GB2254063A

GB2254063A - Method for transporting fluids.

Info

Publication number: GB2254063A
Application number: GB9203744A
Authority: GB
Inventors: David Harrison
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1991-03-25
Filing date: 1992-02-21
Publication date: 1992-09-30
Also published as: GB9203744D0; NO921059D0; FR2674608A1; NO921059L; GB9106297D0

Abstract

A method of transporting a fluid through a body of liquid involves the use of a collapsible hose 7. The fluid is initially compressed 2 to a pressure equal to or slightly above ambient and transported by the hose to a position of shallower depth. During transportation the pressure of the fluid is maintained equal to or slightly above the ambient while the hose rises in the liquid. The method is particularly suitable for the transportation of gas through seawater. For longer distances, the fluid is periodically recompressed and transported from a lower depth to a higher depth e.g. through hose (14). The hose can be laid along a rising seabed, Fig 2 (not shown). <IMAGE>

Description

METHOD FOR TRANSPORTING FLUIDS The present invention relates to a method for transporting a fluid through a body of liquid, in particular to a method for transporting gas through seawater.

It is often necessary to transport crude oil or gas from one facility to another often over long distances. On land conventional pipeline techniques may be used. At sea in shallow waters seabed laid rigid pipelines are used. Alternatively long distance transportation by liquifaction and transport by LNG tanker is the normal method for gas or by VLCC in the case of crude oil.

With rigid pipelines difficulties may be encountered with traversing trenches on the seabed. The use of tankers for transportation requires expensive onshore and shipping facilities.

In addition, with the more recent offshore developments there is the need to transport gas, often co-produced with oil, from deep water offshore facilities resulting in the requirement of pipelines having to bear large deep water pressures.

We have now found that by passing fluids through a collapsible hose at pressures equal to or slightly above ambient they may be transported at depth over long distances through seawater.

Thus according to the present invention there is provided a method for transporting a fluid through a body of liquid, said method comprising: (a) supplying compressed fluid to a first position at a pressure equal to or slightly above the ambient pressure of said liquid, and (b) passing said fluid through a collapsible hose to a second position, situated at a shallower depth than the first position, while said hose rises in the body of liquid to maintain the pressure of said fluid equal to or slightly above the ambient pressure of said liquid.

The method of the present invention is particularly suitable for the transportation of gas through a body of seawater.

The present invention exploits the density difference between the fluid for example oil, gas or mixtures thereof and the liquid for example seawater to enable the fluid to rise naturally to the surface.

Suitable collapsible hoses for use in the method of the present invention are those with a working pressure, relative to ambient, of between 2 and 25 bar.

Particularly suitable is layflat hose, for example fire hose.

The hose may be of a suitable working diameter depending upon the volume of fluid to be transported, for example from 15 cms to 60 cms.

A typical hose construction would have an impermeable liner compatible with the pumped fluid, a reinforcement, for example Kevlar, steel wire, nylon or polyester and a tough water resistant outer cover, for example polyurethane.

Alternatively the collapsible hose may be formed from a series of hinged metallic membranes.

The fluid is initially compressed to a pressure equal to or slightly above that of the ambient pressure of the liquid and within the working pressure of the collapsible hose.

For example in water at a depth of 1000 m the ambient pressure is approximately 103 bar and a hose rated at 5 bar internal pressure relative to ambient may be used to transport fluid at an absolute pressure of between 103 and 108 bar.

The fluid may be compressed at a compressor station positioned at a mid water depth. This may be at the same depth as the start of the collapsible hose or preferably may be at a shallower depth whereby the compressed fluid is then discharged downwards by means of a riser to the start of the hose.

If the fluid to be transported is gas an electrically driven compressor may be used.

The fluid may be transported to the compressor station from an offshore pipeline or production facility by conventional means.

The compressor station may be anchored or dynamically positioned.

The compressor station may typically be located at a depth of approximtely 1000 m with the collapsible hose positioned between 1000-2000 m of water linked to the station if necessary by a riser.

The riser may be of one piece or of jointed construction for ease of assembly and installation.

In a typical situation wherein the fluid is a gas, the gas flows along the collapsible hose which is connected to a second station situated at a shallower depth. This station may be linked to a receiving facility or may be another compressor station if further transportation is requied.

As the gas flows along the hose the pressure falls due to friction losses. Thus over a 100 km line the internal pressure may fall to 100 bar. To ensure transport of gas through the hose and to maintain the hose in equilibrium with the external pressure the hose is required to rise in the water for example from 2000 m to a depth of 1000 m over this distance.

The pressure of the gas is balanced against the external ambient pressure and is thus maintained at or slightly above the ambient pressure to maintain this equilibrium.

The hose may be operated fully or partially inflated over part or all of its length. As the hose pressure falls relative to ambient the hose is squashed and its cross-section is reduced. The gas pressure thus rises in the hose and the hose cross-section is re-established.

As the hose reaches shallower depths and the pressure in the hose falls a gas or multiphase fluid will expand. In such circumstances it may be advantageous to increase the nominal diameter of the hose.

To maintain the hose in position it may be buoyed and weighted in a predetermined form. For example surface buoys may be utilised linked together by means of a caternary structure.

The hose may also be partially suspended off a cable to take some of the tensile loads.

When full of gas the hose and weights may be arranged to be approximately neutrally buoyant.

Alternatively the hose may be suspended from or attached to a conventional pipeline system.

On shutdown the hose will collapse due to the high external pressures. It may be necessary to have some buoyancy and suspension from the surface to maintain the hose position.

In the event of failure the hose will collapse due to the loss of pressure. When the compressors are shut down the hose will collapse and further leakage will not occur and thus reduce the possibility of environmental damage.

If the fluid to be transported is crude oil the compressors may be replaced by electrically driven pumps.

The method of the present invention may also be used offshore to transport well fluids for example oil, gas and water to a production facility which may be a jacket, tension leg platform or a moored vessel.

For example at a wellhead depth of 2000 m the produced well fluids would be pressurized to approximately 205 bar and transported by collapsible hose to the vicinity of the surface for further processing.

In this way a number of subsea wells may be connected by collapsible hoses to the surface. For example at a depth of approximately 50m below the surface the well fluids may enter a vertical separator which may form part of an associated floating production system.

It is envisaged that in this situation a field of diameter between 10 to 20 kms may be exploited using collapsible hoses according to the method of the present invention.

Fluids may also be transported from the seabed to a coastal facility by using the predominantly upward gradient of the sea-bed to guide and support the hose to maintain the hose in position. At a suitable depth the fluids may be brought to onshore facilities by conventional rigid pipeline.

Typically, oil produced from subsea wells flows to a surface facility via conventional towers or risers. Following separation the oil is exported by shuttle tanker or pipeline. The associated gas is compressed and exported to the seabed via rigid riser. The gas then flows via a collapsible hose according to the method of the present invention. The hose may be fully or partially inflated and laid along the upward gradient of the seabed. At a convenient depth the collapsible hose may join a conventional rigid pipeline system which may form part of an existing platform system.

The collapsible hose operates at a low pressure relative to ambient enabling relatively cheap layflat hose to be used rather than high pressure pipeline.

The use of collapsible hose has a significant advantage over conventional steel pipelines as it can easily and quickly be laid in deepwater for example by reeling the collapsible hose and weighting chain off a barge.

The method of the present invention may also be used to transport fluids over greater distances.

Thus according to another aspect of the present invention there is provided a method for transporting a fluid through a body of liquid said method comprising: (a) supplying compressed fluid to a first position at a pressure equal to or slightly above the ambient pressure of said liquid, (b) passing said fluid through a collapsible hose to a second position, situated at a shallower depth than the first position, while said hose rises in said body of liquid to maintain the pressure of said fluid equal to or slightly above the ambient pressure of said liquid, (c) recompressing said fluid to a pressure equal to or slightly above the ambient pressure of said liquid at a third position of greater depth than the second position, (d) directing said fluid downwards to said third position, and (e) repeating steps (b), (c) and (d) as required to complete the transportation of said fluid.

The fluid transportation system may be suspended in mid-water and thus does not have to bear the crushing loads of pipelines situated in very deep water.

The system may be employed in any water depth thus ocean trenches inaccessible to rigid pipelines may be crossed.

The present invention will be further illustrated with reference to the accompanying drawings which represent diagramatic views of the transportation of gas through seawater using the method of the present invention.

In Figure 1 gas from an offshore production facility is transported via pipeline (1) to compressor station (2) situated at a depth of approximately 500 m. The station is connected to a buoyant surface power unit (3) by means of the riser (4). The power unit comprises a gas turbine or other gas driven prime mover which drives a generator, the gas turbine fuel being obtained from the gas in the pipeline. The unit may also contain a battery back up supply and monitoring, control and communication equipment. The riser (4) contains an electic power cable, gas supply line and a control communication cable.

The compressor station (2) comprises a compressor and an electrical drive motor unit.

The gas from line (1) is compressed to approximately 105 bar and exported downwards through the rigid riser (5) to position A situated at a depth of approximately 1000 m. The riser (5) may be of one piece or of jointed construction for ease of assembly.

The compressor station and riser assembly may be positioned if necessary by anchoring at (6) or they may be dynamically positioned.

The gas flows from position A to position B along the collapsible hose (7). The hose may be maintained in position by a series of lines (8) attached to surface buoys (9). Additional support may be given by the weights (10) or the hose may be suspended off a cable (11). Alternatively the hose may be maintained in position by lines (15) attached to the seabed.

The gas arrives at position B with a pressure of approximately 55 bar. If further transportation is required the gas is recompressed at B by means of the compressor station (12), exported downwards by riser (13) to position C and transported onwards through hose (14).

Repeating the cycle allows gas to be transported over long distances irrespective of water depth.

In Figure 2 gas is transported from a seabed location (20) to an onshore facility (21) by means of both a collapsible hose (22) and a rigid pipeline (23).

The gas is introduced at slightly above seabed pressure at (20) by means of a rigid riser (24) linked to a compressor station (25) positioned on the surface. The gas flows along the collapsible hose which may be weighted to the seabed to avoid rising under the buoyancy effect of the gas. This may be by means of sand filled bags (26) or alternatively a steel cable or heavy chain may be used.

The natural gradient of the sea bed supports the hose and assists in maintaining the hose in position. The hose trends generally in the upward direction allowing for small downward contours or trenches (28) to be traversed. Upon reaching a convenient position (27) the gas may be transported to onshore facilities by means of the rigid pipeline (23) which may form part of an existing pipeline system.

Alternatively the gas may be delivered to an existing offshore facility for example a platform (29) where it may be processed and/or further compressed before being introduced into the existing pipeline system (23).

Claims

Claims:

1. A method for transporting a fluid through a body of liquid by means of a collapsible hose comprising: (a) supplying compressed fluid to a first position at a pressure equal to or slightly above the ambient pressure of said liquid, and (b) passing said fluid through said collapsible hose to a second position situated at a shallower depth than the first position while said hose rises in the body of liquid thereby maintaining the pressure of said fluid equal to or slightly above the ambient pressure of said liquid.

2. A method according to claim 1 wherein the fluid is compressed at a shallower depth than said first position and directed downwards to said first position.

3. A method according to either of the preceding claims wherein the collapsible hose has a working pressure relative to ambient in the range 2 to 25 bar.

4. A method according to claim 3 wherein the collapsible hose is layflat hose.

5. A method according to any of the preceding claims wherein the collapsible hose is fully or partially inflated over part or all of its length.

6. A method according to any of the preceding claims wherein the collapsible hose is maintained in position by external means.

7. A method according to claim 6 wherein the external means are provided by buoys, chains or cables.

8. A method according to claim 6 wherein the external means are provided by the predominately upward gradient of a seabed.

9. A method for transporting a fluid through a body of liquid comprising: (a) supplying compressed fluid to a first position at a pressure equal to or slightly above the ambient pressure of said liquid, (b) passing said fluid through a collapsible hose to a second position situated at a shallower depth than the first position while said hose rises in the body of liquid thereby maintaining the pressure of said fluid equal to or slightly above the ambient pressure of said liquid, (c) recompressing said fluid to a pressure equal to or slightly above the ambient pressure of said liquid at a third position of greater depth than the second position, (d) directing said compressed fluid downwards to said third position, and (e) repeating steps (b), (c) and (d) as required to complete the transportation of said fluid.

10. A method according to any of the preceding claims wherein the fluid is gas and the liquid is seawater.

11. A method for transporting a fluid through a body of liquid as hereinbefore described and with reference to the accompanying drawings.