COUNCIL WASHINGTON DC F/G RESEARCH IN SEA ICE ...Donald W. Perkins, Assistant Executive Director...

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Panel on Sea Ice Mechanics

Marine Board

Assembly of Engineering

National Research Council

DISTRIBTMON STAM A(Approved for public relemae

LL. Distribution Unlimited

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MARINE BOARD

of the

ASSEMBLY OF ENGINEERINGNATIONAL RESEARCH COUNCIL

Members

Ben C. (erwick, Jr., Chairman William S. GaitherDepartment of Civil Engineering Dean and Professor

Universitv of California, Berkeley College of Marine StudiesBerkeley, California University of Delaware

Newark, Delaware

Ronald L. Geer, Vice Chairman

Senior Mechanical Engineering Griff C. LeeConsultant Group Vice President

Shell Oil Company McDermott, Inc.Houston. Texas New Orleans, Louisiana

Davton L. Alverson Bramlette McClellandDirector PresidentNatural Resources Consultants McClelland Engineers, Inc.Seattle, lWashington Houston, Texas

H. Ray Brannon, Jr. Leonard C. Meeker.-,;earch Scientist Center for Law and Social PolicyExxon Production Research Company Washington, D. C.

Houston, TexasJ. Robert Moore

'shn D. Custlow, Jr. Director, Marine Science InstituteDuke University Marine Laboratory University of Texas at Austin

Beaufort, North Carolina Austin, Texas

Ira Dyer Hvla S. NapadenskyHead. Department of Ocean lIT Research Institute

Engineering Chicago, Illinois

Massachusetts Institute ofTechnoloov Myron H. Nordquist

Cambridge, Massachusetts Nossaman, Krueger & MarshWashington, D. C.

Phillip EisenbergChairman, Executive Cormmittee David S. Potter

Hydronautics, Inc. Vice PresidentLaurel. Maryland General Motors Corporation

Detroit, Michigan

John F. FlipseDepartment of Civil Engineering Willard F. Searle, Jr.Texas A&M University Chairman

College Station, Texas Searle Consortium, Inc.

Alexandria, VirginiaDavis L. FordSenior Vice President Robert L, WiegelEngineering Science Company Department of Civil Engineering

Austin, Texas University of California, Berkeley

Berkeley, California

Staff

Jack W. Boller, Executive Director

Donald W. Perkins, Assistant Executive Director

Charles A. Bookman, Staff OfficerAurora M. Gallagher, Staff OfficerLinda J. Cannon, Administrative Assistant

Doris C. Holmes, Administrative SecretaryJulia W. Leach, SecretaryTerrie Noble, Secretary

RESEARCH IN SEA ICE M4ECHANICS*--

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4)) ~ ' PANEL N SEA ICE MCHANICS 9 2ASSEMBLY OF ENGINEERING

JN.ATIONAL RESEARCH COUNCILJ

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NOTICE: The project that is the subject of this report was approvedby the Governing Board of the National Research Council, whosemembers are drawn from the Councils of the National Academy ofSciences, the National Academy of Engineering, and the Institute ofMedicine. The members of the panel responsible foT the report werechosen for their special competences and with regard for appropriatebalance. This report has been reviewed by a group pther than theauthors according to procedures approved by a Report Review Committeeconsisting of members of the National Academy of Sciences, theNational Academy of Engineering, and the Institute of Medicine.

The National Research Council was established by the National Academyof Sciences in 1916 to associate the broad community of science andtechnology with the Academy's purposes of furthering knowledge and ofadvising the federal government. The Council operates in accordancewith general policies determined by the Academy under the authorityof its congressional charter of 1863, which establishes the Academyas a private, nonprofit, self-governing membership corporation. TheCouncil has become the principal operating agency of both theNational Academy of Sciences and the National Academy of Engineeringin the conduct of their services to the government, the public, andthe scientific and engineering communities. It is administeredjointly by both Academies and the Institute of Medicine. TheNational Academy of Engineering and the Institute of Medicine wereestablished in 1964 and 1970, respectively, under the charter of theNational Academy of Sciences.

This report rep~esents work supported by Grant NumberN00014-80-G-O 34 be tween the Office of Naval Research and theNational Academy of Sciences.

Copies available in limited quantity from:

Marine BoardAssembly of EngineeringNational Research Council2101 Constitution Avenue, N.W.Washington, D.C. 20418

Printed in the United States of America

PANEL ON SEA ICE MECHANICS

MAX D. COON, Ploy Industries, Incorporated (Chairman)

COLIN B. BROWN, University of Washington

GORDON F. N. COX, U.S. Army, Cold Regions Research and

Engineering Laboratory

TERRANCE D. RALSTON, Exxon Production Research Company

LEWIS SHAPIRO, University of Alaska

WILFORD P. WEEKS, U.S. Army, Cold Regions Research andEngineering Laboratory

PREFACE

For a number of years, the Marine Board has been concerned with thenational ability to support engineering activities in and under theice of the polar oceans. An urgent object of the concern is theprojected increase in operations in Alaskan offshore waters demandingoffshore structures and marine transportation systems, principally

for petroleum exploration and production.An understanding of sea ice mechanics is essential to the design

and integrity of cost-effective structures and fundamental tooperating in the polar oceans. In recognition of this, the NationalResearch Council appointed the Panel on Sea Ice Mechanics, in 1979,

to investigate the available information and research needs in thisarea. The committee defined the scope of its interest to include thewhole state of knowledge of sea ice mechanics, but to stop short ofin-depth analysis of physical and environmental forces, such as wind

and waves, that directly affect the mechanics of sea ice.The panel chose to limit consideration of research needs to the

mechanics of ice masses on the scale of the engineered structuresthat could be introduced into the natural icescape. These may rangein size from a few meters to several hundred meters or longer in thecase of causeways. The panel did not consider mechanics on a largescale of tens of kilometers. The geophysical motions of sea ice, asdriven by winds and ocean currents, that would figure in such large-scale studies are important in determining the loads ultimatelyimposed on engineered structures in a sea-ice environment.Nevertheless, an examination of the mechanics of sea ice and itsoceanic and atmospheric driving forces would involve an investigation

considerably different in character and would emphasize other disci-plines. Likewise, the other extreme of spatial scale, the behaviorof single crystals of sea ice, is not addressed in this study, because

such an investigation would not be linked directly to engineeringactivities.

v

While many of the engineering problems posed by river and lakeIce are similar to those of ice mechanics at sea, the panel chose toaddress basic relationships not made more complex by the variationsattending fresh and salt water or the addition of other constituentsto the sea and ice environment. In regard to the snow-ice (orsoil-ice) problems that can be encountered offshore, the panel choseto treat these as subsets of ice types.

The panel's study was conducted under the guidance of the MarineBoard and with the cooperation of the Polar Research Board. It wassponsored by the U.S. Departments of the Interior, Commerce, State,and Energy, and the U.S. Coast Guard, Navy, the Army Corps ofEngineers, and the National Science Foundation through a generalsupport contract administered by the Office of Naval Research.

The panel met four times--twice in Seattle, Washington, and twiceat the National Academy of Sciences in Washington, D.C.--over aten-month period to review the nature and extent of sea ice mechanicsresearch and to assess the gaps in knowledge of the field that needfurther study in order to advance the safety of structural andfacility design in ice-covered offshore areas. In its assessment,the panel did not suggest research priorities, since many of theinterrelationships between various long-range research efforts andtheir engineering benefits cannot be rigorously established.

This report reviews what is known about the nature of sea ice andits processes, the interaction of ice masses and structures, andrecent and on-going research needed in model ice--using othermaterials to simulate ice characteristics in laboratory-scale tests--and artificially made sea ice. The report draws conclusions andrecommendations in its last section.

vi

SUMMARY

-This report identifies gaps in knowledge and understanding of thephysical and engineering properties of natural and man-made ice. Thepanel offers recommendations for scientific and engineering researchto provide data for the design of offshore and shoreline structures,as well as for the design of marine transportation systems for opera-tions in ice-covered regions. While a leading motivation forincreasing knowledge of sea ice mechanics is Alaskan offshore oil andgas development, research results may be applied to any offshore siteor area affected by ice.i

Sea ice mechanics is-ncerned, in general, with the interactionof environmental forces,P oceanographic and meteorologic origin withan ice cover at se nd the manner in which such forces are trans-mitted to otbt ,;ce masses and eventually to man-made structures.Important aspects of the subject include the mechanical origins ofdef.ortation features, such as cracks, leads, pressure ridges, andu ible fields, and the mechanical properties of ice sheets and the1lk properties of large ice masses.S The interaction of the ice cover with fixed and moving structures

was identified by the panel as an important subject for research.Important ice types include sheet ice, ridges, rubble, fragmentedcovers, frazil ice and brash ice. Additional field observations,analytical studies, and laboratory model studies are needed to betterunderstand the formation, mechanical behavior, an interaction of iceaggregates with engineering structures.--

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Sea ice Is a complicated material, consisting oi trystals of pureice, solid salts, and liquid-filled brine pockets. The mechanicalproperties of sea ice have been studied for many years, but ourunderstanding is still largely qualitative. Most small-scale testresults are inconsistent and gaps remain in the data. Large-scale

properties have proved difficult to measure. Scaling factors for

model tests have not been well defined.

vii

~The panel recommends that laboratory tests be conducted to obtainmechanical characteristics of sea ice with appropriate internalstates. Experiments should be conducted to determine the large-scalemechanical characteristics of natural sea ice cover of known internalstate, and theories should be developed to provide satisfactoryproperties essential for engineering design. \~~---..

The interaction of sea ice types with fixe and moving structuresis fundamental to the basis of the engineering design and operationof offshore structures. The mechanical behavi r of sea ice aggre-gates, as they interact with structures, is no~ well understood.Therefore-, the panel recommends that further ~4owledge of themechanical behavior of sea ice aggregates be, btained through fieldobservations. Laboratory studies should be onducted to betterunderstand the formation and interaction of 'ice aggregates withengineering structures. Analytical studiq~g combining field observa-tions and laboratory experiments with 'basic laws of mechanicsshould be conducted to develop theore cal models of the mechanicalbehavior of various ice types. --

Much of our understandiagc~f ice processes and the interactionsof ice and structures Wil be derived from small-scale model tests.The panel recommends that a systematic research program be conductedto inv~stigate the properties of different materials for modeling icesue-h as synthetics, doped ice, or paraffin. The feasibility of usngmiodel ice to determine the large-scale mechanical properties of ice1features also should be investigated.

The panel concludes that a systematic approach to sea ice researchat the national level will require an integration of government andprivate planning, long-term research, contractual commitments, andthe interpretation of research missions in relation to national needs.The government needs to increase its sea ice research efforts bystating a clear commitment to their pursuit, coordinating the activi-ties of the several agencies with an interest in results, promotingthe dissemination of research results, and attracting more investi-gators Into the field. The panel hopes this report will stimulatethese actions and thus expand the adequacy and availability ofinformation and expertise on sea ice mechanics in the public domain.

The panel recommends that one government agency be designated tolead and coordinate all federal work in sea ice mechanicstechnology. The agency needs to Include in its programs research toanswer, in cooperation with other agencies, the pressing engineeringproblems of sea ice mechanics. To attract additional people to thefield, a long term commitment should be made by the government tosupport sea ice mechanics research. This should include thedevelopment of courses, workshops, symposia, and textbooks In thefield. A single agency should act as a clearinghouse to facilitatepublic access to the results of government and industry research.

viii

TABLE OF CONTENTS

INTRODUCTION 1

SECTION I DESCRIBING SEA ICE 3

Sea Ice as a Material 3Development and Change of Sea Ice Cover 11

SECTION II INTERACTIONS BETWEEN SEA ICE AND ENGINEEREDSTRUCTURES 19

Ice and Modeling Considerations in the EngineeringDesign Process 19

Gravel Islands 22Conical Structures 26Tover/Caisson/Steel Jacket Structures 28Floating Structures 29Submerged Structures 30Artificial Ice and Ice Structures 31Ports and Maritime Operations 32In Situ Stress and Strain Measuring Devices 33

SECTION III STATE OF KNOWLEDGE AND RESEARCH NEEDS 35

Material Properties of Sea Ice 35Properties of Aggregate Masses of Sea Ice 43Mechanical Properties of Model and Artificial Ice 51

SECTION IV GOVERNMENT AND INDUSTRY ACTIVITY INSEA ICE RESEARCH 55

Government Activity 55Industry Activity 57Information and Education 57

SECTION V CONCLUSIONS AND RECOMMENDATIONS 63

Basic Sea Ice Properties 63Properties of Sea Ice Aggregates 64Physical Model Testing Ice 64Government and Industry Activities in Sea Ice

Research 64People and Information 65Additional Studies 65

Ix

REFERENCES 67

FIGURES AND TABLES

Figure I-1 Several Aspects of First-Year Sea Ice Structure 4Figu~re 1-2 Vertical Thin Sections of First-Year Sea Ice 6Figure 1-3 Photomicrography of Horizontal Thin Sections.

of First-Year Sea Ice 7Figure 1-4 Photomicrographs of Sea Ice Showing Details of

Brine Pockets 8Figure 1-5 Typical Salinity Profiles for First-Year Ice of

Various Thicknesses 10Figure 1-6 Variations in Ice Salinity, Temperature,

Modulus, and Strength with Ice Thickness 12Figure II-1 Interactions of a Floating Ice Beam with an

Inclined Structure 21Figure 11-2 Ice-Resistant Platforms for Offshore Operations 23

TABLE 1 Sources of Information on Sea Ice Mechanics 58

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INTRODUCTION

Sea ice covo.rs 10 percent of the earth's ocean during part of theyear (Barry. 1980).1 In spite of the abundance of this naturalmaterial, its mechanics are not well understood. Man, for reasonsof comfort and safety, has preferred to avoid sea ice.

In the next several decades, exploration for energy resources inthe sea off Alaska and Canada, and the possible production of theseresources, will challenge engineering and technology in icyconditions.

The enormity of this region is daunting for its remoteness,environment, and sheer size. The Alaskan coastline is approximatelyas long geographically as the Pacific and Gulf of Mexico coasts ofthe United States.

Recent estimates of the U.S. Geological Survey indicate that ofthe total undiscovered domestic oil and gas reserves, approximately60 percent of the oil and 50 percent of the gas may lie in areas ofoffshore Alaska affected by ice (USGS, 1980).2 The combined areasof the sedimentary basins within which hydrocarbons presumably occur

under the ice-covered arctic waters is several times larger thanthose explored by drilling in the Gulf of Mexico.

The requirements for arctic offshore development emphasize theneed to better understand the nature and interaction of environmentalforces of oceanographic and meteorologic origin and sea ice, and thetransmission of these forces to other ice masses and man-made

structures. Better understanding of sea ice phenomena also wouldadvance shipping, fishing, and other human activities in severelycold climates.

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SECTION IDESCRIBING SEA ICE

This chapter provides a description of sea ice. In order to gain anengineering understanding of sea ice, it is necessary to consider thecharacteristics and mechanics of sea ice at two scales. At the small-scale, sea ice can be described as a material in terms of growth,structure, and composition. At a large-scale, sea ice is an aggre-gate of individual pieces of ice. The aggregate has important uniquecharacteristics and needs to be described in terms of the dynamicsand mechanics of the ice cover.

Sea Ice as a Material

The structure of first-year sea ice is similar to that of a castingot (see Figure I-1). The size and orientation of the firstcrystals that form depend upon the sea state at the time of formation.Turbulence favors the formation of small equigranular crystals whilelarge plate-like crystals grow in calm water. Once an initial iceskin has covered the sea, the crystals grow downward, compete at thegrowing interface, increase in size, and acquire a preferred orienta-tion. Each crystal consists of alternating layers of pure-ice andlayers containing ice and liquid filled brine pockets. The opticalC-axis of the hexagonal sea ice crystal is normal to these layers(see Figure I-1). Those crystals which grow most rapidly have theirC-axis horizontal so that the direction of easiest growth is verticaland parallel to the direction of heat flow. The result is a rapidincrease in grain size in the upper portion of the ice cover associa-ted with a tendency for grains with horizontal or near-horizontalC-axis orientation to become dominant. This portion of the icesheet, where rapid changes in crystal orientation occur, is calledthe transition zone. The base of the transition zone is commonlyless than 30 cm below the upper surface of the ice sheet.

4

Thin Section Fabric

Horizontal Vertical Diagram

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Source: W. F. Weeks

5

Below the transition zone is the columnar zone. It has a fairlyuniform structure with the crystals showing horizontal C-axis orienta-tion and a pronounced crystal elongation parallel to the direction ofbeat flow. Weeks and Assur ( 1967)3 give a number of figures showingthe structure of the initial skin, the transition zone, and the upper-art of the columnar zone. Figure 1-2 shows a vertical thin sectionwhich illustrates these structural elements.

For many years, it was believed that the crystals in the columnarzone invariably showed C-axis horizontal orientations that were randomin the horizontal plane. In fact, such a structure has been document-ed at a number of locations. Such a material would be described astransversely isotropic in that it shows a variation in properties inthe vertical direction, but all directions in the horizontal planeare equivalent. However, recent studies (Weeks and Cow, 1978, 1980;Kovacs and Morey, 1978)4,5,6 have revealed that the great majorityof the ice occurring over the continental shelves of the Arctic showstrong C-axis alignments within the horizontal plane. The apparentfactor in controlling the direction of these alignments is the direc-tion of the current at the ice-sea water interface. These roughlycolinear alignments appear to extend over large distances relative toa typical grain diameter and give the ice a variation in propertiesalong three orthogonal axes.

In addition to these variations in gross crystal shape, size, andalignment, there are pronounced variations in the internal structureof the individual crystals of sea ice. Every crystal is subdividedinto a number of ice platelets that are joined together to produce atype of quasi-hexagonal network as viewed in the horizontal plane.This structure is clearly shown in Figure 1-3, a photomicrograph of ahorizontal thin section of sea ice. Similar substructures are commonin metals produced by the directional solidification of impure alloys(see, for instance, Chalmers, 1964).7 They result from crystalgrowth in which the solid-liquid interface is non-planar, causing theentrapment of impurities within the boundaries of these substructures.In fact, the salt in sea ice is entrapped in brine pockets, or liquidinclusions that are located along these substructures. Figure 1-4shows representative photomicrographs of an array of such brinepockets. The spacing between brine pocket arrays (measured parallelto the C-axis) varies inversely with the growth velocity (Lofgren andWeeks, 1969)8 and is commonly referred to as the plate spacing. Asmight be anticipated, these variations in plate spacing also have aneffect on the mechanical properties of the ice (Weeks and Assur,1963).9

Associated with the variations in the freezing velocity and inthe composition of the sea water being frozen, there are variat!- sin the amount of salt (brine) that is entrapped within sea ice. Theseinterrelations have been studied by Weeks and Lofgren (1967):l0 theyare quite systematic, and are also similar to related occurrences inmetals and ceramics. The salinity of the ice is a linear function ofthe composition of the sea water with very fast growth incorporating

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Photomicrograph of Horizontal Thin Sectionsof First-Year Sea Ice

Source: Schwarz, J. and Weeks, W. F. (1977) "Engineering Propertiesof Sea Ice," J. Glac'iology 19(81), p. 499-531.

8

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Figure 1-4

Photomicrographs of sea ice showing details of brine pockets.The spacing between brine pockets is approximately 0.5 mm.

Source: W. F. Weeks

9

the majority of the salt into the ice and very slow growth resultingin near-total salt rejection. Once salt has been entrapped in seaice, it gradually drains out. This Is a complicated process (Cox andWeeks, 1974, 1975; Niedrauer and Martin, 1979)11,12,13 that is, asyet, only partially understood. Typical salinity profiles forfirst-year ice are given in Figure 1-5. Such profiles show aconsistent C-shape and also change systematically with ice thickness.

As the brine in the ice drains down and out, structural featurescaused by this drainage develop. These are called brine drainagechannels (Lake and Lewis, 1970),14 and can be thought of as tubularriver systems in which the tributaries are arranged with cylindricalsymmetry around the main drainage channels. Near the bottom of thickannual ice, drainage channels appear to occur on a horizontal spacingof 15 to 20 cm and have a diameter of approximately 1 cm. Suchfeatures obviously affect the mechanical properties of sea ice, buttheir effect has, as yet, not been quantified.

The amount of brine in sea ice, or brine volume, v , isdetermined by the ice salinity and temperature. As either the icesalinity or temperature increases, the ice brine volume increasesto maintain phase equilibrium between the ice and brine. Thisvariation is most pronounced near the melting point where smallchanges in temperature result in large changes in the brine volume(Assur, 1958).15

Numerous studies have shown that the strength, modulus, and otherproperties of sea ice are strongly influenced by the ice brine volume(Weeks and Assur, 1969; Schwarz and Weeks, 1977).16,17 The resultsof many sea ice strength tests suggest a relationship of the generalform

ar0

where a ;is the ice failure strength, a0 is the hypotheticalstrength of sea ice at zero brine volume, and c and k are constantswhich depend on the geometry of the brine inclusions. The constant kis commonly found to be equal to 1/2. Thus the strength of sea icevaries as the square root of the brine volume. The results of manymodulus tests also show a brine volume dependency. The modulus ofsea ice, E, varies linearly with the brine volume, that is

E I1- dy

where EO is the hypothetical modulus of sea ice at zero brinevolume, and d is a constant that depends on the geometry of the brineinclusions.

10

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Figure 1-5

Typical Salinity Profiles for First-YearIce of Various Thicknesses

Source: W. F. Weeks

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Since the ice salinity and temperature vary with depth in an icesheet, so do the ice brine volume, strength, and modulus. Figure 1-6illustrates how c~.f and E might vary in 0.2, 0.8, and 3.0 m thick icesheets.

There are also a variety of solid salts that form in cold seaice (Assur, 1958).18 The crystallization temperatures of the twomost common salts are -8.7oC (Na2SO4 .10H20) and -22.70C(NaCl -2H 20). Peyton (1966)19 reported that the crystallizationof these solid salts has a pronounced effect on the mechanical proper-ties of sea ice. However this finding has not been supported bysubsequent work and, until well defined tests are performed, such aneffect will remain speculative (Weeks and Assur, 1969).20

First-year sea Ice, subjected to a period of summer melt, under-goes a pronounced change in salinity. This is largely caused by thepercolation of relatively fresh surface melt water downward into theice (Untersteiner, 1967).21 This iltishing results in salinityprofiles such as that given for the 3.0 m thick ice in Figure 1-6.There is also the possibility of recrystallization in the upper partof such ice floes. Ice that has survived a number of summersultimately becomes a layer cake of the annual layers that are formedduring successive winter periods of ice growth. Ultimately multi-year ice reaches an equilibrium (approximately 3 to 4 m in thecentral Arctic Ocean) such that the thickness ablated during thesummer equals the thickness grown during the winter (Maykut andUntersteiner, 1969).22 Considering the vast amount of multi-yearice present In the high Arctic, Its properties have been littlestudied. In particular, the strength data appear to be contradictoryIn that some results suggest that multi-year ice is appreciablystronger than first-year ice while other data show little difference.

The Impression of differences between the properties of ice andthose of other materials is, to a large part, the result of the factthat sea ice exists in nature and is usually studied at temperatureswithin a few degrees of its melting point while experience with othermaterials is commonly at temperatures far removed from their meltingpoint. Studies of sea ice may provide insight into the behavior ofmany other materials at or near their melting point.

Development and Change of Sea Ice Cover

The traction of wind and currents on an ice cover causes it tomove and deform resulting in the formation of cracks, leads, pressureridges, and rubble fields. Cracks and leads may freeze and producenew areas of thin ice, while pressure ridges and rubble fields mayconsolidate and result in areas of much thicker ice. Icebergs andice islands from glaciers and ice shelves are also incorporated intothe ice and further increase the variation and complexity of the icecover. In coastal areas and shoals, large ice features ground on thesea floor and, as a result of movement of the surrounding ice sheet,they produce gouges on the sea floor. Ice can also pile-up on orcompletely over-ride coastal features.

12

Icti Temperature, *C /.ad0/

* -Temp

I -- SalinityE/2.0- . -E

lee Salinity

Figure 1-6

Variations in Ice Salinity, Temperature,Modulus, and Strength with Ice Thickness

Thickness of 0.2,1 0.8, and 3.0 m, arctic sea ice, 1 Hay.

Source: Schwarz, J. and Weeks, W. F., (1977) "Engineering Propertiesof Sea Ice," J. Glaciology 19(81).

13

Sea ice can take many forms. Level ice occurs in the range ofsizes from continuous sheets to dispersed, fragmented covers ofdiscrete, relatively small floes. Deformed ice occurs as pressureridges, rubble fields, and floebergs. The ice forming any of thesefeatures can be either first-year or multi-year ice. To enable thedesign process, and engineer needs to understand the bulk mechanicalproperties of both first-year and multi-year ice.

The features can be divided into two categories; the firstcategory is consolidated features, such as sheet ice, large floes,multi-year ridges, and floebergs. The properties required for thesefeatures are those of the various constitutive laws that are used todescribe the deformation of a continuum. The remaining features,such as first-year pressure ridges and rubble fields, and fragmentedice covers, constitute a second category which is essentially uncon-solidated aggregates of fragments of ice of various sizes. In thesecases, it is the bulk properties of the aggregate which is ofinterest to the research person and engineer.

Once a sea ice cover has formed, it commonly undergoes a varietyof processes that may produce significant changes in both its externaland internal structure. The most important of these processes aremechanical, as compared to the essentially thermodynamic processesdescribed earlier, and are largely the result of the differentialmotion of one segment of ice relative to another.

If sea ice were motionless, ice thicknesses would be controlledcompletely by the thermal characteristics of the lower atmosphere andupper ocean. Such an ice sheet would have a thickness and physicalproperties that change slowly and continuously from region to region.However, even a casual examination of almost any area of sea icereveals striking lateral changes in ice thicknesses and characteris-tics. These changes are invariably caused by ice movement. For aninitially continuous piece of sea ice to move away from the shore, abreak must occur that will allow separation. This occurs with theformation of cracks. If a crack opens to a width sufficient to saila ship through, the resulting open water area is referred to as alead. There have been few detailed studies of such cracks in sea ice(Evans and Untersteiner, 1971; Evans 1971),23,24 but some knowledgeof their behavior has been gained from field observations. The cracksform very rapidly (possibly propagating at near-sonic speeds); theyappear to be insensitive to ice thickness in that they cut rightthrough areas of thicker ice without noticeable deflections, and theycan extend for tens, and perhaps hundreds, of kilometers. The factthat a crack has formed does not necessarily mean that a lead willdevelop. Many cracks separate a few centimeters and then refreeze.However, this also is a subject that has been little studied. Duringmost of the year in the polar regions, once a lead is formed it isimmediately covered with a thin skin of ice that thickens with time.Therefore, in a region of pack ice where leads are continuallyforming, the development of leads provides a mechanism for generatinga wide variation in ice thicknesses with the thickest undeformed ice

14

being the oldest. The compass orientation of a set of leads maychange over a period of a few hours as the result of the movement ofweather systems; one set of leads may become inactive and be replacedby another set oriented in quite different directions.

Just as important as the opening of leads is their closing. Whenthis occurs, the thin ice in the leads is broken and pushed into avarietie of thlese fe-auresse ofwihes chceristicall nmefrvarietye of piles so-allred, prssue rwiges hrerestialnumbermoin thin ice, some in thick. In a general way, there are two primarytypes of pressure ridges: p-ridges caused by closures essentiallynormal to the lead and s-ridges (so called shear ridges) are producedby motion parallel to the lead. These two ridge types show a numberof characteristic differences. P-ridges have a sinuous surface traceand are composed of blocks having dimensions related to the thicknessof the ice being incorporated into the ridge. Such ridges are alsooften associated with large (many tens of meters) over- andunder-thrusts of the interacting ice sheets. S-ridges are straighterand characteristically have one vertical side. They are composed,primarily, of highly granulated ice a few Centimeters or less indiameter, and variable quantities of rounded ice "boulders" of dimen-sions up to the thickness of the ice sheets from which the ridge wasformed. The properties of the ice in ridges will be discussed later,but a few points should be made now. In a general way, p-ridges areprimarily composed of the thin (or thinner) ice from refrozen leads.This is a simple reflection of the fact that when the ice pack comesunder compression, it is the weakest (thinnest) material that is de-formed first. However, as a lead rarely closes exactly as it opened,there are commonly areas of the thinner lead ice left even afterrather severe deformation. Generally, larger ridges are composed ofthicker ice, and occasionally very thick multi-year floes interacttogether to produce ridges.

Extensive rafting usually occurs in the vicinity of p-ridges. Itis not uncommon to find rafted ice thicknesses 2-4 times the sheetthicknesses within a distance of a few hundred meters of the ridge.In some cases, the raf ted ice is broken into blocks and incorporatedin the ridge.

The heights of ridge sails and the depth of ridge keels have beenstudied using laser and sonar profilometry respectively (Weeks etal., 1971; Tucker et al., 1979).25,26 The distribution functionsare exponential--there are many small ridges and large ridges arerare. The largest free floating sail height and keel depth (not thesame ridge) currently on record are 13 m and 47 m respectively(Kovacs and Mellor, 1974).27 In general, keel depth statisticsappear to be scaled up versions of sail statistics (which have beenmore extensively studied since it is easier to obtain such data) witha depth to height ratio of 4 to 5/1.

15

When pressure ridges survive a melt season, the interbiock voidsthat characterize newly formed ridges are first filled with relativelyfresh water produced by surface melting of the ice. This water thenfreezes, bonding the ice blocks together into massive low-salinitymulti-year pressure ridges. (Kovacs et al., 1973; Wright et al.,1978).28,29

In many cases ice deformation does not result in discrete ridges;instead the complete ice sheet is converted into a wasteland of heapsof ice blocks (Kovacs, 1972).30 This type of chaotic terrain iscalled a rubble field. Many times the surface relief on a rubblefield is only two to four meters high. However, a field may be com-posed of rows upon rows of ridges like the furrows of a plowed field,each row having six to eight meter sails. When such large rubblefields survive a summer, they become welded into extremely massivebodies of low salinity ice. Such formidable ice masses have beenstudied off the Beaufort Sea coast where they were grounded in17-18 m (56-59 ft.) of water and had a maximum freeboard of 12.6 m(41 ft.) (Kovacs, 1976).31 It is generally believed that largerubble fields and shear ridges are more frequent near coasts thanwithin the main pack. However, there are no quantitative observa-tions on this subject.

Recent work has also shown that, at least off the North Slope ofAlaska, the ice within approximately 150 km of the coast is morehighly deformed than the ice in the central pack of the Beaufort Sea(Tucker et al., 1979; Wadhans and Horne, 1978).32,33 Aircraftobservations (Wi.2inann and Schule, 1966;34 suggest that this isgenerally true along the margins of the Arctic Ocean from northeastGreenland to the Chukchi Sea.

A particular problem associated with near shore ice conditions iscaused by grounding. Such groundings plough deep gouges in the seafloor sediments (Barnes and Reimnitz, 1974; Lewis, 1977).35,36Gouges in excess of one meter are fairly common. Lewis (1977)37reported rare gouges in the Canadian Beaufort Sea of up to 7.6 m. inaddition, when an ice pile-up grounds, the fact that it is buildingupon a solid foundation may allow the sail to pile-up to impressiveheights of 30 meters or more. Also, major pile-ups can occur onbeaches, and at times ice can completely override low islands. Auseful summary of observation and theory relative to the occurrenceof such events can be found in Kovacs and Sodhi (1980).*38

Brief mention should also be made of ice islands and icebergseven though they are ice in the sea and not sea ice in the true sense.Nevertheless, they do pose significant hazards in areas where theyoccur. The origins of icebergs are well known, and they are largelya problem of the eastern Arctic, in particular in the region betweenGreenland and Canada, where they present major problems to offshoreoperations. The "icebergs" of the Arctic Ocean are called iceislands. These are, in fact, fragments that have broken off a relictPleistocene ice shelf that exists along the north coast of EllesmereIsland In the Canadian Archipelago. The best known ice island, T-3,

16

has been drifting around the Arctic Ocean for over 30 years. Thethicknesses of ice islands and ice island fragments are quite variableas they gradually ablate during their drift. T-3 showed an initialthickness of approximately 70 m. The lateral dimensions are alsohighly variable, ranging from in excess of 10 km in the case of thevery large examples such as Ward Hunt 5, which at one time completelyblocked Robeson Channel between Greenland and Ellesmere Island, to afew tens of meters for ice island fragments. No detailed studieshave been made of the mechanical properties of the ice comprising iceislands. Much of this ice reportedly has many similarities withglacier ice which has been well studied. However, a wide variety ofice types have been noted (Smith, 1964),39 some of which resemblelake ice and some sea ice. Perhaps the most needed information onice islands is improved observational data on their number, location,and size distribution. This would allow realistic estimates to bemade of the encounter probabilities between such features andoffshore structures.

Finally, some mention should be made of the rates at which seaice could be expected to move against structures. The drift andacceleration rates of an ice mass are the result of wind drag, waterdrag, wave forces, and interaction with other ice masses. Thus,knowledge of ice drift need not be totally dependent on directobservation; it can be bolstered by modeling of these forces. Aswould be expected, there are wide variations in observed sea icedrift rates with mean annual net drift rates varying from 0.4 to4.8 km/day with the actual rates (including loops and other i rre ular-ities) as high a 2.2 to 7.4 km/day (Dunbar and Wittmann, 19 63 ).46Most studies of drift rates involve ice located within the main packof the Arctic Ocean. In general, speeds are low in the winter whenthe ice cover is tight and high in the summer when the pack opensup. Highest drift rates are invariably found near the edge of thepack where the ice is moving under nearly free-drift condi-.1s, th&-iis where there are few ice to ice interactions. For insK,,iee, duringstorms ice drift rates of as much as 40 kilometer per day have beennoted in the Bering Strait (Shapiro and Burns, 1975),41 and similarrates have been observed near the southern part of the East GreenlandDrift Stream (Wadhams, 1980).42 For most offshore operations dataare required for near shore areas, and some information is availableon this subject. For instance, off the Beaufort Sea coast of Alaska,data have been collected by bottom connected systems in areas of fastice (Agerton and Kreider, 1979),43 radar ranting systems in thevery near shore pack (Tucker et al., 1980),4~ and satellitepositioning systems further offshore (Martin et al., 1978).45Offshore from the barrier islands, which line much of this coast,movements during the winter are small and tend to occur in discreteevents associated with strong offshore winds of several daysduration. In protected fast ice, motions are commonly less than afew tens of meters over an entire winter and also usually occur in

17

short discrete events. However, even during the winter, major icemovements can occur at some locations during storms. For instance,Shapiro (1975)46 has observed ice drift velocities of 8 km/hr. fora 5-hour period at Barrow during the winter (associated windvelocities were as high as 90 km/hr.).

SECTION IIINTERACTIONS BETWEEN SEA ICE AND ENGINEERED STRUCTURES

This section describes the interactions between sea ice and engineeredstructu~res from an engineering standpoint. It identifies the physicalproperties and processes that must be considered in designing struc-tures for a sea ice environment. In the next chapter the state ofknowledge of sea ice mechanics is reviewed, and research neels areidentified.

Ice and Modeling Considerations in theEngineering Design Process

When designing for a sea ice environment, the engineer is con-cerned with the mechanical properties of an undeformed ice sheet, andalso with the large scale mechanical properties of the ice cover andbulk properties of ice features. The ice deformation processes andproperties of interest depend on the type of ice feature and type ofstructure, vessel, or equipment in contact with the ice.

Design criteria for new engineering concepts are generallydeveloped by a combination of field measurements and experience,analytical work, and model tests. Field experience applicable toarctic structures is available from the installation and maintenanceof lighthouses and bridge piers in northern waters, the Cook Inletproduction platforms that were installed in the mid 1960's, artifi-cial islands that have been used for exploration drilling in theBeaufort Sea, floating drilling operations in the deeper waters ofthe Canadian Beaufort Sea, and floating ice platforms between theCanadian Arctic islands. The characteristic ice conditions that areapplicable to the design of a particular structure depend on thelocation and intended use of the structure as well as the physicalcharacteristics of the structure.

Analytical estimates of ice forces are usually based on someconstitutive assumptions for the ice, environmental driving force

estimates, a geometric description of the ice, and the boundarycondition between the ice and the structure. Analytical approachesfrequently strive to bound, rather than predict, ice forces, since

19

20

the amount of information needed to establish a bound is substan-tially less than that required for a detailed description of theactual process. If the bound is not overly conservative, it may beadequate to establish a design load.

As an example, consider a floating ice beam that is pushedagainst an inclined structure. There are several possible ways inwhich the ice beam could fail. Three possibilities illustrated inFigure Il-1 are:

1. The ice could fail in bending as it slides up thesurface of the structure.

2. The ice could fail by crushing if the in-plane stressesreach the crushing load.

3. The Ice could fail by buckling if the in-plane stressesreach the buckling load.

Each of these failure modes would require different information onthe ice material characteristics in order to quantify the force thatwould be associated with the failure mode. In order for any of thethree to actually occur, it is necessary that the ice be sufficientlystrong so that other failure modes (including possibilities that maynot be listed) would not occur prior to reaching the load requiredfor the failure mode under consideration. It is also necessary thatthe environmental driving force or driving displacement be suffi-ciently great to create the failure mode. Hence, quantifying theload needed for any of the possible failure modes establishes a boundfor the actual ice failure load, and the lowest calculated load wouldbe the closest to the actual load. The conservativeness of eachbound would depend on the values of the particular parameters. Thebuckling load in the above example is proportional to h 3/2, whereh is the ice beam thickness, and the crushing load is proportionalto h. The slope angle and ice/structure friction coefficientsignificantly affect the bending load. The bending failure modewould be eliminated for a sufficiently steep slope and the bucklingand crushing possibilities would remain. A thin ice beam would beexpected to buckle and a force estimate for the buckling mode wouldbe most appropriate. A thick ice beam would be expected to crushrather than buckle.

The bending failure load would be affected by the frictionalsliding resistance at the ice/structure contact region and by thebending strength of the ice in the presence of the in-plane load.Likewise, the crushing load would depend on the ice crushingstrength. Since ice strengths are rate dependent, both failure loads

21

InclinedStructure

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22

would depend on the rate at which the environmental driving forcesmoved the beam against the structure. The buckling load, on theother hand, does not depend on ice strength properties. It isdetermined by the constitutive behavior of the ice prior to failure,e.g., knowledge of the elastic modulus would be sufficient for anelastic idealization of the buckling load.

Model tests are generally conducted to address a particularfailure mode. The ice properties that are applicable to the failuremode are identified and scaled by the model ice material. It is notgenerally possible to adjust all model ice properties to the samescale factor. However, even if all properties are not exactlyscaled, if the model failure mode is identical to that of theprototype, and the model ice properties are measured, model testresults still provide valuable information for verifying analyticalpredictions. Most model studies have addressed ice bending failureagainst structures with the model material properties adjusted torepresent the bending strength and bending modulus of the ice. Thiswork has been a relatively straightforward extension of prior workfor ships in ice. Modeling difficulties typically arise when aparticular failure mode is not well defined. The process thatcreates an ice rubble pile is an example of a mixed-mode failure inwhich different ice properties may be important at different stagesof the process.

Gravel Islands

Gravel islands, such as that shown in Figure II-2(a) have beenused for exploration drilling platforms in both the Canadian andAlaskan Beaufort Sea. Seventeen man-made islands have been con-structed in the MacKenzie Bay area in water depths ranging from lessthan two meters to as deep as 20 meters. Three man-made gravelislands have been used as drilling platforms in the shallow nearshore area of the Alaskan Beaufort Sea and similar designs have beenproposed for application in the December 1979 lease area (TechnicalSeminar, 1979). .7

The surface geometry of man-made islands have been eithercircular or rectangular, depending on the construction season and thedesired layout of the drilling equipment. Typical working surface'dimensions are of the order of 100 m to 150 m. The slope of theisland beach depends on the construction mode and may range from 1:3for a sandbag protected beach to 1:15 or greater for an unprotecteddredged beach.

Ice conditions near gravel islands consist of extensive movementsof thin ice during freeze-up, of rotten ice during break up, and slowice movements of limited extent in the stable winter landfast ice.The ice activity during freeze-up depends on location. In relativelyprotected areas, freeze-up ice movements may create a small ridge orrubble pile, or some override of thin ice on the island beach.Freeze-up movements can be very extensive in exposed areas, and createice ride-up on island beaches or large rubble piles surrounding the

23

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island. The rubble may be several island-widths in diameter and havean irregular surface topography. Local heights of rubble 13 m abovethe water surface have been observed at some locations (Kry,1977).48

Winter Ice conditions around gravel islands have, in general,been stable, with slow ice movements of limited extent. Typicalevents may consist of ice movement rates of a few feet per hour andexcursions of the order of tens of feet. The movement of the winterice creates an active zone of Ice failure at the exterior of therubble pile. Since the rubble surrounds the island, the total forcecreated by ice failure in the active zone is balanced by the sum ofthe force transferred through the grounded rubble to the sea floorand the force transmitted through the rubble to the island. Fieldmeasurements of ice forces in the smooth ice near the active zonehave been made at a number of islands. However, there Is littleinformation available on transfer of forces through the rubble andinto the sea floor and Island (Technical Seminar, 1979).49

Ice breakup activity usually occurs in late June and early July.At this time, the ice has partially melted and is in a warm deteri-orated state. The deterioration usually begins near river outletsand along the coastlines. After the ice is essentially free of thecoastline, large movements of the deteriorated ice can occur inresponse to the local wind conditions.

Design Considerations and Ice Properties

The two principal ice design considerations for gravel islandsare ice forces and over-ride. During freeze-up the ice propertiesrelevant to the ice/structure interaction include the following:

Ice/Beach-Ice-Rubble Friction CoefficientsThese factors limit: (a) the ability of ice toride up the slope of an island, (b) the extentof Ice rafting, and (c) the ability of an icesheet to over-ride a rubble pile. The ice/beachfriction coefficient would depend on the beachprotection material.

Bending Strength/Buckling PropertiesThe ice rubble formation process creates bendingand buckling failure of the sheet ice.

Unconsolidated Rubble Bulk PropertiesThe bulk strength and deformation characteris-tics of the newly-formed rubble, together withthe local ice movement characteristics, determinemine the geometry of the rubble pile that willsurround an island during the winter season. Theresistance of the grounded portions of the rubbledetermines the extent of rubble formation thatwill remain at the island following Ice movementsf rom several directions.

26

Winter ice loads are developed by movement of the sheet iceagainst the rubble pile that surrounds the island. The properties of

interest are:

Large-Scale Compressive Sheet Ice StrengthsIf the rubble pile is sufficiently competent,the load applied to the rubble/island combi-nation can be limited by the large-scalecompressive failure of the ice. In additionto needing to know the unconfined compressivestrength of sheet ice, the two-dimensionalstrength response that develops under naturalice deformation processes also needs to bedetermined.

Large-Scale Buckling Strength of Sheet IceThe large-scale buckling strength of sheet icealso limits winter ice loads.

Sheet Ice Bending StrengthIf the rubble is not sufficiently competent, thesheet ice will penetrate the rubble and be sub-ject to out-of-plane loads and bending failure.

Ice Rubble Bulk PropertiesThe bulk strength and deformation character-istics of the partially refrozen ice rubbletogether with the rubble pile geometry determinethe relative proportion of the applied ice sheetload that is transmitted to the island and tothe sea floor. The bulk rubble properties alsodetermine whether the ice sheet will penetrate orcompress the rubble or crush at the sheet ice/rubble boundary.

Conical Structures

As indicated in Figure l1-2(b), (page 23), conical structuresemploy an inclined surface to apply vertical loads to floating iceand create bending failure. This approach uses the relative weaknessof ice bending strength compared to its crushing strength and leadsto a significant reduction in forces for conical structures comparedto vertical structures. Field experience with conical-shapedstructures is available from several decades of lighthouse operationsin the St. Lawrence waterway (Danys, 1977)50 and actual fieldmeasurements from an instrumented test structure off the coast ofnorthern Japan (Oshima et al., 1980).51 Conical structuresdeployed in the Arctic would be exposed to considerably thicker icethan that experienced by existing structures and several projects

27

involving model tests, (Arctec, Inc., 1972)52 analysis, (Ralston,1980)53 and design studies (Jazrawi and Khanna, 1977; Hancock andVan Scherpe, 199545 have been conducted in recent years toaddress arctic applications. A typical base dimension of an arcticcone structure is of the order of 100 m. A typical waterlinediameter would be of the order of 30 m. The angle of the conicalslope and its ice/structure frictional characteristics determine theratio of the horizontal and vertical forces that are applied to thestructure. The total ice force can be thought of as consisting of anice breaking force and an ice clearing force. The breaking forcearises from the failure of the floating ice that advances against thestructure while the clearing force results in the removal of thepreviously broken ice from the surface of the cone.


The ice features that a conical structure would be designed toresist depend on the geographical location. First-year ridges,rubble fields, and annual sheet ice represent winter ice designconditions throughout most of the Bering Sea. In the Chukchi andBeaufort Seas, multi-year ice in the form of floes and ridges ispresent along with annual sea ice features. Ice loading can occurat any time of the year in areas that are subject to summer iceinvasions.

The principal design considerations that result from these Iceconditions are the horizontal and vertical ice forces and over-turningmoment and the distribution of local ice pressure in the ice-contactregion. The design of the upper surface of the structure would con-sider the kinematics of ice clearing to minimize ice jamming and todetermine the elevation of the deck. The ice properties of interestinclude the following:

Large Scale Bending StrengthThe full-thickness bending strength of annualsheet ice, consolidated rubble and raftedice, multi-year floes, and multi-year ridgesis directly related to the forces imposed onan inclined structure. The magnitude of thein-plane load that is applied along with thebending moment would depend on the geometryof the structure.

Ice/Structure Friction CoefficientThe sliding resistance between the ice andthe surface material of the cone has asignificant effect on the magnitude of thehorizontal ice force.

28

Ice/Structure Adfreeze StrengthThe advantages of ice bending failure on inclinedstructures may not be achieved if a frozen bondbetween the ice and the structure prevents bend-ing failure. The magnitude of this frozen bondmay be controlled by the use of special coatingsor heating.

Warm Ice Crushing StrengthsThe distribution of ice contact pressures on thesurface of an inclined structure depends on thecrushing behavior of the ice. In particular, thestrength of near-melting sea ice which occurs onthe bottom of a multi-year ridge will determinethe pressure distribution when the ridge initial-ly contacts the structure.

Tower/Caisson/Steel Jacket Structures

In mild sea ice areas, such as the southern Bering Sea, produc-tion platforms similar to those used in Cook Inlet may be constructed.Fourteen permanent production platforms were installed in Upper CookInlet between 1964 and 1968. Thirteen of these platforms resemblethe Jacket-type structures found in the Gulf of Mexico. Eleven arefour-legged structures and two are three-legged structures. To reducethe effects of ice loading, the cross bracing on these structuresIs omitted in the vicinity of the waterline. In order to gain therequired strength, the legs of these structures are larger thannormal--typically of the order of 4.5 meters diameter. The structuresare pile founded with each leg containing from eight to twelve piles.The piles also serve as conductors for the wells. The fourteenthplatform in the inlet is the Monopod, the platform shown in Figure II-2(c), (page 24). The deck of this structure is supported by a singlecylindrical shaft 8.7 meters in diameter, which is attached to a largebase. The foundation support is provided by pilings that are attachedto t'ie base or driven through the walls of the cylindrical shaft.

Cook Inlet is noted for its dynamic winter ice conditions. Icemovement within the inlet is dominated by the 6- to 8-knot tidal cur-rents that move the ice rapidly against the legs of the structures atstrain rates where the ice is well into the brittle range. Thus theentire range of ice strain rate loading conditions Is experiencedwith each tidal cycle. The highest ice forces have been reportedduring the slack tidal period when the ice movements are relativelyslow (Goepfert, 1969).56

The largest ice forces are generated by pressure ridges in CookInlet. These peak loads are reported to be two to three times thesheet ice loads (Blenkarn, 1970).57 Ridge thicknesses of the orderof 3 to 6 m are observed in the Inlet. The ridge forces result from

29

the consolidated, or solid, portion of the ridges. Presumably, theconsolidation is a combined result of the re-freezing and bonding ofrubble in the upper portion of the ridge and the effect of rafted icewithin the ridge.


The principal ice design considerations for tower-like structuresare ice pressure ridge forces, dynamic ice/structure interaction forboth sheet ice and ridges, and possible ice jams between the legs ofmulti-leg structures. The ice properties of interest include thefollowing:

Bulk Pressure Ridge PropertiesThese properties, including those of re-frozenrubble and raf ted ice, are most significant indetermining the maximum ice forces. They alsohelp determine the leg spacing necessary toprevent ice jams from interferring with the iceclearing action between the legs of a multi-legstructure.

Sheet Ice Compressive StrengthsThe rate-dependent aspect of sheet ice strengthin sequential failures is applicable to dynamicice/structure interaction.

Floating Structures

Floating structures have application as exploration and produc-tion platforms in deep ice covered water. The use of floating iceplatforms for exploration drilling between the Canadian Arctic islandsis an example of a floating structure which resists vertically appliedloads but does not resist lateral forces. The mechanics of floatingice structures will be discussed later in this section under the topicof "Artificial Ice and Ice Structures." The use of an ice resistantfloating caisson structure illustrated in Figure I1-2(d), (page 24)has been proposed for deep water drilling and production applications(Gerwick and Jahns, 1979).58 The conceptual design included adouble-cone configuration near the waterline to break ice in bendingand found the downward breaking capability to be most effective inreducing ice loads. Other features, such as actively induced heaving,have also been investigated in model tests.


The major ice design consideration for a floating structure is tocontrol ice forces so that the motion of the structure remains withinits operational limits. The ice features of interest are sheet ice

30

and pressure ridges. The ice properties of interest are similar tothose of conical structures, for example:

Bending Strength (either upward or downward)The bending strength and ice/structures frictioncoefficient would control the sheet ice loadsduring extensive sheet movements.

Adfreeze StrengthIf the ice is allowed to freeze to the structureduring extended periods of no motion, thestrength of the adfreeze bond can be significant.

Bulk Ridge PropertiesThe largest loads would be expected to resultfrom pressure ridge forces.

Submerged Structures

Submerged structures such as pipelines, well-heads, and waterintakes, can either be protected from ice action by burial or bedesigned to withstand submerged ice forces. Exploration wellheads,from floating drilling operations in the Canadian Beaufort Sea, havebeen placed in an 11 meter-deep hole in the seafloor with the top ofthe wellhead 4 meters below the original mud line to prevent damageto ice keels (O'Rourke, 1979).59 The selection of a burial depthfor either pipelines or wellheads depends on the local ice and soilconditions which limit the ice scour depth. An arctic seafloorpipeline has been completed at Panarctic's Drake F-76 gas well offthe east side of Melville Island (Marcellus and Palmer, 1979).60 Inthe near shore area, the pipeline was buried and backfilled. Thepipe bundle was strengthened by circulating a coolant to artificiallyfreeze the soil below the pipeline and the backfill above it. Testsindicated that the compressive strength of the frozen soil was 100times that of the unfrozen soil.


Warm ice properties are of interest for estimating loads appliedto submerged structures. The types of properties (compressivestrength, bending strength, etc.) would depend on the geometry of theice feature and that of the structure, but the temperature of the icewould be expected to be very close to the water temperature. Depend-ing on the geographic location, the properties of both first-year iceand multi-year ice, as well as the bulk properties of ridges andrubble, are of interest to the engineer.

The presence of free-floating ice crystals in the water columnpose potential clogging and other operational problems for waterintakes located in northern environments. Most intake field

31

experience is available for the operation of fresh water rather thansea water intakes; however, plans exist for the construction of a seawater intake near the Prudhoe Bay oil field. Long term developmentof offshore oil reservoirs would possibly require similar sea waterintake capabilities.

Artificial Ice and Ice Structures

Artificial sea ice has proven to be a useful constructionmaterial and a solution to several engineering problems. Artificialice structures are constructed by the controlled flooding of seawater on the ice surface. This technique has been used to build anumber of structures, including ice roads and bridges (Kivisild etal., 1978);61 ice runways and aircraft parking aprons (Dykins etal., 1962;62 Dykins et al., 1962;63 Kingery et al., 1962);64ice wharves (Barthelemy, 1975;5 Kirkpatrick, 1974;6 floatingice drilling platforms (Baudais et al., 1974;67 Masterson andKivisild, 1978);68 and grounded drilling ice islands (Utt,1978;69 and Cox, 1979).10


In many applications, the design and construction of these icestructures requires an understanding of the constructed ice thermaland mechanical properties of the constructed ice, and for floatingice structures, the mechanical behavior of surrounding ice sheet. Aknowledge of the thermal properties is often needed to predict icebuildup rates and the temperature and strength of the resulting ice(MacKay, 1970;71 Cox, 1979).72 As brine has been found to accumu-late at the constructed-natural ice interface (Masterson and Kivisild,1978),73 a better understanding of brine migration and removalwould be helpful. For floating ice structures, the bearing capacityand deflection of the surrounding ice under both static and dynamicloads are important (Masterson and Kivisild, 1978;74 Kivisild etal., 1978) .7 The feasibility and design of grounded ice structuressubjected to lateral ice loads requires a knowledge of the shearstrength of the high brine volume constructed ice near the base ofthe island and the sliding resistance (coefficient of friction) ofthe island on the sea floor (Cox, 1979).76 Specifically, shearstrength data are needed for flooded ice, flooded-snow ice, and iceconstructed by spraying. Tests should be conducted to determine theeffects of strain rate and confinement. Such tests are difficult toperform in that data are needed for warm, saline ice which isdifficult to sample, machine, and test without significant brinedrainage.

32

Ports and Maritime Operations

Ice problems associated with port and terminal operations innorthern environments can be classified as operational problems andstructural problems. The structural problems are similar to thosepreviously discussed for other types of offshore structures. Theoperational problems are mainly those associated with the managementof broken ice to minimize interference with vessel operations(Cammaert et al., 1979).77 Ice management techniques can be thoughtof as procedures for ice suppression, icebreaking, ice removal, andice diversion.

Techniques for ice suppression usually involve transporting heat,providing insulation, or accelerating natural processes (e.g., icedusting) to inhibit or suppress the growth of ice. The thermalproperties of sea ice are relevant to these techniques.

Ice breaking provides a channel for vessel passage. The use oficebreaking vessels is the most proven technique. However otherpossible approaches include air-cushion vehicles, mechanical cutters,chemicals, steam, and explosives. The mechanical properties of icethat are important for these techniques include those relevant toicebreaking as well as the specific cutting and fracture energies.

The mechanical properties of interest in ice removal processesare the bulk properties of ice rubble. The piece sizes of interestmay range from the large pieces produced by an icebreaker to icechips contained in a slurry that may be piped from an ice managementarea.

Ice diversion is generally a rigid body process which would berelatively independent of ice properties, however specific proce-dures, such as the use of ice anchors, may require some knowledge ofice properties.

Vessel operations in ice covered waters are concerned with routeselection and feasibility and the interaction of vessels with theice. The statistical description of ice coverage, ridge distribu-tions, ice physical characteristics, and internal pressure state arerelated to route selection; however these questions are beyond thescope of problems addressed by this panel.

The main thrust of sea ice mechanics in aiding maritime operationsinvolve, the interaction of vessels with the ice. The problems areassociated either with vessels actively moving through an ice coveror with stationary vessels (e.g., moored at a terminal), which areloaded by a moving ice field.

The forces exerted on a vessel moving through a level ice sheethave been thoroughly studied and are reasonably well understood.Route selection procedures generally strive to avoid ice ridges andselect a route through level and, if possible, thin ice. Ice ridgeinteractions with vessels are not as well understood. When avoidanceis not possible, vessels may transit a ridged area by sequentiallyramming the ridges until breakthrough is accomplished. Suchprocedures are costly in terms of both time and fuel.

U11

33

Vessels can be successfully moored in the presence of ice,provided that the ice cover is sufficently veil broken. Cantmar'sfloating drilling operations in the Canadian Beaufort Sea providesome field experience with moored vessels that are subjected tomoving ice fields (O'Rourke, 1979).78 Their operational procedureis to actively create a sufficiently fragmented ice cover such thatthe resultant loads on the vessel are less than the mooring linecapabilities.

In addition to the over-all ice forces, which determine opera-tional capability, local ice pressures on the hull of a vessel areimportant for detailed design consideration. Considerable uncertaintyremains in the specification of local ice pressure magnitudes as afunction of loaded area for ice breaking vessels. This criteriondetermines the local framing and hull thickness needed to transferthe local loads developed on the hull.


Sheet ice properties (bending strength, bending stiffness, andcrushing strength) are of interest for normal vessel operations inlevel ice and for estimating local ice pressures acting on the hull.The aggregate properties of fragmented ice covers are of particularimportance for determining mooring requirements for stationaryvessels subjected to moving ice fields. The bulk properties of iceridges, and a description of the mechanics of their interaction withvessels, are needed for optimal route selection and for performancepredictions for areas that are subjected to ice ridge and rubbleformation.

Significant sea ice mechanics research areas to improve maritimetransportation operations are: the interaction of ridges and non-level ice with vessels; the moving aside of broken ice by a vessel;passage of vessels through broken ice--normal pressures and resultingfriction, in which brash ice is one element; ice pressures on vessels;mechanics of ship propellor-ice interactions; brash ice characteris-tics, including friction, refreezing rates, mechanical properties;and, hull-ice impact forces, mechanisms, and magnitudes.

In Situ Stress and Strain Measuring Devices

Stress and strain measurements made in the field provide funda-mental information on the natural processes that occur in sea ice(Templeton, 1979; Prodanovic, 1978).79,80 The basic measurementproblem--how does the presence of the transducer affect that which isbeing measured--differs for the two kinds of measurements. Strainmeasurements are achieved by measuring displacement over a known gaugelength with the Intent of keeping the disturbance caused by thepresence of the transducer to a minimum. Surface strain measurementsare relatively straightforward, however internal strain measurementspresent some difficulties associated with Installation and interactionof the ice with the transducer.

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In situ stress measurements are made with an embedded sensor.Since the mechanical properties of the sensor are different fromthose of the ice, the transducer represents an inclusion in a hostmaterial. The stress measured by the sensor is the result of theinteraction of the sensor with the ice. The extent of the interactionis generally localized and the undisturbed stress (or that which wouldexist if the sensor were not present) is the quantity that is to beestimated by the measurement.

The nature of the interaction between the sensor and the icedepends on the sensor geometry and the difference in the mechanicalproperties between the sensor and the ice. The effective ice stiff-ness is an important parameter, however its value depends on theloading process. Hence, it is necessary to either desig*n the sensorto be relatively insensitive to ice stiffness variations or be ableto estimate the ice stiffness by some other means in order to inferthe undisturbed stress.


Ice strength and ductility properties are of interest in thedesign of an embedded transducer since the interaction stress fieldcould lead to a structural failure of the ice near the sensor atundisturbed stress levels that do not approach failure. The consti-tutive response of the ice prior to failure is also important, sincethe range of possible effective ice stiffness values is an importantInput parameter in the design of a sensor.

The thermal expansion characteristics of sea ice are important ifsimultaneous in situ measurements of stress and strain are used toobserve natural ice loading events. These events may occur over timeperiods of hours or even days in the landfast ice of the Beaufort Seaand may be accompanied by large temperature changes. Therefore, itis necessary to be able to separate out the thermal and mechanicalcomponents of ice deformation if the stress-strain behavior of theice is to be quantified.

SECTION IIISTATE OF KNOWLEDGE AND RESEARCH NEEDS

This report has described sea ice as a material, and also in theaggregate. It then reviewed the interactions between sea ice andengineered structures and identified the physical properties andprocesses that must be considered in designing structures for a seaice environment. The objective of this chapter is to review thestate of knowledge of sea ice mechanics to determine what is knownrelative to the broader aspects of ice mechanics including the basicneed to understand physical properties and processes, and to identifyresearch needs.

Just as It was necessary in the first section to describe sea iceat two scales--as a material and in the aggregate--this section issimilarly organized. In addition, the mechanical properties of modeland artificial ice are reviewed in a third section.

Material Properties of Sea Ice

The need to establish the mechanical properties of sea ice on thescale of interaction with large structures presents formidable diffi-culties. The scale is so large that it may not be possible todirectly measure any of the desired properties until instrumentedstructures are designed, constructed, and operated for an extendedtime. A more practical approach to the problem is to use smallerscale tests to provide data from which the properties of large icemasses can be established through scaling relationships ormathematical models.

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In general, the smaller the scale at which a particular type oftest is performed, the greater the number of tests which can be runfor a given time or funding level. Because of the large number ofvariables that affect the mechanical properties of sea ice, it islikely that functional relationships will be determined by laboratoryor small-scale field experiments, and the resulting curvescalibrated' to progressively larger scales through a relatively few,selected experimental programs using appropriately large samples.

Few experimental programs have involved samples up to the fullthickness of a late winter ice sheet in the Arctic (AOGA, 1980;Croasdale, 1974; Vaudrey, 1977; Kry, 1975).81,82,83084 Therefore,most of the results discussed in the following paragraphs were drawnfrom small-scale laboratory or in situ beam tests. This discussionnotes numerous problems associated with the organization and inter-pretation of these data which tend to limit their usefulness.

The general characteristics of first-year sea ice, in terms ofsalinity, crystal size and fabric variations, etc., were discussed inSection I. The range and scale of possible structural variations inmulti-year ice is not known. However, considering the combiniationsof thermal and dynamic histories which can affect an ice mass thathas survived one or more melt seasons, the range must be large. Inaddition, superimposed over these variations is a verticaltemperature difference between the top and bottom of an ice masswhich can exceed 400C. Mechanical properties are dependent uponall of these variables to some degree. The strength of sea ice forany loading mode (i.e., compression, tension, shear, bending) is alsohighly dependent upon the rate at which stress is applied. Forengineering applications, knowledge of the strength over severalorders of magnitude of this rate (under confining pressures whereapplicable) is required. This corresponds to a range from ductilefailure at low strength and rate to brittle failure at high strengthand rate. The required data can be obtained from tests in constantstress-rate, constant strain-rate or constant stress (stress-rupture tests). However, there are problems of interpretation incorrelating the results from different test types. The ratedependency also extends to other properties which will require theapplication of theories of visco-elasticity for interpretation.

Some factors that limit the usefulness of many laboratory studiesof the mechanical properties of sea ice done in the past are:

1. The lack of standardization of procedures for sampleselection, preparation, and testing makes it diffi-cult to reconcile results from different test programs.

2. The importance of ice crystal shape and orientation,or, more broadly, the internal structure of the ice, tomechanical properties has only recently been emphasized.

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3. A detailed classification of sea ice is lacking, andsample characteristics have not always been adequatelydescribed. Thus, apparently similar test programs canproduce different results which cannot then be properlyinterpreted.

4. Strain measurements have often been made by monitoringplaten displacement rather than sample strain. Similar-ly, constant strain-rate tests were usually done underconditions of constant platen speed, rather than throughthe use of a closed-loop feedback system based upon sam-ple strain rate.

5. Grain (or crystal) size variations in natural sea ice havenot been accommodated by adjusting sample volume to main-tain a constant ratio of average grain size to samplevolume. There may be some lower limit to the number ofgrains which must be included in a sample in order to giveconsistent results, but, as yet, this has not beenestablished.

These difficulties are recognized, and are accommodated, tovarying degrees, in recent and current work. Standard procedures forsample collection, handling, and testing have been suggested (IAHR,1980).85 However, the lack of a detailed classification of sea icetypes and variations is still a problem, although those proposed forfresh ice by Michel and Ramseier (1971)86 and Cherepanov (1974)87provide a good start. The problem is more acute for multi-year icesince the range of varieties that can occur has not been identifiedand classified.

It should be noted that most of the problems and pitfalls intesting sea ice are applicable to tests involving samples of anysize. Therefore, the effect of natural variability, rate, etc., willbe superimposed on the size effect. Finally, it is important toemphasize that no small laboratory sample can possibly include therange of variations of structure, temperature, and salinity gradientsof a large Ice sheet. Thus, the scaling problem is not simply amatter of compensating for a difference in size of two samples of thesame material. Instead, mathematical development and studies ofdeformation and failure mechanisms may be required to fully establisha correlation. Yet, it is knowledge of the properties of ice sheetson the larger scale that is ultimately of concern for manyengineering applications.

The problems outlined above can be recognized as resulting from

several sources. The first, and perhaps the most formidable, is thenatural variability of the material as it occurs in nature, includingthe properties of a typical ice sheet or ice mass, and the gradientsimposed upon it by the environment. The lack of standardization inprocedures makes each investigation essentially independent of others,

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so that it is difficult to correlate results between investigators.The study of the mechanical properties of sea ice is still ratheryoung, so that several important topics such as the effects of grainsize and orientation have received only minimal attention.

The mechanical properties of sea ice have been reviewed by Weeksand Assur (1967, 1969),88,89 Doronin and Kheisin (1975), 90 andSchwarz and Weeks (1977).91 The following discussions are largelybased on these references.

Stress-Strain Laws for Sea Ice

Stress may be related to the state variables of strain, salinity,temperature, and time. In a given sample a viscoelastic descriptionmay be appropriate, where the strain and strain rate under load aredependent upon the magnitude of applied stress and the rate of stressapplication. Time-dependent effects contribute a significant part ofthe strain for loads applied over time scales that are important inengineering applications. Thus, while elastic or elastic-plasticanalyses can be useful in predicting limiting, or instantaneousrelationships, a complete description of the response of sea ice toapplied stresses will depend upon the development of a comprehensiveviscoelastic stress-strain law.

Few attempts have been made to formulate viscoelastic stress-strain laws for sea ice, particularly for non-linear cases.Understanding of the problem is rudimentary at best; non-linearviscoelastic stress-strain laws are not in general use for othermaterials, and the background required is possibly not widelydistributed among the community of researchers and engineers who areconcerned with sea ice. Only two published papers deal with attemptsto interpret sea ice test data to yield viscoelastic properties; onefor uniaxial compression and bending of small samples (Tabata,1958)92 and the other for the bearing capacity of ice sheets(Vaudrey, 1977). 9 3 The latter can probably be considered to beoperational.

Because of the range of ice properties and environmental vari-ables that can affect the deformational properties of sea ice, it isunlikely that generalized viscoelastic stress-strain laws will beavailable in the near future. However, studies aimed at developingsuch laws are clearly warranted for application in the longer term.

When time is omitted from the constitutive laws for sea ice, anelastic description of the properties is usually attempted. Measure-ments of elastic properties are divided by methodology into twocategories, static and dynamic. The distinction is important becauseof the time-dependence of properties described in the previoussection. In dynamic tests, rapid loading rates are applied bypropagating elastic waves through the samples. The velocities ofvarious phases are measured, and the elastic moduli are then calcu-lated from the data, using the assumption that the ice is homogeneousand isotropic. With the high loading rate and low magnitude of the

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propagating stress, time-dependent effects are negligible. In statictests, loads are applied more slowly so that a contribution fromtime-dependent deformation mechanisms is to be anticipated. Calcu-lation of the moduli is generally done via a linear stress-strain lawin which the material is also assumed to be homogeneous and isotropic.

The available test data for the elastic properties of first-yearsea ice modeled as an isotropic, homogeneous material, can be summar-ized by noting that Young's modulus* is the most studied of theelastic moduli in both the static and dynamic modes. Values fromdynamic tests are reasonably consistent, but those from static testsshow wide variation. The range of possible values over the fullrange of temperatures, salinities, and ice structure that could beencountered in nature, is yet to be determined for either case.Poisson's ratio** has been determined with good repeatibility fromdynamic tests, and the results indicate little variation withtemperature, salinity, and ice fabric. The shear modulus determina-tions from shear wave propagation velocity data are generallyscattered and inconsistent. Neither Poisson's ratio nor the shearmodulus have been measured by static test methods.

Few static test values exist for the elastic properties ofmulti-year ice, and these are not referenced to ice characteristics.

The existing results for creep and elastic properties are incom-plete. Indeed, they pay no attention to the essentially non-isotropicand non-homogeneous characteristics of sea ice. These simplificationsmay be reasonable for applications that do not depend strongly on thedeformation characteristics of ice prior to failure.

Strength of Sea Ice

Strength is defined as the mtaximum stress achieved in any test illwhich the sample is loaded to failure. Strength is a function oftemperature, ice properties, and test parameters such as the rate ofapplication of the stress, or the time that a particular load ismaintained on a sample. For some applications, post-yield behavior(the ability of a sample to sustain a load after the peak stress isreached) is also of interest. In other cases, such as studies ofload bearing capability, the objective is to determine the length oftime that a load can safely be placed on an ice sheet before somecritical stress or deformation is reached.

*Young's modulus: The ratio of the stress in a material to thecorresponding strain.

**Poisson's ratio: The ratio of trarsverse to longitudinal strain ina material under stress tension.

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The strength of sea ice is of interest over the entire range fromductile failure under low constant stress or low rates of stressapplication to brittle failure under high loading rates. Consideringthe number of possible variations in temperature, ice properties,fabrics, loading modes, and other test parameters, it is clear thatto define the strength experimentally will require a large number oftests. The problem is compounded by the absence of any failurecriterion for sea ice with other than a narrow range of application.

Many studies of uniaxial compressive strength have been reportedin the literature, but, to date, tests have not been run over thefull range of temperatures and salinities encountered in nature. Awide variety of test techniques and lack of reported test conditionsfrequently make comparison difficult. Orientation effects in thehorizontal plane have not been thoroughly considered, and a contro-versy exists over the possibility that the strength decreases at highloading rates in the range of brittle failure. No published,systematic studies have appeared on the strength of multi-year ice,although some relevant data are scattered in publications on sea icein general. No studies have been reported in the literature on theeffect of biaxial or triaxial confining pressures on the strength ofsea ice.

Most reported values of tens

COUNCIL WASHINGTON DC F/G RESEARCH IN SEA ICE ...Donald W. Perkins, Assistant Executive Director...

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