A pig is a device which forms a seal within a pipeline so that the pig may be driven down the pipeline by creating a pressure differential across the pig. Pigs are used for many purposes including inter alia to ensure that a newly laid pipeline, when flooded, has a"no air"content, and to check that any debris associated with the pipeline construction is removed from the inside of the pipeline. A pig is often used to carry out many forms of inspection down a pipeline.

Currently there are two known methods commonly used for pigging a subsea pipeline. The first is to pump water, or another suitable liquid or gas, from a vessel or platform at the sea surface, into the pipeline so as to increase the pressure behind the pig and thus create a pressure differential across the pig which propels the pig along the pipeline. This method can be employed where the pig is to be propelled along a newly laid pipeline, filled with air or gas, and also where a pig is to be propelled along a liquid-filled pipeline. The latter case may be applicable where, for example, an older pipeline, filled with liquid (e. g. water) is to be inspected for wear, damage, and/or blockages, or in the case where water has been permitted to enter the pipeline

during installation on the seabed. The main disadvantages of this method are the need for a pumping system, and the fact that significant support vessel time is needed at the sea surface while the pumping is carried out. Support vessel time is costly and an aim of any subsea construction process is to minimise the amount of support vessel time needed.

The second method is specifically for propelling a pig along an air (or gas) filled pipeline and utilises the external water pressure outside the pipeline to propel the pig partially along the length of the pipeline, from a proximal end thereof. The pipeline has inlet and outlet valves at proximal and distal ends thereof which control the flow of fluid in and out of the pipeline. Both valves are closed when the pipeline is installed on the seabed, the installed pipeline thus having air trapped therein. The afore-mentioned second method comprises maintaining the valve at the distal end of the pipeline closed while the valve at the other end of the pipeline is opened: water will enter the pipeline through the open valve and push the pig along the pipeline as the water compresses the air (which is at atmospheric pressure).

The pig continues to travel along the pipe until the air in front of it reaches a pressure equal to the water pressure behind the pig, which in the case of a generally horizontal pipeline would be the external or"ambient"water pressure at the proximal end of the pipeline. At this stage, to further propel the pig so that it reaches the distal end of the pipe, a surface based or subsea pumping system is employed to increase the pressure behind the pig so as to create the necessary pressure differential across the pig (with the valve at the distal end open) to drive it forward. This is not usually a simple procedure and again involves additional support vessel time at the sea surface and/or one or more

divers or remotely operated robots. These requirements introduce huge additional costs to the pipeline construction and pigging process. A further disadvantage is that this method only provides economically beneficial results when the pipeline is deep enough below the surface that a substantial portion of the pipeline is flooded under the action of the external head of water.

It is an aim of the present invention to avoid or minimise one or more of the foregoing disadvantages.

Accordingly, the present invention provides a method for propelling a pig along an underwater pipeline means having inlet means at a proximal end thereof and outlet means at a distal end thereof, and a pig disposed therein at least some distance away from said distal end, the method comprising the steps of opening said inlet means to permit entry of water to the pipeline means at a pressure corresponding generally to the ambient water pressure at said proximal end of the pipeline means, and coupling said outlet means of the pipeline means to a fluid venting system wherein fluid is maintained at a sufficiently low pressure to create a pressure differential across the pig sufficiently large to cause fluid to be vented from the distal end of the pipeline means, whereby the pig is propelled along the pipeline means towards said distal end thereof.

It will be appreciated that the pressure in the venting system as well as on the"outlet"side of the pig inside the pipeline means may be non-uniform. This is particularly the case where the venting system and said pipeline means (at least on the outlet side of the pig) contain water insofar as any such water itself creates a head of pressure thereby creating

pressure gradients. Accordingly, the pressure required to be maintained inside the fluid venting system to create the required pressure differential across the pig may be different in different portions of the fluid venting system, and may depend on the disposition, e. g. any upward or downward inclination, of the pipeline, as will become more clearly apparent from the description of various different embodiments hereinbelow.

An advantage of the present invention is that the pig can be continually propelled along the pipeline means by using the hydrostatic pressure from the head of sea water at the proximal (i. e. inlet) end of the pipeline means, this being achieved by ensuring that a sufficient pressure differential always exists across the pig, at least until the pig reaches a desired end position. Where the pipeline means is an air or gas filled one, there is no requirement for any pumping to create the necessary pressure differential across the pig.

According to another aspect the invention provides a method for propelling a pig along a substantially air-filled underwater pipeline means having inlet means at a proximal end thereof and outlet means at a distal end thereof, and a pig disposed therein at least some distance away from said distal end, the method comprising the steps of opening said inlet means to permit entry of water to the pipeline means at a pressure corresponding generally to the ambient water pressure at said proximal end of the pipeline means, and coupling said outlet means of the pipeline means to a fluid venting system wherein air is maintained at a pressure sufficiently below the ambient water pressure at said proximal end of the pipeline means to create a pressure differential across the pig sufficiently large to cause substantially all air in the

pipeline means, in front of the pig, to be vented from the distal end thereof, whereby the pig is propelled along the pipeline means towards said distal end thereof.

According to yet another aspect, the present invention provides a method for propelling a pig along an underwater pipeline means substantially filled with water and having inlet means at a proximal end thereof and outlet means at a distal end thereof, and a pig disposed therein at least some distance away from said distal end, the method comprising the steps of opening said inlet means to permit entry of water to the pipeline means at a pressure corresponding generally to the ambient water pressure at said proximal end of the pipeline means, and coupling said outlet means of the pipeline means to a fluid venting system wherein fluid is maintained at said pipeline outlet means at a pressure sufficiently below the ambient water pressure at said distal end of the pipeline means to create a pressure differential across the pig sufficiently large to cause water to be vented from said outlet means at the distal end of the pipeline means, whereby the pig is propelled along the pipeline means towards said distal end thereof.

It will be appreciated that where the pipeline contains other liquids e. g. oil, which have a different density to water (fresh water or sea water as appropriate), then the fluid venting system pressure used should take into account such differences in density and the effects thereof on pressure head (s) inside the pipeline and fluid venting system as further discussed hereinbelow.

The pressure differential created across the pig is preferably sufficiently large to overcome any energy losses due to

friction (between the pig and the pipeline means and/or between fluid and the pipeline and/or the inlet and/or outlet means of the pipeline means) as the pig is propelled along the pipeline means towards the distal end thereof.

Preferably, the pressure differential created across the pig is sufficiently large to cause the pig to be propelled along the pipeline means at a predetermined, average velocity, or within a predetermined velocity range.

The pipeline means may follow a substantially horizontal pathway or may be disposed at an angle (e. g. on a sloping seabed). Alternatively, only part of the length of the pipeline means may be disposed at an angle, or the pipeline means may follow an undulating pathway over an undulating seabed.

The inlet means preferably comprises an inlet valve having an open position in which water, or any other fluid, may flow into the proximal end of the pipeline means, and a closed position in which liquid cannot flow into the proximal end of the pipeline means. The outlet means preferably comprises an outlet valve which has a closed position in which fluid from the fluid venting system cannot flow into the distal end of the pipeline means, and an open position in which fluid contents of the pipeline means, namely air/gas or liquid, can flow out of the distal end of the pipeline means (into the fluid venting system).

Optionally, one or both of the inlet and outlet valves may be used to control the rate of flow of water into the pipeline and/or gas or liquid out of the pipeline.

For the avoidance of doubt it should be noted that the inlet means and outlet means referred to above relate generally to the direction of travel of the pig and not to the direction of flow of any fluid (s), for example hydrocarbons, which may flow through the pipeline means during its subsea use. Furthermore, said proximal end and/or said distal end of the pipeline means may be provided with entry means and/or exit means respectively through which the pig may be introduced or removed respectively from the pipeline means. It will be appreciated that said exit means, for example, may be provided in a removable pipeline end closure means, often referred to as a"pig receiver", which incorporates said outlet valve, or may alternatively be provided in a separate portion of said distal end of the pipeline means to said outlet valve.

Similarly, said entry means may be provided in a"pig launcher"at the proximal end of the pipeline means, incorporating said inlet valve, or may be provided in a separate portion of the proximal end of the pipeline means to said inlet valve.

Where the pipeline means is initially substantially filled with air at atmospheric pressure (or, alternatively, another gas having a lower density than the density of water), the method preferably comprises connecting the outlet valve of the pipeline means to a fluid venting system which comprises a conduit, preferably in the form of a hose, connected directly or indirectly to a surface or mid-depth buoy, the system including venting outlet means, conveniently in the form of a venting outlet in the buoy itself, via which air from the pipeline means may vent or discharge, into the surrounding water. The venting outlet means may alternatively be in one end of the hose. Preferably, the venting outlet means is disposed at a higher water level than the proximal (inlet

valve) end of the pipeline means. The pressure on one side of the pig (namely the side nearest the distal end of the pipeline) is thus always lower than the pressure on the other side (which, at any one time, is at the hydrostatic pressure in the pipeline, at the depth of the pig below the surface), creating a pressure differential across the pig. As long as the pressure differential is sufficiently large to overcome any friction losses, the pig will be driven along the pipeline means until it reaches the'distal end thereof. The higher the venting outlet means is above the proximal end of the pipeline means, the greater the pressure differential, the differential being greatest when the venting outlet means is at the surface. The advantage of a buoy disposed part of the way towards the surface, a so-called"mid-depth"buoy, is the avoidance of limitations associated with running a hose all the way from the seabed to the surface (where the pipeline is installed on the seabed). It will be appreciated that the upper end of the hose could, instead of being attached to a buoy, be supported on a platform or vessel at the sea surface, where this option is available.

The height of the venting outlet means above the proximal end of the pipeline means is advantageously chosen so as to achieve a desired rate of travel of the pig along the pipeline means under the action of the resulting pressure differential.

For example, where the venting outlet means is in the buoy, various different pig velocities may be achieved by varying the height of the buoy.

The inlet valve may be opened prior to opening the outlet valve, causing the pig to be propelled along the pipeline means until the air in the pipeline means, in front of the pig, is compressed to a pressure more or less equal to the

water pressure behind the pig. The outlet valve may subsequently be opened to connect the pipeline means to the venting outlet means, causing the pig to be driven the rest of the way along the pipeline means, to the distal end thereof.

This procedure tends to minimise the risk of back pressure from the venting outlet means-particularly in a mid-depth buoy-forcing water into the outlet end of the air-filled pipeline. Alternatively, and preferably, the venting outlet means is provided with a one-way valve mechanism whereby fluid from the pipeline means may only be vented when the pressure of the fluid from the pipeline means exceeds the ambient water pressure at the venting outlet means, and the inlet valve and the outlet valve are opened simultaneously, or the outlet valve is opened prior to opening the inlet valve. In these cases, the air in front of the pig is compressed until it reaches the ambient water pressure at the venting outlet means, at which point said one-way valve opens to allow fluid to be vented from the fluid venting system.

Where the pig is to be conveyed along a liquid-filled pipeline, the method preferably comprises connecting the outlet valve of the pipeline, by means of a conduit, preferably in the form of a hose, directly or indirectly to a surface or mid-depth buoy, either the conduit or the buoy having venting outlet means therein via which contents of the pipeline may discharge into the surrounding water, and introducing gas, which may be air, or alternatively another fluid having a density less than the density of water, into the hose connecting the outlet valve to the venting outlet means. The effect of introducing the air or other fluid into the hose creates a pressure differential between the proximal and distal ends of the pipeline, the pig located therebetween thus being propelled along the pipeline until it reaches the

distal end thereof. The hose, the buoy and the source of the air or other fluid which is introduced into the hose together comprise the fluid venting system. Advantageously, air is introduced into the hose by pumping, or otherwise injecting, pressurised air into the hose via an inlet, preferably a valved inlet, provided in the hose for this purpose.

Alternatively, the air may be pumped into the hose via a multiple valve arrangement comprising the outlet means of the pipeline, or by pumping the air directly into the pipeline via a suitable inlet provided therein for this purpose. The rate of travel of the pig along the pipeline can be controlled by the amount of air which is pumped into the hose. The air may be pumped into the hose by a pump on a surface support vessel or platform or, though perhaps less practical, by a subsea pump which compresses air. Alternatively, cylinders of pressurised air disposed on the seabed may provide the source of pressurised air. The latter arrangement avoids the need for a surface support vessel while pumping is taking place.

In some instances where the method of the invention is being used to propel a pig along a gas or air filled pipeline, the pig may not travel right to the distal end of the pipeline because some (sea) water has entered the portion of the pipeline in front of the pig (i. e. between the pig and the distal end of the pipeline). This may happen when some water from the near (inlet) end of the pipe squeezes past the sides of the pig and/or when water gets into the pipeline via the hose and buoy and/or the outlet valve. In this situation, the afore-described technique for propelling the pig along a liquid filled pipeline (involving introducing air or a lower- than-water-density fluid into the hose) may be used as a supplementary procedure for completing the journey of the pig to the distal end of the pipeline. This supplementary

procedure may be controlled automatically and, preferably, remotely e. g. from a surface vessel, platform, or subsea device.

The afore-described method (s) for propelling a pig along a liquid, namely water, filled pipeline may also be used after the method for propelling a pig down an air filled pipeline, in order to propel a second, third, or further, pig along the pipeline where these additional pigs are separated from one another, and from the first pig, by water or other liquid.

Although the latter method involving introducing compressed air or gas into the hose may involve the use of a pump (unless pressurised gas cylinders are used), this method has distinct potential advantages over the afore-described prior art methods of pigging a pipeline involving the use of pumps. For example, the need for a surface support vessel to support or control pumping at the inlet end of the pipeline throughout the pigging process is avoided in the present invention in which any support vessel which may be required during the pigging process will be required above the far (outlet) end of the pipeline. This method will thus be particularly advantageous where, for example, the near (inlet) end of the pipeline is inaccessible to a support vessel.

According to another aspect the invention comprises a fluid venting system for use in venting of fluid from a fluid- containing underwater apparatus, the system comprising a buoy means and a conduit having one end thereof for connection, in use, to an outlet of said underwater apparatus so as to be in fluid communication therewith, and another end for linking to said buoy means, and venting outlet means via which fluid may exit said fluid venting system, said venting outlet means

being provided with valve means formed and arranged to be in a closed position, substantially preventing fluid flowing into the fluid venting system through said venting outlet means, when the internal pressure in the fluid venting system is less than the external pressure at said venting outlet means, and to be in an open position, allowing fluid in the fluid venting system to vent therefrom, when the internal pressure in the fluid venting system is greater than the external pressure at the venting outlet means, whereby, in use, fluid in the underwater apparatus is vented therefrom into the fluid venting system when said fluid is at a pressure greater than the internal pressure in the fluid venting system from which, in turn, fluid is vented when at a pressure greater than the external pressure at the venting outlet means thereof.

Such a fluid venting system may, for example, be for use in propelling a pig along an underwater pipeline, in which case one end of the conduit is connected to an outlet of one end of the pipeline and the other end linked to the buoy, thus enabling fluid in the pipeline to be vented therefrom.

Said fluid venting system may further include at least one source of compressed air (or, alternatively, another fluid having a density less than water), for connection to said conduit, said conduit preferably being provided with air inlet means for coupling to said compressed air source.

The buoy is preferably hollow, and may be generally bell- shaped, or balloon-shaped, having an inlet connected to one end of said conduit, and a narrow outlet end comprising said venting outlet means. Said valve means in the venting outlet means conveniently comprises a check-valve which controls the flow of fluid through the buoy.

Alternatively, the venting outlet means may be provided in the free end of the conduit which may be suspended from a surface, or mid-depth, buoy. In a further possible embodiment, a lower end of the conduit is connected to the pipeline outlet means and an upper end is mounted in, and in fluid communication with, a venting buffer chamber which is suspended from the buoy, the venting outlet means being provided in the buffer chamber.

It will be appreciated that subsea venting of the contents of the pipeline means offers many advantages. By locating the venting outlet means subsea the present invention can take advantage of the density differences between the ambient water and a gas, for example, air being vented from the pipeline means in order to create the necessary pressure differential across the pig to drive it along the pipeline means. In the case of a pipeline means initially filled with water, by introducing air bubbles into the fluid venting system the density of fluid in the venting system can be effectively reduced below that of the ambient water, again allowing the necessary pressure differential to be achieved. It is by using valve means formed and arranged to prevent ambient water from entering the venting outlet means of the fluid venting system that these pressure differentials can be achieved and maintained.

Furthermore, it will be appreciated that the above-described fluid venting system could be used in other applications in which it is desired to vent fluid contents from an underwater apparatus, not just in the afore-described method for propelling a pig along a pipeline.

Preferred embodiments of the invention will now be illustrated, by way of example only, and with reference to the accompanying drawings in which: Fig. l illustrates schematically a method of propelling a pig through an air filled underwater pipeline according to the invention; Fig. 2 illustrates a modified version of the method illustrated in Fig. l; Fig. 3 is a cross-sectional view of the buoy shown in Figs. 1 and 2; Fig. 4 illustrates schematically a method of propelling a pig through a water-filled subsea pipeline; Fig. 5 illustrates the various pressures acting in the pipeline arrangement shown in Fig. 4, where the pipeline is disposed at an angle to the horizontal; Fig. 6 shows a modified subsea pipeline (in cross-section) with which the method illustrated by Figs. 4 and 5 may be used; Fig. 7 is a part cross-sectional view of a fluid venting system according to one embodiment of the invention; and Fig. 8 illustrates apparatus for injecting chemicals or dyes into a pipeline.

Fig. 1 illustrates a subsea pipeline 1 (in cross-section along its main axis) laid substantially horizontally on the sea bed 2 and containing air at atmospheric pressure PA. Proximal and distal ends 4,6 of the pipeline are sealed by respective closure arrangements 7,8 which are provided with an inlet valve 3 and an outlet valve 5 respectively. The pipeline also contains a pig 9 which is inserted therein at the proximal end 4, usually prior to sinking the pipeline to the seabed. The inlet valve 3 is closed prior to laying the pipeline, so as to prevent sea water W from entering the pipeline at the proximal

end thereof. The outlet valve is connected to one end 13 (the "lower"end) of a hose 10 the other end 14 (the"upper"end) of which is mounted in a buoy 11 floating at the sea surface 12. It will be appreciated that Fig. l is not drawn to scale, the pipeline usually being disposed well below sea level, in some cases in excess of 1000 metres below the surface. The pipeline 1 may itself comprise a single pipe, or a plurality of individual pipes laid end to end.

The buoy 11 is shown in cross-section, in greater detail, in Fig. 3. The buoy 11 is generally bell-shaped having a hollow interior 15 into which the upper end 14 of the hose extends (via an aperture 20 in the buoy, the hose 10 being in sealing engagement with the buoy 11), as shown. A lower portion 16 of the buoy 11 is provided with a narrow outlet end 17 which functions as a venting outlet and in which at least one one-way check valve 18 is fitted to control the flow of fluid (water or air) therethrough. The valve 18 operates so as to prevent flow of water into the hollow interior, or"buffer volume"15 of the buoy 11 when external water pressure at the venting outlet 17 of the buoy exceeds the pressure in the buffer volume 15, and to allow fluid (in Fig. l, namely air), to exit from the buoy, via the outlet 17, when the pressure in the buffer volume 15 exceeds the external water pressure at the outlet 17. Sea water W is thus prevented from entering the buffer volume 15, and therefore the upper end 14 of the hose, by the check valve 18. One or more further such check valves 19a, 19b (indicated in broken lines in Fig. 3) may be included in the venting outlet 17 and/or in the upper hose end 14 to further strengthen the buoy against ingress of sea water through the narrowed outlet 17. Additionally, the upper end 14 of the hose 10 is disposed well above the outlet 17 in the buoy 11. Thus, the"air in"point is higher then the"air out"

point so that should any sea water manage to enter the buoy 11 it is unlikely to enter the hose 10 and, in any case, will be expelled from the buoy on resumption of air over-pressure.

The hose 10 and buoy 11 are connected to the outlet valve 5 prior to the pipeline being sunk to the seabed. The outlet valve is in its open position, sea water being prevented from entering the pipeline via the distal end of the pipeline by the check valves in the buoy. The advantage of having the outlet valve open when the pipeline is installed subsea is that no subsequent intervention is required at the distal end of the pipeline to start the"pigging"process which will now be described.

The method of propelling the pig 9 along the initially air filled pipeline 1, according to the embodiment of the invention illustrated in Fig. l, in which the pipeline 1 is substantially horizontal, comprises opening the inlet valve 3 so as to allow the head of water at the inlet 3 to drive the pig 9 along the pipeline, the air in the pipeline being vented therefrom, via the hose 10, to the buoy 11 (the outlet valve 5 already being open, as afore-described). The invention works due to the hydrostatic pressure Pw from the head of water on one side of the pig (equal to pgHw, where p is the density of water, g is the acceleration due to gravity, and Hw is the depth of the inlet valve 3 below the sea surface) being greater than the pressure on the other side of the pig, which is atmospheric pressure Pal the resulting pressure differential across the pig driving it forward to the distal end 6 of the pipeline 1. In this method, when both the inlet and outlet valves 3,5 are open the pressure of the air in front of the pig (i. e. between the pig and the distal end 6 of

the pipeline) is prevented from rising to the external water pressure at the water depth of the pipeline. Since there is no build up of pressure in front of the pig it will continue to travel along the pipeline (in the direction indicated by the arrow in Fig. l) until all the air has been expelled therefrom.

Fig. 1 in fact shows the pig 9, still in the process of being propelled along the pipeline 1, having been propelled part of the way therealong from the proximal end 4 of the pipeline, with both the inlet and outlet valves 3,5 open.

Fig. 2 shows a modified implementation of the method illustrated in Fig. l. In this embodiment, the buoy 11 is a so- called"mid-depth"buoy, disposed a depth H3 below the sea level 12. This will work in a similar fashion to the Fig. l method, although this time the pressure differential across the pig 9 is equal to the difference in pressure between the hydrostatic pressure P1 (equal to pgHl) at the inlet valve 3 and the hydrostatic pressure P3 at depth H3 below sea level 12 (where P3 = pgH3). Assuming this pressure differential is sufficient to overcome any frictional losses (due to friction between the pig and the pipeline, and associated with fluid flow through the pipeline) the pig 9 will be driven forward under the action of the created pressure differential. When the inlet valve 3 is opened, air pressure in front of the pig will only build up till it equals the hydrostatic pressure P3 at the outlet 17 in the buoy, after which the check valve (s) in the buoy will open and the air will vent out of the buoy, the pig thus continuing to travel along the pipeline until all the air has been expelled therefrom. One advantage of this method is that it avoids the need to run a hose from the sea bed all the way to the surface 12. Moreover, the depth H3 at which the buoy is disposed may be used to determine the

velocity with which the pig 9 will be propelled along the pipeline 1.

If a situation were ever to occur where the venting hose 10 was subjected to an external pressure sufficiently greater than the internal pressure in the hose 10 that the hose collapse pressure is exceeded, the hose is liable to collapse in on itself. This may happen during operation of the apparatus of Figs. l and 2, for example when the pipeline is full of air and the inlet valve 3 is first opened so as to cause the pig to begin travelling towards the distal end of the pipeline. Until the air in front of the pig is pressurised up to the hydrostatic pressure at the outlet valve in the buoy, the pressure in the venting hose 10 may be sufficiently lower than the surrounding subsea hydrostatic pressure that hose collapse can occur. Such hose collapse might, in the worst case scenario, lead to damage to the hose, and/or may prevent or delay the pig 9 being driven further along the pipeline 1 until the hose has been replaced, re- pressurised or"uncollapsed". In order to avoid such problems, the pressure in the hose 10 may be deliberately maintained above a predetermined level at all times. This may be done by, for example, connecting a subsea cylinder 49 of pressurised gas (e. g. pressurised air, Ap) to a dedicated inlet 50 (indicated in hatched outline in Fig. 2) provided in a lower portion of the venting hose, a one-way valve 51 being incorporated in the dedicated inlet 50 which valve opens when the hose pressure drops below said predetermined level. By way of example, in the embodiment of Fig. l, if the venting hose is at a subsea depth of approximately 1000m, the ambient water pressure PW surrounding the venting hose will be approximately 100bar. If the hose collapses when subjected to a pressure differential Pc of, say 10bar, between the outside and inside

of the hose, then in practice as long as the pressure Ph in the venting hose is maintained above Pw ~ Pc (i. e. above 90bar) at all times, hose collapse will be avoided. Where, for example, the check valve CV1 in the buoy 26 is at, say, 950m subsea, the pressure at the CV1 outlet thus being 95bar, and the lower end of the venting hose is at 1000m subsea, in practice as long as the pressure in the hose, Ph, is maintained at, say, 93bar, hose collapse should be avoided.

In this example, fluid in the venting hose can only exit the buoy when it is at a pressure greater than (95bar + pressure needed to physically open the check valve (s) 18,19a, 19b). In most cases the pressure needed to physically open the valves themselves will be negligible.

A further embodiment of the present invention is illustrated in Fig. 4. Like parts to those in Figs. 1 to 3 are indicated by like reference numerals. Here, the horizontal pipeline 1 is already filled with sea water when the pig is inserted into the proximal end 4 of the pipeline (which is illustrated in reverse orientation to that in Figs. 1 and 2). According to this method, pressurised air Apis injected into the hose 10 linked to the buoy 11, at the lower end 13 of the hose.

Instead of air, a fluid (gas or liquid) having a lower density than water could alternatively be used. With both the inlet 3 and outlet 5 valves open, a pressure differential is created across the pig 9, driving the pig forward (in the direction indicated by the arrow in Fig. 4). The air must be injected to the pipeline 1 at a pressure at or above the hydrostatic pressure of the water head at the depth Hw of the inlet valve 3 (and/or outlet valve 5) of the pipeline 1. However, as the pressurised air moves up the hose 10 the hydrostatic pressure acting on it decreases and the air expands (as shown in Fig. 4), so that the pressure in the hose at any given height

is less than the pressure outside the hose at that height.

This result is that, with the inlet and outlet valves 3,5 open, (and assuming the inlet and outlet valves are at the same depth Hwbelow the surface 12), and the pressurised air being injected into the hose, the pressure P2 of the water head in the hose (equal to pXgHw, where Px is the averaged density of fluid in the hose 10 at any one time) is less than the water head pressure Pl (pwgHw where p, is the density of sea water W) at the inlet valve 3 and so a pressure differential exists across the pig which propels the pig forward. (Note that a further reduction in pressure inside the hose is likely to result due to momentum transfer from the accelerating gas to the water in the hose 10).

Instead of the surface buoy 11 illustrated in Fig. 4, a mid- depth buoy, as in Fig. 2, could be used (see Fig. 5, and below).

A check valve 22 may be provided at the point where the air is injected into the hose 10, to stop the contents of the hose (initially sea water) entering the source of pressurised air.

The source of pressurised air (not shown) may be a pump located on a surface support vessel, or may be a subsea pumping unit. In the most preferred embodiment, the pressurised air comes from a subsea compressed air storage facility, for example gas cylinders stored on the seabed.

Fig. 5 illustrates a water-filled pipeline 1 in which the method according to Fig. 4 is implemented, but where the pipeline is not in a substantially horizontal position as in Fig. 4, but is disposed on an inclined seabed 2. For the pig 9, at a depth H2, to continue moving at a desired pig velocity vp, then:

PwSHi = Pwg (Hi-H2) + PwgH3 + pg (H4-R3) + pgHg + (friction losses, at vp) where p, is the density of sea water; p. is the (mean) density of the contents of the hose (a mixture of sea water and air, or other lower density fluid); g is acceleration due to gravity; the"friction losses"represent the pressure losses associated with overcoming frictional forces between the pig and the pipeline (and/or associated with fluid flow in the pipeline) when the pig is travelling at velocity Vp. Hi is the depth of the inlet valve 3, at the proximal end 4 of the pipeline 1; H3 is the depth of the buoy 11; H4 is the depth of the lower end 13 of the hose, where it connects to the distal end 6 of the pipeline; and H. is the difference between H2 and H4 (i. e. H2-H4), as shown in Fig. 5.

Thus, as long as the total pressure in the fluid venting system (i. e. pwgH3 + pxg (H4-H3)) is less than pwgH4 minus friction losses, the pig will move (initially) and will in fact accelerate along the pipeline.

Fig. 6 shows a modification to the arrangement shown in Figs. 4 and 5. In Fig. 6, to prevent localised pressure build-up a pressure vessel 24 with a"buffer volume"25 therein, and which incorporates the gas injection point, is coupled between the lower end 13 of the hose 10 and the outlet valve 5 of the pipeline. In this embodiment the outlet valve 5 in the distal end 6 of the pipeline may be in the form of a check valve which prevents the contents of the pipeline, in front of the pig 9, being pressurised by the air or other fluid Apinjected into the pressure vessel 24. A pressure gauge 26 may be

connected to the pressure vessel 24, if desired, for monitoring pressure in the buffer volume 25.

It will be appreciated that a similar pressure vessel arrangement to that of Fig. 6 could be used where gas injection into the hose is used in connection with the methods of Figs. l and 2 (for pigging an initially air-filled pipeline) in order to prevent hose collapse.

Various additional features may be incorporated in the above- described systems, particularly (but not exclusively) where the system incorporates a subsea compressed air storage facility. For example, a remote control system may be included for controlling injection of the compressed air into the hose 10; fluid flow rate monitoring apparatus for monitoring fluid flow through the outlet valve 5 and/or the inlet valve 3, and a further control system linked to the outlet and/or inlet valves 5,3 for controlling the flow of fluid therethrough; monitoring apparatus for measuring the total amount of gas and/or water entering and/or exiting the pipeline 1, said monitoring system being linked to one or more of the afore- mentioned control systems. A further possible feature of the system would be an automatic activation system, for activating the injection of compressed air into the hose 10, which includes a sensor for detecting any liquid in the pipeline 1 in front of the pig 9 and activating the compressed air injection when any liquid is so detected. (This would be applicable where the pipeline is initially air filled and compressed air injection is used to complete the pigging process, should any liquid get into the pipeline, as described herebelow.) A further possible feature would be a control system linked to the pig 9, and/or any further pigs in the pipeline 1, which co-operates with the fluid venting system

(comprising the hose 10, buoy 11 and compressed air source) to (automatically) control the progress of the pig 9 along the pipeline 1.

The gas injection point may, instead of being in the hose 10, be in the pipeline 1, or into the pressure vessel 25. A pig receiver unit (not shown) may be attached at the distal end 6 of the pipeline, and this unit could alternatively incorporate the gas injection point. (A pig launcher unit may also be attached at the proximal end 4 of the pipeline.) The buoy 11 in any of the afore-described embodiments may additionally be provided with one or more of the following advantageous features: 1) A radio transmitter and/or light may be included in a surface buoy as a shipping sign.

2) An air/water discharge hose may be connected to the outlet 17 in the buoy, and this discharge hose, if used, may be secured to the main hose 10 connected to the pipeline 1, to restrain it.

3) On a subsurface (i. e. mid-depth buoy) a lifting wire may be connected to a smaller surface buoy for ease of recovery of the hose/discharge buoy system.

It will be appreciated that various modifications to the described embodiments are possible without departing from the scope of the invention. For example, in the case of the air filled pipeline, it would be possible to attach the hose connected to the distal end of the pipeline to a suction pump (which may be a subsea pump or a surface pump located on a platform or support vessel) instead of connecting the hose directly to a surface or mid-depth buoy. However this will generally not be preferred since although this may speed up

the flooding of the pig through the pipeline, it is generally the case in practice that the rate of travel of the pig must be kept within a predetermined speed rate which will be satisfactorily achieved with the pressure differential available from a buoy connected to the hose, as afore- described.

It will further be appreciated that in the above-described embodiments, the hose and buoy could be replaced by a suitable conduit, flexible or otherwise, having its upper end mounted or supported on a surface platform or other available construction/support at the surface, or at the desired depth below sea level. It will be appreciated, though, that the hose and buoy arrangement provides the simplest, most convenient way, of implementing the invention. The buoy itself need not be hollow: the upper end 14 of the hose 10 may instead function as the venting outlet 17 via which fluid from the hose is vented, a check-valve being provided in this upper end of the hose. In the latter embodiment, the upper end of the hose may be affixed directly to the buoy, or may be suspended from the buoy (e. g. via a connecting wire) so as to be disposed at a desired depth below the sea surface. In each case the buoy may be a surface or a mid-depth buoy. A further possibility would be to mount the upper end of the hose in a venting chamber which is suspended from a buoy means, the venting chamber having a similar construction to the buoy illustrated in Fig. 3. Such an arrangement is shown in Fig. 7.

In this arrangement the fluid venting system consists of a hose 60 having one end 61 connected to the valved outlet 5 of the distal end 7 of the pipeline 1 (or a pig receiver unit attached to the end of the pipeline and in fluid communication therewith) and the other end 63 mounted in a venting buffer chamber 62. The end 63 mounted in the venting buffer chamber

has a one-way check valve 64 provided therein. The chamber 62 (shown in cross-section in order to reveal the hose 60 inside) has two venting outlets 65,66 which also each have a one-way check valve 67,68 for controlling the flow of fluid therethrough. The venting chamber 62 functions in a similar manner to the buoy 11 illustrated in Fig. 3, the three check valves 64,67,68 operating in a similar manner to the check- valves 18,19 in the buoy venting system of Fig. 3, but in this case the fluid venting system also includes a buoy 70 which floats at the water surface and from which the venting buffer chamber 62 is suspended, as shown. A cable 72 is also provided, extending between the pipeline 1 and the chamber 62 to assist in maintaining the chamber in a steady position above the pipeline 1. The depth H3 of the venting outlets 65,66 below the water surface determines the rate of travel of the pig along the pipeline 1.

The buffer chamber 62 in Fig. 7 provides a"buffer volume"or "buffer zone"similar to that provided by the buoy 11 of Fig. 3. Fluid flowing into the buffer chamber 62 out of the upper end 63 of the hose 60 may only exit the system via the venting outlets 65,66 of the buffer chamber 62. Advantages provided by such a buffer volume have already been described.

One further reason why the buffer zone is important is because, in practical applications, the check valves may not act instantly to pressure reversals. Thus, if an underpressure were to occur in the hose 60 (relative to the ambient surroundings) the buffer zone will prevent water from entering the upper end of the hose. Any ambient water which enters the buffer zone (via the outlets of the buffer chamber 62) during such an underpressure will be expelled on resumption of overpressure in the hose 60.

Furthermore, it will be appreciated that the method described with reference to Figs. 5-6 may be used as a supplementary process, after carrying out the method described with reference to Fig. l or Fig. 2 for an initially air filled pipeline, in order to complete the pigging process where some liquid has managed to ingress into the pipeline 1 in the volume in front of the pig (i. e. between the pig 9 and the outlet valve 5).

The afore-described methods are applicable to flexible pipelines, as well as rigid pipelines.

A further possible feature of the above described methods and apparatus may be the use of a filter at the inlet 3 of the pipeline into which sea water enters the pipeline to push the pig forward along the pipeline. The filter would be arranged to filter water entering the pipeline, preferably prior to it entering the pipeline. Furthermore, apparatus may also be provided for introducing chemicals into the pipeline.

Chemicals are typically introduced into the pipeline, behind the pig, for pipeline corrosion prevention and/or as dyes which may be used to assist in the detection of leaks during pressure testing of the pipeline. A suitable apparatus for introducing such chemicals is illustrated in Fig. 8.

Fig. 8 shows the proximal end 8 of the pipeline 1 (or a pig launching unit connected to the pipeline 1 and in fluid communication therewith), with a valved inlet 80 thereto which is also connected to the outlet of a series of three containers or boxes 82,83,84 in which water soluble chemicals are contained. These three containers are in fluid communication with each other and are linked, via a valved connecting conduit 85, to a filter unit 86. A valve 87 in

this connecting conduit, and the valve 80 between the chemical containing boxes 82-84 and the pipeline 8, are used to control opening of the pipeline to the external hydrostatic pressure.

With both these valves open, and where the pressure in the fluid venting system in fluid communication with the pipeline at the other end thereof is low enough to cause sea water to be drawn into the pipeline, the water is drawn in through the filter unit 86 where it is filtered before passing into the chemical containers. The water soluble chemicals therein are drawn into the pipeline 1, behind the pig 9. There is no particular requirement for the shape of the container or the water soluble chemicals therein, but the following are desired characteristics of the system: (1) The surface area of the chemicals exposed to the flow of water should remain approximately constant to ensure that the rate at which chemicals are introduced to the sea water is constant with respect to the rate of flow of sea water into the pipeline; (2) The surface area and flow path through which the sea water flows through the containers should be designed to achieve, over the expected range of water flow rates, the required chemical dosage rates; (3) The design of the filter unit and containers should minimise pressure losses associated with the water flow therethrough; (4) The filter unit must have a sufficiently large surface area that it is unlikely to become blocked; (5) The containers should hold sufficient chemicals to maintain the required dosage of chemicals throughout the pigging process; (6) further valves may be provided for controlling and/or isolating the flow of water into and out of the containers.