ARCTIC DRILLING PROCESS

Abstract

A process for drilling a well into the seafloor at an offshore drilling location, in particular in arctic regions, the process comprising: drilling a top hole part of the well during a winter season where the water at the drilling location is at least partly covered by ice; and initiating drilling a lower part of the well extending into a hydrocarbon-bearing formation during a subsequent off-winter season where the water is less ice infested than during the winter season.

Claims

1. A process for drilling a well into the seafloor at an offshore drilling location, the process comprising: drilling a top hole part of the well during a winter season where the water at the drilling location is at least partly covered by ice; initiating drilling a lower part of the well extending into a hydrocarbon-bearing formation during a subsequent off-winter season where the water is less ice infested than during the winter season.

2. A process according to claim 1; wherein the winter season comprises a period of the calendar year where the ice concentration at the drilling location is at least 3/10 or higher, such as 5/10 or higher, such as 7/10 or higher, e.g. 8/10 or higher, such as 9/10 or higher.

3. A process according to claim 1 or 2; wherein the winter season comprises a period of the calendar year where ice thickness exceeds 0.1 m, such as 0.3 m, such as 0.5 m, such as 1 m, such as 2 m 0.5 m, such as 1 m, such as 2 m.

4. A process according to any one of the preceding claims, wherein the winter season starts later than 1 October, such as later than 1 November, such as no later than 1 December.

5. A process according to any one of the preceding claims, wherein the off-winter season starts when the ice concentration reaches a level below 4/10, such as below 3/10 such as below 2/10, such as below 1/10.

6. A process according to any one of the preceding claims; wherein the top hole part of the well is a part of the well that only extends through formations above any hydrocarbon-bearing formation from which hydrocarbons may rise through the drilled hole.

7. A process according to any one of the preceding claims wherein initiating drilling the lower part of the well comprises deploying a blow-out-preventer system.

8. A process according to any one of the preceding claims wherein drilling the top hole part of the well comprises one or more acts chosen from the following acts: inserting a conductor pipe into an upper part of the top hole part, optionally cementing the conductor pipe; inserting casing in the top hole part; cementing a casing in the top hole part; establishing a mud-cellar or caisson; covering the top hole part of the well by a removable cover.

9. A process according to any one of the preceding claims wherein drilling the top hole part and initiating drilling the lower part of the well are performed by the same drilling vessel.

10. A process according to any one of the preceding claims; comprising mooring a drilling vessel at the first drilling location prior to the drilling of the top hole part of the first well using a first number of anchor lines; and, prior to initiating drilling the lower part of the well, mooring the drilling vessel at the first drilling location using a second number of anchor lines; wherein the first number of anchor lines is smaller than the second number of anchor lines.

11. A process according to any one of the preceding claims; comprising drilling respective top hole parts of a plurality of wells at respective drilling locations by a drilling vessel during the same winter season; and initiating drilling respective lower parts of one or more of said wells during a subsequent off-winter season where the water at the respective drilling locations is less ice infested than during the winter season; wherein the respective lower parts extend into a hydrocarbon-bearing formation.

12. A process according to claim 11; comprising: mooring the drilling vessel at a first drilling location so as to allow drilling of the top hole part of a first well at the first drilling location during the winter season; repositioning the drilling vessel to and mooring the drilling vessel at a second drilling location so as to allow drilling of the top hole part of a second well at the second drilling location during said winter season.

13. A process according to claim 12; comprising laying out one or more anchor lines for mooring the drilling vessel at the second drilling location prior to completing drilling the top hole part of the first well.

14. A process according to claim 12 or 13; comprising using at least one anchor line for mooring the drilling vessel at the first drilling location and for subsequently mooring the drilling vessel at the second drilling location.

15. A process according to claim 14; wherein repositioning of the vessel is performed while at least one anchor line remains connected to the drilling vessel.

16. A process according to any one of claims 10 through 15; comprising mooring the drilling vessel at a drilling location using a plurality of anchor lines wherein two or more of the anchor lines have an upper end connected to a connecting member; and wherein mooring the drilling vessel comprises connecting the connecting member to the drilling vessel while the upper ends of the two or more anchor lines are connected to the connecting member.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In the following, one or more embodiments of the invention will be described in more detail and with reference to the drawings, where:

[0035] FIG. 1 schematically shows an example of a cross section of a marine vessel.

[0036] FIGS. 2A-D schematically illustrate an embodiment of a drilling process.

[0037] FIGS. 3A-B schematically illustrate an example of the mooring of a drilling vessel during drilling through a marine riser and while drilling a top hole, respectively.

DESCRIPTION OF THE EMBODIMENTS

[0038] FIG. 1 schematically shows an example of a drilling vessel floating in a body of water 105 with drifting ice 127. In particular, FIG. 1 shows a cross section of a drillship, generally designated 100, that is anchored to the seafloor 111 by anchor lines 108. The drillship comprises a hull 101 which may be substantially oblong or ship-shaped; alternatively the drilling vessel may have a different shape, e.g. an off-shore platform. The drillship further comprises a drill floor 102 formed on top of a platform supported by legs 130 or another form of substructure. The platform defines the drill floor from which drilling operations are conducted and spans across a moon pool 107 formed in the hull of the drillship so as to allow equipment to be lowered towards the seafloor. One or more holes in the drill floor, each typically in the form of a rotary table, define one or more well centres through which drilling operations can be performed. The well centre(s) may be located next to or generally under a drilling support structure 104 supporting a hoisting system. In the example of FIG. 1, the drilling support structure is a mast positioned adjacent to the well centre, but other forms of drilling support structures, such as a derrick structure placed over the well centre, are possible as well. The drilling support structure is supported by the legs 130 or a similar substructure and it extends upwardly relative to the drill floor 102. The hoisting system comprises a hook or similar device from which a string of tubulars 109 may be suspended and lowered and raised through the well centre and the moon pool 107. To this end, the hoisting system may comprise a topdrive 103 to which an upper end of the drill string may be connected and which may impart torque on the drill string. The hoisting system may be a draw-works system where the hoisting line is fed over stationary sheaves carried by the drilling support structure or another suitable type of hoisting system such as a hydraulic hoisting system comprising cylinders that extend upwardly from the drill floor and support the load to be lowered or hoisted.

[0039] The drillship is configured to perform drilling operations without a marine riser or through a marine riser string 110 extending from the drillship to a BOP 112 that is placed on the seafloor. Hence, the drillship is connectable to a subsea well via the marine riser string 110. The drillship is moored via a turret 106 and a plurality of anchor lines 108. The turret allows the drill ship to align its longitudinal axis with any given ice drift direction, a direction of a local sea current, a wind direction, and/or the like. To this end, the ship may rotate around the vertical axis defined by the turret 106. The marine riser may be located coaxial with the axis of rotation.

[0040] In general, floating drilling units, such as drillships and semi-submersibles, are frequently used for drilling operations for exploration of hydrocarbon reservoirs, such as oil or natural gas reservoirs, in subsea formations, for establishing bored wells into such reservoirs and/or for subsequent production of hydrocarbons. It will be appreciated that the size and shape of the vessel, its equipment, and/or the type of equipment extending downwards from the vessel may vary according to the specific application.

[0041] When the drillship floats in drifting ice 127 the drillship is typically oriented such that the ice approaches the bow of the hull 101. To this end, embodiments of the hull may have an ice-breaking shape and sufficient strength so as to break the ice. Generally, the hull may comprise one or more features (not explicitly shown) that are shaped and sized so as to prevent ice from submerging below the bottom of the hull. In some embodiments, such features may extend along the bow and/or the sides of the midship section of the hull. The hull may comprise a generally flat bottom, though other hull shapes are possible including hulls having an inclined bottom.

[0042] FIGS. 2A-D schematically illustrate an embodiment of a drilling process. In particular, FIG. 2A shows a drilling vessel 100 positioned and moored at a first drilling location where the subsea formations under the seafloor comprise an oil-bearing formation 216. In the example of FIGS. 2A-D, the drilling vessel is a drillship, e.g. a drillship as described in connection with FIG. 1. The drilling vessel 100 is moored to the seafloor 111 by means of anchor lines 108A and 108B. The anchor lines have a lower end anchored at the seafloor and an upper end attached to the vessel, e.g. to a turret structure as described in connection with FIG. 1. The drilling vessel 100 of FIG. 2A is in the process of drilling a top hole part of a first well 226A, i.e. a part of the well that does not extend into the oil-bearing formation 216. To this end, the drilling vessel drills a hole using a drill string 109 of a suitable diameter, such as 22 or more, such as 36 or more. Drilling the top hole part of the well may include inserting a conductor pipe into the upper part of the top hole, whereafter drilling of a lower part of the top hole continues through the conductor, i.e. where the lower part of the top hole has a smaller diameter than the upper part. A casing pipe may then be inserted into the lower part of the top hole and secured by injecting cement into the annulus surrounding the casing. Drilling the top hole part may further comprise tasks in preparation for the deployment of a blow-out-preventer, e.g. the establishment of a mud cellar or caisson, e.g. as described in U.S. Pat. No. 4,558,744, so as to allow deployment of the BOP into a recess or cavity in the seafloor, e.g. so as to protect the BOP against passing icebergs. Upon completion of the top hole part, a lid or similar cover may be placed on the top hole, so as to protect the top hole during the period until the drilling operations may be resumed during a subsequent off-winter season.

[0043] As the drilling of the top hole part of the well 226A does not involve drilling into the oil-bearing formation 216, this part of the drilling operation may safely be performed during the winter season when the sea surface is completely or partially covered by ice 127.

[0044] Typically, during exploration of an oil field, multiple wells are drilled into the oil-bearing formations 216. Accordingly, it may be desirable to drill, e.g. by a single drilling vessel, top holes of multiple wells within the same Arctic region during a winter season and to continue the drilling of one or more of these wells during a subsequent off-winter season, e.g. using the same drilling vessel. To this end, the process may comprise laying out anchor lines 208A and 208B at a second drilling location before drilling the top hole of the first well 226A at a first drilling location has been completed. The deployment of the anchor lines may e.g. be performed by a supply vessel different from the drilling vessel 100. This is schematically illustrated in FIG. 2A, showing pre-deployed anchor lines 208A and 208B. The predeployed anchor lines may then be picked up again by the drilling vessel or by a supply vessel. For example, the anchor lines 208A, 208B may have lead cables 218 connected to them that can be picked up by a hook, an ROV or the like. To this end, the lead may be connected to a buoy 217 or other floatation device so that it floats above the seafloor, e.g. below the ice 127.

[0045] FIG. 2B shows the drilling vessel 100 after it has completed drilling the top hole part of the first well and has been repositioned to a second drilling location so as to be able to drill the top hole part of a second well 226B. To this end, some anchor lines 108A that were used for mooring the vessel at the first drilling location have been disconnected and, after repositioning the vessel, pre-deployed anchor lines 208A have been recovered and connected to the drilling vessel 100 so as to keep the vessel at the second drilling location. Hence, in some situations, some, or even all, anchor lines used for mooring the vessel at the first drilling location may be re-used for mooring the vessel at the second drilling location without a need for complete recovery and redeployment of the anchor lines. The extent to which this is advantageous may depend on the specific circumstances, e.g. on the distance between the wells. Under some circumstances, it may be beneficial or even necessary to disconnect all anchor lines at the first location and to reconnect an entire set of anchor lines at the second drilling location. In other situations, it may be possible to leave all anchor lines connected to the vessel and to merely adjust their lengths so as to secure the vessel at the second drilling location.

[0046] FIG. 2C shows the drilling vessel after it has completed drilling the top hole part of the second well 226B, repositioned and moored at a third drilling location by anchor lines 208A and 208B and in the process of drilling the top hole part of a third well 226C. Hence, during a single winter season the top hole part of one well or of multiple wells may be drilled in an efficient and safe manner by a single drilling vessel.

[0047] During a subsequent off-winter season, where the water surface 105 is sufficiently free of ice, the drilling vessel 100 may resume drilling operations at one or more of the wells where the top hole part has previously been established.

[0048] To this end, as illustrated in FIG. 2D, the vessel is again moored at the corresponding well 226A. It will be appreciated that, if the vessel continues drilling operations of the latest established top hole, a re-mooring may not be necessary, as the drilling vessel may simply remain at the corresponding drilling location. Hence, for the purpose of the present disclosure, reference to a vessel being again moored refers both to the situation where the vessel remains moored and where the vessel is un-moored and subsequently re-moored at the same location. In any event, in order to be able to drill into the oil-bearing formation 216, the drilling vessel deploys a BOP 112 and a riser 110. In the case of floating vessels, the BOP 112 is typically deployed at the seafloor, as illustrated in FIG. 2Doptionally in a mud cellar or caisson. In this case a marine riser 110 extends between the vessel and the BOP. In any event, the further drilling operations into the deeper oil-bearing formations 216, i.e. deeper than the top hole part that was drilled prior to deployment of the BOP, are performed with the drill string 109 extending through the riser 110 and the BOP 112.

[0049] FIGS. 3A-B schematically illustrate an example of the mooring of a drilling vessel during drilling through a marine riser and while drilling a top hole, respectively. Drilling operations through a riser and the BOP impose stricter requirements on the station keeping of the vessel as the initial drilling of the top hole part that has a relatively larger diameter. In particular, the riser is made up of large-diameter tubular members that extend through the moon pool of the vessel downwards. Large horizontal displacements of the vessel relative to the well centre may cause the riser to impact the walls of the moon pool or the turret, thus involving the risk of damaging parts of the vessel or of the riser.

[0050] Accordingly, during the drilling of the top hole part of a well during winter season, it may be sufficient to moore the vessel using fewer anchor lines than during the further drilling operations through the riser and the BOP during a subsequent off-winter season. This is schematically illustrated in FIGS. 3A-B. FIG. 3A shows the vessel 100 being moored using 16 anchor lines 108 attached to the turret structure 106 of the vessel. FIG. 3B shows the same vessel being moored during the drilling of the top hole part of a well during the winter season, where the vessel is moored by only 8 anchor lines 108. The fewer anchor lines thus allow for a faster repositioning and re-mooring of the vessel at multiple drilling locations during a winter season.

[0051] It will be appreciated that, during the winter and/or the off-winter season, the vessel may be moored using a different number of anchor lines as shown in the example of FIGS. 3A-B, as the exact number of lines may depend on a variety of factors, such as water depths, nature of the seafloor, sea currents, wind conditions, ice drift, etc.

[0052] In order to allow for an even more efficient re-mooring of the vessel during the winter season, some or all of the anchor lines may, prior to or upon deployment, be attached to a connecting member that may be removably attachable to the vessel, e.g. to the turret structure of the vessel. For example, the connecting member may be ring-shaped or have the shape of a ring segment. The diameter of the ring or ring segment may correspond to the dimensions of the turret. When the vessel is positioned at a drilling location, multiple anchor lines may thus be recovered and attached to the vessel in a single efficient process. For example, the connecting member may be formed as a disconnectable rotatable part of a turret moorig system, e.g. as described in U.S. Pat. No. 8,397,655.

[0053] Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In particular, it is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention. In particular, embodiments of the drilling process have mainly been described with reference to a turret mooring system. However, it will be appreciated that embodiments of the process described herein may also be used in connection with other types of positioning systems.

[0054] The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

[0055] It should be emphasized that the term comprises/comprising when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.