Underwater structure

A method of forming an underwater structure which includes fabricating an assembly consisting of an upper deck section, a lower gravity section and an impervious member secured in water-tight fashion to the lower peripheries of the deck and gravity sections. The assembly is floated with the membrane in a folded condition to the site and lowered so that the gravity section rests on the sea or river bed. The space between the sections and the membrane is filled with water and a non-settable particulate material to form a body of such material. The particulate body is drained to enable the external water pressure to exert a confining pressure on the body to render it coherent.

This invention relates to underwater structures, and in particular to 
modifications of the underwater structure described and claimed in our 
copending application now U.S. Pat. No. 4,009,580, the modified structures 
being suitable for protecting underwater installations such as subsea 
completion units at oil well heads.

Referring to FIGS. 1 and 2 of the drawings, the underwater structure 10 
comprises a toroidal membrane 13 which is fabricated on land and then 
transported in a collapsed condition to the site. The interior of the 
membrane may be subdivided into a number of compartments by radially 
extending separate walls 14. On the inner surface of the base of the 
membrane are secured a plurality of submersible pumps 16 having inlets in 
communication with a drainage network consisting of perforated pipes 18; 
the outlets of the pumps are connected by non-perforated tubes 20 to be 
outside of the membrane. At the site location, the membrane is inflated by 
pumping water into its interior through one or more inlets 22. The 
inflated membrane is then sunk to the sea bed so as to surround the 
underwater installation, which in this case (see FIG. 2) is a subsea 
completion unit 24 at an oil well head. A sand and water mixture is then 
fed to inlets 22 to build up a sand body 26 within the membrane. During 
and after the sand/water feed operation, water is removed from the sand 
body by pumps 16 and directed via tubes 20 to the outside of the membrane. 
By drawing water from the sand body, build up of pore water pressure in 
the sand is reduced and this in turn maintains at a suitable level the 
internal shear strength of the partially drained sand body under the 
confinement of the natural hydrostatic pressure acting on the outside of 
the membrane, thereby enabling the underwater structure to effectively 
withstand external forces resulting for example from trawl wires or 
anchors being dragged. 
To sense the progress of forming the sand body, the interior of the 
membrane may be provided with a piezometer stack 28 which monitors to the 
surface reduced internal pressure during filling; systems control for the 
pumps and stack readout is enabled through cable 29. 
The optimum value of the vertical angle .beta. of the membrane will 
probably be between 30.degree. and 70.degree.. 
If a permanent structure is required, a hardening agent such as cement 
would be fixed with sand/water mixture; in this case, the pumps would be 
disconnected after the membrane filling operation. 
Referring to FIG. 3, a breakwater structure 30 comprises an elongate 
membrane 32 perhaps 100 meters or longer which when inflated has a 
cross-sectional shape consisting of a series of (in this case three) 
bulbous sections 32a, 32b and 32c, of progressively smaller area. The 
sections are determined by opposed pairs of clamps 34 secured to the inner 
side walls of the membrane and connected together by chains or ropes 36. 
Within the lowermost section is a continuous perforated flexible drain 38 
attached to the base of the membrane, and manhole openings 40 are provided 
in the top section at regular intervals. 
The above described membrane would be transported to the site is rolled 
form and would be unrolled into position behind a boat. The membrane is 
first inflated with water so that the bottom rests on the seabed and the 
top rises above the water surface, and then with a sand/water mixture to 
build up a coherent sand body 42 within the membrane. Water draining into 
the flexible drain 38 is pumped by submersible pump 44 away from the 
breakwater structure through collector pipes 46 extending from the 
flexible drain through openings 40 to the outside. After body 42 is formed 
the manhole covers may be sealed or can be replaced and a simple wind 
operated pump left to take away any further water entering drain 38. 
The above described structure allows for temporary positioning of the 
breakwater to ascertain its effect of external sand and shore movement. If 
an adverse effect is found, the breakwater can be emptied by pumping out 
its sand fill and reassembled in a different position. If there is no 
adverse effect, the sand fill can be grouted with any type of cementing 
agent if required to provide a permanent structure requiring little or no 
maintenance. 
To increase the length of the breakwater, a number of the above described 
structures may be butt jointed together. 
The membrane may have any number of sections (from 2 up) so long as a side 
slope (that is the slope of a common tangent line touching the sections) 
is maintained between 60.degree. and 70.degree.. 
Referring to FIG. 4, the first stage in the manufacture of the underwater 
structure is to fabricate on land and/or water an assembly 110 consisting 
of an upper deck section 112, a conventional concrete of steel gravity 
section 114 and a prefabricated impervious membrane 116 secured in water 
tight fashion to the lower peripheries of the deck and gravity sections 
117. The assembly is then floated with the membrane in a folded condition 
to the site of the underwater structure and the assembly is lowered so 
that the gravity sections rests on the sea bed. Water is then pumped into 
the membrane to inflate it, followed by a sand and water mixture to build 
up a sand body 18 within the membrane. During and after the pumping 
operation, water draining from the sand body is removed through a suitable 
pump conduit (not shown). By drawing water from the sand body, build up of 
pore water pressure in the sand is reduced and this in turn maintains at a 
suitable level the internal shear strength of the partially drained sand 
body under the confinement of the natural hydrostatic pressure acting on 
the outside of the membrane, thereby enabling the underwater structure to 
effectively withstand large external forces resulting for example from 
collisions between external objects (such as icebergs) and the structure. 
To facilitate draining of the sand body, the body may be composed of 
alternate layers of course and fine sand (as described in our application 
now U.S. Pat. No. 4,009,580,).