CA1063017A - Riser set-aside system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A marine riser system for use in deep water drilling oper-ations from a floating vessel is disclosed. The riser system permits detachment of the lower end of the riser from the wellhead enabling the riser to be set aside to a position which is clear of the wellhead.
Support means adjacent the wellhead can be used to support and position the riser when it is set aside. Tensioners, support mechanisms and guidance means are provided to move the riser back and forth between the wellhead and support means. In this manner, casing and tools having diameters larger than that of the riser can be inserted into the well-head without returning the riser to the surface. Drilling operations with large diameter drill bits can also be conducted with the riser in the set aside position.

Description

1 8ACXGRO~D OF T~E INVENTION

2 1. Field of the In~ention

3 This invention relates to offshore operations conducted from

4 a floating vessel. ~ore specifically, this invention relates to an improved marine riser system for use in drilling oil wells and similar 6 boreholes from floating vessels.
7 2. Description of the Prior Art 8 In recent years the search for oil and gas has extended into g increasinglv deeper waters. Economic conYiderations and physical ln limitations frequently militate against the use of bottom supported 11 platforms in very deep water. Therefore, st offshore dsilling in deep 12 water is conducted from a floating drilling vessel which supports the 13 drill rig and derrick and associated drilling equipment.
14 A r~ser pipe is nor~ally used to interconnect the floating vessel and the wellhead. A drlll string extends from the floating 16 ves~el, through the riser and into the wellhead located on the sea 17 floor. The riser pipe serves to guide the drill string into the wellhead 1~ and to conduct returning drilling fluid back to the vessel during drilling l~ operations in the annulus between the ri~er pipe and drill stsing. The ~ marine riser i9 presently regarded as the limiting element in floating 21 drilling operations since the weight of the riser pipe and the ~tre~es 22 within the pipe increase with water depth. Adding to the stre~s on the 2~ rlser pipe are-bending moment~ caused by the action of wind, wave and 26 sea~cusrents on the riser and b~ vements of the drilling vessel.
25~ To co~nteract riser stres~, riser tensioning devices are --2fi normall~ mounted on the drill ship. These tensioning devices apply a ;~ ccnstant tensile force to the top of the riser pipe, thereby reducing 2~ ~ending stresses on the rlser. The use of flexible ~oints placed 29 ~neërmedl-te the ends of the riser h-s also been suggested to i~crea~e . ~ ... . . .. .

~ ~, ' ,'. .'' , : ' : ' . ' ~
- - ; : : :, :. .. .
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1 riser elexibility. ~ouever, both riser tensioning devices and flex~ble 2 ioints have their limitations as to the amount of riser stres~ which 3 thev can relieve.
4 In ordinarv drilling using conventional risers, the riser pipe

5 is about 17 to 2~ inches in diameter. T~e relatively large diameter of

6 the riser pipe is necessary to permit the drill bit or other large

7 diameter tools used in connecti~n with setting the casing to pass through ~ the riser plpe. Yowever, if a smaller diameter riser (12 to 15 inch a inside diameter~ can be used, the overall weight of the riser would be 1~ less, significantly reducing stress on the riser pipe.
11 The main problem w~th a small diameter riser is that it has to 12 be retrieved everY ti~e it becomes necessary to run casing or tools into 13 the well which have a diameter or width larger than the inside diameter 14 of the riser. Complete retrieval of the riser is called a riser trip 15 and each such trip can take from two to twenty days depending on water 16 depth, weather conditions and other factors. If two or three riser 17 trips are requlred during a drilling operation in deep wates, as much as 18 40 day9 of expensive ri8 time can be lost. Thus, there is a need for a 19 deep water drilling s~stem which permits the use of small diameter 20 ri8ers without the concomitant loss of rig time caused by riser trips.

21 S~MARY OF TEE INVENTION
22 An ob~ect of the present invention i9 to eliminate the need to 23 return a Farine riser to the surface every time drill pipe, casing, or 24 well tools having diameters larger than the inside diameter of the riser 25 have to be placed into a subsea well. The present invention can also be 26 u8ed when it is necessary to change or dify the blowout preventer 27 stac~. Rather than returning the riser to the ~urface, the riser pipe 28 s~8tem of the pre~ent invention pesmits the riser to be set aside by - . ..

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1 movin~ it to a position which is clear of t~e wellhead, there~y per-2 mittin~ insertion o~ tools into the well or modification of wellhead 3 apparatus.
4 In one embodiment of the present invention a riser system includes a riser pipe having its upper end connected to a floating 6 drllling vessel and its lower end detachably connected to a subsea 7 wellhead or to a subsea support mean~ positioned adjacent the wellhead.

8 ~eans are provided for detaching the lower end of the riser fro~ the

9 wel~head and ~or moving it so that the lower end of the riser is in ln supoorting relationship with the support means. Similarly, means are 11 provided ~or returning the riser to the wellhead position. Means for 12 movin~ the riser may include tensioners, support mechanisms and guidance 1~ means.
14 Means for supporting the lower end of the riser ~ay include support posts and frames which are capable of structurally distributing 16 the load of the riser. Tensioning means aboard the drilling vessel may 17 be provided to support the upper end of the riser and to maintain it in 18 tension to prevent buckling. ~ormally, a subsea installation such as a lg blowout preventer stack connects the riser with the wellhead when the 2n riger i9 po dtioned above the wellhead.
21 In accordance with the method of present invention, a riser 2 Dipe initially connected at its upper end to a floating drilling vessel . , 23 and at its lower end to a subsea wellhead is disconnected from the 24 w-llhead. The riser i8 then set aside by ving it to a position which is away from the wellhead and placing the riser on support means located ~6 ad~acent the wellhead. This procedure may be reversed to reconnect the 27 riser to the wellhead. ~uitable support means may also be provided to 28 permit the riser and blowout preventer stack to be set a~ide or to 2~ permit only the riser to ~e set aside. Suitable support means may also 4_ .

-1 be in drillin~ locations where sea currents are sufficiently strong, the 2 riser can ~e set aside bv disconnecting the lower end of the riser and 3 allowing the lower end to drift a safe distance downc~rrent from the 4 wellhead.
~ The method and aDparatus of the present invention permits the 6 efficient use of lighter, smaller diameter risers in deep water. T~hen 7 it becomes necessary to remove the riser to run casing, to insert large 8 diameter drilling bits or tools, or to change the blowout preventer 9 stac~, the riser can be disconnected from the wellhead and set aside on

10 the supPort means or allowed to drift, thereby eliminating a time con- -

11 suming riser trip. The riser set aside syste~ of the present invention

12 therefore has si~nificant advantages over systems existing heretofore.

13 8RIEF ~ESCRIPTIO~-OF TXE DRAWn~GS

14 FIG. 1 is a schematic elevational view partly in section

15 ~howing floatin~ drilling eauipment including the riser system of the

16 present invention in the set aside position.

17 FIG. 2 is an enlarged view of the ~ellhead and lower end of

18 the rlser shown in FIG. 1, illustrating details of the support frame,

19 blowout preventer 9tack, and wellhead.
FIG. 3 is an enlarged view of the l~wer end of the riser in 21 the 9et a~ide po9ition, illu9trating installation of a high pressure 22 blowout preventer stack.
23 ~I~. 4 i9 an enl-rged view of the lower end of the riser, 24 9imilar to FIG. 3, illu8trating the riser po9ition after it has been ~ -25 moved to a po9ition above the blowout preventer stack.
26 FIG. 5 i9 a schematic elevational view partly in sectioD
27 howing floating arilling equipment iDcluding the riser system of the 2~ present inventioD in the conventional drilling position.

. 6 and 7 are schematic elevational views of a ~wo-vessel 2 drillin~ oPeration using the riser system of the present invention.
3 FIG. ~ is an enlarged view of the wellhead, lower end of the 4 riser, and blowout preventer assembly illustrating deeails of another 5 riser sYstem embodiment of the present invention.
6 FIG~. a and 10 are schematic elevational views depicting 7 operation of the riser system embodiment shown in FIG. 8, illustrating 8 set aside positions of the riser.
g FIG. 11 is a sectional view along line 11-11 of FIG. 4 illus-10 tratin~ the riser aligned above the blowout preventer stac~.
11 FIG. 12 is a sectional view along line 12-12 of FIG. 3 illus-12 tratin~ the riser in the set aside position.
13 RIG. 13 is a schematic elevational view of apparatus for 14 another embodiment of this invention illustrating a temporary guide base 15 and e~uipment for drilling a wellbore.
16 FIG. 14 is a schematic elevational view partly in section 17 showin~ installation of a permanent guide base on the temporary guide 1~ base of FIG 13 and installation of casing in the wellbore.
19 FIG. 15 is a sche~atic elevational view illustrating floating

20 drillin~ ea,ui"~ment including a riser and wellhead assembly in conventional

21 drillin~ position.

22 FIG. 16 is a schematic elevational view illustrating the 3 Moating drilling equipment of FIG. 15 with the riser system and a 24 blowout preventer in the set aside position.
F~G. 17 is a schematic elevational vie~ illustrating the 26 ~lowout preventer in conventional operating position and the rises in a 27 ~et adde position.

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28 ~IG. 18 is a s-ctional view along line 18-18 of FIG. 15 illus-2~ trating the blowout preventer assembly in conventional operating posi-3n tion, :: ~
~ - 6 -1 FIG. 19 is a sectional view along line 1a-19 of FIG. 16 2 illustrat~nR the hlowout ~reventer assembly in the set aside position.
3 ~IGS. ?n-?~ are vertical sectional views of a hydraulic pin 4 assembl~ and set aside frame showing a sequence of steps for moving the 5 blowout preventer and riser to the set aside position.
6 FI~. 24 is a schematic view illustrating the blowout prever.ter 7 asse~hlv being set aside.
Q ~IG. 25 is a cross-sectional view of the hydraulic pin assembly 9 alon~ lines 25-?5 of FIG. 22.

ln ~RSCRIPTInM OF T~E P~F~RRED RMBODrMENT
11 ~IG, 1 shows the riser system of the present invention for use 12 with a floating drillin~ vessel ln and a subsea wellhead 11. Drilling 13 vessel 1~ is shown floating on a body of water 12. As illustrated, 14 guide base 13, rests on the sea floor 14 and supports frame 16 which in turn supports ~arine rlser 17 and blowout oreventer stack 18 (hereinafter 16 referred to as BOP stack~. nrill pipe 19 extends from derrick 20 aboard 17 drillinB vessel 1~, d~wn through B~P stack 1~ and into wellbore 15.
1~ For purposes of illustration, one embodiment of present inven-19 tion will be described in connection with the drilling of a subsea well.
2n The descriotlon will include the drilling of a 26 inch diameter hole for 21 20 inch ~tructural ca~ing, a 17-lJ2 inch diameter hole for 13-3/8 inch 22 surface ca~in~, ana a 12-1/4 inch diameter hole for 9-5/8 inch standard

23 casing. Rowever, as will become apparent the riser set aside concept ~4 ma~ be used for other types of drilling operations and procedures.
.5 ~iser 17 is shown in the set aside mode, the operation of 26 which will be explained later. ~he term "set aside" mode or position, ~7 as used herein, means the riser is offset from it~ customarg operatin~
2R po~itlon a~ove wellhead 11. The rises has flexible ~oints 21 anc 22 ;" ' ' ,'"
, _ 7 _ ~, .

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1 attached respectivelv to its uP~er and lower ends. Upper joint 21 2 connects the upper end of the riser to a vertically e~tensible slip ~ ~oint 2~ and lower joint 22 connects the lo~er end of the riser to riser 4 frame 24 which provides load distribution for the riser induced loads.
Lower 10int 22 lessens the transfer of moments from the riser to the 6 riser frame, thus allowing the riser to flex when the drilliag vessel is 7 deflecte~ from the centerline of the wellbore by wind or wave action.
~ ~.arge diameter ball Joints and steel-elastomer flex ~oints have been 9 ~ound to make particularlq effective flexible ~oints for this purpose, 1~ but an~ fle~ible ~oint capable of withstanding high tensile loads is 11 gatisfactorq. Tn addition to the upper and lower ~oints 21 and 22, a l? series of ,~oints (not shown) positioned between the ends of riser 17 may 13 be used to increase riser flexibility.
14 Tensioners (not shown) acting through cables 25 maintain riser 17 in eension and prevent it from buckling. ~ach cable passes over a l~ pulleq 26 and attaches to outer barrel clamp 27 which in turn is attached 17 to outer barrel 28 of slip ~oint 23. Slip ~oint Z3, being vertically 1~ extengible, co~Densates for vessel heave, thereb~ preventing exces~ive lq stresses in the riger. The tensioning system imparts a tensile load to 2n slip ,~oint 23 which is transmitted to the'~hole riser pipe string which 21 includes riser 17, ~oints 21 and 22, slip ~oint 23 and frame 24. The 22 ten~lonerg nay be hydrau}ically or pne~Atically actuated and may reel , ~
23~ca~lo9 25 in and out iD re9pOnge to the vertical motion of vessel 10,

24 thu~ maintainin~ aporo~imately constant tensile load on riser 17.

Pulleys 26 rest upon rail guides 29 and per~it the riser pipe 26 ~trlnR to be laterally moved. Mo~e~ent of the pulleys along the rail 27 ~uldeg alters the ten910ni~g of cableg 25, forcing the riser pipe string '' 2~ to be displaced iateral b in the direction of pulley movement. Tension -29 i~-malntaIned at aIl t~es through cable 25 while the rlser is belng ~ -3~ ~oved. Whe~ lt i~ n~ceJsary to maintain ~he ri~er in a stationary 31 po~ltion oulleys 26 are loc~ed in place.
.~ - .

, 8 -- ,i: . : ,, . - -. . ,. ,: . , , - , : :, . :, . : . .

1 The purpose for initially placing riser 17 in the set aside 2 position is to permit the installation of large diameter surface casing 3 without the use of a correspondinglY large diameter riser. For example 4 drill pipe lq can be used to drill a 26 inch diameter hole through BOP
5 stack 18. Drilling fluids, circulated down through the drill pipe in a conventional manner, return from the wellbore and are diverted into 7 riser 17 by means of diverter line 31 located on the BOP stack. Since ~ riser 17 is onlv u~ed as a return conduit for the drilling fluid it need g not have a large diameter. Thus riser 17 can be lighter weight, small ln d~ameter riser having an inside diameter of about 13 inches.
11 After the 26 inch hole is drilled to a depth of from about 500 12 to 15~0 feet, 20 inch surface casing can be installed and cemented in 13 place- The casing installation is schematically depicted in FIG. 2 - - -14 which also shows a more detailed view of stack isolation frame 16, BOP
15 stack 18 and riser frame 24. Drill pipe l~, carrying wellhead housing 16 32, is used to run casing 33 through BOP stack 18 and into wellbore 15.
17 Also showQ are stinger ~4 and cement shoe 35 which are conventional 1~ equipment used to assist the cementing of casing 33.
1~ ~P stack 18 i9 a large aiameter, low prescure diverter stack.
20 Such a stack normally has a working pressure of about SOO psi and would ~1 be used only for the installation of conductor and surface casing. As 22 illustrated in ~IG. 2, the ~tack consists of two annular preventers 40 ~3 and 41, a blind shear ram 42, and a hydraulic connector 43. The BOP
24 5tack algo include9 crossover spool 44 which diverts fluid flow from the

25 BOP ~tack into diverter line 31, Dump valve 46 i9 provided to permit 2fi rapid espulsion of fluids from the BOP ~tack in the event of an emergency.
27 ~urin~ normal operation, drilling fluids will be diverted b~ crossover ~ Jpool 4 through valve 47 into diverter line 31. Closing off annular 29 preventers 4~ or 41 will prevent fluids from flowing up through BOP
3n stack 18. Alternatively, other sealing means such as a rotating seal 31 can be employed to shut off flow to BOP stack 18.

, , 1 ~iverter line ~1 may per~orm additional important functions 2 besides diverting the flow o~ drilling mud into the riser. As shown in 3 FTG. 2, diverter line 31 further includes throttling valve 48 and hy-4 draulic flowline connector 4~. Throttling valve 48 is used to regulate 5 and maintain wellbore pressure. ~ydraulic flowline connector 49 contains 6 a telescopic piston 50 which inserts into and connects with valve 47 7 after the ~OP stack has been lowered in place. Connector 49 may also 8 ~rovide control functions for the riser lift mechanism 51.
9 Riser lift mechanism 51 consists of a hydraulic cylinder 52 10 and piston t,not shown-, connecting rod 53 and lifting arm 54. Control 11 lines (not shown~ within connector 49 controls the actuation of the lift 12 mechan~sm which provides guidance and lift assistance in ving riser 17 13 to and from the set asiae position.
14 Also shown in FIG. 2 are riser frame 24 and a cut-away view of frame 16 which provides support for 80P stack 18 and riser 17. Frame 16 16 i8 designed to distribute riser induced loads into the structural casing 17 below gulde base 13. '~hen the riser is positioned on top of the BOP
18 stac~ (as shown in FIG. 5) a ma~or portion of the load induced by the 19 ri5er are removed from the BOP clamp and connector by the isolation 2n frame 16, th~s maintaining full pressure integrity within the BOP
21 stack. In the set aside position shown in FIG. 2, the riser loads are 22 distributet by riser frame 24 into support posts 55a and 55b. ~A third ~-23 support post is not in view). Connection of the frame into the posts is 24 made b~ hYdraulic connectorQ 56a and 56c and a third connector (not shown~.

26 ~nce the 20 inch ~urface casing is in place, a 17-1/2 inch

27 hole can then be drilled and cased with 13-3/8 inch casing to a depth of

28 from about 2000 to 4000'feet. ~his phase of the drilling operation

29 would also be completed'with riser 17 in the set aside mode. Ater the 13-3/8 lnch casing has been run and cemented, the drilling operations 31 can be converted to the conventional de with riser 17 positioned above 32 BO~ stack 18.

- m - ' -... . . .

1 As will be illustrated, the conversion to a conventional 2 drilling mote includes replacing the low pressure BOP stack (shown in 3 FIG. 2) with a high pressure stack (shown in FIG. 3). Drilling the 26 4 inch and 17-1/2 inch holes for conductor and surface casing can be 5 performed using sea water or a low density mud as the drilling fluid.
6 Because well control is not generally a problem during the ~nitial 7 phases of the drilling operation, a low pressure BOP stack is used.
8 However, when drilling the remainter of the well to its final depth g through producing formations it is frequently necessary to use a hi8h 10 pre~sure BOP stack with the riser in the conventional drilling position 11 to insure that well control will be maintained.
12 The first step in converting to a conventional drilling de 13 is to di9connect and retrieve the low pressure BOP stack. A drill pipe 14 is lowered and attached to the top of the 80P stack and then, using 15 remote control actuators, the BOP stack is disconnected from the wellhead 16 and diverter line and returned to the surface on the drill pipe. A high 17 pre8sure stack is then lowered on the drill pipe and positioned on the 18 wellhead. This is illustrated in FIG. 3 which hows a high pressure BOP :
19 8tack 60 being attachet to the wellhead. The stack is lowered on drill 20 pipe 61 and attached to wellhead connection 62 by means of hydraulic 21 connector 63. Once the BOP stack i8 in place, drill pipe 61 is detached 22 and rai8ed to the surface. BOP stack 60 is a high pressure stack which 23 permit9 completlon of the well through the producing zones. It normally 24 i8 designed to operate at pressures as high as 10,000 psi. The stack 25 Jhown in FIG. 3 conJists of four ram type preventer~ 64 a, b, c, and d 26 and two annular preventers 65 and 66.
27 FIG. 12, which is a sectional view along line 12-12 of FIG. 3, 28 ghow8 the alignment of the riser support at frame 16 when the riser i9 29 in the set aside de. The rlser is attached to support post 55a, 55b

30 and S5- by hydraulic connectors 56a, 56c ant 56d.

... . . . , .

1 Uith the high pressure 80P stack in place, riser 17 can ~ow be ~ moved from the set aside position (FIG. ~ to the conventional position 3 (FI~. L~ above ~OP stack 6n. ~IG. 11, which is a sectional view along 4 ]ine 11-11 o~ FIG. 4, shows the alignment of riser and support frame 16 S when the riser is in the conventional position. The riser is connectet fi to sup~ort ~osts 55b, 55c, 55d and 55e by connector 56b, 56c, 56d and 7 5~e and connector 5~a now attaches to BOP stack 60. Once again, frame 16 ~ transmits the induced riser loads into the subsurface and structural 9 casing, substantiallv reducing the load on BOP stack 60.
n As mentioned above, the riser is returned by detaching hydraulic 11 connectors 56a, 56c and 56d, uplifting the riser and moving it laterally 12 so that it aligns with BOP stack 60, and then lowering the riser and ~ attaching it to the BOP stack and staek isolation frame 16. Means for 16 uplifting and moving the riser may be provided by the shipboard tensioners 15 and pulley system and the riser lift mechanism. As mentioned previously, lfi llft mechanism 51 assists the tensioners in elevating the riser off the 17 stack isolation fra~e but primarily serves to guide the riser to its 1~ proper position above the BOP stack. ~ydraulic cylinder 52 retracts 19 connecting rod 53 which is connected to liftin~ arm 54. As shown in 2n FIGS. ll and 12, the liftin~ arm 54 is attached to the riser by pins 59 21 which are attached to connector 56a and connecting member 57. The upper ~2 end of the connecting member 57 is rlgidly attached to the riser 17. As 23 the riser i~ moved to the conventional position, lifting arm S4 rotates 24 about sup~ort member 58 of fr2me 16.
~5 Once riser 17 is in the conventional mode shown in FIG. 4, 26 normal arilling operations can be conducted. FIG. 5 schematically ?~7 illustrates drilling operations from vesQel 10 with riser 17 in the 2~ conventional position above ~OP stack 60. Drilling is conducted through 2q rlser 17, the riser seLving to house drill pipe 7n and to conduct drilling 30 fluids ~ack to vessel 1~ in the annulus between the riser and trill

31 plpe.

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1 As can be seen from FIG. 5, pulleys 26 have been moved along 2 rail ~uide 29 to repositon riser 17 beneath the rotary table 71 of 3 derrick 20. In addition to outer barrel 28, slip joint 23 includes an 4 inner barrel 72 slidable within the outer barrel. Seal members (not 5 shown! prevent drilling fluid contained within riser 17 from escaping 5 between the inner and outer barrels. Inner barrel 72 is pivotally 7 connected to vessel 1~.
~ In the preferred embodiment of the present invention riser 17 9 ls a small diameter riser having an lnside diameter of about 13 inches 1~ or less. T~us the well can be completed in a conventional manner by 11 drilllng through this riser with a 12-1/4 inch drill bit and then running 12 and cementinR 9-5l8 inch casing into the wellbore.
1~ The riser set aside system o~ the present invention per~its 14 the efficient use of smsll diameter risers by eliminating the need to 15 return the riser to the surface during the entire drilling operation.
16 ~liminating riser round trips saves from two to twenty da~s depending 17 upon water depth. The resultant savin~ in rlser handling ti~e using the l~ 9et aside system in deep water can be as much as 40 days.
19 Small diameter risers weigh considerably less than large ?,n diameter rlgers in deep water and require significantly ~ess tensioning. ;~
?.1 As illustrated in Table I, which comoares riser weight and shipboard 2~ tension .for 13 inch and 17 inch diameter risers at water depths of 3000 23 eet and gnno feet, the differentials are not proportionate to water 24 depth- ~eduction ln riser weight with the 13 inch riser varies from 100 25 kip8 at ~000 feet to 1450 kips at ~Q00 feet. Thus a three-fold increase .
2fi in water deoth results in ~reater than a fourteen-fold increase in 27 wei~ht differentlal. ~iser tension necessary to support the riser is 2~ reduced 1~0 ki~s at 3000 feet and 700 klps at 9000 feet ~hen t~e ~maller 29 diameter rlser 19 used. ThUJ there i~ a considerable incentive to use ; 3~ smaller dia~eter risers in deep water.

1 TAB~E I
2 13 Inch I.D. Ri9er 17 Inch I.D. Riser 3 ~ater Depth (feet~ 3000 9000 3000 9000 4 Riser Uei~ht (kios*~ 1100 4600 1200 6050 Tension (kips*~~0 1400 960 2100 ~ *l kip eauals 1000 pounds of force.

7 ~nother embodiment of the present invention is schematically ~ illustrated in FIÇ~. 6 and 7. In this embodiment marine riser 80 i9 9 eaui~ed ~ith an inflatable disconnect platform 81 which i9 at a distance ln of about 3nn to sno feet ~elow the surface. Initially drill ship 82 is 11 attached to the riser te.g., as shown in FIG. 1). The drill ship then 12 pumps air into a conventional disconnect structure 81 until the overall 1~ buoyancv of riser 80 per~its it to be self standing. The drill ship then 14 dlsconnects the rlser at the disconnect structure and moves sufficiently far awav to oermit workboat vessel 83 to position itself over riser 80 and lfi connect with it. nrilling fluids are conditioned on vessel 83 and returned 17 to dsill shio 82 through umbilical hose bundle 84. ~ose bundle 84 may also 1~ contain the ~0~ control and hydraulic lines so that BOP stack 85 can be 19 controlled from the drill ship via vessel 83, riser 80 and diverter line . .
2~ R6.

?.1 A urther reinement of this embodiment is shown in FIG. 7.

22 Workboat vessel 83 has inserted two anchors 88a and 88b in the sea floor.

Anchor lines 89a and ~b, respectively attached to anchors 88a and 88b, are , ~ .
24 secured at their uoper ends to riser disconnect platform 81. Rose bundle 2~ 84 is attached directly to the disconnect platform and buoy 90 i9 positioned ?6 above the riser to indicate its location. Once riser 80 i9 secured in ; 27 plac-, orkboat vess-l 83 ~a~ detach from the riser and, as shown, depart 28 to per~or~ other functions.

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1 ~s illustrated in ~IGS. 6 and 7, the two vessel svstem permits 2 riser ~ to be positioned a safe distance away from drill pipe 87. If 3 swift sea currents are prevalent the~ the cursents will cause bending of 4 both the drill pipe and riser when the riser is in the set aside mode.
5 ~owever, drill pipe 87 being lighter and more fle~ible than riser 80 fi will bend a greater distance from the vertical than riser 8~. If the 7 riser and drill pipe are closely ad;acent; as with a single vessel 8 svstem (e.g., See ~IG. 1), then there exists the possiblility that the ~ bendin~ drill pipe will hit against or interfere wlth the riser.
ln Positioning the riser a di~tance from the drill ship necessi-11 tates bending the riser slightly. ~owever, at depths of 9000 feet, it l? is only necessary to flex the riser at an angle of about 3 from the 13 vertical to permit a 50n foot separation between the top of the riser 14 and drill ship. ~aturally, at shallower depths the riser can be posi-15 tioned closer to the drill ship because there will be less bending of 16 the drill string.
17 In waters where sea currents are sufficiently ~trong to move lR the lower end of the riser, another embodiment of the present ~invention 19 may be the preferred approach. In this embodiment the support means for ~n the riser is eliminated. A drilling sequence utilizing this embodiment 21 is shown in FIGS. 8 through 10 and is described below.
22 TIG. 8 shows a high oressure, lQ-3/4 inch 80P stack 112 with 23 four ram preventers attached to and supported on wellhead 110. The BOP
24 stack has a support frame 113 which provides the necessary structural 25 support for the BOP stack when it is in a state of tension under rlser 26 induced loads. Support frame 113 attaches to wellhead support base 114 27 by means of four hydraulic connectors 115a, 115b and two others not in 28 ~iew. ~onnecting the BO~ stack to wellhead 110 is hydraulic connector 29 116. P~ser 117 is supported above BO~ stack 112 by riser frsme 118 30 whlch ls part of ~OP support frame 113.

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1 Drilling of the 26 inch hole can proceed bY drilling through 2 riser 117 with an underreaming tool. nrilllng is conducted in a conven-3 tional manner with drilling fluid circulating down through the drill 4 strin~ and back up the riser. Once the 26 inch hole is drilled it is necessary to remove both BOP stac~ 112 and riser 117 in order to install 6 and cement 20 inch surface casing since both the BOP stack and riser 7 bave inside diameters of less than 20 inches.
~ As is illustrated in FIG. 9, the BOP stack and riser is tem-9 porarilv moved aside bv disconnecting the hydraulic connectors which ln attach RnR stack 112 and its support frame 113 to wellhead 110 and its 11 su~ort base 114. In this manner, the lower end of BOP stack 112 is 12 still attached to riser 117 and riser frame 113.
13 If sea currents are sufficiently strong near the ocean floor, 14 the B~P stac~ and lower end of the riser, if uplifted slightly, will drift downcurrent, awav from the wellhead. ~owever, the 30P stack and 16 frame, which can weigh as much as 400,000 pounds, may have a substantial 17 anchoring effect on the riser. If sea currents are not strong enough to 18 deflect both the riser and BOP stack a safe distance (several hundred 19 feet? from the wellhead lln, then au~iliary propulsion systems can be 2~ emDloved. ~or e~am~le, as shown in FI~. 9, a thruster system can be 21 deploved at the lower ~oint of riser 117. Bypass line 121 can be used 22 to divert a ~luid, such as sea water, under pressure from the riser into 2~ ~et nozzle 122. The fluid is e~ected from nozzle 122 and help~ propel 24 the riser and BOP stack away from the wellhead in the direction indicated.
The nozzle should be a steerably rotable nozzle so that the riser can be 2~ deflected in anv direction.

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1 The up~er end of r~ser 117 i9 laterally ~ovet on drllling vessel 1~0 utilizing a pullev and rail guide system (generally lndicatet ~ by numeral 125~ in the manner previously described. Once the upper end L of the riser is ~oved out from under rotary table 126, 20 inch casing can be run (~arallel to the riser~ down from the drilling vessel and h into the well.
7 After the 20 inch casing has been run into the well and cemented ~ in p1ace, the up~er end of the riser is returned to the rotary table.
9 The BOP stack is then reconnected to the wellhead to return it to the m confi~uration shown in FIG. 8. Peconnection can be accomplished by 11 utilizing some of the recently developed guidelineless re-entry systems.
12 ~or e~ample, syste~s eaui~ped with sonar, television and altimeters can 1~ be incorporated in the subsea structural equipment such as the BOP stack lh and fra~e. Displays and readouts transmitted to the surface by the s~stem would indicate how far and in what direction the drilling vessel lh would hsve to move in order to compensate for the sea current, thereby l? per~itting positioning of the BOP stack directly over the wellhead.
1~ ~nce in position, the 80P stack is reattached to the wellhead.
lq After the BOP stack has been reattached to the wellhead, a 2n 17-1/~ inch hole is drilled to a depth of about 2000 to 4000 feet by 2~ drillinR through the riser, a~ain using an underreaming tool and circu-~ latin~ drillin~ fluids in a conventional manner. To install and cement 23 13-3/8 inch conductor casing in the 17-1/2 hole it is necessary to ~et 24 the riser aside. Since the BOP stack has 8 18-3/4 inch inside diameter the stack can re~ain in ~lace during the second casing installation.
~6 As illustrated in FIG. 10, the ne~t step is the disconnection 27 of riser 117 from riser frame 118. This is accompli~hed by releasing 2R connector 119 and allowing the riser to drift downcurrent a safe distance ~q fro~m the B0P stack and wellhead. Without being weighted down by the BOP
8n st-ck, the riser should be able to drift a safe distance from the BOP

' .

1 stack without the need for propulc~on assistance ~rom nozzle 122. Once ~ again, the upper end of the riser is laterally moved aboard vessel lOO
3 usin~ pulley and rail guide svstem 125.
4 After the riser is set aside, 1~-3/8 inch conductor casing is 5 run into the well from the drilling vessel and then cemented in place.
6 IT~ing the DreviouslY described re-entry techniques, the riser is then 7 returned to its original position and reattached to the BOP stack. From 8 this ~oint on, the well is co~pleted in a conventional manner since the 9 equipment will pass through a 13 inch diameter riser. The riser can m remain in the ~osition shown in FIG. ~ during remaining drilling opera-11 tions, 1~ The advantage of the above embodiment is that it permits the 13 riser to be set aside without the use of support posts. Furthermore, 14 since all trilling is conducted through the riser using underreaming 15 tools it is not necessary to dlvert drilling fluids into the riser when 16 in the set aside de, thereby eliminating the use of a low pressure BOP

17 stack and diverter line. Thus, the riser is only set aside when casing 1~ or tools, having a diameter larger than the tiameter of the riser or BOP

19 ~tack, are to be insertet into the well. This embodiment, however, is 2Q limited to situations in which sea currents are capable of deflecting ?1 the riser a safe distance from the wellhead.

?.2 FI~. 13-25 illustrate another embodiment of this invention wherein both the riser~and BOP are set aside. Referring to FIG. 13, the first ste~ in the ~ractice of this embodiment is to lower a temporary ?.5 guide base 13n from a drilling vessel (not shown) using a conventional 26 double "J" running tool 138 with a ~ilot bit 131 and a 48 inch underreamer 27 1~2. ~be guide base 13~ is defined b~ base plate 133 with attached toe 2~ s~ikes 134. Sup~ort members 135 are arranget about a guide sleeve 136 29 of the temporary guide ba~e and are attached to the base plate 133. At 3~ the uppermo~t end of the guide sleeve is a flared seat 137 which aids in ~-- , . . ;~; . : . .. :

1 running drilling tools and casing through sleeve 136. Sonar reflectors l~q are attached to the hase plate to further aid in guiding equipment ~ through the sleeve 1~ he interior of the guide sleeve 136 includes 4 "J" shaped lu~ slots (not shown) for enRa~ement with the running tool 5 1~.
A 4~ inch bore hole is ooened using the Pilot bit 131 and the 7 underreamer bit 132. The ~ inch hole is drilled to a sufficient depth ~ to enter a comPetent formation, typically a depth of about lO0 feet.
q ~he drilling is desirablY accomplished by using sea water as drilling 1~ ~luid. ~uring drilling with sea water, there generally is no return of 11 cuttings to the 10atln~ vescel.
12 ~eferring to FIG. 14, after the well has been drilled to the 1~ desired deptb, the drill bits and drill strinR are returned to the 14 vessel and a permanent guide structure 145 and 42 inch casing 146 are 1~ lowered with drill pi~e 147 to the temporary ~uide base 130. A retrievable lfi television and sonar head 153 and a ce~ent shoe 152 are attached to the 17 lower end of drill pipe 147. The permanent base 145 and casing 146 are lR lowered until the permanent guide base rests on temporary base 130.
1~ The ~uide base 145 is defined by a sleeve 159, support members 2~ 14q which are arranged about sleeve 15~ to provide support for guide ~ -21 posts 155a, 155b, 155c, 155d, 155e, and 155f (155d, 155e, and 155f are 22 not ghown in view in FIr~. 14), a plate 160 which provides additional 23 ~up~ort ~or the ~uide Posts, and two set aside guides 200. (Only one 24 ~et aside ~uide is in view in FI~. 14.) 2~ After the 42 inch casing has been run into the wellbore, the 2fi casin~ is cemented. A ~6 inch hole is then drilled to a depth of about 27 ~nn feet usin~ a Pilot bit and 3fi inch underreamer. Once the 36 inch 2~ hole i~ drilled, ~n inch casing is run into the 42 inch casing and 36 2~ inch hole. ~he 3~ inch ca~ing is then cemented.

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10630~7 1 After the ca9ing is suitablv cemented, a ~oP assembly, iden-2 tiied in FIG. 1~ bv the nu~eral lfil, is lowered onto permanent guide 3 base 1~5 bv a 14 inch riser 180. The ~OP assembly is defined by frame 4 16R, R~P stack 1~?, set aside guides 2~ running tube 176 and hydraulic 5 assemblies 148. The ~OP frame 168 includes four support columns 167a, 6 lfi7b, 167c, and 167d (FIG. 15 does not show 167c and 167d) which attach ? to sup~ort ~osts 155a, 155c, 155d, and 155f of the guide base 145 bg 8 hvtraulic connectors 174. ~rame 168 transmits the induced riser loads g into the base 145 and casing 146, substantially reducing the load on BOP
1~ stack 162. FI~,. lR, which is a sectional view along line 18-18 of FIG.
11 1-5. shows four ~OP support columns 167a, 167b, 167c, and 167d connected 12 to base 145 bv hydraulie connectors 174a, 174b, 174c and 174d. The 1~ blowout preventer stack 162 ineludes annular blowout preventers 171 and 14 17~, shear ram 163 and pipe rams 164, 165, and 166. Set aside frames lS ~2 are attached to the upper end of the ~OP assembly for use in setting 16 aside the riser. The running tube 176 helps guide tools through the 17 ~ui~e base 145 when the blowout preventer assembly is in a set aside 1~ po~ition. The top of the running tube 17~, as ~hown in FIG. 15, has a 19 frustro-connical pro~ection 177 to acilitate running of tools into the ~-~n runnin~ tube. Also attached to the BOP frame are hydraulic pin assemblies 21 143 which house pins that are designed to egtend and retract.
22 FIG. 25 shows a horizoneal sectional view of a pin assembly 23 143 with pins 140, 141 and 142. This sectional view is taken along 24 li~es ?.5-2S of F}G. 22. These pins may be extended or retractet by .
25 suitable h~draulic cylinders 15Q to engage the set aside frame 200. The 26 pin assemblles and the set aside frames 200 will be describet ln more 27 detall hereinafter. -~
2R A frame 185 t9hown in ~IG. 15) is attached to the lower end of 9 the marine riser 1~ to provite support for equipment used in set aside 30 o the riser. A running tube 183 and two hydraulic pin a~semblies 188 1 are attached to the frame 1~5. The running tube ~9 used in guidln8 ~ tools throu~h the BOP stack when the riser is in the set aside position.
3 ~Jhen the riser is positioned above the BOP assembly, as shown in FIG.
4 1~, the running tube 1~3 is in a~ial alignment with the running tube 176 o~ the BOP stac~. The pin asse~blies 188 aid in setting aside the fi riser. The pin assemblies 188 have substantially the same design and 7 function as the Din assemblies 143 of the BOP assembly.
8 ~he riser, as shown in FIGS. 15 and 16, is ln axial alignment 9 with the BOP stack and is connected to the BOP stack by hydraulic connector 179. The riser also includes a conventional flexible Joint 11 17~ which allows the riser to flex. Conventional kill and cho~e lines 12 181 extend from the BOP stack to the floating vessel (not shown).

1~ Cnce the BOP assembly is in normal operating position, as 14 shown in ~IG. 15, a 26 inch underreamer is run through the riser and 15 wellhead assemblv to drill a 26 inch hole for 20 inch casing. The ter~
lh "wellhead assembly" as used herein includes the temporary guide base 17 13n, per snent guide base 145, and the BOP assembly 161. After the 26 1~ inch hole is drilled to a depth of about 800 feet, the drill string and 19 underreamer are returned to the vessel.
~n Before running 20 inch casing in the wellbore, the 14 inch ?1 riser and the 1~ 2 inch BOP are set aside. The basic steps for setting 22 aside the BOP assembly in d ude detaching hydraulic connectors 174a, 23 174b, 174c, and ~74d, extending pins 140 and 141, uplifting the riser and ~OP assemblv until pins 141 are in the upper position of ~et aside 25 frame (shown in ~IG. 4) retracting pins 140 and extending pins 142, and 2fi then lowering the riser and BOP to the position shown in FIG. 16. Means 27 for upliftin~ and moving the riser may be provided by shipboard systems 28 as previou~ly described.

.

--" 1063017 1 FIG. 18 sh~ws the ~OP and the permanent guide base 145 in 2 conventional operatin~ oogltion. ~IG. 19, which is a sectional view 3 along line 1-19 Of FIG. 16, shows the BOP assemblv and guide base 145 4 when the riser and BOP are in the set aside mode. ~eferring to FIGS. 18 and l~, when the riser and BOP are in the set aside mode, the BOP columns 6 1~7a and 167c are attached to support post 155b, 155e bv connectors 174a 7 and 174b, and the ~OP stack 162 is attached to support column 191 of the ~ ~uide base 145.
4 The mechanism ~or moving the riser and BOP to the set aside ln mode may be more clearlv e~lained by reference to FIGS. 20-23 which 11 depict a vertical ~ectional view of the set aside frames 200. Referring 1~ to FI~. 2n, tbe first step is to extend pins 140 and 141. Pins 141 are 13 extended into grove lL4 of the set aside frames. The second step is to 14 u~lift the riser and BOP until the pins 141 are at the top of slot 144 1~ as shown in FIn. 21. The third step is to retract pins 140 and extend lh pins 142 as sbown in FIG. 22. The 80P and riser are then lowered. Pins 17 142 force pins 141 down the slot 144 in the other side of the frame 200 1~ as shown in FIG. 23.
19 ~ith the riser and BO~ in the set aside position, 20 inch ~ -2n casin~ can be passed through running tube 176, through the guide base 21 145 and into the ~0 inch casing and 26 inch wellbore. The 20 inch 2~. casinR is cemented in a conventionsl manner. Fluids displaced by the 23 cement durin~ the cementing operation mav be returned to the vessel 24 through conduits (not shown) which connect the riser with the casing or, if the fluids are nonpolluting, the fluids may be dumped ~nto seawater.
2fi After the 20 inch casing has been cemented, the BOP assembly 27 and riser are returned to the normal operating position as depicted in 2~ ~J~. 14. ~o return the BOP and riser assembly to this position, the 2q riser and ~OP are u~liftet with the pins in the position shown in FIG.
3n 23. Once pi~s 141 are at the top o~ the slots 144, pins 142 are retracted 31 and pins 14~ are extended. The riser and BOP are then lowered to the

32 conv-entional operatin~ position.

' ~ ' - ., : ' , ,: ., : : ~ . :.

1 With the BOP in conven~ional operating position, drilling is 2 continued. A 17-1/2 inch hole for 13-3/8 inch casing is drilled to a 3 depth of about h,~0~ feet. To run the 13-3/8 inch caqing, it is necessary 4 to again set aside the 14 inch riser. The BOP stac~, however, can 5 remain in its con~entional operating position. The steps for setting 6 aside the riser include detaching the riser from the BOP assembly, 7 extending suitable pins in hqdraulic assembly 1~8 to engage set aside 8 ~uides 2n2, u~lifting the riser, retracting suitable pins in assembly 9 1~ when the pins are at the top of the slot in the frames 202, lowering 1~ the riser to the position shown in FIG. 17 and attaching to the BOP
11 assemblv. ~ins of the hydraulic assembly 188 engage set aside frames 12 2~2 in the same manner ac previou~ly described for set aside of the BOP
13 assemblv. ~7hen the riser is in the set aside positon ~shown in FIG.
14 17~, the running tube 1~3 of the riser frame 185 is in axial alignment 15 with the BOP stack. After the 13-3l8 inch casing is run and cemented, lfi the riser is returned to the ~osition shown in FIG. 15. Drilling is 17 then continued in a conventional manner. The riser does not need to be 1~ 9ee aslde a~ain because casin~ and tools used in further drllling will 1~ pa9s throug~ the riser and the wellhead assembly.
2~ It will be ap~arent from the foregoing that the present inven-21 tion of~ers si~nificant advantages over deep water riser systems previ-22 ou81y known in the art. While the present invention bas been described 23 primarily with re~ard to the foregoing embodiments, it should be under-, 24 9tood that the pre9ent invention cannot be deemed limited thereto but 25 r-ther mu~t be construed as broadly as all or any equivalents or combina-26 tion8 thereof.
; :

~.~"~

~ j _ 23 -.

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Claims (40)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A riser system extending from a floating drilling vessel to a subsea well having a wellhead comprising a substantially vertical riser pipe having its upper end supported on said vessel;
subsea support means positioned in the vicinity of said well-head, the lower end of said riser being supported by said support means; and means for moving the lower end of the riser between a first position on said support means ant a second position on said support means so that the riser in the first posi-tion is in axial alignment with the wellhead and the riser is the second position is laterally removed from axial alignment with the wellhead; and means aboard said vessel for maintaining the ricer pipe in tension.

2. Apparatus defined in claim 1 wherein said wellhead has attached thereto a blowout preventer stack.

3. Apparatus defined in claim 1 wherein said subsea support means includes a frame which provides structural support for said riser pine.

4. Apparatus defined in claim 3 wherein said means for moving said riser includes guidance means interconnecting the lower end of said riser and said frame.

5. Apparatus defined in claim 4 wherein said guidance means includes hydraulically actuated lifting means.

6. Apparatus defined in claim 1 which further includes fluid communication means between said subsea wellhead and said subsea support means.

7. Apparatus defined in claim 6 wherein said fluid commun-ication means includes a diverter pipe which diverts fluid from said wellhead to said subsea support means.

8. A riser system for interconnecting a subsea wellhead and a floating vessel comprising:
(a) a substantially vertical riser pipe having its upper end supported on said vessel and its lower end detachably connected to said wellhead;
(b) subsea support means for supporting said lower end of the riser, the support means positioned adjacent said well-head;
(c) means for detaching the lower end of said riser pipe from said wellhead;
(d) means for moving said riser pipe so that the lower end of said riser pipe can be placed in supporting relation-ship with said subsea support means; and (e) means for returning the lower end of said riser pipe from subsea support means to said wellhead; and (f) means aboard said vessel for maintaining tension in the riser pipe with the riser pipe in supporting relationship with said support means.

9. Apparatus defined in claim 8 wherein said wellhead has attached thereto a blowout preventer stack.

10. Apparatus defined in claim 8 wherein said subsea support means includes a frame which can provide structural support for said riser pipe.

11. Apparatus defined in claim 8 wherein said means for detaching the lower end of said riser pipe includes hydraulic connectors.

12. A riser system for interconnecting a subsea wellhead and a floating vessel comprising:
(a) a substantially vertical riser pipe having its upper end supported on said vessel and its lower end detachably connected to said wellhead;
(b) subsea support means for supporting said lower end of the riser, the support means positioned adjacent said wellhead;
(c) means for detaching the lower end of said riser pipe from said wellhead;
(d) means for moving said riser pipe so that the lower end of said riser pipe can be placed in supporting relation-ship with said subsea support means, said means for moving said riser pipe includes tensioning means aboard said floating vessel which attaches to the upper end of said riser pipe and which maintains said riser in tension; and (e) means for returning the lower end of said riser pipe from subsea support means to said wellhead.

13. Apparatus defined in claim 12 wherein said tensioning means further includes means which are adapted with said tensioning means to permit the lateral movement of the upper end of said riser pipe.

14. Apparatus defined in claim 8 wherein said means for moving said riser pipe includes guidance means attached to the lower end of said riser pipe.

15. Apparatus defined in claim 8 which further includes fluid communciation means between said subsea wellhead and said subsea support means.

16 Apparatus defined in claim 15 wherein said fluid communication means includes a diverter pipe which diverts fluid from said wellhead to said subsea support means

17. In a method of conducting deep water well operations from a floating drilling vessel wherein a riser pipe is attached at its upper end to said floating vessel and at its lower end to a subsea well-head, the improvement comprising:
(a) detaching the lower end of said riser pipe from said subsea wellhead;
(b) laterally moving said riser pipe so that the lower end of said riser pipe is positioned a distance from said subsea wellhead; and (c) lowering casing into the well, said casing having a larger outside diameter than the inside diameter of the riser pipe; and (d) maintaining said riser pipe in tension with said riser pipe positioned said distance from said subsea wellhead.

18 In a method of conducting deep water well operations from a floating drilling vessel wherein a riser pipe is attached at its upper end to said floating vessel and at its lower end to a blowout preventer stack, said blowout preventer stack positioned above and attached to a subsea wellhead, the improvement comprising:
(a) detaching the lower end of said blowout preventer stack from said subsea wellheat;
(b) laterally moving said riser pipe with said blowout preventer stack attached thereto 80 that the lower end of said riser pipe and said blowout preventer stack are positioned a distance from said subsea wellhead; and (c) lowering casing into the well, said casing having a larger outside diameter than the inside diamter of the riser pipe.

19. A wellhead assembly for a subsea well which comprises a frame having first support means adapted to support the lower end of a marine riser in a position which is in general axial alignment with said well and second support means adapted to support the lower end of a marine riser in a position laterally removed from the axially aligned position, thereby permitting the intro-duction of tools into said well by bypassing said marine riser; and means for moving said riser between said axially aligned and laterally removed positions.

20. A wellhead assembly positioned on the floor of a body of water and connected to a floating vessel by a riser, which comprises support means for supporting the lower end of the riser in a first position and then in a second position, said support means with the riser in the first position being adapted to permit the lowering of equipment through the riser and into the well, and said support means with said riser in the second position being adapted to permit the lowering of the equipment into the well without passing through the riser; and means for moving the riser between said first position and said second position.

21. A wellhead assembly for use in well operations positioned on the floor of a body of water and connected to a floating vessel by a riser, which comprises a blowout preventer assembly attached to the lower end of the riser;
guide means attached to the blowout preventer assembly for guiding the lower end of the riser between a first posi-tion and a second position, the riser in said second position being in axial alignment with the well and the riser in said second position being laterally removed from the axis of said well;
a support means positioned below the blowout preventer for supporting the blowout preventer assembly; and a guide means attached to the support means for guiding the blowout preventer assembly between a first position and a second position, the blowout preventer in the first position being in axial alignment with the well and the blowout preventer in the second position being laterally removed from the axis of said well.

22. In a method of conducting deep water well operations from a floating drilling vessel wherein a riser pipe is attached at its upper end to said floating vessel and at its lower end to a subsea support means positioned adjacent a subsea wellhead, the improvement comprising:
(a) maintaining said riser pipe in tension with said riser pipe attached to said support means;
(b) detaching the lower end of said riser pipe from said sub-sea support means;
(c) laterally moving said riser pipe to a position which is in axial alignment with said subsea wellhead; and (d) attaching the lower end of said riser pipe to said subsea wellhead.

23. Method defined in claim 22 wherein said subsea wellhead attached thereto a blowout preventer stack.

24. Method defined in claim 22 wherein said subsea support 18 includes a frame which can provide structural support for said riser pipe.

25. In a method of conducting deep water well operations from a floating drilling vessel wherein a riser pipe is attached at its upper to said floating vessel and at its lower end to a subsea support positioned adjacent a subsea wellhead, the improvement comprising:
(a) detaching the lower end of said riser pipe from said subsea support means;
(b) laterally moving said riser pipe to a position which is in axial alignment with said subsea wellhead;
(c) attaching the lower end of said riser pipe to said sub-sea wellhead;

(d) detaching the lower end of said riser pipe from said subsea wellhead;
(e) returning said riser pipe to a position which is in supporting relationship with said subsea support means; and (f) reattaching the lower end of said riser pipe to said subsea support means.

26. Method defined in claim 25 which further includes the steps of detaching the upper end of said riser pipe from said floating vessel and moving the upper end of said riser pipe a distance away from said floating vessel.

27. Method defined in claim 26 which further includes the step of attaching said upper end of said riser pipe to a workboat vessel which is positioned a distance away from said floating vessel.

28. The method defined in claim 26 wherein said riser has been made sufficiently buoyant to permit it to be self standing after it has been detached from said floating vessel.

29. The method defined in claim 28 wherein said upper end of said riser is secured at a distance away from said floating vessel by the use of anchoring means.

30. In a method of conducting deep water well operations from a floating vessel wherein a rises pipe is attached at its upper end to a floating vessel and at its lower end to a subsea wellhead, and wherein a drill string extends through said riser pipe into said wellbead to perform drilling operations, the improvement comprising:
(a) withdrawing said drill string from said subsea wellhead and said riser pipe;
(b) detaching the lower end of said riser pipe from said subsea wellhead;

(c) laterally moving said riser pipe to a position which aligns in a supporting relationship with a subsea support means positioned adjacent said subsea wellhead, and (d) attaching the lower end of said riser pipe to said subsea support means.

31. Method defined in claim 30 which further includes running casing having a diameter larger than the inside diameter of said riser pipe into said subsea wellhead.

32. Method defined in claim 30 which further includes providing fluid communication between said riser pipe and said subsea wellhead after said riser pipe has been connected to said subsea support means.

33. Method defined in claim 30 wherein fluid communication between said riser pipe and said wellhead is provided by a diverter pipe which diverts fluid from said wellhead to said subsea support means.

34. Method defined in claim 30 which further includes ex-tending a drill string having a drill bit attached thereto into said subsea wellhead, said drill bit having a diameter greater than the diameter of said riser pipe.

35. In a method of conducting deep water well operations from a floating vessel wherein a riser pipe is attached at its upper end to said floating drilling vessel and at its lower end to a subsea wellhead, the improvement comprising:

(a) detaching the lower end of said riser pipe from said subsea wellhead;
(b) laterally moving said riser pipe to a position which is in supporting relationship with a subsea support means positioned adjacent said subsea wellhead; and (c) attaching the lower end of said riser pipe to said subsea support means; and (d) maintaining tension in said riser pipe with said riser pipe attached to said subsea support means.

36. Method defined in claim 35 wherein said wellhead has attached thereto a blowout preventer stack.

37. Method defined in claim 35 wherein said subsea support means includes a frame which can provide structural support for said riser pipe.

38. In a method of conducting deep water well operations from a floating vessel wherein a riser pipe is attached at its upper end to said floating drilling vessel and at its lower end to a subsea wellhead, the improvement comprising:
(a) detaching the lower end of said riser pipe from said subsea wellhead;
(b) laterally moving said riser pipe to a position which is in supporting relationship with a subsea support means positioned adjacent said subsea wellhead;
(c) attaching the lower end of said riser pipe to said support means;
(d) detaching the lower end of said riser pipe from said subsea support means;
(e) laterally moving said riser pipe to return it to a posi-tion which is in axial alignment with said subsea wellhead; and (f) reattaching the lower end of said riser pipe to said sub-sea support means.

39. A method of performing drilling operations in a well from a floating drilling vessel through a subsea wellhead comprising:
(a) lowering a low pressure blowout preventer stack to said wellhead and attaching said low pressure stack to said wellhead;
(b) supporting a riser pipe from said drilling vessel, the lower end of said riser pipe being laterally removed from the axis of said wellhead and in fluid communication with said wellhead;
(c) lowering a drilling string from said vessel through said low pressure stack and wellhead;
(d) drilling a portion of said well including circulating drilling fluid downwardly through said drill string into said well and returning said fluid to said vessel through said riser pipe;
(e) withdrawing said drill string from said wellhead;
(f) lowering casing into the drilled portion of said well;
(g) removing said low pressure stack from said wellhead;
(h) lowering a high pressure blowout preventer stack to said wellhead and attaching said high pressure stack to said wellhead;
(i) moving the lower end of said riser pipe into axial alignment with said high pressure stack and attaching the riser pipe to said high pressure stack:
(j) lowering a drill string from said vessel through said riser pipe, high pressure stack and wellhead; and (k) drilling the remaining portion of said well.

40. A method for performing drilling operations in a well from a floating drilling vessel through a base structure positioned on the bottom of the body of water which comprises lowering a blowout preventer stack to said base structure and attaching said blowout preventer stack to said base structure;
supporting a riser pipe from said drilling vessel, the lower end of said riser pipe being attached to the blowout preventer stack;
lowering a drilling string from said vessel through said riser and said blowout preventer stack;
drilling a first portion of said well;
withdrawing said drilling string from said base structure;
detaching the blowout preventer from the base structure and moving the blowout preventer laterally from the axis of the well;
lowering casing through said base structure and into the drilled portion of the well;
moving the blowout preventer into axial alignment with said well and attaching the blowout preventer to the base structure;
lowering a drill string from said vessel through said riser pipe and blowout preventer stack;
drilling a second portion of said well;
withdrawing said drill string from said blowout preventer stack;
detaching the riser from the blowout preventer stack;
moving the lower end of the riser laterally from the axis of the blowout preventer stack;
lowering casing into the second portion of the well;
moving the lower end of the riser into axial alignment with the blowout preventer stack and attaching the riser to said blowout preventer stack; and drilling the remaining portion of said well.