EP0249422A1

EP0249422A1 - Method and arrangement for raising rigs

Info

Publication number: EP0249422A1
Application number: EP87305033A
Authority: EP
Inventors: Fredrik Waage-Rasmussen; Per Johansson
Original assignee: NEPTUN MARINE CONTRACTING AS; HAUGESUND MEKANISKE VERKSTED AS
Current assignee: NEPTUN MARINE CONTRACTING AS; HAUGESUND MEKANISKE VERKSTED AS
Priority date: 1986-06-09
Filing date: 1987-06-08
Publication date: 1987-12-16
Also published as: FI872565A; PT85040A; FI872565A0; DK278887D0; NO862299D0; NO862299L; NO159006B; DK278887A; NO159006C

Abstract

A method and an arrangement for raising or docking semi-submersible or partly submersible rigs (2) which are supported by pontoons (3) or the like. Two ballastable barges (5) and a ballastable floating body (6) capable of great variation in draught, e.g. a floating dock, are moored together in such a manner that said barges (5) are longitudinally and transversely fixed relative to the floating body (6). Contact means (16) in the shape of fenders or the like are provided between said barges and floating body to form an articulated connection (9) between them during raising and lowering operations. One or a number of stabilizing means (1) may be connected to the barges (5), by e.g. chains, wires, or the like of a desired length. To raise a rig (2) the barges are first ballasted to submerge their ends (11) remote from the body (6), then the body (6) and the other ends (12) of the barges (5) are submerged. The rig (2) is moved into position over the barge (5) and the barges (5) and body (6) deballasted to raise the rig, preferably with longitudinal inclination of the barge deck of e.g. about 2°.

Description

The present invention relates to a method and an arrangement for raising or docking semi-submersible or partly submersible rigs which are supported in water by pontoons or the like on their lower parts.
Semi-submersible drilling rigs, accommodation platforms, service rigs, crane barges, or the like (all to be included in the term "rigs" used hereinafter) are built especially with a view to being able to carry out necessary maintenance operations without docking the rig. Ship register companies do not require docking of rigs, in contrast to what is required for ships, and necessary underwater jobs on e.g. pontoons may nowadays be carried out by various methods dependent on the kind of maintenance task. Diving is much used, e.g. when anodes, etc. are to be replaced. When welding operations are necessary a water-evacuated habitat is often used. During mounting operations of shafts the rig may commonly need to be trimmed and/or heeled. Dry docking is another possibility, but in Europe this is at present only possible at a limited number of docks especially built for large tankers.
It would be desirable to have an alternative to docking in a dry dock which would enable a rig to be raised in the water - preferably entirely above water to permit all kinds of maintenance operations to be carried out without diving, without use of a habitat, and without any need for trimming or heeling the rig.
Additionally, it would be desirable to be able if desired to dry dock the raised rig almost anywhere, with equipment already existing and only requiring slight additions and modifications.
The invention seeks to solve these problems by providing one or more - commonly two - ballastable barges moored so as to be mutually stationary in the longitudinal and transversal direction, also being moored to a ballastable buoyancy body capable of great variations in draught, e.g. a floating dock, or another barge. The relative positions of the barges are adapted to the design and location of rig supporting portions e.g. pontoons, on the rig to be raised. The sterns of the barges are ballasted to sink them to a desired depth; then the bows of the barges and the buoyancy body are ballasted simultaneously so that they sink to a desired depth, but so that part of the buoyancy body always remains above the water surface. The rig is then positioned above the barge or barges, and preferably moored so as to be stationary in the longitudinal as well as the transverse direction, after which the buoyancy body and barge or barges are then raised by de-ballasting them until contact is achieved between rig and barge. Preferably each barge is de-ballasted so that it rises bow first to bring the front part of the barge into contact with the rig, after which the stern of each barge is raised with its position controlled until there is contact between rig and barges along the entire length of the rig. Finally the buoyancy body and the barge(s) are further de-ballasted to raise the rig to a desired level, the stability being maintained. If desired, the barges with the rig on board may be released from the floating body e.g. dock and move to e.g. a fixed dock, quay, or the like. Of course each barge may if desired be moored with its stern rather than its bow towards the buoyancy body.
Stationary barge mooring in the longitudinal and transversal directions, as well as in the vertical direction at the end attached to the buoyancy body, may be achieved by having each barge provided at that end with one or a number of connections in the form of wires, chains, or the like which are secured to the barge and to the buoyancy body, and which are in tension throughout the ballasting and de-ballasting operation. The other end is accordingly moored by one or a number of connections extending on either side of the barge, each connection forming an angle with the longitudinal axis of the barge being anchored to e.g. a point ashore, a buoyancy body, the sea bed, or another barge, so that the resultant of all mooring forces causes each barge separately to exercise a positive force on the buoyancy body. Vertical motions of the barge are thus articulated at the connection between the barge and the floating body.
One or a number of said connections may, advantageously, be provided with load cells for monitoring loads at the connection(s).
Preferably floating bodies or stabilizing members are provided toward the barge's stern at each side, and are connected with the barge by chains, wires, or the like of a length that is adapted to the desired depth of the barge at the time that the rig is to be positioned over it. These bodies or members will function to stabilise the barge when their connections to the barge come into tension.
During de-ballasting of the barges and the floating buoyancy body each barge (and the rig) is preferably trimmed to a longitudinal inclination between 0° and 5° relative to the horizontal, preferably 2°, before the area of contact between the bottom of the rig and the deck of the barge intersects the water line. This helps to ensure further enhancement of stability in this phase.
Most advantageously the barges are moored mutually substantially parallel, and substantially normal to that longitudinal side of the buoyancy body to which they are moored.
The invention also provides an arrangement for carrying out the above method, comprising at least one barge moored to a buoyancy body so as to be articulated thereto in the manner described above.
Generally speaking there are certain problems in connection with raising or transferring a floating load using a submerged barge. The following primary requirements must be fulfilled in a satisfactory manner:

(a) the barges must have a positive stability during submersion and when submerged,
(b) the vertical position of the barges must be controllable,
(c) the load must be correctly positioned on top of the barges,
(d) transfer of the load to the barge must be controlled, and
(e) the arrangement must be stable in all phases of the raising operation, especially in the phase when the bottom of the rig/deck of the barge intersects the water surface.

In this connection it is important to consider that a floating member will lose its capability of self-stabilization as soon as it loses its water line or moves into a submerged state.
The stability and level of submerged barges is commonly controlled by one or more of the following methods:

(a) having a forecastle or turret on the barge intersect the water level,
(b) providing floating tanks with a chain or wire connection to the barge,
(c) grounding one end of the barge, or
(d) submerging the barge with the aid of a floating crane.

In the present invention it is preferred to use one or more of the above mentioned methods, in combination with having the barge or barges connected to a buoyancy body having a greatly variable draught. The buoyancy body ensures the location of the barges in the longitudinal and transverse directions, while the draught and relative vertical alignment of the barges are controlled at the transversal end connected with the buoyancy body. When the barges are submerged their draught may be varied without ballasting or de-ballasting them by varying the draught of the buoyancy body. Mooring of the barge and rig unit is simplified if the buoyancy body has an anchoring/mooring system of its own which is able to absorb great forces.
By way of example, a method and arrangement embodying the invention will now be described with reference to the enclosed drawings, in which:

Figure 1 is a diagrammatical plan view of two barges moored to a floating dock and a quay;
Figures 2 - 5 are diagrammatical side views of a lowering operation, showing one barge;
Figure 6 is a side elevation, perpendicular to Figures 2 - 5, of the floating dock and barges in a floating state;
Figure 7 is the same as Figure 6 but with the barges submerged;
Figures 8 - 11 show diagramatically the raising of a rig using the barges and floating dock;
Figure 12 is a diagrammatical plan view of the rig, placed on top of the barges and raised up out of the water; and
Figure 13 shows how the barges are connected to the floating dock.

Reference is now made to Figure 1 which is a diagrammatical view of a floating dock 6 which is in connection with two barges 5. Floating dock 6 has a large displacement and offers the possibility of great variations of draught, e.g. through 10 meters. The two barges 5 are suitably arranged at about 90° to one long side of floating dock 6. The distance between the barges is adapted to the distance between pontoons 3 on a rig 2 which is to be raised. One transversal end of the barges, the bow or stern, is in direct contact with the floating dock 6 by a fender 16. One of the barges 5 facing the floating dock is moored by one or a number of connections 7 in the shape of wires, chains, or the like. As shown in the Figures, one or a number of connections 7 extend from each side or each corner of the barge, and approximately horizontally towards the floating dock. These connections are tensioned to always provide a positive force from barge 5 toward fender 16 and, thus, to ensure a continuous contact between the barge and fender 16 or the floating dock. End 12 of barge 5 facing the floating dock is furthermore moored by a connection 13 between a centrally located anchoring point 17 on barge 5 and an anchoring point 18 at a higher level on floating dock 6. Barge 5 is de-ballasted to have a substantial load absorbed by connection 13 ensuring tension of said connection during the entire operation. This connection should be provided with a load cell 10 for monitoring the load. Figure 1 also shows moorings 8 in the shape of wires, chains, or the like for the barge 5 facing away from floating dock 6. These moorings are attached at each side of the barge and at a quay 20, a buoyancy body 6, a point on shore or on the bottom of the sea, or at the other barge. Being moored and connected to the floating dock in this manner the barges cannot make any surging, heaving or swaying movement relative to the floating dock. They may, however make a slight rolling movement and a pitching movement, although the last mentioned is not about a midships axis but at the end facing the floating dock, about an axis parallel with the floating dock and the water surface. In this manner an articulated connection is provided between barges 5 and floating dock 6. Figure 1 also shows a number of floating bodies 1 or stabilizing members which are in connection with the barge's sides at the end facing away from the floating dock.
One member at each side is shown, but the number of members may, obviously, be as desired. Members 1 are connected with barges 5 via chains, wires, or the like of a desired length which is adapted to the desired draught to which the barges must be lowered before the rig can be floated on top of them. When having the rig float to over the barges a free space between pontoons and barges is calculated to 0.5 m - 1.0 m. Commonly the barges must be lowered 6.5 - 8.5 m below the water line 4.
Figures 2 - 5 show the immersing operation of a barge 5. As disclosed above, barges 5 are moored at one transversal end, hereinafter called the bow, to floating dock 6. Then the other transversal end, hereinafter called the stern, is ballasted as shown in Figure 3. Stabilizing members 1 remain on the water surface, and the stern of the barge is ballasted until the connections between stabilizing members 1 and the stern 11 are tensioned, as shown in Figure 4. When the stern is suspended with a floating member 1 attached to each side of the stern, the stability of the barge is ensured. Stability during the immersion of the stern is also ensured, part of the barge all the time being kept above water and a waterline area thus being maintained. Now, ballasting of the bow 12 and floating dock 6 is initiated simultaneously and in such a manner as to maintain the tension of the connection 13. Ballasting of the barges stops when they are immersed below the water surface. Immersion of the correct draught is then achieved just by varying the draught of the floating dock. Part of floating dock 6 must always be maintained above the water surface 4 in order to maintain stability of the barges and the floating dock, as shown in Figure 5.
Figure 6 shows how barges 5 are moored to floating dock 6, and to the bottom of the sea, and to a point on shore or on a quay 20, to hold them in a fixed position.
Figure 7 shows the floating dock and the barges in an immersed state.
Figures 8 - 11 show how a raising operation of a rig is carried out. A rig 2 is floated onto barges 5 and correctly positioned relative to the barges. As mentioned above, the mutual distance between barges 5 is adapted to the distance between pontoons 3 of the rig. The first phase of the raising operation may be carried out in two different manners. One manner comprises first de-ballasting the floating dock and the bow of the barge to achieve contact between the forepart of the barges and rig pontoons 3, and then de-ballasting and raising the after body of the barges under position control, until contact is achieved between the rig and the barges all over the length of the rig. Another method for carrying out this phase is to make connections between the stabilizing means and the after end of the barges slightly shorter, and to ballast the barges additionally, ballasting the floating means if desired, in order to achieve the same draught of the stern as mentioned above. In this manner de-ballasting of the barge's forward parts, after parts, and of the floating dock may be carried out in parallel to achieve contact between barges 5 and rig 2 approximately at the same time all along the length of rig 2.
During further de-ballasting of barges 5 and floating dock 6 the barges (and the rig) are advantageously trimmed some degrees in the longitudinal direction, preferably 1 - 2°, before the bottom of the rig and barge deck intersect the water line, in order to ensure stability additionally in this phase. This is indicated in Figure 10.
Figure 11 shows the rig in its final raised position. It may now, if desired, be released from the floating dock and be moved to a fixed dock, to a quay, or to another place.
Figure 12 shows floating dock 6, barges 5, and a rig 2 raised out of the water by the aid of the barges and the floating dock.
Figure 13 shows the connection between barges 5 and floating dock 6 in more detail. A fender 16, as mentioned, is provided between the floating dock and the barge, and when the barge is moored to the floating dock as described above, an articulated connection 9 is formed between barges 5 and floating dock 6. An auxiliary fender 21 is also indicated between pontoon 3 and the floating dock. As mentioned above the forepart 12 of the barge is ballasted always to ensure tension of the connection 13, which is monitored by a loadcell 10. The distance between point of attachment 17 on the barge and point of attachment 18 on the floating dock is preferably such that the connection 13 effectively transfers vertical forces. A suitable vertical distance will be approximately 10 meters. Figure 13 shows the barges and the floating dock in an immersed state with a rig positioned on top of the barges. The barges are suitably provided with keel pads 22 for rest against rig pontoons 3. The keel pads 22 are suitably of such a height and have such mutual distance that operators are able to do work on the underside of pontoons 3.
Even though only stabilizing means 1 that are connected to the barge sides were mentioned above, it is also possible to ground the barges, if desired lowering them with the aid of floating cranes, jacks between the stern of the barge and the bottom, or with turrets on the barges intersecting the water surface to provide additional stability.
It will be understood that the above disclosed raising operation will have to be carried out in a reverse sequence when the rig is to be set afloat again, and forms part of the invention.

Claims

1. A method for raising a rig (2), characterized by:
providing at least one ballastable barge (5) moored at a first end (12) thereof to a ballastable floating body (6) capable of large variation in draught, each barge (5) being so moored as to be substantially fixed longitudinally and transversely relative to the floating body (6);
ballasting each barge (5) so as to submerge to a desired depth a second end (11) thereof, remote from the floating body (6);
subsequently ballasting the floating body (6) and each barge (5) together so as to submerge the first end (12) of each barge (5) to a desired depth and increase the draught of the floating body (6) to which it is moored without submerging said body;
positioning the rig (2) with a lower supporting portion or portions (3) thereof aligned above the barge or respective barges (5), and
de-ballasting each barge (5) and the floating body (6) so as to raise them together, to bring each barge (5) into contact with the supporting portion (3) of the rig (2) and subsequently raise the rig (2) to a desired level.

2. A method for raising or docking semi-submersible or partly submersible rigs (2) having as supports pontoons (3) or the like, which are preferably deballasted before the raising operation, with positive stability and control during the raising operation, characterized in that two ballastable barges (5) are moored, so as to be mutually stationary in the longitudinal and transversal directions, to a ballastable floating body (6) capable of great variation of draught, e.g. a floating dock, with the relative positions of the barges (5) being adapted to the design and location of the rig supports (3), whereupon second ends (11) of said barges remote from the body (6) are ballasted so that said second ends (11) sink to a desired depth, after which first ends (12) of said barges adjacent the floating body (6), and the floating body (6), are ballasted in parallel until the desired depth is achieved, with a portion (14, 15) of said floating body always remaining above the water surface, said rig (2) then being positioned on top of the barges (5) and moored to fix it longitudinally and transversely, and said floating body (6) and said barges (5) then being raised with de-ballasting, under control of their relative positions, until contact is achieved between the rig (2) and the barges (5) along the entire length of the rig, the floating body (6) and said barges (5) are further de- ballasted so as to raise the rig (2) stably to the desired level, said barges (5) with said rig (2) optionally then being released from said floating body (6) and moved to a fixed dock, quay, or the like.

3. A method according to claim 1 or claim 2 wherein the floating body (6) and the first end (12) of each barge (5) are raised first in a controllable manner during de-ballasting until contact is achieved between rig (2) and barge (5), whereafter the second ends (11) of said barge(s) are raised with control of their position until contact is achieved between said rig (2) and barges (5) along the length of the rig.

4. A method according to any one of claims 1, 2 and 3 wherein each barge (5) is at its first end (12) provided with one or a transversely spaced plurality of connections (7) attached to the barge and floating body (6), and in tension during the ballasting and de-ballasting operation, and its second end (11) is moored by one or more connections(8) anchored at a point or points spaced transversely from the barge (5), so that the resultant of all mooring forces causes each barge separately to exert a positive force on said floating body (6) and thereby form an articulated connection (9) between them.

5. A method according to claim 4 wherein one or more of said connections (7, 8) are provided with sensors (10) for monitoring loads on the connection(s).

6. A method according to any one of the preceding claims wherein one or a number of floating bodies (1) or stabilizing means are provided at or towards the second end (11) of each barge(s), at each side, and are connected with the barge by connectors of a length corresponding to a desired depth beneath the surface (4) of said barge(s) for when the rig (2) is positioned above it.

7. A method according to any one of the preceding claims wherein during de-ballasting of each barge (5) and the floating body (6) each barge is trimmed so that its deck has a longitudinal inclination of between 0° and 5.0°, preferably about 2.0° relative to the horizontal before the bottom of the rig and deck of the barge intersect the water surface (4).

8. A method according to any one of the preceding claims, wherein each barge (5) is moored substantially normal to the longitudinal side of said floating body (6) and, where there is a plurality of barges (5), these are substantially mutually parallel.

9. A method of lowering a rig (2) to an operational partially submerged condition, characterized by performing in reverse the steps of a method according to any one of claims 1 to 8.

10. An arrangement for carrying out a method according to any one of the preceding claims, characterized in that it comprises one or more, preferably two, ballastable barges (5) and a ballastable floating body (6) capable of a large variation in draught, e.g. a floating dock, connections (7, 8) mooring said barges to said floating body (6) and in a stationary position longitudinally and transversely relative thereto, and contact means (16) such as fenders between said barges (5) and floating body (6) whereby primary articulations (9) between said barges (5) and floating body (6) are at or adjacent said contact means (16).

11. An arrangement according to claim 10 wherein at least one of said connections (7, 8) is provided with a sensor such as a load cell (10) for monitoring the loads on said connection.

12. An arrangement according to claim 10 or claim 11 wherein each barge (5) has one or more buoyant bodies (1) or stabilizing means connected to it remote from the floating body (6), on each side, by connectors of a predetermined length, which bodies or means (1) function to stabilise the barge when said barge is immersed and the connectors are under tension.

13. An arrangement according to any one of claims 10 to 12 wherein each barge (5) is moored substantially normal to or at 90° to the longitudinal side of said floating body (6) and, where there is a plurality of barges (5), these are substantially mutually parallel.