WO2003031865A1

WO2003031865A1 - Installation of subsea pipelines

Info

Publication number: WO2003031865A1
Application number: PCT/EP2002/010585
Authority: WO
Inventors: Dominique Perinet Marquet; Michel Liogier
Original assignee: Stolt Offshore Sa
Priority date: 2001-10-02
Filing date: 2002-09-26
Publication date: 2003-04-17
Also published as: GB0123627D0

Abstract

There is disclosed improved method and apparatus for installation of deep water subsea pipelines, wherein a pipeline (100) is flooded from one end to drive a pig unit (104) within it along its whole length by the action of hydrostatic pressure acting upon the pig, and without the need for any additional field equipment. A length of pipeline in an initial state contains a gas (102) at a pressure below the ambient hydrostatic pressure, and a pig unit sitting at an end having an entry point (106) for the flood water. The pipeline is flooded between both ends, whereby fluid is admitted progressively by operation of a flood valve (110) into the pipeline to drive the pig unit under the influence of the ambient pressure toward the far end, compressing the gas within the pipeline. An auxiliary chamber (400) is connected to the far end of the pipeline so as to accommodate the gas (102) in its compressed form substantially outside the pipeline itself.

Description

Installation of Subsea Pipelines

This invention relates to methods and apparatus for installation of deep water pipelines, wherein a pipeline is flooded to drive a "pig" along the pipeline by the action of hydrostatic pressure acting upon it.

Pipelines are generally fabricated and laid on the sea bed containing air or other gas at atmospheric pressure. Before connection to other parts of the subsea installation, these pipes are flooded. A "pig" may be driven along the pipeline to expel the air in an orderly fashion. The pig may also carry a gauge plate or other devices to detect anomalies in the bore of the pipe. Pipeline pigging is conventionally achieved by pumping. Extensive on-site equipment is required to support the use of pumps, including hydraulic, power and maintenance systems; all of which are costly and inconvenient to use.

UK patent GB 2303895 B proposes controlled flooding with seawater using the ambient hydrostatic pressure to transport the pig unit along the pipeline. Although this reduces the equipment and expense required, the air within the pipe can only be compressed to a finite volume, at which the pressure difference between water and air will be too close to zero to overcome the substantial friction between pig and pipe. Accordingly, a boost pump is still required in the system of GB 2303895 to drive the pig unit the last part of the way, and to expel the last of the air through a small vent outlet.

The invention aims to provide improved methods and apparatuses for implementing flooding and pigging of subsea installations, particularly to reduce even further the cost and complexity of the equipment required.

The invention provides a method of flooding a length of subsea pipeline between first and second ends, wherein the pipeline in an initial state contains a gas at a pressure below the ambient hydrostatic pressure and a pig unit at the first end, wherein fluid is admitted progressively into the first end of the pipeline to drive the pig unit under the influence of said ambient pressure toward the second end, compressing said gas, and wherein an auxiliary chamber is connected to the second end of the pipeline so as to accommodate the gas in its compressed form substantially outside the pipeline itself.

The provision of the auxiliary chamber, for example a simple tank, allows the pig to travel further than in the prior art, where the pressure of the compressed gas eventually stops the pig well short of the second end.

The volume of the auxiliary chamber may equal or exceed the volume of the contained gas when compressed, thereby permitting the pig to travel completely to the second end of the pipeline.

The auxiliary chamber may be in communication with the pipeline already before flooding commences. It may be coupled to the pipeline before or after laying on the seabed. The contained gas in pipeline and chamber will generally be air at atmospheric pressure, with or without protective additives.

There may be provided plural auxiliary chambers, all comiected at the second end or individually connected at intermediate points on the pipeline.

The method may further comprise recovering the pig. The pig may carry instrumentation for measurement of the pipe between the first and second ends.

The invention further provides an apparatus for use in a method as set forth above, the apparatus comprising a pipeline having first and second ends and containing a gas at a pressure substantially below an ambient hydrostatic pressure at which the pipeline is to be installed, inlet means for causing controlled flooding of the pipeline at the first end under said ambient hydrostatic pressure, and an auxiliary chamber connectable to the second end of the pipeline to receive and hold said gas outside the pipeline when flooded under the influence of said flooding hydrostatic pressure. The apparatus may further comprised a pig unit adapted to be drive from said first end to said second end within the pipeline by said flooding.

Before embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and description or illustrated in the drawings. The invention is capable of being carried out in various other ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of a known apparatus for controlled flooding of a gas-filled underwater pipeline to enable a pig to be transported along the pipeline. The apparatus is shown in an initial state, prior to flooding;

Figure 2 shows the apparatus of Figure 1 at the end of a free-flooding stage of operation;

Figure 3 shows the apparatus of Figures 1 and 2 at the end of a boost pumping stage of operation;

Figure 4 is a diagrammatic representation of an apparatus for controlled flooding of a gas-filled underwater pipeline in accordance with an embodiment of the present invention. The apparatus is shown in an initial state, prior to flooding; and

Figure 5 shows the apparatus of Figure 4 at the end of a free-flooding stage of operation, showing all gas within the pipeline effectively expelled from the pipeline into an auxiliary tank.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 is a diagrammatic representation of the apparatus known from GB 2303895. In its initial state a deep water pipeline 100, filled with gas 102, such as air, containing at its inlet end a pig unit 104, has connected to its inlet a means for flooding 106 the pipeline and a means for pumping liquid 108 into the pipeline. Flood and Pump Valves 110, 112 are applied to each means 106, 108 so that when one is in use the other is disabled. With both valves closed there is no hydrostatic pressure acting upon the rear of the pig unit 104, so it remains motionless at the start of the pipeline.

Figure 2 shows the known apparatus of Figure 1, but after the Flood Valve 110 has been opened and the pipeline 100 has filled with liquid by controlled free-flooding. The flow of liquid into the pipeline has been used to propel the pig unit 104 along the pipeline almost to the far (right hand) end. The driving force that propels the pig unit along the pipeline is the hydrostatic pressure difference between the liquid behind the pig unit and the gas in front of it. Various means are used to control the rate at which the pig unit travels along the pipeline, such as controlling the flow rate of liquid entering the pipeline. These details are not material to an understanding of the present invention, and will not be described further herein.

The hydrostatic pressure difference reduces to a point where there is insufficient pressure to propel the pig unit when the pressure of the liquid behind the pig largely matches the pressure of the gas that has been compressed on the other side. At this point, in 1000m of water for example, the volumetric ratio of liquid to gas in the pipeline is approximately 100:1. Therefore, as a portion of the pipeline remains filled with gas the pig unit cannot be propelled along the remaining 1% of pipeline. To remove the gas completely, in the known system, pumping is employed to increase the hydrostatic pressure behind the pig unit and venting, such as a check valve, is incorporated to release the gas in front of the pig unit, the subsequent pressure differential propelling the pig unit further along the pipeline. Figure 3 shows the same known apparatus, but after the pig unit 104 has been propelled further along the pipeline by the use of a boost pump 108 and check valve 114. The boost pump increases the pressure in the pipeline to reinstate a pressure difference between the rear and the front of the pig unit, and thus propelling the pig unit further along the pipeline. The pressure in front of the pig unit is substantially maintained at deep-water pressure by check valve 114, which releases the gas from the pipeline when the pressure of the gas within the pipeline exceeds the pressure of the surrounding deep-water into which the gas is expelled. The need for boost pump 108, however, adds substantial complexity to the apparatus, requiring complex support systems such as hydraulic, electrical and maintenance systems. In addition, the chances of failure exist, so that a parallel redundant system may also be required. Alternatively, a vessel-based pump must be brought and connected to complete the operation.

Figure 4 shows a novel apparatus for controlled flooding of a gas-filled underwater pipeline to transport a pig unit fully along the pipeline, but without the need for pumping. Shown in its initial state, the apparatus contains the same elements 100 to 110 as the known apparatus described earlier with reference to Figure 1, except for the boost pump 108 and associated valve 112. Connected to the far end of the pipeline, however, is an auxiliary tank 400, also containing gas at atmospheric pressure, or at least a pressure substantially less than that of the surrounding seawater.

Figure 5 shows the novel apparatus after Flood Valve 110 has been opened and pipeline 100 and optionally a portion of auxiliary tank 400 have filled with liquid. The pig and liquid can now be propelled to be stopped only by the end of the pipeline, before reaching any the point where the pressure difference between liquid and compressed gas within the pipeline and auxiliary tank becomes too small. The flow of liquid may optionally be permitted at this point to bypass the pig beyond the end of the pipeline, to complete the flooding and to partially flood the tank 400. In either case, complete flooding of the pipeline has been achieved to propel the pig unit 104 to the end of the pipeline, using only the hydrostatic pressure of the seawater and passive equipment. As a consequence, the improved apparatus and method for flooding and pigging deep water pipelines is more reliable, more cost effective and simpler to support in service. The tanks volume necessary is simply that sufficient to hold the original volume of gas, when compressed to the pressure of the water at the installation depth. With the example of 1000m depth provided earlier, this would amount to approximately 1% of the total volume of the pipeline 100 plus tank 400. The skilled person will readily be able to calculate the percentage volume, and hence the volume of the auxiliary tank, for different situations, by known properties of the gas and seawater, the hydrostatic pressure at the installation depth and so forth.

Those skilled in the art will appreciate that the embodiments described above are presented by way of example only, and that many further modifications and variations are possible within the spirit and scope of the invention.

Claims

1. A method of flooding a length of subsea pipeline between first and second ends, wherein said pipeline in an initial state contains a gas at a pressure below the ambient hydrostatic pressure and a pig unit at the first end, wherein fluid is admitted progressively into the first end of the pipeline to drive the pig unit under the influence of said ambient pressure toward the second end, compressing said gas, and wherein an auxiliary chamber is connected to the second end of the pipeline so as to accommodate the gas in its compressed form substantially outside the pipeline itself.

2. A method as claimed in claim 1, wherein said method further comprises the step of recovering said pig unit.

3. A method as claimed in claims 1 or 2, wherein said auxiliary chamber is coupled to the pipeline after laying said pipeline on the seabed.

4. A method as claimed in any of claims 1 to 3, wherein the initial volume of said auxiliary chamber equals or exceeds the volume of the contained gas when compressed, sufficient for the pig to be transported completely to the second end of the pipeline.

5. A method as claimed in any of claims 1 to 4, wherein said auxiliary chamber is coupled to said pipeline prior to flooding.

6. A method as claimed in any of claims 1 to 5, wherein said auxiliary chamber is coupled to the pipeline after laying on the seabed.

7. A method as claimed in any of claims 1 to 6, wherein said contained gas in pipeline and chamber comprises air.

8. A method as claimed in any of claims 1 to 7, wherein said contained gas in pipeline and chamber comprises a protective gas.

9. A method as claimed in any of claims 1 to 8, wherein said contained gas is held at atmospheric pressure.

10. Subsea pipeline apparatus comprising a pipeline having first and second ends and containing a gas at a pressure substantially below an ambient hydrostatic pressure at which the pipeline is to operate, inlet means for causing controlled flooding of the pipeline at the first end under said ambient hydrostatic pressure, and an auxiliary chamber connectable to the second end of the pipeline to receive and hold said gas outside the pipeline when flooded under the influence of said flooding hydrostatic pressure.

11. Apparatus as claimed in claim 10, wherein the initial volume of said auxiliary chamber equals or exceeds the volume of the contained gas when compressed, sufficient for the pig to travel completely to the second end of the pipeline.

12. Apparatus as claimed in claims 10 or 11, further comprising one or more auxiliary chambers, all connectable at said second end.

13. Apparatus as claimed in any of claims 10 or 11, further comprising plural auxiliary chambers, individually connectable at intermediate points along the pipeline.

14. Apparatus as claimed in any of claims 10 to 13, wherein the or each auxiliary chamber and said pipeline are inter-connected.

15. Apparatus as claimed in any of claims 10 to 14, wherein said contained gas in pipeline and chamber comprises air.

16. Apparatus as claimed in any of claims 10 to 15, wherein said contained gas in pipeline and chamber comprises a protective gas.

17. Apparatus as claimed in any of claims 10 to 16, wherein said contained gas is held at atmospheric pressure.

18. Apparatus as claimed in any of claims 10 to 17, further comprising a pig unit adapted to be driven from said first end to said second end within the pipeline by said flooding.

19. Apparatus as claimed in claim 18, wherein said pig unit carries instrumentation for measurement of the pipe between the first and second ends.

20. An apparatus substantially as hereinbefore described with reference to figures 4 and 5 of the accompanying drawings.

21. A method substantially as hereinbefore described with reference to figures 4 and 5 of the accompanying drawings.