GB2242373A - Crude oil separator - Google Patents

Abstract

A separator 21 located on the seabed separates the solids, gas and liquid phases from the product of an underwater oil well, retaining the solids 25 and discharging the gas to a flare on the surface and the liquid to a surface mooring for transfer to a tanker. The separator may discharge oil and water through separator lines or at different times through one line. The various control valves and discharge pumps are controlled automatically by pressure and level indicators via a common microprocessor which is also located underwater in a separate sealed enclosure (Fig. 7). Separate flowmeters may monitor the flows of the separated fluids, which may thereafter be recombined for passage through a single pipeline (Figs. 5, 6). <IMAGE>

Description

SUBSEA SEPARATOR. STORAGE & PUMPING UNIT AND ITS ASSOCIATED COKTROL SYSTEM The invention relates to the separation of crude oil as it is produced directly from a subsea offshore wellhead, into its constituent parts of petroleum liquids, petroleum gases, produced water and produced solids, utilising processing facilities situated on the seabed. Such processing facilities can be adjacent to the wellhead, and can be capable of metering and pumping the separated fluidic components into a pipeline or riser for export.

In one specific form, the invention relates to an electro-hydraulic control arrangement associated with the facilities described above, whereby process sensing, control intelligence and process actuations are all effected locally and autonomously, in a remote subsea environment by using a microprocessor controller.

Such a control arrangement can eliminate the human operator for most routine and ESD process control functions, the human operator being required only to initiate and curtail production.

The total system is intended to produce "dead" liquid petroleum as the required product, this being known as "dead crude". Dead crude may be defined as crude oil which has been degassed to its atmospheric vapour pressure (or thereabouts) and separated from its constituent aqueous and solid phases, to an acceptable basic sediment and water content (BS & ), suitable for transportation and refining without undergoing further pre-refining treatment.

The separated crude oil constituents (namely dead crude, crude gasses and produced water) are separately transported from a subsea separator, storage and pumping unit by either continuous, intermittent or batch transportation. The separated solids constituents are trapped in the unit and stored for later removal.

In being capable of effecting the aforementioned, the invention is equally disposed to variations as follows: a) in providing a subsea wellhead separation system whereby the produced well fluid is separated into its constituent liquid and gaseous phases thereby enhancing well recovery; b) in providing a subsea separation system, whereby the produced well fluid is separated into its constituent parts of dead crude, produced water and crude gas, and exported immediately into a fixed or floating surface facility; c) in providing a subsea separation and storage system whereby the process, as described in b) above, is augmented by the inclusion of subsea storage facilities for the process liquids phases, as may be required by an intermittent export system eg. a visiting shuttle tanker.

d) in providing a subsea well head test separation system whereby the produced fluid efflux from a single subsea welihead is metered on line, thus providing a measurerrert of well performance, the metered fluids being exported as in sections b) and c) above; e) in providing a subsea multiphase metering system whereby the produced fluid efflux is metered in its separated phases, then recombined for bulk export.

Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig 1 is an overall depiction of a dead crude production system suitable for a large range of seawater depths; Fig 2 is a flow diagram of a wellhead separation system; Fig 3 is a flow diagram of a subsea separation system; Fig 4 is a flow diagram of a subsea separaticn and stowage system; Fig 5 is a flow diagram of a subsea well head test separation system; Fig 6 is a flow diagram of a subsea multiphase metering system; and Fig 7 is a pictorial elevation of the control system located subsea.

Fig 1 illustrates a subsea production system as a whole. A plurality of subsea wells 10 are connected by multiphase pipelines 11 into a centrally disposed subsea separator, storage and pumping unit 12. The separator 12 is in turn connected to a gas riser 14 and surface flare system 15, and by a seabed pipeline 16 to a floating mooring system 17 and shuttle tanker 18, (the latter being for illustrative purposes only). Multiphase crude product is supplied by the subsea wells 10 into the subsea separator 12 whereby separation takes place with gas venting to the surface and being flared, or vented, or compressed and being rejected, or fed into a pipeline or otherwise utilised. Oil and water is pumped to the floating mooring system 17 and thence to the shuttle tanker 18 for export, with solids being retained within the separator 12 for future removal.

Figure 2 illustrates apparatus for obtaining the separation of liquid, gaseous and solid phases of crude wellhead product, for individual and more efficient transportation of these separated phases to remote processing facilities. Crude well head product enters wellhead separator 21 via an inlet pipe 22, which is held at an internal pressure substantially lower than the flowing wellhead pressure. Gas separates from the liquid and solid phases and escapes through pipeline 23 which exits from the top of the wellhead separator 21. Liquids and solids falling to the lower part of the wellhead separator 21 builc up behind a weir 24, where the solids fall to the bottom of a first chamber created by the weir 24.Liquids continue to build up and spill over the weir 24 into a second chamber createc by the weir 24, leaving solids 25 trapped within the first compartment for later removal. Liquids having entered the second compartment are then pumped out of the well head separator 21 by pumps 26 and control valves 27 and 2z, and theme along pipeline 29 to the export gathering facility. The liquid phase pumped to the export facility 17 comprises a mixture of hydrocarbon and aqueous phases. Where it is necessary to separate these two phases for individual export, then the system described below may be used.

The apparatus disclosed in Fig 3 is similar to that in Fig 2, and for this treason elements common to fig 2 and fig 3 carry identical numbers. The difference in Figure 3 is the addition of a second weir 30 and the doubling of liquid pipelines 29 such that produced water 31 (being of a higher specific gravity than the produced crude oil 32) separates by gravity in the intermediate chamber created by the weirs 24 and 30, and is drawn off as the level builds up. Meanwhile the separated crude oil spills over the weir 30 and gathers in the chamber beyond the weir 30 where it may be drawn off separately from the produced water. Pumps 26 and valves 27 and 28 perform liquid draw off functions in the same manner as in Fig 2, but in this Instance the separated liquid phases are pumped along two separate pipelines 29, to the export facili-ty.If it is necessary to house the export storage facility at the same location as the subsea separation unit, then the system described below may be included. (As an alternative, one pump may be used with valves 27 being upstream, and controlled to supply oil or water as alternative batches to the inlet of that pump).

The apparatus disclosed in Fig 4 is similar to that in Fig 2 and Fig 3 in that it combines the separator devices described above with the addition of one or more storage vessels to store the produced water and crude oil. In this instance the well head separator (as described in Fig 2) becomes a high pressure separator 41 and the subsea separator as described in Fig 3 becomes a low pressure separator 42. The addition of one or more storage vessels 43 provides the storage capacity to enable continuous production from the process facilities to intermittent export facilities associated for example with an export shuttle tanker 18.

By constant monitoring of a pressure indicator 45 and a level indicator 46, and constant modulation of control valves 47 and 48, the separation process in the high pressure separator 41 is maintained in a state of equilibrium, whereby the solid, liquid and gaseous phases of the crude product are separated. To safeguard against the over-pressurisation of the high pressure separator, a relief valve 49 vents into the low pressure separator 42.

Separation of the liquid phases and further separation of the gaseous phase are accomplished in the low pressure separator 42 by constantly monitoring the level indicators and controllers 50 and 51, which control respectively the constant modulation of valve 52 to draw off the produced water, and the oil export pump 53.

Dead crude and produced water are provided with buffer storage capacity in storage vessel 43 which is sized to the production rate and export downtime period unique to each production system. By constantly monitoring pressure indicator 55, the wellbeing of the storage vessel is maintained. The indicator 55 initiates cessation of production should abnormally high pressures be detected. By constantly monitoring a level indicator/controller 56, the produced water level Is monitored, initiating export water pump 57 startup and shutdown. (The vessel 43 may be configured as two or more separate storage vessels).

To safeguard the entire system all pressure indicators and level indicators are linked to a central emergency shutdown system causing cessation of production when triggered by abnormal indications, or by input signals from other manual or automatic sources.

The wellhead test separator, Fig 5, provides a means of measuring the rate of production of fluids from a subsea well at a range of well back pressures. For this purpose it would be generally installed on the seabed upstream of a subsea separator unit (Fig 3), or storage separation unit (Fig 4), or a platform or land mounted separator. (As a limited application it can also be installed in a multiphase pipeline as shown specifically in Fig 6).

During operation, crude oil from the well 10 enters through isolation valve 61 and choke 62 which can vary the back pressure provided bypass valve 63 is open. The fluids enter the separation vessel 21 where the gas is separated by gravity and due to pressure changes. The gas flows from the vessel through vent pipe 66 and is measured by a flow measurement device 68 which is upstream of a pressure controlling valve 67. The liquids accumulate by gravity in the base of the vessel 21 where the oil and water are separated by the weir 30 (which operates as described for Fig 3) such that water will drain down pipe 62 to pump 26 in the course of which the flow rate is measured by flowmeter 68, while oil flows down the corresponding pipe 67 and is measured by flowmeter 69 upstream of pump 26A.As a variation of this arrangement the oil and water streams can be recombined before entering a single pump or array of several pumps.

The pump delivery flow passes through valve 28 in each leg, these valves being controlled by the liquid levels measured in old and water compartments of the vessel 21. Used in conjunction with valve 67 and choke 62, valves 28 can be a means of varying the back pressure on the well 10 as described above. For most applications the liquid lines will discharge into the corresponding lines or vessels of the production separator (into which this well head test separator would normally discharge), with the free flowing gas line discharging into the corresponding gas line of the production separator.

The subsea multiphase meter Fig 6 is Intended to separate the liquid and gas phases of the flow from a well such that each phase can be measured by flow measurement devices known to those skilled in the art and shown as 71 in Fig 6, after which the flow is recombined and returned to the pipeline.

In operation, gas and liquids enter through Isolation valve 70 when bypass valve 72 is closed (valve 72 in Figure 6 corresponds to valve 63 in Fig 5). They enter the vessel 21 at the normal flowing pressure without any imposed pressure loss as required by a separator. Gas will vent upwards and be measured; and liquids (produced water and crude oil) will drain downwards at a rate controlled by the vessel level control system, and through the valves 28, during which they will be measured by the flow devices 71A. Both gas and liquids can then be recombined and continue along the pipe to the main separator. (This is a multiphase flow measuring device not a phase separation device).

Reference is now made to Fig 7 to explain a method and apparatus for obtaining control of the process plant previously described in Figures 1, 2, 3, 4, 5 and 6.

The system depicted, in illustrative form only, consists of an outer shell 101 and lower housing 102, known collectively as the control pod, and provides environmental protection for the control system mechanisms by excluding seawater and so negating its detrimental corrosive effects.

Within the protective housing of the control pod 101 & 102, the control mechanisms are conveniently arranged and can be grouped conveniently as a programmable logic controller (PLC) 103, which in turn is housed within its own protective enclosure 104; and electro hydraulic control and sensing devices 105 which are housed within the control pod base 102. The whole of the internal volume of the control pod is occupied by a dielectric mineral oil 106 which serves to provide further protection against the onset of corrosive degradation. The liquid is compatible with operation of electrical systems within it. Alternatively it may be gas, or of near solid consistency, but of specific gravity and composition to prevent mixing, emulsification, or chemical reaction with incoming seawater or other fluid.

In conjunction with a pressure balancing bladder 107, the oil 106 provides pressure equalisation within the pod to that of the surrounding hydrosphere, so eliminating the pressure gradient which would otherwise seek to reach a state of equilibrium by the ingress of seawater. Motive energy in the form of electrical current and pressurised hydraulic control fluid is supplied to the control mechanisms by way of quick-disconnect connectors 108, accompanied by a communications channel to monitor and initiate control.The internal control mechanisms are connected by hydraulic conduits and electrical cables to actuators and sensors located on the subsea separator plant as previously described, such that the process system status is constantly monitored by the control system PLC 103 which autonomously initiates control functions via the electrohydraulic control devices 105 and thence to the process plant actuators. The PLC 103 controls the process plant functions by preprogrammed logic algorithms that may be changed or modified using a surface located micro processor system which is not necessary for the most part of the control cycle. To protect against the confusing or potentially catastrophic effects of random spurious sensor signals, the PLC processor is triplicated, with the decision logic being based upon a 2-out-of-3 voting system fcr control command initiation.

In a variant, the control pod 102 can be a completely sealed enclosure, and the PLC 103 can be in a separated upper enclosure, in whic case the inner shell is not present and the dielectric fluid is confined within the lower enclosure.

Claims (8)

Claims

1. A solid/liquid/gas separation unit for installation on the seabed, the solids being retained in the unit, the liquids pumped to a surface location and the gas being vented to the surface under the pressure in the unit, characterized in that there is provision (21) to split a flow of liquid (17) and gas (14) for flow measurement purposes, using single phase fluid flow measurement devices.

2. A separation unit as claimed in claim 1 characterised in that the liquid and gas fluids are recombined and fed back into a pipeline as multiphase fluid.

3. A separation unit as claimed in claim 1 or claim 2 characterised in that it is used in combination with a floating flare (15) for gas disposal and a moored tanker (18) to receive oil and water products, said tanker being disconnected and sailing away when full, or when sea conditions so dictate, the operation of the seabed unit being continued for as long a period as the well flow rate and storage capacity may dictate.

4. A separation unit as claimed in claim 3 characterised in that the motive power for subsea pumps used to lift liquid products to the tanker is provided by power plant on the tanker by way of a connect/disconnect power cable.

5. A separation unit as claimed in claim 3 or claim 4 characterised in that the floating flare is installed on a conventional monohull ship moored to a conventional mooring buoy which incorporates the flexible riser needed to supply the gas, the gas route to the ship mounted flare being by means of a swivel and flexible hose additional to the conventional mooring line from ship to buoy.

6. A separation unit as claimed In any one of the preceding claims, characterised in that there is a programmable logic controller (plc) (103) installed in a capsule (101) which provides an inert gas environment of suitable pressure and composition to permit operation of the electrical/electronic components, this being installed on the seabed with electrical, hydraulic, optic or similar channels passing by waterproof routes and penetrations through the wall of the said capsule, to operate external valves and connect to sensors which are in the external hyperbaric environment.

7. A separation unit as claimed in claim 6 characterised in that the plc is inside an inner capsule which is itself inside an outer capsule, the space between the walls being filled with liquid at local hyperbaric pressure, as maintained by pressure compensation devices, to avoid ingress of external seawater or other fluid through leaks in the outer wall.

8. A separation unit as claimed in claim 6 or claim 7, characterised in that the plc is used to provide control of valves for pressure and level control and/or the start/stop operation (or speed control) of a pump in response to sensors which indicate required pressure, levels, flow rates, valve status and position, pump speed and status, and any special external input, if any, the whole system and all components being on the seabed with no communication with the surface except for monitoring and available channels for intentional shutdowns or reconfiguration of the control algorithms.