NO177897B - floats - Google Patents

Description

The invention relates to a float, with a submerged floating part of concrete, a column part including one or more concrete columns from the floating part up through the water surface, and a cover part of steel, carried by the column part above the water surface, the said one or the mentioned number of concrete columns above the water surface being extended as a hollow, non-equipable steel column up to the deck section.

A floater is an installation floating in the sea for the exploitation of resources in and under the sea. It can be dynamically positioned or anchored. Typical floats are maritime installations such as drilling platforms and production platform forms, loading buoys etc.

Weight and stability are naturally problems encountered in connection with floats. Thus, the tendency to increase the so-called topside (deck part) and high-placed weights has become an ever-increasing problem. This is connected, among other things, to unforeseen weight increases of deck systems and modules, a common experience from initial design to finished concept.

A float's stability and general motion characteristics are closely related to the height of the material center of gravity, in interaction with the buoyancy center of gravity and the metacenter distance above the buoyancy center of gravity. The height of the metacenter plus the buoyancy center of gravity must thus be defined as positively greater than the height of the material center of gravity in order for the float to have satisfactory stability. It is therefore a given that there will be large optimization gains associated with lowering the total material center of gravity as much as possible. This also means that while the heavy construction of a concrete float is an absolute advantage in the bottom elements, the opposite will be the case in the upper part of the float. A deck part made as a steel structure contributes in a positive direction with regard to stability.

When you choose to combine concrete and steel in floats, as mentioned at the beginning, this is partly economically, partly technically justified. Concrete is competitive in terms of price and is believed to have several advantages compared to steel. The relatively numerous and different types of concrete installations carried out so far in the North Sea have thus far proven to be very resistant to corrosion with minimal maintenance and without special protection. A concrete float is therefore believed to have an advantage in terms of longevity. Another important advantage is the robust design of the concrete structures, which is believed to make them particularly suitable for maritime environments exposed to severe weather and for heavy deck installations.

Studies have shown that the interaction between steel and concrete is one of the conceptual aspects that cause problems. The problem arises when the significant and concentrated static and dynamic loads between the deck part and the column part are to be transferred to the support in the concrete. These large point loads can lead to crushing of the concrete, and such areas will also be strongly subject to fatigue. In order to distribute these forces over a larger area and thus reduce the stress level in a satisfactory manner, it will therefore be necessary to reinforce the concrete with steel structures. Such reinforcements, however, entail relatively large and unfortunate weight increases, especially considering that the weight increase occurs high above the material center of gravity, with the negative stabilization effects this will entail.

The size of these pressure loads will of course be concept-dependent, i.e. dependent on the size of the deck part and on the principles chosen for and the design chosen for the underlying support system's interaction connection with the column part of the float. It will thus be possible to reduce the size of the concentrated pressure loads by careful design selection based on this consideration alone. In such a selection, however, there will be an obvious limitation with regard to the possibilities for technical and economic optimization of a floating concept.

It is a purpose of the present invention to propose measures that can contribute to improving a float's stability and general movement characteristics, with the simultaneous possibility of optimizing construction and equipment time.

This purpose can be achieved by utilizing the concrete's advantages in terms of robustness, weight and corrosion resistance in the underwater, lower parts, i.e. for the part of the float that is below the water surface, in combination with the elasticity/plasticity of the steel and its consequent well documented ability for voltage equalization and distribution, in all parts above the water surface.

According to the invention, a float is therefore proposed as mentioned in the introduction, characterized by the fact that the dividing line concrete/steel in the column is located at a distance in the size range 20-30 m from the deck beam (point of attack of the load) where the stress concentrations from the point loads of the deck support (point of attack of the loads) are distributed along the steel column shell to a low and relatively even level, and at a distance of around 5 m above the calculated water line.

With the invention, the advantages of both the steel and the concrete can be utilized in a favorable way, i.e. the steel/concrete interaction area is placed so that one gets the opportunity for as favorable utilization of the force distribution area as possible.

The use of steel will help reduce weight. At the same time, the steel/concrete interaction area will also advantageously be moved, with the possibility of beneficial utilization of a certain distribution area for the forces, down along the columns. In principle, the distribution area should extend as far down into the columns as possible, but for practical reasons the interaction height should be some distance above the water surface, partly to avoid exposing the interaction connection to a large external water pressure with a theoretical risk of leakage, and partly to ensure access for corrosion inspection and maintenance, which is considered essential, as the floats can have a lifespan of up to 50 years.

By using the inventive concept, the significant concentrated pressure loads from the deck part can be distributed via specially allocated strength parts, built in steel, into the float's cylindrical steel shell (the steel columns). From there, the compressive stresses will spread further down the cylindrical steel shell, in a fan shape that will have a double angle of approx. 45°. Calculations show that both compressive stresses and the induced tensile stresses from eccentricity moments will go from an asymptotically infinite value at the load's point of attack to a low, constant level at a distance approximately equal to the column diameter from the column top and further down. The interaction connection steel/concrete should therefore ideally be placed at a reasonable, but nevertheless the shortest possible distance upwards from this elevation. For a typical float, a column diameter will be around 25 m. According to the invention, the steel column length should therefore be in the same size range, while the concrete/steel connection should be approx. 5 m above the calculated water line, as this will provide reasonable opportunities for periodic inspection and maintenance.

For a float of the aforementioned combined concrete/steel design, two separate construction sites can be used, one for the concrete part and one for the steel part. These two construction sites will be able to work towards a common milestone (date) for completion. The use of steel columns helps to reduce the time until completion, corresponding to the shortened scope of work for the concrete part.

As the upper part of the column section will presumably have built-in all decks for various mechanical equipment and these decks will consequently be built, installed, equipped and then completed and tested, the scope of work for the steel section will be relatively extensive and time-consuming. The all-important part of this manufacturing and completion period constitutes shortening of the total manufacturing time in relation to a uniform design. The concrete/steel design provides major time savings, in addition to taking advantage of the advantages that separate construction sites offer, such as better general access (crane availability etc.) and larger area per operator, which contributes to increasing safety and enabling a more rational utilization of personnel and equipment, reducing the number of work disciplines within a limited area, which is of significant importance for productivity, and

less vulnerability to design changes late in the project, as the fabrication of the steel part begins later than the concrete part.

For a float according to the invention, it will also be possible to achieve the advantage that winches in the float's anchoring system can be placed in one or more of the aforementioned steel columns. This means that this part of the anchoring system will be ready and available as soon as the steel column is installed. Typical anchoring systems for floats are the use of ordinary slack anchoring cables or tension rods.

Particularly in this connection, it could be advantageous if the floater according to the invention includes two steel columns arranged diametrically opposite each other in the column part, as this would give the possibility of an anchoring system where only the two mentioned steel columns are provided with the floater part of the floater's anchoring system (winches , tighter etc). Such a simplified anchoring system is believed to have independent inventive significance.

A float according to the invention can have many different constructive embodiments. Thus, the column part can advantageously consist of a number of tightly arranged columns, a design which, for example, could be particularly relevant for a float that is to be used as a loading buoy. Particularly in this connection, the float according to the invention can have such a design that the submerged floating part is incorporated into the column part. The deck part can also in and of itself be severely restricted and be limited to forming a top part of the column part. A float according to the invention can thus be thought of as realized as a maritime construction where, in terms of appearance, the individual float parts cannot be separated from each other.

The invention will now be explained in more detail with reference to the drawings, where: Fig. 1 shows the installation of a steel column on a

concrete column,

fig. 2 shows a perspective view of a possible

embodiment of a float according to the invention, fig. 3 shows another possible embodiment of a float

according to the invention,

fig. 4 shows a partially sectional view of a

steel column as used in the float in fig. 3, fig. 5 shows in an elevation the steel column in fig. 4

placed on the underlying concrete column, fig. 6 shows an enlarged section from fig. 5,

taken from the concrete/steel interaction area,

and

fig. 7 shows a corresponding section of a

modified version.

1 fig. 1 shows the upper end part of a concrete column 1. This concrete column 1 forms part of a float and projects as shown up through the water surface 2. A steel column 3 is shown lifted into place above the concrete column 1 by means of two crane barges 4 ,5.

The combined column 1,3 can, for example, be included as one of the elements in the float shown in fig. 2. The float in fig.

2 is of a type where the submerged float part is incorporated into the column part, or vice versa, and there is thus no clear distinction between the submerged float part 6 and the float's column part 7. A cover part 8 is indicated by dashed lines. This deck part can have many different designs and can also be almost vanishingly small, for example for a loading buoy only be in the form of a helicopter platform or a suitable top end of the pillar part.

As shown, the float is built up with tightly arranged columns 1,9,10 and 11. The concrete part is cast as a continuous structure, up to a height above the water surface 2, and then continues upwards as steel columns 3,12,13 and 14. The concrete part lines /steel is designated by 15,16 and 17.

Such a floater can be built using two separate construction sites, one for the concrete part and one for the steel part. The steel columns The steel columns can almost be completed before the installation on the concrete columns (fig. 1). Thus, each steel column can be prepared with all tires for various mechanical equipment, and necessary equipment can also be placed in the steel columns before assembly in the float. The one in fig. 2 slack anchored floats will be able to access their part of the anchoring system once the steel columns are in place. That is, the float in fig. 2, for example, in its equipped steel columns 3 and 13 can have in this case the necessary anchoring winches 18,19, so that the indicated anchoring can be quickly established with the help of the slack anchoring cables 20-23. From fig.

2, it appears that the anchoring system can be realized using only two diametrically arranged steel columns in the column section, namely the steel columns 3 and 13. There is no condition that, as in fig. 2 All columns have finishing steel parts. Thus, for example, if it is considered useful or appropriate, the steel columns 12 and 14 can be omitted, and thus the concrete columns 9 and 11 can be terminated at the dividing line 16, or possibly higher or lower than this dividing line. Such a column group can of course also have a larger or smaller number of separate or more or less fused columns.

In fig. 3 shows another possible embodiment of a float according to the invention, here in the form of a tension rod platform. The float in fig. 3 has a submerged floating part 25 made of concrete, designed as a frame part (seen in the floor plan), with upstanding concrete columns 26,27,28 and 29 from each frame corner. The concrete columns 26-29 extend up through the water surface 30, to a level 32,33,34,35. From there, the individual column continues as steel column 36,37,38 and 39. The steel columns carry carriers/trusses 40 for carrying deck modules (not shown) and connecting the columns.

As mentioned, the float in fig. 3 a tension rod platform. The necessary tension rods are indicated at 41,42,43 and 44, and handling/tensioning equipment for the tension cables is arranged in the respective steel columns. This equipment is in fig. 3 denoted by 45,46,47 and 48. The connection between the tie rods and the float is not shown in more detail.

A typical steel column, as used in floats in fig. 3, is shown partially cut through in fig. 4. As can be seen from fig. 3 is the deck part's support structure 40 so that the bearing of the deck part's modules (not shown) will be eccentric in relation to the center lines of the floating columns. Therefore, the steel columns in this case have a special design, in that a strength bulkhead 50 is extended from the periphery of the column and a bulkhead 51 is introduced parallel to this under the carrier system 40 (Fig. 3). Likewise, two parallel bulkheads 52,53 have been introduced between the pairs of bulkheads 50,51. These structural strength parts will primarily act as pressure and torque distribution elements, from the support system 40 into the steel column. At the same time, these parallel bulkheads can be used as storage tanks for e.g. water or diesel oil, as they will be able to be designed with a significant internal storage volume.

Incidentally, it can be seen from fig. 4 that the required number of steel decks 54,55 can be built into the steel column.

In fig. 5 the parting line between concrete and steel is shown, and fig. 6 and 7 show a respective detail of a possible interaction connection between concrete and steel, the sections being taken from the indicated section area 56 in fig. 5. In fig. 5, the concrete column is denoted by 27 (see also fig. 3), and the steel column is denoted by 37 (see also fig. 3).

The interaction connection, which is shown in detail for two possible embodiments in Figs. 6 and 7, includes a thick steel plate 57 which is placed on top of and continuously around the upper part of the concrete column 37. Beneath the steel plate are welded cast iron bolts or other types of bolts 58, which are cast into the concrete. The number and size of these bolts will depend on the tension/compression forces occurring. A shear plate 59 is welded in between the bolts continuously around the circumference. This has the threefold function of absorbing and transmitting horizontal shear forces, securing against water leakage and, moreover, as it consists of so-called bulb-flat steel, it will also be able to absorb and distribute vertical compression/tension forces. In fig. 7, the connection is shown with an alternative design and the bolts have been replaced with two bulb-flat steel 60.

It would be an absolute advantage in terms of leakage protection to have the bulb flat steel and steel plate be a continuous welded connection around the entire circumference, but it would present a handling technical problem to have the elements installed as a continuous ring. Prefabrication should therefore be carried out in sectors suitable for installation purposes, as the sectors are assembled and welded at a suitable height above the concrete edge, temporarily suspended for example in hoists. After welding, the ring can be lowered into a final, precisely adjusted (leveled) position. The steel or top plate 57 can have holes at suitable intervals for (possibly epoxy-based) concrete injection. It will of course be possible with other installation procedures to achieve circular continuity. The steel column 37 has, as can be seen from fig. 5, a somewhat smaller diameter than the concrete column 27. Partly this difference has a strength-related (concrete technological) function, but it will also provide a spacious installation tolerance for the steel column in relation to the very strict building tolerances that are normally set.

The parting line between concrete and steel in the column should ideally be positioned at a reasonable, but nevertheless the shortest possible distance upwards from an elevation where the stresses due to the compressive loads from the deck section have reached a low, essentially constant level. This elevation can be calculated, assuming that the compressive stresses are spread downwards in the cylindrical steel shell of the steel column in a fan shape. Starting from an assumed pressure distribution sector, and using generally known formulas for compressive and tensile stresses as a function of a point load and the thickness of the cylindrical steel shell in the steel column, it will be possible to set up a stress distribution diagram that shows that both compressive stresses and the induced tensile stresses from eccentricity moments will go from an asymptotically infinite value at the point of attack of the load to a low, constant level at a distance from the column top and further down. For a column top diameter of 25 m, the mentioned distance will be approximately equal to the diameter. Another requirement that should be met is that the dividing line is laid at a suitable height above the construction water line, for example approx. 5 m above, as such a location will provide reasonable opportunities for periodic inspection and maintenance, which will be a great advantage, because it is appropriate that the entire steel column is accessible for inspection and maintenance, even if it is assumed that the concrete/steel connection is leak-proof, taking into account that a float can have a specified life expectancy of up to 50 years.

The invention makes use of the concrete version's advantages in terms of robustness, weight and corrosion resistance in the underwater, lower parts, i.e. for the part of the float that is below the water surface, in combination with the elasticity/plasticity of the steel and its consequent well-documented ability to equalize stress and distribution, in all parts above the water surface. The stability and general movement characteristics are improved because the material center of gravity is lowered as much as possible. You can also make full use of the advantage of using two construction sites, with the particular advantage that the steel parts can be fully equipped before establishing the connection with the concrete structure.

The significant and concentrated static and dynamic loads between deck part and column part will be distributed over a larger, favorable area, with a very beneficial reduction of the stress level and a favorable steel/concrete interaction.

Claims (6)

1. Floating, with a submerged floating part (6;25) of concrete, a column part (7;26,27,28,29) including one or more concrete columns from the floating part up through the water surface, and a deck part (8;40) of steel, supported of the column part above the water surface, as the mentioned one or the mentioned number of concrete columns (1,9,10,11;26,27, 28,29) is extended above the water surface as a hollow, non-equipable steel column (3,12,13,14 ;36,37, 38,39) up to the deck section, characterized by the parting line (15,16,17;32, 33,34,35;56) concrete/steel in the column being at a distance in the size range of 20-30 m from the deck bearing (point of attack of the load) where the stress concentrations from the point loads of the deck bearing (point of attack of the loads) are distributed along the shell of the steel column to a low and relatively even level, and at a distance of around 5 m above the calculated waterline. 2. Float according to claim 1, characterized in that winches (18,19;45,46,47,48) in the float's anchoring system (20,21,22,23;41,42,43,44) are placed in one or more of the said steel columns (3,13;36,37,38,39). 3. Floating according to one of the preceding claims, characterized in that it includes two steel columns arranged diametrically opposite each other in the column part (3,13;36,38; 37,39). 4. Floats according to one of the preceding claims, characterized in that the column part consists of a number of tightly arranged columns (1,9,10,11). 5. Floating according to one of the preceding claims, characterized in that the submerged floating part (6) is incorporated into the column part (7) (fig. 2). 6. Floating according to one of the preceding claims, characterized in that the interaction connection in the parting lines between steel and concrete includes a horizontal annular steel plate (57) and an annular shear plate (59) from the steel plate down into the concrete.