CA1054809A - Low adhesional arctic offshore platform - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method and apparatus for reducing ice forces on a marine structure erected in a body of water which becomes frozen through natural weather conditions. The structure has a low-ice-adhesional wall forming its perimetrical surface in the zone where natural ice will freeze onto or impinge against the structure. Enclosed chambers are built within the struc-ture and may be in heat-transmitting relationship with the outer wall. Heat is applied to the chambers to heat and maintain the outer wall at a temperature above the melting point of the natural ice occurring in the water around it. In a preferred embodiment, the marine structure is formed with a heated wall which slopes upwardly and inwardly in the area of ice contact to provide a ramp-like surface upon which sheet of ice will be forced with reduced friction as it moves against the structure. Thus, an edge of the sheet of ice will be prevented from strongly adhering to the structure either initially or subsequently. This sufficiently weakened ice-to-surface bond without heat or reduced heat to the outer wall allows the ice sheet to be lifted above its normal position on the water surface as the ice moves against the structure, causing the sheet to break any weak surface bond so it can be bent and fractured as it rides by the coated ramp-like surface and thereby reducing the force imposed by it on the structure.
An alternate embodiment for reducing ice forces takes the form of a fully or partially coated cone or a cone made from material having low ice-adhesion properties. This embodiment may or may not be in combination with a heated surface.

Description

BACKGROUND OF THE INVENTION
This invention relates to a marine structure for installation in waters upon which thick sheets of ice are formed during the winter season. In Arctic and Antarctic waters the winter ice normally may reach thicknesses of 6 to 10 feet or more and rafting, pressure ridges, and other accumula-tions may cause the thickness of the ice in places to be several times the thickness of the original sheet. The ice sheets are of vast areas; and, although normally they may move relatively slowly with wind and water currents, the mass of ice in movement can cause very high forces on a stationary structure in its path. Such ice may have a compressive strength in the range of about 650 to 1000 pounds per square inch and a structure strong enough to withstand the crushing force of the ice would necessarily be very massive and corres-pondingly expensive to construct.
It has been proposed heretofore that rather than build a structure strong enough to withstand the total crushing force of the ice, i.e., strong enough to permit the ice to be crushed against the structure and thus enable the sheet of ice to flow around it, the structure be built with ramp-like sur-faces which would cause an edge of the moving ice sheet to be forced upwardly above its normal position on the surface of the water as it came into contact with the structure, thus bending the ice sheet and placing a tensile stress in the ice.
Since the ice has a flexural strength of about 85 pounds per square inch, a correspondingly relatively smaller force is placed on the structure as the ice impinging on it fails in tension.
Several forms of structures having a sloping peri-metrical wall for installation in waters where they would be exposed to the forces of moving ice are illustrated in a paper - 1 - ;~

lOS4809 by J.V. Danys entitled "Effect of Cone-Shaped Structures on Impact Forces of Ice Floes," presented to the First Inter-national Conference on Port and Ocean Engineering under Arctic Conditions, held at the Technical University of Norway, Trondheim, Norway, during August 13-30 1971.
Another publication of interest in this respect is a paper by Ben C. Gerwick, Jr., and Ronald R. Lloyd entitled "Design and Construction Procedures for Proposed Arctic Offshore Structures," presented at the Offshore l`echnology Conference meeting in Houston, Texas, during April 1970.
In testlng~in-a laboratory cold room, scale models of offshore structures incorporating the above design principle to investigate the action upon them of sheet ice, it was found that the ramp-type of surface, when moving relative to the ice sheet and in contact with it, caused appreciably less force to be imposed on the platform structure ~han would be the case if the platform wall presented to the ice sheet was disposed vertically to it as would be the situation if, for example, a proportionately larger-diameter pile or caisson was contacted by the moving ice sheet. It was dis-covered, however, that this condition was true only while the ice sheet could move relative to the platform and that, as explained hereinafter, ordinarily much larger forces would be imposed on the marine structure before the bond between it and the ice was broken to permit such relative movement.
In the actual installation of a marine structure in Arc-tic waters, it is proposed to construct and assemble the structure in a shipyard and tow the assembled structure to theoffshore site, where it will be established during the time the waters are open and relatively ice-free. At this time the structure will be lowered into contact with the submerged earth and piles may be driven into the earth to hold the structure in iO54809 place against the horizontal forces imposed upon it. Piles may also be used to assist supporting the vertical loads on the structure.
In the farther-north Arctic waters, such as the waters off the ~orth Slope of Alaska, the open-water season is relatively short, approximately 6 weeks, after which ice begins to form on the open waters. The ice will freeze around and onto the marine structure which has been fixed at the offshore site. This condition has been duplicated in the laboratory to determine what effect the newly frozen ice sheet would have on a scale model of a ramp-sided off-short structure, as described heretofore, and particularly to determine what forces would be imposed on it.
As the ice sheet built up in thickness on the surface of the water surrounding the model structure, it also froze onto the surface of the structure in contact with the water.
When the ice sheet reached the required thickness for the test, it was found that a much greater force was required to start relative motion between the model and the adhering ice sheet than was required to maintain the relative motion after the adhesional bond was broken. For the conditions of the test, approximately 5 to 10 times as much force, depend-ing on specific conditions, was imposed on the model structure by the ice sheet before the bond was broken than was imposed after this relative motion was established.
The amount of force imposed initially on the structure by the ice sheet will, of course, be dependent on the form, dimensions and characteristics of the structure and the dim-ensions and characteristics of the ice. But, in all cases, as the problem is understood now, a much greater force will be imposed initially on the structure before the adhesional bond between it and the ice is broken than will be imposed after the lOS4809 bond is disrupted. Ordinarily it would be necessary under these conditions to build the structure strong enough to withstand the initial forces imposed on it by the ice sheet, even though the forces imposed on it during the major portion of its useful life would not require a structure of such rugged con-struction. A structure built strong enough to withstand the initial ice forces would be correspondingly more expensive to build and more difficult to install than one designed to take only the load of a relatively moving i~e sheet. The present invention is designed to alleviate this condition of initial high loading imposed on the offshore structure by a method and apparatus to be described hereinafter.
The invention will be described hereinafter as applied particularly to an offshore structure used primarily for drilling oil wells or as an adjunct to producing oil from subaqueous oilfields in regions of the earth where the open waters become frozen on the surface with an appreciable thick-ness of ice. For simplicity of description, such a structure may be designated hereinafter as an offshore drilling platform, although it will be appreciated that the principles of this invention may be applied to other types of marine structures such as offshore production platforms, offshore loading and unload-ing stations for petroleum tankers, lighthouses, piers or other structures established in a fixed location and exposed to the forces of ice sheets which move on the surface of the water.
Pursuant to the invention, a method is provided for reducing the force imposed on an offshore structure by the movement against it of ice formed on the surface of a body of water in which the structure is stationari-ly established, which comprises providing the structure with an outer shell support portion against part of the outer surface of which ice will impinge when ambient natural conditions are such as to cause the formation of such ice~ at least said part of the outer surface of the shell sloping upwardly and inwardly of the underwater bottom to receive ice which moves relative to and into contact with the structure whereby such ice will be displaced upward-ly and hence caused to bend and break, and at least said part of the outer surface of the shell being formed from or coated with a material having low -4 ~

1~54809 ice-adhesion properties so as to facili$ate the movement of ice over said part of the outer surface of the shell whereby the force imposed on the structure by the ice is reduced, the adhesion between the ice and a surface of said material being between O and 100 psi.
Preferably the method for reducing these forces on the marine c ,, -4a-~054809 structure includes applying heat to the inner or outer surface of the outer wall or shell of the structure, particularly adjacent the water line, where the natural ice will tend to freeze onto or impinge against the outer surface of the shell to cause the temperature of the outer surface of the shell in the area of ice contact to be maintained above the melting point of the ice.
The heating system is such that heat can be applied continuously to the shell o the platform while ice is present in the water around it, both to prevent adhesior. of ice to the shell and to provide a film of water between the ice and the outer surface of the shell to assist the ice in slipping over and upon this surface when the ice contacts it, both of which features function to reduce the force imposed on the structure by the moving ice sheets.
Examples of a material or coating means that reduces ice adhesion are halocarbon resins like tetrafluoroethylene polymer; tetrafluoroethylene hexafluoropropylene copolymers; chlorotrifluoroethylene polymers; and nylons such as polyamide polymers or copolymers and polylactams. Other similar low or non-ice-adhering materials having an adhesional restraint between ice and the material below 100 psi may also be used. For comparison, the adhesion between ice and steel can be as high as 100 psi at a temperature in the range o 20F. Thus the term "low ice-adhesion properties" is used herein as mean-ing a material having an adhesion between ice and a surface of the material of between O and 100 psi.
In summary, this coating is such that adhesion of ice to the plat-form is negligible, The coating preferably supplants or assists the film of water described above, while at the same time reducing the adhesional bond between the ice and the material. This material can function as a backup in an emergency situation where the heating system fails and a long down-time for repairs is required. Further, this material reduces the amount of heat used during daily operations, since the adhesional bond is significantly reduced as a result of the coatings.
According to the present invention, the apparatus consists of an offshore structure intended for stationary location in a body of water the surface of which can become covered with ice as a result of ambient natural 105~809 conditions, the structure having a support portion comprising an outer shell that slopes upwardly and inwardly of the bottom of the body of water at least in the area of contact of the surface of the body of water with the outer surface of the shell whereby ice moving relative to and in contact with the structure will be displaced upwardly and hence caused to bend and break, at least that part of the outer surface of the shell on which ice would tend to freeze or impinge when the structure is established in said body of water being formed from or coated with a material having low ice-adhesion properties so as to facilitate the movement of ice over the said part of the outer sur-face of the shell whereby the force imposed on the structure by the ice is reduced, the adhesion between the ice and a surface of said material being between 0 and 100 psi.
In a preferred embodiment of the apparatus, the offshore work or drilling platform has a perimetrical outer wall or shell in the form of a frustum of a cone sloping upwardly and inwardly of the platform at an angle of approximately 45 to the horizontal. The conical surface extends from a location below the region of ice formation on the surface of the water to some distance above the level of the water surface. Thus, there is a lower section in the area of ice contact which slopes or converges upwardly and inwardly, a minimal middle cylindrical section which may approach zero above this area of ice contact, and an upper diverging section which slopes upward-ly and outwardly. Since all of these sections are continuous, they will function somewhat in the manner of a ramp upon which an edge of a sheet of ice will be raised above its natural level on the surface of the water as the ice moves toward the platform. As a result, the ice sheet will be flexed in the region of the platform, causing the ice to fracture and break from the tensile forces resulting from the flexural stress. The conical wall is coa~ed with low or non-ice-adhering materials. This coating feature facili-tates breaking the bond of ice to the structure; additionally, it freely permits the flow of ice up the wall.

1059~809 In one preferred embodiment, water-tight compartments, which as water tanks, are constructed within the platform adjacent the outer conical shelll and the latter acts as a cc~mon exterior wall for both the platform structure and the water tanks. The tanks are connected to pumps for circulat-ing water through them and through heat exchangers. The exchangers are in communication with the exhaust gases from the engines which generate the power for operating the machinery and other apparatus on the platform. The tanks are filled with water that is heated an amount to maintain the outer surface of the shell of the platform above the melting point of the natural ice forming around or impinging upon it, thus preventing the ice from freezing onto and adhering to the outer surface of the platform. Preferably the water tanks are of sufficient capacity to contain enough heated water to maintain the critical area of the coated shell of the platform at a temperature above the melting point of the natural ice throughout a period of at least 24 hours if no addi-tional heat is added to the water during that time. This will provide a safe period for making repairs if the source of heat for the water should fail during the critical period when the ice sheet may otherwise be in a condition to freeze onto the platform and start moving. It will be noted that if the source of heat for the water fails during the time when the ice sheet is in motion so that the intact ice sheet does not adhere to the coated shell, any additional load imposed on the platform by the icewillbe conconsiderably less than the force ipposed on the platform by the thrust of an ice sheet which is frozen onto and adhering to the coated shell.
The perimetrical wall may be made entirely of non or low-ice-adhering material, resulting in a substantial reduction in the force derived from breaking the ice bond. A further advantage of this embodiment is a weight savings in the structure itself, with a corresponding reduction in foundation support for the entire platform, since a structurally lighter platform results when forces on it are reduced.
The present invention includes embodiments of the coated exterior wall and the solid perimetrical wall made from the non or low-ice-adhering material with no exterior surface heating.
It is not intended that the invention be limited to heated water tanks placed within the coated shell or shell made from low or non-adhesional material, and other means for performing the method of this invention also will be described hereinafter or will become apparent as the descrip-tion of the invention proceeds in conjunction with the accom-panying drawings which form part of this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation, partily in section and with some elements rearranged in position for clarity of disclosure, of an offshore drilling platform incorporating the features of this invention, and illustrates the embodiment which employs heated water tanks to maintain the critical areas of the coated shell of the structure above the freezing temperature of the natural ice around it.
FIG. 2 is a schematic illustration in sectional plan view taken on the line 2-2 of FIG. 1.
FIG. 3 is a schematic representation in elevation and partly in section of another arrangement of apparatus incor_ porating the features of this invention, and illustrates the use of heat-transfer panels to heat the critical area of the coated perimetrical shell of an offshore structure above the melting point of the natural ice surrounding it.
FIG. 4 is a schematic illustration in plan view and partly in section taken along the line 4-4 of FIG. 3, with portions broken away to expose details of the assembly.
FIG. 5 is a schematic illustration of a marine plat-form composed of an outer shell made from low or non-ice-adhesional material.

FT~. 6 is a schematic illustration in plan view and 359 partly in se_ti~n taken along line 6-6 of FIG. ~, with portions 361 broken away to expose detail~ of the assembly.
Fr~. 7 is a schematic representation, partly in 362 section, of an offshore drilling platform having the feature of 364 a coated exteri~r surface. No exterior surface heating is 365 provided; ho~ever, internal heating for platform personnel is 366 shovn. 367 FI~. 8 is a diagrammatic representation in plan vie~ 368 an~ partly in section t~ken along line 8-8 of PIG. 7, with por- 369 tions broken a~ay to expose details of the assembly. 370 D~SC~IPTION_QP_T~E_PREFEPRED EMBODIMEN 373 FIG. 1 represents an offshore drilling platform lO 375 installed in a body of ~ater 12 in engagement with the under- 376 ~ater bottom 14 to which it is secured temporarily by piles 16. 378 rhe platform is designed p~rticularly for installation in 379 arctic waters up~n which thick sheets of ice 18 will seasonably 380 he formed. The platfor~ h~s a lower support portion 20 which 382 extends into the water ~nd forms a base which supports a deck 383 portion 22 above the surface of the water in an upright 384 position. The lo~er portion of the platform is exposed to the 385 water and ice forces incident to its environment, and is the 386 portion of the platform of principal interest to the present 387 invention. The upper portion of the platform may contain 388 several lev~ls o~ decks and may be enclosed and heated to 389 provide a re~sonably comfort~ble working environment and 390 protection for men and equipment from the ~inter weather, 391 during ~hich the temperature may drop to the range of -60F. 392 qithout a~eguate heatin~ f~cilities in the working areas, the 393 operation of the Arctic drilling platform would become vir- 394 tually impossible during a major portion of the year. 396 g lOS4809 Since it is both expensive and difficult to construct 397 an~ install ~ drillin~ platform in Arctic waters, it is desir- 398 able to provide a platform which is capable of drilling a 399 number of ,rells from the s~me location. For example, the 401 drilling platfor0 illustrated in FIG. ~ may be designed to 402 drill 10 or ~ore wells from the same platform site to a depth 403 of approximately 16,000 feet, and accordingly is made large 404 en~ugh to accomm~date the m~chinery and equipment necessary for 405 this purpose. By ~-ay of illustration only, a platform of this 406 capacity for installation in 40 feet of ~ater may have a bottom 407 diameter of ~pproximately 180 feet and a diameter at the water 408 line of approxim~tely 120 feet. The base portion mav have a 410 height of 85 feet and suppor~ above it decks and other 411 appurtenant aquipment, incluaing ~ drilling derrick reaching to 412 an elevation of ~pproxim~tely 160 feet above the floor of the 413 sea. 414 A ~rilling platform of the above-noted dimensions 415 ~ill weigh sever~l thousand tons before it receives any of the 416 machinery and eguip~ent ne_essary ~or the drilling operation. 417 The weight of the platform ~lill increase proportionately as it 418 is design~ to withstand greater natural forces; and, since the 419 weight of the structure reflects its cost, the cost will in- 420 ~ rease pr~porti~nately as the weight increases. The present 422 invention is directed to~ard a procedure for reducing the 423 forces imposed on the blse portion by natural ice formations, 424 thus permitting less structural material to be incorporated in 425 it and correspondingly reducing its mass as ~eLl as reducing 426 its cost. 427 For a ~rilling platform of the size and drilling 428 capacitq referre~ to, po~er generators adequate to produce 429 appr~ximately 3300 horsepower ~ill be supplied to operate the 430 rotarr table, dr~7dworks, mud pumps and other eguipment and 431 appurtenances necessary for the drillinq operation. In 433 ~ccordance with one exemplary embodiment of this invention, the 434 waste heat from the power-genarating source is used to heat the 435 ~o~ted shell of the pl~tform abov~ the melting point of the 436 natural ice surrounding it in th~ manner and for the purpose to 437 be described in more detail hereinafter. If, for example, the 438 power source selected is a turbine engine and three llO0-hp gas 439 turbines are usel for the power necessary to operate the 440 drilling platform, more th~n 32,000,000 8tu/hr. waste heat 441 from the turbines will be ~vailable for heating the shell of 442 the support stru_ture. This amount of heat is amply adequate 443 to maintain a shell of the dimensions noted above continuously 444 ~t a temperature a~ove the melting point of the natural ice 445 formed in the ~ater in contact with it. 446 ~ he str~cture illustrated in PIG. l represents a 447 drilling pl~tform which is towed to the dril~ing site in a com- 448 pletely assembled and equipped condition and which requires no 449 ad~itional construction at the site except for lowering it into 451 engagement with the sea bottom and, when necessary, securing it 452 with piles. B1llast tanks 24 (FI~S. l and 2) are built into 453 the base portion 20 as an integral part of it to ballast the 454 platform when being towed and to enable it to be lowered 455 through the water into contact ~ith the s~a bottom. The 457 ballast tanks ~re each provi~ed with appropriate sea cocks 26 458 which can be m~nipulated remotely from aboard the platform by 459 means not sho~n, but whi_h can be provided for within the skill 460 of th~ art. Each ballast tank has connected to it a respective 462 blo~down pipe 28 which receives air under pressure from a 463 compressor 30. The sea cocks can be opened to admit ~ater into 464 the ballast tanks or, alt~ernatively, pressurized air can be 465 introduced int~ the tanks to expel water from the tanks through 466 the sea cocks. 467 When the platform is under tow, enough water is admitted to the ballast tanks to give it a draft of about 8 to 10 feet, and the ballast tanks have sufficient volume to provide adequate buoyant space above the ballast water to give the platform stability in its towing condition. The ballast tanks may of course, be trimmed as necessary to compensate for any uneven distribution of weight in the platform.
The drilling platform represented by FIG. 1 is construc-ted to be readily established with full operating capability at a selected drilling site and with the ability to be removed from one site and established at another in operating condition without delay. To assist this mobility, it is desirable that the platform be constructed to be stabilized at an offshore location with a minumum of secondary construc-tion operations, such as the driving of piles, being re-quired to secure it in position against the forces it will be exposed to.
The platform is constructed with a plurality of leg mem-bers 31, which are mounted to be moved vertically relative to the body of the platform by corresponding jacking arrange-ments 33. The legs have expanded foot portions 35 and con-tain internal guides 32 (FIG. 2) for the piles 16. When the platform assembly reaches a drilling site, and while it still is floating, the leg members are lowered into engagement with the sea bottom. While the jacks are engaged with their res-pective legs, the sea cocks 26 are opened to admit additional water to the balast tanks 24, increasing the weight of the platform enough to give it negative buoyancy. The footings 35 are designed to control the penetration of the legs into the sea bottom as the weight of the platform increases.
The jacks 33 are now operated to lower the platform along the legs in a leveled controlled condition until the bottom 37 of the platform is seated on the sea floor. The ballast tanks may now be flooded an additional amount, adding sufficient weight to the platform for it to resist being dis-placed by natural forces.
In locations where the platform wil be exposed to adverse conditions such as the thrust of sheet ice, the piles 16 may be driven into the submerged earth to hold the plat-form securely in position against horizontal loads imposed on it. It is an object of this invention to provide means fori reducing considerably the force which otherwise would be im-posed on a stationary platform by an ice sheet moving against it, thus enabling a structure to be assembled which is more adaptable to the conditions of use described heretofore.
When the platform is to be moved to another location, the piles 16, if such piles have been used, are cut below the ~ootings 35 or otherwise detached from the platform. The compressors 30 are operated to expel water from the ballast tanks until a condition of buoyancy of the assembly is reached which will permit it to be elevated off bottom along the legs 31 under controlled conditions. Preferably during this operation, sufficient water will be kept in the ballast tanks to give the platform some negative buoyancy, and it will be lifted along the legs by the operation of and under control of the jacks 33. The platform is raised to its floating position, the ballast is adjusted, and the assembly is towed to its new location, where it is again set on bottom as described heretofore.
FIG. 1 represents the drilling platform installed at the drilling site and equipped for the drilling operation The derrick, indicated at 39, is enclosed for protections from the weather and extends into the body of the structure to a deck 41, which contains a support 43 for the rotary table. The derrick has a crown block 45 which can be shifted to be aligned vertically ~ith each of th~ several positions 47, FIG. 2, on 54~
bottom Dlate 49, through which separate wells may be drilled. 543 The rotary t~ble, not shown, like~ise is constructed to be 544 move~ into alignment selectively ~ith each of the positions 47. 545 When a well is b~ing drillea, a casing 51 is set into the well 546 bore ~nd sealea ~ith a water-tight connec~ion 53 to the plate 547 49. If the platform is to be moved to another location, the 548 casing will be detached from the platform and a vater-tight 549 cover will be se-ured over the corresponding opening. 551 As previously indicated, gas turbines may be used as 552 the primary source of po~er for the platform. ~wo of the 554 power-generating gas turbines 34 and 36 are illustrated 555 schematically in form and position in FIG. 1. The exhaust from 556 each Surbine is led through a respective conduit 38 and 40 to 557 heat ex_hang2rs as represented by the coils 42 and 44. It is 559 ~ithin the concept of this invention to provide a bank of heat 560 ~xchanaers ~hi~h vill be in communication ~ith the eAhaust 561 ducts of all the po~er turbines or to provide separate heat 562 exch~ngers associated with each respective power turbine.
However, it is i~portant that there be some redundancy in this 563 portion of the apparatus to provide adequate operating heat- 564 exchanqer capacity, if some portion of the power-generating or 565 heat-exchanger apparatus must be closed do~n for maintenance or 566 repair. The perimetrical wall also has a coatiny made from 567 material which reduces ice adhesion and permits a corresponding 568 reduction in heating capacity, since the adhesional force 569 bet~een the coating and ice is less. A further advantage of 570 the coating 131, airectly related to its non-adhesional 571 char~cteristics, is its minimal resistance to ice movement over 572 it. The coating is either mechanically or chemically bonded to 573 the shell. 574 ~OS4809 In this illustrative embodiment of the invention, 57S
after the drilling platform is established in operating condi- 5i6 ~i~n the ballast tanks 24 are substantially filled with a heat- 578 tr~nsfer liquid. ~tmospheric space 48 is left at the top of 579 the tanks to function as a surge chamber and to provide for 580 expansion of the liquia. Other~ise, the ballast tanks may be 582 connected to auxiliary surge tanks, not sho~n, for this 583 purpose. ~ 584 The heat-transfer fluid may be sea water to ~hich an 585 appropriate corr~sion inhibitor has been added to protect the 586 steel surfaces in contact with it. Desirably an antifreeze 588 component is added to the water to prevent it from freezing 589 solid within the ballast tanks and to cause it to remain 590 pumpable if the water is not heated during a period when the 591 shell of the support structure is reduced below the freezing 592 point. Where fresh water is available in sufficient quantity, 593 the ballast tan~s may be purged of any salt water they contain 594 an~ filled with fresh water to which is added a corrosion ~95 inhibitor, an antifreeze component and an algicide to make up a 597 compounded heat~transfer fluid. S98 Antifreeze components available for this purpose 599 would ~e, for example, soluble salts such as sodium chloride 600 and calcium chloride, an alcohol such as methanol, or a glycol 602 such as ethylene glycol, or a glycerol, or any of several other 603 antifreeze subst~nces which are vell known, any of which may be 604 ad~ed to the water in the ballast tanks in a sufficient amount 605 to prevent the water fro~ freezing to a nonpumpable condition 606 throughout ~ preaetermined temperature range at a time ~hen 607 he~t is not being applie~ to the water. A corrosion inhibitor 608 is s~lected to b~ compatible and effective with the chosen 609 antifr~eze c~mponent~ 610 ~OS4809 The he~t exchangers 42 and 44 are connected by appro- 611 priate pumps, as 50 and 52, respectively, to a common manifold 6i2 54 2rom which respective conduits, as 56 and 58, communicate 613 ~ith the top portion of each individua~ tank 24 bel~w the level 614 59 o~ the water contained in it. The lower portion of each 616 tank is in communication with a common manifold 60 through 617 respective lower conduits, as 61 and 62. The heat exchangers 619 42 and 44 are connectea to m~nifold 60, as by respective 620 conduits 63 and 64, and the pumps operate to draw cooler water 621 from the top portion of the tanks and pump it through the heat 622 e~chang~rs and thence into the bottom ~anifold 60, from which 623 it is directed into the bottom portion of the tanks 24. 624 ~lth~ugh a single pu~p may b~ used for circulating the heat- fi26 transfer fluid through the tanks 24, it is advisable to have at 627 le~st a second pump connected in the system, either as an 628 ~perating component or as st~ndby, to insure the continued 629 oper3tion of the system if one of the units should fail to 630 function. ~ppropriate valves placed in the upper and lower 631 conduits such as, respectively, v~lve 65 in conduit 56 and 632 valve 66 in ~onduit 61, pro~ide a means for controlling the 633 ~low o~ heat-transfer fluid through an individual tank 634 in~ependently of the flow through adjoining tanks and also 635 provide a me~ns for isolating an individual tank from the heat- 636 trlnsfer fluid circulating syste~ as may be necessary for 63~
repair or maintenance. 638 ~he ballast tanks for a platform of the dimensions 639 ~escribed heretofore are designed to hol~ in excess of a total 641 of 20,000 barrels of water. The heat-transfer fluid 642 circulating system is aesigned to circulate fluid through these 643 tanks at the rate of approximately 800 ~allons per minute when 644 the platform is in normal operation, and 32,000,000 Btu/hr. 645 will be available from the power-generating turbines to heat 646 ~ 16 -this fluid. ~hen the fluid in the ballast tanks is heated 647 sufficiently to maintain the outer surface of the support 648 struct:ure shell at approximately 33P., there ~ill be enough 649 heat stored in the water in the ballast tanks to keep the shell 650 above the freezing point of the ambient ~ater for a period of 651 24 hours, thus providing a safe period for repairs, or for 652 securing the wells and abandoning the platform i~ the power- 653 generating syste~ on it ~hould fail under conditions which 654 imperil the safety of the platform. 656 The pl~tform sho~n in FIGS. 1 ana 2 indicates, by ~ay 658 of example, six ballast tanks 24. Ho~ever, this is not a 659 critical number, and more or fewer tanks may be appropriate for 660 particular platforms. The tanks illustrated are separated by 661 raaially aisposed ~ater-tight valls or bulkheads 67 and are 662 cl~sed on their radially in~ardly sides by a cylindrical wall 663 or bulkhe~d 68. The radially outer ~all of the tanks is the 664 perimetrical ~11 or shell 70 of the lower portion 20 of the 665 platform. 666 FI,. 1 also indicates the ballast tanks as extending 668 from the water-tight bottom 37 of the platform up to the bottom 669 deck 74 of the upper portion 22, ~ith the heat-transfer fluid 670 in the ball~st tanks in direct heat-transfer contact ~ith the 671 inner surface 76 of outer wall 70 throu~hout substantially all 672 of this region. Ho~ever, for some plat~orms, it ~ill be 673 sufficient to proYide tanks for the heat-exhange fluid ~hich, 674 although of ldequate c~pacity, are of less volume than those 675 indicated in the dra~ings. Such smaller tanks ~ill be 677 distributed around the inner surface 76 of the ~all 70 and be 678 ~onstructed to expose the inner surface to contact ~ith the 679 heat~e%change fluid throughout the area ~here natural ice could 680 freeze to the shell, extending for a distance above and belo~ 681 the surface level of the ambient vater, to maintain this zone 682 - 17 ~

iOS4809 of the shell above the melting temperature of the natural ice in contact with it. By this construction a weighted dry ballast may be used which requires less space than water ballast and thus provides additional dry working area within the platform.
In the illustrated embodiment, the cylindrical bulk-head 68 defines working space at the core of the platform and appropriate decks, as 41, 78 and 80, are provided to support men and machinery. Although this space will be heated to a comfortable working temperature, which normally may be above the temperature of the fluid in the tanks 24, there is nevertheless provided a layer of insulation 84 placed against the radially inner surface 86 of bulkead 68 to reduce heat loss from these tanks if the temperature of the core area 88 should be below that of the tanks.
Preferably a wear plate 90 of a material that reduces ice adhesion is secured to the outer surface of the shea~l in the zone on the platform of ice contact to strengthen this area and to receive the impact and abrasive action of the ice sheet bearing agsint the support structure. Because of the non or low-ice-adhesional nature of the wear plate 90, the adherence of the ice to the shell is minimal in this area.
FIGS. 3 and 4 represent an alternative arrangement of apparatus embodying the present invention and indicate also a modified form of platform to which it is applied. The same numerals as used previously will be used again where applicable in relation to FIGS. ~ and 4 to designate corresponding elements.

In this modification the lower support portion 20 and the upper deck portion 22 may be constructed as separ~te units which will be assembled together at the offshore site. The ql - 1~ -~054809 support portion has pile guides 32 built into it around its - 18a -~1054809 periphery, as well as through its central section, to receive a corresponding number of piles 16.
1he support portion of the platform is towed to a chosen offshore location and sunk into contact with the sea bottom by increasing the ballast weight. Piles 16 are then inserted through the pile guides 32 and driven into the submerged earth. The support section is leveled and the piles are grouted to the pile guides to hold the platform securely in position against the horizontal and vertical loads imposed on it. Subsequently, the upper deck portion 22 is lightered to the location and assembled on the stab-ilized lower portion.
In this modification, as in the platform illustrated in FIG. 1, a water-tight bulkhead 68 surrounds the central area 88 of the platform and defines the inner wall of compartments 100 and 102, which may be used as ballast tanks for trimming the platform under tow and for sinking it at the well site in the manner described heretofore. However, rather than filling the compartments with the heat-transfer fluid to keep the shell of the support section above the freezing point of the ambient water, panels of coils of tubing are fitted to the inner surface of the shell in heat-transfer relation-ship and the panels are manifolded together to receive the heat-transfer fluid from heat exhangers which are exposed to the exhaust gases of the power-generating turbines for the platform in a manner similar to that described heretofore.
In this modification of the invention, after the drilling platform has been secured to the under-water bottom, the water may be displaced from the individual compartments. The compartments are then loaded with sufficient dry weighting material to compensate for whatever residual buoyancy the assembled platform may have. This procedure reduces the cor-rosion problem of the interior surfaces of the compartments caused by - 19 -water containe~ in the tankage and also provides additional dry 756 working or stor~ge space within the confines of the platform. 758 Still referrin~ to FIG. 3, the heating panels 104 are 759 placed again~' the interior surface 76 of the shell 70 with 760 co~ting 131 in heat-transferring relationship throughout the 761 are~ ~hich will be in contact with the ice sheet 18 formed on 762 the surface of the ambient water, and preferably ~ill extend 763 for some distance above and below the thickness of the sheet to 765 assure that this area of the coated shell will be elevated in 766 temperature above the melting point of the surrounding ice. A 767 ~ear plate 90, preferably ma~e from material that reduces ice 768 adhesion, is secured to the outer surface of the shell in this 769 area for the purpose described heretofore. The panels of 770 heating coils are covered on their inward surfaces with a layer 771 of insulatin~ 0~terial 106, for example such as a foamed 772 urethane, to confine the heat from the panels to the shell of 773 the platform in this are~. Preferably the insulating material 774 is in turn c~vera~ by a cover 107 secured in a water-tight 775 manner to the surface 76 to prevent ~ater in the ballast tanks 776 from contacting the heating panels and the insulation. 778 In operation, a heat~transfer fluid of the type 779 descrihed heretofore flo~s from surge tanks, as 108 and 110, 780 into a manifold 54 from ~hich it is ta~en by pu0ps 50 and 52. 781 ~he pumps deliver the fluia to heat exchangers 42 and 44, ~hich 783 receive heat fro~ the exhaust gases of the platform power- 784 generating engilles 34 and 36 through ducts 38 and 40. The 7a5 flui~ flo~s from the heat exchangers to a manifold 112, and from the manifol~ respective conduits 114 conduct the fluid to 787 the heat-transfer panels 104. The fluid is pumped through the 788 tubing 116 of th~ panels and thence flows through respective 789 conduits 118 into a manifold 120, from which it is conducted by 790 piping 122 t~ th~ respective surge tanks, as 108 and 110. 7~2 ~ ppropriate valvin~ is placed in the system to 793 provide for the control of the fluid circulation to any one of 79~
t~e panel se-tions or to any surge tank and to enable these 796 portions of the apparatus to be taken out of the operating 797 system for maintenance or repair. Thus, respective valves 124 798 are placed in the conduits 114 from the manifold 112 to the 799 corresponain~ sections o' the heat-transfer panels 104 and 800 respective valves 126 are placed in the conduits 118 carrying 801 the return fluid from the heat-transfer panels to the manifold 802 120. In like manner, each section of the surge tankage can be 803 isolate~ independently of the others by a respective valve 128 804 placed in the piping 122 which leads from the manifold 120 to 805 the surge ch~ber, and by a corresponding respective valve 65 806 in the conduits, as 56 and 58, from the individual surge 807 ch~mber, as 108 and llO, to the m~nifold 54. 80 Sinc~ in the modification illustrated in ~IG. 3 the 811 heat for the shell 70 of the support section of the platform is 812 concentrate~ in the zone of ice formation in the surrounding 813 water, less tot~l heat uill be required to raaintain this 814 portion of the coated shell above the melting point of the 815 natural ice th~n was used f~r the ~odification of the invention 816 ~escribed in rel~tion to FIG. l, and less heat-generating 817 capacity will be require~ for this purpose. 818 It is within the concept of this invention that, 819 under some c~nditions of ambient weather and platform 820 _onfiguration, the coated shell of the platform in the zone 821 thereon of ice formation can be heated above the melting point 823 of the ice by diverting the exhaust gaseous fluids from the 824 power-generating engines thr~ugh appropriate ducting into heat- 825 tr~nsferring c~ntact with the inner surface of the shell to 826 fun~tion as the heat-transfer fluid. Also, it is within the 827 concept of this invention to provide sufficient power- 828 generating m~ans aboard the platform to generate power for 829 panels of electrical heating elements arranged in a manner 830 similar to that ~scribed ~ith reference to the hsat-transfer 831 panels illustrated in FI~S. 3 ~nd 4. 832 The schematic illustrations of FIGS. 5 and 6 depict a 834 platform whose outer shell is made wholly from a material 137 835 that minimizes ice adhesion to it. Such materials may be 836 halocarbon resins like tetraEluoroethylene hexafluoropropylene 838 _opolymers, tetr~luoroethylene polymers, chlorotrifluoro- 839 e~hylene polymers, or nylons like polyamide polymers or 840 copolymers and polylactams. The platform may also have a 842 he~ted exterior surface, although heating the surface is not 843 necessary. If heating is used, either the heating apparatus 844 illustrated in ~r~s. 5 and 6 or the heating system similar to 845 that sho~n in PIG. 3 can be used. The heating system can 84i function in this moaification as a backup to the non-adhesional 848 outer shell 137. 849 ~ eferring particularly to FTGS. 7 and 8, an embodi- 850 ment of a platform without heating the exterior wall surface of 852 the platform is illustrated. In this embodiment, the ballast 853 tanks 24 are either filled with a mixture of sea ~ater and 854 antifree2e or minimally he~ted in ord~r to avoid freezing the 855 ballast water in them. The heat exchangers 42 and 44 which are 856 -onnected by appropriate pumps 50 and 52 and conduits 63 and 64 857 ~istribute heat within the area enclosed ~y th~ coated shell 858 70, which is use~ hy platform personnel. A benefical 860 ~onsequenc~ of this arrang~ment is -ost savings in piping, fuel 861 and heating apparatus needed to transfer heat to the outer 862 shell surface. 863 ~ f it is desired to keep the platform on location 864 ~fter the ~rilling operations are completed, ~hen it no longer 865 is n~cessary to ~enerate the amount of power required for 866 - 22 _ drilling, auxiliary sources of power may be used directly to supply the heat necessary to prevent ice from adhering to it. Thus, a stem boiler may be used which is designed primarily to supply the heat transfer fluid for the ballast tanks 24 or for the heating panels 104, or the heat may be supplied by a power source external to the platform, as by connecting panels 104 of electrical heating elements to a source of electrical power generated apart from the platform~
The inventive concept is directed to the method and appropriate apparatus for reducing the forces imposed by natural ice on an offshore platform. For platforms of the dimensions recited in the specification, by way of example, the force imposed on such a platform by the movement of a sheet of ice 8 feet thick frozen on and adhering to its steel shell will be approximately 10,000,000 to 20,000,000 pounds total. When the shell is coated and heated above the melting point of the ice and the adhesion is broken, the force of the ice sheet upon the platform will be reduced in the order of five to ten times for a total force of approximately 2,000,000 pounds.
Preferred embodiments of this invention and modifica-tions thereof have been described herein. However, it is apparent that other modifications may be made to the exemplary arrangements of apparatus disclosed herein without departing from the inventive concept, and it is intended that the invention include all the modifications and equivalents within the scope of the appended claims.

Claims (12)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An offshore structure intended for stationary location in a body of water the surface of which can become covered with ice as a result of ambient natural conditions, the structure having a support portion comprising an outer shell that slopes upwardly and inwardly of the bottom of the body of water at least in the area of contact of the surface of the body of water with the outer surface of the shell whereby ice moving relative to and in contact with the structure will be displaced upwardly and hence caused to bend and break, at least that part of the outer surface of the shell on which ice would tend to freeze or impinge when the structure is established in said body of water being formed from or coated with a material having low ice-adhesion properties so as to facilitate the movement of ice over the said part of the outer surface of the shell whereby the force imposed on the structure by the ice is reduced, the adhesion between the ice and a surface of said material being between 0 and 100 psi.

2. An offshore structure as claimed in claim 1 and further comprising heating means for maintaining at a temperature above the melting point of the ice for a required period at least said part of the outer surface of the shell.

3. A structure as claimed in claim 2, comprising one or more water-tight compartments within the support portion and in heat transferable relationship with at least the said part of the outer surface of the shell, and means for circulating heated water through the or each compartment.

4. A structure as claimed in claim 3, wherein a plurality of said compartments are disposed substantially symmetrically about the vertical axis of the structure.

5. A structure as claimed in claim 3 or 4, comprising one or more heat exchangers whereby in use water to be circulated through the or each said compartment can be heated by exhaust gases generated on the structure flowing through the or each heat exchanger.

6. A structure as claimed in claim 2, comprising one or more heating panels in heat transferable relationship with at least the said part of the outer surface of the shell, and means for heating the or each panel.

7. A structure as claimed in claim 6, wherein the or each heating panel comprises one or more heating coils through which heated water can be circulated.

8. A structure as claimed in claim 7, comprising one or more heat exchangers whereby in use water to be circulated through the or each said heating coil can be heated by exhaust gases generated on the structure flow-ing through the or each heat exchanger.

9. A structure as claimed in claim 1, wherein a portion of the outer shell is constructed in the form of a frustum of a cone sloping upwardly and inwardly at an angle of about 45° to the horizontal.

10. A structure as claimed in claim 1, wherein said material is a halo-carbon resin or a nylon resin.

11. A method of reducing the force imposed on an offshore structure by the movement against it of ice formed on the surface of a body of water in which the structure is stationarily established, which comprises providing the structure with an outer shell support portion against part of the outer surface of which ice will impinge when ambient natural conditions are such as to cause the formation of such ice, at least said part of the outer surface of the shell sloping upwardly and inwardly of the underwater bottom to receive ice which moves relative to and into contact with the structure whereby such ice will be displaced upwardly and hence caused to bend and break, and at least said part of the outer surface of the shell being formed from or coated with a material having low ice-adhesion properties so as to facilitate the movement of ice over said part of the outer surface of the shell whereby the force imposed on the structure by the ice is reduced, the adhesion between the ice and a surface of said material being between 0 and 100 psi.

12. A method according to claim 11 wherein at least said part of the outer surface of the shell is maintained at a temperature above the melting point of the ice for a required period.