GB2034378A - Sea bed structure - Google Patents

Abstract

A structure (40) for installation on the sea bed and suitable for piling to provide an anchor comprises an inner portion (42) having a plurality of relatively small diameter apertures (44, 46, 48, 50 and 52) for piles therein and a plurality of connectors (53, 54, 55, 56) for the tethers of a tethered buoyant platform. The structure (40) also has one or more outer portions (80, 82, 84, 86) disposed outwardly with respect to the inner portion (40) having apertures (94, 96, 98, 100) for plies of relatively large diameter. The inner portion (42) is stiff in a vertical direction and the connecting members (60, 62, 64, 66, 68, 70) connecting the inner and outer portions are stiff in a horizontal direction, so that in use vertical loads are distributed to the relatively small diameter piles and horizontal loads to the relatively large diameter piles. <IMAGE>

Description

SPECIFICATION Sea bed structure This invention relates to a structure suitable for installation on the sea bed, to a piled structure installed on the sea bed suitable for an anchor suitable, for example, for a tethered buoyant platform, and to a method of installing said structure.

Tethered buoyant platforms, sometimes called tension leg platforms have been previously proposed for the production of oil and/ or gas from reservoirs beneath the sea bed. A tethered buoyant platform is a platform which in use is moored vertically below its-free floating position by means of tethers to which tension is applied.

It has been previously proposed to hold the lower end of the tethers of tethered buoyant platforms by means of piled anchors. However, there is a risk of failure of the piles due to the combination of vertical and horizontal cyclic and static forces acting on the piles.

It is an object of the present invention to reduce the above mentioned risk.

According to one aspect of the invention a structure suitable for installation on the sea bed to provide, when piled, an anchor has (i) a plurality of first apertures through which piles are to be installed for resisting vertical loads, and (ii) a skirt for penetrating the sea bed to resist horizontal loads and/or a plurality of second apertures of larger diameter than the first apertures through which piles are to be installed for resisting horizontal loads and wherein the stiffness of portions of the structure is designed to distribute the vertical loads to the piles to be installed through the first apertures and the horizontal loads to the skirt and/or to the piles to be installed through the second apertures.

The structure can comprise (i) an inner portion having the first apertures therein to receive piles and also having a plurality of connectors for tethers of a tethered buoyant platform, (ii) one or more outer portions disposed outwardly with respect to the inner portion which outer portions are provided with the skirt and/or the second apertures, and (iii) connecting means connecting the inner and outer portions and wherein the inner portion is stiff in a vertical direction and the connecting means is stiff in a horizontal direction whereby, in use, vertical loads are distributed to the piles installed in the apertures in the inner portion and the horizontal loads are distributed to the skirt and/or piles installed in the second apertures in the outer portions.

Conveniently the connecting means is less stiff in a vertical direction than in the horizontal so that vertical loads are not distributed to the outer portion.

The first apertures can be located on the circumference of a first circle and the second apertures can also be located on the circumference of a second circle of larger diameter and concentric with the first.

The structure can be made up of a plurality of cells.

Conveniently the structure is made so that prior to installation it is buoyant.

In one embodiment the structure can be of a generally annular shape and the piles which resist vertical loads pass through the apertures of relatively small diameter and the piles which resist horizontal loads pass through the apertures of relatively large diameter.

Conveniently the cells having apertures of large diameter are disposed symmetrically, for example, diametrically opposite each other.

The apertures of smaller diameter can also be disposed symmetrically.

The two sets of apertures can be staggered with respect to each other.

The invention also provides a piled structure in which a plurality of piles are installed in the first apertures the first piles being of suitable dimensions for resisting vertical loads.

Further, when piled a plurality of second piles are installed in the second apertures in the second piles being of suitable dimensions for resisting horizontal loads, the first piles being relatively long and of relatively small diameter in relation to the second piles.

According to another aspect of the present invention a method of installing a piled structure to provide an anchor for a tethered buoyant platform comprises transporting a structure to an offshore location lowering the structure to the sea bed and installing a plurality of relatively long, relatively small diameter piles to resist vertical loads, and to resist horizontal loads, penetrating the sea bed with a skirt and/or installing a plurality of relatively short, relatively large diameter piles.

By the term sea bed in the present specification we mean the bottom of any body of water, for example, ocean, river or lake.

The invention is illustrated by reference to the accompanying drawings in which Fig. 1 is a perspective view of the anchor located on the sea bed and Fig. 2 is a plan view of the anchor showing the disposition of piles.

Figure 3 is a plan view part horizontal section of an alternative embodiment of the invention.

Figure 4 is an elevation in the direction indicated by the arrow B in Fig. 3.

Figure 5 is a vertical section through a collar in which is installed a pile of relatively small diameter.

Figure 6 is a vertical section through a collar in which is installed a pile of relatively large diameter.

Figure 7 is a plan view of an anchor post.

Figure 8 is a vertical section through an anchor post on the line A-A of Fig. 7.

Figure 9 is an elevation looking in the direction indicated by the arrow B.

Figure 10 is a partial perspective view of an anchor post.

Figures 11 to 15 are schematic drawings showing various stages of the transportation and installation of the modules.

Referring to Figs. 1 and 2 the anchor, indicated generally by reference numeral 2, comprises a structure in the form of a steel module 4 in the shape of an annulus and of cellular construction. The module 4 has an inherent but adjustable buoyancy to facilitate flotation from fabrication site to offshore location.

The module can be installed by pulling into the water against its buoyancy, for example, by means of cable and pulleys. This has the advantage of reducing the risk of damage to pre-installed well heads or the like in the event of failure of a cable or other mishap during the installation.

The module consists of sixteen cells forming the annulus and has four outer cells 8, 10, 1 2 and 14 attached to every fourth cell in the annulus.

The annulus has four relatively long, rela tiveiy small diameter piles 16, 18, 20 and 22 located in apertures in every fourth cell in the annulus, and four relatively short, relatively large diameter piles 24, 26, 28 and 30 located in the outer cells, the two sets of piles being staggered.

The inner cells are made stiff in a vertical direction (they need not necessarily be stiff in a horizontal direction) so that the vertical loads are distributed to the piles 16, 18, 20 and 22 and the outer cells are made relatively flexible in a vertical direction and relatively stiff in a horizontal direction so that horizontal loads are distributed to the piles 24, 26, 28 and 30.

The relatively long, relatively small diameter piles each have a structural connection to the steel module which acts as a stiff cap. This enables vertical load to be transmitted to the relatively long, relatively small diameter piles and horizontal load to be transmitted by the cap to, and resisted by, the relatively short, relatively large diameter piles.

The relatively large and relatively small diameter piles can be installed by drilling or driving.

Projecting upward from the upper face of the annulus is a plurality, for example, eight, connectors (not shown) for latching with the connectors at the lower ends of the tethers.

Referring to Fig. 3 the steel module indicated generally by numeral 40 comprises an inner annular portion 42 having five equally spaced apertures 44, 46, 48, 50 and 52 to receive piles of relatively small diameter. Each aperture has located close thereto two anchor posts (one on each side) thus making a total of ten anchor posts only five of which 53, 54, 55, 56 and 58 are numbered. The apertures are set on a pitch circle of diameter 8 metres to coincide with the pitch circle of the individual tethers of each tether group.

The annular portion 42 is centred on this circle thereby connecting the tethers to the long relatively small diameter (tensile) piles.

The annular portion is composed of two circumferential steel plates 39 and 41 forming the sides and stiffened radially by steel diaphragms 45. This construction gives the annular portion 42 a very high stiffness, which ensures that vertical load from the tethers is distributed to the piles 1 20. The tethers (not shown) are connected to the anchor posts 53 etc. at the top of the annular portion 42, while the vertical load resisting piles 1 20 are recessed in the apertures 44 etc.

The annular portion 42 is connected to four outer portions of the structure by a framework comprising ten tubular steel members which extend radially with respect to the annular portion 42 only six of the steel members 60, 62, 64, 66, 68 and 70 being numbered.

Located in a square configuration and outwardly with respect to the annular portion 42 are four tubular steel members 80, 82, 84 and 86, to which six of the first mentioned radial steel members are connected, for example, 60, 64, 66 and 68. The remaining four of the radial steel members only one of which 62 is numbered are each connected to a tubular steel member such as 90 disposed at an angle of about 80 thereto.

Located outwardly with respect to the annular portion 42 and at the corners of the square formed by members 80, 82, 84 and 86 are collars 94, 96, 98 and 100 which provide the apertures of relatively large diameter to receive the piles of relatively large diameter.

These apertures 94 etc. are set on a pitch circle of 20 metres diameter to separate the piles by greater than 5 pile diameters to maintain efficiency of pile action. The tubular members of the framework are all 36 inch od steel. This framework has good resistance to axial (horizontal) loads but poor resistance to vertical loads compared to the annular portion 42, so that to a large extent only the horizontal load components are distributed to the large diameter piles which resist these loads in shear.

Referring to Figs. 4 and 6 each collar 94 and 96 comprises a flared portion 102, a portion of enlarged diameter 104 communicating with a pipe 106 for the admission of grout to the annular space between a 60 inch outer diameter driven pile 108 of 50 metres length and the collar. Enclosing each collar is a housing 110 from which extends a steel skirt 11 2 which penetrates the sea bed 114.

Referring to Fig. 5 aperture 44 in the annular portion of module 40 has installed therein a pile assembly indicated generally by 1 20 which comprises a 30 inch diameter steel housing 1 21 with a tubular casing extension 1 22 of 30 metres length and a 20 inch diameter steel housing 1 24 with a tubular casing extension 1 26 of 200 metres length.

Both casings 1 22 and 1 26 penetrate the sea bed 114 as shown. At the mouth of the aperture 44 is a flared portion 1 28.

The pile assembly is installed by drilling and grouting. The purpose of the 30 inch diameter casing is to provide a stable hole for drilling the cavity for the 20 inch casing. The connection of the pile assembly 1 20 to the module 40 is designed to use standard well head equipment. Essentially the 20 inch cas ing 1 26 is hung from the top of the 30 inch casing 1 22 which is in turn hung from the module 40. Positive lock devices are included to prevent flotation of the casing during grouting operations.

Referring to Figs. 7 to 10 a tether anchor post 53 (one of the ten) adjacent aperture 50 projects upwardly from the annular portion 42 and consists of a steel tube having an upper tapered end 1 30 with a notch or groove therein to enable a lower tether terminal (not shown) to latch thereto. Shown adjacent the anchor post 53 in Fig. 8 is a tether haul down pulley 132, located on either side of which are stiffeners 1 34 and 1 36.

Referring to Figs. 11 to 1 5 a number of modules 40 are loaded on to a barge 1 40 and towed by ship 1 42 to an offshore location near drill ship 144 which has drilled production wells using a template 146. At this location, the modules are offloaded from barge 140 and allowed to float in the water.

By flooding and/or hauling down (the latter not being shown) the modules 40 are installed on the sea bed 11 4 and are ready to receive their piles. Alternatively a non-buoyant module can be installed using a crane barge.

Claims (10)

1. A structure suitable for installation on the sea bed to provide, when piled, an anchor the structure having (i) a plurality of first apertures through which piles are to be installed for resisting vertical loads and (ii) a skirt for penetrating the sea bed to resist horizontal loads and/or a plurality of second apertures of larger dismeter than the first apertures through which piles are to be installed for resisting horizontal loads and wherein the stiffness of portions of the structure is designed to distribute the vertical loads to the piles to be installed through the first apertures and the horizontal loads to the skirt and/or to the piles to be installed through the second apertures.

2. A structure as claimed in claim 1 wherein the structure comprises (i) an inner portion having the first apertures therein to receive piles and also having a plurality of connectors for tethers of a tethered buoyant platform, (ii) one or more outer portions disposed outwardly with respect to the inner portion which outer portions are provided with the skirt and/or the second apertures, and (iii) connecting means connecting the inner and outer portions and wherein the inner portion is stiff in a vertical direction and the connecting means is stiff in a horizontal direction whereby, in use, vertical loads are distributed to the piles installed in the apertures in the inner portion and the horizontal loads are distributed to the skirt and/or piles installed in the second apertures in the outer portions.

3. A structure as claimed in claim 2 wherein the connecting means is less stiff in a vertical direction than in the horizontal direction.

4. A structure as claimed in claim 3 wherein the first apertures are located on the circumference of a first circle and the second apertures are also located on the circumference of a second circle of larger diameter and concentric with the first.

5. A structure as claimed in any one of claims 1 to 4 wherein a plurality of first piles are installed in the first apertures the first piles being of suitable dimensions for resisting vertical loads.

6. A structure as claimed in claim 5 wherein a plurality of second piles are installed in the second apertures the second piles being of suitable dimensions for resisting horizontal loads, the first piles being relatively long and of relatively small diameter in relation to the second piles.

7. A structure as claimed in claim 6 wherein the first piles are installed by drilling and the second piles by driving.

8. A method of installing a piled structure to provide an anchor for a tethered buoyant platform which method comprises transporting a structure as claimed in any one of claims 1 to 4 to an offshore location, lowering or pulling the structure to the sea bed and installing a plurality of relatively long, relatively small diameter piles to resist vertical loads and, to resist horizontal loads, penetrating the sea bed with the skirt and/or installing a plurality of relatively short, relatively large diameter piles.

9. A piled structure installed on the sea bed substantially as hereinbefore described with reference to Figs. 1 to 10 of the accompanying drawings.

10. A method of installing a piled structure on the sea bed substantially as hereinbefore described but with reference to Figs. 11 to 1 5 of the accompanying drawings.