WO2011099869A2

WO2011099869A2 - A method of installing a flexible, elongate element

Info

Publication number: WO2011099869A2
Application number: PCT/NO2011/000051
Authority: WO
Inventors: Trygve Holla
Original assignee: Subsea 7 Norway Nuf
Priority date: 2010-02-10
Filing date: 2011-02-10
Publication date: 2011-08-18
Also published as: NO20100203A1; WO2011099869A3; NO332196B1

Abstract

A method of installing an elongate element (8) between a seabed (B) below a body of water (W) and a platform (2) in said body of water, said elongate element comprising a first end (16) and a second end (28), comprising suspending the first end (16) by a second suspension means (25) on the platform; suspending the second end (28) by a first suspension means (15) on the platform; and conveying the first end (16) from the second suspension means (15) to a subsea location (B). The first end (16) may initially be suspended by the first suspension means (15) before it is transferred to the second suspension means (25).

Description

A method of installing a flexible, elongate element

Field of the invention

The invention relates to installation of flexible elongate elements connecting a subsea unit with a topsides platform. More specifically, the invention concerns a method of installing a flexible, elongate element, such as a marine riser, an umbilical, or a cable, as defined in the introduction to claim 1.

Background of the invention

The state of the art includes installation of marine risers, umbilicals, cables, etc, which are useful for a variety of possible applications in the offshore oil and gas industry.

A commonly used riser configuration is termed "reverse pliant wave". Here, the flexible (pliant) riser extends from the platform towards an anchoring point on the seabed and reverses back towards the platform to a subsea structure on the seabed near the platform. An installed reverse pliant wave riser is illustrated in figure 14.

A commonly used method of installing such risers involve the deployment of a clump weight, with a sheave, off the platform on the side opposite of the rises balcony, and connecting this clump weight to a platform winch. The winch wire is connected to the subsea head (static section of the riser). Then, the winch pulls the riser underneath the platform as the installation vessel is paying out on the riser. The deployment of the clump weight and connection of the required wires, etc., are time consuming and thus costly activities. The method is also somewhat imprecise and involves a risk of the subsea head and/or riser interfering with and damaging already installed equipment on the seabed and the platform.

In another method, an auxiliary vessel is positioned near the platform, on the opposite side of the riser balcony side, and uses a winch and wire to pull the subsea head underneath the platform, as the installation vessel is paying out on the riser. While this method is more precise and reliable than the aforementioned method, the requirement for the additional vessel often disqualifies this method for cost reasons.

Summary of the invention

The invention is set forth and characterized in the main claim, while the dependent claims describe other characteristics of the invention. It is thus provided a method of installing an elongate element between a seabed below a body of water and a platform in said body of water, said elongate element comprising a first end and a second end, characterized by

- suspending the first end by a second suspension means on the platform;

- suspending the second end by a first suspension means on the platform;

- and conveying the first end from the second suspension means to a subsea location.

In one embodiment, the first end is initially suspended by the first suspension means before it is transferred to the second suspension means.

In one embodiment, the elongate element is conveyed with its first end first into the water from an installation vessel, and the first end is connected to a first suspension means attached to the platform; and the elongate element is conveyed into the water while the first end is suspended by the first suspension means, whereby an increasing portion of the elongate element is placed on the seabed; said method further comprising the steps of:

a) connecting the first end to a second suspension means attached to the platform; b) extending the first suspension means until the load exerted by the elongate element and the first end on the first suspension means is substantially transferred to the second suspension means, and disconnecting the first suspension means from the first end;

c) connecting the first suspension means to said second end;

d) conveying the second end into the water until the second end and a portion of the elongate element are suspended by the first suspension means;

e) if required, relocate the installation vessel to an appropriate position in relation to the platform;

f) conveying a hoisting means from a vessel, for example the installation vessel, into the water and connecting the hoisting means to the first end;

g) operate the hoisting means until the load exerted by the first end and its associated suspended portion of the elongate element is substantially transferred from the second suspension means to the hoisting means, and disconnecting the second suspension means from the first end; and

h) placing the first end and its associated portion of the elongate element at the desired location on the seabed.

In a preferred embodiment, step c comprises conveying the first suspension means to deployment means on the installation vessel, holding the second end, and then performing the connection. In one embodiment, step d comprises lowering the second end into the water by means of a suspension device on the installation vessel. In one embodiment, buoyancy elements are attached to a portion of the elongate element, and a part of the elongate element is tethered to an anchor on the seabed.

The second end may be pulled in and connected to applicable equipment on the platform, at a suitable time following the completion of step d, and step h may comprise connecting the first end to applicable subsea equipment.

In one embodiment, step a comprises connecting a connecting element between the first end and the second suspension means.

In one embodiment, the elongate element comprises a flexible element. The elongate element may be a flexible tubular element, such as a marine riser. The elongate element may comprise a pliant marine riser which is installed in a reverse pliant wave configuration. The elongate element may comprises an umbilical or a cable.

A purpose of the invention is to achieve an installation method where the tubular element may be pulled underneath platform anchor chains, an other equipment, in a safe, reliable and efficient manner. The installation method saves time and reduces the required installation spread, compared to the prior art, as only one installation vessel is required. The installation vessel is free to return to the platform once the dynamic section of the tubular element has been installed, and may thus perform the remaining installation of the static section.

Brief description of the drawings

These and other characteristics of the invention will be clear from the following description of a preferential form of embodiment, given as a non-restrictive example, with reference to the attached drawings wherein:

Figure 1 is a schematic plan view of an installation vessel and a platform, and other components related to the method of installation;

Figure 2 is a side view of a portion of the installation vessel, particularly identifying the riser deployment equipment;

Figures 3 - 5 are schematic side views, illustrating various stages of the riser installation process;

Figure 6 is a schematic side view of riser suspension equipment on the platform;

Figure 7 is a schematic side view, illustrating a further stage of the riser installation process; Figures 8 - 10 are side views of a portion of the installation vessel, particularly identifying the riser deployment equipment, illustrating further stages of the riser installation process;

Figures 11, 12 and 14 are schematic side views, and figure 13 is a schematic plan view, illustrating further stages of the riser installation process.

Detailed description of a preferential embodiment

Figure 1 is a schematic and simplified illustration of a floating platform 2, indicating platform reference directions (north, east, south, west). The platform 2 comprises a riser balcony 7 and is anchored to the seabed by means of anchor lines or chains 3 (only partly shown) extending in a catenary configuration between each corner of the platform and corresponding anchors on the seabed (not shown). An installation vessel 4, having riser deployment equipment 5, is located in the vicinity of the platform. A number of installed risers 8, tethered to anchors 21 on the seabed, are also shown. The installation method pertains to the installation of such risers.

Figure 2 is an illustration of typical riser deployment equipment 5, positioned in connection with and partly above a so-called moonpool 9 on the installation vessel 4. This equipment, which is well known in the art, comprises lower and upper chutes 14a,b for guiding the riser 8 from e.g. a storage reel and into a more or less straight, vertical, configuration in which it is feed down through the moonpool 9 and into the water W. The hanging riser is supported by upper and lower tensioners 10a,b, which also control the deployment of the riser through the moonpool. Figure 2 also shows an abandonment-and-recovery (A&R) crane 12a, with associated winch 12b and wire 12c. Work platforms 1 la,b are also illustrated, as is a hang-off clamp 31 and a centralizer. A moonpool winch 9a, having a wire 9b, is positioned near the moonpool 9. The riser deployment equipment 5 is generally known in the art.

Figure 3, which is a side view looking in the platform north direction (cf. figure 1), shows a preliminary step of the installation procedure, where the installation vessel 4 is positioned a distance d_\ away from the platform 2, for example about 50 metres. A remotely operated subsea vehicle (ROV) 18, controlled by e.g. the installation vessel via an umbilical 18b, is also shown, as it is being used for executing parts of this embodiment of the method.

A platform pull-in wire 15, being connected to the pull-in winch (not shown) on the platform and comprising a connection element 17 is being lowered to a suitable depth in the water W beneath the surface S. The riser 8, being equipped at its first end with a subsea connection element (hereinafter referred to as a "subsea head") 16 is also being lowered from the installation vessel 4, as explained above with reference to figure 2. The pull-in winch connection element 17 and the subsea head 16 are lowered to approximately the same depth in the water, whereupon they are being connected in a known fashion, e.g. by means of the ROV 18.

Figure 4 shows a stage where the subsea head 16 has been connected to the pull-in wire 15 (via connection element 17) and the installation vessel 4 has moved farther away from the platform while paying out on the riser 8. In the illustrated

embodiment, the distance d₂ between the platform 2 and the installation vessel 4 may be approximately 80 metres, and the distance d₃ between the lowermost portion of the riser 8 and the seabed B may be approximately 20 metres. The skilled person will understand that these distances may be adapted to specific requirements and controlled by either one of e.g. installation vessel surface speed, riser deployment speed and pull-in wire manipulation, by themselves or in combination.

In order to illustrate that the method according the invention also may be executed in between pre-existing risers, figure 4 also shows an installed riser 8', where the topsides end (not shown) has been pulled into the platform riser balcony 7, where buoyancy elements 19 have been installed and the riser 8' is connected to a subsea anchor 21 via a tether clamp 22 and a tether 20. The portion of the riser resting on the seabed and being connected to a subsea structure (not shown in figure 4) is termed the static section 8_S', while the portion of the riser suspended by the platform and describing the known "reverse pliant wave" configuration by virtue of the buoyancy elements is termed the dynamic section 8d'.

Figure 5 depicts three stages of the installation sequence of the riser 8, continued from what was illustrated by figure 4. The preinstalled riser 8' is not shown, for the sake of clarity (In general, already installed risers and other ancillary equipment not essential for the invention, have been omitted from most of the figures for the sake of clarity of illustration). The installation vessel 4 is moving away from the platform 2, as indicated by the bold-faced arrow M in figure 5, while paying out on the riser 8.

In the stage indicated by (a) in figure 5, the installation vessel has moved farther away from the platform than in figure 4 and a portion of the riser 8 is resting on the seabed. Buoyancy elements 19 and a tether clamp 22 have been installed on the riser 8, and a clump weight 23 - appropriately dimensioned for the applicable conditions and requirements - is suspended from the tether clamp 22 via a temporary tether 24.

In the stage indicated by (b) in figure 5, the installation vessel has moved even farther away from the platform, and a longer portion of the riser 8 than in the previous stage is resting on the seabed. In the stage indicated by (c) in figure 5, the installation vessel has moved even farther away from the platform, the clump weight 23 has landed on the seabed in the vicinity of the anchor 21 to which the riser subsequently will be tethered, and an even longer portion of the riser 8 than in the previous stage is now resting on the seabed. The permanent tether 20 may now be installed (indicated by dotted line) and the clump weight 23 and temporary tether 24 may subsequently be removed, all in a manner well known in the art.

During this procedure, illustrated in figure 5, the riser subsea head 16 has been suspended in the water by the pull-in wire 15.

Figure 6 is a schematic side view of a portion of the riser balcony 7 on the platform 2, looking in the direction of platform west (cf. figure 1), and illustrates how the riser 8 subsea head 16 is suspended by the platform's pull-in wire 15. A wire 25, hereinafter referred to as static rigging 25, is suspended from the platform and has a free end in the vicinity of the subsea head 16. At this stage, a sling 26 or similar connection element is connected between the subsea head 16 and the end of the static rigging 25. The distance between the static rigging and the subsea head, and consequently the required length of the sling 26, is determined based on the applicable circumstances. The connection of the sling 26 is preferably performed by an ROV, or similar device.

When this connection between the static rigging 25 and the subsea head 16 (and the riser 8) thus has been established, the platform pull-in wire 15 is lowered further down, until the entire weight of the subsea head and the length of the riser between the seabed and the subsea head (termed the static section, to be discussed below), are suspended by the static rigging 25. At this point, the pull-in wire 15 is no longer under tension from the riser and subsea head, and is disconnected from the subsea head.

Figure 7 illustrates how, after the operation described above with reference to figure 6 has been performed, the subsea head 16 and a portion of the riser 8 now are suspended by the static rigging 25. The actual connection between the static rigging and the platform has been omitted from figure 7 for clarity of illustration of the ensuing steps, and as this connection is illustrated by figure 6 and furthermore is well known to the skilled person.

Following the connection of the tether clamp 22 to the anchor 21 via the permanent tether 20 and removal of the clump weight 23, as described above with reference to figure 5, the installation vessel 4 moves back towards the platform 2 while continuing to feed the riser 8 into the water. This return movement, known in the art as a "flip-over" operation, essentially deploys the dynamic section 8_d of the riser into the water and generates the well known "reverse pliant wave" riser

configuration. The installation vessel 4 continues its movement towards the platform 2 until the two vessels are within operational reach of one another in order to transfer the second end of the riser 8, comprising a platform connection element (hereinafter referred to as a "topsides head") 28, from the installation vessel 4 to the riser balcony 7 on the platform 2.

Figure 8 depicts a state of the riser deployment equipment which could correspond to the installation vessel position shown in figure 7, and illustrates how the riser is suspended by the tensioners 1 la,b and shows the topsides head 28 at the riser's second end. The A&R wire 12c is connected to the topsides head, and the moonpool winch wire 9b is fed down into the moonpool 9 and further into the water.

Referring again to figure 7, the moonpool winch wire 9b is pulled - e.g. by an ROV 18— from the installation vessel and to the end of the pull-in wire 15 which is now hanging freely in the water (following the off-loading of the subsea head, as described above with reference to figure 6). Once the moonpool winch wire 9b has been connected to the pull-in wire 15, the moonpool winch 9a is activated and the pull-in winch is released, whereby the pull-in wire 15 is pulled into the installation vessel's moonpool 9. This process is illustrated in figure 9, also showing how the moonpool winch wire runs via a pulley-and-davit configuration 9c. The topsides head 28, having a pull-in head 28a, has been lowered past the tensioners 10a,b and is suspended above the moonpool by a hang-off clamp 31. A steel wire or sling 33 is connected to the A&R wire 12c, and the pull-in wire 15 is retrieved by the VLS crane wire 34.

In figure 10, both the A&R wire 12c (via the wire 33) and the pull-in wire 15 (extending from the platform and up through the moonpool 9) have been connected to the topsides head 28 (via the pull-in head 28a). The moonpool winch wire 9b has been disconnected from the pull-in winch 15. At this stage, the hang-off clamp 31 is opened and the riser with its topsides head 28 is lowered in a controlled manner through the moonpool and into the water while suspended by the A&R wire 12c being paid out from the A&R crane 12a (illustrated in figure 8).

The topsides head 28 is lowered by the A&R crane into the waters until the topsides head reaches a depth d₄ (see figure 11) corresponding to the extended length of the pull-in wire 15 and the entire weight of the topsides head 28 and the dynamic section 8_d of the riser has been transferred from the A&R crane to the pull-in wire 15. This depth may be determined depending on applicable circumstances and may i.a. be controlled by a hauling up on the pull-in wire as the topsides head is being lowered.

Figure 11 illustrates a state where the entire weight of the topsides head 28 and the dynamic section 8_d of the riser has been transferred from the A&R crane to the pull- in wire 15, and the A&R wire has been disconnected. (The subsea head 16 is still suspended by the static rigging 25, cf. figure 7 and the corresponding description). Both the first end (subsea head 16) and the second end (topsides head 28) of the riser 8 are now suspended by the platform 2, and the installation vessel 4 is free to move.

As the installation vessel now is released from the riser, it may be used in the remaining deployment procedure of the subsea head 16 on the seabed. Another vessel may also be used for this purpose, but it is advantageous from an operational perspective (e.g. cost and time aspects) to also employ the installation vessel 4 for the subsea head installation.

Figure 12 is a schematic side view of a portion of the riser balcony 7 on the platform 2, looking in the direction of platform west (cf. figures 1 and 6), and illustrates how the topsides head 28 is suspended by the pull-in winch 15, at a distance d₅ below the water surface. In a practical application, this distance may be in the order of 10 metres. The topsides head 28 is pulled in and connected to the relevant connections on the platform in a manner which is known to the skilled person.

Figure 12 also illustrates how the installation vessel 4 has moved from the platform east side to the platform south side, and has deployed a crane wire 30 in order to retrieve the subsea head 16. Figure 13 is a top view of essentially the same set-up as in figure 12, and the two figures illustrate how the crane wire 30 is conveyed (preferably by an ROV 18) to the location beneath the riser balcony 7 where the subsea head 16 is suspended by the static rigging 25. Figure 13 indicates how this may be accomplished by going underneath the anchor chains 3. Although not illustrated by figure 13, the installation vessel 4 may also - depending on operational circumstances - be positioned on the platform west side, and the ROV 18 may convey the crane wire 30 underneath the platform. The method according to the invention makes it possible to safely and efficiently manoeuvre the subsea head in the vicinity of the platform without interfering with anchor chains, other risers and other objects connected to the platform.

Figure 12 shows how, in a practical application, a pilot line 30a which is easier to handle with the ROV, is first attached to the crane wire 30, whereupon the actual crane wire 30 is connected to the subsea head while the subsea head is still suspended by the static rigging.

When the crane wire 30 has been connected to the subsea head 16, the subsea head may be positioned in the desired location on the seabed by the installation vessel 4, tied in and connected to a subsea structure 32, schematically illustrated by figure 14. The installation method according to the invention may thus be summarized by the following main points:

• Platform 2 feeds pull-in wire 15 to a desired depth in the water (depth

determined by length of riser static section), and installation vessel 4 feeds riser 8 with subsea head 16 to a depth corresponding to the depth of the pull-in wire (figure 3).

• Subsea head 16 is connected to pull-in wire 15.

• Installation vessel 4 resumes feeding out riser 8, while moving away from

platform 2. Subsea head 16 is suspended by platform pull-in wire 15 (figure 5).

• Increasing length of riser static section 8_S is laid onto seabed. Tether clamp 22, temporary tether 24 and clump weight 23 are attached to riser at the appropriate (predetermined) location. Buoyancy elements 19 are connected to riser 8 above tether clamp 22. Procedure continues until clump weight 23 lands on seabed, in vicinity of permanent tether anchor 21 (figure 5, position (c)).

• Tether clamp 22 is tied to anchor 21 via permanent tether 20. Clump weight 23 and temporary tether 24 are subsequently removed.

• Installation vessel 4 performs "flip-over" manoeuvre, i.e. moves back towards platform 2, deploying riser dynamic section 8_d in a "reverse pliant wave" configuration (figure 7).

• Sling 26 is connected between riser subsea head 16 and static rigging 25 on

platform 2 (figure 6). Platform pull-in winch pays out on pull-in wire 15 until weight of riser subsea end (including subsea head 16) is transferred to static rigging 25. Pull-in winch is disconnected from subsea head 16 (figure 7).

• Pull-in wire 15 is retrieved onto installation vessel 4, e.g. by means of moonpool wire 9b through moonpool 9, via ROV 18 (figures 7 and 9.

• Riser 8 deployment continues until topsides head 28 reaches tensioners 10a,b.

A&R wire 12c on installation vessel is connected to topsides head 28, which is lowered past tensioners (to e.g. hang-off clamp 31). Pull-in wire 15 is connected to topsides head 28 (figure 10).

• Topsides head 28 is lowered into water (by A&R winch wire 12b,c) until weight of riser dynamic section 8_d (including topsides head 28) is transferred to pull-in wire 15. A&R wire is disconnected (figure 11).

Installation vessel 4 moves to desired location and deploys installation crane wire 30 (possibly preceded by pilot line 30a) and connects it to the subsea head 16 which is suspended by static rigging 25. Positioning of crane wire 30 by e.g. ROV 18, going underneath anchor chains 3, etc. Crane wire 30 is tightened, whereby load on static rigging is removed. Static rigging 25 is disconnected from subsea head 16, which now is suspended by crane wire 30 (figure 12 and 13).

• Subsea head 16 positioned on desired seabed location (figure 14) by installation vessel 4 and crane wire 30.

Although the invention has been described with reference to a floating platform, the skilled person will understand that the invention is equally applicable to other marine structures, either floating platforms or structures resting on the seabed.

Although the invention has been described with reference to riser deployment where the riser is feed into the water through a moonpool, the invention shall not be limited to such riser deployment methods and equipment.

The skilled person will understand that the figures are schematic illustrations, not to scale, showing features relevant for understanding the method according to the invention. For example, pre-installed risers and platform anchor chains have been omitted from some of the drawings.

Although the invention has been described with reference to the installation of a marine riser, the method also applies to the installation of other flexible elongate elements, such as umbilicals and cables.

Claims

1. A method of installing an elongate element (8) between a seabed (B) below a body of water (W) and a platform (2) in said body of water, said elongate element comprising a first end (16) and a second end (28), characterized by

- suspending the first end (16) by a second suspension means (25) on the platform;

- suspending the second end (28) by a first suspension means (15) on the platform;

- and conveying the first end (16) from the second suspension means (15) to a subsea location (B).

2. The method of claim 1, wherein the first end (16) is initially suspended by the first suspension means (15) before it is transferred to the second suspension means (25).

3. The method of claim 1 or claim 2, wherein

- the elongate element (8) is conveyed with its first end (16) first into the water (W) from an installation vessel (4), and the first end (16) is connected to a first suspension means (15) attached to the platform (2); and

- the elongate element (8) is conveyed into the water while the first end (16) is suspended by the first suspension means (15), whereby an increasing portion of the elongate element (8) is placed on the seabed (B);

said method further comprising the steps of:

a) connecting (26) the first end (16) to a second suspension means (25) attached to the platform (2);

b) extending the first suspension means (15) until the load exerted by the elongate element (8) and the first end (16) on the first suspension means (15) is substantially transferred to the second suspension means (25), and disconnecting the first suspension means (15) from the first end (16);

c) connecting the first suspension means (15) to said second end (28);

d) conveying the second end (28) into the water until the second end (28) and a portion (8_d) of the elongate element are suspended by the first suspension means (15);

e) if required, relocate the installation vessel (4) to an appropriate position in relation to the platform (2);

f) conveying a hoisting means (30) from a vessel, for example the installation vessel, into the water and connecting the hoisting means (30) to the first end (16); g) operate the hoisting means (30) until the load exerted by the first end (16) and its associated suspended portion of the elongate element (8) is substantially transferred from the second suspension means (25) to the hoisting means (30), and

disconnecting the second suspension means (25) from the first end (16); and h) placing the first end (16) and its associated portion of the elongate element (8) at the desired location (32) on the seabed.

4. The method of claim 3, wherein step c comprises conveying the first suspension means (15) to deployment means (5) on the installation vessel (4), holding the second end (28), and then performing the connection.

5. The method of claim 3 or claim 3, wherein step d comprises lowering the second end (28) into the water by means of a suspension device (12a,b,c) on the installation vessel (4).

6. The method of any one the preceding claims, wherein buoyancy elements (19) are attached to a portion of the elongate element (8), and a part of the elongate element (8) is tethered (22, 20) to an anchor (21) on the seabed.

7. The method of any one the preceding claims 3 - 6, wherein the second end (28) is pulled in and connected to applicable equipment on the platform (2), at a suitable time following the completion of step d.

8. The method of any one the preceding claims 3 - 7, wherein step h comprises connecting the first end (16) to applicable subsea equipment (32).

9. The method of any one the preceding claims 3 - 8, wherein step a comprises connecting a connecting element (26) between the first end (16) and the second suspension means (25).

10. The method of any one the preceding claims, wherein the elongate element (8) comprises a flexible element.

11. The method of any one the preceding claims, wherein the elongate element (8) comprises a flexible tubular element, such as a marine riser.

12. The method of any one the preceding claims, wherein the elongate element (8) comprises a pliant marine riser which is installed in a reverse pliant wave configuration.

13. The method of any one the preceding claims, wherein the elongate element (8) comprises an umbilical or a cable.