An anchoring system for pipeline end termination structures

Abstract

An anchoring system for a pipeline end termination, PLET, comprising a base and a connecting arm for engaging with and supporting the PLET, wherein the connecting arm is pivotally attached to the base for rotation, in use, between a first position, upstanding with respect to the base, and a second position, lying against the base.

Description

[0001] An anchoring system for pipeline end termination structures

[0002] The present disclosure relates to an anchoring system for pipeline end termination structures, PLETs. In particular, to such an anchoring system that provides for a direct connection. It further relates to a method of anchoring a PLET with respect to a seabed and initiation of pipelay away from the anchoring system.

[0003] Suction piles are a type of foundation used primarily in offshore engineering, particularly for the foundations of subsea structures . These piles are designed to be installed on the seafloor by creating a pressure differential between the inside and outside of the pile, allowing the pile to "suction" itself into the seabed.

[0004] Suction piles may be used to provide an “inline anchor” to a pipeline to stop it “walking”, but they are more typically used for the foundations of structures, or as the start-up pile against which the pipeline will initiate.

[0005] Challenges, however, exist with minimising the number of structures needed for the anchoring, foundation and start-up reaction of pipelines, and end terminations thereof.

[0006] The present invention arose as a result of work looking to provide an improved fixing means for subsea pipeline end terminations to their foundations and provide fixity to all degrees of freedom and allow direct initiation (or start-up) of pipelay.

[0007] It should be noted that whilst the present invention is suited to use with suction piles, it is not to be limited as such. The anchoring system may be implemented with alternative foundations such as pin pile or gravity base structures, or combined into larger template or manifold structures, as will be readily appreciated by those skilled in the art. However, suction piles may, by virtue of the disclosed arrangements and connection methods, be used as all of the startup pile, the anchoring pile and the foundation pile simultaneously.Representative features are set out in the following clauses, which stand alone or may be combined, in any combination, with one or more features disclosed in the text and / or drawings of the specification.

[0008] According to the present invention, in a first aspect, there is provided an anchoring system for a pipeline end termination, PLET, comprising a base and a connecting arm for engaging with and supporting the PLET, wherein the connecting arm is pivotally attached to the base for rotation, in use, between a first position, upstanding with respect to the base, and a second position, lying against the base.

[0009] By such an arrangement, the PLET can be installed directly onto the base vertically. Rotation of the connecting arm relative to the base, and its positioning in the first position, greatly simplifies the attachment of the PLET to the connecting arm. Such rotation preferably occurs about an axis that is substantially horizontal in use.

[0010] As noted, whilst not limited as such, the base may comprise a suction pile, allowing for particular utility of the anchoring system in areas with low soil strength. The base may comprise the frame top of a suction pile.

[0011] The connecting arm may be connected to the base at a fixed location or by a lockable swivelling system that allows the directional alignment of the connecting arm to be adjusted. For such purposes, the connecting arm may be connected to the base, such that it may be rotated about a swivel axis that is substantially vertical in use. Regardless of the specific form of the base, i.e. whether it comprises a suction pile or otherwise, it may comprise a first part to which the connecting arm is attached and a second part that is fixed with respect to a seabed in use, wherein the first part is rotatably mounted to the second part for rotation about the swivel axis.

[0012] The connecting arm may be rotated about the swivel axis before and / or after engagement of the PLET with the connecting arm.

[0013] The first part preferably comprises a turntable. The second part preferably comprises foundation base, of any suitable form, as discussed. The turntable ispreferably pivotally attached to the foundation base at its centre and mounted on low friction bearings to allow easy horizontal alignment (by rotation about the swivel axis).

[0014] The first part of the base (and thereby the connecting arm) is preferably rotatable about the swivel axis through a full 360 degrees.

[0015] A suitable base locking mechanism will preferably be provided for selectively locking the first part against rotation about the swivel axis. The locking mechanism may comprise any suitable form of lock or friction brake. It may comprise a removable locking pin, cam locks or a sprung friction plate that can be manipulated (i.e. inserted, removed or activated for locking and unlocking) using a remotely operated vehicle, ROV.

[0016] As will be appreciated, the base locking mechanism may be activated to lock the orientation of the connecting arm with respect to the swivel axis at any desired time during the anchoring of the PLET. It may be before or after the rotation of the connecting arm between the first and second positions (about the substantially horizontal axis). It may be before or after the locking of the connecting arm in its second position.

[0017] Rotation of the connected PLET about the swivel axis to the correct PLET alignment greatly simplifies installation. The base (or foundation), which, as discussed, may comprise a suction pile or otherwise, can be installed with removal of tight tolerances on alignment of the foundation. Following installation of the base, the orientation of the connecting arm may be altered as required. Moreover, an installation vessel can be orientated to best suit the prevailing weather conditions and minimise vessel movement during installation which will increase the limiting sea state for the installation operation.

[0018] A holdback mechanism is preferably provided for keeping the connecting arm in the first position to permit engagement with the PLET. The holdback mechanism may comprise any suitable form of lock or friction brake. It may comprise a removable locking pin or a sprung friction plate that can be manipulated (i.e.inserted, removed or activated for locking and unlocking) using a remotely operated vehicle, ROV.

[0019] There may be a stop provided on the base for limiting rotation of the connecting arm, wherein the holdback mechanism holds the connecting arm with respect to the stop. The holdback mechanism may hold the connecting arm against the stop. The stop may be vertical in use or may, more preferably, be arranged at a predetermined angle to the vertical in use. The stop may set the angle of the connecting arm in the first position.

[0020] Preferably, in use, in the first position, a longitudinal axis of the connecting arm is arranged at an angle to the vertical

[0021] With the connecting arm arranged at an angle to the vertical in the first position vertical movement of the engaged PLET, as a result of vertical movement of a pipeline to which the PLET is attached, may be taken up through rotation of the connecting arm, thereby limiting the transfer of loads and protecting against damage. For such purposes, the holdback mechanism, preferably automatically releases upon application of a predetermined load.

[0022] A locking mechanism is preferably provided to lock the PLET onto the connecting arm during the transfer from the first position to the second position. This locking mechanism will preferably be automatically activated, and may take any conventional form, as will be appreciated by those skilled in the art.

[0023] A further locking mechanism is preferably provided for locking the connecting arm in the second position. The further locking mechanism may comprise any suitable form of lock. It may comprise a removable locking pin or cam lock that can be manipulated (i.e. inserted and removed for locking and unlocking) using a remotely operated vehicle, ROV.

[0024] The locking mechanism or further locking mechanism may lock the PLET in engagement with the base.The PLET and the connecting arm preferably comprise complementary engagement means. The complementary engagement means preferably permit for relative pivotal movement between the connecting arm and the PLET. The complementary engagement means preferably comprise a hook and a hitch, the hitch arranged to be captured by the hook, wherein the PLET comprises the hook and the connecting arm comprises the hitch, or vice versa.

[0025] A hooked engagement allows for particularly straightforward engagement, particularly when the connecting arm is placed in the first position.

[0026] The connecting arm preferably comprises a hinged arm. The hinged arm is preferably pivotally attached to the base in a first end portion of the arm, and the complementary engagement means of the connecting arm is preferably spaced along the arm from the first end portion. It may be provided in an opposed second end portion of the arm.

[0027] Such an arrangement allows for the spacing of the complementary engagement means of the connecting arm away from the base, which makes engagement easier.

[0028] The angle of rotation of the connecting arm between the first and second positions is preferably at least 90 degrees. It may be greater than 90 degrees.

[0029] Preferably, in use, in the second position, a longitudinal axis of the connecting arm is arranged at a horizontal angle but it could be arranged with a slight angle to the horizontal to facilitate a smooth lowering of the connected pipeline to the seabed.

[0030] Preferably, a buoyancy member is removably attachable to the PLET. Such a buoyancy member may comprise a buoyancy element attached to the PLET via a suitable means that could typically be a cable. The buoyancy element may comprise a balloon.

[0031] The buoyancy member is preferably attached to the PLET by a buoyancy attachment member, which projects away from an attachment point with the PLETand comprises an attachment point for the buoyancy element that is spaced from the PLET. The buoyancy attachment member may comprise a post or yoke. The buoyancy attachment member is preferably detachable.

[0032] By use of a buoyancy attachment element, it is possible to suitably maintain an appropriate orientation of the PLET during anchoring, at least during initial engagement of the PLET with the connecting arm. The buoyancy attachment element allows for suitable control based on the centre of gravity of the PLET. The PLET will preferably be substantially vertically oriented during lowering and the initial engagement steps, ending up substantially horizontal when anchored in the second position.

[0033] According to the present invention, in a further aspect, there is provided a method of anchoring a pipeline end termination, PLET, with respect to a seabed, the method comprising: providing a base and a connecting arm for engaging with and supporting the PLET, the connecting arm being pivotally attached to the base, placing the connecting arm in a first position, upstanding with respect to the base, lowering the PLET to the connecting arm and engaging and anchoring the PLET and the connecting arm with one another, and following engagement and anchoring of the PLET and the connecting arm with one another, rotating the connecting arm until a second position, in which the connecting arm is lying against the base, is reached.

[0034] As discussed, the connecting arm may be connected to the top structure of a suction pile, or any other suitable foundation structure, at a fixed location or by a lockable swivelling system that allows the directional alignment of the connecting arm to be adjusted. Regardless of the specific form of the base, it may comprise a first part to which the connecting arm is attached and a second part that is fixed with respect to a seabed in use, wherein the first part is rotatably mounted to the second part for rotation about a swivel axis that is substantially vertical in use.

[0035] The connecting arm may be rotated about the swivel axis before and / or after engagement of the PLET with the connecting arm.The first part preferably comprises a turntable. The second part preferably comprises foundation base, of any suitable form, as discussed. The turntable is preferably pivotally attached to the foundation base at its centre and mounted on low friction bearings to allow easy horizontal alignment (by rotation about the swivel axis).

[0036] The first part of the base (and thereby the connecting arm) is preferably rotatable about the swivel axis through a full 360 degrees.

[0037] A suitable base locking mechanism will preferably be provided for selectively locking the first part against rotation about the swivel axis, i.e. at any desired oreintation. The locking mechanism may comprise any suitable form of lock or friction brake. It may comprise a removable locking pin, a cam lock or a sprung friction plate that can be manipulated (i.e. inserted, removed or activated for locking and unlocking) using a remotely operated vehicle, ROV.

[0038] As will be appreciated, the base locking mechanism may be activated to lock the orientation of the connecting arm with respect to the swivel axis at any desired time during the anchoring of the PLET. It may be before or after the rotation of the connecting arm between the first and second positions (about the substantially horizontal axis). It may be before or after the locking of the connecting arm in its second position.

[0039] As noted, the optional rotation of the connected PLET about the swivel axis to the correct PLET alignment greatly simplifies installation. The base (or foundation), which, as discussed, may comprise a suction pile or otherwise, can be installed with removal of tight tolerances on alignment of the foundation. Following installation of the base, the orientation of the connecting arm may be altered as required.

[0040] Moreover, an installation vessel can be orientated to best suit the prevailing weather conditions and minimise vessel movement during installation which will increase the limiting sea state for the installation operation.

[0041] The method may be implemented using the anchoring system as discussed in any of the above paragraphs.The connecting arm and the PLET are preferably engaged with one another by hooking the connecting arm and the PLET to one another using a hook provided on the connecting arm or the PLET.

[0042] Prior to engagement of the connecting arm and the PLET with one another, the connecting arm is preferably held in the first position. Following rotation, the connecting arm is preferably locked in the second position.

[0043] The PLET is preferably supported by a buoyancy member during connection, at least during the initial engagement of the PLET with the connecting arm, and preferably through until the connecting arm is in the second position and fully anchored.

[0044] Further, preferable, features will be apparent from the dependent claims and the description.

[0045] Non-limiting embodiments of the invention will now be discussed with reference to the following drawings:

[0046] Figure 1 shows a perspective view of an anchoring system, which shows a pipeline end termination, PLET, engaged with a connecting arm that is pivotally attached to a suction pile, the anchoring system is shown in a second, anchored, position with the connecting arm lying substantially horizontally against a top structure of the suction pile;

[0047] Figure 2 shows an alternative perspective view of the anchoring system of Figure 1 ;

[0048] Figure 3 shows a perspective view of the PLET of the anchoring system of Figures 1 and 2;

[0049] Figure 4 shows a side view of the PLET of Figure 3;

[0050] Figure 5 shows a top view of the suction pile of Figures 1 and 2, with the connecting arm in the second position;

[0051] Figure 6 shows a side view of the suction pile of Figure 5 embedded in the sea bed, with the connecting arm in the second position;Figures 7a to 7f show a connection sequence of the anchoring system of Figures 3 and 4.

[0052] With particular reference to Figures 1 and 2, there is shown an anchoring system 1 for a pipeline end termination, PLET, 2 for a pipeline 101. The anchoring system 1 comprises a fixed or lockable swivelling base 3 and a connecting arm 4 for engaging with and supporting the PLET 2. The connecting arm 4 is pivotally attached to the base 3 for rotation, in use, between a first position, as seen in Figures 7a and 7b, in which the connecting arm 4 is upstanding with respect to the base, and a second position, as seen in Figure 1, 2, 5, 6 and 7f, in which the connecting arm 4 lies against the base. In the second position, the connecting arm 4 is preferably substantially horizontal. The axis about which the connecting arm 4 rotates between the first and second positions is preferably substantially horizontal, as seen.

[0053] By such an arrangement, the PLET 2 can be installed directly onto the base 3, which may comprise a suction pile, as shown, or otherwise.

[0054] A method of anchoring the PLET 2 with respect to a seabed (100, Figure 6), using the anchoring system 1 , comprises placing the connecting arm 4 in the first position, as seen in Figure 7a, lowering the PLET 2 to the connecting arm 4 and engaging the PLET 2 and the connecting arm 4 with one another, as seen in Figure 7b, and, following engagement of the PLET 2 and the connecting arm 4 with one another, rotating the connecting arm and PLET 4, as seen in Figures 7c to 7e, and locking the PLET 2 to the connecting arm, the rotation of the connecting arm continuing until the second position is reached, as seen in Figure 7f.

[0055] A holdback mechanism (not shown) is preferably provided for holding the connecting arm 4 in the first position. Fixing the position of the connecting arm 4 in this way will aid, in particular, during engagement of the connecting arm 4 with the PLET 2. The holdback mechanism will be released following engagement to allow for rotation of the connecting arm 4. As will be appreciated by those skilled in the art, the holdback mechanism may take various forms. It preferably comprises a sprung loaded friction plate or one or more removable locking pins of conventional form thatcan be engaged and disengaged, for locking and unlocking respectively, using a remotely operated vehicle, ROV. A stop 5 is preferably provided on the base 3 for limiting rotation of the connecting arm 4 in the vertical plane. In the present case, a pair of stops 5 are provided. However, there may be more or less stops 5 provided. The stops 5 are preferably arranged to abut the connecting arm 4 in the first position, as best seen in Figure 7a. The stops 5 may comprise fixed plates, as seen, but may take any suitable alternative form, as will be appreciated by those skilled in the art. The stop(s) 5, regardless of form, preferably set the angle of the connecting arm 4 in the first position by limiting its rotation. The holdback mechanism preferably holds the connecting arm 4 with respect to the stop(s) 5. The holdback mechanism may hold the connecting arm against the stop(s) 5. The stop(s) 5 may be arranged at a predetermined angle to the vertical in use, wherein the predetermined angle corresponds to a desired angle to the vertical for the connecting arm 4 in the first position. With the connecting arm 4 arranged at an angle to the vertical in the first position, as shown, for example, in Figure 7a, vertical movement of the engaged PLET 2 as a result of vertical movement of a pipeline 101 to which the PLET 2 is attached may be taken up through rotation of the connecting arm 4, whereby the transfer of loads is limited and risks of damage are reduced. For such purposes, the holdback mechanism, preferably automatically releases upon application of a predetermined load to permit rotation of the connecting arm 4 away from the stop(s) 5.

[0056] A locking mechanism is preferably provided for locking the connecting arm 4 in the second position. As will be appreciated by those skilled in the art, the locking mechanism may take various forms. It preferably comprises one or more removable locking pins of conventional form that can be engaged and disengaged, for locking and unlocking respectively, using a remotely operated vehicle, ROV. The locking mechanism preferably locks the PLET 2 in engagement with the base 3, as is the case in the present arrangement, although this need not be the case. The second locking means may alternatively lock the connecting arm 4 in engagement with base 3. In the depicted arrangement, the base 3 is provided with openings 7, as best seen in Figure 5, which preferably comprise slots. Whilst two openings 7 are shown, there may be more or less openings 7 provided. The openings 7 are configured to receive suitably formed engagement members 8 of the PLET 2, as best seen inFigures 1 and 2. The further locking mechanism is preferably engaged following receipt of the engagement members 8 by the openings 7 for preventing removal of the engagement members 8 from the openings 7. By such an arrangement, the locking means will lock the PLET 2 in engagement with the base 3 and lock the connecting arm 4 in the second position. The PLET 2 may comprise trunions 9, as shown. In such case, the further locking mechanism may engage with the trunions 9 to prevent the removal of the engagement member(s) 8 from the opening(s) 7.

[0057] A lockable swivelling system 30 is preferably provided that allows the directional alignment of the connecting arm 4 to be adjusted by rotation about a substantially vertical axis 31 , as shown in Figure 1. The specific form of the lockable swivelling system 30 is not particularly limited, as will be appreciated by those skilled in the art. It may comprise a first part 30a to which the connecting arm 4 is attached and a second part 30b that is fixed with respect to a seabed in use, wherein the first part 30a is rotatably mounted to the second part 30b for rotation about the vertical swivel axis that is substantially vertical in use.

[0058] The first part 30a preferably comprises a turntable. The second part 30b preferably comprises foundation base, of any suitable form, as discussed. The turntable is preferably pivotally attached to the foundation base at its centre and mounted on low friction bearings to allow easy horizontal alignment (by rotation about the swivel axis).

[0059] A suitable base locking mechanism is provided for selectively locking the first part 30a, and thereby the connecting arm 4, against rotation about the swivel axis 31. Accordingly, the orientation of the connecting arm 4 with respect to the swivel axis 31 may be varied and also may be set and fixed as required during use. The locking mechanism may comprise any suitable form of lock or friction brake. It may, for example, comprise a removable locking pin, a cam lock, or a sprung friction plate that can be manipulated (i.e. inserted, removed or activated for locking and unlocking) using a remotely operated vehicle, ROV.

[0060] As will be appreciated, the base locking mechanism may be activated to lock the orientation of the connecting arm with respect to the swivel axis at any desiredtime during the anchoring of the PLET. It may be before or after the rotation of the connecting arm 4 between the first and second positions (about the substantially horizontal axis). It may be before or after the locking of the connecting arm 4 in its second position.

[0061] Rotation of the connected PLET about the swivel axis to the correct PLET alignment greatly simplifies installation. The base, i.e. the first part 30a (or foundation), which, as discussed, may comprise a suction pile or otherwise, can be installed with removal of tight tolerances on alignment of the foundation. Following installation of the first part 30a, the orientation of the connecting arm 4 may be altered as required, by rotation of the second part 30b about the swivel axis.

[0062] Moreover, an installation vessel can be orientated to best suit the prevailing weather conditions and minimise vessel movement during installation which will increase the limiting sea state for the installation operation.

[0063] The PLET 2 and the connecting arm 4 preferably comprise complementary engagement means 10, 11. The complementary engagement means most preferably comprise a hook 10 and a hitch 11 , wherein the hitch 11 is arranged to be captured by the hook 10. In the present arrangement, as is preferred, the PLET 2 comprises the hook 10 and the connecting arm 4 comprises the hitch 11. In alternative arrangements, these may be swapped around. A hooked arrangement allows for particularly straightforward engagement, as best see in Figure 7b. There are two hooks 10 provided in the present arrangement. There may be more or less hooks 10 provided in alternative arrangements.

[0064] The complementary engagement means in alternative arrangements may not comprise a hook and a hitch but may rather comprise a suitable latching mechanism for permitting the ready engagement of the connecting arm 4 and the PLET 2 with one another.

[0065] Irrespective of the specific form of the complementary engagement means 10, 11 , the connecting arm 4 and the PLET 2 are preferably arranged such that following engagement with one another, pivotal movement between the two is possible, as seen, for example, in Figures 7b and 7c, noting that relative pivotal movementoccurs about an axis of the hitch 11 for movement from the position in Figure 7b to the position in Figure 7c. Once in position in figure 7c a lock will preferably engage preventing further rotation of the hook, and thereby the PLET 2, about the axis of the hitch. Such a lock may take any conventional form, as will be readily appreciated by those skilled in the art.

[0066] The connecting arm 4 preferably comprises a hinged arm, as shown. The hinged arm is preferably pivotally / hingedly attached to the fixed or swivelling base in a first end portion 4a of the arm, as shown. The complementary engagement means of the connecting arm 4, i.e. the hitch 11 in the present arrangement, is preferably spaced along the arm from the first end portion 4a. It is most preferably provided in an opposed second end portion 4b of the arm, as shown. Whilst the form of the connecting arm 4 is not particularly limited, in the present arrangement, by way of example only, it comprises a pair of rails 4c, wherein the rails are joined to one another via the hitch 11 , which extends between the rails 4c.

[0067] Regardless of the specific form of the connecting arm 4 it is preferable, in line with the discussion above, that the PLET 2 engages directly with the base 3. For such purposes, it is preferable that the connecting arm comprises an opening 19 for at least partially receiving the PLET 2. The PLET 2 may directly engage with the base 3 through the opening 19. The engagement members 8 may project through the opening, as seen for example in Figures 7d and 7e. In the present arrangement, the opening 19 is provided between the rails 4c. The opening 19 may also help facilitate pivotal movement between the connecting arm 4 and the PLET 2 following engagement with one another, with the PLET 2 moving within the opening 19.

[0068] The axis of rotation of the connecting arm 4 is preferably substantially horizontal in use, as is evident from Figures 7a to 7f.

[0069] For installation of the PLET 2, a buoyancy member 12 is preferably removably attachable to the PLET 2, as seen in broken lines in Figures 7a to 7f. Such a buoyancy member may comprise a buoyancy element 13 attached to the PLET 2 viaa cable 14. The buoyancy element 13 may comprise a balloon or similar floatation element.

[0070] The buoyancy member 12 is preferably attached to the PLET by a buoyancy attachment member 15, which projects away from an attachment point 16 with the PLET 2 and comprises an attachment point 17 for the buoyancy member 12 that is spaced from the PLET 2. The buoyancy attachment member 15 may comprise a post, as shown. It may take alternative forms. It may comprise a yoke. The buoyancy attachment member is preferably detachable from the PLET 2. By such an arrangement, once the PLET 2 is locked in the second position, the buoyancy attachment member 15 may be removed. The buoyancy attachment member 15 may be locked to the PLET 2 by one or more removable locking pins 18 of conventional form that can be engaged and disengaged, for locking and unlocking respectively, using a remotely operated vehicle, ROV.

[0071] By use of the buoyancy attachment element 15, it is possible to suitably maintain an appropriate orientation of the PLET 2 during anchoring, at least during initial engagement of the PLET with the connecting arm, as shown in Figure 7b. The buoyancy attachment member 15 allows for suitable control based on the centre of gravity of the PLET 2. The PLET 2 will preferably be substantially vertically oriented during lowering and initial engagement steps, as seen in Figures 7a and 7b, with its orientation controlled during rotation of the connecting arm 4, as seen in Figures 7c to 7e, until it is substantially horizontal, as seen in Figures 7f when anchored in the second position.

[0072] Numerous alternative arrangements and modifications to the embodiments as described herein will be readily appreciated by those skilled in the art within the scope of the appended claims. In particular, features of the described embodiments may be readily combined.

[0073] When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

[0074] Although certain example embodiments of the invention have been described, the scope of the appended claims is not intended to be limited solely to these embodiments. The claims are to be construed literally, purposively, and / or to encompass equivalents.

Claims

Claims1. An anchoring system for a pipeline end termination, PLET, comprising a base and a connecting arm for engaging with and supporting the PLET, wherein the connecting arm is pivotally attached to the base for rotation, in use, between a first position, upstanding with respect to the base, and a second position, lying against the base2. An anchoring system as claimed in Claim 1 , wherein a holdback mechanism is provided for fixing the connecting arm in the first position to permit engagement with the PLET.

3. An anchoring system as claimed in Claim 2, wherein a stop is provided on the base for limiting rotation of the connecting arm, and the holdback mechanism fixes the connecting arm with respect to the stop.4 An anchoring system as claimed in any preceding claim, wherein a locking mechanism is provided for locking the PLET to the connecting arm during rotation of the connecting arm.

5. An anchoring system as claimed in any preceding claim, wherein a further locking mechanism is provided for locking the connecting arm in the second position6. An anchoring system as claimed in any preceding claim, wherein the further locking mechanism locks the PLET in engagement with the base.

7. An anchoring system as claimed in any preceding claim, wherein the base comprises a first part to which the connecting arm is attached and a second part that is fixed with respect to a seabed in use, wherein the first part is rotatably mounted to the second part for rotation about a swivel axis that is substantially vertical in use.

8. An anchoring system as claimed in Claim 7, wherein a base locking mechanism is provided for locking the first part against rotation about the swivel axis.

9. An anchoring system as claimed in any preceding claim, wherein the PLET and the connecting arm comprise complementary engagement means10. An anchoring system as claimed in Claim 9, wherein the complementary engagement means comprise a hook and a hitch, the hitch arranged to be captured by the hook, wherein one of the PLET and the connecting arm comprises the hook and the other of the PLET and the connecting arm comprises the hitch.

11. An anchoring system as claimed in Claim 9 or 10, wherein the connecting arm is pivotally attached to the base in a first end portion of the arm, and the complementary engagement means of the connecting arm is spaced along the connecting arm from the first end portion.

12. An anchoring system as claimed in any preceding claim, wherein the angle of rotation of the connecting arm between the first and second positions is at least 90 degrees.

13. An anchoring system as claimed in any preceding claim, wherein the angle of rotation of the connecting arm between the first and second positions is greater than 90 degrees.

14. An anchoring system as claimed in any preceding claim, wherein, in use, in the first position, a longitudinal axis of the connecting arm is arranged vertically or at an angle to the vertical.

15. An anchoring system as claimed in any preceding claim, wherein the base comprises a suction pile, pin pile or gravity base.

16. An anchoring system as claimed in any preceding claim further comprising a buoyancy member removably attachable to the PLET.

17. An anchoring system as claimed in Claim 16, wherein the buoyancy member is attached to the PLET by a buoyancy attachment member, which projects awayfrom an attachment point with the PLET and comprises an attachment point for the buoyancy member that is spaced from the PLET.

18. An anchoring system as claimed in Claim 17, wherein the buoyancy attachment member is removably attached to the PLET.

19. A method of anchoring a pipeline end termination, PLET, with respect to a seabed, the method comprising:providing a base, which is fixed with respect to the seabed, and a connecting arm for engaging with and supporting the PLET, the connecting arm being pivotally attached to the base,placing the connecting arm in a first position, upstanding with respect to the base, by pivoting the connecting arm attached to the base,lowering the PLET to the connecting arm in the first position, and engaging the PLET and the connecting arm with one another, andfollowing engagement of the PLET and the connecting arm with one another, rotating the connecting arm until a second position, in which the connecting arm is lying against the base, is reached.

20. A method as claimed in Claim 19, wherein following engagement, the PLET and the connecting arm are pivoted relative to one another.

21. A method as claimed in Claim 19 or 20, wherein the connecting arm and the PLET are engaged with one another by hooking the connecting arm and the PLET to one another using a hook provided on the connecting arm or the PLET.

22. A method as claimed in any of Claims 19 to 21 , wherein, prior to engagement of the connecting arm and the PLET with one another, the connecting arm is fixed in the first position.

23. A method as claimed in any of Claims 19 to 22, wherein, following rotation, the connecting arm is locked in the second position.

24. A method as claimed in Claim 23, wherein the connecting arm is locked in place by locking engagement between the PLET and the base.

25. A method as claimed in any of Claims 19 to 24, wherein the PLET is supported by a buoyancy member during connection.

26. A method as claimed in any of Claims 19 to 25, wherein the base comprises a first part to which the connecting arm is attached and a second part that is fixed with respect to a seabed in use, wherein the first part is rotatably mounted to the second part for rotation about a swivel axis that is substantially vertical in use, and the method further comprises rotating the connecting arm about the swivel axis.

27. A method as claimed in Claim 26, wherein the connecting arm is rotatable about the swivel axis before and / or after engagement of the PLET with the connecting arm.

28. A method as claimed in Claims 26 or 27, wherein a base locking mechanism is provided for selectively locking the first part against rotation about the swivel axis.

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