GB2177745A - Offshore platforms - Google Patents

Abstract

A tower structure support for an offshore platform, comprises corner legs (11) spaced apart by a plurality of levels of horizontal (plan) bracing (15), and by inclined (face) bracing (14) on its faces, the face bracing extending diagonally from the plan bracing approximately midway horizontally between the legs to join the legs at the corners of the tower, and has interstitial bracing (16) positioned such that unsupported lengths of face bracing are reduced. A deck structure for e.g. level (12A) of the platform has at least three upstanding pillar members (111), a first deck level (114) attached to and outstanding from those members near to their feet, a second deck level (115) attached to and outstanding from those members above the first deck level, a level of plan bracing (118) attached to those members near their tops, truss bracing (116) between the first and second deck levels, both between and outstanding from the pillar members; and shear bracing (117) between the second deck level and the level of plan bracing extending only between the pillar members. <IMAGE>

Description

SPECIFICATION Offshore structures The invention relates to offshore platforms, and is particularly concerned with lift installable components for such platforms.

In a first form, the invention relates to tower structures for supporting offshore platforms to be utilised in the exploitation of subsea oil and gas reserves, at locations where the water is deep but the requirements for topside facilities and oil wells are small. (Such tower structures will hereinafter be referred to as being "tower structures of the type described").

Heretofor steel tower structures of the type described have been constructed from lattice frameworks of tubular steel members. These tubular members are designed by many criteria, one of the principal criteria being buckling between semi stiff end fixings.

Optimisation of member slenderness ratios forms a normal part of tower design. It is linked to optimising wall thickness to diameter ratios, spacing of plan levels, selecting forms of bracing (K, X etc.), matching loads to structural form, minimising wave loading and optimising for buoyancy requirements.

Usually the aim is to minimise the weight of the bracing, but this minimisation cannot be taken to extremes at the expense of excessive quantities of temporary steel or overlong fabrication times. The objective is to balance out weight and cost.

Weight efficiency can be judged in two ways. First, how near does the face bracing weight approach that of a stiffened shear web, and second, for plan bracing, how near does the total weight come to that of a shear web to carry shear forces induced by roll motion during transportation. The first criterion would be applied to wave shear due to legs, appurtenances and a nominal tubular face bracing arrangement, and the second to a nominal lateral transportation acceleration of 0.759.

A shear web working at an average stress of 0.4 yield is equivalent to 45" X-bracing working at 0.8 yield, weight for weight. Excluding the provision of plan bracing, the equivalence is the same for all forms of bracing at 45 , which, neglecting buckling, is the most efficient bracing form.

By introducing buckling it is clear how far off theoretical efficiency some bracing arrangements can become. For a length to diameter ratio (L/D) of 40, the material is only working at 25% yield for Grade 50, if Fa (normal allowable stress) is taken as a guide to actual storm stress usage. One way to get good structural efficiency and reduce weight is to reduce slenderness. This can be done by decreasing member length to diameter ratios, or by reducing the effective length factor (k) by consideration of end fixities. The conventional k value for bracing is 0.8. During detailed design it should be possible to reduce this value by detailed analysis on a member-by-member basis.

At this point limiting ratios of diameter to thickness (D/t) interplay with L/D selection. A maximum limit of 50-60 is used on tower structures, with 20 as the lower limit. Even with D/t of 50, many members below an elevation of say --100m will need hydrostatic ring stiffening. Therefore the limit to increasing member diameter for better allowable stress is the minimum wall thickness based on D/t limits.

Member sizing for conventionally braced tower structures (for low topside and well requirements) shows that members sized to the normal minimum values for slenderness and thickness ratio are still under utilised for design wave loads. In effect the bracing is less efficient than the minimum ratios pre-suppose.

Large tower structures of the type described have in the past been assembled in fabrication yards having wharves or quays, over which the tower structures could be loaded out onto barges for transportation to an intended offshore site. When at the intended site, the tower structures have been launched from the barges on launch rails built into the lattice framework of the tower. These launch rails run over slipways constructed on the deck of the barge. This has resulted in a relatively large weight of steel being built into the tower structure to withstand the loads arising from the single act of launching it from the barge-a matter of half a minute in the life of a tower structure which may last for several decades.

With a view to eliminating the steelwork associated with the launch framing, various alternatives have been prqposed to move tower structures offshore, and to found them on the seabed without a need to perform the highly dramatic launch procedure. One such method has been to make the tower structure selffloating, so that its legs, upon which -it will stand when founded, act as hulls during its transportation to the offshore site. This again requires a large quantity of steelwork which may not be fully utilised during the later stages of the life of the structure, although in this case the volume within the legs may subsequently be used for oil storage.

Clearly an alternative form of installation procedure would be desirable for large tower structures of the type described.

The invention also relates to deck structures for offshore platforms.

Deck structures for offshore platforms need to be sufficiently robust to resist different loading conditions at different stages in their lives. Specifically deck structures have to be designed to resist loads arising during construction, loadout from the construction yard onto a transport barge, sea tow from the yard to the offshore location of the-platform, installation by lifting from the barge onto the platform substructure, operation (i.e. forces arising from live and dead loads of the operating facility), and finally, removal. The need to resist all of these different loading conditions requires that steel work be designed into the structure which is only needed for transitionary stages in the life of the deck.

The invention provides tower structure for use as a support for an offshore platform, and comprising corner legs spaced apart by a plurality of levels of horizontal (plan) bracing, and by inclined (face) bracing on its faces, the face bracing extending diagonally from the plan bracing approximately midway horizontally between the legs to join the legs at the corners of the tower, and in which there is interstitial tertiary bracing to stabilise the face bracing, such that unsupported lengths of face bracing are reduced.

More specifically the invention provides tower structure for use as a support for an offshore platform, and comprising four corner legs spaced apart by a plurality of levels of horizontal (plan) bracing, and by inclined (face) bracing on its four faces, the face bracing extending diagonally from the plan bracing approximately midway horizontally between the legs to join the legs at the corners of the tower, and in which there is interstitial tertiary bracing to stabilise the face bracing, such that unsupported lengths of face bracing are reduced.

It is preferred that there is no plan bracing at one or more of the levels at which the face bracing joins the legs.

It is also preferred that the plan bracing levels are adapted to act as lift trusses when the tower is lifted in a prone position (i.e.

with the legs in generally horizontal positions).

(According to one feature of the invention there is a structural component provided specifically to support conductors within the tower, and this component has horizontal bracing levels more closely spaced than the plan bracing levels on the tower, whereby conductor support requirements are isolated from the structural framing requirements of the tower.

It is preferred that conductors are supported within the component.

It is further preferred that the conductors are supported on an external face of said component.

The invention includes an offshore platform having a tower structure as described above any one of the preceeding claims.

The invention also includes a deck structure for an offshore platform as described above and having at least three upstanding pillar members, a first deck level attached to and outstanding from those members near to their feet, a second deck level attached to and outstanding from those members above the first deck level, and a level of plan bracing attached to those members near their tops, in which there is truss bracing between the first and second deck levels, both between and outstanding from the pillar members; and shear bracing between the second deck level and the level of plan bracing extending only between the pillar members.

It is preferred that the plan bracing is used to support an overhead travelling crane for use as a substitute for a drilling derrick tower.

It is also preferred that the tops of the pil lars are adapted to be used as lift points, by which the deck structure may be lifted onto a substructure.

It is further preferred that the shear bracing between the pillars is sufficient to provide lateral support to the deck structure during sea tow.

It is still further preferred that the second deck level outwith the confines of pillars is capable of accepting discrete pallets to be individually placed on top of its upper chords.

The invention further provides a deck structure per se.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a side elevation of a tower structure for use as a support for an offshore platform; Figure 2 is an end elevation of that tower structure; Figure 3 is a composite plan of the tower structure; Figures 4, 5 and 6 are sectional plan views at three elevations in the tower structure, marked IV, V, and VI respectively; Figure 7 is a scrap plan on arrow AA in Figure 1; Figure 8 is a side elevation of a structural component to support conductors within the tower of Figures 1 to 6; Figure 9 is an end elevation of that structural component; Figure 10 is isometric view of a deck structure; Figure 11 is a plan view of that structure; Figure 12 is an end elevation of that structure; and Figure 1 2A is a side elevation of that structure.

As shown in Figure 1, a tower 10 has four legs 11, and tapers upwardly from a base, to be founded on the seabed 12, to a top 12A.

The top 1 2A is to be located above sea level (LAT).

The legs 11 are tubular steel members, which are maintained in their relative positions at the corners of the tower by face bracing 14, located in the planes of the sides of the tower; and by plan bracing 15, shown more particularly in the sectional plan views of Fig ures 4, 5 and 6. These secondary bracing members (14 and 15) act to transmit side loads on the top of the structure (e.g. wave, wind and ship impact loads) down to the seabed 12, where the tower is secured by piling (not shown in the Figures, as not forming part of the present invention).

An arrangement of interstitial tertiary bracing 16 in three-dimensional space can halve the effective lengths of the main face diagonals.

This arrangement works well on tower structures which also feature a trussed conductor support component. The additional stability bracing weighs e.g. just over 100 tonnes but it allows the main brace diameters and wall thicknesses to be sized much more efficiently.

Thus one efficient means of optimisation is to use tertiary props for face bracing.

A typical arrangement of the tertiary bracing 16 is shown in the clouded portion of Figure 1, and in the scrap plan section of Figure 7.

The trussed conductor support component 17 is shown in Figures 8 and 9, and can also be seen in the sections of Figures 3 to 6.

This trussed conductor support component may support say eighteen conductors 18 at frequent intervals up the tower. This is necessary because the conductors 18 (of say 26" OD) require even more lateral stabilisation than the face bracing members 14 which may typically be 900mm OD. The legs 11 typically have 3000mm OD tubulars at the base 12 tapering to 1500mm OD tubulars at the top of the tower 10.

It is a particular feature of the configuration illustrated that the plan bracing at the levels V and VI can also serve (when the tower is lying prone rather standing upright) as lift frames. By this means a Heavy Lift Vessel (not shown) can lift the tower from a transport barge and install it at the required offshore site. The CG lies typically at the location shown. It will be appreciated that using the plan bracing as lifting frames utilises this bracing effectively in two phases of the life of the tower, e.g. at the time of installation, and during subsequent operation.

Secondary Plan bracing 21 and 22 in figures 5 and 6 is provided to locate the trussed conductor support component 17, and the only additional members needed for stability on the barge (i.e. to resist transportation loads) are at 23 and 24.

By using tertiary bracing 16 and a separate conductor support component 17, material cost can be saved, since the structural members are used in modes most appropriate to their optimum utilisation.

Using these methods and current installation vessels, it is possible to design a lift installable oil platform support structure for water depths as great as 200m in the most hostile seas.

As shown in Figures 10 to 12A, a deck 110 for an offshore platform (the remainder of which platform is not shown in Figures 10 to 12A, but which may be as illustrated in Figures 1 to 9 or otherwise) has four upstanding pillar members 111, and these pillar members have feet 112. A first deck level 114 (to become the cellar deck of the platform) is attached to the pillars 111, just above the feet 112, The deck level 114 spans between the pillars, and also extends outwards from the pillars to form an overhang. A second deck level 115 (to become the main deck of the platform) is attached to the pillars 111 to overlie the deck level 114. There is truss bracing 116 between the deck levels 114 and 115, and, as may be seen particularly in Figure 10, this truss bracing 116 extends between the pillars 111, and outward of the pillars to support the overhang.

Upper portions of the pillars 111 (i.e. those parts of the pillars above the deck level 115) are held apart by shear bracing 117, and there is a level of plan bracing 118 near the tops of the pillars.

The effect of providing the upper portions of the pillar members 111 with shear bracing is that the pillars can act efficiently both during the transitionary stages of the life of the deck, and also during its operating phase.

Specifically the tops of the pillars can be used as lift points by which the deck can be lifted onto a substructure; and also to provide lateral support during sea tow (by virtue of the shear bracing).

In the event of that the deck is installed on a drilling platform, an electric overhead travelling crane (or cranes) may be used to replace a conventional drilling derrick.

In another embodiment (not shown) the tops of the pillar members of a first deck component could be removed, and a second deck component could be stacked on top of the first.

Claims (16)

1. Tower structure for use as a support for an offshore platform, and comprising corner legs spaced apart by a plurality of levels of horizontal (plan) bracing, and by inclined (face) bracing on its faces, the face bracing extending diagonally from the plan bracing approximately midway horizontally between the legs to join the legs at the corners of the tower, and in which there is interstitial tertiary bracing to stabilise the face bracing, such that unsupported lengths of face bracing are reduced.

2. Tower structure for use as a support for an offshore platform, and comprising four corner legs spaced apart by a plurality of levels of horizontal (plan) bracing, and by inclined (face) bracing on its four faces, the face bracing extending diagonally from the plan bracing approximately midway horizontally between the legs to join the legs at the corners of the tower, and in which there is interstitial tertiary bracing to stabilise the face bracing, such that unsupported lengths of face bracing are reduced.

3. Tower structure as claimed in Claim 1 or Claim 2 in which there is no plan bracing at one or more of the levels at which the face bracing joins the legs.

4. Tower structure as claimed in any one of Claims 1 to 3 in which the plan bracing levels are adapted to act as lift trusses when the tower is lifted in a prone position (i.e. with the legs in generally horizontal positions).

5. Tower structure as claimed in any one of the preceeding claims in which there is a structural component provided specifically to support conductors within the tower, and this component has horizontal bracing levels more closely spaced than the plan bracing levels on the tower, whereby conductor support requirements are isolated from the structural framing requirements of the tower.

6. Tower structure as claimed in Claim 5 in which conductors are supported within the component.

7. Tower structure as claimed in Claim 5 or Claim 6 in which the conductors are supported on an external face of said component.

8. An offshore platform including a tower structure as claimed in any one of the preceeding claims.

9. Deck structure for an offshore platform as claimed in Claim 8, and having at least three upstanding pillar members, a first deck level attached to and outstanding from those members near to their feet, a second deck level attached to and outstanding from those members above the first deck level, and a level of plan bracing attached to those members near their tops, in which there is truss bracing between the first and second deck levels, both between and outstanding from the pillar members; and shear bracing between the second deck level and the level of plan bracing extending only between the pillar members.

10. A deck structure as claimed in Claim 9 in which the plan bracing is used to support an overhead travelling crane for use as a substitute for a drilling derrick tower.

11. A deck structure as claimed in Claim 9 or Claim 10 in which the tops of the pillars are adapted to be used as lift points, by which the deck structure may be lifted onto a substructure.

12. A deck structure as claimed in any of Claims 9 to 11 in which the shear bracing between the pillars is sufficient to provide lateral support to the deck structure during sea tow.

13. A deck structure as claimed in any of claims 9 to 12 in which the second deck level outwith the confines of pillars is capable of accepting discrete pallets to be individually placed on top of its upper chords.

14. A deck structure as claimed in any one of Claims 9 to 13 per se.

15. A tower structure substantially as hereinbefore described with reference to and as shown in Figures 1 to 9.

16. A deck structure substantially as hereinbefore described with reference to and as shown in Figures 10 to 12A.