The present invention relates a marine pipelaying method, in particular employing J-laying, and a system configured for use in this method.

In a J-lay pipelaying process, a pipeline is launched along a J-lay firing line, wherein in succession new pipe sections are connected to an already launched seagoing part of the pipeline. This connection takes place in a vertical, or nearly vertical orientation of these pipe sections while being supported in line with the firing line on the tower. To this end each new pipe section is to be loaded onto the tower. It is common in the art to employ a dedicated and complex loader device for this purpose, which upends or otherwise partly or completely lifts pipe sections in order to load these on the tower, such that it may be engaged by the head clamp. Examples are shown in US2005100413 and W02012091560.

The invention relates to a marine J-lay system for J-lay pipelaying an offshore pipeline onto the seabed wherein the use of a complex dedicated loader device for this purpose is omitted or at least not necessary.

In W02015/071415 a J-lay system is shown, wherein a hang-off table defines the single firing line of the system. Two pipe-moving devices are provided in the system, that are associated with the single firing line. Each of these devices has a tower that is pivotal between a horizontal pick-up position and a position aligned with the natural angle of the launched pipeline, which may be vertical. At the top end, each tower is provided with a pivotal beam from which the travelling head clamp is suspended by one or more winch driven cables. The pivoting motion of the pivotal beams allows for the pipe-moving devices to operate sequentially; the one bringing a new pipe section in the single firing line after the other pipe-moving device. Each pipe-moving device is provided with holding devices that have gripper members to grip a pipe section. During pick-up the new pipe section to be picked up is located generally along a side of the tower in the horizontal pick-up position thereof, so in a side-by-side arrangement with the now horizontal tower. The new pipe section is then transferred generally sideways towards the tower in order to have the pipe section aligned with the head clamp that is then mated with the adjacent end of the new pipe section. The WO2015/07145 J-lay system is complex. The invention provides a system for J-lay pipelaying an offshore pipeline onto the seabed, which system is described in claim 1.

In contrast to the side-by-side arrangement of the tower and new pipe section during loading in the system of W02010071415, the new pipe section support and guide structure of the inventive system is adapted to be mounted or is mounted on the vessel at a location adjoining the top end of the tower in the horizontal or near horizontal loading position thereof and aligned with the pipe support axis, wherein the one or more pipe section guides are configured to support the new pipe section in a horizontal or near horizontal supply position that is aligned with the pipe support axis in the horizontal or near horizontal loading position of the tower. The transfer of the new pipe section in the inventive system is substantially linearly along the pipe support axis or along a loading axis that is parallel to the pipe support axis, so in general in longitudinal direction of the new pipe section. The J-lay tower and the travelling head clamp are adapted to receive the new pipe section in this manner, wherein the new pipe section travels linearly from the one or more pipe section guides onto the tower such that the trailing end of a new pipe section is located on the pipe support axis of the travelling head clamp and is engageable by the travelling head clamp.

Preferably, the system further comprises a line-up tool, configured for aligning a new pipe section with the launched seagoing part of the pipeline for the purpose of interconnecting, e.g. welding, said new pipe section to the launched seagoing part of the pipeline.

Preferably, the hang-off clamp is mounted to said base frame and is pivotal about a horizontal pivot axis relative to said base frame, wherein the system further comprises a hang-off clamp pivot angle adjustment system adapted to adjust the pivot angle of the hang-off clamp to the angle of the firing line.

Preferably, the tower is provided with a further or lower clamp, e.g. embodied as an external line-up clamp, below the travelling head clamp. Preferably this further or lower clamp is movable along the tower, e.g. over one or more rails, e.g. over one or more rails in common with the travelling head clamp, between a top position and a lower position thereof relative to the tower. ower is provided with one or more first pipe section guides, e.g. provided with one or more rollers adapted to support a new pipe section. These one or more pipe section guides are, in an operative position thereof, aligned with the pipe support axis of the travelling head clamp, e.g. the one or more pipe section guides on the tower being movable to a retracted position, e.g. in order to handle a PLET with the J-lay system.

In embodiments the tower is provided at the top end thereof with two sets of one or more main sheaves for winch driven main cables, wherein the travelling head clamp is provided with two corresponding sets of sheaves, so that the travelling head clamp is suspended at two opposite lateral sides of the tower in a multi-fall cable arrangement, e.g. so that the new pipe section is substantially linearly transferred from said one or more pipe section guides onto the tower whilst passing between said multi-fall cable arrangements.

In embodiments the system further comprises an abandonment and recovery (A& R) system, said A&R system comprising one or more A&R sheaves mounted at the top end of the tower, e.g. two A&R sheaves at opposite lateral sides of the tower or a single A&R sheave that is movable between a retracted position and a deployed position.

In embodiments, the system further comprises a working station supported by the base frame and adapted to provide access for personnel to the top end of the already launched seagoing part of the pipeline whilst held by the hang-off clamp during the J-lay process.

The working station may in embodiments comprise a platform floor that is supported on pivotal arms relative to the base frame, preferably each side of the work station being supported on multiple pivotal arms in a parallelogram mechanism so as to maintain a horizontal orientation of the platform floor for various positions of the parallelogram mechanism.

Furthermore in embodiments, the platform floor may comprise one or more pivotal tiles which are pivotal between a horizontal operative position and a substantially vertical non operative position, wherein in said non-operative position the one or more pivotal tiles provide an opening adapted to receive therein or pass through at least one of the travelling head clamp and, if present, the further or lower clamp.

In embodiments of the system, the length of a new pipe section handled or to be handled thereby is between 25 and 45 foot, e.g. 30 foot or 40 foot. Therefore, in embodiments, the ad clamp along the tower, and thereby the height of the tower, may be in accordance there with.

For example the tower has operational positions that are vertical, as well as within 30 degrees from vertical. Clearly, in embodiments, the tower could be arranged in any position between horizontal and vertical, and possibly also in a through vertical overboarding position, e.g. in view of handling PLET’s or other pipeline accessories.

The invention further relates to a vessel provided with a marine J-lay system according to the invention.

The invention further relates to method for laying a pipeline on the seabed in a J-lay process wherein use is made of a marine J-lay system according to the invention, or of a vessel according to the invention.

The invention further relates to a method for laying a pipeline on the seabed in a J-lay process from a vessel according to claim 14.

The invention will now be described with reference to the drawings. In the drawings:

- figs. 1 - 6 illustrate a first embodiment of a system according to the invention,

- fig. 7 illustrates a second embodiment of a system according to the invention,

- figs.8-17 show the system of figure 7 in subsequent stages for J-laying of a pipeline,

- figs. 18 - 20 illustrate the handling of a first or leading PLET of a pipeline using the system of figure 7.

In the drawings, Figures 1 - 6 show in a front-side perspective view an inventive J-lay system being used in a method according to the invention, namely in six subsequent working positions which accord to different steps of the method.

The figures show a marine J-lay system 1 for J-lay pipelaying an offshore pipeline 11 onto the seabed, configured for being used in the method according to the invention.

The system 1 comprises a tiltable J-lay tower 2, which is configured for launching in J-lay mode a pipeline 11 along a J-lay firing line 12. The J-lay tower allows the pipeline 11 to be laid onto the seabed, further pipe sections 11a to be loaded and supported thereby while extending along a pipe support axis 13, and connected to the seagoing part 11 b of the e pipe support axis 13 collides with the firing line 12, and to continue the launching of the pipeline 11 along the firing line 12 towards the seabed.

The tiltable J-lay tower 2 is at the lower end 21 thereof pivotally mounted to a base frame 3 of the system 1 , which supports the tower 2. It is envisaged that this base frame 3 is supported by a deck of a marine pipelaying vessel (not shown).

The tower 2 is mounted to the base frame 3 such that it is pivotable around a substantially horizontal pivot axis 32 between one or more pipelaying positions, of which an example is shown in Figs.4-6, and a loading position, shown in Figs.1 - 2. As shown in Fig.3, therein the pipe support axis 13 has a pipe section angle a with respect to the horizontal, which is altered by said pivoting.

The system further comprises a tower tilting system 4, which is mounted to the base frame 3 to pivot the tower 2 around the horizontal pivot axis 32, thereby adjusting the pipe section angle a and moving the tower 2 between any pipelaying position and the loading position. The tower tilting system 4 comprises one or more tower tilting actuators 41 , here embodied as two hydraulic cylinders 41. In other, not shown embodiments these may for instance also be embodied otherwise, e.g. as telescopic adjuster devices.

In the one or more pipelaying positions the pipe section pivot axis 13, and thereby the pipeline 11 , or a pipeline section 11a thereof, substantially collides with the firing line 12. This firing line 12 has a sharp or slightly blunt firing line angle b with the horizontal, which firing line angle b is preferably between 45 and 110°, in particular between 70 and 90°. As is preferred, in the pipelaying position shown, the firing line 12 has a sharp firing line angle b of between 70 and 90° with the horizontal.

The system 1 further comprises a hang-off clamp 51 for supporting the weight of the launched pipeline 11 , or the seagoing part 11 b thereof, in a stationary position along the firing line 12. The hang-off clamp 51 is releasably connectable to a trailing end of the pipeline 11 , or the trailing end of a seagoing part 11 b of the pipeline 11. The hang-off clamp 51 is supported by the base frame 3 and, as is preferred, embodied as a friction clamp 51.

In other, not shown embodiments, this hang-off clamp may however also be embodied as a collar clamp, or combined collar and friction clamp. 2 is defined by the axis along which the hang-off clamp supports the pipeline 11 , or the seagoing part 11 b thereof. The hang-off clamp 51 is pivotable around horizontal pivot axis 32 into a position wherein the firing line 12 has a firing line angle b with respect to the horizontal by a hang-off clamp pivoting system 8, which is mounted to the base frame 3. The hang-off clamp pivot angle adjustment system 8 comprises one or more hang-off clamp pivot actuators 81 to realize the pivoting motion.

The pipe support axis 13 is defined by the axis along which a travelling head clamp 52 and preferably a line-up tool 53 on the tower 2 support a pipe section 11a, or the pipeline 11 , on the tower 2. The pipelaying position of the tower 2 is determined as the position thereof in which the pipe support axis 13 substantially collides with the actual firing line 12, at least at the height of the travelling head clamp 52. The tower 2 is shown in the pipelaying position in Figs.4-6 - it may be verified in these figures that the pipe support axis 13 corresponds to the firing line 12.

The collision of the pipe support axis 13 with the firing line 12 is effected by the common horizontal pivot axis 32 of the hang-off clamp 51 and the tower 2, along with the attuned positioning of the hang-off clamp 51 , with the travelling head clamp 52 and the line-up tool 53 on the tower 2 in respect to this axis 32.

The travelling head clamp 52 is suspended from the top end 24 of the tower 2 and is configured for, in its top position, being releasably connected to a trailing end of the pipe section 11a, and supporting a pipe section 11a above the seagoing part 11 b along a pipe support axis 13, and for, in either position thereof, supporting the launched pipeline 11 along the firing line 12 for supporting the weight of the launched pipeline 11 along the firing line 12, or a pipe section 11a along a pipe support axis 13 while being movable between a top position, shown in Figs.1-4, and a bottom position, shown in Fig.6. The travelling head clamp 52 is shown in an intermediate position in Fig.5. The travelling head clamp 52 is releasably connectable to a trailing end of a pipe section 11a.

For controlled vertical motion of the travelling head clamp 52 and the pipeline 11 supported thereby between the top and bottom position of the head clamp 52, the travelling head clamp 52 is in this embodiment integrated with a hoisting block 61 , which is connected via main cables 62 to one or more main winches (not shown), here in a multiple-fall

arrangement. These one or more main winches may for instance be mounted to the tower 2, or be provided elsewhere on the vessel, e.g. on the deck thereof. In this embodiment leaves 63 guide the main cables 62 from the hoisting block 61 parallel to the longitudinal direction of the tower 2 to the top end 24 thereof, from which these extend further to the or more main winches.

The tower 2 is provided with two parallel guide rails 22 extending parallel to the longitudinal direction of the tower 2, which are configured for guiding the hoisting block 61 with the head clamp 52 between the top and bottom position thereof in this direction. Through this arrangement, the travelling head clamp 52 is movable between these positions by operating the main winches.

The line-up tool 53 supported on the tower 2 in between the travelling head clamp 52 and the hang off clamp 51 , for use during continued J-lay operation whenever a pipe section 11 a is being connected to the seagoing part 11 b of the pipeline 11. The line-up tool 53 is configured for aligning a pipe section 11a, and thereby the pipe support axis 13, with the firing line 12, and thereby with the seagoing part 11 b of the pipeline 11 , when this pipe section 11a is supported by the travelling head clamp 52 above the seagoing part 11 b of the pipeline 11 , and the tower 2 is in the pipelaying position.

In this embodiment, the line-up tool 53 is embodied as a line-up clamp, equipped with a separate drive for moving the line-up tool 53 in a direction along the pipe support axis. This enables that, after connecting a pipe section 11a with the seagoing part 11 b of the pipeline 11 at a so-formed junction thereof, a cylindrical sleeve 14 may by means of this clamp be slid over this junction, e.g. a weld along the joined circumference of the seagoing part 11 b of the pipeline and a pipe section 11a, after which the sleeve is injected with a hardening resin. However, in alternative, not shown embodiments, e.g. for uses in which the application of such sleeves is not desired, the line-up tool may also be embodied otherwise, for instance a rollerbox as is known in the art.

In the loading position of the tower 2, shown in Figs.1-2, the tower 2 is able to support a pipe section 11a onto at least the travelling head clamp 52, being in its top position, and a first pipe section guide 55 that is here pivotably mounted to the tower 2, along the pipe support axis 13. In this embodiment this pipe support axis 13 is substantially horizontal, e.g. horizontal, or e.g. nearly horizontal, in the loading position of the tower 2, as may be verified in Fig.2. In the loading position of the tower 2 the first pipe section guide 55 is in the operative position thereof furthermore configured for guiding a pipe section 11a while it is )ng the pipe support axis 13 in the direction from the travelling head clamp 52 towards the hang-off clamp.

Here, the first pipe section guide 55 is embodied as a roller 55, and is pivotable between an operative position, shown in Figs.1-4 in which it engages a pipe section 11a, and a retracted position, in which it is free from the pipe section 11a, namely within the radial contour of the tower 3, shown in Figs.5 - 6. This retracted position enables that the first pipe section guide 55 does not hinder the motion of the hoisting block 61 with the travelling head clamp 52 along the guiding rails 31.

The system 1 further comprises a pipe section support and guide structure 7, comprising second pipe section guides 71 configured for, with the tower 2 in its loading position, supporting a pipe section 11a in a position in line with the pipe support axis 13 of the travelling head clamp 52, and, for guiding the pipe section 11a while it is being moved from this position, along the pipe support axis 13, onto the head clamp 52 and the first pipe section guide 55, until fully supported thereby on the tower 2. Therein, the head clamp 52 is in its top position, and the first pipe section guide 55 in its operative position.

By the pivoting ability of the tower 2 into the loader position, and the ability to supply pipe sections 11a to be supported to the tower 2 by moving these from support and guide structure 7 along the pipe support axis 13, loading these pipe sections 11a onto the tower 2 may be established by the system 1 without requiring a separate loader device for this purpose, which upends or otherwise partly or completely lifts pipe sections 11a to load these on the tower 2, as is common in the art. In fact the need to lift pipe sections 11a to a level higher than the first pipe section guide 55 in its operative position in the loading position of the tower 2 is omitted completely, as from this level only a horizontal movement thereof is required for the loading.

It is envisaged that in alternative embodiments, pipe section guides 55 and 71 are configured to support a pipe section 11a along a loading axis that is higher, e.g. slightly higher, than, and parallel to, the pipe support axis 13 as defined by the travelling head clamp 52 and the line-up tool 53. Therein the pipe section 11a is supported only by the pipe section guides 55 and 71 , while loading the pipe section 11a onto the tower 2. This might for instance further facilitate, and thereby speed up, a sliding movement of the pipe section 11a while loading. Pipe section guides 55 are therein for instance configured to, when the trailing end of the pipe section 11a is supported thereby above the travelling head clamp 3Q section 11a by moving downwardly until the pipe section 11a is supported along the pipe support axis 13 by the travelling head clamp 52 and the line-up tool 53. An alternative lowering mechanism may also be provided.

The system 1 further comprises an abandonment and recovery (A&R) system comprising an A&R cable 65 configured to connect an A&R winch (not shown) via one or more A&R sheaves to the seagoing part 11 b of the pipeline 11 , or to an accessory connected or to be connected thereto, in order to lower the pipeline with the accessory onto the seabed while bypassing the hang-off clamp 51. Therein the A&R system is configured to support the weight of the pipeline including the accessory while lowering the pipeline until the accessory has moved at least past the hang-off clamp 51. Furthermore, therein the tower 2 is tilted to a more upright pipelaying position than shown in Figs.4-6, e.g. wherein the angle a is between 90 and 110°. This lowering procedure using an A&R system is described in EP2281133 of the applicant.

Providing this A&R system enables the accessory to be lowered from the top of the tower 2 towards the seabed along a trajectory at a distance of the original firing line 12 as defined by the hang-off clamp 51. The hang-off clamp 51 is therefore not required to be moved out of the way to enable the lowering of the accessory, so that the need to provide means effecting this functionality is omitted.

In general, an accessory may be provided at one of the ends of the pipeline 11 or at an intermediate position of the pipeline 11. For example Pipeline End Terminals (PLET) are provided either at the leading end or the trailing end of the pipeline 11 , and inline structures (ILS) are to be provided within the pipeline 11.

The system 1 further comprises a working station 9 supported onto the base frame 3. The working station comprises a platform floor 91 which is pivotally connected to and supported by upper ends of arms 94 on the base frame 3 above the hang off clamp 51. The arms 94 are at lower ends thereof pivotally connected to the base frame 3 such that the working station 9 is pivotable around the horizontal pivot axis 32, and pivots along with the hang-off clamp 51. Therein, the supplementary bars 95, extending substantially parallel to the arms 94 and being pivotally connected to the platform floor 91 and the base frame 3 around a pivot axis parallel to the horizontal pivot axis 32, keep the platform floor 91 substantially horizontal during pivoting of the working station 9 and the hang-off clamp 51 around the axis 32. The working station 9 provides a shielded environment for connecting, e.g. welding, as is common, a pipeline section 11a supported by the travelling head clamp 52 and aligned by the line-up clamp 53, and/or an accessory, to the seagoing part 11 b of the pipeline 11.

The floor 91 of the working station 9 comprises two substantially rectangular pivoting floor tiles 92, adjoining each other so as to together define a larger rectangle. The floor tiles 92 are upwardly pivotable away from each other in opposite directions. As is visible in the figures, the adjoining edges of the tiles form a semi-circular indent in a central part of these edges. When in the flat position of Figs.1-4, these together form a circular opening in the plane of the floor 91 that is large enough to pass the pipeline 11 , or a seagoing part 11 b thereof, as held by the hang-off clamp 51 , there through.

Pivoting the floor tiles 92 away from each other to the position shown in Figs.5-6, creates an opening in the plane of the floor 91 that has the size of the larger rectangle, which is large enough to pass the lifting block 61 along with the travelling head clamp 52 there through in a pipelaying position of the tower 2.

In alternative, not shown embodiments, the working station may be embodied to be movable away from the firing line in a sideward direction, that is, in a direction parallel to horizontal pivot axis, wherein the pipeline 11 or a seagoing part 11 b thereof is allowed to pass through a sideward slot.

A possible method seabed according to the invention for J-lay pipelaying an offshore pipeline 11 onto the makes use of the system 1.

Fig.1 illustrates the system 1 in a position prior to a first step of the method.

The hang-off clamp 51 is supporting a seagoing part 11 b of the pipeline 11 in the firing line 12 at the trailing end thereof.

The tower 2 is in its loading position. The travelling head clamp 52 and the line-up tool 53 are opened and able to receive pipe section 11a and support this pipe section 11a such that it extends along pipe support axis 13. The travelling head clamp 52 is therein in its top position. Furthermore the pipe support axis 13 is therein substantially horizontal. 3 section guides 71 of the pipe section support and guide structure 7 support pipe section 11a such that it extends along the pipe support axis 13 and the leading end thereof that is to be connected to the seagoing part 11 b of the pipeline 11 faces the upper end 24 of the tower.

The first step of the method starts from the position shown in Fig.1 and is illustrated by the advancement from Fig.1 to Fig.2. Therein the pipe section 11a is moved along the pipe support axis 13 towards the seagoing section 11 b of the pipeline 11 , therein sliding over the second pipe section guides 71 , the first pipe section guide 55 and/or the travelling head clamp 52 and/or the clamp of the line-up tool 53 while being supported thereby. Therein the pipe section angle a is small, preferably between 0 and 10°, e.g. between 0 and 5°, most preferably 0°. Furthermore, therein the pipe section 11a moves laterally in between the main sheaves 63, until the pipe section 11a is in a position such that the leading end thereof is, after tilting the tower 2 around the horizontal pivot axis 32 until the pipe support axis 13 substantially collides with the firing line axis 14, connectable to the trailing end of the seagoing part 11 b of the pipeline 11.

In an alternative embodiment of the method, prior to the first step thereof the pipe section guides 55 and 71 of the system 1 support a pipe section 11a along a loading axis that is higher, e.g. slightly higher, than, and parallel to, the pipe support axis 13 as defined by the travelling head clamp 52 and the line-up tool 53. In the first step of this embodiment the pipe section 11a is supported only by the pipe section guides 55 and 71 , while loading the pipe section 11a onto the tower 2, moving it along the loading axis until the trailing end of the pipe section 11a is above the travelling head clamp 52, after which the pipe section 11a is lowered while, e.g. by, lowering the pipe section guide(s) 55 or by employing an alternative lowering mechanism, until the pipe section 11a is supported by the travelling head clamp 52 and the line-up tool 53 along the pipe support axis 13.

After the loading of the pipe section 11a onto the travelling head clamp 52 and the clamp of the line-up tool 53, these are closed so as to clamp the pipe section radially, thereby securing it in the axial and radial position thereof along the pipe support axis 13, and so as to arrive at the position shown in Fig.2.

In a second step of the method, starting from the position shown in Fig.2, the tower 2 is tilted upwards to the pipelaying position thereof by means of the tower tilting actuators 41 of the tower tilting system 4 to increase the pipe section angle a up to substantially the actual b in the pipelaying position thereof, in which the pipe support axis 13, at least at the height of the travelling head clamp 52, substantially collides with the firing line 12. Therein the travelling head clamp 52 and the line-up tool 53 retain the pipe section 11a in the same position along the pipe support axis 13. This step is is illustrated in Fig.3, in which the tilting of the tower 2 is in progress: the tower 2 has been tilted to a position in which the pipe section angle a is around half of the actual firing line angle b.

After the tilting the tower 2 to the pipelaying position thereof, at least the line-up tool 53 and the travelling head clamp 52 operated so to fine-tune the alignment of the pipe support axis 13 with the firing line 12 and the position of the leading end thereof relative to the trailing end of the seagoing part 11 b of the firing line such that these are connectable to each other, e.g. by means of manual welding by workers standing on the platform floor 91 of the working station 9.

In a third step of the method, the pipe section 11a is fixedly connected to the seagoing part 11 b of the pipeline 11 , by adjoining the leading end of the pipe section 11a and the trailing end of the seagoing part 11 b so to form a junction over a common circumference thereof.

This fixedly connecting is preferably realized by welding the ends together over the whole circumference thereof, so that the junction is formed by a weld. This welding is therein performed manually by workers standing on the floor 91 of the working station 9 and moving around the circumference of the junction being formed.

As is furthermore preferred, in this step the cylindrical sleeve 14 is by means of the line-up tool 53 slid over the formed junction after which the sleeve is injected with a hardening resin.

After this third step, the system 1 is in the position shown in Fig.4, wherein the pipe support axis 13 collides with the firing line axis 14 and the pipeline 11 is ready for being lowered further towards the seabed along the firing line 12.

A fourth step of the method is illustrated by the advancement from Fig.4 to Fig.5 on to Fig.6. The hang-off clamp 51 is released from the pipeline 11 so that the travelling head clamp 52 supports the weight of the pipeline 11 , and the pivoting floor tiles 92 are moved into their folded-out position as shown. The first pipe section guide 55 is moved into the retracted position thereof, after which the travelling head clamp 52 is lowered to its bottom position by nches of the main cables 62 while the vessel moves in the horizontal direction corresponding to the pipelaying direction on the seabed, namely over a distance corresponding to the length of the pipe section 11a that has been connected to the seagoing part 11b in the third step. Subsequently, the hang off clamp 51 engages the pipeline 11 at a trailing end thereof, and the travelling head clamp 52 is opened, such that the hang-off clamp supports the weight of the pipeline 11.

Fig.5 shows the system 1 during the lowering of the pipeline 11 , wherein the travelling head clamp 52 is in an intermediate position in between its top and bottom position. Fig.6 shows the system 1 after the fourth step, wherein the travelling head clamp 52 is in its bottom position and has been opened, and the hang-off clamp 51 supports the weight of the pipeline 11.

A fifth step of the method is illustrated by the advancement from the position of the system 1 shown in Fig.6 back to that shown in Fig.1. Therein the travelling head clamp 52 is lifted to its top position, the first pipe section guide 55 is moved into its operative position and both the travelling head clamp 52 and the line-up tool 53 are opened. Furthermore the tower 2 is lowered back to the loading position thereof, and new pipe section 11a is positioned onto the second pipe section guides 71.

The steps may be continuously repeated in the discussed order to for continuous pipelaying.

Alternatively, after the fourth step, an accessory may be connected to the seagoing part of the pipeline. Therein the tower 2 is tilted to a more upright pipelaying position than shown in Figs.4-6, e.g. wherein the angle a is between 90 and 110°. This lowering procedure using an A&R system is described in EP2281133 of the applicant.

In Fig.7, an alternative embodiment of the system is shown. Therein the system 101 comprises the same features as the system 1 of Figs.1-6, as indicated by the same reference symbols increased by 100, with a few differences, namely involving the configuration of the work station 109, to which furthermore stairs and bridges 196 are added, and the configuration of the A&R system 165,166,167. These differences may distinctively or in combination also be used in combination with the discussed embodiment of the system 1 of Figs.1-6. i 109, other than that in the previous embodiment, now comprises two boxes at either lateral side of the tower 102 when in its pipelaying position, which may be accessed by workers 15 via bridges and stairs 196 at the rear side of the tower 2. From the boxes a platform at the front side of the work station 109 is accessible, which are equipped with pivoting floor tiles 192 alike the previous embodiment.

The A&R system is configured in a multiple fall arrangement and comprises two pairs of A&R sheaves 166 at either lateral side of the tower 102, whereas the previous embodiment comprised one pair in a single-fall arrangement.

In Figs.8 - 17, the system 101 of Fig.7 is shown in subsequent stages of the discussed steps of the method according to the invention. Therein it is noted that except for the discussed differences of this embodiment 101 of the system with the earlier discussed embodiment 1 , the positions and states of the different parts of the system 101 correspond to those of the system 1 in the same shown stages. These figures are thus illustrative for the method using system 101 as well as the method using system 1 and therefore the earlier discussion of the method is therefore equally applicable. Any differences will be discussed below.

With reference to the earlier discussed steps of the method, the system 101 is shown prior to the first step in Fig.8. The advancement from Fig.8 to Fig.9 illustrates the first step of the method.

The second step is illustrated by the advancement from Fig.9 to Fig.10 and on to Fig.11 involving the tilting, and the alignment of the pipe section 11a with the firing line 12 by the advancement from Fig.11 to Fig.12.

The third step is illustrated in Figs.12-13, wherein Fig.13 shows the sleeve 114 having been slid over the junction of the pipe section and the seagoing part of the pipeline.

The advancement from Fig.13 to Fig.14 illustrates the travelling head clamp 152 being lowered to its bottom position and the hang off clamp 151 having engaged and supporting the pipeline 11 in the fourth step, prior to the opening of the travelling head clamp 152. s the travelling head clamp 152 having been opened to finish the fourth step, after which the travelling head clamp 152 has been lifted to its top position in the fifth step. The advancement to Fig.16 and on to Fig.17 illustrates the finishing of the fifth step.

Figs.18-21 show the system 101 during stages of a PLET-lowering procedure, which procedure may also be executed with the previous embodiment in Figs.1-6 or other embodiments of the system according to the invention. The procedure has been described before in EP228113 by the applicant, and may for example be executed after the fourth step of the method according to the invention. The stages shown are shortly elucidated below.

As shown in the position shown in Fig.18, the tower 102 of the system 101 is provided with accessory supports 156, onto which an accessory 16, in this case a PLET or pipeline end terminal is supported in a loading position of the tower in a first stage of the procedure.

Subsequently the tower is tilted to the position shown in Fig.19, wherein the pipe support axis 113 has an angle larger than 90° relative to the horizontal in order to achieve an overboarding position of the tower. The accessory is, along with a pipe section fixedly connected to the top thereof, then lowered to a height at which the trailing end of this pipe section may be engaged by the hang off clamp 151 , as shown in Fig.20. Fig.21 illustrates the tower being tilted back until the pipe support axis 113 has reached an angle such that the hang off clamp 151 is able to engage the connected pipe section at the trailing end thereof, and the hang off clamp 151 subsequently having engaged this pipe section so that the accessory 16 along with this pipe section is supported thereby in the firing line 112.

According to the same principle of operation of the system, other accessories may be fitted into the pipeline.