GB2291090A

GB2291090A - Installation of an oil storage tank.

Info

Publication number: GB2291090A
Application number: GB9513759A
Authority: GB
Inventors: Morten Diesen; Lars Friisk
Original assignee: Kvaerner Concrete Construction AS
Current assignee: Kvaerner Concrete Construction AS
Priority date: 1994-07-13
Filing date: 1995-07-06
Publication date: 1996-01-17
Also published as: NO942635L; AU684955B2; NO305608B1; AU2493095A; GB9513759D0; NO942635D0

Abstract

A subsea construction with ballast tanks, typically a storage tank (1) for oil, is installed on the seabed by connecting the construction (1) to a buoy (5) and two or more tugboats (3) by means of wires (8) which are attached to the buoy and run from there to pulleys on the construction and on to the tugs. The construction is ballasted until it has a suitable weight, typically 50 tons, and is lowered to a depth at a suitable height above the seabed by paying out wires (8) from winches on the tugs. The construction is lowered the final distance to the seabed by ballasting the construction. <IMAGE>

Description

Installation of an oil storage tank The invention concerns a method for installation of a subsea construction with ballast tanks, typically a storage tank for oil, on the seabed, wherein the construction is ballasted until it floats at a suitable depth in the water, and is towed to the field by means of tugs.

It is a known procedure to instal constructions on the seabed by towing the construction to the position in which is has to be installed by means of one or more tugs, ballasting the construction and lowering it by means of crane ships and/or winches. The drawback with this method, however, is that it requires relatively large crane and/or winch capacity, which entails major expenses.

Moreover, substantial dynamic forces can be transferred both to the construction, the wires, the crane ship and/or the tugs, especially in bad weather conditions, thus increasing the capacity demands on these.

US patent specification No. 4 909 671 describes an installation of a construction on the seabed in which the construction is connected to a buoy, typically with wires, of a predetermined length. The construction is ballasted so that it is given a desired vertical sinking velocity, whereupon it will sink to a predetermined depth at a suitable height above the seabed, determined by the length of the wires and the buoyancy of the buoy. The final lowering is performed by ballasting the buoy, the subsea construction, or both.

US patent specification No. 5 215 410 describes an installation of a construction on the seabed in which weights are attached to the construction, these weights being suspended below the construction, preferably in the form of belts with weights. If the construction itself does not have built-in buoyancy tanks/ballast tanks, it is positioned below a buoyant member with such tanks. The construction is lowered by ballasting the buoyancy tanks, either in the actual construction or in the buoyant member. When the weights reach the bottom the forces from the weights on the construction will be reduced, thus stopping the lowering process and the construction is stabilized at a specific distance from the seabed. The final part of the submersion is performed by further ballasting, thus ensuring a steady and controlled lowering of the construction.

During the submersion of bodies in the water, the increase in the static pressure will cause the construction to be compressed, thus reducing the volume of the construction. According to Archimedes' law, which states that a body submerged in a fluid acquires a buoyancy which is equal to the weight of the volume of fluid it displaces, the buoyancy will consequently also be reduced. In most cases this can be disregarded, but during the installation of large constructions at great depths it is extremely pronounced, and can mean that the buoyancy of the construction is reduced bv several hundreds of tons as it is lowered to the bottom.

This means that installations based on the two above-mentioned U.S. patents will encounter problems in the installation of large constructions at great depths, since in both cases the construction's lowering rate will increase as the construction moves downwards. This can be counteracted by a continuous or gradual deballasting of the construction, which, however, is difficult to achieve in practice. A continuous deballasting of this kind will require constant connection of an umbilical, which will constitute a risk, and will probably make further measures necessary in order to maintain the safety of the operation.

With a submersion operation according to the prior art, represented by the patents mentioned in the introduction, the construction's lowering rate is exclusively a consequence of the balance between buoyancy and ballasting, which means that the movement can only be stopped by deballasting. Due to the inertia of the construction this will take a very long time or require a considerable amount of over-deballasting. If the deballasting continues right up until the construction stops, the construction will turn and move upwards.

The construction will therefore be very difficult to stop, as countercorrections will have to be carried out over and over again. Thus an active ballasting system is necessary, and an advanced control system which observes when the acceleration is turning, thus enabling the deballasting to be stopped in time.

Weather conditions will always be critical for offshore operations, including for the installation of subsea constructions according to known methods, where the entire operation is dependent on the fact that wave height and wind speed do not exceed stated maximum values.

The object of the present invention is to provide an economically beneficial method for the installation of a subsea construction on the seabed, wherein the use of crane ships is avoided and the capacity requirement for the tugs is reduced, while also avoiding the active use of the ballast system in temporary phases. The wish is to provide a method in which the construction is easy to position, and where great flexibility is obtained in the rigidity of the system, thus avoiding major dynamic stresses.

At the same time the object of the invention is to avoid the above-mentioned problems which will necessarily arise at great depths with a method according to the two U.S. patents, due to the fact that the construction's buoyancy is reduced during the lowering process.

A further object of the invention is to provide a method for installation of sub sea constructions offshore, in which there is less dependency on weather conditions in comparison with known methods.

The above-mentioned objects of the invention are achieved by using a combination of a buoy and tugs, characterized by the features indicated in the patent claims.

By using a buoy, and with the wires running from the buoy to the tugs via pulleys on the construction, the buoy which is positioned above the construction will be able to absorb vertical forces as great as those absorbed by the tugs, thereby relieving them of some of the strain. This means that the buoy reduces the tugboat capacity required with regard to vertical forces.

The buoy causes the forces to be centred at a point which lies vertically directly above the construction, thus achieving a simpler positioning than if only tugs had been used.

The buoy will act as a spring element, absorbing most of the tugs' heaving movement, thus preventing these movements from being transferred to the construction. In designing the buoy, account must be taken of rigidity and mass, in such a manner that the interplay of buoylboat results in as small dynamic loads in the wires as possible. In ballasting the buoy it will also be possible to optimize the dynamic properties with regard to the installation concerned. This means easier vertical manoeuvring and reduced dynamic stresses on the construction, wires and tugboats in comparison with a lowering process where crane ships or winches alone are employed.

The buoy will naturally be hollow inside, which means that it can easily be equipped with ballast tanks. The ballast tanks can be used for fine adjustment of vertical position during the final phase of the lowering of the construction, where the construction is placed on the bottom.

The buoy is reasonable to produce compared to the costs involved in extra tugboat capacity, and if the conditions are favourable the buoy can be employed in other similar marine operations, since it can be calibrated for the individual installation by means of pre-ballasting.

By means of a lowering process according to the present invention the construction will be ballasted before submersion by admitting water until it has reached a predetermined weight of, e.g., 25-50 tons. This starting weight is dependent on the extent to which the construction is compressed due to the pressure increase during lowering, as well as the density profile for sea water at the installation site.

If the ballasting of the construction is controlled by means of an umbilical, this will be disconnected and remain disconnected right up until the lowering is stopped at a suitable height above the seabed. Critical balance in the ballasting is avoided during installation, since the increased tension in the wires, due to reduced buoyancy as a result of compression of the construction, is absorbed by the buoy/tug.

If the weight of the construction after ballasting is, e.g., tons and the capacity of the winch is 100 tons, an effective braking power of 75 tons is obtained. The result of this is that the velocity of the construction can be braked at any time causing it to stop, which means that it is completely under control during the entire lowering process. Compared with the known methods, represented by the patents mentioned in the introduction, in which an active ballasting system with an advanced control system is required, a completely different degree of controllability is thus obtained during the lowering process.

It should also be mentioned that a method for installation of a subsea construction according to the invention will be less dependent on the weather conditions than known methods, the construction being capable of lying in a completely submerged state awaiting acceptable weather before lowering starts.

The above-mentioned factors indicate a substantial saving in costs compared with known methods, in which a buoy is not employed.

The invention will now be described in more detail by means of the drawing which illustrates the method according to the invention used during installation of a storage tank for oil on the seabed, and in which: fig. 1 is a plan view of the tank and three tugs, and fig. 2 is a side view of the tank with three tugs and the buoy, where the tank is located immediately below the surface, and fig. 3 is a side view of the tank with two tugs and the buoy, where the tank is installed on the seabed.

A typical area of application for the present invention will be a storage tank for oil which is installed on the seabed. Figure 1 illustrates such a storage tank 1 with six cells 2. The three cells in the middle have compressive strength and are used for ballasting, while the three external cells lack compressive strength, and are open to the water, thereby always having the same pressure as the water. In this context compressive strength or lack of compressive strength refer to whether the cells are designed to withstand the pressure difference which arises between the cells and the environment during the lowering of the tank due to the increase in the hydrostatic pressure.

Figure 1 shows two tugboats 3 which keep the storage tank 1 in place in the surface position by means of the wires 4. The buoy 5 is not connected here.

The wires pass directly from the tugs to the tank's wire fastenings, and will normally be the same wires which are used during the towing. A third tugboat 6 is in the process of ballasting the tank bv means of an umbilical 7.

Figure 2 shows the storage tank 1 15-20 metres below the surface, where it is suspended from the wires 8 which run from the tugboats 3 to pulleys on the tank 1, and on to the buoy 5. Here the tank has been ballasted to a weight of approximately 50 tons. The umbilical 7 is disconnected and will remain disconnected during the lowering process.

The actual lowering is performed by the tugs paying out wires, thus causing the tank to be lowered.

Figure 3 shows the storage tank in the process of being placed on the seabed.

As will be evident from the patent claims, this final part of the operation can be performed in several ways. If the wave motion is moderate, and the requirements with regard to positioning and lowering rate permit it, the final part of the lowering can also be performed by means of the wires. In the event of high waves, and if greater accuracy is required during the final phase of the installation, the final part of the lowering can be performed by deballasting the tank until the tension in the wires is reduced, thus stabilizing the construction by means of the balance between its specific weight, buoyancy and the forces from the buoy and the tugs. One is then faced by two possibilities: either to ballast the tank or to ballast the buoy, thus enabling the tank to sink slowly and in a controlled manner to the bottom.

Once it is installed on the bottom the ballasting valves are opened, resulting in an increase in the weight of the tank and causing it to sink into the seabed.

The wires are than removed.

It will be seen that many modifications are possible within the scope of the invention. For example the number of tugs can vary, the number and size of the tugs naturally being selected from what is the most favourable option economically for the installation concerned. If the buoy is also equipped with pulleys, it will be possible to replace one of the tugs with a fixed installation.

The wires will then be attached in the fixed installation, pass via a pulley on the subsea construction to the buoy, from there via a pulley down again to the subsea construction, and via a pulley to the tugboat's winch. This method could be feasible for an installation in the vicinity of an oil platform, where the use of a tugboat close to the platform can be undesirable for safety reasons.

The greatest possibility of variation will probably be associated with the buoy. The shape, size and possibility of ballasting of the buoy can be varied in a number of different ways. The most obvious shape for the buoy is a vertical cylindrical shape, where the spring constant will be linear, and dependent of the slenderness, i.e. the ratio between length and diameter. A conical form can also be envisaged, which will give a spring constant which increases progressively with the buoy's draught.

These examples of modifications will all lie within the scope of the invention, since, as mentioned above, the essential feature of the invention is the use of the buoy, in that it is connected to the subsea construction by means of wires, which via pulleys on the subsea construction pass on to the tugs.

Claims

PATENT CLAIMS

1. A method for installation of a subsea construction (1) with ballast tanks, typically a storage tank for oil, on the seabed, wherein the construction (1) is ballasted until it floats at a suitable depth in the water, and is towed to the field by means of tugboats (3), c h a r a c t e r i z e d by the following successive stages: a) that valves in the construction are opened for intake of sea water, thus supplying ballast to the construction until its upper section is located approximately on a level with the water line, whereupon the valves are closed; b) the construction (1) is connected to a buoy (5) and tugboats (3) or the like, by means of lowering wires (8) which are attached to the buoy and pass from there to pulleys on the construction and on to the tugboats; c) valves are opened for intake of sea water, thus supplying ballast to the construction until it has a suitable weight, typically 50 tons, and sinks to a suitable depth, typically 15 - 20 metres below the water line, where the weight of the construction is absorbed by the lowering wires (8), whereupon the valves are closed; d) the towing wires (4) are removed; e) the construction is lowered to a depth at a low height above the seabed, by paying out wires (8) from winches on the tugs; f) the construction is lowered the final distance to the seabed; g) valves are opened for intake of sea water in the ballast compartments, thus causing the construction to sink into the seabed whereupon the lowering wires (8) are removed.

2. A method for installation of a subsea construction on the seabed according to claim 1, c h a r a c t e r i z e d in that the wires (8) are attached to the buoy (5) by means of pulleys and that one or more tugboats (3), or other arrangements are employed. for example a platform without a winch, the subsea construction (1) being lowered by means of a continuous wire or wires which are passed from a tugboat or other construction on the surface without a winch, from there to the subsea construction, via a pulley to the buoy, via a pulley back to the subsea construction, an via a final pulley to a tugboat with winch.

3. A method for installation of a subsea construction on the seabed according to claim 1, c h a r a c t e r i z e d in that a buoy (5) is employed with a preferably oblong shape extending in the vertical direction, thus forming a spring element during the submersion of the subsea construction.

4. A method for installation of a subsea construction on the seabed according to claim 1, c h a r a c t e r i z e d in that the construction is lowered to a depth at a suitable height above the seabed, where it is deballasted in order to reduce the tension in the wires, and that the construction is stabilized by means of the balance between its specific weight, buoyancy and the forces from the buoy and the tugs.

5. A method for installation of a subsea construction on the seabed according to claim 4, c h a r a c t e r i z e d in that the final phase of the lowering down to the seabed is performed by ballasting the construction.

6. A method for installation of a subsea construction on the seabed according to claim 4, c h a r a c t e r i z e d in that the final phase of the lowering down to the seabed is performed bv ballasting the buov.