75symp03_polar Classs Ship Hull Design

8/8/2019 75symp03_polar Classs Ship Hull Design

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INTRODUCTIONPolar icebreakers onerate under themost inhospitable ocean ~onditions theworld has to offer. Designed for optimum performance at low temperatures inIce fields varying from uifomn plateice to deep windrowed ridges, they alsotransit areas of intense heat, highwinds, and extreme sea conditions.Their hull structure sees a variety ofloads ranging from those thermally in-duced to concentrated ice Impact loadsto those impossible to design for, suchas grounding on uncharted rock pinnacles .The icebreaker designer must allow forextreme loads on the one hand, but mustpay attention, like all naval architects ,to detail design, with a view towardeliminating structu~al failures frommore subtle causes such as elastic in-stability, brittle fracture, and fatigue.The United States Coast Guard af-fords icebreaker support, both domesticand polar, to a variety of ship opera-tions conducted in areas made hazardous

or impassable by ice. Typical polaricebreaker missions include escort ofvessels supplying outlying military orscientific stations: independent logis-tics SUDDOrt to Similar OUtDOStS . andice and~cean survey operations ;nd suP-port in both polar regions.COaSt Guard icebreakers engaged inthese operations have recentlv consistedof six WIND Class vessels (Fikure 1) andthe GLACIER (Figure 2) Budget reduc-tions and old age will have soon forcedthe decommissioning of all WIND vesselsexceut for WESTWIND and NORTHWIND. eachof w~ich has undergone extensive ~truc-tural and machinery renovation to prolongits service life.

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The need has been apparent sincethe early 19607S for a new class of ice-breaking ships to replace aging membersof the fleet and to undertake more ex-tensive duties as Coast Guard Responsi-bilities change and expand. The recentextension of the search for oil into theArctic region, for instance, appearslikely to require a strong Coast Guardresponse capability in this area. In1966 the Icebreaker Design Project wasestablished in the Naval EngineeringDivision at Coast Guard Headquarters toinitiate preliminary design work on anew polar icebreaker. Tbe initialthrust of the effort was toward a nu-clearpowered cutter, hut this was latermodified to conventional dieselelectricand finally revised to include a gas-turbine mode of operation. During theexistence of the Icebreaker Design Pro-ject, its personnel delved into the si&-nificant aspects of ship design as itapplied to icebreakers Its most signi-ficant contributions to icebreakingtechnology were probably in the areas ofhull form. Dowerin!z predictions. andhull stru;t~re, th; iatter includingmaterial selection. In eXC?SS of 100discrete projects were completed, manydependent on Coast Guard-initiated re-search. Literature searches EIT con.ducted to insure that existing technol-ogy was not neglected. Project membersreceived significant support from contractors, other government agencies, anda large number of Coast Guard personnel.

By the time of the dissolution ofthe Icebreaker Design Project in 1969, abasic preliminary design had been devel-oped. HUI1 fopm and principal dimen-sions had been defined, rough arrange-ments completed, a machinery plant sizeand type selected, a basic structural

Length overall------269or!Length, DWL--------25O ,-0Beam, maximum- -63, -61,Beam, DWL---621 -07,Depth to main deck-- 37T-9l/2,!Draft to DWL-----25!g11Draft , maximum--- 2g,-l? ,,-~-----,.-~ ..

Length overall------jlo t-7ts Displacement to DWL-----7,6OO long tonsLength, DWL---------Z9O, -O?l Displacement, maximum---8, 449 long tonsBeam, maximum-------74 9-3-1/21! Complement-------------- lg7Beam, DWL-----------72 q-611 Speed, knots, crui,sig--l7.6Depth to main deck-- 38v-4qt Propulsion-------------- Diesel-electFicDraft to DWL--------25, -g,,Draft, SHP--------------------- 16,000maximum------28* -611 NwnbeT of screws--------2

Figure 2. GLACIER Profile and Characteristics

Length overall------ OT1)-OT1Length, AWL--------- 352,-O!1 Displacement, maximum--- 13,l79 long tonsComplement --------------l38Beam, maximum-------83, -7rq Speed, knots, cruising--l7Beam, DWL----------78 *-Ott Propulsion--------------Die.sel-electFic orDepth to main deck--49,-3l/21, Gas turbineDraft to DWL--------28, -011 SHP---------------------18 ,000 D-EDraft, rnaximunl------1,-9!t 60,000 G-TEIisplacernentto DWL-11,000 long tons Number of screws------3

Figure 3. POLAR Class Profile and Characteristics

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arrangement chosen, and tentative scant-lings determined. In March of 1970,these early beginnings were transferredto the existing Design Branch of theNaval Engineer-ing Division for contractdesizn development. Completion of thecont~act desi~n in 1971 ;as followed bybid solicitation and award of a contractto Lockheed Shipbuilding and ConstructionCompany, Seattle, to build one icebreak-er, the POLAR STAR (Figure 3) Deliveryof POLAR STAR took place during 1975. Asecond ship, POLAR SEA, will be completedin 1976.STRUCTURAL DESIGN

Structural studies within the Ice-breaker Design Project began with anassessment of existing ships GLACIERand WIND Class experience. totallinasome 220 ship-yeaks of ophration, w~sanalyzed along with that of other ice-breake~s, domestic and foreign, to iden-tify structural inadequacies and to or-ganize this information in a format use-ful to the designer. Accumulated exper-ience reinvested in subsequent designsis, of course, the essence of the clas-SiC shiv desi.m Drocess. and the methodaPPears tO re~ai~ its usefulness as tech-nology advances.The areas of structural concernlisted below were identified as requir-ing special investigation:q Load Definitionq Framing Systemq Allowable Stressesq Analysis Methodsq Hull Materialq Detail DesignAlthough any ship design requiresthat these items be addressed, specialemphasis was imposed in this case be-cause of (1) the lack of documentationfor many decisions reached in designingprevious icebreakers, (2) obvious tech-nological advancements in the interven

ing years , (3) the accumulation of ex-pedience with postwar icebreakers andresultinz conceut realignments . and(4) the ~ealizaiion thai this bro,jectpresented an opportunity to performbenchmark studies that could be refer-enced with confidence in future designworkLOAD DEFINITIONIntroduction

Determining rational design loadsfor a ship of this size and type is aconsiderable undertaking. The need to

research twenty-five years of highlytheoretical icebreaker and ice technolOgY literature was obvious. Further, ifthese studies ei-eto be of any valuethey had not only to be compared to theoriginal design philosophy of the WINDSand GLACIER but also comelated to OToperational experience with these ships.It was also necessary that the investi-gations put in proper perspective thestate of the art i countries such asCanada and Finland, which had been bs-ily building icebreakers since the early1950s.

The design philosophy of the WINDClass took into account only a few majorship parameters such as displacement,shell slope at the ice belt, ship length,and beam, by use of the formula reportedin Volume 54, 1946 of the SNAME Transac-= [11

D D d+zwhere P is the load per foot of water-line perimeter with the ship supportedsolely by ice at the waterline, D is thedisplacement, m is the aerage shellslope, and L is the waterline perimeter.The unit load P readily yields the loadper frame which was in turn used todesign the ice belt plating and framing.The load per frame was envisioned asquasi-concentrated at a pressu~e of3,000 PSi and applied halfway betweentransverse frames in the desizn of theplating. The framing was the; designedto withstand the same load at midapan.The above design philosophy yields astructure in which the framinK is incaD-able of withstanding a unifor~ly distr~-buted pressure of the order of the crush-ing pressure of ice and considerablyless than the distributed pressure thatthe plating can suppo?t. This framingpressure capacity,, is less than 150 psifor the WIND class and about 200 psi forthe GLACIER, both of which have expe~i-enced damage in service.Ice Strength

There was therefore no question ofthe need to establish valid design icepressures based on realistic operatingconditions. A thorough ice technologyliterature search indicated a wide dis-parity in the measur-ed physical proper-ties of ice and, more significantly,showed that most reports did not indi-cate important factors such as samplesize and orientation, temperature, andsalinity (Table I).

This resulted in a decision toundertake an independent ice pressurestudy Assistance was provided by theArmy s Cold Regions Research and Engi-

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uE::cI{lPTIoN THICKNESS

Clear, cold, solid ice Not stated

Not stated Not stated

Clear fresh water ice Not statedpan

River, Kennebec Not stated

Not stated Not stated

St. Lawrence river ice Not statedIce of excellent quality 5-in. X 5-inBlocks loaded to 1,000 test piecespounds in 2 sees

Hard old arctic ice Not stated

Coast Guard criteriabased on temperature. 10-25 feet

STRENGTHTEMP COMPRESSION TENSION SOURCEDEG F PSI PSI

Not stated 3,000 250 H. F. JohnsonSNAME-54 (1946)

Not stated 213 to 388 190 FruhlinglC of Koenigsberg(Markessa) (prior to 1900)Not stated 327 to 1,000 Not stated W. Ludlow(prior to 1900)

-4F 399 to 970 Not stated Prof. Kolster,Helsinki(prior to 1900)

32F 363 Not stated Prof.H. M. MackayProf. H. T. Barnes(1914)

28F 300 Not stated14F 693 Not stated Prof. E. Brown2F 811 Not stated (MCGiIl University)

Not stated 1,000 250 A. Watson, IME(1958)

OF surface?OF bottom 600 pS i ..- Dayton [2]salinity, & ;trengthprofiles

Table 1. Ice Characteristics as re~orted i Volwne 67, 1957 of theSNAME Transactions [3] with Coast Guard data addea

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neering Laboratory (CRREL) and the stud-ies were documented by Dayton [21.The following parameters were consider-ed:

q Sample Size - Due to themeater incidence of faults~id impurities, the averagecompressive strength decreasesas the sample size increases.q Sallnity - Varies through thethickness of sea ice; strengthis In turn a function ofsalinity.q Temperature - Varies throughthe thickness Strength de-creases with increase in temp-erature.From the above, strength profilesand average values for representativethicknesses of ice were calculated(Figure 4). The following average de-sizn uressure values were then arrived

at: c~nsidering data reliability andcontact area factors for each conditionand in both cases departing from a maxi-mum ice sample strength of 1,000 psi:q uniform static load for besetcondition:

Factors forSample size .4Strength profile .75Contact area .5Data reliability 2Design pressure =1,000 psi x .4 x .75 x .5 x 2= 300 psi

q Uniform Impact design pressurefor bow and stern areas :Factors forSample size .4Strength profile 1Contact area 1Data reliability 1.5DesiEn Dressure =l,oolipii x .4 x lx 1 x 1.5= 600 pSi

Load DistributionIt was next necessary to determinethe manner in which these loads were tobe applied to the hull. Since impactsestablish the maximum load to be absoFb-ed by the shell strwcture, studies werecarried out to determie the distribu-tion of Impact loads on the hull undervarying conditions of speed, floe size,ice thickness., and impact location.

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Figure 4. Ice strength profiles, from [2]Work of this nature included thatby Nogid [4], White [5], Kheisin [61,and Tarshis [7]. The Icebreaker DesignProject initiated additional studies,resulting in contributions by Estes [8]and Dayton [2].Dayton applied computer techniquesto Tarshis, p~ocedures to detei-mine im-pact load and area as a function of lo-

cation. The analysis was dependent onship speed, hull shape, mass and rigid-ity of the ship structure relative tothe ice, and entrained mass of water.A siemificant findin!zof this work wasthat-impact loads co~ld realistically beconsidered as uniformly distributed overa large area, typically 48o square feetfo~ an 18,400-ton impact load.Figure 5(a) illustrates the effectof location on total impact load asdetermined by Dayton for one of severalhull designs developed during the piw.liminary design phase. It is apparentfrom the superimposed waterline that thegreatest loads are predicted fop the

shoulder area at about station 2. Fig-ure 5(b) provides a correlation withWIND Class expeI.ience. The two-peakedcurve indicates the relative nunbe~ ofreported damages ersus longitudinallocation, as compiled by D. E. Woodlingand R. A. Yuhas of the Coast Guard. Thehigher peak OCCUFS at the bow screw bos_sing (the bow propeller was neeP in-stalled on most ships of the class andhas been removed fmm the others ), which

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DAMAGE LXS7/2/OUT/ON(25 YE+iSj b$/A/oLASS

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Figure 5. Bow impact locations , POLARClass (predicted) and WIND Class (actual)

provides a stop!t in heavy ramming con-ditions and has pi-ecipitated nume~ousstructu~al ~aflu~es at station 1. (Thefaired stem of the POLAR Class and I.enovated WESTWIND and NORTHWIND IS exoectedto alleviate this problem by redic;igshock loads resulting from sudden energydissipation during compI-essive loadingof the ice by the steD. ) Davtots anal-ysis did not-include ihi bosiing area~nmthus related better to the new hullAs the figure indicates, WINDClass shell failures have also peaked inthe shoulder area whe?e the analysispredicts peak loads.Icebelt Definition

The Icebelt is that portion of theshell plating that is strengthened toaccept the design ice loads. Figure 6indicates the extent of the icebelt onPOLAR Cla.SSships.Longitudinal Bending

Studies were also made to confirmthe magnitude of wave-induced bendingstresses as well as those experiencedwhen the bow is beached on an iceshelf. These studies confirmed previousinvestigations conducted with the MACK-INAW [101 in which it was concluded thatthe stresses induced by these loadingconditions are of secondary importanceIn the determination of the shell anddeck scant lings but nevertheless influ-ence their design. Still-water bending

I-----YDEEP W, L./ LIGHT WL.

Figure 6. POLAR Class icebelttion showing design pressures, cOnfigurafrom [9]

stresses were included along with iceloading in the design of the ice belt.The decks and bottom structure weresized to avoid plate buckling failureunder wave-induced bending and hydro-static loads. The bottom plating thick-ness was further increased to accountfor corrosion and to improve the resis-tance to grounding damage.FRAMING SYSTEM

Various schemes of longitudinalframing systems were considered and dis-carded because of weight considerations.Included among these was a system thatmade use of extrusions incorporating upto two frames and their correspondingareas of shell plating. The studyproved this to be heavier as well ascostlier because of the unique pro-curement and fabrication problemsInvolved.The transverse framing system his-torically used for icebreaking ships wastherefore adopted. Supports for trans-verse frames were readily available Inthe form of decks; the considerable sidetankage pePmitted the use of decks orpartial decks throughout the length anddepth of the hull. The framing wascanted at the bow and stern so as toplace the framing members more perpendi-

cular to the shell for more effectiveload-bearinz abilitv. The 16-inch framespacing agi~nprove; to be the least-cost alternative when compared to smal-ler grillages requiring more frames andlighter plating.Both WIND Class and GLACIER have,within approximately the midships six-tenths length, transverse frames suP-ported by truss structure, Figure 7,


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Figure 7. Truss framing system of WINDClass and GLACIERwhereas other U. S. Coast Guard icebreak-ers (CACTUS. STORIS. etc. ) and mOSt EW-opean icebr;akeps eiploy deep webs andgirders or grillage structure for shellsupport In all cases, however, bow andstern framing consists of grillage structure. Detailed studies of each system,perfomned by the coast Guard and hyLloyd?s Register of Shippin8 [11], cmn-pared the strength and weight of optimized truss and grillage structures.Particular attention was paid to thebehaviour of the structure under owP-load conditions. As expected, the trusswas shown to be the lesse~-weight systemwhen compared on an equal design-strengthbasis It was the response of each tooverloads that led to the selection ofthe grlllage, Figure 8. The truss sys-tem, in response to an overload condi-tion, develops significant bending mo-ments at the connections which, com-bined with the axial loads on the com-pression members, could lead to a pro-gressive failu?e of adjacent framesthrough elastic instability, and subse-quent collapse of the shell framing.This mode of failure has been obsemredon present Coast GuaPd icebreakers. Astable grillage structure, on the otherhand, may experience local plastic de-formation under overloads but will notnecessarily lose its ability to sustainadditional loads at the design loadlevel.

Figure 9(a) illustrates a f~mnlngsystem that was under consideration intothe preliminary design phase of thePOLAR Class This system was consistentwith an early assumption concerning ilII-pact load distribution, namely that the

Figure 8. Grlllage framing systemof POLAR Class

EifD w6e Wfsb Ew o(a)

Figure 9. Alternateframing details, (a)(b) final

MD

POLAR Class shellproposed and

load would be applied to one transverseframe oer it,?length between two decksThe purpose of the web frame-supportedlongitudinal member was to distributethe load onto adjacent frames. As theload criteria were refined, however, itbecame appar-ent that this was not arealistic load assumption and that the


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600 psi impact load could be considereddistributed uniformly over a larger areasupported by many transverse frames.This reduced the basis for selection toa simple one of minimum weight and cost,thus the system of Figure 9(b) waschosen.ALLOWABLE STRESSES

Since the design pressure valuesdiscussed ea~lier contained reliabilityfaCtOFS , it was decided to design theshell structure to the yield strengthwhen loaded amidships to 300 psi, theuniform beset load, or at the ends to600 psi, the uniform impact load.

Plastic deformation will be expect-ed when imposed loads exceed the designloads Daytonts analysis predicts theformation of three plastic hinges at a1,200 psi distributed load. Greensponvsmethods [12] indicate a permanent shellplate set of O.09 inches at a uniformpressure of 1,150 psi.ANALYSIS METHODS

The finite-element method of emal-ysis proved to be an effective tool forevaluating icebreaker hull structure.Genalis [133 inestlgated the applicabi-lity of the Drofnam STRESS1 to two- andthriedimensional frame analysis andcompared WIND Class transverse fPaMeS bythe two methods. He concluded that thetwo-dimensional computer analysis waseffective but that the three-dimensionalapproach, which included the effect oflongitudinal members, was preferable,despite its greater cost, because of theimproved accuracy. He noted that thetwo methods ~redicted frame bendinc mo-ments that w;re different by as mu;h as33 percent. Lloyd$s Register of Ship-ping also used STRESS for the GLACIERanalysis in their GLACIER-MOSCOW comparison in 1967 [11].The program FRANZ was used byI,loydTs for the MOSCOW analysis and byKiesling [16], who studied the applica-tion of available computer methods totbe structural analysis of an icebreak-er. He cmnpa~ed STRESS and FRAN andconcluded that the latter was more suit-able since it could (1) handle more com-plex problems, (2) more easily accountfor temperature effects, and (3) wasnore efficient in the use of machine

time. Kiesling cautioned that furtherdevelopment was necessary in structuralmodeling techniques and in determiningtbe loads acting on an icebreaker, andnoted that reliance on these programs,with their two-dimensional represent a-

- Structural Engineering SystemsSolver [14]2 Framed Structure Analysis Program[15]

tion of plates and shells, is unrealis-tic if quantitative data for such memhers is required.Fe et al r81 studied the bowstruct~re of an-i~ebreaker using SAMIS3,a matrix interpretive code which modeledthe structui-e as a combination of beams,bars, and shell elements. In order tosave time and decrease the possibilityof error, an input data generator was

developed to aid in providing descrip-tive data for nodal coordinates andboundary conditions The WIND Classbow was modeled and it was determinedthat relatively low pressures were suf-ficient to produce yielding; this wasin agreement with p~evious observations .This study also reviewed the various im-pact loading criteria and verified thatthe rise times of these loads were suf-ficiently great that dynamic effectscould be disregarded.The detailed structural desire ofthe POLAR Class utilized both manial andcomputer calculations. The programSTRESS was used to analyze typical trans-verse frames and the Neilsen GrillageComputer Code [18] sa particular appli-cation in tbe design of side shell gril-lages in way of tbe machinery spacesHULL MATERIAL SELECTIONExperience

Shell structural failures experi-enced bv Dresent icebreakers. Drinci -pally W?ND Class ships, havea?ways beencause for concern. Figure 10 is typicalof major damages incurred by these ships.On several occasions of simificant shelldamage, the oDooTtunity wai taken to per-form metallurgical examination oncropped-out hull plate [19, 20, 21, 22].These analyses were invariably charac-terized by comments such as, eXtenSiV?brittle fracture, indicative of brit-tle cleavage type failures, or highlynotchsensitive In at least one in-stance it was noted that catastrophicfailure would have occurred had it notbeen for the local nature of the loadand the lack of a significant tensilefield beyond the immediate damaged area.Table II describes the physicalcharacteristics of samples of platefrom the WIND Class icebreaker USS ATKA(now USCGC SOUTHWIND) in 1965 [22]. Tbematerial is unnormalized Navy High-Ten-

sile Steel (HTS) conforming to [231, theHTS specification in effect in 1943.Refer;nce [19] reported on an analysisof plate samples removed from USCGCEASTWIND in 1956, and concludes, Thecombination of poor notch-toughness, low

3 Structw.al Analysis ad MatrixInterpretive System [17]


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Flgum 10 . WIND Class shellstructure damage

operating temperatures and impact isconsidered responsible for brittle behaviour of the plating dui_ing failure.Nomnalizing of the plate materialii; the laboratory] resulted in a markedimprovement in notch-toughm?ss proper-ties. It may be assumed that bad theplating been normalized (as now requiredby specifications) , the extent of thecasualty would hae been of a less serious nature. The HTS specification hasincluded normalized plate since [24]was introduced in 1953.

By fa~ the greater part of theCoast Guard s structural failure datahas been generated by WIND Class ships.Although GLACIER has incurred damage, itis but one ship versus the WIND Class ,six and it has a somewhat greater designvressure due to its ice frames beirm ofHTS rather than mild steel as on WIiDClass hulls. The present author.? couldlocate no reports of analyses on GLACIERhull steel which is, like the WIND,S,HTS. Whether this steel was normalizedis not mentioned in aailable documents,although the hull construction plansantedate by about one year the ilTSspecification that first ~equimd nornlaliz-ing.

It was appa~ent from WIN3 Classexperience that an improved materialshould be utilized in any new icebreaker.The eni~onmental conditions of low temD-erature and high impact loads, combinedwith a notchsensitive material, promot-ed brittle fracture in extreme icebreak-ing conditionsHull Steel Re~uiTements

Studies by Yuhas and Schumacher [25]of the Icebreaker Design PYoject iclentified the following factors as signifi-cant in the selection of icebreaker hullsteel:q Lowtemperature toughnessThe air temperatures experi-enced by polar icebreakershave been measured as low as500 F. Figuz-e 11 illustratesthe zone of transition from-50 F air temperature to +28F sea water temperature. Thechosen steel must be sufficiently tough at these tempei--

atures to Dreclude brittlefracture. However, as recent-ly noted by Rolfe et al inShips Structure Committee Re-port SSC-224 [26], the combi-nation of toughness, stress

USS ATKA HTS PRESENT-DAY HTS[221 [24](aerag,, of4 samples)

Tensile Strength 81,000 psi 88,OOO max.Yield Point 56,OOO psi 47,000 min.Elongation, % 27 20Charpy V-notch energy,

ft-lb 14 to 26 100 @ +40 F!?+40 F (typical)Table II. USS ATKA shell plate versus p~esent-day HTS

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level, and flaw size togethe~determine the resistance of astructure to brittle fractuie.Fatigue Strength. As in anystructural design, mate~lalselection and detail designgo hand-in-hand in determiningthe resistance of the icebreaker str-ucture to fatigue.Although longitudinal bendingcycles are less than for mostmerchant vessels, Figure 12,and ice impact loads on agiven area occur relativelyinfrequently [2], the stresslevels experienced by an ice-breaker are often sewere. ReSistance to fatigue as a basicmaterial characteristic as-sumes secondary importancewhen compaTed with the low-cycle fatigue strength ofstructural details.Ease of Fabrication and Re-pair. Icebreaklng vesselstypically have heavy plate anddense framing systems andtherefore require a relativelylarge number of constructionman-hours, many of them spentwithin areas of difficult ac_cess. It is to the owner, sadvantage to use a materialthat will not further increasefabrication difficulties dueto more stringent forming orwelding requirements

x

2s

20

15

10

5

0

)+ TR4USITI0N 20NtESEAW4TF2 @ + m FFigurw 11. Minimwn temperatures actingon DolaT icebreaker hullq Yield Point and UltimateStrength Adequate tensileproperties are necessar-y toinsure that the structure willwithstand the basic loads whilenot imposing high wt!i~ht pen-alties and fabrication diffi-culties.q cost . Less is best, but sincematerial characteristics mustbe optimized, cost may be sub-ordinate to other i-eqirements.

WN).IES07A , CANADA4 (*WAG=)C27,28291UOOSd EP. STATE, kOLVEQlhlE3TATE@O YSARS) AS RE$WZTED INVCLUME 73,1965 OF THE S NA W E TRANSACTIONSAND IN SHIPS STRUCTURE COMMITTEERCPORT SSC - 164 [29, mJ

7=0LAE zclzeIEs,AKEIZ30-Y12 LIFE

1I 10 I0 2 K+ !04 10 m 10 Joe lo t4

N= NUb4bEU 0= TIMES THE INDICATED STSE5S WILL bE EKCEEDSDFigure 12. Cumulative frequencydistribution of polar icebreaker admerchant vessels, from [2]

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cantlv meater thanq Corrosion Resistance. Sinceall low-carbon steels tend toexhibit similar cor~osion DI-o-perties in salt water, thisparameter will not generallyinfluence the steel selection.Of greater concern are thecorrosion Dronerties of theheat-a ffecked zone at theshell seams, where acceleratedcorrosion has been noted onWIND Class and GLACIER hulls.Insufficient data of thistYPe was available in 1970 toaid in steel/filler metalselection, and the Coast Guardhas since initiated a loyearprogram to study salt-atercorrosion on welded samplesof Icebreaker plate.

Hull Steel CandidatesThe search for the hull steel thatbest met the above requirements led totwo prime candidates, HY-80 and ASTM-

A537 [Table 1111:HY-80 (MIL-s-16216)The advantages of HY-80 for ice-breaker hull structure are obvious.Cross-rolling provides it with essenti-ally equal properties in all directions,ideal for impact loading normal to theplate. Its low-temperature toughness isoutstanding. Its yield strength of80,000 psi minimwn appears to provideconsiderable potential fo~ weight reduc-tion. But HY-80 has clear disadvantagesalso. Cross-rolling? alloying, and extensive testing requirements increaseits procurement cost to 3.5 times ABSClass B (1964 data as reported in SNAME

T k R Bulletin No. 2-11 [32]) Compared(for now on an equal-thickness basis)with A537, it is less attractive forwelding because:9 A greater preheat/interpasstemperature is required.9 Lower allowable heat input re-sults in lesser depositionrates.q A higher-strength, more costlyelectrode is necessary.q Joint preparation is more dif-ficult due to its greater hard.ness.

Not only do these factors tend to d~ivecost upward, but they also increase thedifficulty of reDair work and make suchwork in remote shipyards less desirableFurthermore, because of fatigue, buckling, and other considerations, full advantage cannot be taken of the highe~yield Strea.sto achieve weis!ht reduction.iensile strem.th. a.me..sure-of overloadabsorption .U..... ..capability, is not signifi -

u. .the final analysis ,that of A537. Intoo little weightreduction and ioo much increased C;stplus the ready availability of ASTM-

A537 were the reasons for the elimina-tion of HY-80 as a hull material.ASTM-A537The steel described by ASTM Speci-fication A537, adopted in 1965, is a

low-carbon pressure vessel steel available in either a normalized (A577A) or;l~:enched and tempered (A53iBj-coidi-Steel of this chemistry was, in1970 as now, seeing wide use in tbe ma-rine field, tYPiCal applications beingin carriers of liquified petroleum gas(LPG) all-hatch ships, and offshoredrilling and production platforms intemperate and arctic regions. It hadbeen considered a leading candidate foruolar icebreaker hull steel since the;nception of the design project due toits excellent low-temperature toughness,good tensile and fatigue properties, andrelative ease of welding. Its materialcost was about 1.Q time~ that of ABSClass B as reported in SNAME T k R Bul-letin No. 2-11 [321.

Fracture mechanics methods indicated that A537B plate had sufficientCharpy V-notch energy in thicknessesthrough 1-1/2 inches to insure through-thickness yielding before fracture, inaccordance with Rolfev s criteria of [33].In addition, the method of [34] was usedto verify that a built-in su~face flawwould not propagate to a dangerous sizeover a 30-year lifetime.Steel Selection and Application

The tensile properties of ASTMA-537 are achieved by allowing carboncontent to a maximum of O.24%. TheCoast Guard decided, however, that resol-ution of the toughnesstensile character-istics trade-off should favor the formerand investigated a carbon reduction toachieve this. The work of Roper andStout published in Ships Structure Com-mittee Report SSC-175 [35] and elsewhere[36] indicated that a reduction to 0.17%would provide thick-plate (to 4 inches)A-577 material with towzhness far suDerior to that of ABS Clas; C while ten; ilestrength was reduced to about 80 KSI,and that underbead c~acking was elimi-nated by using low-hydrogen electrodes .During the polar icebreaker prelim-inary and contract design phases, J. W.

Kime and R. A. Yuhas of the Coast Guardmet with major steel InanufactuTers todetermine the optimum chemistry of anA537-type steel for this application.They then developed a new specificationlimitirm carbon to O.16% and increasingthe Cha~py V-notch requirement to 20 f;lb @ .ho O F transverse (base metal &as-welded) compared with 15 ft lb @ ,-

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-75 F longitudinal for the ASTM mate-rial. This specification was titledllc_A537M,, to indicate a modified A537steel for Coast Guard icebreaker appli-cation. Steel company pricing methodswere such that the identical steel wouldcost less if purchased under a new spec-ification than as modified A537B.

Steel to specification CG-A537M sawits most essential application in thetransition region (Figure 11), whereboth extreme impact loading and lowtemperatures will occur. At higher 10-cations, low temperatures will be thedominant environmental hazard, compli-cated by unavoidable structural discon-tinuities; on the underbody, impactloading at water temperature will pre-vail. The use of CG-A537M was extendedto include the basic hull envelope andother materials were incorporated asshown by Table IV.

Since icebreakers are historicallyunable to retain bottom paint, a cOrrO-sion allowance was added to the icebeltand all other underbody steel, includingthe rudder. consisting of a l/b-inchaddition b~yond that ~equired forstrength. A current Coast Guard R 8 Dstudy is attempting to determine whetherpresent-day materials have potential asabrasive-resistant underbody coatingsfor iceb~eaker application. Such adiscovery would have significant impacton the cost of future icebreakers.POLAR Class ships will, when new, carryaround some 175 tons of sacrificialsteel.DETAIL DESIGN

AlthouEh data of the type presentedschematically in Figure 13 [37, 271would be most useful to the structuralengineer involved in the selection of

APPLICATIONLow-temperatureShell includin!z Icebelt

NuM~Q OF CYC1 ES _

Figure 13. Schematic fatigue designcurves, from [2, 27, 371

details for any ship design, the methodthat it implies has not been rigorouslydeveloped. The limitations are chieflyin the extrapolation of curve A, thecumulative distribution of frequency,to some curve B, the ship equivalentof the S-N curves C, D, and E. Such atransformation involves altering thefatigue load spectrum of curve A into arepresentation of number of load appli-cations versus constant mean stresslevel. Although Nibbering [271 eXPlainsone such method, he is careful to empha-size its approximate nature.A further limitation on the use ofthis method appears to be the lack of

REQUIRED MATERIAL

CG-A537MCG-A577Mce frames -Other shapes, Fabricated CG-A537MRolled CG-A537M orASTM-A537A orASTM-A537BFlight DeckOther Weather Decks (Main k 01)Internal Structure Adjacent toShell or Weather Decks (plate)(rolle~ shapes)

Surrounding Large Deck OpeningsInterior Structure Not Subject toLow TemperaturesSuperstructure

Table IV. POLAR Class steel

HY-80CG-A537MCG-A537MASTM-A537A orASTM-A537BHY-80

ASTM-A131ASTM-A131

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I

F-2 x,03,x10

Ix

r=..,9,mr\-2.46IF.,.0I4,,9$ ,r10 ,Xd

Figure 14. Relative fatigue resistance of structural details, ~rom [271

S-N curve data for specific Material/detail combinations under loading condi-tions identical to those aboard ship.

The data Of [3B] (Figure 14) wasuseful to the extent that it provideda relative ordering of various jointdesigns in terms of fatigue resistance.Despite the low cycle predictions, themost fatigue-resistant details wereselected for structure which would beexpected to absorb impact loads, keep-Lng in mind the usual icebreaker hazards- extreme loads, low temperatures, theremote operating domain - and those nor-mally expected such as material flaws,construction-related structural notches,and corrosion. Figure 15 illustratestYPical jOint designs provided on POLARClass vessels.

A tenet of sound structural designis to insure that instability failuredoes not prevent a member from absorbingits design load. Lateral supports onall heavily loaded framing are incorpor-ated into the POLAR Class structure topreclude such failure.

Additional detail considerationsworth noting are that hull fr~ea arecontinuous through decks and that longi-tudinal structure is continuous throughbulkheads. A general requirement, in-fact, is that plate not be loadedthrough its thickness in order to eliml-nate the possibility of la.mellar tearing.

WAUNC1450 FQ.4M.s #Au ffCL&O ,=%?A#E~OOVC / CEbCLT 47 lC~MLT

Figure 15. Typical POLAR Class shellframing details

COMNENT

More extensive use of Icebreakersand ice-navigating ships is forecast forthe coming years. Various rule-makingbodies have responded with additional m.more severe structural requirements.The Canadian Arctic Shipping PollutionPrevention Regulations requi?e icebreak-er hulls capable of withstanding pl.es-Sures as great as 1,500 psi without ex-ceeding the yield stren~th of the hullstructire. The Amt?I.ica~Bureau of Ship-ping has outllned requirements for new

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Ice Classes requiring shell design icepressures of up to 234.5 psi for navi-gation in extreme ice conditions!t.The latter requirements alone are diffi.cult enough to attain. They are compa~-able to or exceed the actual strengthsof many of today ts !pola~,vieebmakeps.The need for fw.ther resea~ch was nevermore appa~ent than now if these require-ments are intended to specify the trueload-bearing capability of a ship $s hull.As Figure 16 indicates, cum.ent dataon the world, s icebreakers continues topoint to a correlation between ship dis-placement and plating thickness. CurvesA and B describe older and more recenttrends respectively for arctic icebreak-ers, while curve C shows the presenttrend for Baltic and Great Lakes ice_breakers. Note that only curve C tendsto indicate a limit on plate thickness.Neither WIND Class ships nor GLACIERhave sustained damage that can be att~i-buted solely to plating failure. Defor-mation between frames is not apparenteven after thirty years of Arctic service

accompanied by 15 to 25 percent platingdeterioration. It therefore appears thatperhaps this plate has never been loadedto the point that further increases inthickness will be justified. This issuehas immense impact when one considersthat one-eighth inch of shell plate on

.50

POLAR STAR costs some 90 tons and one-quarter-million dollars. Assuming ade-quate strength and ductile behaviour ofthe material, how much plastic deforma-tion should be tolerated - structurallyor even aesthetically - under extremeloads?It appears that the de,signof fFam-ing can likewise be improved. Figure 17indicates the relative strengths of shell

and framing for representative ships,both existing and hypothetical, and com-pares the load-cam.ying ability of plat-ing designed plastically, plating design-ed to yield, and framing designed toyield. The consistent differences be-tween shell and framing st~ength; iewedin the light of Coast Guard experience,aPPeaF tO require additional study. Theframing strengths shown assume that decksand bulkheads provide unyielding supportfor transve~se f~.mnesand lon~ltudinalgirders. This is an optimist~c assump-tion, particularly for icebreakers wheredecks and bulkheads are not necessarilynormal to the shell in the areas ofgreatest load. Furthermore, the framingstrengths of Figure 17 do not account forthe reduction in load-bearing capacitycaused by frame instability under heavyloads . This is due to shapes which areinherently unstable because of sectionasymmetry (such as inverted angles) orto frames which, even though symmetrical,

1111 ]q WGk 76%41LE 3rEEL - & K&i. YIUD II I I I I I I / I 14 & 8 /0 /2 /4 /6 /8 20 22DI SPh I CEMENT I N 70~s x lo Q o

Figure 16. Icebreaker shell plate thickness as a function of displacement

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m FRAMING STRENGTH AT YISLD~ PLATING STRENGTH AT w ELD= PLATING IN PLASTIC CONDITION

i

Figure 17. Relative strengths of icebreaker bow shell structures

cannot be installed normal to the shell.The enlarged detail of Figure 18 illus-trates the situation as found in varyingdegrees throughout the length of anyship, particularly those transverselyframed, yet its effect on the ultimatestrength of the plate-stiffener combina-tion has yet to be assessed through theapplication of research.

Structural fabrication cost must befurther addressed, including cOnsldeFa-tion of weight, shipyard capabilities,and framing system options available tothe deslgnei-. Typical might be a studyof framing systems such as that shown by?Igure 19. This illustrates the concep-tualized framing of an icebreaker ofabout 300 feet owPall len~th, and shosLongitudinally framed grillages in the~ess contmmed areas of the hull, allow-ing more extensive use of standard rolledshapes. While somewhat heavie~, the de-sign minimizes the costly custom fabrica-

E\,*W-IELLBUDFigure 18. Shell framing not normal tothe plate. Combined beam and platesection modulus does not represent thetrue ultimate strength in this situation. .

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Figure 19. Conceptual longitudinal framing system with cant frames fore and aft

tion of frames necessary with a trans-verse framing system.SUMMARY

This paper has only highlighted theextensive research effort that attendedthe POLAR Class structural design. Theresulting CUtterS, although yet unproven,are exoected to accomplish the CoastGuardts missions withthe utmost instructural reliability. This work pro-vides a foundation upon which futuredesiKns. with new and additional require-ment;, !iillbe based.Changing design criteria are impos-ing new constraints on all ship designs .Cost ceilings, once flexible, are nowunyielding. Prevention of pollution ismandatory. More reliable performance isexpected as new missions are added whilemanning levels are reduced. The consci-entious designer can no longer rely onintuition or on the extrapolation of pastconcepts to solve the new problems. He

must actively and objectively seek outsound bases for his choices. The POLARClass design is but one example of hisdependence on the research establishmentfor the necessary answers to his inevit-able questions.ACKNOWLEDGEMENTSThe authors wish to extend their thanksto the following persons who were in-volved in the POLAR Class structuraldesign and who kindly reviewed the draft,DrovldinE valuable comments and encour-agement: - Mr. R. B. Dayton, Jr. , CDR J.W. Kime, CDR L. C. Melberg, Jr. , CAPT K.B. Schumacher, and LCDR R. A. Yuhas . Weare further indebted to MI..Kirk BuI.neP

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2.

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R. ~ , Dayton, tolar IcebreakerPreliminary Structural Design andSpecial Studies, Consultec, inc.Report 006, 1968 (AD 845-560)J. G . GeI.man, ,,esign and Construc-tion of Icebreakers, Transactions ,SNAME, Vol. 67, 1959.L. M. Nogid, tmpact of Ships withIce, (translation) , Transactions,Leningrad Shipbuilding Institute,No. 26, 1959, P. 123.R. M. White, tynamically DevelopedForce at the Bow of an Icebreaker,PhD Dissertation, MIT, 1965.D, ye. Kheysin, vDetermination ofContact Forces when a Ship 1s BowStrikes Ice, Problemy Arktiki IAntarktiki, Vol. 8, 1961, PP 67-74.M. K. Tarshis, tce Loads Acting onShips (translation) , RechnoiTransport, Vol. 16, No. 12, 1957.E. Fey, W. Pilkey, and P. Estes,,,strctural Analysis of Polar Ice-breaker Bows , Illinois Instituteof Technology Research InstituteProject J6127, 1968.L. C. Melberg Jr. , J. W. Lewis,R. Y. Edwards Jr., R. G. Taylor,and R. P. Voelker, The Design ofPolar Icebreakers, Spring Meeting,SNAME, 1970.or his review of the materials section,to M?. Booker Smalley and Mr. WilliamLaw for their illustrations, and to Mm.Donna Bremer for her patient preparationof the manuscript.

4 Num~er~ in parentheses at the end Of Ia reference refer to call numbers Inthe Defense Department Documentation 1Center, Camerok Station, Alexandria, IVirginia, 22314. 1-

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75symp03_polar Classs Ship Hull Design