EP0238535B1

EP0238535B1 - Slug-catcher that can be pigged

Info

Publication number: EP0238535B1
Application number: EP86905447A
Authority: EP
Inventors: Harald Are Asheim
Original assignee: STIFTELSEN FOR INDUSTRIELL OG TEKNISK FORSKNING VED NTH (SINTEF)
Current assignee: STIFTELSEN FOR INDUSTRIELL OG TEKNISK FORSKNING VED NTH (SINTEF)
Priority date: 1985-09-18
Filing date: 1986-09-17
Publication date: 1989-04-26
Also published as: NO157467B; NO157467C; DE3663054D1; NO853656L; EP0238535A1; WO1987001759A1

Abstract

A device for gathering liquid slugs in a pipeline transporting gas (slug-catcher), particularly in the context of the recovery of offshore oil and gas deposits. The pipeline leads along the seabed to an intake unit above the water usually a platform. The pipeline also is linked to a secondary riser (14) which has a smaller cross-section than the cross-section of the main pipeline (12). The main pipeline (12) leads to a loop (15) along the seabed from the junction so that the length is sufficient to accommodate a slug and retard it. The riser (14) is equipped with a regulatory throttle (22) which can provide counter-pressure to empty liquid into the main pipeline.

Description

The invention concerns a slug-catcher of the type described in the introduction to patent claim 1.
A slug is an accumulation of liquid which blocks the cross-section of a pipeline that is mainly filled with gas. Slugs are propelled by the difference in gas pressure at the two edges. Given specific conditions, slugs can contain a large amount of liquid and/or reach high velocities.
Slug-catchers have been developed to prevent slugs entering intake units such as processing plants. These devices are designed to divide the liquid and the gas so that only the gas is permitted to enter the intake unit directly. In addition, the slug has to be retarded to prevent a liquid impact which can cause mechanical stress. A further consideration is that it is desirable to maintain a constant flow of gas into the intake unit.
Slug-catchers are known which are based on the so-called « finger principle (e. g. US-A-4 519 815) where gas and liquid are divided by directing them into a number of parallel branch pipes. This type of device requires a great deal of space and is unsatisfactory for subsea operations. Another difficulty is that it is impossible to clean the unit using pigs, i. e. bodies which are led along a pipe. A second type of slug-catcher exists, this is based on separation in a tank (e. g. US-A-4 160 652) where the capacity of the tank is dimensioned so as to accomodate the largest slug volume that can be expected.
Though this type of slug-catcher requires less space than the first type, regulating the liquid level is an extremely demanding task.
Yet another design for slug-catchers takes the form of relief piping, i.e. an extra loop of pipe. These are frequently used with pigs and assume that incoming slugs are led into the relief pipe by means of a valve.
All existing slug-catchers have one thing in common, the volume must be larger than the slug volume which is the basis for the dimensions. If larger slugs are expected, the slug-catchers must be commensurately larger. This can represent a problem on platforms with limitations upon weight and size. It is therefore desirable to be able to place slug-catchers on the seabed. However, problems with installing the equipment and servicing the valves and regulatory units underwater make this difficult. Internal maintenance work such as the removal of solids also becomes difficult when the equipment is located in deep water.
The main object of the invention is to provide a slug-catcher that functions satisfactorily under water, too and that can be installed under water using normal pipe-laying techniques. A specific object is providing a slug-catcher which can be mounted on the seabed in connection with an intake unit placed above the water, such as on a platform. Such a slug-catcher should have no moveable parts under water, as well as being reliable and easy to maintain.
The invention can be realized by designing the slug-catcher in accordance with the characterizing part of patent claim 1. Such a slug-catcher is constructed in a straight forward manner and can be situated on the seabed and connected to a pipeline from the platform or well. Increasing the size to accomodate large slugs is not problematic and maintenance can be done easily. The slug-catcher according to this invention can be pigged, i. e. cleaned by a pig.
The slug-catcher according to the invention operates as follows :

When a slug passes the secondary riser, a small amount of the liquid will be pressed upwards whilst most will continue along the main loop. After the slug has passed this branching point or junction, the secondary riser will be emptied in that some of the liquid will fall back into the main pipeline and the rest will be forced up into the tank. When there is an open secondary riser, the gas pressure behind the slug is released, this will lead to the slug being retarded by the flow friction in the main loop.

Other advantageous features in the invention are stated in the subclaims.
The invention will now be described more closely by reference to the drawings, where :

Fig. 1 shows a schematic perspective view of an offshore platform which is linked to a slug-catcher designed in accordance with the invention whilst
Fig. 2 is a side view showing the principle of the slug-catcher designed in accordance with the invention.

In fig. 1 a platform 11 is located in an offshore area with a foundation on the seabed. The main pipeline 12 from a remote platform or wellhead (not illustrated) is designed to carry gas to an intake unit on the platform. There is a junction 13 on the main pipe with the outlet of a riser 14 which can have a considerably smaller diameter than the main pipe. The riser 14, which will be called the secondary riser, could for instance have a cross-section which is only 1/4 of that of the main pipeline.
The main pipeline 12 continues from the junction 13 to a pipe loop 15 on the seabed. The pipe loop 15 ensures that there is enough volume to gather liquid from a slug. This loop 15 continues into a riser 16 at the foundations of the platform. This riser 16 which henceforth is referred to as the main riser is led up to the intake unit on the platform.
Fig. 2 is a schematic presentation of the design of the intake unit on the platform 11. The two risers, 14 and 16, are led into a separation tank 17. The separation tank has a liquid drainage tube 18, the drainage being controlled by a valve 19. At the top edge of the separation tank 17 there is an outlet pipe 20 for gas. The pressure in the separation tank 17 is maintained by a valve 21 to regulate pressure in the outlet pipe 20.
When a slug goes past the junction 13, the liquid from the slug will rise up the secondary riser 14. This creates a height potential that prevents anymore liquid entering the secondary riser. The height in the riser will depend on the pressure conditions, the size of the slug and the flow velocity of the slug. The height of slow slugs (3m/s) will be insignificant. Rapid slugs (10 m/s or faster) can produce heights which are 20-30 % of the length of the slug. The amount of liquid which is drawn off equals the product of the height risen and the cross-sectional area of the secondary riser. This can be reduced by reducing the diameter of the latter.
Once the slug has passed the junction 13 the liquid volume which has entered the secondary riser 14 will be emptied, partly by its running back into the main pipeline and also by being pressed through a throttle valve 22 which links the secondary riser 14 to the separation tank 17.
When the secondary riser 14 is emptied of liquids, the difference between the pressure at the front and back of the slug will be evened out by means of the separation tank 17. The kinetic energy of the slug will be reduced by the flow friction until the slug is halted. The main pipe loop 15 can be emptied of any liquid accumulations by regulating the throttle valve 22 so that there will be enough pressure to drive the liquid up along the main riser 16 into the separation tank 17. Under normal conditions, the throttle valve 22 can be regulated so that the pressure it creates compensates for the loss of pressure due to friction in the pipe loop 15 and the main riser 16. This means that the loop 15 will be self- emptying.
A float in the separation tank 17 controls the opening of the valve 19 so that a reasonably constant liquid level is retained in the tank 17. The length of the pipe loop 15 is selected so that apart from accomodating the dimensions of the slugs, sufficient retardation of the said slug is achieved when pressure is equalized. The height of the risers 14, 16 is determined by the height of the platform above the seabed. A height of 150 m will be adequate with almost all conceivable slug sizes. Should this height not be enough, the counter-pressure to limit the entry of liquid in the secondary riser 14 can be increased by adjusting the throttle valve 22.

Claims

1. A device for gathering liquid slugs in a pipeline transporting gas, in the context of the recovery of offshore oil and gas deposits, where the pipeline that on the seabed leads to a topside intake terminal, usually a platform is led into a separation tank, characterized in that the pipeline is linked to a secondary riser (14) which has a cross-section that can be considerably less than the cross-section of the main pipeline (12), the main pipeline being designed so that the volume between the junction (13) and the separation tank (17) is considerably greater than the volume of. the largest slug which can be expected, and that the secondary riser leads to the separation tank through a throttle.

2. The slug-catcher in accordance with claim 1, characterized in that the main pipeline (12) is formed in a loop (15) along the seabed from the junction in a way that the volume in the loop is large enough to accomodate a liquid slug.

3. The slug-catcher in accordance with claim 1 or 2, characterized in that a cross-section in the secondary riser (14) is considerably less than the cross-section of the main pipeline (12) so as to reduce the amount of liquid which is drawn away.

4. The slug-catcher in accordance with one of the claims 1-3, characterized in that a riser (14) is equipped with a regulatory throttle (22) which can be set at a pressure of at least 10 bars to empty the liquid from the main loop (15).

5. The slug-catcher in accordance with one of the claims 1-4, characterized in that a secondary riser (14) is essentially in an almost vertical position.

6. The slug-catcher in accordance with one of the claims 1-5, characterized in that the main pipeline (12), the loop for gathering liquid (15) and the main riser (16) all have the same cross-section so that they can be pigged.

7. The slug-catcher in accordance with one of the claims 4-6, characterized in that a throttle valve (22) on the secondary riser_'(14) can be employed to compensate for any loss of pressure in the loop (15) and the main riser (16), thus ensuring a balanced flow between these areas and the secondary riser (14).

8. A slug-catcher in accordance with one of the claims 4-7, characterized in that the throttle valve (22) is used to limit the entry of liquid into the secondary riser (14), in cases where the height of the secondary riser (14) is insufficient by itself.