GB2139570A

GB2139570A - Jack-up rig

Info

Publication number: GB2139570A
Application number: GB08411017A
Authority: GB
Inventors: George Collingwood Copson
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-04-28
Filing date: 1984-04-30
Publication date: 1984-11-14
Also published as: GB2139570B; GB8411017D0

Abstract

A jack-up rig comprising a deck structure 1 having outer jack-up support legs 5 and a central jack-up support leg 6 of larger cross- sectional area than each of the outer legs. The central leg is preferably capable of supporting the entire deck structure, at least temporarily, and may act as a conductor guideway during drilling and/or piping operations. The central leg supports the deck structure against collapse on damage to or loss of an outer leg. A further embodiment is disclosed which comprises support legs 5 and inner support legs (26 Fig. 5) of similar size to legs 5. <IMAGE>

Description

SPECIFICATION Jack-up rig This invention relates to offshore work platforms, especially to bottom supported stationary platforms and most especially to selfelevating platforms (commonly referred to as "jack-up rigs").

Jack-up rigs are normally towed to the site of operation on their buoyant deck-hulls; support legs at the apices of the deck are lowered to the sea-bed by means of suitable jacks, and the deck then lifted clear of the water surface to a suitable height by the same jacks. Such rigs normally have a square deck area with four outer support legs and employ "over-theside" operation whereby drill conductors, pipelines and cables are lowered to the seabed work-site over the side of the deck. in the event of loss of, or damage to, one of the support legs there is no safety factor and the rig is liable to collapse or capsize.

The present invention provides a jack-up rig comprising a deck structure having outer jackup support legs and a central jack-up support leg, the central leg preferably being of larger cross-sectional area than each of the outer legs. The central support leg is preferably of such cross-sectional size, shape and orientation that its periphery in plan view extends outwardly beyond the outermost line joining the two outer legs adjacent to any given outer leg. On damage to or loss of a given outer leg the central leg will then contribute more to the support provided by the remaining legs against tipping of the deck. Preferably the central leg is of sufficient cross-section and strength that it is capable of supporting the entire deck structure, at least temporarily.

Preferably the deck structure and support legs are polygonal (e.g. square). It may be preferred for the central support leg apices to be skewed relative to the positions of the outer support legs as this can increase the contribution of the centre leg to the rig stability. The central support leg can act as a conductor guideway during drilling and/or piping operations.

The outer support legs may be piled e.g. at each corner, or may be mounted on a sea-bed template or base. The central leg may be likewise piled or mounted.

A jack-up rig according to the invention has a high safety factor in the event of loss or damage to one or more of the outer support legs. It can obviate over-the-side operation without sacrificing useful deck area by utilising the central support leg as a guideway for cables and pipes. Whilst particularly applicable to rigs having four outer support legs, the invention can also have advantage with other numbers of outer support legs.

Embodiments of the invention will be described in detail by way of example only in the accompanying drawings wherein like reference numerals indicate like parts and wherein FIGURE 1 is a plan view of a jack-up rig according to the invention; FIGURE 2 is an elevation view of the Fig. 1 rig in position on the sea-bed; FIGURE 3 is a plan view, similar to Fig.1, of another jack-up rig according to the invention; and FIGURES 4 and 5 are respectively plan and elevation views of a modified jack-up rig according to the invention.

The jack-up rig of Figs.1 and 2 has a square deck area (7) which forms the upper surface of a buoyant hull (2). The deck (1) carries a derrick (3) mounted on a gantry (4).

At the four apices of deck (1) are orifices through which pass truss-type jack-up support legs (5) independently under the control of pneumatic or hydraulic jacks or rack-and-pinion devices (not shown). At the centre of the deck (1) is a further orifice through which passes a central jack-up support leg (6). The central leg (6) is of much larger cross-sectional size than the outer legs (5), and is of sufficiently strong construction that it is capable of supporting the entire deck structure. The central leg (6) may conveniently be fabricated from a number of units (7) which are interconnected as the leg is lowered to the sea-bed section by section.

In use the feet (9) of the support legs (5 and 6) rest on the seabed (10) and the hull (2), deck (1) and operational equipment are supported above the water level (8). The central leg (6) acts as a protective guideway for drill conductors, cables and pipes (12). Such equipment may pass through and be guided by suitable vertically separated spacer plates (not shown). The gantry (4) is moveable on rails (13) so that the derrick (3) may be positioned over the conductor guideway (11).

On the illustrated jack-up rig the deck (1) carries certain of the equipment needed for offshore operations including a helicopter landing area (14), deck cranes (15), and lifeboats (16).

Whilst a particular example of a jack-up-rig according to the invention has been described having a square deck area and square section truss-type support legs at the apices, other geometries, leg positions and construction methods may be employed within the invention.

For example, it may be particularly beneficial to skew the apices of the central leg relative to the positions of the outer legs so that, when the base of the central leg is pinned to the sea-bed at each of its apices, the largest number of pinned apices co-operate to stabilize the rig in the event of loss of support from one outer leg thereby improving stability in comparison to a similar rig having only outer support legs.

Fig. 3 illustrates such a rig with a skewed centre support leg (6); the rest of the deck lay-out is shown as being substantially the same as that of Fig.1, but it may be preferable to rearrange this, e.g. by positioning derrick (3) at a deck corner with rails (13) running parallel to opposed edges of leg (6).

It will be seen that the side of centre leg (6) is longer than the diagonals of outer legs (5).

Whilst not shown, the central and/or outer support legs of the Figs. 1 to 3 embodiments may be piled at their corners and/or mounted on a false sea-bed base.

Central support of the deck and hull allows lighter construction without loss of strength.

Alternatively, for the same weight of superstructure a larger platform may be built providing more work space and greater stability.

In the event of loss or damage to an outer leg the large inner leg cooperates with the remaining outer legs to provide rig stability and so prevent collapse pending outer leg repair or replacement.

By operating through the central conductor guide, pipes and cables are less vulnerable than in the traditional over-the-side arrangement. For towing to the site of operation thc central leg sections may be either stored on deck or carried separately and the outer legs may be in the semi-retracted position so as to lower the metacentric height of the rig and prevent capsize.

In a preferred embodiment the outer legs are also adequate to support the entire deck load, thus providing an improved redundancy safety factor.

There is no restriction to the overall size or depth of operation of a rig according to the invention, but it is currently thought to be particularly applicable to operations in water depths of up to 150 m.

The present invention also provides a jackup rig comprising a (preferably polygonal) deck structure having outer jack-up support legs around the periphery and also having inner jack-up support legs within the perimeter of the deck structure to provide an improved safety factory in the event of loss of, or damage to, a support leg. Such a rig is illustrated in Figs.4 and 5.

Preferably the outer support legs are sufficiently strong that they are capable of supporting the entire deck structure independent of the inner support legs, and suitably the inner legs may independently support the entire deck structure.

Preferably the deck structure is provided with an aperture ("moon-pool") through which drill conductors, pipes and cables may be passed.

In one particularly preferred embodiment of the invention the deck area is square, and has one outer support leg at each apex and one inner support leg positioned on a deck diagonal at a position inboard of each outer leg.

The illustrated jack-up rig of Figs.4 and 5 has a square deck area (1) which forms the upper surface of buoyant hull (2). At the four apices of the deck (1) are orifices through which pass truss-type outer jack-up support legs (5) independently under the control of pneumatic or hydraulic jacks or rack-and-pinion devices (not shown). Disposed inboard of the outer legs (5) are orifices through which pass truss-type inner jack-up support legs (26) which are similarly independently controlled.

It is especially advantageous in enhancing the redundancy safety factor of the rig with respect to loss of or damage to an outer leg (5) if the group of four outer legs (5) alone, and the group of four inner legs (26) alone, are each capable of supporting the entire deck structure.

In use the feet (9) of the support legs (5) and (26) rest on the sea-bed (10) and the deck (1), hull (2) and operational equipment are supported above the water level 8.

The deck (1) is provided with a central moon-pool (21). Drill conductors, cables and pipes may pass through the moon pool (21).

A gantry (4) is provided which is moveable on rails (13) so that a derrick (3) may be positioned over the moon pool (21). On the illustrated jack-up rig the deck (1) carries certain of the equipment needed for offshore operations including a helicopter landing area (14), deck cranes (15) and lifeboats (16).

Support of the deck and hull by two sets of legs (inner and outer) allows for lighter deck construction without loss of strength. Alternatively, for the same weight of superstructure a larger platform may be built providing more work space and greater stability. In the event of loss or damage to an outer leg the remaining legs are sufficient to maintain stability and so prevent collapse.

By operating through a moon pool operational equipment is less vulnerable to damage than in the traditional over the side arrangement. However, it is of course possible to operate a rig according to the invention over the side, with or without the presence of a moon pool.

There is no restriction to the overall size or depth of operation of the Fig. 4, 5 rig according to the invention, but it is currently thought to be particularly applicable to operations in water depths up to 1 50 m. For towing to the site of operation the support legs may be carried in sections on deck or may be in the semi retracted position so as to lower the metacentric height of the rig and prevent capsize.

Whilst a particular example of a rig has been shown in Figs. 4 and 5 as having a square deck area and square section trusstype support legs, other geometries, leg positions and construction methods may be employed within the invention.

Claims

1. A jack-up rig comprising a deck structure having outer jack-up support legs and a central jack-up support leg of larger cross-sectional area than each of the outer legs.

2. A jack-up rig according to claim 1 wherein the central support leg is of such cross-sectional size, shape and orientation that its periphery in plan view extends outwardly beyond the outermost line joining the two outer legs adjacent to any given outer leg.

3. A jack-up rig according to claim 1 or 2 wherein the central leg is capable of supporting the entire deck structure, at least temporarily.

4. A jack-up rig according to any preceding claim wherein the central support leg apices are skewed relative to the positions of the outer support legs.

5. A jack-up rig according to any preceding claim wherein the central support leg acts as a conductor guideway during drilling and/or piping operations.

6. A jack-up rig according to any preceding claim wherein the outer support legs are piled or mounted on a sea-bed template or base.

7. A jack-up rig according to any preceding claim having four outer support legs.

8. A jack-up rig substantially as herein be fore described with reference to Figs. 1 and 2 of the accompanying drawings.

9. A jack-up rig substantially as hereinbefore described with reference to Fig.3 of the accompanying drawings.

1 0. A jack-up rig comprising a deck structure having outer jack-up support legs around the periphery and also having inner jack-up support legs within the perimeter of the deck structure to provide an improved safety factor in the event of loss of, or damage to, an outer support leg.

11. A jack-up rig substantially as hereinbefore described with reference to Figs. 4 and 5 of the accompanying drawings.