WO2008012482A1

WO2008012482A1 - Method and apparatus for erecting a mast

Info

Publication number: WO2008012482A1
Application number: PCT/GB2006/002783
Authority: WO
Inventors: Dicky Robichaux; Ron Henderson; Paul Ray
Original assignee: Psl Energy Services Limited
Priority date: 2006-07-26
Filing date: 2006-07-26
Publication date: 2008-01-31

Abstract

The invention provides an apparatus and a method for erecting a mast on a surface. The method comprises the steps of: (a) providing a support structure and a mast divided into two or more mast portions; (b) engaging a part of a first mast portion with the support structure; (c) arranging the first mast portion in a substantially perpendicular configuration relative to the surface; and (d) engaging a second mast portion with the first mast portion. The method allows a mast to be erected using commonly available lifting equipment in locations where only a small footprint area is available. The invention also provides lifting apparatus for lifting equipment into and out of a wellbore. The apparatus comprises a first lifting assembly and a second lifting assembly. Each lifting assembly is adapted to lift equipment into and out of a wellbore. The first and second lifting assemblies are selectively actuable to lift equipment into and out of a wellbore.

Description

METHOD AND APPARATUS FOR ERECTING A MAST

The present invention relates to a method and apparatus for erecting a mast. In particular, the invention provides a method and apparatus for erecting a mast such as those used in the oil and gas industry, for supporting drillstring and other equipment that is moved into and out of a wellbore. The invention also provides lifting apparatus for lifting equipment into and out of a wellbore.

In the oil and gas industry, masts are typically used for supporting operations such as well intervention, movement of equipment into and out of a well and supporting coiled tubing and wireline operations. Masts offer good stiffness and are generally lighter than conventional derricks and consequently are more suitable for offshore environments where weight is ; a greater concern than for land based rigs. However, the assembly of such masts is restricted by the limited capabilities of equipment and space on an offshore rig.

According to a first aspect of the present invention, there is provided a method for erecting a mast on a surface comprising the steps of:

(a) providing a support structure and a mast divided into two or more mast portions;

(b) engaging a part of a first mast portion with the support structure;

(c) disposing the first mast portion in a substantially perpendicular configuration relative to the surface; and

(d) engaging a second mast portion with the first mast portion.

According to the first aspect of the invention there is also provided an apparatus for erecting a mast, the apparatus comprising a mast divided into two or more mast portions, a support structure having a positioning means for engaging at least a part of the mast portions and moving the mast portions relative to the support structure; and a connecting means to connect the mast portions to one another to thereby erect a mast.

Prior to step (a), the method can include dividing the mast into two or more mast portions, such that each mast portion weighs less than ten metric tons.

Prior to step (a) the method can include dividing the mast into two or more mast portions, such that each mast portion is of dimensions that are selected to allow transportation of the each mast portion to the surface on which the mast is to be erected by a suitable transporting means.

Prior to engaging each mast portion with the support structure, the method can include transporting the mast portions by crane proximate the support structure. Suitable transporting means can include a crane or a helicopter.

According to the embodiment wherein each mast portion weighs less than ten tons, transportation of the mast portion can be achieved using a crane with a lifting capability of ten tons at a reach of ten metres. As a result the method and apparatus according to the invention are suitable for use on an offshore rig or mounted on a trailer where the available cranes can have a limited reach and load capability.

Alternatively, the apparatus comprising the support structure the positioning means and the mast portions can be divided into modules weighing less than a predetermined maximum value. The predetermined maximum value can be calculated on the basis of the lifting capability of a helicopter. Thus, the apparatus can be fully portable allowing the mast to be divided in modules that can be airlifted to sites where a mast is required to be erected. The predetermined maximum value for each module can be less then ten tons. Preferably, the maximum weight of each module can be less than 7 tons and more preferably the maximum weight can be less than 5 tons.

The method can include erecting the mast on a moveable foundation. The surface on which the mast is to be erected is preferably a moveable foundation.

The movable foundation can typically be capable of controlled movement in an at least one of an x- and a y- direction and thus can be capable of movement in a horizontal plane. This is advantageous as the movable foundation allows the, erected mast to be repositioned according to its. intended use. The movable foundation can have hydraulic rams attached thereto and the foundation can be moveable by actuation of the rams.

The support structure can include one or more support towers. Optionally the positioning means can be provided adjacent one or more support towers. At least one of the support towers can be provided with a stairway. The stairway can allow personnel access to the mast portions. Preferably, two support towers can be provided on opposing sides of the positioning means such that the positioning means are actuable to erect the mast therebetween.

The positioning means can comprise a hoisting means. The hoisting means can comprise a pinning mechanism and a pulley and winch arrangement.

According to step (b), the method can include providing a support structure having a positioning means and engaging a lower end of the first mast portion with the positioning means. The first mast portion can be disposed in a substantially perpendicular configuration using the positioning means.

According to step (b), the method can include pinning the mast portion to the support structure, such that the mast portion is pivotable about the pins.

According to step (c), the method can include attaching a hoisting means towards an upper end of the mast portion and actuating the hoisting means.

According to step (c), the method can include the step of pivoting the first mast portion about the part of the first mast portion that is engaged with the support structure so that the first mast portion is disposed in a substantially perpendicular position relative to the surface on which it is erected. Pivoting the first mast portion to a substantially vertical position can be achieved by engaging the hoisting means to the first mast portion and winching the first mast portion substantially perpendicular to the surface on which the mast is to be erected.

The method can include pivoting the first mast portion from a position substantially parallel relative to the surface on which the mast is to be erected to a position substantially perpendicular to the surface. The method can include pivoting each mast portion from a substantially horizontal position to a substantially vertical position relative to the surface on which the mast is to be erected.

The positioning means can also comprise a raising means. The raising means can include a hydraulically operable travelling base plate. Following step (c), the method can include raising the first mast portion relative to the surface on which the mast is to be erected. The first mast portion is preferably raised in a direction perpendicular to the surface on which the mast is to be erected.

The method can include engaging the first mast portion with the support structure above a travelling base plate and raising the mast portion by actuating the travelling base plate.

The method can include retaining the first mast portion in the raised configuration and returning the travelling base plate to its original position prior to engaging the second and subsequent mast portion with the support structure. A retaining means can be provided to restrict movement of the or each mast portion from the substantially perpendicular position relative to the surface.

The mast portions can be retained by friction pads. Alternatively or additionally, the mast portions can be provided with indentations and bolts are located on the support structure for retractably engaging with the indentations on the mast portions.

Prior to step (d), the method can also include engaging a part of the second mast portion with the support structure and disposing the second mast portion in a substantially perpendicular configuration relative to the surface on which the mast is erected and similarly steps (b) and (c) can be repeated for the second mast portion prior to step (d).

The method may include the steps of engaging a part of the second mast portion with the support structure before engaging the two mast portions together. The second mast portion may also be pivoted about the part that is engaged with the support structure so as to be substantially aligned with the first mast portion. This can be achieved by hoisting the second mast portion in a substantially vertical position.

According to step (d), the method can include connecting an upper end of the second mast portion with a lower end of the first mast portion.

The connecting means can be provided towards an end of each mast portion and can include male and female mating ends. The connecting means can also comprise locking pins and an actuating means. The actuating mean can be hydraulicaily actuable to propel the locking pins through a sidewall of the mast portions. ^■

The method can include providing a lower end of the first mast portion with one of a male end connection and a female end connection and providing the upper end of the second mast portion with one of a female or a male end connection for mating with the end connection of the first mast portion and coupling the first and second mast portions by mating the male and female end connections.

The method can include locking the mast portions into engagement by actuating locking pins and passing the locking pins through both mast portions.

Typically, the steps (b) - (d) can be repeated to connect any subsequent mast portions to the previous mast portion to thereby erect a mast.

_*

The method can include maintaining the erected mast in the position perpendicular to the surface on which the mast is erected. Maintaining the mast in its erected position can be achieved using stays that couple part of the mast to a surface on which it is erected. The stays can be rigid and triangulate between the mast and the surface on which the mast is erected.

The support structure can be provided with one or more retractable pads acting as an impediment to movement of the mast in the direction of pivot of the mast.

The method can include locating rack segments within each mast portion and arranging the rack segments of the erected mast to form a continuous rack that engages with one or more pinions for driving the rack relative to the pinion(s) at its lower end.

The method can include coupling one or more counterbalance winches to the mast and thereby increasing the load capability of the mast. The first mast portion can be provided with one or more pulleys towards an upper end thereof to engage with a line coupled to the one or more counterbalance winches to enhance the load capability of the mast.

The present invention is advantageous as it provides a mast that has a greater versatility than conventional masts as it can be erected using commonly available lifting equipment in various locations where only a small footprint area is available.

As used herein, "substantially perpendicular" can be taken to include all configurations that are closer to a perpendicular position than a parallel position. The use of the term "substantially vertical" herein shall be taken to include references to vertical and to other configurations which are not vertical, but in which the mast behaves in the same or a similar way to a mast disposed in a vertical orientation.

According to a second aspect of the invention, there is provided a lifting apparatus for lifting equipment into and out of a wellbore, the apparatus comprising a first lifting assembly and a second lifting assembly, wherein each lifting assembly is adapted to lift equipment into and out of a wellbore and wherein the first and second lifting assemblies are selectively actuable to lift equipment into and out of a wellbore.

Preferably, the first and second lifting assemblies are powered from the -, same source. The first and second lifting assemblies can be independently actuable. The first lifting assembly can be actuable to lift equipment up to a predetermined maximum weight and the second lifting assembly can be actuable to lift equipment up to a predetermined maximum weight. The maximum lift weight of the second lifting assembly can be greater than the maximum lift weight of the first lifting assembly. The second lifting assembly can be actuable to lift weights greater than the maximum lift weight of the first lifting assembly. For example, the maximum lift weight of the first lifting assembly can be in the region of 136 tons (300000 lbs) and the maximum lift weight of the second lifting assembly can be in the region of 227 tons (500000 lbs).

The first and second lifting assemblies can be interchangeably connectable to the equipment to be lifted into and out of the weilbore.

The first lifting assembly can comprise a rack and pinion unit such as that described in WO98/48142, the entire disclosure of which is incorporated herein by reference. The first lifting assembly can be provided within a mast that houses the rack and pinion unit such as that described and assembled with reference to the first aspect of the invention.

The second lifting assembly can comprise two or more sets of gripping means to engage and hold the equipment to be lifted and two or more hydraulic rams coupled to the gripping means and arranged to extend and retract to lift the equipment out of and into a wellbore. The gripping means can comprise slips. Preferably the second lifting assembly comprises four hydraulic rams arranged in a rectilinear configuration to support a load.

Where the first lifting means comprise a rack and pinion unit with a mast according to the first aspect of the invention, the second lifting means is r preferably arranged to transfer the load onto the same surface on which the mast is erected. Preferably the surface on which the mast is erected is the moveable foundation.

Preferably, the control systems for the first and second lifting assemblies are integrated allowing an operator to selectively engage and operate the first or the second lifting assembly. According to the embodiments where the first lifting assembly comprises a rack and pinion unit and the second lifting assembly comprises two or more gripping means and two or more hydraulic rams, the same source of hydraulic fluid can be used to drive both of the lifting assemblies and both of the lifting assemblies can be controlled from the same work station.

Embodiments of the present invention will now be described with reference to and as shown in the following illustrative drawings, in which:- Fig. 1 is a perspective view of a moveable foundation and a support tower prior to installation in its operational position as a support structure;

Fig. 2 is a perspective view of an upper mast portion arranged for engagement with the support structure of Fig. 1 ;

Fig. 3 is a perspective view of a lower end of the upper mast portion connected to the support structure;

Fig. 4 is a perspective view of an upper mast portion in a vertical position; Fig. 5 is a perspective view of an upper mast portion in a raised configuration;

Fig. 6 is a perspective view of a middle mast portion engaged with the support structure; Fig. 7 is a perspective view of the middle mast portion in a vertical position; _: •> ,

Fig. 8 is a perspective view of the middle mast portion connected to the upper mast portion;

Fig. 9 is a perspective view of a lower mast portion engaged with the support structure; Fig. 10 is a perspective view of the lower mast portion in a vertical position;

Fig. 11 is a perspective view of the lower mast portion connected to the middle mast portion;

Fig. 12 is a perspective view of the assembled mast; Figs. 13-15 are perspective views of the erected mast in its operational position with a travelling head attached thereto;

Fig. 15a is a perspective view of part of a rack and pinion unit;

Fig. 16 is a schematic view showing the upper mast portion raised by a travelling base plate; Fig. 17 is a schematic view showing the middle mast portion connected to the upper mast portion; Fig. 18 is a schematic showing a connecting means within the upper mast portion; Fig. 19 is a schematic view showing the lower mast portion connected to the middle and upper mast portions; and Fig. 20 is a side view of a lifting apparatus according to the second aspect of the invention.

Fig. 1 shows a moveable foundation in the form of a sub base 22 on which a mast is to be erected. The sub base 22 is attached to two substantially parallel cross members 28 that act as guide tracks and permit movement of the sub base 22 between and in a direction parallel to the cross members 28. Two rams 26 aligned with the cross members 28 are ^•,-■ arranged to act on the sub base 22 in order to cause movement of the sub base 22 between the cross members 28. Two rams 24 are arranged perpendicular to the cross members 28 to act on the cross members 28 and cause transverse movement of the cross members 28 and the sub base 22 therebetween. As a result of this arrangement, extension and retraction of the rams 26, 24 can cause planar movement of the sub base 22.

The apparatus includes a positioning system 34 and two support towers 16, 18. The support tower 16 has a stairway 16s The support towers 16, 18 are approximately ten metres high. The positioning system 34 is to be provided between the supporting towers 16, 18 and comprises pulleys 36 having hoisting lines 37 attached thereto. The hoisting lines 37 are actuable by means of a winch 35 (Figs. 16, 17). The positioning system 34 also includes a travelling base plate 38 that is adapted to be raised and lowered by a cylinder and cable system in a direction substantially perpendicular relative to the sub base 22. Hydraulically actuated Teflon™ retaining pads 90 are attached to the support towers 16, 18 and are actuable to extend across a portion of the front side of the area between the towers 16, 18.

An upper mast portion 40 is shown in Fig. 2. The upper mast portion 40 has a weight of less than 10 metric tons and is substantially rectangular in section. The interior of the mast is substantially hollow and is provided with a plurality of rack segments 114 (Fig. 15a) extending along the length of the interior of the upper mast portion 40 parallel to the sides. A lower end of the interior of the mast portion 40 houses a connecting means 54, described below. The rear of the upper mast portion 40 is provided with two pad eyes 42 protruding outwardly therefrom. Towards the lower end of the upper mast portion 40, holes 46 extend through the sidewalls on two opposing sides of the mast portion 40 and the end has a female end connection 44 projecting parallel to the longitudinal axis of the upper mast portion 40.

A middle mast portion 60 is illustrated in Fig. 6. Similarly, the middle mast portion 60 has a weight of less than 10 metric tons and is substantially rectangular in section. The upper end of the middle mast portion 60 has a male end connection 65 that is arranged to locate between the female end connection 44 at the lower end of the upper mast portion 40. The interior of the middle mast portion 60 is substantially hollow and accommodates a plurality of rack segments 114 arranged to align with the rack segments 114 provided within the upper mast portion 40. The middle mast portion 60 also houses a connecting means towards its lower end. The connecting means are similar to the connecting means 54 shown in Fig. 18. Holes (not shown) extend through sidewalls on two opposing sides of the mast portion 60. The lower end of the middle mast portion 60 has a female end connection (not shown) projecting parallel to the longitudinal axis of the upper mast portion 40.

The connecting means 54 (shown schematically in Fig. 18) is located within the upper mast portion 40 towards the lower end thereof. The connecting means 54 includes a ram 52 that is actuable to laterally displace locking pins 50 and cause them to protrude through holes (not shown) in the male and female end connections 65, 44 of the middle and upper mast portions 60, 40 respectively.

A lower mast portion 80 having a weight of less than 10 metric tons is shown in Fig. 9. The lower mast portion 80 is hollow and substantially ^: • rectangular in section. The_;lower mast portion 80 houses a machine ^; room containing six sets of hydraulic motors, gears, brakes and control systems. The lower mast portion 80 also contains a plurality of rack segments 114 arranged in two parallel tracks that are linked at their base. The rack segments 114 are arranged at their upper end to align with the rack segments 114 provided within the middle mast portion 60. A set of pinions 122 are also housed within the lower mast portion 80. The rack and pinion mechanism is illustrated in Fig. 15a and the presence of the mast portion 80 has been removed for clarity. The pinions 122 have teeth that engage with the teeth of the rack segments 114 (Fig. 15a). The rack segments 114 comprise jointed I-beams and the outer end of the I-beams are provided with rack teeth. The rack segments 114 are pushed or pulled by stationary AC or hydraulic motors connected to the rack by the pinions 122. Preferably the pinions 122 are fixed and the rack segments 114 move up and down relative to the pinions 122 to support the dolly 120 and maintain a low centre of gravity. The upper end of the middle mast portion 60 has a male end connection (not shown) projecting parallel to the longitudinal axis of the mast portion 80 and for coupling the lower mast portion 80 to the middle mast portion 60 and locating the lower mast portion 80 in an aligned position relative to the middle mast portion 60.

Each mast portion 40, 60, 80 is manufactured from high grade steel to impart strength to the mast when erected. The outer surface of each mast portion 40, 60, 80 has parallel guide tracks extending parallel down the front face of the mast portions 40, 60, 80 along which a dolly 120 can run when driven by the rack and pinion 122 mechanism. According to the present embodiment the cumulative weight of the apparatus making up the mast is around 120 tons.

According to the present invention, the mast is to be erected on an oil and gas rig. Preferably the equipment required to assemble and operate the mast, such as the mast portions 40, 60, 80, positioning system 34, support towers 16, 18 etc are pre-assembled into bundles weighing less than ten metric tons. This allows the equipment to be handled by cranes used offshore and therefore having limited lifting and reach capability due to the size and available space on the platforms on which they operate. The modules containing these components are then delivered, for example, by a vessel to the vicinity of the sub base 22.

Prior to assembly of the mast, the support tower 16 is transported and positioned on the sub base 22 (Fig. 1 ). A crane (not shown) is attached to a basket 19 containing the support tower 18 via a line 32. The crane is used to lift the support tower 18 within the basket 19 to its position proximate the support tower 16 with the stairway 16s. The support tower 18 is then raised to a substantially vertical position and the towers 16, 18 are fixed to the sub base 22 (Fig 2). The positioning means 34 having a pulley lifting mechanism is installed between the two support towers 16, 18 and the travelling base plate 38 is in its lowermost position adjacent the sub base 22.

The crane is then used to position the upper mast portion 40 at the base of the support towers 16, 18 (Fig. 3). Pivot pins 47 (shown schematically in Fig. 16) are actuated to couple the lower end of the mast portion 40 to the positioning means 34. The pivot pins 47 engage the holes 46 at the base of the mast portion 40. Additionally, the line 37 coupled to the winch 35 is connected at an upper end of the lower mast portion 40. The winch 35 is then actuated to reclaim the line 37 guided via the pulleys 36 and thereby pivot the mast portion 40 about the pivot pins 47. In this way, the lower mast portion 40 is moved from a substantially horizontal position to a substantially vertical position as shown in Fig. 4. The retaining pads 90 are then extended to protrude between the support towers across the front side of the upper mast portion 40. The retaining pads 90 act to maintain the upper mast portion 40 in an upright position and restrict the mast from falling forward.

The travelling base plate 38 is then raised by a cylinder and cable system to elevate the upper mast portion 40 (Fig. 5). Once the upper mast portion 40 has been elevated to the required height, securing pins 48 can be actuated hydraulically to engage the upper mast portion 40 with the support towers 16, 18 and retain the mast portion 40 in an elevated position. The upper mast portion can be provided with recesses on an outer surface thereof for receiving the securing pins 48. As an alternative to securing pins 48, friction pads can be energised to apply a lateral force to the upper mast portion 40 and retain the mast portion in an elevated position. Once the upper mast portion 40 is retained in an elevated position by the securing pins 48, the weight is taken off the travelling base plate 38 and the travelling base plate 38 is returned to the base of the support towers 16, 18. The middle mast portion 60 is then placed adjacent the towers 16, 18 by the crane. The pivot pins 47 are then engaged with the holes through the sidewalls of the middle mast portion 60 and the line 37 attached to the winch 35 is attached to the pad eyes 62 (Fig. 6). The middle mast portion 60 is winched to a vertical position (Fig. 7). The travelling base plate 38 beneath the middle mast portion 60 is actuated to raise the mast portion 60. The male end connection 65 of the middle mast portion 60 mates with the female end connection 44 of the upper mast portion 40 to correctly align the mast portions 40, 60. The parallel rack segments 114 extending within the mast portions 40, 60 are thus also aligned to form two rack segment 114 tracks that are continuous within the upper and lower mast portions 40, 60.

The connecting means 54 are then activated to engage the upper and middle mast portions 40, 60. Actuation of the cylinder 52 causes the ram to extend and displace the locking pins 50 laterally. The locking pins 50 are pushed through holes in the sidewalls of the mast portions 40, 60 to lock the mast portions together. Outer ends of the locking pins 50 can be threaded to allow nuts (not shown) to be affixed to the end of the locking pins 50 from the exterior of the mast portions 40, 60 to act as a mechanical failsafe. The nuts can be manually threaded onto the locking pins 50. When the upper and middle mast portions 40, 60 are connected and locked to one another (Fig. 8). The middle mast portion 60 can be retained in an elevated position by actuation of the securing pins 48 in the manner previously described in connection with the upper mast portion 40. The travelling base plate 38 is then lowered. The lower mast portion 80 containing the machine room is then transported, pivotally attached to the positioning means 34 (Fig. 9) and raised to the vertical position as previously described (Fig. 10). The lower mast portion 80 locates with the lower female end connection of the middle mast portion 60 to align the parallel tracks of rack segments 114 within the mast portions 60, 80. The connecting mechanism is then actuated to secure the mast portions 60, 80 to one another (Fig. 11 ) to form an erected mast 100 (Fig. 12).

The erected mast 100 is fixed to the sub base 22 and is supported by rigid supports (not shown) that attach to the rear face of the mast 100 and triangulate between the mast and the surface. A dolly 120 is coupled to the rack segments 114 within the mast 100 and is arranged to move . , .; therewith along outer tracks on a front face of the erected mast 100 driven by the pinions 122.

Following assembly of the mast 100, the sub base 22 can be moved by actuation of the rams 24, 26 to correctly position the mast so that the attached dolly 120 is in the required location for lifting apparatus from the well.

When the mast 100 is required in use to handle equipment from a wellbore (not shown), the rack and pinion mechanism is actuated to enable the dolly 120 to move along the mast 100. In this manner the mast 100 operates as a rigid support structure to facilitate handling of equipment without the risk of the rack segments 14 buckling under the load. The load experienced by the dolly 120 is transmitted by the rack segments 114 and absorbed by the pinions 122 that are positioned beneath the rig floor 98. Although it is not essential to use the rack and pinion mechanism for lifting and retrieving equipment, it is suitable for use with the present invention and its use is preferred since it confers the following benefits. The rack and pinion system is easily maintained and is capable of being accurately controlled. The pinions 122 located beneath the rig floor 98 lowers the centre of gravity of the mast 100 and thus the mast 100 functions to guide the rack segments 98 and prevent buckling, rather than acting as a load bearing structure.

The dolly 120 preferably includes load cells, complete with a manual lockable rotary table and optionally, a hydraulic driven rotary table, two hydraulic power slips and a hydraulic spacer spool. According to a preferred embodiment, the mast 100 is completed with a pipe handling system, utility winches for handling tools on the workbasket and surrounding area, service loops and standpipes to provide maximum, functionality. Figs. 13 and 14 show the dolly 120, pipe handling and work < basket installed Automatic pipe handling can be integrated with the mast 100. Thus, the erected mast 100 and lifting equipment is capable of performing well intervention, completion change outs, reservoir sidetrack penetration and coiled tubing and wireline operations.

The erected mast 100 stands at around 23 metres high and has a load capability of up to around 350 tons, although, the load capability can be enhanced as described below. The racking capacity of the mast 100 is around 8000 metres. Erection of the mast and lifting equipment can be completed in 24 hours.

In order to improve the load capability of the mast 100, two counterbalance winches (not shown) can be mounted on the upper mast portion 40 and connected to the dolly 120. Such an arrangement can increase the load capability by around a further 72.6 metric tons (160000 lbs). Thus modular mast performs a self-erecting function, which minimises the time required for erection of the mast 100. This also enables the mast 100 to be erected using only a small footprint area.

The apparatus and its method of erection is also suitable for use onshore. Alternatively, the apparatus can be truck mounted to enhance portability.

As an alternative, the support structure and the mast can be divided into a greater number of modules of a predetermined maximum weight. This maximum weight can be calculated on the basis of the lifting capability of a helicopter. Thus, one or more helicopters can transport the apparatus in discrete modules from where the mast 100 can be erected in the manner described for the previous embodiment. Thus, the principle of the modular mast construction can be utilised in inaccessible areas where it would otherwise not be possible to erect a conventional mast.

According to another aspect and embodiment of the invention, a hydraulic lifting unit can be incorporated with the modular mast to enhance the overall load capability of the equipment. A suitable hydraulic lifting unit is shown generally at 136 in Fig. 20.

The hydraulic lifting unit 136 is assembled on the sub base 22 of the rack and pinion 122 unit and surrounding the blow out preventor (BOP) stack 158 including two pipe rams 160 arranged above and below a shear seal BOP 162. Load transfer rams 170 have feet located on the sub base 22 and an upper end of the load transfer rams 170 carry a load bearing support plate 168. Four jack legs 180 provided on the load bearing support plate 168 are arranged in a quadrilateral formation surrounding the BOP stack 158. A stationary slip plate 154 is seated on an upper end of the jack legs 180. The stationary slip plate 154 is coupled to a set of slips 152. A jack plate 144 is supported by rods 146 that can retract within and extend from the jack legs 180. Thus the rods are actuable to move the jack plate 144 and attached upper travelling slips 142.

The power supply and hydraulics for the hydraulic lifting unit 136 are coupled to the same source as that used for the rack and pinion mechanism. The controls for the hydraulic lifting unit 136 are also provided in a work basket 150 where the controls for the rack and pinion lifting unit are located.

The dolly 120 is driven by the rack and pinion mechanism as previously described. When the lifting apparatus is required to manipulate loads greater than around 136 metric tons- (300000 lbs), an operator at the work ^• station within the work basket 150 can switch to the hydraulic lifting unit 136.

Initially the rods 146 are in their retracted position within the jack legs 180. The upper travelling slips 142 are engaged with the equipment to be lifted out of a wellbore. The hydraulic lifting unit 136 is then actuated to drive the rods 146 upwardly and thus, equipment is moved from the wellbore. The lower slips 152 can then be engaged to retain the equipment in this position while the rods 146 again retreat within the jack legs 180.

The hydraulic lifting unit 136 has around a 3 metre (10 foot) stroke.

Mounting the hydraulic lifting unit 136 and transferring the load to the sub base 22 via the load transfer rams 170 is advantageous as the load path is not routed via the wellhead (not shown). Integration of the hydraulic lifting unit 136 with the rack and pinion 122 unit increases the load capability of the lifting assembly to 227 metric tons (500000 lbs).

Modifications and improvements can be made without departing from the scope of the invention. For example, the mast can be divided into two, three or any other number of mast portions depending on the environment in which the mast is to be erected.

Claims

1. A method for erecting a mast on a surface, the method comprising the steps of: (a) providing a support structure and a mast divided into two or more mast portions;

(b) engaging a part of a first mast portion with the support structure;

(c) arranging the first mast portion in a substantially perpendicular configuration relative to the surface; and (d) engaging a second mast portion with the first mast portion.

2. A method according to claim 1 , wherein prior to step (a) the method includes dividing the mast into two or more mast portions, such that each mast portion weighs less than ten metric tons.

3. A method according to claim 1 or claim 2, wherein prior to engaging the or each mast portion with the support structure, the method includes transporting the mast portions by crane proximate the support structure.

4. A method according to any preceding claim, including erecting the mast on a surface that is a moveable foundation.

5. A method according to any preceding claim, wherein according to step (b), the method includes providing a support structure having a positioning means and engaging a lower end of the first mast portion with the positioning means.

6. A method according to any preceding claim, wherein according to step (b), the method includes pinning the first mast portion to the support structure, such that the first mast portion is pivotable about its pinned part.

7. A method according to any preceding claim, wherein according to step (c), the method includes attaching a hoisting means towards an upper end of the first mast portion and actuating the hoisting means.

8. A method according to any preceding claim, wherein prior to step (d) the method includes raising the first mast portion relative to the surface on which the mast is to be erected.

9. A method according to any preceding claim, including engaging the part of the first mast portion with the support structure above a travelling base plate and raising the mast portion by actuating the travelling base plate.

10. A method according to claim 9, including retaining the or each mast portion in the raised configuration and moving the travelling base plate towards an end of the support structure proximate the surface prior to engaging the second mast portion with the first mast portion.

11. A method according to any preceding claim, wherein prior to step (d), the method includes engaging a part of the second mast portion with the support structure and arranging the second mast portion in a substantially perpendicular configuration relative to the surface on which the mast is erected.

12. A method according to any preceding claim, including repeating any of the method steps of claims 1 to 11 for the second mast portion prior to step (d).

13. A method according to any preceding claim, wherein according to step (d) the method includes connecting an upper end of the second mast portion with a lower end of the first mast portion.

14. A method according to any preceding claim, including providing locking pins within the first mast portion and locking the first and second mast portions into engagement by actuating locking pins to insert the locking pins through both mast portions.

15. A method according to any preceding claim, including repeating any of the method steps of claims 1 to 14 to connect another mast portion to the previous mast portion.

16. A method according to any preceding claim, including maintaining the mast portions in the erected position substantially perpendicular to the surface on which the mast is to be erected.

17. A method according to any preceding claim, including locating rack segments and one or more pinions within the mast portions, and arranging the rack segments to form a continuous rack within the erected mast and engaging the rack with the one or more pinions for driving the rack.

18. A method according to any preceding claim, including coupling one or more counterbalance winches to the mast and thereby increasing the load capability of the mast.

19. Apparatus for erecting a mast, the apparatus comprising a mast divided into two or more mast portions, a support structure having a positioning means for engaging at least a part of each mast portion and moving each mast portion relative to the support structure; and a connecting means to connect the mast portions to one another to thereby erect a mast.

20. Apparatus according to claim 19, wherein the positioning means comprise a hoisting means including a pulley and winch arrangement.

21. Apparatus according to claim 19 or claim 20, wherein the positioning means further comprises a raising means including a hydraulically actuable travelling base plate.

22. Apparatus according to any of claims 19 to 21 , wherein the support structure further comprises a retaining means for retaining the mast portions in a substantially raised position relative to the surface on which the mast is erected.

23. Apparatus according to claim 22, wherein the retaining means include friction pads for selectively engaging with at least two opposing sides of the mast portions.

24. Apparatus according to claim 22 or claim 23, wherein the mast portions are provided with indentations and the retaining means include bolts located on the support structure for retractably engaging with the indentations on the mast portions.

25. Apparatus according to any of claims 19 to 24, wherein the connecting means include male and female mating ends provided towards respective ends of each mast portion.

26. Apparatus according to any of claims 19 to 25, wherein the connecting means can further comprise at least two locking pins and an actuating means coupled to the locking pins, wherein the actuating means is hydraulically actuable to insert the locking pins through adjacent mast portions.

27. Apparatus according to any of claims 19 to 26, further comprising at least two stays for coupling part of the mast to the surface on which it is erected, wherein the stays are rigid and arranged to triangulate between the mast and the surface.

28. Apparatus according to any of claims 19 to 27, wherein the support structure is provided with one or more retractable pads for supporting at least one face of the mast and to act as an impediment to movement of the mast in a direction of pivot of the mast.

29. Apparatus according to any of claims 19 to 28, wherein the mast is provided with one or more pulleys towards an upper end thereof to engage with a line coupled to one or more counterbalance winches to enhance the load capability of the mast.

30. A lifting apparatus for lifting equipment into and out of a wellbore, the apparatus comprising a first lifting assembly and a second lifting assembly, wherein each lifting assembly is adapted to lift equipment into and out of a wellbore and wherein the first and second lifting assemblies are selectively actuable to lift equipment into and out of a wellbore.

31. A lifting apparatus according to claim 30, wherein the first and the second lifting assemblies are powered from the same source.

32. A lifting apparatus according to claim 30 or claim 31 , wherein first and the second lifting assemblies are independently actuable to lift _. equipment into and out of a wellbore.

33. A lifting apparatus according to any of claims 30 to 32, wherein the first lifting assembly is actuable to lift equipment up to a predetermined maximum weight and the second lifting assembly is actuable to lift equipment up to a predetermined maximum weight and wherein the maximum lift weight of the second lifting assembly is greater than the maximum lift weight of the first lifting assembly.

34. A lifting apparatus according to any of claims 30 to 33, wherein the first and second lifting assemblies are interchangeably connectable to the equipment to be lifted into and out of the wellbore.

35. A lifting apparatus according to any of claims 30 to 34, wherein the first lifting assembly comprises a rack and pinion unit.

36. A lifting apparatus according to any of claims 30 to 35, wherein the first lifting assembly comprises a modular mast housing a rack and pinion unit.

37. A lifting apparatus according to any of claims 30 to 36, wherein the second lifting assembly comprises two or more sets of gripping means to engage the equipment to be lifted and two or more hydraulic rams coupled to the gripping means and arranged to extend and retract to lift equipment out of and into a wellbore.

38. A lifting apparatus according to claim 37, wherein the gripping means comprise slips.

39. A lifting apparatus according to any of claims 30 to 38, wherein the second lifting assembly comprises four hydraulic rams arranged in a rectilinear configuration to support a load.

40. A lifting apparatus according to any of claims 30 to 39, wherein the control systems for the first and second lifting assemblies are integrated allowing an operator to selectively actuate and operate the first or the second lifting assembly.

41. A lifting apparatus according to any of claims 30 to 40, wherein the same source of hydraulic fluid is used to drive the first and second lifting assemblies.