GB2292349A

GB2292349A - Semi-submersible oil platform with double walls in its vertical columns

Info

Publication number: GB2292349A
Application number: GB9516390A
Authority: GB
Inventors: Bjorn Deildok; Oystein Rangnes; Trond Landbo
Original assignee: Kvaerner Concrete Construction AS
Current assignee: Kvaerner Concrete Construction AS
Priority date: 1994-08-15
Filing date: 1995-08-10
Publication date: 1996-02-21
Anticipated expiration: 2015-08-10
Also published as: NO943014D0; NO300921B1; NO943014L; GB2292349B; GB9516390D0

Abstract

A semi-submersible offshore platform, consisting of horizontal pontoons (P) with vertical columns (C) arranged in the pontoons' nodes (N), and a deck construction arranged on top of the columns (C), wherein the pontoons (P) and the nodes (N) are used for ballasting and the columns (C) are used for oil storage and may also be used for ballasting. The columns (C) have double walls, when the outer chambers (cofferdams) (C23, C24, C33, C34) constituting chambers available for ballasting with sea water. The inner chambers (C21, C31) constitute storage tanks for oil. The outer and inner chambers are non-communicating, thus avoiding zones with a mixture of oil and water in the tanks. <IMAGE>

Description

A semi-submersible oil platform with double walls in the columns The invention concerns a semi-submersible offshore platform, consisting of horizontal pontoons with vertical columns arranged in the pontoons nodes, wherein a deck construction is arranged on top of the columns, The pontoons and the nodes are used for ballasting and the columns are employed primarily for oil storage, but may also be used for ballasting.

It is known for oil storage tanks at sea to form a part of a concrete platform foundation which is permanently located on the seabed. In a construction of this kind some of the cells or chambers in the foundation are used as ballast tanks which can be filled with sea water. Some are used as storage tanks for oil. These are always 100% full of liquid with a combination of oil/sea water, and are in communication with the sea.

When the oil is pumped in the water is forced out, and vice versa. Movements in the liquids cause a layer consisting of oil emulsion to be formed between the water which is at the bottom and the oil which is at the top. If the storage tank is overfilled there will be a possibility of pumping produced oil into the sea, which both represents a safety hazard and will give rise to pollution.

Furthermore, the layer of emulsion together with a water buffer against the sea will cause the net oil storage capacity to be much less than the gross storage volume, which gives less flexibility in relation to the production rate and loading frequency.

Another disadvantage of combined oil and ballast tanks is that those parts of the measuring system which measure the amount of oil in each cell are located under water, which makes this equipment inaccessible for maintenance and replacement. Requirements regarding safety and accessibility mean that in manv cases maintenance or replacement which may have to be carried out will result in a shut-down of at least parts of the production system, which is most undesirable for economic reasons.

It should also be noted that the known constructions, where there is only one single wall against the sea, have the drawback that any leakage through the walls of the column means that oil passes directly into the sea. The possibility of a leakage cannot be ruled out, for example in the event of a collision with a boat, which will have consequences both for safety and pollution.

The object of the present invention is to provide a movable oil storage for use offshore where the risk of pumping oil into the sea due to overfilling of the storage is substantially reduced or completely eliminated. A further object is to increase the net oil storage capacity in relation to the gross storage volume. Yet another requirement is to provide a construction where all instrumentation for measuring the amount/level in the tanks is easily accessible for maintenance or replacement. An additional object of the invention is to provide a construction which is more reliable with regard to leakages, oil leakages being captured bv the drainage system instead of ending up in the sea.Finally it should also be mentioned that the aim of making the oil storages movable is to provide oil storages which are economically more profitable than present day fixed oil storages, especially with a view to small oilfields with an economically viable production life which is much shorter than the production life of an oil storage tank made of concrete.

The said objects of the invention are achieved by employing a semi-submersible offshore platform with double walls in the columns, characterized by the features specified in the patent claims.

By separating the oil storage tanks from the ballast tanks there will no longer be any emulsion layer between the water and the oil. The oil tanks are no longer open to the sea, but instead will have the pressure equalized during filling and emptying by being open to the atmosphere. In this manner the risk is eliminated of pumping oil into the sea by overfilling the tanks. The oil will be pumped by means of submerged pumps in the oil tanks. For reasons of safety the oil level in the storage tanks will be measured using a double set of instruments. They will be easily accessible for maintenance and possible replacement, and the filling and emptying will be controlled by a simple system consisting of a combination of manually and automatically operated valves which in practice will make it impossible to overfill the tanks.

The double walls will contribute to the strength of the platform foundation, while at the same time virtually eliminating the risk of oil spillage in the event of leakage through the outer walls of the columns, since the outer chambers which form the intermediate spaces in the double wall construction will never be filled with oil.

By removing the water from the oil tanks, not only can the emulsion layer be disregarded during the operation of the tanks, but it will also be possible to dispense with the water buffer, which is necessary in known constructions in order to avoid the risk of pumping oil into the sea. Both of these factors will contribute to an increase in the available storage volume, the net oil storage capacity thus being approximately equal to the gross storage volume.

The invention will now be described in more detail by means of the drawing which illustrates a preferred embodiment of the invention, and in which: fig. 1 is a plan view, and figs. 2 and 3 are two substantially similar side views of a semi-submersible platform foundation.

The platform foundation consists of 4 horizontal pontoons and 4 vertical columns, connected to the pontoons at the nodes. The foundation is hollow inside, and divided into a number of cells or chambers. Each chamber is labelled with a combination of a letter and a number.

The cells or chambers marked "P" are located in the pontoons and will only be used for ballasting. The chambers marked "N" are located at the nodes between the pontoons and the columns. N14, N24, N34 and N44 will be used as pump rooms with valves, while the remaining N chambers will be used for ballasting.

The chambers marked "C" (column) are located in the columns and are those of interest for the invention. C11, C2 1 C31 and C4 1 are the columns internal chambers, and are used for oil storage. C13, C14 C23, C24, C33, C34, C43 and C44 are the outer chambers and can be used for ballasting. These chambers are normally empty.

Pumps and piping arrangements, including valves and instruments, are not shown neither for oil transfer nor ballasting. Nor is the interplay between instruments and valves, and the logical and sequential control of these shown.

This is prior art, and can be designed by those skilled in the art.

During operation, filling and emptying of oil, together with ballasting and deballasting will be controlled from the platform's central control room. This will be done by operators with the aid of the platform's computer-controlled system for control and monitoring. Instruments for measuring the platform's tilt and draught will assist the operators, thus ensuring that the operations are conducted under proper conditions with regard to operation and safety.

Normally the ballast water in the pontoons and the nodes will be sufficient in order to achieve the desired draught. The outer chambers in the columns will therefore usually be empty.

It will be seen that many modifications are possible within the scope of the invention. For example, the platform foundation can have 3 pontoons and 3 columns. It will also be possible to give the pontoons a different cross sectional shape, for example in the form of a ring or a polygon. It will be possible to vary the number of columns from 1, which may be possible if the pontoon part of the construction is made relatively wide, thus making it substantially responsible for the stability, to a relatively large number, distributed over the pontoon construction.

It will also be seen that the shape of, and the relative proportions between the outer chambers and the inner chambers can be altered, for example by introducing cross walls, thus dividing the chambers up into several smaller chambers. These examples of modifications will all lie within the scope of the invention, since the essential feature of the invention, as mentioned above. is the double wall which separates the oil chambers from the ballast chambers.

Claims

PATENT CLAIMS

1. A semi-submersible offshore platform, consisting of horizontal pontoons with vertical columns arranged in the pontoon's nodes I, wherein a deck construction is arranged on top of the columns , the pontoons and the nodes are used principally for ballasting and the columns have double walls and are used primarily for oil storage, but may also be used for ballasting, wherein the columns' outer chambers (the cofferdams) constitute chambers available for ballasting with sea water, while the inner chambers constitute storage tanks for oil, the outer and inner chambers being noncommunicating, thus avoiding zones with a mixture of oil and water in the tanks, and both the outer and inner chambers are ventilated to the atmosphere.

2. A semi-submersible offshore platform according to claim 1, wherein the outer chambers constitute intermediate spaces in a double-hull construction extending around the entire circumference.

s. A semi-submersible offshore platform according to claim 1 or claim 2 characterized in that the inward-lacing chambers in the nodes are used as pump rooms.

4. A semi-submersible offshore platform substantially as herein described, with reference to figures 1 to 3.