This invention relates to a method of removing trapped hydrocarbons.

To allow for maintenance or decommissioning, it is preferred to remove hydrocarbons from pipelines in a plant or offshore facility. Build-up of residual volatile hydrocarbon vapour within the facilities of the process plant may lead to fire or explosions and it is thus extremely hazardous to both personnel and the environment.

A known safety measure is to flush the pipelines with water to displace their potentially hazardous contents and replace them with water. A main (often horizontal) pipeline usually supplies fluid to up-pipes and down-pipes connected thereto. The water flush displaces hydrocarbons in the main pipeline. A water flush will also displace hydrocarbons in the down-pipes but not in the up-pipes and certain other areas, where hydrocarbons often remain.

If the up-pipes have been fitted with drain lines, these trapped pockets of hydrocarbon fluid may be removed; however, there are many instances where this has not been done. In such cases a hot tap is required. Hot tapping is a known method where a drainage channel is introduced whilst the pipeline or pressure vessel continues to be in operation or contains hazardous substances. By its very nature, a hot tap is hazardous, requires careful planning and skilled and experienced personnel. It is classed as "hot work" that is a process that can be a source of ignition when flammable material is present, or it can be a fire hazard regardless of the presence of flammable material. As such, draining of the trapped hydrocarbon fluid using existing hot tapping techniques can take a long time, incur considerable cost and create hazard and associated risk.

An object of the present invention is to mitigate one or more of the shortcomings of the state of the art.

According to the present invention, there is provided a method of removing trapped hydrocarbons from pipework, the pipework comprising a through pipe extending from an iniet to an outlet, and at least one up-pipe extending upwardly from the through pipe, and in fluid communication therewith, the method comprising: flushing the through pipe with a water-based liquid which has a density less than hydrocarbons. in this way, any hydrocarbons trapped in the at least one up-pipe will be displaced by the lighter water-based liquid, and flushed out of the system.

The at least one up-pipe may he generally vertically orientated (80 - 100 degrees relative to the absolute horizontal), albeit any slight vertical orientation could still entrap hydrocarbons. Similarly, any downpipe may be vertical in orientation - between 80 and 100 degrees relative to the absolute horizontal. Each may, independently, be orientated between 80 and 100 degrees relative to the through pipe.

The water-based liquid may comprise a plurality of rigid containers, the rigid containers each having a sealed void containing a gas.

Preferably the water-based liquid has a specific gravity of less than 0.98 g/cm ³ , more preferably less than 0.90g/cm ³ optionally less than 0.78g/cm ³ . For certain applications it may even be iess than 0.70g/cm ³ , or less than 0.65g/cm ³ .

The rigid containers may be between 5 microns and 5mm in diameter, preferably between 10 microns and 500 micron in diameter and more preferably between 20 micron and 200 micron in diameter.

"Rigid" in this context means that the rigid containers are incompressible at the pressures found in use.

The gas in each rigid container may be air, nitrogen, argon or another gas sufficient to achieve a low bulk density.

Preferably the rigid containers are microspheres.

For certain embodiments, the water-based liquid comprises water and microspheres it may contain miscible solvents such as glycols or alcohols.

The water -based liquid is preferably immiscible with hydrocarbons.

The water-based liquid may include a long chain soluble polymer to act as viscosity; ng agent and preferably imparting non-Newtonian rheological properties to the fluid forming a fiowable gel. Examples of such polymers include Xanthan gum, carboxymethyicellulose and its derivatives, and/or guar and its derivatives.

The viscosity of the water-based liquid may reduce with applied sheer, that is it exhibits thixotropic properties.

Using a Fann 35 type rheometer with Bobl, Rotor 1 and Spring 1, the water-based liquid would, at a 300 rpm diai reading, typically be between 150 (0.15 Pa.s) and 300 (0.3 Pa.s), and at a 3 rpm dial reading typically be between 15 (1.5 Pa.s) and 100 (10 Pa.s). This follows the procedure outlined in "API Recommended Practice for Field Testing Water Based Drilling Fluids, API RP 13B -1/I50 10414- 1". The method may include directing a pipeline pig with the water-based liquid. The pig can he provided in front of, or behind the water-based liquid in this way it can, at least partly, clear the through pipe for the water-based liquid, and/or help drive the water-based liquid through the pipeline.

Preferred embodiments, include a pig train, comprising the water-based liquid and a first pig in front of the water-based liquid and a second pig behind the water-based liquid in this way, the water- based liquid is held between the first and second pipeline pigs.

Any suitable pig can be used.

The pig(s) can be a cylinder comprising a flat base at either end thereof. When compressed along the one axis, preferably it/tbey can tend to expand along another axis, maintaining the same overall volume, and then in the absence of external forces, they have a shape memory, and so return to their previous shape.

Preferably therefore the pigs are cross-linked gel-based pigs with elastomeric properties, that is it substantially regains its shape after being squeezed, within a second of the release of external 'squeezing' forces. Substantially regaining its shape is returning to between 90% and 110% of the previous length of each of the x, y and z axis; usually between 95% and 105% or between 98% and 102%,

The dynamic viscosity of the gei pig is typically more than 2000 Pa.s to essentially solid materials which do not flow and thus have a viscosity of over 5,000,000 Pa.s

The pigs may he a water- based or silicone based.

One suitable composition for the pig comprises: a silicone based oil fluid, a siloxane, silica, microspheres, and a catalyst, the base fluid comprising a polyalphaolefin based oil. The silicone based oil may be a non-reactive, straight chained polydimethylsiloxane. The siloxane may comprise a polydimethylsiloxane, a polysiloxane or a combination of polydimethylsiloxane and polysiloxane. The polydimethylsiloxane may be a vinyl terminated polydimethylsiloxane. The polysiloxane may be a methyl hydrogen polysiloxane.

There may be a plurality of up-pipes between the through pipe inlet and outlet. The volume of hydrocarbons to he displaced in the at least one, or all, up-pipes is normally less than the volume of water-based liquid used to flush the through pipe. For certain embodiments such a volume may be provided between the first and second pig.

A pre-fiush using water (or water- based liquid with a density greater than hydrocarbons) may also be performed. This can remove any hydrocarbons in the through pipe and also remove

hydrocarbons in any down -pipes, the downpipes extending downwardly and in fluid communication with the through pipe.

The through pipe may be provided in various process plants or on an offshore oil and gas facility.

The method may be used as part of a process to make-safe an offshore facility during

decommissioning.

According to a further aspect of the invention there is provided a kit of parts for performing the method described herein, the kit of parts including the water-based liquid.

Optional components and optional features used in the method described herein are also optional component or features which may be included in the kit of parts for example at least one pipeline pig, especially a cross-linked gei based pipeline pig with elastomeric properties.

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

Fig. 1 shows a schematic cross-sectional diagram of a section of a process plant;

Figs. 2 to 4 are a series of sequential views showing a pig train arrangement moving through the Fig, 1 process plant, in accordance with the present invention.

Fig. 1 shows a cross-sectional diagram of a section of a process plant 10 comprising a main through pipe 1 and two up-pipe sections 2a, 2b, After a water flush, hydrocarbon fluid 4 is trapped in the up- pipe sections 2a, 2b due to its lower density compared with water 3. in Fig. 2, a pig train 8 is introduced upstream of the trapped hydrocarbon fluid 4. The pig train 8 comprises two gel pigs 5a, 5b placed such that, along with the main pipe 1, they define a section therebetween which is filled with a water-based liquid 6 having a lower density than the

hydrocarbon fluid 4. in use, the pig train 8 is moved towards the up-pipe sections 2a, 2b by action of pumped water behind the pig 5a. in Fig. 3, the water-based liquid 6 within the pig train 8 is in fluid communication with the first of the up-pipe sections 2a. Since the water-based liquid 6 has a lower density than hydrocarbon fluid 4, the hydrocarbon fluid 4 is displaced by some of the water-based liquid 6, out of the first up-pipe section 2a and into the section of the main pipe 7 between the pigs 5a, 5b, and sealed from the rest of the main pipe 1. in Fig. 4, the pig train 8 comprises hydrocarbon fluid 4 that had been displaced out of section 2a and the remaining water- based liquid 6. The fluids are segregated by the action of gravi ty such that the hydrocarbon fluid 4 is located below the water-based liquid 6 The pig train 8 reaches the second up -pipe section 2b filled with hydrocarbon fluid 4. Similarly, the hydrocarbon fluid 4 is displaced out of the second up-pipe section 2b and into the section of the main pipe 7, and contained between the gel pigs 5a, 5b. At this stage, the pig train 8 comprises hydrocarbon fluid 4 that had been displaced out of sections 2a, 2b, overlaid with the remaining water-based liquid 6.

Once the pig train 8 passes the up-pipe sections 2a, 2b the hydrocarbon fluid 4 is fully displaced therefrom leaving the upright sections 2a, 2b filled with non-hazardous water-based liquid 6. The pig train 3 is then moved away from the upright sections 2a, 2b to conveniently transport the hydrocarbons away from the up-pipes 2a, 2b and out of the through pipe 1 Upon encountering a waste receptacle, the fluids are released thereto from the pig train 8, with the water-based liquid 6 overlaying the hydrocarbon fluid 4, thus rendering the up-pipes of the process plant 10 safe.

The pig train 8 comprising the water-based liquid 6 and gel pigs 5a, 5b provides a safer and more effective method of removing trapped hydrocarbon fluid from up-pipe sections 2a, 2b than the other methods known in the art.

Thus, for certain embodiments, the section of the process plant 10 may be cleaned and rendered safe in a single pass.

The waste receptacle (not shown) is provided at the outlet of the through pipe 1, downstream of the upright sections 2a. 2b. Upon discharge of fluids from the pig train 8 into the waste receptacle, the water- based liquid 6 floats atop the hydrocarbon fluid 4 in the receptacle (because of its lower density) thus acting as a safety barrier to the release of volatile hydrocarbon vapour. The barrier can be increased by adding quantities of the water-based liquid 6 to the waste receptacle. Once the treatment is complete, the waste can he processed, water and hydrocarbon fluid 4 can be drawn off and the water- based 6 fluid can be reused.

The gel pigs 5a, 5b can form a tight seal against the inner walls of the main pipe i, thereby providing a swabbing action. They are also able to contain fluids within the pig train 8, while being movable through long stretches of pipeline of varying diameters and able to navigate through any encountered geometric changes therein. The gel pigs 5a, 5b can negotiate short radius bends of 1,5D with the ability to regain their original shape. With an operating temperature range of -25 and 140°C, the gel pigs 5a. 5b can be propelled through the main pipe 1 by gas or liquids and do not require pig launchers or receivers.

One composition of the gei pigs 5a. 5b is sold by Aubin Ltd under the trade name EVO-Pig LG.

Suitable pigs are described, but not limited to those disclosed in UK Patent Application number GB2538966, the disclosure of which is incorporated herein by reference, in its entirety.

The exact composition of the water-based liquid 6 may be adjusted. Any non-hazardous bulk fluid immiscible with the hydrocarbon fluid may be used as the continuous phase, especially mixtures of water with other water miscible solvents, such as glycols and alcohols. For this embodiment, the water-based liquid 6 comprises: water acting as the continuous phase, a long chain water-soluble polymer which acts as a viscosifying agent preferably imparting non-Newtonian rheological properties to the fluid forming the gel, and microspheres which lower the fluid bulk density to less than 0.7 kg/litre, and therefore below the range of density values characteristic of the trapped hydrocarbon fluid 4. Examples of suitable the long-chain water-soluble polymer Xanthan gum, Carboxymethylcellulose and its derivatives, and Guar and its derivatives. In any case, the water- based liquid is preferably non-hazardous and environmentally acceptable.

The volume of the water-based liquid 6 provides sufficient excess over the sum of the volumes of trapped hydrocarbons, usually of the entire up-pipe sections 2a, 2b. Optionally, sufficient water- based liquid 6 may also be included such as to cover the entire surface area of the inlet of the waste receptacle.

Multiple upright sections can be treated sequentially in this manner, limited only by the volume of water-based liquid 6 provided in the pig train 8. As such, the process may be extended to main pipes with multiple up-pipes which can be treated in one pass. The method may be repeated as necessary for the whole process plant or offshore facility. in another embodiment, the section of the process plant 10 is flushed with the water-based liquid 6 following or before the insertion of a single gel pig Into the main pipe 1. The single gel pig can be one of the gel pigs 5a, 5b. This embodiment has similar advantages to the previous embodiment, but it requires the use of a single pig Instead of two pigs. in yet another embodiment, the section of the process plant 10 Is flushed with the water-based liquid 6 without the use of gel pigs 5a, 5b. This embodiment has similar advantages to the previous embodiments, but it does not require the use of pigs. in alternative embodiments, where pigs are used a cup-pig such as a conical cup-pig may be used instead of the gei pigs shown here.

Modifications and improvements can be incorporated herein without departing from the scope of the invention.