for use as a hold back line. 
     Turning now to FIGS. 15 through 19, the auxiliary vessel  200  moves away from the installation vessel  100  paying out approximately 100 feet of the lowering wire  140 . The auxiliary vessel  200  stops paying out lowering wire  140  and increases tension in the lowering wire. Slowly the lower drum work wire  152  allows the auxiliary vessel  200  to pull the suction anchor  70  out into the water. The A-frame gantry  158  is moved slowly aft, as required, payingout the riser wire  50 . The upper drum work wire  150  lifts the lower end of the suction anchor  70  and the lower drum work wire  152  is disengaged. As shown in FIG. 19, the riser wire  50  is payed out while the auxiliary vessel  200  moves closer to the installation vessel  110 . The upper drum work wire  150  is disengaged, the suction anchor  70  swings down under the stern of the auxiliary vessel  200  and the weight of the suction anchor  70  is transferred to the lowering wire  140 . 
     Turning now to FIG. 20, a remote operation vehicle (ROV)  300  is deployed from the auxiliary vessel  200 . ROV  300  may comprise a Raycal SEA Lion MK II heavy work class ROV having  100  horsepower; however, any of the various commercially available ROV&#39;s having 75 h.p. or more can be used in the practice of the invention. The lowering wire  140  and riser wire  50  are paid out until the suction anchor reaches the ocean floor. Meanwhile the auxiliary vessel also pays out the ROV&#39;s umbilical wire  302 , so that the ROV can observe the suction anchor during its descent. 
     As shown in FIG. 21, the suction anchor  70  is slowly lowered into the seafloor under its own weight. Meanwhile the ROV  300  observes the bullseye level  85  on the top of the suction anchor  70  to assure that the suction anchor remains vertical within established tolerances. Under its own weight, the suction anchor  70  will penetrate about 40%-50% of its length into the seafloor (typical in the-Gulf of Mexico). The ROV  300  next checks the amount of self penetration by reading the penetration marks at the mudline, while the lowering line  140  is slacked off. During the lowering process the evacuation ports  82  and suction port  88  remain open, allowing water displaced by the ocean floor inside the suction anchor to flow outwardly through these ports. 
     As shown in FIGS. 22,  23 , and  24 , the ROV  300  is fitted with a pumpskid  160  which is mounted beneath the ROV. The pumpskid  160  includes a pump  162 , pump manifold valve actuators  164  and  165 , and latching actuators  166 , all powered and controlled by the hydraulic system of the ROV  300 . The pumpskid further includes a male connector  168  for the suction port  88 . 
     Next, the ROV  300  docks and latches onto the suction anchor and its suction port  88  by engagement of the male connector  168  and by engaging the latching actuators onto the clamp down hub of the suction port  88 . Next, the ROV closes the ports  82 . The pump  162  of the pumpskid  160  is started and pumps water out of the interior of the suction anchor  70 , reducing the water pressure inside relative to the outside pressure. This is accomplished by means of actuator  164  which opens valve  170  and closes valve  172  and actuator  165  which opens valve  174  and closes valve  176 , thereby causing water to flow through suction port  88 , valve  174 , pump  162 , and valve  170 , and then out-through opening  178 . 
     The differential pressure under the action of pump  162  acts as a downwards force on the top of the suction anchor  70  pushing the suction anchor further into the seafloor to the desired penetration depth. When the desired penetration has been reached, as determined from the ROV&#39;s depth monitoring system, the ROV disconnects from the top of the suction anchor  70 . Next the ROV checks the suction anchor penetration by reading the penetration marks at the mudline. When the suction anchor  70  penetration is found to be within tolerance, the ROV  300  closes the suction port  88  so that all openings in the top of the suction anchor are closed. As is shown in FIG. 25, the ROV  300  now disconnects lowering line  140  from the recovery buoy  146 . Next the ROV is retrieved by the auxiliary vessel  200 . 
     Turning now to FIG. 26,  27 , and  28 , therein is illustrated a recovery procedure that may be employed to recover the suction anchor  70  after drilling operations at a particular drilling venue are completed. The installation vessel  112  is attached by the riser wire  50  to the suction anchor  20 . The auxiliary vessel  200  lowers the ROV  300  with the pumpskid  160  on the umbilical wire  302 , while at the same time the recovery wire  240  is lowered to the seafloor. The recovery wire  240  is equipped with a special hook, and a submerged buoy  242  some distance above the hook. The submerged buoy  242  isolates the hook from much of the auxiliary vessel&#39;s heave motions. The ROV  300  attaches the hook to the doubled sling on the recovery buoy, or to the doubled sling laying across the suction anchor. The auxiliary vessel takes up tension on the recovery line  240 , and the ROV  300  docks onto the suction anchor top and latches onto the suction port  88 . The ROV  300  pumps water into the interior of the suction anchor by means of the pump  162 . This is accomplished by operating the actuators  164  and  165  to open valve  176 , open valve  172 , close valve  174 , and close valve  170 , thereby causing water to flow through opening  178 , valve  172 , pump  162 , valve  176  and port  88  into anchor  70 . 
     Due to the pump  162 , the water pressure inside becomes greater than the outside water pressure, and the differential pressure results in an upwards force on the suction anchor top. The upwards force, and the pull on the recovery line pulls the suction anchor out of the seafloor. If too much pump pressure is required to pull the suction anchor  70  out of the seafloor, due to too much consolidation of the soil around and inside the suction anchor, the water flow direction of the pump  162  can be reversed instantaneously by changing the positions of valve actuators  164  and  165 . By rapidly changing the water flow direction from pumping in to pumping out, the suction anchor  70  will be alternately pulled out and pushed in. When this is done over time, the soil in contact with the suction anchor cylinder will liquefy, making it easier to pump and pull the suction anchor out off the soil. The suction anchor  70  is raised to the surface by recovery line  240  and is loaded on installation vessel  112  using the riser line  50 . 
     Referring now to FIG. 29, there is shown a catenary mooring leg  342  which may be used in the practice of the invention in lieu of the taut-leg mooring leg  42  illustrated in FIGS. 2 and 3 and described hereinabove in conjunction therewith. The mooring leg  342  may employ a suction anchor of the type illustrated in FIGS. 4,  5 ,  6 ,  7 , and described hereinabove in conjunction therewith. Alternatively, the mooring leg  342  may employ a vertically loaded anchor. Referring momentarily to FIGS. 34,  35 , and  36 , the vertically loaded anchor may comprise a vertically loaded anchor  346  of the type sold by Vryhof under the trademark “STEVMANTA”. Alternatively, the vertically loaded anchor may comprise a vertically loaded anchor  348  of the type sold by Bruce under the trademark “DENIA”. Vertically loaded anchors are particularly adapted to the practice of the present invention for two reasons. First, vertically loaded anchors are designed and adapted to accommodate and withstand relatively high vertical loads and are therefore particularly adapted for use in conjunction with taut-leg mooring legs. Second, vertically loaded anchors are designed and adapted to be recovered after the project for which they are installed has been completed. 
     Referring again to FIG. 29, the mooring leg  342  employs a drag embedment anchor  349 . A 3-inch diameter, 3,300-foot long ORQ ground chain  350  is connected to and extends from the anchor  349 . A 3¼-inch diameter, 9,000-foot long IWRC−EIPS+20% catenary wire  352  is connected to the distal end of the chain  350  and extends upwardly therefrom. A 3-inch diameter, 15-foot long ORQ+20% buoy chain pendant  354  is connected to the distal end of the catenary wire  352 . The 64-foot kip fixed buoyancy submerged buoy  356  is connected to the buoy pendant chain  354 . The buoy  356  is similar to the buoy  54  illustrated in FIGS. 9,  10 , and  11 , and described hereinabove in conjunction therewith except that the buoy  356  comprises a single block of syntactic foam similar to the block  104  of the buoy  54  and does not include removable syntactic foam sections such as the foam sections  106  of the buoy  54 . 
     The buoy  356  is utilized in conjunction with a 57-kip adjustable buoyancy submerged buoy  358  which is identical in construction and function to the buoy  54  illustrated in FIGS. 9,  10 , and  11 . 
     A 3¼-inch diameter, 200-foot long IWRC−EIPS+20% intermediate connection pendant  360  is connected to the upper end of the buoy  356 . A 3-inch diameter, 15-foot long ORQ+20% buoy chain pendant  362  is connected to the distal end of the connection pendant  360 . A 3¼-inch diameter, 400-foot long IWRC−EIPS+20% upper connection pendant  364  is connected to the upper end of the buoy  358 . The upper connection pendant  364  is connected to a 3-inch diameter, 700-foot long ORQ rig chain which extends from the MODU  38 . As will be appreciated by those skilled in the art, the taut-leg mooring leg  342  is typically not employed singly but rather in combination with other, similar taut-leg mooring legs in order to properly stabilize the MODU at a drilling venue. 
     Referring now to FIG. 30, there is shown a catenary mooring leg  372  which may be utilized in the practice of the invention in lieu of the taut-leg mooring leg  42  illustrated in FIGS. 2 and 3 and described hereinabove in conjunction therewith. The mooring leg  372  may employ a suction anchor of the type illustrated in FIGS. 4,  5 ,  6 ,  7 , and  8  and described hereinabove in conjunction therewith. Alternatively, the mooring leg  372  may employ a vertically loaded anchor comprising one of the two types of vertically loaded anchors illustrated in FIGS. 34,  35 , and  36  and described hereinabove in conjunction therewith. As shown, the mooring leg  372  uses a drag embedment anchor  374 . 
     A 3-inch diameter, 3,300-foot long ORQ ground chain  376  is connected to the vertically loaded anchor  374  and extends therefrom. A 3¼-inch, 6,000-foot long IWRC-EIPS+20% catenary wire  378  extends from the distal end of the ground chain  376 . A 3-inch diameter, 15-foot long ORQ+20% buoy chain pendant  380  is connected between the distal end of the catenary wire  378  and a 64-kip fixed buoyancy submerged buoy  382 . The buoy  382  is identical in construction and function to the buoy  360  illustrated in FIG.  29  and described hereinabove in conjunction therewith. 
     A 3¼-inch diameter, 200-foot long IWRC−EIPS+20% intermediate connection pendant  384  is connected to the upper end of the buoy  382 . A 3-inch diameter, 15-foot long ORQ+20% buoy pendant chain  386  is connected between the distal end of the intermediate connection pendant  384  and a 17-kip adjustable buoyancy submerged buoy  388 . A 3¼-inch diameter, 400-foot long IWRC−EIPS+20% upper connection pendant  390  is connected to the upper end of the buoy  388 . The pendant  390  is in turn connected to a 3-inch diameter, 647-foot long ORQ rig chain  392  extending to and connecting from the MODU  38 . Those skilled in the art will appreciate the fact that the mooring leg  372  is typically not employed singly, but rather is employed in conjunction with other, similar mooring legs in order to properly stabilize the MODU at a drilling venue. 
     Referring now to FIG. 31, there is shown a taut-leg mooring leg  402  which may be utilized in the practice of the invention in lieu of the taut-leg mooring leg  42  illustrated in FIGS. 2 and 3 and described hereinabove in conjunction therewith. The taut-leg mooring leg  402  employs a suction anchor  404  which may be of the type illustrated in FIGS. 4,  5 ,  6 ,  7 , and  8  and described hereinabove in conjunction therewith. Alternatively, the taut-leg mooring leg  402  may employ a vertically loaded anchor of the type illustrated in FIGS. 34,  35 , and  36  and described hereinabove in conjunction therewith. A 3-inch diameter, 1,500-foot long K 4  ground chain  406  is connected to the suction anchor  404  and extends upwardly therefrom. A 3½-inch diameter, 8,500-foot long IWRC+20% catenary wire  408  is connected to the distal end of the ground chain  406  and extends upwardly therefrom. A 3-inch diameter, 15-foot long K 4  buoy chain pendant  410  is connected between the distal end of the catenary wire  408  and a 82-kip fixed buoyancy submerged buoy  412 . The buoy  412  is identical in construction and function to the buoy  356  illustrated in FIG.  9  and described hereinabove in conjunction therewith. 
     A 3½-inch diameter, 200-foot long IWRC−EIPS+20% intermediate connection pendant  414  is connected to the upper end of the buoy  412 . A 3-inch diameter, 15-foot long K 4  buoy chain pendant  416  extends between the distal end of the intermediate connection pendant  414  and a 57-kip adjustable buoyancy submerged buoy  418 . The buoy  418  is identical in construction and function to the buoy  54  illustrated in FIGS. 9,  10 , and  11  and described hereinabove in conjunction therewith. 
     A 3½-inch diameter, 400-foot long IWRC−EIPS+20% upper connection pendant  420  is connected to the upper end of the buoy  418 . The distal end of the upper connection pendant  420  is in turn connected to a 3-inch diameter, 500-foot long K 4  rig chain  422  extending from the MODU  38 . Those skilled in the are will appreciate the fact that the taut-leg mooring leg  402  is typically not used singly, but rather is employed in conjunction with other, similar taut-leg mooring legs to properly stabilize the MODU at a drilling venue. 
     Referring now to FIG. 32, there is shown a taut-leg mooring leg  432  which may be used in the practice of the invention in lieu of the taut-leg mooring leg  42  illustrated in FIGS. 2 and 3 and described hereinabove in conjunction therewith. The taut-leg mooring leg  432  employs a suction anchor  434  which may be of the type illustrated in FIGS. 4,  5 ,  6 ,  7 , and  8  and described hereinabove in conjunction therewith. Alternatively, the taut-leg mooring leg  432  may employ a vertically loaded anchor which may be either of the type illustrated in FIG. 34 or of the type illustrated in FIGS. 35 and 36 and described hereinabove in conjunction therewith. 
     A 3-inch diameter, 1,500-foot K 4  ground chain  436  is connected to the anchor  434  and extends upwardly therefrom. A 3½-inch diameter, 6,000-foot long IWRC−EIPS+20% catenary wire  438  is connected to the distal end of the chain  436  and extends upwardly therefrom. A 3-inch diameter, 15-foot long K 4  buoy chain pendant  440  is connected between the distal end of the catenary wire  438  and an 82-kip fixed buoyancy submerged buoy  442 . The buoy  442  is identical in construction and function to the buoy  356  illustrated in FIG.  29  and described hereinabove in conjunction therewith. 
     A 3½-inch diameter, 200-foot long IWRC−EIPS+20% intermediate connection pendant  444  is connected to the upper end of the buoy  442 . A 3-inch diameter, 15-foot long K 4  buoy chain pendant  446  is connected between the distal end of the intermediate connection pendant  444  and a 17-kip adjustable buoyancy submerged buoy  448 . The buoy  448  is identical in construction and function to the buoy  54  illustrated in FIGS. 9,  10 , and  11  and described hereinabove in conjunction therewith. 
     A 3½-inch diameter, 400-foot long IWRC−EIPS+20% upper connection pendant  450  is connected to the upper end of the buoy  448 . The upper connection pendant  450  is in turn connected to a 3-inch diameter, 500-foot long K 4  rig chain  452  extending from the MODU  38 . Those skilled in the art will appreciate the fact that the taut-leg mooring leg  432  is typically not employed singly, but rather is employed in conjunction with other, similar taut-leg mooring legs to properly stabilize the MODU  38  at a drilling venue. 
     Referring now to FIG. 33, there is shown a taut-leg mooring leg  462  which may be utilized in the practice of the invention in lieu of the taut-leg mooring leg  42  illustrated in FIGS. 2 and 3 and described hereinabove in conjunction therewith. The taut-leg mooring leg  462  employs a suction anchor  464  which may be of the type illustrated in FIGS. 4,  5 ,  6 ,  7 , and  8  and described herein in conjunction therewith. Alternatively, the taut-leg mooring leg  462  may employ a vertically loaded anchor, such as the vertically loaded anchor  346  illustrated in FIG. 34 or the vertically loaded anchor  348  illustrated in FIGS. 35 and 36 and described hereinabove in conjunction therewith. 
     A 3½-inch diameter, 4,000-foot long IWRC−EIPS+20% ground wire  466  is connected to the suction anchor  464  and extends upwardly therefrom. A 3-inch diameter, 5-foot long K 4  chain  468  is connected to the distal end of the ground wire  466  and is in turn connected to a 6½-inch diameter, 2,400-foot long catenary segment  470  preferably comprising the polyester rope available from Marlow Ropes of the United Kingdom under the trademark “PolySuper.” A 3-inch diameter, 5-foot long K 4  chain  472  is connected between the “PolySuper” catenary segment  470  and a 6½-inch diameter, 2,440-foot long catenary segment  474  also comprising the “PolySuper” material. A 3-inch diameter, 5-foot long K 4  chain  476  is connected between the “PolySuper” catenary segment  474  and a 6½-inch diameter, 2,400-foot long “PolySuper” catenary segment  478 . A 3½-inch diameter, 5-foot long K 4  chain  480  is connected between the “PolySuper” catenary segment  478  and a 6½-inch diameter, 2,400-foot long “PolySuper” catenary segment  482 . A 3-inch diameter, 20-foot long K 4  chain  484  is connected between the “PolySuper” catenary segment  482  and a 3½-inch diameter, 400-foot long IWRC−EIPS+20% wire for  486 . The wire  486  is in turn connected to a 3-inch diameter, 500-foot long K 4  rig chain  488  which extends from and is connected to the MODU  38 . 
     Those skilled in the art will appreciate the fact the taut-leg mooring leg  462  differs considerably from the taut-leg mooring leg  42  illustrated in FIGS. 2 and 3 and described herein in conjunction therewith and from the taut-leg drilling legs  342 ,  372 ,  402 , and  432  illustrated in FIGS. 29,  30 ,  31 , and  32 , respectively, and described hereinabove in conjunction therewith. This is because the taut-leg mooring leg  462  comprises segments  470 ,  474 ,  478 , and  482  all of which are formed from “PolySuper”, which has a submerged negative buoyancy about 13% that of steel rope of the same strength. The use of more buoyant materials in forming the connection between the anchor  464  and the rig chain  488  extending from and connected to the MODU unit eliminates the necessity of employing discrete submerged buoys within the taut-leg mooring leg  462 , for example, the buoys  54  employed in the taut-leg mooring leg  42  of FIGS. 2 and 3, the buoys  356  and  358  of the taut-leg drilling leg  352  illustrated in FIG. 29, etc. Those skilled in the art will further appreciate the fact that the taut-leg mooring leg  462  is typically not employed singly, but rather is employed in combination with other, similar taut-leg mooring legs in order to properly stabilize the MODU  38  at a drilling venue in the manner illustrated in FIG.  1  and described hereinabove in conjunction therewith. 
     Although preferred and alternative embodiments of the invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements and substitutions of parts and elements without departing from the spirit of the invention.