Offshore structure and method of sinking same

An offshore structure and a method of sinking it to the sea bed. In accordance with one aspect of this invention, the structure is sunk asymmetrically by first sinking a first end portion thereof and then sinking the other end portion. The first end portion is sunk by ballasting it while the other end portion is closed to ballast. The structure is provided with sufficient water plane area while sinking each end portion to maintain stability during the sinking process. In accordance with another aspect of this invention, at least two spaced-apart piles are provided at the end corresponding to the first end portion to absorb the force of impact with the sea bed and to maintain a skirt on the structure out of contact with the sea bed until both ends of the structure have been sunk to the sea bed.

This invention is related to the field of offshore structures and to 
methods of sinking them to the sea bed. 
The cold regions of the world (those regions near the Arctic and Antarctic) 
are considered to be rich sources of hydrocarbons. Although many areas of 
these regions have been explored and some are already being developed, a 
considerable amount of additional exploration and development remains to 
be done. Because the locations for such exploration and development are 
remote and the climate at these locations is severe, the construction of 
offshore ice-resistant platforms can be expensive and time consuming if it 
is required that such construction be performed at these locations. It is 
thus considered desirable for exploration and development of these regions 
that complete platforms be built and tested in more civilized areas, 
floated and towed to the respective locations in which they are to be 
used, and then sunk onto the sea bed. 
Present procedures for installing such structures on the sea bed are not as 
satisfactory as desired. Ballasting such a structure having a rectangular 
base and a central column for sinking thereof symmetrically may cause the 
platform to lose stability as the water plane area changes from that of 
the rectangular base to that of the central column as the base is 
submerged. In addition to being expensive, deck winch operated massive 
anchor blocks used to "pull down" a structure in a level mode still 
require careful control during ballasting of the structure. Thus, loss of 
control during the sinking process is not unlikely with these present 
procedures. Since a platform with its load of expensive machinery and 
equipment typically may weigh as much as 16,000 tons, it is very critical 
that it not be allowed to get out of control. In other words, it is very 
desirable that such a structure be stable throughout the sinking thereof 
so that it may be controlled from accelerating rapidly into the sea bed. 
Anti-aircraft defense forts were installed in estuary waters around the 
British coast during World War II by allowing concrete structures to free 
flood and sink onto the sea bed. These structures were allowed to tilt 
such that one end touched the sea bed first. However, once started, the 
sinking of these forts did not provide for control of their stability or 
for reversing the process. As a result, the speed of descent to the sea 
bottom could not be reduced and a fort would impact with the sea bottom 
with great force. Reinforced concrete "buffers" were provided to crush 
upon impact of a fort with the sea bed. 
It is an object of the present invention to provide an offshore drilling 
and/or production platform which can be installed in a single piece to 
thus minimize offshore construction time. 
It is another object of the present invention to provide a method of 
controllably sinking such a structure to the sea bed. 
It is a further object of the present invention to provide a means for 
protecting the skirt of the platform during the sinking process. 
The above and other objects, features, and advantages of this invention 
will be apparent in the following detailed description of the preferred 
embodiments thereof which is to be read in connection with the 
accompanying drawings.

Referring now to the drawings, FIGS. 1 and 2 represent a marine structure 
generally indicated at 10 for installation in a body of water. This 
structure 10 is provided with a shallow reinforced concrete rectangular 
raft or base 12 which supports at the center a column 14 which carries on 
top a steel box 16 containing various drilling and production machinery 
and facilities such as the crane illustrated at 18. The column is conical 
over its lower portion 20 and cylindrical over its upper portion 22. This 
invention, however, is not restricted to such offshore structures, but is 
suitable for any type of marine structure which it is desired to sink to 
the sea bed in a controlled manner. 
The base 12 of the structure is provided with a plurality of compartments 
24 formed by upper and lower horizontally extending members 27 and 29 
respectively which are connected by vertically extending members 31. These 
compartments 24 may be filled with seawater or other ballast material for 
ballasting thereof or emptied of ballast for deballasting thereof. A 
drill-way 36 is provided vertically through generally the center of the 
base 12. The drill-way 36 may be surrounded in the base by spaces such as 
diesel tanks 34 for storage purposes. Means are provided, in accordance 
with one aspect of the present invention, for closing one end portion such 
as the end portion illustrated at 28 to ballast, while ballasting the 
other end portion illustrated at 26, so that the structure 10 may be sunk 
to the sea bed asymmetrically by first sinking the end portion 26 to the 
sea bed after which the end portion 28 is ballasted for its sinking to the 
sea bed. Such means preferably comprise an individual ballasting and 
deballasting line and valve illustrated at 30 and 32 respectively for each 
individual compartment 24, and each compartment 24 being sealed to the 
flow of ballast to or from any other compartment. Although it is preferred 
that the base portion 12 be provided with a plurality of individual 
compartments 24 which may be separately ballasted and which are closed to 
the flow of ballast from other compartments so as to maximize the amount 
of control which the operator may have on the ballasting process for 
sinking of the structure 10 to the sea bed, the present invention does not 
require a plurality of individual compartments on either end portion of 
the structure 10 or that an individual compartment on one end portion of 
the structure be closed to the flow of ballast from another individual 
compartment on the same end portion. For the purposes of this 
specification and the claims, an end portion is defined as having a width 
equal to one-third of the distance 38 between the respective ends 40 and 
42. The width of end portion 26 is illustrated at 44, and the width of end 
portion 28 is illustrated at 46. 
FIGS. 3 and 4 illustrate the principles used to calculate and achieve 
stability of such a structure at various positions throughout the sinking 
process thereof in accordance with the present invention. These figures 
illustrate an asymmetrical ballasting and sinking of the structure 10 to 
the sea bed illustrated at 48 wherein the end portion 26 (hereinafter 
called first end portion) is sunk to the sea bed 48 as illustrated in FIG. 
3 after which the opposite end portion 28 (hereinafter called second end 
portion) is ballasted and sunk to the sea bed 48 as illustrated in FIG. 4. 
At the beginning of the sinking process of the first end portion 26, as the 
first end portion 26 is being initially ballasted, substantially the 
entire upper horizontal surface 50 of the base portion 12 is above the 
surface illustrated at 52 of the sea thus providing a water plane area 
substantially equal to the entire area of the upper surface 50 of the base 
portion, and the structure 10 should, of course, be stable at the 
beginning of the sinking process. For the purposes of this specification 
and the claims, "water plane" refers to the plane defined by the surface 
of a body of water in which a structure is floating, and "water plane 
area" of a structure floating in the body of water refers to the area or 
areas of a water plane portion bounded by the points of intersection of 
the surface of the body of water with the structure. As the first end 
portion 26 is sunk below the sea surface 52 to the position illustrated in 
solid lines in FIG. 3, the water plane area of the structure 10 is 
decreased substantially so that its water plane area extends in a 
lengthwise direction only over the distance illustrated at 54 provided by 
the conical section 20 breaking the sea surface and over the distance 
illustrated at 56 provided by the opposite end portion 28 breaking the sea 
surface. By "lengthwise direction" is meant a direction, as illustrated at 
57, along a straight line between two ends of a structure whose end 
portions are to be successively sunk. 
It is desirable in accordance with this invention that the structure 10 
have stability at the positions shown in FIGS. 3 and 4 as well as 
throughout the sinking process in order to maintain effective control over 
positioning of the structure 10 on the sea bed 48 and to prevent damage to 
the platform 10 and its equipment which damage may otherwise result if the 
speed of descent of the structure were uncontrolled and the forces of 
impact with the sea bed were consequently excessive. For the purposes of 
this specification and the claims, "stability" of a structure is the 
tendency of the structure to return to its original position in a body of 
water after it has been inclined due to external forces. Whether or not a 
vessel or structure is stable at a particular position is dependent upon 
the location of the center of gravity and the location of the center of 
buoyancy at that position. FIG. 3 illustrates the structure 10 in a first 
position in solid line at 58 and in a second position in dot-and-dashed 
line at 60 superimposed thereon for ease of illustration wherein the 
vessel or structure 10 has been inclined at a small angle illustrated at 
62 from the first position. The first position 58 illustrates an actual 
position to which the structure 10 has been ballasted. The second position 
60 illustrates an assumed deflection of the structure 10 by about 2 
degrees about an assumed pivot point illustrated at 64 at or near the 
second end 42 of the structure. If the structure 10 has stability at the 
first position 58, it will return from the second position 60 to the first 
position 58 upon removal of forces deflecting it to the second position 
60. On the other hand, if the structure 10 were unstable at the first 
position 58, then a deflection of the structure 10 to the second position 
may result in the structure remaining at the second position 60 upon 
removal of forces deflecting it to the second position 60 or in the 
structure deflecting to a greater extent without the application of any 
additional external forces. 
When the structure 10 is in the first position 58, the location of its 
center of gravity is illustrated at 66 and the location of its center of 
buoyancy is illustrated at 68. When the structure 10 has been deflected 
through a small angle 62 to the second position 60 in FIG. 3, although the 
mass of the structure 10 remains the same, the center of gravity has been 
displaced in a lengthwise direction 57 toward the pivot point 64 a 
distance illustrated at 70 to the location illustrated at 72. However, it 
should be realized that, under some platform weight distributions, the 
location of the center of gravity may be displaced away from pivot point 
64. The center of buoyancy of the original water displacement at first 
position 58 has been displaced in a lengthwise direction 57 toward the 
pivot point 64 a distance illustrated at 74 to the location illustrated at 
76. In addition, there is an additional water displacement illustrated by 
the cross-hatched portion 78 which results in additional buoyancy which 
has a moment arm (from the assumed pivot point 64) illustrated at 79. 
Whether or not the structure 10 has stability at the first position 58 in 
FIG. 3 can be determined, in accordance with the present invention, by 
calculating the restoring moment due to the additional water displaced by 
shaded portion 78 upon deflection of the structure 10 through the small 
angle 62 and comparing this calculated restoring moment with the 
respective changes in mass and buoyancy (of original water displacement at 
first position 58) moments due to their lengthwise displacements 70 and 74 
respectively. In other words, taking moments about pivot point 64, for the 
structure 10 to have stability at the first position 58, the amount of 
decrease in the buoyancy moment due to the lengthwise displacement 74 of 
the center of buoyancy of the original water displacement at first 
position 58 toward the pivot point 64 must be less than the moment due to 
the additional displacement of water at shaded portion 78 less the amount 
of decrease in the mass moment due to the lengthwise displacement 70 of 
the center of gravity toward the pivot point 64. These calculations can be 
made using engineering principles of common knowledge to those of ordinary 
skill in the art to which this invention pertains. The term "small angle" 
is a term of art which is of common knowledge to those of ordinary skill 
in the art to which this invention pertains. It is of common knowledge to 
those of ordinary skill in the art to which this invention pertains that 
stability calculations are valid for a "small angle" up to about 3 or 4 
degrees of deflection after which the validity of the calculations becomes 
doubtful. 
In order to insure stability of the structure 10 throughout the process of 
sinking both end portions 26 and 28 thereof to the sea bed 48, the 
stability of the structure 10 is preferably calculated, prior to sinking 
thereof, for a series of positions of the structure 10 in the range over 
which it is to be sunk. If desired, a graph may then be plotted to further 
verify that there is stability throughout the entire sinking process. 
FIG. 4 illustrates the structure 10 after the first end portion 26 has been 
sunk to the sea bed 48 and during the sinking of the second end portion 
28. In this drawing, what will be referred to herein as a buoyancy member, 
illustrated at 80, has been attached to the second end portion 28. A 
buoyancy member may be characterized as a member which is closed to the 
flow of sea water during a stage of sinking of a structure to which it is 
attached in order to provide increased buoyancy of the structure during 
that stage of sinking. Whether or not one or more buoyancy members should 
be added to the structure depends upon whether, during the series of 
calculations over the range of sinking positions of the second portion, 
instability is indicated at any of those positions. For example, the 
calculations may be conducted for the position of the structure 10 
illustrated in solid line at 82 (another actual position of the structure 
which will also be called a "first position") in FIG. 4 to determine 
whether or not the structure 10 has stability when it is in that position 
82 without the buoyancy member 80 attached to the structure 10 by 
comparing the changes in buoyancy and mass moments when the structure is 
deflected through the small angle indicated at 84 to the dot-and-dashed 
line position illustrated at 86 (another assumed deflected position which 
will also be called a "second position") in a similar manner to the manner 
in which the calculations described for the first position 58 of FIG. 3 
are made. In this case, the pivot point for the deflection through the 
small angle is the point illustrated at 88 at which the first end 40 
touches the sea bed 48. In this case, the center of gravity, shown at 67 
when the structure is in first position 82 and at 73 when the structure is 
in second position 86, has been displaced in the lengthwise direction 57 
away from the pivot point 88 a distance illustrated at 90, and the center 
of buoyancy of the original water displacement at first position 82, shown 
at 69 when the structure is in first position 82 and at 77 when the 
structure 10 is in second position 86, has been displaced in the 
lengthwise direction 57 away from the pivot point 88 a distance 
illustrated at 92. When the structure is in second position 86, there is 
also an additional displacement of water indicated by the cross-hatched 
area illustrated at 94 whose center of buoyancy is indicated at 96 and 
whose distance (moment arm) of the center of buoyancy 96 from the pivot 
point 88 is indicated at 98. For the structure 10 to have stability 
without the buoyancy member 80 attached thereto, the amount of increase in 
mass moment due to the increased lengthwise distance 90 of the center of 
gravity from the pivot point 88 must be less than the amount of increase 
in the buoyancy moment due to the increased lengthwise distance 92 of the 
center of buoyancy of the original water displacement when the structure 
is in first position 82 from the pivot point 88 plus the buoyancy moment 
due to the additional water displacement of the shaded portion 94. If the 
calculation indicates instability at the first position 82, then in 
accordance with this invention, one or more buoyancy members such as the 
buoyancy member 80 are attached to the structure 10 to provide additional 
water displacement at locations which are preferably distant from the 
pivot point 88 in order to maximize the resulting additional buoyancy 
moment. With the buoyancy member on the second end portion 28 (preferably 
close to the end 42 so as to be as far from pivot point 88 as practical), 
the changes in buoyancy and mass moments may again be calculated to 
determine whether or not there is stability at the first position 82. In 
this case, the amount of increase in mass moment due to the increased 
distance 90 of the center of gravity from the pivot point 88 must be less 
than the amount of increase in buoyancy moment due to the increased 
distance 92 of the center of buoyancy of the original water displacement 
at the first position 82 from the pivot point 88 plus the buoyancy moment 
due to the added water displacement provided by the portion 94 plus the 
buoyancy moment due to the added water displacement provided by the 
buoyancy member 80 as illustrated by the cross-hatched portion 100 
thereof. 
In order to maintain stability throughout the remainder of the sinking of 
an end portion once it has been determined that a buoyancy member 80 is 
required, it is preferred that the buoyancy member 80 have sufficient 
height when attached to the structure 10 to break to sea surface 52 when 
the structure 10 has been sunk to the sea bed 48 so that sufficient 
additional buoyancy is provided throughout the sinking process. 
The above described stability calculations and additions of buoyancy 
members as required are continued in accordance with this invention until 
the calculated increases in restoring moments are greater than the 
respective increases in mass moments throughout the range of sinking 
positions of the structure. As previously stated, the various stability 
calculations which are described herein may be conducted utilizing 
engineering principles of common knowledge to those of ordinary skill in 
the art to which this invention pertains. 
FIGS. 5 through 10 illustrate in consecutive stages the sinking in a 
controlled manner in accordance with the present invention of the 
structure 10 to the sea bed 48. It is expected that most conventional 
offshore platforms are of such a size that they may be tilted to an angle 
of about 12 to 15 degrees in a controllable and reversible process in 
accordance with the present invention in water depths up to approximately 
150 feet. However, since the angle and water depth are dependent upon the 
size and shape of a structure to be sunk, the present invention should not 
be construed as being limited to such angles or such water depths. 
FIG. 5 illustrates the offshore platform 10 in position for transport to a 
location where it is to be sunk or in position above a location where it 
is to be sunk. Prior to sinking of the structure 10, stability 
calculations should be conducted and the structure 10 provided with 
sufficient water plane area to maintain stability during the sinking of 
each end portion to the sea bed 48 in accordance with the procedures 
discussed above. In accordance with such procedures, it may be determined 
for the platform 10 illustrated in FIG. 5 that additional water plane area 
should be provided on the second end portion 28. Such additional water 
plane area is provided in the form of buoyancy member 80 which is 
preferably provided with sufficient height, as shown in FIG. 9, to break 
the sea surface 52 when the structure 10 is sunk to the sea bed 48 and 
preferably has sufficient cross-sectional area, taken in a horizontal 
plane, at each portion of the buoyancy member 80 to break the surface 52 
during sinking of the respective end portion of the structure 10 to 
maintain stability throughout the sinking of the respective end portion of 
the structure 10. The stability calculations for the sinking of the first 
end portion 26 may show that the platform 10 has stability throughout the 
process of sinking the first end portion 26 without the necessity of 
adding additional buoyancy moment. Thus, the first end portion of the 
platform is shown as not having been provided with any buoyancy member. 
However, the stability calculations may show that a buoyancy member is 
required on the first end portion 26 in which case a buoyancy member is 
provided on the first end portion 26 in accordance with this invention. 
FIG. 6 illustrates the position of the platform as its first end portion 26 
has been partially sunk to the sea bed 48. This is accomplished by 
flooding compartments 24 in the first end portion 26 as shown in FIGS. 1 
and 2 with seawater or other ballast while the compartments 24 in the 
second end portion 28 are closed to ballasting. Thus, in accordance with 
the present invention, the first end portion 26 may be raised or lowered 
in a controlled manner by increasing the ballast in the compartments or 
removing ballast from the compartments of the first end portion 26. 
FIG. 7 illustrates the position of the platform 10 after the first end 
portion 26 has been sunk to the sea bed 48. Two or more pilings 
illustrated at 102 in FIGS. 2 and 7 have made contact with the sea bed 48 
to prevent the base portion 12 of the platform from absorbing the force of 
impact and to maintain the base portion 12 out of contact with the sea bed 
48 until after the second end portion 28 has been sunk to the sea bed 48 
for purposes that will be more fully explained hereinafter. 
FIG. 8 illustrates a position of the platform 10 during sinking of the 
second end portion 28 to the sea bed 48. This is accomplished by 
ballasting the second end portion 28 of the structure by flooding the 
compartments 24 thereof with ballast. Again, the second end portion 28 may 
be raised and lowered in a controlled manner in accordance with the 
present invention by controlling the amount of ballast being pumped into 
or pumped out of the compartments 24. As shown in FIG. 8, additional 
buoyancy moment has been added on the second end portion 28 by the 
addition of buoyancy member 80 to provide stability throughout the sinking 
of the second end portion 28. 
FIG. 9 illustrates the platform 10 after both end portions 26 and 28 have 
been sunk to the sea bed 48 and the pilings 102 have penetrated the sea 
bed 48. 
FIG. 10 illustrates the platform 10 embedded in the sea bed 48 after 
additional ballasting. Although the buoyancy member 80 may be removed from 
the base portion 12 as illustrated in FIG. 10 if the platform 10 is to be 
permanently installed in the sea bed 48, if the platform 10 is an 
exploration platform, it may be desirable to leave the buoyancy member 80 
on the base portion 12 so that the platform 10 may be returned to the 
surface of the water with the platform 10 having stability at each 
position of its return to the surface so that it can be returned to the 
surface of the water in a controlled manner in accordance with the present 
invention. 
Referring to FIG. 11, there is shown in detail the first end 40 of a 
structure 10. Extending downwardly from the bottom of the base portion 12 
and extending around the perimeter thereof is a skirt 104 for enclosing 
the bottom of the structure 10 between lower members 29 and the sea bed 48 
so that grout, which may be pumped therein to fill up voids between the 
lower members 29 and the sea bed 48, may be contained therein. This skirt 
104 is subject to damage if it is caused to absorb the force of impact 
with the sea bed 48 or if the structure 10 is pivoted about the skirt 104 
as a pivot point for sinking of the second end portion 28 of the structure 
to the sea bed 48. It is therefore desirable that the skirt 104 remain out 
of contact with the sea bed 48 until which time both end portions of the 
structure have been sunk to the sea bed 48. In order to achieve this 
objective, in accordance with an aspect of this invention there are 
attached at least two piles 102 such as the generally cylindrical elongate 
pinpile shown in FIG. 11 at the end of the structure 10 corresponding to 
the end portion thereof which is to be sunk to the sea bed 48 first. For 
example, as shown in FIG. 2, the structure 10 is provided with four such 
piles 102. The piles 102 are spaced over the width of the first end 40 of 
the structure 10. Each pile 102 is provided with an annular support 
housing 106 to enclose and fixedly engage the pile 102 in desired 
positions. Several support members 108 fixedly attach the housing 106 to 
the end 40 of the base 12. Means for positioning the piles 102 so that 
they may be elevated to the position shown in FIG. 11 are preferably 
provided so that the structure 10 may be constructed in a drydock and 
floated out in a minimum depth of water. Such means preferably comprise 
one or more annular clamps or inflatable members 110 between each piling 
102 and its respective housing 106. Means such as an air pressure supply 
may be used to inflate these members 110 to fixedly engage the piling 102 
in a desired position. Release of the inflation pressure accordingly will 
allow the pile to fall downwardly by gravity to the position shown in FIG. 
12, with member 112 engaging the top of the housing 106 to act as a stop 
against further downward movement of the pile 102. 
FIG. 12 illustrates the first end portion 26 of the platform making contact 
with the sea bed 48 by means of the pile 102 striking the sea bed. The 
pile clamps or inflatable members 110 at this point grip the pile 102 to 
prevent movement of the pile 102 in vertical directions relative to the 
skirt 104. The pile 102 extends downwardly beyond the skirt 104 to absorb 
the force of impact with the sea bed 48 and to maintain the skirt 104 out 
of contact with the sea bed 48 until after both end portions 26 and 28 of 
the platform have been sunk so that damage to the skirt may be avoided. 
The piles 102 also serve to locate the position of the platform 10 on the 
sea bed 48 and to provide the pivot point 88 for sinking of the second end 
portion 28 of the platform. Referring back to FIG. 2, whether or not there 
is lateral stability during sinking of the first end portion to the sea 
bed 48 is not normally expected to be a problem since the base portion 12 
will usually provide water plane surface over the entire width of the 
platform for sufficient restoring moment. After the first end portion 26 
of the platform has been sunk to the sea bed 48, then the anchoring of the 
piles 102 to the sea bed 48 is expected to normally provide sufficient 
lateral stability during the sinking of the second end portion 28. 
Each pile 102 is provided with a collar 114 which circumferentially engages 
the pile 102 to provide a horizontally extending surface 122 for limiting 
the depth of penetration of the pile 102 into the sea bed 48. The collar 
114 is preferably slidable along the longitudinal axis of the bottom 
portion 116 of the pile 102 between two points illustrated at 118 and 120 
at which points the outer diameter of the pile increases from a smaller 
diameter to a greater diameter to thereby act as stops for the collar 
which stops are both positioned on the pile 102 such as to be lower than 
the skirt 104 when the pile is positioned, as illustrated in FIG. 12, to 
prevent contact of the skirt 104 with the sea bed 48. As shown in FIG. 13, 
the pile 102 has penetrated the sea bed 48 as the collar 114 has slid or 
moved upwardly on the pile 102 until arrested by the stop 118 thereby 
providing a means for controlling the degree of penetration of the sea bed 
by the pile 102. The lower stop 120 is provided to allow positioning of 
the pile 102 with its lower end at substantially the same height as the 
skirt 104 as shown in FIG. 11 so that the structure 10 may be floated in a 
minimum depth of water. In FIG. 13, both end portions 26 and 28 of the 
platform have been sunk to the sea bed 48 and the piles 102 have 
penetrated the sea bed 48 such that the skirt 104 just touches the sea bed 
48. 
Referring to FIG. 14, the grip of the pile clamps or inflatable members 110 
is released and the structure 10 is further ballasted for movement 
straight down so that the skirt 104 is displaced downwardly relative to 
the piles 102 and firmly penetrates the sea bed 48. 
Certain features of this invention may sometimes be used to advantage 
without a corresponding use of the other features. It is also to be 
understood that the invention is by no means limited to the specific 
embodiments which have been illustrated and described herein, and that 
various modifications may indeed be made within the scope of the present 
invention as defined by the claims which are appended hereto.