WO1998004805A1

WO1998004805A1 - An overtrawlable protection envelope

Info

Publication number: WO1998004805A1
Application number: PCT/GB1997/002016
Authority: WO
Inventors: David John Walker
Original assignee: J. Ray Mcdermott S.A.
Priority date: 1996-07-25
Filing date: 1997-07-25
Publication date: 1998-02-05
Also published as: GB9615647D0; AU3700697A

Abstract

An overtrawlable protection envelope (E) for a subsea structure (F, T) which is connected to the seabed wherein the envelope (E) is anchored by connecting it to the subsea structure (F, T) which it is to protect. The envelope (E) may be defined by four legs (3, 14) pivotally deployable from folded positions to deployed positions during installation. Each leg may include separate upper and lower portions (3, 14) when used to protect a two-part structure such as one comprising a flowbase (F) and a tree (T). The legs may also have telescopic parts (4, 15).

Description

AN OVERTRAWLABLE PROTECTION ENVELOPE

This invention relates to an overtrawlable protection envelope

In the field of subsea oil and gas exploitation, it is sometimes desirable or even necessary to provide an overtrawlable protection envelope to provide protection to subsea equipment from accidental damage caused by trawled fishing nets The subsea equipment is usually provided close to the seabed and often includes equipment such as "trees" A large number of trees may be arranged in groups to form "satellite tree systems" Trees are critical assets in subsea development programmes and they are typically provided for a number of different functions well known to persons skilled in the art

As these trees are typically complex items of equipment and often installed at relatively large depths, they are expensive pieces of equipment to purchase and install Trees are valuable assets and it is therefore highly desirable to ensure that individual trees and the system as a whole are as reliable as possible

Unfortunately, as the trees are typically provided on the seabed and often far offshore, their operational location renders them highly susceptible to accidental damage As mentioned above, a principal cause of significant damage is bottom gear towed by trawlers which can impact with and snag on the trees

One solution to this problem would be to designate statutory fishing exclusion zones in the vicinity of subsea installations. However, the designation of these statutory exclusion zones is often not forthcoming, and thus it has been previously proposed to provide a protection structure which is capable of encompassing individual trees of the tree system thereby to protect the trees from accidental damage

Previously proposed protection solutions include a freestanding frame positionable over, but not in contact with, a tree and anchored and mounted on its own dedicated anchoring points and foundations This structure is unduly expensive and awkward to install as it is of such a size that it cannot conveniently be installed from a drilling rig and, thus, the services of a separate specialist installation vessel must be acquired. Furthermore, the manner by which the protection structure is secured to the seabed is such that the installation is complex and usually requires a piling spread, the services of a number of divers and remotely operated vehicles (ROV). This significantly increases the cost of the installation.

An object of the invention is to alleviate some or all of these problems.

In accordance with the present invention, there is provided an overtrawlable protection envelope for a subsea structure which is connected to the seabed wherein the envelope is anchored by connecting it to the subsea structure which it is to protect.

In this way, the invention can provide significant savings in capital expenditure as the protection envelope can, in one exemplary embodiment, be deployed concurrently with the subsea structure which it is desired to protect without requiring the use of separate installation facilities.

Preferably, the envelope comprises means for defining an overtrawlable profile, the defining means being moveable from a folded position wherein the envelope has a reduced footprint to a deployed position wherein the envelope has an enlarged footprint which defines an overtrawlable profile. The ability to reduce the footprint of the envelope can allow the envelope to be deployed through, for example, the moonpool of a drilling rig.

Preferably, the defining means comprises a plurality of members such as legs. The legs may be pivotally deployable from the folded position to the deployed position.

The legs may each comprise an upper portion and a lower portion, the upper and lower portions being independently moveable between the folded position in which they are disaligned and the deployed position in which they are aligned. In this case, the upper portion may be provided with a telescopic portion which is engageable with the lower portion to preserve the overtrawlable profile when the upper and lower portions are aligned.

Preferably, the legs are each provided with a telescopic element slidably mounted therein and extendable to connect with the seabed.

Preferably, a cover assembly is provided for covering at least an upper portion of the subsea structure. The cover can reduce the chance of a trawl net or trawl bottom gear, for example, snagging on the subsea structure.

The legs may be attachable to respective positions on the subsea structure.

The upper portion of each leg may be attachable to an upper tree portion of the subsea structure, and the lower portion of each leg may be attachable to a lower flowbase portion of the subsea structure, in the case that the structure comprises two portions, namely the lower flowbase portion and the upper tree portion.

Preferably, the overtrawlable profile includes a plurality of faces inclined at an angle of between approximately 30° and approximately 70° to the seabed. Preferably, the angle of inclination is substantially 58°. The profile may be substantially square pyramidal or substantially triangular pyramidal.

Preferably, the envelope is recoverable to the surface together with the subsea structure to which it is attached.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 is a front view of an exemplary embodiment of an overtrawlable protection envelope according to the invention;

Figure 2 is a cross-sectional view along the line A-A of Figure 1 ; Figure 3 is a plan view of the envelope of Figure 1 ; and

Figure 4 is a side elevational view of a deployed envelope on the seabed.

The present invention will now be described with particular reference to an exemplary subsea structure which comprises a flow base F and a so-called Christmas tree T. It should be noted that the following description is only exemplary and that the invention may be employed for the protection of subsea structures that include a Christmas tree but do not include a separate flowbase. The invention may also be used with a variety of other subsea structures. Accordingly, the invention is not to be construed as being limited to a particular type or construction of subsea structure. It should also be noted that the envelope may be deployed concurrently with the subsea structure, or may be deployed subsequent to the deployment of the subsea structure. In the present description, it will be assumed that the envelope is to be deployed concurrently with the subsea structure.

It is convenient at this juncture to briefly describe the function of the two parts of the exemplary subsea structure mentioned herein.

In the mentioned structure, the flow base F comprises a system which supports subsea flow line and umbilical tie-ins, a subsea connector which is capable of interfacing with the Christmas tree and interconnecting piping. Typically, during the installation of the subsea structure, the flow base is deployed first and then subsequently connected to and supported by a housing which forms part of a well head.

The Christmas tree T mentioned herein comprises production or injection masters, wing valves, an annulus wing valve and a production or injection choke. The Christmas tree is typically deployed after the flow base and is connected to the housing of the wellhead. Accordingly, in this example, the Christmas tree is positioned above the flow base and both are connected to the wellhead housing. Both the flowbase and the Christmas tree are standard pieces of equipment which will be well known to persons skilled in the art.

With reference to Figure 1 , a protection envelope E according to a preferred embodiment of the invention comprises three sub-assemblies:

1. A lower assembly L connectable to the flowbase F;

2. An upper assembly U connectable to the Christmas tree T; and

3. A cover assembly C mountable on top of the tree. The lower and upper assemblies respectively comprise lower and upper sections of four legs, one of which will be later described. As mentioned above, it would be advantageous if the protection envelope could be deployed from a drilling rig, preferably through a moonpool of the rig. As the envelope is significantly larger than typical moonpools, provision is made for at least some of the legs to be moveable between a folded (stowed) position in which the footprint of the envelope is smaller than the moonpool, and a final deployed position in which the footprint of the envelope is larger than the moonpool. Once free of the moonpool, the stowed legs can be moved from the folded position to the deployed position.

The cover assembly is securable to the tree to provide a smooth profile to the envelope as well as protecting the Christmas tree from damage caused by dropped objects.

With reference to Figure 2, the lower assembly L is connectable to the flow base F and comprises four structurally identical leg lower portions 3, one of which will be described herein. Preferably, each lower leg portion is connected to a respective corner of the flow base. It will be apparent however that three, four or more legs could be provided if desired.

A hinge support bracket 1 is bolted, or otherwise attached, to the underside of the flow base. The bracket supports a hinge pin 2 which locates within a hinge housing 6. The hinge housing 6 is connected to the lower leg portion 3 by a pair of tubular members 9.

The lower leg portion 3 and connected hinge housing 6 are pivotable about the hinge pin 2 so that the lower leg unit can be rotated from a folded pre-deployment position to a final deployed position.

The lower leg portion 3 has a telescopic element 4 slidably provided therein which can extend downwards from inside the lower leg unit to the seabed.

A number of removable pins are provided which, until removed, hold the various components of the lower leg portion in the folded pre-deployment position. A further set of pins enable the members to be locked into the final deployed position.

The hinge housing 6 is locked in the folded position by a pin 7 located at the top of the housing which cooperates with a hole in the hinge pin 2. When the pin 7 is removed, and the lower leg portion is pivoted to its final support position, two load bearing sprung loaded pins 8 are resiliently biased into further holes in the hinge pin

2 thereby to lock the lower leg portion in its final deployed position.

Once the lower leg portion has been pivoted into the final position, a pin 5 can be removed to allow the telescopic element 4 to be extended downwards from inside the lower leg portion to the seabed. The lower leg portion preferably extends by gravity. Alternatively, resilient biasing means, such as a spring, may be provided within the lower leg portion 3 to urge the telescopic element 4 out of the lower leg portion 3.

The upper assembly U is connectable to the Christmas tree T and comprises a plurality of structurally identical upper leg portions 14, one of which will be described herein. Preferably, four leg upper portions are provided, each connected to a respective corner of the Christmas tree.

An upper support plate 10 and a lower support plate 12 are respectively bolted, or otherwise attached, to upper and lower corners of the Christmas tree T. One or more spacer plates 11 may be provided to adjust the position of the upper and lower plates with respect to the Christmas tree.

The upper leg portion 14 is pivotally connected to the upper plate 10 by a locating pin 20, thereby to allow the upper leg portion 14 to be pivoted from a folded pre-deployment position to a final deployed position, wherein the upper leg portion is substantially coincident with the corresponding lower leg portion 3.

The lower support plate 12 supports a hinge pin 13 which locates within a hinge housing 17. The hinge housing 17 is connected to the upper leg portion 14 by a horizontal tube 21. The upper leg portion 14 has a telescopic element 15 with a conical lower end provided therein. In the final deployed position, the telescopic element 15 can be extended downwards from inside the upper leg portion 14 into the lower leg portion 3.

The upper assembly U is provided with a number of removable pins that, until removed, hold the upper leg portion in the folded position. A further set of pins enable the upper leg portion to be locked into the final deployed position.

The hinge housing 17 is locked in the folded position by a pin 18 located at the bottom of the housing. The pin cooperates with a hole in the hinge pin 13 to secure the upper leg portion 14 in the folded position. When the pin 18 is removed, the upper leg portion can be pivoted about a pivot axis defined by the hinge pin 13, and at its upper end by the locating pin 20, to the final deployed position. When pivoted to its final deployed position, two load bearing spring loaded pins 19 are sprung into further holes in the hinge pin 13 to lock the upper leg unit in position.

Once the upper leg unit is pivoted into the final deployed position, a further pin 16 can be removed to allow the telescopic element 15 to descend, preferably under gravity or alternatively by virtue of a resilient biasing means provided within the upper leg portion 14, to enter the top of the lower leg portion 3.

The upper leg portion 14 is provided with an upper part which extends above the upper support plate to provide a continuous profile to the envelope.

The cover assembly C is preferably of a welded tubular steel construction 22 that abuts with a top plate of the Christmas tree T. The cover assembly C is shaped to complete, together with the upper U and lower L assemblies, the overtrawlable profile of the envelope.

A hollow can 25 is preferably provided on top of the cover assembly to provide clearance for a tree cap (not shown) and a debris cap (not shown).

Guide funnels 23 in the four bottom corners of the cover allow the cover assembly C to be located in position, for example by following the exemplary deployment scheme described below. Each guide funnel 23 is split to allow guidelines to pass through as the cover assembly C is lowered into position onto the tree T. Locking pins 24 are provided to lock the cover assembly C to the Christmas tree T.

With reference to Figures 3 and 4, it can be seen that the envelope according to this embodiment, when fully deployed, has a pyramidal shape with four sloping faces each defined by a pair of legs at an angle of approximately 58° to the seabed. This angle of inclination provides the optimum overtrawlable profile whilst also keeping the footprint of the envelope (ie. the area of a surface covered by the envelope) as small as possible when the envelope is in the folded position. It will be apparent that other inclination angles may be chosen in consideration of both of the above mentioned factors. In effect, the inclination angle may be varied from approximately 30° to approximately 70°. Figure 3 also shows, in ghost, the folded position of the lower leg portions (referenced 3') and the folded position of the upper leg portions (referenced 14'). In addition, Figure 3 provides, again in ghost, an indication of the approximate size of a typical moonpool (referenced M). From Figure 4, it can be seen that a trawled net would be guided over the tree, with a greatly reduced chance of snagging, by the profile of the envelope.

A brief description of one exemplary means of deploying the above described embodiment will now be provided.

Prior to lowering through the moonpool on the rig, all pivoting parts of the lower and upper assemblies of the envelope are pinned in the folded positions.

The flowbase F and lower assembly L of the envelope are lowered as an integral unit. First, the flowbase F is installed on the wellhead housing. Guide wires are then connected to guide posts on the flowbase F. The tree T and upper assembly U are then lowered as an integral unit, the guide wires passing through split guide funnels on the tree. The cover assembly C of the envelope is finally lowered as a separate unit with the guide wires passing through its own split guide funnels.

Once the components have been lowered through the moonpool M down to the seabed, each of the leg sections of the tree protection structure is finally positioned, lower assembly first followed by the upper assembly using a remote operated vehicle (ROV) in the below described manner. Then the cover assembly is finally positioned.

To deploy the lower assembly once on the seabed, the ROV is operated to remove the pin 7 located at the top of the hinge housing 6 on the lower assembly. This will leave the lower leg unit free to be pivoted by the ROV to its final installed position. In this position, the load bearing spring loaded pins 8 will be biased into holes in the hinge pin 2 to lock the lower leg unit into position. Pin 5 is removed by the ROV to allow the telescopic element 4 to descend from inside the lower leg portion to the seabed.

To deploy the upper assembly, the ROV is operated to remove the pin 18 located at the bottom of the hinge housing 12 of the upper assembly. This will leave the upper leg unit free to be pivoted by the ROV to its final deployed position. When pivoted to its final deployed position, the two load bearing spring loaded pins 19 will be sprung into holes in the hinge pin 13 to lock the upper leg portion into position. The pin 16 is subsequently removed by the ROV to allow the telescopic element 15 to enter the top of the lower leg unit 3.

The cover assembly C is lowered separately by use of guide wires passing through split guide funnels until it is located on the tree T. The ROV can then push in the locking pins 24 to lock the cover assembly in its final location. The envelope is now completely assembled.

The structure of the envelope is preferably made of steel. However, it is envisaged that other materials may be used. For example, the complete structure, or part thereof, may instead be fabricated from a plastics material such as fibre reinforced plastic.

If necessary, the tree can be recovered to the surface complete with the upper assembly of the envelope Once the tree has been recovered, it is then possible for the flowbase to be recovered to the surface complete with the lower assembly of the envelope. Prior to or during recovery, the leg portions may be moved from their deployed positions to their folded positions to allow the envelope to be recovered through the moonpool.

It will be understood that the invention has been described herein by way of example only and that modification may be made within the scope of the invention.

For example, whilst the preferred embodiment shows four legs, it will be appreciated that three, or more than four legs may be provided if desired. It will also be appreciated that whilst the preferred embodiment provides legs having upper and lower sections, other embodiments may employ legs comprising only a single portion.

Furthermore, whilst the embodiment has been described with simultaneous deployment of an envelope and a subsea structure, it will be apparent that the structure could be deployed first, and the envelope could be deployed later.

It will also be apparent that the legs could have a variety of different configurations. For example, they could be hinged to pivot upwards to a folded position. Alternatively, the legs could be hingedly attached to extend below the subsea structure, adjacent pairs of legs being connected together by a two-part cross member, one part of which is attached to one leg and the other part of which is attached to the other leg. The two parts of each cross member would be pivotally attached to and pivotable about one another. In the folded position, the legs would extend below the structure and the parts of the cross member would be provided at an angle to one another. In the deployed position, the legs would extend to provide a desired profile and the parts of the cross members would substantially horizontally align. A pin may then be provided to restrict further movement of the parts of the cross member. In this example, the legs of the envelope would have a superficially similar configuration to that of a camera tripod, for example.

As a further option, the legs could be attached to the cover assembly, rather than to the subsea structure, the cover assembly being attachable to the subsea structure. As yet another option, the legs could be attached to the subsea structure and the cover assembly without the cover assembly being attached to the subsea structure.

Claims

1. An overtrawlable protection envelope for a subsea structure which is connected to the seabed wherein the envelope is anchored by connecting it to the subsea structure which it is to protect.

2. An envelope according to Claim 1 , comprising means for defining an overtrawlable profile, the defining means being moveable from a folded position wherein the envelope has a reduced footprint to a deployed position wherein the envelope has an enlarged footprint which defines an overtrawlable profile.

3. An envelope according to Claim 2, wherein the defining means comprises a plurality of legs.

4. An envelope according to Claim 3, wherein the legs are pivotally deployable from the folded position to the deployed position.

5. An envelope according to Claim 3 or Claim 4, wherein the legs each comprise an upper portion and a lower portion, the upper and lower portions being independently moveable between the folded position in which they are disaligned and the deployed position in which they are aligned.

6. An envelope according to Claim 5, wherein the upper portion is provided with a telescopic portion which is engageable with the lower portion to preserve the overtrawlable profile when the upper and lower portions are aligned.

7. An envelope according to any of Claims 3 to 6, wherein the legs are each provided with a telescopic element slidably mounted therein and extendable to connect with the seabed.

8. An envelope according to any of Claims 1 to 7, comprising a cover assembly for covering at least an upper portion of said subsea structure.

9. An envelope according to any of Claims 3 to 7, wherein the legs are attachable to respective positions on the subsea structure.

10. An envelope according to Claim 9 when dependent on Claim 5 or Claim 6, wherein the upper portion of each leg is attachable to an upper tree portion of the subsea structure, and the lower portion of each leg is attachable to a lower flowbase portion of the subsea structure.

11. An envelope according to Claim 2, or any of Claims 3 to 10 when dependent upon Claim 2, wherein the overtrawlable profile includes a plurality of faces inclined at an angle of between approximately 30° and approximately 70° to the seabed.

12. An envelope according to Claim 1 1 , wherein the angle of inclination is substantially 58°.

13. An envelope according to Claim 11 or Claim 12, wherein the profile is substantially square pyramidal or substantially triangular pyramidal.

14. An envelope according to any preceding Claim, wherein the envelope is recoverable to the surface together with the subsea structure to which it is attached.