This invention relates to a method for supplying a fill gas to a cargo tank. More particularly, it concerns a method, in which there is preferably used a hydrocarbon gas, referred to below as HC gas, as fill gas in a cargo tank, the gas being supplied in a controlled amount to the cargo tank together with the fluid which is being loaded. The invention also in¬ cludes a device for practising the method.

In a cargo tank for crude oil/gas condensate, typically on board a ship, during filling and storing there will typically be some evolution of gas of volatile organic compounds, VOC, from the load mixing with the gas present in the cargo tank above the liquid crude oil/gas condensate. Among other things, to overcome the risk of igniting one of these gases, it is common to use a fill gas above the liquid load in the cargo tank. The fill gas is used to displace an oxygenic at¬ mosphere, so that this becomes non-flammable. Somewhat mis¬ leading, this gas is normally referred to as inert gas and normally includes, according to the prior art, waste gases from a combustion, the portion of oxygen in the so-called in¬ ert gas typically being less than 8 per cent. According to the prior art the gas above the petroleum load thus includes a mixture of VOC and waste gases from a combus¬ tion process.

As the cargo tanks are filled, or as the evaporation of VOC otherwise increases, the pressure in the cargo tanks will rise. When this pressure exceeds a specific level, the gas is vented from the cargo tank. Among other things, to meet the requirements from the authorities as to reduced emissions into the atmosphere, the gas is treated by VOC gas being separated from the remaining gas and used for various pur¬ poses, possibly being absorbed into the petroleum load again.

A plant for separating VOC from combustion gases is rela¬ tively expensive. Production of waste gases for filling cargo tanks involves combustion of considerable amounts of fuel and thus also adds to increased emission of, among other things, carbon dioxide. Off-shore storage ships must typically be supplied with fuel from shore by means of supply vessels with the cost thereby entailed.

From production plants for crude oil/gas condensate it is known to transfer hydrocarbon gases, HC, as fill gas above liquid crude oil/gas condensate. As only insignificant amounts of oxygen are present, the possibility of igniting the HC gas has been sufficiently reduced.

For storage and cargo ships it has not been practically fea- sible to use pure HC gas as fill gas because, according to the prior art, this will require a separate pipe connection for the HC gas between the production plant and the ship.

The invention has as its object to remedy or reduce at least one of the drawbacks of the prior art. The object is achieved in accordance with the invention through the features specified in the description below and in the following Claims.

The invention is realized through a method for supplying a fill gas to a cargo tank for crude oil/gas condensate. The crude oil/gas condensate flows to the cargo tank through a filling line, fill gas is mixed in in the filling line and flows together with the crude oil/gas condensate through the filling line, at least in a portion of the filling line, into the cargo tank.

It may be advantageous for the fill gas to be separated from the crude oil/gas condensate in or immediately before flowing into the cargo tank.

The best effect of the method is achieved if the amount of fill gas flowing together with the crude oil/gas condensate in the filling line is controlled by means of a pressure gauge measuring the gas pressure of the cargo tank.

The method is particularly suitable in the case of using a hydrocarbon gas as fill gas.

It is a considerable advantage of the method and the device according to the invention that fill gas that becomes super¬ fluous in the cargo tank, can be used directly for the pro¬ duction of electricity, for example.

The invention described above is just as suitable for all kinds of vessels and fixed storage tanks, to/from which crude oil/gas condensates in a liquid form are supplied and dis¬ charged. The term storage ship is chosen for reasons of il¬ lustration and does not limit the scope of the invention to vessels of this kind only. In what follows, there is described a non-limiting example of a preferred method and embodiment which are visualized in the accompanying drawings, in which:

Figure 1 shows a storage ship which is connected to a produc- tion unit, in which crude oil/gas condensate is transferred from the production unit to the storage ship;

Figure 2 shows a principle drawing of a loading system in ac¬ cordance with the invention; and

Figure 3 shows the same as Figure 1, but here a tanker is connected to the storage ship as liquid crude oil/gas conden¬ sate is loaded into the tanker.

In the drawings the reference numeral 1 identifies a storage ship which is moored to the sea bed 2 by means of a connec¬ tion 4 and associated anchoring 6. A filling line 8 extends from a production plant 10 via the connection 4 to the cargo tank 12 of the storage ship 1.

To the production plant 10, see Figure 2, well fluid is flow¬ ing from a petroleum well, not shown, via a well fluid pipe 21 to a processing plant 14. In the processing plant 14 the well fluid is separated into, among other things, a liquid crude oil/gas condensate which flows on through a liquid pipe 16 up to a mixing unit 18, and an HC gas which flows on through a gas pipe 20 up to a distributing valve 22. An in¬ termediate pipe 24 connects the distributing valve 22 to the mixing unit 18.

The filling line 8 is connected to the mixing unit 18 of the production plant 10 and extends, as earlier mentioned, via the connection 4 to the cargo tank 12, where the filling line 8 is connected to a separator 26. From the separator 26 the liquid crude oil/gas condensate flows down into the cargo tank 12 through a pipe 28, whereas the HC gas flows out into the upper portion of the cargo tank 12 via a pipe 30.

A pressure gauge 32 measures the pressure in the cargo tank 12 and communicates by means of a control of a kind known in itself, which is indicated by means of the line 34 in Figure 2, with the distributing valve 22.

While a liquid crude oil/gas condensate is flowing into the cargo tank 12, which has previously been filled with HC gas, the pressure in the cargo tank 12 is measured by means of the pressure gauge 32. If the pressure in the cargo tank 12 falls below a predetermined value, the distributing valve 22 is controlled into directing more HC gas from the gas pipe 20 via the intermediate pipe 24 and the mixing unit 18 into the filling line 8. The HC gas thereby flows together with the liquid crude oil/gas condensate via the filling line 8 to the separator 26. Excess HC gas from the distributing valve 22 flows to other points of consumption, not shown, in the pro¬ duction plant 10.

If the pressure in the cargo tank 12 rises over a predeter- mined value, the inflow of HC gas from the distributing valve 22 to the mixing unit 18 is reduced, while at the same time the consumption of HC gas on the storage ship 1 may possibly be increased.

In a further exemplary embodiment, see Figure 3, a tanker 36 is connected to the storage ship 1. During unloading from the storage ship 1 into the tanker 36 the cargo tanks 12 are re¬ plenished with HC gas from the production plant 10 as the liquid crude oil/gas condensate is flowing into the tanker 36.