GB2598781A - A method and vessel for transporting a semi-stable oil product - Google Patents

Abstract

A method of transporting a semi-stable oil product from an offshore production facility. The method comprises: loading the semi-stable oil product into a storage container 23 on a transportation vessel 17 from the offshore production facility; and transporting the semi-stable oil product stored within the storage container on the transportation vessel. The storage container on the transportation vessel comprises a storage pipe 23. A transportation vessel for use in the method is also disclosed.

Description

A METHOD AND VESSEL FOR TRANSPORTING A SEMI-STABLE OIL

PRODUCT

The present invention relates to a method for transporting a semi-stable oil product. The invention further extends to a vessel for transporting a semi-stable oil product, and to the combination of the vessel with an offshore production facility. Overcoming current economic difficulties in exploiting marginal oil reserves is becoming increasingly important as known large reserves are depleted and it becomes more desirable to exploit smaller reserves in remote locations. Such marginal reserves may be considered as such because they are relatively small and/or spaced at large distances from existing infrastructure that would facilitate their exploitation.

One concept proposed for the exploitation of marginal reserves is to situate a production facility, e.g. a production platform, at the site of the reserve with a limited amount of processing equipment thereon. A conventional production facility at a large reserve would typically comprise sufficient processing equipment such that the fluid produced at the reserve is fully processed in order to separate the produced fluid into stabilised gas fractions and oil fractions, and optionally prepare the oil and gas fractions for final use. As will be appreciated, the equipment required for such a 'full-processing' of the fluid is associated with high operational expenditure, in addition to a large capital expenditure in providing the processing equipment in the first place. Given this high expenditure, such a conventional facility is not seen to be commercially viable for use at a marginal reserve having limited hydrocarbon reserves.

Consequently, it has been proposed to utilise a production facility with limited processing equipment situated at the site of the reserve. This has the advantage that there is a significant reduction in the capital and operational expenditure associated with the production facility.

Applicant's co-pending application, GB 1906716.4, discloses such a production facility. The production facility comprises a limited amount of processing equipment allowing for a part-processing of the fluid to form a semi-stable oil product. The semi-stable oil product is then loaded onto a vessel for subsequent transportation within storage tanks.

US 4446804 discloses a production system wherein no processing equipment is situated at the site of the hydrocarbon reserve. In this document, it is proposed to use a tanker for receiving produced fluid directly from a production well without any processing of the produced fluid prior to receipt on the tanker. The produced fluid is loaded onto the tanker whilst being maintained at high (100 bar) pressure and is stored thereon within storage tanks under pressurised conditions.

The disclosure of US 4446804 differs from that of GB 1906716.4 in that it does not pertain to the transportation of a semi-stable oil product, and there is no suggestion that it would be suited for use in the transportation of such a semi-stable product. The fluid transported in US 4446804 has not been processed at all and thus is not stabilised to any extent.

Transportation of semi-stable hydrocarbon oil-products on vessels, as described in GB 1906716.4, utilises pressurised storage tanks. In fact, storage tanks are typically used for the transportation of hydrocarbon oil products more generally on-board vessels (e.g. as demonstrated in US 4446804 or US 2009/321080). Whilst such storage tanks are technically fit for the purpose of storing hydrocarbon oil products for transportation, the tanks and vessels comprising said tanks are expensive to provide and maintain. This is not least because of the amount of material, typically steel, that is required in order to ensure that the tanks are robust enough to safely maintain the pressurised contents therein. Moreover, the high amount of material required for such tanks makes them extremely heavy. This is disadvantageous as it increases the costs associated with transportation and thus reduces the overall financial return from the transported hydrocarbon product. Furthermore, these tanks are typically cumbersome and irregularly shaped. Thus, there is a poor utilisation of the total space available on the vessel for the transportation of hydrocarbon product thus leading to less than desired financial return on the transported hydrocarbon product on each vessel. . There is thus a desire for an alternative storage solution on-board a vessel for the transportation of an oil product, and in particular a semi-stable oil product. Thus, in a first aspect, the present invention provides a method of transporting a semi-stable oil product from an offshore production facility, the method comprising: loading the semi-stable oil product into a storage container on a transportation vessel from the offshore production facility; and transporting the semi-stable oil product stored within the storage container on the transportation vessel; wherein the storage container on the transportation vessel comprises a storage pipe. 3 -

In its broadest sense, the present invention can thus be seen to provide a method of transporting an oil product (e.g. a semi-stable oil product or a produced hydrocarbon product that has undergone no processing whatsoever). The invention is, however, particularly advantageous in the context of semi-stable oil product transportation.

The use of 'pipe' storage as compared to typical 'tank' storage is advantageous since it is associated with a significantly lower capital and operational expenditure, particularly in the context of transporting semi-stable oil product. As will be appreciated, and as discussed further below, a semi-stable oil product must be transported under pressurised conditions. Maintaining such conditions in typical tank' type storage requires thick steel walled tanks. These tanks are expensive to provide (given the large amount of material, typically, required), and are also expensive to transport given their weight (again, given the large amount of material required). The requisite wall thickness also limits the size of the tank, meaning that the volume, and thus value, of semi-stable oil product that can be transported on a vessel comprising tank storage is limited.

In contrast, pipe storage is relatively cheap to provide given that standard, 'off-the-shelf pipes can be used as the basis for said storage. Moreover, for a given volume of storage, pipe storage can have a comparatively smaller wall thickness, particularly where a plurality of storage pipes are used (more on this below). Thus, an equivalent volume of hydrocarbon product can be transported at a comparatively lower total weight using pipe storage and can be achieved at a lower capital expenditure.

Pipe' storage also better utilises the available space on a vessel as compared to 'tank' storage. Since 'pipe' storage has a regular shape it can be easily arranged to efficiently fill a large portion of the available space on the vessel (particularly in scenarios where a plurality of storage pipes are used -more on this below -since their regular shape allows for the storage pipes to be tightly packed and thereby efficiently fill the volume on board the storage vessel). In contrast, storage tanks have an irregular shape, in part resulting from the flanges, nozzles and welded supports typically provided on the outside of said tanks Thus, given their irregular shape, storage tanks inefficiently use the available volume on board a vessel meaning that comparatively less semi-stable oil product may be transported on said vessel. As such, vessels having tank storage provide poorer capital returns 4 -In the context of the invention, a storage pipe is considered to be a section of pipe with closed ends. Optionally, the ends will be hemispherical in shape, further optionally formed from hemispherical caps attached to the section of pipe.

The section of pipe used to form the storage pipe can be a standard, 'off-the-shelf' component.

The pipe may have a nominal size of between 40-60 inches (100 cm -150 cm). Optionally, the pipe may have a nominal size of 42 inches (110cm) or 56 inches (140 cm), or may have a nominal size in the range of 42 inches (110 cm) to 56 inches (140 cm).

The pipe may be an X52, X56, X60, X65,X70 or X80 pipe in accordance with the API SPEC 5L specification.

The storage pipe may have a length of between 20 m to 30 m, optionally 26 m. Such a storage pipe would then have storage volume in the region of 30 m3 to 40m Pipe storage for the transportation of certain types of hydrocarbon products is known. As demonstrated in, e.g., US 2007/017575 and WO 03/006309, pipe storage for the transportation of a compressed gas liquid hydrocarbon product and a gas product are known. However, transportation of hydrocarbons using pipe storage has been limited to the transportation of gas, and gas derived, products.

The term "semi-stable" herein is used to describe a liquid that has been stabilised to a certain extent, but has not been fully stabilised. This means that, under certain pressure and temperature conditions (in this case, the conditions of loading onto the vessel and storage within the pipe), it will remain in a single (liquid) phase, avoiding evaporation and precipitation (i.e. the precipitation of hydrates in the liquid). However, unlike a fully-stabilised liquid, it must be maintained at a pressure above atmospheric pressure to retain it in that state.

Preferably, the oil product is taken outside of the "hydrate envelope" for the conditions under which it will be held whilst being loaded onto and transported in the pipe storage on the vessel. Hydrates are ice-like crystalline solids composed of water and gas, and hydrate deposition on the inside wall hydrocarbon pipelines, processing equipment, transportation vessels etc. is a serious problem in oil and gas production infrastructure. As discussed below with reference to Figure 4, for a given hydrocarbon fluid, hydrates form at higher pressures and lower temperatures.

When warm hydrocarbon fluid containing water flows through equipment with cold

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surfaces, hydrates will precipitate and adhere to the surfaces of the equipment. This reduces the equipment cross-sectional area, which, without proper counter measures, will lead to a loss of pressure, function and ultimately to a complete blockage of the equipment. The processing and subsequent transportation of a gas-containing product therefore normally requires hydrate control.

An oil product is semi-stabilised by part-processing, and such part-processing typically involves the partial degassing of produced fluid and/or the separation of water from the produced fluid to a certain extent. Degassing of the produced fluid to a certain extent can be considered as a reduction in its gas composition (i.e. of fractions that are gaseous under ambient conditions), in particular a reduction of the gas composition as compared to the produced fluid emanating from the reserve at which the production facility is situated. Most typically, it is the lightest gas fractions (e.g. C1-C4) that are removed from the produced fluid in the formation of the semi-stable oil product. The extent of this processing is preferably dependent on the conditions at which the semi-stable oil product will be held whilst loaded onto and being transported in the vessel, such that it may be taken outside of the hydrate envelope, as noted above. As the fluid will cool whilst being loaded onto and stored on the vessel in the pipe storage, and as its pressure will reduce whilst being loaded and stored (due to frictional losses and imperfect storage), it is necessary to consider conditions during loading and storage. A semi-stable oil product typically still comprises some gas fractions from the produced fluid combined with oil fractions and a small amount of water from the produced fluid in a single liquid phase, wherein the gas fractions remain entrained in the liquid product under pressurised conditions.

In line with the explanation above, the term "part-processing" herein is used to indicate that, whilst the produced fluid is processed, it is only processed to a limited extent and is not processed fully. Full processing would involve the preparation of an oil product and, optionally, a gas product into a fully stabilised state. In contrast, the produced fluid in the present invention is only "part-processed" to form a semi-stable oil product.

The stability of an oil product is often described by its true vapour pressure (TVP). The true vapour pressure of a fully stabilised oil product is typically around 0.97 bar, and such an oil product will be stable under atmospheric conditions. Part-processing of produced fluid to form a semi-stable oil product may lower the TVP of the oil product to below the TVP of fluid in the reservoir, but the TVP of the semi- 6 -stable oil product will remain above 0.97 bar, more usually above 1 bar, and more typically above 1.3 bar. Given that the TVP of a semi-stable oil product is higher than that of a stabilised oil product, the semi-stable oil product may be termed a high TVP (HTVP) oil product.

Producing such a semi-stable oil product is advantageous since the amount of processing of the produced fluid in the vicinity of the production facility (e.g. prior to transportation) is reduced compared to producing a fully stabilised product. Given the semi-stable nature of the oil product in the first aspect of the invention, the loading and subsequent transportation of the semi-stable oil product in the storage pipe may be carried out at pressurised conditions to maintain the semi-stable oil product in its semi-stable state (i.e. to prevent separation of the oil and gas products within the semi-stable oil product). In this context, pressurised refers to a pressure significantly above atmospheric pressure.

The pressure of the semi-stable oil product may be sufficient such that no pumps and/or compressors are required on the vessel for the loading operation of the semi-stable oil product. That is, the pressure of the fluid from the production facility may be sufficient to allow for the step of loading the semi-stable oil product into the storage pipe(s) under its own impetus. At the very least, the pressure of the semi-stable oil product would reduce the demand on compressors and/or pumps for the loading operation and may do away with the requirement for providing a compressor and/or pump on the transportation vessel.

The pressurised conditions may be between 5 bar -400 bar, optionally 10 bar -100 bar, optionally 20 bar -80 bar, and further optionally 40-60 bar, for example 55-56 bar. The exact pressurised conditions may be selected dependent on the nature of the semi-stable oil product, the tolerances of the storage pipe and/or the tolerance of the equipment used for loading the semi-stable product into the storage pipes.

The production facility may comprise a platform, the platform optionally comprising the necessary processing equipment in order to part-process the produced fluid to form the semi-stable oil product. Although the invention may be for transporting the semi-stable oil product from a conventional manned production platform, since only limited processing of the produced fluid is required to form the semi-stable oil product, an unmanned production platform (UPPTM) is both suitable and preferred. The use of a UPPTM greatly improves the commercial viability of 7 -producing at a marginal reserve given the reduced capital and operational expenditure associated therewith.

The offshore production facility from which the semi-stable oil product is being transported may be situated at a marginal reserve (remote oil field) such that it can exploit the hydrocarbon reserves thereat. Such a marginal reserve may be situated more than 60 km from the next nearest reserve and/or production infrastructure, optionally more than 100 km, further optionally more than 200 km away. The marginal reserve may be situated at a distance from the next nearest reserve and/or production infrastructure at which it is not suitable to exploit the hydrocarbon fluids at the marginal reserve via a 'typical' production facility and/or a long distance 'fie-back' solution as is known in the art.

The vessel may be a tanker.

The storage container on the transportation vessel may comprise a plurality of storage pipes into which the semi-stable oil product is loaded and transported.

The vessel may in fact comprise thousands of such pipes into which the semi-stable oil product is loaded and transported, e.g. 2,000 storage pipes. The number of storage pipes may be selected to maximise storage on the vessel. Thus, the method may comprise loading the semi-stable oil product into a plurality of storage pipes on the transportation vessel from the offshore production facility; and transporting the semi-stable oil product stored within the plurality of storage pipes.

Each of the plurality of pipes may be loaded simultaneously with the semi-stable oil product, or there may be a sequential loading of some or all of the storage pipes. The use of a plurality of storage pipes may allow for the transportation of a volume of semi-stable oil product on a vessel that could not technical feasibly achieved by storage tanks given the amount of material that would be required for such storage tanks and thus the weight of the resultant vessel.

The vessel may comprise a plurality of cargo holds. Each cargo hold may have a sub-set of the plurality of storage pipes housed therein, e.g. each cargo hold may comprise up to100 storage pipes. For example, each cargo hold may comprise at least 10, 20, 30, 40, 50, 60, 70, 80 or 90 storage pipes. The cargo holds may be sealed from one another to prevent leakage of liquids (e.g. oil)therebetween. Gas (e.g. an inert gas) however may be able to transfer between the cargo holds.Each cargo hold may be inerted with, e.g., nitrogen. That is, each cargo hold may be filled with a nitrogen gas to provide an inert environment surrounding the sub-set of storage pipes positioned therein. Each cargo hold may be provided with drainage. 8 -

The drainage may allow for any oil product that escapes/leaks from the storage pipes housed within the cargo hold to be removed as appropriate from the cargo hold to be dealt with elsewhere on the vessel. The above described cargo hold arrangement is, as will be appreciated by the skilled person, comparable in this respect to the "tank-in-tank" systems used on conventional crude oil carriers.

The, or each, storage pipe may be connected to one or more vent mast(s) situated on the vessel. The connection of the storage pipe(s) to the vent mast is optionally achieved by appropriate conduits that connect at a top of the storage pipe(s). One or more pressure relief valves (or equivalent) may be situated in the conduits between each storage pipe and the vent mast, at an outlet of the storage pipe(s) and/or at an inlet to the vent mast. The vent mast allows for any gas that separates out of the semi-stable oil product stored within the storage pipe(s) to escape via the conduits and to be vented away safely for further storage on the vessel, for use on the vessel or at the production facility (e.g. as a gas injectant or as a fuel), or to be disposed of. Thus, the vent masts prevent a pressure build up resulting from an accumulation of gas product on the vessel which might otherwise be dangerous.

The pressure relief valve(s) should be biased such that a threshold pressure is maintained in the storage pipe(s) to prevent depressurisation, and thereby separation and stabilisation, of the semi-stable oil product therein. The threshold pressure may be determined to account for the nature of the semi-stable oil product stored within the storage pipe(s) and/or the technical specification and tolerances of the storage pipe(s). The bias of the pressure relief valve(s) should enable gas that does separate out in the storage pipe(s), which would therefore raise the pressure therein, to escape from the storage pipe(s) and through the vent mast(s) once the pressure in the storage pipe(s) exceeds the threshold pressure. This would occur until such a time as pressure in the storage pipe(s) returns to below the threshold pressure set by the pressure release valve(s).

Thus, the method of the first aspect may comprise venting gas separated out from the semi-stable oil product within the storage pipe(s), optionally via one or more vent mast(s). The method may utilise the vent mast(s) and related components as described above.

The storage pipe(s) on the vessel may be capable of storing 10,000 m3 -200,000 m3 semi stable oil product therein. For example, the storage pipe(s) may be capable of storing 64,000 m3 of semi-stable oil product. 9 -

The storage pipe(s) may be vertically arranged on the vessel. Alternatively, the storage pipe(s) may be horizontally arranged on the vessel. Any other suitable orientation of the storage pipe(s) may also be used. In scenarios where there are a plurality of storage pipes, all the storage pipes may be arranged in a common orientation (e.g. all arranged vertically) or a combination of orientations (e.g. some vertical and some horizontal). The exact combination of orientations may be selected to optimise the number on the vessel and thereby the total volume of storage.

Prior to loading the semi-stable oil product into the storage pipe(s) on the vessel, the method may comprise pre-pressurising the storage pipe(s). By pre-pressurising the storage pipe(s), flashing/separation and destabilisation of the semi-stable oil product can be avoided whilst loading the semi-stable oil product into the pipe(s) The pre-pressurisation of the storage pipe(s) may be achieved by filling them with a gas (e.g. carbon dioxide) or a liquid (e.g. water). Optionally, the pre-pressurisation is achieved using a hydrocarbon gas product, for example methane, ethane, propane, butane or a mixture of these gases. Advantageously the pre-pressurisation is achieved using comparatively light gas fractions since it is typically these fractions that are not transported as part of a semi-stable oil product given the technical difficulties associated therewith, and are thus separated out of the produced fluid upon formation of the semi-stable oil product at a/the production facility.

The pre-pressurisation of the storage pipe(s) may occur at a site of the production facility (i.e. after the vessel has arrived at a site of the production facility but prior to loading of the semi-stable oil product). This is particularly advantageous in scenarios where a hydrocarbon gas product (notably a light hydrocarbon gas product) is used as the pre-pressurisation fluid since this can be readily obtained from the production facility, as a by-product of the produced semi-stable oil product or otherwise.

Alternatively, the pre-pressurisation of the storage pipe(s) may occur prior to the vessel arriving at a site of the production facility (e.g. at an end destination of the vessel, whilst the previous cargo of semi-stable oil product is offloaded from the vessel).

-10 -In scenarios where the pipe storage is pre-pressurised with a fluid, the step of loading the semi-stable oil product into the storage pipe(s) will comprise displacing the fluid used for pre-pressurisation with the semi-stable oil product.

The pressure within the storage pipe(s) after pre-pressurisation may be equal to, or slightly lower than, the pressurised conditions at which the semi-stable oil product is to be transported and loaded into the storage pipe(s). As such, when loading the semi-stable oil product into the storage pipes, the fluid used for pre-pressurisation can be displaced from the storage cylinders by the semi-stable oil product with little (or no) external impetus provided to the semi-stable oil product (e.g. as provided by a pump and/or a compressor).

One or more conduit(s) may be connected to the, or each, storage pipe which allows for the fluid used for pre-pressurisation to enter upon pre-pressurisation of the, or each, storage pipe. The same conduit(s), or separate conduit(s), may allow for the pre-pressurisation fluid to escape from the storage pipe(s) after displacement. The conduit(s) used for the pre-pressurisation fluid may be the same, or at least partly overlap with, the conduit(s) used for connection of the storage pipe(s) to the vent mast(s).

In scenarios where the pre-pressurisation fluid is a hydrocarbon gas product, after the gas product has been displaced from the storage pipes through loading of the semi-stable oil product, the method may further comprise conveying the gas product to the production facility for use as fuel for power generation (e.g. for use in a gas turbine engine situated at the production facility), for use as an injectant at a gas injection well or for disposal (e.g. as part of a flare). Additionally and/or alternatively, the method may comprise storing the displaced gas product on the vessel for transportation in one or more storage container(s) and/or using the gas product as fuel for power generation upon the vessel. The optional storage container(s) for the gas product would be separate to the pipe storage used for the semi-stable oil product.

As alluded to above, the semi-stable oil product loaded into the storage pipe(s) may comprise water fractions therein. After loading of the semi-stable oil product into the storage vessel, the water within the semi-stable oil product may begin to settle out at the bottom of the storage pipe(s) from the rest of the semi-stable oil product given its relatively higher density and the effects of gravity. In light of this, the method of the first aspect may comprise the further optional step of drawing water out of the storage pipe(s) that has settled out from the rest of the semi-stable oil product. This water can then optionally be transferred back to the production facility, particularly in a scenario where the water is drawn from the storage pipe(s) whilst the vessel is still connected at a site of the production facility (e.g. whilst the loading operation is still being completed). At the production facility, the water may be treated, used (e.g. as injectant at water injection wells) and/or disposed of. Alternatively, the method may comprise sending the drawn water to a water storage tank on the vessel, for later use or otherwise, treating the water on the vessel, and/or disposing of the water overboard (typically after treatment of the water).

The method may comprise the further step of offloading the semi-stable oil product (and optionally any stored gas product) once the vessel has reached its desired destination (e.g. an onshore receival plant). The offloading may comprise introducing a displacement fluid into the storage pipes to displace the semi-stable oil product stored therein. The displacement fluid may be the same type of fluid used for pre-pressurisation as discussed above.

The present invention extends to a transportation vessel for use in the method of the first aspect. Accordingly, in a second aspect, the invention provides a transportation vessel comprising a storage pipe having semi-stable oil product therein.

The transportation vessel may be in accordance with the transportation vessel as described above in relation to the first aspect, and may thus benefit from the optional features and associated advantages of the vessel as described above.

In a third aspect, the present invention provides a combination of a transportation vessel in accordance with the second aspect and an offshore production facility.

The offshore production facility of the third aspect may be in accordance with the offshore production facility as described above in relation to the first aspect, and may thus benefit from the optional features and associated advantages of the production facility as described above.

In particular, the offshore production facility of the third aspect may comprise or consist of a UPPTM situated optionally at a marginal reserve. The offshore production facility and/or UPPT" may have limited processing equipment situated thereon, e.g. only enough processing equipment to produce the semi-stable oil product for subsequent storage and transportation on the vessel. Such a production facility in combination with the vessel of the second aspect provides a synergy in -12 -that both allow for a significant reduction in the crew required for their operation and are also come at significantly reduced capital and operational cost. Thus, this combination provides a means for producing and retrieving hydrocarbons from, e.g. marginal reserves, at significantly reduced capital and operational expenditure.

Certain embodiments of the present disclosure will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 depicts a process at a production facility of loading a vessel with a semi-stable oil product, the semi-stable oil product being produced at an offshore platform from hydrocarbon fluid produced from a subsea well; Figure 2 is a schematic representation of storage on-board the vessel as depicted in Figure 1; Figure 3 is part-cutaway profile view and cutaway plan view of the vessel of Figures 1 and 2 and Figure 4 shows a generic hydrate-formation phase diagram for an oil product.

Figure 1 depicts a vessel 17 and an offshore production facility 100 at a remote oil and gas field (marginal reserve), whereby the oil and gas field is distanced tens of kilometres from any other production facility.

The production facility 100 comprises a production well 1 connected to a topside of an offshore platform 5 by a conduit 3 that is configured to transport produced hydrocarbon fluid from the well 1 to the platform 5. As shown in Figure 1, the offshore platform 5 is an unmanned production platform (UPPTM) 5. The UPPTM 5 comprises processing equipment (not shown) configured to process the produced fluid received from the well 1 in order to form a semi-stable oil product (see discussion below) and a gas product. The UPPTM 5 is in turn connected to a subsea distribution manifold 9 by a conduit 7, the conduit 7 being configured to transport semi-stable oil product and a gas product from the UPPTM 5 to the distribution manifold 9. (N.B. the conduit 7 may be divided into two hermetically sealed and separate conduits for the separate passage of gas product and semi-stable oil product, respectively). The distribution manifold 9 may optionally comprise a pump as discussed in further detail below.

The distribution manifold 9 has a first riser 11a and a second riser 11b attached thereto, the first and second risers 11a, 11 b being catenary risers 11a, llb that allow for the passage of semi-stable oil product and gas product (e.g. via two hermetically sealed conduits positioned therein c.f. conduit 7). The first riser -13 -lla and the second riser llb are connected to a first buoy 15a floating at sea level and a second buoy 15b floating at sea level respectively, the first and second buoys 15a, 15b being configured to receive the semi-stable oil product and gas product from the first and second risers 11a, 11b. Each of the first buoy 15a and the second buoy 15b are moored by a first set of mooring lines 13a and a second set of mooring lines 13b respectively. The mooring lines 13a, 13b allow for limited movement of the buoys 15a, 15b in both a horizontal and vertical direction so as to account for movement of the buoys 15a, 15b due to changing sea conditions and weather, but otherwise the mooring lines 13a, 13b fix the buoys 15a, 15b substantially in place. Each of the buoys comprise an attachment 18a, 18b that are each configured to connect to a vessel, such as a ship 17, and allow for semi-stable oil product and gas product to pass from the buoys 15a, 15b to a vessel, such as the vessel 17, for transportation.

The vessel 17 is a tanker 17 configured for the transportation of semi-stable oil product produced at the UPPTM 5. The storage on-board the tanker 17 used for transportation of the semi-stable oil product is shown schematically in Figure 2 and comprises a plurality of vertically arranged storage pipes 23 for storage of the semi-stable oil product therein. In Figure 2 only four storage pipes 23 are shown positioned within a single nitrogen inerted cargo hold 20; however, as shown in Figure 3, the tanker 17 comprises thousands of such pipes 23 positioned in several corresponding cargo holds 20 for storage and transportation of the semi-stable oil product.

With further reference to Figure 2, the nitrogen inerted cargo hold 20 provides a sealed and inerted environment about the storage pipes 23, which reduces risks in the event of leakage of hydrocarbons from the storage pipes 23 or associated equipment within the cargo hold 20. A pressure relief valve 20a is provided at the top of the cargo hold 20 and provides a vent to atmosphere for the cargo hold 20 in the event that the pressure inside the cargo hold 20 exceeds the pressure set by the pressure relief valve 20a (e.g. because of a leakage of hydrocarbons from the storage pipes 23 or otherwise).Each storage pipe 23 has a 42 inch nominal size and meets an X70 specification. The storage pipes 23 are configured to store the semi-stable oil product therein at the pressure at which the semi-stable oil product can be maintained in its semi-stable state.

Each of the storage pipes 23 are connected at their lower end to a conduit 21. At their upper end each of the storage pipes 23 are connected to a header 26 -14 -which in turn is connected to a conduit 25 via a control valve 25a and a vent mast 22 via a pressure relief valve 22a The vent mast 22 vents to the environment outside of the cargo hold 20.

The storage on-board vessel 17 additionally comprises storage containers 27 configured for storage and transportation of a gas product. Each of the storage containers 27 are connected at their lower end to a conduit 29. At their upper end each of the containers 27 are connected to a header 24 which in turn is connected to the conduit 29 via a control valve 29a and the vent mast 22 via a pressure relief valve 22b. The vent mast 22 vents to an open environment outside of the cargo hold 20.

Additionally provided on the tanker 17 is a water storage tank 30 connected to a branch of the conduit 21 by a control valve 30a.

In use, fluid is produced at production well 1. The produced fluid is a hydrocarbon fluid comprising a mixture of hydrocarbon fractions, including both gas fractions and oil fractions. The gas fractions (e.g. Ci-C4) comprise those fractions that are typically found in the gas phase at atmospheric pressures and temperatures. However, at least a portion of these gas fractions, particularly the heavier gas fractions, may be in the liquid phase whilst comprised in the produced fluid due to the elevated pressure of the produced fluid, which may for example be 100 bar. The oil fractions (e.g. 05+) of the produced fluid comprise those hydrocarbon fractions that are typically in the liquid phase at atmospheric pressures and temperatures. The produced hydrocarbon fluid from the well 1 may additionally comprise non-hydrocarbon constituents, e.g. water.

The produced fluid is passed from the well 1, via the conduit 3, to the UPPTM 5. On board the UPPTM5 the produced fluid is part-processed by the processing equipment. The part-processing involves a number of sequential separation (including water and gas separation), compression, cooling, scrubbing and recombination steps to form a semi-stable oil product at a pressure of, e.g., 55bar. A gas product is also formed as a resultant by-product of the formation of the semi-stable oil product comprising, e.g., methane and ethane and can be compressed on board the UPPT" 5 to the desired pressure, e.g. 55 bar.

After the semi-stable oil product and gas product have been formed at the UPPTM 5, the semi-stable oil product and/or gas product may be stored on or at a site of the UPPT" (e.g. subsea). Alternatively, the gas product and subsequently the semi-stable product may not be stored prior to being loaded onto the tanker 17 -15 -The loading of the semi-stable oil product on to the tanker 17 is carried out as follows. Firstly the tanker 17 arrives at the site of the UPPT" 5 and connects to either the first attachment 18a of the first buoy 15a or the second attachment 18b of the second buoy 15b. As depicted in Figure 1, the tanker 17 is connected to the attachment 18b of the second buoy 15b. The connection between the tanker 17 and the second attachment 18b provides a passage for gas product and semi-stable oil product to pass from the UPPTM 5, to the second buoy 15b, through the second attachment 18b and onto the tanker 17 for storage thereon.

Once the connection between the tanker 17 and the UPPT" 5 has been established, gas product from the UPPT" 5 is transferred to the tanker 17 via the conduit 7, distribution manifold 9, riser 11 b, buoy 15b and attachment 18b, and is received at the conduit 25. The gas product is then passed into each of the storage pipes 23 via the header 26 by opening of the control valve 25a.

The pressure in the gas product produced by virtue of a compression at a site of the UPPT" 5 may be sufficient to enable loading of the gas product to the tanker 17; however additional compressors to aid in loading the gas product onto the tanker 17 may also be provided (e.g. at the distribution manifold 9 or on the tanker 17 itself).

The transfer of gas product into the storage pipes 23 from the UPPTM 5 is continued until the storage pipes 23 are filled with gas product at a desired pressure, e.g. 55 bar. The pressure in the storage pipes 23 created by the gas product therein is selected to correspond closely to, e.g. be slightly less than or the same as, the pressure at which the semi-stable oil product produced at the UPPT" is to be maintained and stored.

Once the storage pipes 23 have reached the desired pressure with gas product therein further flow of gas product to the storage pipes 23 is prevented by closing the control valve 25a.

Next, the semi-stable oil product from the UPPTM 5 is transferred to the tanker 17 via the conduit 7, distribution manifold 9, riser 11 b, buoy 15b and attachment 18b, and is received at the conduit 21. The semi-stable oil product is then passed into each of the storage pipes 23 by opening of the control valve 21a. At the same time, control valve 25a is allowed to open.

The introduction of semi-stable oil product into the storage pipes 23 displaces the gas product therein. The pressure in the semi-stable oil product produced at the UPPT" 5 may be sufficient to enable displacement of the gas -16 -product in the storage pipes 23 and allow for its own loading into the storage pipes; however additional pumps to aid in displacing the gas product and loading the semi-stable oil product onto the tanker 17 may also be provided (e.g. at the distribution manifold 9 or on the tanker 17 itself).

The displaced gas product leaves the storage pipes 23 through header 26 and passes through control valve 25a to conduit 25 where it can either be returned to the UPPTM 5 for use/disposal there, or it can be re-routed into conduit 29 for storage in containers 27.

Once the storage pipes 23 are full with semi-stable oil product, the valves 21a and 25a are closed to seal the semi-stable oil product within the storage pipes 23. The semi-stable oil product can then be transported on the tanker 17.

In addition to loading the semi-stable oil product onto the tanker 17, a gas product is also loaded onto the tanker 17 from the UPPTm5. As noted above, this may (at least in part) comprise the gas product used to pre-pressurise the storage pipes 23. This gas product is transferred from the UPPTm 5 to the storage containers 27 via the conduit 7, distribution manifold 9, riser 11 b, buoy 15b, attachment 18b, and conduit 29.

Once the vessel 17 is at capacity with semi-stable oil product and gas product, the vessel 17 disconnects from the second attachment 18b. This disconnection prevents further transfer of hydrocarbons between the vessel 17 and UPPT" 5 product through the second attachment 18b. This vessel 17 then transports the semi-stable oil product and gas product to shore and/or another facility for further processing and use.

Shortly before, shortly after or whilst the vessel 17 is disconnected from the second attachment 18b, a further vessel (not shown) may connect to the first attachment 18a and receive semi-stable oil product and gas product therefrom to thereby allow for the continuous, or almost continuous, loading of semi-stable oil product and gas product from the production facility 100 to a vessel.

During the loading of the semi-stable oil product and/or the subsequent transportation of the semi-stable oil product, it is possible for any water contained within the semi-stable oil product to settle out within the storage pipes 23 from the remainder of the semi-stable oil product. If and when this happens, water is drawn out from the storage pipes 23 via conduit 21 (connected at the bottom of each of the storage pipes 23). If the separation of water has occurred whilst the tanker 17 remains connected to the UPPTM 5 then the water is either transferred to the UPPTM -17-for further use and/or disposal there, or it is transferred to water storage tank 30 on board the vessel by opening control valve 30a. If separation occurs after the tanker 17 has disconnected from the UPPTm 5 the water is transferred only to the water storage tank 30. From the water storage tank 30 the water is treated and discharged over board the tanker 17.

At any time during loading and/or transportation, an undesirable increase in pressure within the storage pipes 23 and/or containers 27 may occur by virtue of an excess of gas therein. If this occurs, then the excess of gas can be released to the environment through vent mast 22 as the gas in storage pipes 23 and containers 27 exceeds the bias in pressure regulating valves 22a and 22b respectively.

With reference to Figure 4, a hydrate formation phase diagram of a typical oil product (which may contain oil, water and gas) can be seen, with the temperature and pressure that the oil product may be held at shown on the X and Y axes respectively. There is a hydrate free region 401 on the right hand side of a hydrate dissociation curve 402, a hydrate stable region 403 (i.e. a region where hydrates have formed and are stable in the fluid) on the left hand side of a hydrate formation curve 404 and a metastable region 405 in between the hydrate formation curve and the hydrate dissociation curve where there is a risk of hydrate formation.

An oil product held at low pressure and high temperature will reduce hydrate formation, whereas high pressures and low temperatures increase hydrate formation.

The degassing and separation of water from the product alters the location of the hydrate formation and dissociation curves. Typically, such processing will move the hydrate formation curve to the left of the figure such that the oil product can be held at higher pressures and lower temperatures without the formation of hydrates. Thus, the oil product is said to be more stable, or further stabilised, when gas and water is removed therefrom.

In the embodiments of the invention described herein, the produced oil product has been part-processed such that a semi-stable oil product is formed. The semi-stable oil product produced from the part-processing is taken outside of the hydrate envelope for the conditions of loading onto the vessel, and storage and transportation on the vessel within the pipe storage. As such, the semi-stable oil product does not exhibit significant hydrate formation whilst being loaded onto, and transported on, the vessel.

Claims (23)

1. -18 -Claims 1 A method of transporting a semi-stable oil product from an offshore production facility, the method comprising: loading the semi-stable oil product into a storage container on a transportation vessel from the offshore production facility; and transporting the semi-stable oil product stored within the storage container on the transportation vessel; wherein the storage container on the transportation vessel comprises a storage pipe.
2. 2. A method as claimed in claim 1, wherein the storage container on the transportation vessel comprises a plurality of storage pipes, and wherein the method comprises loading the semi-stable oil product into the plurality of storage pipes on the transportation vessel from the offshore production facility; and transporting the semi-stable oil product stored within the plurality of storage pipes.
3. 3. A method as claimed in claim 1 or 2, wherein the axes of the storage pipe(s) is/are arranged vertically.
4. 4. A method as claimed in any preceding claim, wherein the loading of the semi-stable oil product and the storing of the semi-stable oil product during transportation are carried out at pressurised conditions so as to maintain the semi-stable oil product in its semi-stable state without separation, degassing or flashing of the semi-stable oil product.
5. 5. A method as claimed in any preceding claim, comprising pre-pressurising the storage pipe(s) prior to loading the semi-stable oil product into the storage pipe(s) with a pre-pressurisation fluid, and wherein the step of loading the semi-stable oil product into the storage container thereby additionally comprises displacing the pre-pressurisation fluid.
6. 6. A method as claimed in claim 5, wherein the pre-pressurisation fluid is a hydrocarbon gas product.
7. -19 - 7. A method as claimed in claim 6, wherein the hydrocarbon gas product is loaded into the storage pipe(s) from the offshore production facility.
8. 8. A method as claimed in any preceding claim, comprising, prior to the step of loading the semi-stable oil product into the storage container, part-processing produced fluid at the offshore production facility to form the semi-stable oil product.
9. 9. A method as claimed in any preceding claim, comprising drawing water out of the storage pipe(s) which has settled out from the rest of the semi-stable oil product that has been loaded therein.
10. 10. A method as claimed in any preceding claim, wherein the semi-stable oil product comprises gas fractions from hydrocarbon fluid produced at the offshore production facility combined with oil fractions from the produced fluid in a single liquid phase that has a true vapour pressure (TVP) of greater than 1 bar and a TVP of less than the TVP of the produced fluid from the well.
11. 11 A method as claimed in any preceding claim, wherein the semi-stable oil product has a lower gas and/or water content as compared to hydrocarbon fluid produced at the offshore production facility such that it is taken outside of the hydrate formation envelope for the conditions during the steps of loading and transporting.
12. 12. A method as claimed in any preceding claim, further comprising: loading a gas product into a storage container on the transportation vessel that is separate to the semi-stable oil product storage container; and transporting the gas product stored within the storage container on the transportation vessel.
13. 13. A transportation vessel for use in the method of any preceding claim, the transportation vessel comprising a storage pipe having semi-stable oil product therein.
14. -20 - 14. A transportation vessel as claimed in claim 13, comprising a plurality of storage pipes having semi-stable oil product therein.
15. 15. A transportation vessel as claimed in claim 14, comprising a plurality of cargo holds sealed from one another, wherein a sub-set of the plurality of storage pipes are situated within each cargo hold.
16. 16. A transportation vessel as claimed in claim 15, wherein the cargo holds are nitrogen inerted.
17. 17. A transportation vessel as claimed in any of claims 13 to 16, comprising a water storage tank configured to receive water drawn from the storage pipe(s) which has separated from semi-stable oil product therein.
18. 18. A transportation vessel as claimed in any of claims, comprising water treatment facilities for treating water drawn from the storage pipe(s) for disposal over board.
19. 19. A transportation vessel as claimed in any of claims 13 to 18, wherein the storage pipe(s) is/are arranged vertically.
20. 20. A transportation vessel as claimed in any of claims 13 to 19, comprising storage containers configured for storing a hydrocarbon gas product therein.
21. 21. A combination comprising the transportation vessel as claimed in any of claims 13 to 20 and an offshore production facility.
22. 22. The combination as claimed in claim 21, wherein the offshore production facility comprises an unmanned production platform.
23. 23. The combination as claimed in 21 or 22, wherein the offshore production facility is provided with only enough processing equipment to part process a produced fluid in order to form a semi-stable oil product.