Field of Invention

The present invention refers to a method for tensioning a large diameter pipe (which may include a riser or conductor) by means of a 'Conductor Tensioning Unit', which itself may be located within a tension deck within an offshore drilling unit (which may include a Jack-Up Drilling Rig).

Background to the Invention

Pipes which are part of a pipe string utilised in drilling operations, and particularly those used as part of an offshore drilling unit, are subject to environmental forces, especially in the section of the string which is above seabed level. In addition to the large 'axial' tension applied to the pipe, lateral forces (usually from environmental considerations, like wind or wave) are exerted onto the system. The lateral forces cause bending of the pipe string, which can over time lead to fatigue of the pipe joint.

The present invention addresses the problem, particularly of protecting a pipe string handled by an offshore drilling rig. In broad terms, the invention acts by applying a large axial tension to a pipe (which may include a riser or conductor) through the use of multiple large diameter bore hydraulically actuated cylinders. When the cylinders are extended against a reacting force, they impose a vertical tension on the pipe. Load induced variations from the environmental considerations of the pipe are compensated for by accumulators in a hydraulic tensioning unit, maintaining near identical tension at any given point. All 'axial' and 'lateral' forces are translated through the cylinders and base frame out onto the main structure of the offshore drilling unit.

Summary of the Invention

The present invention applies tension to a large diameter pipe (which may include a riser or conductor) via multiple large bore hydraulic driven cylinders. The two main assemblies of the unit may be separated or joined together, which may be moved into position within the tension deck by means of a hydraulically actuated skid crawler unit. Additionally, this means the unit may be split to accommodate the running of pipe or large machinery, such as a Xmas tree or subsea passage, or indeed moved to one side for stowage.

A load transition plate reacts to both lateral and axial loads to and from the large bore cylinders. The upper work platforms permit access to equipment above and may be positioned at various different heights by means of a lock wheel This is accessible via a means of a ladder and the work platform is fully enclosed with handrails.

A lateral restraint centraliser aids the setting up of the system and is capable of handling a wide variety of pipe sizes.

Several locking methods between the tensioning unit and the skid beam are also discussed.

According to a first aspect of the invention there is provided a tensioning unit for a conductor such as an riser or conductor pipe, the unit housing a load transition plate, the plate defining an aperture within the plate to fit about and grip the outer circumference of a conductor, the plate comprising at least 2 sections securable together by securing means in frictional engagement about a conductor to facilitate fitment, the plate being mounted, to a mounting means, for movement to exert a generally axially directed tensile force on a conductor, the mounting means comprising a plurality of hydraulic cylinders actuable by a hydraulic power unit and hydraulically linked to one or more accumulators.

Preferably, the unit includes a centraliser, the centraliser having a body and a damping element movably secured to the body at a first end, which damping element can be moved between a retracted position and an extended position in which a second end of the damping element engages a conductor to stabilise a conductor against lateral movements. Further preferably, the second end of the damping element supports a resilient member to resiliently engage a conductor and reduce transmitted vibrations and movement. Yet further preferably the damping element is a rod.

Optionally, the body of the damping element is invertibly mounted to the unit enabling different sizes of conductor to be accommodated.

Preferably the stiffness of the accumulator is adjustable to enable the response to lateral movement and vibrations in the conductor to be adjusted, to cope with varying conditions experienced.

The securing means preferably comprises 2 or more pins, a pin further preferably having a non-constant diameter along the pin axis.

Preferably, the unit is mounted for movement on a skid crawler unit, the skid crawler unit comprising a motile unit, mounted for movement along a linear track, the track comprising a plurality of reaction nodes, optionally proud of the track or alternatively below the track surface, one or more hydraulic cylinders mounted to the motile unit and coupled to a reaction element for coupling to a load, actuation of the cylinder against the element inducing a motive force on the motile unit, the motile unit including a plurality of engagement elements, an engagement element to releasably engage a reaction node.

An engagement element preferably comprises an arm mounted at a first end to the motile unit and including at a second end, a hook to retain the engagement element in contact with the track and resist movement of the motile unit. The leading edge of the arm preferably describes a parabolic curve.

The mount of the arm to the motile unit is preferably a pivotal mounted and further preferably allows the arm to pivot 180°.

The reaction nodes are preferably apertures within the track.

According to a second aspect of the invention there is provided a skid crawler unit for moving laterally a large load such as a conductor tensioning unit. The skid crawler unit comprises a motile unit, mounted for movement along a linear track, the track comprising a plurality of reaction nodes, optionally proud of the track or alternatively below the track surface, one or more hydraulic cylinders mounted to the motile unit and coupled to a reaction element for coupling to a load, actuation of the cylinder against the element inducing a motive force on the motile unit, the motile unit including an engagement element to releasably engage a reaction node.

An engagement element preferably comprises an arm mounted at a first end to the motile unit and including at a second end, a hook to retain the engagement element in contact with the track and resist movement of the motile unit. The leading edge of the arm preferably describes a parabolic curve.

The mount of the arm to the motile unit is preferably a pivotal mounted and further preferably allows the arm to pivot 180°.

The orientation of an engagement element is preferably invertible allowing the motile unit to move in either direction along the track.

Brief Description of the Drawings Figure 1 is an isometric view of a tensioning unit within a 'tension deck';

Figure 2 is an isometric view of a tensioning unit, where the Conductor Tensioning Unit's two sections are housed on two opposing large welded beams (also referred to as ‘skid beams') which translate all loads back to the main tension deck;

Figures 3a and 3b illustrate separation of the two halves of the CTU and shows how the work platform assembly may be removed from the stanchions;

Figure 4 illustrates opening of the hatches of a Work Platform to allow for heavy weight items to be loaded out onto a C-Plate;

Figure 5 illustrates a locking mechanism which fits onto removable stanchions, allowing a work platform to be landed out at various heights, making the platform modular and flexible;

Figure 6 shows detail of a load transition 'C-Plate', which is the primary load bearing and load transfer item for the unit;

Figures 7a and 7b detail a lateral restraint centraliser which is used for the set-up of the riser system and is then disengaged;

Figures 8a and 8b are isometric views of two assembled skid crawler units

Figure 9 is a perspective view of a skid crawler unit with the cover removed; and

Figure 10 illustrates stages of the operation of the cylinder and arm.

Detailed Description of the Invention

The aim and purpose of the Conductor Tensioning Unit (CTU) of a first aspect of the present invention is to apply axial tension to a large diameter pipe (e.g. conductor/riser) whilst also reacting to all environmental loads exhibited on this pipe. The loads are, in the exemplified embodiment, transferred back through the CTU onto a set of beams through which the loads are then translated back to the offshore drilling unit on which the CTU is housed.

Referring initially to Figure 1, this illustrates a CTU, generally referenced 10, housed within a tensioning unit 11. The tensioning unit 11 is typically located on a drilling derrick, located offshore, and provides a number of functions. First, the tensioning unit 11 provides a stable platform on which to locate the CTU 10. Second, the walls 12 of the tensioning unit 11 provide protection for the CTU and personnel against wind and rain. Figure 2 shows the CTU 10 comprised of two main sections 13a, 13b, with each main section 13a, 13b being mounted for motion along a track element 14. The sections 13a, 13b can where required be separated from each other allowing, for example, the running of pipe work or large machinery as needed.. Further, each track element 14 is mounted for motion along a further track element 15. The CTU can thus be moved in the x - y plane about the tensioning unit 11.

The CTU thus comprises two nearly-identical assemblies which are drawn together and moved into position using a single or multiple hydraulically actuated skid crawlers. This allows the CTU to be moved into position for operation and allows the CTU to be stowed when not in use. This also allows for the assemblies to be moved apart from each other.

The CTU of the present invention comprises multiple very large bore diameter hydraulic cylinders 20, typically of outer dimension of 16" - 36" (40cm - 92cm) having a large axial and lateral capacity, which cylinders 20 provide the required tension. The cylinders are hydraulically actuated by means of a hydraulic power unit. This hydraulic power unit has multiple accumulators which may be pre-set for the required tension. The effect of this is such that should a lateral load be exerted on the pipe which is being controlled (such as by wave and wind loading), the pipe will draw down cylinders forcing hydraulic fluid back to these accumulators. In one embodiment, the accumulator comprises a bladder element which is pressurised with a pressurising fluid. The pressure held within the bladder can be set to provide different responses to a given load. The pressure within an accumulator depend on the tension applied, but as examples the pressure can be from 70 - 250 bar. The pipe therefore is subject to near constant tension throughout an environmental lateral load cycle without any peak loading nor associated stresses on the pipe and unit. This in turn reduces failure modes and improves the fatigue life of the pipe and unit system in general. A load transition plate (which is also known as a 'C'-Plate) having 2 sections 25a, 25b sits above the cylinders 20, and fits around the pipe when the sections 25a, 25b are brought together. The pipe string around which the C-plate is deployed, rests on an upper surface of the C-plate. For example, a collar or tension-ring fitted about a pipe of the pipe string rests on an upper surface of the C-plate. The cylinders therefore act on the C-plate, exerting pressure thereon which is transmitted to the pipe strings, changing the axial tension therein. A number of methods are known by which to attach the two halves of the load transition plate together.

In one embodiment, the sections 25a, 25b of the C-plate are joined together by two large pins 26 (see Figure 6). The pins 26 can have multiple diameters to ensure that they drop all the way into the load transition plate before binding. There are additional features to aid the removal of a pin 26, such as threaded lift points and crowbar relief edges.

Each section 13a, 13b has an upper work platform section 30a, 30b which combine together to form a platform permitting access to the surface equipment which sits above the tensioned pipe. Typically, in the form of a flange or connector, this platform negates the requirement for scaffolding and reduces down time, lessens the risk of dropped objects, saves expenditure and is safer.

The platform sits on a locking mechanism 31 (also referred to as a 'locking wheel') which is located on a set of tubular stanchions 32. This locking wheel 31 can be lifted and rotated in order to release it, and then re-positioned at a different height. In this way the work platform can be set at varying heights allowing for access to different heights of the surface equipment. The work platform itself is accessed by the user by a ladder 33. There is a set of removable handrails on the work platform to ensure personnel safety. The handrails can be removed and the work platform 'flat packed' in the event the platform is required to be stowed or sent for repair. A fall-arrester cage 34 (also referred to as a 'back scratcher') can be used depending on the height of the work platform and the user preference.

Turning to Figures 7, lateral restraint centralisers 50 are provided for centralising a large diameter pipe within the Conductor Tensioning Unit prior primarily to applying tension. These are separate units which can be removed and replaced as and when necessary. A centraliser 50 comprises a welded unit 51 with a large threaded rod 52 housed for rotation through the centre. At a first end of the rod 52 is housed a pad 53 to engage a pipe section. As the rod 52 is rotated, the rod 52 acts to move the pad 53 either forwards or backwards towards or away from a pipe section. The pad 53 can be formed from various materials, however in this example it is fabricated from a polymeric material. Once the threaded rod 52 is at the required position, relative to the pipe section, it is locked against movement. A number of methods known in the art can be utilised to lock, however in the exemplified embodiment a screw is driven from one side onto the flat portion of the thread, thus locking it. The centraliser 50 is housed on a plate-like support 54. An attachment plate 55 having at one end a hook 56 is secured onto the support 54, with the hook 56 hooked over the end of the support 54. The whole centraliser 50 can be rotated upside down bringing the opposing support 57 and hook 58 into a different position in which the minimum radial distance to the nominal pipe string axis is reduced. Therefore, the range of movement and thus the number of pipe sizes that can be accommodated is increased substantially. Typically pipes of outer dimension of 16" - 36" (40cm - 92cm) can be accommodated.

The CTU is mounted for motion within a tension deck which allows the CTU to be used with different pipe strings in different locations. The tension deck moreover provides a barrier to wind and other environmental factors, reducing their action on a pipe string. To transport the CTU across the tension deck, and in a second aspect of the invention, there is provided a skid beam assembly. It is to be noted that a skid beam can be used more widely to transport other heavy items over relatively short distances in more general situations. A number of methods to lock the Conductor Tensioning Unit to the skid beam are known. The particular means by which the CTU of the first aspect is releasably locked thereto comprises three elements. A welded 'catch' is shown at the front or leading edge of the Conductor Tensioning Unit. A bolted 'catch' is shown at the rear or trailing edge of the Conductor Tensioning Unit. In this way the unit may be slewed into position, catching the leading edge. Subsequently the bolted catch is attached, ensuring that the unit can only be translated along the skid beam and not overturn. Thirdly, a beam clamp may be used which may be locked or unlocked to ensure the unit translates all lateral loads to the skid beams once it has been moved into position. Other locking methods may additionally be used in place of a beam clamp depending on the user's preference. As indicated above, a Conductor Tensioning Unit is preferably movable by means of a hydraulically actuated skid crawler. The skid crawler as set out below in more detail, is described therefore with reference to movement of a Conductor Tensioning Unit, but can be utilised to move other large structures or machinery. The skid crawler may itself be located within a tension deck as part of an offshore drilling unit (which may include a Jack-Up Drilling Rig).

The skid crawler addresses the problem of moving a large piece of machinery in a controlled and efficient manner. There are many different applications known to which this invention could be applied, many within an offshore environment and many onshore as well. The particular method to which the current invention purpose is to move a Conductor Tensioning Unit in two directions within a tension deck within a Jack-Up Drilling rig. This large piece of machinery requires to be located within a specific area to provide support for a large diameter pipe. Therefore, the skid crawler enables the movement in the x and y planes to accommodate this variation.

The purpose of the skid crawler unit, generally referenced 70 is to act as a hydraulically actuated unit which can skid a large structure or piece of machinery over a track. There are numerous applications for this unit, both within offshore and onshore industries.

The skid crawler unit 70 is located on a bespoke designed skid track 71 which has numerous identical reaction nodes 72. These nodes 72 may be either raised from the skid track 71 or sunken into the skid track 71 depending on the requirements and shape of the large structure. Force to move the skid unit 70 is provided by the hydraulically actuated cylinder 73 housed within the cover 74 of the skid unit 70. The hydraulically actuated cylinder 73 is able to exert both a pushing and a pulling force on the object to which it is attached through the actuation rod 75.

The reaction of the hydraulically actuated cylinders 73 is such that the reaction of the force is felt on the nodes 72 of the skid track 71 via a pivotally-mounted reaction arm 76 having at one end a hook 77. The hook 77 has numerous design features to ensure that it can both hook onto the reaction nodes

72 and ride up and out of the node 72 during cylinder retraction. Multiple arms 76can be employed on each cylinder 73 to ensure the item exerts correct loading and keeps on track.

By operating the hydraulically actuated cylinder 73 in push function, the skid crawler unit 70 is able to push the heavyweight item along the track 71, reacting the push force into the hooks 77. By operating the hydraulically actuated cylinder 73 in pull function, the skid crawler unit 70 be able to pull the heavyweight item along the track 71 in a similar way, reacting the pull force into the arms 76. To modify the direction of travel requires the user to rotate the arms 76one hundred and eighty degrees. However, the controls on the hydraulic power unit remain similar, despite the direction of travel. The hydraulic power unit is capable of handling both single and multiple skid crawler units 70 at one time. The hydraulic power unit can be designed to include an additional personnel unit which can be worn by the operator, ensuring increased safety, operability check and flexibility to move with the system.

A single skid crawler unit 70 can be operated, or multiple units at the same time in order to move heavy weight items across multiple tracks (the present examples use both single and duplicate tracks).

The present invention also refers to a method for translating a large structures or piece of machinery (which can include a Conductor Tensioning Unit) by means of a hydraulically actuated Skid Crawler. The large heavy item is located on a skid track 71, which is covered with a thick lubricant. A single skid crawler unit 70 can be operated in isolation on a single trackway, or multiple units used in unison to move large weight items across multiple tracks 71 (the exemplified embodiment can use both single and duplicate tracks). For the push function, the cylinder rod 75 is hydraulically actuated by means of an external hydraulic power unit. The force of the hydraulically actuated cylinder 73 is reacted against a set of hooks 77 that are engaged in the track 71. As such, the skid crawler unit 70 pushes the heavy weight item over the skid track 71 until the end of the cylinder stroke. The cylinder rod 75 is then retracted, dragging the hook 77 into the next available node 72. As the mass of the large structure is greater than that of the skid crawler unit 70 the hydraulically actuated cylinder 73 is pulled towards the stationary large structure. The 'pull' function operates in an identical method with the arms 76 rotated one hundred and eighty degrees. In addition the skid unit 70 is removable from the deck for the following reasons: the skid unit 70 can be stowed easily; the skid unit 70 can be removed from an area which may well receive a lot of viscous returns (which may include drilling mud) and thus protected; the unit may be removed as an obstacle.

The skid crawler unit 70 is made from several different critical components. The bespoke skid track 72 has a pattern of identical reaction nodes 71. The nodes can be constructed by means of one of several methods known in the art. The particular method of the current invention is that the nodes 71 are inverted into the track 72. Alternatively, a node 71 can also be formed protruding out of the track 72 to an extent depending on the design constraints.

A reaction arm 76 has the following features. First, the leading edge 78 has a near parabolic shape made from oblique angles that is bespoke to the nature of the reaction node length, width and depth. Second, the trailing edge has a hook shape 77 to fully catch and push or pull respectively. Third, the hook 77 is designed to ride up and out of the nodes 72 and thus is mounted about a central pivot 79 to enable the arm 76 to perform this rotation.

The central pivot 79 also allows the reaction arm 76 to be rotated one hundred and eighty degrees so the skid crawler unit 70 can be in either push or pull function. The hook 77 is made from a high tensile material to ensure the hook 77 can take the repetitive action of engaging with multiple nodes 72. Multiple hooks 77 are utilised as and when necessary to ensure that the item is kept on track and exerts correct loading.

The hydraulically actuated cylinder 73 is designed with respect to the length and width of the nodes 72 to ensure that only one node is 72 accessed at a time. Therefore, should multiple skid crawler units 70 be used simultaneously (for example, to push a large skid beam across a tension deck), the skid crawler units 70 move in synchronisation with each other.

The process of moving the skid crawler unit 70 along a track 71 is illustrated in Figure 10. In image 1 of Figure 10, the rod 75 is shown in its extended position. The reaction arm 76 rests with the rear surface 90 of the reaction arm 76 engaging a rear wall 91 of a node 72a of the track 71 and preventing movement of the skid unit 70 to the right. In image 2, a force shown by arrow A acts to draw the skid unit 70 to the left. The force arise through retraction of the actuation rod 75 into the skid unit 70. This motion brings the forward edge 92 of the node 72a into contact with the leading edge 78 of the reaction arm 76. The curvature of the leading edge 78 causes the reaction arm 76 to pivot about the central pivot 79 in a clockwise direction B, lifting the reaction arm 76 out of the node 72a. Continued retraction of the actuation rod 75 lifts the reaction arm 76 fully clear of the node 72a as shown in image 3. Eventually, the actuation rod's retraction brings the skid unit 70 sufficiently far to the left that the hook 77 of the reaction arm 76 is over the adjacent node 72b to that which it was originally in, and pivots in the direction shown by arrow C (images 4 and 5) until the leading edge 78 is resting on the leading edge of the adjacent node 72b. The skid unit 70 unit can then be used to push the load to which the end 93 of the actuation rod 75 is attached to the left, by extension of the actuation rod 75. The reaction force to this motion is borne by the engagement of the rear surface 90 of the reaction arm 76 and the rear edge of the node 72b. Any turning force which acts to lift the reaction arm 76 out of the node 72b at this stage is resisted by engagement of the hook 77 with the underside of the track 71. The load can therefore be moved in stages along the track 71. To help keep the present invention on the trackway the unit may employ skid guide. There are many known variations to the skid guide, but the present embodiment of the item is polymer based and hooks over a track. In an alternative embodiment, one of the skid crawlers employs a hook over a raised portion of the track, while the other hook hooks over the track itself.

The hydraulic power unit is capable of handling either a single or multiple skid crawler units at one time. The hydraulic power unit comes as a flexible modular unit which can be easily transported by a variety of means known in the art to the required location. The hydraulic power unit may also come with an additional personnel modular unit which a human operator can wear (by means of a harness or similar). This allows the operator to be able to place themselves in the safest location while having full ability to view the moving object being translated across the trackway. The flexibility of this system increases the safety of the user; ensures that the user can visually check that all the hooks have latched within their respective nodes, and makes sure the user can walk or follow an item along the trackway without being tied to a stationary operating location.

A cover 74 over the skid crawler unit 70 ensures the protection of the hook and hydraulic components therein. The cover 74 has a unique simple catch mechanism to ensure easy opening by the operator who may be wearing gloves at the time. In a similar way, the cover design ensures the operator can join the hoses for the cylinder without removing the cover. For example, quick-connect hydraulic couplings can be used at the rear of the skid crawler unit.

The multifunction work platform removes the need for scaffolding reducing downtime and increasing rig efficiency. This platform is a modular part of the ConductorTensioning Unit. Wheels can be placed at different heights on the stanchions mean that this level can be adjusted as required. The ladder assembly also accommodates for this variation in height. Ultimately, should the rig require much longer stanchions, this can be accommodated for by increasing the length of the stanchions.

The handrails can be removed, so the entire work platform can be 'flat packed' and stored/sent back to storage The hatches within the work platform can be opened as required to allow equipment to be lowered through onto the load transition 'C-plate'. The lateral restraints used for set up of the system can be adjusted using a big threaded push screw for a gap of around 36" to 24" (92 - 61cm). Once in position, the whole assembly can be rotated 180 degrees with the annular gap becoming around 30" to 16" (76 - 40cm). The CTU is slewed into position, hooking the front 'welded' catch and then having a bolted catch at the back with an additional beam clamp to lock the hook in position. The skid crawler can be operated to push a large heavy item over a bespoke designed trackway, the trackway having reaction nodes. The skid crawler is designed so that only one slot may be operated at one time. This safety feature ensures that if multiple skid crawlers are used simultaneously on multiple track beds, the system does not go out of synchronisation. The hook is uniquely designed with respect to the length, height and width of the reaction node. Along one side, the hook has an obliquely angled semi-parabolic shape and a hook element on the other. This ensures that the hook alternately hooks onto the reaction node and also rides up the reaction node depending on the stroke of the cylinder. The skid crawler unit keeps perpendicular to the trackway by means of a skid guide.

The hydraulic power unit allows the operation of single or multiple units at one time. The design of which can include an additional wearable modular unit for the operator. This additional unit ensures the user finds the optimal location to view the skidding operation and provide increased safety.