NO20231033A1 - Mud return riser and methods of deployment and retrieval - Google Patents

Description

MUD RETURN RISER AND METHODS OF DEPLOYMENT AND RETRIEVAL

The present disclosure relates to a mud return riser for offshore drilling and to methods and systems of deploying and retrieving a mud return riser at an offshore vessel.

BACKGROUND

Conventional offshore drilling is typically carried out through a marine drilling riser suspended from a drilling vessel, such as a mobile offshore drilling unit (MODU), and this method has traditionally been the most trusted drilling concept employed for exploration drilling and subsea field production drilling. The marine drilling riser conduit extends from the top of the subsea blow-out preventer (BOP) to the underside of the drill-floor of the vessel. The marine drilling riser has several functions: to serve as a return conduit for cuttings and drilling-fluid from the well, to be used for attachment and support of ancillary lines between the subsea BOP and the MODU for well control, and to bring the subsea BOP to/from the subsea wellhead. The lower end of the drilling riser is attached, via a flexible element, to the top of a lower marine riser package (LMRP), which is connected on top of the lower BOP. A riser tensioning system keeps the riser stable, while, at the same time, compensating passively for vertical motion of the vessel. The riser can be disconnected from the lower BOP with a LMRP high-angle release connector.

In some applications, open water (sometimes also called “riserless”) drilling has been proposed. This method can have advantages for examples in ultra deep water or mature fields with narrow margins between pore-pressures and fracture-gradients in the reservoir, because of depleted and/or pressurised reservoir zones. Open water drilling systems comprise a separate mud return riser deployed separately from the drill string, and having a fluid lift system, such as a subsea pump, to return mud (i.e. drilling fluids, optionally with cuttings) to the drilling vessel.

Publications which may be useful to understand the field of technology include WO 2022/154666 A1; WO 2022/164324 A1; US 8,733,452 B2; and US 10,502,348 B2.

There is a continuous need to improve offshore drilling systems and methods, among other things for more efficient operation or for equipment design which is more compact and/or easier to handle. The present disclosure has the objective to provide such improvements, or at least to provide useful alternatives to the state of the art.

SUMMARY

In an embodiment, there is provided a mud return riser comprising a plurality of interconnected riser sections through which a plurality of pipes extend, each riser section having a lower end flange and an upper end flange for interconnection of the riser sections, wherein a connection between an upper end flange of a first riser section and a lower end flange of an adjacent second riser section is secured by means of a rotatable lock ring, each riser section having a support skirt rigidly fixed to the upper end flange and configured for engagement with a vertical friction support.

In an embodiment, there is provided a mud return riser section for a mud return riser.

In an embodiment, there is provided a mud return riser hang-off system comprising a vertical friction support and a plurality of mud return riser sections.

In an embodiment, there is provided a method of building a mud return riser.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other characteristics will become clear from the following description of illustrative, non-restrictive examples, with reference to the attached drawings, in which:

Fig. 1 illustrates a drilling vessel during an open water (“riserless”) drilling operation.

Figs.2a and 2b provide perspective side views of an upper end flange of a first mud return riser section and of a matching lower end flange of a second mud return riser section according to a first example.

Fig. 3 illustrates an opposite arrangement for interconnection of the upper end flange of the first mud return riser section with the matching lower end flange of the second mud return riser section according to the first example.

Figs.4a and 4b provide perspective side views of an upper end flange of a first mud return riser section and of a matching lower end flange of a second mud return riser section according to a second example.

Fig. 5 schematically illustrates a sectional side view of a mud return riser hang-off system comprising mud return riser sections and a vertical friction support.

Fig. 6 schematically depicts a perspective top view of the vertical friction support.

Fig. 7. schematically illustrates a sectional side view of the vertical friction support.

Fig. 8 provides a perspective side view of an adapter unit of a mud return riser.

DETAILED DESCRIPTION

The following description provides improved mud return riser concepts for so-called riserless drilling arrangements. The described mud return risers, mud return riser sections, mud return riser hang-off systems and methods for building a mud return riser may beneficially facilitate the handling and assembly of mud return risers, thus reducing the process time on mobile offshore drilling units and additionally increasing the process safety and reliability of mud return riser deployment and retrieval.

Fig. 1 illustrates a drilling vessel 100 during an offshore riserless drilling operation. A drill string 11 is led through open water 12 to a subterranean wellbore 13 via a wellhead assembly 14. The wellhead assembly 14 comprises a wellhead established on the sea floor 15 and relevant equipment arranged on the wellhead, such as a blow out preventer (BOP), a rotating control device (RCD) 16 for sealing against the drill string 11, etc.

A mud return riser 20 is suspended from the vessel 100 and fluidly connected to the wellhead assembly 14 below the RCD 16 to receive mud (i.e. drilling fluids pumped into the well from the vessel 100 topside, optionally with cuttings). The fluid connection may be in the form of a flexible riser section 21 comprising at least one flexible hose. A subsea pump 22 is provided to assist in pumping the mud to the vessel 100 via the mud return riser 20. Optionally, a different fluid lift system can be used. The mud return riser 20 is preferably deployed in a lateral distance of several meters from the well center axis in order to ensure that the mud return riser 20 does not interfere with the drill string 11.

The skilled reader will recognise the above as a per se known arrangement for open water (“riserless”) drilling.

The mud return riser 20 comprises a number of riser sections 20a, 20b interconnected in an end-to-end relationship. The mud return riser 20 is, prior to the start of the drilling process, built from the vessel 100 by sequentially attaching riser sections 20a, 20b and lowering the resulting riser string into the water 12. Similarly, after completing the drilling process, the mud return riser 20 can be disassembled by sequentially lifting and disconnecting riser sections 20a, 20b. Each riser section 20a, 20b may be, for example, about 22 m long. Each riser section 20a, 20b may comprise at least one, preferably a plurality of pipes 40 extending axially through the riser section 20a, 20b. For instance, the plurality of pipes 40 may comprise a mud return line 41, choke and kill lines 42, a slops line 43 for return of water to the drilling vessel 100 and conduits 44 for hydraulic supply to the BOP. Furthermore, energy and signal cables for the LMRP, RCD, BOP and subsea pump 22 may be guided via the mud return riser 20.

For interconnecting the riser sections 20a, 20b in an end-to-end relationship, each riser section 20a, 20b comprises a lower end flange 24 and an upper end flange 25, wherein an upper end flange 25 of a first riser section 20a is connectable to a lower end flange 24 of a second riser section 20b and vice versa. Figs.2a, 2b and 3 illustrate end parts of a first riser section 20a and a second riser section 20b depicting the upper end flange 25 of a first riser section 20a (Fig.2a) and the lower end flange 24 of a second riser section 20b (Fig.2b) according to a first example, wherein Fig.3 illustrates an opposite arrangement of said end flanges 24, 25 for interconnection. According to advantageous examples, the lower end flange 24 may provide a male connector part of the riser section 20a, 20b and the upper end flange 25 may provide a female connector part of the riser section 20a, 20b.

As indicated by Figs.2b and 3, a connection between the upper end flange 25 of the first riser section 20a and the lower end flange 24 of the adjacent second riser section 20b may be secured by means of a rotatable lock ring 30. For instance, the upper end flange 25 and the lower end flange 24 may comprise radially outwardly protruding connection faces 26 for engagement with matching lock faces 31 of the lock ring 30. The lock ring 30 may be attached in a rotatable manner to the lower end flange 24 or provided as a separate assembly part and installed on the upper end flange 25 of the first riser section 20a before approximating the lower end flange 24 of the second riser section 20b. For securing the connection between the upper end flange 25 and the lower end flange 24, the lock ring 30 may be rotated in order to engage the lock faces 31 of the lock ring 30 with the connection faces 26 of the end flanges 24, 25. For instance, the connection faces 26 and the lock faces 31 may comprise compatible protrusions and recesses which can form a plurality of bayonet couplings for simultaneous locking upon rotation of the lock ring 30. The protrusions may, for instance, be lugs arranged at regular intervals around a periphery of the end flanges 24, 25. By means of a lock ring 30, the interconnection between the first riser section 20a and the second riser section 20b may be secured in a reliable and robust manner. An index pin 32 may optionally be attached to the lower end flange 24 to prevent the lock ring 30 from rotating to the open position and as an optical marker for visual inspection of the lock ring position and orientation.

As can be furthermore taken from Figs.2a and 3, each riser section 20a, 20b comprises a support skirt 23 rigidly fixed to the upper end flange 25 and configured for engagement with a vertical friction support 50 (described in further detail below). The support skirt 23 is configured to arrest the riser sections 20a, 20b in slips during deployment and retrieval of the mud return riser 20. By providing a support skirt 23 for vertical friction-based support, the handling of the riser sections 20a, 20b is significantly improved. In comparison, conventional horizontal support structures commonly used for riser section assembly may require time-consuming and cumbersome change operations between a supporting mode in which the riser section to be connected is hold and a slipping mode in which the riser section is allowed to move, allowing for a subsequent riser section connection. By means of a vertical support concept, the time and effort for holding and moving mud return riser sections 20a, 20b may be reduced considerably. Since mud return risers 20 are generally lighter and smaller than marine risers configured to enclose the drill string 11, the frictional force provided by a suitable vertical friction support 50 and applied to the support skirt 23 can provide sufficient holding forces for securely keeping the riser section 20a, 20b in position. Since vertical friction may be applied fast, easily and flexibly to the support skirt 23 by activating and deactivating the vertical friction support 50, process times may be significantly reduced for assembly and disassembly of the mud return riser 20. Furthermore, the vertical friction support concept allows for safe and easy rotation of the lock ring 30 relative to the upper end flange 25 of the first riser section 20a comprising the support skirt 23 while preventing rotation of the first riser section 20a by means of the vertical friction support 50 securely fixing the support skirt 23.

As depicted for example in Figs.2a, 3 and 4b, the support skirt 23 preferably comprises an end face 23a providing a circumferential lock ring support surface for the lock ring 30. By providing the lock ring support surface, the handling of the lock ring 30 is facilitated and the lock ring 30 is securely supported on the support skirt 23. The end face 23a may provide a collar-like support surface for circumferential support of the lock ring 30. The end face 23a may provide a substantially flat support surface on which the lock ring 30 may be rotated with low resistance.

Preferably, the lock ring 30 and support skirt 23 may be arranged in a flush-fitting manner on each other for providing a smooth common outer surface and enhanced protection of the pipes 40 and end flanges 24, 25. According to an advantageous example, the support skirt 23 extends over a length of at least 500 mm, thus providing enough surface area for the absorption of friction forces exerted by the vertical friction support 50. The support skirt 23 may comprise a substantially cylindrical shape, for instance having a circular or oval cross-section. With a substantially cylindrical shape, the support skirt 23 may be hold by the vertical friction support 50 independent of a present rotation angle of the support skirt 23 around its longitudinal axis. According to advantageous examples, the support skirt 23 may comprise a friction-enhancing surface, for instance having flutes, grooves, a friction-enhancing coating or form-fit elements, thus increasing the exertable friction forces and the reliability of the vertical friction support concept.

Figs.4a and 4b provide perspective side views of an upper end flange 25 of a first mud return riser section 20a and of a matching lower end flange 24 of a second mud return riser section 20b according to a second example. As can be taken from the figures, the depicted riser sections 20a, 20b comprise a different pipe arrangement compared to the riser sections 20a, 20b of the first example, for instance a slops line 43 is not provided. Thus, the invention may be considered as independent of the types and arrangements of pipes 40 extending through the riser sections 20a, 20b. Furthermore, Fig.4b depicts that the support skirt 23 may comprise a closed part 23c and a frame-like open part 23d, thus reducing the weight of the support skirt 23 and the associated riser section 20a, 20b while at the same time providing a friction area on the closed part 23c large enough to ensure a secure holding of the support skirt 23 by means of a vertical friction support 50. According to an advantageous example, the closed part 23c of the support skirt 23 extends over a length of at least 500 mm, thus providing enough surface area for the absorption of friction forces exerted by the vertical friction support 50. This arrangement provides advantages in that the friction area no the closed part 23c is spaced longitudinally from the upper end flange 25, such that the flange 25 is located farther away from the vertical friction support 50 when engaged with the support skirt 23. This may ease the handling of the riser sections 20a,b, for example providing easier access to the lock ring 30.

Figs.5, 6 and 7 schematically depict an example of a vertical friction support 50, wherein Fig.5 illustrates a vertical friction support 50 supporting a first riser section 20a to which a second riser section 20b is mounted. Fig.6 provides a perspective view and Fig.7 shows a sectional view of the vertical friction support 50. A vertical friction support 50 may be a support structure with at least one vertical friction surface 52 configured to engage and apply friction forces to a supported object. The vertical friction support 50 may be configured to be activated in order to generate the required friction force for holding the supported object and to be deactivated for allowing the support object to move along or through the vertical friction support 50. The vertical friction support 50 may, for instance, be a power slip device which may be activated and deactivated hydraulically, pneumatically or electrically. According to alternative examples, the vertical friction support 50 may be a manual slip device configured to be handled manually. According to an example, the vertical friction support 50 comprises at least two wedges 51, preferably at least three or at least four wedges 51, which may be arranged in an equidistant manner around the riser section 20a, 20b to be supported by the vertical friction support 50. Each wedge 51 may provide a vertical friction surface 52 configured to selectively exert a frictional force to the support skirt 23 of a riser section 20a, 20b. Furthermore, the vertical friction support 50 may comprise a suitable counter bearing 54 for supporting the wedges 51 on the wedge surfaces 53 opposite of the vertical friction surfaces 52. The vertical friction support 50 may comprise a circular structure with an axial opening 55 in the center of the structure and with the wedges 51 distributed circumferentially around the axial opening 55. The distance between the vertical friction surfaces 52 of the wedges 51 may be adjustable in order to configure the vertical friction support 50 for support skirts 23 of riser sections 20a, 20b of different sizes. According to alternative examples, the vertical friction surface 52 of the vertical friction support 50 may be provided by frictional elements other than wedges 51, e.g. cuboid blocks.

A support skirt 23 may be a cover structure attached to the upper end flange 25 of the riser section 20a, 20b, providing a solid shell for the pipes 40 with a vertical mantle surface 23b to which the friction forces may be exerted by the vertical friction support 50. For this, the vertical mantle surface 23b and the vertical friction surface 52 may generally be oriented in parallel to each other in order to exert even friction forces over a large area of the opposing surfaces of the support skirt 23 and the vertical friction surface 52. The vertical friction surface 52 may, for instance, have a flat surface shape or preferably a curved surface shape adapted to the preferably curved vertical mantle surface 23b of the support skirt 23 for increasing the exertable friction forces. The support skirt 23 at least partially, preferably completely encloses the pipes 40 of the riser section 20a, 20b radially and provides force absorption for the applied friction forces, thus preventing force transmission to the pipes 40. According to advantageous examples, the support skirt 23 may be assembled by at least two skirt parts which may be configured as half shell parts fitted around the riser section 20a, 20b and fixed to the upper end flange 22.

The term “vertical” may imply a predominantly vertical extension of the described structures in their use orientation, i.e. during deployment and retrieval of the mud return riser 20. A vertical extension may generally refer to an extension direction of the earth perpendicular. According to a simplified approach, a vertical structure may extend from the drilling vessel 100 in the direction to the sea floor 15.

Now turning again to Fig.1, the handling of the mud return riser 20 may be further enhanced if a suitable transition is provided between the riser sections 20a, 20b and the flexible riser section 21 with flexible hoses configured to fluidly connect the riser sections 20a, 20b with the wellhead assembly 14. Thus, the mud return riser 20 may preferably comprise an adapter unit 27 as for example shown in Fig.8 for connecting the plurality of pipes 40 extending through the interconnected riser sections 20a, 20b to the wellhead assembly 14 via flexible hoses (not shown in the figures) of the flexible riser section 21, the adapter unit 27 having a first interface 27a connectable to the pipes 40 and to the lower end flange 24 of a riser section 20a, 20b, a second interface 27b connectable to the flexible hoses of the flexible riser section 21 and connection lines 27c connecting the first interface 27a with the second interface 27b. The flexible riser section 21 is configured as a conduit between the riser sections 20a, 20b and the wellhead assembly 14. A further technical function of the flexible riser section 21 is to compensate relative motion between the drilling vessel 100 and the wellhead assembly 14. For instance, the flexible riser section 21 may perform heave compensation for vertical vessel movements.

The adapter unit 27 may be beneficially attachable to one of the riser sections 20a, 20b by supporting the riser section 20a, 20b with the vertical friction support 50 while connecting the first interface 27a with the pipes 40 and the second interface 27b with the flexible hoses of the flexible riser section 21, thus facilitating the assembly and disassembly of the mud return riser 20.

Preferably, at least one of the connection lines 27c may be a flexible connection line 27c. Consequently, the flexible hoses of the flexible riser section 21 do not necessarily have to be arranged in the same distribution and order as the pipes 40, thus providing further flexibility for interconnection. Furthermore, the subsea pump 22 as depicted in Fig.1 may be connected to the adapter unit 27 either via the first interface 27a or the second interface 27b, thus providing a defined connector for the subsea pump 22. According to an advantageous example, the mud return pipe 41 of the plurality of pipes 40 may be configured as a load carrying pipe carrying the subsea pump 22, thus enabling a reliable and efficient holding of the subsea pump 22.

Turning again to Fig.5, the vertical friction support 50 and a plurality of mud return riser sections 20a, 20b, each comprising a support skirt 23, may form a mud return riser hang-off system 60 as indicated in Fig.5. The mud return riser hang-off system 60 beneficially facilitates the handling and assembly of mud return risers 20, thus reducing the process time on mobile offshore drilling units and additionally increasing the process safety and reliability of mud return riser deployment and retrieval. The mud return riser hang-off system 60 may further comprise lock rings 30 attachable to and detachable from the riser sections 20a, 20b during riser deployment and retrieval, the lock rings 30 being configured for securing the connections between adjacent riser sections 20a, 20b. The vertical friction support 50 may comprise at least two wedges 51 each having a vertical friction surface 52 configured to selectively exert friction forces to the support skirts 23 of the riser sections 20a, 20b. According to an advantageous example, the vertical friction support 50 is configured as a hydraulically or electronically controllable power slip, thus facilitating the handling of the vertical friction support 50.

In accordance with the mud return riser hang-off system 60 illustrated in Fig.5, a method of building a mud return riser 20 may be performed comprising the following steps:

− providing a vertical friction support 50, a plurality of mud return riser sections 20a, 20b of the described type and a plurality of lock rings 30;

− guiding a first riser section 20a along or through the vertical friction support 50 until a support skirt 23 of the first riser section 20a reaches the vertical friction support 50;

− activating the vertical friction support 50 for exerting friction forces on the support skirt 23;

− connecting an upper end flange 25 of the first riser section 20a to a lower end flange 24 of a second riser section 20b;

− rotating a lock ring 30 placed around the upper end flange 25 and the lower end flange 24 for securing the connection; and

− deactivating the vertical friction support 50 for releasing the first riser section 20a.

The method steps may be repeated with a number of riser sections 20a, 20b until the assembly of the mud return riser 20 is completed. For instance, a following step may comprise guiding the second riser section 20b along or through the vertical friction support 50 until a support skirt 23 of the second riser section 20b reaches the vertical friction support 50. The skilled reader will recognise that the presented method of building a mud return riser 20 may be reversed for retrieval and disassembly of the mud return riser 20, such that an associated method of disassembling the mud return riser 20 may comprise the steps of:

− guiding the second riser section 20b and the first riser section 20a along or through the vertical friction support 50 until a support skirt 23 of the first riser section 20a reaches the vertical friction support 50;

− activating the vertical friction support 50 for exerting friction forces on the support skirt 23;

− unlocking the lock ring 30 placed around the upper end flange 25 and the lower end flange 24 by rotation of the lock ring 30;

− discconnecting an upper end flange 25 of the first riser section 20a from the lower end flange 24 of the second riser section 20b; and

− deactivating the vertical friction support 50 for releasing the first riser section 20a.

The described methods of building and optionally disassembling a mud return riser 20 beneficially facilitate the handling and assembly of mud return risers 20, thus reducing the process time on mobile offshore drilling units and additionally increasing the process safety and reliability of mud return riser deployment and retrieval.

In any of the examples or embodiments described or claimed herein, the lock ring 30 and the support skirt 23 may be arranged to surround or encircle the plurality of pipes 40. All the plurality of pipes 40 may thus be located inside the lock ring 30 and the support skirt 23.

In any of the examples or embodiments described or claimed herein, the support skirt 23 may have a cross-sectional diameter which is substantially the same as the cross-sectional diameter of the lock ring 30, for example wherein the diameter of the support skirt 23 is between 0.8 and 1.2 times that of the lock ring 30, or even more preferably between 0.9 and 1.1 times that of the lock ring 30.

The invention is not limited by the examples described above; reference should be had to the appended claims.

Claims (14)

1. A mud return riser (20) comprising a plurality of interconnected riser sections (20a, 20b) through which a plurality of pipes (40) extend,

each riser section (20a, 20b) having a lower end flange (24) and an upper end flange (25) for interconnection of the riser sections (20a, 20b), wherein a connection between an upper end flange (25) of a first riser section (20a) and a lower end flange (24) of an adjacent second riser section (20b) is secured by means of a rotatable lock ring (30),

each riser section (20a, 20b) having a support skirt (23) rigidly fixed to the upper end flange (25) and configured for engagement with a vertical friction support (50).

2. The mud return riser (20) according to claim 1, wherein the support skirt (23) comprises an end face (23a) providing a circumferential lock ring support surface for the lock ring (30).

3. The mud return riser (20) according to claim 1 or 2, wherein the support skirt (23) extends over a length of at least 500 mm.

4. The mud return riser (20) according to any preceding claim, wherein the support skirt (23) comprises a closed part (23c) and an open part (23d), such as a frame-like open part (23d).

5. The mud return riser (20) according to any preceding claim, wherein the support skirt (23) comprises a friction-enhancing surface, for example wherein the friction-enhancing surface is provided by a coating, by surface treatment, by serrations or by another surface profile.

6. The mud return riser (20) according to any preceding claim, wherein the mud return riser (20) comprises an adapter unit (27) for connecting the plurality of pipes (40) extending through the interconnected riser sections (20a, 20b) to the wellhead assembly (14) via flexible hoses of a flexible riser section (21), the adapter unit (27) having a first interface (27a) connectable to the pipes (40) and to the lower end flange (24) of a riser section (20a, 20b), a second interface (27b) connectable to the flexible hoses of the flexible riser section (21) and connection lines (27c) connecting the first interface (27a) with the second interface (27b).

7. The mud return riser (20) according to claim 6, wherein at least one of the connection lines (27c) is a flexible connection line (27c).

8. The mud return riser (20) according to claim 6 or 7, wherein a subsea pump (22) is connected to the adapter unit (27).

9. The mud return riser (20) according to any preceding claim, wherein a mud return pipe (41) of the plurality of pipes (40) is configured as a load carrying pipe carrying the subsea pump (22).

10. A mud return riser section (20a, 20b) for a mud return riser (20) according to at least one of the claims 1 to 9, the mud return riser section (20a, 20b) comprising a lower end flange (24) and an upper end flange (25) with a radially outwardly protruding connection face (26) for engagement with a lock ring (30), the mud return riser section (20a, 20b) further comprising a support skirt (23) rigidly fixed to the upper end flange (25) and configured for engagement with a vertical friction support (50).

11. A mud return riser hang-off system (60) comprising a vertical friction support (50) and a plurality of mud return riser sections (20a, 20b) according to claim 10.

12. The mud return riser hang-off system (60) according to claim 11, wherein the vertical friction support (50) comprises at least two wedges (51) each having a vertical friction surface (52).

13. The mud return riser hang-off system (60) according to claim 11 or 12, wherein the vertical friction support (50) is configured as a hydraulically or electronically controllable power slip.

14. A method of building a mud return riser (20), the method comprising: − providing a vertical friction support (50), a plurality of mud return riser sections (20a, 20b) according to claim 10 and a plurality of lock rings (30); − guiding a first riser section (20a) along or through the vertical friction support (50) until a support skirt (23) of the first riser section (20a) reaches the vertical friction support (50);

− activating the vertical friction support (50) for exerting friction forces on the support skirt (23);

− connecting an upper end flange (25) of the first riser section (20a) to a lower end flange (24) of a second riser section (20b);

− rotating a lock ring (30) placed around the upper end flange (25) and the lower end flange (24) for securing the connection; and

− deactivating the vertical friction support (50) for releasing the first riser section (20a).