GB2253872A

GB2253872A - Offshore platform

Info

Publication number: GB2253872A
Application number: GB9206155A
Authority: GB
Inventors: Svein Fjeld
Original assignee: Norwegian Contractors AS
Current assignee: Norwegian Contractors AS
Priority date: 1991-03-22
Filing date: 1992-03-20
Publication date: 1992-09-23
Anticipated expiration: 2012-03-20
Also published as: NO172658B; GB9206155D0; NO911166L; NO172658C; GB2253872B; NO911166D0

Abstract

An offshore platform of concrete for exploration of oil/gas, having two or more towers 1 which rest on a fundament slab 2 of concrete having its underside provided with downwardly open skirts 3. The system of a fundament slab and skirts give sufficient strength and rigidity for transferring forces acting on the towers. Riser pipes may be installed wholly within or partly outside the towers and the towers and/or riser pipes may be stiffened by concrete beams within the towers. <IMAGE>

Description

OFFSHORE PLATFORM The invention relates to a concrete offshore platform, especially for production of oil and gas.

More particularly, the invention relates to an offshore platform which, in comparison with known types of offshore platforms which also are prefabricated in a suitable vicinity and are floated out to the installation site to be descended down to a preselected location on the sea bottom, are made with a substantial saving with regard to concrete volumes and building time.

Offshore platforms of concrete are known. Such platforms are composed of one or several towers which support a working platform above the sea surface. The towers are resting on a buoyant body composed of for instance cylindrical, closed chambers which, further to providing the desired strength and rigidity, also are used as ballasting/deballasting chambers during production, towing and descending of the platform.

In addition, such platforms are also provided with skirts for improved contact with the sea bottom.

These ballasting/deballasting chambers constitute a substantial part of the entire platform, and their importance is to a great extent allied to 1) the need for floating ability during production, towing and descent, and 2) need for strength and rigidity of the fundament and the fixation of the towers to the fundament.

It has been shown, however, that the above mentioned floating ability and strength requirements adequately can be met without utilization of a buoyant body as described.

The present invention relates to an offshore platform of concrete, which consists of two or more towers which rest on a concrete fundament slab. The fundament slab is on the underside provided with skirts adapted to penetrate down into the sea bottom, while the fundament slab is positioned closely above the sea bottom. The skirts are substantially closed laterally and in the top during the production, towing, descent and operation on the sea bottom. The system of skirts and fundament slab offers the required buoyancy by establishing a cushion of compressed air between or inside the skirts.

During installation, the skirts may be pressed deeper down into the ground by establishing a sub-pressure between the skirts, and if necessary, carry out a levelling of the fundament slab by applying sub-pressures with varying magnitude below the same. The towers may also be utilized for ballasting purposes, and possibly be divided up into chambers.

In comparison with platforms of concrete of known types, the platform in accordance with the present invention implies an important saving with regard to consumption of concrete 30 % or more - simultaneously as the building time is substantially reduced, and the mechanical equipment is also simplified. The riser pipes for oil wells can be installed entirely or partly on the outside of the towers, or if necessary, be passed through the tower wall. A stiffening of the riser pipes between the towers can be secured by means of bracings in the shape of for instance beam-like elements of concrete, which are bridged between the towers.

The fundament slab together with the skirts offers sufficient rigidity and strength in order to transfer lateral forces on the towers down to the sea bottom. The towers, which in the upper parts may be interconnected with bracing elements, can be made more slender in that the riser pipes are passed up along the outside of the towers. The fact that the towers may be made more slender results in that the forces which must be taken up by the fundament slab and the skirts, will be reduced, and sufficient rigidity is obtained by adapting the wall thickness of the towers and the depth of the skirts and their wall thickness to sustain expected loads.

The fundament slab may be given an area which is greater than the usual area in connection with platforms provided with caisson, and with a correspondingly reduced pressure against the ground masses.

It is important that the fundament slab and the skirts form an integrated structure which has sufficient strength and rigidity to support the towers and the loads which they are subjected to. The rigidity of the fundament slab is secured by among others by providing straight skirt walls between extensions of the towers. These straight skirts walls form triangular structures with high rigidity, both with regard to torque- and bending loads, and secures an homogeneous structural cooperation between the towers.

The invention shall in the following be described in connection with embodiment examples and drawings, wherein Fig. 1 shows a vertical sectional view of a platform in accordance with the invention taken along the plane II-II shown in Fig. 2, Fig. 2 is showing a horizontal section through the skirt portion, Fig. 3 is a perspective view showing the complete platform, including the above-water equipment, and Fig. 4 illustrates how the riser pipes for oil and gas can be arranged.

In Fig. 1 are shown two towers 1 resting on a fundament slab 2 of concrete. The underside of the fundament slab is provided with skirts which are designed to penetrate down into the sea bottom 4 so that the fundament slab just extends above the level of the sea bottom 5 when the platform is installed on the sea bottom. The skirts 3 circumscribe the entire underside of the fundament slab (see Fig. 2), and have for all practical purposes tight walls and top and are downwardly open.

The skirts form in addition to chambers with cylindrical 3,6 or hexagonal 7 cross-section also chambers 12 with triangular cross-section, and the latter contributes especially to a stiffening of the fundament slab and the fixation of the towers.

Fig. 4 shows in a perspective view the complete platform in resting position on a fundament slab 2 of concrete with underneath positioned skirt 3 which penetrates down into the sea bottom, so that the fundament slab is positioned just above the sea bottom. Alternative routes for the riser pipes are shown, some riser pipes 7 are passed up through the fundament slab outside the towers, other riser pipes 8 are passed up through a tower 9 and through the tower wall and from there up to the above-water part 10. The riser pipes can be supported by means of concrete beams 11 which are bridged between the towers.

Traditionally, offshore platforms which are resting on a slab on the sea bottom with skirts have been provided with a layer or body of concrete between the slab and the sea bottom in order to improve the contact. With a platform in accordance with the present invention, such a concrete layer may in many applications be omitted, so that the platform rests on a water cushion between the fundament and the sea bottom.

Claims

Patent claims

1. Offshore platform of concrete resting on the sea bottom for exploration of oil and/or gas from wells in the sea bottom, and which is provided with two or several towers which extend up above the sea surface and support the required equipment for drilling and production of the beforementioned hydrocarbons, c h a r a c t e r i z e d in that said towers rest on a fundament slab (2) of concrete, and which on the underside are provided with a number of downwardly open skirts, so that the system of fundament slab and skirts provide a substantial part of the required strength and rigidity in order to transfer forces acting on the towers, between the towers and from the towers down into the ground.

2. Offshore platform in accordance with claim 1, c h a r a c t e r i z e d in that the platform during construction and transport rests on cushions of compressed air between the skirts.

3. Offshore platform in accordance with preceding claims, c h a r a c t e r i z e d in that the platform during operation partly rests on a water cushion between the fundament slab and the sea bottom.

4. Offshore platform in accordance with preceding claims, c h a r a c t e r i z e d in that riser pipes for oil wells are installed wholly or partly on the outside of the towers, and that the riser pipes if necessary are passed through the tower wall.

5. Offshore platform in accordance with preceding claims, c h a r a c t e r i z e d in that the towers are stiffened relative to each other with concrete beams located at the upper parts of the towers.

6. Offshore platform in accordance with preceding claims, c h a r a c t e r i z e d in that the strengthening elements for stiffening the riser pipes also are used for mutual stiffening of the towers.