As oil and gas wells are drilled in deeper water, the risers become a significant cost. The risers are the pipes carrying fluids from the well to a processing plant which may be on a floating platform. The risers must be capable of withstanding the high pressures of the fluids, which may exceed 600 atmospheres (> 60 MPa). Typically the fluid emerging from the well consists of a mixture of gas, oil, water and sand, and because of its multi-phase nature the pressure drop through the riser may be significant and indeed variable, and the riser diameter must therefore be large. To reduce the cost of the risers and to enhance confidence in their suitability it would be desirable to separate the various components (oil, water, and gas) on the seabed close to the well head. Separation may be undertaken using large vessels in which the phases separate under gravity, and separate smaller diameter risers can then be provided for the oil phase and the gas phase. The water associated with the oil may not be pure enough to be discharged to the environment, but can be re-injected into the well using high-pressure pumps at the well head. The sand that also collects in the separator is more difficult to deal with, because it is also usually contaminated with oil and so should not be discharged directly to the seabed. One solution is to bring the separator module to the surface (say once a year) and remove the sand for further treatment, but this is expensive and may cause disruption to oil production.

According to the present invention there is provided a separator device for use on the sea bed, the separator device comprising a vessel in which sand will be present, immersed in water, in use of the device, the vessel being provided with ultrasonic transducers to subject the contents to intense ultrasonic irradiation, and means to energize the ultrasonic transducers arranged such that the sand is subjected to such irradiation for at least one brief time period of no more than 60 s, so as to remove any oil from the sand.

Energising the transducers tends to disperse the sand in the water, to separate any oil from the surface of the sand particles, and may also tend to emulsify any oil into the water ; this may be due to fluidisation of the water and sand mixture, or cavitation within the mixture, and particle/particle collisions. After the or each period of irradiation the sand separates from the oil/water mixture due to the density difference. This cleaning process can be repeated. The water can subsequently be re-injected into the oil well using a pump. (It will be appreciated that the term"water"in this specification refers to the aqueous phase associated with oil and gas in the oil well, and is perhaps more accurately referred to as a brine. ) The emulsification may be improved if the separator also comprises means to electrolyse brine so as to generate sodium hypochlorite, or caustic (sodium hydroxide). This may enhance removal of the oil from the sand, and dispersion of the oil in the water.

The ultrasonic cleaning step may be performed only when a significant quantity of sand has collected in the vessel. Preferably the sand is then allowed to settle, and the removed oil droplets allowed to float up. If the oil is emulsified into the water, the resulting oil-

contaminated water may be re-injected into the reservoir.

The sand may then be again contacted with water from the well, and the ultrasonic cleaning repeated. This cleaning process is usually performed several times, to reduce the oil contamination to negligible levels.

Alternatively the sand may be subjected to such an ultrasonic cleaning step on a continuous basis, whilst immersed in water. The cleaned sand, dispersed in water from the well, can then be discharged onto the sea bed.

Preferably an array of ultrasonic transducers is provided, each mounted on the outside of the wall of the vessel.

Preferably the separator device also comprises a rechargeable battery to provide electricity for energising the transducers, and for operating water injection pumps, and possibly also to generate caustic or sodium hypochlorite. Preferably the battery is a rechargeable lithium ion battery. Such a battery may be trickle charged through a cable from a generator on a production platform at the surface. Alternatively it may be trickle charged from a seabed generator. Hence the separator device preferably also includes a thermoelectric generator to generate electricity from fluids at two different temperatures. For example the generator might utilize the temperature difference between the fluids from the oil well, which may be at 200°C, and the surrounding sea water which is typically at about 5°C. Alternatively a suitable temperature difference may be generated by causing gas (say from the gas riser) to flow through a vortex tube, so generating higher and lower temperature gas streams, the gas streams subsequently being fed back into the gas riser for example at a venturi.

The invention also provides a process for cleaning sand that is contaminated with oil, the method comprising mixing the sand with an aqueous liquid,. and subjecting this mixture to intense ultrasonic irradiation for at least one brief time period of no more than 60 s, so as to remove any oil from the sand. At least with some types of oil the irradiation period may be less than 30 s, for example 20 s. This is preferably performed using a tubular duct with an array of ultrasonic transducers extending longitudinally and circum-ferentially over its external surface, the sand/liquid mixture being caused to flow along the duct.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a seabed separator device; Figure 2 shows an inlet valve for the separator device of figure 1; Figure 3 shows an alternative seabed separator device; and Figure 4 shows another alternative seabed separator device.

Referring now to figure 1, a separator device 10 is arranged on the seabed 12 adjacent to a wellhead 14 through which emerges a fluid stream comprising oil, gas, an aqueous phase (referred to as water) and some sand, at an elevated pressure. The fluid stream is fed through a vortex flow modulator 16 (in order to adjust flowrate) into a separator tank 18 in which it separates into three layers: gas 20, oil 21 and water 22. The flow rate may

be adjusted in response to a sensor (not shown) of the liquid level in the separator tank 18. Phase separation is enhanced by the vortex flow at the outlet from the modulator 16. The separator tank 18 would typically be- of length between 6 and 9 m, and of diameter between 1.8 and 2.4 m. Risers 24 and 26 carry the gas 20 and oil 21 respectively to a production platform at the surface (not shown). A pump 28 is activated, intermittently, to re- inject the water into the well.

Referring now to figure 2, the vortex flow modulator 16 comprises a vortex chamber 30 with an axial inlet 32 and an axial outlet 34 between which is a baffle plate 36. A plunger 38 may be actuated to adjust the flow rate through the inlet 32. There are also bypass ducts 40 (only one of which is shown) that supply fluid from upstream of the plunger 38 to tangential inlets 42 in the vortex chamber 30. Adjusting the plunger 38 enables the flow rate from the wellhead 14 to be adjusted.

Furthermore, because of the tangential inlets 42, the fluid mixture emerges as a vortex through the outlet 34 (this vortex flow becoming even more vigorous as the plunger 38 restricts the inlet 32). The duct 44 communicating with the outlet 34 has a weir structure 46 defined by a radial gap between a stepped end of the duct 44 and an inner tube 48, and as a consequence of the vortex flow the denser liquids tend to emerge through the weir 46 and the less dense fluid tends to follow the inner tube 48.

Referring again to figure 1, the weir 46 is at a level in the vicinity of the interface between the oil phase 21 and the water phase 22, while the inner tube 48 which primarily carries gas and entrained liquid droplets follows a J-shaped path to emerge in the gas phase 20 just below an impactor plate 50. Hence the vortex flow

modulator 16, the weir 46 and the impactor plate 50 assist in separating the three fluid phases, which also separate within the tank 18 because of their density differences.

The sand is the densest component, and an inclined tray 52 immersed in the water phase 22 is arranged to catch the sand emerging from the weir 46. Several ultrasonic transducers 54 are mounted in an array on the underside of the tray 52, and are connected to a signal generator 56. From the bottom corner of the tray 52 an outlet duct 58 extends to near the seabed 12, incorporating a multi-phase pump 60.

During use of the separator device 10, sand 55 gradually collects in the tray 52. Unfortunately the sand 55 usually has oil associated with it. When the sand 55 has accumulated to a preset level, the transducers 54 are activated so that the ultrasonic energy is focused into the bed of sand and the immediately adjacent water. The ultrasound fluidises the sand 55, and any oil film on the sand particles is disrupted and dislodged by cavitation and particle/ particle collisions. The transducers 54 are activated for a succession of brief pulses, for example pulses of 5 s with intervals of 1 minute between pulses, and between the pulses the displaced oil droplets migrate out of the sand bed, and float up out of the water phase 22. Sodium hydroxide or sodium hypochlorite generated in situ electrochemically (at a cathode, by electrolysis of brine) may be introduced into the oil-contaminated sand prior to ultrasonic cleaning to enhance the effectiveness and completeness of separation. After several such cleaning pulses, for example six or ten pulses, the transducers 54 are again activated to fluidise the sand 55 and at the same time the pump 60 is actuated to pump

the clean sand out onto the seabed 12. (Any overall change of pH can be prevented by subsequently mixing the acidic liquid from near the anode of the electrolysis cell with the aqueous phase in the tank 18).

It will be appreciated that the pressure of the fluids emerging from the wellhead 14 is typically considerably greater than the surrounding pressure of the sea water in the vicinity of the separator 10, for example the fluids may be at between 500 and 1000 atmospheres, whereas at a depth of 100 m the seawater is only at about 10 atmospheres. The sand cleaning process described in the previous paragraph may be carried out online, at this elevated pressure, in which case the pump 60 may be replaced by a valve. Alternatively, the sand cleaning process may be carried out at the seabed ambient pressure; this entails closing the valve at the wellhead 14 or alternatively the flow modulating valve 16, closing valves (not shown) at the bottom of the risers 24 and 26, and opening a pressure equalisation valve (not shown) between the water 22 in the tank 18 and the sea water surrounding it. If it is necessary to maintain production from the wellhead 14 during such a cleaning process, this may be achieved by providing two identical separator tanks 18, so that one may be in use while the other is undergoing this sand-cleaning process.

The electrical power needed for electrolysis and to operate the water reinjection pump 28, the signal generator 56 for the transducers 54, the flow modulating valve 16, and the sand outlet pump 60 is provided by a rechargeable battery 62. This may in principle be trickle charged using a cable (not shown) from the surface, but it is usually preferable if the separator device 10 incorporates means to generate the electricity.

As indicated diagrammatically in figure 1 this may

consist of a stack of thermocouples 64 generating electricity from the temperature difference between the water 22 in the tank 18, and the ambient sea water.

Referring to figure 3 there is shown a modified sand separator device 70, many features of which are the same as in the device 10 of figure 1 and are referred to by the same reference numbers. The separator tank 18 in this case contains a generally funnel-shaped sand collecting tray 72, at least the lower portion of the tray 72 being below the interface between oil 21 and water 22. Ultrasonic transducers 74 are provided around the outlet duct of the funnel-shaped tray 72, and these are continuously energized by the signal generator 56, so that as the sand falls down the outlet duct it is continuously subjected to ultrasonic cleaning while in the water phase 22. A counter-current flow of in-situ electrochemically generated caustic or sodium hypochlorite solution may enhance the effectiveness of the ultrasonic cleaning. A sand collection trap 76 is arranged below the outlet from the funnel-shaped tray 72, and an array of ultrasonic transducers 54 are coupled to the underside of the trap 76. An outlet duct 58 is connected to the base of the sand trap 76, and leads to an outlet valve 78.

The separator device 70 continuously cleans sand as it falls through the outlet duct of the funnel-shaped tray 72, so that the sand 75 in the sand trap 76 is substantially clean. At intervals the transducers 54 are activated by the signal generator 56 so as to fluidise the sand 75 in the trap 76, and the valve 78 is opened to eject a slurry of clean sand and water onto the seabed.

The separator device 70 is intended to operate online, so that the contents of the tank 18 remain at the elevated pressure of the fluids from the wellhead 14 at all times.

It will be appreciated that a separator device may differ from those shown in figures 1 and 3 while remaining within the scope of the invention. For example in the separators 10 and 70 gravitational separation of the three fluids takes place in the same tank 18 as the sand cleaning. As an alternative, referring to figure 4, a separator device 80 may comprise an upright cylindrical tank 82 with a generally conical base 83, separation of the fluids taking place within that tank 82; the conical base 83 has an outlet duct 84 communicating with a sand storage and cleaning vessel 86 below the tank 82.

Ultrasonic transducers 74 may be provided on the wall of the outlet duct 84 between the two vessels, or transducers 54 may be arranged to subject the sand 85 within the storage vessel 86 to intense ultrasonic irradiation. (The wall separating the transducers 74 or 54 from the sand and water is preferably no more than about 10mm thick, to ensure an adequate ultrasonic intensity. ) A water reinjection pump 28 can extract water from the upper part of the sand cleaning vessel 86, the pump 28 being activated only when the sand 85 is not fluidised. This arrangement has the advantage that if the oil becomes emulsified as a result of the ultrasonic treatment, the water and emulsified oil can be immediately disposed of by reinjection. This contaminated water would then. be replaced by comparatively clean water flowing down the duct 84 from the cylindrical tank 82.

At intervals, the clean sand can be fluidised and ejected onto the seabed by opening a valve 78.

It will be appreciated that an apparatus similar to that described in relation to Figure 3 may be used on a beach, for example, to decontaminate oiled sand. The oily sand is mixed with water such as seawater. If the quantity of oil is large, then gravity separation may be

utilized as a first step ; but the significant aspect of the procedure is to pass the oiled sand/water mixture through an ultrasonic irradiation duct with an array of transducers 74 on the wall of the duct. The sand can then be returned to the beach, and the oil (which may be in an emulsion) stored separately for subsequent treatment or disposal.