GB2277547A

GB2277547A - Foundation with downwardly extending skirt

Info

Publication number: GB2277547A
Application number: GB9408600A
Authority: GB
Inventors: Carl Theodre Erbrich; Jonathan Bruce Machin
Original assignee: Kvaerner Earl and Wright
Current assignee: Kvaerner Earl and Wright
Priority date: 1993-04-29
Filing date: 1994-04-29
Publication date: 1994-11-02
Anticipated expiration: 2014-04-29
Also published as: GB9308905D0; GB9408600D0; GB2277547B; MY111379A

Abstract

A plated foundation (or suction anchor) for an offshore structure has a top closure plate 26 and a vertical skirt extending downwardly from a periphery of the closure plate in which the skirt comprises two spaced para axial plates 21, 22 with a relatively narrow gap between these plates, and in which there is means 27, 28 to fluidise soil within the narrow gap at the lower tip of the peripheral skirt. The skirt is made up from a series of vertical compartments open at their base. Gas or liquid is introduced through a plurality of nozzles in a fluid injection system 27. Optionally grout may be injected into the skirt compartments. <IMAGE>

Description

FOUNDATION WITH CELLULAR SKIRT INSTALLATION DEVICE The invention relates to a foundation device, and particularly but not exclusively relates to a foundation device for an offshore structure.

Offshore structures (e.g. for oil or gas drilling/production platforms) have been founded on the seabed by two main methods - piled foundations and gravity foundations.

Piled structures have comprised tubular steel lattice frameworks secured to the seabed by tubular steel piles driven through the frameworks or through pile sleeves attached to those frameworks. The piles have been connected to the frameworks by grouting or swaging. These piles transfer loadings applied to the platform to the subsea strata.

Gravity structures have comprised massive base components, and hollow legs upstanding from those base components. In this case the entire area of the base is used to transfer loadings applied to the platform to the subsea strata.

A third method of founding offshore structures on the seabed has been proposed, but has not yet been implemented to the extent that piled structures and gravity structures have been implemented. This third foundation method uses discrete flat feet with peripheral skirts that are forced downwardly into the subsea strata. These feet are located at the extremities of the planform of the base of the structure, and since they are made of flat plates, rather than tubular piles, they have been referred to as plated foundations. Resistance to overturning uplift is provided by suction generated beneath the feet. Plated foundations have for this reason been compared with suction anchors (which employ active suction).

One application of plated foundations (or suction anchors) has been described in OTC Paper 3203 "Applications of Suction Anchors in Offshore Technology" presented at the 10th OTC at Houston, Texas in May 1978. In that paper an application was outlined for an offshore platform in 300 ft (91.43m) of water, with a 200 ft (60.95m) square planform on the seabed, and suction anchors at its four corners.

Another example of an offshore platform (for much deeper water) and having plated foundations is shown in U.K. Patent Specification 21 92923A, which illustrates a concept described as being 400m tall, and in which mudmats have peripheral skirts. In this case a jacket is secured to the seabed by piles, and also by mudmats with downwardly extending peripheral walls. These mudmats have provision for the injection of pressurised gas, and for the extraction of water from the space between the mudmats and the peripheral walls. This arrangement is described as having the capability to level an offshore structure during the initial installation of that structure. However, the scheme described is dependent upon reliable penetration of the peripheral walls into the subsea strata.

For some practical applications of this concept, a 'suction' system has been utilised to enable peripheral walls or skirts to penetrate down into the seabed. This suction system has consisted of pumps which are used to create a pressure difference between the inside and outside of the foundation unit. The differential pressure acts to provide a driving force to push the skirts down into the seabed. Additionally, in non-cohesive soils, this system will lead to a degradation of the penetration resistance.

One example of the practical application of plated foundations is illustrated in the publication "Offshore Engineer" for October 1992. In this case penetration of the foundation into the seabed is achieved first under the self weight of the platform, and subsequently through suction caused by the evacuation of water from within the skirted foundation. This example was proved to be highly effective in practical tests, given that sufficient suction was applied to achieve adequate penetration into the seabed.

However, in some circumstances, reliable penetration of skirted foundations into the seabed may be sought without the use of suction from within the foundation.

The invention provides a plated foundation (or suction anchor) having a top closure plate and a vertical skirt extending downwardly from a periphery of the closure plate to define a downwardly facing cup, in which the skirt comprises two spaced para axial plates with a relatively narrow gap between these plates, and in which there is means to fluidise soil within the narrow gap at the lower tip of the peripheral skirt.

It is preferred that the gap is of constant width.

It is further preferred that the para axial plates are generally concentric, one within the other.

Preferably the para axial plates are separated by vertical connecting walls, which divide the space between the skirt plates into cellular compartments.

According to one preferred feature of the invention a part of at least one of the para axial plates is broadened outwardly from the gap at the base of the skirt.

According to another preferred feature of the invention the means to fluidise the soil in the gap at the base of the skirt comprises a fluid conduit extending round the base of the skirt, and having a multiplicity of discharge orifices through which pressurised fluid can be directed at soil in the gap.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of an offshore platform; Figure 2 is a perspective view of a plated foundation with its top removed; Figure 3 is a section on the line Ill-Ill in Figure 2; and Figure 4 is a detail of region IV in Figure 2.

As shown in Figure 1 a platform 10 has legs 11, the feet of which are based on plated foundations 1 2 set in to seabed 14. The specific embodiment of the invention is concerned with one plated foundation 1 2.

A cellular skirted foundation consists of a foundation unit (or units), typically spread foundation footings or pads from which a vertical or near vertical cellular skirt hangs.

The cellular skirt penetrates into soil (i.e. the ground or seabed).

The skirt is made up from a series of vertical or near vertical compartments (referred to as cells) which must be open at their base. The cells may be of any shape in plan, although it is envisaged that a rectangular section is easiest to make and offers a high flexural rigidity. Figures 2 to 4 illustrate skirts with rectangular cells as an example.

Optionally, the lower section of any cell can be broadened to act as a friction reducer on one or both of the skirts. One possible arrangement of a friction reducer is shown in Figure 4.

In order to effect installation beyond a skirt penetration into the soil that will occur purely as a result of the applied weight of the skirted foundation (and any additional imposed loading), a soil strength degradation technique is used in the device. This technique involves gas or liquid (or both) injected within each compartment at a velocity sufficient to cause fluidisation of the soil within the cell at or near the base of the skirt and hence reduce the bearing resistance of the soil. The gas or liquid flow (typically air or water) is caused by discharging a jet of gas or liquid from orifices within each cell, at or near the base of the cell. Typically this can be as shown in Figures 3 and 4.

Referring now to the particular configuration illustrated in the drawings, Figure 2 shows a plated foundation of hexagonal planform, with its top removed. The foundation 20 has inner and outer skirt plates 21 and 22, spaced apart by a series of skirt plate connecting walls 24.

Figure 3 shows the plated foundation partly embedded into ground or seabed strata 25. The top of the foundation is designated 26. Following the invention a fluid injection system 27 is used to fluidise soil between the plates 21 and 22 by the injection of pressurised fluid through orifices 28.

Figure 4 illustrates typical details of the cellular compartments. In this case the lower edges of the plates 21 and 22 are splayed outwardly so that a slightly wider section of strata is fluidised. This reduces soil friction on the outer faces of the plates 21 and 22.

The foundation with cellular skirt can, once installed, be used as a foundation in order to transfer load (compression, tension, shear and moment) from a structure into the ground or seabed. Optionally, grout may be injected into the cells through the gas/liquid circulation system or through an independent system, in order to improve the load transfer characteristics.

Claims

1. A plated foundation (or suction anchor) having a top closure plate and a vertical skirt extending downwardly from a periphery of the closure plate to define a downwardly facing cup, in which the skirt comprises two spaced para axial plates with a relatively narrow gap between these plates, and in which there is means to fiuidise soil within the narrow gap at the lower tip of the peripheral skirt.

2. A plated foundation as claimed in Claim 1 in which the gap is of constant width.

3. A plated foundation as claimed in Claim 2 in which the para axial plates are generally concentric, one within the other.

4. A plated foundation as claimed in any one of the preceding claims in which the para axial plates are separated by vertical connecting walls, which divide the space between the skirt plates into cellular compartments.

5. A plated foundation as claimed in any one of the preceding claims, in which a part of at least one of the para axial plates is broadened outwardly from the gap at the base of the skirt.

6. A plated foundation as claimed in any of the preceding claims in which the means to fluidise the soil in the gap at the base of the skirt comprises a fluid conduit extending round the base of the skirt, and having a multiplicity of discharge orifices through which pressurised fluid can be directed at soil in the gap.

7. A plated foundation substantially as hereinbefore described with reference to and as shown in Figures 2 to 4 of the accompanying drawings.