US3603409A

US3603409A - Method and apparatus for balancing subsea internal and external well pressures

Info

Publication number: US3603409A
Application number: US811052A
Authority: US
Inventors: Bruce J Watkins
Original assignee: Regan Forge and Engineering Co
Current assignee: Baker Hughes Holdings LLC
Priority date: 1969-03-27
Filing date: 1969-03-27
Publication date: 1971-09-07
Anticipated expiration: 1988-09-07

Abstract

Method and apparatus for maintaining a pressure balance between internal and external subsea well pressures during underwater drilling, entry and reentry operations conducted from a floating vessel remote from the subsea well comprising injecting gas into the wellhead apparatus in amounts sufficient to cause the density of the well fluid or mud returns below the surface of the sea to approximate the density of sea water and controlling the injection of such gas and thus the internal well fluid pressures by sea water well pressure differential and control means associated with valve means located in gas injection and return lines at the subsea well apparatus.

Description

XR 39603a409 United States Patent [72] Inventor Bruce J. Watkins Palos Verdes Estates, Calif.

(21] Appl. No. 811,052

[22] Filed Mar. 27, 1969 [45] Patented Sept. 7, 1971 [73] Assignee Regan Forge and Engineering Company San Pedro, Calif.

[54] METHOD AND APPARATUS FOR BALANCING SUBSEA INTERNAL AND EXTERNAL WELL PRESSURES 8 Claims, 7 Drawing Figs.

[51] 1nt.Cl ..E21b15/02,

E2lb 41/00 [50] Field of Search 175/7, 8,

Primary Examiner-Stephen .l. Novosad Attorney-Miketta, Glenny, Poms & Smith ABSTRACT: Method and apparatus for maintaining a pressure balance between internal and external subsea well pressures during underwater drilling, entry and reentry operations conducted from a floating vessel remote from the subsea well comprising injecting gas into the wellhead apparatus in amounts sufficient to cause the density of the well fluid or mud returns below the surface of the sea to approximate the density of sea water and controlling the injection of such gas and thus the internal well fluid pressures by sea water well pressure differential and control means associated with valve means located in gas injection and return lines at the subsea well apparatus.

M AERATED 6A5 l MIXTURE I l i I I4 51" V l 2Q 3011 I 50' I: 5 I a 27%- 4 27' i 26 d0 1 a8 i /50 i- T I6 I 56 V 1 2! 7 7 x l 23 a 34 I m ,2 m a 74A PATENTEDSEP H97! 3,603.40?)

sum 2 BF 3 AERATED GAS MIXTURE mzdzjz /zw wmzz A 7- TOE/V5945.

PATENTEDSEP Han 3.603409 SHEET 3 0F 3 EruoD AND APPARATUS FORBALANCINGLSUBSEA .flNTERNAL-hND-ExTERNAL WELLPRESSURES ,BAGKGRQUND OF THE INVENTION Heretoforemost subsea well drilling operations have utilized a largediarneter riser or'conduit extending from a floating vessels drilling gig down to the .ocean floor where awell is to be'dri lledh'llhe riser orlarge diameter conduitYhas-been ,used

to guide the drill string andiassociated, drill-bit ,into blowout preventers associated .with the wellheadas well as to provide a return path-for. drilling .fluids. or .m ud being circulated betweenthe floating vessel and the ,well duringjdrilling operationsaln subsea drilling; particularly in deep water locations,

the use of. such'large .diameterrisers becomes impractical because .of the high stresses imposed on the riser by surface and subsea water currents, weight of the drilling fluids or mud and,u ncontrolled-movement of the floating vessel relative .to the subsea well.3 In order to attempt to overcome these difflculftiesithasbeen common heretofore to .try to maintain the riser intension between the vessel and subsea-well by employing expansive cumbersome devices -.,which have not proved entirely satisfactory.

ln deep jwatersubsea locations, it is desirable to eliminate the -expensive and cumbersome 'devices'referred to above.

topfjofi the'blowout preventer stack-provided at the wellhead.

,However,; the substitution of arrotating blowout preventer at m t s of thewellhead apparatus fortheaforementioned riser .creates at least twomajor problemsJThewell'fluid or mud being more dense thanflsea water in the return lines creates a back pressure inside thev blowout preventer stack, such pressure. frequently being. twice the amount of pressure of the surrounding-sea water. This means thatsuch-rotating blowout :prevcntets, in deepsubsea well installations must operate con- -,tinuously;at'veryhigh pressure-differentials between that of the; internal well fluids, and the a surrounding sea .water. Such pressure differentials increase the wear on the blowout preventer sealsand the associated bearings. Also, there are elements} inf the drill; string suchas tool bitsand stabilizers, as .wen; as. other tools,-.that cannot be; easily stripped through the rotlating blowo,ut.preyenterwhen itis necessary to, remove themfrom the well. It has not been possible-heretofore to merelyopenthe wellhead upyand removesuchtools without .losingall of; the drillir g fluids ormud below sea. level. It has heretofore been necessary to-provide a plurality of blowout presenters spaced a. s utficientv distance apart so that odd shapeddrilling,toolsl anbc pulled-into a space between the blowoutpreventers with the upper one closed while the lower is opened. The lower blowout preventer or-preventers are then .closedand-the upper one or more opened inorder to allow removalof the tools. Such an arrangement requires an increase in the-size, and cost of the subsea. equipmentand also creates serious, instrumentation problems, i.e. manipulating thevarious tools and controls for operating the blowout preventersfrom the remote floatingvessel.

It is therefore the primary object of the present invention to solve the above problems by providing a method and apparatus for equalizing or closely balancing the drilling fluid pressure inside the wellhead with the surrounding sea water pressure sothat the wear on the rotating blowout preventer is reduced and the underwater well can be opened for removal and reentry. of tools without loss at drilling fluid.

It isa further objectof the present invention to provide methods and apparatus for equalizing such well apparatus internal and external pressures by injecting a fluid of lower density than water, suchas-gas, into the well fluids at the wellhead apparatus prior to their-retum through'the mud return lines to reduce thedensity of the drillingfluids to approximately that of the surrounding seawater.

' provided for applying aback pressure on the well fluids in the wellhead apparatus to increase the internal wellhead pressures when they fall below a desired level.

SUMMARY OF THE INVENTION -Generally stated, the method of maintaining a pressure balance between internal and external subseawell pressures at a subsea well apparatus installation-during drilling, entry and reentry operations conductedfrom'a remote floating vessel of ..the present invention comprisesinjecting gas, such as air, into the subsea well in amounts sufficient to cause the density of the well fluids below the surface of the sea to approximate the density of sea watenMore particularly, two or more fluid flow lines are established between the floating vessel and the bore of the subsea well apparatuswith fluid returns from the well being returned through one of the lines. Gas is injected into the bore of the subsea well through the other of said lines in amounts sufficient to lower the density of the drilling fluids in the return line or lines thus exerting a pressure uponthe internal bore of the well apparatus approximately equal to the pressure exerted thereon by the column of sea water above the apparatus. When such balance is achieved, the underwater well vmay be opened for the removal and reentry of tools without theloss of drilling muds or other well fluids. i

Theapparatus of the present invention, in general, includes one or more gas supplylines for injecting gas, such asair, into the subsea well apparatus borefor mixing therein with the drilling fluids or mud being returned to thesurface fluid return line or lines also provided. Valve meansare provided in the gas supply line or lines at a location generally adjacent the subsea-well for controlling the supply of gas to the well bore.

As further contemplated within the present invention, sea

water well pressure'differential sensing means are provided in association with the valve means for operating such valye means in response to a pressure differential between that within the subsea well apparatus bore and that of the surrounding sea water. Also, it is contemplated that similar valve means and pressure differential sensing means may be utilized in association with the drilling fluid or mud return line or lines for applying aback pressure within the well apparatus bore through the return line or lines in the event internal well pressures fallbelow that of thesurrounding sea water.

Further objects, various advantages and a better understanding of the apparatus and methods of the present invention will become apparent to those skilled in the art'from a consideration of the following detailed explanation of an exemplary embodiment thereof. Reference will be made to the appended sheets of drawings in which:

IN THE DRAWINGS FIG. 3 is a somewhat schematic representation of the subsea equipment of FIG. 2, taken partially in section, illustrating the injection of gas into the well bore and drilling mud being returned up themud return line;

FIG. 4 is another-somewhat schematic view of the equipment of FIG. 2 showing the drill string removed, the well open to the surrounding sea water and illustrating how back pressure through the mud return line can be utilized in the balanced pressure drilling system of the exemplary embodiment of FIGS. 1 through 4.

FIG. 5 is a detail view of a portion of the equipment of FIGS. 1 through 4 partially in section, showing the associated valve apparatus in a closed position;

FIG. 6 is a plan view of a portion of the equipment of FIG. taken therein along the plane Vl-Vl; and

FIG. 7 is another section view of the equipment of FIG. 5 showing the valve apparatus in an open position.

The method and apparatus for a balanced pressure drilling system will now be described in detail with reference to the aforedescribed figures. Referring to FIG. I initially, a floating vessel or barge 10 is positioned by suitable anchoring means (not shown) in a body of water or sea 11 over a subsea well formation 12. A conventional drilling rig, indicated at 13, may be provided on the barge or vessel 10 for running a conventional drill string 14 down to the subsea equipment, indicated generally at 15, at the well site.

In accordance with the method and apparatus of the present invention one or more gas supply lines, as line 16, and one or more well fluid or drilling mud return lines, as return line 17, are connected in a manner hereafter described in greater detail between vessel 10 and the subsea equipment indicated generally at 15. A compressor, or other source, of gas or air on vessel 10 may be utilized for injecting or introducing gas into the well fluids or drilling mud being returned up line 17 in order to control the density thereof. Preferably, the gas or air introduced into the well fluids or drilling mud at the subsea equipment via supply line 16 reduces the density of the returns to that approximating the surrounding sea water. Thus, internal pressures within the subsea equipment due to the weight of the well fluid, or mud, returned in line 17 can be balanced against the external well pressures due to the presence of the surrounding sea water.

Referring now to FIG. 2, it can be seen that the drill string 14 is provided in conventional manner with a drill bit 18 at a lower end for drilling a well hole 19 through the well equipment indicated generally at 15. In the preferred exemplary embodiment, the well equipment indicated generally at comprises a stack of blowout preventers and related equipment mounted upon drilling template 20 positioned over the location for drilling well hole 19. The stack of

blowout preventers

21, 22 and 23 are mounted by a connector 24 upon template 20. A rotating blowout preventer 25 is mounted to the top of the aforementioned BOP stack by connector 26. Rotating BOP 25 is illustrated with conventional flanged guides 27, 27' mounted upon

guide arms

28, 28 for running the rotating BOP to the subsea equipment via the

preconnected lines

16 and 17.

The drill string may be run in conventional manner in cooperation with the bit guide 29, the latter having flanged guides 30, 30' mounted on the

guide arms

31, 31, respectively. Choke and kill line-

type valves

32 and 33 may be provided in the

connections

34 and 35, respectively, between the subsea equipment inner bore and the lower ends of the

flow line

16 and 17. Hydraulically or manually operated

valve actuators

36 and 37 may also be provided in known manner.

Valves

32 and 33 may be utilized for conventional choke and kill operations unrelated to the balanced pressure drilling method and apparatus of the present invention.

As particularly contemplated within the present invention, the internal and external subsea well pressures are balanced during drilling, entry and reentry operations conducted from the floating vessel 10. Such balancing of the internal and external well pressures is accomplished by introducing gas, such as air, into the subsea well bore via gas supply line 16 to control the density of the well fluid, such as drilling mud, returns back through return line 17. In accordance with the method and apparatus of the present invention, valve means are introduced into the gas supply line and are controlled by pressure differential sensing means which sense a pressure differential between the internal well bore pressure and the external, surrounding sea water pressure. In the preferred exemplary embodiment, such valve means and associated pressure differential sensing means are indicated generally at 40. Similar valve and sensing means, indicated generally at 50, as also contemplated within the present invention are provided between the well bore and return line 17.

Referring now to FIG. 3, the subsea equipment of FIG. 2 is illustrated partially in section to somewhat schematically show the introduction of gas, such as air, into the well fluids, such as drilling mud, being returned from the well bore up through the return line 17. During a conventional drilling operation, mud is introduced down through the drill string 14, through the drill bit 18 and then circulates back up through the annulus 38 formed between the well bore 19 and the exterior surface of drill string 14. Normally, such drilling fluids or mud have been returned up through the return line 17 to the vessel, with the weight of the dense drilling mud creating considering high pressures within the well bore below the rotating blowout preventer 25. The rotating blowout preventer is thus required to operate under conditions of continuous high differential pressure which is damaging to both the seals and bearings thereof. Further, there are frequently elements in the drill string, such as bits and stabilizers, that cannot be stripped up through the rotating blowout preventer when it is necessary to remove them from the well.

In accordance with the present invention, these problems are solved by the equalization of the pressure inside the wellhead with the surrounding water pressure by controlled introduction of gas or air into the mud returns via the valve and sensing means indicated generally at 40. As seen in FIG. 3, gas is injected down line 16, through valve and sensing means 40 into the well bore 38 where it comingles with the well fluid returns 39. The gas injection into the fluid returns 39 lowers the density thereof and the mixture of gas and returns in line 17. By controlling the amount of gas injected through means 40, the density of returns in line 17 may be controlled to maintain the pressure differential across the rotating blowout preventer 25, and the differential between internal and external well pressures, at a minimum.

Referring to FIG. 5, the exemplary embodiment of valve means and differential pressure sensing means are illustrated generally at 40 connected between the gas supply lines 16 and the well apparatus (blowout preventer 21 in the exemplary embodiment). The valve means of the exemplary embodiment includes the valve body 41 slidably mounted within the valve housing 47. Piston member 42, connected to valve body 41 by stem portion 43, functions to stabilize the sliding movement of body 41 in bore 46 and as described subsequently, forms a part of the pressure sensing means.

Bodies

41 and 42 are provided with fluid seals or O-

ring seals

44 and 45, respectively, for sealing the bodies or

members

41 and 42 within bore 46 of valve housing 47. Bore 46 is enlarged in bore area 48 adjacent the gas inlet conduit 16 and area 49 adjacent the outlet conduit 38 which is in fluid communication with the well apparatus bore 38.

Valve body or member 41 is shown in closed position in FIG. 5. Gas flow from line 16 into the well bore via line 38' is prevented due to the sealing engagement of seal means 44 and the surrounding housing bore 46. However, on an upward movement of valve member 41 into the open position of FIG. 7, gas flow from line 16', through housing bore 46 and line 38 to the well apparatus bore 38 is allowed. The opening and closing of valve member 41 relative to the lines 16' and 38' is controlled by associated pressure differential sensing means as hereinafter described.

Pressure differential sensing and valve actuating means are provided in association with the valve member 41 for operating such valve in response to a pressure differential between the surrounding sea water and the well apparatus bore pressure. In the exemplary embodiment, such pressure sensing means are provided by the upper and

lower members

41 and 42 which act as piston members in housing bore 46. As seen in FIG. 5, valve member 41 is provided with a round pistonlike configuration with an upper end fluid seal 51. The upper end of housing 47 is open to the surrounding sea water pressure through the open end of bore 46. In the exemplary embodiment, an end fitting 52 is shown bolted by bolts 53 onto the upper end of housing 47 with a central threaded aperture or bore 54. While in the preferred exemplary embodiment, the

The lowerend-of h'ousing 47- is also providedwithan' end fitting 55 boltedzto the housing. A conduit56"isconnected'to the valve housing bore 46 via-fitti'ng 57, threaded into end fittingaSSi' The other'end of. conduit 56 is connected'into thelines 38-on 'th'e welliapparatus bore side. ofthevalve housing.

Well apparatus: borepressures in here 38", on-'thej well apparatus' side:ofhousingfiflare thus communicated via-conduit 56" form: line 38 to the bottom end of thesecond valve. memberbrpistonA l ReferringrtdFlGk 7; wellapparatus\bore 38 exceed the surrounding sea-water pressure, such pressure'diffrential betweenf the. upper-and lower ends: of-integral members 4l, 42 an'd'43 'causesan upward movement ofsuch members-from thepositionnof FIG. 5 to that of 'FlG. 73' Gas :is thenjinjected through'line-l6 valve housing- 47 and'line' 38 intotherwell'apparatus bore until the variation of'the wellfluidreducesathe well apparatus bore pressures to a value equal to or slightly less than the surrounding sea'rwater' pressures. Whenthe sea water pressuresexceed that'of'thecin ternal apparatuspressures, valve and piston members4l and 4-2are moved"back toj the-closed positi'onbfFlGl 5 stopping further injectionof'gas into the well fluidreturns;

ltis ldesiredFtomaintain well bore pressures sligh'tly'above th'evalueof the' surrounding sea water pressures-the diameter offther'lowen piston 42 may be; made somewhat smaller than thatof-th'e upper valve member or piston 4f. However, in the preferred exemplary embodiment, the "diameters'of members 41" and-'42,anditheir corresponding-pressure surfaces-exposed within=tore.'-46at-'the upperandlowerend's'of members 41 and 42T- are equal to each other. With" this arrangement, the as sociated =valve' rneans and-pressure differential control means E1683: 5' through 7 ca'nfi be utilized to maintain the; well apparatus"-internal bore pressures and' surrounding; sea water pressures-very nearlybalanced duringth'e' drilling operations.

in the event that the pressure withinthewell apparatusbore 38'rfallsbelowa-thesurrounding seawater pressure, itis contemplatedlwithin ltheipresent invention thata valve means and associated-pressure differential'sensingand valve control 'app'aratus; indicated z generally at 50, constructed and" operated as apparatus 40; may be utilized in association with the mud retumline l'7-asillustrated in 'FlG; Si TheintemaI construction and operation ofthecombination vvalve and pressure differential sensing and valve control means indicated generally at50is the same-asthatfillustrated in-FlGS. 5 and 7; However, the inlet conduit l'Vto valve and pressure control means 50 is connected into'themud return line l7 while the outlet conduit 38 is again connecte'd into the well app'aratusbore 38; as best seemin-FIG; 43 ln=theeventthe pressure-within bore 38 falls below thav of the surrounding sea water pressure, the differential control mea'ns associated with means 50 senses" the difference in presSurebetWeen the well fluids within the conduit'38" and "the surrounding-seawater, opens the associated valve-means and-allows'return'ofmud or=drilling fluids back downline l-7Jinto thejwell; The necessary back pressure in line l7 'may: be obtained'bwpumpihg,drillingfluid back down line 17 inorder tomaintain'the, desired balance between internal and external well pressures:

By maintaining a very nearbalance" between= internal and external =wellbore pressures'as'aforedescribed, the subsea apparatusmay'be opened up to=thesurrounding sea-water as shownin FIG; 4;,and tliedi'ill string lfland associated tool, asbit 18; removed therefrom. As illustrated in FIG. 4, the blowoutpreventers are allin open position with the surrounding sea water pressure in balance with the internal well fluid to maintain'control over the well even though it is open to the surrounding'sea'water. It can be seen'from the foregoing disclosure'that by usingsthe=pressurebalancing apparatus and method 'of the presentinventiong' the instrumentation required and the proeedureszfofentryuand reentry of subsea well apwh en' the well'fluid pressures within" paratus are greatly simplified. Drillingtools may be removed andzinserted'into the subsea welliapparatus with the same caseand conveniencein-the subsea environmentasthoughthc well apparatus'were on land Having thus described'an'exemplary 5 embodiment of the present. invention, what-l claim as my invention-is set forth-"in th'e-followingclaims.

[claim- 1. In a method of maintaining apressure balance between internal and external subseawell pressures during drilling, entry'and'reentry operation conducted from afloating vessel remote fromthe subsea-wellincluding the step of injectingigas into thesubsea well in amountssufficient to cause the density of'the-iwell fluids below-the surface of the-sea to approximate l the=density of seawater, the improvement comprising the'additional steps of;

sensingthe pressure differential at said subsea well between intemalwell borepressureand external waterpressure and;

controlling theamount of gas injected into said well bore to minimize the magnitude of said pressure differential beingsensed.

2. In a method of maintaining a pressure balance between internal and external subsea well pressures during drilling, entry and reentry operations conducted from a floating vessel remote: from the subseawell'wherein drill mudis circulated from the vessel'into the well during drilling operations-and is retumedftothevessel via a mud return line including the steps of establishingztwo ormorefluidflow linesbetween the float- 'ingvessel and the bore of the subsea well, returning drill mud to the floating vessel from the well bore via one or more return lineslof said fluid 'flow'lines and injecting gas into the subsea well through one or more injection'lines of said fluid flow lines in'amounts sufficient'tolower the density of said drill mud in 7 said one or morev return lines to approximate the density of sea water; the improvement comprisingthe additional stepof:

applying a back pressure on said well bore by pumping drill mud down one or more of "said lines to balancethe internal and external well pressures when the internal well pressure fallsbelow that'ofthe surrounding sea water.

3." The .methodof claim 2 comprising the additional step of:

sensing the pressure differential at said subsea well between thepressurewithin saidvone or more return lines'at said well and thesurrounding-water pressure, and

controlling the amount" of drill mud introduced back into said well through said lines to minimize the magnitude of said. pressure differential.

4'5 The method of claim'3comprising the additional step of:

opening the well bore to the surrounding sea water to allow removal of and re-entry of well tools when the internal andexternal well pressures are in balance.

5. In a subsea well drilling apparatus including a drill string anddrilling mud-retum line, each run from a floating vessel 5 5 overa subsea well location to subsea drilling equipment at the well location, a blowout'preventer'means for-sealing the well bore about the drill string within the well during drilling operations and gas supply linevmeans run from said floating vessel and connected into said well bore below said blowout 0 preventer means for injecting gas into the drilling mud to reduce its density as it is returned from said well to the floating- 6. In a subsea well drilling'apparatus including'a drill string and d'rillingmud return line, each run from a floatingvessel therein adjacent said subsea well for controllingthe,

over a subsea well location to subsea drilling equipment at the well location, a blowout preventer means for sealing the well bore about the drill string within the well during drilling operations and gas supply line means run from said floating vessel and connected into said well bore below said blowout preventer means for injecting gas into the drilling mud to reduce its density as it is returned from said well to the floating vessel via said mud return line, the improvement comprising the provision of:

valve means in said mud return line in a location therein adjacent said subsea well for controlling the back flow of mud from said return line back into said well bore; and pressure differential sensing and valve control means in association with said valve means for operating said valve 8. The method of claim 2 comprising the additional steps of:

opening said well bore to the surrounding sea water and running a well tool between said well bore and said floating vessel through said sea water and the opened well bore.

Claims

1. In a method of maintaining a pressure balance between internal and external subsea well pressures during drilling, entry and reentry operation conducted from a floating vessel remote from the subsea well including the step of injecting gas into the subsea well in amounts sufficient to cause the density of the well fluids below the surface of the sea to approximate the density of sea water, the improvement comprising the additional steps of: sensing the pressure differential at said subsea well between internal well bore pressure and external water pressure and controlling the amount of gas injected into said well bore to minimize the magnitude of said pressure differential being sensed.

2. In a method of maintaining a pressure balance between internal and external subsea well pressures during drilling, entry and reentry operations conducted from a floating vessel remote from the subsea well wherein drill mud is circulated from the vessel into the well during drilling operations and is returned to the vessel via a mud return line including the steps of establishing two or more fluId flow lines between the floating vessel and the bore of the subsea well, returning drill mud to the floating vessel from the well bore via one or more return lines of said fluid flow lines and injecting gas into the subsea well through one or more injection lines of said fluid flow lines in amounts sufficient to lower the density of said drill mud in said one or more return lines to approximate the density of sea water, the improvement comprising the additional step of: applying a back pressure on said well bore by pumping drill mud down one or more of said lines to balance the internal and external well pressures when the internal well pressure falls below that of the surrounding sea water.

3. The method of claim 2 comprising the additional step of: sensing the pressure differential at said subsea well between the pressure within said one or more return lines at said well and the surrounding water pressure, and controlling the amount of drill mud introduced back into said well through said lines to minimize the magnitude of said pressure differential.

4. The method of claim 3 comprising the additional step of: opening the well bore to the surrounding sea water to allow removal of and re-entry of well tools when the internal and external well pressures are in balance.

5. In a subsea well drilling apparatus including a drill string and drilling mud return line, each run from a floating vessel over a subsea well location to subsea drilling equipment at the well location, a blowout preventer means for sealing the well bore about the drill string within the well during drilling operations and gas supply line means run from said floating vessel and connected into said well bore below said blowout preventer means for injecting gas into the drilling mud to reduce its density as it is returned from said well to the floating vessel via said mud return line, the improvement comprising the provision of: valve means in said gas supply line means at a location therein adjacent said subsea well for controlling the supply of gas to said well bore through said supply line means and pressure differential sensing and valve control means in association with said valve means for operating said valve means in response to a pressure differential between the pressure in said gas supply line on the well bore side of said valve means and the pressure of the surrounding sea water.

6. In a subsea well drilling apparatus including a drill string and drilling mud return line, each run from a floating vessel over a subsea well location to subsea drilling equipment at the well location, a blowout preventer means for sealing the well bore about the drill string within the well during drilling operations and gas supply line means run from said floating vessel and connected into said well bore below said blowout preventer means for injecting gas into the drilling mud to reduce its density as it is returned from said well to the floating vessel via said mud return line, the improvement comprising the provision of: valve means in said mud return line in a location therein adjacent said subsea well for controlling the back flow of mud from said return line back into said well bore; and pressure differential sensing and valve control means in association with said valve means for operating said valve means in response to a pressure differential between the pressure in said mud return line on the well bore side of said valve means and the surrounding sea water pressure.

7. The method of claim 1 comprising the additional steps of: opening said well bore to the surrounding sea water and running a well tool between said well bore and said floating vessel through said sea water and the opened well bore.

8. The method of claim 2 comprising the additional steps of: opening said well bore to the surrounding sea water and running a well tool between said well bore and said floating vessel through said sea water and the opened well bore. >