GB2186283A

GB2186283A - Treatment of oil

Info

Publication number: GB2186283A
Application number: GB08603195A
Authority: GB
Inventors: Marcus William Raybould
Original assignee: Humphreys and Glasgow Ltd
Current assignee: Humphreys and Glasgow Ltd
Priority date: 1986-02-10
Filing date: 1986-02-10
Publication date: 1987-08-12
Also published as: GB8603195D0

Abstract

A method and apparatus for subsea crude oil separation characterized by (A) flashing of gases from the crude oil, and (B) cooling of the liquid stream resulting from such flashing such that upon piping such liquid stream to a distant facility no significant two phase flow occurs, and (C) cooling of the gas stream resulting from such flashing such that upon piping such gases to a distant facility no significant two-phase flow occurs. d

Description

SPECIFICATION Subsea welihead separation system The present invention relates to offshore oil production.

There are a number of situations where an owner of offshore oilfields wishes to produce oil from a small field which in itself would not normally justify a platform/topside facilities, but the owner has within a distance of 100 km a platform which is under-utilised with respect to production capacity or weight loading.

The system normally considered in such circumstances is to have a subsea template and then to pipe the crude as it comes out of the ground to the under-utilised platform. Unfortunately there are circumstances which render this option undesirable; one of the problems being that the pressure drop is such as to give rise to two-phase flow. The purpose of this invention is to avoid two-phase flow and to allow the distance between the wells and platform to be considerably greater than presently considered economic. The process which is the essence of this invention is the separation of the gas from the crude oil/water, and the cooling of the two streams so as to eliminate the possibility of two-phase flow.

In its simplest form the process is to separate the gas from the liquid phase and to cool both phases to a degree which prevents flashing from the liquid phase or condensation from the gaseous phase during the flow of these phases from the well to the platform.

The cooling of the liquid ensures that even when the liquids condensed from the gas stream are combined with the main liquid stream such added liquids do not flash or cause two-phase flow in the main liquid pipeline.

Figure 1 should be used with the following description of the process. Crude oil leaves the ground through wells (1), may be manifolded (2), and made to flow; with significant pressure drop between the oil in the ground and (3) the separator. Liquid leaving the separator (4) is cooled (5) and pumped (6) into the liquid line (7) to the platform. The gas (8) leaves the separator and is cooled (9) before passing into a knock-out drum (10). Liquids leaving the knock-out drum (11) are recycled back to join up with the liquid stream. The gas (12) leaving the knock out drum (10) may be reheated (5) to gain power before being expanded (13) to cool the gas. The cooled gas is then used (9) to cool the gas stream leaving the separator (8) before being recompressed (14) into the pipeline to the platform.

With reference to Fig. 1 and the above description, the liquid pumping may take place prior to cooling of the liquid.

Fig. 1 shows that cooling of the liquid is effected by cold gas. This need not necessarily be so. The liquid could be cooled by seawater or a closed cycle refrigerant system.

The gas is cooled to a temperature such that, after expansion and recompression and allowing for cooling of the gas along the line, no significardt liquid condenses from the gas.

Another example of the process of this invention is given in Fig. 2. Crude oil flows from the wells (20), possibly via manifold (21), into a separator (22) in a manner that there is a substantial pressure drop between oil in the ground and the separator. Liquid leaving the separator (23) is cooled (31) and then the stream is divided into two parts; one part (24) is cooled by a feed effluent interchanger (25) before being further cooled (26) by a closed cycle cooling device using, e.g., Freon. The further-cooled liquid then enters a direct contact cooling device (27) up which flow the gases (28) which leave the separator. The direct contact cooling condenses out further liquids from the gases leaving the gases in the desired state, i.e. such that after compression (29) liquids will not significantly condense out of the gas during its passage down the pipeline to the platform/shore.Liquid leaving the direct contact cooling device (30) flows back via the feed effluent heat exchanger (25) and joins up with the main stream of liquid leaving the cooler (32).

Of course it would be possible to cycle the liquid around the direct contact cooling device, purging and making up liquid from the main liquid flow. Likewise, it would be possible to utilise the additional cooling effected in (26) within the direct contact cooling device (27).

Some of the gas cooling may be effected by direct contact seawater cooling. Likewise, some of the liquid cooling may be effected by indirect seawater cooling.

One of the advantages given in Fig. 2 is that there is no indirect cooling of the gas stream. Indirect gas cooling normally requires large surface areas because of the low heat transfer co-efficients normally obtainable with gas. Any problems of two-phase gas flow within such a gas cooler are eliminated.

A practical advantage of being able to increase the distance between the welihead and the platform/shore is that an operator may be able to utilise his own platform/shore facility rather than having to have his crude processed by another (nearer) operator's platform/shore facility. The subsea section would normally be designed to allow modular replacement of the units and may incorporate glycol injection facilities at a number of points.

This invention is particularly useful for wells with high gas/oil ratios.

Any pumps, compressors, closed cycle refrigertion systems may be built into a dry sealed module, probably at atmospheric pressure, which will be designed for retrieval and replacement by servicing vessels with minimal diver assistance.

If necessary a water or gas injection pipeline from the platform/shore may be integrated into the subsea facility.

Whilst this patent refers to utlisation of the proceses to allow crude oil to be pumped to under-utilised platforms, the invention may also be used to allow subsea wells to be pumped to the shore.

In this application there is provided an invention which is a subsea crude oil separation process characterised by: A. flashing of gases from the crude oil, and B. the cooling of the liquid stream resulting from such flashing such that upon piping such liquid stream to a distant facility no significant two-phase flow occurs, and C. the cooling of the gas stream resulting from such flashing such that upon piping such gases to a distant facility no significant twophase flow occurs.

Claims

1. Claim 1-A subsea crude oil separation process characterised by: A) flashing of gases from the crude oil, and, B) the cooling of the liquid stream resulting from such flashing such that upon piping such liquid stream to a distant facility no significant two-phase flow occurs, and, C) the cooling of the gas stream resulting from such flashing such that upon piping such gases to a distant facility no significant twophase flow occurs.