GB2222961A - Crude oil separator - Google Patents

Abstract

A subsea crude oil separator comprises a high-pressure gas separation vessel, connected via a pipe and pressure- reducing valve to a much larger, low pressure gas/oil/water gravity separation vessel containing a weir over which flows only substantially aqueous phase free hydrocarbon fluid. The residence time for liquid in the low pressure vessel exceeds 100 minutes. Liquid levels in the vessels are monitored by external radiation detectors which are connected by cables to a dry control pod before the pod is submerged, after which the detectors are attached to the vessel walls.

Description

Subsea Storage Separator Unit This invention relates to the separation of crude oil as it is produced from an offshore welihead, into the constituents of petroleum liquids, petroleum gases, water and solids where present. The required produce being liquid petroleum having a near atmospheric vapour pressure known as "dead crude" to those skilled in the art.

When this "dead crude" is produced from an offshore field the separation equipment is normally installed on an offshore platform so as to be in a normal dry situation well clear of the sea surface. Such platforms can be either fixed to the seabed or floating on a hull or pontoons at the surface. The "dead crude" product is normally exported by a seabed pipeline to the shore or to a nearby loading buoy from which it is removed by an oil tanker.

Such platforms and their accessories are expensive and require a complement of typically 50 to 200 operators according to their size and the additional equipment also carried to enhance production. Where the expected rate of production of crude oil is low the use of such platforms can be uneconomic.

This invention is a subsea crude oil separation device comprising: (a) a high-pressure gas separation vessel, connected via a pipe and valve to (b) a low pressure gas/oil/water gravity separation vessel having an overall residence time of more than 100 minutes and containing a weir over which flows only substantially aqueous phase free hydrocarbon fluid: the overall and interface level sensors for which are separately immersed, preconnected to a dry control pod, and are finally positioned under water.

By this invention the cost of offshore separation is substantially reduced by mounting the separation equipment directly on the seabed and constructing it such that it will operate underwater without direct intervention except by electrical, optic fibre, and/or hydraulic remote control from a small surface station.

Divers will be used for periodic maintenance as required.

Since pipelines are also expensive over extended distances to shore this invention provides for export by tanker as above mentioned. The tanker remains moored to the loading buoy until fill, or for other reasons, thence it is disconnected and sails away with its cargo; the same or another tanker being subsequently reconnected to continue production.

Such systems function more economically and satisfactorily if production can continue while the tanker is away, for which reason a crude oil storage vessel is needed at the well head location. Such arrangements are well known.

This invention is novel in that the storage vessels and capacity are incorporated into the separation unit and contribute to the separation of the oil and water.

Wellhead crude passes into a small vessel known as the high pressure (HP) separator via a pressure let-down valve. The pressure let-down valve is controlled to maintain a pre-set pressure in the high pressure separator which allows most of the gas that was in its liquid phase at wellhead pressure to evolve at this reduced pressure. (Reduced with respect to the oilfield formation pressure.) This has flows to the surface by means of a flexible gas riser connected to a surface buoy, vessel or other structure. The liquid flows into a much larger lower pressure (LP) vessel via a valve which is controlled to maintain a preset liquid level in the HP vessel. This larger LP vessel is in communication with the gas outlet riser manifold both of which are at near atmospheric pressure.Gases which remain in solution at the HP vessel pressure evolve in the LP vessel leaving the remaining liquids as substantially "dead crude". The comparatively large residence time in the LP vessel allows water to separate by gravity. A weir built into the vessel is so sized that the more dense water is contained by it while the less dense oil gravitates upwards and is allowed to flow over the top into an adjacent oil compartment.

Bother water and oil compartments provide substantial storage volume. If additional volume is required it can be provided by additional vessels connected by simple pipework to either the water or oil compartments of the LP separator.

To effect separation of oil/water mixtures normally requires a residence time of about three minutes. By using this invention the storage time in the LP separator is utilised to effect separation. In this invention the residence time in the LP separator is at least one hundred minutes, but may be considerably longer e.g. twelve hours, one day, two days or even a month.

Flow from both oil and water compartments is by gravity until the level in the adjacent oil and water storage vessels is equalised with that in these compartments.

For efficient oil/water separation the natural oil/water interface level in the LP vessel should be at the optimum height relative to the crest of the weir.

This level may be determined by experiment and then controlled by a valve in the water drain which opens and shuts in response to a control indication of this interface level. The oil compartment liquid and that in any (connected) oil storage will find its own level automatically.

Both oil and water are pumped to the surface tanker. The pump or pumps are also controlled by the LP water/oil interface level indication.

Correct operation is therefore driven by the HP/LP pressure difference and by gravity, but relies upon the correct operation of the HP pressure control valve (HP vessel/flare line), the HP level control valve (HP/LP liquid line) and the LP (water compartment) level control valve (LP water compartment/water storage vessel or surface tanker).

These valves may modulate, or they may operate in a fully open/shut manner, to maintain the design pressure and liquid#levels in the unit. They are controlled by servo valves which are operated by a programmable logic controller unit (plc).

The pic and servos are installed, together with pressure transducers and other devices in a dry encapsulated space on the seabed, referred to as the "control pod". An umbilical from this pod to the surface allows continuous surface monitoring, external override if an imposed shutdown is needed and the means of modifying the algorithms in the plc.

The LP separator vessel also incorporates a baffle wall, analagous to a weir, to trap solid particles, mainly sand, produced from the well in a separate compartment which is sized to accommodate all the solids expected to be produced during the life of the field (which may incorporate several wells with their production manifolded prior to entering the HP separator). Alternatively this trap, (or an additional trap) can be located in the entry to the HP stage to minimise wear on the HP vessel level control valve and ductwork.

The complete unit is designed with an electrical power line and with an optic fibre signal channel. These are pre-connected and waterproofed prior to immersion such that no electrical or optic fibre connection is needed underwater.

This applies to the main umbilical line from the surface to the subsea unit, and similarly to the electrical/optic fibre connection between the pod and all other vessels.

For example level control sensor cables connect the dry space to sensor units located on each vessel, with waterproof connections at both ends. The liquid/interface level sensors are operated by detection of radiation through the vessel wall. This feature eliminates underwater mating of electrical or opticchannel which is a known source of unreliability of many subsea control systems.

The nucleonic sensor units are stowed in racks on the exterior of the control pod before the pod is immersed as a separate module onto the subsea separator, these sensor units are then relocated by a diver, with interconnecting cable paid out in the process, to shape-coded receptacles at strategic sites on the external wall of the vessels. This is an important part of this invention.

Any gas flare on the surface is mounted upon a tethered, freely floating, buoyant column platform, or buoy which allows normal catenary mooring with consequent cost savings and ease of maintenance. The column, platform or buoy could also contain equipment such as a generator, hydraulic power unit, fuel tanks, chemical tanks and gas treating units e.g. those required for desulphurisation. These may be located below sea level, but with dry access via a 'conning tower'. Flares are normally mounted on a tower with structural or rigid articulated members connected as a mechanism to a seabed foundation.

The column, platform or buoy may be tethered in the manner of a tensioned leg platform, or may have two or more tethering lines fixed to the sea bed such that they are angled.

Alternatively, the equipment may be located in e.g. a ship, moored to the column, platform or buoy.

Power from the generator may be fed to the subsea separator via the umbilical line (including hydraulic power).

A combination of perimeter skirts and cells at the base, being inverted walls and compartment divisions, may be used to penetrate the seabed and prevent lateral sliding under the influence of tides and wave induced water motion. Extended cantilever arms at four corners or elsewhere, each surmounted by a mud support pad or plate, may be used to help prevent the structure from overturning under the above influences.

The folding nature of the stabilising arms can permit compact stowage on a ship, barge, or similar vessel but can be extended by divers with the support pads adjusted in seabed contact pressure and elevation by screw jacks or similar mechanism, e.g. hydraulic rams, to achieve satisfactory stabilising effect at all four pads equally.

In the North Sea, the seabed temperature is fairly constant at about SOC. At this temperature crude oil can be very viscous. The subsea separator therefore needs insulation.

Pourable/setting insulation (possibly foamed) can be used. The item to be insulated e.g. a vessel or pipe, can be surrounded by cladding, separated from its walls to leave an annular space or cavity. into this cavity may be poured the insulation.

Hitherto this casting technique has been applied to increase the buoyancy of vessels.

In order to overcome any buoyancy effect foamed concrete could be used.

To enhance the insulating effect, before pouring insulation into the cavity, blocks of insulating material, e.g. polystyrene balls, could first be inserted and these set in place using a pourable setting fluid. The advantage of this arrangement is that the blocks can be of high insulating quality which quality is not compromised by the need to be pourable. The pourable fluid also prevents water ingress.

If necessary the subsea separator may be fitted with means to add anti foaming agents.

Claims (1)

1. Claims

   (1) A subsea crude oil separation device comprising: (a) a high-pressure gas separation vessel, connected via a pipe and valve to (b) a low pressure gas7oil/water gravity separation vessel having an overall residence time of more than 100 minutes and containing a weir over which flows Gnl v substantially aqueous phase free hydrocarbon fluid: the overall and interface level sensors for which are separately immersed, preconnected to a dry control pod, and are finally positioned under water.

   (2) A device as claimed in Claim 1 wherein sand contained in the crude oil is trapped in a separate compartment in the high pressure or low pressure vessels or both of them.

   (3) A device as claimed in Claims 1 or 2 wherein gas separated is piped to the surface and flared from a tethered freely floating buoy.

   (4) A device substantially as herein described.

   (5) Oil separated by means of this device.