WO2016089473A1

WO2016089473A1 - Protecting a stationary vessel from encroaching ice

Info

Publication number: WO2016089473A1
Application number: PCT/US2015/054418
Authority: WO
Inventors: Adel H. Younan
Original assignee: Exxonmobil Upstream Research Company
Priority date: 2014-12-04
Filing date: 2015-10-07
Publication date: 2016-06-09
Also published as: RU2665207C1; US20160160463A1

Abstract

Techniques are provided for protecting a stationary vessel from encroaching ice. In an example, a system has a subsea mount disposed on a seafloor and a thruster disposed on the subsea mount under a water surface. The thruster is configured to destabilize a water column under the encroaching ice.

Description

PROTECTING A STATIONARY VESSEL FROM ENCROACHING ICE

CROSS-REFERENCE TO RELATED APPLICATION

[000 Ϊ j This application claims the priority benefit of U.S. Provisional Patent Application 62/087,504, filed 4 December 2014, entitled PROTECTING A STATIONARY VESSEL FROM ENCROACHING ICE, the entirety of which is incorporated by reference herein.

FIELD ΘΘ02] The present techniques are directed to protecting stationary vessels from sea ice. More specifically, the present techniques are for breaking sea ice drifting towards a stationary vessel.

BACKGROUND

[0003] This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art. j 0004 [ Arctic drilling attention has recently transcended shallow waters, e.g., less than about 100 meters (m), into deep waters, e.g., greater than about 100 meters. This, however, multiplies the challenges as arctic and deep-water frontiers are now merged. The challenges generally result from problems in stationkeeping, which is the ability of a vessel to hold a position over a subsea location, such as a well. ΘΘ05] Station keeping is often important to prevent stress on drilling risers and lines that run from a vessel to the seafloor. Station keeping can be performed passively, such as by mooring lines, dynamically using propulsion systems, or a combination of the two. In arctic environments, station keeping can be challenged by sea ice floes. While fixed platforms are in direct contact with the seafloor and can withstand forces up to tens of thousands of tonnes, floating platforms are anchored to the seafloor via mooring systems with the capacity to withstand forces in the range of 1,000-2,000 tonnes.

[0006] While subsea developments may become a viable concept for arctic deep-water development, some operations still need to be conducted at the surface, such as drilling a subsea well or loading crude from subsea storage, among others. With conventional technology, these surface operations would have to rely on open water season. However, in many arctic locations, an open water season can be limited, highly variable, or, in some years, non-existent. Accordingly, systems to protect platforms and other vessels from drifting ice may be useful. Extending the open water season will depend on ice breakers, which may be limited in number and costly to operate.

SUMMARY

[0007] An exemplary embodiment provides a system for protecting a stationary vessel from encroaching ice. The system includes a subsea mount disposed on a seafloor and a thruster disposed on the subsea mount under a water surface. The thruster is configured to destabilize a water column under the encroaching ice.

[0008] Another exemplary embodiment provides a method for protecting a sea surface location from encroaching ice. The method includes detecting encroaching ice and activating a seafloor mounted thruster, wherein the thruster destabilizes the water column below the ice.

[0009] Another exemplary embodiment provides a method for producing hydrocarbons. The method includes positioning a vessel at a location on a sea surface. A thruster attached to a mount on a seafloor is positioned proximate to the location. Hydrocarbons are produced from a well using the vessel. Ice that is encroaching on the vessel is detected. The thruster is activated to destabilize the water column below the ice.

DESCRIPTION OF THE DRAWINGS

[0010] The advantages of the present techniques are better understood by referring to the following detailed description and the attached drawings, in which:

[001 J ] Fig. 1 is a schematic diagram of the use of anchored thrusters to protect a stationary vessel from encroaching ice floes;

[0012] Fig. 2 is a schematic diagram showing thrusters breaking an encroaching ice floe;

[0013] Fig. 3 is a schematic diagram of a single thruster destabilizing a water column under an encroaching ice sheet;

[0014] Fig. 4 is a drawing of a single thruster that may be used for breaking encroaching ice floe;

[0015] Fig. 5 is a schematic diagram of a drilling location showing the placement of sensors and thrusters that may be used to protect a vessel;

[0016] Fig. 6 is a process flow diagram of a method for producing hydrocarbons while using thrusters to break up ice floes before they reach a drilling location; and

[0017] Fig. 7 is a process flow diagram of a method for using thrusters to break up ice floes before they reach a surface location.

DETAILED DESCRIPTION

[0018] In the following detailed description section, specific embodiments of the present techniques are described. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the techniques are not limited to the specific embodiments described below, but rather, include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

[ΘΘ1 ] As discussed above, protecting a vessel or vessels at a sea surface location from encroaching ice may be problematic, especially in deep water operations. Accordingly, embodiments described herein provide protection from ice encroachment for a location at the surface. Thrusters are mounted to locations along the seafloor and used to destabilize the water column under ice floes. Destabilizing the water column may break up the ice floes into fragments that do not apply as much pressure to a vessel. In some embodiments, the thruster may be used to steer larger ice fragments, such as ice ridges or icebergs, away from the vessel.

[0020] The thrusters may be mounted in arcs upstream of the vessel, e.g., in the general direction of encroaching ice floes, to break up or divert ice floes before they reach the vessel. In some embodiments, the subsea mount includes rails (or tracks) mounted to the seafloor. The thrusters may be mounted along the rails, allowing the thrusters to be repositioned to be more effective. For example, the rails may be mounted to mooring pilings, such as suction pilings, used to moor the ship in place. Combinations of these techniques can be used to make multiple layers of protection. For example, fixed thrusters may be mounted in an arc upstream of the vessel, while a rail may be mounted proximate to the fixed thrusters, allowing additional thrusters to be moved into place as needed. Similarly, multiple rails with mounted thrusters can be nested around a location to protect vessels by allowing multiple thrusters on each of the rails to be moved into place as needed.

[0021] The thrusters can be controlled and powered from the vessel being protected or may have a separate power system, control system, or both. In some embodiments, the thrusters may have sensors to detect objects, such as sea life, vessels, ice floes, and the like. The detection of the objects may be used to adjust the depth of the thruster in the water column, to power the thruster down, or both, to avoid collisions and damage.

[0022] Fig. 1 is a schematic diagram of the use of anchored thrusters 102 to protect a stationary vessel 104 from encroaching ice floes 106. The vessel 104 may be a drillship, used to drill a well 108 to a reservoir located below the seafloor 110. However, as discussed herein, any number of other vessels may be protected using the current techniques, including tankers, service vessels, floating storage, offloading, and production (FSOP) vessels, and the like. If the vessel 104 is in deep water, for example, greater than about 100 meters in depth, it may not be in direct contact with the seafloor 110. In this example, station keeping is important to protect equipment and piping that extends between the vessel 104 and the seafloor 110, such as a drilling riser 112.

[0023] The station keeping can be performed using mooring lines 114, for example, coupled to a piling 116 embedded in the seafloor 110. The piling 116 may be driven into the seafloor or may be a suction piling. Suction pilings are pulled into the soft surface of the seafloor by placing an open bottom of the piling in contact with the seafloor and then pumping water out of a chamber located at the top of the piling. In some embodiments, station keeping can be performed by azimuthing thrusters 118 on the vessel 104. Combinations of these techniques may also be used, for example, using a mooring line 114 to generally hold the vessel 104 in place while the azimuthing thrusters 118 hold tension on the mooring line 118 and keep the vessel 104 from lateral motions.

[0024] Encroaching ice floes 106, or ice ridges 107, may approach the vessel 104 and interfere with the station keeping, for example, by forcing the vessel 104 to disconnect the drilling riser 112 from the well 108, pull it up, and move out of the way. The use of the thrusters 102 in a water column 120 can mitigate problems with the ice floes 106. The thrusters 102 may be disposed on the subsea mount by attaching the thrusters 102 to the mount using tethers 122. The subsea mount may include a frame and one or more pilings. The frame, called a template 124, may be attached to pilings 126 set into the seafloor 110. The pilings 126 may be driven into the seafloor 110 or may be suction pilings. In other embodiments, the thruster may be attached to a piling 116 used for mooring the stationary vessel 104. For example, mooring lines for a drilling vessel 104 may be attached to a series of suction pilings located around the vessel. Each of the suction pilings may also be used as attachment points for anchoring thrusters 102. As discussed herein, the pilings 116 may also be used as anchor points for rails that are used as attachment points for anchoring the thrusters 102. This would allow the thrusters to be moved to positions that are between encroaching ice floes 106 and the stationary vessel 104.

[0025] A power and control cable 128 may be attached from the vessel 104 to each tether 122 to provide power to the thruster 102 attached to that tether 122. In some embodiments, a subsea generator may be used to provide power for a thruster 102. Further, power may be provided to the thrusters 102 from an on-shore generating station, depending on the location of the field relative to the shore, e.g., within about 50 miles of the shore or less.

[ΘΘ26] The thrusters 102 can be made from any number of marine propulsion units, such as tunnel thrusters available from Rolls-Royce PLC of London, England, and Thrustmaster of Houston, Texas, USA, among others. The selection of the sizes for the thrusters 102 may depend on the location of the vessel 104 and the likely type of sea ice to be encountered, such as first year, second year, etc. In some embodiments, the thrusters 102 may be about 180 kilowatts (kW) to about 1 megawatt (MW) in power generation, while in other areas, the thrusters 102 may be about 3 to about 8 MW.

[0027] As the thrusters 102 may take water in at the top and eject it from the bottom, the tether 122 will remain in tension. Further, the tether 122 may be coiled to allow the thruster 102 to move vertically in a water column 120 without the tether 122 developing slack. A thruster 102 may change its vertical position along a water column by adjusting its buoyancy. This allows the thrusters 102 to be useful for various water depths and avoid various sea ice keel depths.

[0028] In some embodiments, a thruster 102 may be configured to allow a reversal of flow, e.g., from the bottom of the thruster 102 and out the top. This may allow the thruster

102 to deflect ice floes 106 or ice ridges 107 away from the vessel 104 when they are too large to break up. In this case, the thruster 102 would not be mounted to the template 124 by a flexible tether 122, but would, instead, be disposed on the subsea mount by attaching the thruster with a bar or other rigid part. The bar may be hinged at the bottom to allow the thruster 102 to be lowered in a water column 120, for example, to avoid a collision.

[0029] Fig. 2 is a schematic diagram showing thrusters 102 breaking an encroaching ice floe 202. Like numbered items are as described with respect to Fig. 1. In this example, the thrusters 102 are configured to pull water in through the top 204 and eject the water from the bottom 206, as indicated by arrows. Removing water from underneath the ice floe 202 can cause it to collapse under its own weight, as indicated by the cracks 208 forming in the ice floe 202. The fragments 210 formed by the collapse may be too small to cause problems with the station keeping of the vessel.

[0030] The tethers 122 may be used to set the vertical depth 212 at which the thrusters 102 sit under the ocean surface 214. The depth 212 may be selected by a combination of factors, including the expected size of the ice floes 202, the clearance needed for vessels operating in the area, and the size of the thrusters 102. In various embodiments, the thrusters 102 may be set to be about 5 meters to about 30 meters below the surface.

[0031] Fig. 3 is a schematic diagram of a single thruster 102 destabilizing a water column 300 under an encroaching ice sheet 302. Like numbered items are as described with respect to Figs. 1 and 2. As described herein, the thruster 102 may take inlet water 304 in through the top 204 and eject outlet water 306 through the bottom 206. The flow may lower the pressure in the water column 300 under the ice sheet 302, resulting in a loss of support for the ice sheet 302. The ice sheet 302 may then form fractures 308 and, as the ice sheet moves over the unstable water column 300, the fractures 308 may be propagated through the ice sheet 302, breaking it into smaller fragments.

[0032] The water ejected from the bottom 206 of the thruster 102 may be flowed through a grate or other structure to divert the flow out to the side. This may be useful for decreasing scouring of the ocean bottom below the thruster 102, which could lead to loss of integrity of the supporting piles 126.

[0033] Although the thrusters 102 may cause ice sheets 302 and ice floes 202 to break up, they may also be useful against larger ice structures, such as ice ridges. For example, the flow from the thrusters 102 may be used to change a path of an ice ridge, diverting it away from vessels. This may be caused by the swirling motion and indentation that would form in the surface above the thruster 102, e.g., a whirlpool. The control system may be used to determine how to use the thrusters 102 to divert the larger ice structures.

[0034] Fig. 4 is a drawing of a single thruster 102 that may be used for breaking up an encroaching ice floe. Like numbered items are as described with respect to Figs. 1-3. As described herein, the thruster 102 is coupled to a tether 122, which may also provide power and control. The inlet water 304 is pulled in the open top 204 by thruster blade 402 that is turned by a motor 404. The motor 404 may be an electric motor or may be a hydraulic engine, for example, powered by a sea water flow from a pump on a vessel. The outlet water 306 is ejected through the bottom 206 of the thruster 102. As described herein, a grate 406 may be used to divert the flow of the outlet water 306 away from direct impact with the seafloor, lowering the chance of scouring around the mounting piles or rail.

[0035] The thruster 102 may have buoyancy compartments 408, which may be used to increase or decrease the depth of the thruster 102. The buoyancy compartments 408 may be, for example, hollow vessels that can be partially or completely filled with air from a compressor on the vessel. The buoyancy compartments 408 may not be effective at adjusting the depth when the thruster 102 is operating, as the thrust generated may tend to push the thruster 102 closer to the surface of the water. However, the buoyancy compartments 408 may be useful for lowering the thruster 102 to avoid an impact with an object while the thruster 102 is powered down. Such objects may be detected or identified by any number of means, such as by a sensor 410 mounted to the thruster 102. When an approaching object, such as a ship or a marine mammal, among others, is detected, the thruster 102 may be instructed to shut down, lower its depth in the water, or both.

[0036] The thruster 102 may be mounted to the tether 122 by a framework 412 extending from the tether 122 to the outside shell 414. In addition to the open top 204 and bottom 206, the outside shell 414 may also have other openings 416 that may be used to provide differential thrust if desired. One or more of the side openings 416 may be configured to move the thruster 102 to the side, for example, angled to the side. The amount of thrust provided by these openings 416 may be controlled by louvers 418 that may be opened or closed by a control signal sent to an operator 420 from the vessel.

[ΘΘ37] Fig. 5 is a schematic diagram of a drilling location 500 showing the placement of sensors 502 and thrusters 102 that may be used to protect a vessel 104. Like numbered items are as described with respect to Fig. 1. In this example, two sets of thrusters 102 protect the vessel 104. The first set is mounted to a rail 504 to allow the thrusters 102 to be repositioned as needed, as indicated by an arrow 506. A second set of thrusters 102 is positioned inside the rail 504, for example, in an ice drift direction from the vessel 102. The rail 504 may also decrease the total number of thrusters 102 needed by allowing the thrusters 102 to be moved back and forth across the front of the encroaching ice.

[ΘΘ38] In this example, an ice sheet 510 is moving in a down drift direction 512 towards the vessel 104. Further, ice ridges 514 may be embedded within the ice sheet 510. Three zones can be defined around the vessel 104 to respond to the ice encroachment. In an outer zone defined by a first ring 516, incoming ice, such as the ice sheet 510 and ice ridges 514, are detected and tracked to determine the actions that may need to be taken. The outer zone may have a radius of about 10 to 40 kilometers (km) or greater.

[0039] A middle zone can be defined by a second ring 518, for example, at a radius of about 2 to 10 km. Sensors 502, such as sonar sensors, may be placed along the second line 518 to determine the approach of ice. Encroaching ice may also be detected by other means, such as satellite imagery, optical sensing, or manual sighting. The middle zone may be used as a management zone to deal with the ice. For example, thrusters 102 mounted along the rail 504 may be moved circumferentially into position to break up the ice sheet 510 into fragments 520 as it crosses the thrusters 102. The second set of thrusters 102, mounted inside of the rail 504, may be used to further break up the ice sheet 510 or to divert at least some of the ice ridges 514. The second set of thrusters 102 may also be mounted on a rail to make this operation more efficient. An icebreaker 522 may be used to divert ice ridges 514 that are too large to be diverted by the thrusters 102. The use of the thrusters 102 to break up the majority of the ice sheet 510 may allow the ice breaker 522 to function more efficiently, allowing fewer ice breakers, or even just a single ice breaker 522, to protect the vessel 104 from most encroaching ice.

10040] An inner zone can be defined by a third ring 524. The third ring 524 may be located at about 1 to 2 km from the vessel 104 and may be placed at a distance that gives the vessel 104 sufficient time to secure and detach a drilling riser from a well, or other subsea connections, in order to move away from encroaching ice.

[0041] Fig. 6 is a process flow diagram of a method 600 for producing hydrocarbons while using thrusters to break up ice before reaching an operational location, such as a drilling location. The method 600 begins at block 602 by positioning a vessel at a location on the sea surface. At block 604, a thruster attached to a mount is positioned on the seafloor. At block 606, while hydrocarbons are being produced using the vessel, ice encroaching on the vessel is detected. At block 608, the thruster is activated to destabilize the water column below the ice.

[0042] Fig. 7 is a process flow diagram of a method 700 for using thrusters to break up ice before reaching a surface location. The method 700 begins at block 702 by detecting ice that is encroaching on the surface location. At block 704 a thruster attached to a mount on the seafloor is activated to destabilize the water column below the ice.

[0043] While the present techniques may be susceptible to various modifications and alternative forms, the embodiments discussed above have been shown only by way of example. However, it should again be understood that the techniques are not intended to be limited to the particular embodiments disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

Claims

CLAIMS What is claimed is:

1. A system for protecting a stationary vessel from encroaching ice, comprising: a subsea mount disposed on a seafloor; and

a thruster disposed on the subsea mount under a water surface, wherein the thruster is configured to destabilize a water column under the encroaching ice.

2. The system of claim 1, comprising a cable from the stationary vessel to the subsea mount, wherein the cable carries power, control signals, or both to the thruster.

3. The system of any of claim 1 or claim 2, comprising a sonar sensor configured to detect encroaching ice.

4. The system of claim 1 or any of claims 2 to 3, wherein the subsea mount comprises a template and one or more pilings.

5. The system of claim 4, wherein the one or more piles comprise a suction pile set into the seafloor.

6. The system of claim 1 or any of claims 2 to 5, wherein the subsea mount comprises a rail to allow the position of the thruster to be changed.

7. The system of claim 1 or any of claims 2 to 6, wherein the thruster pulls water in through the top of the thruster and ejects the water downwards towards the seafloor.

8. The system of claim 1 or any of claims 2 to 7, wherein the thruster pulls water in through the bottom of the thruster and ejects water upwards towards the water surface.

9. The system of claim 1 or any of claims 2 to 8, wherein the thruster comprises one or more side openings configured to move the thruster to the side.

10. The system of claim 1 or any of claims 2 to 7 or 9, wherein the thruster comprises a bottom grate to redirect the water outward from the thruster.

11. The system of claim 1 or any of claims 2 to 7 or 9 to 10, wherein the thruster has one or more buoyancy compartments for adjusting the vertical depth of the thruster.

12. The system of claim 1 or any of claims 2 to 11, comprising a sensor on the thruster configured to detect the approach of an object and instruct the thruster to shut down, lower its depth in the water, or both.

13. The system of claim 1 or any of claims 2 to 12, wherein a plurality of subsea mounts are configured to form multiple rails in arcs.

14. The system of claim 1 or any of claims 2 to 7 or 9 to 13, wherein the thruster is disposed on the subsea mount using a tether which is coiled to allow vertical mobility from the mount.

15. The system of claim 1 or any of claims 2 to 14, comprising a subsea generator to provide power for the thruster.

16. A method for protecting a sea surface location from encroaching ice, comprising:

detecting encroaching ice; and

activating a seafloor mounted thruster, wherein the thruster destabilizes the water column below the ice.

17. The method of claim 16, comprising lowering the thruster to avoid impact with an object.

18. The method of claim 16 or claim 17, comprising moving thrusters to positions calculated to intersect the encroaching ice.

19. The method of claim 16 or either of claims 17 or 18, comprising powering the thrusters from a vessel at the sea surface location.

20. The method of claim 16 or any of claims 17 to 19, wherein destabilizing the water column below the ice includes pulling water in through the top of the thruster and ejecting the water downwards towards the seafloor.

21. The method of claim 16 or any of claims 17 to 20, wherein destabilizing the water column below the ice includes pulling water in through the bottom of the thruster and ejecting the water upwards towards the water surface.

22. A method for producing hydrocarbons, comprising:

positioning a vessel at a location on a sea surface;

positioning a thruster attached to a mount on a seafloor;

producing hydrocarbons from a well using the vessel;

detecting ice that is encroaching on the vessel; and

activating the thruster to destabilize the water column below the ice.