US20070221112A1

US20070221112A1 - Solution mining to refloat and dispose of an offshore floating structure

Info

Publication number: US20070221112A1
Application number: US11/389,446
Authority: US
Inventors: James Haney; Shree Akhave
Original assignee: J Ray McDermott SA
Current assignee: J Ray McDermott SA
Priority date: 2006-03-24
Filing date: 2006-03-24
Publication date: 2007-09-27
Also published as: MX2007003412A; NO20071293L

Abstract

An improved fixed ballast for a floating offshore structure. The material used for the ballast may be a dense liquid that is pumped into the fixed ballast tank during ballasting and forced out of the tank during deballasting. The material may also be a soluble material that is ground up into particles and added to water. After a saturated solution is obtained, more particles are added to produce a slurry. The slurry is delivered into the ballast tanks. Removal of the slurry is accomplished by pumping sea water into the ballast tanks until all of the ballast material is dissolved and forced out of the tank.

Description

FIELD AND BACKGROUND OF INVENTION

The invention is generally related to offshore floating structures and more particularly to the refloating and removal of such structures.
A spar buoy is a floating structure that has been used to support drilling and production equipment for oil production in the offshore environment. As seen in FIG. 1, a superstructure 1 that contains the payload is supported on a vertical, buoyant cylinder known as a hard tank 2. A truss 3 extends downward to support a tank that is known as a soft tank 4 (also known as a fixed ballast tank). The function of the soft tank is to carry fixed ballast, which improves the stability and reduces the motions of the spar buoy in response to the action of the waves. The hard tank, truss, and soft tank are fabricated as a unit 5 lying on its side. The superstructure is built elsewhere in the upright position. The unit 5 is loaded out onto a transporter vessel and transported to the vicinity of the installation site. The transporter vessel ballasts down and the unit 5 floats off the transporter vessel, which departs leaving the unit floating horizontally in the water, as seen in FIG. 2. To get the unit into the vertical position the soft tank is flooded. With the loss of buoyancy at the bottom end of the unit, the unit becomes unstable and rotates into a stable vertical position, as seen in FIG. 3.
In the vertical position seen in FIG. 3 the soft tank 4 is fully flooded, ready to receive the fixed ballast. The name fixed ballast refers to the fact that the fixed ballast cannot be moved, once it is installed. The fixed ballast is a slurry of particles of a dense material and water. The slurry is introduced into a pipe above the water surface. This pipe extends downward to a compartment in the soft tank. The slurry runs down the pipe and enters the compartment, displacing sea water from the flooded compartment through an opening at the top of the compartment. As water decants off the slurry, the particles sink to the bottom of the compartment leaving room for more slurry, if needed. Once the fixed ballast is in place the superstructure can be lifted and set on the unit to obtain the completed spar, as seen in FIG. 1.
The soft tank 4 can be divided into compartments so that different quantities of the fixed ballast can be placed in each compartment. As an example, FIG. 4 shows a soft tank 4 that is divided into eight compartments. The square in the central region of the soft tank 4 is not a compartment, but is really a square opening, or center well through the soft tank 4. By doing this the combined center of gravity of the fixed ballast can be moved to a desired position in plan that is not at the center of the soft tank 4. The position of the center of gravity of the fixed ballast is chosen to produce a combined center of gravity of the entire spar buoy platform that is at the center of the structure in plan. The legs of the truss are indicated by numeral 6.
The weights supported by the spar are divided into three categories when center of gravity issues are considered. There are the weights that will never move because they are part of the structure, or there will be no reason to move them. These fixed weights have a known center of gravity that can be compensated by the center of gravity of the fixed ballast. There are weights that are expected to move in normal operations. When these weights move the active ballast system in the hard tank is used to trim the spar. Finally, there are future equipment weights that will be added at a future time. These fixed weights that will be added in the future will cause the center of gravity of the spar to move. Since the fixed ballast cannot be moved, the design of the spar must include sufficient ballasting capability in the hard tank 2 to trim the spar when the future weight is added.
The dense material that is ground up into particles and slurried is usually an enriched iron ore that is chosen because it is economical, dense, and is benign to the environment. If there is a spill during installation of the slurry, or a leak during service, it is essential that the fixed ballast material not be toxic to marine life. Once the water has decanted out of the fixed ballast particle matrix, the fixed ballast will conglomerate into a weak solid mass.
In the prior art, at the end of the economic life of the spar buoy platform, the superstructure is taken off and the rest of the structure is stripped of anything useful and cleaned up. Then the structure is towed to deep water and sunk to get rid of it.
Due to potential environmental concerns there is an increasing interest in retrieving and salvaging entire spar structures instead of sinking them. This means that the unit 3 of FIG. 3 must be rotated into the horizontal floating position of FIG. 2. Thus, the uprighting procedure described above must be reversed. In other words, the unit 5 must be refloated. Once the unit is in the horizontal position it can be towed to shore and salvaged. The problem this presents is that the fixed ballast must be removed before it is possible to refloat the unit. Given that the fixed ballast conglomerates into a solid weak mass over years of service, the fixed ballast is not easily removed and the problem becomes very difficult to solve. Large spar structures have not been in operation for many years and, as a result, the known art has not addressed this problem. While the above discussion is directed to spar structures because the use of large spar structures is relatively new, it should be understood that the problem of removing fixed ballast in order to refloat a structure applies to any floating offshore structure that uses fixed ballast as described above.

SUMMARY OF INVENTION

The present invention addresses the problem of removing fixed ballast and then refloating a floating offshore structure that uses fixed ballast. For new structures, a fixed ballast material is chosen that can be removed by operations conducted above the water surface. Operations are conducted through pipes that are preinstalled during fabrication of the structure. The pipes are used during installation and removal of the fixed ballast. The material used for the ballast may be a dense liquid that is pumped into the soft tank compartments during ballasting and forced out of the tank during refloating. The material may also be a soluble material that is ground up into particles and added to water. After a saturated solution is obtained, more particles are added to produce a slurry. The slurry is introduced into the soft tank through the pipes. Removal of the slurry is accomplished by pumping sea water through one of the pipes until all of the ballast material is dissolved and forced out of the tank.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For a better understanding of the present invention, and the operating advantages attained by its use, reference is made to the accompanying drawings and descriptive matter, forming a part of this disclosure, in which a preferred embodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, forming a part of this specification, and in which reference numerals shown in the drawings designate like or corresponding parts throughout the same:
FIG. 1 illustrates a spar structure in the normal operational position.
FIG. 2 illustrates a truss type spar structure in a horizontal floating orientation.
FIG. 3 illustrates a truss type spar in the vertical floating position with the added detail of flow pipes to the soft tank compartments.
FIG. 4 is a view taken along lines 4-4 in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3 illustrates a floating offshore structure (spar type) wherein pipes 7 and 8 are installed on the unit 5 and run from above the water surface into the soft tank 4. One of the pipes (pipe 7) extends to a point that is near the bottom of the soft tank 4. The other pipe (pipe 8) enters the soft tank 4 and terminates near the top of the soft tank 4. The tops of both pipes 7 and 8 extend above the water surface when the completed structure is in its normal operational position as illustrated in FIG. 1. The pipes 7 and 8 are preinstalled during construction of the structure.
During ballasting operations, the ballast material is pumped through pipe 7 into the soft tank 4. The ballast material, which is heavier than water, displaces the water from the soft tank 4 and forces the water out of the soft tank 4 through pipe 8. The ballast material is pumped into the soft tank 4 until all of the water has been displaced from the soft tank 4 or until the desired ballast weight has been delivered into the soft tank 4.
The ballast material may be a slurry that is formed as follows. A material that is soluble in water is ground up into particles and added to water until a saturated solution is obtained. More of the ground particles are then added to the saturated solution to form a slurry, which produces a ballast material. A slurry of calcium chloride is an example of a suitable slurry material. It is dense enough, soluble in water, and benign to the environment. Another option for ballast material is a liquid that is dense enough to provide the desired ballast effect. The liquid must be denser than water, soluble in water, and benign to the environment. Cesium formate is an example of a liquid that meets these requirements.
Ballasting is carried out as follows. When the slurry option is used, the slurry is delivered into the soft tank 4 through the pipe (pipe 7) that terminates near the bottom of the soft tank 4. The slurry entering the bottom of the soft tank 4 displaces sea water out of the soft tank 4 through pipe 8. Once the desired amount of slurry has been delivered into the soft tank 4, the suspended material in the slurry is allowed to settle. Then more of the slurry material can be added if additional ballast weight is needed. When the dense liquid option is used, the dense liquid is delivered into the soft tank 4 through the pipe (pipe 7) that terminates near the bottom of the soft tank 4. The dense liquid displaces sea water out of the soft tank 4 through pipe 8. For both options, the tops of pipes 7 and 8 are capped after the desired amount of ballast material has been delivered into the soft tank 4.
When removing the slurry ballast to deballast and refloat the structure, water is pumped into the soft tank 4 through either pipe 7 or 8. The water dissolves the slurry material and forces it up the other pipe. Water is pumped into the soft tank 4 until the salinity of the return liquid indicates that the slurry ballast material has been removed. The structure may then be floated into a horizontal position and removed.
When removing the dense liquid ballast, compressed air is introduced into the soft tank 4 through pipe 8. The compressed air displaces the dense liquid through pipe 7. The structure may be floated into a horizontal position once the dense ballast material has been forced from the soft tank 4.
The invention provides another advantage. The invention allows the removal of fixed ballast from the soft tank. Instead of providing extra ballast capacity in the hard tank to trim the structure after additional future equipment is added, the ballast in the soft tank 4 can be moved to trim the structure. Weight can be removed or added to the soft tank compartments to move the center of gravity of the fixed ballast to the desired location.
The invention is applicable to all types of floating offshore structures that use fixed ballast and will encounter the problem of refloating for disassembly as referenced above.
While specific embodiments and/or details of the invention have been shown and described above to illustrate the application of the principles of the invention, it is understood that this invention may be embodied as more fully described in the claims, or as otherwise known by those skilled in the art (including any and all equivalents), without departing from such principles.

Claims

1. In a floating offshore structure having a ballast tank that receives fixed ballast, an improved fixed ballast material, said fixed ballast material comprising a slurry formed from a water soluble material that is benign to the environment.

2. A method for removing the slurry fixed ballast of claim 1, comprising pumping water into the ballast tank containing the slurry until the material has been dissolved and removed from the ballast tank.

3. In a floating offshore structure having a ballast tank that receives fixed ballast, an improved fixed ballast material, said fixed ballast material comprising a liquid that is denser than water and benign to the environment.

4. A method for removing the dense liquid ballast of claim 3, comprising pumping compressed air into the ballast tank to force the liquid from the ballast tank.

5. A method for removing the dense liquid ballast of claim 3, comprising pumping water into the ballast tank to force the liquid from the ballast tank.

6. A method for adding fixed ballast to a floating offshore structure, comprising:

a. dissolving a water soluble material in water to form a saturated solution;

b. forming a slurry by adding additional water soluble material to the saturated solution;

c. pumping the slurry into a ballast tank in the floating offshore structure; and

d. allowing suspended slurry material to settle in the ballast tank.

7. A method for adding fixed ballast to a floating offshore structure, comprising pumping a liquid that is denser than water and benign to the environment into a ballast tank in the floating offshore structure.

8. The improved ballast material of claim 1, comprising calcium chloride.

9. The improved ballast material of claim 3, comprising cesium formate.