GB2300211A - Offshore Platform - Google Patents

Abstract

An offshore platform comprises a superstructure 1 supported above the wave crest level on an elongate caisson 2 founded on a shallow spud-can 3 on the sea floor. Three or more vertically inclined flexible guy lines 4 are attached to said caisson 2, one end of each is affixed near the top of said caisson 2 above the water surface with the other end attached to an anchor system 5 on the sea floor. Three or more arrays of flexible fender lines 6 are attached to said vertically inclined guy lines 4, each array running between two guy lines near the water surface and configured such that an approaching boat will be arrested and its impact energy absorbed before it can strike the platform.

Description

OFFSHORE PLATFORM This invention relates to construction and installation of an offshore platform.

In the drive to reduce costs of production from marginal oil and gas fields offshore, a number of innovative platform structure designs have been developed which are cheaper to construct and install than conventional three or four legged steel jackets.

These innovative platform designs usually comprise a superstructure supported on a single elongate caisson with some form of diagonal bracing arrangement near its base through which piles are driven to secure the structure to the sea floor.

Such braced caisson structures are lighter than conventional jackets, making them cheap to construct. They can also be installed by use of the jack-up drilling rig which will be used for well drilling rather than by a specially chartered floating crane vessel, making them cheap to install.

Principal difficulties encountered in practical construction and installation of braced caisson structures are as follows. Firstly, the jack-up drilling rig operations required for structure installation and pile handling and driving are difficult for all but very shallow water depths and sometimes will require the jack-up drilling rig to be modified and/or be repositioned a number of times to drive all the piles. Secondly, the need to protect product conductors, risers and export lines from accidental boat impact requires either that the conductors, risers and export lines are inside the caisson or, that a boat impact frame is constructed around them outside the caisson. In both cases the hydrodynamic forces acting on the structure are increased and this results in an increase of the structure self weight. This exacerbates the difficulties of installation by jack-up drilling rig.In the former case, when the conductors, risers and export lines are inside the caisson, the spacing between the conductors, etc., must be kept to the minimum, so that the caisson diameter is minimised to reduce hydrodynamic loads and this can result in a constricted layout of the production equipment in the superstructure, and problems in drilling the wells.

The present invention relates to an offshore platform structure that is cheaper and easier to construct and install than a braced caisson structure, since firstly, there is no requirement for piles; secondly, the structure is lighter and simpler than a braced caisson; and thirdly, at least some of the conductors, risers and export lines can be on the outside of the caisson, without the requirement for a boat impact frame. This facilitates a good layout of the production equipment in the superstructure and aids drilling operations.

Thus, according to the present invention there is provided an offshore platform consisting of a superstructure supported on an elongate caisson founded on a shallow spud-can on the sea floor. Said caisson is supported against hydrodynamic loading by means of three or more vertically inclined flexible guy lines each attached at one end to said caisson near its top above the water surface, while the other end is anchored to the sea floor. Product conductors, risers and export lines are supported on said caisson, some on its outside surface by means of guides and some inside said caisson. Ducts are incorporated into said spud-can to allow said conductors etc. to be driven into the seabed.

Protection against boat impact is provided by three or more arrays of flexible fender lines attached to and running between said guy lines near the water surface, such that an approaching boat will be arrested on said fender lines, and its impact energy will be absorbed (mainly as strain energy in said fender lines and guy lines) before it can strike the platform.

Means of tensioning said guy lines and fender lines are provided near the top of said caisson, accessible from the superstructure. Means of slackening, disconnecting and lowering said fender lines are also provided to allow controlled approach of a supply boat to the platform between said guy lines, for platform servicing purposes.

Offshore installation and retrieval of the platform may be by means of a jack-up drilling rig or by a small floating crane vessel.

The invention will now be described by way of example only and with reference to Figures 1 to 5 of the accompanying drawings.

Figure 1 shows an elevation of the platform.

Figure 2 shows a plan view of the platform.

Figure 3 shows an example of the fender lines arrangement to an enlarged scale.

Figure 4 shows on elevation an example of the spud-can arrangement.

Figure 5 shows a plan view of the spud-can arrangement.

Referring to Figures 1 and 2, the platform comprises a superstructure 1 supported above the wave crest level on an elongate caisson 2 founded on a shallow spud-can 3 on the sea floor.

Three or more vertically inclined flexible guy lines 4 are attached to said caisson 2, one end of each is affixed near the top of said caisson 2 above the water surface with the other end attached to an anchor system 5 on the sea floor. Said anchor system 5 may comprise, for example an array of ships anchors, piles, suction anchors or anchor blocks of forms already known.

The horizontal plan arrangement of said guy lines 4 and anchor systems 5 is symmetrical to resist the hydrodynamic loads, as shown in Figure 2. Means of tensioning said guy lines 4 are provided at the top of said caisson 2, for example, in the form of hydraulic jacks attachable to the end of each guy line, accessible from the superstructure 1.

Three or more arrays of flexible fender lines 6 are attached to said vertically inclined guy lines 4, each array running between two guy lines near the water surface and configured such that an approaching boat will be arrested and its impact energy absorbed, mainly by strain energy in said fender lines 6 and guy lines 4, before it can strike the platform.

By way of example only, each fender line array 6 may take the form of three main fender cables 7, 8 in an inverted 'Y' configuration with secondary horizontal cables 9 connected to said main fender cables and guy lines as depicted in Figure 3. The lower legs 7 of said main fender cables are attached to the guy lines 4 below the water surface while the upper fender cable leg 8, which forms the stem of the 'Y' is attached to the caisson 2 near its top such that the fender cable array can be tensioned by applying a jacking force to said upper fender cable leg 8. Said secondary horizontal fender cables 9 may be disconnected from said main fender cables 7, 8, for example, by undoing shackles. Said main fender cables 7, 8 may be slackened by reducing the tension in said upper fender cable leg 8 and may be lowered below the water surface by paying out its length, thus allowing a supply boat to approach the platform.

Connection of said guy lines 4 and main fender cables 8 to said caisson 2 may be by means of cable stoppers or chain stoppers 10 of forms already known.

Product conductors, risers and export lines 11 may be supported on said caisson on its outside surface, by means of guides 12, and inside said caisson 2.

With reference to Figures 4 and 5 said spud-can 3 may take the form of a short downwardly extending cylindrical shell 13 closed by a bearing plate 14 on the top and open at the bottom end which is embedded in the sea floor. Said cylindrical shell 13 is reinforced internally by vertical diaphragm plates 15 which include vertical ducts 16 through which said conductors can be driven into the sea floor and the wells can be drilled.

Said elongate caisson 2 and spud-can 3 are preferably made of steel for lighmess. Said guy lines 4 and fender lines arrays 6 may be steel wire rope, chain or composite fibre construction, or a combination thereof.

Claims (8)

1. An offshore platform consisting of a superstructure supported on an elongate caisson founded on a shallow spud-can on the sea floor, which is supported against hydrodynamic loading by means of three or more vertically inclined flexible guy lines each attached to said caisson above the water surface at one end with the other end anchored to the sea floor, and which is protected against boat impact by three or more arrays of flexible fender lines attached to and running between said guy lines near the water surface such that an approaching boat will be arrested on said fender lines before it can strike said caisson.

2. An offshore platform according to Claim 1 in which said fender line arrays each take the form of three main fender cables in an inverted 'Y' configuration in which the lower two legs of said main fender cables are each attached to a guy line below the water surface while the upper leg of said main fender cables, forming the stem of the inverted 'Y', is attached to said caisson near its top and in which secondary horizontal fender cables are attached to said main fender cables and said guy lines, such that a boat approaching from any direction will be arrested

3. An offshore platform according to Claims 1 or 2 in which means are provided of tensioning or slackening said guy lines and fender cables, accessible from the superstructure.

4. An offshore platform according to Claims 1, 2 or 3 in which product conductors, risers and export lines are supported on said caisson, some on its outside surface by means of guides and some inside said caisson.

5. An offshore platform according to Claims 1, 2, 3 or 4 in which said foundation spud can takes the form of a short downwardly extending cylindrical shell closed by a bearing plate on the top and open at the bottom end which is embedded in the sea floor, in which internal diaphragm plates are provided for reinforcement of said shell and vertical ducts are provided through which said conductors can be driven into the sea floor for well drilling.

6. An offshore platform according to any of the preceding claims in which the platform is used for offshore mineral extraction purposes, habitation, communications, control or observation purposes.

7. An offshore platform according to any of the preceding claims in which said caisson and superstructure are made from metal and said guy lines and fender cables are made from metal wire rope, chain or composite fibres, or a combination thereof.

8. An offshore platform substantially as herein described and illustrated in the accompanying drawings.