US3924896A

US3924896A - Air cushion dredge for use in ice-covered waters

Info

Publication number: US3924896A
Application number: US462749A
Authority: US
Inventors: Owen D Blankenship
Original assignee: Global Marine Inc
Current assignee: Global Marine Inc
Priority date: 1972-07-28
Filing date: 1974-04-22
Publication date: 1975-12-09
Anticipated expiration: 1992-12-09

Abstract

Apparatus for arctic dredging and marine pipelaying during icefree and iced conditions is described. For dredging, a dredging facility is carried on a buoyant platform which is adapted for support on a cushion of air. The dredging facility includes a dredging head which is operable, when engaged with the bed of a body of water to be dredged, for removing material from the bed. A dredgings conductor tube is connectible between the dredging head and the platform for conducting dredged material from the head to the platform. The dredging equipment also includes means carried by the platform for forming a channel through a layer of ice below the platform for passage of the conductor tube through the ice when the dredging apparatus is operated during iced conditions.

Description

1 ited States 1 aten 1191 [11] 3, Blankenship Dec. 9, 1975 AIR CUSHION DREDGE FOR USE IN 3,632,172 1/1972 Robinson et 81.... 299/27 X ICECOVERED WATERS 3,688,850 9/1972 Knorr 180/127 X 3,693,729 9/1972 Blurton et al.... 61/46.5 X lnventorl Blankenship, Houston, 3,763,580 /1973 Kuntz, Jr. ..L 37/65 x Tex. [73] Assignee: Global Marine, Inc., Los Angeles, Primary ne -Clifford D. Crowder Calif. Attorney, Agent, or Firm-Christie, Parker & Hale 22 Filed: Apr. 22, 1974 57 AB T T [21] Appl. No.: 462,749 1 f d d F d l pparatus or arctic re ging an marine pipe aymg Related Apphcatlon Dam during ice-free and iced conditions is described. For [62] Division of Ser. No. 276,110, July 28, 1972, Pat. No. dredging, n dredging facility is carried on a buoyant 313225556 platform which is adapted for support on a cushion of air. The dredging facility includes a dredging head U-S- Cl. is operable when engaged the bed of a 372/195; 61/724; 114/42; 180/127; 299/ body of water to be dredged, for removing material [5 Clo i from the A dredgings conductor tube is connect- Fleld of Search 114/40, ible between the dredging head and theplatform for 114/411 67 67 R; 61/723 465; conducting dredged material from the head to the 299/24, 25, 26, 27, 28, 18; 37/54, 53, 64, platform. The dredging equipment also includes 65, 66, 67, means carried by the platform for forming a channel through a layer of ice below the platform for passage References Clted of the conductor tube through the ice when the dredg- UNITED STATES PATENTS ing apparatus is operated during iced conditions.

3,362,500 H1968 Bliss 180/128 3,473,624 10/1969 Mezhlumov et al. 180/127 6 Clams 6 Drawmg figures US. Patent Dec. 9, 1975 Sheet 1 of4 3,924,896

r l ll II I. ll l l l MM w US. Patent Dec.9, 1975 Sheet2of4 3,924,896

sheet 3 OH 3,924,896

US. Patent Dec. 9, 1975 US. Patent Dec. 9, 1975 Sheet4 0f4 3,924,896

F/EE

AIR CUSHION DREDGE FOR USE IN ICE-COVERED WATERS CROSS REFERENCE TO RELATED APPLICATIONS This is a division of application Ser. No. 276,110, filed July 28, I972, 3,822,558. US. Pat. No. 3,822,558.

FIELD OF THE INVENTION This invention pertains to marine arctic dredging and pipelaying. More specifically, this invention pertains to method and apparatus for performing dredging and pipelaying operations, in waters susceptible to being covered by a layer of ice, during both iced and ice-free conditions.

BACKGROUND OF THE INVENTION Substantial reserves of oil and gas are either known to exist or are suspected to exist in areas lying offshore from northern Alaska and Canada. The precise seaward extent of these reserves from the adjacent shore line has not yet been defined with precision. It is know, however, that many of the established reserves lie under water which is covered by ice for many months of the year and which, during any given year, may be covered yearround by ice.

Oil or gas may be produced from a suitable well only when, and to the extent that the oil or gas can be removed directly from the wellhead to a suitable storage facility or pipeline. In the case of offshore wells, the provision of local storage facilities is not possible, particularly in the arctic. Accordingly, the successful and economic exploitation of the known and suspected oil and gas reserves in the arctic is dependent upon the provision of pipelines from the several wells across the ocean floor to centralized land-based storage and shipment facilities. These facilities may be adjacent a harbor which is ice-free for a sufficient period during every year to enable tankers and the like to collect and remove, within a few months time, all of the oil and gas produced annually from the wells in the area. On the other hand, these facilities may be a central collection point for a large diameter gas or oil pipeline by which the production from the adjacent oil and gas fields may be transported, on a year-round basis, to remote locations of further use or processing.

The economic feasibility and desirability of constructing oil or gas pipelines across the land of northern Alaska and Canada are topics which have not yet been resolved, due primarily to the fact that such pipelines would have to be constructed across the arctic tundra. The tundra presents delicate and complex ecological problems. In the extreme north, the surface vegetation is delicate and, if destroyed, only slowly reestablishes itself. Surface vegetation is particularly vital to the preservation of the arctic terrain due to the presence of permafrost under the surface vegetation. Surface vegetation provides a natural insulative barrier to complete thawing of the permafrost during the arctic summer. The destruction of the surface vegetation leads to rapid and substantial degradation of the underlying permafrost layer during the arctic summer, and such degradation, in turn, produces marked subsidence of the land level and often disastrous interference with the natural water courses in the area. For example, the roadways across which military vehicles moved in Alaska during the arctic summers of World War II are now visible as 2 trenches 8 feet or more in depth and 40 feet or more in width; these trenches are the direct result of the destruction of the surface vegetation by such vehicles.

These ecological considerations, therefore, argue strongly against the construction'by conventional techniques of oil and gas pipelines across the tundra of North America to serve the substantial oil and gas reserves known to exist in northern Alaska and Canada. Similarly, these conditions are :relied upon by ecologists and others to urge that any across-land pipelines in the arctic be fabricated by unconventional, and therefore costly, techniques.

An alternative solution to the problem of removing oil from the northern Alaskan and Canadian petroleum reserves to more southerly points of consumption and refinement is the use of marine pipelines from the oil and gas fields to points sufficiently far south that the ecological problems associated with across-land arctic pipelines are not encountered. Such long distance marine pipelines like the smaller local pipelines necessary to interconnect offshore wells to central collection and storage facilities, would have to be laid under waters which are covered by ice for many months of the year of which, in a given year, may be covered year-round.

The marine pipelaying procedures and devices described in the prior art may be used only to lay pipeline under waters which are ice-free. Therefore, reliance upon existing procedures and equipment to construct long distance marine pipelines in the arctic is feasible during only a few months of the year, and may not be possible at all in some areas where ice may exist on a year-round basis. The construction of long distance marine pipelines in the arctic by existing techniques and equipment would be prohibitively expensive due to the short construction season afforded in these areas and the need to move pipelaying equipment into and out of construction areas during the ice-free period. Dependence upon existing marine pipelaying proce dures and equipment means that if long distance marine pipelines are to be used to provide a solution to the ecological problems reviewed. above, such pipelines would require several years to construct and would be far more costly per mile than any other pipeline now known.

In the view of the foregoing, it is apparent that great need exists for procedures and equipment which will enable the construction on a substantially year-round basis and at reasonable cost of marine pipelines through and under waters which are susceptible to coverage by ice for substantial periods of the year.

SUMMARY OF THE INVENTION This invention provides novel, economic, efficient and practical procedures and equipment which may be used to advantage substantially on a year-round basis in the arctic for the construction of submarine pipelines. The procedures and equipment contemplated and comprehended by this invention may be used during periods when the waters over the path of the pipeline are ice-free or are covered by ice. This invention, therefore, makes possible the economic construction on an essentially year-round basis of marine pipelines which may be used as substitutes for or alternates to cross-land pipelines.

Generally speaking, this invention provides a method for laying a pipeline and the like across the bed of a body of water covered by a layer of ice. The method includes the steps of dredging a trench in the bed along the path of the pipeline ,from a dredging operations station supported on the ice on a cushion of air, and laying the pipeline in the trench from a pipelaying operations station supported on the ice on a cushion of air. Preferably, the dredging and pipelaying operations stations are buoyant so that dredging and pipelaying operations may be performed during periods when the water is icefree as well as during periods when the water is covered by a layer of ice.

This invention also provides a method for dredging in water susceptible to being covered by a layer of ice. A dredging facility is located on a buoyant platform adapted to be supported on a cushion of air. The platform is floated in the water and dredging operations are performed from the platform during periods when the water is not covered by ice sufficient to restrict floating movement of the platform. During periods when the water is covered by ice sufficient to restrict floating movement of the platform, the platform is supported on a cushion of air and dredging operations are performed from the platform through the ice. Preferably, the dredging facility includes a dredging head which is operative to remove material from the bed of the water body, and a dredgings conductor pipe which is connected during dredging operations between the dredging head and the platform. Means are carried by the platform for forming through an ice layer below the platform a channel through which the dredgings conductor pipe may be passed.

DESCRIPTION OF THE DRAWINGS The above-mentioned and other features of this invention are more fully set forth in the following description of certain presently preferred embodiments of the invention, which description is presented with reference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional elevation view of a dredging and pipelaying facility according to this invention;

FIG. 2 is a top plan view of the dredging and pipelaying facility shown in FIG. 1;

FIG. 3 is a cross-sectional elevation view of the mechanism provided in the air cushion platform shown in FIG. 1 for forming a clearance channel through the ice layer for the dredgings conductor pipe and the pipeline;

FIG. 4 is a simplified top plan view of a dredging facility according to this invention in use during iced conditions;

FIG. 5 is an enlarged fragmentary cross-sectional elevation view of a portion of the structure shown in FIG. 4; and

FIG. 6 is a cross-sectional elevation view of another dredging apparatus according to this invention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS An air cushion vehicle 10 is disposed over a sheet of ice 11 formed on the surface 9 of a body of water 12 which has a bottom or bed 13. The vehicle is comprised of a buoyant platform-like hull 14 having a substantially flat deck 15 and a substantially flat bottom 16. A plenum'structure 17 for the distribution of pressurized air extends circumferentially of hull 14 adjacent its gunwale 18. A flexible skirt assembly 19 is connected to the lower portions of the plenum structure and is arranged to depend below the bottom 16 of the hull to enclose a space 20 between the hull and the upper surfaces of ice sheet 11 when vehicle 10 is operated in its are mounted to deck 15 adjacent the aft end thereof,

and are operated by a suitable prime mover 23 (see FIG. 2) which may be an electric motor or a diesel engine, for example.

A forward portion of vehicle 10 defines a dredging station 25 within which a dredging facility 26 is mounted to hull 14. The dredging facility includes a dredging head 27 which is carried by the lower end of a dredgings conductor tube 28 which extends downwardly through hull 14 via a well 29 from a pump 30. The upper end of tube 28 is connected to the suction port of pump 30. A discharge conduit 31 is connected to the discharge port of the pump and extends to ad jacent the gunwale of the hull where it is supported on a boom 32 (see FIG. 2). Boom 32 extends laterally of the side of vehicle 10 from a suitable king post 33 or the like. The boom is provided for supporting conduit 31 above ice surface 11 for a desired distance laterally away from vehicle 10 so that dredgings gathered from bed formation 13 by dredging head 27 and conducted to pump 30 by tube 28 may be discharged via conduit 31 onto the ice surface a suitable distance laterally from vehicle 10. Dredging head 27 preferably is of the suction type which operates in response to the operation of pump 30 to gather the material of bed formation 13 from the bed in the vicinity of the dredging head. Accordingly, as vehicle 10 moves across ice 11 along a desired path, a trench 35 is formed by dredging head 27 in bed formation 13 below vehicle 10.

In order that dredging facility 26 may be operated to form trench 35 during air cushion mode operation of vehicle 10, a suitable seal 36 cooperates between the structure of hull 14 and the dredgings conductor tube within well 29 so that super-atmospheric pressure may effectively be generated and maintained within space 20 by operation of blowers 22.

The dredging facility illustrated in FIGS. 1 and 2 and described above is merely exemplary of a dredging facility which may be provided in any apparatus accord ing to this invention for the performance of the procedures contemplated by this invention.

As shown most clearly in FIG. 2, the area of vehicle 15 aft of dredging station 25 is designated as a pipelaying station 38 and is the location in vehicle 10 where a pipelaying facility 39 is located. The pipelaying facility includes a pipe welding assembly 40, preferably located along the centerline 48 of hull l4 forwardly of a pipe holding and tensioning assembly 41. The pipe welding and pipe holding assemblies may be of conventional configuration and arrangement. Pipe welding station 40 is provided for welding the rear end of a pipe length 42, disposed above deck 15 on suitable supports 43 (see FIG. 1), to the forward end of a plurality of pipe lengths which have been welded together to define a continuum 44 of pipe. The pipe continuum extends from welding assembly 40 through pipe holding and tensionin g assembly 41 aft of vehicle 10 and into trench 35, as shown in FIG. 1. A portion of the pipeline continuum 44 disposed in trench 35 and supported by formation 13 at the bottom of the trench constitutes a portion of a submarine pipeline 45 being laid under body of water 12 by operations carried out on and from the platform defined by vehicle 10. Pipe storage areas 46 are provided laterally from pipe welding assembly 40 as additional components of pipelaying facility 39.

As shown in FIG. 1, pipelaying vehicle 11) is arranged for passage of pipe continuum 44 through hull 14 and into water 12 within the area of air cushion support of hull 14 when operated in its air cushion mode. Accordingly, a passage 47 is provided through hull 14 along the centerline of the hull. Passage 47 slopes downwardly and rearwardly from its upper end at deck 15 to its lower end through bottom 16 of the hull. A suitable seal 49 cooperates between the walls of the passage and the pipe continuum within the length of the passage for preventing undue leakage of pressurized air through the passage from space 20 so that a region of air at superatmospheric pressure may be generated below hull 14 in response to the operation of blowers 22.

It is apparent that when vehicle 10 is operated over ice 11, some mechanism must be provided for forming openings through ice 11 for the passage of dredgings conducting tube 28 and pipe continuum 44 into contact with bed formation 13. Accordingly, pipe-laying vehicle 10 includes an ice cutting assembly 51) for forming an open channel 51 through ice sheet 11 of sufficient width that the dredgings conductor tube and the pipe continuum with its associated supportive stinger assembly 52 may be passed through the ice sheet. As shown best in FIG. 3, ice cutting assembly 50 includes a centerline ice cutting mechanism 54 and a pair of side ice cutting mechanisms 55 disposed on opposite sides of and equidistantly from centerline mechanism 54. Each of mechanisms 54 and 55 is provided in the form of an enlarged chain saw having an elongate blade 56 (see FIG. 1) the circumference of which defines a guide-way for an encircling tooth-carrying chain 57. Each chain carries a plurality of

teeth

58 and 58 adapted upon operation of the associated mechanism for cutting ice with which the teeth may be brought into contact. The blades of ice cutting mechanisms 54 and 55 are adapted to be movable between an operative position in which they depend from the bottom 16 of hull 14 through ice layer 11 and a retracted position in which the blades are disposed in respective elongated recesses 60 formed in hull 14 to open downwardly from the hull into space 20.

Recesses 60 are defined within the interior of corresponding elongate housings 61 which extend upwardly within hull 14 from the bottom of the hull, as shown in FIG. 3. Housings 61 are parallel to each other and extend fore-and-aft within hull 14. The blades 56 of ice cutting mechanisms 54 and 55 are carried by a hollow axle 62 which extends transversely of the hull within the hull and through the upper portions of housings 61. A pair of indexing devices 63 are connected to the axle on either side of central housing 61, and are operable for rotating the axle through an arc of about 90 degrees for moving the ice cutting mechanisms between their operative and retracted positions. Axle 62 is so positioned relative to housings 61, and the housings are so configured in cooperation with the ice cutting mechanisms, that when the ice cutting mechanisms are in their retracted positions within recesses 60, the mechanisms are disposed wholly above the bottom surface 16 of hull 14. A suitable sprocket wheel (not shown, but

well known from the chain saw technology) cooperates with each chain 57 coaxially of axle 62 for driving chains 57 around the circumference of respective blades 56. The sprockets are secured to a drive shaft 64 which extends coaxially of the axle as shown in FIG. 3. The drive shaft is connected at its opposite ends to a pair of drive motors 65 mounted to the structure of bull 14 within the hull.

As shown in FIG. 3, the teeth 58 of central ice cutting mechanism 54 have a greater width transversely of the vehicle 10 than do the teeth 58 of the side ice cutting mechanisms. Teeth 58 are sized to cut a channel 67 through ice layer 11, which channel has a width sufficiently greater than the outer diameter of dredgings conductor tube 28 that the conductor tube may be passed readily through this channel. Teeth 58, associated with side ice cutting mechanisms 55, are sized to cut somewhat narrower channels 63 through ice layer 11 on either side of and parallel to channel 67, thereby to define a pair of strips 69 of ice which are severed from ice layer 11 by operation of the ice cutting mechanisms. The aggregate width of

channels

67 and 68 is greater than the width of stinger assembly 52 provided in conjunction with pipelaying facility 39. Preferably the forward end of the stinger assembly is pointed to wedge ice strips 69 apart from each other to permit the free passage of the stinger assembly, or at least its upper end, through a widened channel 51 which has a width equal to the aggregate of the width of

channels

67 and 68.

It is apparent, therefore, that ice cutting assembly 50 provides a means for forming an open channel through ice sheet 11 as vehicle 10 moves over the ice above the path along which it is desired to lay pipeline 45.

It will be observed from FIG. 1 that ice cutting assembly 50 is disposed wholly within the limits of the air cushion support area for vehicle 10 when it is operated in its air cushion mode. As will be described below, suitable means are provided in cooperation with stinger assembly 52 for sealing skirt assembly 19 to the stinger assembly at the aft end of the vehicle. Accordingly, the front end of channel 51 is at all times defined at a point below hull 14 within skirt assembly 19. Thus, the generation of open channel 51 through ice layer 11 does not provide a source of leakage for air from chamber 20 during air cushion mode operation of the vehicle and the vehicle is effectively supported on ice layer 11 by a cushion of air when blowers 22 are operated.

Under some conditions of ice thickness and structural characteristics, the force applied to the ice by the pressurized air within skirt assembly 19 may be effective to break the ice adequately to enable passage of dredgings conductor tube 28., pipe continuum 44, and stinger assembly 52 through the ice. Ice cutting assembly 50 is not required under such circumstances.

Stinger 52 is an elongate buoyant structure which has a width which is somewhat less than the aggregate width of

ice channels

67 and 68 formed through ice layer 11 by operation of ice cutting mechanisms 54and 55. The stinger has an enlarged upper end 711 and varies appropriately in cross-sectional configuration along its length so that when the stinger assembly is submerged and engaged with pipe continuum 44 as shown in FIG. 1, the buoyant forces developed by the submerged stinger are applied in the desired manner to the pipe continuum. The stinger is provided to impart support to pipe continuum 44 so that the pipe continuum, as it passes from the vehicle into the water, is not bent about 7 a sufficiently small radius of curvature to result in kinking or other damage to the pipe continuum. The support forces applied to the pipe continuum by the stinger also assures that the bend of the continuum as it engages the bottom of trench 35 is not so sharp as to produce kinking or other damage to the pipe.

The upper portions of stinger 52 are transversely dished along the length of the stinger to provide a guide channel 71 (see FIG. along which pipe continuum 44 is passed during pipelaying operations. The interior of the stinger is hollow and preferably is subdivided by partitions 72 to prevent complete flooding of the stinger in the event that a leak should develop through the stinger structure at some localized point. As shown in FIG. I, the upper end 70 of the stinger is secured, as by tether cables 73, to the underside of hull 14 within the perimeter of the space enclosed by skirt assembly 19. The connection of the stinger to the hull is so arranged that the pipe guide channel 71 defined in the upper portions of the stinger defines an extension of the pipe supporting surfaces of passage 47 through the hull when the pipe continuum is engaged with the stinger as shown in FIG. 1.

Stinger 52 is used during pipelaying operations whether or not the pipelaying operations are carried out on an ice layer. In the absence of ice over the intended path of the pipeline, or in the event the water is ice covered but not sufficiently to interfere with floating movement of hull 14, the hull is buoyantly supported in water 12. On the other hand, where the intended path of the pipeline is covered by a layer of ice sufficiently thick to impair or prevent floating movement of hull 14, then pipelaying operations are carried out from vehicle while the vehicle is operated in its air cushion mode. FIG. 5 illustrates that stinger 52 includes structure for maintaining an effective seal around space in the vicinity of ice channel 51 during air cushion mode operation of the vehicle.

Skirt assembly 19 includes an outer sheet 75 which is connected along its upper margin 76 to the lower portions of plenum structure 17 outboard of hull 14. Outer sheet 75 also has a lower edge 77 which, during air cushion mode operation of vehicle 10 is disposed below the bottom 16 of hull 14. The outer sheet of skirt assembly 19 preferably is held during air cushion mode operation of the vehicle in a desired relation to the structure of hull 14 by a plurality of webs 78 interconnected between the hull and the inner surfaces of outer sheet 75. Preferably, outer sheet 75 is fabricated of a fabric impregnated with a suitable material such as latex or a latex-based material to render the same impervious to the flow of air through the fabric.

A yoke member 79 is connected to stinger 52 to straddle channel 71 along which pipe continuum 44 passes during pipe-laying operations. A flap member 80, preferably fabricated of much the same material as outer sheet 75 of skirt assembly 19, is connected to the upper portion of stinger 52 in the vicinity of yoke 79. Flap 80, adjacent the location at which the pipe continuum passes through the yoke, is carried by the exterior portion of the yoke. The clearance within yoke 79 for pipe continuum 44 is relatively small so that a minimum leakage of air from space 20 through the yoke is afforded in those few operational situations where yoke 79 is not fully submerged in water 12. Flap 80 is disposed to extend into skirt assembly 19 along the inner surfaces of outer sheet 75 for a selected distance above the lower edge 77 of the skirt assembly during air cushion mode operation of the vehicle. The increased air pressure within space 20 biases the flap into registration with the inner surfaces of skirt outer sheet 75. Preferably, the lower edge 77 of the skirt assembly is disposed in spaced relation to the proximate portions of stinger 52 during pipelaying operations while the vehicle is operated in its air cushion mode. This spacing between the stinger and the lower portions of the skirt assembly allows the stinger to move relative to the vehicle hull about the connection provided by tether cables 73, and such relative motion is accommodated by flap sliding within skirt assembly 19. Preferably the extent of flap 80 transversely of the length of stinger 52 is a small amount greater than the width of channel 51 within which stinger 52 is disposed during air cushion mode operation of the vehicle. It will be apparent, therefore, that when vehicle 10 is operated in its air cushion mode during pipelaying operations, flap 80 forms an effective seal across the upper portions of channel 51 below skirt assembly 19. In this manner, undue leakage from space 20 to the atmosphere around vehicle 10 is prevented so that blowers 22 may be operated to maintain an effective air cushion within space 20 for the support of hall 14.

Preferably the spacing of skirt assembly partitions 78 at the aft end of hall 14 is selected so that the partitions closest to the centerline of the hull are spaced from each other a distance greater than the extent of flap 80 transversely of stinger 52.

FIG. 4 illustrates the manner in which vehicle 10 preferably is moved across the ice sheet during dredging and pipelaying operations. In FIG. 10, only the dredging facility is illustrated, it being understood that the pipelaying facility, which has been eliminated from FIG. 4 for the purposes of simplicity of illustration, is also present. On the other hand, the principles illustrated in FIG. 4 are applicable to movement of dredging and pipelaying platforms provided separately from each other across an ice sheet for performance of dredging or pipelaying operations, respectively. As shown in FIG. 4, a winch 81 is mounted to hull deck 15 adjacent each corner of the hull; the hull preferably is of rectangular planform configuration. Preferably each winch is disposed so that a mooring cable 82 wound upon the winch drum is conveniently led away from the hull along a line substantially parallel to a diagonal line across the hull. Each mooring cable 82 extends to an anchor point 83 affixed in ice sheet 11. The anchor points may be provided by any suitable means, as by pilings frozen into holes formed in ice sheet 11. Two rows of anchor points 83 are provided parallel to the intended path of vehicle 10 across the ice sheet and on opposite sides equidistantly from the desired path of the vehicle.

Anchor points 83 are installed in ice sheet 11 in advance of the movement of vehicle 10 across the ice sheet. The spacing of the anchor points in each parallel line of anchor points is such that the vehicle may be moved along its intended path of movement by selective reeling in and paying out of mooring cables 82 from winches 81. For example, mooring cables associated with the winches at the forward corners of vehicle 10 are reeled in while the cables extending away from the winches at the aft corners of the vehicle are paid out. In this manner, the vehicle is caused to move along its intended path of motion, and during such movement the platform is maintained by the cables in the desired position on the ice sheet. When the vehicle is advanced to a point at which the forward cables extend substantially transversely of the vehicle to their anchor points (such as anchor points 83), first one and then the other of the forward mooring cables are transferred to the next pair of anchor points more forwardly of the vehicle along its path of motion. The vehicle is moved still further along its path until anchor points 83' are located substantially transversely from, but slightly behind vehicle 10, at which time the aft mooring cables are removed from anchor points 83 and connected to anchor points 83'. In this manner, the vehicle is moved on an air cushion support across the ice sheet while being maintained at all times securely in the desired position over the intended path of pipeline 45. Anchor points 83 left behind vehicle 10 as it moves across the ice sheet may be recovered as desired.

This invention also provides an air cushion dredging vehicle 90. As shown in FIG. 6, vehicle 90 includes a buoyant hull 14 which carries a dredging facility 26. Dredging vehicle 90 is arranged for operation either in a buoyant state, in which hull 14 floats upon water 12, or in an air cushion state, in which the hull is supported on a cushion or air generated within space bounded by skirt assembly 19 disposed circumferentially of hull 14. Air is supplied at superatmospheric pressure to space 20 via skirt assembly 19 through an air distribution plenum structure 17 in response to operation of blowers 22, for example. Dredging facility 26' includes a dredgings conductor tube 91, the upper end of which is connected to the suction port of pump 30 and the lower end of which is connected to a dredging head 92. The dredging head preferably is of the rotary type operated in response to the passage of water upwardly through the conductor tube by operation of pump 30. Conductor tube 91 differs from conductor tube 28, illustrated in FIG. 1, by the inclusion in its length of a flexible section 93 which preferably is disposed sufficiently below hull 14 as to be positioned below the bottom of ice sheet 11 when vehicle 90 is operated in its air cushion mode over ice. Whereas the dredging facility provided in vehicle 10 is adapted primarily for forming trench into which pipeline 45 may be laid, dredging facility 26' of vehicle 90 is adapted for general purpose dredging on a year-round basis in waters susceptible of being covered by an ice sheet of appreciable thickness. The provision of rotary dredging head 92 and flexible section 93 in dredgings tube 91 adapt dredging facility 26' to general purpose dredging operations. To adapt vehicle 90 for dredging operation through ice sheet 11, a single ice cutting mechanism 54 according to the foregoing description is carried by vehicle 90 for forming through ice sheet 11 a single channel 94 of sufficient width that conductor tube 91 and dredging head 92 may be passed through the channel.

As shown in FIG. 6, conductor tube 91 above flexible portion 93 passes through a well 29 formed through hull 14. A suitable seal 36 is provided between the hull and the conductor tube within the well for the purposes described above. To adapt the vehicle for dredging an area in bed formation 13 which is wider than ice channel 94, a collar 95 is secured to conductor tube 91 below flexible portion 93. A plurality of cables 96 are connected between collar 95 and a plurality of winches 97 mounted to hull 14. Preferably winches 97 are dis posed in recesses 98 formed in the flat bottom 16 of hull 14 adjacent conductor tube well 29. Preferably four winches 97 are provided in vehicle 90, and are lo cated fore and aft, port and starboard, respectively,

from the position at which the conductor tube passes through the bottom surface of the hull. Selective operation of winches 97 to reel in or pay out cables 96 produces movement of dredging; head 92 forward, rearwardly, or in either direction laterally relative to hull 14. Such movement of the lower portion of dredging tube 91 is accommodated by the flexible portion of the tube below ice sheet 11.

When the submerged location to be dredged is free of an ice cover or is covered by an ice layer of insufficient thickness to significantly impair floating movement of hull 14, dredging operations are carried out from vehicle while the hull is buoyantly supported in the water. On the other hand, if the location to be dredged is covered by' an ice sheet of sufficient thickness to impair or prevent floating movement of the hull, then vehicle 90 is operated in its air cushion mode and dredging operations are carried out at the desired submerged location through ice channel 94 provided in the ice sheet by operation of ice cutting mechanism 54. As shown in FIG. 6, ice cutting mechanism 54 preferably is movable between the operative position shown in FIG. 6 and a retracted position in which the mechanism is disposed in a recess 60, preferably provided in the manner described above concerning vehicle 10.

The formation of channel 51, for example, through ice layer 11 during pipelaying operations from vehicle 10, for example, provides a discontinuity in the ice. Wind, wave or tidal forces acting on the ice may tend to close channel 51 around stinger 52, for example, or to further open the channel more than may be safe relative to vehicle 10. Accordingly, preferred practice of the pipelaying and dredging operations through ice according to this invention includes the installation across the ice channel behind the vehicle, such as vehicle 10, of means for maintaining the channel at the desired width. Preferably the channel maintaining means are provided in the form of steel bars 100 bent to resemble enlarged staples in that the end portions of the bar are bent at right angles, parallel to each other, relative to the straight central portions of the bar. The central portions of the bars have a length. which exceeds the width of the channel, as initially formed, by a desired amount, and the end portions of the bars have lengths preferably at least equal to the thickness of ice layer 11. The bars are installed across the channel behind the vehicle as it moves along its intended path at appropriate intervals along the length of the channel. The installation of the bars across the channel may be achieved by drilling holes through the ice at aligned locations along the channel and on opposite sides of the channel. The holes preferably are sized to snugly receive the end portions of the bars. The bars are then installed by inserting their end portions into corresponding ones of the aligned holes. When so installed, the bars act to inhibit opening or closing of the channel.

This invention has been described above with reference to certain presently preferred structural arrangements and operational sequences. Those skilled in the art to which this invention pertains will readily realize that the structures illustrated in the accompanying drawings, as well as the procedures and operations described above, may be altered or modified without departing from the scope of this invention. Accordingly, the foregoing description should not be considered as limiting this invention.

What is claimed is:

l. A method for dredging in water susceptible to being covered by a layer of ice comprising the steps of l. locating a dredging facility on a buoyant platform adapted to be supported on a cushion of air,

2. floating the platform in the water and performing dredging operations from the platform during periods when the water is not covered by ice sufficient to restrict floating movement of the platform, and

3. during periods when the water is covered by ice sufficient to restrict floating movement of the platform, forming an opening through ice below the platform, supporting the platform on a cushion of air, and performing dredging operations from the platform through the ice.

2. Apparatus useful for dredging the bed of a body of water covered by a layer of ice, comprising 1. a buoyant platform,

2. a dredging facility carried by the platform including a dredging head operable when engaged with said bed for removing material therefrom, and a dredgings conductor tube connectable between the dredging head and the platform for conducing dredged material from the head to the platform and adapted to be lowered from the platform for engagement of the dredging head with the bed,

3. means carried by the platform for forming a channel through a layer of ice below the platform for passage of the conductor tube therethrough,

4. means carried by the platform and operable for supporting the platform on a cushion of air over the channel.

3. Apparatus for dredging the bed of a body of water covered by a layer of ice comprising a buoyant platform,

means carried by the platform and operable for supporting the platform on a cushion of air,

a dredging facility carried by the platform and including a dredging head engageable with the bed for dredging the same and a conductor tube connectible between the dredging head and the platform 12 for conducting dredged material from the head to the platform, and

means carried by the platform operable for forming through an ice layer below the platform a channel through which the conductor tube is extendible, the means for supporting the platform on a cushion of air including a skirt assembly mounted to the platform for enclosing a space below the platform, the means for forming the channel being operable through the space.

4. Apparatus useful for dredging the bed of a body of water covered by a layer of ice, comprising 1. a buoyant platform,

2. means carried by the platform and operable for supporting the platform on a cushion of air, said means including a skirt assembly mounted to the platform for enclosing a space below the platform and means for supplying air at superatmospheric pressure to the space,

3. a dredging facility carried by the platform includ ing a dredging head operable when engaged with said bed for removing material therefrom, and a dredging conductor tube connectible between the dredging head and the platform for conducting dredging material from the head to the platform and adapted to be lowered from the platform for engagement of the dredging head with the bed, and

4. means carried by the platform operable within said space for forming a channel through a layer of ice below the platform for passage of the conductor tube therethrough,

5. Apparatus according to claim 4 wherein the channel forming means is movable between a retracted position substantially within the platform and an operative position in which it extends into the space into operative engagement with ice below the platform.

6. Apparatus according to claim 5 wherein the channel forming means comprises means for mechanically cutting through the ice.

Claims

1. A method for dredging in water susceptible to being covered by a layer of ice comprising the steps of 1. locating a dredging facility on a buoyant platform adapted to be supported on a cushion of air, 2. floating the platform in the water and performing dredging operations from the platform during periods when the water is not covered by ice sufficient to restrict floating movement of the platform, and 3. during periods when the water is covered by ice sufficient to restrict floating movement of the platform, forming an opening through ice below the platform, supporting the platform on a cushion of air, and performing dredging operations from the platform through the ice.

2. Apparatus useful for dredging the bed of a body of water covered by a layer of ice, comprising

3. Apparatus for dredging the bed of a body of water covered by a layer of ice comprising a buoyant platform, means carried by the platform and operable for supporting the platform on a cushion of air, a dredging facility carried by the platform and including a dredging head engageable with the bed for dredging the same and a conductor tube connectible between the dredging head and the platform for conducting dredged material from the head to the platform, and means carried by the platform operable for forming through an ice layer below the platform a channel through which the conductor tube is extendible, the means for supporting the platform on a cushion of air including a skirt assembly mounted to the platform for enclosing a space below the platform, the means for forming the channel being operable through the space.

3. a dredging facility carried by the platform including a dredging head operable when engaged with said bed for removing material therefrom, and a dredging conductor tube connectible between the dredging head and the platform for conducting dredging material from the head to the platform and adapted to be lowered from the platform for engagement of the dredging head with the bed, and

4. means carried by the plaTform operable within said space for forming a channel through a layer of ice below the platform for passage of the conductor tube therethrough.

4. Apparatus useful for dredging the bed of a body of water covered by a layer of ice, comprising