GB2058181A

GB2058181A - Underwater Structure

Info

Publication number: GB2058181A
Application number: GB8023975A
Authority: GB
Original assignee: Golder Associates Ltd
Current assignee: Golder Associates Ltd
Priority date: 1979-08-14
Filing date: 1980-07-22
Publication date: 1981-04-08
Also published as: GB2058181B

Abstract

An underwater structure comprises a prefabricated deck unit (12), an impervious membrane (14) attached to a periphery of the deck unit, a coherent body of sand (28) filling the membrane so that it is extended and rests on the sea or river bed whereby to support the weight of the deck unit and a pump for removing any build up of water in the sand to maintain the body coherent, and wherein the deck unit is provided with a downwardly depending rigid peripheral skirt 12a in that the impervious membrane is secured to an inner surface of the skirt. <IMAGE>

Description

SPECIFICATION Underwater Structure This invention relates to improvements in and modifications of the underwater structure described and claimed in our earlier U.K. Patent Nos. 1460529 and 1535227.

Briefly, the underwater structures forming the subject of our earlier patents comprise a prefabricated deck unit to the periphery of which is attached an impervious membrane to form a coherent structure which is then floated to the site where the structure is to operate. The membrane is then extended to contact the sea bed and a mixture of water and sand is pumped into the interior of the membrane, the water being extracted from the membrane interior to build therein a coherent body of sand, which is able to adequately support the weight of the deck unit.

For a more complete understanding of the construction and operation of the underwater structures, reference should be made to the specifications of the above mentioned patents.

According to one aspect of the present invention there is provided an underwater structure in which the deck unit is provided with a downwardly depending peripheral skirt and in which the impervious membrane is secured to an inner surface of the skirt.

An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic vertical crosssection through an underwater structure according to the invention and ready for towing, Figure 2 is a diagrammatic vertical section of the underwater structure of Figure 1 in an assembled condition at site, and Figure 3 is an enlarged view of detail B of Figure 2.

Referring to the drawings, the underwater structure 10 includes a prefabricated buoyant deck unit 1 2 of steel or concrete to which is secured an impervious bag-like membrane 14, conveniently made from a high specification nylon fabric coated on both sides with an abrasion resistant Neoprene rubber or similar materials.

Depending downwardly from the main bulk of the deck unit is a rigid peripheral skirt 1 2a, the upper rim of the membrane covers almost the entire inside surface 1 2b of the skirt and is secured as by nuts and bolts 16, and bearing plate 18. To avoid damage to the membrane, the lower edge of plate 1 8 is preferably rounded and the nuts and bolts 1 6 are preferably covered by caps 20. The bottom edge of the skirt is enlarged at 1 2c to provide a rounded footing.

During transport to site, the membrane is folded into the enclosure defined by skirt 1 2a and may be protected during the tow by means of a protective membrane 22.

At the site, the protective membrane 22 is removed and membrane 14 is extended by pumping water into its interior until the membrane is fully inflated with its base on the sea bed. A first fill of ballast is then positioned in the membrane and main well screen or screens 24 are lowered into the interior of the membrane followed by the insertion of further ballast to form a lower drainage layer 26. A mixture of sand and water is now pumped into the bag interior to form therein a sand body 28 which eventually fills the bag, water being removed from the bag by means of a submersed pump 30, and outlet pipe 32.

After the sand filling operation is completed, pump 30 will operate as required to drain the sand to control the build-up of pore water pressure and thereby allow the natural hydrostatic pressure of the surrounding water to confine the sand to increase the internal shear strength of the sand body 28 which is then able to adequately support the weight of the deck unit.

Advantages of the above described structure are as follows:- 1) The membrane can be attached and adequately sealed to the skirt of the deck in the dry on land without the deck being lifted out of the water of the attachment being done below water.

2) After completion of the hydrostatically supported sand structure, the residual sand pressure resulting from the filling process forces the membrane against the inside of the peripheral skirt and thereby improves the seal between the membrane and the skirt. As the sand settles down through the water it temporarily increases the density of the water inside the bag as compared to the water outside the membrane. This forces the membrane out against the inside of the skirt and effects a permanent seal. As the sand fills up and is dewatered it fills the extended membrane and ensures that this residual pressure is maintained. The amount of pressure can be controlled at any time by increasing or decreasing the rate of sand filling.

3) The steel or concrete skirt provides added protection and confinement for the sand in the upper zone where the confining hydrostatic pressure can fluctuate due to varying tide and wave conditions.

4) The deck unit loads are transferred to the sand by the bearing capacity of the base of the deck, the friction between the inside wall of the skirt and the sand mass and the bearing capacity of the rounded footing at the base of the skirt.

5) The effective weight of the deck is increased by the weight of the sand enclosed within the rigid skirt of the deck thereby increasing the structural stability of the entire structure.

6) The steel well casings may be used after construction to post tension the deck to the sand mass by using friction plates 40 on the base of the well screen and post tensioning jacks or equivalent 42 between the top of the deck and the top of the well screen.

7) The structural connection between the steel deck and the sand is improved by the friction between the membrane and the inside of the deck skirt as well as the bolted connection and the post tensioned well screens.

8) The depth of the skirt below the deck unit will be individually designed to ensure that for a combination of low water and largest wave trough likely to be experienced at that particular site, the upper regions of the membrane just below the skirt will always be submerged thereby ensuring confinement of the sand body within those upper regions.

Claims

1. An underwater structure comprising a prefabricated deck unit, an impervious membrane attached to a periphery of the deck unit, a coherent body of sand or other particulate material filling the membrane so that it is extended and rests on the sea or river bed, whereby to support the weight of the deck unit, and means for removing any build up of water in the sand to maintain the body coherent, characterised in that the deck unit is provided with a downwardly depending rigid peripheral skirt and in that the impervious membrane is secured to an inner surface of the skirt.

2. The underwater structure of Claim 1, wherein the lowermost edge of the skirt is enlarged and rounded.

3. The underwater structure of Claim 1 and 2, wherein the membrane covers almost the entire inner surface area of the skirt, and wherein sand fills the interior space of the skirt thereby forcing the membrane against the skirt to improve the seal therebetween.