Three phase separators are commonly used in the hydrocarbon extraction industry for separating a raw flow of gas, hydrocarbon liquid and aqueous liquid into its separate phase constituents for further processing. The present invention provides, in the preferred embodiment thereof, a three phase separator in which the phase separation is carried out in two stages, the first stage comprising the separation of the gaseous phase from the liquid phases, and the second stage comprising the separation of the respective liquid phases from each other.

The division of the phase separation into two stages gives rise to a number of advantages as compared with prior art designs. In particular, the substantially complete removal of the gaseous phase from the liquid phases prior to separation of the liquid phases enables the separation of the liquid phases to be carried out under more controlled conditions than was possible with prior art separator designs.

The preferred embodiments of the present invention are particularly suitable for use on floating production platforms. It is to be understood, however, that the invention is not limited to such applications and may be used in other installations where three phase separation is required.

The invention will be understood from the following description of preferred embodiments thereof, given by way of example only, reference being had to the accompanying drawing wherein: Figure 1 shows in schematic longitudinal cross-section a first preferred embodiment of the invention; Figure 2 is a view corresponding to Figure 1 showing a second embodiment of the invention; and Figure 3 is a view corresponding to Figure 1 showing a third embodiment of the invention.

The embodiment of the invention shown in figure 1 comprises a tank 1 having an inlet 2, an aqueous phase outlet 3, a hydrocarbon liquid outlet 4 and a gas outlet 5. In use, a mixture of gas, hydrocarbon liquid and aqueous liquid is received through the inlet 2 and is separated by the separator into its separate constituent phases for discharge through the outlets 3,4 and 5.

Located centrally within the tank, and connected directly to the inlet 2 and the gas outlet 5 is a gas separator designed to remove gas from the inlet stream. The separator is preferably a compact design of cyclone separator, for example of the type which is the subject of Norwegian patent 180258 and is available from Kvaerner Paladon Limited of Eldon Way, Crick, Northampton, NN6 7SL United Kingdom under the designation"G-Sep CCI". It is to be understood, however, that the exact design of gas separator is not critical to the present invention, and any separator of a suitable size and able to provide the required degree of gas separation may be used.

The separator 6 functions to remove gas from the inflow stream and to discharge that gas directly to the gas outlet 5. It is envisaged that the separator 6 will remove a very high proportion and preferably substantially all (better than 95%) of gas from the incoming stream. It is recognised, however, that a small amount of gas may be carried through the separator 6 by the liquid flow, and accordingly a vent pipe may be provided extending from the upper part of the tank 1 into the gas outlet 5.

Further, the separator 6 and/or the gas outlet 5 may incorporate a demister to remove liquid droplets from the gas stream.

The liquid phases remaining after gas separation are discharged from a distribution box 7 at the bottom of the separator 6 into respective distributor pipes which extend from the box 7 to points close to the respective ends to the tank 1. In the illustrated embodiment of the invention only two such pipes (8,9) are provided.

The pipes 8,9 terminate, at the ends thereof remote from the box 7, in distributors 10 which discharge the liquid phases into the tank close to the ends thereof.

Although in the illustrated embodiment of the invention only two such pipes (8,9) are provided it is to understood that it is possible, and in some instances may be desirable, to have more than one pipe leading from the distribution box 7 to the ends of the tank. Such an arrangement may, for example, be desirable if means are provided in the separator 6 or distribution box 7 for treating the liquid phases to produce some initial separation thereof. For example, it may be possible within the separator 6 or distribution box 7 to separate the liquid phases into an oil rich phase and an aqueous rich phase which can then be discharged via separate pipes to different regions of the end zones of the tank. Alternatively, the aqueous rich phase can be discharged from the apparatus of the present invention directly from the separator 6 or distribution box 7 so that the tank 1 is only required to separate a relatively small volume of aqueous liquid from the bulk of hydrocarbon liquid within the tank.

The liquid phases, after discharge from the distributors 10, flow back towards the centre of the tank, and in the process are separated into the respective phase constituents under the influence of gravity and, optionally, known devices for assisting in phase separation. The tank is provided with vertical distribution baffles 11 and anti-surge baffles 12 and, optionally, may be provided with horizontal anti- surge baffles 13 which reach a short distance, for example 400mm, from the walls of the tank. The effect of the baffles 11,12 and 13 is to limit disruption to the contents of the tank caused by motion of a floating platform or barge upon which the tank is mounted.

In the course of flow from the distributors 10 towards the centre of the tank the aqueous phase will accumulate at the bottom of the tank and will be withdrawn via the aqueous phase outlet 3. Substantially the entire tank above the aqueous liquid/hydrocarbon liquid phase interface will be filled with hydrocarbon liquid. When reaching the centre of the tank the hydrocarbon liquid will flow over control baffles 14 into a well 15 from which hydrocarbon liquid may be removed via the hydrocarbon liquid outlet 4.

The above structure has a number of advantages as compared with conventional three-phase separators. Firstly, because all or substantially all of the gaseous phase is removed from the inflowing liquid by the cyclone separator 6, little or no headspace for the accumulation of the gaseous phase is required within the tank. Accordingly, the tank may be substantially filled with liquid. This arrangement facilitates a compact design of tank and renders the separation process less vulnerable to disruption by movement of the structure upon which the separator is mounted than was the case with separators of the prior art. Further the fact that the liquid phase separation occurs separately from the gas phase separation means that the liquid phases can be better controlled within the liquid phase separation zone. For example, as the liquid phases pass through the distribution pipes 8,9 or the distribution box 7 they may be treated to assist phase separation, for example by the application of an electric field.

A further advantage which derives from delivering the liquid phases from the centre of the tank outwardly to the opposite ends of the tank is that the velocity of liquid flow within the tank is reduced as compared with that which would exist if the entire liquid flow passed from one end of the tank to the other. The reduced flow velocity assists separation of the hydrocarbon and aqueous phases.

Accordingly, the reduced velocity may be used to reduce the tank diameter for a given liquid flow rate or may be used to provide an increased throughput capacity for a tank of given diameter. The fact that the liquid flow is split into two or more paths each of which is constituted by a distribution pipe assists in maintaining consistent flow through the two opposite halves of the tank. Any tendency for more than 50% of the liquid flow from the distribution box 7 to flow towards one end of the tank will result in an increase in pressure drop along the associated distribution pipe or pipes which will in turn reduce flow towards that end of the tank. The liquid distribution system is accordingly to an extent self-balancing and will result in substantially half of the total flow being processed in each half of the tank.

It will be noted that the inlet 2 and the three outlets 3,4 and 5 are all located together and close to the centre of the tank. This substantially simplifies the installation pipework associated with the separator as compared with the prior art arrangements which typically included inlets and outlets at the opposite ends of the tank.

It will also be noted that the position of the hydrocarbon liquid/aqueous liquid interface can be controlled simply by controlling the discharge of aqueous fluid from the tank. In light of this the hydrocarbon liquid can, if desired, be removed from the tank via a simple pipe discharge manifold suitably positioned in the oil phase of the liquid rather than from the well 15.

Although not limited to applications on floating production facilities, the invention is particularly well adapted to such systems since the combination of liquid flow from the opposite extremities to the centre of the tank and the fact that the tank is substantially completely filled with liquid renders the separation of the aqueous and hydrocarbon liquid phases relatively insensitive to tank movement.

Further, flow distribution within the vessel is self-compensated in the event of vessel movement due to pressure losses in the internal pipework system.

Referring now to Figure 2 there is shown a second embodiment of the invention which is substantially the same as that shown in Figure 1 save that the gas outlet 5 of Figure 1 has been omitted so that the separator 6 discharges gas from the upper end thereof directly into the headspace of the tank 1. Accordingly, in this embodiment the level of the upper surface of the hydrocarbon phase is controlled to provide an adequate headspace at the top of the tank to receive gas from the separator 6. Gas from the headspace is removed from the tank via an outlet 5A. In passing from the headspace to the outlet 5A the gas passes through a suitable de-misting device, for example a conventional vane demister 16 located in an enclosure 17 mounted atop the tank 1. This embodiment of the invention preserves the advantages of separating the gaseous phase from the liquid phases before the liquid phases are discharged into the separator tank 1, and accordingly retains the advantages of the embodiment of Figure 1.

As an alternative to the use of a demister 16 located in an enclosure 17 externally of the tank 1, the embodiment of Figure 2 may make use of one or more demisters located within the body of the tank 1 in conventional manner. For example, two demisters can be provided within suitable enclosures within the tank, located adjacent the top of the tank at opposite ends thereof. In this case, gas will be discharged from the tank via pipes connected to the discharge outlets of the demisters.

Referring now to Figure 3, a further embodiment of the invention is illustrated. In this embodiment, the inlet 2 branches, within the tank, to form two branches 2A and 2B. These branches lead to respective separators 6A located adjacent the opposite extremities of the tank 1. The separator 6A are preferably of the type referred to above, namely cyclone separators in accordance with Norwegian patent 180258. The separators 6A are effective to remove a very large portion of the gas from the inflowing mixture and this gas is discharged through the top of the separators 6A into the headspace of the tank 1. The liquid phases are discharged from the bottoms of the separators 6A into the tank and flow from the opposite ends of the tank towards the centre for separation into respective hydrocarbon and aqueous phases, as described above. As with the embodiment of Figure 1, the hydrocarbon phase may be removed through a hydrocarbon outlet 4 and the aqueous phase through an aqueous phase outlet 3.

Gas from the headspace of tank 1 of Figure 3 is removed either via a gas outlet SA mounted on an enclosure 17 which houses a demister 16 or may be removed via one or more demisters located internally of the tank as described above with reference to Figure 2.

In the case of the embodiment of Figure 3 the liquid phases leaving the separator 6 may be a generally homogenous mixture or may be treated prior to discharge into the tank so that a hydrocarbon rich fraction is discharged at a relatively high level with the tank and an aqueous rich fraction is discharged from a relatively low level in the tank.