GB2323402A - Offshore structures - Google Patents

Abstract

An offshore structure 10 utilizes a central tower 14 that is wholly or partly supported by pilings 18 external to the tower 14. The central tower 14 is attached to a pre-installed template 12 on the sea floor 20. Sleeves 28 attached to cantilevers 24 that extend from the central tower 14 each receive and are attached to a piling 18 that is driven into the sea floor 20. The pilings 18 extend up from the sea floor 20 approximately to mid-height of the central tower 14 and are preferably below the elevation of the wave affected zone. Environmental forces are mainly resisted by the pilings 18. Only a small proportion of the environmental and other forces are transferred directly to the template 12.

Description

OFFSHORE STRUCTURES The invention relates to offshore structures, such as those used in drilling and production of hydrocarbons or minerals offshore, and particularly to fixed structures anchored in place by pilings.

In the offshore drilling industry, it is common to use a fixed structure such as a jacket or tower. The jacket or tower is normally held in place by pilings driven into the sea floor through the legs of the structure and possibly also through a template on which the tower is placed.

Exploration and development of hydrocarbons and minerals in deeper waters requires taller towers. The larger towers are more expensive to fabricate and require equipment with increased lifting and weight bearing capacity to accommodate the extra weight of the larger towers. The larger towers also have a tendency to present stability problems during installation.

According to the invention there is provided an offshore structure that utilizes a central tower that is wholly or partly supported by pilings external to the tower and a template on the sea floor. The central tower is attached to a pre-installed template on the sea floor.

Sleeves attached to cantilevers that extend from the central tower each receive and are attached to a piling that is driven into the sea floor. The pilings extend up from the sea floor to approximately mid-height of the central tower and are preferably below the elevation of the wave affected zone. Environmental forces are mainly resisted by the pilings. The central tower transfers only a small proportion of the environmental and other forces to the template.

For a further understanding of the nature of the present invention, reference should be made to the following description, taken in conjunction with the accompanying drawings in which like parts are given like reference numerals, and wherein: Fig. 1 is an elevation view of an embodiment of the invention; Fig. 2 is an elevation view of~another embodiment of the invention wherein a tubular construction is used for the central tower; Fig. 3 is an elevation view of an alternative embodiment of the invention; Fig. 4 is a sectional view of the upper portion of the structure shown in Fig. 3; Figs. 5A - 5D illustrate the installation of an offshore structure utilizing the invention; Figs. 6 - 8 are sectional views of alternative means of attaching a piling to the offshore structure; Fig. 9 is a sectional view that illustrates the mechanical connector that may be used between the piling and sleeve or template; and Fig. 10 is a view taken along lines 10-10 in Fig. 9.

Referring to the drawings, it is seen in Fig. 1 that the invention is generally indicated by the numeral 10. Offshore structure 10 is generally comprised of template 12, central tower 14, deck 16, and pilings 18.

Template 12 is positioned on the sea floor 20 and is used as a guide for the positioning of the central tower 14. Template 12 may be held in position by pilings driven into the sea floor 20 through sleeves in the template. Template 12 is provided with a locking mechanism 22, best seen in Fig. 9 and 10 for forming a mechanical connection between the piling 18 and template 12.

Template 12 is also provided with a plurality of sleeves attached thereto around the outer boundary of the template.

Central tower 14 is also generally known in the offshore drilling industry as a jacket and is formed from a latticed framework. As seen in Fig. 2, central tower 14 may also be of tubular construction. The tower 14 is provided with a plurality of cantilevers 24 along the length and around the outer boundary of the tower. The cantilevers 24 start at approximately the midheight of the tower, extend substantially horizontally from the legs 26 of the tower, and are spaced apart vertically extending down to the bottom or base of the tower. In the embodiment of Fig. 2, the cantilevers 24 are directly attached to and extend from the tubular tower. Each cantilever 24 is provided with a sleeve 28 sized to receive a piling 18 therethrough. The sleeves 28 on cantilevers attached to the same leg of the tower are in coaxial alignment with one another as well as the corresponding sleeves 13 on the template 12. One or more sleeves 28 may also be provided with a locking mechanism 22 which will be described below.

For ease of reference and description, the deck 16 should be understood as generally referring to all of the structures received on top of the central tower 14. Such structures generally include a drilling rig 30, crane 32, and other equipment associated with drilling and producing operations. As seen in the drawings, the tower 14 extends a sufficient distance above the water level 34 such that the deck 16 is above the normal maximum wave height.

The pilings 18 are tubular in construction and are well known in the industry for being driven into the sea floor 20 through jacket legs or templates for securing a jacket or template to the sea floor.

The sectional view of Fig. 4 illustrates that the tubular tower 14 from Fig. 2 may be provided with a plurality of ballast tanks 54, oil storage tanks 56, and impact protection devices 58.

Fig. 5A-5D illustrate the installation of the invention.

In Fig. 5A, the template 12 has been positioned on the sea floor 20 and secured in place with foundations 36, which may be pilings. As seen in Fig. SB, barge 38 is used to place the tower 14 in position on the template 12. In Fig. 5C, the pilings 18 have been installed through the sleeves 28 and are in position for attachment to the foundations 36 by any suitable means such as grouting or mechanical swaging. In Fig. SD, the deck 16 has been installed on top of the tower 14.

Fig. 6-8 illustrate different means that may be used to attach the pilings 18 to the sleeves 36. Fig. 6 illustrates the use of grouting wherein grout 407is pumped into the annulus between the inner diameter of the sleeve 28 and the outer diameter of the piling 18. Grout sleeve 42 is placed between the piling and sleeve at the top and bottom of the sleeve. Fig. 7 illustrates a connection means wherein a portion of the piling 18 is mechanically swaged into recesses 44 that have been machined into the inner surface of the sleeve 28. Fig. 8 illustrates the use of an elastomeric material 46 positioned between the sleeve 28 and piling 18. The friction provided by the elastomeric material 46 holds the piling 18 in position.

Fig. 9 and 10 illustrate a locking mechanism 22 that may be used to engage the central tower 14 to the template 12 or to engage the sleeves 28 to the pilings 18 during the installation or removal process to provide temporary structural integrity.

The locking mechanism 22 is comprised of a plurality of hydraulic rams 48 and pistons 50 that are spaced apart around the circumference of a mounting ring 52 that is attached to the sleeve 28. The hydraulic rams 48 cause radial movement of the pistons 50 such that the pistons 50 bear against the piling 18 and provide sufficient mechanical restraint to inhibit movement of the piling relative to the sleeve.

Fig. 3 illustrates an alternate embodiment of the invention wherein the pilings 18 extend from the sea floor 20 up to the deck 16 and support the deck above the water line 34. The central tower 14 extends from the sea floor 20 approximately to the mid-point of the pilings 18. The drilling and production risers 52 are exposed above the tower 14, which will eliminate the possible accumulation of gas inside the tower 14 and allow inspection and maintenance.

Because many varying and differing embodiments may be made within the scope of the inventive concept herein taught and because many modifications may be made in the embodiment herein detailed, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.

Claims (16)

1. An offshore structure comprising: a template secured to the s;éa floor, said template having a plurality of sleeves attached thereto around the outer boundary of said template; a tower attached to said template; a plurality of sleeves attached to and spaced apart along the length of and around the outer boundary of said tower such that said sleeves are coaxially aligned with the sleeves attached to said template; a piling received through said sleeves on said tower and said template; and means for providing a mechanical connection between said piling and said sleeves on said tower and template.

2. An offshore structure according to claim 1, wherein said pilings extend approximately to the mid-height of said tower.

3. An offshore structure according to claim 1 or claim 2 further comprising means for providing a mechanical connection between said piling and said sleeves on said template.

4. An offshore structure according to claim 1, claim 2 or claim 3 wherein said tower extends above the water level.

5. An offshore structure according to any one of claims 1 to 4, wherein said tower is formed from a latticed framework.

6. An offshore structure according to any one of claims 1 to 4, wherein said tower is formed from a tubular construction.

7. An offshore structure according to any one of the preceding claims, wherein said tower includes buoyancy tanks.

8. An offshore structure according to any one of the preceding claims, wherein said tower includes oil storage tanks.

9. An offshore structure comprising: a template secured to the sea floor, said template having a plurality of sleeves attached thereto around the outer boundary of said template; ~ a tower attached to said template, said tower extending from the sea floor to a height below the water level; a plurality of sleeves attached to and spaced apart along the length of and around the outer boundary of said tower such that said sleeves are coaxially aligned with the sleeves attached to said template; a piling received through said sleeves on said tower and said template, said piling extending above the water level; and means for providing a mechanical connection between said piling and said sleeves on said tower and template.

10. An offshore structure according to claim 9, further comprising means for providing a mechanical connection between said piling and said sleeves on said template.

11. An offshore structure according to claim 9 or claim 10, wherein said tower is formed from a latticed framework.

12. An offshore structure according to claim 9 or claim 10, wherein said tower is formed from a tubular construction.

13. An offshore structure according to any one of claims 9 to 12, wherein said tower includes buoyancy tanks.

14. An offshore structure according to any one of claims 9 to 13, wherein said tower includes oil storage tanks.

15. An offshore structure comprising: a template secured to the sea floor, said template having a plurality of sleeves attached thereto around the outer boundary of said template; a tower attached to said template; a plurality of sleeves attached to and spaced apart along the length of and around the outer boundary of said tower such that said sleeves are coaxially aligned with the sleeves attached to said template; a piling received through said sleeves on said tower and said template such that environmental forces are mainly resisted by said piling; and means for providing a mechanical connection between said piling and said sleeves on stid tower and template.

16. An offshore structure substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.