MARINE RISER SECTION FOR SUBSEA WELLBORE RELATED OPERATIONS

Abstract

A drilling marine riser section comprises a main riser pipe with radially extending flanges and multiple auxiliary pipes at least comprising a choke line and a kill line. One or more clamps retain the auxiliary pipes. Further the riser section has buoyancy members. The choke line and the kill line are connected to each of the flanges in a tensile load sharing arrangement, so that—in vertical use orientation of the riser section in a riser string—weight stress is distributed in the main riser pipe and the choke line and the kill line. The buoyancy members form an exterior of the riser section including diametrically opposed and parallel flat bottom and top stacking faces relative to the axis of the main riser pipe, allowing stacking of riser sections in horizontal orientation. The choke line and the kill line are arranged diametrically opposite from one another and between the flat bottom and top stacking faces.

Claims

1. A marine riser section comprising: a main riser pipe, said main riser pipe having an axis and a length, wherein said main riser pipe is provided with a radially extending flange at each end thereof; multiple auxiliary pipes disposed on the outside of and parallel to the riser pipe, said auxiliary pipes comprising a choke line and a kill line; one or more clamps distributed along the length of the main riser pipe and secured to said main riser pipe, said one or more clamps being adapted to retain said choke line and said kill line relative to the main riser pipe; and buoyancy members, wherein the choke line and the kill line are connected to each of the flanges in a tensile load sharing arrangement with a tensile load transferring connection assembly between each of the choke line and kill line and each of the flanges, so that—in vertical use orientation of the riser section in a riser string—weight stress is distributed in the main riser pipe and the choke line and the kill line, wherein the buoyancy members form an exterior of the riser section including diametrically opposed and parallel flat bottom and top stacking faces relative to the axis of the main riser pipe, allowing stacking of riser sections in horizontal orientation with the flat bottom stacking face resting on the flat top stacking face of an underlying riser section, and wherein the choke line and the kill line are arranged diametrically opposite from one another relative to the axis of the main riser pipe and between the flat bottom and top stacking faces.

2. The marine riser section according to claim 1, wherein a tensile load transferring connection assembly comprises: a multi-stepped bore through the flange, said bore having an axis and said bore having multiple adjoining step portions, each step portion including a peripheral surface and a shoulder surface, with the axially spaced shoulder surfaces of the multi-stepped bore having stepwise decreasing diameter relative to one another when seen in direction of the tensile load on the choke line or kill line; and a multi-stepped end fitting arranged on the respective auxiliary pipe, said end fitting having a multiple adjoining step portions, each step portion including a peripheral surface and a shoulder surface, with the axially spaced shoulder surfaces of the multi-stepped end fitting having stepwise decreasing diameter relative to one another when seen in direction of the tensile load on the choke line or kill line, wherein the shoulder surfaces of the multi-stepped bore and of the multi-stepped end fitting are adapted to simultaneously contact one another so as to distributed the tensile load to be transferred over said shoulder surfaces.

3. The marine riser section according to claim 2, wherein a pair of contacting shoulder surfaces of the multi-stepped bore and end fitting are located in a plane normal to the axis of the bore.

4. The marine riser section according to claim 2, wherein a pair of contacting shoulder surfaces of the multi-stepped bore and end fitting are located in a conical plane that tapers when seen in direction of tensile load on the choke line or kill line, said conical plane having an angle of at most 20° relative to a plane that is normal to the axis of the bore.

5. The marine riser section according to claim 2, wherein a pair of contacting shoulder surfaces of the multi-stepped bore and end fitting are located in a conical plane that widens when seen in direction of tensile load on the choke line or kill line, said conical plane having an angle of at most 20° relative to a plane that is normal to the axis of the bore.

6. The marine riser section according to claim 2, wherein the multi-stepped end fitting is embodied as a nut, and wherein the respective auxiliary pipe is provided with a threaded portion onto which the nut is screwed allowing to adjust the position of the end fitting.

7. The marine riser section according to claim 6, wherein a lock nut is also provided on the threaded portion of the auxiliary pipe allowing to lock the multi-stepped end fitting nut in a desired position.

8. The marine riser section according to claim 2, wherein the bore for the auxiliary pipe in the flange has a length of between 4 inch and 8 .

9. The marine riser section according to claim 2, wherein the flange is provided with two bolt holes near each of the kill line bore and the choke line bore in the flange, one bolt hole on each side of the respective kill line and choke line, and wherein, possibly the flange is furthermore provided with two additional pairs of bolt holes for connector bolts that interconnect adjoining riser sections, said additional pairs each being arranged on opposite sides of the main riser pipe, the bolt holes of each pair being distributed between the bolt holes adjacent the respective choke line and kill line.

10. The marine riser section according to claim 1, wherein the riser pipe is a metal pipe, and wherein a clamp has a metal, discontinuous clamping band with multiple band members that are in direct metal-to-metal contact with the metal main riser pipe, wherein the clamp comprises one or more fasteners to secure the band members to one another and to create a friction clamping of the clamping band onto the main riser pipe, and wherein the friction is such that—when lifting one end of the riser section in the process of upending the riser section from the horizontal orientation into the vertical orientation—the band of the clamp holding the choke line and kill line remains in place relative to the riser pipe, at least concerning its angular position relative the main pipe.

11. The marine riser section according to claim 1, wherein the riser section has a length of at least 100 ft., and wherein the riser section is provided at intermediate locations along the length thereof with two riser gripper engageable portions having a spacing corresponding to the spacing between end portions of a 75 ft. riser section.

12. The marine riser section according to claim 11, wherein each gripper engageable portion comprises a hook member fitted to the riser section.

13. A marine riser section comprising: a main riser pipe, said main riser pipe having an axis and a length, wherein said main riser pipe is provided with a radially extending flange at each end thereof; multiple auxiliary pipes disposed on the outside of and parallel to the riser pipe, said auxiliary pipes comprising a choke line and a kill line; one or more clamps distributed along the length of the main riser pipe and secured to said main riser pipe, said one or more clamps being adapted to retain said choke line and said kill line relative to the main riser pipe, pipe; and buoyancy members, wherein the choke line and the kill line are connected to each of the flanges in a tensile load sharing arrangement with a tensile load transferring connection assembly between each of the choke line and kill line and each of the flanges, so that—in vertical use orientation of the riser section in a riser string—weight stress is distributed in the main riser pipe and the choke line and the kill line, wherein a tensile load transferring connection assembly comprises: a multi-stepped bore through the flange, said bore having an axis and said bore having multiple adjoining step portions, each step portion including a peripheral surface and a shoulder surface, with the axially spaced shoulder surfaces of the multi-stepped bore having stepwise decreasing diameter relative to one another when seen in direction of the tensile load on the choke line or kill line; and a multi-stepped end fitting arranged on the respective auxiliary pipe, said end fitting having a multiple adjoining step portions, each step portion including a peripheral surface and a shoulder surface, with the axially spaced shoulder surfaces of the multi-stepped end fitting having stepwise decreasing diameter relative to one another when seen in direction of the tensile load on the choke line or kill line, wherein the shoulder surfaces of the multi-stepped bore and of the multi-stepped end fitting are adapted to simultaneously contact one another so as to distributed the tensile load to be transferred over said shoulder surfaces.

14. A marine riser section comprising: a main riser pipe, said main riser pipe having an axis and a length, wherein said main riser pipe is provided with a radially extending flange at each end thereof; multiple auxiliary pipes disposed on the outside of and parallel to the riser pipe, said auxiliary pipes comprising a choke line and a kill line; one or more clamps distributed along the length of the main riser pipe and secured to said main riser pipe, said one or more clamps being adapted to retain said choke line and said kill line relative to the main riser pipe; and buoyancy members, wherein the choke line and the kill line are connected to each of the flanges in a tensile load sharing arrangement with a tensile load transferring connection assembly between each of the choke line and kill line and each of the flanges, so that—in vertical use orientation of the riser section in a riser string—weight stress is distributed in the main riser pipe and the choke line and the kill line, wherein the riser pipe is a metal pipe, and wherein a clamp has a metal discontinuous clamping band with multiple band members that are in direct metal-to-metal contact with the metal main riser pipe, wherein the clamp comprises one or more fasteners to secure the band members to one another and to create a friction clamping of the clamping band onto the main riser pipe, and wherein the friction is such that—when lifting one end of the riser section in the process of upending the riser section from the horizontal orientation into the vertical orientation—the band of the clamp holding the choke line and kill line remains in place relative to the riser pipe, concerning its angular position relative the main riser pipe.

15. A method for assembly of a riser string by interconnection of riser sections to compose a riser string that is adapted to extend between a vessel adapted to perform subsea wellbore related operations and a subsea wellbore, which method comprises: upending the riser section according to claim 1 from a horizontal initial orientation by connecting a leading end of the riser section to a riser string lifting tool which connects the riser section to a riser string handling capacity hoisting device of the vessel, then raising the lifting tool, and, bringing the riser section into a vertical or upended orientation in line with a firing line along which the riser string is suspended into the sea.

16. The method according to claim 15, wherein during upending the riser section from a horizontal orientation to a vertical orientation by lifting one end of the riser section the upper one of the choke line and kill line does not carry a compressive load.

17. A riser string extending, or to be assembled and extended, between a subsea wellbore and a subsea wellbore operations vessel, wherein the riser string comprises a first string part composed of interconnected first length riser sections and a second string part composed of interconnected second length riser sections, wherein the first length riser sections are longer than the second length riser sections, and wherein said first length riser sections and said second length riser sections are each provided with buoyancy members, wherein the buoyancy members of the second length riser sections have a greater depth rating than the buoyancy members of the first length riser sections, and wherein the riser string has a length of between 10.000 and 14.000 ft., wherein the first string part composed of first length riser sections extends or is adapted to extend to a depth between 7.000 and 9.000 ft., and the second string part composed of second length riser section extends or is adapted to extend to a depth of at most 12.000 ft., and—when the riser extends or is adapted to extend below 12.000 ft. —, the riser string having a lowermost third string part comprising bare first length riser sections.

18. The riser string according to claim 17, wherein each first length riser section comprises a first series of adjacent buoyancy members and a second series of adjacent buoyancy members, wherein a mid-portion between said first and second series the riser section is bare, said bare mid-portion having a length of between 30 and 60 ft.

19. The riser string according to claim 17, wherein each series of adjacent buoyancy members comprises three adjacent groups of buoyancy members, each group arranged around the main riser pipe and, possibly around one or more auxiliary pipes of the riser section, wherein a hook member is fitted to the riser section between the second and third group of each series seen from the respective end of the riser section, said hook members being adapted to lift the riser section in horizontal orientation thereof.

20. A vessel (1) adapted to perform subsea wellbore related operations involving a riser string between the subsea wellbore and the vessel, said vessel comprising a hull having a deck, said vessel comprising: a riser storage; wherein the riser storage is provided with first length storage racks adapted to store therein—in vertical stacks—single first length riser sections each having a length of at least 100 ft., and wherein the riser storage is provided with second length storage racks adapted to store therein single second length riser sections each having a length of between 50 ft. and 90 ft. which multiple first length riser sections according to claim 1 are stored in the first length storage racks.

Description

[0115] The invention will now be explained with reference to the drawings. In the drawings:

[0116] FIG. 1 shows in longitudinal view a part of a drilling vessel in the process of upending a riser section according to the invention,

[0117] FIG. 2 shows the vessel of FIG. 2 during the process of upending a riser section,

[0118] FIG. 3 shows the vessel of FIG. 1 with the riser section upended in the firing line,

[0119] FIG. 4 shows a an embodiment of a riser section according to the invention,

[0120] FIG. 5 shows one end of the riser section of FIG. 4,

[0121] FIG. 6 shows the other end of the riser section of FIG. 4,

[0122] FIG. 7 shows the end of FIG. 5 partly in cross section,

[0123] FIG. 8 shows the end of FIG. 6 partly in cross section,

[0124] FIG. 9a shows a detail of FIG. 8 on a larger scale,

[0125] FIG. 9b illustrates design options of multi-step bore and end fitting,

[0126] FIG. 10 illustrates an embodiment of a riser string according to the invention,

[0127] FIG. 11 shows an embodiment of a second length, 75 ft. riser section.

[0128] FIG. 1 shows a part of a mono-hull vessel 1 having a hull 2 with a bow, a stern, and a moonpool 5 that extends through the hull 1.

[0129] For example the vessel may have one or more of the features of the vessel disclosed in WO2014/168471 and/or in non-prepublished NL 2013614 which are incorporated herein by reference. E.g. the vessel may have one or more features of the riser storage and/or riser handling as disclosed therein. E.g. use is made of the catwalk machine as described in NL 2013614.

[0130] The vessel 1 is adapted to perform subsea wellbore related operations involving a riser string between the subsea wellbore and the vessel, in particular drilling operations, e.g. for exploratory drilling. The vessel can also perform other subsea wellbore related operations, e.g. wellbore intervention.

[0131] The moonpool 5 has, as is preferred, a rectangular shape with opposed lateral sides, a front side and a rear side.

[0132] A front main deck extends fore of the moonpool 5. A rear main deck 9 extends between the moonpool 5 and the stern of the vessel.

[0133] The vessel is equipped with a tower 10, which is, as is preferred, embodied as a hollow construction mast having a top and having a base that is integral with the hull 2. The base extends between sections of the hull on opposed lateral sides of the moonpool 5 and the base is spaced from each of the front side and the rear side of the moonpool, thereby forming a front moonpool area forward of the mast 10 and a rear moonpool area rearward of the mast 10.

[0134] In this example, drill pipe racks, here embodied as carrousel type racks 14, are located adjacent the lateral sides of the mast 10, as is known in the art.

[0135] At the rear moonpool area, the vessel is provided with a working deck 15 arranged above the rear moonpool area. As is preferred the working deck 15 is a mobile working deck, here liftable along the mast 10 to such a height that a blow-out preventer BOP can be brought and held underneath the working deck 15 in raised position thereof at an elevated position relative to the mast 10. In a lowered, operative position, the working deck 15 preferably, as here, is level with the adjacent main deck area.

[0136] In view of assembly and disassembly of a riser string along a firing line 20 through the rear moonpool area the vessel is equipped with a riser string assembly hanger 17 that is adapted to suspended therefrom a riser string in the firing line 20 into the sea during the riser assembly and disassembly process. As preferred, this hanger 17 is mounted on the working deck 15, e.g. embodied as a riser spider, e.g. provided with a gimballing support so as to allow for angular variation between the riser string and the working deck, e.g. due to sea motion of the vessel.

[0137] The vessel 1 has a riser string handling capacity hoisting device including a riser string lifting tool 25 which is movable up and down relative to the mast 10 and that is adapted to connect to an end of a riser section, and is embodied to support the weight of a riser string in the firing line 20 when released from the riser string assembly hanger 17.

[0138] The riser string lifting tool 25 here is suspended from a travelling hanger device 26 that is movable up and down along the rear side of the mast 10 along one or more vertical rails 27.

[0139] The hanger device 26 is suspended by one or more cables 28 from a sheave arrangement 29 at the top of the mast, which one or more cables 28 are connected to one or more winches, e.g. arranged within the mast 10.

[0140] It is noted that the firing line 20 is outside of the rear side of the mast 10 so that the firing line 20 can be reached without hindrance in the process of upending a riser section from the rear of the vessel.

[0141] In an alternative embodiment, the mast 10 is replaced by a derrick type tower having a latticed frame with corner posts that forms a frame extending over the moonpool. It is then envisaged that the riser storage is outside of the derrick type tower and the derrick is provided with a V-door or similar to allow passage of a riser section or riser stand into and out of the derrick. As will be apparent from this application it is envisaged that a riser section may have a length of 150 ft., thereby requiring a V-door of significant height to allow for passage of the riser section during upending and during reverse motion during tripping of the riser string.

[0142] The vessel also has a second hoisting device having a load attachment device 30 which is movable up and down relative to the mast at a side opposed from the riser firing line 20, so as to allow for handling of items passing through the other moonpool area along a second firing line 21 distinct and spaced from the first firing line 20 where the riser string assembly takes place.

[0143] The second firing line 21 extends through the front moonpool area. Along this firing line 21 primarily drilling operations are performed.

[0144] The second hoisting device is embodied as a drilling drawworks, and is provided with a topdrive 31 suspended from the load attachment device 30 to perform drilling operations. The load attachment device 30 is preferably embodied similar as the travelling hanger device 26.

[0145] A working deck 32, e.g. a mobile working deck, is arranged above the fore moonpool area and may include a rotary table, iron roughneck machine, etc.

[0146] The vessel 1 is thus capable of assembly of a riser string in firing line 20. For transfer of the riser string to the other firing line 21 a riser string support cart is provided that is displaceable within the moonpool, e.g. skiddable over rails along the lateral sides of the moonpool 5.

[0147] The vessel has a riser storage hold 40, here as is preferred, within the hull 2 aft of the moonpool 5.

[0148] The riser storage hold 40 comprises storage racks adapted to store therein parallel stacks of multiple riser sections in horizontal orientation.

[0149] The riser storage hold is provided with first length storage racks adapted to store therein single first length riser sections 85-89 (see FIG. 10) each having a length of at least 100 ft. (30.48 m), e.g. of 120 ft. (36.57 m) or 150 ft. (45.72 m). In the example depicted in the figures the first length is 150 ft.

[0150] The riser storage hold is preferably also provided with second length storage racks adapted to store therein single second length riser sections 95,96 each having a length of between 50 ft. (15.24 meters) and 90 ft. (27.43 meters), e.g. of 75 ft. (22.86 meters). In the example depicted in FIGS. 10, 11 the second length is 75 ft.

[0151] The second length storage racks may be arranged in sets of two, with the two racks being in line with one another and parallel to the adjacent longer first length storage racks.

[0152] A first group of first length storage racks 80 may be arranged adjacent one side of a transfer station and a second group of first length storage racks 80 may be arranged adjacent another side of the transfer station.

[0153] Each storage rack may comprise at ends thereof a pair of adjacent riser end support columns that form a vertical slot which is adapted to receive therein an end portion, e.g. a flange, of a riser section 85-88, 95, 96.

[0154] The riser storage hold has a floor, port and starboard side walls, and a roof. An elongated riser transfer opening is present between the deck 9 and the roof.

[0155] The riser transfer opening extends in a direction parallel to the storage racks and has a length, here of at least 150 ft., and a width so as to allow for transfer of a single riser section in horizontal orientation via the riser transfer opening out of and into the riser storage hold.

[0156] Within the storage 40 a riser transfer station is arranged below the riser transfer opening 45. The station is provided with a transfer elevator that is adapted to raise and lower a single riser section or a single riser stand in horizontal orientation thereof so as to pass the riser section or a riser stand through the riser transfer opening 45.

[0157] In the storage hold 40 an overhead travelling beam crane 60 is arranged.

[0158] The crane 60 is capable of lifting and lowering a single riser section 85-88, 95, 96, either of first length or of second length as described herein, at least allowing for removal of a single riser section from a storage rack and for placing a single riser section 85-88, 95, 96 or into a storage rack respectively. The crane 60 is also capable of transverse transportation of a single riser section 85-88, 95, 96 at least between the transfer station 50 and a position above each of the storage racks in the storage 40.

[0159] The crane 60 comprises:

[0160] a travelling beam extending in a direction parallel to the storage racks and supported at each end thereof on a crane rail perpendicular to the storage racks, here transverse to the hull

[0161] a winch trolley provided with one or more winches and displaceable along the travelling beam,

[0162] an elongated gripper frame suspended by one or more winch driven cables from the winch trolley. The gripper frame is provided with two riser grippers that are each adapted to engage on a single riser section 85-88, 95, 96 at spaced gripping locations thereof.

[0163] The gripper frame 68 is provided with two riser grippers that are adapted and arranged to engage on hooks 95h1, 95h2 that are fitted on the end portions of second length riser section, here on end portions of a 75 ft. riser section. As can be seen in FIG. 11 it is envisaged that at one end there may be two hooks, directed in diametrically opposed directions; one downward and one upward when the riser section is stacked on a flat surface portion thereof. These flat surface portions are formed by buoyancy members of the riser section and are as preferred provided alongside the choke line and the kill line of the riser section.

[0164] The two riser gripper engageable portions, here pairs of hooks 95h1, 95h2, 140, have a spacing the same as the spacing between hooks 85h1, 85h2, etc., arranged on the longer riser sections 85-88 so as to allow said two riser grippers to engage on said gripper engageable portions.

[0165] The crane 60 is also adapted to transfer a second length riser section between each of the second length storage racks and the transfer station, and to transfer a first length riser section between each of the first length storage racks and the transfer station.

[0166] The transfer elevator may comprise one or more elevator units, e.g. two units spaced apart in direction parallel to the storage racks.

[0167] The vessel, e.g. the riser storage hold 40, may be provided with one or more elongated riser workshops, each having a length at least sufficient to receive therein a first length riser section or stand. Each riser workshop has a floor, and, as is preferred also walls and a roof.

[0168] Each riser workshop is preferably arranged parallel to the storage racks and the workshop is adapted to accommodate at least one riser section 85-88, 95, and 96 in horizontal orientation.

[0169] The vessel is preferably provided with movable hatches which in a closed position thereof close the transfer opening 45 and in an opened position thereof open the transfer opening. e.g. pivotal hatches.

[0170] Substantially horizontal rails extend along opposite longitudinal sides of the riser transfer opening

[0171] The vessel comprises a riser horizontal transport device 200 that is mounted on horizontal rails and is adapted to receive and hold a riser section 85-88, 95, 96 that has been raised through said transfer opening by the riser elevator unit or units and to horizontally transport the riser section 85-88, 95, 96 or riser stand so that a leading end thereof is connectable to a riser string lifting tool that is adapted to support the weight of a riser string in the firing line 20 of the vessel.

[0172] The riser horizontal transport device comprises a catwalk machine having a mobile catwalk machine frame that is movable over the horizontal rails 150. The catwalk machine frame has a rear end and a front end and is movable over the horizontal rails 150 at least in a loading position generally above the transfer opening and in a riser upending position closer to the firing line 20.

[0173] The catwalk machine frame may have two parallel and horizontal frame beams. At the rear end the beams may be rigidly and permanently interconnected by a transverse beam. The beams may be less long than the transfer opening and the first length riser section that is stored in the hold 40, e.g. less long than 150 ft. In order to obtain a sturdy frame during transportation of the riser section it is envisaged that, here only at the front end, the frame beams may be interconnected by a mobile transverse connector that is movable between an inactive position allowing for vertical passage of the single riser section or single riser stand and an active position wherein the transverse connector interconnects the frame beams. When lifting and lowering a section of first length the connector is inactive or opened. A shorter second length may be handled with the connector remaining closed as the opening in the frame of the machine is then large enough.

[0174] A skate 206 is supported by the frame beams and travels over the frame beams. As is known in the art the skate 206 comprises a riser end support to support thereon a rearward end of a riser section 85-88, 95, 96.

[0175] As will be appreciated the horizontal frame beams of the catwalk machine frame define between them an opening having a width so as to allow for the vertical passing of a single riser section 85-88, 95, 96 (equipped with buoyancy members) in horizontal orientation through said opening, preferably by means of the transfer elevator unit or units.

[0176] The catwalk machine, in addition to the skate 206, may comprise one or more additional riser support members that are movable between an inactive position allowing for said vertical passage of the single riser section 85-88, 95, 96 or single riser stand and an active position wherein the riser section or riser stand is supported on said riser support member.

[0177] If desired the catwalk machine 200 is provided with a tailing-in arm device 210, e.g. with one tailing arm fitted to the front end of each beam.

[0178] With reference to FIGS. 4-9 now the first, second, third, and fourth aspect of the invention will be illustrated.

[0179] FIG. 4 depicts a marine riser section 85, in this example having, as is preferred, a length of 150 ft.

[0180] The riser section 85, which is also called riser joint in the industry, comprises a main riser pipe 85-1, which main riser pipe has a longitudinal axis and a length. In this example a 21 inch OD riser pipe 85-1 is shown, having an inner diameter of 18.75 inch.

[0181] As is preferred the main riser pipe is a continuous pipe having a wall of solid steel. In an embodiment the main riser pipe, and possibly the flanges, could e.g. be made of aluminium or another metal like titanium.

[0182] For example the main riser pipe is made of X80 steel, e.g. having a yield strength of 555 MPa.

[0183] The main riser pipe 85-1 is provided with a radially extending flange 85-2, 85-3, at each end thereof.

[0184] As is preferred and known in the art each flange 85-2, 85-3 is a steel flange welded to the continuous main riser pipe body at an axial end thereof.

[0185] The flanges could be made of the same steel as the main riser pipe.

[0186] The flanges may each have been manufactured as a single casted metal object with a main riser pipe end piece, that is appropriately machined and is welded or otherwise secured on the end of the main riser pipe body.

[0187] The figures illustrate that multiple auxiliary pipes are disposed on the outside of and parallel to the main riser pipe.

[0188] These auxiliary pipes at least comprise a choke line 85-c and a kill line 85-k.

[0189] In this example, as preferred, also a booster line 85-b is provided. Additionally one or more, here two, hydraulic lines 85-hl1, 85-hl2, are provided. As shown the two hydraulic lines are arranged closely together, diametrically opposite the single booster line of this exemplary riser section.

[0190] As can be seen, the one or more auxiliary lines 85-b, 85-hl1, 85-hl2, additional to the choke line and kill line are arranged close to, here the two hydraulic lines, or in, here the booster line, a horizontal plane intersecting the main riser pipe axis when the riser section 85 is lying horizontally with the choke line and kill line in a vertical plane through said axis.

[0191] The riser section 85 also comprises clamps 85-5 that are distributed along the length of the main riser pipe 85-1 and are secured by clamping to said main riser pipe 85-1. These clamps 85-5 are adapted to retain at least the choke line 85-c and the kill line 85-k relative to the main riser pipe 85-1.

[0192] The riser section 85 is also provided with a series of pairs of buoyancy members 85-6a, b, 85-7a, b . . . , 85-12a, b. Each of the pair of buoyancy members is semi-annular in cross-section so that the pair together makes up an annulus, with some gapes therein, around the circumference of the riser section. For example the buoyancy members are fitted by means of straps encircling the buoyancy members and pressing them onto the main riser pipe, e.g. with a resilient member in between. Alternative securing arrangements are also possible, possibly in combination with straps, e.g. axial stop members can be provided on the main riser pipe, e.g. like collars and/or integrated with the clamps 85-5 that prevent the buoyancy members from sliding axially with respect to the main riser pipe.

[0193] As shown in FIG. 4, in cross section each of the pair of buoyancy members has a semi-circular outer side. That is, the pair of buoyancy member comprises, in cross section, a circular outer edge, wherein the top and bottom of the circular outer edge are levelled off to create parallel flat bottom and top stacking faces relative to the axis of the main riser pipe.

[0194] As explained it is preferred for the riser section 85-88 to have a length of at least 100 ft. (30.48 m), e.g. of 120 ft. (36.57 m), e.g. of 150 ft. (45.72 m).

[0195] At least the choke line 85-c and the kill line 85-k are connected to each of the flanges 85-2, 85-3 in a tensile load sharing arrangement with a tensile load transferring connection assembly between each of the choke line and kill line and each of the flanges, so that—in vertical use orientation of the riser section in a riser string—weight stress is distributed in the main riser pipe and the choke line and the kill line.

[0196] As can be seen the buoyancy members form an exterior of the riser section including diametrically opposed and parallel flat bottom and top stacking faces 85-13, 85-14, relative to the axis of the main riser pipe 85-1, allowing stacking of riser sections 85 in horizontal orientation with the flat bottom stacking face 85-13 resting on the flat top stacking face 85-14 of an underlying riser section 85.

[0197] The choke line 85-c and the kill line 85-k are arranged diametrically opposite from one another relative to the axis of the main riser pipe 85-1 and between the flat bottom and top stacking faces 85-13, 85-14, as is preferred in a plane normal to the flat bottom and top stacking faces.

[0198] As can be seen, in this example, the auxiliary lines protrude at the side of the flange 85-2, whereas the at the other flange 85-3 the auxiliary lines are embodied to receive therein the protruding ends of the auxiliary lines of an adjacent riser section. This interconnection of auxiliary pipes need not be designed to transfer axial load from one auxiliary riser pipe to the next, but in embodiment a clip connector, bayonet connector or the like may be present to mechanically interconnect the auxiliary pipes of adjoining riser sections.

[0199] The tensile load transferring connection assembly between each of the choke line 85-c and kill line 85-k and each of the flanges 85-2, 85-3, is embodied such that—when upending the riser section 85 from a horizontal orientation to a vertical orientation by lifting one end of the riser section—the upper one or the choke line and kill line does not carry a compressive load. This avoid undue loading that may induce buckling of the one line that is on top of the riser section when it is being lifted at one end in the upending process.

[0200] As can be seen, e.g. in FIGS. 8, 9a, b a tensile load transferring connection assembly comprises:

[0201] a multi-stepped bore 90-c, 90-k, through the flange 85-3, which bore has an axis 90-a and which bore has multiple adjoining step portions, each step portion including a peripheral surface 90p1, 90p2, 90p3, and a shoulder surface 90s1, 90s2, 90s3, with the axially spaced shoulder surfaces of the multi-stepped bore having stepwise decreasing diameter relative to one another when seen in direction of the tensile load on the choke line or kill line, here a three stepped bore with three shoulder surfaces,

[0202] a multi-stepped end fitting 91-c, 91-k arranged on the respective auxiliary pipe 85-c, 85-k, here welded onto the end of a steel pipe 85-k.

[0203] The end fitting has multiple adjoining step portions, each step portion including a peripheral surface 91p1, 91p2, 91p3, and a shoulder surface 91s1, 91s2, 91s3, with the axially spaced shoulder surfaces of the multi-stepped end fitting having stepwise decreasing diameter relative to one another when seen in direction of the tensile load on the choke line or kill line, here a three stepped end fitting with three shoulder surfaces.

[0204] The shoulder surfaces 90s1, 90s2, 90s3 of the multi-stepped bore and the shoulder surfaces 91s1, 91s2, 91s3 of the multi-stepped end fitting are adapted to simultaneously contact one another, at least when the auxiliary line is subjected to significant tensile load, so as to distributed the tensile load to be transferred between the flange and the auxiliary pipe over these multiple, here three, pairs of mating shoulder surfaces.

[0205] As is preferred the peripheral and shoulder surfaces of the multi-stepped bore and the mating end fitting have rotational symmetry relative to the coinciding axes of the auxiliary pipe and of the bore through the flange for said auxiliary pipe.

[0206] In an embodiment at least one pair of contacting shoulder surfaces, here 90s3, 91s3, of the multi-stepped bore and end fitting are located in a plane normal to the axis 90-a of the bore.

[0207] In an embodiment at least one pair of contacting shoulder surfaces, here 90s2, 91s2, of the multi-stepped bore and end fitting are located in a conical plane that tapers, here angle α1, when seen in direction of tensile load on the choke line or kill line, said conical plane having an angle of at most 20° relative to a plane that is normal to the axis 90-a of the bore.

[0208] In an embodiment at least one pair of contacting shoulder surfaces, here 90s1, 91s1, of the multi-stepped bore and end fitting are located in a conical plane that widens when seen in direction of tensile load on the choke line or kill line, said conical plane having an angle, here angle α2, of at most 20° relative to a plane that is normal to the axis of the bore.

[0209] In an embodiment, e.g. at one end of a auxiliary pipe, the multi-stepped end fitting may be embodied as a nut, wherein the respective auxiliary pipe is provided with a threaded portion onto which the nut is screwed allowing to adjust the position of the end fitting, e.g. the auxiliary pipe having a steel pipe end piece welded on a steel main body of the auxiliary pipe.

[0210] In an embodiment a lock nut is also provided on the threaded portion of the auxiliary pipe allowing to lock the multi-stepped end fitting nut in a desired position.

[0211] In an embodiment the bore 90-c, 90-k, for the auxiliary pipe in the flange has a length of between 4 inch and 8 inch, e.g. between 5 and 7 inch, e.g. 6 inch.

[0212] The figures also illustrate that the flange 85-2, 85-3, is provided with two bolt holes 92, 93 for connector bolts that interconnect adjoining riser sections, near each of the kill line bore and the choke line bore in the flange, one bolt hole on each side of the respective kill line and choke line. The flange is furthermore provided with two additional pairs of bolt holes 94 for connector bolts that interconnect adjoining riser sections, said additional pairs each being arranged on opposite sides of the main riser pipe, the bolt holes of each pair being distributed between the bolt holes adjacent the respective choke line and kill line.

[0213] As is preferred the riser pipe 85-1 is a metal pipe, preferably a steel pipe, and the clamps 85-5 have a metal, preferably steel, discontinuous clamping band with multiple band members that are in direct metal-to-metal contact with the metal main riser pipe. The clamp comprises one or more fasteners to secure the band members to one another and to create a friction clamping of the clamping band onto the main riser pipe. This friction is such that—when lifting one end of the riser section 85 in the process of upending the riser section from the horizontal orientation into the vertical orientation—the band of the clamp holding the choke line and kill line remains in place relative to the riser pipe.

[0214] As has already been discussed with reference to shorter riser section 95, the riser section 85 has two riser gripper engageable portions, here hooks 85-h1, 85-h2 having a spacing corresponding to the spacing between hooks on the 75 ft. (22.86 meters) riser section.

[0215] A hook may be integrated with a collar that is fitted, e.g. clamped, around the main riser pipe.

[0216] Whilst the aspects of the invention are discussed with reference to riser section 85, it will be appreciated that riser sections 86, 87, 88 may, and preferably do, comprises the same structural features as the riser section 85. The main difference lies in the depth rating of the buoyancy members that are fitted on the riser sections, which are here defined by four sections each corresponding to an additional 2000 ft. of water depth as illustrated by way of example in FIG. 10.

[0217] FIG. 10 also illustrates the presence of a lowermost part of a riser string, made up from one or more bare 150 ft. riser sections 89, e.g. at a depth below 12.000 ft., e.g. to a depth of 13.200 ft.

[0218] Intermediate the 8.000 ft. and 12.000 ft. water depth the example of FIG. 10 illustrates that the lower riser string part is composed of shorter, 75 ft. length riser sections, that may have the same connection between flanges and auxiliary lines, e.g. choke line and kill line as discussed with reference to longer section 85.

[0219] These below 8.000 ft. rated riser sections are provided with buoyancy members that have such a great dry weight that their handling would become cumbersome and stress undue if their length also was 150 ft. Therefore, in view of efficient handling of all elements of the riser string, it is proposed to use the shorter length, e.g. 75 ft. for this lower part of the string. The lowermost bare sections can be longer again, e.g. 150 ft. as they have no buoyancy members due to the insignificant contribution such members would have to the buoyancy of the string.