NZ196534A - Collecting and separating oil gas and water from offshore oil/gas well - Google Patents

Description

•; o •< R I / o J N;Z. PATENT OFFICE , 10 NOV 1983 RECEIVED Patents Form No.5 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "METHOD FOR COLLECTION AND SEPARATION OF OIL, GAS AND WATER FROM AN UNDERSEA OIL/GAS WELL AND A COLUMN FOR USE IN THE SAME" i/WE O.C. OSTLUND A/S, Ovre Stokkavei 21, N-4000, Stavanger, Norway, a Norwegian company, hereby declare the invention, for which f/we pray that a patent may be granted to me/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- i i i i i i i t • ■ t i Priority Dat^a): .. /<?. 2 Of Cor.ip'£i3 Specification Fiicd: 1.

Cfass: ^!P.

Publication Data: .

P.O. Journal Mo: (followed by pag* I A.) lA A / This invention relates to a method by collection and separation of oil, gas and water from an oil/gas well and a column for usage by the same.

During the last years many attempts have been done 5 to control oil and gas streaming out of blowing oil/gas wells on the sea bed. Such attempts are undertaken to avoid pollution of the surrounding sea and sea shores, highly being a danger for damaging marine life and pollutioning large sea shore areas. Additionally high waste economically losses follow 10 blowing of such wells.

Existing equipment of to-day, such as lenses, skimmers, sombreros etc. have proved to be insufficient under prevailing weather conditions. Therefore new ways had to be developed to collect and separate gas and oil blowing uncontrol-15 led from wells at the sea bed.

Means therefore are required, which can collect and commercial utilize gas and oil from such wells during the period of time in which other means are working to control the blow out, e. g. drilling relief wells.

Means in the shape of sombreros, e. g. according to US-PS 3 664 136, have been put into action to avoid pollution of the sea water and the surroundings. The intention of such sombreros has been to collect the oil-gas mixture blowing out of the subsea oil well. Principally reasons for failure of such sombreros have been escape of oil and gas below the edge of the sombrero and the attempt to transfer the oil-gas mixture , usually from the top of the sombrero to the sea surface.

Caused by the proportional expansion of the gas escaping from the oil/gas mixture such attempt have serious problems during the significant expension of the gas volume by transportation of the oil/gas mixture in risers up to the sea surface.

One criteria by controlling a blowing well using a structure arranged on the well is to separate oil from gas and to control the two components individually. Furthermore the pressure increasing within the structure caused by the flowing gas/oil mixture has to be controlled and limited to a pressure which do not exceed the strength of the bearing soil. Otherwise soil layer and damage to the oil/water piping will occur.

Separation of the gas from the oil,as gas and oil 5 have quite different behaviours as to pressure drops and expansion, large pressure fluctations caused by gas bubbles in transportation risers from the sea bed to the sea surface as well as cavitation problems in the pumps,will be required to avoid such problems.

For the requirements it should be demanded for 10 structures as mentioned: Insignificant or no soil failure, limitation of pressure fluctations, insignificant or no damage to flow lines on or near the sea bed, operational independency of water depths, easily and readily installation, realibility and mobility. Furthermore demands to economical construction 15 and maintainance as always will be important.

The problem with soil failure will raise when the structure covering the well head and having an open lower end placed on the sea bed, is filled with the oil/gas mixture from the well, causing pressure differences inside/outside the structure. 20 if the pressure difference outside and inside the structure near the sea bed exceeds S-SmF^O , a breakdown of the soil normally will be expected. Such bjeakdowns normally will cause leakage at the sea bed surface or in the soil. Pressure fluctuations at the sea bed e. g. at 300 meter water depth vary a lot 25 more than the soil limit of 5mHo0. This low pressure difference limit of approximately 5mH20 makes it necessary to be able to minimize the fluctuations in oil/gas pressure within any structure placed open to the sea bed.

While the pressure at the bottom of a structure 30 will be equal the height of the liquid in the structure times the specific gravity of the liquid plus the atmospheric pressure, the gas pressure within such a structure will be the same in all directions.

If gas and oil are transferred together through 35 a riser where the pressure in the riser sections is depending upon the vertical position of the sections, the bottom pressure continuously will vary from time to time with the gas content 1 in the riser as the hydrostatic pressure at the bottom causes the gas to dispLace the oil while expanding during its travel up the riser.

By separating the gas from the oil in such a degree 5 that the gas content in the oil is drastically decreased, the fluctuations in the riser bottom pressure also will be substantially decreased. Such a separation is possible by obtaining a free oil level in a column, establishing an overlaying hat for the gas released from the oil. Such a 10 gas hat thereby forming the top of a structure arranged over the blowing well.

The method and structure of the present invention avoid the failures and disadvantages of prior structures for collecting and/or separating oil and gas from blowing 15 wells. According to the present invention the method comprises lowering of a vertically arranged column over a blowing well by supplying ballast to ballast tanks connected with the column, until the column rests on the sea bed around the well head, guiding oil/gas mixture from the well 20 head into the column, thereby building up an oil column in the interior of the structural column and an upper gas portion in the column, the upwards directed motion of the oil/gas mixture thereby being retarded such that the motion of the mixture at the oil surface being substantially 25 insignificant, the pressure and the amount of oil and gas in the structural column being controlled by actuation of valves for oil and gas outputs, the hydrostatic pressure outside and insid^lower edge of the column resting on the sea bed being substantially equal, thereby maintaining a gas portion in the upper portion of the column by discharging gas from the column through the sea surface and the discharging oil from the oil column also to the sea surface.

A structure according to the present invention may have the shape of a column comprising a vertically arranged tube with an upper closed end having ovfLet means for gas, a lower end of the tube having means for supporting the column at the sea bed and the middle portion of the column having oil outlet means. f An overflow rira may be provided below the upper portion of the tube, whereby remaining gas in the upper part of the oil column is released from the oil well when the oil is flowing over the rim, the outlet means for oil 5 being provided in an overflow channel below the overflow rim and between the rim and the interior wall of the tube.

The interior of the column may be provided with one or more substantially horizontally webs acting as dampers to the upwardly directed motion of the oil/gas 10 mixture.

An outer casing may be provided around the column forming there between ballasting and storing tanks as well as installing spaces for valves, pipelines and tanks provided to control lowering and flotation of the column and the 15 pressures inside the column when the column is in operation. A deck may be arranged, resting on the top of the casing, suitable for supporting necessary equipment for the operation of the column, the equipment on the platform being accessible for divers when the column is in operation at great depths. 20 From the column,oil and gas are separately transferred, to the sea surface where further treatment may take place on barge's, ships, platforms etc. before further transportation, the gas may be burned.

The risers for such transfer of oil and gas may 25 be fixed or flexible, depending on water depths and other circumctances. It is, however, anticipated that flexible risers might be the most economical solution for great depths as such risers also allow the use of the system at different water depths without costly rearrangements. 30 The column principle can be applied for all water depths, subject to establishing a stage of pressure equilibrium, which is set by the following equation: P + $,Hg = P1 + 5>2h2g where P = atmospheric pressure at sea level, ^ - specific gravity of water, H = water depth , 196534 = gas pressure in top of column, = specific gravity of oil in column, h2 = height of oil in column.

In order to limit the possibility of oil leakage under the bottom wall of the column due to small pressure fluctuations , the bottom pressure in the column should be kept with the following ranges: P + 5>1Hg>P1 + 5^h2g^P + 51Hg - Pg giving a pressure control span of P where P = maximum s s soil differential pressure before soil breakdown.

If leakage under the bottom is allowed or controlled by means of skirts penetrated into the sea bed, where flow 15 lines do not prevent this, the bottom pressure in the column can vary as follows: P J^Hg ♦ Pe> P, +S2h2>P \SlHg " Ps giving a control span of 2Pg.

During operation the oil from the well will flow free into the column, the gas will separate and the system will establish its own state of equilibrium. Pressure built up at the bottom is avoided by throttling a bottom valve 25 and/or a top valve. The oil in the column is flowing over the overflow rim prior to entering the transfer risers which will improve the separation of gas.

The dynamic energy in the blowing oil/gas mixture flowing upwards, is dampened by the existing oil and water 30 liquids in the column. To achieve this, the amount, diameter and height of water and oil in the column must be large enough to dampen and absorb the dynamic energy from the blowing oil/ gas mixture. Furthermore, the dimensions must be large enough to let gas bubbles rise and expand without creating large fluctuations in the hydrostatic pressure at the bottom.

This effect will decrease with increasing diameter and height. The required height of the oil/water column might be reduced by installation of mechanical damper or dampers in the column.

N.Z. PATt." 2DECI983 The invention is fully described by way of example with reference to the accompanying drawings/ in which Fig. 1 shows an elevation of the column according to the present invention, Fig. 2 shows the column of Fig. 1 with an outer casing,ballasting and storing tanks and an equipment deck accessible for divers as the column may be in operation on a sea bed, Fig. 3 shows a column constructed for shallow 10 water with a platform deck above the sea surface, and Figs. 4 and 5 show cross-sections according to Fig. 2.

A vertically arranged column 1, in operation covering an oil/gas well 2, comprises a tube 3 having valves 15 4 and 4a and valve 5 constituting parts of outlet means 6 and 7 for gas and oil respectively. The lower end 8 of the column 1 has a lower edge 12 resting on the sea bed 14. In the vertical tube 3 and the lower end 8 of the column 1 an oil column 10 will be built up during operation. An upper 20 end 9 of the column contains an upper gas-filled portion 11 below which an overflow rim 15 is arranged for releasing gas from the oil/gas mixture as the mixture flows over the rim 15 and into an overflow channel 26 from which the oil is transferred to the sea surface by the outlet means 7. 25 The lower end 8 of the column 1 is provided with a valve 18 for discharging water from the lower part of the column 1 and for buoyancy purposes by floating and lowering the column.

Oil/gas mixture flowing out of the valve 2 30 through the well head 22 will be retarded by the fluid of the oil column 10. Gas will be released from the oil/gas mixture and find its way to the gas-filled portion 11.

At the surface of the oil column 10 the motion of the oil/gas mixture is retarded to such an extent that the oil is sub-35 stantially calm and most of the gas is released from the mixture. Possible remaining gas will be released from the mixture when flowing over the overflow rim 15 into the overflow channel 26. Water separated from the oil/gas 7 mixture will establish a water-filled lower portion in the lower end 8 of the column 1, which water can be discharged from the column by valve 18. Valve 18 also may be used as supplementary discharging possibility to the outlet means 5 7 for oil by excessive flow into the column of oil/gas mixture.

Surrounding the column 1 an outer casing 13 is arranged spaced from the tube 3, thereby providing facilities for e. g. piping systems. Surrounding the casing 13 ballasting 10 and storing tanks 27 may be arranged around the lower portion of the casing 13.

Water may be discharged from the lower end 8 of the column 1 through a manifold 17 to the surrounding sea or by valves 18 and 19 to storage tanks 27, from where it may 15 be discharged by pump 2 9 to the surrounding sea. The storage tanks 27 hereby being in operation as separating tanks for oil and water, the oil separated in the upper part of the tanks 27 may be discharged through manifold 16 from which oil may be transferred to the sea surface or to a manifold 20 through which a pump 30 may discharge oil from the overflow channel 26 to the sea surface for further treatment.

To increase the retarding function of the oil column 10, one or more horizontal webs 21 may be arranged in the interior of tube 3, such webs being designed as stiffeners for the tube 3.

The lower edge of the tube 3 may be designed as webs, boxes or sections of which certain ones may be removed to avoid damage on objects on or near the sea bed 14 around the well head 22, such as pipelines. Furthermore, the lower edge 12 may be designed to penetrate the sea bed thereby allowing for a certain pressure difference between the inside and the outside of the lower end 8 of the column 1.

To avoid damage on flowlines and bottom risers at the sea bed, the lower edge 12 of the column I partly or in total may be provided with a water-filled flexible rubber cushion distributing the load of the column evenly on the sea bed. 8 The column 1 may be provided with an equipment deck 31 supporting necessary equipment for the piping systems of the entire column 1 and being accessible for divers when the column is in operation.

Further equipment is installed for purposes of measuring pressures at desired points of the column, the casing, the tanks, pumps, manifolds and the gas- and oil-transmitting devices 23, 24, respectively, and discharging devices for gas and oil, 6, 7, respectively.

At operation the pressure in the lower end 8 of the column 1 by means of the afore-mentioned, equipment and controlling devices, is maintained substantially equal to the pressure outside the column at the sea bed.

The column design is strongly related to the physical conditions under which the column will be in operation and towed offshore to a blowing well. Fullfillment of such requirements consequently will exceed the requirements put forward from the pressure conditions connected with the operation of the column, the column thereby being operational at depths down to more than 300 meters as well as in shallow water.

The column 1 may, if benificial, also be used as an extra safety margin during drilling by placing the column over the bottom installed B.O.P. and drilling through the top of the column which has to be designed and equipped for such a purpose.

For use at shallow water the column may be provided with a fire wall and fire fighting devices withstanding burning oil and gas at the sea surface by installing the column.

Ballasting and storing tanks 27 may be used for trimming the load on the column structure when standing at the sea bed in order to keep it stable dependent on soil conditions of the particular well, in addition to ballasting during submergence and storing purposes for oil and/or water during operation.

During submergence valves 4, 4a, 5 and 18 are open to allow free flow of gas and oil through the column.

After installation at the sea bed and required ballasting, valves carefully are set and oil pumps put into operation, valves, pumps and other equipment being remotely controlled through lifelines from an operation barge, such equipment 5 and piping systems normally being automatically controlled. . 196534

Claims (12)

WHAT WE CLAIM IS:

1. A method of collecting and separating gas, water, and oil which, controlled or not, is flowing out of an oil/gas well on a sea bed, in which: a vertically arranged collection column is lowered over the well until the lower edge of the column rests on the sea bed around the head of the well, oil-gas mixture being guided from the well into the column, thereby building up in the interior of the collection column an elongate oil column and an upper gas-filled portion, the upwards directed motion of the oil-gas mixture being retarded by the oil column; the pressure and the amount of oil and gas in the collection column are controlled by actuation of outlet devices for oil and gas so that the hydrostatic pressure outside and inside the lower edge of the column resting on the sea bed are substantially equal in order to prevent breakdown of the sea bed, thereby maintaining a gas portion in the upper part of the collecting column by discharging gas from the upper gas-filled portion of the collecting column for burning or storing, and discharging oil from the oil column.

2. A balancing column for collecting and separating gas, water, and oil flowing out of an oil/gas well on a sea bed, the column comprising a vertically arranged elongate tube with an upper closed end having gas outlet means, the lower end of the tube having means for supporting the column with N.Z. PATENT OFF ICE 10 NOV 1983 RECEIVED -11- , _ , ' < ' • to £ K ^ ,1 I / u3-' its lower edge on the sea bed, the middle portion of the column having oil outlet means for transferring oil to the sea surface, means being provided for controlling the gas outlet means and the oil outlet means, so that the hydrostatic pressures inside and outside the lower edge of the column when in operation are substantially equal in order to prevent breakdown of the sea bed.

3. A column as claimed in Claim 2, in which an overflow rim is provided below an upper portion of said elongate tube for collection of released gas, whereby remaining gas in the oil will be released when oil is flowing over the rim, and the outlet means for oil is provided with an overflow channel between the interior wall of the tube and the rim and at a level below the rim.

4. A column as claimed in Claim 2 or 3, in which the tube is provided with at least one horizontal interior web for damping the upwardly directed motion of the oil-gas mixture.

5. A column as claimed in any one of Claims 2 to 4, in which the gas outlet means comprises at least one valve for connection with supply means for gas to the sea surface.

6. A column as claimed in any one of Claims 2 to 5, in which the gas outlet means comprises a valve vertically arranged in the top of the column, allowing drilling operations from the sea surface through the valve and the entire height of the column.

7. A column as claimed in any one of Claims 2 to 6, in which the oil outlet means comprises a manifold and at least one valve for connection to supply means for oil to be transferred to the sea surface. N.Z. PATENT OFFICE 1 J -12- 196534

8. A column as claimed in any one of Claims 2 to 7, in which a tubular casing is arranged around the column, \ thereby providing a space between the tube and the casing allowing installation of valves, pumps, and pipes required for operation of the column, the casing protruding upwardly from the lower end of the tube.

9. A column as claimed in any one of Claims 2 to 8, in which the means for supporting the column on the sea bed comprises webs, boxes, or sections of which certain ones may be removed to avoid damage on objects on or near the sea bed around the well head, e.g. pipelines. .

10. A column as claimed in any one of Claims 2 to 9, in which the lower end of the tube is provided with valve means for transfer of liquid into and out of the lower end of the column.

11. A method as claimed in Claim 1, substantially as described with reference to the accompanying drawings.

12. A column as claimed in Claim 2, substantially as described with reference to Figure 1, Figures 2, 4 and 5, or Figure 3 of the accompanying drawings.