EP0045653A1 - Mooring of floating structures - Google Patents
Mooring of floating structures Download PDFInfo
- Publication number
- EP0045653A1 EP0045653A1 EP19810303541 EP81303541A EP0045653A1 EP 0045653 A1 EP0045653 A1 EP 0045653A1 EP 19810303541 EP19810303541 EP 19810303541 EP 81303541 A EP81303541 A EP 81303541A EP 0045653 A1 EP0045653 A1 EP 0045653A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- mooring lines
- temporary mooring
- floating structure
- lines
- temporary
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- 238000007667 floating Methods 0.000 title claims abstract description 49
- 238000000034 method Methods 0.000 claims abstract description 22
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims abstract description 14
- 125000004122 cyclic group Chemical group 0.000 claims description 5
- 230000000712 assembly Effects 0.000 description 14
- 238000000429 assembly Methods 0.000 description 14
- 238000005553 drilling Methods 0.000 description 8
- 238000004458 analytical method Methods 0.000 description 4
- 239000012530 fluid Substances 0.000 description 4
- 239000003550 marker Substances 0.000 description 4
- 238000004519 manufacturing process Methods 0.000 description 3
- 239000013535 sea water Substances 0.000 description 3
- 238000006073 displacement reaction Methods 0.000 description 2
- 239000003643 water by type Substances 0.000 description 2
- 238000010276 construction Methods 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 230000005484 gravity Effects 0.000 description 1
- 238000009434 installation Methods 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 238000005086 pumping Methods 0.000 description 1
- 230000001105 regulatory effect Effects 0.000 description 1
- 238000007789 sealing Methods 0.000 description 1
- 238000006467 substitution reaction Methods 0.000 description 1
- 238000004804 winding Methods 0.000 description 1
Images
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
- B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING
- B63B21/00—Tying-up; Shifting, towing, or pushing equipment; Anchoring
- B63B21/50—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers
- B63B21/502—Anchoring arrangements or methods for special vessels, e.g. for floating drilling platforms or dredgers by means of tension legs
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B41/00—Equipment or details not covered by groups E21B15/00 - E21B40/00
- E21B41/0007—Equipment or details not covered by groups E21B15/00 - E21B40/00 for underwater installations
- E21B41/0014—Underwater well locating or reentry systems
-
- E—FIXED CONSTRUCTIONS
- E21—EARTH OR ROCK DRILLING; MINING
- E21B—EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS
- E21B41/00—Equipment or details not covered by groups E21B15/00 - E21B40/00
- E21B41/10—Guide posts, e.g. releasable; Attaching guide lines to underwater guide bases
Definitions
- This invention relates generally to the mooring of floating structures, and more particularly, but not by way of limitation, to the mooring of a floating structure utilized as an offshore oil and gas drilling and production platform.
- tension leg platform One particular problem which must be overcome with any design of tension leg platform is the manner in which the floating platform is attached to the subsea anchor, i.e. the manner of mooring. Particularly, when locating the platform in deep waters where severe environmental conditions are often present, such as for example in the North Sea, it is desirable that the actual mooring of the platform be accomplished in a relatively short time, e.g. a matter of hours, and that it be accomplished without the need for the use of divers.
- Both of the openings 82 and 41 of Blenkarn et al. include upwardly opening funnel shaped portions.
- the platform of Blenkarn et al. is moved into place with tug boats. After the risers are connected, the temporary mooring cables are released.
- references relating generally to tension leg platforms and/or the lowering of anchors or other objects from floating structures include:
- the invention provides a method of temporarily mooring a floating structure, comprising steps of:
- the invention provides a temporary mooring apparatus for a floating structure, comprising anchor means positioned on a floor of a body of water, a plurality of temporary mooring lines having first ends attached to said anchor means, a plurality of pendant lines connected to second ends of said temporary mooring lines, retrieval means attached to said floating structure for retrieving said plurality of pendant lines and a portion of each of said temporary mooring lines onto said floating structure until said floating structure is located approximately at a position directly above said anchor means; and tension means for tensioning said plurality of temporary mooring lines so that a tension load on each of said temporary mooring lines is greater than a magnitude of cyclic forces exerted on each of said temporary mooring lines.
- a plurality of temporary mooring lines are connected to a corresponding plurality of anchor assemblies.
- a plurality of pendant lines including marker buoys are connected to the temporary mooring lines and are laid on the ocean floor and arranged in a spread pattern.
- the tension leg platform is provided with a plurality of retrieving winches which are connected to the pendant lines to retrieve the pendant lines and a portion of each of the temporary mooring lines onto the floating structure until the tension leg platform is located approximately at a position directly above the anchors. Then the temporary mooring lines are tensioned to temporarily moor the tension leg platform.
- the permanent vertical tethering elements are connected between the tension leg platform and the anchor means by lowering those permanent tethering elements into engagement with connector means attached to the anchors.
- Guide funnels are attached to the anchors above the connector means for guiding the lower ends of the permanent tethering elements into engagement with the connector means.
- a tension leg platform which may be referred to as a floating structure, is generally designated by the numeral 10.
- the tension leg platform 10 is shown floating on a surface 12 of a body of water 14, and is temporarily moored to a floor 16 of the body of water 14 by a plurality of temporary mooring lines 18 connected to a plurality of separate anchor assemblies 20.
- the tension leg platform 10 includes a work deck 22 supported by a buoyant structure including vertical column members 24 and horizontal pontoon members 26.
- FIG. 1 is schematic only and that there are four auxiliary derricks 28, one located above each of the corner columns 24.
- main derrick 32 Also located upon work deck 22 is main derrick 32 which provides a means for performing drilling and production operations.
- FIG. 2 a plan view is thereshown of four anchor assemblies 20 located upon the ocean floor 16 in a predetermined pattern relative to a drilling template 34.
- the drilling template 34 and anchor assemblies 20 are positioned upon the ocean floor 16 prior to the use of the apparatus and method of the present invention for temporarily mooring the tension leg platform 10 to those anchors 20"
- F IG. 2 shows four temporary mooring lines 18, each of which has its first end 36 connected to one of the anchor assemblies 20.
- the temporary mooring lines are laid in a spread pattern upon the ocean floor 16.
- spread pattern it is meant that each of the temporary mooring lines 18 extends outward from its anchor assembly 20 so that second ends 38 of the temporary mooring lines 18 are spaced from each other.
- a plurality of pendant lines 40 are connected to the second ends 38 of the temporary mooring lines 18.
- FIG. 3 illustrates one of the temporary mooring lines 18, with a pendant line 40 attached thereto, being laid upon the ocean floor 16 by second floating structure 44 which is preferably a conventional drilling ship or the like.
- the temporary mooring lines 18' may be attached to the pendant lines 40 prior to the lowering of the temporary mooring lines 18 from the drilling ship 44 and prior to the attachment of the lower ends 36 of temporary mooring lines 18 to the anchor means 20.
- the temporary mooring lines 18 are preferably constructed from four or five-inch link chain having a conventional hydraulic actuated well head connector 46 attached to a lower end thereof.
- the well head connector 46 is actuated by a hydraulic signal transmitted by means of hydraulic lines 48.
- the anchor assembly 20 includes a standard well head type connection 50 for attachment to the well head connector 46.
- the temporary mooring line 18 is lowered by means of a drill string 52 from the drilling ship 44.
- the drill string 52 has. a cage 54 attached to a lower end thereof, which cage includes a bracket means 56 for supporting the lower end 36 of temporary mooring line 18.
- Cage 54 has an underwater television camera 58 disposed therein for observing the connection of temporary mooring line 18 to the anchor assembly 20.
- FIG. 10 illustrates the temporary mooring line 18 attached to the anchor assembly 20.
- the tension leg platform 10 is moved to a position suffi- ciently close to a position directly above anchor means 20 so that the pendant lines 40 may be connected to the tension leg platform 10. Then the pendant lines 40 are attached to a plurality of retrieval means 60 which are located upon the tension leg platform 10.
- the retrieval means 60 preferably includes four separate winches 60.
- Each of the winches 60 includes a conventional drum portion for winding one of pendant lines 40 thereon, and a conventional windlass portion for retrieving one of the mooring lines 18.
- a chain locker is located below the windlass for receiving the mooring line 18 which is a link chain.
- each of those corner column members includes three hause pipes 62 which extend vertically therethrough so that a permanent tethering element 30 may be lowered through each of the hause pipes 62.
- the pendant lines 40 are placed through one of the hause pipes 62 of each of the corner columns 24 and the pendant lines 40 are then attached to the winches 60.
- the view shown in FIG. 4 may be considered to show either the pendant lines 40 or the temporary mooring lines 18 attached to each of the winches 60, and this is indicated by the use of double designations 40, 18 on each of the lines attached to the winches 60.
- the winches 60 are actuated to retrieve the pendant lines, 40 and a portion of each of the temporary mooring lines 18 onto the winches 60 of the tension leg platform 10 until the tension leg platform 10 is located approximately at a position directly above the anchor means 20, such as is shown in FIG. 1.
- the temporary mooring lines 18 are tensioned so that a tension load on each of said temporary mooring lines 18 is greater than a magnitude of cyclic forces exerted on each of said temporary mooring lines 18, thereby preventing any snap loads on the temporary mooring lines 18 which would otherwise occur upon the temporary mooring lines 18 becoming slack.
- the connecting of the temporary mooring lines 18 to anchor means 20, connecting of pendant lines 40 to temporary mooring lines 18, and connecting of pendant lines 40 to retrieval means 60 may be accomplished without the second vessel 44, although the method described above using second vessel 44 is preferred.
- a temporary mooring line 18 could be lowered from the main derrick 32 of platform 10 into engagement with anchor means 20. Then, while the temporary mooring line 18 is supported from main derrick 32, one of the pendant lines 40 could be lowered from one of the auxiliary derricks 28 through one of the hause pipes 62, and its lower end could be attached to the mooring line 18 at connection 38. Then the temporary mooring line 18 is released from main derrick 32, and the pendant line 40 and temporary mooring line 18 are retrieved through the hause pipe 62 on one of the winch means 60.
- the tensioning of the temporary mooring lines 18 may be accomplished in several different ways.
- One manner of tensioning the temporary mooring lines 18 is by the use of a heave compensator 64 engaged with each of the temporary mooring lines 18.
- a heave compensator 64 is shown schematically in FIG. 5 and the position of the heave compensator 64 is also illustrated in FIG. 1.
- the heave compensator 64 includes a hydraulic cylinder 66 having a piston 68 reciprocably disposed therein.
- a rod 70 is attached to piston 68 and has a guide sheave 72 rotatingly attached thereto.
- the temporary mooring line 18 engages guide sheave 72.
- a constant hydraulic pressure is applied to a lower surface 74 of piston 66 from a pressure chamber 76 defined within cylinder 66 below piston 68. Constant pressure hydraulic fluid is supplied to the chamber 76 through a conduit 78 from a pressure transfer cylinder 80.
- pressure transfer cylinder 80 Disposed within pressure transfer cylinder 80 is a floating piston 82 which divides pressure transfer cylinder 80 into a hydraulic fluid chamber 84 and a primary pressure chamber 86..
- the primary pressure chamber 86 is connected to a pressure source 88 by conduit 90 which has a pressure regulator 92 ' disposed therein.
- the pressure source 88 is preferably a source of gas under pressure
- the pressure regulator 92 provides a means for regulating the pressure of the gas within the primary pressure chamber 86 of pressure transfer cylinder 80.
- the pressure of the gas within primary pressure chamber 86 is transmitted to the hydraulic fluid in hydraulic pressure chamber 84 by the floating piston 82.
- This provides a means for applying a constant hydraulic pressure to the piston 68 of cylinder 66 of heave compensator 64, and for varying that constant hydraulic pressure to increase or decrease the same to correspondingly increase or decrease the tension applied to the temporary mooring line 13 by the heave compensator 64.
- Another method of tensioning the temporary mooring lines 18 is to lock each of the temporary mooring lines 18 to the tension leg platform 10 with a conventional chainstopper (not shown) to fix the length thereof, and then to deballast the tension leg platform 10 in a manner well known to those skilled in the art, to increase the buoyancy thereof and thereby increase the tension applied to the temporary mooring lines 18.
- Yet another manner of tensioning the temporary mooring lines 18 is to construct the winches 60 of sufficient capacity so that they may apply the desired tension to the temporary mooring lines 18.
- a solution to this problem is provided by applying a horizontal force to the tension leg platform 10 to move it horizontally away from the position directly above the anchor means 20 by a distance sufficient to apply a temporary tension load to each of the temporary mooring lines 18 great enough to prevent snap loads from being imposed upon the temporary mooring lines 18 due to the cyclical forces of the waves acting upon the tension leg platform 10.
- This horizontal force is preferably applied to the tension leg platform 10 by the use of a conventional tug boat.
- the temporary mooring lines 18 may be permanently tensioned in one of the three manners described above, or in a similar manner, while the horizontal force is maintained upon the tension leg platform 10 by the tug boat. In that manner the permanent tensioning can be accomplished without allowing any snap loads to be applied to the temporary mooring lines 18 during the permanent tensioning.
- F IG. 6 illustrates schematically the forces acting upon tension leg platform 10 when a horizontal force H is applied thereto.
- a tugboat 93 is schematically illustrated as applying the force H.
- Horizontal forces may also be present due to tides, wide currents and the like.
- the non-displaced position of the tension leg platform 10 is shown in phantom lines, and the displaced position of tension leg platform 10 is shown in solid lines, with the platform 10 displaced through a distance X due to the horizontal force H represented by the vector 94.
- Table 1 was prepared from the following analysis of the forces illustrated in FIG. 6. By summing the horizontal and vertical forces acting upon the tension leg platform 10, the following equations 1 and 2, respectively, are obtained.
- a tan -1 x
- T o excess buoyancy
- a wp water plane area
- ⁇ specific weight of sea water (1.026 tonnes/m 3 )
- L 372 ft
- the angle by which the temporary mooring lines 18 are displaced from a vertical position is represented by the symbol ⁇ .
- the excess buoyancy T o is' the weight of water displaced by the tension leg platform 10 in excess of the weight of the tension leg platform 10.
- the water plane area, A is the horizontal area of the tension leg platform 10 at an imaginary horizontal section therethrough at the surface 12 of the body of water 14.
- the specific weight of sea water is represented by the symbol Y.
- the length of the temporary mooring lines 18 for the specific embodiment of tension leg platform 10 for which the calculations and tables 1 and 2 were made, which was based on a design specifically made for use in the Hutton field of the North Sea where the water depth is 485 feet, is given as 372 feet.
- the forces listed in Tables 1 and 2 are in metric tonnes.
- FIG. 7 schematically represents the forces acting upon tension leg platform 10 when the temporary mooring lines 18 are ten feet longer than the desired length. This illustrates the horizontal forces required to pre-tension the temporary mooring lines 18 when the tension leg platform 10 is initially located approximately above the anchor means 20, but not exactly directly above the anchor means 20. It will be understood that the analysis and discussion with regard to FIG. 7 is merely by way of example to illustrate the forces required to pre-tension the temporary mooring lines 18 with a less than perfect initial positioning of the platform 10. These figures are given because it is very possible that the initial positioning of the tension leg platform 10 prior to the permanent tension of the temporary mooring lines 18 will be such that there will be some slight initial offset.
- the initial offset required to straighten the temporary mooring lines 18 is represented by the designation X 0 and is obtained in the following manner:
- the initial angle of-the temporary mooring lines 18 from the vertical is designated as ⁇ o .
- the numbers from Tables 1 and 2 illustrate the .horizontal force H required to achieve a given horizontal displacement X in feet as shown in the left hand column of the tables, for a given excess buoyancy T o listed in the top row of each table.
- the pre-tension force T in the temporary mooring lines 18 corresponding to the horizontal displacement X may be determined by the relationships given with regard to FIGS. 6 and 7.
- the corresponding offset X may be determined from Equation 2 for FIG. 6 and Equation 4 for FIG. 7, and the known relationship between X and a. That value of X may be used to enter Table 1 or 2, and depending upon the value of To for the specific platform under consideration the value of H is shown in the tables.
- the tension leg platform 10 may be permanently moored by attaching a plurality of permanent vertical mooring elements, such as 30, between the tension leg platform 10 and the anchors 20 while the tension leg platform 10 is temporarily moored. This is preferably accomplished in the following manner.
- the following method is particularly useful with a plurality of separate anchor assemblies 20 as disclosed herein, because it provides a means for maneuvering the permanent tethering elements 30 as they are lowered'into engagement with the anchor assemblies 20. This is desirable because of inherent inaccuracies in the positioning of the anchor assemblies 20 upon the ocean floor 16.
- FIG. 8 a plan view is thereshown of one of the anchor assemblies 20.
- the anchor assembly 20 includes three separate connector means 100, 102 and 104, for connecting three of the permanent tethering elements 30 to the anchor 20.
- Each of the guide means 106, 108 and 110 are provided with label indicia means 112, 114 and 116, respectively, so that a proper one of said guide funnels to be engaged by a given one of the permanent vertical tethering elements 30 may be determined by visually observing the guide funnels.
- the indicia means 112 of guide funnel 106 includes the number 1 and a single stripe encircling the connector means 100.
- indicia means 114 includes the numeral 2 and two stripes.
- the permanent vertical tethering elements 30 are lowered from the tension leg platform 10, as is shown in FIGS. 1 and 10, until a lower end 118 of the tethering element 30 is located a relatively short distance above the anchor means 20.
- An underwater television camera is then used to observe the location of the lower end 118 of permanent vertical tethering element 30 relative to the appropriate one of the guide funnels above the appropriate connector means to which it is to be attached. For example, referring to FIG. 10, if the tethering element 30 thereshown is desired to be connected to the connector 102, the tethering element 30 should be located above the guide funnel 108.
- FIG. 1 illustrates a remote controlled vehicle 120 which is connected to the tension leg platform 10 by a command cable 122 and within which is disposed a television camera 124.
- the location of the remote control vehicle 120 within the body of water 114 is controlled by a plurality of thrusting propellers such as 126 which operate in response to signals conveyed down the cable 122.
- FIG. 12 An alternative is shown in FIG. 12, where a.television camera 128 is disposed in an inner passageway 129 of the permanent vertical tethering element 30 so that the television camera 128 looks downward below the tethering element 30.
- the camera 128 may be retrieved after the tethering element 30 is installed.
- the location of the lower end 118 of the permanent vertical tethering element relative to the anchor assemblies 20 may be_observed.
- the lower end 118 of the tethering element 30 may be moved to a position directly above the guide funnel above the connector to which it is to be attached. This can be accomplished by either maneuvering the tension leg platform 10 by applying a lateral force thereto with one or more tug boats, or by rotating the tethering element 30 until a thrusting means 130 thereof is properly directed for moving the lower end 118 in a direction toward a position directly above the guide funnel to which it is desirably attached.
- FIG. 11 illustrates a permanent vertical tethering element 30 with a portion thereof cut away to reveal a plug 132 sealing the inner passageway 129 below the thrusting means 130.
- the thrust means 130 is actuated by pumping a liquid down the inner passage 129 and out the thrust means 130, which is merely a radially directed orifice, as indicated by the jet of fluid 134.
- the rotation of the tethering element 30 may be accomplished manually if the tethering element is suspended from derrick 28 on a swivel.
- the lower end 118 of the permanent vertical tethering element 30 is preferably a standard hydraulically actuated wellhead type connector, and the connector means 110, 102 and 104 are each preferably a standard wellhead.
- the final connection is made by stabbing the lower end 118 of the permanent vertical tethering element 30 into the guide funnel 108.
- the guide funnel 108 guides the lower end 113 of the permanent vertical tethering element 30 into engagement with the connector means 102 and the connection therebetween is accomplished by the conventional hydraulic actuator.
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Abstract
- (a) connecting first ends (36) of a plurality of temporary mooring lines (18) to anchor means (20) positioned on a floor (16) of a body of water (14);
- (b) connecting a plurality of pendant lines (40) to second ends (38) of said temporary mooring lines;
- (c) connecting said pendant lines to retrieval means (60) attached to said floating structure;
- (d) retrieving said pendant lines and a portion of each of said temporary mooring lines onto said floating structure until said floating structure is located approximately at a position directly above said anchor means; and
- (e) tensioning said temporary mooring lines.
Description
- This invention relates generally to the mooring of floating structures, and more particularly, but not by way of limitation, to the mooring of a floating structure utilized as an offshore oil and gas drilling and production platform.
- As offshore exploration for oil and gas from subsea deposits has expanded into deeper and deeper waters, conventional rigid towers setting upon the ocean floor and extending upward to the surface have become more and more impractical.
- One particular solution to this problem is the elimination of the rigid tower and the substitution therefor of a floating platform moored to the ocean floor by a plurality of vertical members which are placed under high tension loads due to excess, buoyancy of the floating platform. Examples of such structures, which are generally referred to as tension leg platforms, are shown in U.S. Patent No. 3,648,638 to Blenkarn and U.S. Patent No. 3,919,957 to Ray, et al.
- One particular problem which must be overcome with any design of tension leg platform is the manner in which the floating platform is attached to the subsea anchor, i.e. the manner of mooring. Particularly, when locating the platform in deep waters where severe environmental conditions are often present, such as for example in the North Sea, it is desirable that the actual mooring of the platform be accomplished in a relatively short time, e.g. a matter of hours, and that it be accomplished without the need for the use of divers.
- One manner of achieving these ends is suggested by U.S. Patent No. 3,919,957 to Ray, et al. and U.S. Patent No. 3,982,492 to . Steddum. Both of those references, which disclose substantially the same structure, use dead weight anchors which are lowered from the floating platform to the ocean floor by the vertical tethering elements.
- Another system is proposed in U.S. Patent No. 3,976,021 to Blenkarn et al. and in U.S. Patent No. 4,062,313 to Stram, wherein a gravity base is lowered on temporary mooring cables from the floating platform and is then attached to the ocean floor by a plurality of piles. Then the permanent tethering elements are lowered from the floating platform and attached to the anchors. The path of the permanent tethering elements as they are lowered into engagement with the anchors is defined by a plurality of guide members, having
guide openings 82 such as shown in FIG. 12 of Blenkarn et al., and vertical guide passages 41 located upon the anchor as shown in FIG. 9 of Blenkarn et al. Both of theopenings 82 and 41 of Blenkarn et al. include upwardly opening funnel shaped portions. The platform of Blenkarn et al. is moved into place with tug boats. After the risers are connected, the temporary mooring cables are released. - Yet another manner of solving the problem of connecting the permanent tethering elements between the anchor and the floating platform is to manufacture the permanent tethering elements integrally with the platform and then attach the tethering elements and the platform to the anchor in one step as is disclosed in U.S. Patent No. 3,611,734 to Mott.
- In U.S. Patent No. 3,955,521 to Mott, individual tethering elements are lowered into engagement with pre-set anchor piles.
-
- Viewed from one aspect the invention provides a method of temporarily mooring a floating structure, comprising steps of:
- (a) connecting first ends of a plurality of temporary mooring lines to anchor means positioned on a floor of a body of water;
- (b) connecting a plurality of pendant lines to second ends of said temporary mooring lines;
- (c) connecting said pendant lines to retrieval means attached to said floating structure;
- (d) retrieving said pendant lines and a portion of each of said temporary mooring lines onto said floating structure until said floating structure is located approximately at a position directly above said anchor means; and
- (e) tensioning said temporary mooring lines.
- Viewed from another aspect the invention provides a temporary mooring apparatus for a floating structure, comprising anchor means positioned on a floor of a body of water, a plurality of temporary mooring lines having first ends attached to said anchor means, a plurality of pendant lines connected to second ends of said temporary mooring lines, retrieval means attached to said floating structure for retrieving said plurality of pendant lines and a portion of each of said temporary mooring lines onto said floating structure until said floating structure is located approximately at a position directly above said anchor means; and tension means for tensioning said plurality of temporary mooring lines so that a tension load on each of said temporary mooring lines is greater than a magnitude of cyclic forces exerted on each of said temporary mooring lines.
- In a preferred method according to the invention for temporarily mooring a tension leg platform, a plurality of temporary mooring lines are connected to a corresponding plurality of anchor assemblies. A plurality of pendant lines including marker buoys are connected to the temporary mooring lines and are laid on the ocean floor and arranged in a spread pattern. The tension leg platform is provided with a plurality of retrieving winches which are connected to the pendant lines to retrieve the pendant lines and a portion of each of the temporary mooring lines onto the floating structure until the tension leg platform is located approximately at a position directly above the anchors. Then the temporary mooring lines are tensioned to temporarily moor the tension leg platform. While the tension leg platform is temporarily moored, the permanent vertical tethering elements are connected between the tension leg platform and the anchor means by lowering those permanent tethering elements into engagement with connector means attached to the anchors. Guide funnels are attached to the anchors above the connector means for guiding the lower ends of the permanent tethering elements into engagement with the connector means.
- An embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which:-
- FIG. 1 is a schematic elevation view of a tension leg platform temporarily moored to the ocean floor.
- FIG. 2 is a schematic plan view of a plurality of temporary mooring lines laid in a spread pattern upon the ocean floor.
- FIG. 3 is a schematic elevation view of a conventional drilling ship laying the temporary mooring lines with pendant lines attached thereto.
- FIG. 4 is a schematic plan view of the work deck of the tension leg platform of FIG. 1, showing winches used for retrieving the pendant lines and temporary mooring lines.
- FIG. 5 is a schematic illustration of a heave compensator utilized with the temporary mooring lines.
- FIG. 6 is a schematic illustration, corresponding to Table 1 of the disclosure, which relates.to horizontal forces required to displace the tension leg platform from a position directly above the anchor means.
- FIG. 7 is a view similar to FIG. 6, and corresponding to Table 2, which relates to horizontal forces required to displace the tension leg platform when the temporary mooring cables have a length longer than a desired length thereof.
- FIG. 8 is a plan view of one of the anchor assemblies to which the tension leg platform is anchored.
- FIG. 9 is a schematic elevation view showing the manner in which a temporary mooring line is attached to an anchor assembly.
- FIG. 10 is a schematic elevation view showing the manner in which a permanent tethering element is attached to an anchor assembly.
- FIG. 11 is a schematic elevation view of a jet thruster means incorporated in a permanent tethering element.
- FIG. 12 is a schematic elevation view of a permanent tethering element having a television camera located in an inner passage thereof.
- Referring now to the drawings, and particularly to FIG. 1, a tension leg platform, which may be referred to as a floating structure, is generally designated by the
numeral 10. Thetension leg platform 10 is shown floating on asurface 12 of a body ofwater 14, and is temporarily moored to afloor 16 of the body ofwater 14 by a plurality oftemporary mooring lines 18 connected to a plurality ofseparate anchor assemblies 20. - The manner of construction and installation of the
anchor assemblies 20 is disclosed in detail in our European Patent Application entitled "Underwater Anchor Assembly" filed concurrently herewith. Thetension leg platform 10 includes awork deck 22 supported by a buoyant structure includingvertical column members 24 andhorizontal pontoon members 26. - Located above each of the four
corner columns 24 is anauxiliary derrick 28 which provides a means for lowering permanent tethering elements, such as the one designated by thenumeral 30 in FIG. 1, through thecorner columns 24. It will be understood that FIG. 1 is schematic only and that there are fourauxiliary derricks 28, one located above each of thecorner columns 24. - Also located upon
work deck 22 ismain derrick 32 which provides a means for performing drilling and production operations. - Referring now to FIG. 2, a plan view is thereshown of four
anchor assemblies 20 located upon theocean floor 16 in a predetermined pattern relative to adrilling template 34. Thedrilling template 34 andanchor assemblies 20 are positioned upon theocean floor 16 prior to the use of the apparatus and method of the present invention for temporarily mooring thetension leg platform 10 to thoseanchors 20" - FIG. 2 shows four
temporary mooring lines 18, each of which has itsfirst end 36 connected to one of theanchor assemblies 20. The temporary mooring lines are laid in a spread pattern upon theocean floor 16. By "spread" pattern, it is meant that each of thetemporary mooring lines 18 extends outward from itsanchor assembly 20 so thatsecond ends 38 of thetemporary mooring lines 18 are spaced from each other. - As can be seen in FIG. 3, a plurality of
pendant lines 40, each of which includes amarker buoy 42, are connected to thesecond ends 38 of thetemporary mooring lines 18. - The left hand side of FIG. 3 illustrates one of the
temporary mooring lines 18, with apendant line 40 attached thereto, being laid upon theocean floor 16 by second floatingstructure 44 which is preferably a conventional drilling ship or the like. - It will be understood that the temporary mooring lines 18' may be attached to the
pendant lines 40 prior to the lowering of thetemporary mooring lines 18 from thedrilling ship 44 and prior to the attachment of thelower ends 36 oftemporary mooring lines 18 to the anchor means 20. - The manner in which the
temporary mooring lines 18 are attached to theanchor assemblies 20 is best illustrated in FIG. 9. - The
temporary mooring lines 18 are preferably constructed from four or five-inch link chain having a conventional hydraulic actuated well head connector 46 attached to a lower end thereof. The well head connector 46 is actuated by a hydraulic signal transmitted by means ofhydraulic lines 48. - The
anchor assembly 20 includes a standard wellhead type connection 50 for attachment to the well head connector 46. - The
temporary mooring line 18 is lowered by means of adrill string 52 from thedrilling ship 44. Thedrill string 52 has. acage 54 attached to a lower end thereof, which cage includes a bracket means 56 for supporting thelower end 36 oftemporary mooring line 18.Cage 54 has anunderwater television camera 58 disposed therein for observing the connection oftemporary mooring line 18 to theanchor assembly 20. - The right hand side of FIG. 10 illustrates the
temporary mooring line 18 attached to theanchor assembly 20. - After all of the
temporary mooring lines 18 have been attached to anchorassemblies 20 and laid upon the ocean floor as shown in FIG. 2, withpendant lines 40 and marker buoys 42 attached thereto as shown in FIG. 3, the system is ready for the arrival of thetension leg platform 10 and the attachment of thetension leg platform 10 to the temporary mooring lines 18. This is accomplished as follows. - The
tension leg platform 10 is moved to a position suffi- ciently close to a position directly above anchor means 20 so that thependant lines 40 may be connected to thetension leg platform 10. Then thependant lines 40 are attached to a plurality of retrieval means 60 which are located upon thetension leg platform 10. - As is best shown in FIG. 4, which is a schematic plan view of the
work deck 22 oftension leg platform 10, the retrieval means 60 preferably includes fourseparate winches 60. Each of thewinches 60 includes a conventional drum portion for winding one ofpendant lines 40 thereon, and a conventional windlass portion for retrieving one of the mooring lines 18. A chain locker is located below the windlass for receiving themooring line 18 which is a link chain. - Of the
vertical column members 24 oftension leg platform 10, there are four of the vertical column members located at the corners ofwork deck 22 which may be referred to ascolumn members 24. Each of those corner column members includes threehause pipes 62 which extend vertically therethrough so that apermanent tethering element 30 may be lowered through each of thehause pipes 62. - When the marker buoys 42 are retrieved, the
pendant lines 40 are placed through one of thehause pipes 62 of each of thecorner columns 24 and thependant lines 40 are then attached to thewinches 60. - The view shown in FIG. 4 may be considered to show either the
pendant lines 40 or thetemporary mooring lines 18 attached to each of thewinches 60, and this is indicated by the use ofdouble designations winches 60. - The
winches 60 are actuated to retrieve the pendant lines, 40 and a portion of each of thetemporary mooring lines 18 onto thewinches 60 of thetension leg platform 10 until thetension leg platform 10 is located approximately at a position directly above the anchor means 20, such as is shown in FIG. 1. - Then the
temporary mooring lines 18 are tensioned so that a tension load on each of saidtemporary mooring lines 18 is greater than a magnitude of cyclic forces exerted on each of saidtemporary mooring lines 18, thereby preventing any snap loads on thetemporary mooring lines 18 which would otherwise occur upon thetemporary mooring lines 18 becoming slack. - Such cyclic loads would be imposed by the undulating motion of the
tension leg platform 10 due to waves and the like acting thereupon. - Before describing the manner in which the
temporary mooring lines 18 are tensioned, it is noted that the connecting of thetemporary mooring lines 18 to anchor means 20, connecting ofpendant lines 40 totemporary mooring lines 18, and connecting ofpendant lines 40 to retrieval means 60 may be accomplished without thesecond vessel 44, although the method described above usingsecond vessel 44 is preferred. For example, atemporary mooring line 18 could be lowered from themain derrick 32 ofplatform 10 into engagement with anchor means 20. Then, while thetemporary mooring line 18 is supported frommain derrick 32, one of thependant lines 40 could be lowered from one of theauxiliary derricks 28 through one of thehause pipes 62, and its lower end could be attached to themooring line 18 atconnection 38. Then thetemporary mooring line 18 is released frommain derrick 32, and thependant line 40 andtemporary mooring line 18 are retrieved through thehause pipe 62 on one of the winch means 60. , - The tensioning of the
temporary mooring lines 18 may be accomplished in several different ways. - One manner of tensioning the
temporary mooring lines 18 is by the use of aheave compensator 64 engaged with each of the temporary mooring lines 18. Such aheave compensator 64 is shown schematically in FIG. 5 and the position of theheave compensator 64 is also illustrated in FIG. 1. - The
heave compensator 64 includes ahydraulic cylinder 66 having a piston 68 reciprocably disposed therein. Arod 70 is attached to piston 68 and has aguide sheave 72 rotatingly attached thereto. Thetemporary mooring line 18 engagesguide sheave 72. - A constant hydraulic pressure is applied to a
lower surface 74 ofpiston 66 from apressure chamber 76 defined withincylinder 66 below piston 68. Constant pressure hydraulic fluid is supplied to thechamber 76 through a conduit 78 from apressure transfer cylinder 80. - Disposed within
pressure transfer cylinder 80 is a floatingpiston 82 which dividespressure transfer cylinder 80 into ahydraulic fluid chamber 84 and aprimary pressure chamber 86.. - The
primary pressure chamber 86 is connected to apressure source 88 byconduit 90 which has a pressure regulator 92 ' disposed therein. - The
pressure source 88 is preferably a source of gas under pressure, and thepressure regulator 92 provides a means for regulating the pressure of the gas within theprimary pressure chamber 86 ofpressure transfer cylinder 80. The pressure of the gas withinprimary pressure chamber 86 is transmitted to the hydraulic fluid inhydraulic pressure chamber 84 by the floatingpiston 82. This provides a means for applying a constant hydraulic pressure to the piston 68 ofcylinder 66 ofheave compensator 64, and for varying that constant hydraulic pressure to increase or decrease the same to correspondingly increase or decrease the tension applied to the temporary mooring line 13 by theheave compensator 64. - Another method of tensioning the
temporary mooring lines 18 is to lock each of thetemporary mooring lines 18 to thetension leg platform 10 with a conventional chainstopper (not shown) to fix the length thereof, and then to deballast thetension leg platform 10 in a manner well known to those skilled in the art, to increase the buoyancy thereof and thereby increase the tension applied to the temporary mooring lines 18. - Yet another manner of tensioning the
temporary mooring lines 18 is to construct thewinches 60 of sufficient capacity so that they may apply the desired tension to the temporary mooring lines 18. - Another problem which is sometimes encountered, while connecting the
temporary mooring lines 18 to thetension leg platform 10, is that wave motion acting upon thetension leg platform 10 causes thetension leg platform 10 to undulate thereby possibly applying snap loads totemporary mooring lines 18 before they can be tensioned in one of the manners just described above. ' - A solution to this problem is provided by applying a horizontal force to the
tension leg platform 10 to move it horizontally away from the position directly above the anchor means 20 by a distance sufficient to apply a temporary tension load to each of thetemporary mooring lines 18 great enough to prevent snap loads from being imposed upon thetemporary mooring lines 18 due to the cyclical forces of the waves acting upon thetension leg platform 10. This horizontal force is preferably applied to thetension leg platform 10 by the use of a conventional tug boat. Then, thetemporary mooring lines 18 may be permanently tensioned in one of the three manners described above, or in a similar manner, while the horizontal force is maintained upon thetension leg platform 10 by the tug boat. In that manner the permanent tensioning can be accomplished without allowing any snap loads to be applied to thetemporary mooring lines 18 during the permanent tensioning. - The feasibility of such temporary tensioning can be appreciated in view of the following analysis with respect to FIGS. 6 and 7.
- FIG. 6 illustrates schematically the forces acting upon
tension leg platform 10 when a horizontal force H is applied thereto. Atugboat 93 is schematically illustrated as applying the force H. Horizontal forces may also be present due to tides, wide currents and the like. The non-displaced position of thetension leg platform 10 is shown in phantom lines, and the displaced position oftension leg platform 10 is shown in solid lines, with theplatform 10 displaced through a distance X due to the horizontal force H represented by thevector 94. -
- Table 1 was prepared from the following analysis of the forces illustrated in FIG. 6. By summing the horizontal and vertical forces acting upon the
tension leg platform 10, the followingequations 1 and 2, respectively, are obtained. - The angle by which the
temporary mooring lines 18 are displaced from a vertical position is represented by the symbol α. The excess buoyancy To, is' the weight of water displaced by thetension leg platform 10 in excess of the weight of thetension leg platform 10. The water plane area, A , is the horizontal area of thetension leg platform 10 at an imaginary horizontal section therethrough at thesurface 12 of the body ofwater 14. The specific weight of sea water is represented by the symbol Y. The length of thetemporary mooring lines 18 for the specific embodiment oftension leg platform 10 for which the calculations and tables 1 and 2 were made, which was based on a design specifically made for use in the Hutton field of the North Sea where the water depth is 485 feet, is given as 372 feet. The forces listed in Tables 1 and 2 are in metric tonnes. - Similarly, FIG. 7 schematically represents the forces acting upon
tension leg platform 10 when thetemporary mooring lines 18 are ten feet longer than the desired length. This illustrates the horizontal forces required to pre-tension thetemporary mooring lines 18 when thetension leg platform 10 is initially located approximately above the anchor means 20, but not exactly directly above the anchor means 20. It will be understood that the analysis and discussion with regard to FIG. 7 is merely by way of example to illustrate the forces required to pre-tension thetemporary mooring lines 18 with a less than perfect initial positioning of theplatform 10. These figures are given because it is very possible that the initial positioning of thetension leg platform 10 prior to the permanent tension of thetemporary mooring lines 18 will be such that there will be some slight initial offset. -
- The data in Table 2 is obtained from FIG. 7 by the following analysis. Horizontal and vertical forces acting upon the
tension leg platform 10 are summed to given the followingequations 3 and 4, respectively: -
- The initial angle of-the
temporary mooring lines 18 from the vertical is designated as αo. - These numbers given in Tables 1 and 2 illustrate the feasibility of utilizing conventional tug boats to achieve this pre-tensioning of the temporary mooring lines 18. Tug boats such as generally used in the North Sea can produce on the order of 50 to 100 metric tonnes of thrust.
- The numbers from Tables 1 and 2 illustrate the .horizontal force H required to achieve a given horizontal displacement X in feet as shown in the left hand column of the tables, for a given excess buoyancy To listed in the top row of each table. The pre-tension force T in the
temporary mooring lines 18 corresponding to the horizontal displacement X may be determined by the relationships given with regard to FIGS. 6 and 7. - For any desired pre-tension T, the corresponding offset X may be determined from Equation 2 for FIG. 6 and
Equation 4 for FIG. 7, and the known relationship between X and a. That value of X may be used to enter Table 1 or 2, and depending upon the value of To for the specific platform under consideration the value of H is shown in the tables. - These values of H, particularly as shown in Table 1 where there is no offset of the platform, are generally on the same order of magnitude as the thrust which may be provided by a typical North Sea tugboat, e.g. 50 to 100 tonnes so that it is feasible to supply the necessary
horizontal force 14 by the use of a reasonable number of tugboats. - After all four of the temporary mooring lines have been connected between the
tension leg platform 10 and theseparate anchors 20, thetension leg platform 10 may be permanently moored by attaching a plurality of permanent vertical mooring elements, such as 30, between thetension leg platform 10 and theanchors 20 while thetension leg platform 10 is temporarily moored. This is preferably accomplished in the following manner. - The following method is particularly useful with a plurality of
separate anchor assemblies 20 as disclosed herein, because it provides a means for maneuvering thepermanent tethering elements 30 as they are lowered'into engagement with theanchor assemblies 20. This is desirable because of inherent inaccuracies in the positioning of theanchor assemblies 20 upon theocean floor 16. - Referring now to FIG. 8, a plan view is thereshown of one of the
anchor assemblies 20. - The
anchor assembly 20 includes three separate connector means 100, 102 and 104, for connecting three of thepermanent tethering elements 30 to theanchor 20. - Located above the connector means 100, 102 and 104 are a plurality of upward opening funnel shaped guide means 106, 108 and 110, respectively.
- Each of the guide means 106, 108 and 110 are provided with label indicia means 112, 114 and 116, respectively, so that a proper one of said guide funnels to be engaged by a given one of the permanent
vertical tethering elements 30 may be determined by visually observing the guide funnels. For example, the indicia means 112 of guide funnel 106 includes thenumber 1 and a single stripe encircling the connector means 100. Similarly, indicia means 114 includes the numeral 2 and two stripes. - The permanent
vertical tethering elements 30 are lowered from thetension leg platform 10, as is shown in FIGS. 1 and 10, until alower end 118 of thetethering element 30 is located a relatively short distance above the anchor means 20. - An underwater television camera is then used to observe the location of the
lower end 118 of permanentvertical tethering element 30 relative to the appropriate one of the guide funnels above the appropriate connector means to which it is to be attached. For example, referring to FIG. 10, if thetethering element 30 thereshown is desired to be connected to theconnector 102, thetethering element 30 should be located above theguide funnel 108. - The television camera may be located in one of two places. FIG. 1 illustrates a remote controlled
vehicle 120 which is connected to thetension leg platform 10 by acommand cable 122 and within which is disposed atelevision camera 124. The location of theremote control vehicle 120 within the body ofwater 114 is controlled by a plurality of thrusting propellers such as 126 which operate in response to signals conveyed down thecable 122. - An alternative is shown in FIG. 12, where
a.television camera 128 is disposed in aninner passageway 129 of the permanentvertical tethering element 30 so that thetelevision camera 128 looks downward below thetethering element 30. Thecamera 128 may be retrieved after thetethering element 30 is installed. - By either of these means, the location of the
lower end 118 of the permanent vertical tethering element relative to theanchor assemblies 20 may be_observed. - Then, if necessary, the
lower end 118 of thetethering element 30 may be moved to a position directly above the guide funnel above the connector to which it is to be attached. This can be accomplished by either maneuvering thetension leg platform 10 by applying a lateral force thereto with one or more tug boats, or by rotating thetethering element 30 until a thrusting means 130 thereof is properly directed for moving thelower end 118 in a direction toward a position directly above the guide funnel to which it is desirably attached. - Such a thrusting
means 130 is schematically illustrated in FIG. 11. FIG. 11 illustrates a permanentvertical tethering element 30 with a portion thereof cut away to reveal aplug 132 sealing theinner passageway 129 below the thrustingmeans 130. The thrust means 130 is actuated by pumping a liquid down theinner passage 129 and out the thrust means 130, which is merely a radially directed orifice, as indicated by the jet offluid 134. The rotation of thetethering element 30 may be accomplished manually if the tethering element is suspended fromderrick 28 on a swivel. - When using the embodiment of FIG. 11 with the thruster means 130, it is necessary to use the remote controlled
vehicle 120 and itscamera 124, rather than a camera disposed within thetethering element 30 as shown in FIG. 12. - The
lower end 118 of the permanentvertical tethering element 30 is preferably a standard hydraulically actuated wellhead type connector, and the connector means 110, 102 and 104 are each preferably a standard wellhead. - The final connection is made by stabbing the
lower end 118 of the permanentvertical tethering element 30 into theguide funnel 108. Theguide funnel 108 guides the lower end 113 of the permanentvertical tethering element 30 into engagement with the connector means 102 and the connection therebetween is accomplished by the conventional hydraulic actuator.
Claims (16)
Applications Claiming Priority (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US175101 | 1980-08-04 | ||
US06/175,101 US4352599A (en) | 1980-08-04 | 1980-08-04 | Permanent mooring of tension leg platforms |
US06/180,767 US4354446A (en) | 1980-08-22 | 1980-08-22 | Temporary mooring of tension leg platforms |
US180767 | 1980-08-22 |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0045653A1 true EP0045653A1 (en) | 1982-02-10 |
EP0045653B1 EP0045653B1 (en) | 1985-04-10 |
Family
ID=26870864
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP19810303541 Expired EP0045653B1 (en) | 1980-08-04 | 1981-08-03 | Mooring of floating structures |
Country Status (2)
Country | Link |
---|---|
EP (1) | EP0045653B1 (en) |
DE (1) | DE3169824D1 (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO1996040548A1 (en) * | 1995-06-07 | 1996-12-19 | Aker Engineering A.S | A method for installing a tension leg platform |
CN113212657A (en) * | 2021-05-26 | 2021-08-06 | 中国能源建设集团广东省电力设计研究院有限公司 | Self-installation offshore platform |
Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB1140796A (en) * | 1966-02-09 | 1969-01-22 | Inst Francais Du Petrole | Device for automatically positioning a floating installation by means of moorings with controlled tension |
FR2413258A1 (en) * | 1977-12-30 | 1979-07-27 | Metalliques Entrepr Cie Fse | Offshore platforms anchored by cables under tension - incorporate connection plates guided to sea-bed and fixed by remotely controlled jacks to bases (NO 30.7.79) |
GB2035935A (en) * | 1978-12-08 | 1980-06-25 | Conoco Inc | Mooring system fopr tension leg platform |
-
1981
- 1981-08-03 EP EP19810303541 patent/EP0045653B1/en not_active Expired
- 1981-08-03 DE DE8181303541T patent/DE3169824D1/en not_active Expired
Patent Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB1140796A (en) * | 1966-02-09 | 1969-01-22 | Inst Francais Du Petrole | Device for automatically positioning a floating installation by means of moorings with controlled tension |
FR2413258A1 (en) * | 1977-12-30 | 1979-07-27 | Metalliques Entrepr Cie Fse | Offshore platforms anchored by cables under tension - incorporate connection plates guided to sea-bed and fixed by remotely controlled jacks to bases (NO 30.7.79) |
GB2035935A (en) * | 1978-12-08 | 1980-06-25 | Conoco Inc | Mooring system fopr tension leg platform |
Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
WO1996040548A1 (en) * | 1995-06-07 | 1996-12-19 | Aker Engineering A.S | A method for installing a tension leg platform |
AU693709B2 (en) * | 1995-06-07 | 1998-07-02 | Aker Engineering As | A method for installing a tension leg platform |
CN113212657A (en) * | 2021-05-26 | 2021-08-06 | 中国能源建设集团广东省电力设计研究院有限公司 | Self-installation offshore platform |
Also Published As
Publication number | Publication date |
---|---|
EP0045653B1 (en) | 1985-04-10 |
DE3169824D1 (en) | 1985-05-15 |
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