NO342003B1 - Blow out preventer bonnet assembly - Google Patents

Abstract

The object of the present invention is to provide an improved hydraulically actuated subsearam type blowout preventer having a ram bonnet having a hydraulically actuated ram actuator. In one embodiment, a blow out preventer is provided having a main body with a main passage therethrough; a ram movable between a first ram position in which the ram is spaced from the main passage and a second ram position in which the ram at least partially interrupts the main passage; a bonnet releasably securedto the main body, the bonnet comprising a hydraulic ram actuator coupled to the ram, the hydraulic ram actuator com prizes: a primary piston disposed within a primary cylinder and fixed to a piston rod; a secondary piston disposed within a secondary cylinder, the secondary piston being slidably arranged on the piston rod; and the primary and secondary pistons arranged to actuate the ram via the piston rod, and the secondary piston having a shorter stroke length than the primary piston.

Description

BLOW OUT PREVENTER BONNET ASSEMBLY

The present invention relates to a bonnet assembly for a blow out preventer and a blow out preventer.

BACKGROUND

Blowout preventers (BOPs) were developed to cope with extreme erratic pressures and uncontrolled flow emanating from a well reservoir during drilling. Known as a "kick", this flow of pressure can lead to a potentially catastrophic event called a blowout. In addition to controlling the downhole well pressure and the flow of oil and gas, blowout preventers are intended to prevent tubular goods used in well drilling, such as, drill pipe, casing, collars, tools and drilling fluid from being blown out of the wellbore when a kick or blowout threatens. Blowout preventers are critical to the safety of crew, drilling rig, and environment, and to the monitoring and maintenance of well integrity; thus blowout preventers are intended to provide an additional and fail-safe barrier to the systems that include them.

Ram-type blowout preventers are part of an overall pressure control system used in oil and gas operations commonly used as pressure containment and unexpected wellbore pressure spikes and well pressure control events. A ram-type BOP is similar in operation to a gate valve, but uses a pair of opposing steel plungers or, rams. The rams extend toward the center of the wellbore to restrict flow or retract open in order to permit flow. The inner and top faces of the rams are fitted with composite steel and elastomeric packers that press against each other, against the wellbore, and around well tubular members running through the wellbore. Outlets at the sides of the BOP housing (body) are used for connection to choke and kill lines or valves. The rams are typically actuated by hydraulic actuators arranged within, or connected to, the BOP housing. The ram type blowout preventer is further integrated with additional well containment and control devices that inclusively make up a subsea blowout preventer stack.

Previously reported solutions and techniques useful for understanding and practicing the present invention include US 7,051,990 B2, US 2,912,214, US 4,969,390 and US 8,596,484 B1, all of which are incorporated herein by reference. Other documents useful for understanding the background include CN 202467749 U, US 4305565 A, US 2006/0000992 A1, US 4504037 A, US 2003/0085040 A1, and US 4438900 A.

The operational requirements for blow out preventers can be very demanding and due to the tendency of the industry to move into harsher and more challenging environments, such as deepwater or arctic areas, this is likely to continue. For this reason, there exists a continuous need for improved techniques and solutions to provide BOPs having increased operational reliability, as well as a design which is compact and permits uncomplicated manufacturing and maintenance.

SUMMARY

The object of the present invention is to provide an improved hydraulically actuated subsea ram type blowout preventer having a ram bonnet having a hydraulically actuated ram actuator.

In one embodiment, there is provided a blow out preventer having a main body with a main passage therethrough;

a ram movable between a first ram position in which the ram is spaced from the main passage and a second ram position in which the ram at least partly interrupts the main passage;

a bonnet releasably secured to the main body, the bonnet comprising a hydraulic ram actuator coupled to the ram, wherein the hydraulic ram actuator comprises:

a primary piston disposed within a primary cylinder and fixed to a piston rod; a secondary piston disposed within a secondary cylinder, the secondary piston being slidably arranged on the piston rod; and

the primary and secondary pistons arranged to actuate the ram via the piston rod, and the secondary piston having a shorter stroke length than the primary piston.

In an embodiment, the rod comprises a mechanical stop and the secondary piston is adapted to engage the mechanical stop to actuate the ram via the piston rod.

In an embodiment, the secondary cylinder comprises a recess configured to receive the mechanical stop for at least a part of a stroke length of the piston rod.

In an embodiment, the primary cylinder is arranged within the bonnet.

In an embodiment, the secondary cylinder is arranged in an actuator unit, the actuator unit being releasably fixed to the bonnet by means of an attachment device.

In an embodiment, the attachment device is fixed to the bonnet by means of a segmented latch ring and a segmented locking ring,

the segmented latch ring comprising at least two latch ring parts, each latch ring part having a serrated outer profile adapted to engage a serrated surface on the bonnet,

the segmented locking ring having at least two locking ring parts, each locking ring part adapted to engage at least one latch ring part,

at least one fastener for fixing each locking ring part to either (i) the bonnet or (ii) the actuator unit.

In an embodiment, the blow out preventer further comprises a locking device operable to selectively lock the ram in the second position,

the locking device being fixed to the actuator unit by means of a second segmented latch ring and a second segmented locking ring,

the second segmented latch ring comprising at least two latch ring parts, each latch ring part having a serrated outer profile adapted to engage a serrated surface on the actuator unit,

the second segmented locking ring having at least two locking ring parts, each locking ring part adapted to engage at least one latch ring part,

at least one fastener for fixing each locking ring part to either (i) the actuator unit or (ii) the locking device.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in greater detail below on the basis of embodiments and of the drawings in which:

Figs 1-5 show various views of blow out preventers.

Figs 6 and 7 show an attachment device for a blow out preventer.

Figs 8-19 show various aspects of bonnet assemblies for a blow out preventer.

DETAILED DESCRIPTION

Figure 1 shows a blow out preventer having a base 100, a main body 101 with a main passage 102 which will, in use, form part of a wellbore and receive tubulars or other equipment used in the wellbore. Bonnets 103 and 104 are releasably attached to the main body 101, for example by means of a set of bolts. The bonnets 103 and 104 comprise ram actuators, as will be described in further detail below. Locking devices 105a-d are provided, the locking devices 105a-d are fixed to the bonnets and arranged to lock the ram actuators in an advanced positon, as will also be described further below. Support rails 106a and 106b support the weight of the respective bonnet when detached from the main body.

Figure 2 shows a blow out preventer having a similar design as that in Fig.1, with the exception that the blow out preventer in Fig.2 has additional auxiliary tandem actuators 107a and 107b arranged between the respective bonnet and the locking devices 105b and 105c. The auxiliary tandem actuators 107a and 107b will be described in further detail below.

In the blow out preventer in Fig.2, the bonnets 103 and 104 are detached from the main body 101 and supported by the support rails 106a and 106b. This permits the bonnets 103 and 104 to be separated from the main body 101 to allow access to components of the blow out preventer such as the rams.

Figures 3-5 show various views of the blow out preventer shown in Fig.2 in an operational configuration. In addition to those components described above, the blow out preventer comprises two sets of rams with associated ram actuators. The ram actuators comprise hydraulically driven piston-cylinder arrangements. An upper pair or rams 110a and 110b are actuated by pistons 108a and 108b, operable in cylinders 109a and 109b, respectively. A lower pair of rams 111a and 111b are actuated by pistons 112a and 112b operable in cylinders 113a and 113b, respectively, and by floating pistons 114a and 114b operating within cylinders 115a and 115b, respectively.

Each ram 110a, 110b, 111a and 111b is movable in a respective guideway 116 and 117 (see Fig.3) transverse to the main passage 102. In the embodiment shown, the upper pair of rams 110a and 110b are pipe rams whereas the lower pair of rams 111a and 111b are blind shear rams. Any type or combination of rams may, however, be used in the blow out preventer, including pipe, variable bore pipe, blind, shear, or blind shear rams.

Figs 3 and 5 show the blow out preventer in an open position, whereas Fig.4 shows the blow out preventer with both set of rams in a closed position. In use, either set of rams will be independently operable, such that the upper pair of rams 110a and 110b can, for example, be operated to close and seal around a pipe or tubular present in the main passage 102. In the case of an emergency, the lower pair of rams 111a and 111b may be operated to shear any object present in the main passage 102 and seal the main passage. This may be done independent of the position of the upper pair of rams 110a and 110b.

The locking devices 105a-d may be fixed to the bonnets 103 and 104 by means of an attachment device. The attachment device may also be used to fix the locking devices 105c and 105d to the respective tandem actuators 107a and 107b, and/or to fix the tandem actuators 107a and 107b to the bonnets 103 and 104.

Figures 6 and 7 show an attachment device having a segmented latch ring 210 and a segmented locking ring 220. The latch ring 210 is made up of two latch ring segments 200 and 201, and the locking ring 220 is made up of two lock ring segments 202 and 203. The latch ring and locking ring may be made up of a larger number of segments, for example, 3, 4, 5, or 6 or more segments. The latch ring and locking ring may be made up of a different number of segments, for example, the latch ring may be made up of three or more segments while the locking ring may be made up of two segments.

The locking device 105a has an annular flange which is inserted into a correspondingly sized circular aperture in the bonnet 103, followed by the latch ring 210 and locking ring 220, so that the annular flange is clamped, and thus held captive, between the bonnet and the latch ring, The segmented latch ring 210 has a series of ridges 204 that engage corresponding ridges 205 on the bonnet or tandem actuator inner diameter to provide an interlocking engagement between male and female profiles. The locking ring 220 engages the latch ring 210 and holds the latch ring 210 in interlocking engagement with the ridges 205. The segmented latch ring 210 contacts a load shoulder 206 on the locking device 105a, or an equivalent load shoulder on the tandem actuator 107a and 107b, and restrains axial movement. Fasteners, in the example shown in Figs 6 and 7 bolts 207, hold the locking ring 220 in place by securing it to either the bonnet 103, the respective locking device 105a-d, the respective tandem actuator 107a and 107b, or to the latch ring 210.

Attachment of the tandem actuators 107a and 107b to the bonnet 103, and the locking device 105b to the tandem actuator 107b, can be arranged equivalently. This attachment device thus ensures a secure connection between the locking devices 105a-d, the tandem actuators 107a and 107b and the bonnet 103 and 104, while allowing simple and fast removal of the attachment device for e.g. repair or maintenance.

Reference is now made to Fig.8, which shows in more detail the bonnet 103 and associated components, and to Fig.9. The bonnet 103 has a support system comprising support rails 106b and 106c arranged on either side of the bonnet 103 for supporting the weight of the bonnet 103 in the detached position (as shown in Fig.2). The support rails 106b and 106c each have a longitudinal axis, and an end connector 304 which is fixed to the main body 101. The support system further comprises a support rail bracket 301 and first and second support rail bearings 302 and 303. The first support rail bearing 302 is arranged on the support rail bracket 301, which again is fixed to the bonnet 103, while the second support rail bearing 303 is fixed directly to the bonnet 103. The support rail 106b engages with the support rail bearings 302 and 303 to support the support rail 106b whilst allowing the support rail 106b to slide along the support rail bearings 302, 303 parallel to its longitudinal axis. The support rails 106b, 106 thus may be used to control the motion of the bonnet 103.

The support system comprises a rail stop 305 which cooperates with a slot 306 in the support rail 106b to define an end position of the bonnet, i.e. a bonnet open position. The location of the support rail bearings 302 and 303 (and the equivalent support rail bearings on the opposite side of the bonnet 103) are spaced apart in the longitudinal direction of the support rails 106b and 106c. Their location is arranged to ensure that the center of gravity of the bonnet 103 lies on an imaginary plane generally perpendicular to the longitudinal axis of the support rail 106b which falls between the two bearings regardless of the bonnet configuration (with or without tandem actuators), bonnet position (open or closed), ram position (open or closed) or state of assembly (locking devices and actuator piston assemblies installed or uninstalled).

The blow out preventer may comprise tandem actuators 107a and 107b. The tandem actuators 107a and 107b may be releasably attached to the bonnet 103 by means of an attachment device, for example according to that above in relation to Figs 6 and 7 or a different type of connection arrangement. Alternatively, the tandem actuators 107a and 107b may be permanently fixed to the bonnet 103.

By providing the tandem actuators 107a and 107b with a releasable attachment device, the blow out preventer may be adapted for any particular use by the operator. For example, during regular drilling operations the blow out preventer may be configured as shown in Fig.3, whereby the tandem actuators 107a and 107b provide additional shearing force for the blind shear rams 111a and 111b to ensure that the drill string can be severed effectively. For other types of operations, for example wireline well intervention, less shearing force may be required and the blow out preventer may be configured as shown in Fig.1, i.e. without the tandem actuators 107a and 107b. The same flexibility will be achieved if using different types of rams in the blow out preventer, for example in certain configurations the blind shear rams 111a and 111b in Fig. 3 may be replaced by an additional set of pipe rams, in which case the tandem actuators 107a and 107b may not be necessary.

Figures 10 and 11 show further details of the ram actuators and the tandem actuators in the configuration shown in Figs 3-5. The bonnet 103 houses two ram actuators. The ram actuators have cylinders 109b and 113b arranged in the bonnet 103, with respective pistons 108b and 112b. A tandem actuator 107b is provided in conjunction with the lower ram actuator. The tandem actuator 107 also has a cylinder 115b and respective piston 114b. The tandem actuator 107b is secured to the bonnet 103 by means of an attachment device having a latch ring 210 and a locking ring 220, as described above. Locking devices 105a and 105b are attached in an equivalent manner to the bonnet 103 and the tandem actuator 107b, respectively.

An upper main rod 411 is fixed to the upper piston 108b and extends out of the bonnet 103. A connector 413 is provided for fixing the upper main rod 411 to a ram, for example pipe ram 110b (see Fig.5). An upper tail rod 412 is fixed to the upper main rod 411 and/or the upper piston 108b and extends into the locking device 105a. When the upper actuator is in the advanced position, the locking device 105a may be operated to engage the upper tail rod 412 and prevent movement of the upper tail rod 412 away from the main passage 102. The locking device 105a may thus be used to lock the upper actuator in the advanced position.

A lower main rod 414 is fixed to the lower piston 112b and extends out of the bonnet 103 to a connector 417 for connecting the lower main rod 414 to a ram, for example blind shear ram 111b (see Fig.5). A connecting rod 415 is fixed to the lower main rod 414 and/or the piston 112b and extends into the tandem actuator 107b. The connecting rod 415 is fixed to a tail rod 416 in the tandem actuator 107b. The tail rod 416 extends into the locking device 105b, and the locking device 105b may be operated to engage the tail rod 416 and to prevent movement of the tail rod 416 away from the main passage 102. The locking device 105b may thus be used to lock the lower actuator in the advanced position.

The piston 114b of the tandem actuator 107b is a floating piston 114b which is slidably arranged on the tail rod 416. The tail rod 416 comprises a mechanical stop 422. The mechanical stop 422 may be a shoulder on the tail rod 416. When the back side of the floating piston 114b is pressurized in cylinder 115b, the floating piston 114b will be urged towards the mechanical stop 422 and thus contribute to actuating the ram via the tail rod 416, connecting rod 415 and lower main rod 414.

The floating piston 114b may have a shorter stroke length than the lower piston 112b. This may be beneficial for example in the arrangement shown in Fig.5. When cutting an object in the main passage 102, for example a drill string, with the blind shear rams 111a and 111b, the highest force requirements will be during the cutting process. After the cut has been done, the final movement of the rams will be to fully close the main passage and actuate the seals. This final movement requires much less actuation force.

By providing the floating piston 114b with a shorter stroke length than the lower piston 112b, the floating piston 114b may contribute actuation force for part of the ram stroke, while not consuming hydraulic fluid during the rest of the actuation stroke (for example during the final movement as noted above). This can be achieved by designing the cylinder 115b such that the floating piston 114b is stopped against an end stop within the cylinder after a pre-determined stroke length.

The cylinder 115b may be provided with a recess 424 being adapted for receiving the mechanical stop 422 during part of the stroke length of the actuator. This allows the end stop for the floating piston 114b to be the end 423 of the cylinder 115b, while the tail rod 416 with the mechanical stop 422 may continue its motion over the final part of the actuation stroke as the floating piston 114b stays in place at the end 423 of the cylinder 115b, the tail rod 416 thus sliding along within the floating piston 114b. This allows the cylinder 115b to be designed with a length substantially equal to the stroke length of the floating piston 114b, thus allowing for a shorter and more compact tandem actuator 107b.

Hydraulic fluid may be provided to the cylinder 115b of the tandem actuator 107 by means of hydraulic supply pipes 420 and 421 (see Fig.8) or, alternatively, by hydraulic channels within the body of the tandem actuator 107.

Various elastomeric seals and bushings, e.g. seals 418 and 419, may be provided to seal around the rods 411, 412, 414, 415 and 416, as appropriate. Similarly, the pistons 108b, 112b, and 114b are sealed against their respective cylinders in the conventional manner. The floating piston 114b may, similarly, be sealed against the tail rod 416 as necessary.

The rods in the embodiment here are made up of individual segments, i.e. main rod, tail rod and connecting rod, however may equally well be formed in one piece as a single rod.

The blow out preventer may further comprise extendible hydraulic supply pipes arranged between the main body 101 and the bonnet 103. Figure 12 shows the bonnet 103 with extendible hydraulic supply pipes 701, 702, 703 and 704 for the bonnet in a fully dismantled position (main body 101 not shown). Figure 2 shows the hydraulic supply pipes 701 and 702 with the bonnet 103 detached and spaced from the main body 101.

The ends of hydraulic supply pipe 701 and hydraulic supply pipe 702 are fixed to a hydraulic supply system (not shown) within the main body 101 and extend from the main body 101 into the bonnet 103. Figure 13 shows the hydraulic supply pipes 701 and 702 extending into a respective passage 705 and 706 in the bonnet 103. The supply pipes 701 and 702 are arranged slidable within the respective passage 705 and 706. Seals 707 and 708 are provided in an interface between the passage 705 and 706 and the respective supply pipes 701 and 702.

The passages 705 and 706 are fluidly connected to the upper ram actuator, specifically the passages 705 and 706 are each connected to the cylinder 109b but on opposite sides of the piston 108b. Providing hydraulic fluid to pipe 701 thereby produces an opening motion of the upper ram actuator, while providing hydraulic fluid to pipe 702 produces a closing motion of the upper ram actuator.

The supply pipes 703 and 704 are arranged equivalently, and connected to the lower ram actuator, i.e. fluidly connected to cylinder 113b. Bonnet 104 is arranged with extendible hydraulic supply pipes equivalently.

The extendible hydraulic supply pipes thus maintain fluid communication between the hydraulic supply system in the main body 101 and the ram actuators at any time, also when the bonnets 103 and 104 are in the detached position and spaced from the main body 101. This allows operation of the ram actuators by the BOP's main hydraulic system regardless of the position of the bonnet 103 or 104, e.g. for moving the rams during maintenance, testing or replacement when the bonnet is in the position shown in Fig. 2.

Referring now to Figs 14-17, the blow out preventer may further comprise a hydraulic bonnet actuator, operable to move the bonnet 103 or 104 relative to the main body 101. Figures 14 and 15 show a top view of the blow out preventer shown in Fig.2, cut centrally between the upper and the lower ram actuator (see also Fig.5).

A first piston rod 601 and a second piston rod 602 are provided, the first and second piston rods being fixed to the main body 101. The first and second piston rods 601 and 602 extend into a respective first cylinder 603 and a second cylinder 604 in the bonnet 104. Piston heads 605 and 606 are provided on the ends of piston rods 601 and 602 and operable within cylinders 603 and 604 such as to create a piston front side 605a and 606a (see Fig.17) and a piston back side 605b and 606b for each of the piston heads 605 and 606. By pressurizing the respective chamber or chambers within cylinders 603 and/or 604, the hydraulic bonnet actuator may move the bonnet 104 relative to the main body 101.

Piston rods 601 and 602 may comprise respective fluid channels 607 and 608 (see Fig. 17) therethrough, whereby hydraulic fluid may be transmitted from a hydraulic fluid supply 609 and 610 in the main body 101 to the cylinders 603 and 604. Fluid channel 607 in piston rod 601 ends in the fluid chamber delimited by piston front side 605a, i.e. the fluid chamber in front of the piston head 605 within cylinder 603. For this purpose an opening 614, extending through piston head 605, is provided. Fluid channel 608 ends in the fluid chamber delimited by piston back side 606b, i.e. the fluid chamber behind piston head 606 within cylinder 604. For this purpose, a fluid channel 613 is provided in the piston head 606.

Communication channel 612 (see Fig.16) provides fluid communication between cylinders 603 and 604 substantially at their outermost ends. Communication channel 611 provides fluid communication between cylinders 603 and 604 substantially at their innermost ends. Since the cylinders 603 and 604 are each divided into two fluid chambers by the pistons 605 and 606, it can be seen that the communication channels 611 and 612 will equalize the pressures acting on the front side of the pistons 605 and 606, and equalize the pressures acting on the back side of pistons 605 and 606.

Providing fluid pressure through channel 607 will therefore provide an actuation force from both cylinder/piston arrangements, and produce a force driving the bonnet 104 away from the main body 101. Conversely, providing a fluid pressure through channel 608 will produce a force driving the bonnet 104 towards the main body 101.

Bonnet 103 may be arranged equivalently to this.

By means of the hydraulic bonnet actuator, it is therefore possible to displace the bonnets 103 and 104 from the main body 101 without the need for external force or support, as well as bring the bonnets 103 and 104 back towards the main body 101 for re-attachment. The bonnets 103 and 104 can be moved relative to the main body 101 independently of the ram actuators or their positions. This can be advantageous for example for testing of the ram actuator functionality, whereby the bonnet actuator will maintain the bonnet in a given position while the ram actuator(s) is/are being operated for testing or other purposes.

The blow out preventer may further comprise an end capture providing an interlocking engagement between male and female profiles to reduce stresses and deflections in the main body 101 and/or the bonnets 103 and 104.

Figures 12, 18 and 19 show the bonnet 103 having a protrusion 501 and a protrusion 502. The protrusion 501 is adapted for interlocking arrangement with a corresponding recess 503 in the main body 101 and the protrusion 502 similarly cooperates with a recess 504 (see Figs 18 and 2.)

The main rod 411 may extend out of the bonnet 103 through the protrusion 502 (see Fig. 12). This may provide the advantage that the interlocking engagement of the protrusion 502 and the recess 504 is provided in a region of the bonnet 103 and main body 101 where tension stresses are high during operation. A support element 505 (see Fig. 18) may, optionally, be provided to further improve stress distribution in this region.

Optionally, hydraulic supply pipes 701 and 702 may extend through the protrusion 501.

According to various embodiments of the present invention, there is thus provided a new and improved subsea ram type blow out preventer. The blow out preventer according to the present invention may provide advantages of design simplicity, ease of manufacturing and maintenance, improved reliability of hydraulic functions and system reliability.

The present invention is not limited to embodiments described herein; reference should be had to the appended claims.

Claims (7)

1. A blow out preventer having a main body (101) with a main passage (102) therethrough;

a ram (110a,b, 111a,b) movable between an open position in which the ram (110a,b, 111a,b) is spaced from the main passage (102) and a closed position in which the ram (110a,b, 111a,b) at least partly interrupts the main passage (102);

a bonnet (103,104) releasably secured to the main body (101), the bonnet (103,104) comprising a hydraulic ram actuator coupled to the ram (110a,b, 111a,b), characterized in thatthe hydraulic ram actuator comprises:

a primary piston (112a,b) disposed within a primary cylinder (113a,b) and fixed to a piston rod (414,415,416);

a secondary piston (114a,b) disposed within a secondary cylinder (115a,b), the secondary piston (114a,b) being slidably arranged on the piston rod (414,415,416); and

the primary (112a,b) and secondary (114a,b) pistons arranged to actuate the ram (110a,b, 111a,b) via the piston rod (414,415,416), and the secondary piston (114a,b) having a shorter stroke length than the primary piston (112a,b).

2. A blow out preventer according to claim 1, wherein the rod (414,415,416) comprises a mechanical stop (422) and the secondary piston (114a,b) is adapted to engage the mechanical stop (422) to actuate the ram (110a,b, 111a,b) via the piston rod (414,415,416).

3. A blow out preventer according to claim 2, wherein the secondary cylinder (115a,b) comprises a recess (424) configured to receive the mechanical stop (422) for at least a part of a stroke length of the piston rod (414,415,416).

4. A blow out preventer according to any one of claims 1 to 3, wherein the primary cylinder (113a,b) is arranged within the bonnet (103,104).

5. A blow out preventer according to any one of claims 1 to 4, wherein the secondary cylinder (115a,b) is arranged in an actuator unit (107a,b), the actuator unit (107a,b) being releasably fixed to the bonnet (103,104) by means of an attachment device (210,220).

6. A blow out preventer according to claim 5, wherein the attachment device (210,220) is fixed to the bonnet (103,104) by means of a segmented latch ring (210) and a segmented locking ring (220),

the segmented latch ring (210) comprising at least two latch ring parts (200,201), each latch ring part (200,201) having a serrated outer profile (204) adapted to engage a serrated surface (205) on the bonnet (103,104),

the segmented locking ring (220) having at least two locking ring parts (202,203), each locking ring part (202,203) adapted to engage at least one latch ring part (200,201),

at least one fastener (207) for fixing each locking ring part (202,203) to either (i) the bonnet (103,104) or (ii) the actuator unit (107a,b).

7. A blow out preventer according to claim 6, further comprising a locking device (105a-d) operable to selectively lock the ram (110a,b, 111a,b) in the second position, the locking device (105a-d) being fixed to the actuator unit (107a,b) by means of a second segmented latch ring (210) and a second segmented locking ring (220), the second segmented latch ring (210) comprising at least two latch ring parts (200,201), each latch ring part (200,201) having a serrated outer profile (204) adapted to engage a serrated surface (205) on the actuator unit (107a,b), the second segmented locking ring (220) having at least two locking ring parts (202,203), each locking ring part (202,203) adapted to engage at least one latch ring part (200,201),

at least one fastener (207) for fixing each locking ring part (202,203) to either (i) the actuator unit (107a,b) or (ii) the locking device (105a-d).