GB2335221A

GB2335221A - Method of forming a substructure for an offshore platform

Info

Publication number: GB2335221A
Application number: GB9827276A
Authority: GB
Inventors: Michael Timothy Smith; David Collier
Original assignee: Ove Arup Partnership Ltd; Kvaerner Oil and Gas Ltd
Current assignee: Ove Arup Partnership Ltd; Kvaerner Oil and Gas Ltd
Priority date: 1997-12-11
Filing date: 1998-12-11
Publication date: 1999-09-15
Anticipated expiration: 2018-12-11
Also published as: GB2335221B; GB2335221A8; GB9726277D0; GB2335221A9; GB9827276D0

Abstract

A method of constructing, transporting and installing a substructure for an offshore platform, comprising the steps of:- forming a generally flat base (10) on the floor of a drydock (14); arranging at least two vertical legs (11) to stand on that base; flooding the drydock so that the upper surface of the base (10) is covered with water with the two legs projecting upwardly through the water surface; floating a vessel (12) over the base (10) and between the two legs (11); connecting the two legs to the vessel; securing the base beneath the vessel so that it can be supported by the vessel; transporting the base to the intended offshore location beneath the vessel; and lowering the base to rest on the seabed at the intended offshore location. The substructure can be removed by reversing the method of installation.

Description

METHOD OF FORMING A SUBSTRUCTURE 2335221 The invention relates to a method

of constructing, transporting and installing a substructure for an offshore platform; and to a substructure so formed.

Substructures for offshore platforms can either be secured to the seabed with piles; or can rest on the sea bed under the influence of gravity alone. This latter type of substructure is known as a "gravity based substructure". It is to this latter type of substructure that the present invention relates.

Heretofore it has been known to construct a Tondeep" type gravity based substructure by forming its base in a dry-dock. When the base has been completed, it has been floated out of the dry-dock. Legs have then been slipformed up to the required height. Slipforming has been carried out at a sheltered deepwater site. The substructure has, while remaining buoyant, sunk down towards the seabed of the site in a controlled manner as slipforming proceeds. A "Condeep" type substructure is described in U.K. Patent Specification 1,417,472 (granted to Olav Mo).

This construction technique has required the availability of a convenient sheltered deepwater site. Additionally it has required the use of slipforming equipment while the partially completed substructure was afloat. Notwithstanding the disadvantages of these requirements, many "Condeep" type substructures have had sufficient buoyancy to carry substantial topside loads out to their intended offshore locations.

It has also been known to construct substantially the whole of a gravity based substructure (such as that for North Ravensburn) in a dry-dock. That substructure had a flat base of generally square planform. The completed substructure was floated out to the intended offshore site. The substructure was then controllably flooded, so that one edge of its base sank down until it rested on the seabed. Further controlled flooding then pivoted the whole substructure about the edge resting on the seabed, so that the base was set in a level attitude on the seabed. The installation of the North Ravensburn platform was described in Offshore Engineer for September 1989 at page 23.

This installation technique allows completion of the substructure in a dry-dock. However, the tilting technique makes it impossible to carry any substantial amount of topsides equipment out to the intended offshore site with the substructure.

(The two examples of prior art substructures described above were made of concrete, However, gravity based substructures can also be made of steel.)

Thus there is a requirement for a substructure for an offshore platform that can be completed in a dry-dock, and which can be set down on the seabed without tilting.

The invention provides a method of constructing, transporting and installing a substructure for an offshore platform, comprising the steps of:- forming a generally flat base on the floor of a dry-dock or other floodable- area; arranging at least two vertical legs to stand on that base; flooding the dry-dock so that the upper surface of the base is covered with water with the two legs projecting upwardly through the water surface; floating a vessel over the base and between the two legs., connecting the two legs to the vessel; securing the base beneath the vessel so that it can be supported by the vessel; transporting the base to the intended offshore location beneath the vessel; and lowering the base to rest on the seabed at the intended offshore location.

The two legs may be cast on the base in the dry-dock, or may be added to the base in the dry-dock.

It is preferred that the base is lowered using strand jacks connected between opposed sides of the vessel and the two legs andlor the base adjacent to respective opposed sides.

It is also preferred that at least a part of a deck is set on either or both of the two legs in the dry-dock, so that the part of the deck can be transported to the offshore location with the substructure.

It is further preferred that a substantially complete deck is set on both of the legs so that the complete deck can be transported to the offshore location with the substructure.

The invention includes a substructure for an offshore platform when constructed, transported and installed in accordance with the above description, and the invention also includes an offshore platform incorporating such a substructure.

The invention also relates to a method of removing a substructure from an offshore site.

A specific embodiment of a method according to the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figures 1 to 8 are a sequence of diagrammatic cross sections illustrating the construction, transportation and installation of a substructure for an offshore platform; and Figure 9 is a perspective view of the completed platform.

Figure 1 shows a substructure 15 for an offshore platform under construction and nearing completion in a dry-dock 14. The substructure has a flat base 10 and cylindrical legs 11. The legs in this instance are of different diameters, but could be identical. The base 10 and legs 11 may be cast of suitably reinforced concrete. Alternatively, the base may be made of concrete, and the legs can be prefabricated of steel. As a third alternative, the whole of the substructure may be made of steel and assembled in the dry-dock.

When the substructure is completed, the dry-dock is flooded, as shown in Figure 2.

Water covers the upper surface of the base, to a depth sufficient for a barge to be floated over the base.

In this situation, a barge 12 is floated over the base 10 between the legs 11, as shown in Figure 3. If the barge 12 is significantly narrower than the space between the legs 11, then outrigger fittings (not shown) may be added to the sides of the barge, so that there are only small gaps between the fittings and the legs. Depending on the weight and balance of the substructure 15, outriggers may be needed on one side only of the barge.

The barge may descend onto the upper surface of the base 10 on a failing tide, as shown in Figure 4.

Strand jacks (not shown) on the sides of the barge are attached to the legs 11 andlor the base 10. As an alternative to allowing the barge to settle onto the base on a failing tide, the strand jacks can be used to lift the base 10 off the floor of the dry- dock. As the weight of the substructure 15 is taken up on the barge 12, the draft of the barge increases.

The substructure 15 is secured in a position directly below the barge 12. This arrangement is illustrated in Figure 5. In this configuration the barge and substructure can be floated out - as shown in Figure 6 - to the intended offshore location for the platform.

At the offshore location, the strand jacks on the barge 12 can be used to lower the substructure 15 to the seabed, as shown in Figure 7. As the weight of the substructure is set on the seabed, the weight is removed from the barge which rises in the water. This situation is shown in Figure 8.

The barge 12 can now be floated away from the substructure 15.

Decks 16 incorporating topsides equipment can be installed on the legs 11, as shown in Figure 9. This completes the platform.

Many variations of this method are possible. For instance, given sufficient reserves of buoyancy for the barge, the decks incorporating topsides equipment could be installed on top of the legs while the substructure is still in the dry-dock.

The topsides equipment may be in two parts connected by walkways (as shown in Figure g), or may be located on a single deck spanning between the two legs.

Two legs upstanding from the base are essential for the method of the invention.

However, there may be more than two legs, depending on the configuration required for the topsides equipment.

While the item 12 has been described as a barge, any floating vessel of suitable shape and size (buoyancy) could be used. In some circumstances a tanker hull might be a suitable vessel. In any case, the vessel could be equipped with strand jacks to engage the legs or base on either side of the vessel.

In some circumstances, it may be possible to remove the substructure by reversing the method described above.

Claims

1 A method of constructing, transporting and installing a substructure for an offshore platform, comprising the steps of:forming a generally flat base on the floor of a dry-dock or other floodable area; arranging at least two vertical legs to stand on that base; flooding the dry-dock so that the upper surface of the base is covered with water with the two legs projecting upwardly through the water surface; floating a vessel over the base and between the two legs; connecting the two legs to the vessel; securing the base beneath the vessel so that it can be supported by the vessel; -transporting the base to the intended offshore location beneath the vessel; and lowering the base to rest on the seabed at the intended offshore location.

2. A method as claimed in Claim 1 in which the two legs are cast on the base in the drydock.

A method as claimed in Claim 1 in which the two legs are added to the base in the drydock 4. A method as claimed in any one of the preceding claims in which the base is lowered using strand jacks connected between opposed sides of the vessel and the two legs andlor the base adjacent to respective opposed sides.

5. A method as claimed in any one of the preceding claims in which at least a part of a deck is set on either or both of the two legs in the dry-dock, so that the part of the deck can be transported to the offshore location with the substructure.

A method as claimed in Claim 5 in which a substantially complete deck is set on both of the legs so that the complete deck can be transported to the offshore location with the substructure.

7. A method substantially as hereinbefore described by way of example with reference to the accompanying drawings.

8. A substructure for an offshore platform when constructed, transported and installed in accordance With any one of the preceding claims.

9. An offshore platform incorporating a substructure as claimed in Claim 8.

10. A method of removing a substructure for an offshore platform from an offshore location, the substructure having a generally flat base and at least two vertical legs upstanding therefrom, and comprising the steps of:arranging a vessel to float over the base and between the legs; raising the base from the seabed to a position beneath the vessel; securing the base beneath the vessel so that it is supported by the vessel; transporting the substructure away from its offshore location to a sheltered shallow water area; lowering the base to rest on the seabed at that area; and releasing the vessel from the substructure.

h Amendments to the claims have been filed as follows 1. A method of assembly of a floating platform comprising in combination a semisubmersible vessel having two or more pontoons, buoyant columns upstanding from those pontoons and a deck supported on the columns (the semi-submersible vessel being of a kind known per se), and a raft having a hull portion with a planform greater than the planform of the vessel; which method comprises the steps of ballasting a raft (of the kind described) so that the raft rests on a prepared subsea surface with the upper surface of its hull submerged to a depth greater than the deballasted draft of the vessel, floating the vessel over the hull of the raft, arranging for the vessel to move downwardly until its pontoons rest on the upper surface of the hull of the raft, deballasting the raft so that the raft floats of the subsea surface and moves upwardly until the pontoons of the vessel are raised aboveWater level, and then securing the vessel to the upper surface of the hull of the raft.

2. A method as claimed in Claim 1 'in which the raft is ballasted down to rest on the prepared subsea surface on a failing tide.

3. A method as claimed in Claim 1 or Claim 2 in which the semisubmersible vessel is floated over the raft at or near high tide, and the vessel is ballasted down to rest on the upper surface of the raft on a failing tide.

A method as claimed in any one of the preceding claims in which the raft is deballasted at or near low water so that on the subsequent rising tide it will raise the pontoons of the sem-isubmersible vessel above water level.

A method as claimed in any one of claims 1 to 3 in which the raft is deballasted on a rising tide to raise the pontoons of the semisubmersible vessel above water level.

A method as claimed in any one of the preceding claims in which the semisubmersible vessel 'is deballasted simultaneously with the raft to raise the pontoons of the vessel above water level..

A method as claimed in any one of the preceding claims in which there are jet nozzels set into the lower surface of the raft, and when the raft is to be debaliasted, water is jetted through those nozzels to break any suction holding the raft down on the prepared subsea surface.