The present invention relates to control of emission of volatile organic compounds (VOCs) from a cargo tank during offshore loading, conventional loading, unloading and transport/holding of crude oil, such as on board a shuttle tanker, in which a blanket gas is used to prevent ingress of oxygen into said cargo tank.

When transporting combustible fluids, such as crude oil, it is essential that the atmosphere above the cargo has a sufficiently low oxygen content so that a combustible gas mixture cannot form. A small overpressure in the tanks is also important to prevent ingress of air.

When transporting crude oil, it is common to use an inert gas with low oxygen content as a blanket gas in the oil tanks. This inert gas is often exhaust gas generated from burning diesel or other fuels. However, VOCs evaporate from the crude oil and so the inert gas in the tank often also contains appreciable quantities of hydrocarbons. The vapour in the cargo tank before loading will typically contain about 20% VOCs, but as more crude oil is pumped in, more VOC evaporates and the last vapour out of the tank can contain as much as 80% or more VOC (the loading operation is the main source of VOC generation). When the tank is loaded with oil, this gas mixture is displaced out of the tank and vented to the atmosphere. However, this has led to large emissions of VOCs.

Emission of VOCs is environmentally harmful and contributes to the greenhouse effect. Therefore, it is of great importance to reduce or eliminate emissions of VOCs during loading of oil tankers.

A solution that has been proposed is to use hydrocarbon gases as the blanket gas in the tanks as a replacement for inert gas. Pure hydrocarbon gas, which may be a mixture of several different pure hydrocarbon gases, offers the same function as the inert gas, i.e. the formation of a sufficiently oxygen-free atmosphere and the prevention of air ingress. Furthermore, pure hydrocarbon gas containing VOCs is considerably less energy demanding to recover than VOCs mixed with the inert gas. Thus, the heavy fraction of the hydrocarbon blanket gas (including any evaporated VOCs) can be recovered when filling the tanks and stored for reuse, rather than being vented to atmosphere. The light fractions of the hydrocarbon blanket gas are used as fuel during the loading operation. This technique has been found to almost completely prevent emission of VOCs during loading and unloading of oil tankers.

During transport, some topping up might be necessary. This toping up gas can be generated by vaporizing some of the stored heavy fraction hydrocarbons condensed during the loading operation. However, as some of the hydrocarbon gases might be dissolved into the crude oil, the amount of liquefied hydrocarbon might be insufficient to generate the necessary blanketing gas required by the regulations. In such events, further blanket gas must be added to the system in order to produce the quantity of blanketing gas as required by the regulations.

In the case of an FPSO (floating production, storage and offloading vessel), a small quantity of produced hydrocarbon gas can be diverted to provide a ready source of hydrocarbon gas. However, for a shuttle tanker, which simply transports crude oil and does not include any separation or production capabilities, a lack of hydrocarbon gas poses a problem because the vessel does not have such a ready source of make-up hydrocarbon gas.

One solution to this problem is proposed in US 2006/243344, in which make-up hydrocarbon gas is produced from an evaporation device that is arranged to heat a small quantity of the crude oil to cause emission of hydrocarbon gas to supplement the blanket gas in the holding tank.

Viewed from a first aspect, the present invention provides a hydrocarbon gas blanketing system for a cargo tank, such as used on an oil tanker, the system comprising: a blanket gas recovery unit for liquefying at least a portion of a hydrocarbon blanket gas received from the cargo tank; a liquefied blanket gas storage tank configured to receive and store liquefied hydrocarbon blanket gas received from the blanket gas recovery unit; a gas vaporisation unit for vaporising at least a portion of the liquefied hydrocarbon blanket gas from the liquefied gas storage tank for supply to the cargo tank; a make-up storage tank storing make-up hydrocarbons, the system being configured so that the make-up storage tank does not receive hydrocarbons from the cargo tank; and a make-up unit for detecting when insufficient hydrocarbon blanket gas is available and responsively supplying make-up hydrocarbons from the make-up storage tank.

In accordance with the described system, a hydrocarbon blanket gas is circulated within a circuit comprising the cargo tank, the blanket gas recovery unit, the liquefied gas storage tank and the gas vaporisation unit. By circulating the blanket gas in this matter, the heavy fractions of the hydrocarbon blanket gas and any VOC from the oil are reused, while the light fractions may be used as fuel. By this system, no VOC emission is released or vented to the atmosphere. However, as some hydrocarbon blanket gas may be absorbed into oil in the cargo hold or may not be fully condensed during recovery, the quantity of hydrocarbon blanket gas in the circuit may reduce over time. To overcome this problem, a separate store of hydrocarbons is used to provide additional blanket gas when the quantity of blanket gas that was recovered is insufficient. This simplifies the recovery process both in complexity and operability, and removes the need to boil off additional VOC from the oil being transported.

Particularly compared to US2006/243344 the described system does not involve any crude oil handling. The hydrocarbons for topping-up the blanket gas are stored is dedicated tanks that are not a part of the cargo tank system. As the make-up hydrocarbons will start to flash as soon as the pressure is lowered, less heating may be required. Conversely, VOC gas generated by boiling crude oil will need to be compressed and further cooled before use as a blanket gas. This requires larger equipment sizes and capacities, including increased power requirements.

In various embodiments, the blanket gas comprises at least 90% volatile organic compounds (VOCs), preferably at least 95% VOCs and most preferably substantially 100% VOCs. That is to say, the blanket gas is preferably

substantially free of inert gases, such as nitrogen and flue gases, which are difficult to separate from the VOCs that vaporise from the crude oil in the cargo tanks during loading operation and in transit.

Preferably the make-up hydrocarbons comprise petroleum gas, which is preferably propane, butane or a propane/butane mixture. Propane is most preferred. In particular, the hydrocarbon make-up gas preferably comprises at least 90% of the petroleum gas, preferably at least 95% of the petroleum gas and most preferably substantially pure petroleum gas, i.e. 100% petroleum gas and unavoidable impurities. The make-up hydrocarbons are preferably stored in the dedicated make-up storage tank as a liquefied gas.

The blanket gas recovery unit preferably comprises a compressor for compressing the hydrocarbon blanket gas received from the cargo tank. The compressor is preferably configured to compress the gas to a pressure of at least 10 bar, and more preferably at least 15 bar. The compressor may be configured to receive the hydrocarbon blanket gas at a pressure of about 1 bar or above. The blanket gas recovery unit may comprise a carry-over separator, e.g. located between the compressor and the cargo hold. The carry-over separator is preferably arranged to separate the hydrocarbon blanket gas from any liquid components, such as liquid carry-over or condensed hydrocarbon blanket gas. The carry-over separator is preferably configured to return a liquid phase to the cargo tank.

The blanket gas recovery unit preferably comprises a cooler to liquefy the hydrocarbon gas. The cooler is preferably configured to cool the hydrocarbon blanket gas to a temperature below 30°C, and preferably below 20°C. The temperature is preferably not below 0°C. The cooler may be configured to operate as a water-gas heat exchanger, preferably wherein sea water is used as a heat exchange medium for cooling the hydrocarbon blanket gas.

The blanket gas recovery unit preferably comprises a liquefied gas separator, which is preferably configured to receive the liquefied hydrocarbon blanket gas from the compressor and/or cooler. The liquefied gas separator is preferably configured to supply the liquefying hydrocarbon blanket gas to the liquefied blanket gas storage tank.

The liquefied gas separator preferably comprises a gas output for outputting non-condensed hydrocarbon gas. The system is preferably arranged to supply the non-condensed hydrocarbon gas to a combustor. For example, the non-condensed hydrocarbon gas may then be used as fuel to provide power and/or heat. The system should be arranged not to vent any of the non-condensed hydrocarbon gas to atmosphere.

The liquefied gas separator may comprise a water outlet. Thus, water may be removed from the blanket gas. Separated water may be stored in a sump or slop tank, e.g. for later processing and disposal, or alternatively returned to the cargo tank.

The gas vaporisation unit preferably comprises a heater for vaporisation of the liquefied hydrocarbon blanket gas. The heater may comprise a heat exchanger, and the heat exchanger may be configured to use steam or brine as a heat exchange medium. The heater is preferably configured to evaporate and heat the hydrocarbon blanket gas so that the amount and conditions of the generated blanketing gas will be according to the quantities required by the relevant regulations. The make-up storage tank may comprise a port for resupply of make-up hydrocarbon to the make-up storage tank. For example, the port may permit the make-up hydrocarbons to be periodically refilled when the vessel is in port.

The make-up unit may detect when insufficient blanket gas is available by monitoring a fluid level in the liquefied blanket gas storage tank and/or a gas pressure within the cargo tank.

The make-up unit is preferably configured to supply the make-up hydrocarbons from the make-up gas storage tank to the gas vaporisation unit. Thus, only a single vaporisation unit may be required. In some embodiment, the make-up unit may be configured to supply the make-up hydrocarbons from the make-up gas storage tank directly to the cargo hold. That is to say, the make-up hydrocarbons may be vaporised by the depressurisation, optionally in combination with heating from a heater separate from the main gas vaporisation unit to maintain a desired temperature.

In a preferred aspect, the present invention further provides an oil tanker, such as a shuttle tanker, comprising a cargo tank for receiving crude oil, and a hydrocarbon gas blanketing system as described above and configured to maintain a hydrocarbon gas blanket within the cargo tank.

In various embodiments, the oil tanker may comprise a plurality of cargo tanks supplied from a single liquefied blanket gas storage tank. Thus, the blanket gas recovery unit may liquefy at least a portion of a hydrocarbon blanket gas received from each of the cargo tanks and/or the gas vaporisation unit may vaporise at least a portion of the liquefied hydrocarbon blanket gas from the liquefied gas storage tank for supply to each of the cargo tanks.

Viewed from a second aspect, the present invention provides a method of maintaining a hydrocarbon gas blanket in a cargo tank, comprising: recovering and liquefying excess hydrocarbon blanket gas from the cargo tank, wherein the liquefied hydrocarbon blanket gas is stored in a liquefied blanket gas storage tank; vaporising at least a portion of the liquefied hydrocarbon blanket gas from the liquefied gas storage tank to maintain a gas blanket within the cargo tank; detecting that insufficient blanket gas is available to maintain a gas blanket within the cargo tank; and supplying make-up hydrocarbons from a make-up storage tank storing the make-up hydrocarbons, wherein the make-up gas storage tank does not receive any hydrocarbons recovered from the cargo tank. As above, the blanket gas preferably comprises at least 90% volatile organic compounds (VOCs), e.g. a mixture of BOG and LPG gases, more preferably at least 95% and most preferably substantially 100%, and is preferably substantially free of non-hydrocarbon inert gases, such as nitrogen.

Preferably the make-up hydrocarbons comprise a petroleum gas, and preferably propane, butane or a propane/butane mixture. Propane is most preferred. The make-up hydrocarbons preferably comprise at least 90% petroleum gas, preferably at least 95% of the petroleum gas and is most preferably

substantially pure petroleum gas, i.e. 100% petroleum gas and unavoidable impurities. The hydrocarbon make-up gas is preferably stored in the dedicated make-up gas storage tank as a liquefied gas (e.g. LPG). The method may comprise vaporising the make-up hydrocarbons together with the liquefied hydrocarbon blanket gas. Alternatively, the make-up hydrocarbons may be separated separately from the liquefied hydrocarbon blanket gas. For example, the make-up

hydrocarbons may be vaporised by depressurisation.

The liquefying preferably comprises compressing the hydrocarbon blanket gas received from the cargo tank, e.g. during loading operations. The hydrocarbon blanket gas is preferably compressed to a pressure of at least 10 bar, and more preferably at least 15 bar.

The recovery may comprise separating the hydrocarbon blanket gas from any liquid components, such as liquid carry-over or condensed hydrocarbon blanket gas, before compression. A liquid phase from the separator is preferably returned to the cargo tank.

The liquification preferably comprises cooling the hydrocarbon blanket gas. The hydrocarbon blanket gas is preferably cooled to a temperature below 30°C, and preferably below 20°C. The temperature is preferably not below 0°C. The cooling may be performed using a water-gas heat exchanger, preferably wherein sea water is used as a heat exchange medium for cooling the hydrocarbon blanket gas.

The method may comprise, after liquification, separating non-condensed hydrocarbon gas from the liquefied hydrocarbon blanket gas. The method may comprise combusting the non-condensed hydrocarbon gas, for example to provide power and/or heat. The method preferably does not comprise venting the non-condensed hydrocarbon gas to atmosphere. The method may comprise, after liquification, removing water from the liquefied hydrocarbon blanket gas. The separated water may be stored in a sump or slop tank, e.g. for later processing and disposal, or alternatively returned to the cargo tank.

The vaporisation preferably comprises heating the liquefied hydrocarbon blanket gas. The heater may comprise heating the liquefied hydrocarbon blanket gas using steam or brine. The hydrocarbon blanket gas has to be evaporated and heated to such an extent that the amount and temperature of vaporized gas is in compliance with the relevant rules.

The method may comprise monitoring a fluid level in the liquefied blanket gas storage tank and/or a gas pressure within the cargo tank, and the detection of insufficient blanket gas availability may comprise determining that a fluid level in the liquefied blanket gas storage tank and/or a gas pressure within the cargo tank is below a threshold level.

The method may comprise resupplying the dedicated make-up gas storage tank with additional hydrocarbon make-up gas (e.g. LPG). That is to say, from a source of hydrocarbon other than from the cargo tank.

Viewed from a third aspect, the present invention provides a method of generating a hydrocarbon gas in a cargo tank, comprising: providing a pressurised crude oil for loading into the cargo tank, the crude oil having an RVP of at least 1 ; and flashing the pressurised crude oil into the cargo tank so as to cause

hydrocarbon gas to evaporate from the crude oil.

In accordance with this method, the cargo tank acts as a final-stage separator for the crude oil, causing hydrocarbon gas to flash evaporate from the crude oil as it is loaded. This release of hydrocarbon gas can effectively refill the blanket gas reserves, ensuring that there is sufficient hydrocarbon gas available to provide blanket gas for the cargo tank. Furthermore, the process may simplify upstream oil processing because the extra production of hydrocarbon gas can be condensed for delivery of liquefied gas to a terminal.

The crude oil may be pressurised to a pressure of at least 2 bara, e.g. about

6 bara. The flashing preferably comprises depressurising the crude oil to cargo tank pressure, e.g. about 0.25 barg.

The method may further comprise recovering and liquefying excess hydrocarbon blanket gas from the cargo tank, wherein the liquefied hydrocarbon blanket gas is stored in a liquefied blanket gas storage tank. This occurs, for example, when oil is loaded because the blanket gas is displaced by the oil and further hydrocarbon gas is released by the flashing of the crude oil. In case there are gas in excess of what is needed for blanketing purposes, this gas may be used for power generation.

The method may comprise vaporising at least a portion of the liquefied hydrocarbon blanket gas from the liquefied gas storage tank to maintain a gas blanket within the cargo tank. Thus, in case the cargo tank pressure decreases, the tanks may be topped up with hydrocarbon gas and thereby the hydrocarbon gas blanket is maintained. This might be the situation in case a portion of the blanket gas dissolves in the crude oil and/or the cargo temperature drops.

As above, the blanket gas preferably comprises at least 90% volatile organic compounds (VOCs) or vaporized LPG gases, more preferaboy at least 95% and most preferably substantially 100%, and is substantially free of inert gases, such as nitrogen.

The liquefying preferably comprises compressing the hydrocarbon blanket gas received from the cargo tank. The hydrocarbon blanket gas is preferably compresses to a pressure of at least 10 bar, and more preferably at least 15 bar.

The recovery may comprise separating the hydrocarbon blanket gas from any liquid components, such as liquid carry-over or condensed hydrocarbon blanket gas, before compression. A liquid phase from the separator is preferably returned to the cargo tank.

The liquification preferably comprises cooling the hydrocarbon blanket gas. The hydrocarbon blanket gas is preferably cooled to a temperature below 30°C, and preferably below 20°C. The temperature is preferably not below 0°C. The cooling may be performed using a water-gas heat exchanger, preferably wherein sea water is used as a heat exchange medium for cooling the hydrocarbon blanket gas.

The method may comprise, after liquification, separating non-condensed hydrocarbon gas from the liquefied hydrocarbon blanket gas. The method may comprise combusting the non-condensed hydrocarbon gas, for example to provide power and/or heat. The method preferably does not comprise venting the non-condensed hydrocarbon gas to atmosphere (except in emergency).

The method may comprise, after liquification, removing water from the liquefied hydrocarbon blanket gas. The separated water may be stored in a sump, e.g. for later processing and disposal. The vaporisation preferably comprises if necessary heating the liquefied hydrocarbon blanket gas. The heater may comprise heating the liquefied hydrocarbon blanket gas using steam or brine. The hydrocarbon blanket gas is heated in order to produce the quantity and quality of blanketing gas as required by the rules.

The method may comprise monitoring a fluid level in the liquefied blanket gas storage tank and/or a gas pressure within the cargo tank, and the detection if insufficient blanket gas availability may comprise determining that a fluid level in the liquefied blanket gas storage tank and/or a gas pressure within the cargo tank is below a threshold level.

Optionally, make-up gas may be supplied from a make-up gas storage tank as in the second aspect. However, it is envisaged that this will be unnecessary because of the additional hydrocarbons released by the flashing of the crude oil.

Certain preferred embodiments of the present invention will now be described in greater detail by way of example only and with reference to the accompanying drawing, which shows a system for recovery of VOC gases released in connection with offshore loading operations of shuttle tankers.

The proposed system is intended for recovery of VOC gases released in connection with offshore loading operations performed by offshore loading of a shuttle tanker. The basis is to use hydrocarbons as a blanketing gas instead of inert gas generated from combustion of fuel oil. During discharging operations, recovered, liquefied VOCs (LVOCs) will be vaporized and used as a blanketing gas. In cases where the quantity of LVOC is insufficient, LPG/propane will be used to supply additional blanketing gas. The non-condensed VOCs, (NCVOCs), will be used for heat or power production.

Dedicated storage tanks for each of the LVOC and the LPG are installed on the main deck of the shuttle tanker. Standard tanks and equipment are used for the LPG vessel(s), i.e. tanks, safety valves, pipes, valves and other relevant equipment that are in compliance with IMO IGC (International Gas Code) requirements for such types of vessel.

An exemplary system incorporating these principles is shown in Figure 1. During oil loading, the VOC gas mixture 14 in the storage tank 10 is displaced by the oil 12. If required liquid droplets from carry over and condensation in piping are removed by a separator 16 before entering a compressor 18. A VOC compressor 18 will compress the displaced VOC from a pressure slightly above 1 bar to desired outlet pressure, for example about 15 bara. A higher pressure will increase the recovery rate of VOC, but increase operating costs. Any suitable compressor 18 may be used for this purpose.

The compressed VOC from the compressor 18 is then cooled by a sea- water based heat exchanger 20 to about 20 °C or less. The amount of recovered LVOC will increase with decreased temperature. By this operation a significant amount of the VOC will be condensed to LVOC.

After compression and cooling, LVOC and probably some water will form. The LVOC phase 24 is separated out by a second separator 22 and transferred to the dedicated LVOC tank 26. If required, water 28 may be separated and drained to a preferred location (e.g. slop tank). The gaseous phase 30 of the VOC (non- condensed VOCs, NCVOCs) is directed to the engine room 32 and can be used for combustion to provide heat or power production.

The LVOC is stored in the dedicated storage tank 26 after separation until required for blanketing purposes.

According to the rules, the capacity of the system supplying blanket gas shall be at least 1.25 times the expected maximum discharging rate. When performing discharging operations, the recovered LVOC from the LVOC storage tank 26 will be vaporized to provide blanketing gas.

The flow of LVOC is controlled by a pressure-controlled flow valve 33 (controlled based on cargo tank pressure). If the cargo tank pressure drops, the valve opens for more supply of LVOC. If the cargo tank pressure increases, the valve closes.

The LVOC will start vaporizing as soon as the LVOC is let to the cargo tank

10 and the pressure drops to approximately atmosphere pressure. A heater 34 is controlled to keep the temperature after pressure reduction above a certain limit. For example if the LVOC was pure propane stored at 20°C and saturation pressure (~8bar), the temperature after expanding to 1.25 bar would be about -35°C. The heater duty of the heater 34 is controlled based on a temperature set point (e.g. 10°C).

If the rate of supplied VOC blanketing gas is insufficient, additional vaporizing may be achieved by using the heater 34. The heater 34 may be used to regulate the supply of the hydrocarbon blanketing gas so as to balance the supply against the maximum allowable cargo tank pressure. The heater 34 may comprise heat exchanger having steam as a heat source, which may be supplied directly to the LVOC heater 34 or a brine circuit may be used to transfer heat from the steam to the LVOC heater 34.

The available quantity of LVOC in the LVOC storage tank 26 will depend on the amount VOC generated from the preceding loading operation and by the recovery efficiency. In certain cases there might be a lack of LVOC for blanketing purposes following the discharge operation. This may be detected, for example, by a controller that detects when a gas pressure within the cargo tank 10 or a liquid level within the LVOC tank 26 falls below a threshold level. In such events additional blanketing gas will be generated from LPG. The LPG (preferably propane) is stored in dedicated LPG storage tanks 36, 38 and is vaporized in the same way as LVOC via the heater 34 before entering the cargo tank 10.

In the proposed system, one dedicated 500m ³ tank 26 is used for LVOC and two dedicated 500m ³ tanks 36, 38 are used for LPG/propane. In order to reduce equipment costs, as well as operation costs, the proposed tanker will use standard equipment complying with the IGC requirements for fully pressurized type-C tanks. The exemplary LPG tanks 36, 38 are standard LPG tanks with a maximum safety valve set-point of 18 barg. The tanks 26, 36, 38 are installed on the main deck of the shuttle tanker.

It will be appreciated that the system is not limited to the specific

configuration of the dedicated storage tanks 26, 36, 38 illustrated, and that other volumes and pressures may be used depending on the specific system

requirements. The intention is that one tank shall contain sufficient LPG to fill all of the cargo tanks with hydrocarbon blanketing gas. Assuming, for example, that propane is used having an expansion factor of 270 between liquid and vapour propane, a volume of 500 m ³ of liquid propane will generate 135,000 m ³ (850.000 bbls) of vaporized propane. Hence, 500 m ³ might be an appropriate size for a standard shuttle tanker of 135,000 m ³ (850.000 bbls). However, the size of the LPG storage tanks should be adjusted according to the actual vessels where they shall be installed.

The LPG tanks 36, 38 may be refilled via line 40 when required at a terminal or from a pressurized LPG vessel.

In special cases, where the shuttle tanker may receive less stabilized crude oil, i.e. with a RVP above 1 , the cargo tank 10 of the shuttle tanker may act as a final stage separator. In such a case, the cargo tanker will act as a large separator in the oil stabilization process. The oil is flashed to the shuttle-tank pressure, such that hydrocarbon gases will flash off due to the pressure reduction. In such a case the hydrocarbon gas generation will be substantially higher than in traditional loading operation and the excess gas will be processed by the VOC processing system discussed above. In such a case it is foreseen that sufficient VOC would be generated during loading and that additional LPG may not be required because NGL/LPG will be produced.