GB2054793A - A method of operating on an immersed pipe - Google Patents

Abstract

The present invention relates to a method of operating on an immersed pipe by temporarily filling a portion (26') of the pipe with water. The portion (26') of the pipe to be filled with water is defined between two closure means (6, 24) which as they are moved to their final positions each moves in a direction towards a part of the pipe which is open to the exterior. Where the portion of the pipe is at or adjacent one end of the pipe which is open to the exterior, the two closure means are introduced from the other end of the pipe. Where the portion (4) of the pipe is intermediate its ends, the two closure means (6, 24) are introduced from opposite ends and a hole (26) made in the portion of the pipe during the final stages of positioning the closure means. <IMAGE>

Description

SPECIFICATION A method of operating on an immersed pipe The invention relates to a method of operating on an immersed pipe by temporarily filling part of the pipe with water, for example when the operation involves the replacement of a portion of a pipe used for carrying a hydrocarbon.

It is particularly important to avoid completely filling a gas pipe line with water; since the operation is long and expensive because, in order to restore the flow of gas after the operation, the entire pipe has to be emptied of water and dried along its complete length, which may be several hundred kilometres.

It has already been proposed to use two closure means capable of moving in the pipe and of being locked at a given place in order to block off a section into which a fluid is introduced through a valve disposed on the pipe. However, this necessitated complicated closure means comprising valves for closing and opening the closure means, the valve remaining secured to the pipe after the operation.

According to the invention there is provided a method of operating on an immersed pipe by partially filling a portion of the pipe with water, using closure means capable of moving in the pipe and of being locked in a given position and brought via the exterior of the pipe into positions defining the ends of the said pipe portion, the closure means being withdrawn after the operation, wherein two closure means are used which are always at least approximately sealingtight and each closure means, as it approaches its final position, advances towards a portion of the pipe which is opened towards the exterior.

It is thus possible to use closure means having a simple design and not needing valves for closing and opening them. Portions of special intermediate fluids can be conveyed in the pipe to one side of the closure means. If the portion of pipe opened towards the exterior is either an end of the pipe or a portion needing replacement, the pipe after the operation will be similar to a newlylaid pipe without any singular points where damage may occur.

If the pipe is only slightly damaged or not at all, the two closure means, with an intermediate fluid between them, may be introduced through a single end of the pipe, the other end of the pipe being open towards the exterior.

If a portion of the pipe is greatly damaged, a first closure means may be brought to its final position, starting from a first end of the pipe, under the action of a first compressed fluid, during which time the second end of the pipe is open, after which a second closure means is moved towards its final position starting from the second end of the pipe under the action of a second compressed fluid, an aperture being formed in the damaged portion of the pipe after the second fluid under pressure has reached a pressure above hydrostatic pressure at the site of operations.

After the operation, therefore, it is sufficient to expel water only from the pipe portion under consideration by using simple pressure to drive out the closure means, which have temporarily been locked and which enclose the water used to balance the pressure between them.

To simultaneously complete the drying of the pipe for re-use the first automatically triggered closure means may be associated with a simple closure means and the space between the two closure means may be filled with a product such as methanol for drying the remaining water by absorption.

To simultaneously complete the return of the pipe to use, e.g. by dissolving any fats capable of covering the inner wall, other ordinary closure means may be provided and a suitable solvent located between them, the assembly comprising the set of closure means and processing products being driven by pressure to one of the ends of the pipe.

Thus, the above described method has the advantage of being very reliable during operations performed on hydrocarbon pipes, since only a small part of the pipe is made wet. Another advantage is that the method can be used to clean the entire pipe.

The invention will be more fully understood from the following description of an embodiment of a method according to the invention, given with reference to the accompanying drawings by way of example only.

In the drawings: Figure 1 is a diagrammatic view of a submerged gas pipe on the sea bed; Figure 2 shows a part of the pipe including a damaged portion after a first closure means has been positioned and during the approach phase of a second closure means; Figure 3 shows the damaged portion of the pipe before the second closure means stops; Figure 4 is a diagram of an automatically triggered closure means and Figure 5 shows a part of the pipe after the operation has been completed and during the phase of restoring the flow therein.

The following is a description of the general method of underwater operation by partially filling a submerged pipe with water although the pipe is not a liquid-conveying pipe. For simplicity, it is assumed that the pipe 1 shown in Figure 1 is a gas pipe connected to an oil rig 2, from which it conveys natural gas coming from an underwater borehole to a storage reservoir 5. The pipe is held on the sea bottom 3 by any suitable method and, to counteract its buoyancy, is covered by concrete (omitted to simplify the drawing). An arrow 4 shows the portion of the pipe at which intervention is required, e.g. because the pipe is dangerously deformed there or because a leak has been detected.

To fix ideas, it will be assumed that the hydrostatic pressure at 4 is of the order of 10 bars and the distance between the damaged part of the pipe from rig 2 is 100 km and the distance between it and the storage reservoir 5 at the other end of the pipe is 50 km.

The first step in the method is to introduce at least one closure means 6 from oil rig 2, the closure means being automatically triggered by an incorporated distance counter and capable of blocking pipe 1 and withstanding pressures of the order of e.g. 40 bars in the present case.

After being repaired, pipe 1 must be ready to convey gas and must be dried before re-use.

Accordingly, closure means 6 is followed by a certain volume 7 of methanol imprisoned between the automatic closure means 6 and an ordinary closure means 8 which separates the methanol from the producer gas used to move the assembly towards the pipe portion 4 where a leak, for example, has been detected.

Closure means 6, diagrammatically shown in Figure 4, may for example be of the kind comprising counting-wheels 9. Each wheel 9, which is designed to move without slipping along the inner wall of pipe 1, is mounted on an articulated arm 10 of a frame 1 1 bearing sealing rings or caps 12, 13 and an inflatable sleeve 14.

Members 12, 13 ensure sealing-tightness with the pipe walls during the motion of the closure means, irrespective of the direction of motion. Sleeve 14, which is normally in the non-inflated position 15, bears against the inner wall of pipe 1 as soon as the reserve of compressed gas in a reservoir 16 (the reserve being sufficient to compensate any leaks) is released by triggering a device, e.g.

opening a valve or igniting hollow charges 17 and 18. To this end, charges 17 and 18 are connected by a control wire 19 to a priming device 20.

Device 20 is triggered by a kilometre counter 21 when the number recorded by the counter reaches a preset value. The means comprising wheels 9 for actuating counter 21 and the means for displaying the preset value of the distance travelled or for triggering the priming device 19 can be of any suitable kind and do not form part of the invention and are therefore not shown in detail. Of course, wheel 9 can trasmit any mechanical or electric signal to counter 21 which, depending on its nature, can directly trigger, e.g. via a spring actuating means, the displacement of a lug normally locked by the display wheels in the counter; a system of slots borne by the wheels correspond to the triggering value and represent the displayed distance and release the lug as soon as the slots are in line.Of course, release can also be brought about by comparing the value stored in an electromechanical or electronic counter with the value stored on an electromechanical or electronic comparison means. In such cases it is convenient to use a number of wheels 9 to prevent any loss in control pulses, thus triggering the priming device 20.

Since, as assumed in the example, the site of pipe portion 4 is 100 km from oil rig 2 and 50 km from station 5, counter 21 is set to trigger the priming device 19 as soon as it displays the value 99.900 m.

The simple closure means 8 preferably comprises sealing means similar to members 12, 13 of closure means 6 so as to ensure sealingtightness irrespective of the direction of travel in the pipe.

When the closure means arrives within 100 metres of the operating site 4, counter 21 triggers the priming device 19 so as to explode the hollow charges 1 7, 18 and release the reserve 1 6 of gas under pressure, thus pressing the cylindrical sleeve 14 of closure means 6 against the wall of pipe 1.

When closure means 6 is secured in the pipe, the result is a pressure increase in that part 22 of the pipe between the gas-distributing oil rig 2 and the closure means 8. The pressure is monitored at rig 2 so as to detect when closure means 6 stops. The next step is to check the position of the closure means, which can be equipped with signalling means 23 transmitting a signal suitable for being picked up by a known suitable device which can be carried on a submarine. Although the increase in pressure in part 22 of pipe 1 can be up to 40 bars without moving the closure means, the pressure is made equal to the hydrostatic pressure at the operating site 4, i.e. 10 bars in the chosen example.

At station 5, a closure means 24 of the same kind as closure means 6 is introduced and moved by pressure after its counter has been adjusted so as to trigger the locking pressure when the counter reaches the value 49.900 m. As soon as the pressure for moving closure means 24 through the part of pipe 25 between closure means 24 and station 5 reaches a preset value, e.g. 20 bars, the gas in the part of pipe 1 near the operating site 4 is evacuated by forming an orifice 26 (Figure 3) of suitable size at the damaged part 4 of the pipe. To this end, a suitable pyrotechnic charge can be placed by submarine on pipe 1. After the explosion, the gas imprisoned between closure means 6 and 24 which is at a pressure above the hydrostatic pressure, escapes through the orifice, so that the motion of closure means 24 can continue at constant pressure in part 25 of the pipe.When closure means 24 reaches the limit of the distance assigned to it, its counter triggers the process of inflating the locking sleeve, which presses against the pipe wall.

Since the gas pressure between closure means 24 and 6 is then equal to the hydrostatic pressure, the part 26' of the pipe between closure means 24 and 6 fills with water. As in the case when closure means 6 is secured, the position of closure means 24 can be checked by means of signals emitted by a signalling device disposed in the closure means or towed by it. It is then possible, accordingly, to take any required action, inter alia to replace a portion of the immersed part 26' of the pipe by a new portion 27 (Figure 5). The operation can be performed in complete safety.

It is thus possible to raise part 26', using any suitable means, to remove the concrete and cut the damaged portion, to prepare one end 29 of the pipe after removing the damaged portion in order to weld on the new portion 27, and carry out the necessary operations of brushing the interior and exterior and finely cutting end 29, after which a closure plug 28 is placed in position for the operations of making a connecting weld along line 32. These operations are known and need not be described further. Similar operations are performed at the other end 30 followed by the insertion of a plug 31 in order to weld on the new portion 27 along line 33.

The result is a pipe (Figure 5) comprising a part 22 containing the natural gas to be conveyed, an ordinary closure means 8 and an automaticallytriggered closure means 6, the two closure means 6, 8 co-operating to partition off a volume of methanol 7 for drying the pipe 1 during the operation of evacuating the introduced sea water, which remains confined between plugs 6 and 28 and between plugs 24 and 31. The region between plugs 28 and 31 is filled with the diving gas used during the process of connecting the new portion 27, and region 25 is filled with the gas used for pushing the closure means 24.

In order to expel the assembly bounded by closure means 8 and 24 without using high pressures, it is preferable to use sleeves 14 which do not withstand prolonged continuous heating, even for a relatively short period. It is then sufficient to increase the gas pressure in part 22 until closure means 6 moves and its sleeve 14 progressively heats and disintegrates. Since the sleeve of closure means 24 disintegrates similarly, the gas pressure at 22 can be reduced. The assembly disposed between closure means 24 and closure means 8 moves towards station 5, where all the components are collected. During this motion, the methanol dissolves the remaining water so that, after being recovered at station 5, the pipe is ready again for conveying gas from rig 2.

If the gas carries fatty products which stick to the wall of pipe 1, an auxiliary closure means 34 similar to means 8 can be disposed downstream of closure means 6 (Figure 5), the space between means 6 and means 34 being filled with an appropriate solvent 35.

Although only a single embodiment of the method according to the invention has been described, it is clear that partial filling with water can be used for operations other than those described, inter alia during tests for laying nonimmersed pipes.

In addition, numerous detailed modifications can be made to the various operations without thereby departing from the invention, the modifications being due to the limitations imposed by the use of available means.

Claims (9)

1. A method of operating on an immersed pipe by partially filling a portion of the pipe with water, using closure means capable of moving in the pipe and of being locked in a given position and brought via the exterior of the pipe into positions defining the ends of the said pipe portion, the closure means being withdrawn after the operation, wherein two closure means are used which are always at least approximately sealingtight and each closure means, as it approaches its final position, advances towards a portion of the pipe which is opened towards the exterior.

2. A method according to claim 1, wherein the two closure means, with an intermediate fluid between them, are introduced through a single end of the pipe, the other end being open towards the exterior.

3. A method according to claim 1, wherein a first one of the closure means is brought towards its final position, starting from a first end of the pipe, under the action of a first compressed fluid, the second end of the pipe being open, after which the second one of the closure means is moved towards its final position, starting from the second end of the pipe, under the action of a second compressed fluid, an aperture being formed in the said pipe portion after the first pressure fluid has reached a pressure above the hydrostatic pressure at said pipe portion.

4. A method according to claim 3, wherein an intermediate fluid is imprisoned between one side of one of the closure means and an auxiliary plug associated with the one closure means.

5. A method according to claim 4, wherein the intermediate fluid is a solvent.

6. A method according to claim 4, wherein the intermediate fluid is methanol and the one closure means is that one which is upstream during the withdrawal of the closure means.

7. A method according to any of the preceding claims, wherein each closure means is locked by triggering a device which causes inflation of a sealing-tight inflatable sleeve which then presses against the inner wall of the pipe,

8. A method according to claim 7, wherein, at the beginning of the withdrawal motion of the closure means, a fluid is first sent at a pressure high enough to move the closure means so as to destroy the inflatable sleeve by heating through friction against the pipe wall.

9. A method of operating on an immersed pipe substantially as herein described with reference to the accompanying drawings.