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Structural Considerations in the Design of the POLAR Class of Coast Guard Icebreakers Bruce H

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Structural Considerations in the Design of the POLAR Class of Coast Guard Icebreakers Bruce H ,,.W,+, THE SOCIETYOF NAvAL ARCHITECTSAND MARINE ENGINEERS 74 TrinityPlace,New York,N.Y.,10006 # *: P,PertobeDre$entedattheShipStructureSymPosiUm 8 :c W,sh,rwton,DC.,October68,1975 010 ~+ 2 Q%,,“,,*>>+. Structural Considerations in the Design of the POLAR Class of Coast Guard Icebreakers Bruce H. Barber, Luis M. Baez and Gary J. North, Members, U.S. Coast Guard Headquarters, Washington, DC. c Cowr,8ht1975 by The SOC’etY.f NavalArc17,tects,nd Msr,!,eEnE’neers Artist’s conception of United States Ceast Guard icebreaker POW STAR (WAGB-10 ) structural failures were undertaken to ABSTRACT aesist in the selection of hull mat- erials, Various finite element analyses The design of the Coast Guard’s were conducted for portions of existing POLAR Class icebreakers incorporated and proposed structural arrangements the results of four years of research, with special emphasis on the forward analysis and testing aimed at optimizing structure. With the coperation of other the structural design. Opration and agent ies, studies of the strength of sea load data were gathered by instrumenting ice led to updated ooncepts of loading existing icebreakers. Exten8ive tests that resulted in significant changes in and studies of available steels as we 11 design phi losophy snd refinements over as a methodical analysis of reported previous icebreaker designs. c-1 — INTRODUCTION The need has been apparent since the early 19607S for a new class of ice- Polar icebreakers onerate under the breaking ships to replace aging members most inhospitable ocean ~onditions the of the fleet and to undertake more ex- world has to offer. Designed for opti– tensive duties as Coast Guard Responsi- mum performance at low temperatures in bilities change and expand. The recent Ice fields varying from u“ifomn plate extension of the search for oil into the ice to deep windrowed ridges, they also Arctic region, for instance, appears transit areas of intense heat, high likely to require a strong Coast Guard winds, and extreme sea conditions. response capability in this area. In Their hull structure sees a variety of 1966 the Icebreaker Design Project was loads ranging from those thermally in- established in the Naval Engineering duced to concentrated ice Impact loads Division at Coast Guard Headquarters to to those impossible to design for, such initiate preliminary design work on a as grounding on uncharted rock pinnacles . new polar icebreaker. Tbe initial The icebreaker designer must allow for thrust of the effort was toward a nu- extreme loads on the one hand, but must clear–powered cutter, hut this was later pay attention, like all naval architects , modified to conventional diesel–electric to detail design, with a view toward and finally revised to include a gas- eliminating structu~al failures from turbine mode of operation. During the more subtle causes such as elastic in- existence of the Icebreaker Design Pro- stability, brittle fracture, and fatigue. ject, its personnel delved into the si&- nificant aspects of ship design as it The United States Coast Guard af- applied to icebreakers Its most signi- fords icebreaker support, both domestic ficant contributions to icebreaking and polar, to a variety of ship opera- technology were probably in the areas of tions conducted in areas made hazardous hull form. Dowerin!z predictions. and or impassable by ice. Typical polar hull stru; t~re, th; iatter including icebreaker missions include escort of material selection. In eXC’?SS of 100 vessels supplying outlying military or discrete projects were completed, many scientific stations: independent logis- dependent on Coast Guard-initiated re- tics SUDDOrt to Similar OUtDOSt S . and search. Literature searches “EIT con. ice and’~cean survey operations ;nd suP- ducted to insure that existing technol- port in both polar regions. ogy was not neglected. Project members received significant support from con– COaSt Guard icebreakers engaged in tractors, other government agencies, and these operations have recentlv consisted a large number of Coast Guard personnel. of six WIND Class vessels (Fikure 1) and the GLACIER (Figure 2) Budget reduc- By the time of the dissolution of tions and old age will have soon forced the Icebreaker Design Project in 1969, a the decommissioning of all WIND vessels basic preliminary design had been devel- exceut for WESTWIND and NORTHWIND. each oped. HUI1 fopm and principal dimen- of w~ich has undergone extensive ~truc- sions had been defined, rough arrange- tural and machinery renovation to prolong ments completed, a machinery plant size its service life. and type selected, a basic structural Y Length over’all------269‘–or! Displacement to DWL--–--5,3OO long tons Length, DWL-–-------25O ,-0” Displacement , maximum–--6,5l5 long tons Beam, maximum––- –––-63, -61, Complement --------------l74 Beam, DWL––––––––---621 -07, Speed, knots, cruising––l6 Depth to main deck-- 37 T-9–l/2,! Propulsion–––––– –––––––– Die,sel-electric Draft to DWL-----–––25!–g11 SHP--------------------- 10,OOO Draft , maximum---– ––2g, -l?< Number of screws --------2 Figure 1. WIND Class Profile and Characteristics + c-2 II“q r A 4 — w? ,.-. r: y-....> ,,- ~----- ,.-~ .. Length overall------jlo t-7ts Displacement to DWL-----7,6OO long tons Length, DWL---------Z9O, -O?l Displacement, maximum---8, 449 long tons Beam, maximum-------74 9-3-1/21! Complement-------------- lg7 Beam, DWL-----------72 q-611 Speed, knots, crui,si”g--l7.6 Depth to main deck-- 38v-4qt Propulsion-------------- Diesel- electFic Draft to DWL--------25, -g,, SHP--------------------- 16,000 Draft, maximum------28* -611 NwnbeT of screws--------2 Figure 2. GLACIER Profile and Characteristics Length overall------ OT1)-OT1 Displacement, maximum--- 13, l79 long tons Length, AWL--------- 352,-O!1 Complement --------------l38 Beam, maximum-------83, -7rq Speed, knots, cruising--l7 Beam, DWL----------–78 *-Ott Propulsion--------------Die.sel-electFic or Depth to main deck--49,-3–l/21, Gas turbine Draft to DWL--------28, -011 SHP---------------------18 ,000 D-E Draft, rnaximunl------31,-9!t 60,000 G-T EIisplacernentto DWL-11,000 long tons Number of screws–-–-----3 Figure 3. POLAR Class Profile and Characteristics c-3 #--- arrangement chosen, and tentative scant- research twenty-five years of highly lings determined. In March of 1970, theoretical icebreaker and ice technol– these early beginnings were transferred OgY literature was obvious. Further, if to the existing Design Branch of the these studies “ei-e to be of any value Naval Engineer-ing Division for contract they had not only to be compared to the desizn development. Completion of the original design philosophy of the WINDS cont~act desi~n in 1971 ;as followed by and GLACIER but also comelated to O“T bid solicitation and award of a contract operational experience with these ships. to Lockheed Shipbuilding and Construction It was also necessary that the investi- Company, Seattle, to build one icebreak- gations put in proper perspective the er, the POLAR STAR (Figure 3) Delivery state of the art i“ countries such as of POLAR STAR took place during 1975. A Canada and Finland, which had been b“s- second ship, POLAR SEA, will be completed ily building icebreakers since the early in 1976. 1950’s. STRUCTURAL DESIGN The design philosophy of the WIND Class took into account only a few major Structural studies within the Ice- ship parameters such as displacement, breaker Design Project began with an shell slope at the ice belt, ship length, assessment of existing ships GLACIER and beam, by use of the formula reported and WIND Class experience. total lin’a in Volume 54, 1946 of the SNAME Transac - some 220 ship-yeaks of ophration, w~s = [11 analyzed along with that of other ice- breake~s, domestic and foreign, to iden- tify structural inadequacies and to or- .—D— D d’+”z ganize this information in a format use- ful to the designer. Accumulated exper- ience “reinvested” in subsequent designs is, of course, the essence of the clas- where P is the load per foot of water- SiC shiv desi.m Drocess. and the method line perimeter with the ship supported aPPears” tO re~ai~ its usefulness as tech- solely by ice at the waterline, D is the nology advances. displacement, m is the a“erage shell slope, and L is the waterline perimeter. The areas of structural concern The unit load P readily yields the load listed below were identified as requir- per frame which was in turn used to ing special investigation: design the ice belt plating and framing. The load per frame was envisioned as ● Load Definition quasi-concentrated at a pressu~e of 3,000 PSi and applied halfway between ● Framing System transverse frames in the desizn of the plating. The framing was the; designed ● Allowable Stresses to withstand the same load at midapan. The above design philosophy yields a ● Analysis Methods structure in which the framinK is incaD- able of “withstanding a unifor~ly distr~- ● Hull Material buted pressure of the order of the crush- ing pressure of ice and considerably ● Detail Design less than the distributed pressure that the plating can suppo?t. This “framing Although any ship design requires pressure capacity,, is less than 150 psi that these items be addressed, special for the WIND class and about 200 psi for emphasis was imposed in this case be- the GLACIER, both of which have expe~i - cause of (1) the lack of documentation enced damage in service. for many decisions reached in designing previous icebreakers, (2) obvious tech- Ice Strength nological advancements in the interven– ing years , (3) the accumulation of ex- There was therefore no question of pedience with post–war icebreakers and the need to establish valid design ice resultinz conceut realignments . and pressures based on realistic operating (4) the ~ealizaiion thai this bro,ject conditions. A thorough ice technology presented an opportunity to perform literature search indicated a wide dis- benchmark studies that could be refer- parity in the measur-ed physical proper- enced with confidence in future design ties of ice and, more significantly, work showed that most reports did not indi- cate important factors such as sample LOAD DEFINITION size and orientation, temperature, and salinity (Table I).
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