GB2118904A

GB2118904A - Offshore structure

Info

Publication number: GB2118904A
Application number: GB08310249A
Authority: GB
Inventors: Seiya Yamashita; Tsuneto Sasaki
Original assignee: IHI Corp
Current assignee: IHI Corp
Priority date: 1982-04-20
Filing date: 1983-04-15
Publication date: 1983-11-09
Also published as: NO160069B; SE8302157L; FR2525176B1; FR2525176A1; NO160069C; SE8302157D0; NO831347L; GB2118904B; GB8310249D0

Abstract

A semisubmersible offshore structure, e.g. for use as an offshore base to exploit and drill oil and/or gas reservoirs under the seabed or used as an offshore terminal to load, and unload cargoes, has buoyant column(s) or leg(s) 1 for supporting a topside provided with a constricted part 2 which is submerged even in the survival condition under bad weather. The constricted part is effective in suppressing wave forces. Either side of the part are tapered surfaces 3, 4 leading to wider cross-sectional areas. Alternatively, the wider parts are cylindrical or the constricted part is provided by recesses in the column. <IMAGE>

Description

SPECIFICATION Offshore structure The present invention relates generally to an offshore structure, more particularly but not exclusively a semisubmersible offshore structure for use as an offshore base to exploit and drill oil and/or gas reservoirs under the seabed or for use as an offshore terminal to load and unload cargoes.

Recently, semisubmersible offshore platforms have been increased in number in order to exploit and drill new submarine oil and/or gas reservoirs.

These semisubmersible offshore structures or platforms are exposed to waves, and the resulting, notably vertical external forces excited by the waves (hereinafter referred to as wave force). When the wave force increases due to bad weather conditions, movement, e.g. heaving, can increase so much that operation must be stopped.

That is, the operations carried out on or by the offshore structures can be adversely affected by weather conditions. In addition, anchoring systems of the offshore structures have to have strength enough to withstand strong wave force under extreme weather conditions.

Natural gas produced at gas fields is usually liquefied and temporarily stored there for subsequent shipment by LNG (Liquefied Natural Gas) vessels to various consumer regions. So far the natural gas recovered from the submarine reserviors has been transported through pipelines to liquefying and storage stations on the ground and then loaded into LNG vessels for shipment, which is very expensive in initial cost especially because of pipelines which must be laid. In addition, a large LNG storage area is required and must be communicated through further piping to a loading port.

In order to overcome these and other problems, there has been an increasing demand for offshore platforms or structures for liquefying, storing and loading LNG in situ, as described above, but due to wave force the operation of such offshore bases also depends upon weather conditions.

A primary object of the present invention is therefore to decrease the wave forces acting on offshore structures and to provide an offshore semisubmersible structure or platform which may be operable longer than conventional ones, preferably using anchoring systems small in capacity and ensuring a high degree of safety.

According to one aspect of the present invention an offshore structure comprises a column adapted to support a work station, the column having a constricted section of reduced cross-sectional area spaced from and below said work station. The column preferably has an upper section of greater cross-sectional area than the said constricted section located above said constricted section. The said upper section preferably links the said constricted section to the work station but may itself on its upper surface provide said work station. In a particularly preferred embodiment the said column comprises upper and lower sections of greater crosssectional area than said constricted section located respectively above and below said constricted section. The said upper section may be frustoconical or have a frustoconical portion extending from said constricted section.Said lower section may further be frustoconical or have a frustoconical portion extending from said constricted section.

The said upper section and/or said lower section and/or said constricted section may each be rectilinear or cylindrical or in cross-section. The column may be substantially rectilinear in crosssection and the constricted section be provided by laterally disposed recesses in sides thereof. The said structure may comprise a plurality of said columns or may comprise a single such colum in conjunction with other columns or supports not as defined above.

The or each column preferably connects the work station to one or more ballast tank.

Alternatively the lower section of the or each said column when present may act itself as a ballast tank. The or each column may be provided with internal space for storage for example of liquefied natural gas, petroleum gas or petroleum.

Structures embodying the present invention may be produced by modification of existing structures having support columns or legs of substantially uniform cross-section to which has been added an upper section of increased crosssectional area, the original column below said upper section thereby providing said restricted section. Said restricted section may extend longitudinally (i.e. vertically with the structure in its normal operating position) between said upper and lower sections (when said lower section is present) or between said upper section and a ballast tank when no lower section is provided, or said constricted section may be a substantially planar junction between two connected frustocones.

The said constricted section in whatever form it appears is located such as to permit it to remain submerged in all normal operating conditions up to and including survival conditions under extremely bad weather. For this purpose the deballasting of the structure to enable it to be moved from site to site is not considered a normal working condition.

The ballast tank may be provided in the form of a lower hull, a footing of the lower section itself or a mat.

The above defined and other objects, effects and features of the present invention will become more apparent from the following description, by way of example, of some preferred embodiments thereof taken in conjunction with the accompanying drawings, in which: Figure 1 shows a column or buoyant leg support of an offshore structure or platform according to the invention Figure2 is a graph used for explanation of the wave force exerted on the column of Figure 1; Figures 3(a), 3(b), 3(c) and 3(d) show further forms of a column or buoyant leg support of an offshore structure or platform according to the invention; Figure 4 is a perspective view structure of the present invention; Figure 5 is a perspective view of a further structure of the present invention;; Figure 6 shows tension leg type anchoring means for a structure of the type shown in Figure 5; and Figure 7 shows a perspective view of a further structure of the present invention.

Referring to the drawings, the most important feature of the present invention resides in the structure of buoyant columns or legs which when submerged are adapted to support topsides such as upper hulls or work stations. Therefore referring first to Figure 1, a buoyant column or leg in accordance with the present invention will be described.

The buoyant column or leg as shown in Figure 1 and generally designated by the reference numeral 1 comprises an inverted truncated cone 3 forming part of the upper section, a constricted section 2 in the form of a cylinder or port 5 and a lower truncated cone 4 forming part of the lower section, all of which are securely joined to each other as shown. It is the constricted part 2 that can considerably reduce wave force exerted to the buoyant column or leg 1.

Figure 2 shows a graph based upon the experimental results as well as the results obtained by the calculation in accordance with the three-dimensional source method (See Journal of the Society of Naval Architects of Japan, Vol. 148, 1980). That is, wave forces exerted on the buoyant column or leg 1 are plotted. The dimensions (in metres) of the model used in experiments and the X- and Y-axes are as follows:

D D1 Dz T ID . D2 T 0.2 0.4 0.08 0.6 where D, D1 and D2 are diameters as shown in Figure 1; and T is the draft.

And X- and Y-axes are defined as follows: 02 D X= . ~ g 2 and

where C9: the frequency of incident wave; gA the amp!itude of incident wave; g: the gravitational acceleration; F: the wave force; p: the fluid density; and rr: the ratio of a circumference of a circle to its diameter Two incident waves with the wave height 2go=0.04 m and 0.08 m were used in the experiments and experimental results were plotted, the results of the former being indicated by o while the results of the latter, by 0. The calculated results are indicated by the smoothed curved line.

Twill be seen from Figure 2 that the buoyant column or leg 1 as shown in Figure 1 is substantially free from the wave force at two wave frequencies. The wave force can be considered to be the sum of two components, the force acting upward and the force acting downward. A frequency at which both these forces are balanced is a frequency at which the wave force becomes zero. The force acting downward is mainly the result of the pressure of waves acting on the inclined surface of the truncated cone 4 (the upper side of the lower bulbous section), and the existence of the constricted part 2 does not have much influence over the full frequency range.On the other hand, the force acting upward results mainly from the wave pressure acting on the inclined surface of the inverted truncated cone 3 (the lower side of the upper bulbous section) and the bottom or underside of the truncated cone 4.

In a low frequency range (long in wavelength), the influence of the constricted part 2 on the wave force is small, but at high frequencies (short in wavelength), the force acting on the inclined surface of the inverted truncated cone 3 becomes relatively large, letting the constricted part 2 demonstrate the effect of increasing the upward acting force considerably. The reason is that when the wavelength is short, the wave pressure is large at and adjacent the free surface, and decreases with the increase in depth.

From the above, let us consider the curves of the upward and downward acting forces as a function of frequencies. First of all, a trend can be revealed that just because of the existence of the lower bulbous part irrespective of the existence of the cylindrical constricted part 2, both the curves cross once at a certain frequency, and that if the cylindrical constricted part 2 is present, the curves cross again at a higher frequency due to the above mentioned effect of the constricted part 2.

It should be noted that columns or buoyant legs as shown in Figures 3(a), 3(b), 3(c) and 3(d) have substantially the same waveforce suppresion effect as the column or buoyant leg 1 as shown in Figure 1.

Offshore structures with buoyant columns or legs designed and constructed based upon the above-explained underlying principle of the present invention will now be described.

First referring to Figure 4, an offshore platform comprises a work station topside or upper hull 5 upon which are installed drilling rigs, cranes and the like equipment (not shown), supported by parallel rows of buoyant columns or legs 1 and 1' which in turn are supported by parallel lower hulls 7 each in the form of a ship hull.

Each of the lower hulls 7 functions as a ballast tank and by controlled flooding of the lower hulls 7, the draft of the offshore structure can be controlled depending upon whether it is in operative condition, in inoperative condition because of storms or the like, or is being towed or moving. The adjacent buoyant columns or legs 1 or 1' on opposite hulls 7 are interconnected with cross or reinforcing members 8.

When this offshore structure is towed to an installation or working site, the lower hulls 7 are deballasted so that only they are submerged. The offshore structure may also be moved by for example propulsion systems (not shown) installed in the lower hulls 7. At an installation or working site, the lower hulls are flooded so that the upper hull 6 may be positioned at a required predetermined height above the sea level and the offshore structure is then anchored by anchoring means (not shown). In the cases of bad weather conditions or storms, the lower hulls 7 are again deballasted so that the upper hull 6 is higher than the crest of maximum wave expected, whereby damage to or vibrations or oscillations of the upper hull 6 can be avoided.The constricted section 2 is located such that even in the case of survival conditions, with the upper hull 6 very high above normal flotation level, it is completely submerged.

As described above, the buoyant columns or legs 1 and 1' are provided with the constricted parts 2 which have the advantageous effect of suppressing wave forces on the structure when submerged. That is, the inverted truncated conical surface immediately above the constricted part 2 has the function as described above in conjunction with the inverted truncated cone 3 as shown in Figure 1 while the truncated conical surface immediately below the constricted part 2 and above lower hull 7 has the function as described above in conjunction with the lower truncated cone 4 as shown in Figure 1.

In a further embodiment of the invention, the advantages of wave force suppression can be brought to an existing offshore structure of the type having lower hulls and a substantially uniform column by adding a cylinder on a part of the column adjacent to the water surface, to form a column for example as shown in Figure 3(b). In this way the embodiment shown in Figure 4 may be modified as described below. The upper cylindrical part and the upper inverted truncated cone 3 are replaced by a large cylindrical part of substantially uniform diameter and the constricted part 2 is extended and directly joined to the lower hull 7, thus eliminating the lower truncated cone 4. In this modification, the lower hull 7 has the function of the lower truncated cone 4 as shown in Figure 1.

In Figure 5 a further embodiment of the present invention is shown in which each column or leg 1 has a disc-shaped footing 9 which functions as a ballast tank so that the lower hulls 7 as shown in Figure 4 are eliminated. Controlled flooding of the footings 9 with the sea water controls the draft of the whole structure depending upon whether it is in operative or working position, in inoperative position in case of bad weather or storm or if it is being moved.

Furthermore, as is the case of the embodiment of Figure 4, the buoyant columns or legs 1 are interconnected with cross or reinforcing members 8 and the constricted parts 2 are completely submerged in the case of bad weather or storm as well as in the case of operative condition.

When the offshore structure of the type described is moved to an installation site, the footings 9 are deballasted so that only they are submerged. At an installation site, the footings 9 are flooded in such a way that the upper hull is posited at a required predetermined height above the sea level and securely anchored through anchoring means 10 in the form of anchors and cables. In the case of bad weather or a storm, the footings 9 are suitably deballasted so that the upper hull 6 may be positioned higher than the crest of a maximum wave expected, whereby damage to and vibrations of the upper hull 6 by waves may be avoided. This embodiment has also the constricted parts 2 so that wave forces can be considerably reduced.

In Figure 6 is shown an alternative anchoring means 10 in the form of tension legs for anchoring the offshore structure. Such anchoring means is very effective in handling extremely variable loads due to behaviour of waves which other anchoring means cannot handle, and can substantially prevent heave of the offshore structure because it pulls the floating offshore structure down so far it never goes slack even in the trough of the maximum wave expected.

In Figure 7 is shown a third embodiment of the present invention; that is, an offshore loading station or a floating plant. The upper hull 6 upon which is installed for instance a gas liquefaction plant or the like (not shown) is supported by a main column 11 and four corner columns 12 all of which are supported by a mat 13 which has a function of a ballast tank. This structure is securely anchored by suitable anchoring means (not shown).

The main column 11 has the constricted part 2 and is hollow so that it may store a liquefied gas.

Flooding of the mat 13 is controlled to maintain the draft of the offshore structure at a predetermined level regardless of the quantity of the liquefied gas stored in the main column 11. In the case of bad weather or a storm, the mat 13 is deballasted so that the upper hull 6 may be positioned higher than the crest of the maximum wave expected. Therefore, damage to and vibrations of the upper hull 6 by waves can be avoided.

Since the main column 11 is also provided with the constricted part 2, wave forces exerted to the offshore structure can be considerably reduced. This embodiment can be used as a structure or a platform on which a plant or the like is mounted.

In this embodiment, instead of the liquefied gas, petroleum, petroleum gas or the like may be stored in the hollow main column 11.

Claims

1. An offshore structure comprising a column adapted to support a work station, the column having a constricted section of reduced crosssectional area spaced from and below said work station.

2. A structure as claimed in Claim 1 wherein said column comprises an upper section of greater cross-sectional area than said constricted section located above said constricted section.

3. A structure as claimed in Claim 1 or Claim 2 wherein said column comprises upper and lower sections of greater cross-sectional area than said constricted section located above and below said constricted section.

4. A structure as claimed in any of Claims 1 to 3 wherein said upper section is frusto-conical or has a frusto-conical portion extending from said constricted section.

5. A structure as claimed in any of Claims 1 to 4 wherein said lower section is frusto-conical or has a frusto-conical portion extending from said constricted section.

6. A structure as claimed in any of Claims 1 to 5 wherein the upper section is rectilinear or cylindrical in cross-section.

7. A structure as claimed in any of Claims 1 to 6 wherein the constricted section is rectilinear or cylindrical in cross-section.

8. A structure as claimed in any of Claims 1 to 7 wherein the lower section is rectilinear or cylindrical in cross-section.

9. A structure as claimed in any of Claims 1 to 3 wherein said column is substantially rectilinear in cross-section and wherein said constricted section is provided by laterally disposed recesses therein.

10. A structure as claimed in any of the preceding claims comprising a plurality of said columns.

11. A structure as claimed in any of the preceding claims wherein the or each column connects the work station to one or more ballast tanks.

12. A structure as claimed in any of Claims 3 to 10 wherein said the or each lower section act as a ballast tank.

13. A structure as claimed in any of the preceding claims wherein the or each said column has space therein for storage.

14. An offshore structure as claimed in Claim 1 and substantially as described herein with reference to Figure 1, Figure 3(a), 3(b), 3(c) or 3(d), Figure 4, Figure 5, Figure 6 or Figure 7 of the accompanying drawings.

1 5. A column for an offshore structure as claimed in any of the preceding claims comprising an upper section and a constricted section of lower cross-sectionai area than the said upper section.

1 6. A column for an offshore structure as claimed in Claim 1 5 and substantially as described herein with reference to Figure 1, Figures 3(a), 3(b), 3(c) or 3(d), Figure 4, Figure 5, Figure 6 or Figure 7 of the accompanying drawings.