Apparatus for Anchoring an Offshore Structure

The present invention concerns, broadly, maneuvering large and heavy equipments onto and from the sea floor. More particularly, the invention concerns a support structure for use in drilling and completing subsea oil and/or gas wells and methods for installing the structure on and removing the structure from the sea floor. As drilling and production water depths increase beyond the point of optimum diver utilization placing such structures on and removing such structures from the ocean floor must be conducted remotely from a floating vehicle. Motions induced by wind, wave and current create vessel-equipment interactions which, if not properly controlled, can render conventional handling techniques worthless. In addition, such structure placement and removal operations should include utilization of a minimum number of marine vessels, the ability to work under normal weather conditions, a relatively rapid completion of the operations to preclude extended waits for lengthy periods of good weather, and the ability to place the structure in the correct geographical location and attitude. The structure should also be designed for easy installation, should be capable of withstanding the abnormal rigors of sea floor existence, and should be retrievable at the conclusion of its working life. The structure and methods of the present invention meets all of the aforementioned requirements.

The present invention involves a subsea structure which comprises a plurality of horizontal and vertical structural tubes arranged to provide support for the subsea oil and/or gas drilling and production equipment. Certain of the tubes are segregated to form compartmented ballast chambers or tanks capable of being selectively flooded or dewatered to achieve desired negative or positive buoyancies, respectively, so that the weight of the submerged structure can be controlled to facilitate carrying out the various required operations. The elevated location of the uppermost circumferential ring makes it very useful in generating a large water plane area for yielding excellent floating stability and provides, also, a high center of buoyancy in the submerged conditions which also contributes to stability. In addition, this ring serves as a fender to protect the equipment surrounded by the ring from damage by dragging anchors or other submerged objects. Selected vertical structural tubes form pile sleeves used in securing the structure to the sea floor. Guide means are provided on the ring to aid in guiding equipment into the pile sleeves. Such equipment aids in anchoring the subsea structure to the sea floor and for releasing the subsea structure from the sea floor. Orienting tubes are mounted on the periphery on the subsea structure through which lines are extendible to a surface drilling vessel and to surface working vessels. The drilling and production equipment on the subsea structure includes a track for a manipulator which surrounds a series of well bays, an antipollution pan, flowline valves, a framework for flowline connectors, electrical-hydraulic units, a separator and pump unit and a buoyancy control manifold having flood and vent lines extending to the ballast chambers.

The method involved in maneuvering the subsea structure to install it on the sea floor comprises the steps of towing the structure to adjacent a drilling vessel; connecting lines between the vessel and the structure and the structure and one or more working vessels, ballasting to trim the structure level at the surface; ballasting the structure to a negative buoyancy; keelhauling the structure to beneath the drilling vessel; lowering the structure to the sea floor; orienting the structure geographically; pile founding the structure onto the sea floor; cementing the piles in the sea floor and then leveling the structure. The structure is removable by severing the piles and dewatering the structure to make it positively buoyant.

FIG. 1 is a schematic perspective view of the subsea structure of the invention;

FIG. 1A is a fragmentary view illustrating buoyancy control means;

FIGS. 2, 3 and 4 are respectively plan, side and front views of the subsea structure of FIG. 1;

FIGS. 5 to 11, inclusive, illustrate schematically towing the subsea structure into the launch area, launching the structure from the tow barge into the water, connecting the structure to the drilling vessel and then keelhauling the structure to a position for lowering it to the sea floor;

FIG. 12 is a schematic illustration of the lowering of the subsea structure to the sea floor;

FIGS. 13 to 15 inclusive illustrate the pile and means associated therewith for placing the pile in position and for cementing thereof;

FIGS. 16 to 18 illustrate the pile sleeve and lifting tool for leveling the subsea structure;

FIGS. 19 to 25 illustrate schematically the steps employed in cementing the piling in the ground underlying the water and in cementing the piling in the piling sleeve; and

FIGS. 26 to 33 illustrate schematically the steps employed in removing the subsea structure from the ocean floor.

As shown in FIGS. 1 and 1A, a large boxlike structure or template 10 designed for subsea use in drilling and producing submerged oil and/or gas wells includes a truss or framework of interconnected vertical and horizontal steel tubular members 11 and 12, respectively, which are segregated to form compartmented tanks which function as ballast chambers, and other steel tubular members, which form pile sleeves 32 and cross support members 9 and 9a. The large uppermost circumferential tubular members 13 also form a protective ring or guard fender for the equipment supported on structure 10. That equipment includes production manifolding 14 on which is mounted an antipollution pan 15 and which surrounds a number of well bays 16, a track 17 surrounding the production manifolding and having a straight track section 18 on which is positioned an anchor 19 for a manipulator and a releasable buoy 20 for use in guiding the manipulator to the track, framework 21 for flowline connectors, flowline valves 22, a separator and pump unit 23, a buoyancy control manifold 24, flood (water) and vent (air) lines connected to buoyancy control manifold 24 and the ballast chambers, deballast tube 110 mounted on one of the corner support tubes 9, and an electric-hydraulic unit 25. As shown particularly in FIGS. 1 to 4 template 10 is rectangularly shaped and has a bow 26 and a stern 27. A space 28, is formed in the bow side of tubular ring 13 to accommodate flowline connector apparatus and a space 29, is formed in the stern side of tubular ring 13 to accommodate a power cable which extends from the surface to the separator and pump unit 23. Power for control system purposes is delivered through a separate umbilical cable 25a extending through space 28.

Two L-shaped orienting tubes 30, each having funneled ends and each being located in opposite diagonal corners of template 10, are used in orienting template 10 and orienting line 31 is threaded through each of the orienting tubes 30 as shown.

Four of the vertical tubular members 32 form pile sleeves. Adjacent each pile sleeve are two guide post sleeves 33.

Vertical tubular members 34 at the extremities of template 10 are flooded as necessary to level the template while it is floating at the surface to ensure that it will submerge level. Preselected portions of the lower horizontal tubular members 35 are flooded to change the template from a positive to a negative buoyancy at which buoyancy the weight is maintained at a sufficiently low value to permit handling of the template by conventional apparatus. Thus, the template will remain at such negative buoyancy during lowering operations, orienting, pile setting, cementing and leveling. Divers operate the valves on the buoyancy control manifold to control the ballasting operations while the template is at or near the water's surface prior to lowering it to the sea floor. Tubular members not flooded prior to or during the keel-hauling and the change from positive to negative buoyancy, including centrally located members 36 and buoyancy or fender ring 13, are flooded after template 10 is on bottom, leveled and piles set. The manipulator operates the valves on the buoyancy control manifold to control fill and vent operations once the template is positioned on the sea floor. Upon an acoustic command from the surface vessel, a buoy is released from the template and the manipulator which is positively buoyant upon launch hauls itself down, lands, and latches on the track encircling the manifold. Once latched on the track the manipulator is in a position to remove malfunctioning parts of, install replacements parts of and resurface carrying used components of the pretested equipment initially installed on the template.

As shown in FIGS. 2 and 3 more particularly a single tilt angle beacon 37 used to measure the angular displacement of template 10 during leveling is positioned in an array with two locator beacons 38 which are used to measure azimuth. Alternative similar arrays are shown and may be desirable as spares to forestall replacement delays on deep-water systems. A backup hardware telemetry system 137, 138 is also shown.

FIG. 5 shows template 10 being towed on a barge 42 by a vessel 43 to the site of launch. As seen in FIG. 6 template 10 is connected to a drilling vessel 44 by a control line 45 and to a work boat, not shown, by another control line 46. As indicated, end tanks of barge 42 are flooded to permit template 10 to slide into the water. In the position shown in FIG. 7 the vertical trim tanks 34 are flooded, as necessary, to level template 10 at the water's surface. As seen in FIG. 8 template 10 is warped alongside drilling vessel 44. Pneumatic bumpers 47 are positioned between template 10 and drilling vessel 44. Keel haul bridles 48 and 49 are connected to the bow and stern of the drilling vessel and to the bow and stern of the template. The warping lines are removed and the main lowering sling assembly 50 is attached to template 10. The main line 51 of sling assembly 50 is keelhauled through the drilling well (moonpool) 52 of drilling vessel 44 and connected to a preferably heave-compensated hook 55 (as indicated in FIG. 8) in the derrick. One work boat 56 pulls template 10 away from drilling vessel 44 a specified distance 53 to permit template 10 to swing under and clear vessel 44 as it is submerged. A work boat 57 may be connected to template 10 by line 58 and it may proceed upstream to a current control anchor if local current conditions demand such additional control. Anchors are set in a line suitable for azimuth positioning of template 10 utilizing winches on drilling vessel 44. FIGS. 10 and 11 show template 10 in position for lowering relative to drilling vessel 44. The weight of the template as it swings under drilling vessel 44 is supported by lowering sling assembly 50. Template 10 is raised slightly by line 51 and sling assembly 50 and keelhaul lines 48 and 49 are removed.

FIG. 12 shows the relationship between drilling vessel 44 and template 10 as it is lowered through the water to the subsea floor 60. The lines from anchors 63 which are the aforementioned orienting lines 31 are taken from the work boats and passed through orienting tubes 30 bow and stern of the template and connected to winches on the bow and stern on drilling vessel 44. A pendant line 61 connected to a buoy 62 at the surface is connected to each anchor 63 as shown. Hydrophones 64 located on the underside of drilling vessel 44 in association with beacons 37 and 38 continuously monitor the azimuth positions as the template is lowered through the water. Azimuth readings are made just before setting the template on the ocean floor. Tensioning on orienting lines 31 can rotate template 10 to the desired orientation at which point template 10 can be set on the ocean floor.

When properly landed lowering sling assembly 50 may be removed from template 10 by hydraulic lines connected to pelican hooks (not shown) on the sling or by mechanical release cables (not shown) operated from drilling vessel 44. Control lines 156 and 58, if used, are removed by running go-devils down the lines to releasable connection 157 for retrieval by the work boats. Orienting lines 31 are retrieved by picking up anchors 63 with pendant lines 61 and moving towards template 10 while reeving in lines 31 with winches mounted on drilling vessel 44. As shackle 65 reaches drilling vessel 44 the upper segment of line 31 will be replaced with a synthetic fiber rope 31a. The work boat will then proceed away from template 10 drawing orienting line 31 back down through orienting tube 30. Line 31 will be tensioned with line 31a so as not to foul template 10. As line 31 clears the template 10, line 31a will be jettisoned with buoy 31b attached to prevent line 31a from fouling the template. As buoy 31b impinges on orienting tube 30, tensioning line 31a will be strained and parted. Lines 31a and 31 will then be retrieved by work boat as will be buoy 31b.

The heading for the drilling vessel mooring is chosen to accommodate current and sea conditions at the launch site. The drilling vessel should be aligned with the surface current during keelhauling. Selection of the drilling vessel's heading is made to permit turning the vessel in its moorings during placement of the template to accommodate variables of current, wind, etc. Anticipation of such variables will establish whether the drilling vessel is moored bow or stern to the current and the relative position of the template.

Referring to FIGS. 13, 14 and 15 and with particular reference to FIG. 13 there is shown the pile to be inserted into and through piling sleeve 32 and the means attached to the pile 70 for placing and cementing pile 70 in place. Piling 70 includes a cementing shoe 71, a lower retainer basket 72, an upper retainer basket 73, slips 74 and an upper funnel-shaped end 75. A spacer pipe 76 is connected into a collar 77 (see FIG. 14) to which is connected a length of plastic pipe 78 which extends down through piling 72. Spacer pipe 76 has mounted on it a hang-off tool 79 to which is attached drill pipe 80.

Referring now to FIGS. 16 to 18 in which the lifting tool for leveling the template is shown in operating position relative to the pile sleeve. Piling sleeve 32 has overflow slots 32a positioned just below buoyancy and fender ring 13. The top of pile sleeve 32 is provided with two sets of J-slots 85 and a pile sleeve guide cone 86. The purpose of the overflow slots are to prevent cement slurry from spilling over on production and other equipments 14-24 and from entering the upper end of the piling sleeve adjacent the J-slots. A lifting tool 87 having a sleeve portion 87a and tapered plate members 87b thereon is attached to drill pipe 89. A pair of J-lugs 87c positioned 180.degree. apart are fixed on and extend outwardly from sleeve portion 87a. A stringer or pipe 87d extends downwardly from the lower end of tool 87. Plate members 87b guide and center the tool 87 into the coneshaped top 86 of pile sleeve 32 and allow the lugs 87c to engage J-slots 85. Guide cables 90 extend to the surface from guide posts 90a positioned in sleeves 33 and guide frame members 91 positioned about the cables 90 and connected to lifting tool 87 guide the lifting tool down into pile sleeve 32. On larger templates only three leveling pile sleeves may be used with several non-leveling pile sleeves.

The cementing and leveling operations are illustrated in FIGS. 19 to 25 inclusive. Referring to FIG. 19 operations are begun on the lowest sleeve as indicated by tilt beacon 37. A drill string 100 including a drill bit 101, drill collars 102 and drill pipe 103 is guided from drilling vessel 44 into pile sleeve 32 by means of the guide means 91 and cables 90 and the pile hole 104 is drilled to a desired depth. The hole is conditioned by displacing seawater with viscous drilling fluid to preserve the hole. The drill string is then recovered to the drill vessel. As illustrated in FIGS. 20 and 21 the pile assembly shown in FIG. 13 is run into the pile hole through the pile sleeve on drill pipe until hang-off tool 79 reaches pile cone 86. Slips 74 engage the inner wall of pile sleeve 32 and allows upward movement of piling sleeve 32 but prevents downward movement thereof. Spacer pipe (drill pipe) 76 positions the top of the pile or slips a distance D (approximately 10 feet) above the bottom of pile sleeve 32. The spacer pipe 76 connects into pile sleeve 32 by means of the adapter collar 77 (FIG. 14) which is threaded into pile 70 by left-hand threads. Plastic pipe section 78 extends from the underside of the adapter collar 77. The upper and lower retainer baskets 73 and 72, respectively, prevent primary cement from entering the pile sleeve annulus and the secondary cement from going below retainer basket 73. Plastic pipe 78 permits the primary cement slurry to remain free of seawater as it is pumped down the drill pipe and through the plastic pipe. In the event the plastic pipe becomes cemented in it can be twisted off when the adapter and drill pipe are removed and left in piling 70. A logging tool 105 (or detector) is run through the drill pipe and through the piling assembly to detect cement as it passes uphole in the piling annulus 106. The cement slurry is mixed with radioactive material in the first batch thereof sufficient to provide a reading on the detector when the cement passes the detector and also when it moves up in the annulus 106. This assures that the slurry is not being lost to formations. Cement is then pumped down the drill pipe and through the piling and the cement shoe as illustrated in FIGS. 24 and 25 until the cement is just below the top of hole 104 as indicated by raising logging tool 105. The cement is then permitted to set and the drill pipe and adapter plus the plastic pipe, if not cemented in, are removed from the hole. If the plastic pipe is cemented in then it is twisted off and left in the pile.

As illustrated in FIGS. 22 and 23 lifting tool 87 is run on drill pipe with a stinger and stabbed into pile sleeve 32 to engage lugs 87c into J-slots 85. The drill pipe is then pulled up to raise the lowest corner of template 10 while taking readings indicated by tilt beacon 37. After each upward movement of the template the tilt beacon is allowed to steady and is again read. The template pile sleeve is raised until template 10 is as near level as can be achieved with the first or lowest sleeve 32. As shown in FIG. 23 the first sleeve 32 has been raised a distance D'. Lifting tool 87 and drill pipe 89 are then removed. The pipe slips 74 will support the template 10 in this position. The same operations are then performed on the next lowest pile sleeve 32 as indicated by the tilt beacon readings. The procedure for leveling is repeated for each remaining pile sleeve until the template is level.

FIGS. 24 and 25 illustrate the secondary cementing procedure. Secondary cementing is begun in the last pile used to level template 10. Cement is pumped through the drill pipe connected to the lifting tool through the lifting tool and into the top of piling 70 from which the secondary element overflows and fills the upper end of pile sleeve 32 until the cement flows out slots 32a in the pile below the fender ring 13. Cement also surrounds slips 74 and the upper portion of piling 70 down to the top of retainer basket 73. The cement is permitted to equalize and then the J-tool and pipe are raised up and flushed with seawater. Then a bailer is run through the drill pipe to detect and sample the top of the cement. The secondary cementing operations are repeated on each of the remaining pile sleeves. The secondary cement acts as a plug in the top of the pile sleeve and above the pile and also acts as a backup for slips 74. The secondary cement permits recovery of the upper portion of the piles 70 and slip units 74 when the templates is salvaged after its use in that particular location has terminated. In addition, the secondary cement prevents the template from moving off piles 70 during deballasting operatings when cutting or severing the piles as hereinafter described.

After all of the piles have been set and cemented in, tubular structure members thus far remaining dry on the template can be flodded to bring the template to full submerged weight. Drilling of the wells through the well bays in the template is commenced and completion and production operations are carried out.

The remaining Figs. concern salvaging the template. The wells are plugged and the well casings are cut loose from the submerged production system equipment on template 10. Referring to FIG. 26 then the secondary cement is drilled out of pile sleeve 18 to adjacent the top of pile 70 in the pile sleeve which is nearest to the deballasting tube 110 (see FIG. 29) which connects to the ballast manifold 24. The pile is then drilled out to 10 feet below the cutoff point of the pile as illustrated in FIG. 27. The drill pipe is then pulled and a cutting tool assembly 112 on which is mounted a marine swivel 113 for seating in pile cone 75 is run into pile 70 and the pile is cut off as indicated at 114. The drill pipe 103 and cutting tool 112 attached to it are then pulled.

Before proceeding to cut a second pile deballasting operations are begun. Referring to FIG. 29 a compressed air adapter stab unit 115 is connected to the lower end of a drill pipe 116 (which contains a jar 117 just above adapter 115) on the drilling vessel. The air hose 118 to the work boat is keelhauled from the compressor on the work boat under the drilling vessel through the moon well and attached to the adapter unit then the adapter 115 and drill pipe 116 are lowered together with guide frame 119 and the adapter is stabbed and locked into the deballast tube 110. Air is then pumped through tube 110 to the buoyancy control manifold 24 and then to the ballast tanks formed by the tubular members to force water out of them. A release sub 120 contains a shear pin. That shear pin is sheared by jar 117 to release the drill pipe connection from the adapter 115 and the drill pipe 116 is removed. As seen in FIG. 30 the air continues to deballast the ballasting tanks.

Then the diagonally positioned pile sleeve 32 and the pile therein are drilled out as discussed above and the pile is cut and removed in the same manner as discussed above.

The remaining two piles are preferably cut explosively. Referring to FIGS. 31 and 32 after drilling out the cement as described above a telescoping tool or locator sleeve 120 provided with at its upper end a landing head 121 and having a shear pin connection 122 to the lower end of lifting tool 87 to which is connected a release sub 123 on drill pipe 124 is run into pile 70 until landing head 121 lands on the pile cone 75 as shown. An electrical cable 125 extends from the work boat into the locator sleeve 120 and a cable 126 connects the lifting tool to the work boat. A sub 127 on the locator sleeve 120 is provided with outlet ports 128. Locater sleeve 120 contains an explosive compounds such as a plastic explosive compound. The locater sleeve 120 positions the outlet ports at the desired cutoff point in pile 70. After head 121 lands in cone 75 pin 122 is sheared and lifting tool 87 is lowered until lugs 86c on the lifting tool are locked in the J-slots of the pile sleeve. This downward movement forces the explosive compound through the ports into direct contact with the inner wall of pile sleeve 32. Such telescopic movement also arms the unit for firing. The explosive compound is moved through outlet ports 128 as indicated in FIG. 32. Thereafter the drill pipe 124 is released from sub 123 above lifting tool 87. A similar operation is performed in the diagonally positioned pile sleeve, i.e. the pile sleeve is cleaned out and another locater sleeve containing explosive is run into that pile. Both the electric cable and the other cable 126 are connected at the surface to a second work boat.

Deballasting operations are halted. The drilling vessel is removed from the area. The deballasting is brought to a desired state. The charges are fired remotely from the work boats at short intervals. The released template is controlled by control lines and the work boats. As shown in FIG. 33 the template rises to the water surface. When template 10 is afloat all the valves are manually closed by divers and the template is towed to port.

Changes and modifications may be made in the specific illustrative embodiments of the invention shown and/or described herein without departing from the scope of the invention as defined in the appended claims. Thus, as mentioned previously, instead of four piles two or three piles or more than four piles may be used. In addition, the manner of ballasting and deballasting the manifold may vary according to desired operations. Further the manner of salvaging the template may be changed. All of the piles may be explosively cut or all may be mechanically cut.