JPS6158610B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a ballast device for offshore structures and a method for installing offshore structures.

BACKGROUND OF THE INVENTION In recent years, offshore oil exploration and production has expanded to the Arctic and icy water regions such as northern Alaska and Canada. These water zones are
It is covered with a layer of ice that is more than a month wide. The ice layer
It can reach from 1.5m (5feet) to more than 3m (10feet) and weighs approximately 14.5Kg/cm ² to 70.4Kg/cm ² (200 to 1000pounds).
per square inch). Although appearing stationary on the ice layer, the ice layer actually moves laterally to wind and water currents, thus potentially exerting significant forces on stationary structures in its path. There is. An even more severe problem encountered in Arctic waters is the presence of pressure ridges, ice floes, or large iceberg-like bodies. Large ice packs exert greater forces on offshore structures than normal ice sheets. Therefore, the possibility that a large ice mass will cause major damage or catastrophic destruction to offshore structures is very high.

Stationary structures are made sufficiently strong to resist the crushing forces exerted by impinging ice. That is, such structures are made strong enough to allow ice to impact and break up the structure, and to allow ice to flow around the structure. However, such structures are very material intensive and therefore expensive to construct. It has therefore been proposed to construct structures for use in icy water regions so that their outer surfaces slope converging upward and inward, rather than perpendicularly to the impinging ice. When the ice contacts the sloped outer surface, it is forced upwardly from its normal position, which places tension on the ice and causes a flexural failure. The deflection strength of ice is 6.0Kg/cm ² (85pound per square
inch), so by allowing the ice that hits the structure to break through deflection rather than compression, the force exerted on the structure is reduced.

Several configurations of conical offshore structures with sloped external surfaces are presented in a paper by G.V. Danies entitled ``Effect of conical structures on ice floe impact forces.'' This paper was published on August 13, 1971.
Presented to the 1st International Conference on Port and Marine Industry in Extremely Cold Conditions, held at the Norwegian Institute of Technology, Norway-Londheim, for ~30 days. Other papers of interest from this point of view include the 1970
A paper submitted to the Offshore Technology Conference in Hyuston, Texas, April 2013, by Benny Gerwig Giuniar and Ronald Earl Lloyd, entitled ``Design and Construction Methods for Conventional Offshore Structures.'' .

Cone-shaped structures designed for use in ice-covered water areas can easily be operated at full operating capacity in a given location and can be removed from one location and then operated without delay in another location. It is expected that the building will be constructed in such a way that the conditions can be met. For this purpose, a ballast system is provided to provide adequate stability when towing the structure and to allow the structure to be lowered into the water, brought into contact with the seabed, and removed stably from the seabed. It's summery. However, these structures must have special stability characteristics, which must be compensated for by ballasting devices.

When an offshore structure with a sloped exterior surface is unballasted and therefore floating at a shallow draft, the structure has a large waterline area. Although the center of gravity of this structure is rather high, this structure is very stable. Introducing ballast into a structure causes the structure to float lower in the water, reducing the waterline area. The waterline area decreases rapidly as the draft of the structure increases. If the draft of the structure increases and therefore the waterline area of the structure decreases, the structure reaches a point of instability and at a given draft, the structure becomes unstable given the overall dimensions of the structure. It becomes stable and begins to rock or lean from one side to the other. More specifically, as the waterline area of a structure decreases, the distance between the center of buoyancy and the structure's center of inclination decreases, as this decrease in distance is faster than the decrease in the lift of the center of gravity. , the structure becomes unstable at a certain draft.

Previously proposed designs for ballast devices for conical structures, such as those disclosed in U.S. Pat. It is inadequate in that it uses a relatively small number of relatively large ballast tanks of the same volume designed to fill up Such ballast system designs make it difficult to stably position and re-float structures.

Problems to be Solved by the Invention and Means for Solving the Problems Therefore, the present invention is directed to a ballast device that can stably tow a structure having an inclined outer surface, position it on the water bottom, and remove it from the water bottom. The present invention provides a ballast device with extremely high flexibility in controlling ballast introduction and removal operations by giving the ballast device a special shape.

Generally speaking, the present invention includes a ballast system designed for use with an offshore structure having an outer shell that converges generally upwardly and inwardly to form an angled outer surface. The ballast system of the present invention includes a plurality of radially and circumferentially arranged ballast tanks housed within the outer shell of the structure. These ballast tanks form a plurality of concentrically arranged separate ballast tank rings. The ballast tank is arranged in fluid communication with the water in which the structure is disposed, and is adapted to selectively introduce ballast into and selectively remove ballast from the tank. Also provided is a device for transferring ballast from a tank located on one side of the structure to a tank located on the other side.

To position an offshore structure at a particular location, ballast is introduced into the tank and the structure is lowered substantially horizontally until the maximum stable draft of the structure is achieved. Ballast is then transferred from a tank on one side of the structure to a tank on the other side until the structure is tilted sufficiently such that only a portion of the bottom of the structure is in contact with the seabed. Additional ballast is introduced into the tank to keep the structure in place at the bottom of the water.

If it is desired to resurface the structure, the above method is performed in reverse. If ballast is selectively removed from the tank and the structure floats substantially horizontally until the structure is tilted from the vertical by the same angle as when it first made contact with the sea bed, the structure is given a maximum stable draft. The structure is designed so that only a portion of the bottom of the structure is in contact with the ocean floor. Ballast is then transferred from the ballast tank on the side of the structure in contact with the seabed to the tank on the other side, allowing the structure to rise to its maximum stable draft while remaining substantially parallel. Additional ballast is removed from the tank so that the structure is substantially parallel and at tow draft and ready for movement to a new location.

According to the above configuration of the present invention, the ballast device for an offshore structure having an outer shell that converges upwardly and inwardly to form an inclined outer surface has a structure in which a plurality of ballast tanks arranged in a radial and circumferential direction are provided. To form a plurality of concentrically arranged separate ballast tank rings housed within the outer shell of an object and to stably position and remove the structure from the sea bed.

Effects of the Invention According to an arrangement of the invention, a plurality of separate ballast tank rings are housed within the outer shell of the structure and arranged radially and circumferentially to form a plurality of separate ballast tank rings, which rings are arranged concentrically and Since a plurality of ballast tanks are arranged radially relative to each other, it is possible to introduce and remove ballast and stably tow a structure with an inclined outer surface. These work controls are extremely flexible and can be removed.

Now referring to the drawings, FIG.
An offshore structure 15 is shown. The offshore structure is held in place on the seabed 14 by its own weight plus the weight of the added ballast, as detailed below. A skirt structural member 180 is disposed at the bottom 16 of the offshore structure to help hold the offshore structure in place against horizontal and vertical forces exerted by impinging ice masses. The skirt structure transmits additional shear resistance between this base and the sea bed, preventing soil movement from beneath the offshore structure, thereby keeping the offshore structure relatively anchored above the sea bed. It is designed to hold it in place.
Of course, suitable means such as stakes can be used to help hold the structure in place.

The working platform 10 of the offshore structure 15 is
This deck 42 is shown in FIG. 1 as being equipped with a drill rig 45. Although not shown, other conventional drilling equipment may be located on the work platform 10. The invention is not limited to offshore structures used to support drill rigs. The invention is suitable for any type of offshore work carried out on seabed supported offshore structures designed to be towed from one location to another. The outer surface of this offshore structure generally converges upward and inward to form an inclined surface. For example, the shape of an offshore structure is such that its sloped outer surface has a continuous curved surface, such as a simple cone, multiple cones, or a portion of a hyperboloid of revolution.

As shown in FIG. 1, the offshore structure 15 has a conical lower part 4 and a conical upper part 6, which are arranged coaxially with respect to each other and which
It is designed to accept ice blocks that shift on contact. The walls of the upper part 6 form inclined surfaces inclined with respect to the horizontal, and these surfaces converge upwardly and inwardly of the lower part 4, but the angle of inclination from the horizontal is equal to the angle of inclination 4 of the lower part 4.
It's made to be bigger. Similarly, the walls of the lower part 4 converge inwardly above the seabed, forming an inclined plane with respect to the horizontal. Furthermore, the cross-sectional diameter of the walls of the lower part 4 is larger than the cross-sectional diameter of the walls of the upper part 6. That is, the diameter of the base of the cone forming the upper part 6 is not greater than the diameter of the top of the cone forming the lower part 4.

As mentioned above, drill rig 45 is positioned on deck 42 along with other conventional drilling equipment (not shown) used to drill wellbore 90 underwater.
Moonpool or drill path 49 thus extends from deck 42 down through the structure to water bottom 14 such that drill string 92 extends into wellbore 90 . Because it is expensive and difficult to construct and install structures underwater in the Arctic, it is desirable that the structures have the ability to drill multiple wells at any location.
For example, the structure is approximately 6096 m (20000 feet) 2
Designed to drill more than one well. Therefore,
The structure must be of sufficient size to accommodate the equipment necessary for this purpose. For purposes of illustration, an offshore structure 15 installed in water at a depth of approximately between 6 and 18.3 m (20 and 60 feet) has a bottom diameter of 76.2 m.
(250 feet), with a lower part 4 of 10.7 m (35 feet) in height. Upper part 6 has a height of 13.7 m (45 feet) and a bottom diameter of 3.5
m (110 feet), and the height from the seabed to the deck is approximately 47.2 m (155 feet).

The offshore structure 15 is configured to readily provide full operating capacity at a given drilling location and to move from one drilling location to achieve operating conditions at another location without delay. . To this end, as shown in FIGS. 1 and 2, the ballast arrangement according to the invention comprises ballast tanks 62a-62x, 64a-64x and 66a.
~66x is integrally formed inside the lower part of the offshore structure and is used during towing of the offshore structure, when the structure is lowered into the water and comes into contact with the seabed, and for transportation to a new location. is constructed to provide adequate stability during removal from the seabed.

Ballast tanks are concentric with each other3
They form two separate rings, with the outer ring containing the ballast rings 66a-66x, the middle ring containing the ballast tanks 64a-44x, and the inner ring containing the ballast tanks 62a-62x. The ballast tanks within each ring are a series of compartments separated by a radially disposed watertight wall or bulkhead indicated at 20 and a circumferentially disposed watertight wall or bulkhead indicated at 25. Arranged in FIG. 1 also shows that the ballast tank extends from the watertight bottom or keel 16 of the structure to the outer surface of the lower conical portion of the structure.

In addition to the ballast tank, the lower part of the offshore structure 4
includes suitable pumps 50-52 and 56-58 located in pump chambers 60 and 65, respectively. Pumps 50, 51 and 52 in pump chamber 60
are connected to the ballast tanks 62g-62x, 64g-64x and 6 through appropriate pipes and valves indicated generally by reference numerals 100 and 120.
Communicate with 6g to 66x. These tanks are located on one side of the structure when the structure is divided into two parts on opposite sides along the line Y--Y. Similarly, the pumps in pump chamber 65 are connected to a common manifold 72. Manifold 7
2 is a ballast tank 62 located on the other side of the structure via suitable pipes and valves, indicated generally by reference numerals 110 and 130.
a-62f, 64a-64f and 66a-66
communicates with f. Furthermore, manifolds 70 and 7
2 are in fluid communication with each other through conduit arrangement 150.

To introduce ballast into the tank, seawater is pumped through sea chests 26 and 26A. Chiest 26 and 2
2a communicates by suitable conduit arrangements (not shown) with pumps located in pump chambers 60 and 65, respectively. Ballast is introduced into designated tanks by manifolds, pipes and valves in each pump chamber. For example, pumps 50, 51 and 52
The seawater discharged by the SeaTest 26 and through appropriate conduits to the manifold 70 is supplied to all tanks located on the same side of the offshore structure, i.e. tanks 62g to 62x, 64g.
~64x, and 66g~66x. Ballast tank 62
If you want to put seawater into pipe 100
Valve 120a is opened to allow flow through tank 62x. In a similar manner, ballast is discharged from a given tank through the seat est. If ballast is to be removed from tank 62x, valve 12 is configured such that the pump in pump chamber 60 drains water from the tank through pipe 100a communicating with seat est 26 and suitable conduit arrangement.
0a is opened.

The ballast equipment we have discussed so far is designed to transfer ballast from either tank or set of tanks located on one side of an offshore structure to either tank or set of tanks located on the other side. has been done. For example, the ballast is transferred from the ballast tank 62c through the pipe 110g, the open valve 130g, the manifold 72, the conduit device 150, the manifold 70, the open valve 120x, and the pipe 100x. tank 62i
pumped to. Or, for example, ballast can be routed from tank 62i through pipe 100x, open valve 120x, manifold 70, conduit 150, manifold 72, both open valves 130f and 130u, pipes 110f and 110u to tanks 66b and 62f.
each by a pump. At the same time, the device moves to transfer ballast from the tank pair or pairs on one side of the structure to the tank pair or pairs on the other side. For example, ballast is transferred from tanks 62c and 62d to diametrically opposed tanks 62i and 62j. This work requires a valve of 130g and 130g.
x, 120g and 120x, and transfer the ballast from tanks 62c and 62d to their respective pipes, 110g and 110x, and manifold 7.
2. Through conduit 150, into manifold 70, and from manifold 70 through respective pipes to tanks 62j and 62i, 10.
This is done by pumping to 0g and 100x. Note that for other examples, the ballast described above is transferred from tanks 64a, 64b, 64c, and 64d to tanks 62g, 62h, 64i, and 64j.

Therefore, the ballast device of the present invention can be used to balance offshore structures while towing them, and in other cases to compensate for non-uniform distribution of weight within the structure. It can be seen that this provides a great deal of flexibility in the control and operation of objects. This uneven weight distribution occurs, for example, if the fuel and drill water tanks 82 and 86 located inside the lower part 4 are loaded asymmetrically. moreover,
This ballast system provides a highly effective method for stably operating a generally conical offshore structure when positioning and removing the structure from a particular location underwater.

The offshore structure 15 with dimensions as described above has a depth of approximately 7 m, excluding the depth of the skirt structural member 180.
Becomes unstable at drafts exceeding (23 feet).
Therefore, the maximum draft when the structure is in a stable state is 7 m.
(23 feet).

If an offshore structure is placed in water less than the maximum stable draft, i.e. less than 7 m (23 feet), there will be no stability problem and the structure can be installed by uniformly filling the ballast tanks. be able to. However, the underwater area where the structure is placed is 7
m (23 feet), the structure becomes unstable and rocks as it is lowered into the water.
Also lean to one side. Positioning a structure on the seabed in such an uncontrolled manner not only causes significant damage to the structure, but also to equipment and personnel on the structure.

Accordingly, in accordance with the present invention, a method is provided for positioning a movable offshore structure having a sloped exterior surface on the seabed. After towing the structure into position, ballast is selectively deposited in ballast tanks and is adapted to be lowered while maintaining the structure substantially horizontal until a maximum stable draft is reached. At this point, the ballast is transferred in a controlled manner from a tank located on one side of the structure to a tank located on the other side, tilting the structure enough so that the bottom of the structure contacts the seabed. I come to do it. After a portion of the bottom of the structure contacts the seabed, additional ballast is contained in the tank, allowing the structure to be placed in position on the seabed.

Referring now to FIG. 3, the above-described method of positioning an offshore structure 15 on the seabed is shown. In position, the structure floats in the water above well section D with a tow draft B of between approximately 14 feet and 17 feet, excluding the depth of the skirt member. The towing draft of a structure depends on the weight of the structure and the weight within the structure, including ballast. Ballast is added to the tank to balance the structure during towing so that the structure floats substantially horizontally. Dimension A indicates the height of the outer shell of the structure, which, as previously stated, is approximately 80 feet. Also, as mentioned above, the structure is designed to be placed in water no more than 60 feet deep. Therefore, the amount of ice-like material contacting parts of the structure above the shell is greatly reduced. The minimum depth at which the structure can operate depends on the towing draft plus the depth of the approximately 61 cm skirt structure located at the bottom of the platform. Considering a safety factor of 1 to 2 meters (several feet), the structure can be placed in about 20 feet of water.

In the position of the sequence shown in FIG.
b~64e, 66b~66e, 62h~62k,
64h~64k, 66h~66k (see Figure 2)
The maximum stable draft of offshore structures is C (7
m) while maintaining the structure substantially horizontally. At draft C, the tank is filled to approximately 1/2 of its full capacity. In terms of total capacity, the outer ring ballast tank is ^388m3
(13728 cubic feet), while the middle and inner ring ballast tanks have a volume of 698 ^m3 (24653 cubic feet) and
It has a volume of 747 m ³ (26415 cubic feet).

In position, ballast is transferred from the inner ring pair of tanks 62i and 62j to diametrically opposed tanks 62c and 62d;
The offshore structure tilts enough so that a portion of the bottom of the structure (designated E) comes into contact with the seabed. The angle of tilt of the structure from the vertical is represented by θ. The value of this angle is directly proportional to the depth in the water at which the structure is located. In 18.3 meters (60 feet) of water, the structure tilts approximately 13 degrees from the vertical, so that part of the bottom of the structure is in contact with the seabed. To create the moment necessary to tilt the structure 13 degrees, tanks 62i and 62
Approximately 40% of the ballast of tank 62c and 62
It is necessary to move to d. Preferably, ballast is transferred from the inner ring of the tank to provide the necessary moment to tilt the structure. This is because these tanks have short moment arms,
Therefore, it is less sensitive to increases in ballast than the outer two ring tanks. Therefore, by using the inner ring tank pair,
Fine control can be exercised during the tilting of the structure, allowing the structure to come into contact with the seabed relatively gently.

Additional ballast tank 62h in position
~62k, 62h~64k and 66h~66k
will be introduced and offshore structures will be installed at predetermined locations on the seabed. All ballast tanks are then filled to substantially full capacity to hold the structure in place. It should be understood that filling other combinations of tanks with different amounts of ballast and transferring ballast from different tanks will produce the same results as described above. Similar modifications are possible with respect to the methods described below for removing structures from the waterbed.

If it is necessary to remove an offshore structure from well section D, the structure can be resurfaced in the reverse manner that the structure was installed. In the position of FIG. 4, the structure is in place on the ocean floor. position until the structure tilts to the same tilting angle θ from vertical as it had when it was first placed on the seabed.
A sufficient amount of ballast is removed from the ballast tank. In this position, the position E of the bottom of the structure
Only the seafloor is in contact with the ocean floor. Ballast is transferred from a tank located on the side of the structure in contact with the seabed to a tank located on the other side, as shown in the position, until the maximum stable draft C of the structure is achieved and the structure is It will essentially float horizontally. At this point, more ballast is removed from the tank, lowering the draft of the structure and allowing the structure to rise to towing draft B. The ballast is removed so that the structure floats substantially horizontally and is ready for transport to a new location.

As shown in the steps in Figure 4, when the offshore structure 15 is placed 18.3 m (60 feet) underwater, the tilt angle θ is the same as when the structure was installed.
That is, it is tilted to a tilting angle θ from the vertical having a value of about 13 degrees, and the ballast is removed from the tank until only a portion E of the bottom of the structure is in contact with the seabed. In order to tilt the structure in this way, basically all the ballast is transferred to the tanks 62a and 64.
a, 66a, 62x, 64x, 66x, 62g,
Excluded from 64g, 66g, 62f, 64f and 66f. Also, ballast is tank 66b~6
6e, 64b~64b, 66h~66k, 64h
~64k, 62h, 62k, 62b and 62e
approximately 1/2 of the total capacity of these tanks
It is now filled only with Additionally, approximately 10% of the ballast in tanks 62c and 62d is removed, and that of tanks 62i and 62j is removed.
90% are eliminated. Next is the ballast tank 62c
and 62d to tanks 62i and 62j, making the structure float substantially horizontally at a maximum stable draft C. We are beginning to do this. To do this, about 40 ml of ballast in tanks 62c and 62d
% is transferred to tanks 62i and 62j. Additionally, ballast is removed from the appropriate ballast tanks, the draft of the structure is reduced, and the structure is raised to towing draft while remaining substantially level, ready for movement to a new location. .

It should be noted that the bottom of the structure is designed such that the same portion of the bottom 16 of the offshore structure is always in contact with the seabed during ballast introduction and removal operations. In this case, the skirt member 18 can be removed from this section and the bottom of the structure in this section can also be structurally reinforced.

The circumferential and radial arrangement of multiple tanks reduces the chance of a catastrophic failure in which some of the tanks become damaged due to corrosion, causing water to flood into the tank. In the outer and middle rings, 2
It is rare for more than one tank to corrode;
Even if the four tanks overflow, this offshore structure can remain stable. This particular arrangement of the tanks limits damage to the tanks to a relatively small number, thus reducing the cost of repairing the ballast system.

The offshore structure 15 shown in FIGS. 1 and 2 shows that the ballast system of the present invention includes three separate rings each containing twelve ballast tanks. However, changes can be made to the total number of ballast tanks, the number of rings, and the number of tanks housed in each ring. It is also possible to use different shaped ballast devices for different sized platforms with different geometries.
Similarly, regarding the combination of tanks for introducing, transferring and removing ballast, it is possible to change the ballast introduction and removal method of the structure 15, and if this is taken into consideration,
The piping and manifold arrangement of the ballast equipment can be changed.

The ballast device of the present invention provides a device that can stably and widely control the ballast introduction and removal work having a substantially conical structure.

Although the invention has been described with respect to particular embodiments only, the invention is not limited to such embodiments, but rather by the scope of the following claims.

[Brief explanation of the drawing]

FIG. 1 is a schematic front view, partially in section, illustrating a preferred embodiment of the invention. FIG. 2 is a schematic cross-sectional view taken along line 2--2 of FIG. FIG. 3 is a schematic diagram illustrating the method of positioning the preferred embodiment structure on the bottom of the water. FIG. 4 is a schematic diagram illustrating a preferred embodiment method for removing structures from the bottom of the water. 4...Lower conical part, 6...Upper conical part, 1
0...Platform, 14...Underwater, 15...
Offshore structure, 25...Bulkhead, 30...Underwater, 42...Deck, 45...Drill rig, 49
...Moon pool, 50, 51, 52, 56, 5
7,58...pump, 60,65...pump room,
62a-62x, 64a-64x, 66a-66
x... Ballast tank, 70, 72... Manifold, 82, 86... Drill water tank, 90...
Well hole, 100... Pipe, 120, 130...
Valve, 150... conduit.

Claims (1)

[Scope of Claims] 1. A ballast device for a movable offshore structure having an outer shell converging generally upwardly and inwardly to form an inclined outer surface, comprising: A plurality of ballast tanks arranged,
a plurality of ballast tanks, wherein the ballast tanks form a plurality of separate ballast tank rings, the rings of ballast tanks being arranged concentrically and radially relative to each other; a device for forming fluid communication between a ballast tank on one side of the structure and a ballast tank on the other side of the structure; a device for selectively transferring ballast from a ballast tank on one side of the ballast tank to a ballast tank on the other side of the ballast tank; and a device for selectively introducing ballast into the ballast tank and removing ballast from the ballast tank. A ballast device for an offshore structure characterized by: 2. A ballast according to claim 1, characterized in that an outer ring of the ballast tank and at least two additional rings of the ballast tank are arranged radially inside the outer ring. Device. 3. The ballast tank forming the inner ring has a larger volume than the ballast tank forming the middle ring, and the ballast tank forming the middle ring has a larger volume than the ballast tank forming the outer ring. A ballast device according to claim 2, characterized in that it has: 4. A ballast arrangement according to claim 3, wherein each of said rings includes at least twelve ballast tanks. 5. A ballasting system for a movable offshore structure, wherein said structure has conical upper and lower portions in coaxial relationship, the walls of both said upper and lower portions being inclined with respect to the horizontal. the ballast device, wherein the angle of inclination from the horizontal of the lower part is smaller than the angle of inclination from the horizontal of the upper part, and the cross-sectional diameter of the upper part is not larger than the cross-sectional diameter at the top of the lower part, a first outer ring of the ballast tank concentrically disposed within the conical lower portion of the structure; and a first outer ring of the ballast tank concentrically disposed within the conical lower portion of the structure and concentrically with said outer ring; a second middle ring of ballast tanks disposed adjacent to the inner side, wherein the number of ballast tanks forming the outer ring and the middle ring is equal, and the ballast tanks forming the middle ring are larger than the outer ring; said middle ring having a larger volume than said ballast tank forming a ballast tank; a third inner ring of a ballast tank, the number of ballast tanks forming the middle ring and the inner ring being equal, and the ballast tanks forming the inner ring forming the middle ring; said inner ring having a larger volume; a device for forming fluid communication between said ballast tank and the water in which the structure is disposed; and a device for forming fluid communication between radially opposed ballast tanks. a device for selectively transferring ballast from any one of the ballast tanks contained in one of the rings to a diametrically opposite ballast tank contained in the same ring; , a device for selectively transferring ballast to the ballast tank and removing ballast from the ballast tank. 6. A method of installing an offshore structure at a predetermined position underwater, the structure comprising: a plurality of ballast tanks arranged circumferentially within the outer shell; are arranged concentrically, and the water depth at the predetermined location is greater than the towing draft of the structure, but less than the height of the outer shell of the structure, wherein the structure reaches a maximum stable draft. introducing ballast from the water into at least some of the ballast tanks on each side of the structure to lower the structure while remaining substantially level until the structure is tilted sufficiently; transferring ballast from a ballast tank on one side of the trench structure to a ballast tank on the other side until a portion of the bottom of the structure is in contact with the sea bed; and placing the structure in position on the sea bed. 1. A method for installing an offshore structure, comprising the steps of: introducing additional ballast from underwater into at least some of the ballast tanks in order to do so. 7. Said offshore structure is arranged in such a way as to give said structure a maximum stable draft when said offshore structure is floating substantially horizontally, and so that only a portion of the bottom of said structure is in contact with the seabed. removing said structure from the seabed by expelling a sufficient amount of ballast from at least some of the ballast tanks into the water until the structure is tilted at a predetermined angle from vertical; transferring ballast from a ballast tank on the side of the structure in contact with the seabed to a ballast tank on the other side until the draft of the structure is achieved and the structure floats substantially horizontally; additional ballast from at least some of the ballast tanks on each side of the structure so that the structure is brought up to towing draft while remaining substantially level and ready for movement to a new location. 7. A method according to claim 6, further comprising the steps of: expelling the water into the water.