This invention relates to a foundation arrangement for an offshore framework (jacket) construction or subsea installation for use in oil or gas operations and the like, in particular where the seabed consists of relatively solid sand or of clay, and the framework construction or subsea installation comprises a number of separate legs being rigidly connected to each other and having each a separate foundation on the seabed. Moreover, the invention comprises particular methods associated with the employment of certain embodiments of the arrangement.

Installations such as framework structure (jacket) platforms or seabed templates and the like on the seabed are most frequently anchored by means of piles. The piles are driven into the seabed by means of big hammers and pile guides attached to the installations. The piles are cast or pressed to be fixed in the pile guides.

An alternative manner of anchoring platform install- ations is to make their foundation area so large that the installation is stable when placed on the seabed. This principle is used in connection with gravitation platforms. In order to secure the seabed, which can consist of sand, against digging-out (erosion) under such foundations, they are often provided with an edge of steel or concrete, which can be an extension of the walls, so that the edge ("the skirt") is pressed down into the seabed. If such platforms shall be located on a soft bottom, for example a clay bottom, they are provided with longer skirts which can penetrate into harder layers in the seabed so as to secure stability. Such platforms as a rule will be large and they can be ballasted so as to have a sufficient weight for the skirts to be pressed down into the seabed.

If it is necessary to press the skirts through a combi- nation of soft and hard layers in the seabed, it is possible also in a known manner to employ a suction effect in combi¬ nation with ballast for installing the platform. The employ¬ ment of suction will establish an underpressure within the

platform skirt, so that the platform is pressed down in the seabed because of a pressure difference between the skirt and the outer water pressure. In the case of a soft bottom anchoring systems for other offshore installations have also been based on the use of suction.

Known methods as discussed above may also be found in the patent literature, as seen from the following comments: Norwegian patent application No. 89.0005 relates to a method of installing jack-up platforms based on the employ- ment of suction effect in chambers or skirts mounted under¬ neath the platform legs. Jack-up platforms, however, in the present connection, have quite different properties from framework structure platforms or subsea installations, such as templates. This in the first place consists therein that jack-up platforms considered as a whole do not constitute any rigid structure which can lead to upwardly directed forces in any platform leg for example in the case of lateral stress effect (waves, current, wind) against the platform. Nor does the patent application contain anything more particular about establishing and controlling suction effect.

Use of the suction principle, whereby anchoring (for example the skirts) comprise means for evacuation water being entrapped in the elements, is known from Norwegian patent application 86.2200. This, however, does not describe any method of establishing suction. This in particular is criti¬ cal when lowering light installations to a relatively hard seabed, and the present invention is specifically directed to framework structure platforms being such typically light installations. This applies both to steel framework struc- tures and to concrete framework structures. In this context the invention is directed to establishing a sufficient suction effect in permeable seabed material which can be sand or clay.

Norwegian patent 135,677 primarily relates to a gravi- tational foundation which in operation will always be under compressive stress, whereby the forces are absorbed by the spherical shells and skirts described. The fact that there is the question of a gravitational platform also appears from

the characteristic three concrete columns of a special design.

In clear contrast to this Norwegian patent the present invention is primarily directed to the foundation of frame- work structure platforms. These platforms are characterized by a separate foundation for each individual leg. Hitherto these platforms have been provided with foundations exclu¬ sively based on piles. The piles are driven into the seabed with heavy hydraulic hammers. This is a time consuming and expensive operation. It is not known that platforms of the framework structure type have had foundations by means of skirts wherein suction effect is incorporated as an active factor both during installation and for the purpose of the permanent supporting capacity of the foundation. The instal¬ lation time when using this foundation method is considered to be much shorter and it is not required to rely on heavy and expensive installation equipment. The method is also very interesting for the construction of removable framework structure platforms and possibly subsea installations. British patent 1,088,804 also relates to a gravitation platform whereby the pure compressive stresses occurring are absorbed by a skirt foundation interconnected into a triangular basis. The structure comprises one or more steel columns of special design. Suction effect is provided for through separate pipes to the surface, and associated pumps are located above the sea surface. Again this is a case of gravitational foundation without the occurrence of upward forces through columns or legs, as may occur with the lighter framework structure platforms. Additional examples of known techniques in this connec¬ tion may be found in

Norwegian patent application No. 87.2128 British patent application No. 2,079,826 British patent No. 1,417,471 - British patent 1,451,537

US patent No. 3,863,457 US patent No. 3,911,687 US patent No. 3,928,982 US patent No. 4,257,721 (pile)

US patent No. 3,263,641 (anchor). On the background of the known techniques the present invention provides novel and specific features as mentioned above, and as stated more closely in the appended claims. The invention involves the possibility of significant cost savings for the foundation of framework structure platforms and possibly subsea installations, compared to conventional foundation methods for such types of structures and install¬ ations. The upward forces discussed above, which in particular are intended to be brought under control by means of the invention, may also occur in connection with templates and similar subsea installations, when anchors or other equipment being pulled up or towed, can be caught thereby. When employing this invention for framework structure platforms, subsea installations and the like, an installation with legs having an anchoring element consisting of a chamber/skirt and installed by employing suction effect, will to a great extent be able to replace piles even for permanent anchoring of such installations on moderately hard to hard clay or sand bottoms. Thereby expensive offshore piling operations can be avoided, and there can be established sufficiently strong forces for anchoring the installation with the skirt foundation on the seabed. The installations of interest here usually do not have a sufficient weight to penetrate the skirts into the seabed without employing suction or other auxiliary forces. For moderately hard to hard seabeds a particularly strong suction effect may be required. The magnitude of the suction force, however, must be controlled very carefully, so that liquifaction failure can be avoided during.the installation phase.

In the following description the invention shall be explained more closely with reference to the drawings, in which: Fig. l in schematic elevation shows a framework structure or jacket platform located on the seabed, Fig. 2 in sectional view shows an embodiment of the arrangement according to the invention, within and

at the lower end of one of the platform legs in Fig. 1, and Fig. 3 in perspective view shows a typical example of a subsea installation in the form of a template for subsea wells, provided with an arrangement accor¬ ding to the invention. Reference is first made to Fig. 1 of the drawings. On the seabed 1 there is located an installation 2 of the framework structure platform or jacket type of steel or concrete. The installation has three or four legs 8, each leg being provided with an anchoring element (skirt/chamber) 3 which is open downwards and closed with a roof 4. In order for these skirts 3 to be able to penetrate down into the seabed so that the platform will be securely anchored, a necessary prerequisite is that suction effect or another auxiliary force is employed. Initially the installation has been pressed down into the seabed as much as possible by its own weight, so that a sealing is established as a consequence of the penetration depth 5 of the skirt 3 into the seabed 1. According to the invention it may be advantageous to provide the installation with one or more tanks 6 which are filled for example with concrete when the installation has been landed on the seabed, whereby an external additional force is established. For other operations the tank or tanks 6 can serve as buoyancy tanks. In the case of framework structure platforms also the legs 8 can be filled with concrete (tanks can be regarded as integral parts of legs 8) , and when subsea installations are concerned the skirts can extend above the seabed and be filled with ballast. Then water is carefully pumped out of the skirts. The under¬ pressure (suction) which is created in the skirts in view of the fact that the external water pressure is higher than the pressure in the skirts, leads to the pressing of the skirts into the seabed. This suction effect can be controlled in various ways and represents an important aspect of this invention.

The chambers within the skirts 3 are connected to sepa¬ rate pumps 7, so that the underpressure (suction) can be controlled and can be different for each chamber. In this

way it will be possible to maintain control so that the platform 2 can be adjusted in the vertical direction if any dis-alignment develops during the installation phase or if the seabed 1 consists of inhomogeneous materials or has a slope.

Advantageously the chambers 3 can communicate with cavities 12 in the platform legs 8, where an underpressure can be gradually established. Thereby a particularly large volume will be formed, where there is a differential pressure with respect to the external water pressure. The suction effect provided thereby is particularly great, and as a result thereof the skirts will penetrate into the seabed. The speed of penetration is controlled by means of the magni¬ tude of the suction effect. It is remarked that the suction is necessarily controlled so that open channels are not inadvertantly established for water inflow underneath the skirt edge 9 or other forms of failure in the bottom material. The cavities mentioned are preferably in the form of pipes or conduits 12 extending through a substantial portion of each leg, usually all the way to the top of the leg.

Based on extensive large scale tests in connection with this invention, new theorethical models are established for calculating the required suction in sand of various proper- ties. On the basis of these calculations the pumps 7 can be located at a level Δh underneath the sea surface 20, so that the required underpressure is obtained. The underpressure can be established by operating the pumps 7 to pump out water in the platform leg 8. Then via the pipe system 12 and associated valves 13 the underpressure is established within the skirt chambers 3. Different underpressures in the respective skirt chambers can be established by active mani¬ pulation of the valves 13 and the water level in the legs 8. The underpressure can also be established by operating the pumps 7 to pump water directly out of the skirt chambers 3. Possibly the pumps can be injector pumps.

The chambers or skirts 3 can be provided with nozzles 10 from which water can be ejected (for example taken from a tank 11 at the top of the platform) , so that the sea bottom

underneath the skirt edge 9 or along the edges of the skirts, is loosened before the skirts are moved downwardly. This reduces the resistance to penetration very significantly. The nozzles 10 mentioned above can also be employed ultimately during removal of the platform 2 by ejecting water through the nozzels, whereby the contact between the bottom layers around the skirts and the skirts themselves is reduced. Thus, it will be possible to lift the platform 2 from the seabed 1 by means of a sufficient external lifting force, established for example by means of crane barges. An important contribution in this connection can also be provided for by pumping water into the above cavities 12 in the legs 8 in order to create a relative overpressure in the chambers 3. Throughout the time period when the installation or construction concerned is in regular operation at a field, there may occur influences or stress effects as mentioned above, which induce a tendency for pulling-up one or more legs. In the case of such attempts or initiation of pulling- up there will be developed an enhanced suction effect because of the underpressure in the chambers described, this effect strengthening the resistance against further pulling-up. This is a very substantial and specific effect in connection with the present invention. In the example of an underwater installation illustrated in Fig. 3, being as a whole designated with reference numeral 32, there are four vertical legs 34A, 34B, 34C and 34D. At the lower end of each of these legs there is provided a chamber 35A, 35B and 35C respectively, as shown at three of the legs. The fourth leg 34D is also considered to be provided with such a chamber (not shown) . Nor is there in Fig. 3 shown any closer details with respect to pumps and pipes etc. in association with the respective legs 34A-D. As will be apparent to experts in the field, such an install- ation, for example in the form of a template as shown in Fig. 3, may be subjected to upwardly directed forces resulting from for example anchors, trawler equipment or the like, which accidentally can get caught in the installation. The chamber with skirts as described above will give a sufficient

and secure anchoring of such underwater installations, even without using any piles, which has been common practice hitherto.

Various modifications are possible in relation to the embodiment which as an example has been described and illustrated in the drawing. Thus, the controlled, relative underpressure or suction in each chamber 3 can be generated in a way known per se by arranging for cavities in the associated leg 8 to be kept empty of water, and for a number of valves in pipes corresponding to the pipes 12 to be selec¬ tively operated for establishing a water column of desired height and thereby a corresponding relative pressure in the chamber. The shape and the size of the chambers apparently can be varied within wide limits, but will be determined inter alia by stress loads and forces which may occur.