GB1593145A - Method of offshore construction - Google Patents

Description

(54) METHOD OF OFFSHORE CONSTRUCTION (71) We, A/S AKERS MEK.

VERSKSTED, of Munkedamsveien 45, Oslo 2, Norway, a Norwegian company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of mounting a large structural part on an offshore structure, where a structural part is brought to rest on an offshore structure, a number of plastically deformable elements first being placed between the structural part and the offshore structure.

Such methods are used for instance in the building of large platform structures for use in offshore oil production. Such structures are often built in two parts, usually a deck structure and a supporting or base structure for the deck structure. The base structure may be equipped with upwardly extending columns upon which the deck structure is brought to rest, for instance by floating the deck structure in position over the columns of the base structure while the latter is in a floating, almost entirely submerged condition, whereupon the base structure is raised so that the columns are brought into contact with the deck structure and raises the latter.

With the large sizes in question it is very difficult to make the top surfaces of the columns and the corresponding contact surfaces of the deck structure absolutely even and parallel. Consequently, very large variations in the stress distribution in the contact surfaces between columns and deck may occur, which in turn leads to high stress concentrations in these structures. Especially when one or both of these structures are made of concrete, such stress concentrations may lead to damage and may upset the calculation basis for sizing the prestressed reinforcing steel of the concrete and the stress and buckling safety factor in structures of steel.Therefore, it has been necessary to make the contact surfaces of such structures with very high accuracy and, in addition, to size these structures to be able to tolerate high stress concentrations and eccentric forces, the result being increased cost, increased weight and increased building time.

According to the present invention there is provided a method of mounting a large structural part on an offshore structure, where a structural part is brought to rest on an offshore structure, wherein a number of plastically deformable elements in the form of pieces of metal pipe are placed between the structural parts and the offshore structure prior to bringing the structural part and the offshore structure together.

For a better understanding of the present invention and to show more clearly how it may be carried into effect reference will now be made, by way of example, to the accompanying drawings in which: Fig. 1 illustrates a deck structure during transfer to the columns of a base structure of an offshore structure; Fig. 2 shows a top view of one of the columns of the base structure during the performance of the method according to the present invention; Fig. 3 shows a further embodiment of the top of one of the columns during the performance of the method according to the present invention Figs. 4a and 4b show a section along the line IV IV in Fig. 2 for two stages of the compression of a deformable element used in the method according to the present invention;; Fig. 5 graphically illustrates the deformation characteristic for the deformable element; and Figs. 6a, 6b and 7a, 7b show stages of deformation for two alternative embodiments of the deformable element used in the method according to the present invention In Fig. I reference numeral I designates a deck structure supported by two floating barges 2. The deck structure rests in a mounting position above the upwardly extending columns 3 of a submerged base structure (not shown). Downwardly facing surfaces 4 on the deck structure are here located opposite corresponding upwardly facing surfaces 5 on top of the columns 3.

The surfaces 4 and 5 should be as plane and parallel as possible in order to prevent large stress concentrations when the deck structure is brought to rest on the columns. However, due to the large dimensions and weights in question, it is impossible to obtain completely even and co-planar contact surfaces.

Furthermore, it is in practice impossible to obtain absolute parallelism between the surfaces 4 and 5. inter alia because the deck structure has a natural deflection which changes when the deck structure is transferred from the barges 2 to the columns 3.

Thus, it is clear that by direct transfer of the deck structure I to the columns 3 large local and uncontrollable stress concentrations in the column tops and the adjacent parts of the deck structure will result.

Figs. 2 and 3 illustrate schematically how the present invention solves this problem in a very simple fashion. These figures show the top of a column 3 seen from above. The column is hollow and therefore presents an annular top surface 5 which during installation is to be brought into contact with a corresponding surface on the deck structure 1. On the top surface 5, a number of plastically deformable elements 6 are placed, said elements taking the form of relatively thick walled pieces of pipe in an especially simple and suitable embodiment of the invention. In Fig. 2, the pieces of pipe are arranged with their axes tangential to the column while in Fig. 3 they are directed radially with respect to the cross section of the column.

Fig. 4a shows how a plastically deformable element is placed in between the top surface 5 on the column 3 and the corresponding surface 4 on the deck structure 1. The latter is made of steel while the column 3 is made of concrete. In order to protect the concrete at the surface 5, a base plate 7 is inserted beneath the element 6 in order to distribute the transmitted forces.

The number of deformable elements and their dimensions are adapted so that they will all be deformed when the weight of the deck structure 1 is transferred to the columns 3.

Fig. 4b illustrates how the plastically defor- mable element 6, here in the form of a piece of pipe, changes form during the transfer operation.

The forces transmitted by the piece ofpipe in Figs. 4a and 4h between the deck structure and the column during this operation appear from Fig. 5. Here, the variation of the force as a function of the deformation or compression in the vertical direction of the piece of pipe is graphically illustrated. The deformation curve has an elastic section e and a plastic section p which shows a slightly increasing force with increased deformation.

The pieces of pipe 6 are dimensioned so that the average force per piece of pipe will lie on the curve section p. Due to the slight slope of this curve section, each piece of pipe will transfer approximately the same force regardless of the degree of compression. This means in turn that relatively large deviations in the planeness and parallelism of the surfaces 4 and 5 may be tolerated without resulting in appreciable variations in the surface pressure.

When the entire weight of the deck structure I has been transferred to the columns 3 so that the pieces of pipe 6 are squeezed more or less flat, the remaining opening between the surfaces 4 and 5 and the pieces of pipe 6 is filled with concrete which may be prestressed.

Figs. 6a. 6b and 7a, 7b show alternative embodiments of plastically deformable elements and how these are deformed. These are also based on pieces of pipe or sections of such. The deformable elements 6 are in these cases welded to the base plates 7.

A further advantage of the elements shown is that they are plastically deformable in the vertical direction without loosing the ability to withstand horizontal and vertical forces. It is very important that the ability to withstand horizontal forces is not reduced when the vertical load is increased.

Even though Figs. 2 and 3 show elements 6 which are all directed tangentially and radially, respectively, with respect to the cross section of the columns, it will be obvious to a skilled person that the elements may be positioned in other ways or in combinations of such ways without loosing the desired effect.

It has been found that elements of relatively thick walled pipe give good results. In order to give a suitable deformation the ratio between the outer diameter of the pipe and its wall thickness should lie in the range 5:1 to 15:1 and preferably in the range 7:1 to 10:1. It has been found that steel is a suitable pipe material, but metals having similar plastic properties may also be used.

WHAT WE CLAIM IS: 1. A method of mounting a large structural part on an offshore structure, where a structural part is brought to rest on an offshore structure, wherein a number of plastically deformable elements in the form of pieces of metal pipe are placed between the structural part and the offshore structure prior to bringing the structural part and the offshore structure together.

2. A method according to claim 1, wherein pieces of pipe are used having a ratio of outer diameter to wall thickness in the range of 5:1 to 15:1.

3. A method according to claim 2, wherein the ratio is in the range of7:1 to 10:1.

4. A method according to claim 1, 2 or 3, wherein pieces of steel pipe are used.

5. A method according to any preceding

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. parallel as possible in order to prevent large stress concentrations when the deck structure is brought to rest on the columns. However, due to the large dimensions and weights in question, it is impossible to obtain completely even and co-planar contact surfaces. Furthermore, it is in practice impossible to obtain absolute parallelism between the surfaces 4 and 5. inter alia because the deck structure has a natural deflection which changes when the deck structure is transferred from the barges 2 to the columns 3. Thus, it is clear that by direct transfer of the deck structure I to the columns 3 large local and uncontrollable stress concentrations in the column tops and the adjacent parts of the deck structure will result. Figs. 2 and 3 illustrate schematically how the present invention solves this problem in a very simple fashion. These figures show the top of a column 3 seen from above. The column is hollow and therefore presents an annular top surface 5 which during installation is to be brought into contact with a corresponding surface on the deck structure 1. On the top surface 5, a number of plastically deformable elements 6 are placed, said elements taking the form of relatively thick walled pieces of pipe in an especially simple and suitable embodiment of the invention. In Fig. 2, the pieces of pipe are arranged with their axes tangential to the column while in Fig. 3 they are directed radially with respect to the cross section of the column. Fig. 4a shows how a plastically deformable element is placed in between the top surface 5 on the column 3 and the corresponding surface 4 on the deck structure 1. The latter is made of steel while the column 3 is made of concrete. In order to protect the concrete at the surface 5, a base plate 7 is inserted beneath the element 6 in order to distribute the transmitted forces. The number of deformable elements and their dimensions are adapted so that they will all be deformed when the weight of the deck structure 1 is transferred to the columns 3. Fig. 4b illustrates how the plastically defor- mable element 6, here in the form of a piece of pipe, changes form during the transfer operation. The forces transmitted by the piece ofpipe in Figs. 4a and 4h between the deck structure and the column during this operation appear from Fig. 5. Here, the variation of the force as a function of the deformation or compression in the vertical direction of the piece of pipe is graphically illustrated. The deformation curve has an elastic section e and a plastic section p which shows a slightly increasing force with increased deformation. The pieces of pipe 6 are dimensioned so that the average force per piece of pipe will lie on the curve section p. Due to the slight slope of this curve section, each piece of pipe will transfer approximately the same force regardless of the degree of compression. This means in turn that relatively large deviations in the planeness and parallelism of the surfaces 4 and 5 may be tolerated without resulting in appreciable variations in the surface pressure. When the entire weight of the deck structure I has been transferred to the columns 3 so that the pieces of pipe 6 are squeezed more or less flat, the remaining opening between the surfaces 4 and 5 and the pieces of pipe 6 is filled with concrete which may be prestressed. Figs. 6a. 6b and 7a, 7b show alternative embodiments of plastically deformable elements and how these are deformed. These are also based on pieces of pipe or sections of such. The deformable elements 6 are in these cases welded to the base plates 7. A further advantage of the elements shown is that they are plastically deformable in the vertical direction without loosing the ability to withstand horizontal and vertical forces. It is very important that the ability to withstand horizontal forces is not reduced when the vertical load is increased. Even though Figs. 2 and 3 show elements 6 which are all directed tangentially and radially, respectively, with respect to the cross section of the columns, it will be obvious to a skilled person that the elements may be positioned in other ways or in combinations of such ways without loosing the desired effect. It has been found that elements of relatively thick walled pipe give good results. In order to give a suitable deformation the ratio between the outer diameter of the pipe and its wall thickness should lie in the range 5:1 to 15:1 and preferably in the range 7:1 to 10:1. It has been found that steel is a suitable pipe material, but metals having similar plastic properties may also be used. WHAT WE CLAIM IS:

1. A method of mounting a large structural part on an offshore structure, where a structural part is brought to rest on an offshore structure, wherein a number of plastically deformable elements in the form of pieces of metal pipe are placed between the structural part and the offshore structure prior to bringing the structural part and the offshore structure together.

2. A method according to claim 1, wherein pieces of pipe are used having a ratio of outer diameter to wall thickness in the range of 5:1 to 15:1.

3. A method according to claim 2, wherein the ratio is in the range of7:1 to 10:1.

4. A method according to claim 1, 2 or 3, wherein pieces of steel pipe are used.

5. A method according to any preceding

claim, wherein the space between adjacent surfaces of the structural part and the offshore structure and said elements, following the plastic deformation of the latter, are filled with concrete.

6. A method according to any preceding claim, where at least one of the structural part and the offshore structure is made of concrete, wherein base plates are arranged between said at least one of the structural part and the offshore structure and said elements.

7. A method of mounting a large structural part on an offshore structure, substantially as hereinbefore described with reference to the accompanying drawings.