GB2290334A

GB2290334A - Offshore platform and method of installation

Info

Publication number: GB2290334A
Application number: GB9512086A
Authority: GB
Inventors: Michael Timothy Smith
Original assignee: Kvaerner Earl and Wright
Current assignee: Kvaerner Earl and Wright
Priority date: 1994-06-14
Filing date: 1995-06-14
Publication date: 1995-12-20
Anticipated expiration: 2015-06-14
Also published as: GB9512086D0; GB2290334B; GB9411920D0

Abstract

An offshore platform having at least three spaced feet (12), each foot including a pile sleeve (14) through which a tubular pile (19) can be inserted into the subsea strata, said pile sleeve comprising upper and lower portions, the upper portion of the pile sleeve (16) having an internal cylindrical surface for connecting the pile sleeve to the exterior surface of the pile (e.g. by grouting, swaging or any other form of connection), and the lower portion of the pile sleeve (17) being arranged as part of a joint to connect the pile sleeve to the remainder of the platform. The platform may be reinstalled elsewhere by cutting the pile below its connection to the upper portion, removing the platform from the water, and then fixing another upper portion to the lower portion. <IMAGE>

Description

OFFSHORE PLATFORM AND METHOD OF INSTALLATION/REINSTALLATION The invention relates to offshore platforms, and to methods of installing, and of installing, recovering and reinstalling such platforms.

Heretofore it has generally been impractical or uneconomic to use a substructure for a fixed offshore platform at more than one location.

Difficulties have arisen because the designs of substructures for such platforms have been optimised for particular locations. Moreover, substructures have been fixed to the seabed with tubular piles driven (or drilled and grouted) into the subsea strata. A substructure of very specific dimensions, optimised for whole life costs in specific environmental conditions (winds, waves and currents) with a specified topside load and with foundations designed for specific subsea strata, has been difficult to reuse effectively at another location. The advent of a braced monotower (described and claimed in our U.K. Patent Specification 2267525), has made reuse a practical possibility.

One proposal for enabling a substructure to be reused is described and claimed in our U.K. Patent Specification 2278871. This shows a substructure which has pile sleeves around its base that can react compressive loads in direct bearing on preinstalled piles. Tensile loads are reacted through grout between the external surface of a lower part of a pin extending downwardly within each pile sleeve, and an internal surface of a corresponding pile. The substructure can be removed by cutting a connection between the pin (connected by grout to the pile) and the pile sleeve (connected by shear plate to the substructure). This arrangement saves steel in that pile lengths are reduced, and enables the substructure to be removed, but has the disadvantage of leaving a certain amount of residual steelwork on the seabed.

The invention provides an offshore platform having at least three spaced feet, each foot including a pile sleeve through which a tubular pile can be inserted into the subsea strata, said pile sleeve comprising upper and lower portions, the upper portion of the pile sleeve having an internal cylindrical surface for connecting the pile sleeve to the exterior surface of the pile (e.g. by grouting, swaging or any other form of connection) and the lower portion of the pile sleeve being arranged as part of a joint to connect the pile sleeve to the remainder of the platform.

Preferabiy the axes of all the pile sleeves are vertical when the platform stands on the seabed.

In one preferred form the lower portion of the pile sleeve is the body of a nodal joint arranged to accept angled tubular bracing members from other parts of the platform.

In an alternative preferred form the lower portion of the pile sleeve forms part of a complex joint constituting a foot of the platform.

In a further preferred form, in the upper portion of the pile sleeve, the wall thickness is reduced along at least one part of its length between two thicker parts on either side, so that the pile can be swaged onto the thinner part.

The invention also provides a method of installing an offshore platform having at least three spaced feet, each foot including a pile sleeve having an upper portion with an internal cylindrical surface for connecting the pile sleeve to the exterior surface of the pile, and a lower portion forming part of a joint to connect the pile sleeve to the remainder of the platform, in which method the platform is placed on the seabed, a tubular pile is inserted through the pile sleeve and into the seabed, and the pile is connected to the upper portion of the pile sleeve so to secure the platform to the seabed.

More specifically the invention provides a method of removing and reinstalling a platform previously installed by the method described above, which comprises the further steps of severing the connection between the platform and the pile by cutting the pile below its connection to the upper portion of the pile sleeve, removing the platform from the water, cutting the pile sleeve above the position at which the upper portion joins the lower portion, fixing another upper portion to the lower portion, and then repeating the method of installation described above.

Two specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure lisa side elevation of a 3-leg braced monotower substructure for an offshore platform; Figure 2 is a plan view of that substructure; Figure 3 is a detailed side elevation of one foot of that substructure; Figure 4 is a sectionai view of an upper portion of a pile sleeve shown in Figure 3, relating to a swaged connection detail.

Figure 5 is a side elevation of another substructure for an offshore platform; and Figure 6 is a detailed side elevation of one foot of the substructure shown in Figure 5.

As shown in Figures 1 and 2, a braced monotower substructure 10 has three angled legs 11 and three spaced feet 1 2. One of the feet 1 2 is shown in more detail in Figure 3.

Each foot 12 comprises a pile sleeve 14 set on a mudmat 15. The pile sleeve 14 is a cylindrical steel tube, and in this example its axis is arranged vertically. The pile sleeve 14 has an upper portion 1 6 and a lower portion 1 7.

The lower portion 1 7 of the pile sleeve 14 forms the body of a nodal joint to which the leg 11 is attached. Additional bracing members (typically 18) are also attached to the incoming leg at the nodal joint. The projected axes of the leg 11 and bracing members 1 8 intersect with the axis of the pile sleeve 14.

Following the invention, the upper portion 1 6 of the pile sleeve 14 is adapted to surround and engage part of the length of a pile 1 9 driven through the pile sleeve. The connection between pile 19 and pile sleeve 14 may be by grouting or swaging, and in the latter case the wall thickness of the upper portion 1 6 of the pile sleeve is varied lengthwise of the pile sleeve.Details of the pile sleeve wall thickness for a swaged connection can be seen in Figure 4. (Specific details of the swaged connection which are proprietary to the firm Hydro-lok form no part of the present invention.) The arrangement is such that the two portions of the pile sleeve can fulfil their functions independently of each other (i.e. the upper portion can secure the pile to the pile sleeve, and the lower portion can form part of the nodal joint between the pile sleeve and the remainder of the substructure).

The upper portion 16 of the pile sleeve 14 is surmounted by a guide cone 21 to direct the pile 19 into alignment with the pile sleeve during initial engagement. The pile 19 is then installed so that its upper end is just above the top of the upper portion 1 6 of the pile sleeve 14.

The invention is particularly applicable to cases where platforms are to be installed, used for a limited period, and then removed before the substructure life has reached any where near its limit. In such a case it may be advantageous to reuse the substructure.

With the substructure of the present example, the pile 1 9 would be severed at or below seabed level, and the complete substructure 10 would be removed. Referring particularly to Figure 3, the upper portion 16 of the pile sleeve 14 (with the top of the pile 1 9 still inside it and connected thereto) would be cut off where the lower portion 1 7 runs into the upper portion 1 6. When the platform had dried out - either on a barge or at a quayside - another upper portion 1 6 would be attached in its place.

The substructure could then be reinstalled at another site where the water depth was equal to or shallower and/or the sea states equal to or less severe than on its original site, so that the loads on the substructure at its new site were no more onerous than at the original site. New piles 1 9 would be driven through the pile sleeves to secure the substructure at its new site.

It will be understood that the invention may be applied to other forms of substructure for offshore platforms in addition to braced monotowers. For instance this invention may be applied to conventional jackets or spread based jackets, and the pile sleeves may be slightly angled to the vertical as well as being truly vertical.

An example of an application of the invention to a conventional jacket is shown in Figures 5 and 6.

In this case a substructure (or jacket) 100 has four tubular legs 101, and each leg terminates at its lower end in a foot generally indicated as 102, and illustrated in more detail in Figure 6.

Each foot comprises the bottom of a leg 101, a yoke plate 103, a mudmat 104, one or more shear plate(s) 105, and the lower portions 106 of one or more pile sleeve(s).

The lower portion(s) 106 extends between the yoke plate and the mudmat, and forms part of a complex joint at the foot of the jacket. Following the invention, each pile sleeve has an axially aligned upper portion 107 attached to the upper end of the lower portion 106. The whole pile sleeve (i.e. lower portion 106 and upper portion 107) is arranged to surround a pile 108. When the jacket 100 has been set on the seabed, a pile 108 is driven through each of the pile sleeves 106/107. The piles 108 are then fixed to their respective pile sleeves by grouting the space between the pile 108 and the upper portion 107. The space between the pile 108 and the pile sleeve lower portion 106 is not grouted.

When the jacket is to be removed, the pile 108 is severed at the seabed level, and the complete jacket 100 is removed (e.g. by lifting or by using detachable auxiliary buoyancy). Each of the pile sleeve upper portions 107 can then be cut off with the grout and the top part of the pile still inside it. Another pile sleeve extension (upper portion 107) could be welded on to the pile sleeve lower portion 106, and the jacket could then be reinstalled at another offshore site where the water depth was equal to or shallower and/or the sea states equal to or less severe than on its original site, so that the loads on the substructure at its new site were no more onerous than at the original site.

Claims

1. An offshore platform having at least three spaced feet, each foot including a pile sleeve through which a tubular pile can be inserted into the subsea strata, said pile sleeve comprising upper and lower portions, the upper portion of the pile sleeve having an internal cylindrical surface for connecting the pile sleeve to the exterior surface of the pile (e.g. by grouting, swaging or any other form of connection), and the lower portion of the pile sleeve being arranged as part of a joint to connect the pile sleeve to the remainder of the platform.

2. A platform as claimed in Claim 1 in which the axes of all the pile sleeves are vertical when the platform stands on the seabed.

3. A platform as claimed in Claim 1 or Claim 2 in which the lower portion of the pile sleeve is the body of a nodal joint arranged to accept angled tubular bracing members from other parts of the platform.

4. A platform as claimed in Claim 1 or Claim 2 in which the lower portion of the pile sleeve forms part of a complex joint constituting a foot of the platform.

5. A platform as claimed in any one of the preceding claims in which in the upper portion of the pile sleeve, the wall thickness is reduced along at least one part of its length between two thicker parts on either side, so that the pile can be swaged onto the thinner part.

6 A method of installing an offshore platform having at least three spaced feet, each foot including a pile sleeve having an upper portion with an internal cylindrical surface for connecting the pile sleeve to the exterior surface of the pile, and a lower portion forming part of a joint to connect the pile sleeve to the remainder of the platform, in which method the platform is placed on the seabed, a tubular pile is inserted through the pile sleeve and into the seabed, and the pile is connected to the upper portion of the pile sleeve so to secure the platform to the seabed.

6. A method of removing and reinstalling a platform previously installed by the method of Claim 6, which comprises the further steps of severing the connection between the platform and the pile by cutting the pile below its connection to the upper portion of the pile sleeve, removing the platform from the water, cutting the pile sleeve above the position at which the upper portion joins the lower portion, fixing another upper portion to the lower portion, and then repeating the method of installation of Claim 5.

8. An offshore platform substantially as hereinbefore described with reference to and as shown in Figures 1 to 4 and in Figures 5 and 6 of the accompanying drawings.

9. A method of installing (and reinstalling) an offshore platform substantially as herein before described with reference to Figures 1 to 4 and Figures 5 and 6 of the accompanying drawings.

Amendments to the claims

flows 1. An offshore platform having at least three spaced feet, each foot including a pile sleeve through which a tubular pile can be inserted into the subsea strata, said pile sleeve comprising upper and lower portions, the upper portion of the pile sleeve having an internal cylindrical surface for connecting the pile sleeve to the exterior surface of the pile (e.g. by grouting, swaging or any other form of connection), and the lower portion of the pile sleeve being arranged as part of a joint to attach the pile sleeve to the remainder of the platform, in which only the upper portion of the pile sleeve is connected to the pile.

6 A method of installing an offshore platform having at least three spaced feet, each foot including a pile sleeve having an upper portion with an internal cylindrical surface for connecting the pile sleeve to the exterior surface of the pile, and a lower portion forming part of a joint to connect the pile sleeve to the remainder of the platform, in which method the platform is placed on the seabed, a tubular pile is inserted through the pile sleeve and into the seabed, and the pile is connected only to the upper portion of the pile sleeve so to secure the platform to the seabed.

7. A method of removing and reinstalling a platform previously installed by the method of Claim 6, which comprises the further steps of severing the connection between the platform and the pile by cutting the pile below its connection to the upper portion of the pile sleeve, removing the platform from the water, cutting the pile sleeve above the position at which the upper portion joins the lower portion, fixing another upper portion to the lower portion, and then repeating the method of installation of Claim 6.

9. A method of installing (and reinstalling) an offshore platform substantially as hereinbefore described with reference to Figures 1 to 4 and Figures 5 and 6 of the accompanying drawings.