METHOD AND COLUMN FOR COLLECTION AND SEPARATION OF OIL,GAS AND WATER FROM BLOWING WELLS AT THE SEA BED

Abstract: Method of collection and separation of oil, gas and water from an offshore oil/gas well (2) and a column (1) for usage by the same, The column (1) comprising a vertically arranged tube (3) with a lower end (8) resting on the sea bed (14) and an upper closed end (9) from which gas may be discharged by gas outlet means (6). Oil-gas mixture flowing out of a well head (22) in operation of the column (1) will be retarded by an oil column (10) in the tube (3), thereby releasing gas which is collected in the upper portion (11) of the column. Motion of the oil at the surface of the oil column (10) will be very small, oil thereby flowing over an overflow rim (15) into an overflow channel (26), from where oil is transferred to the sea surface by oil outlet means (7). The motion of the mixture may be additionally dampened by horizontal webs. The column (1) may be operated at sea depths more than 300 meters and at shallow water where the column may be constructed as part of a platform.

Description: Method and column for collection and separation of oil, gas and water from blowing wells at the 10 sea bed 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. 15 During the last years many attempts have been made 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 20 large sea-shore areas. Additionally high waste economic losses follow blowing of such wells. Existing equipment of to-day, such as booms, skimmers, sombreros etc. have proved to be insufficient under pre¬ vailing weather conditions. Therefore, new ways had to •25 be developed to collect and separate gas and oil blowing uncontrolled from wells at the s-ea bed. Means therefore are required, which can collect and commercially utilize gas and oil from such wells during the period of time in which other means are working to 30 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 pol¬ lution of the sea water and the surroundings. The inten¬ tion of such sombreros has been to collect the oil-gas * ^ 5 mixture blowing out of the subsea oil well. Principal 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 has serious problems during the significant expan¬ sion of the gas volume bytransportation of the oil-gas mixture in risers up to the sea surface. 5 One criterion 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 does not exceed the strength of the bearing soil. Otherwise damage to soil layer and oil/water piping will occur. Separation of the gas from the oil, as gas and oil have quite different behaviours as to pressure drops and 15 expansion, large pressure fluctuations caused by gas bubbles in transportation risers from the sea bed to the sea sur¬ face as well as cavitation problems in the pumps, will be required to avoid such problems. For the requirements it should be demanded of 20 structures as follows: Insignificant or no soil failure, limitation of pressure fluctuations, insignificant or no damage to flσwlines on or near the sea bed, operational independency of water depths, easy and ready installation, reliability and mobility. Furthermore, demands to 25 economical construction and maintainance as always will be important. The problem of soil failure will arise when the structure covering the well head and having an open lower end placed on the sea bed, is filled with the oil-gas 30 mixture from the well, causing pressure differences inside/outside the structure. If the pressure difference outside and inside the structure near the sea bed exceeds 3 - 5 m H^O, a breakdown of the soil normally will be expec¬ ted. Such breakdowns normally will cause leakage at the sea 35 bed surface or in the soil. Pressure fluctuations at the sea bed, e.g. at 300 m water depth, vary a lot more than the soil limit of 5 m H„0. This low pressure difference limit of approximately 5 m H„0 makes it necessary to be able to mini¬ mize the fluctuations in oil/gas pressure within any structure OMPI placed open to the sea bed. While the pressure at the. bottom of a structure will be equal to the height of the liquid in the structure times the specific gravity of the liquid plus the atmospheric pres- sure, the gas pressure within such a structure will be the same in all directions. If gas and oil are transferred together through a riser where the pressure in the riser sections is depen¬ ding upon the vertical position of the sections, the bottom pressure continuously will vary from time to time with the gas content 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 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, estabilishing an overlaying hat for the gas released from the oil. Such a 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/όr separating oil and gas from blowing wells. According to the present-invention the method com¬ prises 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 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 insignifi¬ cant, 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 inside the 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 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 outlet 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. An overflow rim 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 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 horizontal webs acting as dampers to the upwardly directed motion of the oil-gas mixture. Ah outer casing may be provided around the column forming therebetween ballasting and storing tanks as well as installing spaces for valves, pipelines and tanks pro¬ vided to control lowering and flotation of the column and the pressure inside the column when the column is in ope- ration. 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. From the column, oil and gas are separately trans¬ ferred to the sea surface where further treatment may take place on barges, ships, platforms etc. before further tran¬ sportation, or the gas may be burned. The risers for such transfer of oil and gas may be fixed or flexible, depending on water depths and other circumstances. 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. The column principle can be applied for all water depths, subject to establishing a stage of pressure equi¬ librium, which is set by the following equation: where P = atmospheric pressure at sea level, P - - specific gravity of water _ H = water depth, P 1 = gas pressure in top of column, p ? = specific gravity of oil in column, _ = 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 within the following ranges: P + p t Hg> P 1 + p 2 h 2 h P + Pl Hg - T?_ giving a pressure control span of P S where P5 = maximum 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 lines do not prevent this, the bottom pressure in the column can vary as follows: P + p.Hg + P s >P 1 + p 2 h 2 >P + Pl Hg - _>_ giving a control span of 2P . During operation the oil from the well will flow freely intothecolumn, 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 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 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. 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 ope¬ ration on a sea bed, Fig. 3 shows a column constructed for shallow 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 cover¬ ing an oil/gas well 2, comprises a tube 3 having valves 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 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. 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 O PI SUBSTITUTE WIPO lowering the column. Oil-gas mixture flowing out of the well 2 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 substantially 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 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 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 facili- ' ties for e.g. piping systems. Surrounding the casing 13 ballasting 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 be discharged by a pump 29 to the surrounding sea. The s€orage 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 a mani¬ fold 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 the tube 3, such webs being designed as stif- feners for the tube 3. The lower edge of the tube 3 may be designed as webs, -g- S - E XcT O PI SUBSTITUTE SH E 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 lowe edge 12 may be designed to penetrate the sea bed thereby allowing 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 1 partly or totally may be provided with a water-filled flexible rubber cushion ' distributing the load of the column evenly on the sea bed. 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 " the dis- charging devices 6, 7 for gas . nd oil, respectively. In 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. Fulfilment 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 operatio¬ nal at depths down to more than 300 meters as well as in shallow water. The column 1 may, if beneficial, 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 in shallow water the column may be provided with a fire wall and fire fighting devices withstanding burning oil and gas at the sea surface when 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, the valves carefully are set and the oil pumps put into operation, the valves, pumps and other equipment being remotely controlled through lifelines from an operation barge, such equipment and piping systems normally being automatically controlled. OMPI IPO

Claims: Patent claims 1. Method by collection and separation of gas, water and oil which, controlled or not, is flowing out of an 5 oil/gas well on a sea bed , c h a r a c t e r i z e d in that a vertically arranged column (1) is lowered over the well (2) by supplying ballast to ballast tanks (27) connected with the column (1) until the column rests on - 10 the sea bed around the head (22) of the well (2) , oil-gas mixture being guided from the well (2) into the column (1), thereby building up an oil column (10) in the interior of the column (1), and an upper gas-filled portion (11) of the column (1), the upwards directed motion 15 of the oil-gas mixture thereby being retarded by the oil column (10), the pressure and the amount of oil and gas in the column being controlled by actuation of outlet devices (6, 7) for oil and gas, the hydrostatic pressure outside 20 and inside the lower edge (12) of the column resting on the sea bed, being equal, thereby maintaining a gas por¬ tion (11) in the upper part (9) of the column (1) by dis¬ charging gas from the upper gas-filled portion (11) of the column (1) for burning or storing, and discharging oil •25 from the oil column (10). 2. Balancing column for collection and separation of gas, water and oil which, controlled or not, is flowing out of an oil/gas well on a sea bed, c h a r a c ¬ t e r i z e d in that the column comprises a vertically 30 arranged tube (3) with an upper closed end (9) having out¬ let means (6) for gas, a lower end (8) of the tube (3) having means (12) for supporting the column (1) on the sea bed (14), the middle portion of the column having oil outlet 35 means (7) for transferring oil to the sea surface and the lower end (8) of the tube (3) having valve devices (18) for liquid passage out of and into the column for purposes as to equalize the hydrostatic pressures OMP inside and outside the lower edge (12) of the column when in operation and for ballasting and buoyancy purposes when floating or lowering the column (1). 3. Column according to claim 2, c h a r a c - t e r i z e d in that an overflow rim (15) is provided below an upper portion (11) of the tube (3) for collection, of released gas, whereby remaining gas in the oil will be released when oil is flowing over the rim (15), and the out¬ let means (7) for oil is provided with an overflow channel (26) between the interior wall of the tube and the rim and at a level below the rim (15). 4. Column according to claim 2 or 3, c h a r a c¬ t e r i z e d in that the tube (3) is provided with at least one horizontal interior web (21) for damping the upwardly directed motion of the oil-gas mixture. 5. Column according to claims 2 - 4, c h a r a c¬ t e r i z e d in that the gas outlet means (6) comprises at least one valve (4) for, connection with supply means (23) for gas to the sea surface. 6. Column according to claims 2 - 4, c h a r a c¬ t e r i z e d in that the gas outlet means (6) further comprises a valve (4a) vertically arranged in the top of the column, allowing drilling operations from the sea surface through the valve (4a) and the entire height of the column (1). 7. Column according to claims 2 - 6, c h a r a c¬ t e r i z e d in that the gas outlet means (7) comprises a manifold (20) , at least one valve (5) for connection to supply means (24) for oil to be transferred to the sea surface. 8. Column according to claims 2 - 7, c h a r a c¬ t e r i z e d in that a tubular casing (13) is arranged around the column (1), thereby providing a space (25) between the tube (3) and the casing (13) allowing instal- lation of valves, pumps and pipes required for operation of the column (1), the casing (13) protruding upwardly from the lower end (8) of the tube (3) . 9. Column according to claims 2 - 8, c h a r a c- OMPI t e r i z e d in that the means (12) for supporting the column (1) on the sea bed (14) comprises 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), e.g. pipelines. 10. Column according to claims 2 - 9, c h a r a c¬ t e r i z e d in that the lower end (8) of the tube (3) is provided with valve devices (18) for transfer of liquid into and out of the lower end (8) of the tube (3) . OMPI : κ* . WIPO

Inventors: OSTLUND O
Applicants: OSTLUND AS, OSTLUND O, OC OSTLUND AS
Application year: 1980
Main classification: E21B7
IPC codes: E21B43