The present invention relates to a workover unit or hydraulic workover unit, optionally comprising a snubbing unit, and that is capable to operate as a service rig that is typically used for downhole operations or proceedings of oil and gas wells, for example.

From practice workover units are known that rely on a crane, more specifically a mobile crane, for lifting and positioning the workover unit over a wellhead. This requires delicate maneuvering. Also, there is a risk of damaging the wellhead during this maneuvering.

Also known from practice are so-called (mobile) derrick rigs that are pivoted from a mobile platform, such as a trailer, to an upright position while being positioned directly over a wellhead. This also requires accurate positioning in an often relatively rough terrain. Incidents when performing the lifting and/or pivoting maneuvers may result in damaging the wellhead. This forms an unwanted risk in conventional workover units.

An object of the invention is to obviate or at least reduce the above mentioned problems and to provide a workover unit that can be positioned and/or operated more effectively and/or more efficiently with reduced risks.

This object is achieved with the workover unit according to the invention, the unit comprising:

- a basic structure;

- a BOP-stack provided in the basic structure; and

- a skidding system configured for positioning the basic structure being in an upright working position over a wellhead.

The workover unit according to the present invention is preferably hydraulically operated and is also referred to as a hydraulic workover unit. The workover unit according to the present invention can be operated as a workover rig that enables repair, piping/tubing replacement, or other downhole proceedings of an oil or gas well, for example. Preferably, the workover unit is capable of pulling sufficient weight, dealing with upwards acting forces during live well operating due to a well pressure, and/or withstanding wind gusts, for example.

The workover unit comprises a basic structure that comprises a BOP-stack having a Blow

Out Preventer (BOP) and can be applied to well with or without pressure. Preferably, the BOP- stack comprises a swivel for aligning the stack. The basic structure acts as a frame for the overall workover unit. Preferably, the first structure is configured such that one or more further structures can be positioned on top of it. This enables a modular approach and modular assembly of the workover unit according to the present invention. As a further effect this enables optimization of the logistic operation when a workover unit needs to be installed at a well, for example. The workover unit according to the present invention further comprises a skidding system. This skidding system is configured for positioning the basic structure, when brought in an upright working position, over a wellhead.

More specifically, this skidding system enables placing the basic structure of the working unit next to the wellhead in an upright working position. Thereafter, the basic structure is moved in this upright working position with the skidding system and positioned over the wellhead. This enables accurate positioning of the basic structure of the workover unit over the wellhead, thereby also enabling an accurate connection thereto.

In addition, as the skidding system enables placing the basic structure next to the wellhead in an upright position, the risk of damaging the wellhead in case of incidents or accidents when performing maneuvering operations on the basic (or further) structure, such as pivoting and lifting, is significantly reduced. This reduces the overall risks of workover units according to the present invention.

The workover unit according to the present invention with the BOP-stack and skidding system further enables operation of the workover unit as a so-called hydraulic workover unit that can be used in oil and gas wells, geothermal wells, salt solution mining, and gas storage, for example. Also these operations with the workover unit according to the present invention can be performed more safely as compared to conventional units.

In a further preferred embodiment according to the present invention the workover unit further comprises a further structure, with the basic structure comprising an integrated crane configured for lifting the further structure to a working position on top of the basic structure.

Providing a further structure next to the basic structure of the work unit enables a modular configuration of the workover unit. The individual basis and/or further structures can be transported separately having optimal dimensions suitable for efficient and effective transport.

Providing an integrated crane to the basic structure enables lifting a further structure to a working position on top of the basic structure. This prevents the need for providing a separate crane or lifting facility during rig up of the workover unit. Also, this prevents transport of such separate crane or lifting facility to the wellhead location. This is especially advantageous in case the wellhead location is an (impassable) remote location such that the logistic operation can be relatively complex and expensive. Also, some of these locations do not hold sufficient footprint for easy use of a separate crane or lifting facility.

Preferably, the integrated crane comprises a first telescopic element enabling reduction of the transport dimensions. When in use the extended telescopic element enables lifting and positioning of the further structure.

Optionally, the workover unit according to the invention comprises three or more further structures. The crane may also involve additional or alternative pivoting elements to increase its working height when in use and reduce its dimensions in a transport mode. Optionally, the integrated crane according to the invention can also be applied to conventional workover units, optionally without applying the skidding system according to the invention.

Preferably, the crane comprises a further crane element configured for enabling lifting operations when the work unit is in use. Such further crane element preferably comprises a second telescopic crane element that enables pipe handling, for example. This provides the integrated crane of the basic structure with a dual action mode. In a first assembling mode, the crane is used for assembling the workover unit according to the present invention. In a second operational mode, the integrated crane enables further operations when the work unit is in use, such as pipe handling. This enables an effective use of the integrated crane.

In a presently preferred embodiment according to the present invention the crane is connected to the basic structure with a pivoting shaft enabling rotating the crane over a working angle around the pivoting shaft that extends in a substantially horizontal direction in a working position of the work unit with the basic structure being in the upright working position.

Providing a pivoting shaft enables rotating the integrated crane around a substantially horizontal shaft or axis such that the crane can be rotated in a substantially vertical plane. This provides a relatively simple crane configuration that is preferably capable to perform the aforementioned dual mode operation. For example, a further structure that is positioned next to the wellhead with the basic structure being positioned over the wellhead can be lifted after which the crane pivots and next positions the further structure on top of the basic structure. This relates to using the crane in the assembling mode. Similar crane movements can also be used in the operational mode to enable pipe handling, for example.

In a presently preferred embodiment according to the present invention the work unit further comprises a snubbing unit.

The snubbing unit that is preferably provided in or on the basic structure enables (heavy) well intervention, for example on oil and gas wells. The snubbing unit is typically configured for running in and pulling out pipes to and from the well. Furthermore, such snubbing unit enables performing operations with the well still being under pressure. For example, this enables effective pipe replacement and efficient well operation. Preferably, the snubbing unit includes the BOP to improve safety when performing operations with the workover unit, while the well is under pressure. In that case the snubbing unit may enable bringing pipes, tubing and bottom hole assemblies in and out of a well, for example.

In a presently preferred embodiment according to the present invention the workover unit further comprises a pre -tensioning system configured for applying a pre-tension to a wellhead.

Providing a pre-tensioning system enables application of a pre-tension onto a wellhead.

This may eliminate the use of so-called guywires and anchoring dead-weights. This reduces the amount of operations that is required when installing a workover unit. Also, as some locations do not hold sufficient footprint for the deploying anchoring dead-weights, these locations can be operated effectively with the workover unit according to the present invention.

Furthermore, by providing a pre-tensioning system the so-called wellhead loading can be substantially eliminated. Also, the use of the pre-tensioning system prevents transport of deadweights to a remote location thereby rendering the overall logistic operation is simpler.

Preferably, the pre-tensioning system comprises one or more hydraulic cylinders to press the BOP towards the wellhead. In this preferred embodiment according to the present invention, the BOP is positioned movable in the basic structure of the workover unit. Such BOP is preferably part of the snubbing unit. This enables an effective operation of the BOP with the pre-tensioning system.

The invention further relates to a mobile workover unit, the mobile workover unit comprising:

- a workover unit as described earlier; and

- a trailer, truck or mobile platform comprising a self-propelling device.

The mobile work unit provides the same effects and advantages as described for the workover unit.

The mobile platform, including a trailer or truck, comprises a self-propelling device that is configured to pivot the basic structure from the mobile platform to an upright working position next to the wellhead. This self-propelling device may comprise a number of hydraulics cylinders and a hinge or pivoting shaft enabling pivoting the basic structure, for example. Similar maneuvers can be performed with additional further structures.

The invention further also relates to a method for providing a workover unit, the method comprising:

- providing a workover unit or mobile workover unit as described earlier;

- positioning the workover unit; and

- connecting the workover unit.

The method provides the same effects and advantaged as mentioned to the workover unit and the mobile workover unit that optionally are provided with a snubbing unit. Specifically, the method has the advantage that the basic structure can be positioned in an upright working position next to the wellhead, where after the basic structure in the upright position can be moved horizontally with the skidding system over the wellhead and be connected thereto. This provides a relatively simple and safe approach to provide a workover unit, while reducing risks associated with maneuvering a component of such workover unit, for example a basic structure thereof.

In a preferred embodiment of the present invention the positioning of the workover unit involves placing a basic structure over a wellhead, providing a second structure that is placed next to a wellhead in an upright position, where after the further structure is placed on top of the basic structure using a crane or lifting facility, preferably an integrated crane of the basic structure. This reduces the need for providing a separate crane or lifting facility.

In addition, the risks associated with maneuvering the (sub) structures are significantly reduced.

In a presently preferred embodiment according to the present invention the method further comprises the step of connecting the workover unit to a wellhead using a pre-tensioning system such that in use the pre-tensioning system applies a pre-tension onto the wellhead. By applying a pre-tension onto the wellhead so-called wellhead loading can be eliminated and/or reduced.

Furthermore, applying a pre-tension enables more stable positioning of the workover unit.

Further advantages, features or details of the invention are elucidated on basis of the preferred embodiments thereof, wherein reference is made to the accompanying drawings, wherein:

- figure 1 shows a truck for transporting a basic structure of the work unit according to the invention;

- figure 2 shows the truck of figure 1 when pivoting the basic structure to an upright position;

- figures 3 and 4 show the basic structure being moved over the wellhead with a skidding system; and

- figures 5-7 show maneuvering a further structure relative to the basic structure with an integrated crane;

- figure 8A-B illustrates the pre-tensioning system with the movable BOP; and

- figure 9 shows a further detail of the pre-tensioning system of figure 8.

Mobile workover unit 2 (figure 1) is in the illustrated embodiment transported with truck 4. Trailer 6 carries basic structure 8. Having arrived at the desired location, basic structure 8 of workover unit 2 needs to be maneuvered in an upright position (figure 2). In the illustrated embodiment trailer 6 of truck 4 comprises self-propelling device 10. Self-propelling device 10 comprises hinge 12, first hydraulic cylinder 14 and second hydraulic cylinder 16. Self-propelling device 10 enables pivoting a basic structure 8 comprising an optional snubbing unit 18 and integrated crane 20 from a substantially horizontal transport position into a substantially vertical working position.

A basic structure 8 of workover unit 22 is pivoted from trailer 6 and is positioned next to well 24 and wellhead 26 thereof (figure 3). Wellhead 26 comprises wellhead connector 28 comprising a connecting flange. Basic structure 8 optionally comprises snubbing unit 18.

Furthermore, basic structure 8 comprises telescopic crane 20 and is positioned on ground or floor 30. In the illustrated embodiments basic structure 8 further comprises a BOP 32 with aligning swivel that can be integrated with the optional snubbing unit 18. Integrated crane 20 is connected with rotating shaft 34 to basic structure 8. In the upright position of structure 8 rotating shaft 34 extends in a substantially horizontal direction. Crane 20 is connected to shaft 34 with connecting plate 36. Crane 20 comprises two first crane elements or arms 38 that are substantially parallel. In the illustrated embodiment crane 20 further comprises second telescopic elements 40 and third telescopic elements 42. The two elements or arms of crane 20 are connected with a cross beam carrying hoisting device 44.

Skid 46 (figure 4) enables moving basic structure 8 in a substantial horizontal direction towards wellhead 26. In the illustrated embodiment, after positioning basic structure 8 with skid 46, a further structure 48 can be positioned next to wellhead 26 (figure 5). Crane 20 can be rotated over working angle a to enable lifting further structure 48 with hoist 44 (figure 6). After reaching the desired height with structure 48 crane 20 pivots around shaft 34 to its substantially vertical position and connects further structure 48 to the top of basic structure 8 (figure 7). In the illustrated embodiment on top of basic structure 8 there is provided first platform 50 and on top of further structure 48 there is provided second platform 52. This enables a safe working environment.

Hydraulic pre-tensioning system 54 (figure 8A-B) comprises first hydraulic cylinder 56 and second hydraulic cylinder 58 (figure 9) enabling movement of BOP 32 in basic structure 8 along guiding elements 59. BOP connector 60 is moved towards wellhead connector 28 to connect BOP 32 to wellhead 26. In addition, pre-tensioning system 54 is capable of applying a pre -tension onto wellhead 26.

Assembling a workover unit 22 starts with transporting basic structure 8 and further structure 48 to well 24. After arriving at the desired location basic structure 8 is pivoted from mobile platform, such as trailer 6, with self-propelling device 10 from a substantially horizontal transport position to a substantially vertical working position, such that basic structure 8 is positioned next to wellhead 26. After having put basic structure 8 into an upward position structure 8 is moved with skidding system 46 and positioned over wellhead 26.

Further structure 48 can be placed in a similar way next to well head 26. By providing basic structure 8 with integrated crane 20, further structure 48 can be lifted and positioned on top of basic structure 8. Preferably, integrated crane is capable of performing a pivoting or rotating movement around shaft 34 to change working angle a. In addition, integrated crane 20 can be used to handle pipes when workover unit 22 is in operational use. BOP 32 is connected to wellhead 26 with pre-tensioning system 54 that moves BOP 32 downwards such that BOP connectors 60 engages wellhead connector 28. Pre-tensioning system 54 preferably applies a pre-tension onto wellhead 26 to eliminate guywires, dead-weights, and wellhead loading. The present invention is by no means limited to the above-described preferred embodiments thereof. The rights sought are defined in the following claims within the scope of which many modifications can be envisaged.