Marine structures

An earthquake-resistant off-shore gravity structure which can stand on the sea bed under its own weight and the weight of any ballast carried by the structure has coupled to its undersurface a separately formed substantially flat concrete foundation grid which underlies at least a major part of the area of the undersurface of the structure, which is of a thickness substantially less than the height of the base and which has a downwardly extending projection for penetrating the sea bed. The coupling between the structure and the foundation grid is such that vertical loads on the structure are transferred through the foundation grid to the sea bed and is such that, for horizontal forces on the structure less than a predetermined maximum value, relative movement between the foundation grid and the structure is substantially prevented and, for a horizontal force equal to or in excess of said maximum value, limited relative movement between the foundation grid and the structure is permitted. Preferably, when the structure is subjected to a horizontal force of gradually increasing value, as may occur in the event of an earth tremor, when the value of the horizontal force reaches said predetermined maximum value, limited relative movement between the foundation grid and the structure is permitted without any further increase in horizontal force on the structure.

This invention relates to a marine structure of the kind which stands on 
the bed of the sea or other body of water (hereinafter referred to as the 
sea) under its own weight and the weight of any ballast carried by the 
structure. Marine structures of this kind will hereinafter, for 
convenience, be referred to as "offshore gravity structures of the kind 
described". The invention is especially, but not exclusively, concerned 
with an offshore gravity structure of the kind described which is in the 
form of a platform including a hollow base and, upstanding from the base, 
an upper structure slimmer than the base, which platform can form a 
floating body for movement from one place to another and which, when 
required, can be sunk to stand on the bed of the sea with the upper 
structure protruding above the surface of the sea. 
When an offshore gravity structure of the kind described standing on the 
sea bed is subjected to horizontal forces caused by wind, waves, current 
or other normal environmental conditions, the direct contact between the 
base of the structure and the sea bed enables these forces to be 
transferred through the base into the sea bed. To provide for transfer of 
such horizontal forces and vertical loads to the sea bed it is the general 
practice to provide on the undersurface of the base a downwardly extending 
projection or downwardly extending projections, usually in the form of a 
skirt or skirts, which penetrates or penetrate into the sea bed. 
Where an offshore gravity structure of the kind described is standing on 
the sea bed in an area that is prone to earth tremors, there is a 
substantial risk that, in the event of a severe earthquake, the resulting 
seismic forces may be so large that the sea bed in the vicinity of the 
structure will move to such an extent as to cause inertia forces on the 
interface between the base and the sea bed of such magnitude that failure 
of the structure will occur. 
It is an object of the present invention to provide an improved offshore 
gravity structure of the kind described which is or can be so founded on 
the sea bed that risk of failure of the structure under severe earthquake 
conditions is substantially reduced. 
According to the invention the improved offshore gravity structure has 
coupled to the undersurface of its base a separately formed substantially 
flat concrete foundation grid which underlies at least a major part of the 
area of the undersurface of the structure, which is of a thickness 
substantially less than the height of the base and which has one or more 
than one downwardly extending projection for penetrating the sea bed, the 
coupling between the base and the or each foundation grid being such that 
vertical loads on the structure are transferred through the foundation 
grid to the sea bed and being such that, for horizontal forces on the 
structure less than a predetermined maximum value, relative movement 
between said foundation grid and the base is substantially prevented and, 
for a horizontal force equal to or in excess of said maximum value, 
limited relative movement between said foundation grid and the base is 
permitted. 
Preferably, the predetermined maximum value which must be equalled or 
exceeded by a horizontal force on the structure for limited relative 
movement between the foundation grid or bodies and the base of the 
structure to be permitted is the maximum horizontal force caused by wind, 
waves and current to which the structure will be expected to be subjected 
under normal environmental conditions. In this way, during normal 
environmental conditions, relative movement between the base of the 
structure and the foundation grid is substantially prevented. In the event 
of a severe earthquake causing such movement of the sea bed that the 
structure is subjected to a horizontal force of gradually increasing 
value, when the value of this horizontal force reaches said predetermined 
value the foundation grid is permitted to move to a limited extent 
relative to the base of the structure so that the horizontal force to 
which the structure is subjected is limited to the maximum value the 
structure is designed to withstand. 
Preferably, the coupling arrangement between the base of the structure and 
the foundation grid is such that, when a horizontal force on the structure 
reaches said predetermined maximum value, limited movement between the 
foundation grid and the base will occur without any further increase in 
horizontal force on the structure. 
Preferably, where the base of the structure is of hollow form, the 
foundation grid may be of a thickness approximating to that of the bottom 
wall of the hollow base. Preferably the foundation grid is made of 
concrete. 
The or each downwardly extending projection on the foundation grid is 
preferably a skirt open at its lowermost end. 
Preferably, the base of the structure is coupled to the foundation grid at 
each of a plurality of positions mutually spaced over the undersurface of 
the base by substantially fluid-tight coupling means. The coupling means 
at each of these positions preferably comprises at least one substantially 
rigid elongate member which engages in aligned holes in the base and in 
the foundation grid, transversely-deformable means in one or both of said 
aligned holes and substantially fluid-tight sealing means positioned 
between the base and foundation grid and enclosing the rigid elongate 
member and transversely-deformable means. The arrangement is such that, 
under normal environmental conditions, vertical loads on the structure are 
transferred to the sea bed through bearings, which have a low resistance 
against horizontal displacement and which may or may not be incorporated 
in the coupling means, and horizontal forces on the structure, which are 
insufficient to deform the transversely-deformable means, are transferred 
by said means via the rigid elongate members to the sea bed so that 
relative movement between the base of structure and the foundation grid is 
prevented, and that, when the horizontal forces to which the structure is 
subjected reach said predetermined maximum value--as may arise in the 
event of an earth tremor--said transversely-deformable means are deformed 
to such an extent as to permit limited relative movement between the 
foundation grid and the base of the structure to take place. 
In one preferred embodiment of the invention, the substantially rigid 
elongate member and transversely deformable means of each coupling means 
are of composite form and comprise a tube having such a wall thickness and 
transverse cross-sectional shape that it will be transversely deformed 
when the horizontal forces on the structure reach said predetermined 
maximum value. 
In another preferred embodiment of the invention, the transversely 
deformable means of each coupling means is separately formed with respect 
to the substantially rigid elongate member and in this case the part of 
the elongate member that engages in a hole in the base of the structure 
and/or in a hole in the foundation grid is surrounded by a plurality of 
separately formed tubes arranged with their axes substantially parallel to 
the axis of the member and interposed between the member and the boundary 
wall of the hole, each of said tubes having such a wall thickness and 
transverse cross-sectional shape that it will be transversely deformed 
when the horizontal forces on the structure reach said predetermined 
maximum value. 
Where the base of the structure is a hollow body, the hole is the base 
associated with each coupling means may extend through the bottom wall of 
the base. Each hole may be closed by a detachable cover whereby access is 
provided to the coupling means for inspection and/or replacement of the 
transversely-deformable means. 
Although provision may be made for replacement of the 
transversely-deformable means of a coupling means, preferably the 
transversely-deformable means is of such a form that it will not be 
rendered completely inoperative by the horizontal forces likely to arise 
from an earth tremor and will still accommodate for limited relative 
movement between the foundation grid and the base of the structure in the 
event of a subsequent earth tremor at a later date. 
In some circumstances, the foundation grid may be expendable and the 
coupling means coupling the base of a structure to the separately formed 
expendable foundation grid may be of such a form that the structure can be 
uncoupled from the foundation body or bodies and, if desired, moved to 
another site where it is coupled to another foundation grid before it is 
refounded on the sea bed. This arrangement avoids the necessity of 
disengaging the foundation grid and its downwardly-extending projection or 
projections from the sea bed.

Referring to FIGS. 1 and 2, the off-shore gravity structure is in the form 
of a platform 1 comprising a hollow concrete base 2, an upper concrete 
structure 3 slimmer than and upstanding from the base and, supported on 
the upper structure 3, a deck 4 on which the equipment, apparatus and 
accommodation associated with an oil rig can be supported. The interior of 
the hollow base 2 is subdivided into a plurality of cells by intersecting 
vertical walls 6 extending between the bottom wall 5 and the top wall of 
the base and coupled to the bottom wall 5 of the base 2 at each of a 
plurality of positions mutually spaced over the undersurface of the base 
by a coupling device 8 is a substantially flat concrete foundation grid 7 
which has skirts 9 extending downwardly from the grid and penetrating the 
sea bed. 
One form of coupling device 10 that may be employed to couple the 
foundation grid 7 to the bottom wall 5 of the base 2, is shown in FIGS. 3 
and 4 and comprises a substantially rigid steel tube 14 which engages in 
aligned holes 11 and 12 in the bottom wall 5 and in the grid 7. The tube 
14 is a tight fit in the hole 12 and has an integral flange 15 which bears 
against the upper surface of the grid 7. The hole 11 in the bottom wall 5 
has a diameter substantially greater than that of the tube 14 and is lined 
by a steel cup 16 having a flange 17 which bears against the undersurface 
of the bottom wall. Fluid-tight bearings 18 interposed between the flanges 
15 and 17 seal the coupling device. Spacing the tube 14 from the cup 16 
lining the hole 11 are six steel tubes 19 of circular cross-section. The 
hole 11 may, in some circumstances, extend throughout the depth of the 
bottom wall 5 and may be closed by a detachable cover which can be removed 
to provide for inspection and/or replacement of the tubes 19. 
Under normal environmental conditions vertical loads on the structure are 
transferred to the sea bed through the bearings 18 and horizontal forces 
on the structure, which are less than a predetermined maximum value, are 
transferred by the tubes 19 via the tubes 14 to the sea bed so that 
relative movement between the base 2 of the structure and the foundation 
grid 7 is prevented. Each of the tubes 19 is of such a wall thickness that 
when the horizontal forces to which the structure is subjected reach said 
predetermined maximum value, as may arise in the event of an earth tremor, 
the tubes 19 are deformed to such a extent as to permit limited relative 
movement between the foundation grid 7 and the base 2 to take place. 
In the second form of coupling device 20 shown in FIGS. 5 to 7 that may be 
used to couple the foundation grid 7 to the bottom wall 5 of the base 2 of 
the structure, a substantially rigid steel tube 23 whose wall is so 
corrogated that it has a plurality of flutes 29 extending along its length 
engages in aligned holes 21 and 22 in the bottom wall 5 and foundation 
grid 7. The hole 21 is lined by a steel cup 26 having a flange 27 which 
bears against the undersurface of the bottom wall 5 and the hole 22 is 
lined by a steel cup 24 having a flange 25 which bears against the upper 
surface of the foundation grid 7. Fluid-tight bearings 28 interposed 
between the flanges 25 and 27 effectively seal the coupling device. The 
hole 21 in the bottom wall 5 extends throughout the depth of the wall and 
is closed by a detachable cover 30 which can be removed to provide for 
inspection and or replacement of the tube 23 and, if desired, uncoupling 
of the structure from the foundation grid 7. 
As in the case of the coupling device shown in FIGS. 3 and 4, under normal 
environmental conditions vertical loads on the structure are transferred 
to the sea bed through bearings 28; horizontal forces on the structure, 
which are less than a predetermined maximum value, are transferred by the 
flutes 29 via the tubes 23 to the sea bed so that relative movement 
between the base 2 and the foundation grid is prevented. When the 
horizontal forces to which the structure is subjected reach said 
predetermined maximum value, the wall thickness of the tubes 23 is such 
that the tubes are deformed to such an extent as to permit limited 
relative movement between the foundation grid 7 and the base 2 to take 
place. 
The concept of the present invention is especially, but not exclusively, 
suitable for use with an off-shore gravity structure which is in the form 
of a platform for the drilling for oil or other substances from the bottom 
of the sea and which is to be installed in areas that may be prone to 
earth tremors.