GB2309246A

GB2309246A - Pile Sleeve Assembly

Info

Publication number: GB2309246A
Application number: GB9700911A
Authority: GB
Inventors: Murray Charles Ferguson; Michael Timothy Smith
Original assignee: Kvaerner Oil and Gas Ltd
Current assignee: Kvaerner Oil and Gas Ltd
Priority date: 1996-01-17
Filing date: 1997-01-17
Publication date: 1997-07-23
Anticipated expiration: 2017-01-17
Also published as: GB9600939D0; GB2309246B; GB9700911D0

Description

PILE SLEEVE ASSEMBLY The invention relates to a pile sleeve assembly for an offshore tower structure; to an offshore tower structure having such an assembly; and to a method of installation, and a method of installation, removal and reinstallation.

In particular, the invention relates to a pile sleeve assembly which facilitates reuse of a piled offshore tower structure. Such structures are secured to the seabed using piles extending down into the seabed strata. These piles are fixedly connected to the offshore tower structure by means of corresponding pile sleeve assemblies.

In this specification the part of an offshore tower structure between the seabed and just above the sea surface will be termed a substructure.

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

Difficulties have arisen in reusing substructures for offshore tower structures because the designs of substructures have been optimised for particular locations. Moreover, the substructures have been fixed permanently 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 piled foundations designed for specific subsea strata - has been difficult to reuse effectively at another location.

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 bearing on preinstailed 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. However, it has the disadvantage of leaving a certain amount of residual steelwork (i.e. preinstalled piles and grouted pins) on the seabed.

Another proposal for enabling a substructure to be reused is described and claimed in our U.K. Patent Specification 2290334. This shows a pile sleeve through which a tubular pile can be inserted into the subsea strata. The pile sleeve comprises upper and lower portions. The upper portion of the pile sleeve has an intemal 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 is arranged as part of a joint to connect the pile sleeve to the remainder of the platform. The substructure can be removed by cutting off the upper portion of the pile sleeve.

These concepts enable a substructure to be removed from the seabed and modified for re-use (or in the case of the concept of U.K. Patent Specification 2290334 to be reused once without modification; and then modified for further re-use). However, there is in general an unavoidable delay incurred between removal of a substructure from the seabed and the reinstallation of that substructure at another site.

The concepts described above incur a delay because the parts of the pile sleeve assemblies which have been grouted or swaged to the piles need to be cut off, and new pile sleeves welded on to the substructure in order to accept new piles suitable for the new location. These operations require the substructure to be lifted clear of the water, and landed on a barge or offshore operations vessel. The cutting and welding are difficult and potentially dangerous tasks to be performed offshore, and lead to expensive delays.

In view of the large expenses incurred during offshore operations (due to equipment hire charges and crewing costs) it is desirable to minimise the time between removal and reinstallation.

The present invention is concemed with reducing the delay between removal and reinstallation.

The invention provides a pile sleeve assembly for securing a substructure for an offshore platform to the seabed using piles extending down into the seabed strata, the assembly comprising: a sleeve support element having inwardly facing arcuate support portions disposed on a surface of a cylinder having a generally vertical axis, a replaceable intermediate sleeve, having an extemal surface adapted to fit within the arcuate support portions, and having an intemal surface dimensioned for engagement with a pile when that pile is inserted within the sleeve, and readily releasable means to secure the intermediate sleeve to the sleeve support element.

It is preferred that the inwardly facing arcuate support portions of the sleeve support element are joined up to form a single part cylindrical surface.

It is also preferred that the inwardly facing arcuate support portions of the sleeve support element are arranged all around the circumference of the cylinder on which those arcuate portions are disposed.

In one particular form the replaceable intermediate sleeve has a fixed outwardly projecting flange to abut one end of the cylinder, and a removable outwardly projecting flange to abut the other end of the cylinder, whereby on removal of the removable flange, the replaceable intermediate sleeve can be withdrawn axially of the cylinder in the direction towards the fixed flange.

In this form it is preferred that the removable flange is at the lower end of the cylinder.

It is further preferred that the removable flange is arranged to fit in a circumferential groove on the extemal surface of the sleeve.

It is still further preferred that the removable flange is made of two semi circular halves which can be bolted together.

The invention also provides a substructure for an offshore platform including at least three pile sleeve assemblies as described above.

The invention also provides a method of installing a substructure for an offshore platform having at least three spaced feet, each foot including a pile sleeve assembly comprising a sleeve support element and a replaceable intermediate sleeve releasably fixed to that support element; in which method the substructure is placed on the seabed, a tubular pile is driven downwardly through each replaceable intermediate sleeve into the seabed, and the tubular pile is permanently fixed within the replaceable intermediate sleeve.

The invention further provides a method of removing and reinstalling a platform previously installed by the method described above, which comprises the further steps of severing the piles at or below the seabed, removing the substructure from the water, releasing each replaceable intermediate sleeve from its respective support element, replacing that replaceable intermediate sleeve with another replaceable intermediate sleeve, and then repeating the method described above.

A specific embodiment of the invention and three variants thereof will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side view of a substructure for an offshore platform; Figure 2 is a diagrammatic cross section through the part of the substructure circled in Figure 1, and showing a first embodiment of the invention; Figure 3 is a cross section similar to Figure 2 showing a first variant of the invention; Figure 4 is a view on the line A-A of Figure 3; Figure 5 is a cross section similar to Figure 2 showing a second variant of the invention; Figure 6 is a view on the line B-B of Figure 5; and Figure 7 is a cross section similar to Figure 2 showing a third variant of the invention.

The subject matter of Figures 3 to 7 is provided for the generality of its disclosure only. No claim is made for the specific arrangement of the bolted connections shown in those Figures.

As shown in Figure 1, a tripod tower structure 10 has three legs 11 and three feet 12. This form of substructure would be particularly suitable for removal and reinstallation, but the invention could also be applied to other types of substructure such as conventional jackets. In particular, the angles at which tubular braces intersect with the pile sleeve assembly may differ to suit the configuration of the substructure.

Figures 2 to 7 show diagramatically how a substructure is secured to its piles. It must be emphasised that these figures are illustrative only, and do not necessarily show practical wall thicknesses or clearances.

Figure 2 shows a pile sleeve assembly for securing a substructure to the seabed. In this case two substructure (jacket) braces 21 and 22 are welded into a node formed by a cylindrical sleeve support element 23. Following the invention there is a replaceable intermediate sleeve 24 within the support element 23. The sleeve 24 is cylindrical, and at its upper end there is a fixed outwardly projecting flange 25, which is arranged to abut the upper end face of the sleeve support element 23. At the lower end of the sleeve 24 there is an outwardly projecting removable flange 26, which is arranged to abut the lower end face of the sleeve support element 23. When the removable flange 26 is in place, the sleeve 24 is held securely within the sleeve support element 23.

The replaceable intermediate sleeve 24 acts as a conventional pile sleeve, and is sized to surround a tubular pile 27.

The interior surface of the replaceable intermediate sleeve 24 and the exterior surface of the pile 27 have weld beads 28. These weld beads 28 form shear keys on the respective surfaces, so that grout introduced between the sleeve 24 and pile 27 will secure them together. Near to the lower end of the sleeve 24 there is a wiper 29 to retain grout in the annulus between pile and sleeve until that grout is set. As an altemative to grouting, the pile and replaceable intermediate sleeve may be swaged together.

Use of the replaceable intermediate sleeve 24 allows the substructure to be removed from the seabed, and reinstalled elsewhere very soon afterwards. The substructure is freed by severing the piles 27 at a level below the sea floor. The substructure is then lifted out of the water onto a vessel. On the vessel, each foot of the substructure is freed from the pile stub grouted within thesleeve 24 by removal of the flange 26 and withdrawal of the pile stub, grout, and sleeve 24 from the support element 23. The substructure can then be fitted with another replaceable intermediate sleeve 24 secured within the element 23, and reinstalled on the seabed.

In this way there is minimal delay between removal and reinstallation.

Figures 3 and 4 show a variant of the invention. In this case tubular braces 31 and 32 are welded into a node body comprising a half cylinder 33 forming a sleeve support element. A replaceable intermediate sleeve 34 is cylindrical, and fits within the half cylinder 33. The sleeve 34 is in this case secured to the half cylinder 33 by a series of vertically aligned horizontal bolts 35.

These horizontal bolts extend through corresponding lugs 36 and 37 outstanding from the half cylinder 33 and the sleeve 34 respectively. In this way the sleeve 34 can receive a pile 38 which is secured within the sleeve by grouting (as shown) or by swaging.

On removal of the substructure, the pile stubs and sleeves can be unbolted and new replaceable intermediate sleeves rebolted for reinstallation with minimal delay.

Figures 5 and 6 show a second variant of the invention. In this case the sleeve support element is formed as a node body comprising a half cylinder 43 (similar to the half cylinder 33 of Figures 3 and 4). However, instead of lugs 36 and 37 to be joined by bolts 35, the half cylinder 43 is connected directly to a replaceable intermediate sleeve 44 by radially extending horizontal bolts 45. A pile 48 can be secured within the sleeve 44 by grouting (as shown) or by swaging.

In Figures 2 to 6 the sleeve support elements 23, 33 and 43 have been described as cylinders or half cylinders. It will be understood that the cylinders or half cylinders do not need to have continuous uninterrupted cylindrical surfaces. The material may be cut away to save weight, or the cylindrical surface (to fit round the replaceable intermediate sleeves 24, 34 and 44 respectively) may be built up e.g. from longitudinal strips joined by peripheral rings. The essential feature is that the support elements 23, 33, 43 join the respective sleeves 24, 34, 44 securely to the substructure in a manner that can quickly be released.The importance of the cylindrical surface being made up of arcuate portions is so that the interior surface so defined will fit round the extemal periphery of a cylindrical sleeve (i.e. the replaceable intermediate sleeve) which must itself fit around a tubular pile.

Figure 7 shows a third variant of the invention. In this case a cylindrical sleeve support element 53 forms a node barrel into which braces 51 and 52 are welded. A replaceable intermediate sleeve 54 is bolted on top of the support element 53. Radial lugs 56 and 57 are fixed to and extend outwardly from the peripheries of the top of the support element 53 and bottom of the sleeve 54 respectively. These lugs 56 and 57 are connected vertically oriented tension bolts 55. In this case the pile 58 is grouted only to the intermediate sleeve 54, so that when the substructure is removed, the freeing of bolts 55 will allow the substructure to be reused with a new replaceable intermediate sleeve 54.

Claims

1. A pile sleeve assembly for securing a substructure for an offshore platform to the seabed using piles extending down into the seabed strata, the assembly comprising: a sleeve support element having inwardly facing arcuate support portions disposed on a surface of a cylinder having a generally vertical axis, a replaceable intermediate sleeve, having an external surface adapted to fit within the arcuate support portions, and having an intemal surface dimensioned for engagement with a pile when that pile is inserted within the sleeve, and readily releasable means to secure the intermediate sleeve to the sleeve support element.

2. A pile sleeve assembly as claimed in claim 1 in which the inwardly facing arcuate support portions of the sleeve support element are joined up to form a single part cylindrical surface.

3. A pile sleeve assembly as claimed in claim 1 or claim 2 in which the inwardly facing arcuate support portions of the sleeve support element are arranged all around the circumference of the cylinder on which those arcuate portions are disposed.

4. A pile sleeve assembly as claimed in claim 3 in which the replaceable intermediate sleeve has a fixed outwardly projecting flange to abut one end of the cylinder, and a removable outwardly projecting flange to abut the other end of the cylinder, whereby on removal of the removable flange, the replaceable intermediate sleeve can be withdrawn axially of the cylinder in the direction towards the fixed flange.

5. A pile sleeve assembly as claimed in claim 4 in which the removable flange is at the lower end of the cylinder.

6. A pile sleeve assembly as claimed in claim 4 or claim 5 in which the removable flange is arranged to fit in a circumferential groove on the extemal surface of the sleeve.

7. A pile sleeve assembly as claimed in claim 6 in which the removable flange is made of two semi circular halves which can be bolted together.

8. A pile sleeve assembly substantially as hereinbefore described with reference to and as shown in Figures 1 and 2 of the accompanying drawings.

9. A substructure for an offshore platform including at least three pile sleeve assemblies in accordance with any one of the preceding claims.

10. A method of installing a substructure for an offshore platform having at least three spaced feet, each foot including a pile sleeve assembly comprising a sleeve support element and a replaceable intermediate sleeve releasably fixed to that support element; in which method the substructure is placed on the seabed, a tubular pile is driven downwardly through each replaceable intermediate sleeve into the seabed, and the tubular pile is permanently fixed within the replaceable intermediate sleeve.

11. A method of removing and reinstalling a platform previously installed by the method of claim 10, which comprises the further steps of severing the piles at or below the seabed removing the substructure from the water, releasing each replaceable intermediate sleeve from its respective support element, replacing that replaceable intermediate sleeve with another replaceable intermediate sleeve, and then repeating the method of claim 10.

12. A method substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings.

Amendments to the clalms have been filed as follows CLAIMS 1. A pile sleeve assembly for securing a substructure for an offshore platform to the seabed using piles extending down into the seabed strata, the assembly comprising: a sleeve support element having inwardly facing arcuate support portions disposed on a surface of a cylinder having a generally vertical axis, a replaceable intermediate sleeve, having an external surface adapted to fit within the arcuate support portions, and having an internal surface dimensioned for engagement with a pile when that pile is inserted within the sleeve, and mechanically releasable means arranged around the periphery of the sleeve support element to secure the intermediate sleeve to the sleeve support element.

6. A pile sleeve assembly as claimed in claim 4 or claim 5 in which the r,$rrtval*arLge is arranged to fit in a circumferential groove on the external surface of the sleeve.

b 7. A pile sleeve assembly as claimed in claim 6 in which the removable flange is made of two semi circular halves which can be bolted together.

10. A method of installing a substructure for an offshore platform having at least three spaced feet, each foot including a pile sleeve assembly comprising a sleeve support element and a replaceable intermediate sleeve secured to the support element by mechanically releasable means arranged around the periphery of that support element; in which method the substructure is placed on the seabed, a tubular pile is driven downwardly through each replaceable intermediate sleeve into the seabed, and the tubular pile is permanently fixed within the replaceable intermediate sleeve.