Information
-
Patent Grant
-
6499418
-
Patent Number
6,499,418
-
Date Filed
Monday, September 17, 200122 years ago
-
Date Issued
Tuesday, December 31, 200221 years ago
-
Inventors
-
Original Assignees
-
Examiners
Agents
-
CPC
-
US Classifications
Field of Search
US
- 114 264
- 114 265
- 114 2301
- 166 352
- 405 2231
-
International Classifications
-
Abstract
A method of connecting two or more elongate connection members (105) between the seabed and a floating vessel (106) carrying a connector (102) which is suspended from the vessel by at least two spaced apart suspension members for relative displacement of the connector with respect to the vessel, at least one suspension member being connected to tensioning elements (109, 109′) for exerting an upward force on the connector. The method comprises the steps of: a) attaching one or more connection members to the connector, b) increasing the tensioning force of the tensioning elements, or vice versa, and c) repeating steps a and b until the connection members are installed between the vessel and the seabed.
Description
The invention relates to a method of connecting two or more elongate connection members, such as risers or tendons, between the seabed and a floating vessel. The invention also relates to a vessel for carrying out this method and to a vessel carrying a connector suspended from the vessel by at least two spaced apart suspension members for relative displacement of the connector which respect to the vessel, one or more elongate connection members, such as risers or tendons, being attached with one end to the connector and with their other end to the seabed.
From U.S. Pat. No. 4,272,059 a riser tensioning system is known wherein a riser, such as a drilling riser, is at its upper end provided with a tension ring which is connected via cables to sheaves on the drilling vessel. The sheaves are mounted on the free end of piston rods of hydraulic cylinders, the second end of the cables is being attached to the vessel. Upon heave, roll or pitch of the vessel the tensional forces on the riser are maintained generally constant by movement of the piston rods against the hydraulic pressure in the cylinders. This system has as a disadvantage that it carries only a single riser and that tensional forces exerted on the riser will vary with the buoyancy of the vessel. In order to obtain a relatively large stroke of the cylinders, these cylinders should be relatively long and therefore take up a lot of space, which in view of the moving nature of the cylinders cannot be effectively used. Furthermore, the hydraulic system is relatively complex.
From U.S. Pat. No. 3,681,928 a barge supporting a drilling rig is known, in which a platform is movably suspended from two mounting arms above deck level of the barge. The platform is connected to the seabed via two parallel tendons which pass through openings in the platform and through a central well in the barge. By this construction the platform remains in a horizontal position and at a constant height above the seabed when the vessel moves vertically due to wave motion. Under the influence of the dependant counterweights, the tendons are kept taut.
It is an object of the present invention to provide a method and a vessel for attaching multiple risers or tendons to the seabed and to the vessel. It is especially an object of the present invention to provide a method of drilling a stitch hydrocarbon well and attaching multiple risers with one end to the wellhead and with their other end to the vessel. Thereto the method is characterised in that it comprises the steps of:
a. attaching one or more connection-members to the connector,
b. increasing the tensioning force on the tensioning means, or vice versa, and
c. repeating steps a and b until the connection members are installed between the vessel and the seabed.
The present invention is based on the insight that a constant tensioning force can be easily maintained on the tendons or risers when they are being connected to the wellhead one after the other, by a stepwise increase of the tensioning force of the tensioning means. Each time after a drilling operation, one or more additional risers are attached to the connector. In case of hydraulic tensioning means, it is envisaged that the oil pressure in the hydraulic system or the air pressure in case of a pneumatic system, is stepwise increased when the number of risers and tendons attached to the connector increases. In case of the connector being suspended from a cable or rod, the tensioning force can be increased in a stepwise manner by adding extra tensioning weights or buoyancy members to the second end of the cable or rod. The increase in the tensioning force according to step b above can be carried out either prior to or after connection of additional connection members to the connector.
The method of the present invention of stepwise increase of the tensioning force is especially suitable for drilling methods in deep water. In this case the risers may have a length of 1000 meters or more and may have a weight of between 40 and 60 tons in submerged conditions. Each time one or more risers are installed, the force exerted by the connector of the present invention is increased a little over 40-60 tons, or a multiplicity thereof corresponding to the number of risers or tendons that are added, such that a substantially constant tension is maintained in the risers or tendons at all times. A further advantage is that, depending on the prevailing current, the tensioning force can be varied to compensate for the current factor and prevent the risers from buckling.
In one embodiment, the connector may be formed by a deck structure suspended from two or more oppositely located pivoting arms, which on their free ends are provided with connection means for attaching a multiplicity of separate weight elements to the free end of the arms.
The connector may also be suspended from cables running along sheaves, the free end of the cables carrying a counterweight. Additional counterweight elements may be added around the cable to be suspended from the free end to increase the tensioning force. As is used herein, the term “cable” comprises ropes, wires, chains, lines, combinations thereof and any equivalent means.
In one preferred embodiment, the cables have a first section extending vertically downward from the vessel to below water level and a second section which extends in a loop back upwards from the first section to a pulling device on the vessel, tensioning weights being comprised on the first and second cable sections, The tensioning force on the connector can be varied by varying the length of the first and second cable sections via the pulling device. When the loop is made large, the tensioning weight will be placed on the first cable section and will act completely on the connector. When the second cable section is shortened, the tensioning weight will be placed along the second cable section such that less weight is dependant from the connector via the first cable section. The tensioning weight may be comprised of clump-weights added to the cable or may be formed by the weight of the cable section itself, which may for instance be formed by a chain.
In another embodiment, the cables are guided along a cable guide means, such as a sheave, and are with one end attached to the seabed. A take-up device is connected to the cables for varying the cable length. The cable may be comprised of an anchor line such as a polyester line, the tension of which can be varied by the take-up device, which can for instance be formed by a winch. In another embodiment the cables may comprise a chain part carrying clump weights. By varying the length of the cable, the clump weights may be lifted from the seabed one after the other, such that the tensioning force on the connector is increased. The take-up device may be comprised of a winch and chain stopper assembly of the type known in the state of the art.
The varying tensioning force on the connector can in another embodiment be exerted by a buoyancy tank which is attached to the free end of cables from which the connector is suspended. The cables may be guided via a sheave lo a cable guide means located below keel level, such that the upward buoyancy force acts on the cable. The buoyancy tanks may for instance exert a maximum upwards force of 600 tons each, three buoyancy tanks being attached to the connector. Pneumatic lines may be attached to the buoyancy tanks for ballasting or deballasting the tanks. In another embodiment, the position of the cable-guide means with respect to keel level of the vessel can be varied such that the tensioning force is increased or decreased.
In a further alternative embodiment, the tensioning force on the suspension members can be varied by movement of the pivot arms from which the risers or tendons are suspended, with respect to pivot points and/or by movement of a counterweight along the pivot arms. The counterweight may be moved for instance by means of a rack and pinion construction.
A vessel according to the present invention, which comprises a connector suspended from at least two spaced apart suspension members, is characterised in that a supporting deck is situated above the connector, a first section of a connection member extending from the seabed to the connector and being detachably connected to a second section of the connection member via coupling device, the second section extending to the supporting deck. Preferably the connector carries a blow-out preventor, the supporting deck carrying a drilling rig. The drill string that is attached to the drilling rig, may be disconnected from rig during high seas, when drilling is suspended. The casing is suspended from the blow-out preventor. When the relative motions between the connector and the deck are again within certain limits, the drill string can be reconnected to the drillling rig and drilling may be resumed. In this way it is not necessary to dismantle the total drill string and drilling riser, because the drill string and drilling riser can after disconnection move independently from the drilling rig. In this way drilling down time in stormy conditions is reduced compared to constructions in which the blow-out preventor is situated on the seabed and disconnection of the drilling riser is effected near the seabed, at the position of the blow- out preventor.
Different embodiments of the method and vessel according to the present invention will be explained in detail with reference to the accompanying, non-limiting drawings. In the drawings:
FIG. 1
shows a vessel comprising a riser tensioning deck attached to pivoting arms, to which weight elements can be added,
FIGS. 2 and 3
show a side view and a plan view, respectively, of a drilling barge comprising four riser tensioning decks, each deck being suspended from three mounting-arms and from three sets of three cables each,
FIG. 4
shows an embodiment wherein a varying tensioning force is exerted by a loop configuration of a cable carrying tensioning weights,
FIGS. 5 and 6
show an embodiment wherein the tensioning weight is formed by a chain section which is placed in a loop configuration, in a side view and a plan view, respectively,
FIG. 7
shows an embodiment wherein the tensioning weight is formed by clump-weights which are lifted from the seabed,
FIG. 8
is a detail of
FIG. 7
showing the take up device,
FIG. 9
shows buoyancy means for exerting a tensioning force,
FIG. 10
shows an embodiment wherein the varying tensioning force is exerted by displacement of a counterweight along pivot arms,
FIG. 11
shows an embodiment wherein a supporting deck is mounted above the connector, carrying a drilling rig, with a disconnectable drill string,
FIG. 12
shows an embodiment of a vessel of the same type as shown in
FIG. 11
, which is moored to the seabed via a turret having full weathervaning capacities, and
FIG. 13
shows an embodiment comprising hydraulic tensioning means.
FIG. 1
shows a drilling barge
1
with a supporting deck
2
, carrying a drilling rig
3
. From the drilling rig
3
, a drilling riser, schematically indicated with the dash and dot line
4
, is introduced into the seabed for drilling a hydrocarbon well. The risers
5
,
6
which are connected to the wellheads, are suspended from a connector, or riser tensioning deck
7
. The riser tensioning deck
7
is supported from cables or rods
8
,
9
connected to the free ends of pivot arms
10
,
11
, which are supported on pivot mountings
10
′,
11
′. At their second end the pivot arms
10
,
11
are provided with weight elements
12
,
12
′,
13
,
13
′. The weight elements can be individually connected to or detached from the arms
10
,
11
, each time a riser is connected to or is disconnected from the deck
7
. Tensioning deck
7
maintains a substantially constant tension in the risers
5
,
6
during wave-induced motions of the barge
1
, by pivoting movements of the arms
10
,
11
. When the risers
5
,
6
are successively connected to the tensioning deck
7
, the total number on risers connected to the deck varying between 2 and 50 and the weight of each riser, which can have a length of about 1000-3000 meters, varying between 40 and 180 tons, the number of weight elements
12
,
12
′,
13
,
13
′ at the end of the pivoting arms
10
,
11
is increased.
A blow-out-preventor
16
is supported from the supporting deck
2
for closing of the drilling riser upon a certain pressure increase. Positioning the blow-out-preventor in an accessible location on the supporting deck
2
, above water level, facilitates repair and change out of parts.
FIG. 2
shows a side view of an embodiment wherein the riser tensioning deck
20
is supported from a suspension member comprising sheaves
21
,
22
along which cables
23
,
24
are guided. The cables
23
,
24
are with one end connected to the riser tensioning deck
20
and with the other end to tensioning weights
25
,
26
. The weights
25
,
26
can slide up and down in guide shafts
27
,
28
. The weights
25
,
26
may be provided with rolling guide elements such as wheels, which contact the walls of the guide shafts
27
,
28
. As can be seen in
FIG. 3
, the barge
29
comprises four riser tensioning decks
20
,
20
′,
20
″,
20
′″. Each deck is suspended from three sheaves
21
,
22
,
30
. Each sheave carries three cables for increased safety.
FIG. 4
shows a barge
30
carrying a drilling rig
31
and a riser tensioning deck
32
. In
FIG. 4
two risers
33
,
34
are connected to the riser tensioning deck
32
. The riser tensioning deck
32
is suspended from cables
35
,
36
. Each cable has a first cable section
37
,
38
extending through a well in the barge to below keel level. Each cable
35
,
36
comprises a loop
39
,
40
and a second cable section
41
,
42
extending upwards from the loop
39
,
42
through a well in the barge to a pulling device, such as a winch
43
,
44
. At the position of the winches
43
,
44
a chain stopper may be provided. When the length of cable sections
37
,
38
,
41
,
42
is increased, the weight elements
45
,
46
and
47
,
48
can be lowered and can all be placed on the first cable section
37
,
38
. In this way, the tensioning force on the riser tensioning deck
32
is increased when more risers are added to the tensioning deck
32
in addition to risers
33
,
34
. By shortening the first and second cable sections
37
,
38
and
41
,
42
, the weight on the tensioning deck
32
can be decreased as the weights will then be distributed along the second cable sections
41
,
42
. The weight elements
45
,
46
,
47
,
48
may be combined with flushable buoyancy elements for varying the weight thereof.
FIG. 5
shows an embodiment of a barge
50
wherein the cables
51
,
52
are each comprised of relatively long chains sections with a length of between 100 and 1000 meters. The chains
51
,
52
are guided via sheaves
53
,
54
projecting beyond the perimeter of the barge
50
, and are via a pulling device (not shown in the figure) collected in chain lockers
55
,
56
.
As can be seen from
FIG. 6
, the riser tensioning deck
57
is placed over a central well
58
in the vessel and is supported from eight sheave combinations
59
.
FIG. 7
shows a barge
62
in which the riser tensioning deck
63
is supported from two lines
64
,
65
. The first ends
66
or
67
of each line are attached to the riser tensioning deck
63
, a second end
68
,
69
being placed on the seabed
70
. Each line
64
,
65
is attached to a take-up device
71
,
72
, such as a winch and chain stopper combination. The tensioning force on the riser tensioning deck
63
can be increased by shortening the lines
64
,
65
via the take-up device
71
,
72
to lift clumpweights
73
,
74
, attached to the second ends
68
,
69
of the lines
64
,
65
from the seabed
70
. By paying out the lines
64
,
65
, the clumpweights
73
,
74
will come to rest on the seabed, such that the tension in the lines
64
,
65
is decreased. The upper part
75
of the lines
64
,
65
may be formed by a steel cable part. The middle section
76
, even as the lower ends
68
,
69
may be comprised of a chain. An intermediate section
77
may be formed by a cable or polyester mooring line part.
FIG. 8
shows an enlarged detail of a part of the barge
62
of
FIG. 7
showing the upper section
75
of line
65
which is guided along a sheave
78
,
78
′ and which is attached to the riser tensioning deck
63
. At one end of upper section
75
, a chain stopper
71
is connected. By pulling the middle chain section
76
through a well
80
in the barge
62
and through chain stopper
71
and storing it in a chain locker
79
, the clump weights
73
are lifted from the seabed
70
.
FIG. 9
shows barge
82
wherein the riser tensioning deck
83
is suspended from cables
84
,
85
. The cables
84
,
85
extend along cable guide means
86
,
87
at the end of pivot beams
88
,
89
. Buoyancy tanks
90
,
91
are attached to the end of cables
84
,
85
. Before installation, the pivot beams
88
,
89
may be placed into the position which is indicated with the dash and dot lines. When the weight of the risers attached to the deck
83
increases, the pivot beams
88
,
89
may be lowered to the position shown in
FIG. 9
with the solid lines, for increasing the tension in the cables
84
,
85
. In an alternative embodiment, the arms
88
,
89
are fixed and the buoyancy tanks
90
,
91
are ballastable and flushable.
FIG. 10
shows an embodiment of a barge
92
wherein the riser tensioning deck
93
is suspended from cables
99
,
100
which are attached to the ends of pivot arms
94
,
95
. The pivot arms
94
,
95
are hingeingly connected to the barge
92
via hinges
94
′,
95
′. Counterweights
96
,
97
are movable along the arms
94
,
95
for instance by a rack
97
and pinion
98
construction for varying the tensioning force on the cables
99
,
100
.
FIG. 11
shows an embodiment of a barge
106
wherein the blow-out preventor
101
is supported on the riser tensioning deck
102
. A drilling rig
107
is placed on a supporting deck
108
. The riser tensioning deck
102
is suspended from cables which are tensioned by weights
109
,
109
′, which may be ballastable for varying the tensioning force. Via a coupling member, which may be a usual coupling of a drill string segment
103
, the drill string
104
may be detached from the drilling rig
107
. During stormy conditions, the drill string
104
is detached from the rig
107
, while the lower part of the drill string and drilling riser
105
are hung off the inside of the blow out preventor
101
. When sea conditions return to within specific limits, the drill string
104
is reconnected to the drilling rig
107
, without the need to dismantle the total drill string and drilling riser (
105
) and with a minimum down time.
FIG. 12
shows a barge
110
which comprises a turret
111
which is anchored to the seabed via a chain table
112
. The riser tensioning deck
113
is suspended above or within the turret
111
. The supporting deck
114
and drilling rig
115
are connected to the vessel which can weather vane fully around the turret
111
. This construction provides full weathervaning and drilling capacities, such that the barge
110
can adjust its position according to prevailing wind and current conditions.
FIG. 13
shows an embodiment wherein two risers
120
,
121
are suspended from hydraulic cylinders
122
,
123
, that are connected via a flow line
124
. The upward force exerted by the cylinders can be increased by increasing the pressure or by increasing the number of interconnected cylinders. Any of the tensioning methods described above can be used either seperately or in combination with one or more other tensioning methods that are illustrated. Furthermore, active tensioning systems using winches or hydraulic pressure may also be used, either as an alternative to, or in combination with the passive tensioning methods described above.
Claims
- 1. Method of connecting an elongate connection member (5, 6; 33, 34; 105) between the seabed and a floating vessel (1, 29, 30, 50, 62, 82, 92, 106), the vessel carrying a connector (7, 20, 32, 57, 63, 83, 93, 102) which is suspended from the vessel by at least two spaced apart suspension members (10, 11, 21, 22) for relative displacement of the connector with respect to the vessel, at least one suspension member being connected to tensioning means (12, 12′, 13, 13′; 25, 26; 45, 46, 47, 48; 51, 52; 73, 74; 90, 91; 96, 97) for exerting an upward force on the connector, characterized in that, the connector is adapted to carry a plurality of connection members, each connection member being suspendable from the connector by an upper end part, the method comprising the steps of:a. attaching one or more connection members to the connector, the added connection members having a predetermined weight; b. increasing the tensioning force of the tensioning means before or after attaching the one or more connection members by an amount that is dependent on the weight of said one or more connection members; and c. repeating steps a and b. until the plurality of the connection members are installed between the vessel and the seabed.
- 2. Method according to claim 1, wherein the suspension members comprise a cable (23, 24, 35, 36) or arm (10, 11) having a first end attached to the connector (7, 20, 32, 57) and a second end attached to a tensioning weight, wherein additional tensioning weights (12, 12′, 13, 13′, 45, 46, 47, 48, 51, 52) are added to the cable or arm.
- 3. Method according to claim 2, the cable (35, 36) having a first section (37, 38) extending vertically downward from the vessel to below water level, and a second section (41, 42) which extends via a loop (39, 40) back upward from the first section (37, 38) to a pulling device (43, 44) on the vessel, tensioning weight (45, 46, 47, 48, 51, 52) or buoyancy members being comprised on the first and on the second cable sections (37, 38, 41, 42), the tensioning force on the connector being varied by varying the length of the first and second cable sections via the pulling device.
- 4. Method according to claim 1, wherein the suspension members comprise a cable guide member (78, 78′), a cable (64, 65) being placed along the cable guide member (78, 78′), and having one end attached to the connector (63) and another end attached to the seabed (70), a take-up device (71, 72) being connected to the cable (64, 65) for varying t he length of the cable.
- 5. Method according to claim 4, wherein weights (73, 74) are attached to the end of the cable (64, 65) resting on the seabed, the tensioning force being increased by pulling in the cable and lifting one or more of the weights from the seabed.
- 6. Method according to claim 4, wherein the cables (64, 65) comprise at least one elastic cable section (77), the tensioning force on the connector being increased by increasing the tension of the elastic cable section.
- 7. Method according to claim 1, wherein the suspension members comprise a cable (84, 85) having a first end attached to the connector (83) and having a second end attached, via a cable guide means (86, 87) that is located below water level, to a buoyancy device (90, 91) below water level.
- 8. Method according to claim 7, the buoyancy of the buoyancy device being varied.
- 9. Method according to claim 7, the cable guide means (86, 87) being displaced for varying the tensioning force on the connector.
- 10. Method according to claim 1, the suspension members comprising pivot arms (10, 11, 94, 95), which are attached to the vessel (1, 92) in a pivot point (10′, 11′, 94′, 95′) and which are with one end attached to the connector (7, 93), the tensioning force being varied by displacing a tensioning weight (12, 12′, 13, 13′, 96, 97) along the pivot arms and/or varying the relative posit ion of the pivot point with respect to the arms.
- 11. Vessel (1, 29, 30, 50, 62, 92, 106) for carrying out the method according to claim 1, comprising a connector (7, 20, 32, 57, 63, 83, 93, 102), for supporting an elongate connection member extending from the seabed, which connector is suspended from the vessel by at least two spaced apart suspension members (10, 11, 21, 22) for relative displacement of the connector with respect to the vessel, each suspension member comprising tensioning means (12, 12′, 13, 13′; 25, 26; 45, 46, 47, 48; 51, 52; 73, 74; 90, 91; 96, 97) for exerting an upward force on the connector, characterized in that, the connector is attachable to a plurality of connection members, the tensioning means comprising tension varying means for successively increasing the tensioning force of the tensioning means before of after attaching additional connection members to the connector, the additional connection members having predetermined weight, by an amount dependent on the weight of the additional connection members.
- 12. Vessel (106) according to claim 11, comprising a supporting deck (108) located above the connector (102), a first section of a connection member (105) extending from the seabed to the connector (102), and being detachably connected to a second section (104) of the connection member via a coupling device (103), the second section (104) being supported by the supporting deck (108).
- 13. Vessel according to claim 12, wherein the connector carries a blow-out preventor (101), the supporting deck (108) carrying a drilling rig (107).
Priority Claims (1)
Number |
Date |
Country |
Kind |
99200823 |
Mar 1999 |
EP |
|
PCT Information
Filing Document |
Filing Date |
Country |
Kind |
PCT/EP00/02363 |
|
WO |
00 |
Publishing Document |
Publishing Date |
Country |
Kind |
WO00/58597 |
10/5/2000 |
WO |
A |
US Referenced Citations (8)
Foreign Referenced Citations (2)
Number |
Date |
Country |
2 109 036 |
May 1983 |
GB |
WO 9931345 |
Jun 1999 |
WO |