The present invention relates to a method of storing ammonia and to a corresponding storage. The invention further extends to transporting ammonia using said storage.

Ammonia (NH3) is a compound having broad reaching applications. For example, it is well known that ammonia is prevalently used as the basis of many fertilisers. Ammonia also has applications in hydrocarbon production and associated operations, and the use of ammonia in this field is growing.

Ammonia can be stored and transported in a gaseous state relatively viably since at standard state conditions ammonia has a boiling point of -33.3 °C ammonia and hence at ambient/atmospheric pressure and temperature conditions ammonia is a gaseous compound. However, this offers poor storage density and poor efficiency. Therefore, conventionally, ammonia is stored and transported in an anhydrous (i.e. containing no water) liquid state. Obtaining liquid ammonia requires the ammonia to be cooled to very low temperatures well below ambient temperature conditions and/or to be pressurised to well above ambient pressure conditions. An appropriate storage tank or vessel is therefore required to maintain the requisite high pressures and/or low temperatures involved.

Improved storage and transportation of ammonia is desired.

In a first aspect, the invention provides a method of storing ammonia, the method comprising storing the ammonia in a plurality of storage pipes in a liquid state (i.e. as a liquid) at ambient temperature conditions.

The term ‘storage pipe’ refers to a storage container formed from (i.e. comprising) a length of pipe, which has been closed at each end, optionally by a hemispherical cap or dome that has, for example, been welded to the end of the pipe. Accordingly, the storage pipes are highly elongate, typically having a length- to-diameter ratio of at least 20.

The method of the first aspect makes use of storage pipes to permit storage of the ammonia as a liquid at ambient temperature conditions. Using storage pipes compared to, e.g., conventional tank storage (i.e. vessel storage) as the basis for storage of ammonia is advantageous since it is associated with a significantly lower capital and operational expenditure, particularly in the context of storing ammonia at elevated pressures and ambient temperature conditions.

Conventional ‘tank’ type storage solutions, either when being used to store liquid ammonia at elevated pressures or at very low temperatures, require 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 (and hence weight of the tank) also limits the size of the tank that can be used, meaning that the volume of ammonia stored therein is limited. The large wall thickness also occupies a large volume of space, which in turn results in tank type storage utilising available space for storage inefficiently.

In contrast, pipe storage is relatively inexpensive to provide because standard, ‘off-the-shelf’ pipes may be used to manufacture them. Moreover, for a given volume of storage, pipe storage can have a comparatively smaller wall thickness. Thus, a given volume of ammonia can be stored using a comparatively lower total weight of storage tank material using pipe storage and can be achieved at a lower capital expenditure. The relatively smaller wall thickness of pipe storage means that, for a given volume of ammonia, the total space occupied using the pipe storage solution is smaller. Thus, pipe storage provides a more viable solution.

The method of the first aspect requires that the ammonia is stored in a liquid state at ambient temperature conditions. Ambient temperature conditions may be any temperature between 0°C and 40 °C. The skilled person will appreciate that in order to maintain the ammonia as a liquid at ambient temperature conditions it requires the ammonia to be pressurised to above ambient pressure conditions. The pressure conditions that the ammonia is stored at may therefore be between 5 barg - 20 barg, for example at 5 barg, 10 barg, 15 barg or 20 barg, with the exact pressurised condition required being determined by the ambient temperature and the requirement that the ammonia must be in a liquid state. The pressurised conditions selected may further be affected by other factors, e.g. the tolerances of the storage pipe and/or the tolerance of the equipment used for loading and unloading the ammonia into the storage pipes.

The liquid state may be an anhydrous liquid state.

It is advantageous to store ammonia as a liquid at ambient temperature conditions as opposed to ambient pressure conditions (and very low temperature conditions). This is because it is technically more challenging and labour intensive to liquefy and store ammonia as a liquid at ambient pressure conditions given the very low temperatures involved. There is significant complexity and expenditure, both operational and capital, associated with the equipment, personnel and processes required to produce and maintain ammonia as a liquid at such temperature conditions and at ambient pressure conditions. Moreover, in certain settings, e.g. an offshore scenario as discussed as an optional feature of the invention below, limited space also means that it may not be viable to provide the necessary infrastructure to produce liquefied ammonia at ambient pressure conditions. The advantageous storage pipes of the invention, offering an effective storage solution, are also not suitable for storing liquefied ammonia at ambient pressure conditions given they cannot provide the requisite temperature control.

Each of the storage pipes may have a nominal diameter of between 40-60 inches (1.0m - 1.5m). Optionally, the pipe may have a nominal size of 42 inches (1.1 m) or 56 inches (1 ,4m), or may have any nominal size in the range of 42 inches (1.1m) to 56 inches (1.4m).

A vessel having a nominal diameter greater than about 56-60 inches (1.4 m -1.5m) would typically be considered by the skilled person as a conventional tank (or pressure vessel) that is distinct from a pipe. This consideration is also true in the context of the current application, whereby any vessel having a nominal diameter of greater than about 56-60 inches (1.4m -1.5m) would not be considered as a pipe.

Each storage pipe may comprise or be formed from a pipe that is an X42, X52, X56, X60, X65, X70 or X80 pipe in accordance with the API SPEC 5L specification.

As noted above, the storage pipes are highly elongate. Accordingly, each storage pipe may have a length of between 10 m to 30 m, for example 12 m, 24 m or 26 m.

Each of the storage pipes may be formed from rolled pipes with, optionally, a single longitudinal seam. Such pipes are commonly available as ‘off-the-shelf’ type components and are typically inexpensive.

The storage pipes will be configured for storing ammonia at an elevated pressure given the ambient temperature conditions of the liquid ammonia to be stored. The storage pipes may be configured to store the ammonia at between 5 barg - 20 barg, for example at 5 barg, 10 barg, 15 barg or 20 barg. The exact pressurised conditions that the storage pipes are configured to store the ammonia at may be selected dependent on the ambient temperature of the liquid ammonia to be stored therein, the tolerances of the storage pipe and/or the tolerance of the equipment used for loading and unloading the ammonia into the storage pipes. Each of the storage pipes may be arranged vertically (i.e. the primary axis of the storage pipes may be arranged vertically or substantially vertically) or horizontally (i.e. the primary axis of the storage pipes may be arranged horizontally or substantially horizontally). The storage pipes may comprise a combination of horizontally and vertically arranged storage pipes. Each or some of the storage pipes may be arranged in any other orientation between horizontal and vertical.

The method may comprise the step of liquefying ammonia at ambient temperature conditions prior to the step of storing the ammonia.

The plurality of storage pipes may be comprised as part of a hydrocarbon production system. To say this another way, the method may comprise storing the ammonia in a plurality of storage pipes at a hydrocarbon production system in a liquid state at ambient temperature conditions. As noted above, ammonia has applications for hydrocarbon production and therefore it is beneficial to be able to store ammonia at a hydrocarbon production system in accordance with the method of the first aspect.

The hydrocarbon production system may be land based (i.e. onshore). Alternatively, the hydrocarbon production system may be offshore, e.g. it may be or comprise an offshore hydrocarbon production facility. The hydrocarbon production facility may be a hydrocarbon production platform, e.g. an unmanned hydrocarbon production platform, or a floating production, storage and offloading (FPSO) facility.

It will be understood by the skilled person that a hydrocarbon production system is a system that is specifically configured for the production and/or processing of hydrocarbons (e.g. oil, natural gas, etc.). As such, a hydrocarbon production system will comprise a degree of production equipment and/or a degree of processing equipment configured for processing or part-processing the produced hydrocarbons.

The hydrocarbon production system may be a brownfield production facility or a greenfield production facility.

The step of storing the ammonia may take place on/at a single, modular unit of the hydrocarbon production system. The modular unit may be self-contained, separate and separable from the remainder of the hydrocarbon production system. For instance, this unit may be a separate facility from the portion of the hydrocarbon production system where hydrocarbon producing operations are carried out. In an offshore scenario, the separate modular unit may be a separate floating unit/facility situated adjacent, e.g., a production platform or FPSO that constitutes the remainder of the hydrocarbon production system.

By modular it is meant that the separate unit may be considered to constitute a module that is self-contained and that can be readily conjoined with and split apart from the remainder of the hydrocarbon production system. That is to say, a modular unit is not integral with the remainder of the hydrocarbon production system. Any connections between the modular unit and the remainder of the hydrocarbon production system may hence be readily reversible (i.e. they are designed to be reversible) such that the modular unit is readily disconnected from the remainder of the hydrocarbon production system (e.g. through electrical/power connections, utilities connections, walkways, etc.).

Having a separate, modular unit is advantageous since it can be easily installed/retrofitted in combination with an already existing hydrocarbon production system (i.e. a brownfield facility) to provide for ammonia storage without significant or any downtime in production at the already existent hydrocarbon production system. This also means that minimal modification is required to the already existing hydrocarbon production system. It furthermore means the separate modular unit can be moved and used in combination with other already existing hydrocarbon production systems.

The single, modular unit may form part of a plurality of hydrocarbon production systems. That is to say, a plurality of different hydrocarbon production systems may comprise a common single, modular unit. The common, single modular unit may provide storage of liquid ammonia for each of the plurality of hydrocarbon production systems. For example, in an offshore scenario, the single modular unit may take the form of a floating unit/facility situated adjacent and connected to a plurality of production platforms, production facilities and/or FPSOs and may provide ammonia storage for each of the connected production platforms, production facilities and/or FPSOs. In such a scenario, the single, modular unit may be termed a hub given that it acts as storage for a plurality of systems.

The single modular unit is not essential however, and the step of storing ammonia may be carried out at the same facility as the remainder of the hydrocarbon production system. This may be the situation for, e.g., a greenfield production facility where the facility is initially built to operate in accordance with the method of the first aspect.

The ammonia may be grey ammonia, green ammonia and/or blue ammonia. In a second aspect, there is provided a method of transporting ammonia, the method comprising: loading the ammonia onto a transportation vehicle comprising a plurality of storage pipes; storing the ammonia in accordance with the first aspect of the invention; and transporting the ammonia stored on the transportation vehicle.

The method of the second aspect may comprise/be in accordance with any compatible optional features of the first aspect. In particular, the storage pipes of the second aspect may be in accordance with the storage pipes as described above.

The vehicle may be, e.g., a heavy goods vehicle, light good vehicle, etc. in an onshore/land based scenario.

In an offshore scenario, the vehicle may be a transportation vessel, e.g. a tanker or a ship.

The method of the second aspect may comprise liquefying the ammonia before or after loading the ammonia onto the transportation vessel.

The method of the second aspect may comprise transporting the ammonia stored on the transportation vehicle to a hydrocarbon production system and unloading the liquid ammonia from the transportation vessel to the hydrocarbon production system. The method may further comprise storing the liquid ammonia at the hydrocarbon production system in a second set of plurality of storage pipes in a manner equivalent to the first aspect of the invention, optionally inclusive of any optional features thereof. The hydrocarbon production system of the second aspect of the invention may be in accordance with the hydrocarbon production system as described above in connection with the first aspect of the invention.

In a third aspect of the invention, there is provided a plurality of storage pipes configured to store liquid ammonia at ambient temperature conditions. The plurality of storage pipes may be in accordance with the plurality of storage pipes of the first and/or second aspect of the invention as described above, and may comprise any optional features thereof.

The storage pipes of the third aspect may comprise liquid ammonia at ambient temperature conditions.

In a fourth aspect, there is provided a transportation vehicle comprising the plurality of storage pipes according to the third aspect, optionally inclusive of any optional feature thereof. The transportation vehicle may be in accordance with the transportation vehicle described above in respect of the first and/or second aspects. For example, the vehicle may be a transportation vessel, e.g. a tanker or a ship.

In a fifth aspect, there is provided a hydrocarbon production system comprising a plurality of storage pipes according to the third aspect, optionally inclusive of any optional features thereof.

The hydrocarbon production system of the fifth aspect may be in accordance with the hydrocarbon system described above in connection with any of the above aspects.

In a sixth aspect, there is provided an offshore floating facility comprising a plurality of storage pipes according to the third aspect, optionally inclusive of any optional features thereof.

The offshore floating facility of the sixth aspect may be in accordance with the offshore floating facility described above (i.e. the optional, offshore form of the separate modular unit) in connection with any of the above aspects.

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

Figure 1 is a schematic depiction of an offshore floating facility connected to a tanker and to three separate production facilities;

Figure 2 is a perspective view of the offshore floating facility of Figure 1;

Figure 3 is a cross-sectional view of the offshore floating facility of Figures 1 and 2;

Figure 4 shows a gas engine of a generator positioned on the offshore floating facility of Figures 1 to 3;

Figure 5 is a part-cutaway side view and cutaway plan view of the tanker of Figure 1; and

Figure 6 is a pressure enthalpy chart for ammonia.

Figure 1 shows an offshore floating facility 1 that is connected by three separates cables 3a, 4a, and 5a to three separate production facilities 3, 4, 5. Production facility 3 is an offshore production platform 3, production facility 4 is another offshore production platform 4, whilst production facility 5 is a floating, production, storage and offloading (FPSO) 5. Each production facility 3, 4, 5 is situated at its own respective hydrocarbon reservoir and is arranged to permit production of hydrocarbons from its respective reservoir via suitable production and processing equipment. The cables 3a, 4a, 5a are each electrical power cables 3a, 4a, 5a that are configured for carrying electrical power from the offshore floating facility 1 to each of the production facilities 3, 4, 5 in order to provide power thereto, as will be described in further detail below.

Also connected to the offshore floating facility 1 is a tanker 2. The tanker 2 is connected to the offshore floating facility 1 via two separate conduits 2a, 2b that are each arranged to carry a fluid, specifically ammonia, between the tanker 2 and the offshore floating facility 1 as will be described in greater detail below. The conduits 2a and 2b are reversibly connected to the tanker 2 as discussed in further detail below.

As can be seen in greater detail with reference to Figures 2 and 3, the offshore floating facility 1 has a predominantly ship shaped hull 101 and, situated at a top of the hull 101 , a landing pad 102 for helicopters to permit personnel access to the facility 1 for maintenance or otherwise. Housed with the hull 101 are a plurality of vertically oriented storage pipes 103. The storage pipes 103 are arranged to store liquid ammonia therein at ambient temperature conditions and, consequently, at pressures of between 5 barg - 20 barg (the exact pressure of storage being dependent on the ambient temperature conditions as discussed below with reference to Figure 6). Each storage pipe 103 is formed from a section of pipe having an X42 specification and that is sealed at either end via a suitable hemispherical cap. As shown, several hundred storage pipes 103 are housed within the hull 101 of the offshore floating facility 1.

The hull 101 also comprises a plurality of generators 105 that each comprise a gas engine 107. Each gas engine 107, further details of which can be seen in Figure 4, is a V18 engine which is specifically configured to combust gaseous ammonia therein. Motive power produced by each engine 107 drives electricity generation at each generator 105.

Each of the generators 105 are connected to the cables 3a, 4a, 5a in a manner that permits electrical power produced at the generators 105 to be passed via the cables 3a, 4a, 5a to the production facilities 3, 4, 5.

Figure 5 shows further details of the tanker 2. The tanker 2 comprises a plurality of storage pipes 23 (again, several hundred storage pipes 23) divided between several cargo holds 25 on the tanker 2. The storage pipes 23 are comparable to storage pipes 103 situated on the offshore floating facility 1 in that they are vertically oriented on-board the tanker 2 and are arranged to store liquid ammonia therein at ambient temperature conditions and, consequently, at pressures of between 5 barg - 20 barg (the exact pressure of storage being dependent on the ambient temperature conditions). Each storage pipe 23 is formed from a section of pipe having an X42 specification and that is sealed at either end via a suitable hemispherical cap.

In use, the storage pipes 23 on-board the tanker 2 are loaded with ambient temperature liquid ammonia at an onshore site that is remote from the offshore floating facility 1 and the production facilities 3, 4, 5. The tanker 2 then travels to a site of the offshore floating facility 1 (e.g. as shown in Figure 1) whilst the liquid, ambient temperature ammonia is maintained as such within the storage pipes 23. This is achieved by ensuring that the storage pipes 23 remain suitably pressurised during transit of the tanker 2. Once at the site of the offshore floating facility 1 , the conduits 2a, 2b are connected to the tanker 2. Liquid ammonia is then offloaded from the storage pipes 23 on the tanker 2 to the offshore floating facility 1 via the conduit 2a and is transferred into the storage pipes 103 on-board the offshore floating facility 1 for storage therein.

The transfer between the storage pipes 23 and the storage pipes 103, and the subsequent storage therein, is carried out whilst maintaining, as far as possible, the ammonia as a liquid at ambient temperature conditions. This requires the transfer and subsequent storage in the pipes 103 to be carried out at suitably pressurised conditions. It may not however be possible to maintain all ammonia in a liquid state during this transfer and subsequent storage. For example, prior to introduction of liquid ammonia from the tanker 2 to the storage pipes 103, the interior of the storage pipes 103 may be under ambient pressure conditions and be filled with air and/or gaseous ammonia left over as a remnant from a previous stock of ammonia. As such, initial introduction of the pressurised liquid ammonia at ambient temperature conditions into the storage pipes 103 will result in an initial depressurisation of some liquid ammonia such that it is vaporised into a gaseous form. Soon after however, the storage pipes 103 will pressurise sufficiently such that further introduced ammonia will remain in a liquid state at ambient temperature conditions.

In the event of vaporisation of any portion of the ammonia during its transfer between the tanker 2 and the offshore floating facility 1 (e.g. after initial introduction into the storage pipes 103 as discussed above), then the vaporised portion of the ammonia is separated off from the liquid ammonia at the offshore floating facility 1 and is transferred back to the tanker 2 via the conduit 1b. This gaseous ammonia can then be re-condensed on board the tanker 2 at ambient temperature conditions and subsequently transferred back to the offshore facility 1 via the conduit 1a for storage thereon. Alternatively, the gaseous ammonia can be stored aboard the tanker 2 for transit back to, e.g., an on shore site. Once all of the liquid ammonia has been offloaded from the tanker 2, the conduits 2a, 2b are disconnected. The tanker 2 then travels away from the offshore floating facility, optionally back to an onshore site for further loading and transport of ammonia.

The liquid ammonia stored within the storage pipes 103 aboard the offshore floating facility 1 is used as a fuel to power the gas engines 107 comprised as part of generators 105. As such, the storage pipes 103 on-board the floating facility 1 serve as a buffer storage for fuel for the generators 105.

Specifically, the liquid ammonia stored within the storage pipes 103 is sent from the storage pipes 103 to an inlet of the engines 107. During transit between the storage pipes 103 and the gas engine 107 the liquid ammonia is vaporised, either passively as a natural result of depressurisation that occurs during transit between the pipes and engine and/or actively via a suitable vaporiser, into a gaseous form. In its gaseous form, the ammonia can then be appropriately introduced into the gas engine 107 for combustion therein.

Combustion of the ammonia in the engines 107 drives electricity generation at the generators 105. The electrical power (e.g. rated at 7 MW) produced at the generators 105 is then transferred via the cables 3a, 4a, 5a from the offshore floating facility 1 to the production facilities 3, 4, 5 and is used to power operations at the production facilities 3, 4, 5, including powering the production and processing equipment situated thereat.

The offshore floating facility 1 thus acts as an energy hub for a plurality of hydrocarbon production facilities 3, 4, 5 and serves to provide power for production and other operations that occur at each facility. The floating facility 1 is thus comprised as part of a plurality of hydrocarbon production systems, wherein the remainder of each system is formed by a respective production facility 3, 4, 5.

Figure 6 shows a pressure-enthalpy chart for ammonia. Pressure is represented along the y-axis whilst enthalpy is represented along the x-axis. The region below the saturation line 61 (termed ‘saturation dome’) represents where any addition of enthalpy will cause additional liquid ammonia to vaporize instead of raising the temperature of the ammonia. The red lines on the chart indicate constant temperatures and, whilst not depicted therein, it can be deduced from the temperature values given either side of the saturation dome that the temperature lines would be at a complete horizontal through the saturation dome. The green lines on the chart each represent a constant ratio of vapour mass to total mass of the ammonia. The region 63 shown in blue represents where ammonia is at ambient temperature conditions (i.e. 0°C - 40 °C) within the saturation dome. Within region 63 the vast majority of ammonia would be in a liquid state whilst being at ambient temperature conditions as can be deduced from the ratios of vapour mass to total mass of the ammonia in this region. The ambient temperature liquid ammonia that is stored in the storage pipes of the invention would typically fall within region 63 of the pressure-enthalpy chart.