WO1999030963A1

WO1999030963A1 - Submerged disconnectable anchor buoy and ship

Info

Publication number: WO1999030963A1
Application number: PCT/NO1997/000339
Authority: WO
Inventors: Knut BØRSETH
Original assignee: Petroleum Geo-Services Asa
Priority date: 1996-06-13
Filing date: 1997-12-15
Publication date: 1999-06-24
Also published as: NO962506L; NO310604B1; NO962506D0

Abstract

A method for connecting an anchor line (15) to an attachment point (14) on a submerged anchor buoy (11) for a floating vessel (1) having a vertical shaft (4) provided at the lower end with receiving means (10, 12) for the buoy comprises the steps of: connecting the buoy (11) to the vessel (1) so as to permit the attachment point (14) to turn about a vertical axis of said shaft (4); attaching a connecting element (18) at the free end of a pre-installed anchor line (15) to a receptacle (31) at the end of a pulling line (28) running through the bottom (3) of said vessel at a point closer to said axis than said attachment point (14); pulling the connecting element (18) by means of the receptacle (31) and the pulling line (28) into close proximity to said attachment point (14), adjusting the relative position of the connecting element (18) and the attachment point (14) and joining them together, and removing the receptacle (31) from the connecting element (18). Also disclosed is a vessel (1) provided with equipment (24, 27-36) for performing the method.

Description

SUBMERGED DISCONNECTABLE ANCHOR BUOY AND SHIP

The present invention relates to a method for connecting an anchor line to an attachment point on a submerged anchor buoy for a floating vessel having a vertical shaft provided at the lower end with receiving means for the buoy.

It is well known in the art to moor vessels like hydrocarbon production ships and shuttle tankers at an offshore location by means of a buoy anchored to the ocean floor by a number of catenary anchor lines connected to anchors arranged in a predetermined pattern around the buoy . The buoyancy of the buoy and the weight of the anchor lines are such that the buoy will assume an equilibrium position at a predetermined depth below the water surface when not connected to the vessel. When mooring the vessel, the buoy is pulled up to the vessel by means of a suitable rope and is locked in place in the receiving means in the bottom of the vessel.

Such mooring systems are known e.g. from US-A-4 604 961 and WO 93/11030, these publications being incorporated by reference. This prior art is silent on how the anchor lines are attached to the buoy upon initial installation. To the applicant's knowledge, the usual procedure is first to install all the anchors and anchor lines in a predetermined pattern and with the anchor lines having a predetermined length, the free end of the anchor lines being connected to a retrieving line extending from the sea floor to the surface where it is supported by a marker float. The connection of the mooring lines to the buoy is normally done while the buoy is located on board an auxiliary vessel. This procedure is quite difficult to perform satisfactorily because of the considerable tension in the anchor lines and problems associated with keeping the already connected anchor lines freely suspended while new ones are connected. After some time of operation, the anchor lines will stretch somewhat, particularly those that are stressed the most due to the prevalent wind direction. Therefore, in time there will be a need for readjusting the length of the mooring lines. Besides, such a need may be present at the time of initial installation since in some cases it may be difficult to predict the exact location and length of the anchors and anchor lines, respectively.

An object of the present invention is to provide a method for connecting an anchor line to an attachment point on a submerged anchor buoy that can be performed in a simple and safe manner without considerable auxiliary equipment.

It is also an object of the invention to provide such a method which, in addition, will permit easy and accurate adjustment of the length of the anchor lines both at initial installation and at any time during the life of the mooring system.

According to the invention, the first object is obtained by a method as defined in the introductory paragraph, the method being characterized by - connecting the buoy to the vessel so as to permit the attachment point to turn about a vertical axis of said shaft, attaching a connecting element at the free end of a pre-installed anchor line to a receptacle at the end of a pulling line running through the bottom of said vessel at a point closer to said axis than said attachment point, pulling the connecting element by means of the receptacle and the pulling line into close proximity to said attachment point, adjusting the relative position of the connecting element and the attachment point and joining them together, and removing the receptacle from the connecting element.

Since the buoy during the connection process will be held in its intended place in the vessel to be moored by it, there will be no interference between the anchor lines and little danger of overstressing them during the installation. Furthermore, the method permits measuring the tension of the lines during installation, thus allowing for immediate fine adjustment of their length.

It will be understood that the second object mentioned above can be obtained by disconnecting an anchor line using some of the method steps above in reverse, adjusting the length of the anchor line, and then reinstalling it by using the above method, even while the vessel is in normal use.

The invention also relates to a floating vessel equipped to allow performing said methods. Such a vessel is defined in claim 5.

Further advantageous features of the method and vessel are defined in the dependent claims.

For better understanding of the invention, it will be described in more detail with reference to the figure on the appended drawing, said figure showing schematically part of a vessel according to the invention partly in vertical section.

The vessel, which may be an oil production ship, is generally designated by reference numeral 1. The vessel has a main deck 2 and a bottom 3 , between which a vertical shaft 4 extends. A turret 5 is located in the shaft 4 and is supported axially and radially by a suitable bearing arrangement 6 at the main deck 2 level. The turret 5 is also radially supported by a self-lubricating sliding bearing 7 in the lower part of the shaft 4. At this level, the turret is also provided with a chain stopper 8 and an operating mechanism 9 for a locking arrangement 10 at the bottom of the turret for locking an anchor buoy 11 to the turret .

At the bottom, the turret 5 is also provided with a main connector 12 for the buoy and a plurality of riser con- nectors 13, only one of which is shown.

The anchor buoy 11 is provided with a plurality of attachment points 14 for anchor lines 15, the attachment point devices comprising a fork-like structure having a trans- verse pin 16 and a shaft journalled in a bearing 17. The bearing or the pin may contain a load cell arrangement in order to register the tension of the respective anchor line 15. The anchor line is shown in an unconnected condition and is at its free end provided with a dual hook 18 having an upper hook and a lower hook.

In its upper part, the turret 5 is provided with a turret turning system 19 and a swivel arrangement 20 having a central passage therethrough. The swivel arrangement is rotatably connecting production piping on the turret 5 and vessel 1, respectively, in a well-known manner.

The main deck 2 of the vessel supports a gantry 21 carrying a sheave 22 at its free end. The sheave is located above the central passage of the swivel arrangement 20. A rope 23 extends from a winch 24 over the sheave 22 and down through the swivel arrangement to a short chain 25 connected to the top of the anchor buoy 11. A plurality of buoyancy elements 26 are arranged with a loose fit on the rope 23 so as to be slidable along the rope. The purpose of these buoyancy elements is to keep the free end of the rope floating on the surface when the buoy 11 and rope 23 has been released from the vessel 1, a stopper for the buoyancy elements having been installed at the free end of the rope 2 before said release. Thus, the outer dimensions of the buoyancy elements 16 are sufficiently small for them to pass through the central passage of the chain stopper 8 and main connector 12.

The vessel 1 is provided with a second shaft 27 extending between the main deck and bottom of the vessel, this second shaft accommodating a pulling wire 28 passing from the winch 24 via a sheave 29 down through the second shaft and through a J-type sliding fairlead 30 at the bottom of the shaft. The free end of the pulling wire 28 is connected to a receptacle 31 generally in the form of a steel ring which can be hooked into the upper hook of the dual hook 18 at the end of the anchor line 15. The receptacle 31 is also connected to the free end of a supporting wire 32, which extends through a double J-type sliding fairlead 33 at the bottom of a third shaft 34, up through said shaft and over a sheave 35 to a second winch (not shown) . A television camera 36 is located in the bottom 3 of the ship between the shafts 27 and 34, preferably directly above the attachment point 14 on the anchor buoy 11.

The method for connecting the anchor lines 15 to the buoy 11 assumes that the buoy has been brought into the position shown in the drawing, for example by suitably ballasting the buoy while having the rope 23 attached to the pulling wire of the winch 24 and then raising the buoy into contact with the receiving means 10, 12 of the turret. The method further assumes that the various anchors and anchor lines 15 have been deployed on the ocean floor and that retrieving lines with marker floats are connected to the free ends (dual hooks 18) of the anchor lines. It is also assumed that the receptacle 31 has been lifted to the deck of the vessel 1 while concurrently slacking the pulling wire 28 and supporting wire 32 as necessary. This may be done by means of a permanently or diver connected handling line.

An auxiliary vessel, for instance an anchor handling tug, will deliver a messenger line to the production vessel 1. This messenger line is connected to the receptacle 31 , which is hauled over to the tug and brought by the tug to the location of the marker float of the anchor line to be installed. Pulling wire 28 and supporting wire 32 are paid out as necessary during this operation. The anchor line is pulled to the surface and the upper hook of its dual hook 18 is connected to the receptacle 31. The anchor line and receptacle is subsequently lowered by means of a double part wire.

On the production vessel 1, the turret turning system 19 is used to move the attachment point 14 on the buoy 11 somewhat to the side of the line of the pulling wire 28 so as not to interfere therewith when, in the next step, the pulling wire 28 is heaved in in order to pull the anchor line 15 with its dual hook 18 into a position as indicated by the dash-dot-line 37. The slack in the supporting line 32 will be taken up as necessary during this operation.

With the receptacle 31 in the position shown with the dual hook 18 connected thereto, the supporting wire 32 is tightened so as to lift the anchor line 15 with the hook 18 approximately to the level shown. With the line and hook in this position, the turret turning system 19 is operated so as to move the attachment point 14 directly below the hook 18, e.g. with the aid of the television camera 36. By first paying out on the supporting wire 32 and then on the pulling wire 28, the lower hook of the dual hook 18 can be made to engage the transverse pin 16 of the attachment point 14. By further slacking the pulling wire 28 and tightening the supporting wire 32, the receptacle ring 31 may be disengaged from the upper hook of the dual hook 18, and the receptacle 31 is subsequently brought up to the deck level of the vessel 1 to be ready for the installation procedure of the next anchor line.

The second anchor line will preferably be one diametrally opposed to the first anchor line 15 installed. In order to bring the proper attachment point 14 in position below the television camera 36, the vessel is turned 180° while the turret turning system 19 is moving the turret through the same angle so that the anchor line 15 already installed will maintain its position with respect to the sea floor. Thereafter, the procedure outlined above is repeated.

If the load cells in the attachment points or other factors indicate that the length of one or more of the anchor lines 15 should be either shortened or lengthened, the vessel 1 and turret are turned so as to bring the attachment point 14 in question directly below the television camera 36. By operating the pulling wire 28 and supporting wire 32 appropriately, the receptacle 31 can be hooked onto the upper hook of the dual hook 18, and by pulling further on the pulling wire 28, the dual hook 18 may be disengaged from the pin 16 of the attachment point device. After raising the hook 18 by means of the supporting wire, the vessel 1 or turret 5 is turned somewhat so that the anchor line 15 and hook 18 will clear the attachment point 14 when the pulling wire 28 is paid out. By means of the messenger line attached to the receptacle 31, the hook 18 and the end of the anchor line 15 are brought on board the tug, where the anchor line may be shortened or lengthened appropriate- ly before it is again installed on the attachment point using the same procedure as followed during the initial installation.

The diameter of the second shaft 27 is sufficiently large for the J-type fairlead 30 along with the receptacle 31 to be pulled up to the level of the main deck 2 of the vessel for maintenance or replacement. Furthermore, the attach- ment point 14 may also be removed for maintenance or replacement by using the pulling wire 28 and supporting wire 32. Diver assistance may be used to connect the wires to the attachment point, or the receptacle 31 may be replaced by suitable remotely operated grabbing equipment to lock onto the attachment point device 14.

It will be understood that the invention is not in any way limited by the exemplifying embodiment detailed above but may be modified and varied in a number of ways within the scope of the appended claims. For instance, the pulling wire 28 may have its own winch instead of sharing the winch 24 with the lifting rope 23 of the buoy. Another alternative is for the lifting rope 23 and pulling wire 28 to have separate drums on the same winch. Furthermore, the winch 24 may be used when installing flexible risers 38 in the buoy 11. This is indicated by the pulling line 39 passing over guide rollers 40 and 41. Before such connection can be done, the rigid internal riser 42 is lifted by means of the cylinder 43. Since the rigid riser 42 is pivotably connected to pivot arms 44, it will swing laterally out of the way when lifted. Such lifting is permitted by the flexible connection 45 between the rigid riser and piping leading to the swivel arrangement 20. It will also be understood that load cells or other instrumentation may be connected to the vessel, e.g. through the locking arrangement 10, so as to permit continuous monitoring of anchor line tension.

Claims

C A I M S

1. A method for connecting an anchor line (15) to an attachment point (14) on a submerged anchor buoy (11) for a floating vessel (1) having a vertical shaft (4) provided at the lower end with receiving means (10,12) for the buoy, characterized by connecting the buoy (11) to the vessel ( 1 ) so as to permit the attachment point (14) to turn about a vertical axis of said shaft (4), attaching a connecting element (18) at the free end of a pre-installed anchor line (15) to a receptacle (31) at the end of a pulling line (28) running through the bottom (3) of said vessel at a point closer to said axis than said attachment point (14), pulling the connecting element (18) by means of the receptacle (31) and the pulling line (28) into close proximity to said attachment point (14), adjusting the relative position of the connecting element (18) and the attachment point (14) and joining them together, and removing the receptacle (31) from the connecting element (18).

2. A method according to claim 1, characterized in that a second pulling line (32) is used in adjusting said relative position.

3. A method according to claim 1 or 2, characterized in that a dual hook (18) is used for said connecting element.

4. A method according to any of the preceding claims, characterized in that in connecting the buoy to the vessel, a rope (23) is used which is provided with a plurality of buoyancy elements (26) which are slidably arranged on the rope (23) and are of sufficiently small outer diameter to pass through a central opening in said receiving means (10,12) for the buoy (11) when the buoy is pulled up to or released from the vessel ( 1 ) .

5. A floating vessel comprising a vertical shaft (4) provided at the lower end with receiving means (10,12) for a normally submerged anchor buoy (11), said buoy being provided with attachment points (14) spaced along the outer periphery of the buoy and below the bottom ( 3 ) of the vessel for the connection of anchor lines (15), a first winching arrangement (22,24) for handling a rope (23) attached to the buoy (11) for pulling the buoy from a submerged position into contact with the receiving means (10,12), and an arrangement (19) for turning at least a part of the buoy (11) in relation to the vessel ( 1 ) about the vertical axis of the buoy, characterized in that the vessel (1) is further comprising a second winching arrangement (24,29) for handling a pulling wire (28) emerging from the bottom of the ship at a point (30) closer to said axis of the buoy (11) than its attachment points (18), the free end of the pulling wire being connected to a receptacle (31) for releasable cooperation with connecting elements (18) on the anchor lines (15), and a third winching arrangement (35) for handling a supporting wire (32) emerging from the bottom (3) of the ship at a point (33) farther away from said axis of the buoy (11) than its attachment points (14), the free end of the supporting wire (32) also being connected to said receptacle (31).

6. A vessel according to claim 5, characterized in that said first and second winching arrangements ( 22, 24; 24, 29) shear the same winch (24).

7. A vessel according to claim 6, characterized in that said sheared winch (24) has separate drums for the first and second winching arrangements .

8. A vessel according to claim 5, 6 or 7, characterized in that a television camera (36) is arranged in the bottom (3) of the vessel at a location having substantially the same distance from the axis of the buoy as its attachment points (14).

9. A vessel according to any one of claims 5 - 8, characterized in that said receptacle (31) is substantially comprised by a steel ring adapted for cooperation with connecting elements (18) in the form of dual hooks on the anchor lines (15).

10. A vessel according to any one of claims 5 - 9, characterized in that a turret (5) is provided in said shaft (4) of the vessel (1), the turret having at least one rigid riser (42) which is pivotally connected to spaced pivot arms (44) so as to move laterally when lifted.