Method for operating a submarine, rotating device and an apparatus for said device The invention relates to a method as disclosed in the preamble of the independent method claim. The invention also relates to an arrangement as disclosed in the preamble of the independent apparatus claim.

The gas-filled electric motor must be kept as dry as possible to avoid corrosion as a result of the reaction between water and hydrogen sulphide and dilution of the lubricant with hydrocarbon condensate. These problems have been discussed in other documents including NO 172075 and NO 173197, which describe respectively the use of a suitable dry foreign gas that is fed to the common pressure shell of a compressor and an electromotor, and the use of a part of the gas pressurised in a compressor as seal gas passed to the seals of the compressor and the gas atmosphere of the electromotor, with cooling of this branched-off gas en route to the device consisting of a compressor and its electromotor.

The object of the present invention is to improve the last-mentioned, known method outlined above by ensuring that a cooled gas is provided which will remain dry during the subsequent heating in the device or motor, under all conditions and especially also during shutdowns and long-term stoppages (dew point control). During such stoppages the gas in the motor may be cooled to the seawater temperature, which typically can be in the range of +10° to-2°C.

The known cooling during heat exchange with the surrounding seawater is, according to the invention, followed by a further cooling using the known Joule-Thomson effect.

During the heat exchange with the seawater, the branched-off gas is cooled to about 30°C, above the hydrate temperature, and during the subsequent throttling utilising the Joule-Thomson effect the gas temperature is brought down to zero and lower, for example minus 5°C. A cold gas of this kind will behave like a dry gas under all the conditions that will be encountered in the subsea station.

According to the invention there is therefore proposed a method and an arrangement as defined in the independent claims.

The invention will be described in more detail with reference to the schematic drawing, which shows arrangement according to the invention.

The drawing shows a subsea station having a rotating device 1, for example, a compressor or a wet gas compressor, arranged in a common pressure shell 2 with an electromotor 3. For more details, reference is made to the aforementioned NO patents, although the invention is of course not limited specifically to such embodiments. The pressure shell 2 is supplied with dry gas from a circuit 4 comprising a line 5 which branches off from the outlet 11 of the device 1. The line 5 runs to a heat exchanger 6, where the gas branched off through the line 5 is cooled in heat exchange with the surrounding seawater. After the cooler 6 there follows a throttle 7, where the already cooled gas is cooled further utilising the known Joule-Thomson effect. After the throttle 7 there follows a scrubber 8, i. e. , a cyclone, a filter, a precipitation chamber or the like, where as much liquid as possible is separated and removed from the cooled gas. From the scrubber 8 there runs a line 9 to the pressure shell 2.

The hydrocarbon stream may, for example, have a pressure of 50 bar at the inlet 10 and a pressure of 100 bar at the outlet 11. The pressure is reduced to 60 bar in the throttle 7.

In the cooler 6 the gas temperature is reduced to about 30°C, i. e. , above hydration temperature, and after the throttle the gas has a temperature close to minus 5°C. After the scrubbing in the scrubber 8, there will be a dry gas which will remain dry under all conditions, even during shutdowns and stoppages, provided that the scrubber is of some known type that removes water and hydrocarbon liquid in an effective manner.

An optional injection 12 of a hydrate inhibitor is indicated upstream of the scrubber 8.