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Title:
CONNECTOR WITH REDUCIBLE LOAD CAPACITY AND METHOD FOR DISCONNECTING A STRUCTURE FROM A REMOTELY LOCATED ATTACHMENT
Document Type and Number:
WIPO Patent Application WO/2017/123097
Kind Code:
A1
Abstract:
An overload-releasable connector (1) in which a connector pin (1) comprises a locking dog housing (21) accommodating several locking dogs (23, 25) which are adapted to lockingly engage with a connector housing (3) and, in a locking position, are resting against a dog actuator (22) provided with a first piston (221) and a dog-locking element (222) connected to a second piston (2222), the pistons (221, 2222) being displaceable in the axial direction of the connector pin (2) and bounding a first and a second cylinder chamber (V1, V2), respectively, wherein several secondary locking dogs (25) are radially fixed by the first piston (221); several primary locking dogs (23) are radially displaceable into the locking dog housing (21); the first cylinder chamber (V1) has a larger cross-sectional area than the second cylinder chamber (V2); and the cylinder chambers (V1, V2) are in fluid communication with a hydraulic-fluid source (224). A method of disconnecting a structure (4) from a remotely located attachment (6), the structure (4) being connected to the attachment (6) via an elongated body (5) and the overload-releasable connector (1).

Inventors:
STEINHOVDEN KNUT OVE (NO)
Application Number:
NO2017/050010
Publication Date:
July 20, 2017
Filing Date:
January 12, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
STEINHOVDEN KNUT OVE (NO)
International Classes:
F16G15/04; B63B21/20; B63B21/60; B63G8/00
Foreign References:
US3905190A1975-09-16
US4559890A1985-12-24
US4307678A1981-12-29
CH118371A1927-01-03
US3413030A1968-11-26
US6070548A2000-06-06
Attorney, Agent or Firm:
HÅMSØ PATENTBYRÅ ANS (SANDNES 4301, NO)
Download PDF:
Claims:
C l a i m s

An overload-releasable connector (1) connected to an elongated body (5) connecting a structure (4) to a remotely located attachment (6), in which a connector pin (1) comprises a locking dog housing (21) accommodating several locking dogs (23, 25) which are adapted to lockingly engage with corresponding grooves (32, 33) internally on a side- wall (31) of a connector housing (3) and, in a locking position, are resting against a dog actuator (22) provided with a first piston (221) and a dog-locking element (222) connected to a second piston (2222), said pistons (221 , 2222) being displaceable in the axial direction of the connector pin (2) and bounding a first and a second cylinder chamber (V1 , V2), respectively, c h a r a c t e r i z e d b y

several secondary locking dogs (25) being radially fixed by the first piston (221); an abutment surface (254) on each of the secondary locking dogs (25) resting supportingly against the first piston (221) and being displaceable in the axial direction of the connector (1);

several primary locking dogs (23) being radially displaceable into the locking dog housing (21) on the displacement of the dog-locking element (222) away from the first piston (221);

the first cylinder chamber (V1) having a larger cross-sectional area than the second cylinder chamber (V2); and

said first and second cylinder chambers (V1 , V2) being in closable fluid communication with a hydraulic-fluid source (224).

The overload-releasable connector (1) according to claim 1 , wherein the hydraulic-fluid accumulator (224) is arranged on the dog-locking element (222), and the fluid communication with the first and second cylinder chambers (V1 , V2) is formed of one or more hydraulic-fluid channels (2224) extending through the dog-locking element (222).

The overload-releasable connector (1) according to claim 1 , wherein the second piston (2222) is formed as a rod portion projecting from the dog-locking element (222) into a bore (2212) extending through the first piston (221).

The overload-releasable connector (1) according to claim 1 , wherein a third piston (215) is arranged to abut against a bottom portion (34) of the connector housing (3) and bounds a third cylinder chamber (V3) in an end wall (21 1) of the locking dog housing (21).

The overload-releasable connector (1) according to claim 4, wherein the third cylinder chamber (V3) is in closable fluid communication with the first and second cylinder chambers (V1 , V2). Epno The overload-releasable connector (1) according to claim 4, wherein the third cylinder chamber (V3) is in closable fluid communication with the first and second cylinder chambers (V1 , V2) via a hydraulic-fluid passage (212) through the end wall (21 1) of the locking dog housing (21).

The overload-releasable connector (1) according to claim 6, wherein the hydraulic-fluid passage (212) is arranged to accommodate, in a fluid-sealing manner, a rod portion (2223) projecting from the second piston (2222).

The overload-releasable connector (1) according to any one of claims 1 -7, wherein the hydraulic-fluid source is a hydraulic-fluid accumulator (224) arranged in the connector pin (1).

The overload-releasable connector (1) according to claim 4, wherein the third cylinder chamber (V3) is in closable fluid communication with a second hydraulic-fluid source (224') via a second hydraulic-fluid channel (2227) extending through a rod portion (2223') projecting from the second piston (2222).

The overload-releasable connector (1) according to claim 9, wherein the second hydraulic-fluid source is a second hydraulic-fluid accumulator (224') arranged in the connector pin (1).

The overload-releasable connector (1) according to claim 8, wherein the hydraulic-fluid accumulator (224) is connected to a remotely operated control valve (225) arranged at least to open to fluid communication between the hydraulic-fluid accumulator (224) and the first and second cylinder chambers (V1 , V2).

The overload-releasable connector (1) according to claim 10, wherein the second hydraulic-fluid accumulator (224') is connected to a remotely operated second control valve (225') arranged to open to fluid communication between said hydraulic-fluid accumulator (224') and the third cylinder chamber (V3).

The overload-releasable connector (1) according to claim 4, wherein the bottom portion (34) of the connector housing (3) is provided with a raised piston abutment (341).

A method of disconnecting a structure (4) from a remotely located attachment (6), the structure (4) being connected to the attachment (6) via an elongated body (5) and an overload-releasable connector (1) according to claim 1 , c h a r a c t e r i z e d i n that the method comprises the steps

whenever there is a need for quick disconnection, activating a dog actuator (22) arranged in a connector pin (2) which is securely held in a connector housing (3), by supplying hydraulic fluid to a first cylinder chamber (V1) in the dog actuator (22); Epno relieving a set of primary locking dogs (23) by axially displacing a first piston (221) of the dog actuator (22) in order thereby to displace a set of secondary locking dogs (25) in the axial direction of the connector pin (2);

supplying the hydraulic fluid to a second cylinder chamber (V2) in the dog actuator (22);

releasing the connector housing (3) from the engagement of a set of primary locking dogs (23) with the connector housing (3) by axially displacing a dog-locking element (222) away from supporting abutment against the primary locking dogs (23); and subjecting the secondary locking dogs (25) to a shear force sufficiently large to break loose dog fragments (255) from an engagement portion (252) of the secondary locking dogs (25).

The method according to claim 14, wherein the method includes the further step

supplying the hydraulic fluid to a third cylinder chamber (V3) in the dog actuator (22) in order thereby to displace a third piston (215) into abutment against a bottom portion (34) and subject the connector pin (2) to a push force directed axially, in order thereby to apply an increased shear force to the secondary locking dogs (25).

Epno

Description:
CONNECTOR WITH REDUCIBLE LOAD CAPACITY AND METHOD FOR DISCONNECTING A STRUCTURE FROM A REMOTELY LOCATED ATTACHMENT

The invention relates to an overload-releasable connector connected to an elongated body connecting a structure to a remotely located attachment, in which a connector pin includes a locking dog housing accommodating several locking dogs which are adapted to lockingly engage with corresponding grooves internally on a side wall of a connector housing and, in a locking position, are resting against a dog actuator provided with a first piston and a dog-locking element which is connected to a second piston, said pistons being displaceable in the axial direction of the connector pin and bounding a first and a second cylinder chamber, respectively. The invention also relates to a method of disconnecting a structure from a remotely located attachment, the structure being connected to the attachment via an elongated body and the overload-releasable connector.

In connection with the attachment of large structures, for example in connection with anchor systems for floating installations on oil- or gas-field installations offshore, there is sometimes a need to be able to release the structure quickly if a critical situation arises. For that purpose, connectors having a weak link, which will break when the load exceeds a certain limit value, are often used. A drawback of the prior art is that this load limit value is far higher than the load values normally induced in the attachment by the structure. This may lead to great strains on the structure and the attachment points, for example the anchors on the sea floor, and it is difficult to dimension the weak link of the connector in such a way that the connector will release at a predetermined load while, at the same time, the connector will not represent a risk of unintended release of the structure.

An example of a releasable connector for use on an anchor chain for a floating drilling rig is Rig Anchor Release (RAR) model 6600 from InterOcean Systems, Inc., San Diego, CA 92123, USA.

The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art.

The object is achieved through features, which are specified in the description below and in the claims that follow.

P27484PC00DEpno An overload-releasable connector, especially a connector for releasing a structure from a remotely located, inaccessible attachment, comprises a connector pin provided with several locking dogs, which, in an active position, are in engagement with one or more grooves in an internal sidewall of a connector housing enclosing a substantial portion of the connector pin. One of the connector pin and the connector housing is secured to a first elongated body, for example a first anchor-chain length, and the other one of the connector pin and connector housing is secured to a second elongated body, for example a second anchor-chain length, or said attachment, for example an anchor. A dog actuator arranged in the locking dog housing prevents the dogs from being retractable into the locking dog housing from their active, locking positions. The dog actuator may disengage a set of primary dogs from engagement with the connector housing, by a hydraulic fluid being supplied to cylinder chambers in the dog actuator. The hydraulic fluid is preferably supplied from an associated accumulator through a remotely operated valve. A set of secondary locking dogs remains engaged with the connector housing.

The reduction in the number of locking dogs forming the locking connection between the connector housing and the connector pin may be a sufficient weakening of the connector, so that the shear forces imposed on the secondary locking dogs by the tensile forces generated in the elongated body by the structure attached may be sufficient to break the connection between the connector housing and the connector pin.

In an advantageous embodiment of the connector according to the invention, the actuator is pro- vided with an actuating element, which, after the release of the primary locking dogs from locking engagement with the connector housing, applies a push force to the connector pin relative to the connector housing in order thereby to hasten the shearing of the secondary locking dogs.

The invention is defined by the independent claims. The dependent claims define advantageous embodiments of the invention. In a first aspect, the invention relates more specifically to an overload-releasable connector connected to an elongated body connecting a structure to a remotely located attachment, in which a connector pin includes a locking dog housing accommodating several locking dogs which are adapted to lockingly engage with corresponding grooves internally on a side wall of a connector housing and, in a locking position, are resting against a locking dog actuator provided with a first piston and a dog-locking element connected to a second piston, said pistons being displaceable in the axial direction of the connector pin and bounding a first and a second cylinder chamber, respectively, characterized by

several secondary locking dogs being radially fixed by the first piston;

an abutment surface on each of the secondary locking dogs resting supportingly against the first piston and being displaceable in the axial direction of the connector;

several primary locking dogs being radially displaceable into the locking dog housing on the displacement of the dog-locking element away from the first piston;

P27484PC00DEpno the first cylinder chamber having a larger cross-sectional area than the second cylinder chamber;

and

said first and second cylinder chambers being in closable fluid communication with a hy- draulic-fluid source.

The hydraulic-fluid accumulator may be arranged on the locking dog-locking element, and the fluid communication with the first and second cylinder chambers may be formed of one or more hydraulic-fluid channels extending through the locking dog-locking element.

The second piston may be formed as a rod portion projecting from the dog-locking element into a bore extending through the first piston.

A third piston may be arranged to abut against a bottom portion of the connector housing and bound a third cylinder chamber in an end wall of the locking dog housing.

The third cylinder chamber may be in closable fluid communication with the first and second cylinder chambers. The third cylinder chamber may, more specifically, be in closable fluid communica- tion with the first and the second cylinder chamber via a hydraulic-fluid passage through the end wall of the locking dog housing.

The hydraulic-fluid passage may be arranged to accommodate, in a fluid-sealing manner, a rod portion projecting from the second piston.

The hydraulic-fluid source may be a hydraulic-fluid accumulator arranged in the connector pin. The third cylinder chamber may be in closable fluid communication with a hydraulic-fluid source via a second hydraulic-fluid channel extending through the rod portion projecting from the second piston.

The second hydraulic-fluid source may be a second hydraulic-fluid accumulator arranged in the connector pin. The hydraulic-fluid accumulator may be connected to a remotely operated control valve arranged at least to open to fluid communication between the hydraulic-fluid accumulator and the first and second cylinder chambers.

The second hydraulic-fluid accumulator may be connected to a remotely operated second control valve arranged to open to fluid communication between said hydraulic-fluid accumulator and the third cylinder chamber.

The bottom portion of the connector housing may be provided with a raised piston abutment.

P27484PC00DEpno In a second aspect, the invention relates more specifically to a method of disconnecting a structure from a remotely located attachment, the structure being connected to the attachment via an elongated body and an overload-releasable connector as described above, characterized by the method comprising the steps

- whenever there is a need for quick disconnection, activating a dog actuator arranged in a connector pin which is securely held in a connector housing, by supplying hydraulic fluid to a first cylinder chamber in the dog actuator;

relieving a set of primary locking dogs by axially displacing a first piston of the dog actuator in order thereby to displace a set of secondary locking dogs in the axial direction of the connector pin;

supplying the hydraulic fluid to a second cylinder chamber in the dog actuator;

releasing the connector housing from the engagement of a set of primary locking dogs with the connector housing by axially displacing a dog-locking element away from a supporting abutment against the primary locking dogs; and

- subjecting the secondary locking dogs to a shear force sufficiently large to break loose dog fragments from an engagement portion of the secondary locking dogs.

The method may include the further step

supplying the hydraulic fluid to a third cylinder chamber in the dog actuator in order thereby to displace a third piston into abutment against a bottom portion and apply a push force directed axially to the connector pin in order thereby to subject the secondary locking dogs to an increased shear force.

In what follows, an example of a preferred embodiment is described, which is visualized in the accompanying drawings, in which:

Figure 1 shows a principle drawing in perspective of an anchoring system with a releasable connector according to the invention;

Figure 2 shows a side view on a larger scale of the assembled connector;

Figure 3 shows a plan view of the assembled connector;

Figure 4 shows a side view of a connector pin;

Figure 5 shows a plan view of the connector pin; Figures 6-9 show, on a larger scale, axial sections A-A according to figures 3 and 5 of a first embodiment of the connector in different phases of the disengagement of primary locking dogs and separation of the connector pin and connector housing on the rupture of secondary locking dogs;

P27484PC00DEpno Figure 10 shows a second embodiment of the connector in a phase of the disengagement of the primary locking dogs, the connector being provided with two hydraulic-fluid sources (shown schematically); and

Figure 1 1 shows, on a smaller scale, a partially sectioned side view of an embodiment of the connector pin and the connector housing integrated in respective casings with attachment lugs for connection to an attachment element.

Reference is first made to figure 1 in which a floating structure 4, shown here as a drilling rig, is secured via attachment elements in the form of elongated bodies 5, typically anchor chains, to respective attachments 6, typically bottom anchors. A releasable connector 1 is arranged between the elongated body 5 and the attachment 6.

In figures 2, 3, 4 and 5 the connector is shown in greater detail, as a connector pin 2 is releasably held in a connector housing 3 by means of several locking dogs 23, 25.

Reference is now made to figure 6 in particular. The centre axis of the connector 1 is indicated by the reference numeral 1 1 . The connector pin 2 comprises a locking dog housing 21 accommodat- ing a dog actuator 22. In radial recesses 213 in the locking dog housing 21 , a set of primary locking dogs 23 is arranged, in this embodiment altogether six primary locking dogs 23 grouped in pairs, and a set of secondary locking dogs 25, in this embodiment three secondary locking dogs 25 arranged between the groups of primary locking dogs 23. The locking dogs 23, 25 are attached to the locking dog housing 21 at respective hinge joints 24 and 26, respectively, pivotable in the radial direction of the locking dog housing 21 . The hinge joints 24, 26 are formed with radial play, which allows vertical displacement of the locking dogs 23, 25. In a normal state, the locking dogs 23, 25 lie with end surfaces 233 and 253, respectively, supportingly abutting against a dog abutment 214 bounding the dog recesses 213 in the locking dog housing 21 remotely from the hinge joints 24 and 26, respectively, as shown in figure 6. The locking dogs 23, 25 are provided with outward-facing engagement portions 232 and 252, respectively, which are complementary to internal grooves 32, 33 arranged internally in a sidewall 31 of the connector housing 3.

In a normal state of the connector 1 , as is shown in figure 6, the locking dogs 23, 25 are in engagement with the grooves 32, 33 of the connector housing 3. The secondary locking dogs 25 are provided with a ridge 251 extending in towards a first piston 221 in the locking dog actuator 22, an abutment surface 254 at the end portion of the secondary locking dog 25 being arranged to rest against an annular seat face 221 1 extending in the radial direction of the first piston 221 .

In the locking dog housing 21 , a first cylinder chamber V1 is formed between an end wall 21 1 and the first piston 221 . The end wall 21 1 is provided with a raised middle portion 21 1 1 surrounded by an annular end portion 2213 of the first piston 221 , said end portion 2213 bounding the first cylinder

P27484PC00DEpno chamber V1 in a fluid-sealing manner in the radial direction. The first piston 221 is arranged in an axially displaceable manner in the locking dog housing 21 . From the annular seat face 221 1 of the first piston 221 , a raised middle portion 2214 extends in the axial direction. A bore 2212, shown here as a centre bore, extends through the first piston 221 . A dog-locking element 222 is arranged concentrically with the first piston 221 axially displaceable in the locking dog housing 21 . A peripheral face of the dog-locking element 222 forms an abutment surface 2225 for ridges 231 of the primary locking dogs 23. The abutment surface 2225 is broken by radial recesses 2226 arranged to accommodate the ridges 251 of the secondary locking dogs 25. An annular end portion 2221 of the dog-locking element 222 is adapted to enclose the raised middle portion 2214 of the first piston 221 . A rod-shaped portion 2222 projects in the axial direction from the dog-locking element 222 into the centre bore 2212 of the first piston 221 and forms a second piston, the rod portion 2222 having a fluid-sealing fit against the wall of the centre bore 2212. A second cylinder chamber V2 is defined by the first cylinder chamber V1 , the centre bore 2212 and the second piston 2222.

A rod end portion 2223 with a reduced cross section is arranged to extend in a fluid-sealing manner into a fluid passage, shown here as a centre bore 212 in the end wall 21 1 of the locking dog housing 21 , coinciding axially with the bore 2212 of the first piston 221 .

The dog actuator 22 is connected to a hydraulic-fluid source, shown here as a hydraulic-fluid ac- cumulator 224 projecting up from the dog-locking element 222 and being in fluid communication with the first cylinder chamber V1 and the second cylinder chamber V2 via hydraulic-fluid channels 2224 extending through the dog-locking element 222 and the second piston 2222. A control valve 225 is arranged between the hydraulic-fluid actuator 224 and the hydraulic-fluid channels 2224. The control valve 225 is remotely operated, indicated here by the control valve 225 being provided with a valve actuator 2251 , a transceiver 2252 and an energy source (battery) 2253.

In the embodiment of the connector 1 shown, a third cylinder chamber V3 is formed in the end wall 21 1 of the locking dog housing 21 , in a recess 21 12 opposite the raised middle portion 21 1 1 of the end wall 21 1 . The third cylinder chamber V3 is bounded by a third piston 215, which is axially displaceable in the recess 21 12 and abuts in a fluid-sealing manner against the periphery of the re- cess 21 12. The third piston 215 is arranged to abut supportingly against a piston abutment 341 projecting up from an end wall 34 in the connector housing 3. The third cylinder chamber V3 is in closable fluid communication with the first and second cylinder chambers V1 , V2 through the centre bore 212 in the end wall 21 1 . Said fluid communication is controlled by the dog-locking element 222 through the axial position of the rod end portion 2223. A piston recess 2151 is arranged to accommodate a portion of the rod end portion 2223.

P27484PC00DEpno When there is a need to break the connection between the attached structure 4 and the attachment 6, the control valve 225 of the connector 1 is activated. Hydraulic fluid flows from the hydraulic-fluid accumulator 224 and through the hydraulic-fluid channels 2224 in the dog-locking element 222 into the first cylinder chamber V1 and the second cylinder chamber V2. Because of the difference in cross-sectional areas of said cylinder chambers V1 , V2, the first piston 221 will be displaced first. Because of the contact between the abutment surface 254 and the seat surface 221 1 and the radial play in the secondary dog hinge joints 26, the secondary locking dogs 25 will be displaced in the axial direction of the connector. Thereby, by the secondary locking dogs 25 being in engagement with the grooves 32, 33 of the connector housing 3, the connector housing 3 is displaced relative to the connector pin 2 so that the primary locking dogs 23 are relieved. The first cylinder chamber V1 has reached its maximum size. See figure 7.

When the axial displacement of the first piston 221 has ceased, the dog-locking element 222 is then displaced in the axial direction away from its abutment against the primary dog ridges 231 by means of the hydraulic pressure against the second piston 2222, as the secondary cylinder cham- ber V2 expands, the primary locking dogs 23 thereby being able to swing freely inwards in their recess 213. The connector housing 3 and the connector pin 2 are now held together by a reduced number of locking dogs 25. In this state, the load applied to the connector 1 by the structure 4 through the elongated body 5 may be sufficiently great for the secondary locking dogs 25 to shear and the structure 4 to be released from the attachment 6. Figure 7 shows the dog-locking element 222 in the process of being displaced and figure 8 shows the connector housing 3 disengaged from the primary locking dogs 23.

By the connector 1 in the embodiment shown being provided with a further, third piston 215, there will be a supply of hydraulic fluid to the third piston 215 on the axial displacement of the dog-locking element 222 in a last phase of the movement of the dog actuator 22, as the rod end portion 2223 pulls out of the centre bore 212 of the end wall 21 1 and opens to fluid communication between the first cylinder chamber V1 and the third cylinder chamber V3, as is shown in figure 8. By the third piston 215 abutting against the piston abutment 341 of the connector housing 3, the load on the secondary locking dogs 25 increases and this ensures the shearing of the secondary locking dogs 25 and release of the connector 1 as is shown in figure 9, in which dog fragments 255 have been shorn from the secondary locking dog 25.

On account of the joining-together of the connector 1 , the connector housing 3 is dividable (not shown), so that at least the first piston 221 can be connected to the rest of the dog actuator 22 after the secondary locking dogs 25 have engaged with grooves 32, 33 in the connector housing 3.

The end wall 21 1 of the locking dog housing 21 is preferably detachable to enable extraction of the dog actuator 22 from the connector pin 2 for the connector pin 2 to be adapted for other load conditions, for example by pistons 221 , 2222 and locking dogs 23, 25 being replaced.

P27484PC00DEpno Reference is now made to figure 10. In a second embodiment of the connector 1 , the second piston 2222 of the dog actuator 22 is provided with a further hydraulic-fluid channel 2227 having its mouth at the free end of an extended rod end portion 2223'. In this embodiment, the length of the rod end portion 2223' is so large that the hydraulic-fluid channel 2227 ends in the bore 212 of the end wall 21 1 when the locking dog-locking element 222 has been displaced to its axially extreme position with abutment against the locking dog housing 21 as is illustrated in figure 8. In this embodiment, the supply of hydraulic fluid is directed to the third cylinder chamber V3 independently of the supply to the first and second cylinder chambers V1 , V2, by the hydraulic-fluid channels 2224, 2227 each being arranged with a control valve 225 and 225', respectively; shown here as a tripar- tite directional valve with a closed centre. When a hydraulic-fluid accumulator is used as the hydraulic-fluid source, it is an advantage to arrange a hydraulic-fluid accumulator 224 and 224', respectively, for each of the control valves 225, 255', as is shown in figure 8, to ensure that there will be sufficiently high hydraulic pressure in every operative situation.

Even though it is an advantage to arrange hydraulic-fluid sources in the form of hydraulic-fluid ac- cumulators 224, 224' integratedly in the connector 1 , this does not exclude the use of a remotely located hydraulic-fluid source, arranged, for example, on the attached structure 4 and connected to the connector 1 via a conduit (not shown) extended along the attachment element 5. This may be applicable in particular when the structure 4 is in shallow water and the distance from the structure 4 to the connector 1 is short. Figure 1 1 shows the connector 1 arranged in outer casing sections 27, 35 provided with attachment lugs 271 , 351 for connection to the attachment element 5. The locking dog housing 21 is re- leasably attached internally in a first, outer casing section 27, for example with threads (not shown). The connector housing 3 is releasably attached internally in a second, outer casing section 35, shown here by means of a locking ring 353 resting against an end of the connector housing 3 and typically being connected to the second, outer casing section 35 by means of threads (not shown). To prevent the locking dogs 23, 25 from being subjected to unfavourable strain due to torsion via the attachment element 5, the casing sections 27, 35 are provided with engagement elements, shown here as pins 272 extending in the axial direction of the first casing section 27 for engagement in corresponding recesses 352 in the second casing section 35. It should be noted that all the above-mentioned embodiments illustrate the invention, but do not limit it, and persons skilled in the art may construct many alternative embodiments without departing from the scope of the attached claims. In the claims, reference numbers in parentheses are not to be regarded as restrictive.

The use of the verb "to comprise" and its different forms does not exclude the presence of ele- ments or steps that are not mentioned in the claims. The indefinite article "a" or "an" before an element does not exclude the presence of several such elements.

P27484PC00DEpno The fact that some features are indicated in mutually different dependent claims does not indicate that a combination of these features cannot be used with advantage.

P27484PC00DEpno