US3675429A

US3675429A - Arctic ice platform

Info

Publication number: US3675429A
Application number: US25464A
Authority: US
Inventors: Bertram T Wlliman
Original assignee: Exxon Production Research Co
Current assignee: ExxonMobil Upstream Research Co
Priority date: 1970-04-03
Filing date: 1970-04-03
Publication date: 1972-07-11
Anticipated expiration: 1989-07-11

Abstract

A method of forming a permanent offshore platform in areas where a mass of ice rests on the earth''s surface in winter but thaws during more moderate weather. The platform is formed by insulating the surface of the ice proximate the drilling site to prevent any substantial thawing of the ice.

Description

United States Patent Willman July 11, 1972 [54] ARCTIC ICE PLATFORM [7 2] Inventor: Bertram T. Willman, Marsa Bregha, Libya [73] Assignee: Esso Production Research Company [22] Filed: April 3, 1970 [21] Appl. No.: 25,464

[52] US. Cl ..6l/46 [51] Int. Cl ..E02d 27/04 [58] Field of Search ..61/34, 36 R, 36 A, 46, 46.5, 61/54, 50; 62/260; 165/45; 9/8 P [5 6] References Cited UNITED STATES PATENTS 3,135,097 6/1964 Scheinberg ..6l/50 3,195,310 7/1965 Schroeder ..61/36 AX 3,217,791 11/1965 Long ..62/26 OX 3,295,328 1/1967 Bishop ..6l/36 AX OTHER PUBLICATIONS Humble News March 1969 Humble Oil Co. pg 8 Ice island Sept. 14, 1970 The Oil & Gas Journal pgs 60 & 61

Schroeder ..61/34 X Primary Examiner-J. Karl Bell Attorney-James A. Reilly, John B. Davidson, Lewis H. Eatherton, James E. Gilchrist, Robert L. Graham and James E. Reed [5 7] ABSTRACT A method of forming a permanent offshore platform in areas where a mass of ice rests on the earth's surface in winter but thaws during more moderate weather. The platform is formed by insulating the surface of the ice proximate the drilling site to prevent any substantial thawing of the ice.

6 Clains, 2 Drawing Figures PATENTEDJUL 1 1 m2 INVENTOR BERTRAM I W/LLMAN FIG. 2

ATTORNEY ARCTIC ICE PLATFORM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to drilling platforms for use in arctic areas and is particularly concerned with a method for forming a permanent drilling platform out of a mass of ice.

2. DESCRIPTION OF THE PRIOR ART In certain parts of the world, exploitation of deposits of crude oil and natural gas is hampered by ice that forms during severe winter weather. This is particularly true in the Arctic, where ice in shallow bodies of water may become bottom fast during the winter months and then completely thaw during the summer. Offshore drilling sites that are accessible by landbased vehicles in cold weather can often be reached only by boats in milder weather. Drilling and production operations carried out in such areas have therefore generally been conducted from pile supported platforms that extend above the water surface.

Bottom founded drilling and production platforms intended for use in arctic areas normally have to be designed to withstand the forces developed by ice freezing in the winter, ice breakup in milder weather, and floating ice masses. In addition, they must resist ice push, a phenomenon caused by the wind driving the offshore ice pack shoreward. Moreover, these offshore structures are frequently bottomed in permafrost zones which may soften and yield as the ice melts. Such platforms must therefore not only have rigid substructures that will withstand the forces imparted by moving ice but also must extend to depths sufficient to penetrate that portion of the permafrost zone that will soften during mild weather. Such platforms are thus massive, extremely expensive structures.

SUMMARY OF THE INVENTION The method of this invention provides a relatively inexpensive platform that alleviates the problems outlined above in arctic areas where ice rests on the surface of the earth during the winter and thaws during more moderate weather. Broadly stated, the method involves insulating the surface of the ice proximate the drilling site to prevent the ice from thawing. This method is particularly useful in shallow, offshore waters adjacent arctic land masses. Platforms formed in this manner will remain fixed to the ocean bottom through summer and winter and therefore provide a relatively inexpensive, safe, stationary location for drilling and production operations. An additional advantage is that the permafrost zone is maintained in a frozen condition. Accordingly, the method of the invention offers significant advantages over the prior art.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective of an ice platform formed in accordance with the invention.

FIG. 2 is a cross-sectional elevation of the platform of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A platform constructed in accordance with the invention is shown in perspective in FIG. 1 and in elevation in FIG. 2. An ice mass 11, situated in a body of water 12, extends from the water surface to the submerged bottom 13. Four wells are located in a somewhat central position on the body of ice. Each well extends downwardly through the ice into an oil or gas bearing subsurface formation. An insulating material 17 covers a substantial area of the surface of the ice mass proximate the well cluster and has also been placed in trenches 19 which extend downwardly to the submerged bottom.

Formation of the permanent platform of the invention normally requires the existence of an ice mass that rests on the earth's surface. Where the platform is to be formed in a shallow water, offshore area, holes can readily be drilled through the ice to determine its thickness. Estimation of the rigidity of the connection between the ice mass and the subsea bottom can be determined similarly, rigidity depending upon bottom topography as well as thickness and areal extent of the ice mass.

Once it is established that the ice is rigidly affixed to the bottom, the areal extent of the ice mass required to form the platform should be estimated. The areal extent needed will be governed largely by anticipated climatic conditions, by the thickness and thermal properties of the insulating material, and by the likelihood of exposure to ice push and the like.

The insulation material should be a nonaqueous waterproof material, and preferably should be of a type that may be readily applied, such as a foam. Preferred materials are foam plastics, e.g., polyurethane. Other insulating materials which will not deteriorate under arctic weather conditions would, of course, also be suitable. .It will frequently be necessary to anchor the insulating material to the ice mass to prevent its movement away from the protected area. One convenient way of anchoring the insulating material is to drill a number of shallow wells on the surface of the ice and to extend the foam downwardly into these wells, thereby providing a number of anchor legs. Pilings, fences, and the like may also be utilized.

The thickness of the insulating material required at a given location will depend in part upon the thermal properties of the insulating material and by ambient weather conditions. The flow of heat across the insulating material, assuming steady state conditions, is governed by the following equation:

Thus the rate of heat flow for a unit area is directly proportional to the thermal conductivity of the insulation material and the difference in temperature between the ice and the atmosphere, and inversely proportional to the thickness of the insulating material. Techniques for determining the thermal conductivities of materials are well known to those skilled in the art. Much information on insulating materials is available in the literature. Historical data on climatic conditions has also been published for most geographical areas. From such information the seasonal variation of the difference between ambient temperature and ice temperature can be estimated. With this data and the thermal conductivity of the insulation material, the thickness of insulation necessary to prevent any substantial melting of the ice caused by heat conduction through the insulation material can be determined. In most instances, it appears that a 12-inch layer of polyurethane will suffice.

An ablation study of the Alaskan Gulkana Glacier to evaluate the importance of radiation showed solar radiation to be the most important energy exchange process to melt snow and ice on the glacier. This work is reported by Mayo and Pewe in their article entitled Ablation and Net Total Radiation, Gulkana Glacier, Alaska," in Ice and Snow, Cambridge, Mass, the M.I.T. Press, 1962, p. 633-643. Accordingly, it is desirable that the insulation material employed possess a low absorptivity and a high reflectivity to minimize the effect of solar radiation. In that regard, in some instances, it may be desirable to spray the upper surface of the insulation material with a highly reflective material that will adhere to the insulation material.

It will be recognized that it will frequently not be practicable to insulate the ice platform sufiiciently to completely prevent melting. In general, however, the heat flow that results in the ice melting during mild weather will be balanced by the flow of heat in the opposite direction during cold weather that causes refreezing. In those geographical locations where such =the temperature gradient a balance is not achieved it may be desirable to refreeze the What is claimed is: ice, as required, vby partially removing the insulating material 1. A method of forming a permanent ice platform in a shalduring cold weather. This could also be accomplished by inlow body of water which freezes to form a bottom-fast'mass of stalling refrigeration equipment, and periodically removing ice in the winter and thaws during moderate weather compris-; heat, asrequired, to refreeae the ice. 5 ing: g v m 7 I The effect of ice thickness and climatic conditions on'the V app y g a y" of a "omiquemls insulating matfl'ia! to areal extent of insulating material required will be governed the Surface of Said mass Ofice a time W a dm f by 'a-"numberof-factors. For example when the insulating ice is fixed 10 the o Said y r being S M material covers only the upper surface of V the ice mass, he thick to prevent any substantial melting of saidmass of periphery of the platform maybe subject tothawing during 10 8 Weather; I 1 warm weather. it will thereforebe desirable to extend the inexcavating a trench around the P ip i n u e e s f I -s ul ating material a substantial distance away from the well to ice l and therfiaafteri; v i V insure that ifany portion of the ice melts-,that portion will not v applymg a lf of msulatmg m the Peripheral affect the wells drilled through the insulated block of ice. Swim Ofthe we mass- I f While theamquntrequired willvary fr om location to location, e p's g' method defined b clam 1 further 9'F P. h i 'I extendingthe insulating material to a radius of at least 300 meters from the well'site' will probably be adequate in most im 7 7 stances. Alternatively, a.trenchrnay'be excavatedaround the cold weather to refreeze any i that has F.9d 9 insulatedareaandtilledwith insulating material toprotectthe b f F l n I m i 2; zit-areas: lame P "1 ia? P 9 a ""T 3; The rseth'od defined by elem "1 further comprising the V Pmbkm 99 e-" 93* mass whcn W "step of refrigerating the ice beneath said insulating material, as I pa-ctgd'by naturaliforcegmay aficct'th'e an?! of the {n' re uired to refreeze an icethat has melted fi?" j fi PQ t b i bs-m of an ice 5 i A niethod of forming a permanentice platformin ashal-r a.' removing said insulating material, f as required, during m l s i s Oat m' S f l beigovemed in large a 'low body of water which freezes to forma bottom-fast mass of p fl l l qt n s a -3 extent ofirtheiyiycey n ice in the winter and thaws during moderate weather compristopography of the earths surface on twhichit rests. Where the f f v icb'is ll y th h S is q t Smooth, a t a. applying a layer of nonaqueous insulatingmateri :the ly' greatareal eXtent is desirable. on theother hand, if the botfa f id 3 1 i at ajtirfl fwhn aidnfi of'iq t ws b y' -r s t e is thicha m l areal-e is fixed to the bottom, said layerbein'g sufficientlythick to h extentwillsuffice. Where problems withice floes and rafting revent any substantial meltingof icelduringni A t e t are anticipateirit may desirabletoelevate the surface of a weather; i g the platform a substantial'distance above sea level to prevent b. coating the surface of said layerflof insulatirlg-rnaterial :1 r the moving ice from sweeping the equipment off the'platform. with a substanceiwhich adheres"'t0 said =ma'terial and Thisrnay be accomplished in a'fashion well known to those reflects radiant energy; s f V V sltilledin the 'artbypumping water to the topof the platform c. excavating a trench around the periphery of the -insulated and allowing it tofreeze once a sufficient height is attained, ice surfac e;and thereafter, r v T theinsulating material mayibe applied in the fashion discussed 7 d. applying a ye q j fl nglm to the P ip above I v t f 40 urfaceoftheicemesso 'Once'the' immobilityof'tlie ice massi'has been established T m h le y CIaimi'Y rth r comprising 1 and tthe insulating material requirements have been "deterstepsQ t T h 1' I .j if Jmine d, theinsulating material is applied and the wells are" .Femovingsaid u ng" le Teqlllled, t sq, I

drilled from atop the icedownwardly; into theearth below. cold e er to a y, that-has m and Such 1 drilling' operations-'1 maybe carried out in the-same hfl g fi 1 V general manner conventionally usej'd'to drill wells for crude oil p m Insulating matenalqto Pm h l m g I and natural gas onshorein arctic areas. Completion practices a ,meltindufins wal-imlweatherf v h i would also parallel those presently "used in arctic ,environ- -1 9 mF QQ in 'by m 9 E p lg h w mg Alternatively theinsulatihghmaterifl ma be applied step ofrefrrgeratmg the ice beneath said lnsulatlngmaterial, as,

" after the wells are drilled and completedrand the rig-,hasbeen d fs s 9: that has l' movedout r n:

Claims

1. A method of forming a permanent ice platform in a shallow body of water which freezes to form a bottom-fast mass of ice in the winter and thaWs during moderate weather comprising: a. applying a layer of a nonaqueous insulating material to the surface of said mass of ice at a time when said mass of ice is fixed to the bottom, said layer being sufficiently thick to prevent any substantial melting of said mass of ice during moderate weather; b. excavating a trench around the periphery of the insulated ice surface; and thereafter, c. applying a layer of insulating material to the peripheral surface of the ice mass.

2. The method defined by claim 1 further comprising the steps of: a. removing said insulating material, as required, during cold weather to refreeze any ice that has melted; and thereafter, b. replacing said insulating material to prevent ice from melting during mild weather.

3. The method defined by claim 1 further comprising the step of refrigerating the ice beneath said insulating material, as required, to refreeze any ice that has melted.

4. A method of forming a permanent ice platform in a shallow body of water which freezes to form a bottom-fast mass of ice in the winter and thaws during moderate weather comprising: a. applying a layer of nonaqueous insulating material to the surface of said mass of ice at a time when said mass of ice is fixed to the bottom, said layer being sufficiently thick to prevent any substantial melting of ice during moderate weather; b. coating the surface of said layer of insulating material with a substance which adheres to said material and reflects radiant energy; c. excavating a trench around the periphery of the insulated ice surface; and thereafter, d. applying a layer of insulating material to the peripheral surface of the ice mass.

5. The method defined by claim 4 further comprising the steps of: a. removing said insulating material, as required, during cold weather to refreeze any ice that has melted; and thereafter, b. replacing said insulating material to prevent the ice from melting during warm weather.

6. The method as defined by claim 4 further comprising the step of refrigerating the ice beneath said insulating material, as required, to refreeze any ice that has melted.