WO2013144724A2

WO2013144724A2 - Pipeline and method of laying a pipeline

Info

Publication number: WO2013144724A2
Application number: PCT/IB2013/001131
Authority: WO
Inventors: Roberto Jourdan De Aquino; Jean-Paul FERRAZ
Original assignee: Technip France
Priority date: 2012-03-29
Filing date: 2013-03-25
Publication date: 2013-10-03
Also published as: BR112014023617B1; GB2500677A; GB201205596D0; GB2500677B; WO2013144724A3

Abstract

A pipeline for laying in a sea comprising a plurality of conjoined pipeline sections having end fittings, wherein the pipeline has at least two block valves, and at least two of the pipeline sections include a block valve therebetween. It is a particular advantage of the present invention that where pipeline sections have block valves therebetween, any damage or other accident to such a pipeline section during laying, generally resulting in flooding into the pipeline section, can be limited to the or each pipeline section between the block valves. Thus, not the entire length of the pipeline would be flooded, which could result in a serious increase in the top tension capacity, and possible a significant effect of the pipe- laying vessel.

Description

Pipeline and Method of Laying a Pipeline The present invention relates to a pipeline and a method of laying a pipeline in a sea, particularly but not exclusively for laying in ultra-deep water.

Laying pipelines, especially flexible pipelines, in ultra-deep water is a challenge. "Ultra-deep water" can be defined as being at least >1000m, usually > 2000m, and sometimes even > 3000m below the level of the water surface, generally sea-level.

A major problem to the installer is to take account of the tension ("T") created on the laying vessel from the weight of the pipeline being held but strung out from the vessel and extending through the water depth, prior to reaching the sea bed, and to take account of the capacity of the vessel to cope with such tension. This is sometimes termed the "top tension capacity" or "T_c". Naturally, even with achieving T<T_C, significant safety limits apply, such that especially with laying pipelines in deep water, few vessels are available with sufficient T_c capacity for such laying within the required safety limits.

The assessment of the top tension capacity of pipeline-laying vessels, especially with laying in deep water, and in particular for laying in ultra-deep water, requires the pipeline to be laid 'empty', i.e. not-flooded. Generally, there is a closed valve at the lower end of the pipeline. However, to cope with the accidental case when the pipeline may become accidentally flooded, and the effect of the sudden increase in pipeline weight and tension (caused by the seawater entering the full length of the pipeline underwater at the time) on the top tension capacity of the pipe laying vessel (i.e. T>T_C] , buoyancy modules are fixed along the length of the pipeline to reduce the amount of laying tension, and thus keep the top tension capacity within the capability of the laying vessel. Buoyancy modules, generally in the form of the buoys, allow laying tension to be reduced by introducing a 'floatability' element along the pipeline. However, the use of buoyancy modules also has several disadvantages. These include:

- the need for a large amount of buoyancy modules all along the pipeline to allow all of the pipeline to be installed in a similar manner;

- the cost of buoyancy modules;

- the installation time and effort needed for buoyancy modules;

- the space required to store buoyancy modules when not in use:

- the immediate redundancy of buoyancy modules if they are in any way damaged: and

- stability issues on the seabed which may occur due to the effect of the current, etc. (whose influence could be magnified by the presence of buoyancy modules).

Also, buoyancy modules cannot be used for installing a riser in a free hanging configuration.

The actual laying of a pipeline, in particular a flexible pipeline, under water, such as in a sea, can be carried out using a laying vessel or barge having horizontal or vertical laying equipment. For deepwater installation, the vertical laying method is particularly suited: firstly because the flexible pipeline is not subjected to a combination of tension and bending strains during the installation, and secondly, because it makes the installation of intermediate or terminal equipment (such as end fittings, pipeline end terminal "PLET", etc.] easier to handle. As described in the WO 91/15699, a flexible pipeline can comprise rigid accessories, for example anode clamps, end fittings, and other connectors, with an external diameter greater than external diameter of said flexible pipeline. Such a flexible pipeline is reeled out from the storage basket and driven into a guiding means or a chute. Then, the flexible pipeline is moved within a derrick structure by a tensioning means, comprising 2, 3 or 4 tracks disposed symmetrically and exerting a tightening force on the external surface of the flexible pipeline in order to place it vertically up to the ocean floor. Once the flexible pipeline is vertically placed above seawater, it is guided, still by tensioning means, through a moon pool made on the laying vessel's deck, and then laid onto the seabed. In WO 99/35429, the derrick used to transporting the flexible pipeline from the laying vessel to the seabed has a vertical structure. However, for deepwater laying installation, the derrick can be tilted up to an angle of 30° from the vertical axis to make the lay easier.

Whichever installation method is used, the pipeline is laid empty, but still combined with buoyancy modules along its length in case the pipeline is damaged and becomes full of water, and in order to achieve the best floatability in comparison with the top tension capacity of the installation vessel.

It is an aim of the present invention to provide an improved pipeline and method of laying a pipeline in a sea.

According to one aspect of the present invention, there is provided a pipeline for laying in a sea comprising a plurality of conjoined pipeline sections having end fittings, wherein the pipeline has at least two block valves, and at least two of the pipeline sections include a block valve therebetween.

The use of two or more block valves, generally positioned between pipeline sections or integrated within end fittings, allows the pipeline to be installed in a 'dry' condition.

It is a particular advantage of the present invention that where pipeline sections have block valves therebetween, any damage or other accident to such a pipeline section during laying, generally resulting in flooding into the pipeline section, can be limited to the or each pipeline section between the block valves. Thus, not the entire length of the pipeline would be flooded, which could result in a serious increase in the top tension capacity, and possibly a significant effect of the pipe- laying vessel. Thus, the skilled man can consider the use of pipe-laying vessels having a lower tension-laying capacity, which vessels are more common and therefore more readily available for use, especially for pipe-laying in deep and ultra-deep water.

The pipeline is preferably a riser or a liquid injection flowline or hydrocarbon flowline, generally having one or more types of elongated or longitudinal elements, such as layers, sheaths, tubes, hoses, cables and strength members, and optionally one or more internal pipelines for the transport of a hydrocarbon fluid such as oil and/or gas, or other liquid fluid such as water. One or more of the elements may be sheathed, and the pipeline may include one or more armour layers.

The laying of a pipeline in a sea is a sufficiently complex operation that the skilled man will be aware of the parameters involved in the laying process, including the weight and floatability of the pipeline, for the assessment of the top tension capacity, and the capacity of the pipe-laying vessel to cope with such tension.

According to one embodiment of the present invention, the or each block valve is part of an end fitting.

For example, such a block valve could be formed as part of, optionally integrally in, the end fitting, in the form of an end fitting connector or end fitting vault or end fitting body. A particular benefit of the or each block valve being part of an end fitting is that the offshore installation is simplified and less time consuming.

According to a second embodiment of the present invention, the or each block valve is within or otherwise part of a valve-unit added between the pipeline sections.

The or each valve-unit could be adapted to fit with the pipeline section end fittings, such as with matching flanges, etc. This is particularly advantageous in allowing the use of existing pipeline sections and existing end fittings, with the simple addition of valve-units thereinbetween. Such a pipeline could be formed onshore or offshore (or both). In particular, such a pipeline could be formed offshore, where each of the pipeline sections are conjoined as the pipeline is being laid (such as using the stove-pipe method], with the simple addition and installation of each valve-unit as the end of each pipeline section is being prepared for laying from the laying vessel.

According to one embodiment of the present invention, one block valve is located at the end of the pipeline, in particular the end of the pipeline to be laid first, or to be connected to a subsea installation or other piece of equipment to which the pipeline is to be connected in subsequent use.

At a minimum, the present invention comprises a pipeline with two block valves therein, providing at least one 'dry' section thereinbetween to increase the floatability of the overall pipeline, and to limit flooding between such block valves should there be any accidental damage to that section or part of the pipeline between the block valves.

Optionally, that section of the pipeline between the uppermost block valve during pipeline laying and the surface is also 'dry', thus as a minimum providing at least two dry sections in the pipeline during laying.

According to another embodiment of the present invention, the pipeline comprises at least 3 pipeline sections, optionally in the range 3-20 pipeline sections, and at least 3 block valves, optionally in the range 3-20 block valves. That is, the pipeline comprises a number of consecutive pipeline sections, with a number of block valves thereinbetween, forming a number of 'dry' sections along the length of the pipeline between each block valves. The amount of the pipeline comprising consecutive pipeline sections conjoined by block valves need not be continuous or sequential, and may be non-continuous or intermittent, whilst still achieving the benefit of the present invention by increasing the floatability of the overall pipeline. Thus, the benefit of the present invention can still be achieved by having a pipeline having one or more parts thereof being flooded and one or more parts thereof being dry. According to another embodiment of the present invention, the method of the present invention comprises conjoining all of or substantially all of the pipeline sections together with block valves thereinbetween. Substantially all" as used herein can refer to at least 90%, more commonly at least 95%, of the pipeline sections forming the pipeline being conjoined using block valves.

Optionally, the block valves are one or more of the group comprising: part of the pipeline section end fittings, part of valve-units added between the pipeline sections, or both. Block valves are known in the art, generally being one or more moveable valves in one or more valve housings, able to provide a passageway for a fluid, and having at least an "open" position and a "closed" position; usually a fully open position and a fully closed position. Block valves can have one or more manual, semiautomatic or automatic mechanisms, and/or remote valve-opening mechanisms, including taps, actuators, handles, motors, etc. optionally solenoid operated. Operation of the opening mechanism could be carried our remotely, such as for example an acoustic signal transmitted from the surface to a receiver positioned next to the block valve, or could be carried out manually, such as for example by an ROV or a diver, etc. Springs, hydraulic accumulators and/or gear boxes could also be used for energising one or more opening mechanisms or actuators.

The present invention is not limited to the material, shape, size, design or form of block valve, and the present invention may use one or more different block valves at different positions along the pipeline. The or each block valve may be open or in an open position, or closed or in a closed position, for the present invention. The present invention is not limited to all of the block valves being closed or in a closed position during laying of the pipeline. However, according to a preferred embodiment of the present invention, most or all of the block valves used between the pipeline sections are closed or in a closed position during laying of the pipeline.

After laying the pipeline, the or each block valve that is closed can be opened. This provides a full flowline through the pipeline for the subsequent transportation of a fluid along the pipeline, generally being hydrocarbons such as oil and/or gas or liquid water.

The pipeline may fully or partly comprise a plurality of pipeline sections, sometimes also termed pipeline "stalks". The pipeline sections may be the same or different. The pipeline may include one or more other or further elements, units, items, or other pipeline or pipe-based sections. This can include different inter-pipeline end fittings, anode sections, terminations, etc., known in the art and not further discussed herewith.

The pipeline sections may have a variable length without limitation to the present invention, and generally based on the nature of the pipeline and the hydrocarbon extraction site. Production sites are all generally different, and the present invention is not limiting on the installation conditions regarding the depth, the relief of the seabed, the hydrocarbon pressure, the oil well layout, etc.

Optionally, the present invention comprises pipeline sections as described above wherein each pipeline section has the same or similar length. The end of each pipeline section can be conjoined to another pipeline section via any suitable pipe connectors, such as API flanges (for example segmented flanges for API 6A and swivel flanges for API 17D) or any other kind of connection. Suitable pipe connectors are well known in the art, and include those already known to be fitted or mounted at the end of a pipeline or pipeline section.

The pipeline may comprise one or more further valves or fittings along its length, including one or more further valves for redundancy reasons, or for easier access at certain locations along the length of the pipeline.

The interface of any block valve units and the pipeline sections, could be based on any suitable mechanism, including but not limited to API flange, SPO flange and Grayloc.

The pipeline of the present invention can be used for laying in any depth of water, being shallow or deep water. The pipeline of the present invention is particularly suitable for laying in ultra-deep water. This is because of the pipeline is laid in 'dry' condition, with conjoining a few or all of the pipeline sections together through operated block valves. The present invention avoids the pipeline being completely flooded if any damage occurs during the installation period, and also eliminates vessel accidents because of decreasing the top tension force T exerted by the pipeline on the laying vessel.

According to a preferred embodiment of the invention, the pipeline is flexible.

According to a second aspect of the present invention there is provided a method of laying a pipeline in a sea comprising at least the steps of:

(a) providing a plurality of pipeline sections having end fittings;

(b) conjoining the pipeline sections together to form the pipeline, with at least two block valves, and wherein at least two of the pipeline sections have a block valve therebetween; and

(c) laying the pipeline of step (b) in the sea.

After laying, the method preferably comprising the further step of: (d] opening the block valvefs].

The method of the present invention is advantageous for laying the pipeline in ultra- deep water as described above.

Preferably, all of the pipeline sections are conjoined together with block valves therebetween.

The block valves used in the method of the present invention may be one or more of the group comprising: part of the pipeline section end fittings, part of valve-units added between the pipeline sections, or both.

According to another embodiment of the present invention, the method comprises at least the steps of:

[i] providing a first pipeline section having an end fitting;

(if) conjoining the end fitting with a block valve unit;

(iii) conjoining the block valve unit with a second pipeline section to form the pipeline;

(iv) optionally repeating steps (ii) and (iii) to lengthen the pipeline;

(v) laying the pipeline in the sea;

(vi] opening the or each block valve.

According to a third aspect of the present invention there is provided a method of recovering a pipeline as defined herein from a sea comprising at least the steps of: (a) removing any liquid within the pipeline;

(b) closing the or each block valve; and

(c) recovering the pipeline from the sea.

Generally, the pipeline to be recovered is on a seabed, and is a flexible pipeline. Optionally, step (a] comprises blowing a pressurised gas along the pipeline. Such gas is usually an inert gas such as nitrogen, and can be added under pressure so as to help move liquid within the pipeline to its furthest or most distal end. Optionally, at least one block valve, possibly all of the block valves, is then closed with the pressurised gas still in the pipeline.

Alternatively, step (b) comprises closing the most distal block valve after blowing the pressurised gas along the pipeline; and releasing the pressurised gas from the pipeline prior to closing any further block valve. The pressurised gas may be released by a vent or by opening one or more other valves above the sea level, and generally closer to the end of the pipeline to be recovered first, which may or may not already be held by a suitable pipe-recovery vessel. In this way, the method of recovering a pipeline is wholly or substantially the reverse of the method of laying a pipeline as described herein. Again, any accidental damage or leaking of a portion or section of the pipeline leads only to flooding of that portion or section between closed block valves, avoiding any catastrophic increase in the weight of the pipeline (due to flooding of the complete pipeline where no block valves exist).

This reduces the safety margin of error required for in the top tension capacity T_c of the pipe-recovery vessel. And again, this allows a pipe recoverer either to use pipe- recovery vessels with a lower top tension capacity, or for the same pipe-recovery vessels to recover pipelines from deeper water, and in particular from ultra-deep water.

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

Figure 1 is a diagrammatic side view of a prior art method of laying a pipeline in a sea from a vessel; Figure 2 is a diagrammatic side view of a pipeline and method of laying a pipeline in a sea according to embodiments of the present invention;

Figure 3 is a diagrammatic side view of the opening of the pipeline laid in Figure 2 ready for use;

Figure 3a is an enlarged portion of Figure 3;

Figure 4 is a side part cross-sectional view of a part of a pipeline according to another embodiment of the present invention;

Figures 5a and 5b are schematic drawings of "normal situation" and "accidental situation" respectively in a prior art method of laying or recovering a pipeline in a sea;

Figures 6a, b and c are a normal situation and two accidental situations respectively for laying or recovering a pipeline in a sea according to a further embodiment of the present invention;

Figures 7a and 7b are a normal situation and an accidental situation respectively for a method of laying or recovering a pipeline in a sea according to a further embodiment of the present invention; and

Figures 8a and 8b are normal situation and accidental situation respectively of a method of laying or recovering a pipeline in a sea according to a further embodiment of the present invention.

Referring to the drawings, Figure 1 shows a prior art method of laying a pipeline in a sea from a vessel. The vessel 2 has a pipeline 4 extending therefrom towards the bottom of a sea 6 and onto a seabed 8. The distal end of the pipeline 4 is closed to the sea 6.

Along the length of the pipeline 4 there are located buoyancy modules 10, which are added in order to increase the floatability of the flooded pipeline 4. This not only reduces the top tension capacity acting on the vessel 2 in general, but ensures no serious failure of the vessel should there be any flooding of the pipeline 4, thereby ensuring that the top tension is well within the capacity of the vessel 2. The number of buoyancy modules 10 required obviously depends upon their degree of floatability, generally depending on their size. Figure 2 shows a pipeline 14 as one embodiment of the present invention, as well as a method of laying the pipeline 14 from a vessel 12 in a sea 16 according to another embodiment of the present invention.

The pipeline 14 comprises a plurality of conjoined pipeline sections having end fittings, wherein at least two of the pipeline sections include a block valve therebetween. Figure 2 shows four pipeline sections 20, either in full or partial view, having end fittings 30, and conjoined thereinbetween by three block valve units 22. The pipeline 14 is to be laid onto a sea bed 24.

During the laying the pipeline 14, the block valve units 22 are 'closed' or in a closed position, so that the pipeline 14 is not flooded, or is empty, or is otherwise 'dry', in comparison with the flooded pipeline 4 being laid according to the method shown in Figure 1. In this way, the gas, generally air, within the pipeline 14, increases the floatability of the overall pipeline 14, and therefore reduces the tension T acting on the vessel 12 This keeps the installation tension T within the laying vessel capacity Tc in case of any accidental pipe flooding. The block valve units 22 could be installed into the pipeline 14 on the laying vessel 22 during the connection of the pipelines sections 20, as each pipeline section 20 is being prepared for laying. Thus no different procedure or equipment is required by the installer. Suitable underwater block valve units 22 are known in the art and can be easily fitted and tested when installed between the pipeline sections 20. The end fittings 30 added to the end of each pipeline section 22 have familiar matings for the end or ports of a block valve unit 22. The connection between the block valve units 22 and the pipeline end fittings 30 can be tested onshore prior to be laid. The sealing integrity of the interface between a end fitting flange and the block valve units can be verified by carrying out a gaseous nitrogen (N₂)_g test via a special outlet port hollowed into end-fitting flange frame. One or more flanges or other intermediate means could be added between the end fittings 30 and the block valve units 22, such as API flanges, Grayloc, or other kinds of interfaces.

Once the pipeline 14 is laid, the block valve units 22 are opened to allow the passage of fluid along the pipeline 14, such an oil and/or gas from a seabed installation, to a surface or onshore. The block valve units 22 could be arranged to be opened remotely, for example by a suitable signal, such as an acoustic signal, or manually, such as by a diver or an ROV known in the art.

Figure 3 shows an example of the opening of a block valve unit 22 using an ROV 26 operating on an external handle 34. Figure 3a shows an enlarged portion of the Figure 3, wherein each end of the two pipeline sections 20 has a suitable pipe end fitting 30. The pipeline 14 is connected to a suitable floating platform 32 ready for use once the block valve units 22 are all open. Multiple handles at different angles, such as at 90°, could be added for suitable access whichever way the pipe becomes laid on the sea bed 24. Optionally, a swivel system can be used in between the block valve 22 and the end fittings 30 in order to allow the ROV 26 access to the block valve units 22 if they get hidden or stuck into the seabed 24.

After a time, it may be desired to recover the pipeline 20 from the seabed 24. To achieve this, a pressurised gas such as nitrogen could be blown through the pipeline 20 to expel any fluids therein. Then, the block valve at the pipe end, generally the most distal block valve, can be closed, optionally with the pipeline 20 still full of the pressurised gas. Considering Figure 3, a valve above sea level on the platform 32 could then be opened to release the pressurised gas, followed by closing of all the other block valves 22 in the opposite manner to their opening as described hereinabove. Once all the block valves 22 are closed, recovery of the pipeline 20 by a suitable pipe- recovery vessel can be carried out wholly or substantially in the opposite manner to its method of laying.

In this way, any failure or leakage in a section of the pipeline 20 between two block valves 22 will only affect that section, and the expected additional weight caused by any such flooding can be pre-calculated by the pipe recoverer to fit within the limitations imposed by the maximum top tension capacity of the pipe recovery vessel. That is, any such partially flooded pipeline will remain within the vessel handling capacity. Figure 4 shows a second pipeline 40 according to an embodiment of the present invention, comprising a plurality of conjoined pipeline sections 42 having end fittings 44, wherein at least two of the pipeline sections 42 include a block valve 46 therebetween and also a block valve 46 conjoined at the lower end of the pipeline 40.

In the second pipeline 40, the block valves 46 are formed within one of the end fittings 44. The block valves 46 may be formed integrally with one or more of the end fittings 44, or added to existing or standard end fittings 44. Optionally, a block valve 46 is included in the end fitting 44 at only one end of each pipe section 42, as shown in Figure 4, so as to provide one block valve 46 per pipeline section conjunction. However, the invention is not limited thereto.

Each block valve 46 includes actuators 48 such as external handles, for activating the block valve 46 between closed and open positions. Thus, once the pipeline 40 is laid in a sea as described above, actuation of the actuators 48 opens the block valves 46 to allow the flow of a fluid or liquid therethrough. Optionally, the actuators 48 are housed within the overall dimensions of the end fittings 44, so as not to extend therebeyond, and allow the forming, transportation and laying of the pipeline 40 to be carried out in a similar manner to the laying of a multi pipeline-section having only 'standard' end fittings in a manner known in the art. In particular, this includes reel-transportation and laying, which is generally a faster method of laying a pipeline than conjoining pipeline sections on a laying vessel. Thus, the incorporation of the block valves 46 as shown in the pipeline 40 in Figure 4 can be carried out either onshore, offshore, or both, particularly onshore. Figure 5a shows a first schematic of a known method of laying or recovering a pipeline 50 in a sea 51 in a 'normal situation. The pipeline 50 has a top valve 52 above the sea surface 54, and a bottom closed valve 56 in connection with a subsea installation 58. In this way, there is a "dry" section between the top and bottom valves 52, 56, such that the installation tension T is at least less than the top tension capacity T_c of the vessel (not shown] laying or recovering the pipeline 50.

If there were no problems laying the pipeline 50, this normal situation could be provided by any suitable laying vessel wherein T <T_C . But, any accidental damage or leak to the pipeline 50 under the sea level 54 will result in flooding of the entire pipeline 50a between the top and bottom valves 52, 56, which will very usually lead to the installation tension T now exceeding the top tension capacity T_c, with obvious consequences on the laying vessel. Thus, the laying vessel must have a safety margin of its top tension capacity able to accommodate the flooded weight of the entire pipeline 50a as shown in Figure 5b. This safety margin gap or margin obviously increases the greater the sea depth, to a point where few pipe-laying vessels are available to cope.

Figure 6a shows an embodiment of the present invention comprising a pipeline 60 in a sea 61. The pipeline 60 has a top block valve 62 above a sea surface 63 and is attached to a vessel (not shown], a bottom block valve 66 involved in the connection of the pipeline 60 to a subsea installation 67, and an intermediate block valve 64 thereinbetween. This time, and as shown in Figure 6b, any accidental damage or leak to the pipeline 60 between the top valve 62 and the intermediate block valve 64 only causes flooding 68a thereinbetween, such that the installation tension T of the part-flooded pipeline shown in Figure 6b is still less than the top tension capacity T_c of the vessel.

Similarly, Figure 6c shows that any accidental damage or leakage between the intermediate block valve 64 and the bottom block valve 66 only causes flooding 68b thereinbetween, still leaving T<T_C.

Figure 7a shows another embodiment of the present invention, wherein a pipeline 70 includes a top block valve 72 above a sea level 73, and attached to a vessel (not shown), six intermediate block valves 76, and a bottom block valve 74 involved in the connection of the pipeline 70 to an underwater installation 75. Any accidental damage or leak between two of the block valves 72, 74 and 76 only results in flooding 78 thereinbetween, as shown in Figure 7b. This decreases, in comparison with the situation shown in Figures 6b and 6c, the increase in the installation T after flooding, therefore maintaining T<T_C. Figure 8a shows another embodiment of the present invention based on a pipeline 80 comprising a top valve 82 above a sea level 83, a bottom valve 84, and a number of intermediate block valves 86. In the example shown in Figure 8a, the top and bottom valves 82, 84 are not block valves, and allow the flooding of the pipeline 80 up to the first of the block valves 86, both from the sea level 83, and from the subsea installation 87 to which the pipeline 80 is attached. Any partial flooding 85 is deliberately intended to still maintain the installation T<T_C, whilst not requiring block valves at one or both ends of the pipeline 80.

Figure 8b shows accidental damage or leakage between two of the block valves 86, leading to a flooded portion 88. However, T is still <T_C. All of Figures 6, 7 and 8 are examples of alternative arrangements for the location of block valves along a pipeline formed of a plurality of pipeline sections, so as to reduce the increase in weight of the pipeline should there be any accidental damage or leaking during installation or recovery. Clearly, the number of block valves located along the pipeline reduces the length between such block valves, and therefore reduces the increase in any weight due to accidental flooding between such block valves.

The number of pipeline sections and block valves required will be based on the nature and installation of the pipeline, in particular the depth of the water. However, in all of Figures 6, 7 and 8, the top tension capacity of the pipeline vessel can be reduced compared to that required for the situation as shown in Figure 5a (and more importantly in case of the situation as shown in Figure 5b), making the number of pipe laying vessels able to lay such a pipeline more commercially available, and/or increasing the depth to which such a pipeline can laid.

The present invention allows integration of mechanical protection for the valve itself, or its components, like actuators. The block valves could additionally or alternatively be opened using one or more actuators (not shown), and the number and disposal of suitable actuators could vary for redundancy reasons or for easier access, in particular to make it easier to open a block valve with equipment laid on the sea bed. For example, there could be two actuators 180° apart, or there could be three actuators 120° apart. The present invention provides a pipeline and a method of laying a pipeline, especially in deep or ultra-deep water, whilst not being overbearing to the top tension capacity, especially taking into account any accidental flooding that may occur during installation which may affect the top tension capacity. The present invention also allows a broader range of installation vessels from the installer's fleet of ships, or other available vessels, to be able to carry out the pipe laying and installation. Further, it reduces the installation time, as well as being cheaper and safer. Indeed, the block valve units 22 are standard equipment contrary to buoyancy modules. The use of block valve units 22 reduces installation time risk of damages of the pipeline 14 and thereby reduces the cost of offshore operation Various modifications and variations to the described embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined herein. Although the invention has been described in connection with specific preferred embodiments it should be understood that the invention as defined herein should not be unduly limited to such specific embodiments.

Claims

1. A pipeline for laying in a sea comprising a plurality of conjoined pipeline sections having end fittings, wherein the pipeline has at least two block valves, and at least two of the pipeline sections including a block valve therebetween.

2. A pipeline as claimed in claim 1 wherein at least one block valve is part of an end fitting.

3. A pipeline as claimed in claim 1 wherein at least one block valve is part of a valve-unit added between the pipeline sections.

4. A pipeline as claimed in any one of the preceding claims wherein the pipeline comprises at least three pipeline sections, optionally in the range 3-20 pipeline sections, and at least 3 block valves, optionally in the range 3-20 block valves.

5. A pipeline as claimed in any one of the preceding claims wherein the pipeline comprises all of or substantially all of the pipeline sections conjoined together with block valves therebetween.

6. A pipeline as claimed in claim 4 or claim 5 wherein the block valves are one or more of the group comprising: part of the pipeline section end fittings, part of valve-units added between the pipeline sections, or both.

7. A pipeline as claimed in any one of the preceding claims wherein one block valve is located at the end of the pipeline.

8. A pipeline as claimed in any one of the preceding claims wherein the pipeline is a flexible pipeline.

9. A pipeline as claimed in any one of the preceding claims wherein the pipeline is a riser or a liquid injection flowline or a hydrocarbon flowline.

10. A method of laying a pipeline in a sea comprising at least the steps of:

(a providing a plurality of pipeline sections having end fittings;

(b) conjoining the pipeline sections together to form the pipeline with at least two block valves, wherein at least two of the pipeline sections have a block valve therebetween; and

(c) laying the pipeline of step (b) in the sea.

11. A method as claimed in claim 10 comprising the further step of:

(d) opening the block valve(s].

12. A method as claimed in any one of claims 10-11 for laying the pipeline in ultra-deep water.

13. A method as claimed in any one of claims 10-12 wherein all of the pipeline sections are conjoined together with block valves therebetween.

14. A method as claimed in any one of claims 10-13 wherein the block valves are one or more of the group comprising: part of the pipeline section end fittings, part of valve-units added between the pipeline sections, or both.

15. A method as claimed in any one of claims 10-14 comprising at least the steps of:

(i) providing a first pipeline section having an end fitting;

(ii) conjoining the end fitting with a block valve unit;

(iv) optionally repeating steps (iij and (iiij to lengthen the pipeline;

(v) laying the pipeline in the sea; (vi) opening the or each block valve.

16. A method of recovering a pipeline as defined in any one of claims 1-9 from a sea comprising at least the steps of:

(a) removing any liquid within the pipeline;

(b) closing the or each block valve; and

(c) recovering the pipeline from the sea.

17 A method as claimed in claim 16 wherein step (a) comprises blowing a pressurised gas along the pipeline.

18. A method as claimed in claim 17 wherein step (b) comprises:

closing the most distal block valve after blowing the pressurised gas along the pipeline; and

releasing the pressurised gas prior to closing any further block valves.