US20100206213A1

US20100206213A1 - Method for Constructing a Floating Unit

Info

Publication number: US20100206213A1
Application number: US12/703,486
Authority: US
Inventors: Thomas Kullander; Tommy Lager; Daniel Åstrand
Original assignee: GVA Consultants AB
Current assignee: GVA Consultants AB
Priority date: 2009-02-13
Filing date: 2010-02-10
Publication date: 2010-08-19
Also published as: SE535055C2; WO2010093315A1; SE0900185A1

Abstract

The present invention relates to a method for constructing a unit adapted to float in a body of water, the method comprising the steps of:

- providing a cavity in the unit such that the cavity is open to the environment surrounding the unit, wherein at least a portion of the cavity is adapted to be located below a still water surface when the unit floats in the body of water, and
- determining a first value of at least a first parameter relating to required hydrostatic properties for the unit.

The present invention also relates to a unit adapted to float in a body of water. The unit comprises a cavity such that the cavity is open to the environment surrounding the unit. At least a portion of the cavity is adapted to be located below a still water surface when the unit floats in the body of water. Moreover, the present invention relates to a use of a cavity in a unit adapted to float in a body of water.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Patent Application No. 61/152,293 which was filed on Feb. 13, 2009 and SE 0900185-0 which was filed on Feb. 13, 2009, the entirety of which is incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a method for constructing a unit adapted to float in a body of water. The method comprises the steps of:
providing a cavity in the unit such that the cavity is open to the environment surrounding the unit, at least a portion of the cavity being adapted to be located below a still water surface when the unit floats in the body of water, and
determining a first value of at least a first parameter relating to required hydrostatic properties for the unit.

BACKGROUND

The present invention also relates to a unit adapted to float in a body of water. The unit comprises a cavity such that the cavity is open to the environment surrounding the unit. At least a portion of the cavity is adapted to be located below a still water surface when the unit floats in the body of water. Moreover, the present invention relates to a use of a cavity in a unit adapted to float in a body of water.
Units adapted to float in a body of water—such as ships or vessels for drilling and/or production of hydrocarbons—are generally designed to carry load, i.e. to have a load carrying capacity. As such, in order to carry the aforesaid load, the unit is designed so as to provide appropriate buoyancy in terms of the magnitude of the buoyancy as well as the location of the centre of buoyancy. In addition, the buoyancy of the unit—when floating in a body of water—should also be designed to carry the weight of the unit per se, which weight is often referred to as a light unit weight, as well as the weight of inter alia operational fluids—e.g. ballast water and fuel—required for the unit to function properly.
In addition to providing appropriate buoyancy, the unit should also be designed to provide appropriate stability characteristics of the unit. Depending on inter alia the vertical centre of gravity of the unit as well as the magnitude and location of the unit's surface exposed to the wind, the unit should be designed so as to provide a water plane area the magnitude and location of which provide an appropriately large righting moment for the unit.
Parameters relating to: the buoyancy, the centre of buoyancy as well as the magnitude and location of the water plane area of the unit, when the unit is floating in a body of water, may be regarded as parameters relating to the hydrostatic properties of the unit. As such, when designing and constructing a unit adapted to float in a body of water, it is generally of interest to ensure that the actual hydrostatic properties of the unit meet the hydrostatic properties required for the unit in order to function properly, e.g. in order to be able to provide a specific load carrying capacity.
However, during a design and/or construction phase of a unit, it is quite often realized that the initial hydrostatic properties of the unit may have to be modified in order to meet new requirements for the unit. Traditionally, the need for modified hydrostatic properties is often occasioned by—but is not limited to—the fact that: the magnitude and centre of gravity of the light unit weight of the constructed unit do not correspond to the values assumed when designing the unit or that the magnitude and/or centre of gravity of the load carrying capacity is changed during the design and/or construction phase of a unit. It should also be noted that the specified load carrying capacity of a unit may sometimes be changed after the completion of the construction phase of a unit.
In order to modify the hydrostatic properties of a unit during the construction thereof, prior art proposes that the unit be furnished with extensions, such as sponsons or fenders, which extensions protrude from the original unit and which are adapted to be at least partially immersed in water when the unit is floating in a body of water. Depending on the location of the extensions, at least one parameter relating to the hydrostatic properties, such as the buoyancy and/or water plane area, of the unit may be modified such that the modified unit presents hydrostatic characteristics which meet the new requirements for the unit.
However, although the extensions are useful for obtaining desired hydrostatic properties, the extensions also often introduce other problems for the unit. For instance, the extensions generally result in that the unit will be subjected to increased environmental loads, e.g. loads from waves and currents, and these increased loads may in turn result in the need for reinforcements of the unit. Moreover, additional arrangements of the unit, such as propulsion and/or mooring arrangements may also have to be modified in order to meet the increased environmental loads emanating from the aforesaid extensions.
Additionally, the unit generally has to be located in a safe location, such as a dock or by a quay, when furnishing the unit with the extensions. Thus, the provision of the extensions of the unit generally results in an increased construction time for the unit, which in turn may result in increased construction costs.
As may be realized from the above, there is a need for improving the procedure of modifying the hydrostatic properties of a unit.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a method for constructing a unit adapted to float in a body of water, which method provides for that the hydrostatic properties of the unit may be modified in a simple manner, even at a late stage of the construction phase of the unit.
A second object of the present invention is to provide a method for constructing a unit adapted to float in a body of water, which method provides for that the hydrostatic properties of the unit may be modified in such a way that other properties, such as hydrodynamic properties, of the unit are not unduly effected.
A third object of the present invention is to provide a method for constructing a unit adapted to float in a body of water, which method provides for that the hydrostatic properties of the unit may be modified outside a dock or a quay.
A fourth object of the present invention is to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
At least one of the above objects is solved by a method for constructing a unit adapted to float in a body of water according to claim 1.
As such, the present invention relates to a method for constructing a unit adapted to float in a body of water. The method comprises the steps of:
providing a cavity in the unit such that the cavity is open to the environment surrounding the unit, at least a portion of the cavity being adapted to be located below a still water surface when the unit floats in the body of water;
determining a first value of at least a first parameter relating to required hydrostatic properties for the unit.
As used herein, the expression “hydrostatic properties” encompasses, but is not limited to, at least one of the following properties of the unit: the buoyancy, the centre of buoyancy, the water plane area and the moment of inertia of the water plane area.
According to the present invention, the method further comprises the step of fixedly sealing at least a portion of the cavity from the surrounding environment to thereby form an enclosed volume such that a second value of the first parameter is obtained, such that the absolute value of the difference between the first value and the second value is below a predetermined value.
Thus, by providing a method for constructing a unit which method comprises the step of modifying a parameter relating to the unit's hydrostatic properties by sealing at least a portion of a cavity, the hydrostatic properties of the unit may be modified at a late stage of the construction phase, and even after the completion of the construction of the unit in a dock or by a quay, in a simple and straightforward manner.
Moreover, since the hydrostatic properties of the unit—according to the method of the present invention—are modified without having to furnish the unit with protruding extensions, the hydrodynamic properties of the unit are generally only marginally modified, which reduces, and often even removes, the need for reinforcements of the unit and/or modifications of additional arrangements, such as mooring arrangements, due to the modifications of the hydrostatic properties of the unit.
The expression “modifying the hydrostatic properties of a unit” is intended to mean that a first parameter may be increased, decreased or re-distributed throughout the unit.
According to a preferred embodiment of the present invention, the method comprises the step of providing a plurality of cavities in the unit.
By the provision of a plurality of cavities in the unit, the flexibility as regards how to modify the hydrostatic properties of the unit is enhanced.
According to another embodiment of the present invention, the step of fixedly sealing at least a portion of the cavity is performed for at least two of the plurality of cavities. This further enhances the flexibility regarding how to modify the hydrostatic properties of the unit.
According to a further embodiment of the present invention, the floating unit comprises a float adapted to be located under the still water surface, the floating unit further comprises a plurality of support columns, each one of the support columns extending from the float and being adapted to intersect the still water surface, wherein the cavity is provided on at least one of the support columns.
According to another embodiment of the present invention, at least one cavity is provided in each one of the support columns.
According to a further embodiment of the present invention, the step of determining the first value comprises a step of determining the weight and/or centre of gravity of the unit.
According to a further embodiment of the present invention, the step of determining the first value comprises a step of determining the buoyancy and/or centre of buoyancy of the unit.
According to another embodiment of the present invention, the step of determining the first value comprises a step of determining a load carrying capacity of the unit.
According to a further embodiment of the present invention, the step of fixedly sealing at least a portion of the cavity comprises the steps of providing the unit with a sealing member, such as a panel, and fixedly attaching the sealing member to the unit.
A second aspect of the present invention relates to a unit adapted to float in a body of water, the unit comprising a cavity such that the cavity is open to the environment surrounding the unit, at least a portion of the cavity being adapted to be located below a still water surface when the unit floats in the body of water. According to the second aspect of the present invention, the cavity is adapted to receive a sealing member to thereby fixedly sealing at least a portion of the cavity from the surrounding environment.
According to a preferred embodiment of the second aspect of the present invention, the unit comprises guide means for guiding the sealing member into position in the cavity.
A third aspect of the present invention relates to a use of a cavity in a unit adapted to float in a body of water, the cavity being open to the environment surrounding the unit, at least a portion of the cavity being adapted to be located below a still water surface when the unit floats in the body of water, wherein the use comprises modifying at least one hydrostatic property of the unit by fixedly sealing at least a portion of the cavity from the surrounding environment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be further explained by means of non-limiting examples with reference to the appended figures wherein:

FIG. 1 is a schematic side view of a floating unit according to the second aspect of the present invention;

FIG. 2 is a sectional view of a portion of the FIG. 1 unit;

FIG. 3 is a sectional view of a portion of the FIG. 1 unit illustrating a step of the method of the present invention;

FIG. 4 a is a sectional view of a portion of the FIG. 1 unit when a sealing member has been attached to the unit;

FIG. 4 b is a top cross section view of a portion of the FIG. 4 a unit;

FIG. 5 is a sectional view of a portion of the FIG. 1 unit illustrating a step of the method of the present invention;

FIG. 6 is a sectional view of a portion of another embodiment of the unit of the present invention;

FIG. 7 is a sectional view of a portion of a further embodiment of the unit of the present invention;

FIG. 8 is a sectional view of a portion of yet another embodiment of the unit of the present invention;

FIG. 9 is a top view of a portion of the FIG. 8 unit;

FIG. 10 illustrates a perspective view of a unit of ring wall type.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will be described using examples of embodiments. It should however be realized that the embodiments are included in order to explain principles of the invention and not to limit the scope of the invention, defined by the appended claims.
FIG. 1 illustrates a unit 10 adapted to float in a body of water 12, during the execution of the construction method of the present invention. In FIG. 1, the unit 10 is a semi-submersible unit 10 but is should be noted that the construction method of the present invention is also applicable for other types of floating units, such as for instance ships or spar buoys (not shown in FIG. 1). Purely by way of example, the buoyancy of the unit 10 may be above 10 000 metric tonnes and may in some cases even be above 100 000 metric tonnes.
The FIG. 1 semi-submersible unit 10 comprises a float 14, a deck structure 16 and at least one support column 18 extending from the float 14 to the deck structure 16. A support column 18 generally has the shape of a cylinder the cross-section of which generally is circular or rectangular, although there may of course be other types of cross-sections. The unit 10 in FIG. 1 has four support columns, wherein a first 18 and a second 20 support column are visible. The unit 10 in FIG. 1 has a longitudinal extension indicated by an arrow L and a vertical extension indicated by arrow V.
As may be realized from FIG. 1, when the unit 10 is floating in the water 12 having a still water surface 22, the float 14 is adapted to be located at least partially beneath the still water surface 22 and the deck structure 16 is adapted to be located at least partially above the still water surface 22. In the position illustrated in FIG. 1, the whole of the float 14 is located beneath the still water surface 22 and the deck structure 16 is located completely above the still water surface 22. Moreover, each one of the columns 18, 20 intersects the still water surface 22 resulting in a water plane area WPA′, WPA″ for each one of the columns which are comprised in the total water plane area WPA of the unit 10.
Moreover, in FIG. 1, the unit's 10 centre of buoyancy CoB is indicated. As may be realized by a person skilled in the art, when the unit 10 is floating in a body of water 12, the buoyancy B of the unit 10 is defined as the mass of the water displaced by the unit 10. Moreover, the centre of buoyancy CoB is the centre of gravity of the aforesaid displaced water.
FIG. 1 also illustrates the location of the centre of gravity CoG of the unit 10. Generally, the mass M of the unit 10—which mass M is also associated with the centre of gravity CoG—comprises inter alia the light unit weight, the weight of operational fluids required for the unit to function properly as well as the load carrying capacity of the unit 10.
As may be realized by a person skilled in the art, in order for the unit 10 to float at an even keel, there needs to be a balance between the mass M and the buoyancy B of the unit 10. For instance, if there is a misalignment between the centre of gravity CoG and the centre of buoyancy CoB in a direction parallel to the longitudinal extension L of the unit 10, the unit 10 will be subjected to an inclination. Such an inclination is generally undesired, for instance since at least a portion of the deck structure 16 will then be located closer to the still water surface 22 than in an “even keel” position, which makes this portion prone to being subjected to wave loading. As previously discussed, such misalignments between the centre of gravity CoG and centre of buoyancy CoB have previously been corrected by furnishing for instance the float 14 with one or more sponsons (not shown in FIG. 1), i.e. one or more buoyant extensions protruding from the float 14.
Moreover, if the mass M of the unit exceeds the buoyancy B of the unit 10 when the unit is floating at a draught 24 at which the unit 10 is designed to float, the unit 10 will decline further into the body of water 12 until a balance between the mass M and the buoyancy B is obtained, resulting in that the unit 10 will float at a new draught 24′ as indicated by a dotted line in FIG. 1.
Additionally, without going into details, the unit's 10 stability—i.e. capability of withstanding heeling moments—is dependent on inter alia the centre of gravity CoG, the centre of buoyancy CoB, the magnitude of the moment of inertia of the water plane area which in turn is dependent on the water plane area as well as the distances, which distances are measured in a plane parallel to the still water plane, between the individual water plane areas WPA′, WPA″. To this end, it should be noted that a high vertical centre of gravity generally requires a large moment of inertia of the water plane area. This may for instance be achieved by a large water plane area WPA—i.e. a water plane area having a large moment of inertia per se—and/or a water plane area WPA constituted by a plurality of individual water plane areas WPA′, WPA″ the distances between which are large.
As such, if it for instance is realized that the vertical centre of gravity CoG of the unit is higher than what was assumed when designing the unit 10, the unit 10 may have to be modified in order to compensate for the aforesaid increase of the vertical centre of gravity CoG. To this end, prior art proposes that fenders (not shown in FIG. 1) are attached to at least one of the support columns 18, 20 wherein the fender is buoyant and adapted to intersect the still water surface 22 in order to provide an additional water plane area WPA′″ to the unit 10.
As may be realized from the above, during its construction, the unit 10 may have to be modified for a plurality of different reasons in order to obtain required hydrostatic properties of the unit 10. In order to modify the unit 10 in a preferred manner, the present invention presents a method for constructing a unit 10, which method comprises the step of providing a cavity in the unit such that the cavity is open to the environment surrounding the unit 10. At least a portion of the cavity is adapted to be located below a still water surface 22 when the unit 10 floats in the body of water 12.
The unit 10 in FIG. 1 is provided with a plurality of cavities, wherein two of the cavities 26, 28 are located in the supporting columns 18, 20 of the unit 10 whereas two cavities 30, 32 are located in the float 14. Preferred implementations of the cavities 26, 28, 30, 32 will be explained in detail hereinbelow. However, as a general remark, the cavities are preferably designed so as to prevent fluid passage through the unit 10, for example through the support columns 18, 20 or the float 14. As such, a component of the unit 10, such as a support column 18, 20 or the float 14, provided with a cavity to be used in the method of the present invention preferably has a closed circumference throughout the area of the location of the cavity. In other words, a cavity to be used in the method of the present invention does preferably not comprise a through opening in the unit 10.
FIG. 2 illustrates a perspective view of a section of the FIG. 1 unit 10, wherein two of the unit's 10 cavities 26, 30, namely a first 26 and a second cavity 30, are shown. As may be gleaned from FIG. 2, the first cavity 26 is located on the first support column 18 and the first cavity 26 is adapted to intersect the still water surface 22, indicated with dotted lines in FIG. 2, when the unit 10 floats in a body of water. Furthermore, FIG. 2 illustrates that the first cavity 26 is located by the outermost corner of the first support column 18, i.e. the corner 32 of the first support column 18 being located at the largest distance from the centre of the unit 10. This is a preferred location of the first cavity 26 since a subsequent step of sealing at least a portion of the first cavity 26 from the surrounding environment will result in a substantial increase of the stability of the unit 10. However, in other implementations of the unit 10, the first cavity 26 may be located in other positions of the first support column 18.
FIG. 2 further illustrates that the first cavity 26 is delimited by a plurality of panels, namely a bottom panel 34, a top panel 36 and a first and a second side panel 38, 40. As may be realized when studying FIG. 2, of the panels delimiting the first cavity 26, the bottom panel 34 is located closest to the float 14 in the vertical direction V whereas the top panel 36 is located farthest away from the float 14 in the vertical direction V. Moreover, the first and second side panels 38, 40 extend from the bottom panel 34 to the top panel 36. The panels are preferably steel plates and the panels are preferably attached to one another by means of tight joints, such as weld joints.
Purely by way of example, the volume of the first cavity 26 may be within the range of 0.02-0.001, preferably 0.01-0.004, of the total volume displaced by the unit 10 when the unit 10 floats at an operational draught. Moreover, again purely by way of example, the horizontal cross sectional area of the first cavity may be in the range of 0.1-0.005, preferably 0.07-0.01, of the total water plane area of the unit 10.
In a similar manner as the first cavity 26, the second cavity 30 is delimited by a plurality of panels forming a notch in the float 14. The second cavity 30 is preferably located on the outside of the float 14 such that a subsequent step of sealing at least a portion of the second cavity 30 from the surrounding environment generally will result in a substantial change of the centre of buoyancy CoB of the unit 10. Purely by way of example, the volume of the second cavity 30 may be within the range of 0.1-0.001, preferably 0.01-0.004, of the total volume displaced by the unit 10 when the unit 10 floats at an operational draught.
The method of the present invention also comprises a step of determining a first value of at least a first parameter relating to required hydrostatic properties for the unit 10.
As has been discussed hereinabove, the hydrostatic properties may be related to a plurality of properties. As such, the step of determining the aforesaid first value, may in some embodiments of the method of the present invention comprise a step of determining the mass M and/or centre of gravity CoG of the unit 10. This step may be performed by actually weighing the unit 10, by procedures known by a person skilled in the art, or by assembling information as regards the mass and centre of gravity for components—which components are considered relevant from a weight point of view—forming a part of the unit 10.
In addition to, or instead of, the step presented above, in embodiments of the method of the present invention, the step of determining the first value may comprise a step of determining a load carrying capacity of the unit 10.
It should be noted that none of the steps above, be it a step of determining the mass or the load carrying capacity of the unit, needs to be performed when the unit is in a dock or by a quay. Instead, the steps as presented hereinabove may be performed after the unit 10 has left the construction site and in some embodiments of the method of the present invention, the step of determining the first value of a first parameter may actually be performed when the unit is in its operating location, e.g. travelling at sea or being moored to a specific operating location, and even when the unit 10 is in an operating condition.
Irrespective of when the step of determining the first value is performed, the first value is preferably compared to the actual value of the first parameter relating to the hydrostatic properties of the constructed unit 10, i.e. the unit including the cavities 26, 30. If it, from the aforesaid comparison, is realized there is a misalignment between the first value and the actual value of the first parameter, the unit 10 may have to be modified in order to correct the aforesaid misalignment.
The method steps above are exemplified hereinbelow by means of non-limiting examples.
In a first example, the first parameter relating to required hydrostatic properties is the buoyancy B₁of the unit 10. As such, the step of determining a first value of the parameter may comprise a step of determining the mass M of the unit 10 and the first value B₁should thus preferably correspond to the mass M. If the mass M is larger than the actual buoyancy B_Aof the unit 10, when the cavities are open, at least a portion of at least one of the cavities is sealed such that a second value of the buoyancy B₂is obtained and such that the difference |B₂-B₁| between the first and second values are below a predetermined value, which predetermined value may be regarded as a tolerance for the method.
In a second example, the first parameter is the horizontal centre of buoyancy HCB₁of the unit 10. As such, the step of determining a first value of the parameter may comprise a step of determining the horizontal centre of gravity HCG of the unit 10 and the first value HCB₁should generally substantially correspond to the horizontal centre of gravity HCG. If the horizontal centre of gravity HCG differs from the actual horizontal centre of buoyancy HCB_Aof the unit 10, when the cavities are open, at least a portion of at least one of the cavities is sealed such that a second value of the horizontal centre of buoyancy HCB₂is obtained and such that the difference |HCB₂—HCB₁| between the first and second values is below a predetermined value.
It should be noted that in the second example above, the horizontal centre of buoyancy HCB is generally constituted by two components, a longitudinal centre of buoyancy LCB and a transversal centre of buoyancy TCB. Correspondingly, the horizontal centre of gravity HCG is generally also constituted by two components, a longitudinal centre of gravity LCG and a transversal centre of gravity TCG. However, in some implementations of the second example, the first parameter may be chosen so as to only relate to only one of the aforesaid components, e.g. either the longitudinal centre of buoyancy LCB or the transversal centre of buoyancy TCB which thus should be compared to the corresponding component of the horizontal centre of gravity.
In a third example, the first parameter relating to required hydrostatic properties of the hull is water plane area WPA₁—for instance both magnitude and location of the water plane area—of the unit 10. As such, the step of determining a first value of the parameter may comprise a step of determining the vertical centre of gravity VCG of the unit 10 and from that information determining the required magnitude and position of the water plane area WPA₁in order to obtain a unit 10 with sufficient stability characteristics. If the water plane area WPA₁differs from the actual water plane area WPA_Aof the unit 10, when the cavities are fully open, at least a portion of at least one of the cavities is sealed such that a second value of the water plane area WPA₂is obtained and such that the difference |WPA₂-WPA₁| between the first and second values are below a predetermined value.
It should be noted that the first parameter relating to required hydrostatic properties of the unit may in some embodiments of the present invention be determined by combining some or all of the parameters from the above examples.
As previously mentioned, the aforesaid predetermined value may be regarded as a tolerance for the method. The magnitude of the predetermined value may be selected from case to case based on inter alia the design of the unit as well as the hydrostatic property concerned. Purely by way of example, the predetermined value may be selected as a percentage of the first value of the first parameter. As such, again purely by way of example, the predetermined value may be set to be 10%, preferably 5%, more preferably 1% of the first value.
The method of the present invention further comprises a step of fixedly sealing at least a portion of the cavity 26, 30 from the surrounding environment to thereby form an enclosed volume such that a second value of the first parameter is obtained, such that the absolute value of the difference between the first value and the second value is below a predetermined value. Examples of how this is done are presented in FIG. 3.
FIG. 3 illustrates how a first panel 42 is inserted into the first cavity 26. The first panel 42 preferably is a metal plate of a metal which is similar or the same as the metal of the column 18 and/or the panels 34, 36, 38, 40 partially delimiting the first cavity 26. The first panel 42 may be inserted into the first cavity 26 by using a lifting arrangement such as a crane (not shown). Optionally, the first panel 42 may be inserted into the first cavity 26 when the unit 10 is floating in a body of water at an appropriate draught such that the first panel 42 may be floated into position, for instance using a barge (not shown). In order to facilitate the insertion of the first panel 42 into the first cavity 26, the first column 18 preferably comprises guide means (not shown), such as outwardly extending pins, for guiding the panel into the first cavity 26. In addition, the first panel 42 may be provided with auxiliary guide means (not shown), such as openings, adapted to interact with the guide means of the first column. FIG. 3 also illustrates that the unit 10 is provided with a second panel 44 in order to seal the second cavity 30.
The first 42 and second 44 panels may be fixedly attached to the unit by means of one or more joints. Purely by way of example, such a joint may be a weld joint and/or a bolt joint. When attaching a panel 42, 44 to a portion of the unit 10 being located at least partially beneath the still water level, a habitat may be used in order to provide a substantially dry environment for the attachment operation.
FIG. 4 b illustrates a cross section from above of the first column 18 when the first panel 42 has been inserted into the first cavity 26. Moreover, the first panel 42 has been attached to the first column 18 by means of tight joints. As may be gleaned from FIG. 4 b, once the first panel 42 is attached to the first column 18, an enclosed volume 45 is formed in the first cavity 26. The enclosed volume 45 will be buoyant when at least partially submerged into a body of water, thus the enclosed volume 45 increases the buoyancy, as well as the water plane area WPA, of the unit 10. Moreover, as may be realized from FIG. 4 b, a portion 46 of the cavity 26 is still open to the environment surrounding the unit 10. The open portion 46 may be regarded as additional buoyancy and/or water plane area reserve, which may be fixedly sealed at a later stage of the life of the unit 10, should increased buoyancy and/or water plane area be subsequently required.
FIG. 5 illustrates an optional procedure of fixedly sealing at least a portion of a cavity from the surrounding environment. In FIG. 5, the second cavity 30 is used as an example although it should be realized that the procedure is equally applicable for any one of the cavities of the unit 10. As may be gleaned from FIG. 5, rather than closing the entire second cavity 30 by a panel, only a portion—in this case in the longitudinal direction L—of the second cavity 30 is fixedly sealed from the surrounding environment. To this end, in the embodiment of the method of present invention illustrated in FIG. 5, the method comprises a step of providing a coffer 48—or end piece—which is inserted into the second cavity 30 and subsequently fixedly attached to the float 14. Preferably, the coffer 48 is buoyant such that it may be floated into the second cavity 30.
Instead of, or in addition to, the provision of the coffer as discussed with reference to FIG. 5 hereinabove, in order to be able to close only a portion of a cavity in its longitudinal extension—which extension may coincide with the longitudinal extension L of the unit 10 as is the case for the second cavity 30 in FIG. 5—the cavity may be divided into a plurality of compartments. An example of a cavity provided with a plurality of compartments is presented in FIG. 6, wherein the second cavity 30 contains a first 50 and a second 52 compartment. As such, in the implementation illustrated in FIG. 6, the second cavity 30 has been partitioned by an additional panel 54 or partition wall. Thus, either one, or both, of the first 50 and second 52 compartments may later on be fixedly sealed by corresponding sealing members. In FIG. 6, the sealing members are exemplified by two sealing panels 56, 58, but the sealing members may in other embodiments of the method of the present invention be coffers (not shown in FIG. 6) similar to the one illustrated in FIG. 5 for instance.
To this end, FIG. 7 illustrates another implementation of a sealing means which may be used for sealing the first compartment 50. The FIG. 7 sealing means is a coffer 60 the depth d of which may be smaller than the depth D of the first compartment 50. Moreover, the FIG. 7 coffer 60 comprises an outer flange 62 provided with openings 64 for fastening means such as bolts (not shown in FIG. 7). The FIG. 7 coffer 60 will thus act as a plug when inserted in the first compartment 50 and attached to the unit by means of e.g. a bolt joint. One advantage of the FIG. 7 coffer 60 is that it may be used for sealing the first compartment 50 in a straightforward manner, even if the first compartment is located below the still water surface. A method of sealing the first compartment 50 by means of the coffer 60 may comprise the steps of: filling the coffer 60 with water such that it submerges, guiding the coffer into the first compartment 50, attaching the coffer to the unit 10 and removing water from the first compartment 50 (and possibly also from the coffer 60).
FIG. 8 and FIG. 9 illustrate an optional implementation of the cavity 26. As may be realized from FIG. 8, instead of providing a cavity the opening of which is substantially of the same size as the cavity itself, the FIG. 8 cavity 26 is open to the surrounding environment by means of a plurality of openings 66, 68 in a panel 70 outwardly delimiting the cavity 26. In the implementation illustrated in FIG. 8 and FIG. 9, the panel 70 has two openings. As such, if the two openings 66, 68 would not have been present in the panel 70, the panel 70 would have formed a part of the outer skin of the unit. The openings 66, 68 are sufficiently large so as to allow a free sea water flow in and out of the cavity 26. If the additional buoyancy and/or water plane area of the cavity 26 is required, the cavity 26 is sealed from the surrounding environment by fixedly sealing the openings 66, 68, for instance by using sealing members such as sealing panels (not shown).
Moreover, it should be noted that the step of fixedly sealing the at least a portion of the cavity from the surrounding environment may comprise a step of at least partly emptying the portion of the cavity from sea water. Such a step may for instance be performed after a sealing member has been attached to the unit 10 for sealing the portion of the cavity. The step of emptying the portion of the cavity from sea water may typically be performed for a cavity at least a portion of which is located beneath the still water surface 22 during the attachment of the sealing member to the unit.
As regards the embodiments of the method present invention as presented hereinabove, it should be noted that although a semi-submersible unit 10 comprising a plurality of columns 18, 20 has been used as an example of a unit 10 of the present invention, the method as claimed in claim 1 is also applicable for other types of units 10. As an example, FIG. 10 illustrates a unit 10 of a so-called ring wall type having an inner 60 and an outer wall 62 forming a closed hollow wall structure 64. The FIG. 7 unit 10 is provided with a first 26 and a second 30 cavity, wherein the first cavity 26 is adapted to intersect a still water surface when the unit 10 is floating in a body of water whereas the second cavity 30 is adapted to be located below the still water surface. As such, it should be realized that the present invention is not limited to the embodiments described hereinabove and illustrated in the drawings. Rather, a person skilled in the art will realize that many changes and modifications may be performed within the scope of the appended claims.

Claims

1. A method for constructing a unit adapted to float in a body of water, said method comprising the steps of:

providing a cavity in said unit such that said cavity is open to the environment surrounding said unit, at least a portion of said cavity being adapted to be located below a still water surface when said unit floats in said body of water;

determining a first value of at least a first parameter relating to required hydrostatic properties for said unit; and

fixedly sealing at least a portion of said cavity from the surrounding environment to thereby form an enclosed volume such that a second value of said first parameter is obtained, such that the absolute value of the difference between said first value and said second value is below a predetermined value.

2. The method according to claim 1, wherein the method comprises the step of providing a plurality of cavities in said unit.

3. The method according to claim 2, wherein said step of fixedly sealing at least a portion of said cavity is performed for at least two of said plurality of cavities.

4. The method according to claim 1, wherein said unit comprises a float adapted to be located under said still water surface, said floating unit further comprises a plurality of support columns, each one of said support columns extending from said float and being adapted to intersect said still water surface, wherein said cavity is provided on at least one of said support columns.

5. The method according to claim 4, wherein at least one cavity is provided in each one of said support columns.

6. The method according to claim 1, wherein said step of determining said first value comprises a step of determining at least one of a weight and centre of gravity of said unit.

7. The method according to claim 1, wherein said step of determining said first value comprises a step of determining at least one of buoyancy and centre of buoyancy of said unit.

8. The method according to claim 1, wherein said step of determining said first value comprises a step of determining a load carrying capacity of said unit.

9. The method according to claim 1, wherein said step of fixedly sealing at least a portion of said cavity comprises the steps of providing said unit with a sealing member, such as a panel, and fixedly attaching said sealing member to said unit.

10. The method according to claim 1, further comprising modifying at least one hydrostatic property of said unit by fixedly sealing at least a portion of said cavity from the surrounding environment.

11. A unit adapted to float in a body of water, said unit comprising a cavity such that said cavity is open to the environment surrounding said unit, wherein at least a portion of said cavity is adapted to be located below a still water surface when said unit floats in said body of water, wherein said cavity is adapted to receive at least one sealing member to thereby fixedly seal at least a portion of said cavity from the surrounding environment.

12. The unit according to claim 11, wherein said unit comprises guide means for guiding said sealing member into position in said cavity.

13. The unit according to claim 11, further comprising a plurality of cavities in said unit.

14. The unit according to claim 13, wherein at least a portion of at least two of said plurality of cavities are fixedly sealed from the surrounding environment.

15. The unit according to claim 11, further comprising a float located under said still water surface, said floating unit further comprising a plurality of support columns, each one of said support columns extending from said float and adapted to intersect said still water surface, wherein said cavity is provided on at least one of said support columns.

16. The unit according to claim 15, wherein at least one cavity is provided in each one of said support columns.