The present invention relates to a method and a device for collection of gas that is displaced during loading of hydrocarbon containing liquid to one or more tanks, such as tanks onboard ships, and reuse of the collected hydrocarbon containing gas as a blanket gas in tanks that are being emptied of liquid, as can be seen in the introduction to the subsequent claims 1 and 10, respectively. The invention also relates to an application of the method and the device.

Consequently, the present invention concerns a method for control of discharges of volatile gases (VOC) from a storage tank that is being filled with liquid, such as filling an oil tank onboard an oil tanker, in which a system is used for collecting and intermediate storing of gas or steam that is displaced from the tank during loading, for later use as an inert gas environment (blanket gas) in tanks for transport of liquid.

During transport of volatile and/or inflammable liquids, such as crude oil, condensate or other hydrocarbon containing liquids, an inert gas is normally supplied to the tanks to fill the volume of the tank that is not taken up by liquid, so that air/oxygen can not enter the tanks and lead to an inflammable gas mixture. This gas environment is called a blanket gas. As a blanket gas, exhaust gas is normally used, but hydrocarbon gases or other gases that have a low content of oxygen can also be used.

In filling of a liquid to a tank where a blanket gas is used such as, for example, during loading of a tanker for transport of crude oil, condensate or petroleum products, the blanket gas is displaced by the incoming volume of liquid if pressure

in the tank is not permitted to build-up. Previously, the displaced gas was vented directly into the air, but since the gas contains considerable amounts of hydrocarbons, it constitutes a risk factor and is harmful for personnel and the environment.

In the same way as the gas is displaced during filling of a liquid to a tank, inert blanket gas must be supplied to the tank when the liquid is drained off (un-loaded). If not, a vacuum will arise that can lead to ingress of air/oxygen.

The inert blanket gas can be exhaust gas from the propulsion machinery or generators for exhaust gas, or hydrocarbon gas. In the use of exhaust gas as blanket gas above a volatile liquid, such as crude oil, condensate or other petroleum products, it has been found that a considerable part of the volatile components in the liquid evaporates.

Over the last years, recovery plants have been installed on some crude oil tankers or at export terminals to recover the hydrocarbon vapours during loading of crude oil or condensate to tankers that use exhaust gas as a blanket gas in the tanks. The recovery solutions are typically based on absorption, condensation, adsorption or a combination thereof. These recovery plants are large and costly and the disadvantage is that they only manage to recover a part of the oil vapour from the blanket gas.

The invention described here is related to a method for limiting these discharges of harmful materials, such as hydrocarbon vapour, in connection with loading volatile liquids to a tanker, in that the displaced gas is collected and led to a storage space.

The storage space can be one or more large storage tanks or caves/caverns. Said storage space can be a store for a liquid corresponding to the one that is delivered to the ship's tanks or other tanks, or a store for a completely different purpose. The storage space can also be specially made for the collection of said gas.

The collected gas is used as a blanket gas in the emptying of a liquid from a tanker instead of exhaust gas. By using a blanket gas that is already rich in hydrocarbons, the loss of hydrocarbon products from the load due to evaporation is reduced, and the total amount of hydrocarbon gas in the empty tank will be

approximately equal to the amount that would have evaporated from the load if an exhaust gas had been used as a blanket gas.

The tanks that are emptied and filled, respectively, can be onboard different ships or they can be the same tanks. In the latter case, a tanker will empty a cargo from the tanks where gas collected previously is used as a blanket gas. During filling of a new cargo, the blanket gas is collected again to be used later.

The collected gas is not compressed to a large extent, and is stored in the storage tanks or caves/caverns at a relatively low pressure. Therefore, there is no need for a separation plant for hydrocarbons or volatile components that condense or are separated out by increasing the pressure. To overcome flow losses and possible pressure difference, it can be necessary, however, to install equipment for increasing the pressure for the transport of gas between the storage tanks and the tanks onboard the ships. This can, for example, be a gas-blowing machine or a fan.

In Norwegian patent 310.377 a solution is described where gas from a process plant, which normally is burned in a flame, is collected and led to one or more low- pressure stores. Gas from the low-pressure store(s) is then led back to the process. The patent 31.0.377 does not relate to gas that is released in the loading of transport means during normal operation. When it comes to known solutions, reference is especially made to US Patent 5,367,882, US Patent 6,604,558 and WO 2004/081440.

In US Patent 5,524,456 a process is described where a blanket gas from a tanker is collected, compressed and cooled down in several steps to separate out volatile organic components, thereafter to store the remaining gas at high pressure to be used later as a blanket gas in tankers.

The method according to the invention is characterised in that the gas from the tanks that are being filled with liquid is collected in a collection arrangement and is led to one or more storage spaces for intermediate storage, as the collected gas can be led back to the same tanks or other tanks that are being emptied of liquid and functions as a blanket gas to prevent ingress of air. The further preferred embodiments of the invention appear in the method claims 2-9.

The device according to the invention is characterised by a collection arrangement for the displacement of gas, one or more storage spaces for intermediate storage, and a device for recycling of gas as a blanket gas in tank(s). The preferred embodiments appear in the product claims 11-14. The method and device according to the invention are preferably used in connection with an installation that comprises a crude oil or raw product storage at a processing plant, such as for handling of stabilised or non-stabilised oil, condensate, naphtha or other petroleum distillation products as described in claim 15. .

In contrast to the solution that is described in US Patent 5,524,456, the principle of the present invention is to collect and store untreated blanket gas as a high content of hydrocarbon gives a smaller evaporation of the cargo with the use of the gas as a blanket gas in a tanker. In the present invention, there is no use of a treatment plant for the collected gas ejther, nor is there pressure tanks for storage of the collected gas.

The present invention shall now be described in more detail with the help of figure 1 , which shows a main principle diagram of the invention. A tank 10, especially onboard a ship, for transport of volatile liquids is connected via a pipe 12 to a collection arrangement for a gas mixture. The pipe 12 leads the gas mixture to a large storage tank/cave/cavern 14. To take care of the transfer and the transportation of the gas from the ship's tank 10 to the storage tank 14, it may be necessary to use pressure increase equipment 13 connected to the pipe 12, for example, a gas-blowing machine or a fan to increase the pressure of the gas that is being led to the tank 14. To control the gas pressure in the tank 10, the suction volume of gas for the pressure increase machine 13 can be varied, or a pressure control valve can be used.

During loading of liquid through the pipe 20 to the first ship's tank 10, a gas volume 11 in the tank will be displaced by the incoming liquid. The displaced gas is collected in the collection arrangement through the pipe 12 and transported with the help of the gas-blowing machine 13 to the storage hall 14. Here, the gas is temporarily stored for later use, or it is used immediately as a blanket gas for the emptying of other tanks, for example on other ships.

To the left in the figure, a ship's tank 18 is illustrated schematically, which initially is fully loaded with liquid 21 (for example oil). During the emptying of the ship's tank 18, such as through the pipe 30, the tank must be filled with a blanket gas 11 to replace the liquid volume 21 that is removed. The ship's tank 18 is then connected to the storage tank 14. The gas 11 in the storage tank 14 is then transported through a pipe system 15 and if necessary with the help of pressure increase equipment (pump) 16 up to the tank 18. A valve 17 connected to the line/pipe 15 can be used to control the pressure in the ship's tank 18. The suction volume of the pressure increase equipment can also be varied to vary the flow of gas into the tank 18.

Embodiments of the invention.

From the above, the transport of the gases from and to, respectively, the two tank types (one that is being emptied of liquid and one that is being loaded with liquid, respectively) is executed for many different situations.

The storage volume 11 can be used for simultaneous storage of the collected gas and a liquid corresponding to the liquid that is loaded into the ship's tanks, or it can be used for simultaneous storage of the collected gas and a liquid different from the liquid that is loaded into the ship's tanks.

Alternatively, the gas can be collected from one or more ship's tanks and be led to the storage space, and that the gas is led further to other ship's tanks that are being emptied of liquid.

The storage space can be made up from the crude oil or raw product store of the plant, such as stabilised or non-stabilised oil, condensate, naphtha or other petroleum distillate.

The gas can be collected from one or more ship's tanks and be led to the storage space for intermediate storage, and that the gas at a later stage is used as a blanket gas in ship's tanks in connection with emptying of liquid from these tanks.

The above mentioned pressure increase equipment can, in connection with the storage space, be used for pressure increase to transport the stored gas directly to a tank where it is used as a blanket gas. The pressure increase equipment can,

at the same time, be used for the transport of the gas itself and to regulate the pressure in the gas.

It is also an alternative that the gas from the storage space is not used as a blanket gas, but is used for another appropriate purpose.

With the present invention a solution is provided which implies that a blanket gas can be used in a number of ways from a starting point of a store that has a large volume. Furthermore, large storage volumes that are meant for a different purpose can be used, for example, for storage of hydrocarbon containing liquids.

Even if pressure setting equipment is used when needed to control the gas pressure and to transport the gas, the storage tank and the complete pipe system can, on the whole, be operated at low pressures, in contrast with what is proposed according to prior art as a consequence of US Patent 5,524,456.