GB2084529A

GB2084529A - Stabilizing element

Info

Publication number: GB2084529A
Application number: GB8129480A
Authority: GB
Original assignee: Akers Mek Verksted AS
Current assignee: Akers Mek Verksted AS
Priority date: 1980-10-01
Filing date: 1981-09-30
Publication date: 1982-04-15
Also published as: NL190546C; NL190546B; GB2084529B; NO147176B; DK153750C; DK153750B; SE8105780L; NO802912L; SE442736B; NL8104472A; NO147176C; DK431581A

Abstract

Semi-submersible platform structures for drilling for natural resources in pelagic areas may be equipped with so much fixed equipment at deck level that their stability becomes insufficient. In order to improve the stability, such a platform is provided with one or more stabilizing elements (6) which are placed aft on the platform. The stabilizing element (6) extends between a submerged buoyancy body (1) of the platform and its deck (4) and has a constant cross-section up to the deck (4). For example, it can comprise a cylinder (7) with a tapering lower portion (9). The connection between the lower portion (9) of the element (6) and the buoyancy body (1) can have a connection (13) which is arranged to rupture before the buoyancy body can be damaged should the stabilizing element (6) be subjected to strong collision forces. The stabilizing element (6) can be connected at the top to the deck (4) of the structure by means of a connecting link (8) which permits a certain resilient rotation about horizontal axes. <IMAGE>

Description

SPECIFICATION Stabilizing element This invention relates to a stabilizing element for a semi-submersible structure.

The present invention more particularly relates to a stabilizing element for a semi-submersible platform structure comprising horizontally extending buoyancy bodies and columns extending upwardly from said bodies and supporting the deck of the structure. The stabilizing element is elongate and connected in an upright position to one of the buoyancy bodies of the structure spaced from the columns.

Such platform structures are used for drilling for natural resources in pelagic areas. The platforms are typically equipped with various fixed equipment such as a drilling tower, drilling machinery, windlasses, crew accommodation etc.

The remaining part of the load carrying capability of the platform is used for equipment, the amount of which varies, such as drill piping, casing piping, drilling mud, cement, fuel etc.

However, experience has shown that such platforms often are provided with more fixed equipment than originally assumed and, furthermore, this additional equipment is often placed at deck level. This not only leads to a reduction of the capacity of the platform for variable payloads, but also reduces stability. It is therefore not uncommon to modify already existing platforms of this type in order to increase their stability. This is done by increasing the moment of inertia or the cross-sectional area of the water line cross section, for example by providing at least some of the columns of the platform with protrusions in the form of closed tanks or the like in the water line area.Such protrusions on the columns do, however, require relatively long installation time and require that the platform must be taken out of service for a correspondingly long period, increased cost being the result. It has further been realised that such protrusions on the columns require a relatively large amount of steel as compared to the additional cross-sectional area provided.

There are also known stabilizing elements in the form of buoy-like hollow bodies which are attached in the water line area by means of struts extending between the deck of the platform and buoyancy bodies or pontoons. Such buoys may also be pivotable to an extended position in order to give maximum efficiency when in use, while being capable of being swung out of the way during towing. It is further known to arrange such buoy-like stabilizing elements extending out from the fore corners of the platform structure, said elements being attached by means of struts extending at an angle to the upper and lower ends of the fore columns. Such stabilizing elements, however, are very prone to damage by collision with supply ships or the like, and under more unfortunate impact angles the columns may be damaged at the attachment points of the struts.

Thus, such stabilizing elements have had to be removed.

What is desirable is a stabilizing element of the type mentioned at the beginning which does not suffer from the above noted drawbacks and deficiencies. Furthermore, a stabilizing element should be capable of absorbing shocks without causing damage to the platform, and in addition, should provide reserve buoyancy in case part of the normal buoyancy of the platform should be lost.

According to the present invention there is provided, in, or for use in, a semi-submersible structure which includes one or more buoyancy bodies, a stabilizing element which is, or is to be, secured to a buoyancy body of the semisubmersible structure extending generally upwardly therefrom, and which has a constant cross-section from a normal water line up to the level of a deck of the semi-submersible structure.

Preferably, the stabilizing element is secured to a buoyancy body of a semi-submersible structure, which comprises a plurality of horizontallyextending buoyancy bodies, a plurality of columns extending upwardly from the buoyancy bodies and the deck supported on the columns.

Preferably, the stabilizing element is spaced apart from the columns.

Advantageously, a lower end of the stabilizing element is, or is to be, secured to a buoyancy body via a connecting member with the mechanical strength of a joint between the connecting member and the buoyancy body being greater than the mechanical strength of a joint between the stabilizing element and the connecting member.

Since the stabilizing element has a relatively large cross-section ail the way up to the deck level of the structure, it will provide a substantial additional buoyancy in case of heeiing of the structure and thus contribute to preventing the structure from capsizing should its normal buoyancy bodies be damaged. The large crosssection also provides sufficient stiffness in the stabilizing element for it to withstand relatively high impact forces without collapsing. Collision forces exceeding a certain limit will, however, lead to deformation and possibly bending of the stabilizing element, but since the element is spaced from the columns of the platform, such deformation will not damage the columns.The provision of a weaker portion at the lower end of the stabilizing element assures that this portion will be torn loose from the buoyancy body or pontoon without causing it damage in the event of particularly strong collisions. According to an advantageous embodiment of the invention, the stabilizing element has a downwardly tapering portion which ends near the connecting member when present. This facilitates the making of a connection between the buoyancy body and the stabilizing element, which is to transmit forces in three orthogonal directions, but not appreciable moments. Furthermore, the tapering portion of the stabilizing element will give less buoyancy and therefore reduce the need for compensating ballast.

At the upper end, the stabilizing element can advantageously be connected to the deck of the platform structure by means of a connecting link which is arranged so as only to transmit substantially horizontal forces. Thereby the upper end of the element can rotate to a certain extent and thus provide a better resilient effect during possible collisions. By letting the connecting link be formed generally like a plate, one obtains a relatively simple, economical and quickly installable attachment between the platform structure and the upper end of the stabilizing element.

Advantageously, the connecting member between the buoyancy body and the lower end of the stabilizing element comprises a base which is intended for welding to both horizontal and vertical portions of the buoyancy body. This provides substantial security against fatigue and laminar tearing of the plate material of the buoyancy body after prolonged use, and concurrently one will be more certain that the stabilizing element will break where it is supposed to during a possible strong collision without damaging the buoyancy body.

By placing the stabilizing element behind a rearmost column extending up from the respective buoyancy bodies, one will in many cases succeed in giving the platform structure maximum longitudinal stability. Furthermore, the reserve buoyancy of the stabilizing element will come to its best advantage since the aft portions of the buoyancy bodies often contain the machine and pump room which necessarily are big and air filled and which, if they should be filled with water, could lead to a dangerous reduction in the buoyancy and stability of the platform.

For a better understanding of the invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a side view of part of a semisubmersible platform structure provided with a stabilizing element according to the present invention; Figure 2 shows a plan view of a portion of the platform structure of Figure 1; and Figure 3 shows perspectively and on a larger scale, a partial section of the connection between a lower end of the stabilizing element and the platform structure.

The semi-submersible platform structure shown in the drawing comprises two elongate, parallel buoyancy bodies in the form of pontoons 1, only one being shown in Figure 2. Columns 2, 3 extend up from the pontoon 1 and support a deck 4 of the platform. Only the aft portion of the platform is shown in the figures.

A tapering part 5 of the pontoon 1 lying aft of the column 2 contains the machine room of the platform. A stabilizing element according to the invention generally designated 6, extends upwardly from the machine room. The stabilizing element 6 has a cylindrical portion 7 which comprises 4/5 of its length and extends up to the level of the deck 4 of the platform and is connected to the deck 4 by means of a plate-like, horizontally arranged connecting link 8. The lower 1/5 of the stabilizing element is comprised by a conically tapering portion 9 which at its lower end is welded to a connecting member formed as base 10, which in turn is welded to the tapering part 5 containing the machine room.

Further details of the design of the base 1 0 and the connection between the base 10 and the conical portion 9 of the element 6 are shown in Figure 3. It will be seen that the base 10 comprises ribs 11, 12 which are welded to both the horizontal and vertical portions of the tapering part 5, thus providing a strong and stable anchorage for the base 10.

The lower end of the conical portion 9 of the stabilizing element 6 has a substantially smaller diameter than the cylindrical portion 7 of the element. A connection 1 3 between this lower end and the base 10 will thus not have any appreciable ability to resist bending loads as compared to the remainder of the element 6. The connection 1 3 is primarily designed to transmit forces in the directions of the three orthogonal axes.Thus, buoyancy forces and limited collision forces that might act on the element 6 will be transmitted to the base 1 0. Should the stabilizing element 6 be subjected to collision forces exceeding a predetermined limit, however, the connection 13 will rupture without the attachment of the base 10 to the tapering part 5 suffering damage, since the mechanical strength of the joint or connection between the base 10 and the tapering part 5 of pontoon 1 is greater than the mechanical strength of the joint or connection 13 between the stabilizing element 6 and the base 10.

The plate-like connecting link 8 at the upper end of the element 6 has no appreciable ability to transmit vertical forces, nor moments about horizontal axes. The strength in the horizontal plane is high, however, and the connecting link 8 will therefore effectively hold the upper end of the element 6 away from the column 2 during a possibie collision with the element 6 while a resilient rotation of the end is permitted. Since the stabilizing element 6 is spaced a certain distance from the column 2, the element 6 can be subjected to bending in the middle without coming into damaging contact with the column 2.

The stabilizing element 6 is furthermore provided with suitable buffers 14 in the lower part of the cylindrical portion, i.e. in the normal water line area, in order to prevent damage during weak collisions.

It will be understood that the stabilizing element 6 according to the invention, including the connecting link 8 and the base 10, can be prefabricated. Thus, the installation can be performed relatively quickly, with a corresponding reduction in deadtime for the platform. It will also be understood that the form of the stabilizing element 6 can be varied within the frame of the following claims. Particularly, the form of the upper portion of the stabilizing element 6 will be the result of a balancing of the desired reserve buoyancy against the permissible steel weight.

Claims

1. In, or for use in, a semi-submersible structure which includes one or more buoyancy bodies, a stabilizing element which is, or is to be, secured to a buoyancy body of the semi-submersible structure extending generally upwardly therefrom, and which has a constant cross-section from a normal water line up to the level of a deck of the semi-submersible structure.

2. A stabilizing element as claimed in claim 1, wherein the stabilizing element is secured to a buoyancy body of a semi-submersible structure, which comprises a plurality of horizontallyextending buoyancy bodies, a plurality of columns extending upwardly from the buoyancy bodies and the deck supported on the columns.

3. A stabilizing element as claimed in claim 2, wherein the stabilizing element is spaced apart from the columns.

4. A stabilizing element as claimed in claim 1, 2 or 3, wherein a lower end of the stabilizing element is, or is to be, secured to a buoyancy body via a connecting member with the mechanical strength of a joint between the connecting member and the buoyancy body being greater than the mechanical strength of a joint between the stabilizing element and the connecting member.

5. A stabilizing element as claimed in claim 4, wherein the connecting member comprises a base arranged for welding to both horizontal and vertical portions of a respective buoyancy body.

6. A stabilizing element as claimed in any preceding claim, wherein a lower end of the stabilizing element has a downwardly tapering portion, which terminates at the connecting member, when present.

7. A stabilizing element as claimed in claim 2, or in claim 3, 4, 5 or 6 when appendant to claim 2, wherein the stabilizing element is placed behind the rearmost column.

8. A stabilizing element as claimed in claim 2 or 7, or in claim 3, 4, 5 or 6 when appendant to claim 2, wherein an upper end of the stabilizing element is connected to the deck of the structure by means of a connecting link which is arranged so as to transmit substantially horizontal forces only.

9. A stabilizing element as claimed in claim 8, wherein the connecting link is generally plateshaped.

10. A stabilizing element substantially as hereinbefore described with reference to, and as shown in the accompanying drawings.

11. A semi-submersible structure, to which a stabilizing element is secured, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.