Abstract:
A subsea mud pump can be used to return heavy drilling fluid to the surface. In order to provide a less stringent requirement for such a pump and to better manage the bottom hole pressure in the case of a gas kick or well control event, the gas should be separated from the drilling fluid before the drilling fluid enters the subsea mud pump and the pressure within the separating chamber. The mud pump suction should be controlled and kept equal or lower than the ambient seawater pressure. This can be achieved within the cavities of the subsea BOP by a system arrangement and methods explained. This function can be used with or without a drilling riser connecting the subsea BOP to a drilling unit above the body of water.

Description:
TECHNICAL FIELD 
     The present invention relates to the field of oil and gas exploitation, more specifically to systems and methods for well control, especially for well pressure control in wells with hydrocarbon fluids, as defined in the enclosed independent claims. 
     BACKGROUND ART 
     Drilling for oil and gas in deep waters or drilling through depleted reservoirs is a challenge due to the narrow margin between the pore pressure and fracture pressure. The narrow margin implies frequent installation of casing, and restricts the mud circulation due to pressure drop in the annulus between the wellbore and drill string or in other words the increase in applied or observed pressure in the borehole due to the drilling activity such as circulation of drilling fluid down the drill pipe up the annulus of the well bore. Reducing this effect by reducing the circulating flow rate, will again reduces drilling speed and causes problems with transport of drill cuttings in the borehole. 
     Normally, in conventional floating drilling with a marine drilling riser installed, two independent pressure barriers between a formation possibly containing hydrocarbons and the surroundings are required. In conventional subsea drilling operations, normally, the main (primary) pressure barrier is the hydrostatic pressure created by the drilling fluid (mud) column in the borehole and drilling riser up to the drilling installation. The second barrier comprises the Blow-Out Preventer (BOP) connected to the subsea wellhead on seabed. 
     A conventional drilling system is shown in  FIG. 1   a.    
     If a formation is being drilled where the hydrostatic pressure of the drilling fluid is not sufficient to balance the formation pore pressure, an influx of formation fluids that may contain natural gas could enter the wellbore. The primary barrier is now no longer effective in controlling or containing the formation pore pressure. In order to contain this situation, the subsea Blow Out Preventer (BOP) must be closed. In a conventional drilling system the oil and gas industry has developed certain standard operational well control procedures to contain the situation for such an event. These are well established and known procedures and will here only be described in broad general terms. 
       FIG. 1   a  illustrates a conventional subsea drilling system. If the pressure in the borehole  1  due to the hydrostatic pressure from the drilling fluid is lower than the pore pressure in the formation being drilled, an influx into the well bore might occur. Since the density of the influx is lower (in most cases) than the density of the drilling fluid and now occupy a certain height of the wellbore, the hydrostatic pressure at the influx depth will continue to decrease if the well can not be shut in using the BOP. By shutting in the well by closing one of several elements  15   a, b, c, d ,  16  in the subsea BOP stack  3  and trapping a pressure in the well  14 , the influx from the formation can be stopped (see  FIG. 1   b ). The procedures of containing this situation and how the influx is circulated out of the well by pumping drilling fluid down the drillstring  8  out of the drillbit  10  and up the annulus of the wellbore  14  is well established. The valves in the choke line  25  is opened on the subsea BOP to the high pressure (HP) choke line  24  and the bottom hole pressure controlled by the adjustable choke  22  on top of the coke line on the drilling vessel above the body of water. Downstream the adjustable choke valve, the well stream is directed to a mud-gas separator  42 . This is a critical operation, particularly in deep water areas as there are very narrow margins as to how high the surface pressure upstream the surface choke can be before the formation strength is exceeded in the open hole section. 
     Floating drilling operations are often more critical compared to drilling from bottom supported platforms, since the vessel is moving due to wind, waves and sea currents. This means that the floating drilling vessel and the riser may be disconnected from the subsea BOP and wellbore below. If heavier than seawater drilling fluid is being used, this will result in a hydrostatic pressure drop in the well. Generally, a riser margin is required. A riser margin is defined as the needed density (specific gravity) of the drilling fluid in the borehole to over-balance any formation pore pressure after the drilling riser is disconnected from the top of the subsea BOP near seabed in addition to the seawater pressure at the disconnect point  20 . When disconnecting the marine drilling riser from the subsea BOP, the hydrostatic head of drilling fluid in the bore hole and the hydrostatic head of sea water should be equal or higher than the formation pore pressure (FPP) to achieve a riser margin. Riser margin is difficult to achieve, particular in deep waters. The reason is that there can be substantial pressure difference between the pressure inside the drilling riser due to the heavy drilling fluids and the pressure of seawater outside the disconnect point on the riser. To compensate for the pressure reduction in the open hole falling below the pore pressure when the riser is disconnected, would require drilling with a very high mud weight in the well bore and riser. So when drilling with this heavy mud weight all the way up to the spill point on the rig  5 , normally being between 10 to 50 m above sea level, the bottom hole pressure would be higher than the formation strength is able to support. Hence the formation strength would be exceeded and mud losses would occur. It would no longer be possible to circulate and transport the drill cuttings out of the borehole and the drilling operation would have to stop. 
     Riser Less Drilling, Dual Gradient Drilling and drilling with a Low Riser Return System (LRRS), have been introduced to reduce some of the above mentioned problems. The LRRS is described in, e.g., WO2003/023181, WO2004/085788 and WO2009123476, which all belong to the present applicant. 
     In dual gradient (DG) drilling systems a high density drilling fluid is used below a certain depth in the borehole, with a lighter fluid (for example sea water or other lighter fluid) above this point. When drilling with a riser, a dual gradient effect could be achieved by diluting the drilling riser contents with a gaseous fluid for example, or another lighter liquid, U.S. Pat. No. 6,536,540 (de Boer). Another method could be to install a pump on the seabed or subsea and keep the riser content full or partially full of seawater instead of mud while the returns from the well bore annulus is pumped from seabed up to the drilling installation in a return path external from the main drilling riser. Hence there are two different density liquids in addition to the atmospheric pressure creating the hydrostatic pressure on the underground formation. References are made to prior art, U.S. Pat. No. 4,813,495 (Leach) and U.S. Pat. No. 6,415,877 (Fincher et. al.). 
     Another technology that could create a riser margin is the single mud gradient, Low Riser Return System (LRRS) belonging to the applicant. Here, a pump is placed somewhere between the sea level and sea bed and connected to the drilling riser. The drilling mud level is lowered to a depth considerable below the sea level. Due to the shorter hydrostatic head (height) of the drilling fluid acting on the open hole formation, the density of the drilling mud could be increased without exerting excess pressure acting on the formation. If this heavy drilling mud was carried all the way back to the drilling rig, as the case would be in a conventional drilling operation, the hydrostatic pressure would exceed the formation strengths, and hence mud losses would occur. 
     In riserless drilling, there is simply not a riser installed hydraulically connecting the seabed installed BOP to the drilling rig trough a marine drilling riser. Normally, the top of the wellbore (subsea BOP) is kept open to seawater pressure during drilling; hence the hydrostatic wellbore pressure is made up of the seawater pressure acting on the well at seabed, plus the hydrostatic pressure of the drilling fluid in the well below this point, also described in U.S. Pat. No. 4,149,603 (Arnold). 
     Several other concepts have been introduced and are in the public domain. 
     Other systems have introduced a closing element on top of the subsea BOP that can isolate the seawater pressure at seabed from acting on the borehole annulus (U.S. Pat. No. 6,415,877). Such closing element could be a so called Rotating Control Device (RCD) or a rotating BOP. These are somewhat different from an annular preventer in that it is possible to rotate the drill string while sealing pressure from below or above (seawater). It is not recommended practice to rotate the drillstring while a conventional annular BOP is closed during drilling due to excess wear on the rubber element. If such a system is used in combination with a subsea mudlift pump at seabed or mid sea, the suction pressure of the mud pump below the RCD in addition to the drilling fluid height and dynamic pressure loss in the annulus, directly control the pressure in the borehole. 
     Common for all these drilling systems is that the drilling fluid returning from the well cannot be returned through high pressure choke or kill lines in a conventional manner due to limited formation strength when the BOP is closed after an influx has occurred. Due to the heavy mud weight required or used, this mud will be displaced out of the wellbore annulus ahead of the lighter influx, hence the formation strength cannot support to be hydraulically in contact with the surface installation when the annulus of the wellbore and the conduit (kill or/and choke lines) back to surface are filled with the heavy drilling fluid. This effect will restrict the use of earlier systems or will put severe strain and requirement on the equipment and processes in a well control event. 
     In dual Gradient Drilling and riserless drilling, many types of Subsea Lift Pumps (SLP) can normally not handle a significant amount of gas from the well, as the case may be in a well control event for a gas kick. There are several reasons for this. In normal operations these pumps must handle a significant amount of drill cuttings and rocks in addition to the fine solid particles of the weight materials used in the drilling mud. If a gas influx is introduced into the wellbore at a considerable depth and pressure, this gas will expand when circulated up the bore hole to the seabed or mid-ocean where the pump is located. If this return path of fluids from the well has to go directly into the pump, it will put severe strain on the pump system. 
     Secondly, the bottom hole pressure will be a direct function of the fluid head in the annulus, the dynamic pressure loss in the annulus and the pump suction pressure. It will be extremely difficult to achieve a stable and controllable suction pressure on the pump when you will have slugs of high concentration hydrocarbon gas flowing directly into the pump system. As a consequence it will be a great advantage if the hydrocarbon gas and drilling fluid could be separated from each other subsea, before liquid drilling fluid and solids being diverted and pumped to surface by the subsea pump. This was also envisioned by Gonzales in U.S. Pat. No. 6,276,455. 
     Thirdly as the subsea pump in earlier systems is in direct communication with the annulus, the return lines and the pump system must be of the same high pressure rating as the BOP itself. This put severe requirements on the pump system to handle internal pressures. 
     Subsea Choke Systems. 
     Prior art exists in an attempt to compensate for the excessive pressure in the borehole acting on the well when circulating out a kick in a conventional manner through high pressure small bore choke line and a surface choke on the upper part of this line. U.S. Pat. No. 4,046,191 (Neath) and U.S. Pat. No. 4,210,208 (Shanks) introduced a surface controlled subsea choke where the flow from below a closed Subsea BOP was directed into the main bore of the drilling riser through a subsea choke. 
     Neath envisioned a conventional drilling system where the riser was full of conventional weighted drilling fluid. If such a system was used in a situation where dual gradient drilling technology was used, the pressure on the downstream of the adjustable choke could become too high due to the high mud weight used. Also since the riser was initially full of drilling mud, gas introduced into the base of the riser at great water depth could introduce further problems since the riser have limited collapse and internal pressure ratings. 
     SUMMARY OF THE INVENTION 
     In order to overcome challenges with prior art in conducting well control operations during riserless drilling and other dual gradient drilling technology, a method of controlling wellbore pressure in a controlled fashion will be explained. 
     Several alternatives for creating a subsea separation system within a subsea BOP will be explained below. Reference numbers refer to the accompanying drawings, as examples only. 
     Subsea BOP Gas Separation System 
     A riser joint used may be particularly designed to function as a separator where the separated gas is vented to the surface via the riser and the liquid is pumped to the surface via an exterior return path from the main drilling riser ( FIG. 2  and  FIG. 3 ). The main difference here with prior art is that the mud/liquid level in the riser is controlled and located at a considerable level below the sea level. In this fashion it is prevented that drilling fluids or liquids will be unloaded from the top of the riser if gas is being released into the base of the riser. 
     In another embodiment, a BOP extension joint (BOP-EJ) located between lower and upper annular preventer is so designed that with 2 different BOP elements closed, a chamber or cavity will be formed where gas can be separated from liquids by gravity and the separated gas vented via a conventional choke line or a separate conduit line, or alternatively via a riser to the surface. The liquid is pumped to the surface by the subsea mud pump controlling the liquid level in the cavity. 
     Another alternative would be a separate unit for separation where the separated gas is vented via a conventional choke line and the liquid is pumped to the surface through a separate liquid conduit line (not shown here). 
     A representation of a new riserless drilling system is shown in  FIG. 4 . In this system a subsea mud pump  11  is installed on seabed or some distance above and hydraulically connected to the well so that the drilling fluid and drill cuttings are pumped up to the drilling installation in a separate return flow path  12 . The interphase between the drilling fluid and the seawater is then somewhere in the vicinity of the Subsea BOP. 
     BOP-Extension Joint vs. Riser Joint for Mud/Gas Separation 
     A conventional subsea BOP is normally equipped with two annular preventers on modern rigs. The lower annular preventer  16  in  FIG. 1   a  is normally the uppermost closing element in the lower BOP stack  3  which consists of a series of ram type preventers stacked on top of each other  15   a, b, c, d  and the said BOP stack  3  installed with a special connector either to a High Pressure Wellhead (HP WH)  52  or a Horizontal Christmas-Tree (HXT) (not shown here). The total height of the lower subsea BOP is in the vicinity of 7 to 10 meter. The height of the HP WH is approximately 1 meter. The HP WH is normally installed on what is defined as the surface casing which normally sticks 2-3 meter above the seabed. The upper annular preventer  19  is normally installed in what is termed the Lower Marine Riser Package (LMRP). However, some rigs may have both annular preventers above the riser BOP disconnect point  20 ,  FIG. 1   b , in the LMRP. The interface between the lower BOP stack and the LMRP is normally designed a hydraulic remote operated disconnect point between the lower marine riser package (riser) and the lower subsea BOP. Hence the distance between the lower annular preventer on the BOP and the upper annular preventer in the LMRP is normally approximately 1.5-2.5 meters. In order to create a longer distance between the 2 annular preventers an extension joint could be installed to create more space. 
     If the mud and gas could be separated in a BOP cavity and/or BOP Extension Joint thereby creating a gas phase in the upper part of the BOP, this would allow a surface choke to control the gas pressure if connected to the cavity between the two closing elements, hydraulically either by flexible or fixed lines (no gas vent through riser). 
     BOP-Extension Joint can then be used for fluid-mud/gas separation in drilling with and without the riser. 
     If and when using the Low Riser Return System in another embodiment of this invention, the upper annular preventer can be closed during a drill pipe connection to avoid fluid level adjustment in the riser where in this case, the fluid level in the choke line is used to control and regulate the annulus pressure in order to compensate for the equivalent circulating density (ECD) effect (time saving). This is also explained in WO2009/123476, belonging to the applicant. The downside of having the liquid separated from the gas close to seabed as opposed to higher up in the riser is the longer pump suction line needed in deep water and the higher differential pressure capacity of the subsea pump system. 
     Another feature of this arrangement is the possibility to control bottom hole pressure while drilling (lower annular open) and when circulation out a well kick (lower annular closed), by controlling the liquid mud level in the choke line (subsea choke fully open) ( FIG. 6 ). In this case the upper annular could be substituted with a rotating BOP (RBOP or RCD)  19  where the mud pressure in the borehole annulus  1  is regulated by the liquid mud level in the choke line  51  ( FIG. 6 ). The pressure in the BOP and or BOP extension is now a function of the liquid level  51  in the choke line and the gas/air pressure above. This gas can either be ventilated to atmospheric pressure or controlled and regulated by the surface choke  22 . This will create a softer and more dynamic process than having the pump suction pressure (only liquid) directly controlling the wellbore pressure. When low compressibility liquid is contained in a closed loop system, it will create a very stiff system. Small changes will affect well bore pressure immediately, while a level control of drilling fluid, mud and/or seawater in the choke line will be a slower and more controllable process. 
     While drilling, this could set up a unique method of pressure control. An influx into the borehole between the open hole and drillstring could have a self regulating effect. An influx into the wellbore has a density higher than air in top of the choke line and for the case of example 8½″ hole and 6″ drill collars would have a capacity of minimum 17.8 liter per meter hole section. The capacity of most choke lines (3″-5) is between 4.56 liter per meter to 12.6 liter per meter. An influx of a certain magnitude would increase the level in the smaller capacity choke line to a higher level than the influx constitute in the openhole—drillstring annulus, hence an influx progressing would be stopped just by the higher hydrostatic pressure created by a higher liquid level  51  in the choke line  17 . 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1   a  illustrates a conventional subsea drilling system in normal drilling operations 
         FIG. 1   b  illustrates conventional subsea drilling system in well control mode 
         FIG. 2  illustrates a first embodiment of the present invention, including a riser, in drilling mode. 
         FIG. 3  illustrated the embodiment of  FIG. 2  in well control mode. 
         FIG. 4  illustrates a second riserless embodiment of the present invention in drilling mode. 
         FIG. 5  illustrates the embodiment of  FIG. 4  in well control mode. 
         FIG. 6  illustrates the system of  FIGS. 4 and 5  performing an alternative method for well control. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  illustrates a first embodiment of the subsea drilling system of the invention. It comprises a well having a well bore  1 . The well bore may be partially cased. Above the seabed level  2  is arranged a subsea BOP  3  with a BOP extension joint  3   a  which is equipped with several pressure sensors and several inlets and outlets. A riser  4  is connected to the BOP and extends to a vessel  5  above the sea level  6 . The riser  4  has a slip joint  7  to accommodating heave of the vessel  5  and a riser tensioning system  7   a ,  7   b . Above the diverter housing and diverter outlet is a low pressure gas stripper installed  53  to prevent low pressure gas escaping to the drill floor of the drilling rig. The diverter line  36  is ventilated to the atmosphere or the mud gas separator (not shown). The flow line valve  35  is closed as the drilling fluid now is returned via the subsea pump  11  and return line  12 . 
     Drill string  8  extends from a top drive  9  on the platform  5  and into the well bore  1 . The lower end the drill string  8  is equipped with a drill bit  10 . 
     A liquid return line  12  is connected to the BOP extension  3   a  at a first side port  13  and extends to the water surface. The liquid return line has a subsea lift pump  11  for aiding mud return to the surface vessel  5 . The liquid return line has a valve  49  in the branch between the first side port  13  and the pump  11 . 
     A gas return line  17  is also connected to the BOP  3  or BOP extension  3   a  by a second side port  18 . The gas return line  17  extends to the water surface and drilling vessel  5 . The gas return line has a first valve  21  close to the second side port  18  and a choke valve  22  near the water surface  6  or on the drilling unit. Both the liquid return line  12  and the gas return line  17  are at their upper ends connected to a collection tank  23  via a mud gas separator  42  on the drilling rig. 
     The BOP has a main bore  14  through witch the drill string  8  extends. A plurality of safety valves  15 , rams  15   a ,  15   b ,  15   c , are adapted to close the main bore  14  around the drilling tubular or to seal the wellbore completely  15   d , to prevent a blow-out. 
     Above the safety valves  15  and below the first side port  13  the BOP  3  has a lower annular valve  16 , which is adapted to close around the drilling tubulars  8 . 
     The BOP has an upper annular valve  19  above the second side port  18 . This annular valve may be a so-called rotating BOP, enabling drilling while the valve is closed. 
     A by-pass line  24  extends from the lower BOP (here two side ports  25  and  26  are shown) below the lower annular valve  16  to a third side port  27  between the first and the second side ports  13  and  18 . The by-pass also has a branch  29  connecting to the gas return line  17  here defined as the gas line or choke line. The bypass line  24  has lower valves  28  to close off the lower part of the by-pass line  24 , a first upper valve  30  to close off the branch  29  and a second upper valve  31  to close off the connection to the port  27 . In addition, there is a choke valve  32  in this bypass line. 
     The system also has a kill line  33 , which is also included in a conventional system. 
     At the top of the riser is a mudflow line  34  with a flow line valve  35  and a overboard line (diverter)  36  with a valve  37 , which are also according to a conventional system. 
     As also according to a conventional system, there are several mud pumps  38  pumping mud from the collection tank  23  to the top drive  9  through a line  39 . A valve  40  is included in the line  39  close to the top drive. 
     In addition, there is a booster line  41  extending from a mud pump  38  to a fourth side port  42  in the Lower Marine Riser Package or a circulating line connected below the first side port  13 . The line  41  is equipped with at least 1 valve  50  close to the side port  42 . This can be a backpressure valve and or a 2 way shut-off valve. This line may also be used to inject low density fluid or gas into the return path downstream the subsea choke valve installed close to the subsea BOP. 
     The system as described above in connection with  FIG. 2  is basically the same for all the embodiments described hereinafter. In the following only the items deviating from the arrangement in  FIG. 2  will be described in detail. 
     The system of  FIG. 2  can be used for drilling with and without marine drilling riser.  FIG. 4  shows a system without a riser. Except for the lack of a riser, the system is identical with the system described in  FIG. 2 . 
     The operation of the system according to the invention will now be described: 
       FIG. 2  illustrates normal drilling mode of the system. During normal drilling with a riser, both the lower and upper annular valves  16 ,  19  in the BOP  3  are open. The mud level  45  in the BOP or BOP extension or riser is controlled using the subsea mud lift pump  11 , which is hydraulically connected to the lower part of the BOP extension joint or riser. Any drill gas or background gas is vented off through the marine drilling riser, i.e. through the gas vent line  36 . Suspended and small gas bubbles may for the most case follow the liquid mud phase into the pump system  11  and be pumped to the surface. At surface the returns can be directed to the shale shakers  43  directly or via a valve  47  to the mud gas separator  42 . The system allows the mud level  45  to be adjusted for control of the bottom hole pressure. The fluid above the mud in the riser can be any type of liquid or gas, including air. 
       FIG. 3  shows the system in a well control event. The drill string rotation is stopped and the lower and upper annular valves  16 ,  19  are closed. This creates a cavity  46  between the lower and upper annular valves  16 ,  19 . The well fluid is diverted from below the lower annular valve  16  to below the upper annular valve  19 , i.e. to within the cavity  46 , through the bypass line  24  containing the choke valve  32 . Separation of the fluids in the cavity  46  in the BOP extension joint will take place due to gravity. The outlet  13  to the subsea lift pump  11  is arranged below the inlet level  27  for the well fluid, and the gas is vented off to the surface through the choke or gas return line  17  connected to the outlet  18  located above the fluid inlet  27  from the well. Normally, the gas/liquid interface level  45  will be located below the level for the gas line  17 . A surface choke  22  is used to control the pressure of the gas phase. The level  45  in the BOP cavity can be measured either by pressure transducers, gamma densitometries, sound, or other methods. 
     In this circulation and well pressure control method the surface drill pipe pressure can be regulated by regulating the subsea choke  32 , the subsea pump  11  can be used to regulate the liquid level  45  in the BOP cavity and the pressure in the cavity can be regulated by the pressure in the surface choke  22 , pressure in the BOP cavity, or the liquid level  51  in  FIG. 6  (or combination of the two). 
       FIGS. 4 and 5  show riserless drilling, and well control mode in riserless drilling, respectively. 
     During riserless drilling, the annular valves  16 ,  19  in the BOP  3  are open as illustrated in  FIG. 4 . The mud/sea water level  45  in the BOP  3  is controlled using the subsea mud lift pump  11  and pressure sensors in the extension joint  3   a  between the two annulars  16 ,  19 . Any small amount of drill gas or background gas may escape to sea from the open top of the BOP extension. However, most of the drill gas will follow the return liquids through the pump system  11 . In a well control event, the drill string  8  rotation is stopped and the lower and upper annular valves  16 ,  19  are closed, as illustrated in  FIG. 5 . The well fluid is diverted from below the lower annular  16  to below the upper annular valve  19  through the bypass line  24  containing the choke valve  32 . The choke valve  32  will now control the bottom hole pressure and the pressure downstream the choke  32  will be much lower than the upstream pressure. This will improve the separation process. 
     Separation of the fluids in the BOP extension joint  3   a  will take place due to gravity. An outlet  13  to the subsea lift pump  11  is arranged below the inlet level  27  for well fluid, and any free gas is vented off to surface through the flexible or fixed choke line  17  to above the water surface. Normally, the gas/fluid level  45  will be located below the outlet level  18  for the vent line  17 . A surface choke  22  is used to control the pressure of the gas phase. 
       FIG. 6  illustrates the subsea separator in an alternative mode. Here the subsea choke  32  is used to control bottom-hole pressure (BHP). The separator with the vent line  17  is used to remove the gas from the liquid before entering the subsea lift pump. However, the liquid is allowed to enter the vent line  17  and establish a liquid/gas interface  51  in the vent line  17 . The head of this liquid column and any pressure above the liquid/gas interface defines the pressure in the separator cavity  46 . By regulating the pressure above the fluid level and the level of the interface  51 , the pressure in the cavity  46  can be adjusted as illustrated in  FIG. 6 . 
     The pressure in the cavity  46  can be increased by pumping mud from the surface through the boost line  41 . This will quickly raise the interface  51  and hence increase the pressure in the cavity  46 . The pressure in the cavity  46  can be lowered by increasing the pump rate of the subsea return pump  11 . This will quickly reduce the level of the interface  51  and hence the pressure in the cavity  46 . This provides a means for rapidly adjusting the pressure in the cavity  46  and hence the back pressure against the well fluid entering the cavity  46  from the by-pass line  24  if the choke is fully open. 
     In the case of a subsea pump failure or as an option, a low density fluid or gas may be injected into the return lines or choke line, downstream of the subsea choke valve, so as to keep the pressure immediately downstream the subsea choke valve  32  substantially lower than the pressure upstream the subsea choke valve. In this manner the well pressure can be controlled accurately by the subsea choke. 
     Means to Reduce Pressure Fluctuations: 
     In order to avoid slug flow and large pressure variations, a choke valve  32  can be used to control the flow of fluids into the separator  48  and avoid or reduce the pressure fluctuations. Pressure fluctuation downstream of the subsea choke valve  32  could also affect the upstream pressure of the subsea choke (well pressure). However, keeping the gas/fluid level within the separator allows large gas flow rates to the handled. 
     Increasing the diameter of the choke line (6-8 inches) allows the liquid to enter the vent line  17  and separate from the gas without excessive pressure fluctuation in the BOP cavity. Since a subsea choke valve reduces the pressure, a low pressure choke line may be used. 
     In an effective riserless subsea separation system, the liquid/gas interface level may be kept within the separator and a surface choke valve to control the separator pressure may be introduced. 
     When keeping the pressure in the separator equal to or just below the ambient seawater pressure, the normal drilling operations can be conducted without major adjustments to the separator pressure. With only gas in the choke line, the size can be reduced (2-3 inches). This system will also reduce the gas separated from the liquid before entering the subsea lift pump. The pressure will reduce the subsea pump differential pressure needed to bring the return fluid back to the drilling vessel. Gas bleed off may take place at high rates. 
     This means that the remaining gas still contained in the liquids has to be separated at surface. So, the gas from the choke line, and the mud and gas from the subsea lift pump can be diverted through the mud gas separator/Poor Boy degasser  42  and vented off through the vent line in the derrick.