Technical field The present invention relates to pipes, and in particular pipeline repair or modification. Background

Pipelines are used in many industrial processes to convey fluids from one location to another. Some pipelines, dependent for example upon their application and conditions of use, may over time develop a need for modification or may develop faults or become damaged such that the original or intended performance of the pipeline cannot be maintained. There may then be a need to recondition, modify or replace parts of a pipeline in order to repair or modify the pipeline so that it can continue to operate effectively. Various repair or modification operations may then need to be performed such as welding, cutting, joining a new section or branch, sealing, applying a coating, scraping off deposits, or the like.

A pipeline repair process may involve a number of steps.

One technique for repairing damage involves flooding the pipeline with water. In pipelines that carry an operational fluid in the form of a hydrocarbon product, particularly offshore pipelines, it is typical to remove the hydrocarbon product and flood the pipeline with water in order to safely allow the pipeline to be cut and a modified section be inserted or repaired. After such a modification or repair, the pipeline will then be dewatered and refilled with the hydrocarbon product again in order to resume operation. Such a technique may use pigs that are passed through the damaged section. The operational fluid may be a fluid that is conveyed in the pipeline during normal operation of the pipeline or system in which the pipeline is used, when not being repaired or modified.

Furthermore, it may be desirable to isolate an operational fluid contained in the pipeline from the location or section along the pipeline where the repair or modification is to be performed, such that a repair or modification operation can be performed safely at that location or section without interference of the operational fluid. It is known to do so by providing a mechanical plug in the pipeline, to prevent the operational fluid from flowing past the plug to the repair or modification location. These plugs are typically installed by pigging them into place from one end of the pipeline.

However, in situations where the damage to the pipeline section is such that it is not possible for a pig to pass through the damaged section, a pig based solution cannot be applied. Alternative solutions have been devised for non-through piggable damage where the pipeline is pigged from both ends and hydrocarbon fluid is discharged and disposed of at the point of damage via a hot tap.

There are drawbacks associated with existing pipeline repair or modification processes. They can be time consuming and costly requiring significant preparation and planning, and effort in mobilization of equipment and performance. Furthermore, the pipeline can remain non-operational for extensive periods of time. Typically, pipelines each have unique repair requirements, making cost effective pre-planning and pre-investment difficult. They may involve operating large temporary pumping spreads.

These are significant issues in hydrocarbon removal and water flooding operations, in particular where a damaged section is not through-piggable. Such operations rely on having sources of large volumes of water, which may not be easy to access, particularly at onshore locations. Moreover, some pipeline configurations may altogether prevent the pipeline from being pigged from both ends. Summary of the invention

There is provided according to one aspect of the invention a method of repairing or modifying a pipeline and according to another aspect of the invention apparatus for repairing or modifying a pipeline, as set out in the accompanying claims.

Each and any of the above aspects may include further features, as set out in the claims appended hereto or in the present description. It will be appreciated that features mentioned in relation to any of the above aspects, whether in the claims or in the description, may be combined with each other and between the different aspects in any appropriate combination. Brief description of the drawings

Figure 1 is a representation of the preparation of a pipeline for performing a repair operation according to an embodiment of the invention. Figure 2 is a representation of the replacement of a pipeline prepared as shown in Figure 1 .

Figure 3 is a representation of the resumption of operation of the pipeline upon performing the repair from Figures 1 and 2.

Detailed description of the invention

In the example of Figure 1 , a pipeline 1 is located at the sea bed 2. The pipeline contains inside it an operational fluid in the form of hydrocarbon fluid which during normal operation of the pipeline is passed through pipeline 1 as indicated by arrow 3. Downstream of the pipeline shown in Figure 1 , the fluid may pass into further pipelines or into other processing equipment for processing the hydrocarbon fluid. The hydrocarbon fluid may come from a well. A section 4 of the pipeline is damaged at a damage location 5 and requires to be replaced in order to repair the pipeline. The replacement may be performed as follows, firstly with reference to Figure 1 :

The pipeline is depressurised from normal operating pressure to the ambient pressure of the pipeline surroundings at the point of damage.

First and second hot tap fittings 6, 7 are connected to the pipeline on each side of the damage location 5 to provide fluid communication to the inside of the pipeline through respective apertures in the pipeline wall. Flow lines 8, 9 from a vessel 10 extend subsea and connect respectively at their far ends to the hot tap fittings 6, 7, and at their near, opposing, ends to the vessel 10. The fittings may include a valve acting as a tap, allowing gel from the flow lines when connected thereto, to be inserted controllably into the pipeline through the pipeline wall. Gel is injected through the flow lines 8, 9 from the vessel 10, through the hot tap fittings 6, 7 and into the pipeline 1 , to form plugs of gel in the pipeline. The injection of the gel through the first hot tap fitting 6 produces a first plug of gel 12 which extends inside and along the pipeline away from the damage location 5. The injection of gel through the second hot tap fitting 7 produces a second plug of gel 13 which extends inside and along the pipeline, away from damage location 5 in the other direction. In this example, the two plugs of gel are separated from each other by damaged sub-section 16 at the damage location 5. The subsection may contain trapped hydrocarbon fluid between the two plugs 12, 13. It will be appreciated that in other embodiments, the gel plugs may be positioned differently along the pipeline. In other embodiments, a single hot tap fitting may for example be provided, and a gel plug may be formed in the pipeline that extends across the subsection or the damage location.

The first and second gel plugs 12, 13 prevent hydrocarbon fluid contained in the pipeline in upstream and downstream end sections 14, 15 from accessing the section to be replaced 4 and its sub-section 16. Thus, each plug provides a fluid seal against an internal surface of the pipeline, preventing a flow of fluid along the pipeline across the plug.

Back flow tests may be performed to check the condition of the plugs 12, 13. Such tests may include applying different fluid pressures to the plug and measuring any fluid flow across the plug to determine the pressure or resistance to fluid flow of the plugs. The injection of the gel is performed so that the plugs can at least withstand an expected differential pressure across the plug during the repair process. Expected differential pressure values might be obtained based on data from pressure measurements made on the pipeline, or from other comparable pipelines or repair operations. In order to increase sealability, additional gel can be inserted to increase the length of the plug along the pipeline. The ability of the gel to support pressure is substantially proportional to the length of the plug. The appropriate length of the plug along the pipeline may depend upon the type of repair operation. It can also depend on pipe dimensions. Plug length may vary from 10 m to 1 km or more. In general the larger diameter pipelines would require a longer plug length.

Pipeline pressure may be measured at both ends of the pipeline, for example in sections 14 and 15, to monitor the pressure of fluid acting on the plugs.

With further reference to Figure 2, the pipeline is cut (a repair or modification operation) in first and second cut locations 17, 18 (repair or modification locations) such that the section to be replaced 4 can be removed. The plugs isolate the cut locations from the hydrocarbon fluid contained in sections 14, 15. Each plug extends lengthways along the pipeline past and to either side of each cut location, such that discharge of hydrocarbons and ingress of water through the pipeline to the cut location is prevented. As can be seen in Figure 2, cuts made through the pipeline at the repair location are made through the respective gel plugs. A remote operated vehicle (ROV) 19 is connected to the vessel 10 via a control umbilical 20 and may be used for performing the cuts. The ROV may be equipped with repair equipment for performing the cutting or other repair or modification operations or separate equipment may be deployed.

The pipeline section 4 is then removed. The removal of the pipeline section 4 may be carried out using the vessel and/or the ROV.

A replacement pipeline section 21 is then connected to the pipeline to replace the removed section 4. The replacement pipeline section has a plug of gel 22 formed therein. The plug prevents ingress of seawater through the replacement section, across the plug 22. Typically, the plug 22 extends the full length of the replacement section. In practice, as illustrated in the Figures 2 and 3, a short length within the pipeline at each end of replacement section may contain sea water. In Figure 2, this is shown by the plug 22 extending within the replacement section 21 at each end to a location which just short of the end of the section 21 .

Pipe connectors 23, 24 are provided on the ends 25, 26 of the replacement section and applied to the cut ends 27, 28 of the pipeline, to connect the replacement section to the pipeline. The connection of the replacement section to the pipeline may be performed by welding or other suitable means. The OV may carry tools and be used for performing the joining operation or separate equipment may be deployed.

With further reference to Figure 3, the pipeline 1 is then re-pressurised to at least normal operating pressure to resume normal operation. Re-pressurisation is performed initially by pressurising, substantially equally, the hydrocarbon fluid contained in the end sections 14, 15 of the pipeline to an elevated pressure above the ambient pressure. The hydrocarbon fluid in the pipeline is then allowed to flow downstream through the pipeline to resume normal operation as indicated by arrow 3. The plug of gel 22 and plug remnants 12b, 13b of plugs 12, 13 remaining in the pipeline after cutting are pushed through the pipeline by the hydrocarbon fluid. The gel may dissolve or disintegrate and be processed along with the hydrocarbon fluid further downstream. In this example, a pig 28 is disposed inside the pipeline and is used to sweep through the repaired pipeline to remove any residual gel remaining in the pipeline from the plugs. There may be a collector provided at an outlet of the pipeline 1 downstream that may be used to collect residual gel. After a period of time, normal operation of the pipeline 1 can continue without contamination of gel from the plugs.

It can be noted that the gel used for forming the plug have formulations compatible with the hydrocarbon fluid. Suitable gels are commercially available. A suitable gel may include a thixotropic gel. The gel is selected to have a yield strength that is exceeded when pumping the gel through the supply lines into the pipeline. Thus, the gel can flow into place in the pipeline. When static in the pipeline, the gel develops a yield strength that allows the pipe diameter to fill with gel and is capable of supporting a predetermined differential pressure, to thereby provide an effective seal.

In other embodiments still, replacement of a pipeline section may not be required. Instead, a repair or modification operation may be performed to repair the pipe without removal or replacement of a section. For example, repair or modification operations such as grinding and a restorative treatment may be applied successively to repair the pipe at a single repair location. Thus, it will be appreciated that the point of damage may be substantially coincident with the repair location at which a repair operation is performed.

The method provides a way to provide a temporary plug at a repair location without the need to drain hydrocarbon fluid from the pipeline or flood the pipeline with water. The gel plugs provide isolation of the hydrocarbon contents from the repair location to allow cutting and welding during repair or modification of a subsea pipeline, without the risk of hydrocarbon discharge or seawater ingress. The use of a hot tap connection allows injection and placement of the gel plug directly at the repair location. An entry point by way of an aperture in the wall of the pipe for delivering gel into the pipe can be installed when the need arises upon identification of damage or a fault. The entry point can be made at a suitable, desired location along the pipe. A hot tap installation allows this to be accomplished. This reduces or avoids the degradation of gel due to contamination by materials in the pipe which may otherwise be experienced by transport of gel over long distances, for example several hundreds of kilometres. By using hot tap connections and forming apertures through the pipe wall, one may select the location of the passageway to minimise such degradation. The location of the aperture may therefore preferably be selected to be as close as possible to the repair location (where for example a cut is required), the point of damage, and / or desired location for the plug. The aperture through which gel is injected is provided as close to the desired plug set location as is practical, typically within a few ten of meters. This may include an aperture at a location of less than 40, 30 or 20 m away. More specifically, the aperture may be less than 10 m away from such a location.

The formation of a hot tap connection may be considered to include the penetration of the pipe wall to allow injection of the gel to form a gel plug without the risk of uncontrolled release of hydrocarbons to the environment or the ingress of seawater. Hot tapping may include forming a T-connection in the pipe, and a passageway through the wall of the pipe. Various hot tapping processes are known in the art.

The method may facilitate:

1 ) Reduced down time of the pipeline during the repair or modification;

2) Removal of potential need to remove hydrocarbons at the damage location; 3) Simplified pre-planning for damage repair;

4) Reduced level of pre-investment for damage repair; and

5) Faster response time for damage repair. A further advantage is that it does not require availability of large volumes of water.

Various changes or improvements may be made without departing from the scope of the claimed invention. For example, although the description accompanying the drawings refers particularly to performing a repair at a repair location, it will be appreciated that other embodiments may equally relate to performing a modification of the pipeline at a modification location. It will also be appreciated that instead of there being damage or a fault at a damage location, it may in certain embodiments, for example in those where a modification is performed, simply be the case that the pipeline may not be operating as desired or not be operating optimally at a certain location on the pipeline.